Abstract IA15: PARP inhibitor resistance and acquired vulnerability in breast cancer

Large-scale genomic studies have demonstrated that some breast cancers, especially triple-negative breast cancers (TNBCs), harbor genetic and epigenetic alterations in homologous recombination repair (HRR) pathway genes. The most commonly altered HRR genes are BRCA1 and BRCA2, followed by other Fanconi anemia genes including FANCN/PALB2, FANCO/RAD51, FANCJ/BRIP, and FANCA. Loss of HRR causes genomic instability, hyperdependence on alternative DNA repair mechanisms, and enhanced sensitivity to platinum analogues, topoisomerase inhibitors, and PARP inhibitors (PARPi). The synthetic lethal interaction with PARPi is being exploited therapeutically in diverse clinical contexts and most notably in ovarian cancer where the PARPi olaparib is FDA approved for use in patients with germline BRCA1/2 mutations. PARP inhibitor 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. However, PARPi resistance may still 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 PARP inhibitor resistance by limiting the nucleolytic degradation of their stalled replication forks. We have recently made the surprising observation that BRCA2-deficient tumors 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. A molecular understanding of PARP inhibitor 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 PARP inhibitors to more tumor types. Citation Format: Alan D. D’Andrea. PARP inhibitor resistance and acquired vulnerability in breast cancer [abstract]. In: Proceedings of the AACR Special Conference: Advances in Breast Cancer Research; 2017 Oct 7-10; Hollywood, CA. Philadelphia (PA): AACR; Mol Cancer Res 2018;16(8_Suppl):Abstract nr IA15.