Defective repair of gamma-ray induced DNA damage in xeroderma pigmentosum cells

We used the bromouracil-photolysis technique to estimate the sizes of the repaired regions in normal human and xeroderma pigmentosum (XP) cells irradiated by gamma-rays aerobically or anoxically. After 1 1/2 hours of incubation, single-strand breaks were repaired and the repaired regions were small--one to two BrUra residues--for cells irradiated aerobically or anoxically. After a 20-hour incubation, the repaired region in normal cells showed a component mimicking U.V.-repair. There were large patches (approximately 30 BrUra residues) in the approximate ratios of one per six chain breaks for aerobic irradiation and one per three chain breaks for anoxic irradiation. XP cells, however, only showed large patches at 20 hours if they had been irradiated aerobically. We could not detect such regions in XP cells irradiated anoxically. These results indicate (1) that some part of ionizing damage mimics excision of U.V. damage in that the repair patches are large and the repair takes an appreciable time; (2) the types of such damage depend on whether the irradiation is done aerobically or anoxically; and (3) XP cells are defective in repairing a component of anoxic damage.