Hypercholesterolemia (LDLR,
APOB)

Frequently Used Abbreviations: Apo B-100: apolipoprotein B-100; CHD: coronary heart disease; FDB: familial defective apolipoprotein B-100; FH: familial hypercholesterolemia; LDL: low-density lipoprotein; LDLR: low-density lipoprotein receptor;

Introduction

Hypercholesterolemia, which affects about 18% of the US population (1), is known to increase the risk for coronary heart disease (CHD). In about 0.2% and 0.1% of the population, loss-of-function mutations in the gene for the low-density lipoprotein receptor (LDLR) or the gene for apolipoprotein B-100 (APOB), respectively, lead to an increased risk for hypercholesterolemia (2-5). Both LDLR-related Familial Hypercholesterolemia (FH) and APOB-related Familial Defective Apolipoprotein B-100 (FDB) are dominantly inherited conditions associated with premature CHD and myocardial infarction (6, 7). Early detection of FH or FDB is important, since atherosclerosis often begins in childhood (8) and prevention or treatment of hypercholesterolemia can significantly lower the risk of CHD (9). In addition, knowledge of a genetic predisposition for hypercholesterolemia may provide powerful motivation for lifestyle changes.

FH/FDB is known to be underdiagnosed and is typically not diagnosed until middle age, after overt clinical symptoms develop or when hypercholesterolemia is detected during a routine physical (9). The most efficient strategy for detecting FH/FDB in children and young adults is cascade screening. Cascade screening starts with the identification of an index patient and radiates out through first, second, and then third-degree relatives. Several nationwide programs have demonstrated that cascade screening for FH leads to improved rates of diagnosis and treatment of affected individuals (10, 11).

Genetic testing for disease-associated mutations in LDLR or APOB has been shown to be the most reliable method for family screening for FH and FDB, respectively (10, 11).

Types and Causes of Monogenic Hypercholesterolemia

Familial Hypercholesterolemia (FH)

FH is due to loss-of-function mutations in the low-density lipoprotein receptor (LDLR), which mediates clearance of low-density lipoprotein (LDL), the main plasma reservoir of cholesterol, from the plasma (see box below for details on Cholesterol Homeostasis and Function of Lipoproteins). Binding of LDL to LDLR expressed on the cell surface leads to internalization of the LDLR-LDL complex. While LDLR is recycled to the membrane, LDL is disassembled into its components. Cholesterol derived from disassembled LDL then inhibits cholesterol biosynthesis within the cell.

Loss-of-function mutations in LDLR lead to increased cholesterol biosynthesis and decreased plasma clearance of total and LDL-cholesterol. Mutations that completely prevent synthesis of a functional LDLR molecule are believed to cause a more severe elevation in plasma cholesterol than mutations that lead to synthesis of a defective LDLR with reduced activity.

Familial Defective Apolipoprotein B-100
LDL-LDLR complex formation is mediated by apolipoprotein B-100 (Apo B-100), the principal protein component of LDL. Loss-of-function mutations in Apo B-100 can decrease the efficiency of LDL binding to LDLR, leading to increased plasma levels of total and LDL-cholesterol. All FDB-associated mutations in Apo B-100 identified to date are clustered within a 200-nucleotide region in exon 26 of APOB.

Other Types

• Autosomal Recessive Hypercholesterolemia (12).
• Autosomal dominant hypercholesterolemia due to mutations in PCSK9 coding for neural apoptosis regulated convertase I (NARC-I) (13)

Cholesterol Homeostasis

Most tissues are able to synthesize cholesterol de novo. In addition, cholesterol can be absorbed from the diet and transported through the plasma to various tissues. Both cholesterol biosynthesis and cellular uptake of cholesterol from the plasma are subject to feedback inhibition from intracellular cholesterol. Cholesterol clearance occurs only in the liver, through secretion either as free cholesterol or as bile acids in the bile.

Function of Lipoproteins

Lipoproteins allow the transport of hydrophobic cholesterol and other lipids through aqueous plasma. These globular particles are composed of a core of triglycerides and cholesteryl esters and a surface layer of phospholipids and free cholesterol, into which one or more proteins (apolipoproteins) are inserted. Classification of lipoproteins is based on their specific density, which reflects their protein and lipid composition. Low-density lipoprotein (LDL) is particularly rich in cholesterol.

Hypercholesterolemia and Coronary Heart Disease

Excess plasma LDL can accumulate in arterial walls, where it becomes chemically modified and is taken up by macrophages. As the macrophages become engorged with modified LDL, they initiate the development of an atherosclerotic lesion, which, over time, can grow into an atherosclerotic plaque composed of cholesterol, cellular debris, and fibrous tissue. If the plaque ruptures, a blood clot can form and completely obstruct the artery; in a coronary artery, such an event leads to myocardial infarction.

