Disorders of lipoprotein metabolism are collectively referred to as “dyslipidemias.” Dyslipidemias are generally characterized clinically by increased plasma levels of cholesterol, triglycerides, or both, variably accompanied by reduced levels of HDL cholesterol. Because plasma lipids are commonly screened (see below), dyslipidemia is frequently seen in clinical practice. The majority of patients with dyslipidemia have some combination of genetic predisposition (often polygenic) and environmental contribution (lifestyle, medical condition, or drug). Many, but not all, patients with dyslipidemia are at increased risk for ASCVD, the primary reason for making the diagnosis, as intervention may reduce this risk. In addition, patients with substantially elevated levels of triglycerides may be at risk for acute pancreatitis and require intervention to reduce this risk.
Although literally hundreds of proteins influence lipoprotein metabolism and may interact to produce dyslipidemia in an individual patient, there are a limited number of discrete “nodes” that regulate lipoprotein metabolism. These include: (1) assembly and secretion of triglyceride-rich VLDLs by the liver; (2) lipolysis of triglyceride-rich lipoproteins by LPL; (3) receptor-mediated uptake of apoB-containing lipoproteins by the liver; (4) cellular cholesterol metabolism in the hepatocyte and the enterocyte; and (5) neutral lipid transfer and phospholipid hydrolysis in the plasma. The following discussion will focus on these regulatory nodes, recognizing that in many cases these nodes interact with and influence each other.
DYSLIPIDEMIA CAUSED BY EXCESSIVE HEPATIC SECRETION OF VLDL
Excessive production of VLDL by the liver is one of the most common causes of dyslipidemia. Individuals with excessive hepatic VLDL production usually have elevated fasting triglycerides and low levels of HDL cholesterol (HDL-C), with variable elevations in LDL cholesterol (LDL-C) but usually elevated plasma levels of apoB. A cluster of other metabolic risk factors are often found in association with VLDL overproduction, including obesity, glucose intolerance, insulin resistance, and hypertension (the so-called metabolic syndrome, Chap. 422). Some of the major factors that drive hepatic VLDL secretion include obesity, insulin resistance, a high-carbohydrate diet, alcohol use, exogenous estrogens, and genetic predisposition.
Secondary Causes of VLDL Overproduction
Dietary carbohydrates are converted to fatty acids in the liver. Some of the newly synthesized fatty acids are esterified forming triglycerides (TGs) and secreted as constituents of VLDL. Thus, excessive intake of calories as carbohydrates, which is frequent in Western societies, leads to increased hepatic VLDL-TG secretion.
Regular alcohol consumption inhibits hepatic oxidation of free fatty acids, thus promoting hepatic TG synthesis and VLDL secretion. Regular alcohol use also raises plasma levels of HDL-C and should be considered in patients with the unusual combination of elevated TGs and elevated HDL-C.
OBESITY AND INSULIN RESISTANCE
(See also Chaps. 416 and 417) Obesity and insulin resistance are frequently accompanied by dyslipidemia characterized by elevated plasma levels of TG, low HDL-C, variable levels of LDL-C, and increased levels of small dense LDL. The increase in adipocyte mass and accompanying decreased insulin sensitivity associated with obesity have multiple effects on lipid metabolism, with one of the major effects being excessive hepatic VLDL production. More free fatty acids are delivered from the expanded and insulin-resistant adipose tissue to the liver, where they are reesterified in hepatocytes to form TGs, which are packaged into VLDLs for secretion into the circulation. In addition, the increased insulin levels promote increased fatty acid synthesis in the liver. In insulin-resistant patients who progress to type 2 diabetes mellitus, dyslipidemia remains common, even when the patient is under relatively good glycemic control. In addition to increased VLDL production, insulin resistance can also result in decreased LPL activity, resulting in reduced catabolism of chylomicrons and VLDLs and more severe hypertriglyceridemia (see below).
(See also Chap. 335) Nephrotic syndrome is a classic cause of excessive VLDL production. The molecular mechanism of VLDL overproduction remains poorly understood but has been attributed to the effects of hypoalbuminemia leading to increased hepatic protein synthesis. Effective treatment of the underlying renal disease often normalizes the lipid profile, but most patients with chronic nephrotic syndrome require lipid-lowering drug therapy.
(See also Chap. 406) Endogenous or exogenous glucocorticoid excess is associated with increased VLDL synthesis and secretion and hypertriglyceridemia. Patients with Cushing’s syndrome frequently have dyslipidemia especially characterized by hypertriglyceridemia and low HDL-C, although elevations in plasma levels of LDL-C can also be seen.
Primary (Genetic) Causes of VLDL Overproduction
Genetic variation influences hepatic VLDL production. A number of genes have been identified in which common and low-frequency variants likely contribute to increased VLDL production, likely involving interactions with diet and other environmental factors. The best recognized inherited condition associated with VLDL overproduction is familial combined hyperlipidemia.
FAMILIAL COMBINED HYPERLIPIDEMIA (FCHL)
FCHL is generally characterized by elevations in plasma levels of TGs (VLDL) and LDL-C (including small dense LDL) and reduced plasma levels of HDL-C. It is estimated to occur in approximately 1 in 100–200 individuals and is an important cause of premature coronary heart disease (CHD); approximately 20% of patients who develop CHD under age 60 have FCHL. FCHL can manifest in childhood but is usually not fully expressed until adulthood. The disease clusters in families, with affected family members typically have one of three possible phenotypes: (1) elevated plasma levels of LDL-C, (2) elevated plasma levels of TGs due to elevation in VLDL, or (3) elevated plasma levels of both LDL-C and TG. The lipoprotein profile can switch among these three phenotypes in the same individual over time and may depend on factors such as diet, exercise, weight, and insulin sensitivity. Patients with FCHL almost always have significantly elevated plasma levels of apoB. The levels of apoB are disproportionately high relative to the plasma LDL-C concentration, indicating the presence of small, dense LDL particles, which are characteristic of this syndrome.
