Causality between autoimmune diseases and breast cancer: a two-sample Mendelian randomization study in a European population

Abstract Background The incidence of autoimmune diseases and breast cancer is significantly higher in women compared to men. Previous observational studies have not conclusively determined the relationship between these two conditions. This study utilizes the Mendelian randomization approach to investigate the genetic association between autoimmune diseases and breast cancer. Method Two-sample Mendelian randomization was conducted on a European population using the GWAS database. The inverse variance-weighted method served as the primary analytical approach. The MR-PRESSO test was applied to detect horizontal pleiotropy. To ensure result robustness, the FDR correction method was used. Result The study revealed that Sjögren’s syndrome lowers the overall risk of breast cancer (OR 0.96, 95% CI [0.93–0.99], p = 0.011). Idiopathic inflammatory myopathy shows a protective effect against overall breast cancer (OR 0.98, 95% CI [0.97–0.99], p = 0.035). An association was identified between rheumatoid arthritis and overall breast cancer (OR 0.98, 95% CI [0.96–1.00], p = 0.050). No causal link was found between systemic lupus erythematosus, systemic sclerosis, and overall breast cancer. The study also suggests that Sjögren’s syndrome, rheumatoid arthritis, and idiopathic inflammatory myopathy might reduce the risk of developing HER + breast cancer. Specifically, Sjögren’s syndrome (OR = 0.90, 95% CI [0.83–0.98], p = 0.02), rheumatoid arthritis (OR = 0.94, 95% CI [0.91–0.98], p = 0.006), and idiopathic inflammatory myopathy (OR = 0.96, 95% CI [0.93–0.99], p = 0.036). Additionally, systemic lupus erythematosus was found to lower the risk of HER- breast cancer (OR = 0.95, 95% CI [0.91–0.99], p = 0.046). The study did not establish a causal relationship between these five autoimmune diseases and ER + or ER- breast cancer. Conclusion This study found that autoimmune diseases may act as protective factors against breast cancer risk.