Chapter 400: Disorders of Lipoprotein Metabolism

Lipoproteins are complexes of lipids and proteins that are essential for transport of cholesterol, triglycerides (TGs), and fat-soluble vitamins in the blood. Disorders of lipoprotein metabolism include primary and secondary conditions that substantially increase or decrease specific circulating lipids (e.g., cholesterol or TGs) or lipoproteins (e.g., low density or high density lipoproteins, see below). The demonstration that cholesterol-lowering therapy significantly reduces the clinical complications of atherosclerotic cardiovascular disease (ASCVD) makes it important for clinicians to be familiar with the diagnosis and treatment of lipoprotein disorders. This chapter reviews normal lipoprotein physiology, the pathophysiology of disorders of lipoprotein metabolism, the effects of genetic and environmental factors on lipoprotein metabolism, and the clinical approaches to the diagnosis and management of lipoprotein disorders.

LIPOPROTEIN CLASSIFICATION AND COMPOSITION

Lipoproteins are large macromolecular complexes composed of lipids and proteins that transport poorly soluble lipids (primarily TGs, cholesterol, and fat-soluble vitamins) through body fluids (plasma, interstitial fluid, and lymph) to and from tissues. Lipoproteins play an essential role in the absorption of dietary cholesterol, long-chain fatty acids, and fat-soluble vitamins; the transport of TGs, cholesterol, and fat-soluble vitamins from the liver to peripheral tissues; and the transport of cholesterol from peripheral tissues to the liver and intestine.

Lipoproteins contain a core of hydrophobic lipids (TGs and cholesteryl esters) surrounded by a shell of hydrophilic lipids (phospholipids, unesterified cholesterol) and proteins (called apolipoproteins) that interact with body fluids. The plasma lipoproteins are divided into five major classes based on their relative density (Fig. 400-1 and Table 400-1): chylomicrons, very-low-density lipoproteins (VLDLs), intermediate-density lipoproteins (IDLs), low-density lipoproteins (LDLs), and high-density lipoproteins (HDLs). Each lipoprotein class comprises a family of particles that vary in density, size, and protein composition. Because lipid is less dense than water, the density of a lipoprotein particle is primarily determined by the amount of lipid per particle. Chylomicrons are the most lipid-rich and therefore least dense lipoprotein particles, whereas HDLs have the least lipid and are therefore the most dense lipoproteins. In addition to their density, lipoprotein particles can be classified according to their size, determined either by nondenaturing gel electrophoresis or by nuclear magnetic resonance profiling. There is a strong inverse relationship between density and size, with the largest particles being the most buoyant (chylomicrons) and the smallest particles being the most dense (HDL).

The proteins associated with lipoproteins, called apolipoproteins (Table 400-2), are required for the assembly, structure, function, and metabolism of lipoproteins. Apolipoproteins provide a structural basis for lipoproteins, activate enzymes important in lipoprotein metabolism, and act as ligands for cell surface receptors. ApoB is a very large protein and is the major structural protein of chylomicrons, VLDLs, IDLs, and LDLs; one molecule of apoB, either apoB-48 (chylomicron) or apoB-100 (VLDL, IDL, or LDL), is present on each lipoprotein particle. The human liver synthesizes the full-length apoB-100, whereas the intestine makes the shorter apoB-48, which is derived from the same APOB gene by post-transcriptional mRNA editing. HDLs have different apolipoproteins that define this lipoprotein class, most importantly apoA-I, which is synthesized in both the liver and intestine and is found on virtually all HDL particles. ApoA-II is the second most abundant HDL apolipoprotein and is on approximately two-thirds of the HDL particles. ApoC-II, apoC-III, and apoA-V regulate the metabolism of TGs-rich lipoproteins. ApoE plays a critical role in the metabolism and clearance of TG-rich particles. Most apolipoproteins, other than apoB, exchange actively among lipoprotein particles in the blood. Apolipoprotein(a) [apo(a)] is a distinctive apolipoprotein that results in the formation of a lipoprotein known as lipoprotein(a) [Lp(a)], and is discussed more below.

TRANSPORT OF INTESTINALLY DERIVED DIETARY LIPIDS BY CHYLOMICRONS

One critical role of lipoproteins is the efficient transport of dietary lipids from the intestine to tissues that require fatty acids for energy or store and metabolize lipids and of intestinal cholesterol to the liver (Fig. 400-2). Dietary lipids are hydrolyzed by lipases within the intestinal lumen and emulsified with bile acids to form micelles. Dietary cholesterol, fatty acids, and fat-soluble vitamins are absorbed in the proximal small intestine. Cholesterol and retinol are esterified (by the addition of a fatty acid) in the enterocyte to form cholesteryl esters and retinyl esters, respectively. Longer-chain fatty acids (>12 carbons) are incorporated into TGs and packaged with apoB-48, cholesteryl esters, retinyl esters, phospholipids, in a process that requires the action of the microsomal TGs transfer protein (MTP), to form chylomicrons. Nascent chylomicrons are secreted into the intestinal lymph and delivered via the thoracic duct directly to the systemic circulation, where they are extensively processed by peripheral tissues before reaching the liver. The particles encounter lipoprotein lipase (LPL), which is anchored to a glycosylphosphatidylinositol-anchored protein, GPIHBP1, that is attached to the endothelial surfaces of capillaries in adipose tissue, heart, and skeletal muscle (Fig. 400-2). The TGs of chylomicrons are hydrolyzed by LPL, and free fatty acids are released. ApoC-II and apoA-V are apolipoproteins that are transferred to circulating chylomicrons from HDL in the post-prandial state; apoC-II acts as a required cofactor for LPL activation and apoA-V serves as a facilitator of LPL activity. The released free fatty acids are taken up by adjacent myocytes or adipocytes and either oxidized to generate energy or reesterified and stored as TG. Some of the released free fatty acids bind albumin before entering cells and are transported to other tissues, especially the liver. The chylomicron particle progressively shrinks in size as the hydrophobic TG core is hydrolyzed and the hydrophilic lipids (cholesterol and phospholipids) and apolipoproteins on the particle surface are transferred to HDL, ultimately creating chylomicron remnants.

