In vivo studies on DNA repair and turnover with age

Since the capacity for DNA repair relative to other cellular processes should be an important parameter of mutagenesis, carcinogenesis, and also aging, this capacity should preferably be studied in intact animals. Thus, we developed autoradiographic techniques for measuring DNA repair directly in vivo. By these methods unscheduled DNA synthesis (UDS) was detected quantitatively as silver grains on epithelial cells of mouse skin after treatment with chemical carcinogens or UV irradiation, and on cerebral ganglion cells of aquarium fish after treatment with various chemical carcinogens. Several interesting findings so far obtained are presented. Possible age-related change in the UDS response was examined by the skin technique with mice of 2 and 18 months old. Similar dose-dependent induction of UDS was observed in mice of both ages after treatment with 4-hydroxyaminoquinoline 1-oxide; their levels of UDS at each dose were not significantly different. The dose-response curves for young and aged animals after UV irradiation showed similar increases to a plateau at low doses, but their responses to high doses were very different: in aged mice the UDS level decreased markedly with increase in the dose, whereas in young mice it remained at the same level. This suggests that in aged animals, high doses of UV irradiation cause deterioration of DNA repair systems, and that aged animals cannot repair extensive DNA damage efficiently. It is generally thought that DNA has a stable structure and a much slower turnover than other cellular components. Although the effect of DNA repair on DNA turnover may be insignificant, accumulation of repaired DNA in cells should result in detectable DNA turnover. Therefore, we investigated DNA turnover in postmitotic ganglion cells of rat retina. However, careful autoradiographic studies on pairs of eyes showed no detectable DNA turnover up to nearly their median life span (2 years). This result suggests that the DNA of post-mitotic cells, which are not replaced throughout the life span of the animal, is very stable and is possibly protected in some special way.