Chapter 132. The Porphyrias

The porphyrias are among the most intriguing human diseases. Widely variable, even bizarre in their clinical manifestations, these disorders of porphyrin or porphyrin-precursor metabolism result from aberrations in the control of the heme biosynthetic pathway. Heme is essential for oxygen binding and transport (as in hemoglobin and myoglobin), for electron transport (as in cytochromes), and for monooxygenases such as cytochrome P450. Chlorophyll, a magnesium-chelated porphyrin, is another important tetrapyrrole that is critical for photosynthesis, the specialized energy-storing system found in plants in which the conversion of light energy into stabilized chemical energy is achieved with a sequence of oxidation-reduction reactions. The corrin ring, a cobalt-chelated tetrapyrrole, is a major constituent of vitamin B12, the lack of which results in pernicious anemia. Therefore, porphyrins are ubiquitous and essential biochemical constituents of living beings. The biologic importance of the porphyrins and their iron complexes lies in their capacity to facilitate metabolic reactions, either as oxidative components in the metabolism of steroids, drugs, and environmental chemicals or by enhancing gas exchange, such as oxygen and carbon dioxide, between the environment and the tissues of the body.

Daily synthesis of porphyrins and heme in humans occurs in amounts sufficient to provide for the body's metabolic requirements. The control of heme synthesis is so precise that, under normal circumstances, only microgram quantities or less of pathway intermediates are present in plasma, red blood cells (RBC), urine, and stool (Table 132-1).

The porphyrias are a clinically and genetically heterogeneous group of metabolic diseases, which result from an either inherited or acquired dysfunction of enzymes crucial for heme biosynthesis (Fig. 132-1). Heme synthesis is controlled by eight enzymes along the heme biosynthetic pathway. Deficient activity of seven of these eight enzymes can give rise to a specific type of porphyria (Fig. 132-1).1–3 A gain of function of the first enzyme in the pathway, δ-aminolevulinic acid synthase (ALAS) is responsible for X-linked dominant protoporphyria (XLDPP), a recently recognized type of porphyria.4 Each of these enzymes will be discussed separately below, in the context of the corresponding type of porphyria.

Biochemically, the different porphyrias are characterized by particular patterns of accumulation and excretion of porphyrins and/or their precursors. In general, the porphyrins excreted in a specific type of porphyria are the irreversibly oxidized substrate(s) of the deficient enzyme (Table 132-2). These intermediates, when present in excess amounts, exert toxic effects that are responsible for the cutaneous and neurological signs and symptoms of clinically overt porphyria. The porphyrias are of particular dermatologic interest because several types exhibit cutaneous manifestations that may permit diagnosis from clinical signs alone.

The major sites of heme synthesis in the body are the bone marrow and the liver. Heme is the prosthetic group for a number of proteins, including among others hemoglobin, myoglobin, mitochondrial cytochromes, microsomal cytochromes (including cytochrome P450), catalase, peroxidase, tryptophanpyrrolase, prostaglandin endoperoxide synthase, and the soluble form of guanylate cyclase. Approximately 85% of heme synthesis occurs in the bone marrow where it is utilized for the production of hemoglobin; the majority of the rest occurs in the liver for making cytochrome P450, catalase, and various mitochondrial cytochromes. Heme is a critical cellular constituent essential for a variety of metabolic processes, primarily because of its unique ability to take up and release oxygen and to facilitate electron transport. Heme synthesis is regulated by the interplay of a number of factors and is directly dependent upon its concentration within cells and upon the requirements of the cell for production of the various hemoproteins described above. Many of these have rapid turnover times (minutes to hours), thereby necessitating continuously high rates of hepatic heme synthesis. For example, cytochrome P450, an important membrane-bound enzyme in the liver involved in the detoxification and metabolism of drugs, has a half-life of 90–180 minutes.

Regulation of Heme Biosynthesis

Heme synthesis begins in the mitochondrion of the cell where succinate and glycine (single molecules of glycine and succinyl CoA) are conjugated by ALAS to form a five-carbon aminoketone, δ-aminolevulinic acid (ALA). This reaction requires pyridoxal 5′-phosphate as a cofactor. The regulation of heme biosynthesis (and indeed the ability to synthesize heme at all) is dependent upon the serial interaction of eight intracellular enzymes (Figs. 132-1 and 132-2). The first, as well as the last three enzymes involved, coproporphyrinogen oxidase (CPOX), protoporphyrinogen oxidase (PPOX), and ferrochelatase (FECH) are mitochondrial, whereas the remaining enzymes including δ-aminolevulinic acid dehyratase (ALAD), porphobilinogen deaminase (PBGD), uroporphyrinogen synthase (UROS), and uroporphyrinogen decarboxylase (UROD) are localized in the cytosol. The degradation of heme is catalyzed by the microsomal enzymes NADPH-cytochrome C reductase, heme oxygenase 1 and 2. Heme oxygenase 1 is a -signature of oxidative tissue injury and is a potent antioxidant. Heme oxygenase 2 is an oxygen sensor. The metabolic by-products of these enzymes are the linear tetrapyrrole biliverdin IXα and carbon monoxide and also possess antioxidant properties. cDNAs for each of the eight enzymes involved in heme biosynthesis as well as the enzymes responsible for heme catabolism have been cloned in mammalian cells.1–3 Abnormal regulation of heme synthesis may result from defects in enzymes of the synthetic pathway, and this may occur as the result of inherited and/or environmental factors leading to the accumulation in the body of one or more heme pathway intermediates, such as the porphyrins or their precursors. Although a diagnosis of porphyria can often be made from a careful medical history (including the family history) and physical examination, the definitive diagnosis requires measurement of: (1) porphyrins and/or porphyrin precursors in urine, stool, plasma, or RBCs, or (2) the residual activity of specific enzymes in the heme pathway, or (3) the identification of mutations in the genes encoding these enzymes (Fig. 132-1). Modern molecular biological techniques have helped to clarify the mechanistic basis of the porphyrias.3–8 The steps involved in the biosynthesis of porphyrins and heme and their regulatory control are summarized in Figs. 132-1, 132-2, and 132-4.

δ-Aminolevulinic Acid Synthase

Traditionally, most classifications of the porphyrias are based on the primary site of expression of the specific enzymatic defect, thereby distinguishing erythropoietic and hepatic types. From a dermatologic perspective, the porphyrias might also be classified into cutaneous and noncutaneous forms. However, from a general clinical perspective, we have chosen to classify the porphyrias into two subtypes: (1) nonacute and (2) acute thereby distinguishing patients in whom dermatologic findings predominate from those susceptible to potentially life-threatening acute neurovisceral attacks (Tables 132-3 and 132-4). We will employ this classification throughout this chapter. It should be pointed out that two of the acute porphyrias, variegate porphyria (VP) and hereditary coproporphyria (HCP), are also referred to as neurocutaneous porphyrias since patients with these diseases may have both neurovisceral and cutaneous manifestations.

An acute porphyric attack can present a constellation of symptoms and signs, among them, intense abdominal pain, vomiting, electrolyte dysregulation (hyponatremia), constipation, tachycardia, hypertension, muscle pain and weakness, seizures, paresis of the upper and lower extremities, paralysis, and a variety of other neurological and psychiatric symptoms.11 These myriad clinical manifestations inevitably mimic other diseases, thereby challenging the diagnostic acumen of even the most experienced clinicians particularly since the porphyrias are rare disorders.12 It is known that acute porphyric attacks can be precipitated by a variety of external factors, in particular porphyrinogenic drugs, ethanol, hormonal fluctuations, and diminished caloric intake from dieting or fasting.11,13 Yet in practice, it is often impossible to identify the immediate cause of an acute attack in an individual patient. Timely diagnosis of an acute porphyric attack can be lifesaving because several complications may have fatal consequences if not recognized and treated. It is crucial to keep in mind that many drugs are capable of inducing heme synthesis in the liver.11,13 Thus, in a patient an acute porphyria with episodic abdominal pain, treatment with certain analgesics may prolong or aggravate the attack. Diffuse abdominal pain can mimic acute appendicitis, diverticulitis, intestinal obstruction, or other painful gastroenterological disorders that may necessitate urgent surgical intervention.14,15 After administration of certain drugs (e.g., barbiturates) or during general anesthesia, acute attacks may ensue.1 A particularly problematic complication is muscle weakness. This can range from mild paralysis of small muscle groups to flaccid paralysis of multiple muscle groups resulting in paraplegia and respiratory compromise. These findings may be accompanied by severe sensory loss and amyotrophy.16 In some cases, paralysis may progress rapidly and can affect cranial nerves or develop into a Guillain–Barré-like syndrome.17

(See Tables 132-3 and 132-4)

Patients with all forms of nonacute porphyria manifest cutaneous findings predominantly on sun-exposed body areas. The nonacute porphyrias include porphyria cutanea tarda (PCT), the most prevalent type of porphyria overall, hepatoerythropoietic porphyria (HEP), the homozygous variant of PCT, erythropoietic protoporphyria (EPP), XLDPP, the most recently recognized type of porphyria, and congenital erythropoietic porphyria (CEP) (Table 132-3). The nonacute porphyrias can present with variable cutaneous features, including mild-to-severe photosensitivity, increased skin fragility, vesicles and bullae, scarring with milia formation, burning and stinging, edema, pruritus, hypertrichosis, hyperpigmentation, and mild-to-severe scleroderma-like changes with tissue calcification.4

The acute porphyrias include acute intermittent porphyria (AIP), VP, HCP, and δ-aminolevulinic acid dehydratase (ALAD) deficiency porphyria (also known as plumboporphyria) (Table 132-4). Whereas AIP is the most prevalent form of acute porphyria worldwide, in some countries other acute forms may predominate, as is the case for VP in South Africa.1,2,18,19 Besides neurovisceral symptoms, individuals suffering from VP or HCP can also present with cutaneous findings, including increased photosensitivity, abnormal skin fragility, and bullous skin lesions on sun-exposed areas, erosions, chronic scarring, and postinflammatory pigmentary change. Thus, VP and HCP are also referred to as neurocutaneous porphyrias, to distinguish them from AIP and ALAD deficiency porphyria in which there are no cutaneous abnormalities.4,20

It is of interest that ethanol and estrogens, which can trigger acute porphyric attacks in AIP, VP, and HCP, can also exacerbate PCT, although acute attacks do not occur in this disease and the mechanisms likely differ.21

Pathophysiology of Cutaneous Signs and Symptoms in the Nonacute Porphyrias

The most common cutaneous manifestation of the nonacute porphyrias is photosensitivity; it is observed in patients with several of these disorders (PCT, HEP, EPP, XLDPP, CEP) as well as in patients with VP and HCP who have both cutaneous and neurovisceral findings. The sine qua non of photosensitivity in porphyria is increased plasma and tissue porphyrins. Patients with AIP, in whom only nonphotosensitizing porphyrin precursors (ALA, PBG) are formed in substantially increased amounts, do not have photosensitivity nor do any patients with ALAD deficiency porphyria.

Patients with the erythropoietic porphyrias frequently complain of a painful and intense burning sensation and pruritus during or following sun exposure. A “priming” effect of sun exposure on cutaneous photosensitivity in EPP has been described in which a threshold exposure on one day that evokes virtually no reaction seems to augment the response to subsequent exposure on the following day.22 The cause of this is unknown, but the phenomenon suggests that repair of damage to skin by photosensitized porphyrins may be prolonged. These acute symptoms are notably absent in patients with the chronic hepatic porphyrias such as PCT and VP. Although there is some information about the pathophysiology of photosensitivity and sclerodermoid skin changes in the chronic hepatic porphyrias, the causes of the pigmentary alterations and hypertrichosis seen in these patients remain to be elucidated.

Porphyrins have certain unique photobiologic and spectroscopic properties that make them potent photosensitizers. Importantly, metal-chelated porphyrins (e.g., heme or FePROTO) show no fluorescence and are not photosensitizers. Porphyrins that are chelated to other paramagnetic metals, such as Mn2+, Co2+, or Zn2+ (e.g., chlorophyll, ZnPROTO), also do not fluoresce. For this reason patients with lead intoxication, in which ZnPROTO is elevated, do not have photosensitivity.23,24 However, metal-free porphyrins including, Uroporphyrin (URO), Coproporphyrin (COPRO), and Protoporphyrin (PROTO) in acidic solutions show a major absorption peak in the 400- to 410-nm region (Soret band); they also exhibit four additional absorption bands with decreasing intensity between 500 and 700 nm. Exposure of porphyrins to the Soret band spectra results in fluorescence emission peaks between 550 and 680 nm (Table 132-5). Although there is no single clearly defined pathway that can currently explain the photosensitization evoked by porphyrins and light, there are a number of potential cellular and soluble factors that are likely to be involved. Among them, reactive oxygen species (ROS) and their effect upon certain cells (erythrocytes, mast cells, polymorphonuclear cells, and fibroblasts), and soluble mediators (the complement system and matrix metalloproteinases) are discussed (Table 132-6). It is likely that a combination of these factors will prove to be responsible for the pathogenesis of the cutaneous lesions in the porphyrias.

