Genetic Testing in Hyperlipidemia Ozlem Bilen, MDa,1, Yashashwi Pokharel, MD, MSCRb,c,1, Christie M. Ballantyne, MDc,d,e,* KEYWORDS Dyslipidemia Hereditary lipid disorder Familial hypercholesterolemia Genetic testing
KEY POINTS Cascade screening of family members with lipid profile should be widely implemented for identification of familial hypercholesterolemia (FH) cases. Using cholesterol levels and other clinical information for FH diagnosis and screening can be specific but less sensitive. Some existing FH diagnostic criteria already incorporate genetic information for FH diagnosis. Identification of specific mutation or mutations in the affected individual with a focused screening of the mutation in family members can be quick and less expensive. Employment and insurance implications of genetic screening are important. Randomized clinical trials and cost-effectiveness analyses comparing the incremental benefit of genetic testing with clinical criteria are needed.
Levels of certain plasma lipids and lipoproteins, such as low-density lipoprotein (LDL) cholesterol (LDL-C) and lipoprotein(a) (Lp[a]), are key risk factors for cardiovascular disease (CVD).1,2 Although plasma lipids are determined largely by environmental and genetic factors, for some individuals levels are primarily determined by genotype. The Fredrickson classification of lipid disorders was
based on common phenotypes (ie, abnormal lipid and lipoprotein subclasses) (Table 1).3 Some of these phenotypes are frequently due to monogenic defects that directly affect lipoproteins and their function, and others are associated with polygenic abnormalities with multiple genetic variations.3 Familial hypercholesterolemia (FH), which most commonly has a Fredrickson IIa phenotype, is the most common hereditary lipid disorder, resulting in elevated blood cholesterol levels
Disclosures: O. Bilen: Nothing to disclose. Y. Pokharel: Supported by American Heart Association SWA Summer 2014 Postdoctoral Fellowship Award. C.M. Ballantyne: Grant/Research support (All paid to institution, not individual): Abbott Diagnostic, Amarin, Amgen, Eli Lilly, Esperion, GlaxoSmithKline, Merck, Novartis, Pfizer, Regeneron, Roche Diagnostic, Sanofi-Synthelabo, National Institutes of Health, American Heart Association. Consultant: Abbott Diagnostics, Amarin, Amgen, Astra Zeneca, Cerenis, Esperion, Genentech, Genzyme, Kowa, Merck, Novartis, Pfizer, Regeneron, Sanofi-Synthelabo. Advisory panel: Merck, Pfizer. a Department of Medicine, Baylor College of Medicine, 3131 Fannin Street, Houston, TX 77030, USA; b Section of Cardiovascular Research, Department of Medicine, Baylor College of Medicine, 6565 Fannin Street, Suite B157, Houston, TX 77030, USA; c Center for Cardiovascular Disease Prevention, Methodist DeBakey Heart and Vascular Center, 6565 Fannin Street, M.S. A-601, Houston, TX 77030, USA; d Section of Cardiovascular Research, Department of Medicine, Baylor College of Medicine, 6565 Fannin Street, M.S. A-601, Suite 656, Houston, TX 77030, USA; e Section of Cardiology, Department of Medicine, Baylor College of Medicine, 6565 Fannin Street, M.S. A-601, Suite 656, Houston, TX 77030, USA 1 Both authors contributed equally. * Corresponding author. 6565 Fannin Street, M.S. A-601, Suite 656, Houston, TX 77030. E-mail address: [email protected] Cardiol Clin 33 (2015) 267–275 http://dx.doi.org/10.1016/j.ccl.2015.02.006 0733-8651/15/$ – see front matter Ó 2015 Elsevier Inc. All rights reserved.
