Abstract B018: Impact of 13q.14 loss in prostate cancer progression and sensitivity to targeted agents

Abstract Purpose of the study: Chromosomal aberrations are frequent in cancer and play a pivotal role in disease initiation and progression. In prostate cancer (PCa), deletions at chromosome 13q14 are common and enriched in advanced disease, suggesting selective advantage in tumor progression. Beyond BRCA2, this region harbors other tumor suppressor genes, including RB1 and RNASEH2B, which impact sensitivity to androgen receptor inhibitors (ARi) and PARP inhibitors (PARPi). This study aims to elucidate the effect of individual vs concomitant loss of RB1 and RNASEH2B in response to dual AR and PARP inhibition. Methods: To explore the prevalence of deletions in 13q we interrogated copy number alterations in the Hartwig Medical Foundation (HMF) PCa WGS dataset (n=486). In vitro, we generated CRISPR-Cas9-mediated knock-outs (KO) of RB1, RNASEH2B, or both genes in PCa cell lines (LNCaP, C4-2, 22Rv1, DU145). Gene editing was confirmed by Sanger, and western blot analysis validated protein loss. Antitumor activity of enzalutamide (ARi), and olaparib or saruparib (PARPi) was determined using CCK8 kit and colony formation assays. Results: RB1 deletions were present in 42% of HMF PCa samples (205/486), being mostly heterozygous deletions (200/205). RB1 deletions co-occurred with RNASEH2B loss in 94% of cases (192/205) and with BRCA2 deletions in 42% of cases (87/205). In vitro, RB1-KO increased resistance to ARi and PARPi in hormone-sensitive lines LNCaP and C4-2, while no change in sensitivity was detected in AR-independent models 22Rv1 and DU145. Loss of RNASEH2B increased sensitivity to PARPi in all models. Next, we generated RB1 and RNASEH2B double KO models in LNCaP and C4-2 by sequentially editing single KO populations. After single-cell seeding, we obtained: (1) RB1-KO (homozygous) clones with complete protein loss; (2) RNASEH2B-heterozygous (RNASEH2B-Het) models with partial protein expression; and (3) RB1-KO/RNASEH2B-Het models. No clones with complete RNASEH2B loss were obtained, suggesting that RNASEH2B-loss population is transient and that clones with complete loss do not proliferate. RB1-KO models maintained resistance to both treatments. RNASEH2B-Het cells showed increased sensitivity to PARPi, although to lower extent than the population with complete RNASEH2B loss. Importantly, introducing RB1-loss in RNASEH2B-Het cells restored PARPi resistance, and combined AR/PARP inhibition did not overcome resistance in RB1-KO or RB1-KO/RNASEH2B-Het cells. Conclusions: Our findings indicate that 13q14 deletions in PCa predominantly occur as heterozygous events encompassing key genes influencing ARi and PARPi responses. Heterozygous loss of RNASEH2B confers sensitivity to PARPi, although to a lesser extent than in models with complete RNASEH2B loss. RB1-loss confers higher resistance to ARi and PARPi, restoring resistance to PARPi in RNASEH2B heterozygous models. These results highlight the need for models that better mirror the heterozygous nature of 13q14 deletions to clarify their biological effects and refine therapeutic strategies for advanced PCa. Citation Format: Victor Esquefa, Pablo Cresta Morgado, Richard Norris, Lara de Llobet, Julian Brandariz, Sara Arce-Gallego, Teresa Casals, Manuel Ramos, Daniel Aguilar, Gisela Mir, Irene Casanova-Salas, Francisco M. Barriga, Joaquin Mateo. Impact of 13q. 14 loss in prostate cancer progression and sensitivity to targeted agents 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 (2Suppl): Abstract nr B018.