Coronary artery disease and its clinical sequelae, including myocardial infarction, together constitute the single greatest cause of death worldwide (27
). CHD has a complex etiology, with multiple genes and environmental factors believed to be involved in its pathogenesis. Although the heritability of CHD is estimated to be 40%–60% (29
), with a risk to first-degree relatives that is 5–7 times higher than that for members of the general population (30
), the reproducible identification of genetic variants underlying this increased susceptibility has been difficult.
A genome-wide association study conducted by the WTCCC compared the frequency of 469,557 SNPs in approximately 2,000 cases and 3,000 shared controls for 7 different complex diseases, including CHD (2
). The CHD cases in the WTCCC study included only persons with premature cardiovascular events occurring before the age of 66 years, to maximize the likelihood of detecting a genetic component contributing to disease causation. Seven novel variants were identified, with the strongest association being found for an SNP (rs1333049) located in the same region of chromosome 9p21 that was independently reported to harbor polymorphisms conferring increased risk of myocardial infarction in 5 different Caucasian populations (12
). However, the ascertainment scheme chosen by the WTCCC may also have resulted in the choice of CHD cases that were not representative of the population from which they were drawn; thus, SNPs identified using this type of study design may not be associated with CHD outside of the selected subgroup. Our aim in the current investigation was to determine whether any of the 7 SNPs associated with CHD in the initial case-control study (2
) and the 5 additional SNPs identified in replication studies conducted by the WTCCC (17
) predicted incident CHD in the large, biracial, population-based ARIC cohort.
Two of the sequence variants described by the WTCCC in case-control studies (rs1333049 and rs501120) were significantly associated with incident CHD for white participants in the ARIC Study. Results from the Cox proportional hazards models used to estimate the risk of CHD showed that the 2 intergenic SNPs were independently associated with CHD even after adjustment for multiple risk factors. One of these SNPs (rs1333049) maps to a region of chromosome 9p21 previously reported to be associated with risk of myocardial infarction among whites in the ARIC cohort (12
). The 58-kilobase genomic interval defining the risk allele is located near the cyclin-dependent kinase inhibitor 2A, cyclin-dependent kinase inhibitor 2B, and cyclin-dependent kinase inhibitor antisense RNA loci, while there are no annotated genes or micro-RNAs within the region itself (Appendix Table).
The rs501120 variant is located approximately 127 kilobases downstream of the gene encoding stromal cell-derived factor 1 (SDF-1
) on chromosome 10q11. SDF-1 is a member of the family of chemoattractant cytokines known as chemokines and is the ligand for cell-surface chemokine receptor 4. SDF-1
has been implicated in a wide variety of biologic processes, including trafficking of hematopoietic stem and progenitor cells (31
), migration of lymphocytes and monocytes (32
), and the recruitment of endothelial progenitor cells derived from bone marrow to ischemic tissues in animal models (33
has also been reported to be involved in the induction of platelet aggregation, with high expression in smooth muscle cells, endothelial cells, and macrophages in human atherosclerotic plaques (35
). Although the latter function suggests that the rs501120 T allele could confer susceptibility to CHD by playing a role in the regulation of thrombus formation after plaque rupture, this would require rs501120 to exert its effect at a considerable distance from the SDF-1
coding region (Appendix Table).
One additional SNP (rs599839) was associated with incident CHD in whites when the results were adjusted only for age and gender, but this relation was abolished after either LDL or HDL cholesterol was added as an additional covariate (P
0.556 and P
0.109, respectively). Since the rs599839 polymorphism was recently reported to be significantly associated with serum LDL cholesterol concentration in 2 genome-wide association studies conducted in European Caucasian populations (25
), these results suggest that the influence of this SNP on CHD risk in the ARIC cohort may operate through its effect on plasma lipid levels. The noncoding rs599839 variant is located near 3 coding genes on chromosome 1p13, including sortilin, which is involved in the receptor-mediated binding of lipoprotein lipase on the surface of adipocytes (36
). A second SNP that is in linkage disequilibrium with rs599839 (rs646776; r2
0.89 in HapMap Utah residents with Northern and Western European ancestry) has been shown to be correlated with the expression of sortilin in human liver cells (25
), providing a possible link to known events in lipoprotein metabolism (Appendix Table).
When incident CHD was examined in African Americans, one of the 12 SNPs (rs7250581) showed a nominally significant relation after adjustment for age and gender that was abrogated after the addition of multiple covariates known to be associated with cardiovascular disease. However, since the observed genotype frequencies for this variant did not meet Hardy-Weinberg equilibrium expectations, the possibility of a false-positive association must also be considered. As shows, there was 80% power to detect a hazard rate ratio of 1.2–1.3 for 8 of the polymorphisms studied in African Americans, and a hazard rate ratio of 1.1–1.2 could be detected for all of the genetic variants in whites. Therefore, the failure to identify significant associations between most of the SNPs described in a population of Northern European origin in the WTCCC case-control study and incident CHD in African Americans could be due to differences in linkage disequilibrium in the genomic regions where the respective genetic markers reside; the SNPs identified by the WTCCC may be correlated with a true causative variant in whites but not in populations of African origin. Another possibility is that there may be variation in additional, as-yet-unknown genetic or environmental factors that might modify the effect of the risk variants, although this would need to be examined in a larger population of sufficient size to allow detection of such gene-environment or gene-gene interactions (37
). Alternatively, the SNPs identified by the WTCCC were associated with prevalent CHD case status, so for both white and African-American participants, at least some of the polymorphisms analyzed here might play a role in CHD only after it has become well-established rather than in earlier events in disease pathogenesis.
Cox Regression Hazard Rate Ratios for Incident Coronary Heart Disease That Were Detectable at 80% Power, by Race, Atherosclerosis Risk in Communities Study, 1987–2004
The odds ratios of 1.2 found for the 2 SNPs showing a significant association with CHD in whites under an additive genetic model were in accordance with the modest effects reported for most common sequence variants influencing complex disease in genome-wide association studies (38). However, homozygous carriers of the susceptibility alleles for the rs1333049 and rs501120 variants comprised 22.5% and 75.2% of white persons in the ARIC cohort, respectively, so the public health impact of these polymorphisms could be substantial, and further investigation in other community-based cohorts is warranted. Neither the WTCCC case-control studies nor this study was designed to test alternative hypotheses of disease causation (such as the contribution of copy number variation or of rare variants with large effects) that might help to further explain the observation that family history of CHD is a strong predictor of disease risk.