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INTRODUCTION
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Table 1 Genetics underlying Fredrickson phenotypes ICD 10 Codes
Fredrickson Phenotype
[ Lipid(s)
[ Lipoprotein(s)
Genetic Mutations
E78.3 Hyperchylomicronemia
I
TG
CM
E78.0 Pure hypercholesterolemia
IIa
Chol
LDL
E78.2 Mixed hyperlipidemia
IIb
Chol, TG
VLDL, LDL
E78.2 Mixed hyperlipidemia
III
Chol, TG
IDL
E78.1 Pure hyperglyceridemia E78.3 Hyperchylomicronemia
IV V
TG Chol, TG
VLDL VLDL, CM
Monogenic; autosomal recessive: LPL, APOC2; other forms: APOA5, LMF1, ?GPIHBP1 w90% polygenic, w10% monogenic; heterozygous: LDLR, APOB, PCSK9; homozygous: LDLR, LDLRAP1 Polygenic; some cases due to USF1, APOB, LPL; w35% have APOA5 S19W or –1131T>C Polygenic; APOE or homozygosity for E2 allele of APOE necessary but not sufficient; w40% have APOA5 S19W or –1131T>C Polygenic; w35% have APOA5 S19W or –1131T>C Polygenic; w10%: LPL, APOC2, APOA5; w55% have APOA5 S19W or –1131T>C; small effects from APOE, TRIB1, CHREBP, GALNT2, GCKR, ANGPTL3
Abbreviations: 1131T>C, a T-to-C conversion at position –1131; ANGPTL3, angiopoietin-like 3; APOA5, apolipoprotein A-V; APOB, apolipoprotein B; APOC2, apolipoprotein C-II; APOE, apolipoprotein E; Chol, cholesterol; CHREBP, carbohydrate response element binding protein (also known as MLXIPL); CM, chylomicron; GALNT2, UDP-N-acetyl-a-D-galactosamine:polypeptide N-acetylgalactosaminyltransferase 2; GCKR, glucokinase regulator; HLP, hyperlipoproteinemia; IDL, intermediate-density lipoprotein; LDLRAP1, LDL receptor adaptor protein 1 (also known as ARH); LPL, lipoprotein lipase; PCSK9, proprotein convertase subtilisin/kexin type 9; S19W, serine to tryptophan conversion at amino acid 19; TG, triglyceride; TRIB1, tribbles homologue 1 (Drosophila); USF1, upstream transcription factor 1; VLDL, very low density lipoprotein.
Genetic Testing in Hyperlipidemia and premature atherosclerotic disease. FH has an autosomal-dominant inheritance with rare autosomal-recessive forms also described.4 The most common cause of FH is mutation in the LDL receptor (LDL-R), with greater than 1600 different genetic mutations associated with FH.5 Other causes include defects in apolipoprotein B (apoB), gain-of-function mutations in proprotein convertase subtilisin/kexin 9 (PCSK9), and other rare genetic abnormalities resulting in the FH phenotype.6–8 FH can be heterozygous (HeFH), affecting only one allele; homozygous (HoFH), affecting 2 identical alleles; or compound heterozygous, affecting 2 different alleles. HeFH is among the most common metabolic disorders, affecting 1 in 300 to 1 in 500 individuals.9 In founder populations such as French Canadians, Dutch, and Lebanese, HeFH is even more prevalent (1 in 50 to 1 in 100 individuals).10–12 FH has a gene-dose effect such that in patients with HoFH, LDL-C level often exceeds 500 mg/dL. Patients with HeFH typically have LDL-C levels greater than 160 mg/dL as children and greater than 190 mg/dL as adults.13 In addition to lifestyle changes, high-intensity statins are the pharmacotherapy of first choice, and if adequate response is not obtained, other lipidlowering medications and LDL apheresis are available. In general, response to cholesterol-lowering medications that lead to increased LDL-R activity is determined by the degree of LDL-R function.14 In patients with HeFH, the risk of premature coronary heart disease increases by about 20-fold compared with individuals without FH.15 Very often, a myocardial infarction is the first presenting sign in FH patients.16 Approximately 20% of myocardial infarctions before the age of 45 years and 5% before the age of 60 years can be attributed to FH.14 It is estimated that the risk of having CVD before age 50 is about 50% in men and 30% in women with FH. Despite the risk burden, most individuals with FH remain undiagnosed and either untreated or inadequately treated.9,17 Furthermore, it has been shown that after FH patients attain optimal LDL-C levels by using lifestyle and pharmacologic therapies (such as high-intensity statins), the risk for ischemic events is reduced to that in non-FH populations.18,19 Therefore, an improved screening and diagnostic tool for FH would be of immense public health value. Lp(a) is another atherogenic lipoprotein, and higher levels increase the risk for ischemic cardiovascular events independent of other risk factors, including LDL-C.20 Elevated Lp(a) is also one of the most commonly inherited dyslipidemias and is primarily genetically determined, but unfortunately, no approved therapies that lower Lp(a)
also lower ischemic vascular events.21 Currently, there is no clear role of genetic testing in routine practice in the management of individuals with elevated levels of Lp(a). In this article genetic testing in the management of lipid disorders is reviewed, with a focus on FH.
