Section 5: Disorders of Intermediary Metabolism

CLASSIFICATION OF CONNECTIVE TISSUE DISORDERS

Some of the most common conditions that are transmitted genetically in families are disorders that produce clinically obvious changes in the skeleton, skin, or other relatively acellular tissues that have been loosely defined as connective tissues. Because of their heritability, some of the disorders were recognized as potentially traceable to mutated genes soon after the principles of genetics were introduced into medicine by Garrod and others. About half a century later, McKusick emphasized the specificity of many of the diseases for selective connective tissues and suggested that they were probably caused by mutations in genes coding for the major proteins found in those tissues. In the last several decades, many of the disorders have been linked to mutations in several hundred different genes expressed in connective tissues. However, classifying the disorders on the basis of either their clinical presentations or the mutations causing them is continuing to present a challenge for both the clinician and the molecular biologist.

The information on the disorders has continued to develop on two levels. The initial clinical classifications suggested by McKusick and many others had to be refined as more patients were examined. For example, some patients had skin changes similar to those commonly seen in Ehlers-Danlos syndrome (EDS), but this feature was over-shadowed by other features such as extreme hypotonia or sudden rupture of large blood vessels. To account for the full spectrum of presentations in patients and families, many of the disorders have been re-classified several times and each has been divided into a series of sub-types. The task was daunting. For example, a recent effort to classify all the heritable disorders that alter the skeleton defined 436 distinctive conditions that were divided into 42 major groups.

The identification of mutations causing the diseases has developed on a parallel track. The first genes cloned for connective tissues were the two genes coding for Type I collagen, the most abundant protein in bones, skin, tendons and several other tissues. Some of the first assays in patients with osteogenesis imperfecta (OI) revealed mutations in Type I collagen genes. And biochemical data developed primarily with cultures of skin fibroblasts from the patients demonstrated that the mutations dramatically altered the synthesis or structure of collagen fibers. The results stimulated efforts to identify additional mutations in genes coding for structural proteins. Genes for collagens provided an attractive paradigm to search for mutations, since a series of different types of collagens were found in different connective tissues and the collagen genes were readily isolated by their unique signature sequences. Also, the collagen genes were vulnerable to a large number of different mutations because of unusual structural requirements of the protein. The search for mutations in collagen genes proved fruitful in that mutations were found in most patients with OI, in many patients with hyper-extensible skin, in some patients with dwarfism, and in patients with other disorders, including some such as the Alport syndrome (AS) that were not initially classified as disorders of connective tissue. Also mutations in collagen genes were found in subset of patients with a diagnosis of osteoarthritis (OA) and a subset of patients with the diagnosis of osteoporosis. However, the search for mutations quickly expanded to hundreds of other genes that included genes for other structural proteins, for the post-translational processing of the structural proteins, for growth factors and their receptors, and other genes whose functions are still not fully understood.

In many instances, the mutations helped to define the clinical sub-type of the disorder. In some, however, it did not. Some patients with the same clinical presentations were found to have mutations in different genes. Also, some patients with different manifestations were found to have mutations in same genes. In addition, it was difficult to establish whether a change in the structure of a gene caused the phenotypic changes in the patients and was not simply a neutral polymorphism. Therefore, there has been a continuing debate as to whether the disorders should be classified by their clinical presentations or by the genes at fault. As an illustration of the problems, mutations in 324 genes have been found associated with the 436 defined disorders of the skeleton. The latest nosology for the disorders remains a “hybrid between a list of clinically defined disorders, waiting for molecular clarification, and an annotated database documenting the phenotypic spectrum produced by mutations in a given gene.” A simpler system of classification proved feasible for one rare heritable disorder of skin, epidermatolysis bullosa. The disorder was first defined by the presence of friction-induced blister. It was then divided into subtypes that were defined by the ultrastructural layers of the skin that cleaved and blistered. Most patients in each subtype were subsequently shown to have mutations in genes expressed in the corresponding layer of skin. Even with these patients, however, the strength of the genotype-phenotype correlation varies and mutations have not yet been found in every patient.

The best pathway through this maze of information is probably to begin by matching the signs and symptoms in a patient with the presentations that define each clinical classification. A major focus should be on the most common disorders, recognizing that the signs and symptoms may vary among different individuals and family members with the same diagnosis. Then, attempt to reach a decision, in consultation with the patient, parents and probably a specialist, as to whether a DNA analysis for the probable mutation is indicated. Among the considerations are the cost, the rigor with which the clinical classification has been linked to mutated genes, the reassurance the diagnosis can bring to patients and their families, the use of the information for prenatal diagnosis, and the possibility that mutation-specific therapies may be developed in the future. For patients with the most severe forms, it is probably best to consult a specialist in the disease to determine a program for therapy. Patient help groups have formed for many of the diseases and are an important source of information.

Patients with the most common forms of the disorders have mutations in a limited number of genes. This chapter will focus primarily on these. Also, it will provide a brief summary of biosynthesis and structure of connective tissues that may help guide the physician from the nature of the mutations to their clinical presentations.

COMPOSITION OF CONNECTIVE TISSUES

Connective tissues such as skin, bone, cartilage, ligaments, and tendons are the critical structural frameworks of the body. They consist of a complex interacting extracellular matrix network of collagens, proteoglycans, and a large number of non-collagenous glycoproteins and proteins. While these precise combinations of up to ~500 potential extracellular matrix building blocks provide tissue-specific function, there are many overarching similarities in composition such as the role of composite collagen fibrils in providing strength and form, elastin fibrils and proteoglycans and other interacting proteins, and glycoproteins that fine-tune function (Table 406-1). The most abundant components of many connective tissues are three similar fibrillar collagens (Types I, II, and III). They have a similar tensile strength that is comparable to that of steel wires. The three fibrillar collagens are distributed in a tissue-specific manner: Type I collagen accounts for most of the protein of dermis, ligaments, tendons, and demineralized bone; Type I and Type III are the most abundant proteins of large blood vessels; and Type II is the most abundant protein of cartilage.

BIOSYNTHESIS AND TURNOVER OF CONNECTIVE TISSUES

Connective tissues are among the most stable components in living organisms, but they are not inert. During embryonic development, connective tissue membranes appear as early as the four-cell blastocyst to provide a structural scaffold for the developing embryo. With the development of blood vessels and skeleton, there is a rapid increase in the synthesis, degradation, and resynthesis of connective tissues. The turnover continues at a slower, but still rapid pace throughout postnatal development and then spikes during the growth spurt of puberty. During adulthood, the metabolic turnover of most connective tissues is slow, but it continues at a moderate pace in bone. With age, malnutrition, physical inactivity, and low gravitational stress, the rate of degradation of most connective tissues, especially in bone and skin, begins to exceed the rate of synthesis and the tissues shrink. In starvation, a large fraction of the collagen in skin and other connective tissues is degraded and provides amino acids for gluconeogenesis (Chap. 327). In both OA and rheumatoid arthritis, there is extensive degradation of articular cartilage collagen. Glucocorticoids weaken most tissues by decreasing collagen synthesis. In many pathologic states, however, collagen is deposited in excess. With most injuries to tissues, inflammatory and immune responses stimulate the deposition of collagen fibrils in the form of fibrotic scars. In humans, as distinct from many other species, the deposition of the fibrils is largely irreversible and prevents regeneration of normal tissues in diseases such as hepatic cirrhosis, pulmonary fibrosis, atherosclerosis, and nephrosclerosis.

Structure and Biosynthesis of Fibrillar Collagens

The tensile strength of collagen fibers derives primarily from the self-assembly of protein monomers into large fibril structures in a process that resembles crystallization. The self-assembly requires monomers of highly uniform and relatively rigid structure. It also requires a complex series of posttranslational processing steps that maintain the solubility of the monomers until they are transported to the appropriate extracellular sites for fibril assembly. Because of the stringent requirements for correct self-assembly, it is not surprising that mutations in genes for fibrillar collagens cause many of the diseases of connective tissues.

The monomers of the three fibrillar collagens are formed from three polypeptide chains, called α chains, that are wrapped around each other into a rope-like triple-helical conformation. The triple helix is a unique structure among proteins, and it provides rigidity to the molecule. It also orients the side chains of amino acids in an “inside out” manner relative to most other proteins so that the charged and hydrophobic residues on the surface can direct self-assembly of the monomers into fibrils. The triple-helical conformation of the monomer is generated because each of the α chains has a repetitive amino acid sequence in which glycine (Gly) appears as every third amino acid. Each α chain contains about 1000 amino acids. Therefore, the sequence of each α chain can be designated as (-Gly-X-Y-)_n, where X and Y represent amino acids other than glycine and n is >338. The presence of glycine, the smallest amino acid, in every third position in the sequence is critical because this residue must fit into a sterically restricted space in the middle of the helix where the three chains come together. The requirement for a glycine residue at every third position explains the severe effects of mutations that convert any of the glycine residues to an amino acid with a bulkier side chain (see below). Many of the X- and Y-position amino acids are proline and hydroxyproline, which, because of their ring structures, provide additional rigidity to the triple helix. Other X- and Y-positions are occupied by charged or hydrophobic amino acids that precisely direct lateral and longitudinal assembly of the monomers into highly ordered fibrils. Mutations that substitute amino acids in some X- and Y-positions can, in rare instances, also produce genetic diseases.

The fibers formed by the three fibrillar collagens differ in thickness and length, but they have a similar fine structure. As viewed by electron microscopy, they all have a characteristic pattern of cross-striations that are about one-quarter the length of the monomers and reflect the precise packing into fibrils. The three fibrillar collagens, however, differ in sequences found in the X- and Y-positions of the α chains and therefore in some of their physical properties. Type I collagen is composed of two identical α1(I) chains and a third α2(I) chain that differs slightly in its amino acid sequence. Type II collagen is composed of three identical α(II) chains. Type III collagen is also composed of three distinctive but identical α1(III) chains.

To deliver a monomer of the correct structure to the appropriate site of fibril assembly, the biosynthesis of fibrillar collagens involves a large number of unique processing steps (Fig. 406-1). The monomer, first synthesized as a soluble precursor called procollagen, contains an additional globular domain at each end. As the pre-proα chains of procollagen are synthesized on ribosomes, the free N-terminal ends move into the cisternae of the rough endoplasmic reticulum (ER). Signal peptides at the N-termini are cleaved, and additional posttranslational reactions begin. Proline residues in the Y-position of the repeating -Gly-X-Y- sequences are converted to hydroxyproline by the enzyme prolyl hydroxylase. The hydroxylation of prolyl residues is essential for the three α chains of the monomer to fold into a triple helix at body temperature. The enzyme requires ascorbic acid as one of its essential cofactors, an observation that explains why wounds fail to heal in scurvy (Chap. 326). In scurvy, some of the underhydroxylated and unfolded protein accumulates in the cisternae of the rough ER and is degraded. Lysine residues in the Y-position are also hydroxylated to hydroxylysine by a separate lysyl hydroxylase. Many of the hydroxylysine residues are glycosylated with galactose or with galactose and glucose. A large mannose-rich oligosaccharide is assembled on the C-terminal propeptide of each chain. The proα chains are assembled by interactions among these C-terminal propeptides that control the selection of the appropriate partner chains to form hetero- or homotrimers and provide the correct chain registration required for subsequent formation of the collagen triple helix. After the C-terminal propeptides assemble the three proα chains, a nucleus of triple helix is formed near the C-terminus, and the helical conformation is propagated toward the N-terminus in a zipper-like manner that resembles crystallization. The folding into the triple helix is spontaneous in solution, but as discussed below, identification of rare mutations causing OI demonstrated that the folding in cellulo is assisted by a number of ancillary proteins which also prevent collagen fibril formation with the ER. The fully folded protein is then transported to the Golgi via a specific COPII vesicle process. After further modifications in the Golgi stack, the procollagen is secreted. After secretion, procollagen is processed to collagen by cleavage of the N-propeptides and C-propeptides by two specific proteinases. The release of the propeptides decreases the solubility of the protein about 1000-fold. The entropic energy that is released drives the self-assembly of the collagen into fibrils. Self-assembled collagen fibers have considerable tensile strength, but their strength is increased further by cross-linking reactions that form covalent bonds between α chains in one molecule and α chains in adjacent molecules. The resulting fibers, comprised of hundreds or thousands of triple-helical monomers, have some of the properties of a crystal but have innate imperfections that make them highly flexible.

Although the assembly of collagen monomers into fibers is largely a spontaneous reaction, the process in tissues is modulated by the presence of less abundant collagens (Type V with Type I, and Type XI with Type II) and by other components such as a series of small leucine-rich proteins (SLRPs). Some of the less abundant components alter the rate of fibril assembly, whereas others change the morphology of the fibers or their interactions with cells and other molecules. The presence of these other components is one explanation for why, in some tissues, the fibers are further assembled into large tendons; in others, into sheets; and in still others into complex structures such as the hexagonal array of fibers that provide both the strength and transparency of the cornea.

Collagen fibers are resistant to most proteases, but during degradation of connective tissues, they are cleaved by specific matrix metalloproteinases (collagenases) that cause partial unfolding of the triple helices into gelatin-like structures that are further degraded by less specific proteinases.

OTHER COLLAGENS AND RELATED MOLECULES

The unique properties of the triple helix are used to define a family of at least 28 collagens that contain repetitive -Gly-X-Y- sequences and form triple helices of varying length and complexity. The proteins are heterogeneous both in structure and function, and many are the sites of mutations causing genetic diseases. For example, the Type IV collagen found in basement membranes is composed of three α chains synthesized from any of six different genes. Mutations in any of the six genes can cause AS.

Fibrillin Aggregates and Elastin

In addition to tensile strength, many tissues such as the lung, large blood vessels, and ligaments require elasticity. The elasticity was originally ascribed to an amorphous rubber-like protein named elastin. Subsequent analyses, largely sparked by discoveries of mutations causing the Marfan syndrome (MFS), demonstrated that the elasticity resided in thin fibrils composed primarily of large glycoproteins named fibrillins. The fibrillins contain large numbers of epidermal growth factor–like domains interspersed with characteristic cysteine-rich domains that are also found in latent transforming growth factor β (TGF-β) binding proteins. The fibrillins assemble into long beadlike strands that also contain numerous other components including small and variable amounts of elastin, bone morphogenic proteins (BMPs), and microfibril-associated glycoproteins (MAGPs). The principles whereby the fibrils provide elasticity to tissue and their biosynthetic assembly are still under investigation. As well as contributing to extracellular matrix structure, the fibrillins play a major role in TGF-β signaling.

Proteoglycans

The resiliency to compression of connective tissues such as cartilage or the aorta is largely explained by the presence of proteoglycans. Proteoglycans are composed of a core protein to which are attached a large series of negatively charged polymers of disaccharides (largely chondroitin sulfates). At least 30 proteoglycans have been identified. They vary in their binding to collagens and other components of matrix, but specific functions have not been assigned to most. The major proteoglycan of cartilage, called aggrecan, has a core protein of 2000 amino acids that is decorated with about 100 side chains of chondroitin sulfate and keratin sulfate. The core protein, in turn, binds to long chains of the polymeric disaccharide hyaluronan to form proteoglycan aggregates, one of the largest soluble macromolecular structures in nature. Because of its highly negative charge and extended structure, the proteoglycan aggregate binds large amounts of water and small ions to distend the three-dimensional arcade of collagen fibers found in the same tissues. It thereby makes the cartilage resilient to pressure.

SPECIFIC DISORDERS

OSTEOGENESIS IMPERFECTA

The central feature of OI is a severe decrease in bone mass that makes bones brittle. The disorder is frequently associated with blue sclerae, dental abnormalities (dentinogenesis imperfecta), progressive hearing loss, and a positive family history. Most patients have mutations in one of the two genes coding for Type I collagen.

Classification

OI was originally classified into two subtypes of congenita and tarda depending on the age of onset of the symptoms. Sillence suggested a series of subtypes based on clinical, radiological findings, and mode of inheritance. As with the other disorders discussed here, the description of rare recessive forms of OI and discovery of mutations in new genes has opened a debate as to whether the disorders should be classified by the clinical phenotypes or by the genes at fault. For the moment, the classification based on the clinical presentations seems the most useful (Table 406-2).

Type I is the mildest subtype and can produce either mild or no apparent deformities of the skeleton. Most patients have distinctly blue sclerae. Type II produces bone so brittle that it is lethal in utero or shortly after birth; it can be subclassified into Types II A, B, and C, depending on radiologic findings. Of the nonlethal forms, Type III is progressively deforming with moderate to severe bone deformity and Type IV (common variable OI with normal sclerae) has mild to moderate bone fragility.

The classifications of patients by types of OI do not consistently predict the clinical course of the disease. Some patients appear normal at birth and become progressively worse; others have multiple fractures in infancy and childhood, improve after puberty, and fracture more frequently later in life. Women are particularly prone to fracture during pregnancy and after menopause. A few women from families with mild variants of OI do not develop fractures until after menopause, and their disease may be difficult to distinguish from postmenopausal osteoporosis.

Incidence

Type I OI has a frequency of about 1 in 15,000–20,000 births. Type II OI has a reported incidence of about 1 in 60,000. Only a limited number of patients with the severe forms of OI have been reported, and the combined incidence of the severe forms that are recognizable at birth (Types II, III, and IV) may be much higher than 1 in 60,000.

Skeletal Effects

In Type I OI, the fragility of bones may be severe enough to limit physical activity or be so mild that individuals are unaware of any disability. Radiographs of the skull in patients with mild disease may show a mottled appearance because of small islands of irregular ossification. In Type II OI, ossification of many bones is frequently incomplete. Continuously beaded or broken ribs and crumpled long bones (accordina femora) may be present. For reasons that are not apparent, the long bones may be either thick or thin. In Types III and IV, multiple fractures from minor physical stress can produce severe deformities. Kyphoscoliosis can impair respiration, cause cor pulmonale, and predispose to pulmonary infections. The appearance of “popcorn-like” deposits of mineral in x-rays of the ends of long bones is an ominous sign. Progressive neurologic symptoms may result from basilar compression and communicating hydrocephalus. Type V OI is recognized by the presence of dislocated radial heads and hyperplastic callus formation.

In all forms of OI, bone mineral density is decreased. However, the degree of osteopenia may be difficult to evaluate because recurrent fractures limit exercise and thereby diminish bone mass. Surprisingly, fractures appear to heal normally.

Ocular Features

The sclerae can be normal, gray, slightly bluish, or bright blue. Blue sclerae, however, are an inherited trait in some families who do not have increased bone fragility.

Dentinogenesis

The teeth may be normal, moderately discolored, or grossly abnormal. The enamel generally appears normal, but the teeth may have a characteristic amber, yellowish brown, or translucent bluish gray color because of a deficiency of dentin that is rich in Type I collagen. The deciduous teeth are usually smaller than normal, whereas permanent teeth are frequently bell-shaped and restricted at the base. In some patients, the teeth readily fracture and need to be extracted. Similar tooth defects, however, can be inherited without any evidence of OI.

