Prevention 2014: Genetic Assessments of Lipid Disorders

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Svati H. Shah, MD

1st Annual Duke Preventive Cardiology Symposium
Saturday, April 26, 2014
The overall goal of this activity is to review the latest advancements in the management of lipids in clinical practice, including the new American Heart Association and American College of Cardiology guidelines on lipids announced in November 2013. Topics include learning about evaluation and treatment options in lipids and lipoprotein disorders, as well as focusing on new prevention guidelines, physical activity, nutrition, drug therapies, advanced lipoprotein testing, special patient populations, and new technologies for lifestyle management.

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  • --resulting in macrophage lipid accumulation and foam cell formationPCSK9 is a serine protease that is secreted by the liver into the plasma that binds to an extracellular domain of the LDL receptor primarily on liver cells. This binding prevents LDL receptors from recycling to the cell surface and ultimately accelerates their destruction inside liver cells. Fewer LDL receptors lead to less clearance of LDLC and higher serum levels. GOF mutations more rare; LOF mutations more common
  • AS due to atherosclerotic involvement of the aortic root; incidence of AS lower in heterozygoesHypercholesterolemia often is detectable atbirth or shortly thereafter, and total cholesterol levelseventually rise to 350–500 mg/dL in many persons.Tendon xanthomas, especially in the Achilles tendonsand the extensor tendons of the hands, are typical. FH carries increased risk of premature CHD; CHDcommonly occurs in men by the fourth or fifth decade,and about 10 years later in women. T
  • LDLR negative means no gene functionIn individuals with the LDLR mutation IVS14+1G-A (606945.0063), the phenotype can be altered by a SNP in the APOA2 gene (107670.0002), a SNP in the EPHX2 gene (132811.0001), or a SNP in the GHR gene (600946.0028). Genetic testing can identify genetic mutations associated with FH, but physicians order such testing infrequently. “We’ve only done a few hundred of the tests available,” says Michael W. Henry, vice president of business development at Athena Diagnostics, a CLIA-accredited lab and a subsidiary of Thermo Fisher Scientific, in Worcester, Mass. Henry is referring to Athena Diagnostics’ LDLR (Hypercholesterolemia) DNA Sequencing Test and APOB (Hypercholesterolemia) Mutation Analysis, which were launched in 2005 under license with Correlagen Diagnostics. The LDLR test identifies autosomal dominant loss-of-function mutations in the LDLR gene, which codes for the LDLR. The APOB analysis identifies autosomal dominant loss-of-function mutations in the APOB gene, which codes for apolipoprotein B-100, the principal protein component of LDL
  • Hypercholesterolemia often is detectable atbirth or shortly thereafter, and total cholesterol levelseventually rise to 350–500 mg/dL in many persons.Tendon xanthomas, especially in the Achilles tendonsand the extensor tendons of the hands, are typical. FH carries increased risk of premature CHD; CHDcommonly occurs in men by the fourth or fifth decade,and about 10 years later in women. T
  • MTP responsible for intracellular assemby of apo B & lipids in liver & intestineNew genes being discovered are potential therapeutic targets
  • Family clustering of elevated triglycerides withoutincreased serum cholesterol levels characterizes familialhypertriglyceridemia.82,946,947 Persons with familialhypertriglyceridemia seemingly do not carry as high a risk for premature CHD as do those with familialcombined hyperlipidemia.954,955 This is not surprisingbecause the former generally have lower levels of LDLcholesterol than the latter. Many persons with familialhypertriglyceridemia also manifest obesity,956 but insome, triglycerides are elevated without obesity or any other evidence of the metabolic syndrome. Theselatter persons may have a defect in catabolism ofTGRLP (e.g., an abnormality in lipoprotein lipaseactivity).957,958
  • Upstream transcription factor 1encodes transcription factor known to regulate several genes involved in glucose and lipid metabolism
  • Hypercholesterolemia often is detectable atbirth or shortly thereafter, and total cholesterol levelseventually rise to 350–500 mg/dL in many persons.Tendon xanthomas, especially in the Achilles tendonsand the extensor tendons of the hands, are typical. FH carries increased risk of premature CHD; CHDcommonly occurs in men by the fourth or fifth decade,and about 10 years later in women. T
  • PCSK9 also has LOF mutations that lead to lower LDL levels and decreased risk of ischemic heart diseaseApoE is required for receptor-mediated clearance of chylomicron and VLDL remannts from the circulationApoE4 has a higher affinity for the LDL receptor than for the other apoE isoforms; enhanced lipid binding leads, via negative feedback, to downregulation of LDL receptor synthesis and secondary rise in LDLC
  • One gene that may contribute is CETP, which plays a central role in reverse cholesterol transport. CETP moves cholesterol from peripheral tissues to the liver by transferring cholesteryl ester from HDL-C to Apo B containing lipoproteins with triglyceride transfer in the opposite direction; increased CETP activity may be proatherogenic and is associated with this phenotype BHigh hepatic lipase activity associated with this phenotype.
