Relationship of DNA repair to chromosome aberrations, sister-chromatid exchanges and survival during liquid-holding recovery in X-irradiated mammalian cells.

The repair of X-ray-induced DNA single strand breaks and DNA--protein cross-links was investigated in stationary phase, contact-inhibited mouse cells by the alkaline-elution technique. Approx. 90% of X-ray-induced single strand breaks were rejoined during the first hour of repair, whereas most of the remaining breaks were rejoined more slowly during the next 5 h. At early repair times, the number of residual non-rejoined single strand breaks was approx. proportional to the X-ray dose. DNA--protein cross-links were removed at a slower rate (T1/2 approx. 10--12 h). Cells were held in stationary growth for various periods of time after irradiation before subculture at low density to score for colony survival (potentially lethal damage repair), chromosome aberrations in the first mitosis, and sister-chromatid exchanges in the second mitosis. Both cell killing and the frequency of chromosome aberrations decreased during the first several hours of recovery, reaching a minimum level by 6 h; this decrease correlated temporally with the repair of the slowly rejoining DNA-strand breaks. Relatively few sister-chromatid exchanges were observed when the cells were subcultured immediately after X-ray. The exchange frequency rose to maximum levels after a 4-h recovery interval, and returned to control levels after 12 h of recovery. The possible relationship of DNA repair to these changes in survival, chromosome aberrations, and sister-chromatid exchanges during liquid-holding recovery is discussed.