Genetics of Familial Hypercholesterolemia

1. Genetics of Familial Hypercholesterolemia 张咸宁 zhangxianning@zju.edu.cn Tel：13105819271; 88208367 Office: C303, Teaching Building 2016/03

2. 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。

3. Required Reading ● Thompson &Thompson Genetics in Medicine, 8th ed. 2016 ● pp.226-230; ● Clinical Case Studies-16 Familial Hypercholesterolemia ● 陈竺，主编：医学遗传学，第3版。 pp.212-214。人民卫生出版社，2015

4. Atherosclerotic cardiovascular disease (CVD)动脉粥样硬化性心血管疾病and its major clinical manifestation, coronary heart disease (CHD)冠心病, are common conditions responsible for 1 in 3 deaths in the United States claiming more than 2200 lives every day • A number of risk factors for CAD have been identified, including obesity, cigarette smoking, hypertension, elevated cholesterol level, and positive family history

5. Estimates of the heritability for CVD range from 30% to 60% • Some studies suggesting that a number closer to 60% may be more accurate, including one large (n > 20,000) twin study that used over 30 years of follow-up data

6. Risk for atherosclerotic cardiovascular disease (CVD) ? 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)

7. CHALLENGES TO GENETICSTUDIES OF coronary heart disease (CHD) (1) the presence of numerous heterogeneous CHD phenotypes异质性 (myocardial infarction心肌梗死, chronic stable angina慢性稳定性心绞痛, coronary artery spasm冠状动脉痉挛, …) and use of surrogate替代 phenotypes (coronary artery calcium冠状动脉钙化, carotid intimal-media thickness颈动脉内膜中层增厚, …) (2) complex non-Mendelian inheritance in most cases of CHD with complex gene–gene and gene– environment underpinning (3) failure of mouse models to translate into accurate depictions描述 of disease in humans

13. Overcoming the Challenges • A combination of different techniques including animal models, GWAS, family studies, and whole-exome and whole-genome studies will be needed to help further clarify individual rare and common genetic contributions and their interactions with each other and environmental factors

14. The Nobel Prize in Medicine1985: Brown MS & Goldstein JL "for their discoveries concerning the regulation of cholesterol metabolism"

15. 5 4 3 2 1 6 General Concepts of Metabolism and Etiologies of enzymopathy (inborn errors of metabolism) membrane C B A Ai holoenzyme Co-factror apoenzyme D • Mechanisms: • Membrane transport • Enzyme deficiency • Alternative pathways • Cofactor deficiency • Feedback control • Inhibition by by-product E F

16. DEFECTS IN RECEPTOR PROTEINS • Familial hypercholesterolemia (FH)：A Genetic Hyperlipidemia • FH have raised cholesterol levels with a significant risk of developing early CHD • 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 (LDLR)leading to increased levels of endogenous cholesterol synthesis

17. Clinical Synopsis of FH • INHERITANCE: AD. • 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 Aa, during first 4 years of life in AA; (2)Planar xanthomas in AA • LABORATORY ABNORMALITIES: Hypercholesterolemia, 350-550 mg/dl in heterozygotes, 650-1 000 mg/dl in homozygotes. (normal: ~300-400 mg/dl) • MISCELLANEOUS: Incidence, 1/500 Aa, 1/106 AA • MOLECULAR BASIS: Caused by mutations in LDLR

19. Arcus lipoides类脂环、老年环

20. Xanthomas黄瘤(fatty deposit)

21. Xanthomas黄瘤(fatty deposit)

22. Xanthomas黄瘤(fatty deposit)

25. 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 targets LDL receptors for lysosomal degradation, preventing them from recycling back to the plasma membrane

26. Gene dosage in low-density lipoprotein (LDL) deficiency. Shown is the distribution of total plasma cholesterol levels in 49 patients homozygous for defi ciency of the LDL receptor, their parents (obligate heterozygotes), and normal controls

27. 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

28. LDLR gene → FH (OMIM: 143890) • 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 • Exons：18

29. LDL receptor:a membrane-bound 160-kD Pr. of 839 AAs (LDLR: 18 exons)

30. 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

31. 6 principal classes of LDLR mutations: (1) receptor null mutations resulting from lack of receptor protein synthesis in the endoplasmic reticulum, (2) defective intracellular transport to the Golgi apparatus, (3) defective extracellular ligand binding, (4) defective endocytosis, (5) failure to release the LDL molecules inside the endosome (recycling-defective mutations) and (6) defective targetingof the mutant receptor to the basolateral membrane • > 1000 unique mutationshave beenrecorded in LDLR. 65% are missense mutations caused by substitutions, 24% are small DNA rearrangements,and 11% are large rearrangements • Familial Hypercholesterolemia (FH) Variant Database(www.ucl.ac.uk/ugi/fh)

34. The PCSK9 Protease, a Potential Drug Target for Lowering LDL Cholesterol

35. Nature. 2013; 496(7444):152-155

36. 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.

37. Age- and Sex-specific rates (%) of CAD and death in FH heterozygotes

38. 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.

39. Therapy for FH homozygotes • Homozygotes have few or no LDLRs • Liver transplants, which provide hepatocytes that have normal LDLRs, 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 ~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