Individuals with this phenotype generally share the same metabolic defect, namely overproduction of VLDL by the liver. The molecular etiology of this condition remains poorly understood, and no single gene has been identified in which mutations cause this disorder. It is likely that defects in a combination of genes can cause the condition, suggesting that a more appropriate term for the disorder might be polygenic combined hyperlipidemia.
The presence of a mixed dyslipidemia (plasma TG levels between 200 and 600 mg/dL and total cholesterol levels between 200 and 400 mg/dL, usually with HDL-C levels <40 mg/dL in men and <50 mg/dL in women) and a family history of mixed dyslipidemia and/or premature CHD strongly suggests the diagnosis. Individuals with this phenotype should be treated aggressively due to significantly increased risk of premature CHD. Decreased dietary intake of simple carbohydrates, aerobic exercise, and weight loss can all have beneficial effects on the lipid profile. Patients with diabetes should be aggressively treated to maintain good glucose control. Most patients with FCHL require lipid-lowering drug therapy, starting with statins, to reduce lipoprotein levels and lower the risk of cardiovascular disease.
Lipodystrophy is a condition in which the generation of adipose tissue generally or in certain fat depots is impaired. Lipodystrophies are often associated with insulin resistance and elevated plasma levels of VLDL and chylomicrons due to increased fatty acid synthesis and VLDL production, as well as reduced clearance of TG-rich particles. This disorder can be especially difficult to control. Patients with congenital generalized lipodystrophy are very rare and have nearly complete absence of subcutaneous fat, accompanied by profound insulin resistance and leptin deficiency, and accumulation of TGs in multiple tissues including the liver. Some patients with generalized lipodystrophy have been treated successfully with leptin administration. Partial lipodystrophy is somewhat more common and can be caused by mutations in several different genes, most notably lamin A. Partial lipodystrophy is usually characterized by increased truncal fat accompanied by markedly reduced or absent subcutaneous fat in the extremities and buttocks. These patients generally have insulin resistance, often quite severe, accompanied by type 2 diabetes, hepatosteatosis, and dyslipidemia. The dyslipidemia is usually characterized by elevated TGs and cholesterol and can be difficult to manage clinically. Patients with partial lipodystrophy are at substantially increased risk of atherosclerotic vascular disease and should therefore be treated aggressively for their dyslipidemia with statins and, if necessary, additional lipid-lowering therapies.
DYSLIPIDEMIA CAUSED BY IMPAIRED LIPOLYSIS OF TRIGLYCERIDE-RICH LIPOPROTEINS
Impaired lipolysis of the TGs in TG-rich lipoproteins (TRLs) also commonly contributes to dyslipidemia. As noted above, LPL is the key enzyme responsible for hydrolyzing the TGs in chylomicrons and VLDL. LPL is synthesized and secreted into the extracellular space from adipocytes, myocytes, and cardiomyocytes. It is then transported from the subendothelial to the vascular endothelial surfaces by GPIHPB1. LPL is also synthesized in macrophages. Individuals with impaired LPL activity, whether secondary or due to a primary genetic disorder, have elevated fasting TGs and low levels of HDL-C, usually without elevation in LDL-C or apoB. Insulin resistance, in addition to causing excessive VLDL production, can also cause impaired LPL activity and lipolysis. A number of common and low-frequency genetic variants have been described that influence LPL activity, and single-gene Mendelian disorders that reduce LPL activity have also been described (Table 421-3).
Secondary Causes of Impaired Lipolysis of TRLs
OBESITY AND INSULIN RESISTANCE
(See also Chaps. 415e, 416, and 417) In addition to hepatic overproduction of VLDL, as discussed above, obesity, insulin resistance, and type 2 diabetes have been reported to be associated with variably reduced LPL activity. This may be due in part to the effects of tissue insulin resistance leading to reduced transcription of LPL in skeletal muscle and adipose, as well as to increased production of the LPL inhibitor apoC-III by the liver. This reduction in LPL activity often contributes to the dyslipidemia seen in these patients.
Primary (Genetic) Causes and Genetic Predisposition to Impaired Lipolysis of TRLs
FAMILIAL CHYLOMICRONEMIA SYNDROME
As noted above, LPL is required for the hydrolysis of TGs in chylomicrons and VLDLs, and apoC-II is a cofactor for LPL. Genetic deficiency or inactivity of either protein results in impaired lipolysis and profound elevations in plasma chylomicrons. These patients can also have elevated plasma levels of VLDL, but chylomicronemia predominates. The fasting plasma is turbid, and if left at 4°C (39.2°F) for a few hours, the chylomicrons float to the top and form a creamy supernatant. In these disorders, collectively called the familial chylomicronemia syndrome, fasting TG levels are almost invariably >1000 mg/dL. Fasting cholesterol levels are also elevated but to a lesser degree.
LPL deficiency has autosomal recessive inheritance and has a frequency of approximately 1 in 1 million in the population. ApoC-II deficiency is also recessive in inheritance pattern and is even less common than LPL deficiency. Multiple different mutations in the LPL and APOC2 genes cause these diseases. Obligate LPL heterozygotes often have mild-to-moderate elevations in plasma TG levels, whereas individuals heterozygous for mutation in apoC-II do not have hypertriglyceridemia.
Both LPL and apoC-II deficiency usually present in childhood with recurrent episodes of severe abdominal pain due to acute pancreatitis. On funduscopic examination, the retinal blood vessels are opalescent (lipemia retinalis). Eruptive xanthomas, which are small, yellowish-white papules, often appear in clusters on the back, buttocks, and extensor surfaces of the arms and legs. These typically painless skin lesions may become pruritic. Hepatosplenomegaly results from the uptake of circulating chylomicrons by reticuloendothelial cells in the liver and spleen. For unknown reasons, some patients with persistent and pronounced chylomicronemia never develop pancreatitis, eruptive xanthomas, or hepatosplenomegaly. Premature CHD is not generally a feature of familial chylomicronemia syndromes.