GENETIC TESTING IN FAMILIAL HYPERCHOLESTEROLEMIA Current Clinical Criteria for Diagnosis of Familial Hypercholesterolemia It is currently estimated that only about 15% to 20% of patients with FH are actually diagnosed.9 There are no internationally accepted criteria for FH diagnosis. However, the 3 commonly used criteria are the Dutch, Simon Broome, and US MedPed criteria (Table 2). Unlike the US criteria, which use total cholesterol levels and family history of FH, the Simon Broome and the Dutch criteria integrate personal and family lipid profiles, history of premature CVD (onset in men before age 55 years and in women before age 65 years), physical examination findings such as tendon xanthomas for the index person and family members, and genetic information.22–24 The Simon Broome and the Dutch criteria classify definitive, probable, and possible FH. According to the Simon Broome criteria, to make a definitive FH diagnosis, either a positive genetic test or elevated cholesterol levels accompanied by tendon xanthomas in self or family are needed, whereas according to the Dutch criteria (which uses a scoring system), either a positive genetic test or a constellation of the aforementioned nongenetic criteria qualify for a definitive FH diagnosis (see Table 2). The first steps in the assessment of a hereditary dyslipidemia in a clinic are to take a thorough history, including family history that covers at least 3 generations’ history of CVD (including premature onset) and risk factors, family members’ lipid profile (if available), performing a focused physical examination, and ordering a lipid profile. The presence of tendon xanthomas should be sought by careful inspection and palpation of the tendons commonly affected, such as the Achilles, finger extensor, and patellar tendons. Corneal arcus, if present in a patient under the age of 45 years, can indicate FH. Similarly, xanthelasma or tuberous xanthomas in a young patient should raise a concern for FH, although these are not FH specific.
Genetic Testing to Improve Diagnostic Accuracy It is obvious that identification of the mutated gene or genes provides a definite diagnosis of FH, which
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Table 2 Clinical criteria in diagnosis of familial hypercholesterolemia Simon Broome Familial Hypercholesterolemia Register Diagnostic Criteria for Familial Hypercholesterolemia: Criteria (Description) TC >290 mg/dL in adults or >260 mg/dL in children aged 190 mg/dL in adults or >155 mg/dL in children (A) Tendinous xanthomata in the patient or a firstdegree relative (B) Positive DNA test (C) Family history of premature CVD (D) Family history of: TC >290 mg/dL in a first-degree or second-degree relative or >260 mg/dL in child or sibling aged
KEY POINTS Cascade screening of family members with lipid profile should be widely implemented for identification of familial hypercholesterolemia (FH) cases. Using cholesterol levels and other clinical information for FH diagnosis and screening can be specific but less sensitive. Some existing FH diagnostic criteria already incorporate genetic information for FH diagnosis. Identification of specific mutation or mutations in the affected individual with a focused screening of the mutation in family members can be quick and less expensive. Employment and insurance implications of genetic screening are important. Randomized clinical trials and cost-effectiveness analyses comparing the incremental benefit of genetic testing with clinical criteria are needed.
Levels of certain plasma lipids and lipoproteins, such as low-density lipoprotein (LDL) cholesterol (LDL-C) and lipoprotein(a) (Lp[a]), are key risk factors for cardiovascular disease (CVD).1,2 Although plasma lipids are determined largely by environmental and genetic factors, for some individuals levels are primarily determined by genotype. The Fredrickson classification of lipid disorders was
based on common phenotypes (ie, abnormal lipid and lipoprotein subclasses) (Table 1).3 Some of these phenotypes are frequently due to monogenic defects that directly affect lipoproteins and their function, and others are associated with polygenic abnormalities with multiple genetic variations.3 Familial hypercholesterolemia (FH), which most commonly has a Fredrickson IIa phenotype, is the most common hereditary lipid disorder, resulting in elevated blood cholesterol levels
Disclosures: O. Bilen: Nothing to disclose. Y. Pokharel: Supported by American Heart Association SWA Summer 2014 Postdoctoral Fellowship Award. C.M. Ballantyne: Grant/Research support (All paid to institution, not individual): Abbott Diagnostic, Amarin, Amgen, Eli Lilly, Esperion, GlaxoSmithKline, Merck, Novartis, Pfizer, Regeneron, Roche Diagnostic, Sanofi-Synthelabo, National Institutes of Health, American Heart Association. Consultant: Abbott Diagnostics, Amarin, Amgen, Astra Zeneca, Cerenis, Esperion, Genentech, Genzyme, Kowa, Merck, Novartis, Pfizer, Regeneron, Sanofi-Synthelabo. Advisory panel: Merck, Pfizer. a Department of Medicine, Baylor College of Medicine, 3131 Fannin Street, Houston, TX 77030, USA; b Section of Cardiovascular Research, Department of Medicine, Baylor College of Medicine, 6565 Fannin Street, Suite B157, Houston, TX 77030, USA; c Center for Cardiovascular Disease Prevention, Methodist DeBakey Heart and Vascular Center, 6565 Fannin Street, M.S. A-601, Houston, TX 77030, USA; d Section of Cardiovascular Research, Department of Medicine, Baylor College of Medicine, 6565 Fannin Street, M.S. A-601, Suite 656, Houston, TX 77030, USA; e Section of Cardiology, Department of Medicine, Baylor College of Medicine, 6565 Fannin Street, M.S. A-601, Suite 656, Houston, TX 77030, USA 1 Both authors contributed equally. * Corresponding author. 6565 Fannin Street, M.S. A-601, Suite 656, Houston, TX 77030. E-mail address: [email protected] Cardiol Clin 33 (2015) 267–275 http://dx.doi.org/10.1016/j.ccl.2015.02.006 0733-8651/15/$ – see front matter Ó 2015 Elsevier Inc. All rights reserved.
