PROJECT SUMMARY/ABSTRACT Prostate cancer (PCa) is a clinically and molecular heterogenous disease, with biologically distinct subtypes driven by characteristic genomic alterations in both early, untreated disease and treatment resistant castration- resistant prostate cancer (CRPC). However, whether early alterations affect fidelity to prostate lineage, shape the specific resistance patterns that emerge with treatment, and the underlying mechanisms, remain unclear. We hypothesize that specific molecular features of early, untreated PCa establish distinct pathways to progression and therapeutic resistance through transcriptional control and fidelity to luminal prostate lineage. Preliminary data generated by our multi-institutional, multidisciplinary, collaborative group suggest specifically that the subclass of PCa defined by recurrent mutations in SPOP maintain strong fidelity to prostate lineage, and are therefore preferentially reliant on androgen receptor (AR) signaling and possibly resistant to conversion to AR-indifferent subtypes of CRPC. The overall objective of this proposal is to define the propensity of prostate cancers harboring SPOP mutations to progress to specific subtypes of treatment resistant CRPC, and to define the mechanisms that shape these resistance patterns. Using novel models and human prostate cancer samples, our preliminary data demonstrate that SPOP mutation reprograms AR function, altering chromatin accessibility and transcription driven by AR and making these cancers highly reliant on AR activity. In contrast, N-Myc induction combined with RB1-loss is a strong driver of AR-indifferent CRPC that functions to rewire and deactivate the AR transcriptional program. Our central hypothesis is that opposing effects on rewiring of the AR-directed epigenomic and transcriptional programs mediate the downstream impact on biology and therapeutic sensitivity shown with SPOP mutation and drivers of AR-indifferent disease. This project will elucidate the