Incorporation of dUMP into DNA is a major source of spontaneous DNA damage, while excision of uracil is not required for cytotoxicity of fluoropyrimidines in mouse embryonic fibroblasts.

Uracil may arise in DNA as a result of deamination of cytosine or through incorporation of dUMP instead of dTMP during replication. We have studied the steady-state levels of uracil in the DNA of primary cells and mouse embryonic fibroblast (MEF) cell lines from mice deficient in the Ung uracil-DNA glycosylase. The results show that the levels of uracil in the DNA of Ung(-/-) cells strongly depend on proliferation, indicating that the uracil residues originate predominantly from misincorporation during replication. Treatment with 5-fluoro-2'-deoxyuridine (5-FdUrd) or 5-fluorouracil (5-FU) gives rise to a dose-dependent increase of uracil in Ung(-/-) MEFs (up to 1.5-fold) but not in wild-type cells. Interestingly, Ung(-/-) MEFs accumulate AP-sites as well as uracil in response to 5-FdUrd but not to 5-FU. This accumulation of repair intermediates suggests a loss of tightly co-ordinated repair in the absence of Ung, and correlates with stronger inhibition of cell proliferation in response to 5-FdUrd, but not to 5-FU, in Ung(-/-) MEFs compared with wild-type cells. However, other cytotoxic effects of these fluoropyrimidines are comparable in both wild-type and Ung-deficient cells, demonstrating that excision of uracil from DNA by the Ung uracil-DNA glycosylase is not a prerequisite for obtaining cytotoxicity.