Abstract 887: N-Myc drives neuroendocrine prostate cancer

Emerging observations from clinical trials suggest that a subset of castration resistant prostate adenocarcinomas (CRPC) eventually evolve or progress to a predominantly neuroendocrine phenotype (NEPC). This transition is emerging as an important mechanism of treatment resistance. This cell plasticity is characterized by loss of androgen receptor (AR) and prostate specific antigen (PSA), and significant over-expression and gene amplification of MYCN (encoding N-Myc). While N-Myc is a bona fide driver oncogene in several rare tumor types, the molecular mechanisms that underlie N-Myc driven NEPC have yet to be characterized. Integrating a novel genetically engineered mouse (GEM) model of prostate specific N-Myc overexpression, human prostate cancer cell line modeling, and human prostate cancer transcriptome data, we found that N-Myc over-expression leads to the development of poorly differentiated, invasive prostate cancer that is molecularly similar to human NEPC tumors. To determine if N-Myc plays a causal role in driving the NEPC phenotype, we generated GEM lines that carry a CAG-driven lox-stop-lox human MYCN gene integrated into the ROSA26 (LSL-MYCN) locus and either a Tmprss2 driven tamoxifen-activated Cre recombinase (T2-Cre) or probasin (Pb)-Cre. Since PTEN deletion is a frequent alteration in CRPC and PI3K/AKT signaling can enhance N-Myc protein stability we also engineered the mice with a floxed Pten locus. N-Myc over-expression in the context of Ptenf/+ at 3 months post-induction leads to focal mouse high-grade prostatic intraepithelial neoplasia (mHGPIN). T2-Cre; Ptenf/f; LSL-MYCN+/+ mice develop highly proliferative, diffuse mHGPIN which consists of proliferations of cells with nuclear atypia that expand the glands, imparting irregular borders and inducing a mild stromal response, mitotic figures, and incipient necrosis. RNAseq data from N-Myc these mHGPIN lesions show they are molecularly similar to NEPC based on RNAseq data from 203 human CRPC and NEPC samples. At 6 months, Pb-Cre; Ptenf/f; LSL-MYCN+/+ mice develop poorly differentiated, highly proliferative, invasive prostate cancer. Based on the RNAseq data from the N-Myc GEM line, GEM-derived mouse prostate cancer organoid cultures and isogenic cell lines, we found that N-Myc regulates a specific NEPC-associated molecular program that includes a repression of AR signaling, enhanced AKT signaling and repression of Polycomb Repressive Complex 2 target genes. We further showed that N-Myc inter