Linking uracil base excision repair and 5-fluorouracil toxicity in yeast

5-fluorouracil (5-FU) is a widely used anticancer drug thatdisruptspyrimidinenucleotidepool balances and leads to uracil incorporation in DNA, which is then recognized and removed by the uracil base excision repair (BER) pathway. Using complementary bio-chemical and genetic approaches we have examined the role ofuracilBER in thecell killing mechanism of5-FU. A yeast strain lacking the enzyme uracil DNA glyc-osylase (Ung1), which excises uracil from the DNA backbone leaving an abasic site, showed significant protection against the toxic effects of 5-FU, a G1/S cell cycle arrest phenotype, and accumulated massive amounts of U/A base pairs in its genome (4 % of T/ A pairs were now U/A). A strain lacking the major aba-sic site endonuclease of Saccharomyces cerevisiae (Apn1) showed significantly increased sensitivity to 5-FU with G2/M arrest. Thus, efficient processing of abasic sites by this enzyme is protective against the toxic effects of 5-FU. However, contrary to expecta-tions, the Apn1 deficient strain did not accumulate intact abasic sites, indicating that another repair path-way attempts to process these sites in the absence Apn1, but that this process has catastrophic effects on genome integrity. These findings suggest that new strategies for chemical intervention targeting BER could enhance the effectiveness of this widely used anticancer drug