Causal plasma metabolites for breast cancer risk: a two-sample Mendelian randomization study with colocalization evidence

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BACKGROUND: Breast cancer pathogenesis involves complex metabolic dysregulation, yet causal biomarkers remain elusive. This study aimed to assess causal effects of 1,400 human plasma metabolites on breast cancer (BC) risk using a two-sample Mendelian randomization (MR) framework. METHODS: We employed a rigorous two-sample Mendelian randomization framework with tiered quality control (Bonferroni correction, sensitivity analyses, meta-analyses) to investigate causal metabolite-BC associations. Colocalization (PPH4 > 0.80) and phenome-wide MR (2,099 FinnGen phenotypes) validated mechanistic specificity and clinical safety profiles. RESULTS: Five genetically determined plasma metabolites were identified as the potential causal biomarkers for BC risk: 3,5-dichloro-2,6-dihydroxybenzoic acid (odds ratio [OR]: 0.90; 95% confidence interval [CI] 0.87-0.94; p < 0.001), carnitine C14 (OR: 0.72; 95% CI 0.64-0.83; p < 0.001) and epiandrosterone sulfate (OR: 1.04; 95% CI 1.01-1.06; p < 0.001), Glyco-beta-muricholate (OR: 0.95; 95% CI 0.93-0.97; p < 0.001), N4-acetylcytidine (OR: 0.93; 95% CI 0.91-0.96; p < 0.001). Colocalization analysis showed strong evidence for Glyco - beta - muricholate and Epiandrosterone sulfate with BC risk (PPH4 = 1). PheWAS-MR revealed metabolite-specific safety profiles, with carnitine C14 showing broadest phenotypic associations (96 outcomes). CONCLUSIONS: This study establishes carnitine C14 as a novel protective biomarker and epiandrosterone sulfate as a risk biomarker for breast cancer, with colocalization evidence supporting their therapeutic targeting. The metabolic risk profile provides a foundation for precision prevention strategies.