NEW YORK (GenomeWeb News) – Three new studies — including the largest meta-analysis yet of coronary artery disease — have identified dozens of coronary artery disease risk loci in European, South Asian, and Han Chinese populations. All three papers appeared online yesterday in Nature Genetics.
For the first meta-analysis, members of a large international consortium known as the Coronary Artery Disease Genome-wide Replication and Meta-Analysis study, or CARDIoGRAM, sifted through data on more than 135,000 individuals from the UK, US, Europe, Iceland, and Canada. In so doing, they tracked down nearly two-dozen new and previously reported coronary artery disease risk loci.
Because only a few of these loci have been linked to other heart disease-related risk factors such as high blood pressure, those involved say the work points to yet unexplored heart disease pathways.
"[W]e have discovered several new genes not previously known to be involved in the development of coronary heart disease, which is the main cause of heart attacks," co-corresponding author Nilesh Samani, a cardiology researcher affiliated with the University of Leicester and Glenfield Hospital, said in a statement. "Understanding how these genes work, which is the next step, will vastly improve our knowledge of how the disease develops, and could ultimately help to develop new treatments."
Samani and his co-workers identified the loci by bringing together data on 22,233 individuals with coronary artery disease and 64,762 unaffected controls. The participants, all of European descent, had been sampled through 14 previous genome-wide association studies and genotyped at an average of about 2.5 million SNPs each. The team then assessed the top candidate SNPs found in this initial analysis in another 56,582 individuals (roughly half of whom had coronary artery disease).
The search not only confirmed associations between coronary artery disease and 10 known loci, but also uncovered associations with 13 other loci. All but three of these were distinct from loci previously implicated in other heart disease risk factors such as hypertension or cholesterol levels, researchers noted.
Consequently, those involved in the study say that exploring the biological functions of the newly detected genes could offer biological clues about how heart disease develops — along with strategies for preventing and treating it.
The genetic complexity of coronary artery disease being revealed by such studies has diagnostic implications as well, according to some.
"Each new gene identified brings us a small step closer to understanding the biological mechanisms of cardiovascular disease development and potential new treatments," British Heart Foundation Medical Director Peter Weissberg, who was not directly involved in the new studies, said in a statement. "However, as the number of genes grows, it takes us further away from the likelihood that a simple genetic test will identify those most of risk of suffering a heart attack or a stroke."
Meanwhile, researchers involved with Coronary Artery Disease Genetics Consortium did their own meta-analysis using data collected from four GWAS to find five coronary artery-associated loci in European and South Asian populations.
The group initially looked at 15,420 individuals with coronary artery disease — including 6,996 individuals from South Asia and 8,424 from Europe — and 15,062 unaffected controls. Participants were genotyped at nearly 575,000 SNPs using Illumina BeadChips. Most South Asian individuals tested came from India and Pakistan, researchers noted, while European samples came from the UK, Italy, Sweden, and Germany.
For the validation phase of the study, the team focused in on 59 SNPs at 50 loci from the discovery group that seemed most likely to yield authentic new disease associations. These variants were assessed in 10 replication groups comprised of 21,408 individuals with coronary artery disease and 19,185 individuals without coronary artery disease.
All told, researchers found five loci that seem to influence coronary artery disease risk in the European and South Asian populations: one locus each on chromosomes 7, 11, and 15, along with a pair of loci on chromosome 10.
The team didn't see significant differences in the frequency or effect sizes of these newly identified variants between the European and South Asian populations, though they emphasized that their approach may have missed some potential risk variants, particularly in those of South Asian descent.
"[C]urrent genome-wide arrays may not capture all important variants in South Asians," they explained, "Nevertheless, all of the known and new variants were significantly associated with [coronary artery disease] risk in both the European and South Asian populations in the current study, indicating the importance of genes associated with [coronary artery disease] beyond the European ancestry groups in which they were first defined."
Finally, using a three-stage discovery, validation, and replication GWAS approach, Chinese researchers identified a single coronary artery disease risk variant in the Han Chinese population.
In this first phase of that study, researchers tested samples from 230 cases and 230 controls from populations in Beijing and in China's Hubei province that were genotyped at Genentech and CapitalBio using Affymetrix Human SNP5.0 arrays.
From the nearly three-dozen SNPs identified in the first stage of the study, they narrowed in on nine suspect variants. After finding linkage disequilibrium between two of the variants, they did validation testing on eight of these in 572 individuals with coronary artery disease and 436 unaffected controls, all from Hubei province.
That analysis implicated a single chromosome 6 SNP called rs6903956 in coronary artery disease — a finding the team ultimately replicated in another group of 2,668 coronary artery disease cases and 3,917 controls from three independent populations in Hubei, Shandong province, and northern China.
The team's subsequent experiments suggest that the newly detected polymorphism, which falls within a putative gene called C6orf105 on chromosome 6, curbs the expression of this gene. The functional consequences of this shift in expression, if any, are yet to be determined.
Because C6orf105 shares some identity and homology with an androgen hormone inducible gene known as AIG1, those involved in the study argue that it may be worthwhile to investigate possible ties between C6orf105 expression, androgen signaling, and coronary artery disease.
"Androgen has previously been reported to be associated the pathogenesis of atherosclerosis," they wrote. "Future studies are needed to explore whether C6orf105 expression can be induced by androgen and to further determine the potential mechanism of [coronary artery disease] associated with decreased C6orf105 expression."