cardiology.theclinics.com
INTRODUCTION
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Bilen et al
Table 1 Genetics underlying Fredrickson phenotypes ICD 10 Codes
Fredrickson Phenotype
[ Lipid(s)
[ Lipoprotein(s)
Genetic Mutations
E78.3 Hyperchylomicronemia
I
TG
CM
E78.0 Pure hypercholesterolemia
IIa
Chol
LDL
E78.2 Mixed hyperlipidemia
IIb
Chol, TG
VLDL, LDL
E78.2 Mixed hyperlipidemia
III
Chol, TG
IDL
E78.1 Pure hyperglyceridemia E78.3 Hyperchylomicronemia
IV V
TG Chol, TG
VLDL VLDL, CM
Monogenic; autosomal recessive: LPL, APOC2; other forms: APOA5, LMF1, ?GPIHBP1 w90% polygenic, w10% monogenic; heterozygous: LDLR, APOB, PCSK9; homozygous: LDLR, LDLRAP1 Polygenic; some cases due to USF1, APOB, LPL; w35% have APOA5 S19W or –1131T>C Polygenic; APOE or homozygosity for E2 allele of APOE necessary but not sufficient; w40% have APOA5 S19W or –1131T>C Polygenic; w35% have APOA5 S19W or –1131T>C Polygenic; w10%: LPL, APOC2, APOA5; w55% have APOA5 S19W or –1131T>C; small effects from APOE, TRIB1, CHREBP, GALNT2, GCKR, ANGPTL3
Abbreviations: 1131T>C, a T-to-C conversion at position –1131; ANGPTL3, angiopoietin-like 3; APOA5, apolipoprotein A-V; APOB, apolipoprotein B; APOC2, apolipoprotein C-II; APOE, apolipoprotein E; Chol, cholesterol; CHREBP, carbohydrate response element binding protein (also known as MLXIPL); CM, chylomicron; GALNT2, UDP-N-acetyl-a-D-galactosamine:polypeptide N-acetylgalactosaminyltransferase 2; GCKR, glucokinase regulator; HLP, hyperlipoproteinemia; IDL, intermediate-density lipoprotein; LDLRAP1, LDL receptor adaptor protein 1 (also known as ARH); LPL, lipoprotein lipase; PCSK9, proprotein convertase subtilisin/kexin type 9; S19W, serine to tryptophan conversion at amino acid 19; TG, triglyceride; TRIB1, tribbles homologue 1 (Drosophila); USF1, upstream transcription factor 1; VLDL, very low density lipoprotein.
