Ablating UNG activity in a mouse model inhibits colorectal cancer growth by increasing tumor immunogenicity

Uracil DNA glycosylase (UNG) excises uracil and 5-fluorouracil bases from DNA and is implicated in fluorodeoxyuridine (FdU) resistance. Here we explore the effects of inhibiting UNG activity, or depleting the UNG protein, in 2 mouse syngeneic models for colorectal cancer. Overexpressing the small UNG inhibitor protein (UGI) in mismatch repair-deficient (MMR-deficient) MC38 cells injected into C57BL/6J mice delayed tumor growth and prolonged survival when combined with FdU. Combining UNG inhibition with FdU numerically increased CD4+ T lymphocytes and B cells compared with FdU or UNG inhibition alone, suggesting an immune component to the effects. In contrast, shRNA depletion of UNG in the absence of FdU treatment resulted in 70% of mice clearing their tumors, and a 3-fold increase in overall survival compared with FdU. Analysis of MC38 tumor-infiltrating immune cells showed UNG depletion increased monocyte and dendritic cell populations, with CD8+ T cells also numerically increased. shRNA depletion of UNG in MMR-proficient CT-26 cells injected into BALB/c mice produced minimal benefit; the addition of anti-PD-1 antibody synergized with UNG depletion to increase survival. Cytotoxic T cell depletion abolished the benefits of UNG depletion in both models. These findings suggest UNG inhibition and/or depletion could enhance antitumor immune responses in humans.