Hearing Loss

Hearing loss usually begins during the second decade of life and occurs in >50% of individuals aged >30. The loss can be conductive, sensorineural, or mixed, and it varies in severity. The middle ear usually exhibits maldevelopment, deficient ossification, persistence of cartilage in areas that are normally ossified, and abnormal calcium deposits.

Other Features

Changes in other connective tissues can include thin skin that scars extensively, joint laxity with permanent dislocations indistinguishable from those of EDS, and occasionally, cardiovascular manifestations such as aortic regurgitation, floppy mitral valves, mitral incompetence, and fragility of large blood vessels. For unknown reasons, some patients develop bouts of a hypermetabolic state with elevated serum thyroxine levels, hyperthermia, and excessive sweating.

Molecular Defects

Of the ~1720 unique gene mutations now described in OI, >86% are heterozygous mutations in either COL1A1 and COL1A2, the genes coding for proα1 or proα2 chain of Type I procollagen (Table 406-2).

Most patients with Type I OI and blue sclerae have mutations that reduce the synthesis of proα1 chains to about one half. Mutations that reduce the synthesis of proα2 chains produce slightly more severe phenotypes and skin defects similar to EDS.

In contrast to the null mutations found in Type I OI, most of the severe variants (Types II, III, and IV) are caused by mutations that produce structurally abnormal proα chains that have compromised assembly or abnormal folding of the triple helix. As with collagen mutations in other connective tissue diseases, these structural mutations generally fall into two functional categories. The relatively rare mutations in the C-propeptide domain can prevent or seriously impair initial assembly of the procollagen trimers. These misfolded chains are retained in the ER and targeted for degradation by the ER-associated proteasomal pathway. Because these mutations induce an ER-stress response, the unfolded protein response (UPR) may have many downstream effects on cells. A possible role for ER stress in the pathology of OI is highlighted by the recent finding that a mutation in CREB3L1, the gene for OASIS, a member of the ATF6 family of ER stress sensors, produces severe recessive OI. ER stress is a new concept in the pathophysiology of connective tissue disease and has been best characterized for chondrodysplasias (CDs) (see below).

The most common Type I collagen mutations, however, are single base substitutions that introduce an amino acid with a bulky side chain for one of the glycine residues that appear as every third amino acid in the triple helix. In effect, any of the 338 glycine residues in the helical domain of either the proα1 or proα2 chain of Type I procollagen is a potential site for a disease-producing mutation. These mutations compromise the structural integrity of the triple helix, causing disruption to helix folding, retention of the mutant trimers in the ER, and increased posttranslational hydroxylation and glycosylation of lysines. Collagen-containing helix mutations can form insoluble aggregates in the ER that are degraded by the autophagosome-endosome system, rather than the proteasomes. The precise nature of any resulting ER stress response remains to be characterized.

A similar sequence of events occurs with less common mutations that produce partial gene deletions, partial gene duplications, and splicing mutations. In addition to their intracellular effects, the structurally abnormal mutant-containing collagen that is secreted by the cell can also have important extracellular affects. For example, the presence of one abnormal proα chain in a procollagen molecule can interfere with cleavage of the N-propeptide from the protein. The persistence of the N-propeptide on a fraction of the molecules interferes with the self-assembly of normal collagen so that thin and irregular collagen fibrils are formed. Furthermore, if structurally abnormal collagens are incorporated into fibrils, they may have a destabilizing effect and be selectively degraded, or they may alter the interactions of collagen with other connective tissue components, disturbing architecture and stability.

Several generalizations can be made about mutations in Type I collagen genes. One is that unrelated patients rarely have the same mutation in the same gene. Glycine substitutions in the N-terminal region of the triple helix tend to produce milder phenotypes, apparently because they have less effect on the zipper-like propagation of the triple-helical conformation from the C terminus. Rare substitutions of charged amino acids (Asp, Arg) or a branched amino acid (Val) in X- or Y-positions produce lethal phenotypes, apparently because they are located at sites for lateral assembly of the monomers or binding of other components of the matrix.

The search for mutations causing the less common and autosomal recessive forms of OI identified mutations in genes for a series of proteins that are essential for the timely folding of the procollagen monomer: cartilage-associated protein (CRTAP), prolyl-3-hydroxylase (LEPRE1/P3H1), cyclophilin B (PPIB), collagen chaperone-like protein HSP47 (SERPINH1), and the procollagen chaperone protein FKBP65 (FKBP10). Recently, mutations have been characterized the collagen vesicular secretion pathway (SEC24D) and in additional downstream components of the collagen fibrillogenesis pathway; BMP1, the gene coding for a metalloproteinase that cleaves the C-propeptide of Type I procollagen; SPARC (osteonectin), a collagen binding protein involved in extracellular matrix assembly; and PLOD2 (LH2, lysyl oxidase 2) which is involved in establishing collagen cross-links. In addition to these mutations that affect the collagen assembly pathway, mutations have been characterized in genes involved in the regulation of bone formation and mineralization such as SP7 (osterix), IFITM5, WNT1, and TMEM38B (Table 406-2).

Inheritance and Mosaicism in Germ-Line Cells and in Somatic Cells

Type I OI is inherited as an autosomal dominant trait. However, some patients with Type I OI appear to represent sporadic new mutations or a diagnosis that was missed in earlier generations. Most lethal OI is the result of sporadic mutations that occur in the germ line in one of the parents. Because of the possibility for germ-line mosaicism for newly generated mutations, there is about a 7% probability that a second child could inherit a severe variant of OI.

Diagnosis

OI is usually diagnosed on the basis of clinical criteria. The presence of fractures together with blue sclerae, dentinogenesis imperfecta, or family history of the disease is usually sufficient to make the diagnosis. Other causes of pathologic fractures must be excluded, including battered child syndrome, nutritional deficiencies, malignancies, and other inherited disorders such as CDs and hypophosphatasia that can have overlapping presentations. The absence of superficial bruises can be helpful in distinguishing OI from battered child syndrome. X-rays usually reveal a decrease in bone density that can be verified by photon or x-ray absorptiometry. Bone microscopy can be helpful in the diagnosis. The diagnosis, like other genetic disorders, is now routinely conducted using targeted candidate gene sequencing but whole genome sequencing is becoming increasingly common.

TREATMENT

EHLERS-DANLOS SYNDROME

EDS is characterized by hyper-extensible skin and hypermobile joints but the category includes rare patients with other distinctive features. Mutations in different types of collagen are found in many patients but other genes are at fault in rare forms. Contrary to initial expectations, no patients have been found with mutations in the gene for elastin in EDS.

Classification

Several types of EDS have been defined, based on the extent to which the skin, joints, and other tissues are involved, mode of inheritance, and by molecular and biochemical analysis (Table 406-3). Classical EDS includes a severe form of the disease (Type I) and a milder form (Type II), both characterized by joint hypermobility and skin that is velvety in texture, hyper-extensible, and easily scarred. In hypermobile EDS (Type III), joint hypermobility is more prominent than skin changes. In vascular-type EDS (Type IV), the skin changes are more prominent than joint changes, and the patients are predisposed to sudden death from rupture of large blood vessels or other hollow organs. EDS V is similar to EDS II but is inherited as an X-linked trait. The ocular-scoliotic type of EDS (Type VI) is characterized by scoliosis, ocular fragility, and a cone-shaped deformity of the cornea (keratoconus). The arthrochalasic type of EDS (Type VII A and B) is characterized by marked joint hypermobility that is difficult to distinguish from EDS III except by the specific molecular defects in the processing of Type I procollagen to collagen. The periodontotic-type EDS (Type VIII) is distinguished by prominent periodontal changes. EDS IX, X, and XI were defined on the basis of preliminary biochemical and clinical data. EDS due to tenascin X deficiency has not been assigned a type; it is an autosomal recessive form of the syndrome similar to EDS II. The cardiac valvular form of EDS has similar features to EDS II, but also involves severe changes to the aorta. The progeroid form of EDS displays features of both EDS and progeria. Because of overlapping signs and symptoms, many patients and families with some of the features of EDS cannot be assigned to any of the defined types.

Incidence

The overall incidence of EDS is about 1 in 5000 births, with a higher rate for blacks. Classical and hypermobile types of EDS are the most common. Patients with milder forms frequently do not seek medical attention.

Skin

Skin changes vary from thin and velvety to skin that is either dramatically hyperextensible (“rubber person” syndrome) or easily torn or scarred. Patients with classical EDS develop characteristic “cigarette-paper” scars. In vascular-type EDS, extensive scars and hyperpigmentation develop over bony prominences, and the skin may be so thin that subcutaneous blood vessels are visible. In the periodontotic type of EDS, the skin is more fragile than hyperextensible, and it heals with atrophic, pigmented scars. Easy bruisability occurs in several types of EDS.

Ligament and Joint Changes

Laxity and hypermobility of joints vary from mild to unreducible dislocations of hips and other large joints. In mild forms, patients learn to avoid dislocations by limiting physical activity. In more severe forms, surgical repair may be required. Some patients have progressive difficulty with age.

Other Features

Mitral valve prolapse and hernias occur, particularly with Type I. Pes planus and mild to moderate scoliosis are common. Extreme joint laxity and repeated dislocations may lead to degenerative arthritis. In the ocular-scoliotic type of EDS, the eye may rupture with minimal trauma, and kyphoscoliosis can cause respiratory impairment. Also, sclerae may be blue.

Molecular Defects

Subsets of patients with different types of EDS have mutations in the structural genes for collagens (Table 406-3). These include mutations in the COL1A1 gene in a few patients with moderately severe classical EDS (Type I); mutations in COL1A2 in rare patients with an aortic valvular form of EDS; mutations in two of the three genes (COL5A1 and COL5A2) for Type V collagen, a minor collagen found in association with Type I collagen, in about half the patients with classical EDS (Types I and II); mutations in the COL3A1 gene for Type III collagen that is abundant in the aorta in patients with the frequently lethal vascular EDS (Type IV).

Some of the Type I collagen-related mutations alter processing of the protein or genes for the processing enzymes. Arthrochalasic EDS (Type VII) is caused by mutations in the amino acid sequence that make Type I procollagen resistant to cleavage by procollagen N-proteinase or by mutations that decrease the activity of the enzyme. The persistence of the N-propeptide causes the formation of collagen fibrils that are thin and irregular. Some of the patients have fragile bones and therefore a phenotype that overlaps with OI. The ocular-scoliotic type of EDS (Type VI) is caused by homozygous or compound heterozygous mutations in the PLOD 1 gene, which encodes procollagen-lysine 5-dioxygenase (lysyl hydroxylase 1), an enzyme required for formation of stable cross-links in collagen fibers.

Some patients with the hypermobile EDS (Type III) and a few with mild EDS (Type II) have mutations in the TNXB gene, which encodes tenascin X, another minor component of connective tissue that appears to regulate the assembly of collagen fibers. Mutations in proteoglycans have been found in a few patients. The progeroid and musculocontractural forms of EDS result from mutations in the key enzymes of proteoglycan biosynthesis: B4GALT7, the gene for β-1, 4-galactosyltransferase 7, CHST14, dermatan 4-O-sulfotransferase 1 and DSE, dermatan sulfate epimerase.

Diagnosis

The diagnosis is based on clinical criteria and increasingly on DNA sequencing. Correlations between genotype and phenotype can be challenging, but gene or biochemical tests are particularly useful for the diagnosis of vascular-Type IV EDS with its dire prognosis.

As with other heritable diseases of connective tissue, there is a large degree of variability among members of the same family carrying the same mutation. Some patients have increased fractures and are difficult to distinguish from OI. A few families with heritable aortic aneurysms have mutations in the gene for Type III collagen without any evidence of EDS or OI.

TREATMENT

CHONDRODYSPLASIAS

CDs, also referred to as skeletal dysplasias, are heritable skeletal disorders that are characterized by dwarfism and abnormal body proportions (See also Chap. 405). The category also includes some individuals with normal stature and body proportions who have features such as ocular changes or cleft palate, which are common in more severe CDs. Many patients develop degenerative joint changes; and mild CD in adults may be difficult to differentiate from primary generalized OA. An undefined number of patients have mutations in either the most abundant collagen in cartilage (Type II) or the less abundant collagens (Types X or XI). Other patients have mutations in genes that code for other components of cartilage or for proteins required for the embryonic development of cartilage, including a common mutation in a gene for a fibroblast growth factor receptor.

Classification

Over 200 distinct types and subtypes have been defined based on criteria such as “bringing death” (thanatophoric), causing “twisted” bones (diastrophic), affecting metaphyses (metaphyseal), affecting epiphyses (epiphyseal), affecting spine (spondylo-), and producing histologic changes such as an apparent increase in the fibrous material in the epiphyses (fibrochondrogenesis). Also, a number of eponyms are based on the first or most comprehensive case reports. Severe forms of the diseases produce dwarfism with gross distortions of most cartilaginous structures and of other structures including the eye. Mild forms are more difficult to classify. Among the features are cataracts, degeneration of the vitreous, and retinal detachment, high forehead, hypoplastic facies, cleft palate, short extremities and gross distortions of the epiphyses, metaphyses, and joint surfaces. Patients with Stickler syndrome (hereditary arthro-ophthalmopathy) have been classified into three types based on a combination of the ocular phenotype and mutated genes.

Incidence

The overall incidence of all forms of CD ranges from 1 per 2500 to 1 per 4000 births. Data on the frequency of individual CDs are incomplete, but the incidence of the Stickler syndrome is 1 in 10,000. Therefore, the disease is probably among the more common heritable disorders of connective tissue.

Molecular Defects

Mutations in the COL2A1 gene for the Type II collagen of cartilage are found in a fraction of patients with both mild and severe CDs. For example, a mutation in the gene substituting a cysteine residue for an arginine was found in three unrelated families with spondyloepiphyseal dysplasia (SED) and precocious generalized OA. Mutations in the gene were also found in some lethal CDs characterized by gross deformities of bones and cartilage, such as those found in SED congenita, spondyloepimetaphyseal dysplasia congenita, hypochondrogenesis/achondrogenesis Type II, and Kniest syndrome. The highest incidence of COL2A1 mutations, however, occurs in patients with the distinctive features of the Stickler syndrome, which is characterized by skeletal changes, orofacial abnormalities, and auditory abnormalities. Most of the mutations in COL2A1 are premature stop codons that produce haploinsufficiency. In addition, some of the patients with the Stickler syndrome or a closely related syndrome have mutations in two genes specific for Type XI collagen, which is an unusual heterotrimer formed from α chains encoded by the gene for Type II collagen (COL2A1) and two distinctive genes for Type XI collagen (COL11A1 and COL11A2). Mutations in the COL11A1 gene are also found in patients with Marshall syndrome, which is similar to classic Stickler syndrome, but with more severe hearing loss and dysmorphic features, such as a flat or retracted midface with a flat nasal bridge, short nose, anteverted nostrils, long philtrum, and large-appearing eyes.

CDs are also caused by mutations in the less abundant collagens found in cartilage. For example, patients with Schmid metaphyseal CD have mutations in the gene for Type X collagen, a short, network-forming collagen found in the hypertrophic zone of endochondral cartilage. The syndrome is characterized by short stature, coxa vara, flaring metaphyses, and waddling gait. As with other collagen genes, the most common mutations are of two types: Nonsense mutations that lead to haploinsufficiency and structural mutations that compromise collagen assembly. In Type X collagen all the structural mutations detected occur in the C-terminal NC1 domain that coordinates the formation of the trimers. This NC1 domain is functionally equivalent to the C-propeptide of the fibrillar collagens. These mutations disturb the structure of the NC1 domain, leading to misfolding and initiation of cellular ER stress via the UPR. While the UPR evolved to allow cells to adjust their ER folding capacity to differing protein folding loads, it is deployed by cells when mutant misfolded proteins accumulate in the ER. Activation of the UPR attenuates protein translation and activates mutant protein degradation pathways such as ER-associated degradation. If these strategies do not sufficiently reduce the stress response, cell death may occur. In Schmid metaphyseal CD, mutant misfolded Type X collagen induces the UPR, resulting in downstream consequences that contribute to the pathophysiology. This general mechanism may also contribute to pathology in other chondrodyplasias (and in other connective tissues disorders) where gene mutations lead to protein structural abnormalities.

Some patients have mutations in genes for proteins that interact with collagens. Patients with pseudoachondroplasia or autosomal dominant multiple epiphyseal dysplasia have mutations in the gene for the cartilage oligomeric matrix protein (COMP), a protein that interacts with both collagens and proteoglycans in cartilage. However, some families with multiple epiphyseal dysplasia have a defect in one of the three genes for Type IX collagen (COL9A1, COL9A2, and COL9A3) or in matrilin-3, another extracellular protein found in cartilage. With misfolding mutations in COMP and matrilin-3, the activation of the UPR has been described, providing further evidence that the UPR is a component of pathology of these conditions.

Some CDs are caused by mutations in genes that affect early development of cartilage and related structures. The most common form of short-limbed dwarfism, achondroplasia, is caused by mutations in the gene for a receptor for a fibroblastic growth factor (FGFR3). The mutations in the FGFR3 gene causing achondroplasias are unusual in several respects. The same single-base mutation in the gene that converts glycine to arginine at position 380 in the FGFR3 gene is present in >90% of patients. Most patients harbor sporadic new mutations, and therefore this nucleotide change must be one of the most common recurring mutations in the human genome. The mutation causes unregulated signal transduction through the receptor and inappropriate development of cartilage. Mutations that alter other domains of FGFR3 have been found in patients with the more severe disorders of hypochondroplasia and thanatophoric dysplasia and in a few families with a variant of craniosynostosis. However, most patients with craniosynostosis appear to have mutations in the related FGFR2 gene. The similarities between the phenotypes produced by mutations in genes for FGF receptors and mutations in structural proteins of cartilage are probably explained by the observation that the activity of FGFs is regulated in part by binding of FGFs to proteins sequestered in the extracellular matrix. Therefore, the situation parallels the interactions between transforming growth factors (TGFs) and fibrillin in MFS (see below).

Other mutations involve the proteoglycans of cartilage, aggrecan (AGC1) and perlecan (HSPG2), and in the proteoglycan posttranslational sulphation pathway (DTDST, PAPSS2 and CHST3). Mutations in >45 other genes have been defined in CDs.