Genetic Testing in Hyperlipidemia and premature atherosclerotic disease. FH has an autosomal-dominant inheritance with rare autosomal-recessive forms also described.4 The most common cause of FH is mutation in the LDL receptor (LDL-R), with greater than 1600 different genetic mutations associated with FH.5 Other causes include defects in apolipoprotein B (apoB), gain-of-function mutations in proprotein convertase subtilisin/kexin 9 (PCSK9), and other rare genetic abnormalities resulting in the FH phenotype.6–8 FH can be heterozygous (HeFH), affecting only one allele; homozygous (HoFH), affecting 2 identical alleles; or compound heterozygous, affecting 2 different alleles. HeFH is among the most common metabolic disorders, affecting 1 in 300 to 1 in 500 individuals.9 In founder populations such as French Canadians, Dutch, and Lebanese, HeFH is even more prevalent (1 in 50 to 1 in 100 individuals).10–12 FH has a gene-dose effect such that in patients with HoFH, LDL-C level often exceeds 500 mg/dL. Patients with HeFH typically have LDL-C levels greater than 160 mg/dL as children and greater than 190 mg/dL as adults.13 In addition to lifestyle changes, high-intensity statins are the pharmacotherapy of first choice, and if adequate response is not obtained, other lipidlowering medications and LDL apheresis are available. In general, response to cholesterol-lowering medications that lead to increased LDL-R activity is determined by the degree of LDL-R function.14 In patients with HeFH, the risk of premature coronary heart disease increases by about 20-fold compared with individuals without FH.15 Very often, a myocardial infarction is the first presenting sign in FH patients.16 Approximately 20% of myocardial infarctions before the age of 45 years and 5% before the age of 60 years can be attributed to FH.14 It is estimated that the risk of having CVD before age 50 is about 50% in men and 30% in women with FH. Despite the risk burden, most individuals with FH remain undiagnosed and either untreated or inadequately treated.9,17 Furthermore, it has been shown that after FH patients attain optimal LDL-C levels by using lifestyle and pharmacologic therapies (such as high-intensity statins), the risk for ischemic events is reduced to that in non-FH populations.18,19 Therefore, an improved screening and diagnostic tool for FH would be of immense public health value. Lp(a) is another atherogenic lipoprotein, and higher levels increase the risk for ischemic cardiovascular events independent of other risk factors, including LDL-C.20 Elevated Lp(a) is also one of the most commonly inherited dyslipidemias and is primarily genetically determined, but unfortunately, no approved therapies that lower Lp(a)
also lower ischemic vascular events.21 Currently, there is no clear role of genetic testing in routine practice in the management of individuals with elevated levels of Lp(a). In this article genetic testing in the management of lipid disorders is reviewed, with a focus on FH.
GENETIC TESTING IN FAMILIAL HYPERCHOLESTEROLEMIA Current Clinical Criteria for Diagnosis of Familial Hypercholesterolemia It is currently estimated that only about 15% to 20% of patients with FH are actually diagnosed.9 There are no internationally accepted criteria for FH diagnosis. However, the 3 commonly used criteria are the Dutch, Simon Broome, and US MedPed criteria (Table 2). Unlike the US criteria, which use total cholesterol levels and family history of FH, the Simon Broome and the Dutch criteria integrate personal and family lipid profiles, history of premature CVD (onset in men before age 55 years and in women before age 65 years), physical examination findings such as tendon xanthomas for the index person and family members, and genetic information.22–24 The Simon Broome and the Dutch criteria classify definitive, probable, and possible FH. According to the Simon Broome criteria, to make a definitive FH diagnosis, either a positive genetic test or elevated cholesterol levels accompanied by tendon xanthomas in self or family are needed, whereas according to the Dutch criteria (which uses a scoring system), either a positive genetic test or a constellation of the aforementioned nongenetic criteria qualify for a definitive FH diagnosis (see Table 2). The first steps in the assessment of a hereditary dyslipidemia in a clinic are to take a thorough history, including family history that covers at least 3 generations’ history of CVD (including premature onset) and risk factors, family members’ lipid profile (if available), performing a focused physical examination, and ordering a lipid profile. The presence of tendon xanthomas should be sought by careful inspection and palpation of the tendons commonly affected, such as the Achilles, finger extensor, and patellar tendons. Corneal arcus, if present in a patient under the age of 45 years, can indicate FH. Similarly, xanthelasma or tuberous xanthomas in a young patient should raise a concern for FH, although these are not FH specific.
Genetic Testing to Improve Diagnostic Accuracy It is obvious that identification of the mutated gene or genes provides a definite diagnosis of FH, which
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Table 2 Clinical criteria in diagnosis of familial hypercholesterolemia Simon Broome Familial Hypercholesterolemia Register Diagnostic Criteria for Familial Hypercholesterolemia: Criteria (Description) TC >290 mg/dL in adults or >260 mg/dL in children aged 190 mg/dL in adults or >155 mg/dL in children (A) Tendinous xanthomata in the patient or a firstdegree relative (B) Positive DNA test (C) Family history of premature CVD (D) Family history of: TC >290 mg/dL in a first-degree or second-degree relative or >260 mg/dL in child or sibling aged