Of note, porphyrin abnormalities can also occur in lead poisoning, sideroblastic, hemolytic and iron deficiency anemia, renal failure, cholestasis, liver disease, and gastrointestinal hemorrhage. Of these, however, cutaneous photosensitivity has only been documented in rare cases of sideroblastic anemia.

Pathophysiology of Neurovisceral Signs and Symptoms in the Acute Porphyrias

Two acute porphyrias, AIP and ALAD deficiency porphyria, do not manifest cutaneous symptoms. In these diseases, the dysfunctional enzymes function early in heme biosynthesis and their substrates are nonphototoxic porphyrin precursors. However, patients with both AIP and ALAD deficiency porphyria as well as VP and HCP can manifest life-threatening acute neurovisceral attacks. Although the exact pathogenesis of these attacks is not well understood, it is known that the porphyrin precursors ALA and PBG are massively excreted from the liver during an acute attack and in particular ALA is known to be have neurotoxic properties. Furthermore, as a result of porphyria-related enzyme defects the net result may be heme deficiency in nerve tissue. Thus, the autonomic and peripheral nervous systems are particularly susceptible to porphyrin precursor induced neuropathy.25

Cellular and Soluble Factors Contributing to Photosensitivity

(See Table 132-6)

Reactive Oxygen Species

Porphyrins such as URO, COPRO, and PROTO absorb light intensely around 400 nm and in the longer visible spectrum between 580 and 650 nm. Absorption results in the generation of “excited” state porphyrin molecules (Fig. 132-5). The initial excited state porphyrin generated has an extremely short half-life of less than 0.01 μs. Singlet excited state porphyrins may spontaneously convert to a triplet, another excited state that has a lower energy level but a longer half-life (in the order of microseconds to milliseconds). Because of their longer half-life, triplet-state porphyrins have a higher probability of reacting with biologic substrates and are likely candidates to mediate most porphyrin-associated photobiological reactions. Excited state porphyrin molecules ultimately must return to their normal ground states by releasing their absorbed energy in the form of light (fluorescence is emitted by singlet state molecules and phosphorescence by triplet states), heat, or by transferring energy to cell constituents (cell membranes, organelles, proteins, DNA) (Figs. 132-5 and 132-6).

Excited porphyrins in their singlet and triplet states can also transfer their absorbed energy to oxygen molecules thereby creating reactive “excited” oxygen states.26 Cellular and tissue damage induced by photoactivated porphyrins is believed to occur primarily as a result of the formation of reactive singlet oxygen (1O2), as illustrated in Figs. 132-5 and 132-6.27,28 In some cases, the sensitized porphyrin may further react with oxygen to yield hydrogen peroxide (H2O2) or with water to form hydroxyl radicals (•OH). Those processes in which activated oxygen species play a role in photosensitization are referred to as photodynamic reactions.29 Most of the porphyrin-mediated cutaneous photosensitization reactions are essentially photodynamic (oxygen-dependent) and can be minimized or prevented if ROS are eliminated by inactivation processes known as quenching or scavenging.

Although these concepts regarding excited porphyrins and ROS are valid in simple systems and several hypotheses have been advanced regarding the mechanism of porphyrin-induced photosensitivity, their applicability to complex tissues such as the skin remains to be established.

Erythrocytes

Photoexcited PROTO elicits peroxidation of cholesterol groups in erythrocyte membranes, whereas URO or COPRO has no such effect. This difference is also true of mast cells, polymorphonuclear leukocytes (PML), and fibroblasts, and most likely relates to variations in lipid-water partitioning of the different porphyrins, which may in turn relate to their polarity. Lipophilic PROTO, a 2-carboxyl porphyrin, is more damaging to lipid-rich membranes as compared to the more hydrophilic COPRO and URO.30

Mast Cells

Phototoxic damage to mast cells is associated with elevated serum histamine levels and dermal mast cell degranulation,31 and the phototoxic response can be suppressed by pretreatment with antihistamines (H1 blockers) or by intradermal injection of a mast cell secretagogue (compound 48/80).32

Mast cell degranulation occurs in the exposed skin of patients with EPP.33 Irradiation of PROTO-treated mast cells in vitro induces release of mast cell-derived mediators, although exposure to lower doses of PROTO and radiation results in inhibition of secretagogue-induced histamine release from these cells.34 In contrast, no mediator release is detectable when cells are radiated in the presence of URO. These findings may help to explain the differences in the clinical presentation of patients with EPP and PCT. The elevated PROTO in patients with EPP may induce the release of mast cell mediators in vivo following sun exposure, resulting in painful, pruritic erythema and edema. The absence of these changes in patients with PCT may be explained by the apparent inability of photoexcited URO to damage cutaneous mast cells.

Polymorphonuclear Cells

Exposure of human PML to PROTO and radiation in vitro results in membrane damage, whereas photoexcited URO induces no such alterations.35 These results are strikingly similar to those obtained in studies with mast cells (see above). In an animal model, porphyrin-induced phototoxicity was associated with a dermal PML infiltrate31,36 and the response was markedly suppressed in leukopenic animals.32 These results appear to suggest that PML may be necessary but not sufficient to induce porphyrin-induced phototoxicity.

Fibroblasts

Differential phototoxic effects of various porphyrins have also been verified in studies using dermal fibroblasts. An increase in collagen biosynthesis is observed following incubation of fibroblasts with URO.37 This effect is independent of irradiation and may partly explain the sclerodermoid changes observed in patients with PCT, which can occur in sun-exposed and in sun-protected areas. In contrast, PROTO induces photolysis of fibroblasts in vitro.38,39

Matrix Metalloproteinases

Photoexcited URO has been shown to coordinately induce interstitial collagenase (MMP-1), 72-kDa type IV collagenase (MMP-2) and stromelysin-1 (MMP-3) in human dermal fibroblasts in vitro.40 The induction of the MMPs by porphyrin photosensitization suggests that activation/release of these enzymes could contribute to the blistering process.

Complement System

The complement system is activated by photoexcited porphyrins such as URO and PROTO. In vitro exposure to Soret band radiation activates the complement cascade and releases anaphylatoxins.41,42 Exposure of the skin of patients with EPP and PCT to Soret band energy in vivo also generates anaphylatoxins.43 Granular and homogeneous deposits of the complement membrane attack complex C5b9 have been observed in skin biopsies obtained from the skin of patients with PCT.44 It is of interest that target tissue heme pathway enzymes may be a crucial determinant of skin susceptibility to porphyrin photosensitization. Using a mouse model of EPP it has been shown that in transplanting bone marrow from EPP animals to wild-type mice the skin of the recipients was resistant to photosensitivity despite very high levels of plasma porphyrins suggesting that the normal FECH levels in the recipients somehow protected them against phototoxicity.45

In summary, cutaneous phototoxicity in the porphyrias is directly related to the interaction of porphyrins with light of the Soret and other bands in the solar spectrum, resulting in the generation of ROS. This in turn can induce lipid peroxidation and cell membrane alterations. Release of mediators and enzymes from cells such as mast cells and PML contributes to the inflammatory response. Variations in solubility influenced by the lipid–water partitioning of porphyrins may account for the striking patterns of the skin findings observed in various types of porphyria. It is important to emphasize that porphyrinogens (reduced porphyrins) are the true intermediates in heme synthesis. The irreversibly oxidized porphyrins, with the exception of PROTO, do not function as substrates for the enzymes of the pathway. Thus, porphyrins are actually heme pathway by-products, which are of special interest to dermatologists because of their unique photosensitizing properties.

(See Table 132-3)

Porphyria Cutanea Tarda (PCT)

Epidemiology

PCT occurs throughout the world and is the most common of all the porphyrias.46 The prevalence is estimated at 1:10,000.47 The disease most often begins in middle-aged individuals but can develop earlier. Prior to the widespread use of oral contraceptives, PCT developed predominantly in males.48,49 In contrast, others have emphasized that the sex incidence is approximately equal.50 The rising incidence of PCT in females is probably due to the widespread ingestion of estrogens in oral contraceptives or in hormone supplements. It should be noted that males treated with estrogens, for example, as adjunctive therapy for carcinoma of the prostate, have also developed PCT.

Etiology and Pathogenesis

PCT is due to either an inherited or acquired deficiency of UROD, the fifth enzyme in the heme pathway (Figs. 132-1 and 132-2) and it has a molecular mass of about 42 kDa.51 UROGEN I and III each contain eight carboxyl groups as side chains, four of which are acetate (–CH2COOH) and four of which are propionate (–CH2–CH2–COOH) moieties (Fig. 132-2). The soluble cytosolic enzyme UROD catalyzes the sequential oxidative decarboxylation of the four carboxyl groups of the acetate side chains to methyl groups to form COPROGEN (Fig. 132-2). Decarboxylation first occurs on ring D, after which the enzyme turns around to decarboxylate rings A, B, and C in a clockwise fashion. This converts the original 8-carboxyl porphyrinogen (UROGEN I or III) first to 7-carboxyl, then to 6-carboxyl, and 5-carboxyl porphyrinogen. The 5-carboxyl porphyrinogen can then undergo decarboxylation of its last acetyl group to form the 4-carboxyl porphyrinogen that is known as coproporphyrinogen (COPROGEN I or III) (Fig. 132-2). These intermediates are also referred to as hepta-, hexa-, penta-, and tetracarboxylate porphyrinogens. COPROGEN I cannot be further metabolized to heme.

Human UROD is encoded by a single copy of the gene of the same name that is located on chromosome 1p34 spreads over a genomic distance of 3 kb, and contains ten exons.51 The human UROD cDNA has been isolated, and the deduced sequence was found to be equivalent to 367 amino acids, consistent with the molecular mass and the amino acid composition of the purified protein.

Most classifications of PCT separate the disorder into at least two broad categories, both associated with decreased UROD activity: (1) acquired PCT, also referred to as sporadic or type I PCT; and (2) hereditary PCT, also referred to as familial or type II PCT.

In acquired (type I) PCT, the enzyme is deficient only in the liver and no mutations have been detected in the coding or promoter sequences of the UROD gene (Table 132-2).46,52 Some of the precipitating substances (e.g., alcohol and estrogen) may provoke PCT only in selected individuals and others [e.g., hexachlorobenzene (HCB)] in practically all exposed individuals (Table 132-7).

Hereditary (type II) PCT is an autosomal dominant disorder and the residual UROD activity is decreased approximately 50% in all tissues, including RBC and cultured skin fibroblasts.53–56 Decreased enzyme concentration appears to follow a bimodal distribution, suggesting two overlapping groups of patients: a large group (>80%) with normal UROD concentration and a small group (<20%) in who the amount of detectable enzyme is about half normal. Some patients with type II PCT were found to have UROD activity at the lower end of the normal range. The clinical penetrance of type II PCT is relatively low (approximately 20%), so that the majority of individuals with the inherited enzyme defect do not manifest the disease, suggesting that additional genetic or nongenetic factors are needed for disease expression. To date, more than 50 mutations in the UROD gene have been identified in type II PCT, mostly in exons 5–10, which encode 75% of the protein that either decrease the stability of the enzyme or produce defective pre-mRNA splicing.3,46,47,51,56–59

It is important to emphasize that not all patients with a positive family history of PCT will have type II disease. In support of this notion, several patients have been described with one or more relatives with type I PCT but with normal RBC UROD activities.60 This latter category of PCT has been designated as type III by some investigators.54 These patients could either have inherited some form of UROD that is immunochemically indistinguishable from the normal enzyme but which is uniquely susceptible to inhibition in the liver, or they may have a second inherited enzyme deficiency unrelated to UROD. These possibilities require further investigation.

It is likely that some patients who were initially reported as having hereditary PCT actually had VP, another dominantly inherited porphyria. PCT and VP may occur in different members of the same family, in the so-called dual porphyrias.61–63 Another form of dual porphyria in which PCT and AIP coexist has also been described.64

Numerous agents are known to contribute to the development of PCT (type I, acquired, or sporadic) including alcohol, estrogens, iron, viral infections (hepatitis C and HIV), polychlorinated hydrocarbons, particularly HCB and TCDD, and hemodialysis in patients with renal failure (Table 132-7). Each of these precipitating factors is discussed briefly below.