Diagnosis

The diagnosis of CDs is made on the basis of the physical appearance, slit-lamp eye examinations, x-ray findings, histologic changes, and clinical course. Targeted gene and exome sequencing or more global sequencing strategies are used for molecular diagnosis. Given the wide spectrum of CD phenotypes, these genes tests are becoming critical diagnostic tools. For Stickler syndrome, more precise diagnostic criteria have made it possible to identify Type I variants with mutations in the COL2A1 gene with a high degree of accuracy. It has been suggested that the Type II variant with mutations in the COL11A1 gene can be identified on the basis of a “beaded” vitreous phenotype, and the Type III variant with mutations in the COL11A2 gene can be identified on the basis of the characteristic systemic features without the ocular involvement. Prenatal diagnosis based on analysis of DNA obtained from chorionic villus or amniotic fluid is possible.

TREATMENT

MARFAN SYNDROME (MFS)

MFS includes features that primarily affect the skeleton, the cardiovascular system, and the eyes. Most patients have mutations in the gene for fibrillin-1 (FBN1).

Classification

MFS was initially characterized by a triad of features: (1) skeletal changes that include long, thin extremities, frequently associated with loose joints; (2) reduced vision as the result of dislocations of the lenses (ectopia lentis); and (3) aortic aneurysms. An international panel has developed a series of revised “Ghent criteria” that are useful in classifying patients.

Incidence and Inheritance

The incidence of MFS is among the highest of any heritable disorder: about 1 in 3000/5000 births in most racial and ethnic groups. The related syndromes are less common. Mutations are generally inherited as autosomal dominant traits, but about one-fourth of patients have sporadic new mutations.

Skeletal Effects

Patients have long limbs and are usually tall compared to other members of the same family. The ratio of the upper segment (top of the head to the top of the pubic ramus) to the lower segment (top of the pubic ramus to the floor) is usually 2 SDs below mean for age, race, and sex. The fingers and hands are long and slender and have a spider-like appearance (arachnodactyly). Many patients have severe chest deformities, including depression (pectus excavatum), protrusion (pectus carinatum), or asymmetry. Scoliosis is frequent and usually accompanied by kyphosis. High-arched palate and high pedal arches or pes cavus are common. A few patients have severe joint hypermobility similar to EDS. CT or MRI examinations of the lumbar sacral region frequently reveals enlargement of the neural canal, thinning of the pedicles and laminae, widening of the foramins, or anterior meningocele (dural ectasia).

Cardiovascular Features

Cardiovascular abnormalities are the major source of morbidity and mortality (Chap. 274). Mitral valve prolapse develops early in life and progresses to mitral valve regurgitation of increasing severity in about one-quarter of patients. Dilation of the root of the aorta and the sinuses of Valsalva are characteristic and ominous features of the disease that can develop at any age. The rate of dilation is unpredictable, but it can lead to aortic regurgitation, dissection of the aorta, and rupture. Dilation is probably accelerated by physical and emotional stress, as well as by pregnancy. Patients usually differ from patients with familial aortic aneurysms who tend to develop aneurysms in the abdominal aorta. The location of the aneurysms, however, is somewhat variable, and the high incidence of aortic aneurysms in the general population (1 in 100) makes the differential diagnosis difficult unless other features of MFS are clearly present.

Ocular Features

Upward displacement of the lens is common. It is usually not progressive but may contribute to the formation of cataracts. The ocular globe is frequently elongated, and most patients are myopic, but with adequate vision. Retinal detachment can occur.

Other Features

Striae may occur over the shoulders and buttocks. A number of patients develop spontaneous pneumothorax. Inguinal and incisional hernias are common. Patients are typically thin with little subcutaneous fat, but adults may develop centripetal obesity.

Molecular Defects

More than 90% of patients clinically classified as having MFS by the “Ghent criteria” have a mutation in the gene for FBN1. Mutations in the same gene are found in a few patients who do not meet the Ghent criteria. Also, a few MFS patients without mutations in the FBN1 gene have mutations in the gene for TGF-β receptor 2 (TGFBR2). In addition, mutations in either TGFBR2 or TGFBR1 are found in the related Loeys-Dietz syndrome which is characterized by aortic aneurysms, cleft palate, and hypertelorism. Mutations in the FBN2 gene, which is structurally similar to the FBN1 gene, are found in patients with MFS-like syndrome of congenital contractual arachnodactyly.

FBN1 gene mutations are scattered throughout its 65 coding exons. Most are private mutations, but ~10% are recurrent new mutations that are largely located in CpG sequences known to be “hot spots.” Most severe mutations are located in the central codons (24–32). About one-third of the mutations introduce premature termination codons, and about two-thirds are missense mutations that alter calcium-binding domains in the repetitive epidermal growth factor–like domains of the protein. Rarer mutations alter the processing of the protein. As in many genetic diseases, the severity of the phenotype cannot be predicted from the nature of the mutation.

The discovery that syndromes similar to MFS are caused by mutations in TGFBR1 and TGFBR2 refocused attention on structural similarity between FBN1 and TGF-β binding proteins that sequester TGF-β in the extracellular matrix. As a result, some of the manifestations of MFS have been shown to arise from alterations in binding sites that modulate TGF-β bioavailability during development of the skeleton and other tissues. Likewise, TGFBR1 and TGFBR2 mutations in Loeys-Dietz syndrome alter TGF-β signaling. In both MFS and Loey-Dietz syndrome, the pathogenic mechanisms involve increased TGF-β signaling which contributes to aneurysm formation.

Diagnosis

All patients with a suspected diagnosis of MFS should have a slit-lamp examination and an echocardiogram. Also, homocystinuria should be ruled out by amino acid analysis of plasma (Chap. 413). The diagnosis of MFS according to the international Ghent standards places emphasis on major criteria that include presence of at least four skeletal abnormalities: ectopia lentis; dilation of the ascending aorta with or without dissection; dural ectasia; and a blood relative who meets the same criteria, with or without a DNA diagnosis. A final diagnosis is based on a balanced assessment of the major criteria together with several minor criteria. The absence of ocular changes suggests the Loeys-Dietz syndrome, and the presence of contractures with some of the signs of OI suggests congential contractual arachnodactyly.

Diagnostic tests based on gene sequencing or detection of protein defects are available. These results are unlikely to alter the treatment or prognosis, but are helpful to inform the patients and families and to rapidly exclude the diagnosis in unaffected family members.

TREATMENT

ELASTIN-RELATED DISEASES

Mutations in the elastin gene (ELN) have been found in patients with supravalvular aortic stenosis and skin that hangs in loose and redundant folds (cutis laxa). As indicated in Table 406-3, patients with several forms of EDS have similar changes in skin that were initially thought to reflect changes in elastin.

EPIDERMOLYSIS BULLOSA (EB)

EB has been defined as the category of heritable disorders involving skin that is specifically characterized by blistering as a result of friction. Using this criterion, it was possible to define subtypes by the ultrastructural layer of skin in which the cleavage and blistering occurred. These functional and anatomical criteria made it possible to establish that most patients with a specific subtype have mutations in genes coding for a structural protein or a cell adherence protein expressed in the corresponding layer of skin.

Classification and Incidence

The four major types of EB are: (1) EB simplex in which cleavage occurs within the epidermis, (2) junctional EB in which cleavage occurs within the lamina lucida, (3) dystrophic EB in which cleavage occurs within the sub-lamina densa, and (4) Kindler syndrome with a mixed level of cleavage in different layers. Patients are then separated into major and minor subtypes based on clinical features and analysis of mutations.

The incidence of EB in the United States is about 1 in 50,000.

Molecular Defects

The distinctive anatomic locations in skin have made it possible to relate the clinical subtypes of EB to mutations for specific components. In EB simplex, mutations are found primarily in the genes for the major keratins of basal epithelial cells (keratins 5 and 14), and the cell adhesion proteins plectin, plakophilin-1, desmoplakin, and dystonin. Mutations in exophilin-5 and transglutaminase 5, both of which impact keratin filaments, have been reported. Patients with the related syndrome, epidermolytic ichthyosis, have mutations in keratin 1 and keratin 10. In junctional EB, mutations occur in Type XVII collagen, a laminin (laminin-332) and α6β4 integrin. In the severe syndrome of dystrophic EB, mutations are found in the gene that codes for Type VII collagen that forms long loops anchoring the epidermis to the dermis. Patients with more complex features of what is classified as the Kindler syndrome have mutations in Kindlin-1, a. focal adhesion protein involved in integrin activation.

Diagnosis and Treatment

The diagnosis is based on skin that readily breaks and forms blisters from minor trauma. EB simplex is generally milder than junctional EB or dystrophic EB. Dystrophic EB variants usually have large and prominent scars. Precise classification within subtypes usually requires immunofluorescent mapping. DNA diagnostic tests have been developed as research tools but are not readily available. The treatment is symptomatic. Novel therapeutic approaches such as gene therapy, protein replacement therapy and cell therapy are being explored.

ALPORT SYNDROME

AS is an inherited disorder characterized by hematuria and several associated features. It was not initially considered as a disorder of connective tissue. However, the search for mutations in the genes coding for collagens found that most patients had mutations in collagen found in basement membranes (Type IV). Four forms of the AS are now recognized: (1) classic AS, which is inherited as an X-linked disorder with hematuria, sensorineural deafness, and conical deformation of the anterior surface of the lens (lenticonus); (2) an X-linked form associated with diffuse leiomyomatosis; (3) an autosomal recessive form; and (4) an autosomal dominant form. Both autosomal recessive and dominant forms can cause renal disease without deafness or lenticonus.

Incidence

The incidence of AS is about 1 in 10,000 births in the general population and as high as 1 in 5000 in some ethnic groups. About 80% of AS patients have the classical X-linked variant.

Molecular Defects

Most patients have mutations in four of the six genes for the chains of Type IV collagen (COL4A3, COL4A4, COL4A5, and COL4A6). The genes for the proteins are arranged in tandem pairs on different chromosomes in an unusual head-to-head orientation and with overlapping promoters; i.e., the COL4A1 and COL4A2 genes are head-to-head on chromosome 13q34, the COL4A3 and COL4A4 genes are on chromosome 2q35–37, and the COL4A5 and COL4A6 genes are on chromosome Xq22. The X-linked variants are caused by either mutations in the COL4A5 gene or by partial deletions of both of the adjacent COL4A4 and COL4A5 genes. The autosomal recessive variants are caused by mutations in either the COL4A3 or COL4A4 genes. The mutations responsible for the autosomal dominant variants are still unknown, but they have been mapped to the same locus as the COL4A3 and COL4A4 genes.

Diagnosis and Treatment

The diagnosis of classic AS is based on X-linked inheritance of hematuria, sensorineural deafness, and lenticonus. The lenticonus together with hematuria is pathognomonic of classic AS. The sensorineural deafness is primarily in the high-tone range. It can frequently be detected only by an audiogram and is usually not progressive. Because of the X-linked transmission, women are generally underdiagnosed and are usually less severely affected than men. The hematuria usually progresses to nephritis and may cause renal failure in late adolescence in affected males and at older ages in some women. Renal transplantation is usually successful.

FURTHER READING

DEFINITION

Hemochromatosis results from a relatively common inherited genetic mutation in European populations. Once thought to be a single disease entity it is now known to be an iron-storage disorder with genetic heterogeneity but with a final common metabolic pathway resulting in inappropriately low production of the hormone hepcidin. This leads to an increase in intestinal iron absorption and the deposition of excessive amounts of iron in parenchymal cells with eventual tissue damage and organ failure. Thus, the term hemochromatosis now refers to a group of genetic diseases that predispose to iron overload, potentially leading to fibrosis and organ failure. Cirrhosis of the liver, diabetes mellitus, arthritis, cardiomyopathy, and hypogonadotropic hypogonadism are the major clinical manifestations.

The following terminology is widely accepted.

1. Hereditary hemochromatosis is most often caused by a mutant gene, termed HFE, which is tightly linked to the HLA-A locus on chromosome 6p. Persons who are homozygous for the mutation are at increased risk of iron overload and account for 80–90% of clinical hereditary hemochromatosis in persons of northern European descent. In such subjects, the presence of hepatic fibrosis, cirrhosis, arthropathy, or hepatocellular carcinoma constitutes iron overload–related disease. Rarer forms of non-HFE hemochromatosis are caused by mutations in other genes involved in iron metabolism (Table 407-1). The disease can be recognized during its early stages when iron overload and organ damage are minimal. At this stage, the disease is best referred to as early hemochromatosis or precirrhotic hemochromatosis.

2. Secondary iron overload occurs as a result of an iron-loading anemia, such as thalassemia or sideroblastic anemia, in which erythropoiesis is increased but ineffective. In the acquired iron-loading disorders, massive iron deposits in parenchymal tissues can lead to the same clinical and pathologic features as in hemochromatosis.

PREVALENCE

Although HFE-associated hemochromatosis mutations are common, the prevalence varies in different ethnic groups. It is most common in populations of northern European extraction in whom ~1 in 10 persons are heterozygous carriers and 0.3–0.5% are homozygotes. However, expression of the disease is variable and modified by several factors, especially alcohol consumption, dietary iron intake, blood loss associated with menstruation and pregnancy, and blood donation. Recent population studies indicate that ~30% of homozygous men develop iron overload–related disease and about 6% develop hepatic cirrhosis; for women, the figure is closer to 1%. Presumably there are as yet unidentified modifying genes responsible for expression. Nearly 70% of untreated patients develop the first symptoms between ages 40 and 60. The disease is rarely evident before age 20, although with family screening (see “Screening for Hemochromatosis,” below) and periodic health examinations, asymptomatic subjects with iron overload can be identified, including young menstruating women.

In contrast to HFE-associated hemochromatosis, the non-HFE-associated forms of hemochromatosis (Table 407-1) are rare, but they affect all races and young people (juvenile hemochromatosis).

These result from mutations in one or more of the genes for proteins in the hepcidin pathway (Fig. 407-1), i.e., hemojuvelin, transferrin receptor 2 (TfR2), or ferroportin. The resultant clinical disease is very similar to HFE-related disease because they all lead to hepcidin deficiency, which is the final common pathway (Fig. 407-1).

A rare autosomal dominant form of hemochromatosis results from two types of mutations in the gene for the iron transporter ferroportin. Firstly, loss of function mutations decrease the cell surface localization of ferroportin, thereby reducing its ability to export iron (“ferroportin disease”). A second mutation abolishes the hepcidin-induced ferroportin internalization and degradation resulting in a “gain-of-function.” Here the tissue iron distribution is similar to that in HFE-related disease (e.g., in parenchymal cells).

GENETIC BASIS

A homozygous G to A mutation in the HFE gene resulting in a cysteine to tyrosine substitution at position 282 (C282Y) is the most common mutation. It is identified in 85–90% of patients with hereditary hemochromatosis in populations of northern European descent but is found in only 60% of cases from Mediterranean populations. A second, relatively common HFE mutation (H63D) results in a substitution of histidine to aspartic acid at codon 63. Homozygosity for H63D is not associated with clinically significant iron overload. Some compound heterozygotes (e.g., one copy each of C282Y and H63D) have mild to moderately increased body-iron stores but develop clinical disease only in association with cofactors such as heavy alcohol intake or hepatic steatosis. Thus, HFE-associated hemochromatosis is inherited as an autosomal recessive trait; heterozygotes have no, or minimal, increase in iron stores. However, this slight increase in hepatic iron can act as a cofactor that may modify the expression of other diseases such as porphyria cutanea tarda (PCT) or nonalcoholic steatohepatitis (NASH).

Thus, mutations in other genes involved in iron metabolism are responsible for non-HFE-associated hemochromatosis, including juvenile hemochromatosis, which affects persons in the second and third decades of life (Table 407-1). Mutations in the genes encoding hepcidin, transferrin receptor 2 (TfR2), and hemojuvelin (Fig. 407-1) result in clinicopathologic features that are indistinguishable from HFE-associated hemochromatosis. However, mutations in ferroportin, responsible for the efflux of iron from enterocytes and most other cell types, result in iron loading of reticuloendothelial cells and macrophages as well as parenchymal cells.

PATHOPHYSIOLOGY AND THE ROLE OF HEPCIDIN

Normally, the body-iron content of 3–4 g is maintained such that intestinal mucosal absorption of iron is equal to iron loss. This amount is ~1 mg/d in men and 1.5 mg/d in menstruating women. In hemochromatosis, mucosal absorption is greater than body requirements and amounts to ≥4 mg/d. The progressive accumulation of iron increases plasma iron and saturation of transferrin and results in a progressive increase of plasma ferritin (Fig. 407-2). The key regulatory hormone that allows the liver to communicate with the bone marrow was discovered quite serendipitously and has transformed our understanding of the coordination of absorption, mobilization, and storage of iron to meet the requirements of erythropoiesis. It was called hepcidin based upon its anti-bacterial activity (“HEPatic bacterioCIDal proteIN”). This liver-derived peptide represses basolateral iron transport in the intestine and iron release from macrophages and other cells by binding to ferroportin. Hepcidin, in turn, responds to signals in the liver mediated by HFE, TfR2, and hemojuvelin (Fig. 407-1). The development of minihepcidins, i.e., small peptides that mimic the action of hepcidin, is promising for the development of new therapeutic approaches for iron overload disorders caused by low hepcidin levels.

The HFE gene encodes a 343-amino-acid protein that is structurally related to MHC class I proteins (HFE). The basic defect in HFE-associated hemochromatosis is a lack of cell surface expression of HFE (due to the C282Y mutation). The normal (wild-type) HFE protein forms a complex with β₂-microglobulin and transferrin receptor 1 (TfR1). The C282Y mutation completely abrogates this interaction. As a result, the mutant HFE protein remains trapped intracellularly, reducing TfR1-mediated iron uptake by the intestinal crypt cell. This impaired TfR1-mediated iron uptake leads to upregulation of the divalent metal transporter (DMT1) on the brush border of the villus cells, causing inappropriately increased intestinal iron absorption (Fig. 407-1). In advanced disease, the body may contain 20 g or more of iron that is deposited mainly in parenchymal cells of the liver, pancreas, and heart. Iron deposition in the pituitary causes hypogonadotropic hypogonadism in both men and women. Tissue injury may result from disruption of iron-laden lysosomes, from lipid peroxidation of subcellular organelles by excess iron, or from stimulation of collagen synthesis by activated stellate cells.

Secondary iron overload with deposition in parenchymal cells occurs in chronic disorders of erythropoiesis, particularly in those due to defects in hemoglobin synthesis or ineffective erythropoiesis such as sideroblastic anemia and thalassemia (Chap. 94). In these disorders, iron absorption is increased. Moreover, these patients require blood transfusions and are frequently treated inappropriately with iron. PCT, a disorder characterized by a defect in porphyrin biosynthesis (Chap. 409), can also be associated with excessive parenchymal iron deposits. The magnitude of the iron load in PCT is usually insufficient to produce tissue damage. However, some patients with PCT also have mutations in the HFE gene, and some have associated hepatitis C virus (HCV) infection. Although the relationship between these disorders remains to be clarified, iron overload accentuates the inherited enzyme deficiency in PCT and should be avoided along with other agents (alcohol, estrogens, haloaromatic compounds) that may exacerbate PCT. Another cause of hepatic parenchymal iron overload is hereditary aceruloplasminemia. In this disorder, impairment of iron mobilization due to deficiency of ceruloplasmin (a ferroxidase) causes iron overload in hepatocytes.

