Protein–protein interactions shape trans-regulatory impact of genetic variation on protein expression and complex traits

Most genetic variants influence complex traits by affecting gene regulation. Yet, despite comprehensive catalogs of molecular quantitative trait loci (QTLs), linking trait-associated variants to biological functions remains difficult. By re-analyzing large maps of protein QTLs (pQTLs), we found that genes with trans-pQTLs but no cis-pQTLs are under strong selective constraints and are particularly informative in interpreting genome-wide association study (GWAS) loci. We observed that trans-pQTLs and their target proteins are frequently involved in protein-protein interactions (PPIs). Notably, trans-pQTLs are enriched in missense variants and at PPI interfaces, suggesting a key role of PPIs in the trans-regulation of proteome. Using PPI annotations to guide trans-pQTL mapping, we identified 17,662 trans-pQTLs affecting 961 PPI clusters after accounting for blood cell composition effects. These trans-pQTLs colocalized with 36% GWAS loci per trait on average for 27 complex traits, helping in many cases to link GWAS loci to cellular function. Finally, we identified trans-pQTL effects at multiple autoimmune GWAS loci that converge to the same PPIs, pinpointing protein complexes and signaling pathways that show promising therapeutic target potential.