Chylomicron remnants are rapidly removed from the circulation by the liver through a process that requires apoE as a ligand for receptors in the liver. Consequently, few, if any, chylomicrons or chylomicron remnants are generally present in the blood after a 12-h fast, except in patients with certain disorders of lipoprotein metabolism.

TRANSPORT OF HEPATICALLY DERIVED LIPIDS BY VLDL AND LDL

Another key role of lipoproteins is the transport of hepatic lipids from the liver to the periphery (Fig. 400-2) to provide an energy source during fasting. During the fasting state, lipolysis of adipose TGs generates fatty acids that are transported to the liver, and the liver is also capable of synthesizing fatty acids through de novo lipogenesis. These fatty acids are esterified by the liver into TGs, which are packaged into VLDL particles along with apoB-100, cholesteryl esters, phospholipids, and vitamin E in a process that, like for chylomicron assembly, requires MTP. VLDL thus resemble chylomicrons in that they are “triglyceride-rich lipoproteins,” but they contain apoB-100 rather than apoB-48, are smaller and less buoyant, and have a higher ratio of cholesterol to TG (~1 mg of cholesterol for every 5 mg of TG). After secretion by the liver into the plasma, as with chylomicrons, the TGs of VLDL are hydrolyzed by LPL, especially in muscle, heart, and adipose tissue. After the relatively TG-depleted VLDL remnants dissociate from LPL, they are referred to as IDLs, which contain roughly similar amounts of cholesterol and TG. The liver removes ~40–60% of IDL by receptor–mediated endocytosis via binding to apoE, which is acquired through transfer of this protein from HDL. The remainder of IDL is further remodeled by hepatic lipase (HL) to form LDL. During this process, phospholipids and TG in the particle are hydrolyzed, and most of the remaining apolipoproteins except apoB-100 are transferred to other lipoproteins. Approximately 70% of LDL is removed from the circulation by receptor–mediated endocytosis (primarily the LDL receptor) in the liver with apoB-100 serving as the ligand for the LDL receptor. It should be noted that apoB-48 does not contain the LDL receptor-binding ligand region and, therefore, clearance of apoB-48-containing chylomicron remnants is dependent on apoE-mediated clearance as noted above. Some LDL particles are lipolytically processed to “small dense LDL” particles.

Lp(a) is a lipoprotein similar to LDL in lipid and protein composition, but it contains an additional distinctive protein called apolipoprotein(a) [apo(a)]. Apo(a) is synthesized in the liver and attached to apoB-100 by a disulfide linkage. The major site of clearance of Lp(a) is the liver, but the uptake pathway is not known.

HDL METABOLISM AND REVERSE CHOLESTEROL TRANSPORT

All nucleated cells synthesize cholesterol, but only hepatocytes and enterocytes can effectively excrete cholesterol from the body, into either the bile or the gut lumen, respectively. In the liver, cholesterol is secreted into the bile, either directly or after conversion to bile acids. Cholesterol in peripheral cells is transported from the plasma membranes of peripheral cells to the liver and intestine by a process termed “reverse cholesterol transport” that is facilitated by HDL (Fig. 400-3).

Nascent HDL particles are synthesized by the intestine and the liver. Newly secreted apoA-I rapidly acquires phospholipids and unesterified cholesterol from its site of synthesis (intestine or liver) via cellular efflux promoted by the membrane protein ATP-binding cassette protein A1 (ABCA1). This process results in the formation of discoidal HDL particles, which then recruit additional unesterified cholesterol from cells or circulating lipoproteins. Within the HDL particle, the cholesterol is esterified to cholesteryl ester (CE) through the addition of a fatty acid by lecithin-cholesterol acyltransferase (LCAT), a plasma enzyme associated with HDL; the hydrophobic CE forms the core of the mature HDL particle. As HDL acquires more CE, it becomes spherical, and additional apolipoproteins and lipids are transferred to the particles from the surfaces of chylomicrons and VLDLs during lipolysis.

HDL cholesterol is transported to hepatocytes by two major pathways. HDL CE can be “selectively” taken up by hepatocytes via the scavenger receptor class B1 (SR-B1), a cell surface HDL receptor that mediates the selective transfer of CE from HDL with subsequent dissociation and “recycling” of the HDL particle. In addition, HDL CE can be transferred to apoB-containing lipoproteins in exchange for TG by the cholesteryl ester transfer protein (CETP). The CE esters are then removed from the circulation by LDL receptor–mediated endocytosis. HDL-derived CE taken up by the hepatocyte through these pathways is hydrolyzed and much of the cholesterol is ultimately excreted directly into the bile or converted to bile acids with excretion to bile, providing a biliary route into the intestinal lumen. There is also evidence that, under certain conditions, HDL cholesterol can be transported directly into the intestinal lumen without requiring a transhepatobiliary route, a process known as “’transintenstinal cholesterol excretion.”

