HEAT-EFFECTS ON THE REPAIR OF DNA DOUBLE-STRAND BREAKS IN CHO CELLS

The effect of heat (43-45 degrees C) on the induction and the repair o f DNA double-strand breaks (dsb) was studied in CHO cells after 60 Gy of X-rays using constant-field gel electrophoresis. Heat given prior t o irradiation was found not to alter the number of dsb when measured i mmediately after irradiation. In non-heated cells, about 80% of all ds b were rapidly repaired with a half-time of 4 min, while 20% were repa ired more slowly with T-1/2=160 min. These kinetics were grossly alter ed by heat. Both the fast and the slow process were retarded. However, the main effect of heat was an increase in the number of slowly rejoi ned dsb. This increase was shown to result from the additional formati on (up to 1.4-fold the initial number) of dsb early during the repair course. It is suggested that the additional dsb arose from base damage , the repair of which was unbalanced by heat. No evidence was found fo r apoptosis being involved in this process. The kinetics of the additi onal dsb was found to correlate with thermal radiosensitization.