Genetically elevated non-fasting triglycerides and calculated remnant cholesterol as causal risk factors for myocardial infarction

CLINICAL RESEARCH European Heart Journal (2013) 34, 1826–1833 doi:10.1093/eurheartj/ehs431 Genetics/lipids Genetically elevated non-fasting triglycerides and calculated remnant cholesterol as causal risk factors for myocardial infarction Anders Berg Jørgensen 1, Ruth Frikke-Schmidt 1,2, Anders Sode West1, Peer Grande 3, Børge G. Nordestgaard 2,4,5, and Anne Tybjærg-Hansen 1,2,5* 1 Department of Clinical Biochemistry KB3011, Section for Molecular Genetics, Rigshospitalet, Copenhagen University Hospitals and Faculty of Health Sciences, University of Copenhagen, Denmark; 2The Copenhagen General Population Study, Herlev Hospital, Copenhagen University Hospitals and Faculty of Health Sciences, University of Copenhagen, Copenhagen, Denmark; 3Department of Cardiology, Rigshospitalet, Copenhagen University Hospitals and Faculty of Health Sciences, University of Copenhagen, Copenhagen, Denmark; 4Department of Clinical Biochemistry, Herlev Hospital, Copenhagen University Hospitals and Faculty of Health Sciences, University of Copenhagen, Copenhagen, Denmark; and 5The Copenhagen City Heart Study, Frederiksberg Hospital, Copenhagen University Hospitals and Faculty of Health Sciences, University of Copenhagen, Copenhagen, Denmark Received 13 July 2012; revised 1 November 2012; accepted 20 November 2012; online publish-ahead-of-print 17 December 2012 Aims Elevated non-fasting triglycerides mark elevated levels of remnant cholesterol. Using a Mendelian randomization approach, we tested whether genetically increased remnant cholesterol in hypertriglyceridaemia due to genetic variation in the apolipoprotein A5 gene (APOA5) associates with an increased risk of myocardial infarction (MI). ..................................................................................................................................................................................... Methods We resequenced the core promoter and coding regions of APOA5 in individuals with the lowest 1% (n ¼ 95) and and results highest 2% (n ¼ 190) triglyceride levels in the Copenhagen City Heart Study (CCHS, n ¼ 10 391). Genetic variants which differed in frequency between the two extreme triglyceride groups (c.-1131T . C, S19W, and c.*31C . T; P-value: 0.06 to ,0.001), thus suggesting an effect on triglyceride levels, were genotyped in the Copenhagen General Population Study (CGPS), the CCHS, and the Copenhagen Ischemic Heart Disease Study (CIHDS), comprising a total of 5705 MI cases and 54 408 controls. Genotype combinations of these common variants associated with increases in non-fasting triglycerides and calculated remnant cholesterol of, respectively, up to 68% (1.10 mmol/L) and 56% (0.40 mmol/L) (P , 0.001), and with a corresponding odds ratio for MI of 1.87 (95% confidence interval: 1.25–2.81). Using APOA5 genotypes in instrumental variable analysis, the observational hazard ratio for a doubling in non-fasting triglycerides was 1.57 (1.32–2.68) compared with a causal genetic odds ratio of 1.94 (1.40 –1.85) (P for comparison ¼ 0.28). For calculated remnant cholesterol, the corresponding values were 1.67(1.38 –2.02) observational and 2.23(1.48 –3.35) causal (P for comparison ¼ 0.21). ..................................................................................................................................................................................... Conclusion These data are consistent with a causal association between elevated levels of remnant cholesterol in hypertriglyceridaemia and an increased risk of MI. Limitations include that remnants were not measured directly, and that APOA5 genetic variants may influence other lipoprotein parameters. ----------------------------------------------------------------------------------------------------------------------------------------------------------Keywords Genetics † Lipoproteins † Triglycerides † Remnant cholesterol † Myocardial infarction Introduction Elevated non-fasting plasma triglyceride is a marker of elevated remnant cholesterol,1 – 3 and associated with an increased risk of ischaemic cardiovascular disease.1 – 7 Remnant cholesterol is the cholesterol content of triglyceride-rich remnant lipoproteins, which in the fasting state comprise very low-density lipoproteins (VLDLs) and intermediate-density lipoproteins (IDLs), and these two lipoproteins together with chylomicron remnants in the nonfasting state. Remnant lipoproteins share with low-density lipoproteins (LDLs) the potential to infiltrate the arterial intima, and thus to accumulate and cause atherosclerosis due to their cholesterol * Corresponding author. Tel: +45 3545 4159, Fax: +45 3545 4160, Email: Published on behalf of the European Society of Cardiology. All rights reserved. & The Author 2012. For permissions please email: 1827 Genetically elevated non-fasting triglycerides content,8 – 10 whereas triglycerides per se are unlikely to cause atherosclerosis.4 Apolipoprotein A-V (apoA-V) is located on the surface of triglyceride-rich lipoproteins, where it modulates activation of lipoprotein lipase (LPL)11,12 and increases liver uptake of remnant particles.13 Studies of Apoa5 knockout mice show a 400% increase in triglyceride levels,14 and genetic deficiency of apoA-V in humans is associated with hypertriglyceridaemia.14 – 18 Furthermore, genomewide association studies have consistently shown that polymorphisms in or near the apolipoprotein A5 gene (APOA5), located in the APOA1/APOC3/APOA4 gene cluster, are among the strongest genetic determinants of plasma triglyceride levels.19,20 Resequencing APOA5 to identify variants with an effect on triglyceride levels therefore provides an excellent instrument to explore causality between elevated non-fasting triglycerides, remnant cholesterol, and risk of myocardial infarction (MI), even if the exact role of APOA5 genetic variants in the regulation of plasma triglycerides and remnant cholesterol is not known.21 Using a Mendelian randomization approach, we tested the following hypotheses: (i) genetic variants in APOA5 affect levels of non-fasting triglycerides and remnant cholesterol in the general population; (ii) genetic variants in APOA5 associate with risk of MI to the extent predicted by their effects on plasma levels of nonfasting triglyceride and remnant cholesterol; and (iii) remnant cholesterol is causally associated with risk of MI, using instrumental variable analysis. A Mendelian randomization approach circumvents most confounding and reverse causation,21 but may have limitations such as a need for huge statistical power, and for genetic instruments without major pleiotropic effects. This may pose a particular challenge when studying triglycerides and remnant cholesterol because of the inverse association with high-density lipoprotein (HDL) cholesterol.22 These hypotheses were tested in white individuals of Danish descent from Copenhagen, including 5705 MI cases and 54 408 controls. Importantly, on purpose we did not m (...truncated)