HDL particles undergo extensive remodeling within the plasma compartment by a variety of lipid transfer proteins and lipases. The phospholipid transfer protein (PLTP) transfers phospholipids from other lipoproteins to HDL or among different classes of HDL particles and is a regulator of HDL metabolism. After CETP- and PLTP-mediated lipid exchange, the TG-enriched HDL becomes a much better substrate for HL, which hydrolyzes the TGs and phospholipids to generate smaller HDL particles. A related enzyme called endothelial lipase (EL) hydrolyzes HDL phospholipids, generating smaller HDL particles that are catabolized faster. Remodeling of HDL influences the metabolism, function, and plasma concentrations of HDL.

Disorders of lipoprotein metabolism are collectively referred to as “dyslipidemias.” Dyslipidemias are generally characterized clinically by increased plasma levels of cholesterol, TGs, or both, variably accompanied by reduced levels of HDL cholesterol. Unusually low levels of cholesterol also fall within the broad scope of lipoprotein disorders. 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 (diet, lifestyle, medical condition, or drug). Many, but not all, patients with dyslipidemia are at increased risk for ASCVD, which is the primary reason for making the diagnosis, as intervention can substantially reduce this risk. In addition, patients with markedly elevated levels of TGs 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” or pathways that regulate lipoprotein metabolism and are dysfunctional in specific dyslipidemias. These include: (1) assembly and secretion of TG-rich VLDLs by the liver; (2) lipolysis of TG-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 TGs and low levels of HDL cholesterol (HDL-C), with variable elevations in LDL cholesterol (LDL-C). 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. 401). Some of the major factors that drive hepatic VLDL secretion include a high-carbohydrate diet, excessive alcohol use, obesity and insulin resistance, nephrotic syndrome, and genetic factors.

Secondary Causes of VLDL Overproduction

HIGH-CARBOHYDRATE DIET

Dietary carbohydrates are utilized as a substrate for fatty acid synthesis in the liver. Some of the newly synthesized fatty acids are esterified, forming TGs, and secreted in VLDL. Thus, excessive intake of calories as carbohydrates, which is frequent in Western societies, leads to increased hepatic VLDL-TG secretion.

ALCOHOL

Excessive 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 relatively unusual combination of elevated TGs and elevated HDL-C.

OBESITY AND INSULIN RESISTANCE

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 (See also Chaps. 395 and 396). 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 re-esterified 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).

NEPHROTIC SYNDROME

Nephrotic syndrome is a classic cause of excessive VLDL production (See also Chap. 305). 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.

CUSHING’S SYNDROME

Endogenous or exogenous glucocorticoid excess is associated with increased VLDL synthesis and secretion and hypertriglyceridemia (See also Chap. 379). 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 probably 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 ~1 in 100–200 individuals and is an important contributor to premature coronary heart disease (CHD); ~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, and 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 have substantially elevated plasma levels of apoB, often 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 in a simple Mendelian fashion. 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 dyslipidemia and/or premature CHD suggests the diagnosis. Measurement of apoB levels can help support the diagnosis if they are substantially elevated relative to the LDL-C level. Individuals with this phenotype should be treated aggressively due to significantly increased risk of premature CHD. Decreased dietary intake of simple carbohydrates, increase aerobic exercise, and weight loss can all have beneficial effects on the lipid profile. Patients with type 2 diabetes should be aggressively treated to maintain good glucose control. Most patients with FCHL require lipid-lowering drug therapy, starting with statins, to reduce apoB-containing lipoprotein levels and lower the risk of cardiovascular disease.

LIPODYSTROPHY

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. Patients with congenital generalized lipodystrophy—a recessive disorder caused by mutations in the AGPAT2 and BSCL2 genes—are very rare. These patients have nearly complete absence of subcutaneous fat, accompanied by profound insulin resistance and leptin deficiency, severe hypertriglyceridemia, and accumulation of TGs in multiple tissues including the liver. Patients with generalized lipodystrophy can often be effectively treated with recombinant leptin administration. Partial lipodystrophy is a dominantly inherited disorder that is somewhat more common than the generalized form. It is caused by mutations in several different genes, including lamin A/C (LMNA), PPAR gamma (PPARG), perilipin (PLIN1), and AKT2. Partial lipodystrophy is characterized by markedly reduced subcutaneous fat in the extremities and buttocks, accompanied by increased facial, neck, and truncal fat. These patients generally have insulin resistance, often quite severe, accompanied by type 2 diabetes, hepatosteatosis, and dyslipidemia. The dyslipidemia, attributed mostly to increased VLDL production but also possibly due to other factors, is usually characterized by substantially 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 TG-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, skeletal myocytes, and cardiomyocytes. It is then transported from the subendothelial to the vascular endothelial surfaces by GPIHPB1, which helps dock it to the endothelial surface. 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, low-frequency, and rare genetic variants have been described that influence LPL activity, and single-gene Mendelian disorders that reduce LPL activity have also been described (Table 400-3).

