Determination of apoptosis, uracil incorporation, DNA strand breaks, and sister chromatid exchanges under conditions of thymidylate deprivation in a model of BER deficiency

Thymidylate synthase (TS) is an important target of several chemotherapeutic agents. During TS inhibition, dTTP levels decrease with a subsequent increase in dUTP. Uracil incorporated into the genome is removed by base excision repair (BER). BER has been hypothesized to play a role in the response to thymidylate deprivation, despite a lack of direct evidence. We previously found that β-pol null murine fibroblasts were ∼six-fold more resistant than wild-type cells to raltitrexed, a folate-based inhibitor specific for TS. In this study, a number of endpoints were determined to understand the influence of BER and β-pol during raltitrexed treatment. Raltitrexed induced apoptosis in wild-type cells to a greater extent than in β-pol null cells. A PARP inhibitor decreased the sensitivity to raltitrexed, although the extent was not different between wild-type and β-pol null cells. No evidence was seen for extensive strand break formation that preceded apoptosis, although raltitrexed induced more sister chromatid exchanges in wild-type cells. Increased levels of uracil in DNA were detected following treatment in wild-type and β-pol null cells. However, uracil levels were only ∼two-fold higher in DNA from treated cells compared to untreated. Uracil DNA glycosylase activity was slightly higher in β-pol null cells, although not sufficiently different to explain the difference in sensitivity to raltitrexed. Taken together, the data suggest that the sensitivity of the wild-type cells to raltitrexed is not associated with activation of PARP-1 dependent BER, extensive uracil incorporation into DNA and persistent strand breaks, but rather with changes suggestive of DNA recombination.