The diagnoses of LPL and apoC-II deficiency are established enzymatically in specialized laboratories by assaying TG lipolytic activity in postheparin plasma. Blood is sampled after an IV heparin injection to release the endothelial-bound LPL. LPL activity is profoundly reduced in both LPL and apoC-II deficiency; in patients with apoC-II deficiency, it normalizes after the addition of normal plasma (providing a source of apoC-II). Molecular sequencing of the genes can be used to confirm the diagnosis.
The major therapeutic intervention in familial chylomicronemia syndrome is dietary fat restriction (to as little as 15 g/d) with fat-soluble vitamin supplementation. Consultation with a registered dietician familiar with this disorder is essential. Caloric supplementation with medium-chain TGs, which are absorbed directly into the portal circulation, can be useful, but there is uncertainty about their hepatic safety with prolonged use. If dietary fat restriction alone is not successful in resolving the chylomicronemia, fish oils have been effective in some patients. In patients with apoC-II deficiency, apoC-II can be provided by infusing fresh-frozen plasma to resolve the chylomicronemia in the acute setting. Management of patients with familial chylomicronemia syndrome is particularly challenging during pregnancy when VLDL production is increased. A gene therapy approach, called alipogene tiparvovec, is approved for LPL deficiency in Europe; it involves multiple intramuscular injections of an adeno-associated viral vector encoding a gain-of-function LPL variant, leading to skeletal myocyte expression of LPL.
Another apolipoprotein, ApoA-V, facilitates the association of VLDL and chylomicrons with LPL and promotes their hydrolysis. Individuals harboring loss-of-function mutations in both APOA5 alleles develop hyperchylomicronemia. Heterozygosity for variants in APOA5 that reduce its function contributes to the polygenic basis of hypertriglyceridemia.
Homozygosity for mutations that interfere with GPIHBP1 synthesis or folding cause severe hypertriglyceridemia by compromising the transport of LPL to the vascular endothelium. The frequency of chylomicronemia due to mutations in GHIHBP1 has not been established but appears to be very rare.
FAMILIAL HYPERTRIGLYCERIDEMIA (FHTG)
FHTG is characterized by elevated fasting TGs without a clear secondary cause, average to below average LDL-C levels, low HDL-C levels, and a family history of hypertriglyceridemia. Plasma LDL-C levels are often reduced due to defective conversion of TG-rich particles to LDL. In contrast to FCHL, apoB levels are not elevated. The identification of other first-degree relatives with hypertriglyceridemia is useful in making the diagnosis. Unlike in FCHL, this condition is not generally associated with a significantly increased risk of CHD. However, if the hypertriglyceridemia is exacerbated by environmental factors, medical conditions, or drugs, the TGs can rise to a level at which acute pancreatitis is a risk. Indeed, management of patients with this condition is mostly geared toward reduction of TGs to prevent pancreatitis.
Individuals with this phenotype generally have reduced lipolysis of TRLs, although overproduction of VLDL by the liver can also contribute. No single gene has been identified in which mutations cause this disorder, whereas combinations of gene variants have been shown to cause this phenotype. A more appropriate term for this condition might be polygenic hypertriglyceridemia.
It is important to consider and rule out secondary causes of the hypertriglyceridemia as discussed above. Increased intake of simple carbohydrates, obesity, insulin resistance, alcohol use, estrogen treatment, and certain medications can exacerbate this phenotype. Patients who are at high risk for CHD due to other risk factors should be treated with statin therapy. In patients who are otherwise not at high risk for CHD, lipid-lowering drug therapy can frequently be avoided with appropriate dietary and lifestyle changes. Patients with plasma TG levels >500 mg/dL after a trial of diet and exercise should be considered for drug therapy with a fibrate or fish oil to reduce TGs in order to prevent pancreatitis.
DYSLIPIDEMIA CAUSED BY IMPAIRED HEPATIC UPTAKE OF APOB-CONTAINING LIPOPROTEINS
Impaired uptake of LDL and remnant lipoproteins by the liver is another common cause of dyslipidemia. As discussed above, the LDL receptor is the major receptor responsible for uptake of LDL and remnant particles by the liver. Downregulation of LDL receptor activity or genetic variation that reduces the activity of the LDL receptor pathway leads to elevations in LDL-C. One major factor that reduces LDL receptor activity is a diet high in saturated and trans fats. Other medical conditions that reduce LDL receptor activity include hypothyroidism and estrogen deficiency. In addition, genetic variation in a number of genes influences LDL clearance, and mutations in some of these genes cause several discrete Mendelian disorders of elevated LDL-C (Table 421-3).
Secondary Causes of Impaired Hepatic Uptake of Lipoproteins
(See also Chap. 405) Hypothyroidism is associated with elevated plasma LDL-C levels due primarily to a reduction in hepatic LDL receptor function and delayed clearance of LDL. Thyroid hormone increases hepatic expression of the LDL receptor. Hypothyroid patients also frequently have increased levels of circulating IDL, and some patients with hypothyroidism also have mild hypertriglyceridemia. Because hypothyroidism is often subtle and therefore easily overlooked, all patients presenting with elevated plasma levels of LDL-C, especially if there has been an unexplained increase in LDL-C, should be screened for hypothyroidism. Thyroid replacement therapy usually ameliorates the hypercholesterolemia; if not, the patient probably has a primary lipoprotein disorder and may require lipid-lowering drug therapy with a statin.
(See also Chap. 335) Chronic kidney disease (CKD) is often associated with mild hypertriglyceridemia (<300 mg/dL) due to the accumulation of VLDLs and remnant lipoproteins in the circulation. TG lipolysis and remnant clearance are both reduced in patients with renal failure. Because the risk of ASCVD is increased in end-stage renal disease, subjects with hyperlipidemia, they should usually be aggressively treated with lipid-lowering agents, even though there is inadequate data at present to indicate that this population benefits from LDL-lowering therapy.