Secondary Causes of Impaired Lipolysis of TRLs

OBESITY AND INSULIN RESISTANCE

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 (See also Chaps. 394, 395, and 396). 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 exacerbates the effects of increased VLDL production and 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. Genetic deficiency or inactivity of LPL results in impaired lipolysis and profound elevations in plasma chylomicrons, causing familial chylomicronemia syndrome. While chylomicronemia predominates, in fact these patients often have elevated plasma levels of VLDL as well. The fasting plasma is turbid, and if left undisturbed for several hours, the chylomicrons float to the top and form a creamy supernatant layer. Fasting TG levels are almost invariably >1000 mg/dL. Fasting cholesterol levels are also elevated but to a lesser degree. The most common cause of FCS involves mutations in the LPL gene. LPL deficiency has autosomal recessive inheritance (loss of function mutations in both alleles) and has an estimated frequency of ~1 in 1 million, though its true prevalence is unknown. Heterozygotes with LPL mutations often have moderate elevations in plasma TG levels and increased risk for CHD.

Familial chylomicronemia syndrome can be caused by mutations in genes other than LPL. For example, apoC-II is a required cofactor for LPL. ApoC-II deficiency due to loss of function mutations in both APOC2 alleles results in functional lack of LPL activity and severe hyperchylomicronemia that is indistinguishable from LPL deficiency. It is also recessive in inheritance pattern and much rarer than LPL deficiency. Individuals heterozygous for a mutation in APOC2 do not generally have hypertriglyceridemia. Another apolipoprotein, apoA-V, facilitates the association of VLDL and chylomicrons with LPL and promotes hydrolysis of the TGs. Individuals harboring loss-of-function mutations in both APOA5 alleles causing ApoA-V deficiency develop a form of familial chylomicronemia syndrome. Heterozygosity for variants in APOA5 that reduce its function contributes to the polygenic basis of hypertriglyceridemia. GPIHBP1 is required for transport and tethering of LPL to the endothelial luminal surface. Homozygosity for mutations in GPIHBP1 that interfere with its synthesis or folding cause familial chylomicronemia syndrome. Autoantibodies to GPIHBP1 have also been reported to cause severe hyperchylomicronemia.

Familial chylomicronemia syndrome can present in childhood or adulthood with recurrent episodes of severe abdominal pain due to acute pancreatitis. In this setting, the diagnosis should be suspected if a fasting TG level is >750 mg/dL. Eruptive xanthomas, which are small, yellowish-white papules, may appear in clusters on the back, buttocks, and extensor surfaces of the arms and legs. On funduscopic examination, the retinal blood vessels may be opalescent (lipemia retinalis). Hepatosplenomegaly is sometimes noted as a result of uptake of circulating chylomicrons by reticuloendothelial cells in the liver and spleen. Premature CHD is not generally a feature of familial chylomicronemia syndromes.

The diagnosis of familial chylomicronemia syndrome is a clinical diagnosis based on persistence and severity of hypertriglyceridemia in the setting of a history of proven or suspected acute pancreatitis. While LPL activity can be measured in “postheparin plasma” obtained after an IV heparin injection to release the endothelial-bound LPL, this assay is not widely available. Molecular sequencing of the candidate FCS genes can be used to confirm the diagnosis, but is not required for making the clinical diagnosis.

Because of the risk of pancreatitis, it is important to consider the diagnosis and institute therapeutic interventions in familial chylomicronemia syndrome. The goal is to prevent pancreatitis by reducing fasting TG levels to <500 mg/dL. Dietary fat intake should be markedly restricted (to as little as 15 gm/day), often with fat-soluble vitamin supplementation. Consultation with a registered dietician familiar with this disorder is essential. Usually dietary fat restriction alone is not successful in resolving the chylomicronemia, in which case fish oils have been modestly effective in some patients; fibrates (such as fenofibrate) may be tried but are also unlikely to be effective. A new therapeutic approach involving the suppression of APOC3 with an antisense oligonucleotide is a promising approach for patients with FCS. 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.

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 lipoproteins 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 focused on 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. While this disorder runs in families, often with a dominant pattern of inheritance, a molecular etiology has not been established. Combinations of gene variants have been shown to cause this phenotype and therefore 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. Down-regulation 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 400-3).

Secondary Causes of Impaired Hepatic Uptake of Lipoproteins

HYPOTHYROIDISM

Hypothyroidism is associated with elevated plasma LDL-C levels due primarily to a reduction in hepatic LDL receptor function and delayed clearance of LDL (See also Chap. 375). 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.

CHRONIC KIDNEY DISEASE

Chronic kidney disease (CKD) is often associated with mild hypertriglyceridemia (150–400 mg/dL) due to the accumulation of VLDLs and remnant lipoproteins in the circulation (See also Chap. 305). TG lipolysis and remnant clearance are both reduced in patients with renal failure. Because the risk of ASCVD is increased in CKD, patients should usually be treated with lipid-lowering agents, particularly statins.

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, but statins are often indicated in these patients, with careful attention to the potential for 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 due to multiple genetic variants exerting modest LDL-raising effects. 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. Importantly, single-gene (Mendelian) causes of elevated LDL-C are relatively common and should be considered in the differential diagnosis of elevated LDL-C (Table 400-3).

