Formation and repair of thymine damage in DNA of mammalian cells exposed to gamma radiation

Chinese hamster ovary (CHO) cells growing exponentially in tissue culture were exposed to gamma rays and subsequently incubated at 370C for various periods of time. The TCA-soluble intracellular and extracellular material was analyzed for both damaged and undamaged thymine residues. It was found that a major class of thymine radiation product--ring-saturation derivatives of the 6-hydroxy(hydroperoxy)-5,6-dihydrothymine type--appears in cellular acid-soluble material and in the postirradiation incubation medium within minutes after irradiation. A radiation-specific release of unaltered thymine residues occurs concomitantly, indicating the operation of a controlled process of DNA degradation. Three actively-growing mammalian cell lines--two of human and one of rodent origin--were irradiated with high doses of gamma rays and their DNA analyzed, following various periods of postirradiation incubation, for the presence of 5,6-dihydroxydihydrothymine-type radiation products. Diploid human fibroblasts (WI-38) exhibited a rapid and selective excision of damaged thymines from the DNA. A similar process occurred in both virally-transformed human fibroblasts (VA13) and in cells of the permanent CHO line. Corresponding appearance of thymine radiation products was detected in acid-soluble material during postirradiation incubation