PROJECT SUMMARY- PROJECT 2 Castration resistant prostate cancer (CRPC) is a lethal disease. Epigenetic dysregulation, including overexpression of the enhancer of zeste homolog 2 (EZH2) drives treatment resistance, and EZH2 inhibitors are in clinical trials. Beyond its canonical role as a member of the PRC2 complex as a transcriptional repressor, we previously identified a non-canonical role of EZH2 in activating AR signaling, as well as in driving lineage plasticity and neuroendocrine prostate cancer. Current data suggests that single agent activity may be limited and that combination strategies should be pursued. We recently discovered a novel interaction between EZH2 and DNA repair processes and a synergy between EZH2 and PARP inhibition in prostate cancer. We hypothesize that EZH2 drives downstream molecular programs through canonical and non-canonical mechanisms, and that these downstream effects are context dependent. Combination therapy with EZH2 and PARP inhibition will therefore have differential biologic impact in CRPC based on the underlying genomic, epigenomic, and phenotypic context. We will elucidate the mechanisms by which EZH2 regulates DNA repair and define how DNA repair pathways impact EZH2 drug sensitivity. We will evaluate the biologic and clinical impact of EZH2 and PARP inhibition across the heterogeneous spectrum of CRPC through extensive preclinical and clinical studies. We will conduct a first-in-field, investigator-initiated clinical trial of EZH2i (tazemetostat) plus PARPi (talazoparib) with extensive translational correlates. Results from this project will provide the basis for a new therapeutic strategy for men with CRPC.