Oxidative damage to DNA has been proposed to have a role in cancer and
ageing1. Oxygen-free radicals formed during normal aerobic cellular
metabolism attack bases in DNA, and 7,8-dihydro-8-oxoguanine (8-oxoG) is one
of the adducts formed2,
3. Eukaryotic replicative DNA polymerases
replicate DNA containing 8-oxoG by inserting an adenine opposite the lesion4; consequently, 8-oxoG is highly mutagenic and causes G:C to T:A
transversions5. Genetic studies in yeast have indicated a role
for mismatch repair in minimizing the incidence of these mutations. In
Saccharomyces cerevisiae, deletion of OGG1, encoding a DNA glycosylase
that functions in the removal of 8-oxoG when paired with C, causes an increase
in the rate of G:C to T:A transversions6. The ogg1
msh2 double mutant displays a higher rate of CAN1S
to can1r forward mutations than the ogg1
or msh2 single mutants, and this enhanced mutagenesis is primarily
due to G:C to T:A transversions7. The gene RAD30 of
S. cerevisiae encodes a DNA polymerase, Pol, that efficiently replicates
DNA containing a cis-syn thymine-thymine (T-T) dimer by inserting two
adenines across from the dimer8. In humans, mutations in the
yeast RAD30 counterpart, POLH, cause the variant form of xeroderma
pigmentosum9,
10 (XP-V), and XP-V individuals suffer from a
high incidence of sunlight-induced skin cancers. Here we show that yeast and
human POL replicate DNA containing 8-oxoG efficiently and accurately
by inserting a cytosine across from the lesion and by proficiently extending
from this base pair. Consistent with these biochemical studies, a synergistic
increase in the rate of spontaneous mutations occurs in the absence of POL
in the yeast ogg1 mutant. Our results suggest an additional
role for Pol in the prevention of internal cancers in humans that would
otherwise result from the mutagenic replication of 8-oxoG in DNA.
msh2
, that efficiently replicates
DNA containing a cis-syn thymine-thymine (T-T) dimer by inserting two
adenines across from the dimer