FAMILIAL HYPERCHOLESTEROLEMIA (FH)

FH, also known as autosomal dominant hypercholesterolemia (ADH), is an autosomal co-dominant disorder characterized by elevated plasma levels of LDL-C in the absence of hypertriglyceridemia. FH is caused by mutations that lead to reduced function of the LDL receptor, with the most common being mutations in the LDLR gene itself. 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. 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 LDLR alleles (FH homozygotes, or compound heterozygotes) have much higher LDL-C levels than those with one mutant allele (FH heterozygotes).

Although mutations in the LDLR are the most common cause of FH, mutations in at least two other genes, APOB and PCSK9, can also cause ADH. ApoB-100 is the critical structural protein in LDL and contains a ligand for binding to the LDL receptor. Mutations in the LDL receptor–binding domain of apoB-100 cause a form of FH, also known as ADH type 2 or familial defective apoB (FDB). The mutations reduce the affinity of LDL binding to the LDL receptor, such that LDL is removed from the circulation at a reduced rate. Of note, truncating mutations in APOB cause low LDL-C levels (see below). The proprotein convertase subtilisin/kexin type 9 (PCSK9) is a secreted protein that binds to the LDL receptor and targets it for lysosomal 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 circulating PCSK9 binds the receptor, the complex is internalized and the receptor is directed to the lysosome, rather than to the cell surface, reducing the number of active LDL receptors. Gain-of-function mutations in PCSK9 that enhance the activity of PCSK9 cause a form of FH, also known as ADH type 3. Of note, loss-of-function mutations in PCSK9 markedly lower LDL-C levels (see below).

The population frequency of heterozygous FH was originally estimated to be 1 in 500 individuals, but recent data suggest it is ~1 in 250 individuals, making it one of the most common single-gene disorders in humans. FH has a much higher prevalence in certain founder populations, such as South African Afrikaners, Christian Lebanese, French Canadians, and Lancaster County Amish. Heterozygous FH is characterized by elevated plasma levels of LDL-C (~190–400 mg/dL) and usually relatively normal levels of TGs. Patients with heterozygous FH have hypercholesterolemia from birth, and disease recognition is often based on detection of hypercholesterolemia on routine screening, or a notable family history of hypercholesterolemia, or premature coronary heart disease. Inheritance of FH is dominant, meaning that the condition is inherited from one parent, and ~50% of the patient’s siblings and children can be expected to have FH. For this reason, family-based “cascade screening” can be very effective in identifying additional persons with FH. The family history is frequently positive for premature CHD on the side of the family from which the mutation was inherited. Physical findings in some, but not all, patients with heterozygous FH include corneal arcus and/or 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.

The diagnosis of FH is generally a clinical diagnosis based on substantial hypercholesterolemia with LDL-C >190 mg/dL in the absence of a secondary etiology, and a family history of hypercholesterolemia and/or premature coronary disease. Secondary causes of significant hypercholesterolemia such as hypothyroidism, nephrotic syndrome, and obstructive liver disease should be excluded. Sequencing of the FH genes (LDLR, APOB, PCSK9) to confirm the diagnosis is available and worthy of consideration; persons with confirmed FH are at higher risk of CVD than those with similar LDL-C levels who don’t have FH and therefore may benefit from more aggressive treatment of hypercholesterolemia.

FH patients should always be actively treated to lower plasma levels of LDL-C, preferably starting in childhood. Initiation of a diet low in saturated and trans fats is recommended, but heterozygous FH patients require pharmacologic therapy for effective control of their LDL-C levels. Statins are the initial drug class of choice, and usually a more potent member of the class. Many heterozygous FH patients cannot achieve adequate control of their LDL-C levels even with high-intensity statin therapy, and a cholesterol absorption inhibitor (ezetimibe), a PCSK9 inhibitor, or a bile acid sequestrant are the next-line classes of drugs.

Homozygous FH (hoFH) is caused by mutations in both alleles of the LDL receptor or double heterozygosity for mutations in two FH genes. Patients with homozygous FH have been classified into those 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. Some 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. While the diagnosis is usually made on clinical grounds, specific mutations can usually be identified by DNA sequencing. Patients with homozygous FH must be treated aggressively to delay the onset and progression of CVD. Although receptor negative patients have no response to statins and PCSK9 inhibitors, receptor defective patients can have modest responses to these medicines and they should be tried in patients with hoFH. 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 patients with homozygous FH and should be considered in patients who have insufficient response to statins and PCSK9 inhibitors. LDL apheresis, a physical method of purging the blood of LDL in which the LDL particles are selectively removed from the circulation, should be considered in hoFH patients who have persistently elevated LDL-C levels despite attempts at drug therapy. Liver transplantation is effective in decreasing plasma LDL-C levels in this disorder and is sometimes used as a last resort. Liver-directed gene therapy is under development for hoFH.

FH is an autosomal dominant disorder. There are a few rare conditions that cause an FH-like phenotype in an autosomal recessive manner and should be considered in patients with substantial hypercholesterolemia who do not report a dominant family history of hypercholesterolemia or premature CHD.

