Project Summary/Abstract Prostate cancer arises as an androgen driven disease and therefore androgen receptor (AR) targeting therapies have been a major focus of prostate cancer treatment. Lineage lose mechanism cancer histologic transformation from an AR-positive prostate adenocarcinoma to an AR-negative small cell carcinoma that expresses neuroendocrine markers, often referred to as neuroendocrine prostate cancer (NEPC). NEPC is clinically aggressive and prognosis is poor. Therefore, effective treatment for NEPC patients remains an unmet clinical need. A thorough molecular understanding of NEPC progression is needed for the development of effective treatments for this lethal disease. Although NEPC tumors arise clonally from prostate adenocarcinoma and share genomic alterations, there is significant epigenetic deregulation during the transformation process. However, mechanistically, we still do not know how these epigenetic alterations arise and how best to leverage these alterations as a therapeutic opportunity. Our preliminary and published data from in vivo, in vitro and ex vivo models (NEPC-patient-derived organoids) suggest that the N-Myc transcriptome and cistrome is androgen-dependent and drives epithelial plasticity and the acquisition of clinically-relevant, NEPC molecular program and a reprogramming of the epigenome. Most recently, based on data from a new genetically engineered mouse model (GEMs), we found that N-Myc induction synergizes with RB1 loss to deregulate DNA methylation readers, writers (e.g. DNMT1 and DNMT3B) and erasures (e.g. TET1). Interestingly, we and others have shown that specific molecular or pharmacological interventions can revert NEPC phenotype to a luminal—more clinically manageable—adenocarcinoma phenotype. Our over-arching hypothesis, which is based on our published and preliminary data, is that specific molecular alterations (e.g. MYCN induction/RB1 loss) in prostate cancer cells drive lineage plasticity through epigenetic reprogramming (i.e., DNA methylation) as a mechanism of resistance to anti-AR therapy and this leads to transformation to NEPC. To address this hypothesis, we will employ patient-derived organoids and xenograft and novel genetically engineered mouse models to elucidate the role and specificity of DNMTs/TET1 in establishing the NEPC-related DNA methylation program (Aim 1), characterize the upstream regulation of DNMTs expression in the progression to NEPC (Aim 2) and to assess the therapeutic potential of DNMTs inhibition alone or in combination with AR targeted therapy to block the transition to or maintenance of NEPC (Aim 3). Successful completion of these Aims will provide unique insights into NEPC development, identify key and potential targetable mediators of lineage plasticity, and provide rationale for future clinical strategies to target the underlying epigenetic mechanisms that drive the transition from prostate adenocarcinoma to NEPC. plasticity, a process by which differentiated ...