RT Journal Article SR Electronic T1 Genomically Incorporated 5-Fluorouracil that Escapes UNG-Initiated Base Excision Repair Blocks DNA Replication and Activates Homologous Recombination JF Molecular Pharmacology JO Mol Pharmacol FD American Society for Pharmacology and Experimental Therapeutics SP mol.115.100164 DO 10.1124/mol.115.100164 A1 Amelia M Huehls A1 Catherine J Huntoon A1 Poorval M Joshi A1 Carly A Baehr A1 Jill M Wagner A1 Xiaoxiao Wang A1 Marietta Y Lee A1 Larry M Karnitz YR 2015 UL http://molpharm.aspetjournals.org/content/early/2015/10/22/mol.115.100164.abstract AB 5-fluorouracil (5-FU) and its metabolite 5-fluorodeoxyuridine (FdUrd; floxuridine) are chemotherapy agents that are converted to FdUMP and FdUTP. FdUMP inhibits thymidylate synthase and causes the accumulation of uracil in the genome, whereas FdUTP is incorporated by DNA polymerases as 5-FU in the genome; however, it remains unclear how either genomically incorporated U and 5-FU contribute to killing. We show that depletion of the uracil DNA glycosylase UNG sensitizes tumor cells to FdUrd. Furthermore, we show that UNG depletion does not sensitize cells to the thymidylate synthase inhibitor (raltitrexed), which induces uracil but not 5-FU accumulation, thus indicating that genomically incorporated 5-FU plays a major role in the anti-neoplastic effects of FdUrd. We also show that 5-FU metabolites do not block the first round of DNA synthesis but instead arrest cells at the G1/S border when cells again attempt replication and activate homologous recombination (HR). This arrest is not due to 5-FU lesions blocking DNA polymerase δ, but instead depends, in part, on the glycosylase TDG. Consistent with the activation of HR repair, disruption of HR sensitized cells to FdUrd, especially when UNG was disabled. These results show that 5-FU lesions that escape UNG repair activate HR, which promotes cell survival.