Investigating the shared genetic architecture between multiple sclerosis and inflammatory bowel diseases

An epidemiological association between multiple sclerosis (MS) and inflammatory bowel disease (IBD) is well-established, but whether this reflects a shared genetic aetiology, and whether consistent genetic relationships exist between MS and the two predominant subtypes of IBD, ulcerative colitis (UC) and Crohn9s disease (CD), remains unclear. Here, we used genome-wide association study (GWAS) summary data to estimate genetic correlations (rg) between MS and each of IBD, UC and CD, finding that the rg between MS and UC was approximately twice that between MS and CD. On the basis of these genetic correlations, we performed cross-trait meta-analysis of GWAS summary data for MS and each of IBD, UC and CD, identifying a total of 42 novel SNPs shared between MS and IBD (N=19), UC (N=14), and CD (N=18). We then used multiple Mendelian randomization (MR) methods to investigate causal relationships between these diseases, finding suggestive but inconclusive evidence for a causal effect of MS on UC and IBD, and no or weak and inconsistent evidence for a causal effect of IBD or UC on MS. There was also no evidence for causality in bidirectional analyses of MS and CD. We also investigated tissue- and cell-type-specific enrichment of SNP heritability for each disease using stratified LD score regression. At the tissue level, we observed largely consistent patterns of enrichment for all four diseases in immune system-related tissues, including lung, spleen and whole blood, and in contrast to prior studies, small intestine. At the cell-type level, we identified significant enrichment for all diseases in CD4+ T cells in lung, and for MS, IBD and CD in CD8+ cytotoxic T cells in both lung and spleen, and regulatory T cells in lung. Our study sheds new insights into the biological basis of comorbidity between MS and both UC and CD.