AUTOSOMAL RECESSIVE HYPERCHOLESTEROLEMIA (ARH)

ARH is a very rare autosomal recessive disorder that was originally reported in individuals of Sardinian descent. The disease is caused by mutations in the gene LDLRAP1 encoding the protein LDLR adaptor protein (also called the ARH protein) which is required for LDL receptor–mediated endocytosis in the liver. LDLRAP1 binds to the cytoplasmic domain of the LDL receptor and links the receptor to the endocytic machinery. In the absence of LDLRAP1, 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 often require additional therapy to lower plasma LDL-C to acceptable levels.

SITOSTEROLEMIA

Sitosterolemia is a rare autosomal recessive disease that is caused by biallelic 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 both 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, reducing their absorption and promoting their excretion. 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 and 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 clinical picture of severe hypercholesterolemia, often accompanied by 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 fails to respond to statin therapy. Sitosterolemia can be diagnosed by a laboratory finding of a substantial increase in plasma sitosterol and/or other plant sterols, and should be confirmed by gene sequencing of ABCG5 and ABCG8. It is important to make the diagnosis, because diet, bile acid sequestrants, and cholesterol-absorption inhibitors are the most effective agents to reduce LDL-C and plasma plant sterol levels in these patients. Of note, heterozygosity for mutations in ABCG5 or ABCG8 is now recognized to cause a moderate form of hypercholesterolemia.

LYSOSOMAL ACID LIPASE DEFICIENCY (LALD)

LALD, also known as cholesteryl ester storage disease, is an autosomal recessive disorder caused by loss-of-function variants in both alleles of the gene LIPA encoding the enzyme lysosomal acid lipase (LAL). 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. LALD is characterized by elevated LDL-C, usually in association with low HDL-C and with variably elevated TG levels, together with progressive fatty liver ultimately leading to hepatic fibrosis. 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 most severe form of this disorder, Wolman’s disease, presents in infancy and is rapidly fatal. The etiology of the elevated LDL-C levels is primarily due to impaired LDL receptor–mediated clearance of LDL. LALD should be 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. LALD is underdiagnosed; it is critically important to suspect it and make the diagnosis because enzyme replacement therapy is now available and is highly effective in treating this condition.

The above conditions primarily cause elevations in LDL due to impaired catabolism of LDL from the blood. There are a few forms of primary dyslipidemia that impair the catabolism of “remnant” TG-rich lipoproteins (after their processing by LPL) and therefore cause elevations in both cholesterol and TGs due to remnant accumulation.

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. 400-2). 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. Certain “dominant negative” 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 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 PCSK9 inhibitors 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 400-4) have been described above. Additional considerations are discussed here.

LIVER DISORDERS

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 (See also Chap. 329). 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 often associated with hypercholesterolemia. 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.

DRUGS

Many drugs have an impact on lipid metabolism and can result in significant alterations in the lipoprotein profile (Table 400-4). 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.

Abetalipoproteinemia

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 the APOB gene. 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.

Familial Combined Hypolipidemia

Nonsense mutations in both alleles of the gene Angiopoietin-like 3 (ANGPTL3) lead to low plasma levels of all three major lipid fractions—TG, LDL-C, and HDL-C, a phenotype termed familial combined hypolipidemia. ANGPTL3 is a protein synthesized by the liver and secreted into the bloodstream. It inhibits LPL, thus delaying clearance of TRLs from the blood and increasing TRL blood concentrations. Deficiency of ANGPTL3, therefore, raises LPL activity and predominantly lowers blood TG. ANGPTL3 deficiency is associated with a reduced risk for CHD. Therapies to antagonize ANGPTL3 are in development and initial human studies show that inhibition of ANGPTL3 by either an antisense oligonucleotide or a monoclonal antibodies lower blood levels of TG and LDL-C.

PCSK9 Deficiency

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 more 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. 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 CHD risk. The discovery of this condition led to the development of therapies that antagonize PCSK9, thus reducing LDL-C levels and risk of CHD. Two antibodies against PCSK9 are currently on the market (Table 400-5).

Low levels of HDL-C are very commonly encountered in clinical practice. Low HDL-C is an important independent predictor of increased cardiovascular risk and has been used regularly in standardized risk calculators. However, it is doubtful that low HDL-C is directly causal for the development of ASCVD. HDL metabolism is strongly influenced by TG metabolism, insulin resistance, and inflammation, among other environmental and medical factors. Thus the HDL-C measurement integrates a number of cardiovascular risk factors, potentially explaining its strong inverse association with ASCVD.

The majority of patients with low HDL-C have some combination of genetic predisposition and secondary factors. Variants in dozens of genes have been shown to influence HDL-C levels. Even more important quantitatively, obesity and insulin resistance have strong suppressive effects on HDL-C, and low HDL-C in these conditions is widely observed. Furthermore, the vast majority of patients with elevated TGs have reduced levels of HDL-C. Most patients with low HDL-C who have been studied in detail have accelerated catabolism of HDL and its associated apoA-I as the physiologic basis for the low HDL-C. Importantly, although HDL-C remains an important biomarker for assessing cardiovascular risk, it is not currently a direct target of intervention for raising the level in order to reduce cardiovascular risk.

INHERITED CAUSES OF VERY LOW LEVELS OF HDL-C

Mutations in genes encoding proteins that play critical roles in HDL synthesis and catabolism can result in reductions in plasma levels of HDL-C. Unlike the genetic forms of hypercholesterolemia, which are invariably associated with premature coronary atherosclerosis, genetic forms of hypoalphalipoproteinemia (low HDL-C) are often not associated with clearly increased risk of ASCVD.

