Causal association between autoimmune diseases and risk of breast cancer: A bidirectional Mendelian randomization study

The potential causal relationship between autoimmune diseases (AIDs) and breast cancer (BC) remains a topic of debate. In this study, we conducted Mendelian randomization (MR) analysis between common AIDs and BC using publicly available genome-wide association studies data from the MR Base platform. Initially, we conducted a two-sample MR analysis of 10 AIDs and BC using the inverse variance weighted method. Subsequently, a series of sensitivity analyses were performed to validate the robustness of our findings. Lastly, we utilized the reverse MR analysis to evaluate the potential for reverse causality. Among the 10 types of AID studied, 3 were found to be associated with an elevated risk of BC. These include ankylosing spondylitis (odds ratio [OR]: 1.21; 95% confidence interval [CI]: 1.04-1.41; P = .013), inflammatory bowel disease (OR: 1.05; 95% CI: 1.01-1.09; P = .012), and rheumatoid arthritis (OR: 1.04; 95% CI: 1.01-1.07; P = .023). Additionally, BC was found to be linked with a heightened risk of inflammatory bowel disease (OR: 1.112; 95% CI: 1.022-1.210; P = .014). Our study's findings support a bidirectional causal relationship between AIDs and BC, offering novel insights into the developmental mechanisms underlying the interaction between AIDs and BC.