Trans-ethnic fine-mapping of lipid loci identifies population-specific signals and allelic heterogeneity that increases the trait variance explained

Abstract

Genome-wide association studies (GWAS) have identified ~100 loci associated with blood lipid levels, but much of the trait heritability remains unexplained, and at most loci the identities of the trait-influencing variants remain unknown. We conducted a trans-ethnic fine-mapping study at 18, 22, and 18 GWAS loci on the Metabochip for their association with triglycerides (TG), high-density lipoprotein cholesterol (HDL-C), and low-density lipoprotein cholesterol (LDL-C), respectively, in individuals of African American (n = 6,832), East Asian (n = 9,449), and European (n = 10,829) ancestry. We aimed to identify the variants with strongest association at each locus, identify additional and population-specific signals, refine association signals, and assess the relative significance of previously described functional variants. Among the 58 loci, 33 exhibited evidence of association at P<1 × 10(-4) in at least one ancestry group. Sequential conditional analyses revealed that ten, nine, and four loci in African Americans, Europeans, and East Asians, respectively, exhibited two or more signals. At these loci, accounting for all signals led to a 1.3- to 1.8-fold increase in the explained phenotypic variance compared to the strongest signals. Distinct signals across ancestry groups were identified at PCSK9 and APOA5. Trans-ethnic analyses narrowed the signals to smaller sets of variants at GCKR, PPP1R3B, ABO, LCAT, and ABCA1. Of 27 variants reported previously to have functional effects, 74% exhibited the strongest association at the respective signal. In conclusion, trans-ethnic high-density genotyping and analysis confirm the presence of allelic heterogeneity, allow the identification of population-specific variants, and limit the number of candidate SNPs for functional studies.

Figure 1. LDL-C locus PCSK9 exhibited seven signals in African Americans.

Initial association in the main analysis (A). Residual association in sequential conditional analysis by sequentially adding the lead SNPs into the regression model (B–G). Each SNP was colored according to its LD (r²) in the PAGE consortium, with the strongest SNP colored in purple and symbols designating genomic annotation defined in the ‘annotation key’. Genomic coordinates refer to build 36 (hg18).

Figure 2. Trans-ethnic high-density genotyping narrows the association signal at the HDL-C locus PPP1R3B.

Association in Europeans (A), East Asians (B), African Americans (C) and in a combined trans-ethnic meta-analysis (D). Index SNP rs6601299 colored in purple is the variant showing strongest evidence of association in the combined trans-ethnic meta-analysis.