Repair kinetics in CHO cells of X-ray induced DNA damage and chromatid aberrations during a cell cycle extended by transient hypothermia

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Abstract
Transient hypothermia was employed to extend the G2 phase of CHO K1 cells in order to facilitate study of the repair of X-ray induced chromatid and DNA damage. Thus G2 + 1/2M at 37°C of 2.9 h was lengthened to 5.7 h at 33°C and 7.3 h at 29°C. While chromatid break kinetics remained essentially unaltered at 33°C, at 29°C there was an initial shoulder followed by a decrease in breaks similar to that at 37 and 33°C. Although fewer exchanges were observed at 33 and 29°C than at 37°C, a similar kinetic involving a sharp initial rise followed by a plateau was observed at 33 and 29°C, and, as far as could be judged, also at 37°C. The failure of G2 prolongation to influence the rate of break disappearance was taken as evidence in support of the view that the disappearance of chromatid breaks represented a repair process rather than the decline of chromosomal radiosensitivity throughout this phase, though the possibility of a reduced sensitivity close to the G2/M border remained open. This hypothesis was supported by the mainly flat kinetics of exchanges. The data were taken as further evidence that chromatid rejoining and misjoining (exchanges) are essentially different processes. The rates of repair of DNA double-strand breaks as measured by neutral filter elution were similar at 37 and 33°C, while there was evidence of inhibition at 29°C. Our results suggest that hypothermia at 33°C and the use of short colcemid exposure times can be used successfully to enhance the study of chromatid break kinetics in Chinese hamster cells where the short G2 phase has hitherto precluded such analysis.