Defective Repair of Gamma-ray-induced DNA Damage in Xeroderma Pigmentosum Cells

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Abstract
The bromouracil-photolysis technique was used 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 h of incubation, single-strand breaks were repaired. The repaired regions were small, 1-2 BrUra residues, for cells irradiated aerobically or anoxically. After a 20 h incubation, the repaired region in normal cells showed a component mimicking UV-repair. There were large patches (.apprx. 30 BrUra residues) in the approximate ratios of 1 per 6 chain breaks for aerobic irradiation and 1 per 3 chain breaks for anoxic irradiation. XP cells only showed large patches at 20 h if they were irradiated aerobically. Such regions in XP cells irradiated anoxically were not detected. Some part of ionizing damage mimics excision of UV damage in that the repair patches are large and the repair takes an appreciable time. The types of such damage depend on whether the irradiation is done aerobically or anoxically, and XP cells are defective in repairing a component of anoxic damage.