Platinum-based chemotherapy remains a cornerstone of treatment for triple-negative breast cancer (TNBC), yet the molecular determinants governing platinum response remain poorly defined. By leveraging the randomized Phase II INFORM trial, which compared neoadjuvant cisplatin to anthracycline-based therapy in BRCA1/2-mutant breast cancer, we identified miR-362-3p as a specific regulator of cisplatin sensitivity. Higher plasma miR-362-3p expression was exclusively associated with favorable clinical outcome in the cisplatin arm, with no association observed in the AC arm, decoupling platinum-specific vulnerability from general chemotherapy response. We used gain- and loss-of-function TNBC models to establish that miR-362-3p functions as a potent sensitizer to cisplatin in vitro and in vivo. Integrated TCGA analysis and experimental validation identified BCLAF1, a key regulator of DNA damage response, as a direct repression target of miR-362-3p. We uncovered a novel role for the miR-362-3p/BCLAF1 axis in overcoming platinum resistance in TNBC.Competing Interest StatementJKC is partially supported by Mirxes through a manpower secondment scheme from the National University of Singapore. LZ HC are employees of Mirxes. PM was formerly employed by Mirxes and is currently employed by Amgen. Mirxes had no role in the study design or manuscript preparation. ELM declares consulting/advisory role with Genentech, Lilly, Novartis, and AstraZeneca. JEG declares consulting/advisory role with Novartis, Kronos Bio, GV20 Therapeutics, Belharra Therapeutics, Inc, and Earli, Inc, and research funding from Novartis, Ambry Genetics, InVitae, and Amgen. NMT declares institutional research funding from AstraZeneca, and GlaxoSmithKline. ALK is a co-founder and CEO of LigamiR Therapeutics, and an inventor on U.S. patent PCT/US2017/061997. GMW declares stock/other ownership from Cartesian Therapeutics, a consulting/advisory role with Totus Medicines, research funding from Seagen, Totus Medicines, Boundless Bio, Gilead Sciences, Celgene, Mersana, PureTech, Pfizer, Eikon, and Acrivon Therapeutics, and inventor on patent 8129131, Pin 1 inhibitors. The other authors declare no competing financial or non-financial interests.Clinical TrialNCT01670500, NCT01982448Funding StatementFunding and Acknowledgements: This work was supported by NIH R35CA232105 (FJS), R01CA235740 (FJS), R01CA226259 (ALK), R01CA205420 (ALK), the Ludwig Center at Harvard (GMW), AGA/Jenzabar research fund (GMW), and Breast Cancer Research Foundation (NMT, GMW, JEG & SJS). Mirxes provided the ID3EAL Cancer miRNA Knowledge Panels. The Detection Unit, Precision RNA Medicine Core at BIDMC (RRID:SCR_024819), performed the ID3EAL assays. BIDMC Spatial Technologies Unit digitized the slides (RRID:SCR_024905) and the Histology Core provided histology services (RRID:SCR_009669). The authors thank the administrative staff for compiling the INFORM and TBCRC 030 trial data.Author DeclarationsI confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained.YesThe details of the IRB/oversight body that provided approval or exemption for the research described are given below:The protocols were approved by the institutional review board of the Dana-Farber/Harvard Cancer Center and of each participating site, as well as by the central committee of the TBCRC.I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals.YesI understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance).Yes I have followed all appropriate research reporting guidelines, such as any relevant EQUATOR Network research reporting checklist(s) and other pertinent material, if applicable.YesAll data produced in the present work are contained in the manuscript.

Background We investigated the impact of gender-affirming testosterone therapy (TT) on breast cancer (BC) risk and tumor progression. Materials and methods 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 Pik3caf/wt n=149 and K14-Cre Brcaf/fTp53f/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). Results 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%). Conclusion 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.Competing Interest StatementThe authors have declared no competing interest.Funding StatementThis work was supported by the National Cancer Institute (NCI) R56CA284564 (YJH and GMW), R21CA267088 (YJH and GMW), R01CA226776 (GMW), P50CA168504 SPORE Career Enhancement Program (YJH), and Columbia Innovation Grant (LCH and KC). GMW is supported by the Breast Cancer Research Foundation 24-177. The University of Virginia Center for Research in Reproduction Ligand Assay and Analysis Core is supported by the NICHD R24HD102061.Author DeclarationsI confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained.YesThe details of the IRB/oversight body that provided approval or exemption for the research described are given below:This study was approved by the BIDMC Committee for Clinical Investigations (2018P000814), Columbia University IRB (AAAT0129), and BIDMC IACUC protocol (#052-2020-23). The Dana Farber/Harvard Cancer Center Office for Human Research Studies IRB determined that the medical records review of the four new invasive BC cases meets the criteria for exemption from IRB review (25-223).I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals.YesI understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance).Yes I have followed all appropriate research reporting guidelines, such as any relevant EQUATOR Network research reporting checklist(s) and other pertinent material, if applicable.YesRNASeqdata are available at the National Center for Biotechnology Information Gene Expression Omnibus (GSE306236). All other data that support the findings of this study are available from Dr. Jan Heng.