Alcohol

Alcohol ingestion has long been recognized to exacerbate PCT. Ethanol has been shown to induce hepatic ALAS1 in patients with PCT.65 Erythrocyte UROD activity is diminished in healthy subjects following acute ethanol ingestion and in chronic alcoholics.66 Ethanol can also inhibit the activity of other enzymes in the heme pathway, including FECH and ALAD. Chronic alcoholism leads to suppression of erythropoiesis67 and increased absorption of dietary iron, perhaps linked to inherited mutations associated with hemochromatosis (see below). The fact that ALAS1 is increased in patients with hepatic cirrhosis without porphyria raises questions concerning the relevance of alcohol effects on ALAS1 in the clinical expression of PCT.68

Estrogens

The widespread use of estrogens as contraceptive agents or as hormone supplements for postmenopausal replacement therapy in females and as adjunctive hormonal therapy in males with prostatic carcinoma has been associated with PCT.50 The mechanism of the estrogen effect on the expression of PCT has not been elucidated. Although diethylstilbestrol, an estrogen, induces hepatic ALAS1,69 this would not explain the distinctive porphyrin excretion pattern found in PCT. The vast majority of patients receiving estrogens do not manifest the biochemical abnormalities associated with PCT.

Hexachlorobenzene

This fungicide caused an “epidemic” of a PCT-like syndrome in Southeastern Turkey in the 1950s.70 It was added as a preservative to wheat intended for planting, but, because of a famine, several thousand individuals of diverse ethnic origin, mostly children, ingested the seed wheat and subsequently developed typical PCT. Over 4,000 cases of this syndrome were reported from 1956 to 1961. The porphyrin excretion pattern and the cutaneous findings in these patients were quite similar to those seen in PCT evoked by ethanol or estrogens. The outbreak of PCT in Turkey caused by ingestion of HCB indicated that the disease could occur in nongenetically predisposed individuals.

Twenty-five years later, the most common clinical findings in these HCB-poisoned individuals were those of chronic porphyria, including sclerodermoid scarring (84%), hyperpigmentation (78%), hirsutism (49%), thyromegaly, and increased skin fragility (38%).71 A painless arthritis was seen in two-thirds of affected individuals, and a variety of neurologic signs and symptoms occurred in the majority. Stool and urine porphyrins remained elevated in many patients.

Studies have shown that the chronic administration of HCB to experimental animals produces excessive porphyrin accumulation in the liver in a pattern quite similar to that seen in PCT in humans.72 These data are consistent with the hypothesis that chlorinated hydrocarbons, such as HCB, or their metabolites inhibit hepatic UROD, leading to excessive hepatic storage of URO and other acetate-substituted porphyrins.73,74 Experimental studies have also shown that HCB can inactivate UROD, thereby abolishing catalytic activity without changing the amount of immunoreactive enzyme protein.75 Chemical porphyria, similar to PCT, is caused by other chlorinated hydrocarbons such as the polychlorinated biphenyls (PCBs) and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), a by-product in the synthesis of the herbicide 2,4,5-trichlorophenoxyacetic acid.76 Additional studies of the porphyrinogenic effects of chlorinated hydrocarbons suggest that metabolic activation of the compounds mediated by cytochrome P450 1A2 and involving iron-generated ROS is associated with an attack on the catalytic site of UROD.56,77

Tetrachlorodibenzo-p-Dioxin

TCDD is a toxic environmental pollutant chemical. Among its numerous effects are chloracne, liver damage, and hepatic porphyria in experimental animals and perhaps also in humans.78 It has been shown that the hepatic porphyrinogenic effect of TCDD can be abolished in mice by first depleting them of iron.79 Furthermore, it is known that highly inbred mouse strains vary in their susceptibility to induction of hepatic porphyria by TCDD, indicating that the porphyrogenic effect of this hydrocarbon is modulated by as yet undefined genetic factors.80

Iron

Serum iron and ferritin concentrations are often elevated or in the upper range of normal in PCT, confirming the important role of iron in the pathogenesis of the disease.81 Hepcidin is a 25-amino acid peptide that is encoded in humans by the HAMP gene and it is secreted by the liver and inhibits iron transport by binding to the iron channel ferroportin located on the surface of gastrointestinal enterocytes and the plasma membrane of reticulo-endothelial cells. The net result is that iron is trapped in these cells and cannot be absorbed. Ajioka et al showed that hepatic HAMP expression is decreased in patients with PCT suggesting that the hepatic siderosis associated with PCT likely results from dysregulated HAMP.82 Hepatic iron overload accompanies clinical PCT in practically all cases, and elevation of plasma iron is found in one-third to one-half of patients.50 In PCT, the quantity of iron that can be mobilized by phlebotomy indicates that total body iron stores are approximately twice normal. Ferrokinetic studies in patients with PCT are said to be normal. The long remissions that follow repeated phlebotomy and the apparent ineffectiveness of this treatment if supplemental iron is administered concomitantly suggest that iron plays a role in the excessive hepatic porphyrin production of PCT.83 PCT is particularly common where alcoholism and iron overload occur together.

The role of iron in the pathogenesis of PCT is complex, and several hypotheses have been proposed to explain it. Iron may directly inhibit UROD. However, studies with purified UROD prepared from human erythrocytes show that the purified enzyme is not inhibited by Fe2+ or Fe3+.84 Chronic iron overload can produce peroxidative damage to lipid-rich mitochondrial and microsomal membranes in the liver of experimental animals, but the relationship of this toxic effect to changes in hepatic heme synthesis has not been clearly defined.85 An increased frequency of the hemochromatosis (HFE) gene C282Y mutation has been found in British patients with sporadic PCT.86,87 This mutation is responsible for much of the iron overload in populations of European descent. A second mutation in the HFE gene, H63D, may also be associated with PCT in some populations.88 PCT is rare in heterozygotes for each of these mutations but 20% of British patients with PCT are C282Y homozygotes. C282Y homozygosity is an important susceptibility factor for both type I and type II PCT.57 However, iron stores are increased in many patients with PCT without mutations in the HFE gene.46

Iron may have a permissive effect on the inhibition of UROD by halogenated hydrocarbons, and it can also enhance the induction response of hepatic ALAS1 to drugs.89,90 Although such an iron-augmented increase in ALAS1 activity could lead to enhanced porphyrinogenesis, this alone would not explain the porphyrin excretion pattern seen in PCT. It is known that addition of ferrous iron to liver in vitro causes a marked increase in porphyrin synthesis and inhibits UROS activity, providing an explanation for the URO isomer I excess characteristic of PCT.91 A mouse model of type II PCT has been described in which one allele of UROD is disrupted and the animals bred to mice null for HFE thereby creating a PCT-like phenotype.92

Viral Infections

There is an association between PCT and hepatitis C virus (HCV) infections and combined HCV and HIV infections.56,93,94 The role of these viruses in the pathogenesis of PCT is unclear, although some connection with the HFE gene mutation H63D has been suggested. It is also possible that the connection is fortuitous and secondary to nonspecific hepatotoxic effects of these viruses.

From these known effects of alcohol, estrogens, chlorinated hydrocarbons, iron, and viral infections on the heme pathway, it is clear that each of these could contribute to the excessive hepatic porphyrinogenesis characteristic of PCT.56 There is growing evidence that hepatic siderosis is the critical pathologic endpoint in PCT and that the other agents somehow intensify the ability of iron to attack the catalytic site of UROD. The clinical expression of PCT is therefore dependent upon the interaction of a number of factors, both genetic and environmental. However, it is important to note that the ingestion of drugs that have been associated with inducing acute neurovisceral attacks in the acute porphyrias is not known to exacerbate similar neurological crises in PCT.

Vesicles and bullae followed by erosions and crusting occur predominantly in areas subject to repeated trauma (Figs. 132-7 and 132-8). There is increased skin fragility, usually on the dorsa of the hands (Fig. 132-8). The traumatized skin becomes crusted and, as the lesions resolve, areas of scarring may ensue. Numerous small milia can develop, particularly on the fingers and hands. These pearly white to yellow papules occur in each of the hepatic porphyrias with cutaneous photosensitivity (PCT, VP, and HCP) (Fig. 132-7). Although the cutaneous lesions are primarily seen on the light-exposed areas, patients are often unaware that sunlight plays a role in producing their lesions because the acute painful and burning photosensitivity, so characteristic of the erythropoietic porphyrias, is rare in PCT. However, most patients do recognize that their skin condition worsens in the spring and summer and seems to improve in the fall and winter.

Other skin changes seen in PCT include hyperpigmentation and hypopigmentation that may be mottled, resembling chloasma. There may be an associated purplish-red (“heliotrope”) suffusion of the central part of the face, particularly involving the periorbital areas, which may bear a striking resemblance to the plethora seen in polycythemia rubra vera (Fig. 132-9). This is not seen in the porphyrias of bone marrow origin.

Hypertrichosis (nonvirilizing) is a useful diagnostic sign that often brings the female patient to the dermatologist (Fig. 132-10). Facial hypertrichosis develops gradually and occurs in both males and females. The hair may vary in texture between fine and coarse and in color between light and dark. These hairs are particularly prominent along the temples and the cheeks, but may occasionally involve the trunk and extremities in severe cases. Such hair may continue to grow, darken, and thicken, particularly on the cheeks, the forehead between the eyes, and at the hairline of the scalp. Males may complain that shaving is more difficult and that the growth pattern of their beard has changed. A particularly severe form of hypertrichosis may occur in younger children with PCT and HEP. In the reports of HCB poisoning in Turkey, some of the children were described as “monkey-like” because of marked hypertrichosis.95 The mechanism of this phenomenon is unknown; androgen levels are reported to be normal. It is possible that surface receptors or growth factors for hair bulb keratinocytes are activated by the dual action of light and porphyrins. The hypertrichosis of PCT usually improves slowly following depletion of excessive hepatic iron stores.

Sclerodermoid plaques can occur in PCT and in HEP and typically develop on both light-exposed and light-protected body areas (Figs. 132-11A and 11B). These are usually scattered, waxy yellow to white, indurated plaques that closely resemble, both clinically and histopathologically, morphea or scleroderma. There is some evidence to indicate that PCT may occur concomitantly with true scleroderma but this seems to be quite rare. As discussed earlier, URO I stimulates collagen synthesis in human skin fibroblasts.37 In some patients, calcification has developed in these sclerodermoid plaques, necessitating surgical excision and grafting.

PCT-like syndromes are occasionally seen in association with hepatic tumors and lupus erythematosus.96,97 Subepidermal bullous dermatoses mimicking PCT clinically and histologically have been described (see Section “Pseudoporphyria”). A number of cases of true PCT have occurred in patients with renal failure undergoing hemodialysis.98 Confirmation of the diagnosis rests on detection of markedly elevated plasma porphyrins (usually 5–100-fold) and increased ISOCOPRO in the feces.

Laboratory Tests

Patients with PCT excrete increased amounts of porphyrins in the urine, which rarely may exhibit characteristic pink–red fluorescence when examined with a Wood's lamp. The porphyrin excretion pattern of PCT has three main features: (1) increased urinary excretion of URO and of other acetate-substituted porphyrins; (2) a distinctive pattern of excretion of isomer series I and III porphyrins; and (3) increased excretion of fecal ISOCOPRO.56,99,100 PCT patients excrete greatly increased amounts of urinary 8-carboxyl URO and also porphyrins with 7-, 6-, and 5-carboxyl groups; 4-carboxyl porphyrin (COPRO) is also increased but to a lesser extent than URO and rarely surpasses 600 μg per 24 hours (Table 132-8). In PCT, the hepatic UROD deficiency results in the accumulation of 5-carboxylporphyrinogen III (Figs. 132-1 and 132-2). This can be utilized as a substrate by the enzyme CPOX and thereby forming dehydroisocoproporphyrinogen, which in turn is oxidized to ISOCOPRO, resulting in the characteristic elevation of this compound in the feces of these patients.

The 8-carboxyl URO and 7-carboxyl porphyrins are the predominant urinary porphyrins in PCT (>90% of total porphyrins). The urinary porphyrin excretion pattern is a mixture of type I and type III isomers. URO is about 60% type I isomer and 40% type III; the 7- and 6-carboxyl porphyrins are >90% type III and <10% type I isomer; the 5- and 4-carboxyl porphyrins are about 50% each isomer. This distinctive isomer pattern is found consistently in patients with PCT.

In general, only trace amounts of URO are present in the stools of normal individuals. The porphyrin content of stool is increased in PCT and consists primarily of ISOCOPRO (type III), 7-carboxyl porphyrin, and lesser amounts of URO and COPRO. The total daily 24-hour fecal porphyrin excretion may exceed total urinary porphyrin excretion.

The ratio of URO to COPRO in the urine is often helpful in differentiating PCT and VP. Thus, in PCT, the URO:COPRO ratio usually exceeds >3:1, whereas in VP the ratio is typically less than 1:1. Occasionally, 24-hour urine porphyrins will be normal or only slightly increased in a patient with the cutaneous findings of PCT. This should alert the physician to suspect the diagnosis of VP and thus to evaluate stool porphyrins, as these are almost always elevated in patients with VP (see Section “Variegate Porphyria”).

