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Somatic mutations in mitochondrial DNA (mtDNA) are thought to play an important role in both aging and neurodegenerative diseases although their specific contributions remain a subject of intense debate. We analyzed somatic mutations in the mtDNA control regions in the liver of Wistar rats. The mutation rate was found to be high and increased with age from 5.3×10−4mutations per position to 4.48×10−3mutations per position at 3 and 12 months of age, respectively. The vast majority of nucleotide substitutions are transitions (∼95%) with A:TG:C transitions being the most frequent type of substitution (50%). In 3-month-old Wistar rats, approximately 40% of somatic mutations in the control region of mtDNA are significantly consistent with the model of dislocation mutagenesis which is a signature of error-prone DNA synthesis by mtDNA polymerase γ. The results are consistent with the previous hypothesis that normal intramitochondrial dNTP pool asymmetries, which have been shown to reduce the fidelity of mtDNA polymerase γ, substantially contribute to somatic mutagenesis of the rat mtDNA.

Production of free radicals in animals is accompanied with a number of pathologic conditions, some of which may be manifested through DNA damage. Studies of mechanisms of oxidative DNA damage by free radicals in vivo are hindered by the lack of good animal models with significant overgeneration of or increased sensitivity to free radicals. An inbred rat strain (OXYS) is characterized by inherited overgeneration of free radicals, lipid peroxidation, protein oxidation, DNA rearrangements, and pathological conditions paralleling several human degenerative diseases. We have used monoclonal antibodies against a common pre-mutagenic base lesion 8-oxoguanine (8-oxoG) in combination with indirect immunofluorescence microscopy and image analysis to follow the relative age-dependent amounts and distribution of 8-oxoG in liver cells from OXYS and Wistar rats. 8-OxoG increased with age in both strains of rats, with OXYS rats always displaying statistically significantly higher levels of oxidative DNA damage than Wistar rats. Statistical analysis indicates that 8-oxoG does not uniformly accumulate in all cells with advancing age or increasing free radical load, but rather concentrates in a minor fraction of cells with a high damage level.