  • Laboratory and clinical manifestations of familial defective apolipoprotein B-100 are identical to those of familial hypercholesterolemia; distinction can only be made by molecular biology techniques.
  • National payers get lower unit prices on genetic tests, and many European countries routinely screen their citizens for FH using the so-called Simon Broome criteria, which include “DNA-based evidence of an LDLR mutation or FDB.” Both Hopkins and Shamburek talk about the FH screening programs in the Netherlands, Italy, Norway, France, and Spain with admiration.Cascade screening is a mechanism for identifying people at risk for a genetic condition by a process of systematic family tracing. The National Institute for Health and Clinical Excellence in the United Kingdom recommends cascade screening of close biological relatives of people with a clinical diagnosis of FH in order to effectively identify additional FH patients. The ultimate goal of this testing is to reduce morbidity and mortality from heart disease in persons with FH through early diagnosis and effective disease management. The goal of this article is to outline the available evidence on the clinical validity and utility of cascade screening for FH, while emphasizing the availability, usefulness, and recommendation for including DNA testing (if the disease-causing mutation has been identified)The NICE guidelines on the identification and management of FH recommend cascade screening using a combination of genetic testing and LDL cholesterol concentration measurement “is recommended to identify affected relatives of those index individuals with a clinical diagnosis of FH. This should include at least the first- and second- and, when possible, third-degree biological relatives” [3][22] .Published studies on the clinical validity of diagnosing relatives using DNA-based criteria compared to clinical criteria have shown that a clinical diagnosis is less sensitive and leads to under-diagnosis [15][30][31][32] , raising the possibility that relatives diagnosed by clinical criteria may, in fact, have another form of dyslipidemia (i.e., they may have been misdiagnosed).
  • The main lipids in lipoproteins are free and esterified cholesterol (C) and triglyceride (TG). The metabolism of TG, low-density lipoprotein (LDL) cholesterol and high-density lipoprotein (HDL) cholesterol is shown. In TG metabolism, hydrolysed dietary fats enter intestinal cells (enterocytes) via fatty acid (FA) transporters. Reconstituted TG is packaged with C ester and the apolipoprotein B (APOB) isoform B48 into chylomicrons (CMs) by microsomal TG-transfer protein (MTTP) through a vesicular pathway. CMs, secreted via the lymphatic system, enter the vena cava and circulate until they interact with lipoprotein lipase (LPL), the secretion of which depends on lipase-maturation factor 1 (not shown), and which is secured to endothelium by proteoglycans and glycosylphosphatidylinositol-anchored HDL-binding protein 1 (not shown). CMs contain apoliproteins, including APOA5 (A5), APOC2 (C2) and APOC3 (C3). Released free FAs incompletely enter peripheral cells. In adipocytes, enzymes including acyl CoA:diacylglycerolacyltransferase (DGAT) resynthesize TG, which is hydrolysed by adipose TG lipase (ATGL) and hormone sensitive lipase (HSL). CM remnants (CMRs) are taken up by hepatic LDL receptor (LDLR), in the absence of LDLR they are taken up by LDLR-related protein-1 (LRP1). In liver cells (hepatocytes), TG is packaged with cholesterol and the APOB isoform B100 into very low-density lipoprotein (VLDL); the TG contained in VLDL is hydrolysed by LPL, releasing FAs and VLDL remnants (IDL) that are hydrolysed by hepatic lipase (HL), thereby yielding LDL. In LDL cholesterol metabolism, sterols in the intestinal lumen enter enterocytes via the Niemann-Pick C1-like 1 (NPC1L1) transporter and some are resecreted by heterodimeric ATP-binding cassette transporter G5/G8 (ABCG5/G8). In enterocytes, cholesterol is packaged with TG into CM. In hepatocytes, cholesterol is recycled or synthesized de novo, with 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGCR) being rate-limiting. LDL transports cholesterol from the liver to the periphery. LDL is endocytosed by peripheral cells and hepatocytes by LDLR, assisted by an adaptor protein (AP). Proproteinconvertasesubtilisin/kexin type 9 (PCSK9), when complexed to LDLR, short-circuits recycling of LDLR from the endosome, leading to its degradation (X). In HDL cholesterol metabolism, HDL, via APOA-I (A1), mediates reverse cholesterol transport by interacting with ATP-binding cassette A1 (ABCA1) and ABCG1 transporters on non-hepatic cells. Lecithin-cholesterol acyltransferase (LCAT) esterifies cholesterol so it can be used in HDL cholesterol, which, after remodelling by cholesterol ester transfer protein (CETP) and by endothelial lipase (LIPG), enters hepatocytes via scavenger receptor class B type I (SRB1).
  • Prevention 2014: Genetic Assessments of Lipid Disorders

    1. 1. Genetic Assessments of Lipid Disorders Svati H. Shah, MD, MS, MHS, FACC Associate Professor of Medicine Vice-Chief, Translational Research Director, Adult Cardiovascular Genetics Clinic Associate Director, Cardiovascular Fellowship Program Division of Cardiology, Department of Medicine Duke Molecular Physiology Institute
    2. 2. Disclosures/Funding • Consultancies/Speakers’ Bureau – Biosense Webster – Biotronic – Spectranetics – Boston Scientific – St Jude • Patent held by speaker • Research supported by: – NHLBI R01HL095987-01 (Shah) – NHLBI P01 (Williams, Shah) – NHLBI RC2HL101621-01 (Kraus) – American Heart Association FTF (Shah) – MEDUSA Award from Medtronic, Inc. (Shah)
    3. 3. Outline • Overview of genetics • Mendelian lipid disorders • “Common” complex lipid disorders with a genetic basis • Clinical considerations • Future directions
    4. 4. Outline • Overview of genetics • Mendelian lipid disorders • “Common” complex lipid disorders with a genetic basis • Clinical considerations • Future directions
    5. 5. Mendelian vs. Complex Disease Genetics • Mendelian Disease – Single gene – Rare – High penetrance – Autosomal dominant, recessive or X-linked – Minimal to mild environmental influences – Often gross pertubations in gene/protein function • Complex Disease – Polygenic – Common – Variable penetrance – Variable, unclear modes of inheritance – Gene-environment interactions – Minor common variations, regulatory and modifying changes
    6. 6. What causes dyslipidemia?
    7. 7. Lipids are Genetically Determined • LDL, HDL and TG are highly heritable • >50% of total inter-individual variation in serum lipids can be explained by genetics • 70% of patients with premature CAD and lipid abnormality have a familial disorder1 • Overall, inheritance is likely polygenic • How those genetics manifest in lipid levels is influenced strongly by environmental factors 1. Genest JJ et al. Circulation 1992.
