Infidelity of DNA Synthesis as a Cause of Mutagenesis
The concept underlying these studies is that a major determinant of mutagenesis involves perturbations in the fidelity of DNA replication. i.e., the accuracy by which DNA polymerases copy DNA templates. To investigate this relationship, we have designed in vitro assays to measure the accuracy of DNA replication and used these systems to screen for and to quantitate factors that promote errors in DNA synthesis. Using DNA polymerase from bacteria, the frequency of mistakes with X174 DNA as a template approaches 10-7 and is similar to the spontaneous mutation rates in bacterial cells. In contrast, DNA polymerases from animal cells are more error-prone.
The differences in fidelity among mammalian DNA polymerases which lack error-correcting mechanisms suggest that these enzymes enhance accuracy by improving base-selection. Thus, mutants in DNA polymerase-α might be altered in base-selection. Chinese hamster V79 cell mutants selected by resistance to aphidicolin, a specific inhibitor of DNA polymerase-α, have been reported (Somatic Cell Genet., 7: 235-253, 1981). DNA polymerase-a was purified from mitochondria-free crude extracts of these mutants by sequential column chromatography using DEAE- cellulose and phosphocellulose. DNA polymerase-α purified from one of the mutants is 10-fold more resistant to aphidicolin than the same enzyme purified from the parental cells. Moreover, the apparent K m for dCTP is 1.0±0.4 μ m for the mutant polymerase and 10±4 μ m for the parental enzyme. These observed differences are in accord with the known competition between aphidicolin and dCTP, and provide a mechanism for the aphidicolin resistance of the mutant, i.e., the decrease in K m for dCTP. The elevated spontaneous and induced mutation rate exhibited by this mutant could be mediated by the alteration in DNA polymerase-α.
With DNA replicating enzymes from a variety of sources, enhancement of mutagenesis has been demonstrated by alteration in precursor pools, damage to DNA templates, loss of nucleotide bases on DNA, metal ions that interact with nucleotide bases, and organic compounds that intercalate into DNA. The alterations of deoxynucleoside triphosphate pools also occur after treatment of animal cells with known 268mutagens. This observation may provide a new mechanism for mutagenesis by these agents independent of alterations in DNA.