Abstract 1650: Non-invasive imaging tool to predict BRCA2 silencing in the context of prostate cancer
Authors: Kalidindi Teja , Lee Sang Gyu , Schoder Heiko , Jehanae Lina E. , Hirani Rahim , Chakraborty Goutam , Kantoff Philip W. , Pillarsetty Naga Vara Kishore 
Published: September 2020

Abstract
Background: We recently demonstrated that biallelic as well as monoallelic loss of the DNA damage response (DDR) gene BRCA2 in prostate cancer (PC) cell lines and mCRPC organoids leads to an aggressive form of PC and early androgen deprivation therapy (ADT) resistance (Chakraborty et al.). Clinical studies have established that DDR deficiency is generally associated with a poor prognosis. Very recently, immunohistochemical analysis of mCRPC patient samples by Paschalis et al. revealed that defects in DDR genes (in particular BRCA2 and ATM) are associated with increased prostate-specific membrane antigen (PSMA; folate hydrolase, FOLH1) expression on the cell membrane. PSMA expression can be measured non-invasively in pre-clinical models and human subjects using one of the several positron emission tomography (PET) imaging agents such as [68Ga]-PSMA11 or [124I]-MSK-PSMA11 that are being evaluated in pre-clinical and/or clinical setting. Therefore, we hypothesized that upregulation of PSMA expression can be a marker for BRCA2 loss and this increased expression can be quantified using PET agents both in vitro and in vivo.
Experimental design: We investigated the effect of BRCA2 deletion on PSMA expression in the castration sensitive human PC cell line LNCaP at the transcriptional and translational level and quantified the changes using saturation binding assays with [124I]-MSK-PSMA11. Using CRISPR-Cas9 and RNAi-based methods, we silenced BRCA2 in the castration sensitive cell line LNCaP and evaluated its effect on PSMA at the transcriptional and translational level. We carried out saturation binding assay using [124I]-MSK-PSMA11 to measure changes in cell surface PSMA receptor density.
Results: BRCA2 knockout was achieved successfully using CRISPR-Cas9 based methods. Immunoblotting analysis revealed that BRCA2 loss resulted in a significant increase in PSMA levels when compared to control LNCaP cell line. Immunohistochemical analysis confirmed this obervation. Cell binding assays demonstrated that BRCA2 null LNCaP cell lines have about 5-6 fold higher uptake of the PET tracer [124I]-MSK-PSMA11. We will be conducting in vivo studies to demonstrate that BRCA2 deletion leads to a significant increase in PSMA signal in mice xenograft models.
Conclusions: Our results indicate that BRCA2 silencing leads to significant upregulation of PSMA expression in PC cell lines, which can be imaged using a PSMA targeted PET tracer. These studies were partly supported by DOD-PCRP-Grant # W81XWH-19-1-0536 and PCF Young Investigator Award to Goutam Chakraborty. References: Chakraborty G et al. Clin Can Res 2019 DOI:10.1158/1078-0432.CCR-19-1570; Paschalis A et al. Eur Urol. 2019 Oct; 76(4): 469-478.
Citation Format: Teja Kalidindi, Sang Gyu Lee, Heiko Schoder, Lina E. Jehanae, Rahim Hirani, Goutam Chakraborty, Philip W. Kantoff, Naga Vara Kishore Pillarsetty. Non-invasive imaging tool to predict BRCA2 silencing in the context of prostate cancer [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 1650.