    8. 8. Outline • Overview of genetics • Mendelian lipid disorders • “Common” complex lipid disorders with a genetic basis • Clinical considerations • Future directions
    9. 9. Mendelian Lipid Diseases • Require few or no environmental factors (genetic mutation itself is enough) • High risk of CVD at a young age • Few patients in a general practice have Mendelian lipid disorders, but common enough that they need to be recognized • Includes – Familial hypercholesterolemia (FH) – Familial defective apolipoprotein B100 – High lipoprotein(a)
    10. 10. Familial Hypercholesterolemia (FH) • Most important genetic dyslipidemia to be able to recognize • Heterozygous FH has prevalence of 1/500 in Caucasians (1/70 in Afrikaners, 1/150 in French Canadians) • To be differentiated from polygenic hypercholesterolemia and familial combined hyperlipidemia (FH is monogenic) • Characterized by LDL-C>95th% from birth (i.e. >190 mg/dL), premature atherosclerosis, tendon xanthomas, early onset CAD • Autosomal dominant but with gene dosage effect
    11. 11. Genetics of Familial Hypercholesterolemia (FH) • 95% of patients carry mutation in one of 3 genes • LDL receptor (LDLR, 93%) – Reduced clearance of LDL from the circulation – Increased uptake of modified LDL by macrophage scavenger receptors • Apolipoprotein B (APOB, 5%) • Impaired binding of LDL particles to the LDL receptor • Proprotein convertase subtilisin kexin 9 (PCSK9, GOF mutations, 2%) • Leads to decreased LDL metabolism From Soutar et al. Nature Clin Prac, 2007.
    12. 12. FH: LDLR Genetic Mutations • Over 1600 different mutations • Diagnostic criteria do not include genetic testing • Patients with FH classified into two major groups based on amount of LDLR activity – Risk of CVD based on severity of the genetic defect • FH inherited with gene dosing effect – 2 copies (homozygotes) worse than 1 copy (heterozygotes) • Homozygous FH rare (1/1,000,000) – CAD and florid xanthomatosis occur in childhood – Also prone to supravalvular aortic stenosis (50%) • Heterozygous FH more common (1/500) – Coronary artery calcification as early as 11-23 yo
    13. 13. FH: LDLR Genetic Mutations • Severity of disease affected by genetic mutation – LDLR-negative have higher LDLC (295 vs 258) than LDLR- defective mutations – LDLR negative have higher prevalence of premature CAD (36 vs. 7%) • Recommendations for screening (LDL-C and possible genetic testing) if: – Person or family with known FH – Adult with total cholesterol >310 – Child with total cholesterol >230 – Premature CAD – Tendon xanthomas – Sudden premature cardiac death
    14. 14. Familial Defective Apolipoprotein B-100 • Autosomal dominant • Like FH, impaired binding of LDL particles to the receptor • Differs from FH in that defect is localized to apo B-100 ligand on the LDL particle (not receptor) • Net effect is that clearance of LDL is reduced • Most due to Arg3500 mutation heterozygosity • Arg3500 prevalence 1/1000
    15. 15. Elevated Lipoprotein(a) • Lp(a) is a modified form of LDL • Lp(a) interferes with fibrinolysis & binds macrophages to promote foam cell formation & deposition of cholesterol into atherosclerotic plaques • Primarily genetically determined • In families without FH, >90% of variability in Lp(a) levels due to apo(a) (i.e. LPA gene) variants • Consider testing it in – Patients with CHD & no other dyslipidemia – Patients with strong FHx & no other dyslipidemia – Patients with hypercholesterolemia refractory to therapy
    16. 16. Low LDL Genetic Disorders • Abetalipoproteinemia – Rare recessive disorder – Mutation in MTP (microsomal transfer protein) – LDL-C levels almost zero – Results in impaired transport of fat-soluble vitamins (A, D, E, K) – Manifests in infancy with mental retardation, growth abnormalities and peripheral neuropathies • Hypobetalipoproteinemia – Mutation in gene encoding apo B – Low plasma apo B levels, low LDL (25-40), low VLDL cholesterol – Manifest with intestinal fat malabsorption, hepatic steatosis, fat soluble vitamin deficiencies – New genes being discovered: ANGPTL3 • PCSK9 – Two LOF functions occur in US blacks 1/50markedly low LDL & protection against CHD
