The causality between Type 2 diabetes and breast cancer: a bidirectional two-sample Mendelian randomization study

Objective: Observational studies showed that Type 2 diabetes increased the risk of breast cancer, and vice versa. However, it is uncertain whether the link is causal or just due to confounding factors. Using bidirectional Mendelian randomization analysis, we assessed the bidirectional causal relationship from a genetic level. Methods: Large genome-wide association studies yielded summary-level data for Type 2 diabetes and breast cancer. Results: Genetically predicted Type 2 diabetes presented no statistically significant association with overall breast cancer or its subtypes. Similarly, genetically predicted overall breast cancer or its subtypes had no causal effect on Type 2 diabetes. Sensitivity analyses yielded similar results. Conclusion: Our bidirectional Mendelian randomization studies revealed no causal links between Type 2 diabetes and breast cancer.
Type 2 diabetes had no causal effect on the development of overall breast cancer, estrogen receptor (ER)+ or ER- subtypes.
Overall breast cancer, ER+ and ER- subtypes had no causal effect on the development of Type 2 diabetes.
The previously observed substantial relationships between Type 2 diabetes and breast cancer in observational studies could be explained by failing to account for confounders completely.