Excessive iron ingestion over many years rarely results in hemochromatosis. An important exception has been reported in South Africa among groups who brew fermented beverages in vessels made of iron (see later). Hemochromatosis has been described in apparently normal persons who have taken medicinal iron over many years, but such individuals probably had genetic disorders.

The common denominator in all patients with hemochromatosis is excessive amounts of iron in parenchymal tissues. Parenteral administration of iron in the form of blood transfusions or iron preparations results predominantly in reticuloendothelial cell iron overload. This appears to lead to less tissue damage than iron loading of parenchymal cells.

In the liver, parenchymal iron is in the form of ferritin and hemosiderin. In the early stages, these deposits are seen in the periportal parenchymal cells, especially within lysosomes in the pericanalicular cytoplasm of the hepatocytes. This stage progresses to perilobular fibrosis and to fibrous septa due to activation of stellate cells. In the advanced stage, a macronodular or mixed macro- and micronodular cirrhosis develops. Hepatic fibrosis and cirrhosis correlate significantly with hepatic iron concentration.

Histologically, iron is increased in many organs, particularly in the liver, heart, and pancreas, and, to a lesser extent, in the endocrine glands. The epidermis of the skin is thin, and melanin is increased in the cells of the basal layer and dermis. Deposits of iron are present around the synovial lining cells of the joints.

CLINICAL MANIFESTATIONS

C282Y homozygotes can be characterized by the stage of progression as follows: (1) a genetic predisposition without abnormalities; (2) iron overload without symptoms; (3) iron overload with symptoms (e.g., arthritis and fatigue); and (4) iron overload with organ damage—in particular, cirrhosis. Thus, many subjects with significant iron overload are asymptomatic. For example, in a study of 672 asymptomatic C282Y homozygous subjects—identified by either family screening or routine health examinations—there was hepatic iron overload (grades 2–4) in 56% and 34.5% of male and female subjects, respectively; hepatic fibrosis (stages 2–4) in 18.4% and 5.4%, respectively; and cirrhosis in 5.6% and 1.9%, respectively.

Initial symptoms are often nonspecific and include lethargy, arthralgia, skin pigmentation, loss of libido, and features of diabetes mellitus. Hepatomegaly, increased pigmentation, spider angiomas, splenomegaly, arthropathy, ascites, cardiac arrhythmias, congestive heart failure, loss of body hair, testicular atrophy, and jaundice are prominent in advanced disease.

The liver is usually the first organ to be affected, and hepatomegaly is present in >95% of symptomatic patients.

Manifestations of portal hypertension and esophageal varices occur less commonly than in cirrhosis from other causes. Hepatocellular carcinoma develops in ~30% of patients with cirrhosis, and it is the most common cause of death in treated patients—hence the importance of early diagnosis and therapy. The incidence increases with age, it is more common in men, and it occurs almost exclusively in cirrhotic patients.

Excessive skin pigmentation is present in patients with advanced disease. The characteristic metallic or slate-gray hue is sometimes referred to as bronzing and results from increased melanin and iron in the dermis. Pigmentation usually is diffuse and generalized.

Diabetes mellitus occurs in ~65% of patients with advanced disease and is more likely to develop in those with a family history of diabetes, suggesting that direct damage to the pancreatic islets by iron deposition occurs in combination with other risk factors. The management is similar to that of other forms of diabetes.

Arthropathy develops in 25–50% of symptomatic patients. It usually occurs after age 50 but may occur as a first manifestation or long after therapy. The joints of the hands, especially the second and third metacarpophalangeal joints, are usually the first joints involved, a feature that helps to distinguish the chondrocalcinosis associated with hemochromatosis from the idiopathic form (Chap. 365). A progressive polyarthritis involving wrists, hips, ankles, and knees may also ensue. Acute brief attacks of synovitis may be associated with deposition of calcium pyrophosphate (chondrocalcinosis or pseudogout), mainly in the knees. Radiologic manifestations include cystic changes of the subchondral bones, loss of articular cartilage with narrowing of the joint space, diffuse demineralization, hypertrophic bone proliferation, and calcification of the synovium. The arthropathy tends to progress despite removal of iron by phlebotomy. Although the relation of these abnormalities to iron metabolism is not known, the fact that similar changes occur in other forms of iron overload suggests that iron is directly involved.

Cardiac involvement is the presenting manifestation in ~15% of symptomatic patients. The most common manifestation is congestive heart failure, which occurs in ~10% of young adults with the disease, especially those with juvenile hemochromatosis. Symptoms of congestive heart failure may develop suddenly, with rapid progression to death if untreated. The heart is diffusely enlarged; this may be misdiagnosed as idiopathic cardiomyopathy if other overt manifestations are absent. Cardiac arrhythmias include premature supraventricular beats, paroxysmal tachyarrhythmias, atrial flutter, atrial fibrillation, and varying degrees of atrioventricular block.

Hypogonadism occurs in both sexes and may antedate other clinical features. Manifestations include loss of libido, impotence, amenorrhea, testicular atrophy, gynecomastia, and sparse body hair. These changes are primarily the result of decreased production of gonadotropins due to impairment of hypothalamic-pituitary function by iron deposition.

DIAGNOSIS

The association of (1) hepatomegaly, (2) skin pigmentation, (3) diabetes mellitus, (4) heart disease, (5) arthritis, and (6) hypogonadism should suggest the diagnosis. However, as stated above, significant iron overload may exist with none or only some of these manifestations. Therefore, a high index of suspicion is needed to make the diagnosis early. Treatment before permanent organ damage occurs can reverse the iron toxicity and restore life expectancy to normal.

The history should be particularly detailed in regard to disease in other family members; alcohol ingestion; iron intake; and ingestion of large doses of ascorbic acid, which promotes iron absorption (Chap. 326). Appropriate tests should be performed to exclude iron deposition due to hematologic disease. The presence of liver, pancreatic, cardiac, and joint disease should be confirmed by physical examination, radiography, and standard function tests of these organs.

The degree of increase in total body iron stores can be assessed by (1) measurement of serum iron and the percent saturation of transferrin (or the unsaturated iron-binding capacity), (2) measurement of serum ferritin concentration, (3) liver biopsy with measurement of the iron concentration and calculation of the hepatic iron index (Table 407-2), and (4) magnetic resonance imaging (MRI) of the liver. In addition, a retrospective assessment of body-iron storage is also provided by performing weekly phlebotomy and calculating the amount of iron removed before iron stores are exhausted (1 mL blood = ~0.5 mg iron).

Each of these methods for assessing iron stores has advantages and limitations. The serum iron level and percent saturation of transferrin are elevated early in the course, but their specificity is reduced by significant false-positive and false-negative rates. For example, serum iron concentration may be increased in patients with alcoholic liver disease without iron overload; in this situation, however, the hepatic iron index is usually not increased as in hemochromatosis (Table 407-1). In otherwise healthy persons, a fasting serum transferrin saturation >45% is abnormal and suggests homozygosity for hemochromatosis.

The serum ferritin concentration is usually a good index of body-iron stores, whether decreased or increased. In fact, an increase of 1 μg/L in serum ferritin level reflects an increase of ~5 mg in body stores. In most untreated patients with hemochromatosis, the serum ferritin level is significantly increased (Fig. 407-2 and Table 407-1), and a serum ferritin level >1000 μg/L is the strongest predictor of disease expression among individuals homozygous for the C282Y mutation. However, in patients with inflammation and hepatocellular necrosis, serum ferritin levels may be elevated out of proportion to body-iron stores due to increased release from tissues. Therefore, a repeat determination of serum ferritin should be carried out after acute hepatocellular damage has subsided (e.g., in alcoholic liver disease). Ordinarily, the combined measurements of the percent transferrin saturation and serum ferritin level provide a simple and reliable screening test for hemochromatosis, including the precirrhotic phase of the disease. If either of these tests is abnormal, genetic testing for hemochromatosis should be performed (Fig. 407-3).

The role of liver biopsy in the diagnosis and management of hemochromatosis has been reassessed as a result of the widespread availability of genetic testing for the C282Y mutation. The absence of severe fibrosis can be accurately predicted in most patients using clinical and biochemical variables. Thus, there is virtually no risk of severe fibrosis in a C282Y homozygous subject with (1) serum ferritin level <1000 μg/L, (2) normal serum alanine aminotransferase values, (3) no hepatomegaly, and (4) no excess alcohol intake. However, it should be emphasized that liver biopsy is the only reliable method for establishing or excluding the presence of hepatic cirrhosis, which is the critical factor determining prognosis and the risk of developing hepatocellular carcinoma. Biopsy also permits histochemical estimation of tissue iron and measurement of hepatic iron concentration. Increased density of the liver due to iron deposition can be demonstrated by computed tomography (CT) or MRI, and with improved technology, MRI has become more accurate in determining hepatic iron concentration.

SCREENING FOR HEMOCHROMATOSIS

When the diagnosis of hemochromatosis is established, it is important to counsel and screen other family members (Chap. 457). Asymptomatic and symptomatic family members with the disease usually have an increased saturation of transferrin and an increased serum ferritin concentration. These changes occur even before the iron stores are greatly increased (Fig. 407-2). All adult first-degree relatives of patients with hemochromatosis should be tested for the C282Y and H63D mutations and counseled appropriately (Fig. 407-3). In affected individuals, it is important to confirm or exclude the presence of cirrhosis and begin therapy as early as possible. For children of an identified proband, testing for HFE of the other parent is helpful because if normal, the child is merely an obligate heterozygote and at no risk. Otherwise, for practical purposes, children need not be checked before they are 18 years old.

The role of population screening for hemochromatosis is controversial. Recent studies indicate that it is highly effective for primary care physicians to screen subjects using transferrin saturation and serum ferritin levels. Such screening also detects iron deficiency. Genetic screening of the normal population is feasible but is probably not cost effective.

TREATMENT

PROGNOSIS

The principal causes of death are cardiac failure, hepatocellular failure or portal hypertension and hepatocellular carcinoma.

Life expectancy is improved by removal of the excessive stores of iron and maintenance of these stores at near-normal levels. The 5-year survival rate with therapy increases from 33 to 89%. With repeated phlebotomy, the liver decreases in size, liver function improves, pigmentation of skin decreases, and cardiac failure may be reversed. Diabetes improves in ~40% of patients, but removal of excess iron has little effect on hypogonadism or arthropathy. Hepatic fibrosis may decrease, but established cirrhosis is irreversible. Hepatocellular carcinoma occurs as a late sequela in patients who are cirrhotic at presentation. The apparent increase in its incidence in treated patients is probably related to their increased life span. Hepatocellular carcinoma rarely develops if the disease is treated in the precirrhotic stage. Indeed, the life expectancy of homozygotes treated before the development of cirrhosis is normal.

The importance of family screening and early diagnosis and treatment cannot be overemphasized. Asymptomatic individuals detected by family studies should have phlebotomy therapy if iron stores are moderately to severely increased. Assessment of iron stores at appropriate intervals is also important. With this management approach, most manifestations of the disease can be prevented.

ROLE OF HFE MUTATIONS IN OTHER LIVER DISEASES

There is considerable interest in the role of HFE mutations and hepatic iron in several other liver diseases. Several studies have shown an increased prevalence of HFE mutations in PCT patients. Iron accentuates the inherited enzyme deficiency in PCT and clinical manifestations of PCT. The situation in NASH is less clear, but some studies have shown an increased prevalence of HFE mutations in NASH patients. The role of phlebotomy therapy, however, is unproven despite an intriguing fall in liver enzyme levels. In chronic HCV infection, HFE mutations are not more common, but some subjects have increased hepatic iron. Before initiating antiviral therapy in these patients, it is reasonable to perform phlebotomy therapy to remove excess iron stores, because this reduces liver enzyme levels.

HFE mutations are not increased in frequency in alcoholic liver disease. Hemochromatosis in a heavy drinker can be distinguished from alcoholic liver disease by the presence of the C282Y mutation.

End-stage liver disease may also be associated with iron overload of the degree seen in hemochromatosis. The mechanism is uncertain, although studies have shown that alcohol suppresses hepatic hepcidin secretion. Hemolysis also plays a role. HFE mutations are uncommon.

Whether subjects homozygous for C282Y are at increased risk of breast and colorectal cancer is controversial.

GLOBAL CONSIDERATIONS

The HFE mutation is of northern European origin (Celtic or Nordic) with a heterozygous carrier rate of ~1 in 10 (1 in 8 in Ireland). Thus, HFE-associated hemochromatosis is quite rare in non-European populations, e.g., Asia. However, non-HFE-associated hemochromatosis resulting from mutations in other genes involved in iron metabolism (Fig. 407-1) is ubiquitous and should be considered when one encounters iron overload.

African iron overload occurs primarily in sub-Saharan Africa and was previously thought to be due to the consumption of an iron-rich fermented maize beverage. However, recent evidence suggests that it is primarily the result of a non-HFE-related genetic trait that is exacerbated by the dietary iron loading. A similar form of iron-overload has been described in African Americans. Further research is needed to clarify this condition.

FURTHER READING

INTRODUCTION

Wilson’s disease is an autosomal recessive disorder caused by mutations in the ATP7B gene, which encodes a membrane-bound, copper-transporting ATPase. Clinical manifestations are caused by copper toxicity and primarily involve the liver and the brain. Because effective treatment is available, it is important to make this diagnosis early.

The frequency of Wilson’s disease in most populations is about 1 in 30,000–40,000, and the frequency of carriers of ATP7B mutations is ~1%. Siblings of a diagnosed patient have a 1 in 4 risk of Wilson’s disease, whereas children of an affected patient have about a 1 in 200 risk. Although a large number of inactivating mutations have been reported in the ATP7B gene, mutation screening for diagnosis is now available, and a definitive diagnosis can be obtained in 50–75% of patients, depending on the population. DNA haplotype analysis can be used to genotype siblings of an affected patient. A rare multisystem disorder of copper metabolism with features of both Menke’s and Wilson’s diseases has been reported. It is termed the MEDNIK (mental retardation, enteropathy, deafness, neuropathy, ichthyosis, keratodermia) syndrome and is caused by mutations in the AP1S1 gene, which encodes an adaptor protein necessary for intracellular trafficking of copper pump proteins ATP7A (Menke’s disease) and ATP7B (Wilson’s disease).

PATHOGENESIS

ATP7B protein deficiency impairs biliary copper excretion, resulting in positive copper balance, hepatic copper accumulation, and copper toxicity from oxidant damage. Excess hepatic copper is initially bound to metallothionein; liver damage begins as this storage capacity is exceeded, sometimes by 3 years of age. Defective copper incorporation into apoceruloplasmin leads to excess catabolism and low blood levels of ceruloplasmin. Serum copper levels are usually lower than normal because of low blood levels of ceruloplasmin, which normally binds >90% of serum copper. As the disease progresses, nonceruloplasmin serum copper (“free” copper) levels increase, resulting in copper buildup in other parts of the body (e.g., in the brain, with consequent neurologic and psychiatric disease).

CLINICAL PRESENTATION

Hepatic Features

Wilson’s disease may present as hepatitis, cirrhosis, or hepatic decompensation. Patients typically present in the mid- to late teenage years in Western countries, although the age of presentation is quite broad and extends into the fifth decade of life. An episode of hepatitis may occur—with elevated serum aminotransferase levels, with or without jaundice—and then spontaneously regress. Hepatitis often recurs, and most of these patients eventually develop cirrhosis. Hepatic decompensation is associated with elevated serum bilirubin, reduced serum albumin and coagulation factors, ascites, peripheral edema, and hepatic encephalopathy. In severe hepatic failure, hemolytic anemia may develop because large amounts of copper derived from hepatocellular necrosis are released into the bloodstream. The association of hemolysis and liver disease makes Wilson’s disease a likely diagnosis.

Neurologic Features

The neurologic manifestations of Wilson’s disease typically occur in patients in their early twenties, although the age of onset extends into the sixth decade of life. MRI and CT scans reveal damage in the basal ganglia and occasionally in the pons, medulla, thalamus, cerebellum, and subcortical areas. The three main movement disorders include dystonia, incoordination, and tremor. Dysarthria and dysphagia are common. In some patients, the clinical picture closely resembles that of Parkinson’s disease. Dystonia can involve any part of the body and eventually leads to grotesque positions of the limbs, neck, and trunk. Autonomic disturbances may include orthostatic hypotension and sweating abnormalities as well as bowel, bladder, and sexual dysfunction. Memory loss, migraine-type headaches, and seizures may occur. Patients have difficulty focusing on tasks, but cognition usually is not grossly impaired. Sensory abnormalities and muscular weakness are not features of the disease.

Psychiatric Features

Half of patients with neurologic disease have a history of behavioral disturbances with onset in the 5 years before diagnosis. The features are diverse and may include loss of emotional control (temper tantrums, crying bouts), depression, hyperactivity, or loss of sexual inhibition.

Other Manifestations

Some female patients have repeated spontaneous abortions, and most become amenorrheic prior to diagnosis. Cholelithiasis and nephrolithiasis occur with increased frequency. Some patients have osteoarthritis, particularly of the knee. Microscopic hematuria is common, and levels of urinary excretion of phosphates, amino acids, glucose, or urates may increase; however, a full-blown Fanconi’s syndrome is rare. Sunflower cataracts and Kayser-Fleischer rings (copper deposits in the outer rim of the cornea) may be seen. Electrocardiographic and other cardiac abnormalities have been reported but are not common.

DIAGNOSIS

Diagnostic tests for Wilson’s disease are listed in Table 408-1. Serum ceruloplasmin levels should not be used for definitive diagnosis, because they are normal in up to 10% of affected patients and are reduced in 20% of carriers. Kayser-Fleischer rings (Fig. 408-1) can be definitively diagnosed only by an ophthalmologist using a slit lamp. They are present in >99% of patients with neurologic/psychiatric forms of the disease and have been described very rarely in the absence of Wilson’s disease. Kayser-Fleischer rings are present in only ~30–50% of patients diagnosed in the hepatic or presymptomatic state; thus, the absence of rings does not exclude the diagnosis.

Urine copper measurement is an important diagnostic tool, but urine must be collected carefully to avoid contamination. Symptomatic patients invariably have urine copper levels >1.6 μmol (>100 μg) per 24 h. Heterozygotes have values <1.3 μmol (<80 μg) per 24 h. About half of presymptomatic patients who are ultimately affected have diagnostically elevated urine copper values, but the other half have levels that are in an intermediate range between 0.9 and 1.6 μmol (60–100 μg) per 24 h. Because heterozygotes may have values up to 1.3 μmol (80 μg) per 24 h, patients in this range may require a liver biopsy for definitive diagnosis.