Gene Deletions in the APOA5-A1-C3-A4 Locus and Coding Mutations in APOA1

Complete genetic deficiency of apoA-I due to a complete deletion of the APOA1 gene results in the virtual absence of circulating HDL and appears to increase the risk of premature ASCVD. The genes encoding APOA5, APOA1, APOC3, and APOA4 are clustered together on chromosome 11. Some patients with no apoA-I have genomic deletions that include other genes in the cluster. ApoA-I is required for LCAT activity. In the absence of LCAT, free cholesterol levels increase in both plasma (not HDL) and in tissues. The free cholesterol can form deposits in the cornea and in the skin, resulting in corneal opacities and planar xanthomas. Premature CHD is associated with apoA-I deficiency.

Missense and nonsense mutations in the apoA-I gene are present in some patients with low plasma levels of HDL-C (usually 15–30 mg/dL), but are a rare cause of low plasma HDL-C levels. Most individuals with low plasma HDL-C levels due to missense mutations in apoA-I do not appear to have premature CHD. Patients who are heterozygous for an Arg173Cys substitution in apoA-I (so-called apoA-I_Milano) have very low plasma levels of HDL-C due to impaired LCAT activation and accelerated clearance of the HDL particles containing the abnormal apoA-I. Despite having very low plasma levels of HDL-C, these individuals do not have an increased risk of premature CHD. A few selected missense mutations in apoA-I and apoA-II promote the formation of amyloid fibrils, which can cause systemic amyloidosis.

Tangier Disease (ABCA1 Deficiency)

Tangier disease is a rare autosomal co-dominant form of extremely low plasma HDL-C levels that is caused by mutations in the gene encoding ABCA1, a cellular transporter that facilitates efflux of unesterified cholesterol and phospholipids from cells to apoA-I (Fig. 400-3). ABCA1 in the liver and intestine rapidly lipidates the apoA-I secreted from the basolateral membranes of these tissues. In the absence of ABCA1, the nascent, poorly lipidated apoA-I is immediately cleared from the circulation. Thus, patients with Tangier disease have extremely low circulating plasma levels of HDL-C (<5 mg/dL) and apoA-I (<5 mg/dL). Cholesterol accumulates in the reticuloendothelial system of these patients, resulting in hepatosplenomegaly and pathognomonic enlarged, grayish yellow or orange tonsils. An intermittent peripheral neuropathy (mononeuritis multiplex) or a sphingomyelia-like neurologic disorder can also be seen in this disorder. Tangier disease is probably associated with some increased risk of premature atherosclerotic disease, although the association is not as robust as might be anticipated, given the very low levels of HDL-C and apoA-I in these patients. Patients with Tangier disease also have low plasma levels of LDL-C, which may attenuate the atherosclerotic risk. Obligate heterozygotes for ABCA1 mutations have moderately reduced plasma HDL-C levels (15–30 mg/dL), and their risk of premature CHD remains uncertain.

Familial LCAT Deficiency

This rare autosomal recessive disorder is caused by mutations in LCAT, an enzyme synthesized in the liver and secreted into the plasma, where it circulates associated with lipoproteins (Fig. 400-3). As reviewed above, the enzyme is activated by apoA-I and mediates the esterification of cholesterol to form cholesteryl esters. Consequently, in familial LCAT deficiency, the proportion of free cholesterol in circulating lipoproteins is greatly increased (from ~25% to >70% of total plasma cholesterol). Deficiency in this enzyme interferes with the maturation of HDL particles and results in rapid catabolism of circulating apoA-I.

Two genetic forms of familial LCAT deficiency have been described in humans: complete deficiency (also called classic LCAT deficiency) and partial deficiency (also called fish-eye disease). Progressive corneal opacification due to the deposition of free cholesterol in the cornea, very low plasma levels of HDL-C (usually <10 mg/dL), and variable hypertriglyceridemia are characteristic of both disorders. In partial LCAT deficiency, there are no other known clinical sequelae. In contrast, patients with complete LCAT deficiency have hemolytic anemia and progressive renal insufficiency that eventually leads to end-stage renal disease. Remarkably, despite the extremely low plasma levels of HDL-C and apoA-I, premature ASCVD is not a consistent feature of either LCAT deficiency or fish eye disease. The diagnosis can be confirmed in a specialized laboratory by assaying plasma LCAT activity or by sequencing the LCAT gene.

Primary Hypoalphalipoproteinemia

The condition of low plasma levels of HDL-C (the “alpha lipoprotein”) is referred to as hypoalphalipoproteinemia. Primary hypoalphalipoproteinemia is defined as a plasma HDL-C level below the tenth percentile in the setting of relatively normal cholesterol and TG levels, no apparent secondary causes of low plasma HDL-C, and no clinical signs of LCAT deficiency or Tangier disease. This syndrome is often referred to as isolated low HDL. A family history of low HDL-C facilitates the diagnosis of an inherited condition, which may follow an autosomal dominant pattern. The metabolic etiology of this disease appears to be primarily accelerated catabolism of HDL and its apolipoproteins. Some of these patients may have ABCA1 mutations and therefore have heterozygous Tangier disease. Several kindreds with primary hypoalphalipoproteinemia and an increased incidence of premature CHD have been described, although it is not clear if the low HDL-C level is the cause of the accelerated atherosclerosis in these families. Association of hypoalphalipoproteinemia with premature CHD may depend on the specific nature of the gene defect or the underlying metabolic defect that either directly or indirectly causes the low plasma HDL-C level.

