The role of dUTPase and uracil-DNA repair in cancer chemotherapy

Thymidylate metabolism is an important target for chemotherapeutic agents that combat a variety of neoplastic diseases including head and neck, breast and gastrointestinal cancers. Therapeutic strategies applied to this pathway target the thymidylate synthase (TS) reaction that catalyzes the reductive methylation of deoxyuridylate (dUMP) to form thymidylate (TMP). This reaction represents the sole de novo source of TMP required for DNA replication and repair. Inhibitors of this pathway include the widely utilized fluoropyrimide and antifolate classes of anti-cancer agents. Studies attempting to elucidate the molecular mechanisms of cell killing mediated by inhibitors of the TS reaction suggest that cytotoxicity results from a process known as "thymineless death". This term describes the extreme TTP pool depletion observed following TS inhibition. Although depletion of TTP pools is clearly involved in this process, there is now considerable evidence implicating aberrant uracil-DNA metabolism as an important mechanism of toxicity. Upon TS inhibition, dUTP pools may accumulate, inducing repeated cycles of uracil misincorporation into DNA and repair-mediated DNA damage. Central to the uracil-misincorporation pathway are the enzymes deoxyuridine nucleotidohydrolase (dUTPase) (EC 3.6.1.23) and uracil-DNA glycoslyase (UDG) (EC 3.2.2.3). dUTPase catalyzes the hydrolysis of dUTP to form dUMP and pyrophosphate thereby eliminating dUTP and preventing its utilization by DNA polymerases during replication and repair. UDG initiates the base excision repair pathway effectively removing any uracil residues that may arise in DNA. Under normal conditions, uracil is precluded from DNA by the combined actions of dUTPase and UDG. However, during TS inhibition, dUTP pools may accumulate and overwhelm dUTPase, resulting in repeated cycles of uracil in is incorporation and detrimental repair leading to strand breaks and cell death. Because dUTPase plays a pivotal role in regulating cellular dUTP pools, this enzyme could have profound effects on the efficacy of agents that target thymidylate biosynthesis. This article reviews our current understanding of the role of aberrant uracil-DNA metabolism as a contributing mechanism of cytotoxicity initiated by chemotherapeutic agents that target de novo thymidylate metabolism. The role of dUTPase expression in modulating therapeutic response is presented including evidence from yeast and mammalian cell culture models and clinical studies. The regulation of human dUTPase isoforms in normal and neoplastic tissues will be reviewed as well as the role of dUTPase expression as a prognostic marker for overall survival and response to therapy in colon cancer.