Testing for mutations in ATP7B can be done and is recommended for a definitive diagnosis. Several hundred inactivating mutations have been described and genetic testing laboratories can now make a definitive diagnosis in about 50–75% of patients, depending on the population.

The gold standard for diagnosis remains liver biopsy with quantitative copper assays. Affected patients have values >3.1 μmol/g (>200 μg/g [dry weight] of liver). Copper stains are not reliable. False-positive results can occur with long-standing obstructive liver disease, which can elevate hepatic and urine copper concentrations and rarely causes Kayser-Fleischer rings.

TREATMENT

MONITORING ANTICOPPER THERAPY

When trientine or penicillamine is first used, it is necessary to monitor for drug toxicity, particularly bone marrow suppression and proteinuria. Complete blood counts, standard biochemical profiles, and a urinalysis should be performed at weekly intervals for 1 month, then at twice-weekly intervals for 2 or 3 months, then at monthly intervals for 3 or 4 months, and at 4- to 6-month intervals thereafter.

The anticopper effects of trientine and penicillamine can be monitored by following “free” serum copper levels. Changes in urine copper levels are more difficult to interpret because excretion reflects the effect of the drug as well as body loading with copper. Free serum copper is calculated by subtracting the ceruloplasmin copper from the total serum copper. Each 10 mg/L (1 mg/dL) of ceruloplasmin contributes 0.5 μmol/L (3 μg/dL) of serum copper. The normal serum-free copper value is 1.6–2.4 μmol/L (10–15 μg/dL); the level is often as high as 7.9 μmol/L (50 μg/dL) in untreated Wilson’s disease. With treatment, the serum-free copper should be <3.9 μmol/L (<25 μg/dL).

Zinc treatment does not require monitoring of blood or urine for toxicity. Its only significant side effect is gastric burning or nausea in ~10% of patients, usually with the first morning dose. This effect can be mitigated if the first dose is taken an hour after breakfast or if zinc is taken with a small amount of protein. Because zinc mainly affects stool copper, 24-h urine copper can be used to reflect body loading. The typical value in untreated symptomatic patients is >3.1 μmol (>200 μg) per 24 h. This level should decrease during the first 1–2 years of therapy to <2.0 μmol (<125 μg) per 24 h. A normal value (0.3–0.8 μmol [20–50 μg]) is rarely reached during the first decade of therapy and should raise concern about overtreatment (copper deficiency), the first sign of which is anemia and/or leukopenia.

GLOBAL CONSIDERATIONS

The age of onset of clinical disease may be considerably younger in India and the Far East; in these regions, onset often occurs in children at only 5 or 6 years of age. The incidence of the disease may be increased in certain populations as a result of founder effects. For example, in Sardinia, the incidence may be 1 in 3000. In countries where penicillamine, trientine, and zinc acetate (as Galzin) are not available or are unaffordable, zinc salts such as gluconate or sulfate provide an alternative treatment option.

FURTHER READING

THE PORPHYRIAS: INTRODUCTION

The porphyrias are metabolic disorders, each resulting from the deficiency or increased activity of a specific enzyme in the heme biosynthetic pathway (Fig. 409-1 and Table 409-1). These enzyme disorders are inherited as autosomal dominant, autosomal recessive, or X-linked traits, with the exception of porphyria cutanea tarda (PCT), which is usually sporadic (Table 409-1). The porphyrias are classified as either hepatic or erythropoietic, depending on the primary site of overproduction and accumulation of their respective porphyrin precursors or porphyrins (Tables 409-1 and 409-2), although some have overlapping features. For example, PCT, the most common porphyria, is hepatic and presents with blistering cutaneous photosensitivity, which is typically characteristic of the erythropoietic porphyrias (EPPs).

The major manifestations of the acute hepatic porphyrias are neurologic, including neuropathic abdominal pain, peripheral motor neuropathy, and mental disturbances, with attacks often precipitated by dieting, certain porphyrinogenic drugs, and hormonal changes. While hepatic porphyrias are symptomatic primarily in adults, rare homozygous variants of the autosomal dominant hepatic porphyrias usually manifest clinically prior to puberty. In contrast, the erythropoietic porphyrias usually present at birth or in early childhood with cutaneous photosensitivity, or in the case of congenital erythropoietic porphyria (CEP), even in utero as nonimmune hydrops fetalis. Cutaneous sensitivity to sunlight results from excitation of excess porphyrins in the skin by long-wave ultraviolet light, leading to cell damage, scarring, and disfigurement. Thus, the porphyrias are metabolic disorders in which environmental, physiologic, and genetic factors interact to cause disease.

Because many symptoms of the porphyrias are nonspecific, diagnosis is often delayed. Laboratory measurement of porphyrin precursors (5′-aminolevulinic acid [ALA] and porphobilinogen [PBG]) in the urine or porphyrins in the urine, plasma, erythrocytes, or feces is required to confirm or exclude the various types of porphyria (see below). However, a definite diagnosis requires demonstration of the specific gene defect (Table 409-3). The genes encoding all the heme biosynthetic enzymes have been characterized, permitting identification of the mutations causing each porphyria (Table 409-2). Molecular genetic analyses now make it possible to provide precise heterozygote or homozygote identification and prenatal diagnoses in families with known mutations.

In addition to recent reviews of the porphyrias, informative and up-to-date websites are sponsored by the American Porphyria Foundation (www.porphyriafoundation.com) and the European Porphyria Initiative (www.porphyria-europe.org). An extensive list of unsafe and safe drugs for individuals with acute porphyrias is provided at the Drug Database for Acute Porphyrias (www.drugs-porphyria.com).

GLOBAL CONSIDERATIONS

The porphyrias are panethnic metabolic diseases that affect individuals around the globe. The acute hepatic porphyrias—acute intermittent porphyria (AIP), hereditary coproporphyria (HCP), and variegate porphyria (VP)—are autosomal dominant disorders. The frequency of symptomatic AIP, the most common acute hepatic porphyria, is ~1 in 20,000 among Caucasian individuals of Western European ancestry, and it is particularly frequent in Scandinavians, with a frequency of ~1 in 10,000 in Sweden. However, a recent study using genomic/exomic databases showed an estimated frequency of pathogenic variants in the HMBS gene as ~1 in 1,700. Thus, the penetrance of AIP, and likely the other acute hepatic porphyrias, is low, about 1–10% of those with pathogenic mutations experiencing acute attacks (see below).

VP is particularly frequent in South Africa, where its high prevalence (>10,000 affected patients) is in part due to a genetic “founder effect.” The autosomal recessive acute hepatic porphyria, ALA-dehydratase-deficient porphyria (ADP), is very rare, and <20 patients have been reported worldwide.

The EPPs—CEP, EPP, and X-linked protoporphyria (XLP)—also are panethnic. EPP is the most common porphyria in children, whereas CEP is very rare, with about 200 reported cases worldwide. The frequency of EPP varies globally because most patients have the common low expression ferrochelatase (FECH) mutation that varies in frequency in different populations. It rarely occurs in Africans, is present in about 10% of whites, and is frequent (~30%) in the Japanese. The reported prevalence of EPP in the Caucasian population ranges from 1 in 75,000 to 1 in 152,000.

The autosomal recessive porphyrias—ADP, CEP, hepatoerythropoietic porphyria (HEP)—are more frequent in regions with high rates of consanguineous unions. PCT, which is typically sporadic, occurs more frequently in countries in which its predisposing risk factors such as hepatitis C and HIV are more prevalent.

HEME BIOSYNTHESIS

Heme biosynthesis involves eight enzymatic steps in the conversion of glycine and succinyl-CoA to heme (Fig. 409-2 and Table 409-2). These eight enzymes are encoded by nine genes, as the first enzyme in the pathway, ALA-synthase, has two genes that encode unique housekeeping (ALAS1) and erythroid-specific (ALAS2) isozymes. The first and last three enzymes in the pathway are located in the mitochondrion, whereas the other four are in the cytosol. Heme is required for a variety of hemoproteins such as hemoglobin, myoglobin, respiratory cytochromes, and the cytochrome P450 (CYPs) enzymes. Hemoglobin synthesis in erythroid precursor cells accounts for ~85% of daily heme synthesis in humans. Hepatocytes account for most of the rest, primarily for the synthesis of CYPs, which are especially abundant in the liver endoplasmic reticulum, and turn over more rapidly than many other hemoproteins, such as the mitochondrial respiratory cytochromes. As shown in Fig. 409-2, the pathway intermediates are the porphyrin precursors, ALA and PBG, and porphyrins (mostly in their reduced forms, known as porphyrinogens). At least in humans, these intermediates do not accumulate in significant amounts under normal conditions or have important physiologic functions.

The first enzyme, ALA-synthase, catalyzes the condensation of glycine, activated by pyridoxal phosphate and succinyl-coenzyme A, to form ALA. In the liver, this rate-limiting enzyme can be induced by a variety of drugs, steroids, and other chemicals. Distinct nonerythroid (e.g., housekeeping) and erythroid-specific forms of ALA-synthase are encoded by separate genes located on chromosome 3p21.1 (ALAS1) and Xp11.2 (ALAS2), respectively. Defects in the erythroid gene ALAS2 that decrease its activity cause an X-linked sideroblastic anemia (XLSA). Gain-of-function mutations in the last exon (11) of ALAS2 that increase its activity cause an X-linked form of EPP, known as XLP.

The second enzyme, ALA-dehydratase, catalyzes the condensation of two molecules of ALA to form PBG. Hydroxymethylbilane synthase (HMB-synthase; also known as PBG-deaminase) catalyzes the head-to-tail condensation of four PBG molecules by a series of deaminations to form the linear tetrapyrrole, HMB. Uroporphyrinogen III synthase (URO-synthase) catalyzes the rearrangement and rapid cyclization of HMB to form the asymmetric, physiologic, octacarboxylate porphyrinogen, uroporphyrinogen (URO’gen) III.

The fifth enzyme in the pathway, uroporphyrinogen decarboxylase (URO-decarboxylase), catalyzes the sequential removal of the four carboxyl groups from the acetic acid side chains of URO’gen III to form coproporphyrinogen (COPRO’gen) III, a tetracarboxylate porphyrinogen. This compound then enters the mitochondrion via a specific transporter, where COPRO-oxidase, the sixth enzyme, catalyzes the decarboxylation of two of the four propionic acid groups to form the two vinyl groups of protoporphyrinogen (PROTO’gen) IX, a decarboxylate porphyrinogen. Next, PROTO-oxidase oxidizes PROTO’gen to protoporphyrin IX by the removal of six hydrogen atoms. The product of the reaction is a porphyrin (oxidized form), in contrast to the preceding tetrapyrrole intermediates, which are porphyrinogens (reduced forms). Finally, ferrous iron is inserted into protoporphyrin to form heme, a reaction catalyzed by the eighth enzyme in the pathway, FECH (also known as heme synthase or protoheme ferrolyase).

REGULATION OF HEME BIOSYNTHESIS

Regulation of heme synthesis differs in the two major heme-forming tissues, the liver and erythron. In the liver, the concentration of “free” heme regulates the synthesis and mitochondrial translocation of the housekeeping form of ALA-synthase 1. Heme represses the synthesis of the ALA-synthase 1 messenger RNA (mRNA) and interferes with the transport of the enzyme from the cytosol into mitochondria. Hepatic ALA-synthase 1 is increased by many of the same chemicals that induce the CYPs enzymes in the endoplasmic reticulum of the liver. Because most of the heme in the liver is used for the synthesis of CYPs enzymes, hepatic ALA-synthase 1 and the CYPs are regulated in a coordinated fashion, and many drugs that induce hepatic ALA-synthase 1 also induce the CYP genes. The other hepatic heme biosynthetic enzymes are presumably expressed at constant levels, although their relative activities and kinetic properties differ. For example, normal individuals have high activities of ALA-dehydratase, but low activities of HMB-synthase, the latter being the second rate-limiting step in the pathway.

In the erythron, novel regulatory mechanisms allow for the production of the very large amounts of heme needed for hemoglobin synthesis. The response to stimuli for hemoglobin synthesis occurs during cell differentiation, leading to an increase in cell number. In contrast, the erythroid-specific ALA-synthase 2 is expressed at higher levels than the housekeeping enzyme, and erythroid-specific control mechanisms regulate other pathway enzymes as well as iron transport into erythroid cells. Separate erythroid-specific and nonerythroid or “housekeeping” transcripts are known for the first four enzymes in the pathway. As noted above, housekeeping- and erythroid-specific ALA-synthases are encoded by genes on different chromosomes, but for each of the next three genes in the pathway, both erythroid and nonerythroid transcripts are transcribed by alternative promoters from their single respective genes (Table 409-2).

CLASSIFICATION OF THE PORPHYRIAS

As mentioned above, the porphyrias can be classified as either hepatic or erythropoietic, depending on whether the heme biosynthetic intermediates that accumulate arise initially from the liver or developing erythrocytes, or as acute or cutaneous, based on their clinical manifestations. Table 409-1 lists the porphyrias, their principal symptoms, and major biochemical abnormalities. Three of the five hepatic porphyrias—AIP, HCP, and VP—usually present during adult life with acute attacks of neurologic manifestations and elevated levels of one or both of the porphyrin precursors, ALA and PBG, and are thus classified as acute porphyrias. Patients with ADP have presented in infancy and adolescence, and typically have elevated ALA with normal or slightly elevated PBG levels. The fifth hepatic disorder, PCT, presents with blistering skin lesions. HCP and VP also may have cutaneous manifestations similar to PCT.

The erythropoietic porphyrias—CEP, EPP, and XLP—are characterized by elevations of porphyrins in bone marrow and erythrocytes and present with cutaneous photosensitivity. The skin lesions in CEP resemble PCT but are usually much more severe, whereas EPP and XLP cause a more immediate, severe, painful, and nonblistering type of photosensitivity. EPP is the most common porphyria to cause symptoms before puberty. Around 20% of EPP patients develop minor abnormalities of liver function, with up to about 5% developing hepatic complications that can become life-threatening. XLP has a clinical presentation similar to EPP causing photosensitivity and liver disease.

DIAGNOSIS OF PORPHYRIA

A few specific and sensitive first-line laboratory tests should be used whenever symptoms or signs suggest the diagnosis of porphyria (Table 409-3). If a first-line test is significantly abnormal, more comprehensive testing should follow to establish the type of porphyria, including the specific causative gene mutation.

Acute Porphyrias

An acute porphyria should be suspected in patients with neurovisceral symptoms after puberty. Symptoms include acute abdominal pain, nausea, vomiting, tachycardia, hypertension, and motor neuropathy. As these symptoms are common, other causes should be ruled out. The diagnosis is made by measuring urinary porphyrin precursors (ALA and PBG) in a spot sample of urine (Fig. 409-2). Urinary PBG is always increased during acute attacks of AIP, HCP, and VP and is not substantially increased in any other medical condition. Therefore, this measurement is both sensitive and specific. Results from spot (single-void) urine specimens are highly informative because very substantial increases in PBG are expected during acute attacks of porphyria. A 24-h collection can unnecessarily delay diagnosis. The same spot urine specimen should be saved for quantitative determination of ALA, PBG, and creatinine, in order to confirm the qualitative PBG result and also to detect patients with ADP. Urinary porphyrins may remain increased longer than porphyrin precursors in HCP and VP. Therefore, it is useful to measure total urinary porphyrins in the same sample, keeping in mind that urinary porphyrin increases are often nonspecific. Measurement of urinary porphyrins alone should be avoided for screening, because these may be increased in disorders other than porphyrias, such as chronic liver disease, and misdiagnoses of porphyria can result from minimal increases in urinary porphyrins that have no diagnostic significance. Measurement of erythrocyte HMB-synthase is not useful as a first-line test. Moreover, the enzyme activity is not decreased in all AIP patients, a borderline low normal value is not diagnostic, and the enzyme is not deficient in other acute porphyrias.

Cutaneous Porphyrias

Blistering skin lesions due to porphyria are virtually always accompanied by increases in total plasma porphyrins. A fluorometric method is preferred, because the porphyrins in plasma in VP are mostly covalently linked to plasma proteins and may be less readily detected by high-performance liquid chromatography (HPLC). The normal range for plasma porphyrins is somewhat increased in patients with end-stage renal disease.

Although a total plasma porphyrin determination will usually detect EPP and XLP, an erythrocyte protoporphyrin determination is more sensitive. Increases in erythrocyte protoporphyrin occur in many other conditions. Therefore, the diagnosis of EPP must be confirmed by showing a predominant increase in free protoporphyrin rather than zinc protoporphyrin. In XLP, both free and zinc protoporphyrin are markedly increased. Interpretation of laboratory reports can be difficult, because the term free erythrocyte protoporphyrin sometimes actually represents zinc protoporphyrin.

More extensive testing is justified when an initial test is positive. A substantial increase in PBG may be due to AIP, HCP, or VP. These acute porphyrias can be distinguished by measuring urinary porphyrins (using the same spot urine sample), fecal porphyrins, and plasma porphyrins. Assays for COPRO-oxidase or PROTO-oxidase are not available for clinical testing. More specifically, mutation analysis by sequencing the genes encoding HMB-synthase, COPRO-oxidase, and PROTO-oxidase will detect almost all disease-causing mutations, and will be diagnostic even when the levels of urinary ALA and PBG have returned to normal or near normal. The various porphyrias that cause blistering skin lesions can be differentiated by measuring porphyrins in urine, feces, and plasma. These porphyrias also should be confirmed at the DNA level by the demonstration of the causative gene mutation(s). It is often difficult to diagnose or “rule out” porphyria in patients who have had suggestive symptoms months or years in the past, and in relatives of patients with acute porphyrias, because porphyrin precursors and porphyrins may be normal. In those situations, detection of the specific gene mutation in the index case can make the diagnosis and facilitate the diagnosis and genetic counseling of at-risk relatives. Consultation with a specialist laboratory and physician will assist in selecting the heme biosynthetic gene or genes to be sequenced.

THE HEPATIC PORPHYRIAS

Markedly elevated plasma and urinary concentrations of the porphyrin precursors, ALA and/or PBG, which originate from the liver, are especially evident during attacks of neurologic manifestations of the four acute porphyrias—ADP, AIP, HCP, and VP. In PCT, excess porphyrins also accumulate initially in the liver and cause chronic blistering of sun-exposed areas of the skin.

ALA-DEHYDRATASE-DEFICIENT PORPHYRIA

ADP is a rare, autosomal recessive, acute hepatic porphyria caused by a severe deficiency of ALA-dehydratase activity. To date, there are only a few documented cases, some in children or young adults, in which specific gene mutations have been identified. These affected homozygotes had <10% of normal ALA-dehydratase activity in erythrocytes, but their clinically asymptomatic parents and heterozygous relatives had about half-normal levels of activity and did not excrete increased levels of ALA. The frequency of ADP is unknown, but the frequency of heterozygous individuals with <50% normal ALA-dehydratase activity was ~2% in a screening study in Sweden. Because there are multiple causes for deficient ALA-dehydratase activity, it is important to confirm the diagnosis of ADP by mutation analysis.