It should be emphasized that in patients with clinical signs of PCT a fluorescent screening test for urinary porphyrins is often negative. In such patients, it is absolutely essential to measure plasma porphyrin levels and perform quantitative 24-hour urine URO and COPRO determinations and quantitative stool PROTO and COPRO determinations preferably using high-performance liquid chromatography, which often permit differentiation of PCT from VP. Rarely, some patients with VP will have normal fecal porphyrin excretion, and bile porphyrin measurements may be helpful in evaluating such patients.101

Virtually all patients with PCT have excessive total body iron stores manifested as increased serum iron, ferritin, and/or hepatocellular iron.46,56 Occasionally, there is mild erythrocytosis. An abnormal glucose tolerance test occurs in a minority of patients.

Biochemical tests for liver function often reveal elevated serum transaminases and γ-glutamyltranspeptidase levels. Serum iron and ferritin concentrations may be elevated. The measurement of erythrocyte UROD is useful for the detection of patients with type II PCT. Mutational analysis is an essential part of evaluating these patients (see below).

Histopathology

The characteristic histopathologic finding in PCT is a subepidermal blister (Fig. 132-12). Bullae characteristically show a corrugated, undulating base that has been termed festooned.102 There is little or no inflammatory infiltrate. PAS stain reveals a mild degree of thickening of the papillary vessel wall, not nearly so marked as that seen in patients with EPP. Reticulin staining demonstrates slight proliferation of fibers along the basement membrane. Direct immunofluorescence studies reveal deposition of C3, C5b-9, and IgG in a granular pattern at the dermal–epidermal junction and in and around vessel walls in affected individuals.44,103 These changes are most apparent in sun-exposed areas of patients with active disease and high urinary porphyrin excretion, and they decrease substantially in patients after appropriate treatment. It is possible that the deposition of immunoglobulins and complement is a nonspecific result of injury to the cutaneous tissue. Damage of the upper dermal vessels and at the dermal–epidermal junction suggests that structural changes evoked by porphyrin photosensitivity may be responsible for the unique skin fragility seen in PCT.

Regardless of these findings, it should be emphasized that histopathologic examination does not substantially contribute to confirming the diagnosis of PCT. This is also true for the other cutaneous porphyrias and, consequently, it is not essential to obtain a skin biopsy if one of the cutaneous porphyrias is suspected, mainly for two reasons. First, simple noninvasive biochemical laboratory techniques (see above) can usually permit presumptive diagnosis of porphyria and, second, external trauma (such as a biopsy or excision) inevitably constitutes an avoidable risk for delayed and/or dysfunctional wound healing that is a characteristic feature of all cutaneous porphyrias.

Differential Diagnosis

Other dermatoses that can be confused with PCT include VP, HCP, mild forms of HEP and/or CEP, pseudoporphyria, scleroderma, and epidermolysis bullosa acquisita (EBA). Each of these can be differentiated by appropriate porphyrin studies. Careful evaluation of urine, stool, and plasma porphyrins will almost always permit confirmation of the diagnosis of PCT. Nonsteroidal anti-inflammatory drugs such as naproxen and antibiotics such as the tetracyclines and nalidixic acid as well as a variety of other agents may rarely produce bullous lesions closely resembling PCT but in contrast show no evidence of abnormal porphyrins (see Section “Pseudoporphyria”).

Treatment

(See Box 132-1). Initially, a careful history should be taken in an effort to identify an environmental toxin, for example, alcohol, estrogen, or chlorinated hydrocarbon, which may have triggered the disease.46,56 If HCV or HIV infection is present these should be managed appropriately. Elimination of toxin exposure alone may result in gradual improvement. However, in most patients with PCT, more aggressive treatment is usually appropriate to accelerate therapeutic benefit and this currently consists of either repeated phlebotomy56,104,105 or orally administered antimalarial drugs, either chloroquine or hydroxychloroquine106,107 or a combination of both.108 Other forms of treatment that have been described include administration of iron chelators109,110 and the oral administration of cholestyramine.111

Phlebotomy

Phlebotomy is still the treatment of choice for PCT. Numerous reports have confirmed the safety and efficacy of this form of therapy112 which was introduced by Ippen.104 Phlebotomy is effective because it depletes the excessive hepatic iron stores characteristic of PCT. Biochemical remission of PCT has occurred in patients treated with phlebotomy with iron overload as well as in patients with quantitatively normal iron stores. Replenishment of iron following phlebotomy-induced remission of PCT has resulted in biochemical and clinical exacerbation of the disease. Abstinence from the environmental triggers alone, especially alcohol, may induce a clinical and biochemical remission, although this may take months to years.112

The efficacy of phlebotomy may relate to several effects:

1. Iron effect on ALAS1. Iron can enhance the induction response of ALAS1 to drugs and the hepatic porphyrinogenic response to HCB in experimental animals, and its depletion could render ALAS1 less inducible and thereby diminish hepatic porphyrinogenesis.

2. Iron depletion effects on other heme pathway enzymes. Ferrous iron inhibits UROD and increases the rate of hepatic porphyrin synthesis from ALA or PBG, suggesting that removal of iron can allow UROD activity to return to normal and/or reduce excessive porphyrinogenesis.81,91 This concept has been verified by the creation of genetically engineered mice in which iron overload inhibits the enzyme.

3. Iron effects on hepatic lipid peroxidation secondary to ROS. Iron depletion could reverse this response of the liver to iron overload.85

4. Iron effects on oxidation of URO. Removal of iron could result in a diminished capacity of the ferrous metal to irreversibly oxidize the UROGEN substrate to nonmetabolizable porphyrins.113 The first product in the oxidation of UROGEN to URO is uroporphomethene in which one methane bridge is oxidized.46 This metabolite is an inhibitor of UROD.

Phlebotomy is carried out as an ambulatory procedure. The total amount of blood removed varies widely, usually ranging from 1,500 to 12,000 mL. It is most convenient to use plastic blood-drawing bags available in any blood bank. Approximately 500 mL of blood is removed at weekly or biweekly intervals until the hemoglobin decreases to approximately 10 g/dL or until the serum iron drops to 50–60 μg/dL. Some believe that phlebotomies should be continued until serum ferritin falls to the lower range of normal. Patients are strongly encouraged to discontinue or decrease exposure to porphyrinogenic agents as this usually hastens clinical and biochemical remission.

It is particularly important to reassure the patient that clinical improvement may not become apparent for variable intervals after beginning the phlebotomies. Porphyrin excretion continues to fall long after phlebotomies are discontinued. Urinary URO excretion reaches normal levels (<100 μg per 24 hours) after 5–12 months in more than 90% of patients treated with regular phlebotomy. Blistering is the first sign to disappear, followed by improvement in skin fragility and in hypertrichosis over a period of 3–18 months. Even sclerodermoid changes can resolve slowly, although this may take several years. There are few published data on long-term follow-up of treatment, but most relapsed patients have again responded to repeated courses of phlebotomies.105 The length of remission induced by phlebotomy varies widely and ranges from 6 months to more than 10 years. At least 10%–20% of patients will relapse within 1 year.

Phlebotomy is a safe, effective, and relatively simple form of therapy with minimal associated morbidity. A few patients may complain of mild-to-moderate fatigue and weakness during the treatment period, but this usually resolves as the hemoglobin returns to normal. Phlebotomy remains the treatment of choice for uncomplicated PCT. The availability of more effective oral iron chelators such as deferiprone and deferasirox suggest that these agents may prove to be a useful alternative to phlebotomy in managing the iron overload of PCT.114

Antimalarials

In some patients, phlebotomy is not recommended or is contraindicated owing to the presence of anemia or cardiopulmonary disorders or HIV infection. In such cases low-dose antimalarial therapy is a useful alternative. The antimalarial aminoquinolines, chloroquine, and hydroxychloroquine, are used.106,107 The cutaneous signs of the disease cleared within 1 year in one patient who received 500 mg daily for several months. However, such doses may trigger severe hepatotoxicity in many PCT patients and this is no longer considered an acceptable approach.

Instead, low-dose chloroquine therapy at a dose of 125 mg twice weekly is effective in treating PCT and obviates the hepatotoxic effects of high-dose therapy.115 Beside the successful therapeutic outcome in single patients and small cohorts,116 low-dose chloroquine (125 mg twice weekly for 8–18 months) was employed successfully in more than 100 patients with PCT.117 Liver function tests and urinary porphyrins are monitored quarterly, and the medication continued until urinary URO is <100 μg per 24 hours. This usually requires 6–12 months of treatment. Studies comparing the therapeutic efficacy of phlebotomy with that of low-dose chloroquine suggest that they are equally efficacious and, likewise, the clinical and biochemical remission of PCT obtained with chloroquine appears to be identical in all respects to that evoked by phlebotomy. However, it has been reported that rapid relapse occurred in several patients treated with hydroxychloroquine118 and, therefore, chloroquine may be preferable. The mechanism of action of chloroquine may relate to formation of water-soluble drug-porphyrin complexes that are then excreted from the liver.119 However, Taljaard et al feel that chloroquine chelates iron in the hepatocyte and that the bound iron is then eliminated.116

The combination of phlebotomy and chloroquine treatment may reduce the severity of the hepatotoxic response to chloroquine and also accelerate remission of the disease.108 Patients are treated with a series of one to four phlebotomies prior to starting chloroquine (250 mg daily for 7 days). The procedure is repeated when signs of biochemical or clinical relapse develop, which may occur in 1–2 years. It should be emphasized that despite the tendency of the antimalarials to evoke hepatotoxic responses in patients with PCT, there is no evidence to suggest that the changes in hepatocellular pathology characteristic of PCT worsen as a result of treatment with these drugs.120 The antimalarials may cause retinopathy, and the low-dose regimen helps to minimize the risk of this complication. Pretreatment and semiannual ophthalmologic examinations should be obtained on patients treated with these drugs. In patients with PCT and chronic renal failure standard phlebotomy and antimalarial therapy are not feasible. The combination of high-dose recombinant erythropoietin and small volume phlebotomy has been successful in selected patients.121

Hepatoerythropoietic Porphyria

HEP is the homozygous/compound heterozygous variant of PCT and results from a profound deficiency of UROD due primarily to a G281E mutation (GGG → GAG, codon 281). Measurement of the enzyme in hemolyzed whole blood or skin fibroblasts from three unrelated patients with the disease showed that it was reduced to less than 5% of normal levels. There is no evidence as yet to show that clinical expression of HEP is related to exposure to environmental drugs or chemicals, as is true for PCT. In one family clinical, biochemical, and enzymatic studies of three generations in which a second-generation 31-year-old male was shown to have HEP revealed that both of the proband's parents and each of his three children had a moderate deficiency of UROD.122 Additional cases of HEP with UROD gene mutations other than G281E have been reported in which there is 30% residual UROD activity associated with a milder phenotype.

Epidemiology

Patients with HEP have been reported in Europe (predominantly in Spain), America, and Japan.

Clinical Manifestations

The disease typically manifests in early childhood and dark-red urine is often the first sign. Severe cutaneous photosensitivity with blistering and pruritus then ensues. The photosensitivity may diminish with age and is followed by hypertrichosis, hyperpigmentation, and scleroderma-like scarring similar to that seen in CEP (Günther's disease) (Fig. 132-13). Ocular manifestations include ectropion associated with cutaneous sclerosis and scleromalacia perforans. Splenomegaly has occurred in a small percentage of affected individuals, particularly after the age of 10, and hemolytic anemia has been documented as well. In some patients, liver function tests have been abnormal, but serum iron and iron binding are normal. In summary, the clinical manifestations of HEP resemble a combination of PCT and CEP.

Laboratory Findings

The urinary porphyrin pattern is similar to that found in PCT (see Table 132-8). However, elevated RBC PROTO in HEP helps to distinguish it from PCT. Elevated urinary URO I and III (URO:COPRO ratio>5∼1) and 7-carboxyl porphyrins (>90% isomer III), elevated fecal COPRO and ISOCOPRO, and elevated RBC PROTO (zinc-chelated) have been observed in all patients. These findings suggest that there is abnormal porphyrin synthesis in both the liver and the bone marrow. In contrast to PCT patients, serum iron concentrations are usually normal in HEP.

Histopathology

Skin biopsies of blistering lesions show subepidermal bullae resembling those of PCT and PAS-positive material is seen in and around dermal capillaries. However, skin biopsy is not essential for making the diagnosis of HEP.

Differential Diagnosis

These include childhood porphyrias (e.g., the rare homozygous or compound heterozygous variants of VP and HCP), clinically severe forms of PCT, and CEP. In contrast to HEP, CEP manifests erythrodontia and mutilating scarring.

Treatment

There is no known treatment aside from careful photoprotection, including protection from visible light.

