Rejoining of DNA double-strand breaks induced by accelerated nitrogen ions

Abstract

Rejoining of radiation-induced DNA double-strand breaks (dsb) was measured in cultured cells with pulsed-field gelelectrophoresisafterradiationdosesin therangeof5-30Gy. Human glioma, U-343MG and Chinese hamster, V79, cells were irradiated with either accelerated nitrogen ions of high linear energy transfer, LET 125 keV mum, or photons from 60Co. The induction frequencies of dsb were similar for the two radiation qualities with a relative biological effectiveness, RBE, of 0.90 and 0.89 for the human and hamster cell lines respectively. The biphasic rejoining kinetics differed significantly between the two radiation qualities when studied in the human glioma cells. The difference was seen within the first hour after irradiation and after 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 20-22h after irradiation, the nitrogen gamma ions gave 2.5-2.9 times more residual dsb than the photons. The results for the hamster V79 cells were, up to 2 h after irradiation, similar, but the 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 cell typedependent differences when the rejoining of these breaks were compared.