Exploring the relationship between gut microbiota and breast cancer risk in European and East Asian populations using Mendelian randomization.

Background: Several studies have explored the potential link between gut microbiota and breast cancer; nevertheless, the causal relationship between gut microbiota and breast cancer remains unclear.

Methods: We utilized summary statistics from genome-wide association studies (GWAS) of the gut microbiome from the MiBioGen project with summary data from GWAS on breast cancer from the FinnGen consortium and the IEU database, with the IEU data sourced from the Biobank Japan. Preliminary statistical analyses were conducted using inverse variance weighting (IVW), supplemented by various sensitivity analysis methods, including MR-Egger regression, weighted median, weighted mode, simple median, and simple mode, to ensure the robustness of our findings. Heterogeneity and pleiotropy were assessed to avoid misleading conclusions caused by unconsidered confounders or non-specific effects of genetic variants, ensuring that the results reflect a genuine causal relationship.

Results: In European populations, four types of gut microbiota were associated with breast cancer. The genus Erysipelatoclostridium was positively associated with the risk of breast cancer, with an odds ratio (OR) of 1.21 (95% confidence interval [CI] 1.083-1.358), false discovery rate (FDR) = 0.0039. The class Coriobacteriia, order Coriobacteriales, and family Coriobacteriaceae, which belong to the same phylogenetic system, showed a consistent inversely association with breast cancer risk, with an OR of 0.757 (95% CI 0.616-0.930), FDR = 0.0281. In East Asian populations, three types of gut microbiota were related to breast cancer. The Eubacterium ruminantium group was positively associated with breast cancer risk, with an OR of 1.259 (95% CI 1.056-1.499), FDR = 0.0497. The families Porphyromonadaceae and Ruminococcaceae were inversely associated with breast cancer risk, with ORs of 0.304 (95% CI 0.155-0.596), FDR = 0.0005, and 0.674 (95% CI 0.508-0.895), FDR = 0.03173, respectively. However, these two taxa had limited instrumental variables, restricting the statistical power and potentially affecting the interpretation of the results.

Conclusion: This MR analysis demonstrated a probable causal link between specific gut microbiota and breast cancer. This study, through Mendelian randomization analysis comparing European and East Asian populations, reveals that gut microbiota may influence breast cancer risk differently across populations, providing potential directions for developing targeted prevention and treatment methods.