The Level of Induced DNA Double-strand Breaks Does Not Correlate with Cell Killing in X-irradiated Mitotic and G₁-phase CHO Cells

Summary

The neutral (pH 9·6) filter elution technique was used to evaluate DNA damage induced in CHO cells irradiated at mitosis or in G₁-phase under various incubation and postirradiation treatment conditions. Mitotic and G₁/S border cells were more sensitive to radiation than G₁ cells with respect to cell killing, but showed similar (G₁/S) or lower (M) DNA elution dose–response curves. Similar cell survival and DNA/elution dose–response curves were obtained with plateauphase cultures containing mainly G₁-cells, as well as with G₁ cells obtained after division of mitotic cells in either fresh or conditioned medium. However, survival of plateau-phase cells could be modified substantially by delayed-plating or postirradiation treatment with araA. These results, together with previously published observations, indicate that induction of DNA dsb cannot be invoked as an explanation for the variations in radiosensitivity observed through the cycle, or as an explanation for the formation of the survival curve shoulder. It is proposed that repair and fixation of radiation-induced DNA damage, expressed at the cell survival level as repair and fixation of alpha-PLD, are responsible for these effects.