Repair of ionizing radiation induced DNA double-strand breaks (dsb) at the c-myc locus in comparison to the overall genome

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Abstract
Purpose: The aim of this study was to determine the repair of radiation-induced DNA double-strand breaks (dsb) in actively transcribed regions 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 14C-labelled DNA 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 repaired by a fast mechanism with a repair half time of 9.4 3.2 min in comparison to the overall genome where all dsb induced at 400 Gy were repaired with a half time of 86 23 min. The fraction of residual dsb was about 30% higher in the c-myc locus than in the overall genome. Conclusions: This study demonstrates intragenomic heterogeneity in half times of dsb repair with faster repair at the c-myc locus. In addition, differences in the residual dsb were found to represent region specific heterogeneity in residual damage or to possibly be attributed to the different assays used. The approach with gene probing can distinguish between correct and incorrect rejoining of dsb within the resolution of the experiments (20 kbp), in contrast to the assay at the overall genome.

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