Age-related and tissue-specific accumulation of oxidative DNA base damage in 7,8-dihydro-8-oxoguanine-DNA glycosylase (Ogg1) deficient mice

Abstract

Mutations that influence the repair of oxidative DNA modifications are expected to increase the steady-state (background) levels of these modifications and thus create a mutator phenotype that predisposes to malignant transformation. We have analysed the steady-state levels and repair kinetics of oxidative DNA modifications in cells of homozygous ogg1^–/– null mice, which are deficient in Ogg1 protein, a DNA repair glycosylase that removes the miscoding base 8-hydroxyguanine (8-oxoG) from the genome. Oxidative purine modifications including 8-oxoG were quantified by means of an alkaline elution assay in combination with Fpg protein, the bacterial functional analogue of Ogg1 protein. In primary hepatocytes of adult ogg1^–/– mice aged 9–12 months, the steady-state level of the lesions was 2.8-fold higher than in wild-type control mice. In contrast, no difference between ogg1^–/– and wild-type mice was observed in splenocytes, spermatocytes and kidney cells. In hepatocytes of ogg1^–/– mice, but not in wild-type controls, the steady-state levels increased continuously over the whole lifespan. No significant accumulation of the oxidative base modifications was observed in ogg1^–/– fibroblasts in culture when they were kept confluent for 8 days. Both in confluent and proliferating ogg1^–/– fibroblasts, the global repair of additional oxidative base modifications induced by photosensitization was 4-fold slower than in wild-type cells. The results suggest that the consequences of an Ogg1 defect are restricted to slowly proliferating tissues with high oxygen metabolism such as liver, because of a back-up mechanism for the repair of 8-oxoG residues that is independent of transcription and replication.