DNA Degradation and Excision Repair in γ-Irradiated Chinese Hamster Ovary Cells

Abstract

THE removal from the DNA of cyclobutane-type photo-dimers following exposure to ultraviolet irradiation has been demonstrated in human cells^1,2. No excision of photo-dimers has, however, been observed in rodent cells^3–6 or in cells from patients with the hereditary disease xeroderma pigmentosum^2,7. Recently, it has been recognised that damage to the nucleic acid bases may also constitute a major portion of the damage introduced into DNA by ionising radiation (refs 8, 9 and J. L. Swinehart, W.-S. Lin, J. Roti Roti, R. B. Painter and P. A. C., unpublished). The following evidence suggests that excision repair processes may occur following exposure to ionising radiation. The insertion of short patches of nucleotides into the DNA in γ-irradiated mammalian cells has been observed in the form of repair replication and unscheduled DNA synthesis¹⁰. These processes are best understood in molecular terms if it is assumed that damaged and undamaged residues are first removed from the DNA in early steps of postirradiation repair. Endonuclease activities which appear to recognise γ-ray-induced nucleotide damage in in vitro systems have recently been identified. A partially purified preparation from Micrococcus luteus has been studied by Carrier and Setlow¹¹, and an endonuclease activity in crude HeLa cell extracts by Brent¹². Such enzyme activities could be involved in the excision repair of γ-ray damaged residues. We report here that Chinese hamster ovary cells (CHO) undergo limited DNA degradation following exposure to γ rays and we demonstrate by direct chemical analysis the excision of damaged thymine residues from the DNA during post-radiation incubation.