Clinical Features

The clinical presentation depends on the amount of residual ALA-dehydratase activity. Four of the documented patients were male adolescents with symptoms resembling those of AIP, including abdominal pain and neuropathy. One patient was an infant with more severe disease, including failure to thrive beginning at birth. The earlier age of onset and more severe manifestations in this patient reflect a more significant deficiency of ALA-dehydratase activity. Another patient developed an acute motor polyneuropathy at age 63 that was associated with a myeloproliferative disorder. He was heterozygous for an δ-aminolevulinic acid dehydratase (ALAD) mutation that presumably was present in erythroblasts that underwent clonal expansion due to the bone marrow malignancy.

Diagnosis

All patients had significantly elevated levels of plasma and urinary ALA and urinary coproporphyrin (COPRO) III; ALAD activities in erythrocytes were <10% of normal. Hereditary tyrosinemia type 1 (fumarylacetoacetase deficiency) and lead intoxication should be considered in the differential diagnosis because either succinylacetone (which accumulates in hereditary tyrosinemia and is structurally similar to ALA) or lead can inhibit ALA-dehydratase, increase urinary excretion of ALA and COPRO III, and cause manifestations that resemble those of the acute porphyrias. Heterozygotes are clinically asymptomatic and do not excrete increased levels of ALA but can be detected by demonstration of intermediate levels of erythrocyte ALA-dehydratase activity or a specific mutation in the ALAD gene. To date, molecular studies of ADP patients have identified point mutations, splice-site mutations, and a two-base deletion in the ALAD gene (Human Gene Mutation Database; www.hgmd.org). The parents in each case were not consanguineous, and the index cases had inherited a different ALAD mutation from each parent. Prenatal diagnosis of this disorder is possible by determination of ALA-dehydratase activity and/or gene mutations in cultured chorionic villi or amniocytes.

TREATMENT

ACUTE INTERMITTENT PORPHYRIA

This hepatic porphyria is an autosomal dominant condition resulting from the half-normal level of HMB-synthase activity. The disease is widespread but is especially common in Scandinavia and Great Britain. Clinical expression is highly variable, and activation of the disease is often related to environmental or hormonal factors, such as drugs, diet, and steroid hormones. Attacks can be prevented by avoiding known precipitating factors. Rare homozygous dominant AIP also has been described in children (see below).

Clinical Features

Induction of the rate-limiting hepatic enzyme ALA-synthase in heterozygotes who have half-normal HMB-synthase activity is thought to underlie the acute attacks in AIP. The disorder remains latent (or asymptomatic) in the great majority of those who are heterozygous for HMBS mutations, and this is almost always the case prior to puberty. In patients with no history of acute symptoms, porphyrin precursor excretion is usually normal, suggesting that half-normal hepatic HMB-synthase activity is sufficient and that hepatic ALA-synthase activity is not increased. However, under conditions where heme synthesis is increased in the liver, half-normal HMB-synthase activity may become limiting, and ALA, PBG, and other heme pathway intermediates may accumulate and be excreted in the urine. Common precipitating factors include endogenous and exogenous steroids, porphyrinogenic drugs, alcohol ingestion, and low-calorie diets, usually instituted for weight loss.

The fact that AIP is almost always latent before puberty suggests that adult levels of steroid hormones are important for clinical expression. Symptoms are more common in women, suggesting a role for estrogens or progestins. Premenstrual attacks are probably due to endogenous progesterone. Acute porphyrias are sometimes exacerbated by exogenous steroids, including oral contraceptive preparations containing progestins. Surprisingly, pregnancy is usually well tolerated, suggesting that beneficial metabolic changes may ameliorate the effects of high levels of progesterone. Table 409-4 provides a partial list of the major drugs that are harmful in AIP (and also in HCP and VP). Extensive lists of unsafe and safe drugs are available on websites sponsored by the American Porphyria Foundation (www.porphyriafoundation.com) and the European Porphyria Initiative (www.porphyria-europe.org), and at the Drug Database for Acute Porphyrias website (www.drugs-porphyria.com). Reduced intake of calories and carbohydrate, as may occur with illness or attempts to lose weight, can also increase porphyrin precursor excretion and induce attacks of porphyria. Increased carbohydrate intake may ameliorate attacks. Studies in a knockout AIP mouse model indicate that the hepatic ALAS1 gene is regulated by the peroxisome proliferator-activated receptor γ coactivator 1α (PGC-1α). Hepatic PGC-1α is induced by fasting, which in turn activates ALAS1 transcription, resulting in increased heme biosynthesis. This finding suggests an important link between nutritional status and the attacks in acute porphyrias. Attacks also can be provoked by infections, surgery, and ethanol.

Because the neurovisceral symptoms rarely occur before puberty and are often nonspecific, a high index of suspicion is required to make the diagnosis. The disease can be disabling but is rarely fatal. Abdominal pain, the most common symptom, is poorly localized, but may be associated with cramping, ileus, abdominal distention, and decreased bowel sounds. However, increased bowel sounds and diarrhea may occur. Abdominal tenderness, fever, and leukocytosis are usually absent or mild because the symptoms are neurologic rather than inflammatory. Nausea; vomiting; constipation; tachycardia; hypertension; mental symptoms; pain in the limbs, head, neck, or chest; muscle weakness; sensory loss; dysuria; and urinary retention are characteristic. Tachycardia, hypertension, restlessness, tremors, and excess sweating are due to sympathetic overactivity.

The peripheral neuropathy is due to axonal degeneration (rather than demyelinization) and primarily affects motor neurons. Significant neuropathy does not occur with all acute attacks; abdominal symptoms are usually more prominent. Motor neuropathy affects the proximal muscles initially, more often in the shoulders and arms. The course and degree of involvement are variable and sometimes may be focal and involve cranial nerves. Deep tendon reflexes initially may be normal or hyperactive but become decreased or absent as the neuropathy advances. Sensory changes such as paresthesia and loss of sensation are less prominent. Progression to respiratory and bulbar paralysis and death occurs especially when the diagnosis and treatment are delayed. Sudden death may result from sympathetic overactivity and cardiac arrhythmia.

Mental symptoms such as anxiety, insomnia, depression, disorientation, hallucinations, and paranoia can occur in acute attacks. Seizures can be due to neurologic effects or to hyponatremia. Treatment of seizures is difficult because most anti-seizure drugs can exacerbate AIP (clonazepam may be safer than phenytoin or barbiturates). Hyponatremia results from hypothalamic involvement and inappropriate vasopressin secretion or from electrolyte depletion due to vomiting, diarrhea, poor intake, or excess renal sodium loss. Persistent hypertension and impaired renal function may occur. When an attack resolves, abdominal pain may disappear within hours, and paresis begins to improve within days and may continue to improve over several years.

Homozygous dominant AIP is a rare form of AIP in which patients inherit HMBS mutations from each of their heterozygous parents and, therefore, have very low (<2%) enzyme activity. The disease has been described in a Dutch girl, two young British siblings, and a Spanish boy. In these homozygous affected patients, the disease presented in infancy with failure to thrive, developmental delay, bilateral cataracts, and/or hepatosplenomegaly. Urinary ALA and PBG concentrations were markedly elevated. All of these patients’ HMBS mutations (R167W, R167Q, and R172Q) were in exon 10 within five bases of each other. Studies of the brain magnetic resonance images (MRIs) of children with homozygous AIP have suggested damage primarily in white matter that was myelinated postnatally, while tracks that myelinated prenatally were normal. Most children with homozygous AIP die at an early age.

Diagnosis

ALA and PBG levels are substantially increased in plasma and urine, especially during acute attacks. For example, urinary PBG excretion during an attack is usually 50–200 mg/24 h (220–880 μmol/24 h) (normal, 0–4 mg/24 h, [0–18 μmol/24 h]), and urinary ALA excretion is 20–100 mg/24 h (150–760 μmol/24 h) (normal, 1–7 mg/24 h [8–53 μmol/24 h]). Because levels often remain high after symptoms resolve, the diagnosis of an acute attack in a patient with biochemically proven AIP is based primarily on clinical features. Excretion of ALA and PBG decreases over a few days after intravenous hemin administration. A normal urinary PBG level before hemin effectively excludes AIP as a cause for current symptoms. Fecal porphyrins are usually normal or minimally increased in AIP, in contrast to HCP and VP. Most AIP heterozygotes with no history of symptoms have normal urinary excretion of ALA and PBG and are classified as latent. Patients can also have high levels of urine PBG and ALA with no clinical symptoms. These patients may have a previous history of an acute attack. These patients are classified as “asymptomatic high excretors” (ASHE). Therefore, the detection of the family’s HMBS mutation will diagnose asymptomatic family members.

Patients with HMBS mutations in the initiation of translation codon in exon 1 and in the intron 15′-splice donor site have normal enzyme levels in erythrocytes and deficient activity only in nonerythroid tissues. This occurs because the erythroid and housekeeping forms of HMB-synthase are encoded by a single gene, which has two promoters. Thus, the enzyme assay may not be diagnostic, and genetic testing should be used to confirm the diagnosis.

More than 410 HMBS mutations have been identified in AIP, including missense, nonsense, and splicing mutations and insertions and deletions, with most mutations found in only one or a few families (Human Gene Mutation Database, www.hgmd.org). The prenatal diagnosis of a fetus at risk can be made with cultured amniotic cells or chorionic villi. However, this is seldom done, because the prognosis of individuals with HMBS mutations is generally favorable.

TREATMENT

PORPHYRIA CUTANEA TARDA

PCT, the most common of the porphyrias, can be either sporadic (type 1) or familial (type 2) and can also develop after exposure to halogenated aromatic hydrocarbons. Hepatic URO-decarboxylase is deficient in all types of PCT, and for clinical symptoms to manifest, this enzyme deficiency must be substantial (~20% of normal activity or less); it is currently attributed to generation of an URO-decarboxylase inhibitor in the liver, which forms uroporphomethene in the presence of iron and under conditions of oxidative stress. The majority of PCT patients (~80%) have no UROD mutations and are said to have sporadic (type 1) disease. PCT patients heterozygous for UROD mutations have familial (type 2) PCT. In these patients, inheritance of a UROD mutation from one parent results in half-normal enzyme activity in liver and all other tissues, which is a significant predisposing factor, but is insufficient by itself to cause symptomatic PCT. As discussed below, other genetic and environmental factors contribute to susceptibility for both types of PCT. Because penetrance of the genetic trait is low, many patients with familial (type 2) PCT have no family history of the disease. HEP is an autosomal recessive form of porphyria that results from the marked systemic deficiency of URO-decarboxylase activity with clinical symptoms in childhood.

Clinical Features

Blistering skin lesions that appear most commonly on the backs of the hands are the major clinical feature (Fig. 409-3). These rupture and crust over, leaving areas of atrophy and scarring. Lesions may also occur on the forearms, face, legs, and feet. Skin friability and small white papules termed milia are common, especially on the backs of the hands and fingers. Hypertrichosis and hyperpigmentation, especially of the face, are especially troublesome in women. Occasionally, the skin over sun-exposed areas becomes severely thickened, with scarring and calcification that resembles systemic sclerosis. Neurologic features are absent.

A number of susceptibility factors, in addition to inherited UROD mutations in type 2 PCT, can be recognized clinically and can affect management. These include hepatitis C, HIV, excess alcohol, elevated iron levels, and estrogens. The importance of excess hepatic iron as a precipitating factor is underscored by the finding that the incidence of the common hemochromatosis-causing mutations, hemochromatosis gene (HFE) mutations C282Y and H63D, are increased in patients with types 1 and 2 PCT (Chap. 407). Excess alcohol is a long-recognized contributor, as is estrogen use in women. HIV is probably an independent but less common risk factor that, like hepatitis C, does not cause PCT in isolation. Multiple susceptibility factors that appear to act synergistically can be identified in the individual PCT patient. Patients with PCT characteristically have chronic liver disease and sometimes cirrhosis and are at risk for hepatocellular carcinoma. Various chemicals can also induce PCT; an epidemic of PCT occurred in eastern Turkey in the 1950s as a consequence of wheat contaminated with the fungicide hexachlorobenzene. PCT also occurs after exposure to other chemicals, including di- and trichlorophenols and 2,3,7,8-tetrachlorodibenzo-(p)-dioxin (TCDD, dioxin).

Diagnosis

Porphyrins are increased in the liver, plasma, urine, and stool. The urinary ALA level may be slightly increased, but the PBG level is normal. Urinary porphyrins consist mostly of uroporphyrins and heptacarboxylate porphyrin, with lesser amounts of coproporphyrin and hexa- and pentacarboxylate porphyrins. Plasma porphyrins are also increased, and fluorometric scanning of diluted plasma at neutral pH can rapidly distinguish VP and PCT (Table 409-3). Isocoproporphyrins, which are increased in feces and sometimes in plasma and urine, are diagnostic for hepatic URO-decarboxylase deficiency.

Type 2 PCT and HEP can be distinguished from type 1 by finding decreased URO-decarboxylase in erythrocytes. URO-decarboxylase activity in liver, erythrocytes, and cultured skin fibroblasts in type 2 PCT is ~50% of normal in affected individuals and in family members with latent disease. In HEP, the URO-decarboxylase activity is markedly deficient, with typical levels of 3–10% of normal. Over 120 mutations have been identified in the UROD gene (Human Gene Mutation Database; www.hgmd.org). Of the mutations listed in the database, ~65% are missense or nonsense and ~10% are splice-site mutations. Most UROD mutations have been identified in only one or two families.

TREATMENT

HEREDITARY COPROPORPHYRIA

HCP is an autosomal dominant hepatic porphyria that results from the half-normal activity of COPRO-oxidase. The disease presents with acute attacks, as in AIP. Cutaneous photosensitivity also may occur, but much less commonly than in VP. HCP patients may have acute attacks and cutaneous photosensitivity together or separately. HCP is less common than AIP and VP. Homozygous dominant HCP and harderoporphyria, a biochemically distinguishable variant of HCP, present with clinical symptoms in children (see below).

Clinical Features

HCP is influenced by the same factors that cause attacks in AIP. The disease is latent before puberty, and symptoms, which are virtually identical to those of AIP, are more common in women. HCP is generally less severe than AIP. Blistering skin lesions are identical to PCT and VP and begin in childhood in rare homozygous cases.

Diagnosis

COPRO III is markedly increased in the urine and feces in symptomatic patients, and often persists, especially in feces, when there are no symptoms. Urinary ALA and PBG levels are increased (but less than in AIP) during acute attacks, but may revert to normal more quickly than in AIP when symptoms resolve. Plasma porphyrins are usually normal or only slightly increased, but they may be higher in cases with skin lesions. The diagnosis of HCP is readily confirmed by increased fecal porphyrins consisting almost entirely of COPRO III, which distinguishes it from other porphyrias.

Although the diagnosis can be confirmed by measuring COPRO-oxidase activity, the assays for this mitochondrial enzyme are not available and require cells other than erythrocytes. To date, >65 mutations have been identified in the CPOX gene, 67% of which are missense or nonsense (Human Gene Mutation Database; www.hgmd.org). Detection of a CPOX mutation in a symptomatic individual permits the identification of asymptomatic family members.

TREATMENT

VARIEGATE PORPHYRIA

VP is an autosomal dominant hepatic porphyria that results from the deficient activity of PROTO-oxidase, the seventh enzyme in the heme biosynthetic pathway, and can present with neurologic symptoms, photosensitivity, or both. VP is particularly common in South Africa, where 3 of every 1000 whites have the disorder. Most are descendants of a couple who emigrated from Netherlands to South Africa in 1688. In other countries, VP is less common than AIP. Rare cases of homozygous dominant VP, presenting in childhood with cutaneous symptoms, also have been reported.

Clinical Features

VP can present with skin photosensitivity, acute neurovisceral crises, or both. In two large studies of VP patients, ~60% had only skin lesions, 20% had only acute attacks, and ~20% had both. Acute attacks are identical to those in AIP and are precipitated by the same factors as AIP (see above). Blistering skin manifestations are similar to those in PCT, but are more difficult to treat and usually are of longer duration. Homozygous VP is associated with photosensitivity, neurologic symptoms, and developmental disturbances, including growth retardation, in infancy or childhood; all cases had increased erythrocyte levels of zinc protoporphyrin, a characteristic finding in all homozygous porphyrias so far described.

Diagnosis

Urinary ALA and PBG levels are increased during acute attacks, but may return to normal more quickly than in AIP. Increases in fecal protoporphyrin and COPRO III and in urinary COPRO III are more persistent. Plasma porphyrin levels also are increased, particularly when there are cutaneous lesions. VP can be distinguished rapidly from all other porphyrias by examining the fluorescence emission spectrum of porphyrins in plasma since VP has a unique fluorescence peak at neutral pH.

Assays of PROTO-oxidase activity in cultured fibroblasts or lymphocytes are not widely available. Over 180 mutations have been identified in the PPOX gene from unrelated VP patients (Human Gene Mutation Database; www.hgmd.org). The missense mutation R59W is the common mutation in most South Africans with VP of Dutch descent. Five missense mutations were common in English and French VP patients; however, most mutations have been found in only one or two families.

TREATMENT

THE ERYTHROPOIETIC PORPHYRIAS

In the erythropoietic porphyrias, excess porphyrins from bone marrow erythrocyte precursors are transported via the plasma to the skin and lead to cutaneous photosensitivity.

X-LINKED SIDEROBLASTIC ANEMIA

XLSA results from the deficient activity of the erythroid form of ALA-synthase (ALA-synthase 2) and is associated with ineffective erythropoiesis, weakness, and pallor.

Clinical Features

Typically, males with XLSA develop refractory hemolytic anemia, pallor, and weakness during infancy. They have secondary hypersplenism, become iron overloaded, and can develop hemosiderosis. The severity depends on the level of residual erythroid ALA-synthase activity and on the responsiveness of the specific mutation to pyridoxal 5′-phosphate supplementation (see below). Peripheral blood smears reveal a hypochromic, microcytic anemia with striking anisocytosis, poikilocytosis, and polychromasia; the leukocytes and platelets appear normal. Hemoglobin content is reduced, and the mean corpuscular volume and mean corpuscular hemoglobin concentration are decreased. Patients with milder, late-onset disease have been reported recently.

Diagnosis

Bone marrow examination reveals hypercellularity with a left shift and megaloblastic erythropoiesis with an abnormal maturation. A variety of Prussian blue-staining sideroblasts are observed. Levels of urinary porphyrin precursors and of both urinary and fecal porphyrins are normal. The activity of erythroid ALA-synthase 2 is decreased in bone marrow, but this enzyme is difficult to measure in the presence of the normal ALA-synthase 1 housekeeping enzyme. Definitive diagnosis requires the demonstration of mutations in the erythroid ALAS2 gene.