Pseudoporphyria

This term is used to describe patients who clinically exhibit cutaneous manifestations resembling PCT without the characteristic abnormal porphyrin profile. This disorder may develop in association with ingestion of certain drugs such as furosemide,123 nalidixic acid.124 tetracycline,125 naproxen,126 pyridoxine,127 and isotretinoin.128

In the drug-induced type of pseudoporphyria, the blistering process is subepidermal with little or no dermal inflammation. Staining with PAS reveals little or no positive deposition around upper dermal blood vessels and capillary walls. Indirect immunofluorescence studies conducted on split skin reveal no dermal deposition of fluorescent material in pseudoporphyria. Direct immunofluorescence studies have shown patchy granular deposition of IgG and C3 at the basement membrane zone in pseudoporphyria, PCT, and in epidermolysis bullosa acquisita.

A bullous dermatosis that is morphologically and histologically indistinguishable from PCT has also been observed in patients with chronic renal failure treated with long-term hemodialysis. Such patients have moderately elevated plasma porphyrins (two- to fourfold) secondary to impaired renal clearance as well as diminished clearance through dialysis filters.

Epidemiology

Although the exact incidence of EPP is unknown, it has been reported with increasing frequency since its initial recognition in 1961, indicating that it is likely the second most common type of cutaneous porphyria after PCT. The disease occurs globally with a prevalence estimated between 1:75,000 and 1:200,000.129 A recent review of the demographic, clinical, biochemical and genotypic features of EPP in Sweden showed that the prevalence was 1: 180,000 and the most commonly reported age of onset of symptoms was the first year of life and the mean age at diagnosis was 22 years.130

Etiology

The specific defective enzyme in EPP is FECH, which is decreased in various tissues of affected patients, including RBC, mitogen-stimulated lymphocytes, and cultured skin fibroblasts of patients and unaffected carriers.131–134 FECH is located at the matrix face of the inner mitochondrial membrane and catalyzes the final step in the formation of heme, the incorporation of ferrous iron (Fe2+) into PROTO (Figs. 132-1 and 132-2). This lysine-rich enzyme has a molecular mass of 63 kDa, a pH optimum of 7.8, and is also referred to as heme synthase or proto-heme-ferro-lyase. Unlike other enzymes in the heme biosynthesis pathway, that require reduced forms of the porphyrins (porphyrinogens) as substrates, FECH, utilizes PROTO, the oxidized isomer of PROTOGEN. The endproduct of the pathway, ferrous-PROTO (FePROTO) or heme, diffuses out of the mitochondrion into the cytoplasm, where it is available to function as a prosthetic group by combining with appropriate apoproteins (Figs. 132-1, 132-3, and 132-4). Through rate of consumption of heme in the cytoplasm, heme thus controls its own synthesis based upon the needs of the cell.

The cDNA for human FECH has been cloned, sequenced, and shown to have 88% homology to the mouse enzyme. Human FECH mRNA is encoded by a single nuclear gene of the same name, ferrochelatase (FECH), that is located on chromosome 18q21.3 and spans about 45 kb of genomic DNA.134 The gene consists of eleven exons and ten introns. Molecular genetic studies have been conducted in patients with EPP and revealed more than 110 different mutations of various types with deletions and insertions being the most common. The molecular epidemiology of EPP involves three patterns of inheritance.135 In the majority, the disease is inherited in a pseudodominant manner such that disease expression only occurs in the setting of a FECH mutation that is inherited trans to a hypomorphic allele (FECH IVS3–48C, c.315–48C). This additional nucleotide alteration in trans leads to modulation of splicing and the use of a constitutively aberrant acceptor splice site. The aberrantly spliced mRNA is degraded by a nonsense-mediated decay mechanism, producing a decreased steady-state level of mRNA. This results in an additional in vivo decrease in residual FECH activity of about 15%–40% that diminishes catalytic activity to the point where the EPP phenotype is expressed.136 The second pattern of inheritance of EPP (approximately 5%) is autosomal recessive in which a FECH mutation occurs on both alleles and the third pattern is due to C-terminal deletions in the ALAS2 gene resulting in gain of function that causes EPP without anemia or iron overload. This is inherited in an X-linked dominant manner (see below).5

Clinical Manifestations

Skin

Typically, the disease begins early in life and is characterized by acute episodes of cutaneous photosensitivity including burning, stinging (smarting), and pruritus affecting light-exposed skin, particularly the nose, cheeks, and dorsal aspects of the hands. These are followed by erythema, edema (Fig. 132-14), urticarial lesions, and less commonly purpura. Symptoms may occur within minutes of sun exposure; it is often seasonal, starting early in the spring, continuing through the summer, and diminishing in the winter. The skin lesions often resolve slowly, leaving small, atrophic, waxy, or pitted scars (Fig. 132-15). There may be some pursing of perioral skin (pseudorhagades). The skin of the knuckles and fingers, particularly over the metacarpophalangeal and interphalangeal joints, often appears thickened, wrinkled, and waxy, resembling the features of aging (so-called old knuckles, in a child). This subtle change is pathognomonic. Superficial scarring over the bridge of the nose and small annular shallow scars occur on the face. Vesicular or bullous lesions are rare in temperate climates, although they have been reported in patients exposed to tropical sunlight. In most affected individuals cutaneous involvement persists throughout life, although some patients seem to become less symptomatic over time.

Liver

Severe life-threatening hepatotoxicity occurs in approximately 5% of patients with EPP. PROTO is a lipophilic molecule that is cleared by the liver and these patients are at increased risk for cholelithiasis and obstructive hepatic disease that ultimately compromises liver function. This may be aggravated by hemolysis of PROTO-laden RBCs creating a vicious cycle of increased erythropoiesis and increased PROTO loading of the liver. The net result of these alterations is terminal hepatic failure in about 5% of EPP patients.137 Affected individuals become severely jaundiced, develop hepatic cirrhosis, and may go into hepatic coma and die. Some EPP patients with end-stage liver disease may develop axonal neuropathy resembling that in the acute hepatic porphyrias despite normal serum ALA and PBG.138 Risk factors for EPP-associated hepatic failure remain unknown but parenchymal crystallization of incompletely cleared hepatic PROTO is a likely explanation. Mitoferrin1 (Mfrn1) is 1 of 2 homologous mitochondrial iron transporters and is required for mitochondrial iron delivery in developing erythroid cells. Deletion of Mfrn1 in hepatocytes has no phenotype or biochemical effect under normal conditions but in the presence of increased porphyrin synthesis, Mfrn1 deletion in hepatocytes results in decreased ability to convert protoporphyrin IX into heme, leading to protoporphyria, cholestasis, and bridging cirrhosis. Altered heme synthesis can cause hepatotoxicity.139 Experimental studies have shown that perfusion of rat liver with PROTO induces a dose-dependent cholestasis that could contribute to the hepatotoxicity of PROTO.140 Liver failure has also occurred in two siblings who inherited EPP in an autosomal recessive pattern as compound heterozygotes, suggesting that this genotype may somehow augment the risk of this complication.141 This is debatable since some compound heterozygotes maintain normal liver function while others develop liver failure.142

The cutaneous and hepatic symptoms described above are the major findings in EPP. There is no hypertrichosis, milia, sclerodermoid change, or hyperpigmentation as in PCT and no erythrodontia as in CEP. Hemolytic anemia is decidedly uncommon, although about 11% of patients with EPP have a mild anemia of unknown cause.129 Gallstones have been reported in some patients with EPP at a relatively early age; in one series, 12% of patients had cholelithiasis, of whom three underwent cholecystectomy.143

Laboratory Findings

The diagnosis of EPP is based on the detection of elevated free PROTO in the RBC and/or feces (Table 132-8). Controversy exists concerning the tissue origin of the excessive PROTO. Some have shown that RBC PROTO levels alone are sufficient to explain the increased levels of PROTO,24,144 whereas others have suggested that hepatic PROTO production contributes to the excessively high levels of circulating PROTO.145 In addition, there may be increased plasma PROTO, increased fecal COPRO, and occasionally slightly increased RBC COPRO. In patients with deteriorating hepatic function, the fecal excretion of PROTO may decrease as plasma levels and cutaneous photosensitivity increase.146

Examination of a blood smear under a fluorescent microscope reveals red-fluorescing RBC (5%–30%) (Fig. 132-16). Since the fluorescence is often transient and quite light sensitive the procedure should be carried out in subdued light or preferably in the dark. However, this is not a reliable diagnostic test, since RBCs from patients with chronic lead intoxication also show fluorescence.

A rapid quantitative microfluorometric assay for free erythrocyte and plasma PROTO may be used to screen for suspected EPP.147 In one series of 32 patients with EPP, the RBC PROTO levels ranged from 131 to 1617 μg/dL of RBC (normal <90 μg/dL of RBC).148 It has been shown that erythrocyte accumulation of PROTO in EPP patients influences hematologic and iron status. Patients with EPP had a mild anemia and thrombocytopenia that positively correlated with the amount of erythrocyte PROTO. Iron and transferrin saturation saturation negatively correlated with erythrocyte PROTO.

Histopathology

Skin

Immediately following irradiation, endothelial cell damage, mast cell degranulation, and polymorphonuclear cells are seen in the dermis.33 In sun-exposed areas, there is often marked eosinophilic homogenization and thickening of vessels in the papillary dermis due to the accumulation of an amorphous, homogeneous, slightly basophilic (hyaline-like) substance in and around the vessel walls.129 The perivascular deposits of concentric eosinophilic layers of hyaline-like material stain strongly positive with periodic acid-Schiff (PAS) and are diastase positive (Fig. 132-17). Histochemical studies suggest that this material may be a neutral glycoprotein with smaller amounts of acid mucopolysaccharide and lipids.149 The histologic findings are similar to those of lipoid proteinosis (hyalinosis cutis et mucosae).150 Electron microscopic studies in EPP show that the amorphous material consists of a multilayered partially fragmented basement membrane and finely fibrillar material of moderate density that permeates and surrounds the vessel walls.151 Other studies have shown that type IV collagen and laminin as well as amyloid P and fibronectin are deposited in the walls of dermal blood vessels.152

Liver

Light microscopy of liver biopsies has revealed slight portal and periportal fibrosis and deposition of brown pigment, which may occlude bile canaliculi and ducts; it is also present in hepatocytes, in Kupffer cells, and in periportal macrophages. The pigment is birefringent on polarization microscopy.153 Hepatocytes may also contain cytoplasmic or mitochondrial inclusions, which at the ultrastructural level appear as needle-like crystals (Maltese crosses), probably due to precipitated PROTO (Fig. 132-16).154 Biliary excretion of PROTO is coupled to secretion of phospholipid, translocated from the inner to outer canalicular membrane leaflet by the mdr2P glycoprotein.135 PROTO excretion from the liver requires bile acids and excessive amounts of the porphyrin inhibit its transport out of the hepatocyte.

Differential Diagnosis

EPP must be differentiated from other photosensitivity diseases, primarily, polymorphous light eruption, drug-induced phototoxicity, actinic prurigo, hydroa vacciniforme, idiopathic solar urticaria, and other types of porphyria, in particular the newly identified variant XLDPP. Photoallergic and phototoxic contact dermatitis and angioedema should also be considered.

In polymorphous light eruption (PMLE), the lesions are characteristically papules, plaques, and papulovesicles. A family history of photosensitivity is less common. Burning and stinging of the skin during or soon after sun exposure is unusual in PMLE, whereas it is a common feature of EPP. If doubts persist, RBC PROTO measurement or plasma porphyrin determination will differentiate the two. The same is true for idiopathic solar urticaria. Airborne contact dermatitis may involve nonlight-exposed areas such as the skin folds and the submental areas. Patch testing with suspected allergens and negative porphyrin values help to differentiate this condition. The lesions of angioedema may occur anywhere on the body, including the mucous membranes. Because the discomfort described by patients with EPP is often disproportionate to visible lesions, the disease can be confused with psychoneurosis or even malingering. Porphyrin determinations will clarify this issue.

Treatment

(See Box 132-2). Photoprotection is absolutely essential although the use of topical sunscreens, such as products containing mexoryl, titanium, or zinc oxides are at best only marginally helpful because absorption and reflection of visible light is modest. Other agents such as orally administered antimalarial drugs, cholestyramine, and vitamins E and C have all been suggested but are of unproven benefit.

