Abstract 3629: Balance shift between error-free and error prone DNA double-strand break repair as a novel mechanism of radiosensitization by nucleoside analogs

Abstract

Abstract Nucleoside analogs (NAs) represent a large group of anticancer drugs, several members of which have been shown to sensitize cells to ionizing radiation (IR). Among the lesions induced in cells by IR, the DNA double-strand break (DSB) is considered to be the most detrimental. Therefore we focused on interactions between NAs and pathways of DSB repair. We opted for post-irradiation treatment with NAs to avoid complications arising from redistribution of cells throughout the cycle prior to IR exposure. We compared the effects of 4h treatments post-IR with vidarabine (ara-A), fludarabine (F-ara-A) or gemcitabine (dFdC) on the survival of A549 lung carcinoma cells. We found that post-irradiation treatment with ara-A and F-ara-A, but not dFdC, resulted in cell radiosensitization to killing. Radiosensitization by ara-A was superior to that by F-ara-A. We employed immunofluorescence detection of IR-induced foci and I-SceI-based reporter cell lines to identify the pathways of DSB repair that are inhibited by ara-A and cause the observed radiosensitization. We found that the formation of IR-induced Rad51-foci was compromised by ara-A suggesting inhibition of homologous recombination repair (HRR). Repair of I-SceI-induced DSBs by HRR was also suppressed by ara-A. Furthermore, A549 cells deprived of Rad51 by RNAi could not be efficiently radiosensitized by ara-A. On the other hand, we could not detect impairment by ara-A of non-homologous end-joining (NHEJ), as measured by pulsed-field gel electrophoresis (PFGE) at high doses of IR (20 Gy). Notably, we observed an increase in the frequency of mutagenic NHEJ events in several I-SceI-based repair reporter systems, indicative of processing by error-prone repair pathways such as alternative/backup NHEJ (B-NHEJ). In addition, we could show that treatment with ara-A induces a dramatic increase in B-NHEJ in plateau phase Lig4-/- MEFs. Treatment with F-ara-A also inhibited HRR, but was less proficient in promoting mutagenic NHEJ. We conclude that inhibition of HRR and promotion of B-NHEJ represent two important mechanisms of radiosensitization by NAs. The strong impact of ara-A on both of these repair pathways underpins its superior radiosensitizing potential. Targeting of both HRR and B-NHEJ may provide a promising new strategy for radiosensitization. Citation Format: George Iliakis, Simon Magin. Balance shift between error-free and error prone DNA double-strand break repair as a novel mechanism of radiosensitization by nucleoside analogs. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 3629. doi:10.1158/1538-7445.AM2015-3629