Patients with solid organ transplants often have increased lipid levels due to the effect of the drugs required for immunosuppression. These patients can present a difficult clinical management problem, since statins should be used cautiously in these patients due to untoward muscle-related side effects.
Primary (Genetic) Causes of Impaired Hepatic Uptake of Lipoproteins
Genetic variation contributes substantially to elevated LDL-C levels in the general population. It has been estimated that at least 50% of variation in LDL-C is genetically determined. Many patients with elevated LDL-C have polygenic hypercholesterolemia characterized by hypercholesterolemia in the absence of secondary causes of hypercholesterolemia (other than dietary factors) or a primary Mendelian disorder. In patients who are genetically predisposed to higher LDL-C levels, diet plays a key role; indeed increased saturated and trans fats in the diet shifts the entire distribution of LDL levels in the population to the right. Inheritance of several variants that together elevate LDL-C, coupled with diet, is generally the cause of this condition; <10% of first-degree relatives themselves have hypercholesterolemia. However, single-gene (Mendelian) causes of elevated LDL-C are relatively common and should be considered in the differential diagnosis of elevated LDL-C.
FAMILIAL HYPERCHOLESTEROLEMIA (FH)
FH, also known as autosomal dominant hypercholesterolemia (ADH) type 1, is an autosomal co-dominant disorder characterized by elevated plasma levels of LDL-C in the absence of hypertriglyceridemia. FH is caused by loss-of-function mutations in the gene encoding the LDL receptor. The reduction in LDL receptor activity in the liver results in a reduced rate of clearance of LDL from the circulation. The plasma level of LDL increases to a level such that the rate of LDL production equals the rate of LDL clearance by residual LDL receptor as well as non-LDL receptor mechanisms. More than 1600 different mutations have been reported in association with FH. The elevated levels of LDL-C in FH are primarily due to delayed removal of LDL from the blood; in addition, because the removal of IDL is also delayed, the production of LDL from IDL is also increased. Individuals with two mutated LDL receptor alleles (FH homozygotes, or compound heterozygotes) have much higher LDL-C levels than those with one mutant allele (FH heterozygotes).
Heterozygous FH is caused by the inheritance of one mutant LDL receptor allele. The population frequency of heterozygous FH due to LDL receptor mutations was originally estimated to be 1 in 500 individuals, but recent data suggest it may be as high as approximately 1 in 250 individuals, making it one of the most common single-gene disorders in humans. FH has a higher prevalence in certain founder populations, such as South African Afrikaners, Christian Lebanese, and French Canadians. Heterozygous FH is characterized by elevated plasma levels of LDL-C (usually 200–400 mg/dL) and normal levels of TGs. Patients with heterozygous FH have hypercholesterolemia from birth, and disease recognition is usually based on detection of hypercholesterolemia on routine screening, the appearance of tendon xanthomas, or the development of symptomatic cardiovascular disease. Inheritance is dominant, meaning that the condition was inherited from one parent and ~50% of the patient’s siblings can be expected to have hypercholesterolemia. The family history is frequently positive for premature CHD on the side of the family from which the mutation was inherited. Physical findings in many, but not all, patients with heterozygous FH include corneal arcus and tendon xanthomas particularly involving the dorsum of the hands and the Achilles tendons. Untreated heterozygous FH is associated with a markedly increased risk of cardiovascular disease. Untreated men with heterozygous FH have an ~50% chance of having a myocardial infarction before age 60 years, and women with heterozygous FH are at substantially increased risk as well. The age of onset of cardiovascular disease is highly variable and depends on the specific molecular defect, the level of LDL-C, and coexisting cardiovascular risk factors. FH heterozygotes with elevated plasma levels of Lp(a) (see below) appear to be at greater risk for cardiovascular disease.
No definitive diagnostic test for heterozygous FH is available, except in certain founder populations where selected mutations predominate. Most LDL receptor mutations are private and require sequencing of the LDL receptor gene for identification. Sequencing for clinical diagnosis is available but not standard of care and is rarely performed in the United States, because the clinical utility of identifying the specific mutation has not been demonstrated. A family history of hypercholesterolemia and/or premature coronary disease is supportive of the diagnosis. Secondary causes of significant hypercholesterolemia such as hypothyroidism, nephrotic syndrome, and obstructive liver disease should be excluded.
Heterozygous FH patients should be aggressively treated to lower plasma levels of LDL-C, starting in childhood. Initiation of a diet low in saturated and trans fats is recommended, but heterozygous FH patients virtually always require lipid-lowering drug therapy for effective control of their LDL-C levels. Statins are effective in heterozygous FH and are clearly the drug class of choice, and usually a more potent member of the class. However, some heterozygous FH patients cannot achieve adequate control of their LDL-C levels even with high-dose statin therapy and require additional drugs; a cholesterol absorption inhibitor and/or a bile acid sequestrant are the next-line classes of drugs. Currently, heterozygous FH patients whose LDL-C levels remain markedly elevated (>200 mg/dL with cardiovascular disease [CVD] or >300 mg/dL without CVD) on maximally tolerated drug therapy are candidates for LDL apheresis, a physical method of purging the blood of LDL in which the LDL particles are selectively removed from the circulation; LDL apheresis is usually performed every 2 weeks. A new class of drugs known as PCSK9 inhibitors is under clinical development and has the potential to effectively control LDL-C levels in the vast majority of patients with heterozygous FH who are inadequately controlled on a statin alone or who are statin intolerant.
