REPAIR OF IONIZING-RADIATION INDUCED DNA DOUBLE-STRAND BREAKS (DSB) AT THE C-MYC LOCUS IN COMPARISON TO THE OVERALL GENOME

Purpose: The aim of this study was to determine the repair of radiatio n-induced DNA double-strand breaks (dsb) in actively transcribed regio ns and in the overall genome. Materials and methods: Pulsed-field gel electrophoresis was performed on Sfi I restriction enzyme digested DNA , labelled with a c-myc probe and on non-specifically C-14-labelled DN A of the adenocarcinoma cell line COLO320HSR after 400 Gy irradiation with 7 MeV electrons and repair incubation. Results: At the 130 kbp c- myc locus 68 +/- 5% of all dsb induced at a dose of 400 Gy were repair ed by a fast mechanism with a repair half time of 9.4 +/- 3.2 min in c omparison to the overall genome where all dsb induced at 400 Gy were r epaired with a half time of 86 +/- 23 min. The fraction of residual ds b was about 30% higher in the c-myc locus than in the overall genome. Conclusions: This study demonstrates intragenomic heterogeneity in hal f times of dsb repair with faster repair at the c-myc locus. In additi on, differences in the residual dsb were found to represent region spe cific heterogeneity in residual damage or to possibly be attributed to the different assays used. The approach with gene probing can disting uish between correct and incorrect rejoining of dsb within the resolut ion of the experiments (less than or equal to 20 kbp), in contrast to the assay at the overall genome.