Age dynamics of DNA damage and CpG methylation in the peripheral blood leukocytes of mice.

Certain types of DNA damage are known to accumulate with age. Here we present the quantitative data describing the extent of the spontaneous DNA damage in 144 SHK mice of various ages. In each animal, we assessed double strand breaks, single strand breaks and alkali-labile sites, as well as amounts of oxidized purines, oxidized pyrimidines and misincorporated uracils. In addition, overall levels of genome DNA methylation were evaluated. The amounts of oxidized pyrimidines were correlated with age in males only, while the amounts of double strand breaks (DSB) in DNA samples were correlated with age in females only (R=0.26; p<0.035). No age-related accumulation of single-strand breaks (SSB) was observed. The hypomethylation of DNA was significant in aging females, but not in aging males. Various types of DNA damage were correlated to each other. In attempt to develop more stable indicator of age-dependent alterations in DNA, the DNA Damage Level Differential (DDLD) indices was developed for comet assaying of peripheral blood leukocytes. As expected, DDLD index was shown to be better correlated with age than any single quantitative measure reflecting certain type of DNA damage. A variability of effectiveness of various kinds of DNA repair in individual animals was larger than expected. This conclusion may have a substantial impact on subsequent studies of the mutagens and other kinds of environmental stressors in animal populations. Nor DDLD neither individual quantitative measures of DNA damage were capable of prediction post-sampling survival time.