Although not confirmed by double-blind placebo-controlled trials, tolerance to sunlight in some patients with EPP may improve with the use of orally administered β-carotene.155 Many patients claimed to triple their tolerance to sunlight after a course of β-carotene exceeding 2 months. However, these results are based on limited controlled laboratory testing and uncontrolled clinical impressions. Indeed, a recent review concluded that the available data are insufficient to prove the efficacy of any treatments for dermal photosensitivity in EPP.156,157 The mechanism of the putative photoprotective effect of β-carotene is not precisely known. Recent studies in cultured keratinocytes showed that β-carotene can suppress induction of matrix metalloproteinases (MMPs) including MMP-1, MMP-3, and MMP-10, and that these effects are mediated by quenching singlet oxygen-mediated induction of MMPs.158 In further studies using microarrays it was shown that β-carotene inhibited UVA-induced extracellular matrix degradation.159

The amino acid cysteine has been shown to improve photosensitivity.160 Hematin infusions may decrease the production of heme temporarily and are associated with a decrease in fecal and plasma PROTO in some patients.144 The use of RBC exchange with autologous washed cells has also been explored for its ability to induce clinical and biochemical remission of EPP. In one patient with EPP, such therapy resulted in a marked decrease in photosensitivity associated with a decline in free erythrocyte PROTO levels.161 In summary, the currently available treatments for EPP are minimally effective and do not address the underlying molecular pathogenesis of the disease.

Liver Disease

Management of EPP-associated hepatic failure is particularly challenging. Orally administered bile salts may diminish the enterohepatic recirculation of the excessive PROTO and thereby enhance the clearance capacity of the liver.162 Orally administered iron was attempted in one patient who manifested incipient hepatic failure as well as elevated erythrocyte and plasma PROTO levels with concomitant iron deficiency anemia but proved ineffective.163 In another patient oral iron therapy exacerbated the disease.164 Liver transplantation, bone marrow transplantation, and a combination of both have been used successfully in EPP, but in the case of liver transplantation recurrence of EPP hepatic disease is a significant risk.165–167 Future therapy could include recombinant enzyme replacement of defective FECH. The feasibility of obtaining detectable levels of the enzyme in fibroblasts from EPP patients by adding the cDNA for normal FECH employing a retroviral vector has been demonstrated. Gene replacement has been achieved in a mouse model of EPP in which ex vivo preselection of hematopoietic stem cells transduced with a polycistronic retrovirus expressing human FECH resulted in complete correction of skin photosensitivity.168

ALAS is the mitochondrial enzyme that catalyzes the formation of ALA and is a homodimer of two identical catalytically active subunits of molecular mass 55 kDa, linked to catalytically inactive substrates of higher molecular mass.3,8,9

The major significance of this enzyme is its role in controlling the rate of heme synthesis in the liver via a negative feedback mechanism. Heme acts at multiple sites to regulate hepatic ALAS. Currently, two differentially expressed isoforms of ALAS are known, one with ubiquitous expression, including the liver ALAS1, and the other with exclusive expression in erythroid tissues, ALAS2. It is thought that two pools, one cytosolic and one nuclear, regulate the respective isozymes. Two genes encoding different, tissue-specific ALAS mRNAs have been found. The human housekeeping gene is located on chromosome 3p21 and the erythroid gene has been assigned to a distal subregion of chromosome Xp21-Xq21. The erythroid preprotein reveals more than 75% homology with the hepatic protein. Immunochemical differences exist between the hepatic and erythroid enzyme in various animal species.9,10 In contrast to deficiencies of the second to eighth enzyme in the heme biosynthetic pathway, a reduction in residual ALAS activity is not associated with a specific type of porphyria.

Mutations in the ALAS2 gene were previously associated with X-linked sideroblastic anemia (see below.). However, eight families have been described with ALAS2 deletions resulting in frameshifts that cause replacement or deletion of the 19–20 terminal residues of the enzyme that in turn creates gain-of-function of the enzyme and markedly increases its catalytic activity.5 A characteristic biochemical feature in these patients is elevation of both free and zinc-chelated PROTO in the erythrocytes. The clinical manifestations of XLDPP closely resemble those of EPP (see above).

Congenital Erythropoietic Porphyria

Epidemiology

CEP occurs globally; fewer than 500 cases have been reported. The disease nearly always begins in childhood, although apparent adult onset has occurred in 13 cases.169 CEP also occurs in animals, including swine, cattle, and cats, and these animal models have greatly aided research in this disease.

Etiology

CEP is an autosomal recessive disorder that results from a variable but profound deficiency of UROS, the fourth enzyme in the heme synthetic pathway, which catalyzes the formation of uroporphyrinogen III from hydroxymethylbilane (HMB) or from PBG if PBGD is also present. The only difference between the porphyrinogen isomers I and III is the reversal of the side chains on the “D” ring of this tetrapyrrole. By simple inversion of one PBG molecule during synthesis of UROGEN I, the III isomer is formed (Fig. 132-2). This apparently minor structural alteration is of considerable biologic importance as only III isomers can form heme. The formation of UROGEN III is catalyzed by the cytoplasmic enzyme UROS, closely linked to PBGD. This enzyme has been purified from human erythrocytes, is monomeric, and has a molecular mass of approximately 29.5 kDa; it is thermolabile and exhibits maximum catalytic activity at pH 7.4.170 In most tissues, UROGEN III synthase is present in considerable excess as compared to PBGD, thus assuring efficient conversion to the III isomer. The hepatic and erythroid forms of the enzyme are identical.

Human UROS is a cytosolic protein encoded by a single copy gene of the same name, UROS, which consists of ten exons, and is located on chromosome 10q25.2-q26.3 Human UROS cDNA has been cloned and sequenced and encodes a protein of 265 amino acids.171 To date, approximately 30–40 mutations in the UROS gene have been identified including several in the erythroid-specific promoter. UROS deficiency results in the accumulation of HMB that mostly converts spontaneously to URO-1. CEP has also been reported in association with thalassemia secondary to a novel germ line mutation in the X-linked erythroid-specific transcription factor GATA binding protein 1 (GATA-1).172

Clinical Manifestations

The disease usually develops during the first few months of life with moderate-to-severe cutaneous photosensitivity associated with pink to red urine.173 The patients have excessive URO I and COPRO I in RBC, plasma, and skin.

Cutaneous manifestations include increased skin fragility, vesicles and bullae, which may contain pink fluorescent fluid. Secondary infection, delayed healing, and scarring is common. This may lead to loss of acral tissue, such as the tips of the ears, the nose, and fingers (Figs. 132-18A and 132-18B), and facial mutilation. CEP is the most mutilating of the cutaneous porphyrias. Hirsutism, with long, dark, lanugo-like hair, may occur and is particularly evident in light-exposed areas such as the face, neck, and extremities. The scalp may develop a cicatrizing alopecia. Other chronic findings include eye changes (photophobia, keratoconjunctivitis, ectropion, symblepharon, and even loss of vision) and irregular hyper- and hypopigmentation of the skin. Erythrodontia (red-stained teeth) is a common finding in both deciduous and permanent teeth and is virtually pathognomonic of CEP. The urine may also fluoresce reddish pink.

Associated, noncutaneous findings include splenomegaly, porphyrin-rich cholelithiasis, and fluorescent bone marrow normoblasts. Hemolytic anemia associated with shortened erythrocyte life span and hypersplenism (36 vs. 120 days) occurs. Whether the hemolytic anemia is due to “photohemolysis” of circulating porphyrin-laden erythrocytes or to an associated intracorpuscular red cell defect is unresolved.174

Laboratory Findings

The primary defect in CEP is decreased UROS activity resulting in accumulation of predominantly type I porphyrins in the tissues. The pink to burgundy red color of the urine from excess URO I is often visible on inspection. Marrow normoblasts exhibit relatively stable fluorescence and contain markedly elevated URO I, COPRO I, and PROTO. Urinary excretion of ALA and PBG is normal. COPRO I may be present in large amounts in the feces. The typical biochemical findings of CEP are summarized in Table 132-8.

Histopathology

The bullous lesion of CEP is subepidermal with minimal inflammation. Thickening of collagen bundles may be seen in areas of scarring. Perivascular deposits of porphyrin can be found when the unstained sections of skin or liver are viewed with a fluorescence microscope.

Differential Diagnosis

The diagnosis of CEP can usually be made from the early onset of severe cutaneous photosensitivity associated with reddish-pink fluorescent urine and erythrodontia. Distinguishing CEP from other congenital photodermatoses is important. Cutaneous findings in patients with xeroderma pigmentosum, epidermolysis bullosa, hydroa vacciniforme, and bullous pemphigoid may mimic those of patients with CEP, but porphyrin analysis will distinguish them.

Patients with chronic hepatic porphyrias such as PCT and VP have normal RBC PROTO. The cutaneous manifestations of HEP may be strikingly similar to those of CEP and measurement of RBC UROS (decreased in CEP) and UROD (decreased in HEP) will distinguish the two. The dual occurrence of HCP and CEP in a single patient who inherited the HCP trait from her mother and the CEP trait from both of her parents has been reported.175

Treatment

The treatment of CEP is essentially preventive and symptomatic and includes avoidance of sun exposure, surveillance of the anemia, and treatment of recurrent skin infections. Absolute protection from sunlight using photoprotective clothing may be of substantial benefit. Topical sunscreens have no proven efficacy. Orally administered β-carotene is not effective. Splenectomy performed to relieve intractable hemolytic anemia has occasionally resulted in marked improvement of both the anemia and cutaneous photosensitivity. Suppression of erythropoiesis by transfusion of packed erythrocytes can diminish porphyrin production and excretion176; however, secondary iron overload is of concern. Oral administration of activated charcoal to a single patient (60 g three times a day) for 9 months decreased porphyrin levels in plasma and skin with complete remission of photosensitivity during the treatment period.177 Charcoal is thought to interfere with the enterohepatic recirculation of the porphyrin. However, this beneficial effect has been disputed.178 Allogeneic bone marrow transplantation has been curative for a small number of patients and, thus, seems to be the current first-line treatment.179,180

(See Table 132-4)

Acute Intermittent Porphyria

Epidemiology

This form of porphyria occurs globally and the incidence in the human population is approximately 1.5:100,000, although it is much higher in Scandinavia, particularly Lapland (1:1,000).181 AIP rarely, if ever, manifests before puberty. Most published series emphasize the female predominance of affected patients, ranging from ratios of 1.5–2.0:1.7

Etiology

AIP is due to a deficiency of PBGD also known as hydroxymethylbilane synthetase (HMBS), the third enzyme in the heme biosynthetic pathway (Figs. 132-1 and 132-2). This enzyme combines four molecules of the monopyrrole PBG to form the linear tetrapyrrole HMB, which cyclizes spontaneously to form the initial porphyrinogen, or tetrapyrrole, known as uroporphyrinogen I (UROGEN I) (Fig. 132-2). Depending on the manner in which the PBG molecules are arranged, several different isomers of the tetrapyrroles are possible. The sole difference between type I and type III porphyrinogens is that one of the four pyrrole rings is “flipped over.” Only two of them (labeled I and III) are known to occur in the mammalian heme synthetic pathway. PBGD was purified from human RBC and shown to have a molecular mass of approximately 37 kDa and a pH optimum of 8.2.182 In AIP patients, PBGD activity is approximately 50% of that in unaffected normal individuals, consistent with an autosomal dominant mode of inheritance. Diminished PBGD partially blocks heme synthesis, which in turn decreases the regulatory heme pool and leads to derepression of ALAS mRNA synthesis.

The PBGD gene has been cloned and characterized.183 Two isoforms, one ubiquitous, and one erythroid-specific, are encoded by a single gene localized to chromosome 11q23→11qter.3 It comprises 15 exons spread over 10 kb of DNA. The two isoforms of the enzyme are encoded by two distinct mRNAs that arise from two overlapping transcription units, the first of which (upstream) is active in all tissues and its promoter has some features of a housekeeping promoter, whereas the second, located 3 kb downstream, is active only in erythroid cells and displays structural homology with β globin gene promoters. Differential splicing gives rise to two isozymes of slightly different molecular size.8 To date, more than 200 mutations have been reported at the PBGD locus, including deletions, insertions, missense, nonsense, and splicing mutations.2,184

PBGD is present in the second lowest concentration of any enzyme in the heme pathway; ALAS is the lowest. This suggests that additional factors (genetic or acquired) that influence PBGD activity may have important regulatory influences on the rate of heme synthesis. Consequently, if excessive PBG is formed because of increased ALAS activity, as is often seen in the acute hepatic porphyrias, there may be only partial conversion of this monopyrrole to UROGEN I. These factors account for the elevated urinary PBG characteristic of attacks of the acute hepatic porphyrias. PBGD activity is known to be reduced by about 50% in the tissues of patients with AIP and is currently thought to be the primary enzymatic abnormality in this autosomal dominant disorder. There is a variant form of AIP (perhaps 10% of cases) in which the PBGD defect is restricted to nonerythropoietic tissues due to mutations in exon 1 that is absent from the erythroid mRNA.