SCREENING

Hypercholesterolemia is a cause of premature CHD that is highly treatable and therefore persons should be actively screened. Plasma lipid levels should be measured, preferably after a 12-h overnight fast, in all adults; guidelines suggest that screening of children between 9 and 11 years of age is also recommended. In most clinical laboratories, the total cholesterol and TGs in the plasma are measured enzymatically, and then the cholesterol in the supernatant is measured after precipitation of apoB-containing lipoproteins to determine the HDL-C. The LDL-C is then estimated using the following equation (the Friedewald formula):

(The VLDL cholesterol content is estimated by dividing the plasma TG by 5, reflecting the ratio of TG to cholesterol in VLDL particles.) This formula is reasonably accurate if test results are obtained on fasting plasma and if the TG level does not exceed ~200 mg/dL; by convention it cannot be used if the TG level is >400 mg/dL. LDL-C can be directly measured by a number of methods. Further evaluation and treatment are based primarily on the clinical assessment of absolute cardiovascular risk using risk calculators such as the AHA/ACC risk calculator based on a large amount of observational data.

DIAGNOSIS

A critical first step in managing a lipoprotein disorder is to attempt to determine the class or classes of lipoproteins that are increased or decreased in the patient. Once the dyslipidemia is accurately classified, efforts should be directed to rule out any possible secondary causes (Table 400-4). A careful social, medical, and family history should be obtained. A fasting glucose should be obtained in the initial workup of all subjects with an elevated TG level. Nephrotic syndrome and chronic renal insufficiency should be excluded by obtaining urine protein and serum creatinine. Liver function tests should be performed to rule out hepatitis and cholestasis. Hypothyroidism should be ruled out by measuring serum thyroid-stimulating hormone.

Once secondary causes have been ruled out, attempts should be made to diagnose a primary lipid disorder, because the underlying genetic defect can provide important prognostic information regarding the risk of developing CHD, the response to drug therapy, and the management of other family members. Obtaining the correct diagnosis often requires a detailed family medical history, lipid analyses in family members, and sometimes specialized testing.

Severe Hypertriglyceridemia

If the fasting plasma TG level is >750 mg/dL, the patient may have chylomicronemia. If the elevated TG levels are persistent and the TG-to-total cholesterol ratio is >8, particularly in the setting of a history of pancreatitis, familial chylomicronemia syndrome should be considered. While LPL activity measured in postheparin plasma can support diagnosis, genetic testing for FCS genes may be indicated. Most individuals with persistent severe hypertriglyceridemia do not have a single gene disorder (FCS) but instead are genetically predisposed and have secondary factors (diet, obesity, glucose intolerance, alcohol ingestion, estrogen therapy) that contribute to the hyperlipidemia. Such patients are still at risk for acute pancreatitis and should be treated to reduce their TG levels and thus their risk of pancreatitis.

Severe Hypercholesterolemia

If the levels of LDL-C are very high (greater than a ninety-fifth percentile for age and sex) in absence of secondary causes, familial hypercholesterolemia should be considered, particularly if there is a family history of hypercholesterolemia and/or premature CHD. While FH is a clinical diagnosis, genetic sequencing is now widely available as may be considered in order to determine the molecular diagnosis; a finding of a causal mutation may appropriately result in earlier and more aggressive therapy to lower LDL-C and could also promote family-based cascade screening. Recessive forms of severe hypercholesterolemia are rare, but if a patient with severe hypercholesterolemia has parents with normal cholesterol levels, ARH, sitosterolemia, and LALD should be considered. Patients with more moderate hypercholesterolemia that does not segregate in families as a monogenic trait are likely to have polygenic hypercholesterolemia.

Combined Hyperlipidemia

Elevations in the plasma levels of both cholesterol and TGs are seen in patients with increased plasma levels of both VLDL and LDL or of remnant lipoproteins. A β-quantification to determine the VLDL cholesterol/TG ratio in plasma (see discussion of FDBL), an NMR lipoprotein profile, or a direct measurement of the plasma LDL-C should be performed at least once prior to initiation of lipid-lowering therapy to determine if the hyperlipidemia is due to the accumulation of remnants or to an increase in both LDL and VLDL. Measurement of plasma apoB levels can help identify patients with FCHL who may require more aggressive treatment.

Given the prevalence of primary and secondary dyslipidemias and the clinical benefits of early diagnosis and initiation of therapy, it is essential that physicians screen lipids systematically, rule out secondary causes of dyslipidemia, suspect inherited disorders of lipoprotein metabolism where appropriate, actively promote family-based cascade screening, and be knowledgeable about the existing therapeutic options, including PCSK9 inhibitors. The field of “clinical lipidology” has matured and is moving toward a more systematic clinical application of genomic medicine. Diagnostic DNA sequencing or genotyping in patients with suspected FH, FCS, and FDBL has the potential to enhance molecular diagnosis, facilitate appropriate therapeutic interventions, and promote family-based cascade screening.

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