Homozygous FH is caused by mutations in both alleles of the LDL receptor and therefore much rarer than heterozygous FH. Patients with homozygous FH have been classified into those patients with virtually no detectable LDL receptor activity (receptor negative) and those patients with markedly reduced but detectable LDL receptor activity (receptor defective). LDL-C levels in patients with homozygous FH range from about 400 to >1000 mg/dL, with receptor-defective patients at the lower end and receptor-negative patients at the higher end of the range. TGs are usually normal. Many patients with homozygous FH, particularly receptor-negative patients, present in childhood with cutaneous xanthomas on the hands, wrists, elbows, knees, heels, or buttocks. The devastating consequence of homozygous FH is accelerated ASCVD, which often presents in childhood or early adulthood. Atherosclerosis often develops first in the aortic root, where it can cause aortic valvular or supravalvular stenosis, and typically extends into the coronary ostia, which become stenotic. Symptoms can be atypical, and sudden death is not uncommon. Untreated, receptor-negative patients with homozygous FH rarely survive beyond the second decade; patients with receptor-defective LDL receptor defects have a better prognosis but almost invariably develop clinically apparent atherosclerotic vascular disease by age 30, and often much sooner. Carotid and femoral disease develops later in life and is usually not clinically significant.
Homozygous FH should be suspected in a child or young adult with LDL >400 mg/dL without secondary cause. Cutaneous xanthomas, evidence of CVD, and hypercholesterolemia in both parents all are supportive of the diagnosis. Although the specific mutations in the LDL receptor can usually be identified by DNA sequencing, this is not generally performed, and the diagnosis is usually made on clinical grounds.
Patients with homozygous FH must be treated aggressively to delay the onset and progression of CVD. Receptor defective patients sometimes respond to statins and other LDL-lowering drug classes such as a cholesterol absorption inhibitor or a bile acid sequestrant, which upregulate the LDL receptor activity. Two drugs that reduce the hepatic production of VLDL and thus LDL, a small-molecule inhibitor of the microsomal TG transfer protein (MTP) and an antisense oligonucleotide to apoB, are approved in the United States for the treatment of adults with homozygous FH and can be considered. PCSK9 inhibitors, which work through increasing LDL receptor availability, appear to have some benefit in receptor-defective patients and are under clinical development. LDL apheresis is used to lower plasma LDL levels in these patients and can promote regression of xanthomas as well as slow the progression of atherosclerosis. Because the liver is quantitatively the most important tissue for removing circulating LDLs via the LDL receptor, liver transplantation is effective in decreasing plasma LDL-C levels in this disorder but is infrequently used because of the associated problems with immunosuppression.
FAMILIAL DEFECTIVE APOB-100 (FDB)
FDB, also known as autosomal dominant hypercholesterolemia (ADH) type 2, is a dominantly inherited disorder that clinically resembles heterozygous FH with elevated LDL-C levels and normal TGs. FDB is caused by mutations in the gene encoding apoB-100, specifically in LDL receptor–binding domain of apoB-100. Several different mutations have been identified, but a single mutation predominates: substitution of glutamine for arginine at position 3500. The mutation results in a reduction in the affinity of LDL binding to the LDL receptor, so LDL is removed from the circulation at a reduced rate. FDB is less common than FH but is more prevalent in individuals of central European descent; the Lancaster County (United States) Amish are a founder population in which the prevalence of FDB is as high as 1 in 10 individuals. FDB is characterized by elevated plasma LDL-C levels with normal TGs; tendon xanthomas can be seen, although not as frequently as in FH, and there is an associated increase in risk of CHD. Patients with FDB cannot be clinically distinguished from patients with heterozygous FH, although patients with FDB tend to have somewhat lower plasma levels of LDL-C than FH heterozygotes, presumably due to the fact that IDL clearance is not impaired in this disorder. Homozygotes for FDB mutations have higher LDL-C levels than FDB heterozygotes but are not as severely affected as homozygous FH patients. The apoB-100 gene mutations can be detected directly through sequencing of the receptor-binding region of the apoB gene or genotyping for the most common mutation, but genetic diagnosis is not generally performed because there is no direct implication for clinical management. As with FH, patients are treated with statins first and, if necessary, with additional classes of LDL-lowering drugs.
AUTOSOMAL DOMINANT HYPERCHOLESTEROLEMIA DUE TO MUTATIONS IN PCSK9 (ADH-PCSK9 OR ADH3)
ADH-PCSK9, also known as autosomal dominant hypercholesterolemia (ADH) type 3, is a very rare autosomal dominant disorder caused by gain-of-function mutations in proprotein convertase subtilisin/kexin type 9 (PCSK9). PCSK9 is a secreted protein that binds to the LDL receptor, targeting it for degradation. Normally, after LDL binds to the LDL receptor, it is internalized along with the receptor, and in the low pH of the endosome, the LDL receptor dissociates from the LDL and recycles to the cell surface. When PCSK9 binds the receptor, the complex is internalized and the receptor is directed to the lysosome, rather than to the cell surface. The missense mutations in PCSK9 that cause hypercholesterolemia enhance the activity of PCSK9. As a consequence, the number of hepatic LDL receptors is reduced. Patients with ADH-PCSK9 are similar clinically to patients with FH. They may be particularly responsive to PCSK9 inhibitors in clinical development. Loss-of-function mutations in PCSK9 cause low LDL-C levels (see below).
AUTOSOMAL RECESSIVE HYPERCHOLESTEROLEMIA (ARH)
ARH is a very rare disorder that is mostly seen in individuals of Sardinian descent. The disease is caused by mutations in a protein, ARH (also called LDLR adaptor protein, LDLRAP), which is required for LDL receptor–mediated endocytosis in the liver. ARH binds to the cytoplasmic domain of the LDL receptor and links the receptor to the endocytic machinery. In the absence of LDLRAP, LDL binds to the extracellular domain of the LDL receptor, but the lipoprotein-receptor complex fails to be internalized. ARH, like homozygous FH, is characterized by hypercholesterolemia, tendon xanthomas, and premature coronary artery disease (CAD). The levels of plasma LDL-C tend to be intermediate between the levels present in FH homozygotes and FH heterozygotes, and CAD is not usually symptomatic until the third decade. LDL receptor function in cultured fibroblasts is normal or only modestly reduced in ARH, whereas LDL receptor function in lymphocytes and the liver is negligible. Unlike FH homozygotes, the hyperlipidemia responds to treatment with statins, but these patients usually require additional therapy to lower plasma LDL-C to acceptable levels.