TREATMENT

CONGENITAL ERYTHROPOIETIC PORPHYRIA

CEP, also known as Günther’s disease, is an autosomal recessive disorder. It is due to the markedly deficient, but not absent, activity of URO-synthase and the resultant accumulation of URO I and COPRO I isomers. CEP is associated with hemolytic anemia and cutaneous lesions.

Clinical Features

Severe cutaneous photosensitivity typically begins from birth. The skin over light-exposed areas is friable, and bullae and vesicles are prone to rupture and infection. Skin thickening, focal hypo- and hyperpigmentation, and hypertrichosis of the face and extremities are characteristic. Secondary infection of the cutaneous lesions can lead to disfigurement of the face and hands. Porphyrins are deposited in teeth and in bones. As a result, the teeth are brownish and fluoresce on exposure to long-wave ultraviolet light. Hemolysis is due to the marked increase in erythrocyte porphyrins and leads to splenomegaly. Adults with a milder later-onset form of the disease also have been described.

Diagnosis

URO and COPRO (mostly type I isomers) accumulate in the bone marrow, erythrocytes, plasma, urine, and feces. The predominant porphyrin in feces is COPRO I. The diagnosis of CEP can be confirmed by demonstration of markedly deficient URO-synthase activity and/or by the identification of specific mutations in the UROS gene. The disease can be detected in utero by measuring porphyrins in amniotic fluid and URO-synthase activity in cultured amniotic cells or chorionic villi, or by the detection of the family’s specific gene mutations. Molecular analyses of the mutant alleles from unrelated patients have revealed the presence of >50 mutations in the UROS gene, including four in the erythroid-specific promoter of the UROS gene. Genotype/phenotype correlations can predict the severity of the disease. The CEP phenotype may be modulated by sequence variations in the erythroid-specific ALA-synthase 2, mutation of which typically causes XLP. One mutation (p.ArgR216WTrp) in GATA1, encoding the X-linked erythroid-specific transcription factor GATA binding protein 1 (GATA1), has been identified in an individual with CEP, thrombocytopenia, and β-thalassemia.

TREATMENT

ERYTHROPOIETIC PROTOPORPHYRIA

EPP is an autosomal recessive disorder resulting from the deficient activity of FECH, the last enzyme in the heme biosynthetic pathway. EPP is the most common erythropoietic porphyria in children and, after PCT, the second most common porphyria in adults. EPP patients have FECH activities as low as 15–25% of normal in lymphocytes and cultured fibroblasts. Protoporphyrin accumulates in bone marrow reticulocytes and then appears in plasma, is taken up in the liver, and is excreted in bile and feces. Protoporphyrin transported to the vessels in the skin causes the nonblistering phototoxicity. In most symptomatic patients (~90%) with this disorder, a deleterious mutation in one FECH allele was inherited with the relatively common (~10% of Caucasians) intronic 3 (IVS3) alteration (IVS3–48T>C) on the other allele that results in the low expression of the normal enzyme. In about 2% of EPP families, two FECH deleterious mutations have been found.

XLP is a less common condition with the same phenotype in affected males, including increased erythrocyte protoporphyrin levels resulting from gain-of-function mutations in the last exon of the erythroid-specific form of 5-aminolevulinate-synthase 2 (ALAS2). These mutations delete ALAS2 C-terminal amino acids resulting in its increased activity and the subsequent accumulation of protoporphyrin. Manifestations in female heterozygotes with XLP can range from asymptomatic to as severe as their affected male relatives. The variation in the presence and severity of manifestations in XLP heterozygotes results primarily from random X-chromosomal inactivation. XLP accounts for ~2–10% of cases with the EPP phenotype in Europe and North America.

Clinical Features

In EPP and male XLP patients, skin photosensitivity, which differs from that in other porphyrias, usually begins in childhood and consists of pain, tingling, and itching occurring within minutes of sunlight exposure (Fig. 409-4). Photosensitivity is associated with substantial elevations in erythrocyte protoporphyrin and occurs only in patients with genotypes that result in FECH activities below ~35% of normal. Vesicular lesions are uncommon. Redness, swelling, burning, and itching can develop shortly after sun exposure and resemble angioedema. Pain symptoms may seem out of proportion to the visible skin involvement. Sparse vesicles and bullae occur in ~10% of cases. Chronic skin changes may include lichenification, leathery pseudovesicles, labial grooving, and nail changes. Severe scarring is rare, as are pigment changes, friability, and hirsutism. Unless hepatic or other complications develop, protoporphyrin levels and symptoms of photosensitivity remain remarkably stable over many years in most patients. Factors that exacerbate the hepatic porphyrias play little or no role in EPP or XLP.

The primary source of excess protoporphyrin is the bone marrow reticulocytes. Erythrocyte protoporphyrin is free (not complexed with zinc) and is mostly bound to hemoglobin. In plasma, protoporphyrin is bound to albumin. Hemolysis and anemia are usually absent or mild.

Although EPP is an erythropoietic porphyria, up to 20% of EPP patients may have minor abnormalities of liver function, and in about 5% of these patients the accumulation of protoporphyrins causes chronic liver disease that can progress to liver failure requiring transplantation. Protoporphyrin is insoluble, and excess amounts form crystalline structures in liver cells (Fig. 409-4) and can decrease hepatic bile flow. Studies in the mouse model of EPP have shown that the bile duct epithelium may be damaged by toxic bile, leading to biliary fibrosis. Thus, rapidly progressive liver disease appears to be related to the cholestatic effects of protoporphyrins and is associated with increasing hepatic protoporphyrin levels due to impaired hepatobiliary excretion and increased photosensitivity. The hepatic complications also are often characterized by increasing levels of protoporphyrins in erythrocytes and plasma as well as severe abdominal and back pains, especially in the right upper quadrant. Gallstones composed at least in part of protoporphyrin occur in some patients. Hepatic complications appear to be higher in autosomal recessive EPP due to two FECH mutations and in males with XLP.

Diagnosis

A substantial increase in erythrocyte protoporphyrin, which is predominantly free and not complexed with zinc, is the hallmark of EPP. Protoporphyrin levels are also variably increased in bone marrow, plasma, bile, and feces. Erythrocyte protoporphyrin concentrations are increased in other conditions such as lead poisoning, iron deficiency, various hemolytic disorders, all homozygous forms of other porphyrias, and sometimes even in acute porphyrias. In all these conditions, however, in contrast to EPP, protoporphyrin is complexed with zinc. Therefore, after an increase in erythrocyte protoporphyrin is found in a suspected EPP patient, it is important to confirm the diagnosis by an assay that distinguishes free and zinc-complexed protoporphyrin. Erythrocytes in EPP also exhibit red fluorescence under a fluorescence microscopy at 620 nm. Urinary levels of porphyrins and porphyrin precursors are normal. FECH activity in cultured lymphocytes or fibroblasts is decreased (<30% of normal mean). DNA diagnosis by mutation analysis is recommended to detect the causative FECH mutation(s) and/or the presence of the IVS3–48T>C low expression allele. To date, >190 mutations have been identified in the FECH gene, many of which result in an unstable or absent enzyme protein (null alleles) (Human Gene Mutation Database; www.hgmd.org).

In XLP, the erythrocyte protoporphyrin levels appear to be higher than in EPP and the proportions of free and zinc protoporphyrins may reach 50%. To date, four ALAS2 mutations, three deletions of one to four bases, and one novel nonsense mutation have been described, which markedly increase ALA-synthase 2 activity and cause XLP. XLP accounts for about 2% of patients with the EPP phenotype in Western Europe. Recent studies show that about 10% of North American patients with the EPP phenotype have XLP.

TREATMENT

ACKNOWLEDGMENT

The authors thank Dr. Karl E. Anderson for his review of the manuscript and helpful comments and suggestions. This work is supported in part by the Porphyrias Consortium (U54 DK083909), a part of the National Institutes of Health (NIH) Rare Disease Clinical Research Network (RDCRN), supported through collaboration between the NIH Office of Rare Diseases Research (ORDR) at the National Center for Advancing Translational Science (NCATS), and the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK). The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health. MB is supported by a career development award K23-DK-095946.

FURTHER READING

INTRODUCTION

Purines (adenine and guanine) and pyrimidines (cytosine, thymine, uracil) serve fundamental roles in the replication of genetic material, gene transcription, protein synthesis, and cellular metabolism. Disorders that involve abnormalities of nucleotide metabolism range from relatively common diseases such as hyperuricemia and gout, in which there is increased production or impaired excretion of a metabolic end product of purine metabolism (uric acid), to rare enzyme deficiencies that affect purine and pyrimidine synthesis or degradation. Understanding these biochemical pathways has led, in some instances, to the development of specific forms of treatment, such as the use of allopurinol and febuxostat to reduce uric acid production.

URIC ACID METABOLISM

Uric acid is the final breakdown product of purine degradation in humans. It is a weak diprotic acid with pK_a values of 5.75 and 10.3. Urates, the ionized forms of uric acid, predominate in plasma, extracellular fluid, and synovial fluid, with ~98% existing as monosodium urate at pH 7.4.

Plasma is saturated with monosodium urate at a concentration of 405 μmol/L (6.8 mg/dL) at 37°C. At higher concentrations, plasma is therefore supersaturated—a situation that creates the potential for urate crystal precipitation. However, plasma urate concentrations can reach 4800 μmol/L (80 mg/dL) without precipitation, perhaps because of the presence of solubilizing substances.

The pH of urine greatly influences the solubility of uric acid. At pH 5.0, urine is saturated with uric acid at concentrations ranging from 360 to 900 μmol/L (6–15 mg/dL). At pH 7.0, saturation is reached at concentrations from 9840 to 12,000 μmol/L (158–200 mg/dL). Ionized forms of uric acid in urine include monosodium, disodium, potassium, ammonium, and calcium urates.

Although purine nucleotides are synthesized and degraded in all tissues, urate is produced only in tissues that contain xanthine oxidase, primarily the liver and small intestine. Urate production varies with the purine content of the diet and with rates of purine biosynthesis, degradation, and salvage (Fig. 410-1). Normally, two-thirds to three-fourths of urate is excreted by the kidneys, and most of the remainder is eliminated through the intestines.

The kidneys clear urate from the plasma and maintain physiologic balance by utilizing specific organic anion transporters (OATs), including urate transporter 1 (URAT1, SLC22A12) (Fig. 410-2). In humans, OAT1 (SLC22A6), OAT2 (SLC22A7), and OAT3 (SLC22A8) are located on the basolateral membrane of renal proximal tubule cells. OAT4 (SLC22A11), OAT10 (SLC22A13), and URAT1 are located on the apical brush-border membrane of these cells. The latter transporters carry urate and other organic anions into the tubular cells from the lumen in exchange for intracellular organic anions. Once inside the cell, urate must pass to the basolateral side of the lumen in a process controlled by voltage-dependent carriers, including glucose transporter 9 (GLUT9, SLC2A9). Uricosuric compounds (Table 410-1) directly inhibit URAT1 on the apical side of the tubular cell (so-called cis-inhibition). In contrast, antiuricosuric compounds (those that promote hyperuricemia), such as nicotinate, pyrazinoate, lactate, and other aromatic organic acids, serve as the exchange anion inside the cell, thereby stimulating anion exchange and urate reabsorption (trans-stimulation). The activities of URAT1, other OATs, and sodium anion transporters result in excretion of 8–12% of the filtered urate as uric acid.

Most children have serum urate concentrations of 180–240 μmol/L (3–4 mg/dL). Levels begin to rise in males during puberty but remain low in females until menopause. The most recent mean serum urate values for men and premenopausal women in the United States are 415 and 360 μmol/L (6.14 and 4.87 mg/dL), respectively, according to National Health and Nutrition Evaluation Survey (NHANES) data for 2007–2008. After menopause, values for women increase to approximately those for men. In adulthood, concentrations rise steadily over time and vary with height, body weight, blood pressure, renal function, and alcohol intake.

HYPERURICEMIA

Hyperuricemia can result from increased production or decreased excretion of uric acid or from a combination of the two processes. Sustained hyperuricemia predisposes some individuals to develop clinical manifestations including gouty arthritis (Chap. 365), urolithiasis, and renal dysfunction (see below).

In general, hyperuricemia is defined as a plasma (or serum) urate concentration >405 μmol/L (>6.8 mg/dL). The risk of developing gouty arthritis or urolithiasis increases with higher urate levels and escalates in proportion to the degree of elevation. The prevalence of hyperuricemia is increasing among ambulatory adults and even more markedly among hospitalized patients. The prevalence of gout in the United States more than doubled between the 1960s and the 1990s. Based on NHANES data from 2007 to 2008, these trends continue, with an approximate prevalence of gout among men of 5.9% (6.1 million) and among women of 2.0% (2.2 million). Mean serum urate levels rose to 6.14 mg/dL among men and 4.87 mg/dL among women, with consequent hyperuricemia prevalences of 21.2 and 21.6%, respectively (with hyperuricemia defined as a serum urate level of >7.0 mg/dL [415 μmol/L] for men and >5.7 mg/dL [340 μmol/L] for women). These numbers represent a 1.2% increase in the prevalence of gout, a 0.15-mg/dL increase in the serum urate level, and a 3.2% increase in the prevalence of hyperuricemia over figures reported in NHANES-III (1988–1994). These rises are thought to be driven by increased obesity and hypertension and perhaps also by better medical care and increased longevity.

CAUSES OF HYPERURICEMIA

Hyperuricemia may be classified as primary or secondary, depending on whether the cause is innate or an acquired disorder. However, it is more useful to classify hyperuricemia in relation to the underlying pathophysiology—i.e., whether it results from increased production, decreased excretion, or a combination of the two (Fig. 410-1, Table 410-2).

Increased Urate Production

Diet contributes to the serum urate concentration in proportion to its purine content. Strict restriction of purine intake reduces the mean serum urate level by ~60 μmol/L (~1 mg/dL) and urinary uric acid excretion by ~1.2 mmol/d (~200 mg/d). Foods high in nucleic acid content include liver, “sweetbreads” (i.e., thymus and pancreas), kidney, and anchovy.

Endogenous sources of purine production also influence the serum urate level (Fig. 410-3). De novo purine biosynthesis is a multistep process that forms inosine monophosphate (IMP). The rates of purine biosynthesis and urate production are predominantly determined by amidophosphoribosyltransferase (amidoPRT), which combines phosphoribosylpyrophosphate (PRPP) and glutamine. A secondary regulatory pathway is the salvage of purine bases by hypoxanthine phosphoribosyltransferase (HPRT). HPRT catalyzes the combination of the purine bases hypoxanthine and guanine with PRPP to form the respective ribonucleotides IMP and guanosine monophosphate (GMP).

Serum urate levels are closely coupled to the rates of de novo purine biosynthesis, which is driven in part by the level of PRPP, as evidenced by two X-linked inborn errors of purine metabolism (Table 410-3). Both increased PRPP synthetase activity and HPRT deficiency are associated with overproduction of purines, hyperuricemia, and hyperuricaciduria (see below for clinical descriptions).

Accelerated purine nucleotide degradation can also cause hyperuricemia—i.e., with conditions of rapid cell turnover, proliferation, or cell death, as in leukemic blast crises, cytotoxic therapy for malignancy, hemolysis, or rhabdomyolysis. Hyperuricemia can result from excessive degradation of skeletal muscle ATP after strenuous physical exercise or status epilepticus and in glycogen storage disease types III, V, and VII (Chap. 412). The hyperuricemia of myocardial infarction, smoke inhalation, and acute respiratory failure may also be related to accelerated breakdown of ATP.

Decreased Uric Acid Excretion

More than 90% of individuals with sustained hyperuricemia have a defect in the renal handling of uric acid. For any given plasma urate concentration, patients who have gout excrete ~40% less uric acid than those who do not. When plasma urate levels are raised by purine ingestion or infusion, uric acid excretion increases in patients with and without gout; however, in those with gout, plasma urate concentrations must be 60–120 μmol/L (1–2 mg/dL) higher than normal to achieve equivalent uric acid excretion rates.

Diminished uric acid excretion could theoretically result from decreased glomerular filtration, decreased tubular secretion, or enhanced tubular reabsorption. Decreased urate filtration does not appear to cause primary hyperuricemia but does contribute to the hyperuricemia of renal insufficiency. Although hyperuricemia is invariably present in chronic renal disease, the correlation among serum creatinine, urea nitrogen, and urate concentrations is poor. Extrarenal clearance of uric acid increases as renal damage becomes more severe.

Many agents that cause hyperuricemia exert their effects by stimulating reabsorption rather than inhibiting secretion. This stimulation appears to occur through a process of “priming” renal urate reabsorption through the sodium-dependent loading of proximal tubular epithelial cells with anions capable of trans-stimulating urate reabsorption. The sodium-coupled monocarboxyl transporters SMCT1 and 2 (SLC5A8, SLC5A12) in the brush border of the proximal tubular cells mediate sodium-dependent loading of these cells with monocarboxylates. A similar transporter, SLC13A3, mediates sodium-dependent influx of dicarboxylates into the epithelial cell from the basolateral membrane. Some of these carboxylates are well known to cause hyperuricemia, including pyrazinoate (from pyrazinamide treatment), nicotinate (from niacin therapy), and the organic acids lactate, β-hydroxybutyrate, and acetoacetate. The mono- and divalent anions then become substrates for URAT1 and OAT4, respectively, and are exchanged for uric acid from the proximal tubule. Increased blood levels of these anions result in their increased glomerular filtration and greater reabsorption by proximal tubular cells. The increased intraepithelial cell concentrations lead to increased uric acid reabsorption by promoting URAT1-, OAT4-, and OAT10-dependent anion exchange. Low doses of salicylates also promote hyperuricemia by this mechanism. Sodium loading of proximal tubular cells also provokes urate retention by reducing extracellular fluid volume and increasing angiotensin II, insulin, and parathyroid hormone release. Additional OAT1, OAT2, and OAT3 are involved in the movement of uric acid through the basolateral membrane, although the detailed mechanisms are still being elucidated.

GLUT9 (SLC2A9) is an electrogenic hexose transporter with splicing variants that mediate co-reabsorption of uric acid along with glucose and fructose at the apical membrane (GLUT9ΔN/SLC2A9v2) as well as through the basolateral membrane (SLC2A9v1) and thus into the circulation. GLUT9 has recently been identified as a high-capacity urate transporter, with rates 45–60 times faster than its glucose/fructose transport activity. GLUT9 may be responsible for the observed association of the consumption of fructose-sweetened soft drinks with an increased risk of hyperuricemia and gout. Genome-wide association scanning suggests that polymorphisms in SLC2A9 may play an important role in susceptibility to gout in the Caucasian population. The presence of one predisposing variant allele increases the relative risk of developing gout by 30–70%, most likely by increasing expression of the shorter isoform, SLC2A9v2 (GLUT9ΔN). Notably, though, genetic polymorphisms explain only ~6% of the differences in serum uric acid levels in Caucasians. Clearly, gout is polygenic and complex, and at this time the utility of genetic testing for relevant polymorphisms remains investigational and of no clinical utility.

