Concurrent deletion of BRCA2 and RB1 and aggressive prostate cancer.

Abstract

241 Background: Pathogenic variants of BRCA2 have been observed in a substantial subset of men with metastatic castration resistance prostate cancer (mCRPC). Prostate cancer (PC) patients with germline mutations of BRCA2 experience more rapid progression of their localized PC to mCRPC. This stands in contrast to other cancers where BRCA2 alterations do not appear to be associated with a worse prognosis. We identified homozygous and hemizygous deletions of BRCA2 in a subset of primary PC, which had been previously unrecognized. BRCA2 deletion in PC more frequently co-exists with RB1 deletion rather than alone. BRCA2-RB1 co-deletion in primary PC (TCGA and Taylor cohort) is associated with a shorter disease free survival and increased genomic instability in patients, indicating that BRCA2-RB1 null tumors are likely very aggressive in nature. Methods: To determine the underlying molecular and genomic consequences of BRCA2- RB1 loss, we CRISPR/shRNA-out these genes from human PC cell lines and subjected them to various in vitro assays, RNA-seq and kinase arrays. We applied a 3-color FISH assay to identify the deletion of BRCA2 and RB1 in PC. Results: BRCA2-RB1 null LNCaP cells exhibit androgen independence as evidenced by relative resistance to enzalutamide, and increased growth in absence of androgen but show enhanced sensitivity towards PARPi or platinum. Moreover, the null cell induces an aggressive EMT like phenotype, which is associated with enhanced migration and invasion. RNA-seq and array results show significant activation of EMT related signaling pathways including an unexpected activation of WNK1 upon co-deletion of BRCA2-RB1. FISH assay revealed significant co-deletion of BRCA2-RB1 in ADT resistant aggressive PC tumor cells. More importantly these cells also show greater sensitivity towards PARPi or platinum. Conclusions: Our finding suggests that concurrent deletion of BRCA2-RB1 is most likely is a driver of therapy resistant aggressive PC rather than the consequence of exposure to therapy. We propose that screening for BRCA2-RB1 deletion early could be implemented to identify those at highest risk of aggressive PC and provide an opportunity for early intervention and alternative treatments.