Clinical Presentation of FH/FDB

FH and FDB are clinically indistinguishable (14), although hypercholesterolemia due to FDB tends to be less severe (15, 16). FH/FDB is typically characterized by a two to threefold increase in plasma levels of total and LDL-cholesterol. Plasma triglycerides are not elevated. While hypercholesterolemia may be present from birth, symptoms of CHD appear on average at age 45 in men and at age 55 in women. Atherosclerosis, however, can already be detected in children and adolescents with FH (8). Tendon xanthoma, xanthelasma, and premature arcus corneae are present in some individuals with FH/FDB, but may not develop until later in life. The severity of clinical symptoms may depend on lifestyle and diet, as the same mutations in LDLR that lead to FH in Chinese Canadians did not cause any clinical symptoms in Chinese living in China (17).

Homozygosity for an FH-associated mutation in LDLR leads to much more severe hypercholesterolemia, with a three to sixfold increase in total plasma cholesterol and a more than fivefold increase in plasma LDL-cholesterol. Homozygotes for FH-associated mutations in LDLR typically show planar xanthomata by age six, may exhibit symptoms of CHD by age 10, and, if untreated, usually die of myocardial infarction by age 20.

Diagnosis of FH/FDB

Clinical diagnosis of FH/FDB is based mainly on presence of elevated total plasma cholesterol levels. The diagnostic cut-off points vary depending on age and family history of hypercholesterolemia or premature CHD (5). Tendon xanthomata confirm a diagnosis of FH/FDB, but do not occur in all individuals with FH/FDB and may not develop until later in life.

Genetic testing allows diagnosis of FH/FDB at any age and before overt clinical symptoms develop. It is therefore particularly valuable for family screening, and has been shown to be significantly more accurate in detecting affected family members than biochemical testing (11). In one study, a diagnosis of FH would have been missed in 18% of affected individuals if based on plasma cholesterol levels (10).

Treatment of FH/FDB

Treatment of FH/FDB is based on depleting liver cells of cholesterol, which acts as a feedback inhibitor of LDLR expression. Lower levels of cholesterol in the liver cells lead to an increase in the number of LDLR molecules expressed on the cell surface, improving plasma clearance of LDL. Treatment options include dietary restriction of cholesterol and saturated fat and administration of bile acid sequestrants, which prevent re-absorption of bile acids from the gut. In addition, statins have proven to effectively lower LDL concentrations in most cases of FH/FDB by inhibiting the enzyme HMG-CoA reductase, which catalyzes the rate-limiting step in cholesterol biosynthesis. Statins may be combined with ezetimibe, which selectively inhibits intestinal absorption of dietary and biliary cholesterol (18). If statins are not tolerated or not effective (eg, in individuals without residual LDLR activity), ileal bypass surgery to decrease re-absorption of bile acids from the gut or extracorporeal pheresis combined with LDL immunoabsorption provide ways to treat hypercholesterolemia.

Use of Statins in Children

A number of small clinical trials have shown the safety and efficacy of statins in children with FH (19-23). The most long-term study involved two years of pravastatin therapy in 104 children 8 to 18 years of age. No adverse effects on growth, sexual maturation, hormone levels, or liver or muscle tissue were observed, and a significant regression of pre-existing carotid atherosclerosis was documented (22).

Genetics of FH

FH is caused by autosomal dominant loss-of-function mutations in LDLR, the gene for LDLR. FDB is due to autosomal dominant loss-of-function mutations in APOB, the gene for Apo B-100. While heterozygotes usually express the phenotype, homozygotes are much more severely affected. Compound heterozygosity for an FH-associated mutation in LDLR and an FDB-associated mutation in APOB also leads to more severe symptoms (24, 25).

Genetic Testing for FH/FDB

The Hypercholesterolemia Evaluation detects mutations in LDLR and APOB and can diagnose FH an FDB at any age. Such early diagnosis is critical, since prevention or timely treatment of hypercholesterolemia may prevent or delay the onset of CHD.

How Is Genetic Testing for FH/FDB Performed?

DNA for sequencing is obtained from leukocytes present in a small blood sample. The entire coding sequence of LDLR and the region of APOB containing known FDB-associated mutations are amplified in a highly specific manner through a polymerase chain reaction (PCR), and all PCR products are fully sequenced. Sequencing results are interpreted, and a detailed result report is sent to the patient’s physician.

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