Identifying druggable proteins of the association of chronotype on breast cancer using Mendelian randomization.

Emerging evidence shows that morning chronotype is associated with a reduced risk of breast cancer, yet biological mechanisms remain unclear. This study aimed to identify circulating proteins that mediate this association. We employed a two-step Mendelian randomization approach using European-ancestry genome-wide association studies. Exposure data on self-reported morning chronotype were from the UK Biobank and 23andMe (372,765 cases, 278,530 controls). Mediators included 4907 plasma proteins measured by SomaScan in 35,559 Icelandic individuals. Outcome data for overall breast cancer and subtypes were sourced from the Breast Cancer Association Consortium (133,384 cases, 113,789 controls) and FinnGen DF 11 (20,586 cases, 201,494 controls). We identified mediators through concordant step 1 and step 2 associations, followed by colocalization (posterior probability > 0.80) and mediation analyses. Single-cell RNA sequencing in human breast cancer tissues was used to assess biological relevance. Genetically predicted morning chronotype is associated with a lower risk of breast cancer (odds ratio, 0.93; 95% confidence interval, 0.89 to 0.98). Morning chronotype is associated with 895 plasma proteins, of which seven proteins show inverse associations with overall breast cancer (false discovery rate <0.05). Colocalization support five mediators sharing causal variants for protein levels and breast cancer susceptibility. Mediation and single-cell RNA sequencing analyses further confirm their biological roles. These findings uncover plausible biological pathways linking morning chronotype to reduced breast cancer risk, nominating ADAM15, BTN2A1, CASP8, PDCD6, and RSPO3 as potential therapeutic targets. This work bridges epidemiological observations with mechanistic insights, offering a translational roadmap for chronotype-based interventions.