Abstract 3718: Sensitization of human tumor cells to chemotherapy drugs by antisense downregulation of RAD51: Targeting DNA repair to induce synthetic lethality (2016)

Abstract The inherent genomic instability of cancer cells has been exploited as a tumor-selective drug target to treat tumors that are deficient in specific mechanisms of DNA repair. Inhibitors of poly(ADP-ribose) polymerase (PARP) are routinely used clinically against tumors deficient in BRCA1 or BRCA2, due to the poor capacity of these cells to undergo homologous recombination repair (HRR). This exploitation of a tumor cell deficiency to enhance selectivity to a particular drug is “synthetic lethality” (Nature 434: 913, 2005). To make use of this phenomenon in tumors that may not be inherently hypersensitive to a particular treatment, we have sought to induce synthetic lethality by down-regulating essential components of DNA repair, in particular BRCA2, to sensitize cells to chemotherapy drugs [Mol Oncol 8(8): 1429-1440, 2014]. Given that an important function of BRCA2 is to modulate the action of RAD51 in HRR, we determined whether antisense knockdown of RAD51 could enhance tumor cell sensitivity to the PARP inhibitor olaparib and the DNA-crosslinking agent cisplatin. Four different anti-RAD51 siRNA molecules (Dharmacon), targeting coding sequences, were tested against cell lines representative of different tumor types in an in vitro assay of proliferation (non-small cell lung cancer line A549b, colon carcinoma line HT-29, and prostate carcinoma lines DU145 and LNCaP). The siRNAs, as single agents, inhibited proliferation in a concentration-dependent fashion and to varying degrees, and sensitized tumor cells to olaparib. A sequence that targeted region 1169-1187 of the RAD51 cDNA (NM_002875.4) was utilized for further studies. At concentrations of anti-RAD51 siRNA 51a that inhibited proliferation of cell lines by less than 50%, 51a enhanced cytotoxicity of olaparib by over 90% and of cisplatin by 60-90%. In all cell lines except LNCaP (with mutant BRCA2) the combination of siRNAs against RAD51 and BRCA2 acted cooperatively to enhance cytotoxicity of olaparib and cisplatin. Notably, when used together, each siRNA down-regulated expression of its respective target mRNA, as demonstrated by quantitative RT-PCR, without interfering with the activity of the other. RAD51 can be exploited clinically as a target for inherent or induced synthetic lethality to DNA-damaging agents (e.g., cisplatin) or inhibitors of DNA repair (e.g., olaparib). Such treatment can include tumors with BRCA2-deficiency, either inherent or induced, to yield at least an additive anticancer effect. MR is a scholar of the CIHR Strategic Training Program in Cancer Research and Technology Transfer (CaRTT) and a recipient of the CIHR Banting and Best Canada Graduate Scholarship. Citation Format: Peter J. Ferguson, Mateusz Rytelewski, Mark D. Vincent, James Koropatnick. Sensitization of human tumor cells to chemotherapy drugs by antisense downregulation of RAD51: Targeting DNA repair to induce synthetic lethality. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 3718.