The causal effects of inflammatory bowel disease on primary biliary cholangitis: A bidirectional two‐sample Mendelian randomization study

Abstract Background Observational studies have indicated that the incidence of primary biliary cholangitis (PBC) is higher in inflammatory bowel disease (IBD) patients than that in healthy people. However, whether the correlation is causal remains unclear. Methods The genetic associations with IBD were obtained from publicly available genome‐wide association studies (GWAS) of European ancestry with 31 665 cases and 33 977 controls, consisting of 17 897 Crohn's disease (CD) and 13 768 ulcerative colitis (UC) cases. The genetic associations with PBC were obtained from a European GWAS with 2764 cases and 10 475 controls. A bidirectional two‐sample Mendelian randomization (MR) design was implemented to determine the causal relationship between IBD and PBC. In the forward MR, the IBD was treated as the exposure while the PBC was the exposure in the reverse MR. The inverse‐variance‐weighted (IVW) method was utilized as the main statistic method, and a series of sensitivity analyses were performed to detect heterogeneity and horizontal pleiotropy. Results A total of 99 valid instrumental variables (IVs) were selected for IBD and the number of IVs for PBC was 18. The forward MR analysis indicated that genetically predicted IBD (UC and CD) was significantly associated with an increased risk of PBC (IVW OR = 1.343; 95% CI: 1.220–1.466). Similar casual associations were observed in UC (IVW OR = 1.244; 95% CI: 1.057–1.430) and CD (IVW OR = 1.269; 95% CI: 1.159–1.379). Such results were still consistent in multiple MR methods. The reverse MR analysis implicated that genetic susceptibility to PBC might not alter the risk of IBD (IVW OR = 1.070; 95% CI: 0.984–1.164). Conclusion Our study found that genetically predicted IBD can increase the risk of PBC while not vice versa in the European population, which may enlighten the aetiology of PBC, together with the IBD patient management.