CORRELATION BETWEEN THERMAL RADIOSENSITIZATION AND SLOWLY REJOINED DNA STRAND BREAKS IN CHO CELLS

The effect on the repair of slowly rejoined strand breaks was studied in CHO cells using the alkaline unwinding technique. Heat (45 degrees C, 20 min) combined with a X-ray dose of 9 Gy was found to result in a n increased half-time of repair but also in an increased number of slo wly rejoined strand breaks. when a time interval at 37 degrees C was i nserted between irradiation and heat, the half-time of repair was not altered, whereas the number of slowly rejoined strand breaks as measur ed 300 min after irradiation decreased with increasing time interval b etween the two treatments. The half-time of 18 +/- 2 min suggested tha t the additionally formed, slowly rejoined strand breaks arise from a certain type of radiation-induced DNA base lesions with repair of whic h is modified by heal. The effect of X-irradiation combined with heat was also studied for cell survival. When irradiation and heat were sep arated by an incubation at 37 degrees C, cell survival increased with a half-time of 20 +/- 2 min, which is similar to that measured for the number of additional, slowly rejoined strand breaks. For a great vari ety of combined treatments, the reduction in cell survival correlates well with the enhanced number of slowly rejoined strand breaks measure d 300 min after irradiation. This positive correlation and the similar ity in the half-times mentioned above suggests that thermal radiosensi tization results from the number of additional, slowly rejoined strand breaks formed when irradiation was combined with heat.