Abstract 976: The regulation of FOLH1/PSMA in prostate cancer
Authors: White Margaret E. , Pranzatelli Thomas , Moore Xavier , Bowman Joel , Chiorini Jay , Choyke Peter , Kelly Kathy 
Published: April 2023

Abstract
Prostate Specific Membrane Antigen (PSMA) is a prominent biomarker in Prostate Cancer (PC) and has evolved into a useful target for both imaging and therapy. But how PSMA becomes overexpressed upon PC development, its modulation by androgen deprivation therapy (ADT), and its absence in neuroendocrine-differentiated PC is not well understood. The exact mechanism by which PSMA is regulated has not been defined. Understanding PSMA regulation in PC patients for diagnostic, prognostic, and treatment purposes would be extremely valuable and could be exploited for PSMA-targeted therapies. To address this, we sought to characterize the epigenetic landscape of FOLH1 (the gene for PSMA) and determine the regulatory factors involved in FOLH1 transcription. Based on histone marker and protein ChIP-Seq data sets alongside ATAC-seq, we found that FOLH1 is a Super Enhancer (SE) and is collectively bound by an array of transcription factors (TFs), including AR, FOXA1, and HOXB13. We developed a hidden Markov model (HMM) with seven chromatin states built on average TF binding in the FOLH1 landscape and trained on the FOLH1 gene and known enhancer regions. Chromatin states were annotated based on features in the FOLH1 gene. With a simple regression model and k-fold cross-validation, mean occupancy of the three TFs and H3K27Ac in each state accurately predicts FOLH1 expression (ρ &gt; 0.9) across PC samples. To assess the functionality of the enhancer regions predicted by our computational HMM, we performed a CRISPR interference (CRISPRi) screen with a nuclease dead-Cas9 protein conjugated to the KRAB transcriptional repressor and directed by a pool of gRNAs that tile across the AR protein-bound regions within the FOLH1 landscape. A sliding window analysis of our CRISPRi screen revealed that the top scoring gRNAs delineate 5 Regulatory Regions (RRs) found within the SE region of FOLH1 that control FOLH1/PSMA expression. Motif identification in these RRs have provided a list of potential TF candidates involved in regulating FOLH1, and knockdown experiments are currently being conducted to evaluate individual and combinations of TF contribution to FOLH1 expression. There is evidence for a constellation of TFs responsible for regulating FOLH1, and context-specific modifications in FOLH1 regulation will need to be evaluated in various models and conditions. These results suggest FOLH1 is regulated by epigenetic control in the SE region and may be altered in progression or with ADT. Characterizing the regulation of FOLH1/PSMA will result in a deeper understanding of the meaning of alterations in uptake on PSMA imaging and may enable some degree of control over expression, thus aiding in PSMA-directed treatments.
Citation Format: Margaret E. White, Thomas Pranzatelli, Xavier Moore, Joel Bowman, Jay Chiorini, Peter Choyke, Kathy Kelly. The regulation of FOLH1/PSMA in prostate cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 976.