Abstract IA17: PARP inhibitor resistance in prostate cancer

Abstract

Abstract Large-scale genomic studies have demonstrated that approximately 15% of metastatic castration-resistant prostate cancer (mCRPC) harbors genetic and epigenetic alterations in homologous recombination repair (HRR) pathway genes. The most commonly altered HRR genes are BRCA2, BRCA1, and ATM, followed by other Fanconi anemia genes including FANCN/PALB2 and FANCA. Loss of HRR causes genomic instability, hyperdependence on alternative DNA repair mechanisms, and enhanced sensitivity to platinum analogues and PARP inhibitors (PARPi). The synthetic lethal interaction with PARPi is being exploited therapeutically in diverse clinical contexts and most notably in BRCA2-deficient mCRPC where the PARPi olaparib has demonstrated improvements in progression-free survival. PARPi resistance has already emerged as a vexing clinical problem for the treatment of BRCA1/2-deficient tumors. The most prevalent mechanism of PARPi resistance is secondary events that cancel the original HRR alteration and restore HRR proficiency. These somatic reversions of mutant BRCA2 alleles can be monitored by sequencing of ctDNA. However, PARPi resistance may also develop without restoration of HRR proficiency via disruption of multiple proteins, such as PTIP or CHD4, that leads to replication fork (RF) stabilization. Importantly, this latter mechanism—namely, the restoration of RF stability—appears to be a highly prevalent mechanism of PARP inhibitor resistance in vitro and in vivo, particularly in tumor cells with an underlying BRCA2 deficiency. Due to their underlying deficiency in BRCA2 and inability to generate RAD51 nucleofilaments, these tumor cells are unable to restore HRR mechanisms. Instead, these cells acquire may PARPi resistance by limiting the nucleolytic degradation of their stalled replication forks. We have recently made the surprising observation that BRCA2-deficient ovarian and breast cancer cells can become resistant to PARPi by downregulating the expression of the polycomb repressive complex PRC2, a methyltransferase complex containing EZH2, SUZ12, EED, and RbAp48. Importantly, downregulation of PRC2 results in the reduced recruitment of the nuclease MUS81 to the RF, thereby providing a novel mechanism of RF protection and PARPi resistance. Whether or not the downregulation of the PRC2/MUS81 pathway is also a mechanism of PARPi resistance in mCRPC remains to be determined. A molecular understanding of PARPi resistance mechanisms may allow the generation of a new class of drugs, or a repurposing of existing drugs, which may reverse this resistance and extend the use of PARPi to more tumor types. Citation Format: Alan D. D’Andrea. PARP inhibitor resistance in prostate cancer [abstract]. In: Proceedings of the AACR Special Conference: Prostate Cancer: Advances in Basic, Translational, and Clinical Research; 2017 Dec 2-5; Orlando, Florida. Philadelphia (PA): AACR; Cancer Res 2018;78(16 Suppl):Abstract nr IA17.