Abstract B011: PRAC1 epigenetic silencing in castration resistant prostate cancer and its novel role in androgen receptor biology
Authors: Low Jin-Yih , Esopi David M. , Lim Yiting , Vaghasia Ajay M. , Tsai Harrison , Zheng Qizhi , Hicks Jessica , Wyhs Nicolas , Lotan Tamara L. , Isaacs William B. , DeMarzo Angelo M. , Nelson William G. , Hsieh Andrew C. , Yegnasubramanian Srinivasan , Haffner Michael C. 
Published: June 2023

Abstract
Background Resistance to androgen receptor (AR) directed therapies is one of the key drivers of prostate cancer mortality. Although several molecular alterations involved in castration resistant prostate cancer (CRPC) have been catalogued, a deep mechanistic understanding of the diverse resistance pathways operative in CRPC is still lacking. Here, we describe the role of a previously uncharacterized gene, PRAC1 in treatment resistance and the mechanism that involves epigenetic silencing of the AR co-regulator PRAC1. Methods We applied a reductionist approach by performing whole genome methylation analyses using MBD-seq on paired androgen dependent and castration resistant cell lines to identify candidate gene loci with differential methylation in CRPC. Results from this screen were validated in CRPC cohorts. In vitro and in vivo loss and gain of function experiments were used to delineate molecular interactions, phenotypic and transcriptomic alterations. Results We demonstrate that the PRAC1 gene locus, which encodes for a 6 kDa peptide, frequently undergoes CpG methylation mediated epigenetic silencing. Loss of PRAC1 expression was tightly associated with resistance to AR directed therapies and was present in ~30% of advanced CRPC but not in hormone naïve tumors. Furthermore, tumors with PRAC1 loss showed a significantly shorter time to progression on AR signaling inhibitors (ARSI). Mechanistically, PRAC1 loss allowed for prostate cancer cell growth in the absence of androgens or presence of ARSIs, suggesting that PRAC1 is a key mediator of resistance. Importantly, PRAC1 binds to the AR and was necessary for efficient AR transactivation and using biochemical methods, we showed that these two molecules interact at the molecular level. Additionally, PRAC1 loss dampened canonical AR signaling but induced an AR dependent pro-proliferative expression program. Restoring PRAC1 expression in ARSI resistant models re-established sensitivity to AR directed therapies. Conclusion Our data suggests that PRAC1 is a novel AR co-regulator that undergoes epigenetic silencing in CRPC. Loss of PRAC1confers resistance to AR directed therapies. Restoring PRAC1 function in CRPC could represent a novel therapeutic approach in advanced CRPC.
Citation Format: Jin-Yih Low, David M. Esopi, Yiting Lim, Ajay M. Vaghasia, Harrison Tsai, Qizhi Zheng, Jessica Hicks, Nicolas Wyhs, Tamara L. Lotan, William B. Isaacs, Angelo M. DeMarzo, William G. Nelson, Andrew C. Hsieh, Srinivasan Yegnasubramanian, Michael C. Haffner. PRAC1 epigenetic silencing in castration resistant prostate cancer and its novel role in androgen receptor biology [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 B011.