Abstract B017: Lineage-specific PRC2 targets and response to EZH2 inhibition in neuroendocrine prostate cancer

Abstract Lineage plasticity is an emerging mechanism of treatment-resistance in prostate cancer, which can manifest clinically as histologic transformation from prostate adenocarcinoma to small-cell neuroendocrine prostate cancer (NEPC). NEPC is associated with poor prognosis, and new treatment strategies are urgently needed. Epigenetic dysregulation has been implicated as a key driver of prostate cancer lineage plasticity, mediated in part by EZH2. EZH2 is a component of the polycomb repressive complex (PRC2) regulating the transcriptional repressive mark H3K27me3. EZH2 also controls cell-fate determination during normal development via bivalent promoters bearing both repressive H3K27me3 and active H3K4me3 marks. EZH2 is targetable, and several EZH2 inhibitors are in clinical development; these trials are currently in biomarker-unselected prostate cancer populations. A thorough understanding of the mechanism of action of EZH2 is imperative for predicting optimal response to EZH2i and for the development of robust combination treatment strategies. Cell viability studies across a diverse panel of prostate cancer models representing adenocarcinoma and NEPC indicated variable responses to tazemetostat (EZH2i) treatment. The castration-resistant prostate adenocarcinoma (CRPC) cell line LNCaP-abl showed a greater than 50% reduction in cell viability upon EZH2i (5uM) while LNCaP and 22Rv1 showed moderate response with 15-25% reduction in cell viability. However, none of our 5 NEPC patient-derived models showed significant response to EZH2i which was also validated in vivo. Hierarchical clustering of RNA-seq profiles of responder and non-responder models revealed distinct cell-type specific EZH2 targets and clusters of genes that may be associated with response to EZH2i. Approximately 15% of the EZH2 targets in the NEPC organoid WCM154 carried bivalency in their promoters including neuronal lineage determining genes such as ASCL1 and LHX2. Gene set enrichment analysis of EZH2 targets bearing bivalency in NEPC revealed association with neurogenesis and cell proliferation pathways. Further upregulation of neuronal-lineage PRC2 targets in NEPC models upon EZH2i may provide justification for lack of lineage reversal and maintenance of terminal differentiation. While there is evidence that EZH2i leads to lineage reversion in adeno-to-NEPC transitioning preclinical models, our data shows that EZH2i was associated with modest response in NEPC models and did not revert lineage plasticity. Our results provide insights into a potential role for bivalent promoters in EZH2-mediated lineage reprogramming and sustenance of terminal differentiation upon EZH2i in NEPC. Understanding lineage-specific action of EZH2i may inform biomarkers in ongoing clinical trials. Genes and pathways dysregulated because of their bivalency might also be exploited as potential candidates to be co-targeted with EZH2i. Citation Format: Varadha Balaji Venkadakrishnan, Richa Singh, Yasutaka Yamada, Kei Mizuno, Adam G. Presser, Sheng-Yu Ku, Henry Long, David Rickman, Himisha Beltran. Lineage-specific PRC2 targets and response to EZH2 inhibition in neuroendocrine prostate cancer [abstract]. In: Proceedings of the AACR Special Conference: Advances in Prostate Cancer Research; 2023 Mar 15-18; Denver, Colorado. Philadelphia (PA): AACR; Cancer Res 2023;83(11 Suppl):Abstract nr B017.