Abstract 3893: Mechanism of radiosensitization by inhibitors of poly(ADP-ribose) polymerase (PARP)

Abstract

Abstract Inhibitors of poly(ADP-ribose) polymerase (PARP) are in clinical trials for cancer therapy based on specific lethality to cancer cells deficient in BRCA1 or BRCA2. In addition, pre-clinical work has shown that PARP inhibition induces radiosensitization in cancer cells. To provide a basis for the clinical application of PARP inhibitors as radiosensitizers, we conducted mechanistic studies to elucidate the pathways by which PARP inhibitors influence radiation response. Cells treated or not with PARP inhibitors and treated or not with siRNAs or other reagents to manipulate gene expression were assayed for radiation response by clonogenic survival assays. DNA double strand break repair was assayed using a chromosomally integrated GFP-based reporter construct. Although it was previously thought that PARP plays a role in DNA repair and radiation response primarily via the base excision repair (BER) pathway and recruitment of BER factors to sites of damage, we found that inhibitors of PARP can disrupt repair of double strand breaks (DSBs) by suppressing expression of BRCA1 and RAD51, key factors in homology-dependent repair (HDR) of DSBs. Treatment of human cancer cells with PARP inhibitors or with siRNAs targeted to PARP-1 yielded reduced levels of BRCA1 and RAD51. Radiosensitization by PARP inhibition was also seen in cells in which BER was already attenuated (via expression of a dominant negative variant of polymerase beta). Direct measurement of HDR using a GFP-based chromosomal reporter assay demonstrated reduced HDR in cells treated with PARP inhibitors. This work demonstrates that PARP inhibition can suppress the HDR pathway of DSB repair by down-regulation of BRCA1 and RAD51 expression. As such, it identifies a novel mechanism by which PARP regulates DNA repair and promotes radiosensitization, providing further rationale for therapeutic strategies combining PARP inhibition and radiation. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 3893. doi:10.1158/1538-7445.AM2011-3893