Abstract 2509: RB1-loss in castration-resistant prostate cancer reprograms androgen receptor signaling and confers vulnerability to LSD1 inhibition

Background: Retinoblastoma protein (Rb) functions to repress the transcription activity of E2Fs by forming an Rb-E2F repressor complex and the hyperphosphorylation of Rb during G1/S cell cycle transition results in its disassociation with E2Fs, thus inducing E2F-dependent transcription of DNA replication and cell cycle. Genomic RB1-loss is frequently occurred in castration-resistant prostate cancer (CRPC) (~10-15%) and is significantly associated with poor clinical outcomes. RB1-loss is also more frequently found in CRPC with neuroendocrine features (CRPC-NE) (~45-50%). However, it remains unclear on the global activities of Rb-E2F repressor complex functions in modulating transcriptional networks in CRPC. In this study, we performed a comprehensive analysis to functionally characterize the activities of Rb-E2F in multiple CRPC models and aimed to identify vulnerabilities in CRPC with RB1-loss. Methods: C4-2 cells (PTEN-loss, p53 positive) stably infected with doxycycline-inducible lentiviral shRNA against RB1 were generated. VCaP cells (PTEN positive, p53 R248W mut) that transfected with siRNAs against RB1 (siRB) were also examined. The chromatin landscape of Rb-E2F complex was determined by Rb ChIP-seq. Rb-E2F transcriptome was obtained from RNA-seq analysis in the C4-2 stable line treated with/out doxycycline and VCaP cells with siRB. An integrated analysis combines the ChIP-seq and RNA-seq data was also performed to characterize the direct targets of Rb-E2F. Furthermore, we have also assessed the effect of RB1 loss on AR transcriptional program using AR ChIP-seq and RNA-seq in cells treated with/out DHT. Results: Using an integrated cistromic and transcriptomic analysis, we have characterized Rb-E2F activities in multiple CRPC models by identifying the Rb-E2F directly regulated genes. Our results revealed that Rb-E2F has distinct binding sites and targets in TP53-mutated CRPC, and that the expression of Rb-E2F directly repressed genes is increased in the neuroendocrine (NE) subtype of CRPC. Furthermore, we have found that loss of Rb expression redistributes the chromatin binding of androgen receptor (AR), leading to the transcriptional activation of a subset of genes that are increased in CRPC-NE. Remarkably, we have found that the chromatin binding of E2F1 is regulated by an epigenetic factor LSD1/KDM1A (a lysine-specific demethylase that demethylates histone and non-histone proteins) and loss of Rb sensitizes the E2F signaling to the regulation by LSD1. Importantly, LSD1 inhibitor treatment was more effective in CRPC xenograft tumors with RB silencing. Conclusion: Taken together, these results characterized the direct transcriptomic changes in RB1-loss CRPC, indicated a reprogramming of AR signaling by loss of RB1, and suggested LSD1 as a potential therapeutic target in RB1-loss CRPC or CRPC-NE by modulating chromatin binding of E2Fs. Citation Format: Wanting Han, Mingyu Liu, Dong Han, Muqing Li, Anthia A. Toure, Susan C. Patalano-Salsman, Jill A. Macoska, Shuai Gao, Changmeng Cai. RB1-loss in castration-resistant prostate cancer reprograms androgen receptor signaling and confers vulnerability to LSD1 inhibition [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 2509.