Masculinizing Testosterone Therapy Reduces the Incidence of PIK3CA-Mutant/ER⁺ Breast Cancer but Not BRCA1-Associated Triple-Negative Breast Cancer.

We investigated the impact of gender-affirming testosterone therapy (TT) on breast cancer (BC) risk and tumor progression. We leveraged a large human breast tissue dataset (n=417) to assess TT and terminal duct lobular unit (TDLU) involution, complemented with tissue markers (ER, PR, AR, and Ki67; n =24) and transcriptome profiling ( n =8). Preclinical models assessed the effect of TT on BC incidence ( MMTV-Cre Pik3ca f/wt n =149 and K14-Cre Brca f/f Tp53 f/f n =153), murine mammary gland architecture ( n =60), and tumor transcriptome ( n =10). Lastly, we discuss trans masculine invasive BC cases and summarize tumor characteristics in this population ( n =24). TT promotes TDLU involution by reducing epithelial proliferation via altered estrogen signaling and increases ER+, PR+, and Ki67+ extralobular stromal cells. In mice, TT similarly reduced mammary gland ductal branching and terminal end buds. TT decreased Pik3ca -related ER+ BC incidence by 81% compared to female controls (adj RR 0.19, 95% CI 0.08-0.45), but did not affect Brca1 -related triple negative BC incidence. TT did not influence tumor progression in either model but shaped the Pik3ca -related ER+ tumor microenvironment toward a pro-tumor phenotype. Most trans masculine BC cases were ER+ (83.3%), small and node-negative, but were also moderately to poorly differentiated (70.8%). TT reduces ER+ BC risk but does not eliminate risk, and has a negligible impact on BRCA1 -related triple-negative BC risk. TT does not affect tumor growth once tumors are established but modulates the tumor microenvironment. Our work supports the need for breast cancer screening in TT users. Highlights: TT reduces but does not completely ablate the breast epithelium. TT decreases PIK3CA -related ER+ breast cancer incidence by 81% compared to female control mice (adj RR 0.19, 95% CI 0.08-0.45), but does not affect BRCA1 -related triple negative breast cancer incidence. TT does not affect tumor progression once the tumor is established.Trans masculine breast tumors are mostly ER+ (83.3%), small and node-negative, but are also moderately to poorly differentiated (70.8%).Tailored risk assessment and ongoing surveillance strategies are key for the care of transmasculine individuals who use TT.