Abstract B037: N-Myc driven cell plasticity in castrate-resistant prostate cancer

An emerging concept of anti-AR therapy resistance is the induction of epithelial plasticity in castration-resistant prostate adenocarcinoma (CRPC) cells that eventually evolve to a neuroendocrine phenotype (e.g., low to no AR signaling and expression of neuroendocrine markers). Neuroendocrine prostate cancer (NEPC) is clinically aggressive and carries a poor prognosis with an average survival of less than one year. We and others have identified and validated new therapeutic targets and drivers of cell transformation from CRPC to NEPC. These include induction of MYCN (encodes N-Myc) or RB1/TP53 loss of function. While N-Myc is a bona fide driver oncogene in several rare tumor types, the N-Myc-driven molecular reprogramming and acquisition of cell plasticity remains poorly understood. We have previously shown that intact N-Myc+/Pten-null genetically engineered mice (GEM) develop poorly differentiated, highly proliferative, invasive prostate cancer that is molecularly similar to human NEPC tumor (1). Similar morphologic features were observed in organoids derived from our GEM model. Flow analysis of these organoids revealed that N-Myc+/Pten-null GEM-derived organoids are comprised of a heterogeneous population of cells with luminal, basal, or features of both lineages compared to more homogeneous population of cells from Pten-null control organoids. Recently, we also found that N-Myc overexpression induced a faster recurrence in the context of castration in vivo, based on the GEM model and 22Rv1 isogenic xenografts. This is consistent with the significant N-Myc induced increased castrate-resistant tumor growth based on data from GEM organoids grown in vivo as allografts. In the GEM, we found that castration led to invasive prostate tumors that metastasize to multiple locations including the liver and that express the EMT marker vimentin and the NEPC marker neural cell adhesion molecule (NCAM1). Based on RNAseq data from 22Rv1 xenografts in castrated mice, we identified stem cell signatures including neural crest stem genes (GSEA q value = 7.0E-19) that were significantly enriched in the N-Myc upregulated genes including SOX2, SOX10, and SOX11, genes implicated in neural development and NEPC associated genes (e.g., NCAM1 and chromogranin A [CHGA]). N-Myc ChIP-Seq data confirmed N-Myc occupancy enriched at promoters of genes implicated in stem cell-associated pathways. RNAseq data from human NEPC organoid following MYCN depletion revealed similar N-Myc-associated gene expression changes. Altogether, these data led to a model in which N-Myc overexpression induces a castrate-resistant, lineage-plastic phenotype that would give rise to NEPC, consistent with the phenotype induced by RB1/TP53/PTEN loss [2-4]. References: 1. Dardenne E., et al. N-Myc induces an EZH2-mediated transcriptional program driving neuroendocrine prostate cancer. Cancer Cell 2016:30(4):563-77. 2. Ku SY, et al. Rb1 and Trp53 cooperate to suppress prostate cancer lineage plasticity, metastasis, and antiandrogen resistance. Science 2017:355(6320):78-83. 3. Mu P, et al., SOX2 promotes lineage plasticity and antiandrogen resistance in TP53- and RB1-deficient prostate cancer. Science 2017;355(6320):84-8. 4. Zou M, et al. Transdifferentiation as a mechanism of treatment resistance in a mouse model of castration-resistant prostate cancer. Cancer Discov 2017. Citation Format: Etienne Dardenne, Adeline Berger, Katie Gayvert, Loredana Puca, Jamal Elkader, Brian Robinson, Olivier Elemento, Beltran Himisha, David Rickman. N-Myc driven cell plasticity in castrate-resistant prostate cancer [abstract]. In: Proceedings of the AACR Special Conference: Prostate Cancer: Advances in Basic, Translational, and Clinical Research; 2017 Dec 2-5; Orlando, Florida. Philadelphia (PA): AACR; Cancer Res 2018;78(16 Suppl):Abstract nr B037.