Clinical Manifestations

(See Table 132-4). Less than 10% of latent carriers of the PBGD defect ever express the clinical phenotype. Hence, the gene defect alone seems to be inconsequential unless the individual is exposed to additional triggering factors, of which certain drugs that induce hepatic cytochromes (CYPs), are particularly important (Table 132-9).185,186 The clinical signs and symptoms of AIP may also relate to the effects of excessive porphyrin precursors or to heme depletion in the autonomic nervous system.5

The acute porphyric attack is characterized by abdominal pain (in 80%–90% of AIP patients) and neurologic and psychiatric symptoms that can mimic numerous disorders. The pain may be vague or localized and is often intermittent and spastic. Vomiting and constipation are frequently associated. Mild fever and leukocytosis may occur, making differential diagnosis extremely difficult. This is one reason many patients with AIP may have undergone multiple exploratory laparotomies prior to establishing the correct diagnosis. Between attacks patients are often remarkably symptom free. Attacks are often precipitated by ingestion of drugs such as those listed in Table 132-9. Acute attacks of AIP may be accompanied by seizures, especially in patients with hyponatremia resulting from vomiting and inappropriate fluid therapy. Peripheral neuropathy is a major part of the clinical syndrome in many patients varying from sensory (localized pain) to motor (weakness progressing to generalized flaccid paralysis).12,187 Patients may die during an attack, usually due to respiratory failure, or may improve slowly, although residual muscle weakness may be permanent.

Cutaneous photosensitivity does not occur in AIP. This is logical as the abnormal excretion pattern of the disease consists mostly of the nonphotosensitizing porphyrin precursors ALA and PBG.

Laboratory Findings

The primary gene defect in AIP results in PBGD deficiency, which causes excessive urinary excretion of ALA and PBG. Urinary porphyrins may also be slightly elevated (Table 132-8). Urinary excretion of ALA and PBG may be as high as 100 mg/24 hours in an acute attack. During remission urinary excretion of ALA and PBG decreases but usually remains above normal values. This is in contrast to patients with VP and HCP who often exhibit normal urinary excretion of ALA and PBG between acute attacks.

Two rapid screening tests are available to test freshly voided urine for increased PBG. The first is simply to expose the urine to bright sunlight for several hours. Darkening to a deep-red color suggests but does not prove that excessive PBG is present since porphobilin, another dark pigment, can also be photocatalytically formed in urine from AIP patients. The second rapid screening procedure, known as the Hoesch test is more definitive and involves a simple assay for detecting excessive urinary PBG.188 Two drops of fresh urine are added to 2 mL of Ehrlich's reagent (3 g of p-dimethylaminobenzaldehyde dissolved in 125 mL of acetic acid and 24 mL of perchloric acid). A uniform cherry-red color of the sample indicates a positive reaction. This test is based upon the formation of a chromogen by PBG and Ehrlich's aldehyde reagent that produces a red pigment with strong absorbance at 552 nm. The classic Watson–Schwartz test is based on this same principle. These qualitative tests while potentially useful for screening must be confirmed by quantitative 24-hour measurement of urinary ALA and PBG using ion-exchange chromatography or high-performance liquid chromatography.189

Histopathology

AIP does not manifest cutaneous symptoms.

Differential Diagnosis

The clinical manifestations of AIP are protean and resemble so many different diseases that Waldenström has used the term little imitator to describe it.12,181 Several excellent reviews have summarized the differential diagnosis of AIP and because this disorder has no cutaneous findings it will not be discussed further here.3,184,190

Treatment

There is no cure for AIP. A major component of managing patients with the acute hepatic porphyrias including AIP is focused on prevention of the acute attacks. This is accomplished by minimizing exposure to porphyrinogenic drugs and chemicals.11 Management of the acute attack utilizes multiple approaches including glucose loading (2 liters of 20% glucose over 24 hours in divided doses of 500 mL through a central venous line).120,190,191 Administration of a gonadotropin-releasing hormone analogue has also been recommended.192 Intravenous infusion of hematin in the form of heme arginate (Normosang®, Orphan Europe, Paris, or Panhematin®, Ovation Pharmaceuticals, Deerfield, Il) is effective in shortening attacks.190 Precipitating factors such as drugs (Table 132-9), sex steroid hormones, starvation, etc, should be avoided.11 AIP patients should wear medical warning bracelets and be given lists of drugs to avoid. Drugs thought to be safe for AIP patients are listed in (Table 132-10).

Currently, recommended therapeutic regimens for the treatment of an acute porphyric attack and associated prophylactic/preventive measures are described below in detail. These procedures are applicable for all patients with acute porphyrias presenting with neurovisceral manifestations.

Treatment of the Neurovisceral Attacks in the Acute Porphyrias

(See Box 132-2). Most patients with acute porphyria experiencing an attack can now be treated successfully and life-threatening complications like muscle paralysis, respiratory failure, and coma can be avoided if appropriate aggressive treatment is implemented immediately. Untreated or inappropriately treated patients suffering an acute porphyric attack may have a 10%–15% risk of mortality.11 Today with mechanism-based therapeutic intervention the mortality rate of the acute attack has decreased to approximately 2%. There are four major steps in treating an acute porphyria attack.

First, all potentially porphyrinogenic drugs should be stopped immediately and the patient should be monitored carefully and admitted to an intensive care unit if possible. Several sources are available that categorize drugs as relatively “safe” or “unsafe” in patients with acute porphyria.13 Additional information is available on the Internet at the Web site of the European Porphyria Initiative (EPI) at: http://www.porphyria-europe.com/. See also Tables 132-9 and 132-10.

Second, symptomatic treatment of neurovisceral manifestations like abdominal pain, vomiting, tachycardia, and seizures should be initiated promptly. These recommendations originate from different sources including recently published guidelines from the EPI and a consortium of North American porphyria specialists.

Third, intravenous infusion of high doses of carbohydrates, preferably 400–500 g of glucose/24 hours using 5% or 10% glucose solutions. Simultaneously, diuresis should be induced by administration of furosemide with careful monitoring of serum electrolytes. Patients experiencing an acute attack are often unable to eat or drink because of severe nausea and recurrent vomiting. The rationale for high doses of glucose is based on experimental evidence showing that carbohydrates suppress the induction of ALAS, the first and rate-limiting enzyme of the heme biosynthetic pathway. Recent studies have shown that ALAS is regulated by the peroxisome proliferator-activated receptor γ coactivator 1α (PGC-1α).193 Elevation of PGC-1α in mice increases the levels of heme precursors in vivo as observed in acute porphyric attacks, whereas the induction of ALAS by fasting is lost in liver-specific PGC-1α knockout animals, as is the ability of porphyrogenic drugs to dysregulate heme biosynthesis. These data show that PGC-1α links nutritional status to heme biosynthesis and acute hepatic porphyria and provides a rationale for high-dose glucose infusions. Unfortunately, in many patients this regimen is only partially helpful and, thus, glucose infusions are considered to be adjuvant therapy until hematin preparations are available.

Fourth, intravenous administration of heme arginate (Normosang®), currently non-FDA approved, is the most efficient method to suppress heme synthesis. The rationale for this approach is based on intravenous administration of high doses of the endproduct heme to repress the synthesis of ALAS.194 Initially the available hematin preparations were quite unstable and were prothrombotic.120,195 One derivative that is now available, heme arginate, consists of human hemin and l-arginine as an additive to increase the solubility and stability of the product. This preparation does not affect coagulation and fibrinolysis and the frequency of thrombophlebitis is markedly reduced.196 Heme arginate exerts its specific action by correcting the underlying heme deficiency and restoring the negative feedback control of heme on ALAS, thereby decreasing the excessive hepatic production of porphyrins and porphyrin precursors. Heme arginate should be administered intravenously as a short infusion at a dose of 3 mg/kg body weight/day over a period of 4 days. Further, administration can be continued if the attack is not aborted; however, use of heme arginate for more than 7 days is not recommended. In the United States, an alternative form of heme known as hemin (Panhematin®) can be administered for the treatment of an acute attack. It is currently recommended that Panhematin should be considered only after an appropriate period of alternate therapy (i.e., 400 g glucose/day for 1–2 days).

Prevention of Attacks

Patients should be advised to avoid crash dieting, to abstain from alcohol, and to be fully conversant with porphyrinogenic drugs, such as barbiturates, sulfonamides, and estrogens to name several.11,13,189 Lists of safe and unsafe drugs are available on the Web site of the America Porphyria Foundation: http://www.porphyriafoundation.com/about_por/drugs/drugs02.html and, in ten different languages, on the Web site of the EPI (see above).

Using appropriate screening procedures, asymptomatic family members of individuals with known acute porphyria should be screened for enzymatic deficiency/mutations to help define inheritance patterns and to identify individuals at risk.

Variegate Porphyria

(See Table 132-9)

Epidemiology

The disease occurs worldwide but is most prevalent in South Africa and Chile, mainly due to founder effects in both countries.5,18,197–199 Within South Africa, this form of porphyria is quite common among the Caucasian and so-called Cape-Colored South Africans. A high proportion of the present Caucasian population in South Africa is descended from a pair of early Dutch settlers who emigrated from the Netherlands in 1688. The incidence of VP in South Africa is the highest in the world, with approximately 1:300 individuals in the white population.

Etiology

VP results from decreased PPOX activity and is an autosomal dominant disorder.200–202 PPOX is an integral protein of the inner mitochondrial membrane and catalyzes the oxidation of PROTOGEN IX to PROTO thereby removing six hydrogens from PROTOGEN IX (Figs. 132-1, and 132-2). This reaction can also occur nonenzymatically, but the enzyme appears to be necessary for normal heme synthesis. The enzyme acts specifically on PROTOGEN IX, the penultimate step in heme synthesis. COPROGEN I or III or UROGEN I or III are not substrates for this enzyme. PPOX isolated from rat liver mitochondria has been shown to have a molecular mass of 51 kDa and requires molecular oxygen to catalyze the oxidation of PROTOGEN to PROTO.

Human PPOX is encoded by a single copy gene of the same name, PPOX, which has been mapped to chromosome 1q22–23, spans 8 kb, and consists of 13 exons. Northern blot analyses from a variety of tissues indicate that there is a single transcript of 1.8 kb encoding a protein of 477 amino acids.203 Multiple mutations have been identified, including founder mutations in South Africa (R59W) and Chile (1239delTACAC).19,204

Clinical Manifestations

The clinical manifestations of VP encompass those of both PCT and the other acute hepatic porphyrias, which can occur simultaneously or separately in the same individual. The major difference between VP and PCT is that skin signs and symptoms usually develop at an earlier age in VP (second and third decades) as compared to PCT (fourth and fifth decades). The major difference between VP and the other acute hepatic porphyrias (AIP and HCP) is that acute attacks are much less common in VP. In South Africa, excessive intake of home-brewed spirits (Kaffir beer) and dietary iron overload from cooking vessels have been considered to be important factors in the development of clinically overt disease. Thus, VP occurring in the descendants of the Dutch immigrants of South Africa is quite distinct from the PCT seen in the native black population. As pointed out by Dean in a group he designated as Cape-Colored, (descendants of white European and Indian immigrants who intermarried with black natives), VP and PCT may cosegregate in the same family.205

Skin

The cutaneous lesions of VP are indistinguishable from those of PCT and HCP. These include bullae and erosions on light-exposed skin (Figs. 132-19 and 132-20). Hyperpigmentation, milia, hypertrichosis, and increased skin fragility are also seen. They appear to be more common in the hot climate of South Africa and are less frequently seen in affected individuals in colder climates. Blisters often contain blood, heal slowly, and result in scarring and milia formation. Occasionally, the patient gives a history of acute sun sensitivity occurring during or soon after a period of exposure; this may include burning, erythema, and edema. In its chronic state, the skin changes may include thickening and scarring. Some believe that melanin pigmentation may reduce the severity of the photocutaneous findings. The skin manifestations do not correlate with the acute attacks in most patients and, in general, males are more likely to have the PCT-like skin findings, whereas females are more prone to AIP-like acute attacks.

Neurovisceral Symptoms

These are identical to those occurring in AIP and HCP consisting of attacks of abdominal pain, constipation, vomiting, muscle weakness, and neuropsychiatric manifestations of stupor and coma. Factors that trigger attacks of AIP and HCP also appear to induce VP (various drugs, starvation, etc.). All patients with the skin findings of PCT should be screened for VP because of the life-threatening potential of the acute neurovisceral attacks.

