Genetics of Familial Hypercholesterolemia 张咸宁 Tel ： ; Office: A705, Research Building 2013/03

Learning Objectives l. 掌握家族性高胆固醇血症致病基因的有 关知识。 2. 了解 LDL 在细胞内的相关转运和代谢途 径。

Required Reading Thompson &Thompson Genetics in Medicine, 7 th Ed （双语版， 2009 ） ● P ; ● Clinical Case Studies-14 Familial Hypercholesterolemia

Cardiovascular Disorders Heart disease is the leading cause of death worldwide, and it accounts for approximately 25% of all deaths in the US. The most common underlying cause of heart disease is coronary artery disease (CAD), which is caused by atherosclerosis. A number of risk factors for CAD have been identified, including obesity, cigarette smoking, hypertension, elevated cholesterol level, and positive family history.

Risk for CAD? The risk is higher: if there are more affected relatives. if the affected relative is female (the less commonly affected sex) rather than male. if the age of onset in the affected relative is early (before 55 years of age).

What part do genes play in the familial clustering of CAD?

Familial hypercholesterolemia (FH) have raised cholesterol levels with a significant risk of developing early CAD. Cells normally derive cholesterol from either endogenous synthesis or dietary uptake from LDL receptors on the cell surface. High cholesterol levels in FH are due to deficient or defective function of the LDL receptors leading to increased levels of endogenous cholesterol synthesis.

Clinical Synopsis of FH INHERITANCE: AD. (Prevalence: 1/500) HEAD AND NECK: Eyes: (1) Corneal arcus; (2) Xanthelasma. CARDIOVASCULAR: Heart: CAD presenting after age 30 years in heterozygotes, in childhood in homozygotes. SKIN, NAILS, HAIR: Skin: (1)Tendinous xanthomas presenting after age 20 years in heterozygotes, during first 4 years of life in homozygotes; (2)Planar xanthomas in homozygotes. LABORATORY ABNORMALITIES: Hypercholesterolemia, mg/dL in heterozygotes, mg/dL in homozygotes. (normal: ~ mg/dL) MISCELLANEOUS: Incidence, 1/500 heterozygotes, 1/10 6 homozygotes. MOLECULAR BASIS: Caused by mutations in LDLR.

Arcus lipoides

Xanthomas (fatty deposit)

FH

Cholesterol levels in affected families are variable and lipid assays do not necessarily identify those with mutations. There is therefore interest in the introduction of widespread genetic testing, though most mutations are missense, which may pose problems of interpretation.

LDLR gene → FH (OMIM: ) An important advance was the isolation and cloning of the gene (1984) that encodes the low-density lipoprotein (LDL) receptor. Heterozygosity for a mutation in LDLR (19p13.2) roughly doubles LDL cholesterol levels and is seen in ~1 in 500 persons --FH -- accounting for ~5% of myocardial infarctions (MIs) in persons <60 years.

The 4 Genes Associated with FH Mutant Gene Product Pattern of Inheritance Prevalence Effect of Disease- Causing Mutations Typical LDL Cholesterol Level (Normal Adults: ~120 mg/dL) LDL receptor AD (19p13.2) HTZs: 1/500 HMZs: 1/10 6 Loss of function HTZs: 350 HMZs: 700 Apo B-100AD (2p24) HTZs: 1/1000* HMZs: 1/10 6 * Loss of function HTZs: 270 HMZs: 320 ARH adaptor Pr. AR (1p36-p35) Very rare†Loss of function HMZs: 470 PCSK9 protease AD (1p34.1-p32 ) Very rareGain of function HTZs: 225

The 4 proteins associated with FH

The LDL receptor binds apo B-100. Mutations in the LDL receptor binding domain of apo B-100 impair LDL binding to its receptor, reducing the removal of LDL cholesterol from the circulation. Clustering of the LDL receptor–apo B-100 complex in clathrin- coated pits requires the ARH adaptor protein, which links the receptor to the endocytic machinery of the coated pit. Homozygous mutations in the ARH protein impair the internalization of the LDL:LDL receptor complex, thereby impairing LDL clearance. PCSK9 protease activity leads to degradation of the LDL receptor.

LDL receptor: a membrane-bound 160-kD Pr. of 839 AAs

The structure of the LDL receptor gene showing its 5 domains and the effect on the receptor of mutations in these domains that lead to FH

Each class of mutations reduces the number of effective LDL receptors, resulting in decreased LDL uptake and hence elevated levels of circulating cholesterol

Common PCSK9 Variants Associated with Low LDL Cholesterol Levels Sequence Variant Population Frequency of Heterozygotes Mean Reduction in LDL Cholesterol Impact on Incidence of CAD p.Tyr142X or Cys679X p.Arg46Leu African Americans: 2.6% Whites: 3.2% 28% (38 mg/dL) 15% (20 mg/dL) 90% reduction 50% reduction

The Nobel Prize in Medicine 1985: Brown MS & Goldstein JL "for their discoveries concerning the regulation of cholesterol metabolism"

FH ~75% of men with FH developed CAD, and 50% had a fatal MI, by age 60 years. The corresponding percentages for women were lower (45% and 15%, respectively). Most homozygotes experience MIs before 20 years of age, and an MI at 18 months of age has been reported. Without treatment, most FH homozygotes die before the age of 30 years.

Age- and Sex-specific rates (%) of CAD and death in FH heterozygotes MalesFemales Age CADDeathCADDeath 305—0— —

Therapy for FH heterozygotes Dietary reduction of cholesterol has only modest effects. The administration of bile-acid absorbing resins, such as cholestyramine. However, the decrease in intracellular cholesterol also stimulates cholesterol synthesis by liver cells, so the overall reduction in plasma LDL is only about 15% to 20%. This treatment is much more effective when combined with one of the statin drugs (e.g., lovastatin, pravastatin), which reduce cholesterol synthesis by inhibiting 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase. Serum cholesterol levels in FH heterozygotes can often be reduced to approximately normal levels.

Therapy for FH homozygotes Homozygotes have few or no LDL receptors. Liver transplants, which provide hepatocytes that have normal LDL receptors, have been successful in some cases. Plasma exchange, carried out every 1 to 2 weeks, in combination with drug therapy, can reduce cholesterol levels by about 50%. However, this therapy is difficult to continue for long periods. Somatic cell gene therapy, in which hepatocytes carrying normal LDL receptor genes are introduced into the portal circulation, is now being tested.