Sitosterolemia is a rare autosomal recessive disease that can result in severe hypercholesterolemia, tendon xanthomas, and premature ASCVD. Sitosterolemia is caused by loss-of-function mutations in either of two members of the ATP-binding cassette (ABC) half transporter family, ABCG5 and ABCG8. These genes are expressed in enterocytes and hepatocytes. The proteins heterodimerize to form a functional complex that transports plant sterols such as sitosterol and campesterol, and animal sterols, predominantly cholesterol, across the biliary membrane of hepatocytes into the bile and across the intestinal luminal surface of enterocytes into the gut lumen. In normal individuals, <5% of dietary plant sterols are absorbed by the proximal small intestine. The small amounts of plant sterols that enter the circulation are preferentially excreted into the bile. Thus, levels of plant sterols are kept very low in tissues. In sitosterolemia, the intestinal absorption of sterols is increased and biliary and fecal excretion of the sterols is reduced, resulting in increased plasma and tissue levels of both plant sterols and cholesterol. The increase in hepatic sterol levels results in transcriptional suppression of the expression of the LDL receptor, resulting in reduced uptake of LDL and substantially increased LDL-C levels. In addition to the usual clinical picture of hypercholesterolemia (i.e., tendon xanthomas and premature ASCVD), these patients also have anisocytosis and poikilocytosis of erythrocytes and megathrombocytes due to the incorporation of plant sterols into cell membranes. Episodes of hemolysis and splenomegaly are a distinctive clinical feature of this disease compared to other genetic forms of hypercholesterolemia and can be a clue to the diagnosis.
Sitosterolemia should be suspected in a patient with severe hypercholesterolemia without a family history of such or who responds dramatically to dietary therapy and/or ezetimibe but not statins. Sitosterolemia can be diagnosed by a laboratory finding of a substantial increase in the plasma level of sitosterol and/or other plant sterols. It is important to make the diagnosis, because bile acid sequestrants and cholesterol-absorption inhibitors are the most effective agents to reduce LDL-C and plasma plant sterol levels in these patients.
CHOLESTERYL ESTER STORAGE DISEASE (CESD)
CESD, also known as lysosomal acid lipase deficiency, is an autosomal recessive disorder characterized by elevated LDL-C, usually in association with low HDL-C, together with progressive fatty liver ultimately leading to hepatic fibrosis. Plasma TG levels can also be mild to moderately increased in this disorder. The most severe form of this disorder, Wolman’s disease, presents in infancy and is rapidly fatal. Both Wolman’s disease and CESD are caused by loss-of-function variants in both alleles of the gene encoding lysosomal acid lipase (LAL; gene name LIPA). LAL is responsible for hydrolyzing neutral lipids, particularly TGs and cholesteryl esters, after delivery to the lysosome by cell-surface receptors such as the LDL receptor. It is particularly important in the liver, which clears large amounts of lipoproteins from the circulation. Genetic deficiency of LAL results in accumulation of neutral lipid in the hepatocytes, leading to hepatosplenomegaly, microvesicular steatosis, and ultimately fibrosis and end-stage liver disease. The etiology of the elevated LDL-C levels is uncertain; one study suggested that VLDL production is increased, but impaired LDL receptor–mediated clearance of LDL is also likely.
CESD should be particularly suspected in nonobese patients with elevated LDL-C, low HDL-C, and evidence of fatty liver in the absence of overt insulin resistance. The diagnosis can be made with a dried blood spot assay of LAL activity and confirmed by DNA genotyping for the most common mutation, followed if necessary by sequencing of the gene to find the second mutation. Liver biopsy is required to assess the degree of inflammation and fibrosis. It is important to make the diagnosis because it has implications for liver monitoring and potentially for therapeutic approaches under development.
FAMILIAL DYSBETALIPOPROTEINEMIA (FDBL)
FDBL (also known as type III hyperlipoproteinemia) is usually a recessive disorder characterized by a mixed hyperlipidemia (elevated cholesterol and TGs) due to the accumulation of remnant lipoprotein particles (chylomicron remnants and VLDL remnants, or IDL). ApoE is present in multiple copies on chylomicron remnants and IDL, and mediates their removal via hepatic lipoprotein receptors (Fig. 421-2). FDBL is due to genetic variants of apoE, most commonly apoE2, that result in an apoE protein with reduced ability to bind lipoprotein receptors. The APOE gene is polymorphic in sequence, resulting in the expression of three common isoforms: apoE3, which is the most common; and apoE2 and apoE4, which both differ from apoE3 by a single amino acid. Although associated with slightly higher LDL-C levels and increased CHD risk, the apoE4 allele is not associated with FDBL. Individuals who carry one or two apoE4 alleles have an increased risk of Alzheimer’s disease. ApoE2 has a lower affinity for the LDL receptor; therefore, chylomicron remnants and IDL containing apoE2 are removed from plasma at a slower rate. Individuals who are homozygous for the E2 allele (the E2/E2 genotype) comprise the most common subset of patients with FDBL.
Approximately 0.5% of the general population are apoE2/E2 homozygotes, but only a small minority of these individuals actually develop hyperlipidemia characteristic of FDBL. In most cases, an additional, sometimes identifiable, factor precipitates the development of hyperlipoproteinemia. The most common precipitating factors are a high-fat diet, diabetes mellitus, obesity, hypothyroidism, renal disease, HIV infection, estrogen deficiency, alcohol use, or certain drugs. The disease seldom presents in women before menopause. Other mutations in apoE can cause a dominant form of FDBL where the hyperlipidemia is fully manifest in the heterozygous state, but these mutations are very rare.