Alcohol promotes hyperuricemia because of increased urate production and decreased uric acid excretion. Excessive alcohol consumption accelerates hepatic breakdown of ATP to increase urate production. Alcohol consumption can also induce hyperlacticacidemia, which blocks uric acid secretion. The higher purine content in some alcoholic beverages may also be a factor. Consumption of beer confers a greater risk of gout than liquor, and moderate wine intake does not increase gout risk. Intake of red meat and fructose increases the risk of gout, whereas intake of low-fat dairy products, purine-rich vegetables, whole grains, nuts and legumes, less sugary fruits, coffee, and vitamin C reduces the risk.

EVALUATION

Hyperuricemia does not necessarily represent a disease, nor is it a specific indication for therapy. The decision to treat depends on the cause and the potential consequences of hyperuricemia in each individual.

Quantification of uric acid excretion can be used to determine whether hyperuricemia is caused by overproduction or decreased excretion. On a purine-free diet, men with normal renal function excrete <3.6 mmol/d (600 mg/d). Thus, the hyperuricemia of individuals who excrete uric acid above this level while on a purine-free diet is due to purine overproduction; for those who excrete lower amounts on the purine-free diet, it is due to decreased excretion. If the assessment is performed while the patient is on a regular diet, the level of 4.2 mmol/d (800 mg/d) can be used as the discriminating value.

COMPLICATIONS

The most recognized complication of hyperuricemia is gouty arthritis. NHANES 2007–2008 found a prevalence of gout among U.S. adults of 3.9%, with figures of ~6% for men and ~2% for women. The higher the serum urate level, the more likely an individual is to develop gout. In one study, the incidence of gout was 4.9% among individuals with serum urate concentrations >540 μmol/L (>9.0 mg/dL) as opposed to only 0.5% among those with values between 415 and 535 μmol/L (7.0 and 8.9 mg/dL). The complications of gout correlate with both the duration and the severity of hyperuricemia. For further discussion of gout, see Chap. 365.

Hyperuricemia also causes several renal problems: (1) nephrolithiasis; (2) urate nephropathy, a rare cause of renal insufficiency attributed to monosodium urate crystal deposition in the renal interstitium; and (3) uric acid nephropathy, a reversible cause of acute renal failure resulting from deposition of large amounts of uric acid crystals in the renal collecting ducts, pelvis, and ureters.

Nephrolithiasis

Uric acid nephrolithiasis occurs most commonly, but not exclusively, in individuals with gout. In gout, the prevalence of nephrolithiasis correlates with the serum and urinary uric acid levels, reaching ~50% with serum urate levels of 770 μmol/L (13 mg/dL) or urinary uric acid excretion >6.5 mmol/d (1100 mg/d).

Uric acid stones can develop in individuals with no evidence of arthritis, only 20% of whom are hyperuricemic. Uric acid can also play a role in other types of kidney stones. Some individuals who do not have gout but have calcium oxalate or calcium phosphate stones have hyperuricemia or hyperuricaciduria. Uric acid may act as a nidus on which calcium oxalate can precipitate or lower the formation product for calcium oxalate crystallization.

Urate Nephropathy

Urate nephropathy, sometimes referred to as urate nephrosis, is a late manifestation of severe gout and is characterized histologically by deposits of monosodium urate crystals surrounded by a giant-cell inflammatory reaction in the medullary interstitium and pyramids. The disorder is now rare and cannot be diagnosed in the absence of gouty arthritis. The lesions may be clinically silent or cause proteinuria, hypertension, and renal insufficiency.

Uric Acid Nephropathy

This reversible cause of acute renal failure is due to precipitation of uric acid in renal tubules and collecting ducts that obstructs urine flow. Uric acid nephropathy develops following sudden urate overproduction and marked hyperuricaciduria. Factors that favor uric acid crystal formation include dehydration and acidosis. This form of acute renal failure occurs most often during an aggressive “blastic” phase of leukemia or lymphoma prior to or coincident with cytolytic therapy but has also been observed in individuals with other neoplasms, following epileptic seizures, and after vigorous exercise with heat stress. Autopsy studies have demonstrated intraluminal precipitates of uric acid, dilated proximal tubules, and normal glomeruli. The initial pathogenic events are believed to include obstruction of collecting ducts with uric acid and obstruction of the distal renal vasculature.

If recognized, uric acid nephropathy is potentially reversible. Appropriate therapy has reduced the mortality rate from ~50% to near zero. Serum levels cannot be relied on for diagnosis because this condition has developed in the presence of urate concentrations varying from 720 to 4800 μmol/L (12–80 mg/dL). The distinctive feature is the urinary uric acid concentration. In most forms of acute renal failure with decreased urine output, urinary uric acid content is either normal or reduced, and the ratio of uric acid to creatinine is <1. In acute uric acid nephropathy, the ratio of uric acid to creatinine in a random urine sample or a 24-h specimen is >1, and a value that high is essentially diagnostic.

HYPERURICEMIA AND METABOLIC SYNDROME

Metabolic syndrome (Chap. 401) is characterized by abdominal obesity with visceral adiposity, impaired glucose tolerance due to insulin resistance with hyperinsulinemia, hypertriglyceridemia, increased low-density lipoprotein cholesterol, decreased high-density lipoprotein cholesterol, and hyperuricemia. Hyperinsulinemia reduces the renal excretion of uric acid and sodium. Not surprisingly, hyperuricemia resulting from euglycemic hyperinsulinemia may precede the onset of type 2 diabetes, hypertension, coronary artery disease, and gout in individuals with metabolic syndrome.

TREATMENT

HYPOURICEMIA

Hypouricemia, defined as a serum urate concentration <120 μmol/L (<2.0 mg/dL), can result from decreased production of urate, increased excretion of uric acid, or a combination of both mechanisms. This condition occurs in <0.2% of the general population and <0.8% of hospitalized individuals. Hypouricemia causes no symptoms or pathology and therefore requires no therapy.

Most hypouricemia results from increased renal uric acid excretion. The finding of normal amounts of uric acid in a 24-h urine collection from an individual with hypouricemia is evidence for a renal cause. Medications with uricosuric properties (Table 410-1) include aspirin (at doses >2.0 g/d), losartan, fenofibrate, x-ray contrast materials, and glyceryl guaiacolate. Total parenteral hyperalimentation can also cause hypouricemia, possibly a result of the high glycine content of the infusion formula. Other causes of increased urate clearance include conditions such as neoplastic disease, hepatic cirrhosis, diabetes mellitus, and inappropriate secretion of vasopressin; defects in renal tubular transport such as primary Fanconi syndrome and Fanconi syndromes caused by Wilson’s disease, cystinosis, multiple myeloma, and heavy metal toxicity; and isolated congenital defects in the bidirectional transport of uric acid. Hypouricemia can be a familial disorder that is generally inherited in an autosomal recessive manner. Most cases are caused by a loss of function mutation in SLC22A12, the gene that encodes URAT-1, resulting in increased renal urate clearance. Individuals with normal SLC22A12 most likely have a defect in other urate transporters. Although hypouricemia is usually asymptomatic, some patients suffer from urate nephrolithiasis or exercise-induced renal failure.

SELECTED INBORN ERRORS OF PURINE AND PYRIMIDINE METABOLISM

More than 30 defects in human purine and pyrimidine metabolic pathways have been identified thus far (See also Table 410-3, Table 410-4, Fig. 410-3, and Fig. 410-4). Many are benign, but about half are associated with clinical manifestations, some causing major morbidity and mortality. Advances in genetics, along with high-performance liquid chromatography and tandem mass spectrometry, have facilitated diagnosis.

PURINE DISORDERS

HPRT Deficiency

The HPRT gene is located on the X chromosome. Affected males are hemizygous for the mutant gene; carrier females are asymptomatic. A complete deficiency of HPRT, the Lesch-Nyhan syndrome, is characterized by hyperuricemia, self-mutilative behavior, choreoathetosis, spasticity, and mental retardation. A partial deficiency of HPRT, the Kelley-Seegmiller syndrome, is associated with hyperuricemia but no central nervous system manifestations. In both disorders, the hyperuricemia results from urate overproduction and can cause uric acid crystalluria, nephrolithiasis, obstructive uropathy, and gouty arthritis. Early diagnosis and appropriate therapy with allopurinol can prevent or eliminate all the problems attributable to hyperuricemia without affecting behavioral or neurologic abnormalities.

Increased PRPP Synthetase Activity

Like the HPRT deficiency states, PRPP synthetase overactivity is X-linked and results in gouty arthritis and uric acid nephrolithiasis. Neurologic hearing loss occurs in some families.

Adenine Phosphoribosyltransferase (APRT) Deficiency

APRT deficiency is inherited as an autosomal recessive trait. Affected individuals develop kidney stones composed of 2,8-dihydroxyadenine. Caucasians with the disorder have a complete deficiency (type I), whereas Japanese individuals have some measurable enzyme activity (type II). Expression of the defect is similar in the two populations, as is the frequency of the heterozygous state (0.4–1.1 per 100). Allopurinol treatment prevents stone formation.

Hereditary Xanthinuria

A deficiency of xanthine oxidase causes all purine in the urine to occur in the form of hypoxanthine and xanthine. About two-thirds of deficient individuals are asymptomatic. The remainder develop kidney stones composed of xanthine.

Myoadenylate Deaminase Deficiency

Primary (inherited) and secondary (acquired) forms of myoadenylate deaminase deficiency have been described. The primary form is inherited as an autosomal recessive trait. Clinically, some persons may have relatively mild myopathic symptoms with exercise or other triggers, but most individuals with this defect are asymptomatic. Therefore, another explanation for the myopathy should be sought in symptomatic patients with this deficiency. The acquired deficiency occurs in association with a wide variety of neuromuscular diseases, including muscular dystrophies, neuropathies, inflammatory myopathies, and collagen vascular diseases.

Adenylosuccinate Lyase Deficiency

Deficiency of this enzyme is due to an autosomal recessive trait and causes profound psychomotor retardation, seizures, and other movement disorders. All individuals with this deficiency are mentally retarded, and most are autistic.

Adenosine Deaminase Deficiency and Purine Nucleoside Phosphorylase Deficiency

See Chap. 344.

PYRIMIDINE DISORDERS

The pyrimidine cytidine is found in both DNA and RNA; it is a complementary base pair for guanine. Thymidine is found only in DNA, where it is paired with adenine. Uridine is found only in RNA and can pair with either adenine or guanine in RNA secondary structures. Pyrimidines can be synthesized by a de novo pathway (Fig. 410-4) or reused in a salvage pathway. Although >25 different enzymes are involved in pyrimidine metabolism, disorders of these pathways are rare. Seven disorders of pyrimidine metabolism have been discovered (Table 410-4), three of which are discussed below.

Orotic Aciduria

Hereditary orotic aciduria is caused by mutations in a bifunctional enzyme, uridine-5′-monophosphate (UMP) synthase, which converts orotic acid to UMP in the de novo synthesis pathway (Fig. 410-4). The disorder is characterized by hypochromic megaloblastic anemia that is unresponsive to vitamin B₁₂ and folic acid, growth retardation, and neurologic abnormalities. Increased excretion of orotic acid causes crystalluria and obstructive uropathy. Replacement of uridine (100–200 mg/kg per day) corrects anemia, reduces orotic acid excretion, and improves the other sequelae of the disorder.

Pyrimidine 5′-nucleotidase Deficiency

Pyrimidine 5′-nucleotidase catalyzes the removal of the phosphate group from pyrimidine ribonucleoside monophosphates (cytidine-5′-monophosphate or UMP) (Fig. 410-4). An inherited deficiency of this enzyme causes hemolytic anemia with prominent basophilic stippling of erythrocytes. The accumulation of pyrimidines or cytidine diphosphate choline is thought to induce hemolysis. There is no specific treatment. Acquired pyrimidine 5′-nucleotidase deficiency has been reported in lead poisoning and in thalassemia.

Dihydropyrimidine Dehydrogenase Deficiency

Dihydropyrimidine dehydrogenase is the rate-limiting enzyme in the pathway of uracil and thymine degradation (Fig. 410-4). Deficiency of this enzyme causes excessive urinary excretion of uracil and thymine. In addition, this deficiency causes nonspecific cerebral dysfunction with convulsive disorders, motor retardation, and mental retardation. No specific treatment is available.

Medication Effect on Pyrimidine Metabolism

A variety of medications can influence pyrimidine metabolism. The anticancer agents fluorodeoxyuridine and 5-fluorouracil and the antimicrobial agent fluorocytosine cause cytotoxicity when converted to fluorodeoxyuridylate, a specific suicide inhibitor of thymidylate synthase. Fluorocytosine must be converted to 5-fluorouracil to be effective. This conversion is catalyzed by cytosine deaminase activity. Fluorocytosine’s action is selective because cytosine deaminase is present in bacteria and fungi but not in human cells. Dihydropyrimidine dehydrogenase is involved in the degradation of 5-fluorouracil. Consequently, deficiency of this enzyme is associated with 5-fluorouracil neurotoxicity.

Leflunomide, which is used to treat rheumatoid arthritis, inhibits de novo pyrimidine synthesis by inhibiting dihydroorotate dehydrogenase, resulting in an antiproliferative effect on T cells. Allopurinol, which inhibits xanthine oxidase in the purine metabolic pathway, also inhibits the activity of orotidine-5′-phosphate decarboxylase, a step in UMP synthesis. Consequently, allopurinol use is associated with increased excretion of orotidine and orotic acid. There are no known clinical effects of this inhibition.

ACKNOWLEDGMENT

The authors are grateful to Robert L. Wortmann for contributions to this chapter in previous editions of the book.

FURTHER READING

INTRODUCTION

Lysosomes are heterogeneous subcellular organelles containing specific hydrolyses that allow selective processing or degradation of proteins, nucleic acids, carbohydrates, and lipids. There are more than 50 different lysosomal storage diseases (LSDs), classified according to the nature of the stored material (Table 411-1). Several of the most prevalent disorders are reviewed here: Tay-Sachs disease, Fabry disease, Gaucher disease, Niemann-Pick disease, lysosomal acid lipase deficiency (LALD), the mucopolysaccharidoses, and Pompe disease. LSDs should be considered in the differential diagnosis of patients with neurologic, renal, or muscular degeneration and/or unexplained hepatomegaly, splenomegaly, cardiomyopathy, or skeletal dysplasias and deformations. Physical findings are disease specific, and enzyme assays or genetic testing can be used to make a definitive diagnosis. Although the nosology of LSDs segregates the variants into distinct phenotypes, these are heuristic; in the clinic, each disease exhibits—to varying degrees—a continuous spectrum of manifestations, from severe to attenuated variants.

PATHOGENESIS

Lysosomal biogenesis involves ongoing synthesis of lysosomal hydrolases, membrane constitutive proteins, and new membranes. Lysosomes originate from the fusion of trans-Golgi network vesicles with late endosomes. Progressive vesicular acidification accompanies the maturation of these vesicles; this gradient facilitates the pH-dependent dissociation of receptors and ligands and also activates lysosomal hydrolases. Lysosomes are components of the lysosome/autophagy/mitophagy system, which are disrupted in the LSDs.

Abnormalities at any biosynthetic step can impair enzyme activation and lead to a lysosomal storage disorder. After leader sequence clipping, remodeling of complex oligosaccharides (including the lysosomal targeting ligand mannose-6-phosphate as well as high-mannose oligosaccharide chains of many soluble lysosomal hydrolases) occurs during transit through the Golgi. Lysosomal integral or associated membrane proteins are sorted to the membrane or interior of the lysosome by several different peptide signals. Phosphorylation, sulfation, additional proteolytic processing, and macromolecular assembly of heteromers occur concurrently. Such posttranslational modifications are critical to enzyme function, and defects can result in multiple enzyme/protein deficiencies.

The final common pathway for LSDs is the accumulation of specific macromolecules within tissues and cells that normally have a high flux of these substrates. The majority of lysosomal enzyme deficiencies result from point mutations or genetic rearrangements at a locus that encodes a single lysosomal hydrolase. However, some mutations cause deficiencies of several different lysosomal hydrolases by alteration of the enzymes/proteins involved in targeting, active site modifications, or macromolecular association or trafficking. All LSDs are inherited as autosomal recessive disorders except for Hunter (mucopolysaccharidosis type II), Danon, and Fabry diseases, which are X-linked. Substrate accumulation leads to lysosomal distortion/dysfunction, which has significant pathologic consequences. In addition, abnormal amounts of metabolites may also have pharmacologic effects important to disease pathophysiology and propagation, particularly activation of the innate immune responses.

For many LSDs, the accumulated substrates are endogenously synthesized within particular tissue sites of pathology. Other diseases have greater exogenous substrate supplies. For example, substrates are delivered by low-density lipoprotein receptor–mediated uptake in Fabry and LALD or by phagocytosis in Gaucher disease type 1. The threshold hypothesis refers to a level of enzyme activity below which disease develops; small changes in enzyme activity near the threshold can lead to or modify disease. A critical element of this model is that enzymatic activity can be challenged by changes in substrate flux based on genetic background, cell turnover, recycling, or metabolic demands. Thus, a set level of residual enzyme may be adequate for substrate in some tissues or cells but not in others. In addition, several variants of each LSD exist at a clinical level. These disorders therefore represent a continuum of manifestations that are not easily dissociated into discrete entities. The molecular/genetic bases for such variations have not been elucidated in any detail.

SELECTED DISORDERS

TAY-SACHS DISEASE

About 1 in 30 Ashkenazi Jews is a carrier for Tay-Sachs disease (total hexosaminidase A [Hex A] deficiency), resulting from defective α-chains. The infantile form is a fatal neurodegenerative disease with macrocephaly, loss of motor skills, increased startle reaction, and a macular cherry red spot. The juvenile-onset form presents as ataxia and dementia, with death by age 10–15 years. The adult-onset disorder is characterized by clumsiness in childhood; progressive motor weakness in adolescence; and additional spinocerebellar and lower-motor-neuron signs and dysarthria in adulthood. Intelligence declines slowly, and psychiatric disorders are common. Screening for Tay-Sachs disease carriers is recommended in the Ashkenazi Jewish population. Sandhoff disease, due to a deficiency in both Hex A and Hex B resulting from defective β-chains, is phenotypically similar to Tay-Sachs disease, but also includes hepatosplenomegaly and bony dysplasias.

FABRY DISEASE