Abstract PR004: Differential coregulator usage mediates Androgen Receptor Splice Variant 7 activity in castration resistant prostate cancer

Abstract Background: Inhibiting androgen receptor (AR) signaling is the standard-of-care for advanced prostate cancer. While this is initially effective, the cancer almost always develops resistance and progresses to the more aggressive castration resistant prostate cancer (CRPC). One major resistance mechanism is the expression of constitutively active AR splice variants, most commonly AR-V7. These variants lack the ligand-binding domain and are intrinsically resistant to current therapies. While its expression has been extensively linked to worse prognosis, how AR-V7 drives CRPC remains unclear. Although AR-V7 binds many of the same genomic loci as full-length AR (AR-FL), it drives a distinct transcriptional program, suggesting that there is a unique molecular mechanism. Methods and Results: To determine the AR-V7 mechanism of action, we quantified endogenous enhancer activity using self-transcribing active regulatory regions sequencing (STARR-Seq) combined with selective knockdown (KD) of either AR-FL or AR-V7. Unexpectedly, no AR-V7 specific enhancer activity was detected across >4000 clinical AR binding sites (ARBS). Supporting these results, we found that endogenous AR-V7 could either repress or support AR-FL activity in different models. In contrast, overexpressed AR-V7 showed strong enhancer activity, though the enhancers activated and the extent of activity varied between cell lines. Given the lack of distinct AR-V7 enhancer activity, we next investigated whether altered coregulator interactions for AR-V7 could explain its divergent activity from AR-FL. Through proximity labeling, we identified high-confidence AR-V7-preferential interacting partners (n=133) including multiple members of the chromatin remodeling PBAF complex. By cross-referencing CRISPR dropout screen data, we found 16 proteins selectively essential for AR-V7 expressing cells, including the PBAF targeting subunit PBRM1. Next, we validated the endogenous PBRM1/AR-V7 interaction, then further mapped the interaction sites and mechanism of gene transcription. Upon PBRM1/AR-V7 KD, we observed significant overlap in the genes they regulate and strong co-localization of AR-V7/PBRM1 binding at the promoters of AR-V7 target genes. Conclusions: Our findings demonstrate that AR-V7 mediated transcription is not solely driven by enhancer activity and suggest that coregulator usage modulates AR-V7 function. Targeting these coregulators offers a novel therapeutic opportunity in CRPC. Citation Format: Pak Lok Ivan Yu, Sila Akdogan, Chia-Chi Flora Huang, Daniel Robinson, Umut Berkay Altintas, Bengul Gokbayrak, Shreyas Lingadahalli, Hans H. Adomat, Tunc Morova, Ugur Meric Dikbas, Kevin Xiao, Colin Collins, Adam Sharp, Nathan A. Lack. Differential coregulator usage mediates Androgen Receptor Splice Variant 7 activity in castration resistant prostate cancer [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: Innovations in Prostate Cancer Research and Treatment; 2026 Jan 20-22; Philadelphia PA. Philadelphia (PA): AACR; Cancer Res 2026;86(2_Suppl):Abstract nr PR004.