Patients with FDBL usually present in adulthood with hyperlipidemia, xanthomas, or premature coronary or peripheral vascular disease. In FDBL, in contrast to other disorders of elevated TGs, the plasma levels of cholesterol and TG are often elevated to a similar degree, and the level of HDL-C is usually normal or reduced. Two distinctive types of xanthomas, tuberoeruptive and palmar, are seen in FDBL patients. Tuberoeruptive xanthomas begin as clusters of small papules on the elbows, knees, or buttocks and can grow to the size of small grapes. Palmar xanthomas (alternatively called xanthomata striata palmaris) are orange-yellow discolorations of the creases in the palms and wrists. Both of these xanthoma types are virtually pathognomonic for FDBL. Subjects with FDBL have premature ASCVD and tend to have more peripheral vascular disease than is typically seen in FH.
The definitive diagnosis of FDBL can be made either by the documentation of very high levels of remnant lipoproteins or by identification of the apoE2/E2 genotype. A variety of methods are used to identify remnant lipoproteins in the plasma, including “β-quantification” by ultracentrifugation (ratio of directly measured VLDL-C to total plasma TG >0.30), lipoprotein electrophoresis (broad β band), or nuclear magnetic resonance lipoprotein profiling. The Friedewald formula for calculation of LDL-C is not valid in FDBL because the VLDL particles are depleted in TG and enriched in cholesterol. The plasma levels of LDL-C are actually low in this disorder due to defective metabolism of VLDL to LDL. DNA-based methods (apoE genotyping) can be performed to confirm homozygosity for apoE2. However, absence of the apoE2/E2 genotype does not strictly rule out the diagnosis of FDBL, because other mutations in apoE can (rarely) cause this condition.
Because FDBL is associated with increased risk of premature ASCVD, it should be treated aggressively. Other metabolic conditions that can worsen the hyperlipidemia (see above) should be managed. Patients with FDBL are typically diet-responsive and can respond favorably to weight reduction and to low-cholesterol, low-fat diets. Alcohol intake should be curtailed. Pharmacologic therapy is often required, and statins are the first line in management. In the event of statin intolerance or insufficient control of hyperlipidemia, cholesterol absorption inhibitors, fibrates, and niacin are also effective in the treatment of FDBL.
HEPATIC LIPASE DEFICIENCY
Hepatic lipase (HL; gene name LIPC) is a member of the same gene family as LPL and hydrolyzes TGs and phospholipids in remnant lipoproteins and HDL. Hydrolysis of lipids in remnant particles by HL contributes to their hepatic uptake via an apoE-mediated process. HL deficiency is a very rare autosomal recessive disorder characterized by elevated plasma levels of cholesterol and TGs (mixed hyperlipidemia) due to the accumulation of lipoprotein remnants, accompanied by elevated plasma level of HDL-C. The diagnosis is confirmed by measuring HL activity in postheparin plasma and/or confirmation of loss-of-function mutations in both alleles of HL/LIPC. Due to the small number of patients with HL deficiency, the association of this genetic defect with ASCVD is not entirely clear, although anecdotally patients with HL deficiency who have premature CVD have been described. As with FDBL, statin therapy is recommended to reduce remnant lipoproteins and cardiovascular risk.
Additional Secondary Causes of Dyslipidemia
Many of the secondary causes of dyslipidemia (Table 421-4) have been described above. Additional considerations are discussed here.
TABLE 421-4Secondary Causes of Dyslipidemia |Favorite Table|Download (.pdf) TABLE 421-4 Secondary Causes of Dyslipidemia
|LDL ||HDL ||VLDL Elevated ||IDL Elevated ||Chylomicrons Elevated ||Lp(a) Elevated |
|Elevated ||Reduced ||Elevated ||Reduced |
|Hypothyroidism ||Severe liver disease ||Alcohol ||Smoking ||Obesity ||Multiple myeloma ||Autoimmune disease || |
Chronic kidney disease
Exposure to chlorinated hydrocarbons
DM type 2
DM type 2
Glycogen storage disease
|Monoclonal gammopathy ||DM type 2 || |
|Acute intermittent porphyria ||Chronic infectious disease ||Drugs: estrogen ||Gaucher’s disease ||Nephrotic syndrome |
|Autoimmune disease || ||Orchidectomy |
Drugs: thiazides, cyclosporin, carbamazepine
Drugs: niacin toxicity
| || |
Cholesteryl ester storage disease
Drugs: anabolic steroids, beta blockers
Drugs: estrogen, beta blockers, glucocorticoids, bile acid binding resins, retinoic acid
|Hypothyroidism || || |
Drugs: growth hormone, isotretinoin
(See also Chap. 357) Because the liver is the principal site of formation and clearance of lipoproteins, liver disorders can affect plasma lipid levels in a variety of ways. Hepatitis due to infection, drugs, or alcohol is often associated with increased VLDL synthesis and mild to moderate hypertriglyceridemia. Severe hepatitis and liver failure are associated with dramatic reductions in plasma cholesterol and TGs due to reduced lipoprotein biosynthetic capacity.
Cholestasis is associated with hypercholesterolemia, which can be very severe. A major pathway by which cholesterol is excreted from the body is via secretion into bile, either directly or after conversion to bile acids, and cholestasis blocks this critical excretory pathway. In cholestasis, free cholesterol, coupled with phospholipids, is secreted into the plasma as a constituent of a lamellar particle called LP-X. The particles can deposit in skinfolds, producing lesions resembling those seen in patients with FDBL (xanthomata strata palmaris). Planar and eruptive xanthomas can also be seen in patients with cholestasis.
