Adiponectin GWAS loci harboring extensive allelic heterogeneity exhibit distinct molecular consequences

by Cassandra N. Spracklen, Apoorva K. Iyengar, Swarooparani Vadlamudi, Chelsea K. Raulerson, Anne U. Jackson, Sarah M. Brotman, Ying Wu, Maren E. Cannon, James P. Davis, Aaron T. Crain, Kevin W. Currin, Hannah J. Perrin, Narisu Narisu, Heather M. Stringham, Christian Fuchsberger, Adam E. Locke, Ryan P. Welch, Johanna K. Kuusisto, Päivi Pajukanta, Laura J. Scott, Yun Li, Francis S. Collins, Michael Boehnke, Markku Laakso, Karen L. Mohlke

Loci identified in genome-wide association studies (GWAS) can include multiple distinct association signals. We sought to identify the molecular basis of multiple association signals for adiponectin, a hormone involved in glucose regulation secreted almost exclusively from adipose tissue, identified in the Metabolic Syndrome in Men (METSIM) study. With GWAS data for 9,262 men, four loci were significantly associated with adiponectin: ADIPOQ, CDH13, IRS1, and PBRM1. We performed stepwise conditional analyses to identify distinct association signals, a subset of which are also nearly independent (lead variant pairwise r²<0.01). Two loci exhibited allelic heterogeneity, ADIPOQ and CDH13. Of seven association signals at the ADIPOQ locus, two signals colocalized with adipose tissue expression quantitative trait loci (eQTLs) for three transcripts: trait-increasing alleles at one signal were associated with increased ADIPOQ and LINC02043, while trait-increasing alleles at the other signal were associated with decreased ADIPOQ-AS1. In reporter assays, adiponectin-increasing alleles at two signals showed corresponding directions of effect on transcriptional activity. Putative mechanisms for the seven ADIPOQ signals include a missense variant (ADIPOQ G90S), a splice variant, a promoter variant, and four enhancer variants. Of two association signals at the CDH13 locus, the first signal consisted of promoter variants, including the lead adipose tissue eQTL variant for CDH13, while a second signal included a distal intron 1 enhancer variant that showed ~2-fold allelic differences in transcriptional reporter activity. Fine-mapping and experimental validation demonstrated that multiple, distinct association signals at these loci can influence multiple transcripts through multiple molecular mechanisms.