Abstract IA016: Epigenetic crosstalk associated with prostate cancer lineage plasticity
Authors: Rickman David , Singh Richa , Venkadakrishnan Varadha Balaji , Yamada Yasutaka , Beltran Himisha 
Published: June 2023

Abstract
Background: 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 plasticity, a process by which differentiated cells lose their identity and acquire alternative lineage programs, has recently been identified as an emerging mechanism of resistance to targeted therapies in prostate cancer. This plasticity can manifest as histologic transformation from an AR-positive prostate adenocarcinoma to neuroendocrine prostate cancer (NEPC). NEPC is clinically aggressive and prognosis is poor. 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. Methods: To model DNA methylation program associated with the transition from CRPC to NEPC, we performed reduced-representation bisulfite sequencing (RRBS) in a variety of clinically relevant models including a novel genetically engineered mouse model and patient-derived organoids/xenografts (PDO/X). We performed genetic (CRISPR/Cas9 technology) and pharmacologic inhibition of DNA methylation writers (DNMTs), erasures (TETs) and polycomb repressive complex 2 histone methyltransferase EZH2 to query their role in the establishment of the DNA methylation program. To assess EZH2 enzymatic activity, we performed H3K27me3 ChIP-seq in our murine and human models. Results: The transcriptome from adenocarcinoma or NEPC foci in our murine model closely resembled clinical CRPC or NEPC transcriptome, respectively. We also found increased expression of DNA methylation regulators (DNMT1/3B and TET1) in NEPC foci. A clinically relevant methylome signature based on murine model was found to segregate clinical samples into CRPC and NEPC subtypes which was also validated in androgen-deprived isogenic LNCaP cells as well as PDO/Xs. Interestingly, AR-target genes (e.g., FKBP5 and TMPRSS2) were found to be hypermethylated and downregulated while cell fate decision regulators (e.g., FOXA2 and SLIT2) and EZH2 targets were hypomethylated and upregulated in NEPC foci. While we found that genetic or pharmacologic inhibition of DNA methylation writers and erasures impacted the NEPC-associated DNA methylation program, inhibition of EZH2 resulted in significant alterations to the DNA methylation program. In turn, we also found that that DNMT inhibition led to equally significant alterations to EZH2 activity. Conclusion: Our data suggest that there is an interplay between EZH2 and DNA methylation that is associated with the molecular reprogramming associated with the progression from AR-positive adenocarcinoma towards NEPC as well as for the maintenance of the NEPC molecular program. More mechanistic studies are now needed to fully characterize this interplay and to determine if we can harness this synergy for therapeutic potential.
Citation Format: David Rickman, Richa Singh, Varadha Balaji Venkadakrishnan, Yasutaka Yamada, Himisha Beltran. Epigenetic crosstalk associated with prostate cancer lineage plasticity [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 IA016.