Many drugs have an impact on lipid metabolism and can result in significant alterations in the lipoprotein profile (Table 421-4). Estrogen administration is associated with increased VLDL and HDL synthesis, resulting in elevated plasma levels of both TGs and HDL-C. This lipoprotein pattern is distinctive because the levels of plasma TG and HDL-C are typically inversely related. Plasma TG levels should be monitored when birth control pills or postmenopausal estrogen therapy is initiated to ensure that the increase in VLDL production does not lead to severe hypertriglyceridemia. Use of low-dose preparations of estrogen or the estrogen patch can minimize the effect of exogenous estrogen on lipids.
INHERITED CAUSES OF LOW LEVELS OF APOB-CONTAINING LIPOPROTEINS
Plasma concentrations of LDL-C <60 mg/dL are unusual. Although in some cases LDL-C levels in this range may be reflective of malnutrition or serious chronic illness, LDL-C <60 mg/dL in an otherwise healthy individual suggests an inherited condition. The major inherited causes of low LDL-C are reviewed here.
The synthesis and secretion of apoB-containing lipoproteins in the enterocytes of the proximal small bowel and in the hepatocytes of the liver involve a complex series of events that coordinate the coupling of various lipids with apoB-48 and apoB-100, respectively. Abetalipoproteinemia is a rare autosomal recessive disease caused by loss-of-function mutations in the gene encoding microsomal TG transfer protein (MTP; gene name MTTP), a protein that transfers lipids to nascent chylomicrons and VLDLs in the intestine and liver, respectively. Plasma levels of cholesterol and TG are extremely low in this disorder, and chylomicrons, VLDLs, LDLs, and apoB are undetectable in plasma. The parents of patients with abetalipoproteinemia (obligate heterozygotes) have normal plasma lipid and apoB levels. Abetalipoproteinemia usually presents in early childhood with diarrhea and failure to thrive due to fat malabsorption. The initial neurologic manifestations are loss of deep tendon reflexes, followed by decreased distal lower extremity vibratory and proprioceptive sense, dysmetria, ataxia, and the development of a spastic gait, often by the third or fourth decade. Patients with abetalipoproteinemia also develop a progressive pigmented retinopathy presenting with decreased night and color vision, followed by reductions in daytime visual acuity and ultimately progressing to near-blindness. The presence of spinocerebellar degeneration and pigmented retinopathy in this disease has resulted in some patients with abetalipoproteinemia being misdiagnosed as having Friedreich’s ataxia.
Most of the clinical manifestations of abetalipoproteinemia result from defects in the absorption and transport of fat-soluble vitamins. Vitamin E and retinyl esters are normally transported from enterocytes to the liver by chylomicrons, and vitamin E is dependent on VLDL for transport out of the liver and into the circulation. As a consequence of the inability of these patients to secrete apoB-containing particles, patients with abetalipoproteinemia are markedly deficient in vitamin E and are also mildly to moderately deficient in vitamins A and K. Patients with abetalipoproteinemia should be referred to specialized centers for confirmation of the diagnosis and appropriate therapy. Treatment consists of a low-fat, high-caloric, vitamin-enriched diet accompanied by large supplemental doses of vitamin E. It is imperative that treatment be initiated as soon as possible to prevent development of neurologic sequelae, which can progress even with appropriate therapy. New therapies for this serious disease are needed.
Familial Hypobetalipoproteinemia (FHBL)
FHBL generally refers to a condition of low total cholesterol, LDL-C, and apoB due to mutations in apoB. Most of the mutations causing FHBL result in a truncated apoB protein, resulting in impaired assembly and secretion of chylomicrons from enterocytes and VLDL from the liver. Mutations that result in VLDL particles containing a truncated apoB protein are cleared from the circulation at an accelerated rate, which also contributes to patients with this disorder having low levels of LDL-C and apoB. Individuals heterozygous for these mutations usually have LDL-C levels <60–80 mg/dL and also tend to have lower levels of plasma TG. Many FHBL patients have elevated levels of hepatic fat (due to reduced VLDL export) and sometimes have increased levels of liver transaminases, although it appears that these patients infrequently develop associated inflammation and fibrosis.
Mutations in both apoB alleles cause homozygous FHBL, an extremely rare disorder resembling abetalipoproteinemia with nearly undetectable LDL-C and apoB. The neurologic defects in this form of hypobetalipoproteinemia tend to be less severe than is typically seen in abetalipoproteinemia. Homozygous hypobetalipoproteinemia can be distinguished from abetalipoproteinemia by examining the inheritance pattern of the plasma LDL-C level. The levels of LDL-C and apoB are normal in the parents of patients with abetalipoproteinemia and low in those of patients with homozygous hypobetalipoproteinemia.
Another inherited cause of low LDL-C results from loss-of-function mutations in PCSK9. PCSK9 is a secreted protein that binds to the extracellular domain of the LDL receptor in the liver and promotes the degradation of the receptor. Heterozygosity for nonsense mutations in PCSK9 that interfere with the synthesis of the protein are associated with increased hepatic LDL receptor activity and reduced plasma levels of LDL-C. Such mutations are particularly frequent in individuals of African descent. Individuals who are heterozygous for a loss-of-function mutation in PCSK9 have an ~30–40% reduction in plasma levels of LDL-C and have a substantial protection from CHD relative to those without a PCSK9 mutation, presumably due to having lower plasma cholesterol levels since birth. This observation led to the development of PCSK9 inhibitors as a new strategy for reducing LDL-C levels and cardiovascular risk. Homozygotes for these nonsense mutations have been reported and have extremely low LDL-C levels (<20 mg/dL) but appear otherwise healthy. A sequence variation of somewhat higher frequency (R46L) is found predominantly in individuals of European descent. This mutation impairs, but does not completely destroy, PCSK9 function. As a consequence, the plasma levels of LDL-C in individuals carrying this mutation are more modestly reduced (~15–20%); individuals with these mutations have a 45% reduction in ASCVD risk.