    17. 17. Non-LDL Monogeneic Disorders • High TG (>200 mg/dL) – Familial lipoprotein lipase deficiency – Familial apolipoprotein C-II deficiency – Familial hypertriglyceridemia (can have TG>500) – Familial dysbetalipoproteinemia (rare, apoE2) – Very high triglycerides >500 mg/dL (apo C-II, LPL) • Low HDL – APOA1, LCAT, ABCA1 – Autosomal recessive – Rare
    18. 18. Outline • Overview of genetics • Mendelian lipid disorders • “Common” complex lipid disorders with a genetic basis • Clinical considerations • Future directions
    19. 19. Familial Combined Hyperlipidemia • Relatively common – 1-2% of general population – accounts for 30-50% of familial causes of CHD and 10% of premature CHD cases • Presents with hypercholesterolemia and/or hypertriglyceridemia • Variable presentation even within a given family • Increased RR of CHD (1.7) • Thought to be genetically complex disease – Usually need coinheritance of 2+ genetic variants affecting lipoprotein metabolism – Candidates: USF1, LPL
    20. 20. Familial Combined Hyperlipidemia • Fredrickson phenotypes – Type IIb: combined ↑TG and ↑TC – Type IIa: ↑LDL – Type IV: Isolated hypertriglyceridemia (induced by rise in VLDL) • Diagnosis made by LDL to apoB ratio of <1.2 (normal is >1.4)
    21. 21. Hyperapobetalipoproteinemia • Characterized by overproduction of apo B • May be a variant of familial combined hyperlipidemia • Manifest with premature CHD (esp if concurrent hypertriglyceridemia), xanthelasma, obesity • LDL species enriched in apo B-100 – Elevation in apo B (>135), but normal LDL-C (<160) – LDL to apo B ratio < 1.2 (normal >1.4)
    22. 22. “Common” Lipid Disorders: Polygenic Hypercholesterolemia • >50% of total inter-individual variation in serum lipids can be explained by genetics but Mendelian lipid disorders explain little of the heritability • Most patients have multiple genetic mutations • Each contributes in a small way • Aka “polygenic hypercholesterolemia” • Can be hard to distinguish from heterozygous FH without genetic testing • Like FH, familial aggregation of moderate hypercholesterolemia & premature CHD • ↑LDL, usually normal TG • Tendon xanthoma not seen
    23. 23. Genes Implicated in Polygenic Hypercholesterolemia • Genetics poorly understood • Candidates include: – PCSK9, LDLR (milder mutations) – ApoE4, CELSR2, APOB, ABCG8, HMGCR, CETP • Advances in genomic technologies have allowed large scale GWAS • Genes with rare Mendelian mutations of large effect size also harbor common variants of more modest effect size
    24. 24. Small Dense LDL (LDL phenotype B) • 3 phenotypic patterns of LDL – Pattern A: large particle size (>26.3 nm) – Pattern B: small particle size (<25.8 nm) – Pattern I: intermediate size (25.8-26.3) • Small dense LDL particles (LDL phenotype B) associated with increased apoB and triglyceride levels, lower HDL, & increased risk of CHD – Enhanced oxidative susceptibility – Reduced clearance by LDL receptors in the liver with increased LDL receptor-independent binding in the arterial wall – Endothelial dysfunction that is independent of concentrations of other lipids • Partially genetically determined – CETP (cholesteryl ester transfer protein) – Hepatic lipase gene promoter
    25. 25. Distinguishing Heterozygous FH from Polygenic Hypercholesterolemia Disorder Lipid Levels Confirmatory Studies Physical Findings Familial hypercholesterolemia (FH) *High TC *TG usually normal Genetic testing *Tendon xanthomata in patient (or in relative) *Corneal arcus *Xanthelasma Familial combined hyperlipidemia *TC and/or TG ≥90th %ile in patient AND relatives (one 1st or two 2nd degree relatives) *apo b≥90th %ile LDL/apo B ratio <1.2 *Xanthelasma *Corneal arcus Familial hyperapobetalipoproteinemia *apo b>90th%ile *At least two relatives with similar profile LDL/apo B ratio <1.2 *Xanthelasma Polygenic hypercholesterolemia *TC>90th %ile *TG<90th %ile *Dx excluded if tendon xanthomata present in patient or family members Modified from UpToDate Data from Rosenson RS et al. Dis Mon 1994.