These clinical features rarely develop before puberty, except in the rare homozygous patients who may develop cutaneous signs and symptoms at or shortly after birth. Beside the early onset of skin symptoms, these patients frequently manifest short stature and small hands with shortening of the digits.206–208

Laboratory Findings

Urinary ALA and PBG are elevated during acute attacks of VP (when the Watson–Schwartz test may be positive) but characteristically fall to normal levels between attacks (Table 132-8), whereas in AIP patients, the urinary ALA and PBG are elevated, both during and between attacks. Another distinguishing feature of the two disorders is stool porphyrin excretion. Asymptomatic patients with VP commonly have marked elevations of stool PROTO and COPRO between attacks.209 These may fluctuate somewhat during acute attacks and PROTO typically exceeds COPRO. In AIP, fecal porphyrins are not elevated between attacks and usually increase only slightly during attacks. It is said that certain patterns of fecal and urinary porphyrin excretion may help in distinguishing VP from PCT.210 URO in the urine is only moderately elevated in VP and urinary COPRO typically exceeds URO. This is in marked contrast to active PCT where the reverse is seen, i.e., urinary URO is usually several-fold higher than COPRO. Again in VP, fecal porphyrins usually exceed 500 μg/g dry weight in >90% of patients, whereas in PCT this amount of stool porphyrin is excreted by only 1% of patients. Furthermore, the ratio of stool PROTO to COPRO in VP usually exceeds 1.5:1, whereas in PCT the ratio is almost always <1:1 due to the increased ISOCOPRO content characteristic of the latter disease. Stool porphyrin excretion patterns consistent with both VP and PCT have been found in different members of single families.61,62 These types of findings have led to the suggestion that measurement of bile porphyrins may be decisive in confirming a diagnosis of VP when excretory porphyrin patterns are ambiguous.101 Saline-diluted plasma specimens from patients with VP have characteristic fluorescence emission spectra (626-nm emission peak) that can be used to differentiate this disease from other forms of acute porphyria, PCT, EPP, and lead poisoning (Table 132-6).211

Histopathology

The histopathologic alterations in VP are indistinguishable from those of PCT.

Differential Diagnosis

The differential diagnosis of VP should be considered from two perspectives. The skin lesions of VP are identical to those of PCT and HCP, and, as such, the differential diagnosis includes the bullous diseases as well as other photosensitivity disorders. HEP must also be considered, but this usually begins in early childhood and is characterized by markedly deficient erythrocyte UROGEN decarboxylase activity and elevated erythrocyte PROTO as well as increased stool ISOCOPRO. HCP is characterized by increased excretion of COPRO III in urine and feces. Markedly elevated (usually 10–100-fold) fecal COPRO, more than 90% of which is the III isomer, is present at all times in these patients. The presence of skin lesions rules out AIP. The determination of urinary ALA and PBG concentrations may help to rule out PCT and HEP. Screening of family members by measuring fecal porphyrins may also be helpful in differentiating VP from PCT and HCP.

Acute attacks of abdominal pain and neurologic signs and symptoms are identical to those described for AIP. Fecal and biliary PROTO and COPRO determinations and plasma fluorescence spectra are decisive in making the diagnosis. In HCP, there may be identical acute attacks, but markedly elevated fecal COPRO III is diagnostic for this disease.

Treatment

Preventive and symptomatic treatment of the acute attacks in VP is identical to that described in AIP, including avoidance of inducing drugs (Table 132-4) and glucose loading as well as hematin infusions. In contrast to PCT, there is no evidence of iron overload in VP and therefore phlebotomy and the antimalarials are ineffective.212

Hereditary Coproporphyria

(See Table 132-9)

Epidemiology

Although apparently rare, the disease occurs worldwide and, like AIP and VP, has a female preponderance. To date, fewer than 75 patients have been reported. Beside classical HCP, an extremely rare form of porphyria, designated as harderoporphyria, has been described. This disease is a homozygous variant of HCP in which CPOX activity is very low (10% of control values).213–215

Etiology

HCP is an autosomal dominant disorder resulting from a decrease in the residual catalytic activity of CPOX (Figs. 132-1 and 132-2).216,217 The enzyme is localized in the mitochondrial intermembrane space, has a molecular mass of 74 kDa and catalyzes the conversion of COPROGEN III to PROTOGEN. COPROGEN III has four carboxyl groups, each of which is part of a propionate side chain. COPROGEN oxidase catalyzes the sequential oxidative removal of the carboxyl group from two of the propionate groups, forming first the 3-carboxyl porphyrinogen known as, harderoporphyrinogen (HARDEROGEN), and next the 2-carboxylporphyrinogen, known as PROTOGEN (Fig. 132-2).

CPOX is encoded by a single copy gene of the same name, CPOX, as suggested by Southern blot analysis of human restriction fragments. The CPOX gene is located on chromosome 3q12, spans a genomic distance of 14 kb, and consists of seven exons and six introns.218 Multiple mutations have been found in patients with heterozygous and homozygous HCP. These include point mutations resulting in base substitutions or exon skipping. This results in increased excretion of COPRO in the urine and stool suggesting the presence of a single gene.218

Clinical Manifestations

As in VP, the spectrum of clinical symptoms includes both cutaneous manifestations and potentially life-threatening acute neurological attacks. Cutaneous symptoms apparently occur in less than 10% of the patients and are clinically indistinguishable from those seen in PCT and VP.189 (Fig. 132-21).

The acute attacks are similar to those of AIP, and neurovisceral symptomatology predominates in HCP. This includes abdominal pain, vomiting, constipation, neuropathy, and psychiatric symptoms. The oral hypoglycemic agent glipizide has been reported to exacerbate a coproporphyria-like syndrome that was reversible following discontinuation of the drug.219

Laboratory Findings

HCP is characterized by increased excretion of COPRO III in urine and feces. Markedly elevated (usually 10–100-fold) fecal COPRO, more than 90% of which is the III isomer, is present at all times in these patients. In addition, the feces may also contain increased amounts of hepta-, hexa-, and pentacarboxylic porphyrins. Urinary COPRO III is also raised, as are the precursors ALA and PBG, during attacks. The latter may be normal between attacks (Table 132-8).

Histopathology

None

Differential Diagnosis

In HCP, cutaneous signs are uncommon and if they do occur are most often seen in homozygous patients with onset in childhood and the skin manifestations resemble those seen in PCT and VP (Fig. 132-21). Fecal predominance of COPRO III is more suggestive of HCP than VP, in which PROTO and COPRO are usually increased.

Acute attacks are similar to those of AIP. The differential diagnosis rests on stool porphyrin determinations and measurement of CPOX in fibroblasts or leukocytes. DNA analysis for mutations in the CPOX gene can also be performed.

Treatment

(See Table 132-4). Same as for AIP and VP described above and in Table 132-4.

(See Table 132-9)

This is a very rare autosomal recessively inherited type of acute hepatic porphyria, and not more than ten patients have been reported.220–223 Clinically, these patients have symptoms similar to AIP. Alcohol ingestion is reported to precipitate attacks.

The disease results from a profound deficiency of ALAD, the second enzyme in heme biosynthesis, i.e., <5% of normal. ALAD is a cytosolic enzyme and catalyzes the formation of PBG from two molecules of ALA.

The enzyme is encoded by the ALAD gene that is located on chromosome 9q34 and the mutations described to date are very heterogeneous. Although it has been suggested that ALAD deficiency porphyria may represent a homozygous ALAD defect, cloning of a mutant ALAD cDNA in a patient showed that the patient was compound heterozygous for two independent mutant alleles of the enzyme.221

In the few patients studied, the urinary porphyrin excretion shows elevated ALA, COPRO, and, to a lesser extent, URO. Fecal porphyrins revealed mildly elevated COPRO and PROTO in one patient, elevated COPRO in another, and a normal profile in two others. Erythrocyte PROTO was elevated in all patients. Increases of plasma ALA, COPRO, and PROTO have been reported.224 The porphyrin excretion pattern in these patients is unexplained.

Disease management consists of a combination of obtaining a thorough medical history (particularly a complete family history), clinical examination, and biochemical determination of porphyrins and porphyrin precursors in urine, feces, and blood. In specialized laboratories, these diagnostic procedures can be extended to the measurement of specific enzymatic activities in erythrocytes, lymphocytes, or fibroblasts of affected patients and family members. Molecular techniques now provide the most precise diagnostic information based on DNA isolation from peripheral blood followed by polymerase chain reaction (PCR) and automated DNA sequencing. Furthermore, these methodologies permit detailed analyses of genes and their function and provide affected individuals and their family members with genetic counseling.

But even with the use of these sophisticated diagnostic tools, there are times when the results may remain inconclusive. This is particularly true for the so-called dual porphyrias.

In the dual porphyrias, laboratory tests may point to the simultaneous deficiency of two enzymes in the heme biosynthetic pathway either in one individual or within one family. To date, approximately 15 patients and families with this constellation of findings have been reported, indicating that these porphyria variants are very rare.

The majority of the so-called dual porphyrias encompass combined deficiency of UROD, with PBGD, CPOX, or PPOX, respectively, and this is not surprising since the prevalence of PCT exceeds that of any other type of human porphyria. In three families, the simultaneous deficiency of CPOX and either PBGD, UROS, or ALAD was detected. In one patient with CEP, the UROS defect was accompanied by a deficiency of UROD.

Until recently, the diagnosis of dual porphyria in all patients and families was established based on biochemical measurement of porphyrins and/or porphyrin precursors in urine and feces and enzymatic assays.

Porphyria patients have been described in whom DNA analysis unequivocally confirmed the presence of mutations in two different genes encoding distinct enzymes in the heme synthetic pathway. Harraway et al described a young female patient who developed skin symptoms on the sun-exposed areas of the body and had increased urinary PBG excretion.225 The initial diagnosis of VP, however, could not be confirmed and biochemical analysis of urine, feces, and plasma suggested a combined deficiency of PBGD and UROD. Subsequent automated sequencing of the PBGD and UROD genes revealed mutations in both confirming the diagnosis of dual porphyria. Akagi et al studied a patient who suffered from acute porphyric attacks accompanied by increased urinary excretion of ALA, PBG, and COPRO.226 Although these findings were suggestive of HCP, the authors observed marked elevation of ALA and increased erythrocyte zinc PROTO, suggesting ALAD deficiency. Subsequent automated sequencing of both the CPOX and ALAD genes led to the detection of disease-causing mutations in each gene. Both genetic defects were confirmed by in vitro expression experiments, establishing the molecular basis of dual porphyria in a single individual consisting of simultaneous CPOX and ALAD deficiencies.

These reports emphasize that in patients with biochemical and enzymatic studies indicating coexistent deficiency of two enzymes in the heme biosynthetic pathway, molecular genetic analysis of the corresponding genes should be carried out to identify the disease-causing mutations underlying a particular variant of dual porphyria.

Some cases of sideroblastic anemia may be associated with abnormal porphyrin profiles.227–229 These patients may have cutaneous photosensitivity closely resembling that of EPP and they subsequently develop sideroblastic anemia; in some cases, the two conditions may develop within a few months of each other. All patients have elevated PROTO in RBC and/or plasma; many also have elevated stool PROTO levels. Urinary COPRO and URO are also elevated. The activities of FECH and, less frequently, of ALAS may be decreased. Chromosomal abnormalities, which include deletions in chromosomes 18 and 20, have been described.

In the various types of porphyrias, discrepancies between clinical findings and the results of biochemical tests or enzyme assays often lead to doubtful or incorrect diagnoses. In the case of the acute porphyrias, this can have fatal consequences for the patient if an accurate diagnosis is not established in a timely manner.

Once a mutation is identified in a patient with porphyria, the detection of asymptomatic gene carriers within the patient's family is easily accomplished using standard molecular biological techniques. In the acute porphyrias, the identification of these “silent” carriers is the first and most important step in the prevention of life-threatening acute attacks. Genetic analysis provides an effective way to make an early diagnosis thereby enhancing the likelihood of providing patients and asymptomatic gene carriers in their family with advice regarding the avoidance of precipitating factors.

Animal models for EPP and PCT have been described.92,168,230 These models afford unique opportunities to explore the pathogenesis of these types of porphyria using in vivo models, and to study the feasibility of achieving phenotypic reversion by gene therapy. Retrovirus-mediated gene transfer of human FECH cDNA into deficient fibroblasts from an EPP patient has been achieved.167 In transduced cells, FECH catalytic activity increased from 7% to 550% of the normal value, and partial metabolic correction of the phenotype was demonstrated. In CEP, correction of the underlying enzymatic defect in cultured cells was also accomplished by retrovirus-mediated gene transfer leading to partial normalization.231 Excessive porphyrin levels in the transduced cells were reduced to normal. These investigations demonstrate the potential utility of somatic stem cell gene therapy for the treatment of different types of human porphyria.

Human porphyrias often present with overlapping clinical and laboratory findings making precise diagnosis a challenging proposition, especially in the acute porphyrias. The clinical signs and symptoms are highly variable in different patients and even in the same patient at different times. Biochemical analyses of urinary and/or fecal porphyrins/precursors often display overlapping values. These methods, as well as the measurement of enzymatic activities in fibroblasts or lymphocytes, are somewhat imprecise, and it is not uncommon to find values that do not distinguish between clinically unaffected gene carriers (so called “silent” carriers), and normal controls. Once a DNA-based diagnosis is made, patients and their families can be provided with genetic counseling and with advice regarding the avoidance of precipitating factors. In case of a life-threatening acute attack, a prior genetic diagnosis permits the attending physician to begin appropriate treatment immediately.189