Induction of DNA double-strand breaks by ionizing radiation at the c- myc locus compared with the whole genome: a study using pulsed-field gel electrophoresis and gene probing

Ionizing radiation-induced double-strand breaks (dsb) in a human colon carcinoma-derived cell line COLO320HSR were determined from the fragment size distribution of non-specifically labelled DNA and Sfi I restriction enzyme-digested DNA uniformly labelled with a c-myc probe. The dose-effect relation for the induction of DNA dsb was linear with no significant difference between slopes for the curves in the whole genome (7.2 +/- 0.3 x 10(-9) dsb/bp/Gy) and in the 130 kbp restriction fragments containing c-myc (6.5 +/- 0.5 x 10(-9) dsb/bp/Gy). The size distribution of the c-myc fragments showed deviations from the random-breakage model, indicating heterogeneity of dsb induction at this locus.