Studies were undertaken to determine how a line of mutant Syrian hamster melanoma cells (HAB-2E) that displays unlimited growth potential when all of the thymine residues in nuclear DNA are replaced by bromouracil (BrUra) could avoid the deleterious effects of bromodeoxyuridine (BrdUrd) mutagenicity. It was found that BrdUrd could be mutagenic to these cells. However, there was a nonlinear relationship between mutagenicity and the amount of BrUra in the DNA of the HAB-2E cells. With these cells, mutagenicity apparently is determined by the concentration of BrdUrd to which the cells are exposed rather than the amount of BrUra in DNA. These results were obtained with both the induction of ouabain resistance and thioguanine resistance as markers for mutagenesis. The dependence of BrdUrd mutagenicity on BrdUrd concentration was also observed for the parental melanoma cells.

No abstract

The halogenated thymidine (dT) analogue, 5-bromodeoxy-uridine (BUdR), has a variety of effects on mammalian cells, including toxicity, suppression of differentiation, and mutagenesis. Although it is generally assumed that the effects of BUdR are due primarily to its presence in DNA, results from our laboratory have raised doubts about such assumptions. We have shown, for example, that BUdR mutagenesis in mammalian cells is determined by the concentration of BUdR in the medium rather than in DNA, and that mutagenesis can be suppressed by deoxycytidine (dC) without changing the amount of BUdR in DNA. BUdR has also been shown to induce sister chromatid exchanges (SCEs) in mammalian cells. Initial results suggested that the relationship between BUdR and SCEs might not be explained by a single factor, and various correlations between BUdR and SCEs have been proposed. However, the results to date have been inconclusive, because the experiments did not resolve as independent variables the concentration of BUdR in the medium and the amount of BUdR incorporated into nuclear DNA. We have now carried out experiments to resolve these two factors; the results indicate that the major factor in determining the frequency of SCEs is the concentration of BUdR in the medium.

The effects of purine deoxyribonucleosides on bromodeoxyurdine (BrdU) mutagenesis in Syrian hamster melanoma cells were determined. Both deoxyguanosine (dG) and deoxyadenosine (dA) were found to stimulate mutagenesis without changing the amount of BrdU in DNA. In addition, the stimulation of mutagenesis by dG and dA was suppressed by the addition of deoxycytidine (dC). These results suggest that BrdU mutagenesis involves the perturbation of dC metabolism, which perturbation is enhanced by dGTP and dATP. The mutagenic activity of dG in the absence of BrdU was tested, as was that of thymidine (dT), which we had shown previously to stimulate BrdU mutageneis. With dG alone, no increase above the spontaneous mutation frequency was detected. However, at extremely high concentration, dT in the absence of BrdU was slightly mutagenic, and the mutagenesis by dT was enhanced by dG and suppressed by dC.