Rejoining of DNA fragments produced by radiations of different linear energy transfer

Purpose : To analyse the rejoining of DNA double-strand breaks (dsb) produced by high-linear energy transfer (LET) ions, with the specific focus on the influence, on the rejoining estimates, of the way dsb are distributed along chromosomes. Material and methods : Low passages of normal human fibroblasts (GM5758) were irradiated with 60 Co photons, 40 keV/mum helium ions or nitrogen ions with LETs of 80, 125, 175 and 225 keV/mum. The double-stranded DNA fragment distributions, ranging from 5 kbp to 5.7 Mbp, were assayed by pulsed-field gel electrophoresis after repair incubation for 0-22 h. Results : The rejoining was biphasic and the half-times of the two phases were 15 min and 2-3 h, respectively, and were independent of LET. Although the majority of breaks were rejoined by the fast phase, both the fraction of dsb rejoined by the slow phase and the fraction of unrejoined dsb at 20-22 h increased with increasing LET. Conclusions : DNA fragment analysis detected LET-dependent differences in the amount of rejoining while the half-times were independent of LET. The majority of dsb were rapidly rejoined even after high-LET irradiation. If fragment-size distribution is not taken into account, both the fraction of breaks rejoined by slow kinetics, and the fraction of unrejoined breaks, can be overestimated when the LET is increased.