Effect of Uracil DNA Glycosylase Activity on the Efficacy of Thymidylate Synthase Inhibitor/HDAC Inhibitor Combination Therapies in Colon Cancer

Human uracil DNA glycosylase (UNG2) is responsible for removal of uracil bases from DNA and initiates base excision repair pathways. Accumulation of uracil or its fluorinated analogs in DNA is one of the killing mechanisms of thymidylate synthase (TS) inhibitors in cancer cells, and depletion of UNG2 often enhances the toxicity of these anticancer drugs. We used CRISPR to knockout UNG2 from HT29 colon cancer cells and confirmed the absence of protein by western blot and uracil excision assays. We tested the effect of UNG2 KO on the efficacy of multiple TS inhibitors (5‐fluorouracil, fluorodeoxyuridine, pemetrexed, and raltitrexed), and we determined that only fluorodeoxyuridine and raltitrexed were significantly more potent in UNG2 KO cells compared to wild‐type HT29 cells (fluorodeoxyuridine IC50: 2 mM (wt) vs. 3 nM (KO); raltitrexed IC50: 14 nM (wt) vs. 2 nM (KO)). Interestingly, UNG2 protein levels can also be depleted by the HDAC inhibitors SAHA and MS275, providing a pharmacologic strategy to reduce UNG2 activity in cells. Unexpectedly, the HDAC inhibitors synergized with 5‐fluorouracil, but not fluorodeoxyuridine, in both wild‐type and UNG2‐knockout cells. This suggested that HDAC inhibitors sensitized cells to 5‐fluorouracil through an UNG2‐independent mechanism. Moreover, cell death pathways activated by fluorodeoxyuridine and regulated by UNG2 activity are not sensitized by HDAC inhibitors. Our combined genetic and pharmacologic strategies targeting UNG2 activity in cells are defining cell death mechanisms for combination therapies of TS inhibitors and HDAC inhibitors. Future work will examine these drug combinations in additional cell lines to understand optimal therapeutic combinations and to further refine mechanisms of cell death.