Abstract 4486: 5-Hydroxymethyl-2’-deoxyuridine, but not temozolomide, enhances the selective synthetic lethality in BRCA1 and BRCA2-deficient cells caused by PARP inhibition.

Abstract

Abstract Inhibition of poly(ADP-ribose) polymerase (PARP) has been determined as an attractive chemotherapeutic treatment for homologous recombination (HR)-deficient tumors. PARP is involved in repairing DNA single strand breaks (SSBs) via the base excision repair (BER) pathway; therefore, PARP inhibitor (PARPi) leads to an accumulation of DNA double strand breaks (DSBs) at the sites of SSBs during DNA replication. While the cytotoxic effect of PARPi was demonstrated to be potentiated by addition of genotoxic alkylating agent temozolomide (TMZ), it is not clear that this combined treatment further increases selectivity to cause cell death in HR-deficient cells, such as BRCA-deficient cells, over HR-proficient cells. TMZ randomly increases SSBs at scattered locations in the genome; in contrast, incorporation of 5-hydroxymethyl-2’-deoxyuridine (5-HmedU) in DNA has a potential to cause multiple SSBs within a short distance during BER. We hypothesized that a combination treatment of PARPi and 5-hmedU could increase selectivity of PARPi-mediated cytotoxicity in HR-deficient cells more so than the PARPi and TMZ combination. Here we show that in single chemical treatment experiments, the IC50 of 5-HmedU was eight times lower in BRCA1-/- and BRCA2-/- mutant cells, but in TMZ treatments the IC50 was only two times lower compared HR-proficient cells. Furthermore, the 5-HmedU and PARPi agent Olaparib combination enhanced a synergistic cell killing effect in BRCA1-/- and BRCA2-/- mutant cells with higher selectivity than the PARPi and TMZ combination. Advantages of the 5-HmedU and PARPi combination is as follows: 1] 5-HmedU causes DNA damage through vast genome incorporation in highly proliferating cells. Thus, 5-HmedU is more effective at selecting highly proliferating tumor cells; 2] 5-HmedU single treatment more selectively targeted BRCA1-/- and BRCA2-/- mutant cells than TMZ. This selectivity could be due to more efficient DSB formation by 5-HmedU through accumulation of clustered SSBs; and 3] combination treatment of 5-HmedU and PARPi produces a synergistic selectivity to kill HR-deficient BRCA1-/- and BRCA2-/- mutant cells whereas TMZ and PARPi combination does not. A possible mechanism of the selective cell killing effect by 5-HmedU and PARPi could result from an accumulation of closely located DSBs with PARP-DNA strand break complexes in HR-deficient cells. Citation Format: Jean Clement, Jun Nakamura. 5-Hydroxymethyl-2’-deoxyuridine, but not temozolomide, enhances the selective synthetic lethality in BRCA1 and BRCA2-deficient cells caused by PARP inhibition. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 4486. doi:10.1158/1538-7445.AM2013-4486