High accumulation of oxidative DNA damage, 8-hydroxyguanine, in Mmh/Ogg1 deficient mice by chronic oxidative stress

8-Hydroxyguanine (8-OH-G) is a major pre-mutagenic lesion generated from reactive oxygen species. The Mmh/Qgg1 gene product plays a major role in maintaining genetic integrity by removing 8-OH-G by way of the base excision repair pathway. To investigate how oxidative stress influences the formation of 8-OH-G in Ogg1 mutant mice, a known oxidative agent, potassium bromate (KBrO3), was administered at a dose of 2 g/l in the drinking water to Ogg1(+/+), Ogg1(+/-) and Qgg1(-/-) mice for 12 weeks. Apurinic (AP) site lyase activity, measured by the excision of 8-OH-G from synthetic oligonucleotides, remained unchanged in kidney cell extracts isolated from Ogg1 mutant mice when the mice were pre-treated by KBrO3. The levels of 8-OH-G in kidney DNA tremendously increased in a time-dependent manner following exposure of Ogg1(-/-) mice to KBrO3. Of particular note, the amount of 8-OH-G in kidney DNA from Ogg1(-/-) mice treated with KBrO3 was similar to70 times that of KBrO3-treated Ogg1(+/+) mice. The accumulated 8-OH-G did not decrease 4 weeks after discontinuing treatment with KBrO3. KBrO3 treatment for 12 weeks gave rise to increased mutation frequencies at the transgenic gpt gene in Ogg1(+/+) mice kidney. Absence of the Ogg1 gene further enhanced the mutation frequency. Sequence data obtained from gpt mutants showed that the accumulated 8-OH-G caused mainly GC --> TA transversion and deletion. Other mutations including GC --> AT transition also showed a tendency to increase. These results indicate that 8-OH-G, produced by chronic exposure to exogenous oxidative stress agents, is not repaired to any significant extent within the overall genome of Qgg1(-/-) mice kidney.