Non-reparable DNA strand breakage was measured by means of alkaline unwinding technique in CHO, CHO K1, xrs1 and xrs5 cells for X-ray doses ranging from 15 to 180 Gy. For comparison, cell survival was recorded for doses up to 9 Gy using the colony forming assay. DNA strand breaks determined 24 h after irradiation were considered as non-preparable, since it was shown previously that repair processes are completed by 20 h after irradiation and the level reached remained constant for at least another 10 h. The number of non-reparable strand breaks was found to increase with dose. This increase was much steeper for xrs1 and xrs5 cells as compared with that for the parental strain CHO K1. This difference correlates with the respective cellular radiosensitivities. For the three cell lines the ratio of non-reparable strand break/lethal event is about 15:1. For the original CHO cells, which showed the same radiosensitivity as K1 cells, much less non-reparable breaks were found when compared with K1 cells. For CHO cells the ratio of residual strand breaks/lethal event is about 1:1. From these data it is concluded that about one non-reparable DNA strand break is sufficient for cell kill, and that the higher number of non-reparable breaks found for CHO K1 cells and xrs mutants results from unrejoined breaks which do not affect lethality. These breaks might result from DNA degradation.
