REJOINING OF DNA DOUBLE-STRAND BREAKS INDUCED BY ACCELERATED NITROGEN-IONS

Rejoining of radiation-induced DNA double-strand breaks (dsb) was meas ured in cultured cells with pulsed-field gel electrophoresis after rad iation doses in the range of 5-30 Gy. Human glioma, U-343MG and Chines e hamster, V79, cells were irradiated with either accelerated nitrogen ions of high linear energy transfer, LET approximate to 125 keV/mu m, or photons from Co-60. The induction frequencies of dsb were similar for the two radiation qualities with a relative biological effectivene ss, RBE, of 0.90 and 0.89 for the human and hamster cell lines respect ively. The biphasic rejoining kinetics differed significantly between the two radiation qualities when studied in the human glioma cells. Th e difference was seen within the first hour after irradiation and afte r 6 h there were considerable differences in both the total amount of unrejoined dsb and the fraction of dsb rejoined during the slow phase. When rejoining was analysed 20-22 h after irradiation, the nitrogen i ons gale 2.5-2.9 times more residual dsb than the gamma photons. The r esults for the hamster V79 cells were, up to 2 h after irradiation, si milar, but thr difference between the two radiation qualities was less accentuated. In summary, similar initial yields of dsb after exposure of cells to high or low LET resulted in both radiation quality and ce ll type-dependent differences when the rejoining of these breaks were compared.