PROJECT SUMMARY Prostate cancer (PCa) is the most commonly diagnosed cancer among American men and metastatic Castration- Resistant Prostate Cancer (mCRPC) is the mostly deadly form of PCa. mCRPC is currently treated with AR targeted therapy, however resistance to the current therapies for mCRPC is unfortunately inevitable, underscoring the critical demand for elucidating the mechanisms of resistance and identifying actionable therapeutic targets to overcome resistance. We have identified the loss of Synaptotagmin Binding Cytoplasmic RNA Interacting Protein (SYNCRIP) as a top candidate conferring therapy resistance through a in vivo library screening examining genomic alterations which confer resistance. We discovered that the frequent deletion of SYNCRIP, occurring in approximately 14% of patients, is strongly associated with poor clinical outcomes of mCRPC patients receiving AR targeted therapy. Our data has demonstrated that depletion of SYNCRIP promotes resistance to AR targeted therapy both in vitro and in vivo. Mechanistically, we found significantly upregulated APOBEC-mediated DNA mutation patterns in both PCa cell lines and mCRPC patients with SYNCRIP depletion. The DNA deaminase APOBEC1, presumably sequestered by SYNCRIP, is required and sufficient for the AR targeted therapy resistance observed in SYNCRIP-depleted tumor cells. Thus, the overall objectives of this application are to define the molecular role of SYNCRIP in PCa tumorigenesis and AR targeted therapy resistance, and to elucidate the molecular mechanisms through which the loss of this protein promotes tumorigenesis and resistance, using human PCa cells, organoids, and mouse models. We proposed to elucidate the role of this putative tumor suppressor gene by testing the following central hypothesis: Loss of SYNCRIP confers resistance to AR targeted therapy through the ectopic APOBEC-mediated mutagenesis of key oncogenes and the activation of downstream signaling pathways. We will examine this hypothesis by pursuing three specific aims: In Aim 1, we will define whether SYNCRIP depletion promotes PCa tumorigenesis, including both primary PCa and mCRPC, and AR targeted therapy resistance using multiple human PCa cell lines, organoids, and xenograft models. We will also examine whether SYNCRIP depletion adjusts AR dependency and lineage plasticity. In Aim 2, we will elucidate the role of APOBEC-mediated mutagenesis in mediating AR targeted therapy resistance in cells with SYNCRIP depletion, and identify the mutated resistant driver genes. In Aim 3, we will determine the role of SYNCRIP in the initiation, progression, and response to AR targeted therapy of primary PCa using various mouse models. These proposed aims will capitalize on the expertise of our lab and of our collaborators to comprehensively define the role of SYNCRIP in PCa tumorigenesis and therapy resistance, and to elucidate the molecular mechanism through which the loss of this tumor suppressor confers resistance. The...