DNA repair in mouse embryo fibroblasts. I. Decline in ultraviolet-induced incision rate accompanies cell transformation

Abstract

In this paper we examine DNA repair in the mouse. Using stringently applied inhibitors of DNA synthesis (hydroxyurea and 1-beta-D-arabinofuranosylcytosine) after ultraviolet irradiation we have quantitated an early step in the repair process - the frequency of single-strand DNA breaks, products of enzymic incision—in fibroblasts and epithelial cells grown in vitro from adults and embryos of different ages. We find that initial rates of incision are very similar in the earliest-passage fibroblasts, regardless of donor age, resembling the rates measured in other rodents while being considerably less than in diploid human fibroblasts. Consistently higher rates of incision are found in adult kidney epithelial cells. With increasing passage number the embryo cultures change with respect to growth rate and chromosome constitution, undergoing spontaneous transformation to established heteroploid cultures. This sequence is accompanied by characteristic changes in initial rates of inhibitor-sensitive incision. An early increase in incision rate, at a time when the cultures are enriched with tetraploid cells, is succeeded by progressive reduction in rate as the cultures become overtly transformed into permanent lines and heteroploidy is established. The lowest rate of incision occurs in a transformed line cloned from a late-passage culture. The decline in incision rate associated with transformation is accompanied by reduced unscheduled DNA synthesis (UDS). The sensitivity to inhibitors of UDS levels in the mouse cultures has been found to vary considerably; inhibitors are most effective in slowly proliferating cultures and apparently not at all when cultures are growing rapidly despite their ability to accumulate DNA breaks in these cells. Caution is needed, therefore, in the interpretation of repair data obtained in the presence of these commonly used inhibitors of DNA synthesis.