    26. 26. Outline • Overview of genetics • Mendelian lipid disorders • “Common” complex lipid disorders with a genetic basis • Clinical considerations • Future directions
    27. 27. General Lipid Screening • Standard lipid profile in first-degree relatives of patients with MI (especially if early onset, i.e. <55 in women, <50 in men • If normal, then consider lipoprotein(a), apolipoprotein B, A-1 testing, NMR lipoproteins • 25% of patients with early-onset CAD and “normal” standard lipids have abnormalities in one of these factors
    28. 28. Genetic Testing for FH • Most guidelines vague • CDC and NICE support genetic testing – Insurance coverage & cost ($1200-$1500) – Many clinically available labs • Genetic testing is high yield in FH (>85%), but not in other lipid disorders • LDLR mutation identification would change management (lifelong therapy) and help initiation of therapy early in childhood • Would also help with identification of other family members (clinical diagnosis somewhat underdiagnoses) • Cascade screening important regardless (population screening not cost-effective)
    29. 29. Treatment of Genetic Dyslipidemias From NCEP ATPIII guidelines
    30. 30. All Rights Reserved, Duke Medicine 2008 Duke Cardiovascular Genetics Clinic • Overview: Multi-disciplinary clinic offering comprehensive genetic evaluation for inherited cardiovascular disease. – Staffed by cardiology, electrophysiology, and genetic counseling. • Services: – Thorough personal and family history and physical evaluation; – Risk assessment and genetic counseling; – Genetic testing and insurance coordination; – Screening recommendations to referring provider, patient, and at-risk family members. • Indications for Referral: – Personal or family history of suspected or diagnosed inherited disease, such as: • LQTS, Brugada, CPVT, ARVD/C, other arrhythmia • HCM, DCM, early-onset CAD, connective tissue disease • Pharmacogenetics, DTC genetic testing results • Questions? Call our genetic counselor at (919)660-2278 for information.
    31. 31. Outline • Overview of genetics • Mendelian lipid disorders • “Common” complex lipid disorders with a genetic basis • Clinical considerations • Future directions
    32. 32. Figuring Out Additional Genes Polygenic hypercholesterolemia Mendelian genetic disorders
    33. 33. Practical Summary • Family hypercholesterolemia (FH) and familial combined hyperlipidemia are two of the most common types of dyslipidemia • For screening, recommend doing lipid screen on first- degree relatives of patients with MI (especially if early- onset) • If lipid screen normal, consider lipoprotein(a), apolipoprotein B, A-1, NMR lipoprotein profile • If Mendelian dyslipidemia (i.e. heterozygous FH) suspected (LDL>190 in multiple family members): – Consider genetic testing – Consider referral to specialty lipid and/or genetics clinic – Cascade screening of family members – Enrollment in registry (CASCADE FH)
    34. 34. Thank you! Questions? Shah
    35. 35. Online Resources • http://www.ncbi.nlm.nih.gov/SNP/ • http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?d b=OMIM • http://www.genecards.org/index.shtml • http://statgen.iop.kcl.ac.uk/gpc/cc2.html • http://www.ensembl.org/index.html • http://www.hapmap.org/ • http://www.geneclinics.org/ • http://ca.expasy.org/
    36. 36. Duke Resources • Sarah W. Stedman Center for Nutrition and Metabolism – Chris Newgard – metabolomics core facility • Center for Human Genetics – http://www.chg.duke.edu/ – has large scale DNA bank, core genotyping and sequencing facility – resource for study design setup, statistical consults, etc. • Duke Institute for Genome Sciences and Policy – http://www.genome.duke.edu/ – has DNA bank, core genotyping & sequencing facility • Proteomics Core facility – run by Art Moseley, Ph.D – http://www.genome.duke.edu/cores/proteomics/ • DHMRI (David H. Murdock Research Institute) – Kannapolis will soon be up and running – http://www.dhmri.org/
    37. 37. Sitosterolemia • Autosomal recessive • Hyperabsorption of cholesterol & plant sterols from the intestine • Genetic mutation in ATP-binding cassette G5 (ABCG5) or ABCG8 • Genes expressed in the liver and intestine and are upregulated by cholesterol feeding (limit intestinal sterol absorption normally)
    38. 38. Cholesterol Components and Metabolism

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