Epigenetic and expression analysis of ankylosing spondylitis association loci point to key cell types driving disease

<b>Introduction</b>\ud \ud Susceptibility to ankylosing spondylitis (AS) is primarily genetic; thus far 113 susceptibility variants for AS have been identified. However, most of the AS associated SNPs do not directly affect protein-coding genes. Studies of disease- and trait-associated SNPs suggest they may act by affecting gene regulatory regions in specific cell types or tissues. Therefore, identifying the AS relevant cell types is crucial for further mechanistic studies.\ud \ud <b>Material and Methods</b>\ud \ud We applied several bioinformatics methods to utilize epigenetic, gene and protein expression information to identify the primary relevant cell types through which genetic variants associated with AS operate. In total, there are 113 AS associated loci; 39 of them show genome-wide significance in AS-only analyses, whereas the remainder are genome-wide significant in analyses leveraging pleiotrophy with other related diseases (inflammatory bowel disease (IBD), psoriasis, primary sclerosing cholangitis (PSC) and ulcerative colitis (UC)) (1).\ud \ud <b>Results</b>\ud \ud Epigenetic analysis suggests that AS-associated SNPs operate primarily in immune cell types including monocytes, CD4+ and CD8+ T cells, NK cells, regulatory T cells, and B cells. Gene expression studies showed enrichment of AS associated loci in genes specifically expressed in monocytes and NK cells while protein expression study shows protein products of AS associated loci were significantly enriched in CD8+ T cells. Epigenetic analyses also showed evidence that AS-associated signals operate in gut cell types including in mucosa from the small intestine, sigmoid colon and rectum. These findings particularly relate to leiotropic loci also associated with IBD, psoriasis, and PSC.\ud \ud <b>Conclusions</b>\ud \ud These findings highlight the role of key immune cell types in the mechanism by which genetic associations with AS drive the disease, as well as providing further evidence for the involvement of the gut in the pathogenesis of AS