Induction and rejoining of large DNA fragments after ion irradiation

Radiation-induced DNA double-strand breaks (DSBs) were analyzed by separati ng large DNA fragments by pulsed-held gel electrophoresis. Human U-343MG gl ioma and K562 erythroleukemia cells were irradiated with Co-60 gamma rays o r nitrogen ions with high linear energy transfer (125 keV/mu m). By compari ng the fraction of DNA released into the gel below different size threshold s, corresponding to megabase-pair-sized DNA fragments, the relative effecti veness of the nitrogen ions was found to be dependent on both dose and the threshold size used in the evaluation. This dose dependence was most eviden t for the smallest threshold (6 Mbp) and was due to a linear dose response for release of the fragments for the ions compared to the curvilinear respo nse for the gamma rays. The two curves intersected, and the relative yield of fragments (nitrogen ions/gamma rays) decreased from more than 3 below 1. 5 Gy to 0.8 at 30 Gy. For the larger sizes (6-10.5 Mbp), the relative yield was constant at around 0.7. Thus the,ion-induced fragments were shifted to smaller sizes compared to the Co-60 gamma rays, and the data for nitrogen ions could not be fitted to random fragment distributions at doses less tha n or equal to 20 Gy. From these results, we-conclude that a substantial fra ction of the DSBs induced by heavy ions were nonrandomly distributed, corre lated with DSBs within a region of less than or equal to 2 Mbp. After a dos e of 20 Gy, the rejoining curves for ion-induced DSBs were different for ea ch fragment size, resulting in different levels of unrejoined breaks after 6 h. (C) 1999 by Radiation Research Society.