Repair of chromosome and DNA breaks versus cell survival in Chinese hamster cells.

Clonogenic and non-clonogenic parameters of cell survival were compared in irradiated Chinese hamster cells. Clonogenic survival, chromatid break and repair kinetics, as well as DNA damage and repair, were assessed in synchronized cells in different parts of the cell cycle. C2 chromatid damage and repair was examined in metaphase chromosomes of cells irradiated during S and G2 phase, treated with or without inhibitors of DNA repair. Bromodeoxyuridine labelling of S phase cells starting at the time of irradiation made it possible to determine precisely, while scoring metaphase chromosomes, whether cells were irradiated in mid S, late S, or G2 phases of the cycle. The results showed that chromatid breaks induced in S phase are efficiently repaired until the moment cells progress into G2, when repair stops abruptly. Chromatid damage in G2 phase is not repaired. On the other hand, DNA double-strand breaks are repaired in all phases of the cycle, even during G2 phase which has no concurrent chromatid break repair. Finally, there is no consistent correlation between chromatid damage and repair, DNA damage and repair, and cell survival, thus indicating that the interaction of different parameters of radiosensitivity must be better understood for them to be useful predictors of cell survival.