Extent of excision repair before DNA synthesis determines the mutagenic but not the lethal effect of UV radiation.

Excision repair-proficient diploid fibroblasts from normal persons (NF) and repair-deficient cells from a xeroderma pigmentosum patient (XP12BE, group A) were grown to confluence and allowed to enter the G0 state. Autoradiography studies of cells released from G0 after 72 h and replated at lower densities (3-9 x 10(3) cells/cm2) in fresh medium containing 15% fetal bovine serum showed that semi-conservative DNA synthesis (S phage) began approximately 24 h after the replating. To determine whether the time available for DNA excision repair between ultraviolet irradiation (254 nm) and the onset of DNA synthesis was critical in determining the cytotoxic and/or mutagenic effect of UV in human fibroblasts, we released cultures of NF or XP12BE cells from G0, allowed them to reattach at lower densities, irradiated them in early G1 (approximately 18 h prior to the onset of S) or just prior to S phase, and assayed the frequency of mutations to 6-thioguanine resistance and the survival of colony-forming ability. The XP12BE cells, which are virtually incapable of excising UV-induced DNA lesions, showed approximately the same frequency of mutations and survival regardless of the time of UV irradiation. In NF cells, the slope of the dose response for mutations induced in cells irradiated just prior to S was about 7-fold steeper than that of cells irradiated 18 h earlier. However, the two sets of NF cells showed no significant difference in survival. Neither were there significant differences in the survival of NF cells released from G0, plated at cloning densities and irradiated as soon as they had attached and flattened out. (approximately 20 h prior to S) or 4, 8, 12, 16, 20, or 24 h later. We conclude that the frequency of mutations induced by UV is dependent upon the number of unexcised lesions remaining at the time of semi-conservative DNA replication. However, the amount of time available for excision of potentially cytotoxic lesions is not determined primarily by the period between irradiation and the onset of S phase.