Abstract 2828: MMR status affects efficiency of homologous recombination repair of F10-induced DNA DSBs

Abstract

Abstract Background: DNA mismatch repair (MMR) is an important stratification factor in colorectal cancer (CRC). 10-20% of sporadic CRC cases display high microsatellite instability (MSI-H) due to DNA mismatch repair (MMR) deficiency, and these are refractory to current fluoropyrimidine (FP) chemotherapy. Our studies with DNA-directed FP polymers, such as F10, indicate they are highly effective regardless of MMR-deficiency consistent with inducing MMR-independent cell death. We investigate the role of MMR-status on potency of F10 and induction of DNA damage and activation of the DNA damage response as part of an overall effort to determine if fluoropyrimidine polymers can be used effectively in the treatment of MMR-deficient CRC. Methods: Studies were conducted using MMR-deficient HCT-116 CRC cells and HCT-116 3-6 cells. Potency was assessed by cell-titer glo and clonogenic assays. Replication stress was evaluated by DNA fiber combing. DNA damage and activation of the DNA damage response was evaluated by Western blots for gH2AX, Chk1 activation, and proteins important for homologous recombination repair. Results: F10 is highly potent to CRC cells regardless of MMR status and is ~1,000-fold more potent than 5-FU to MMR-deficient HCT-116 cells. The increased potency of F10 parallels increased efficiency at causing replication fork slowing as assessed by DNA fiber analysis. F10 causes DNA double strand breaks (DSBs) and activates Chk1 consistent with causing replication fork collapse. DNA DSBs, Chk1 activation, and upregulation of FANCD2, which mediates HR-repair of DNA DSBs all show a marked MMR-dependence consistent with hMLH1 being a factor in HR-mediated repair of F10-induced DNA DSBs. Conclusions: F10 is highly potent to CRC cells regardless of MMR status via a mechanism involving replication fork slowing and increased replication stress. MMR status affects basal levels of proteins involved in HR and the upregulation of these proteins in response to F10 treatment. Our studies indicate FP polymers are likely to be generally effective in CRC regardless of MMR status however MMR status may be an important factor in a personalized medicine approach to their use as combination with Chk1 inhibitors and other drugs modulating the DNA damage response may depend on MMR status. Citation Format: William H. Gmeiner, Chinnadurai Mani, Komaraiah Palle. MMR status affects efficiency of homologous recombination repair of F10-induced DNA DSBs [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 2828.