Repair of I-SceI Induced DSB at a specific site of chromosome in human cells: influence of low-dose,low-dose-rate gamma-rays

We investigated the influence of low-dose, low-dose-rate gamma-ray irradiation on DNA double strand break (DSB) repair in human lymphoblastoid TK6 cells. A single DSB was introduced at intron 4 of the TK+ allele (chromosome 17) by transfection with the I-SceI expression vector pCBASce. We assessed for DSB repair due to non-homologous end-joining (NHEJ) by determining the generation of TK-deficient mutants in the TK6 derivative TSCE5 (TK +/−) carrying an I-SceI recognition site. We similarly estimated DSB repair via homologous recombination (HR) at the same site in the derived compound heterozygote (TK−/−) cell line TSCER2 that carries an additional point mutation in exon 5. The NHEJ repair of DSB was barely influenced by pre-irradiation of the cells with 30 mGy γ-rays at 1.2 mGy h⁻¹. DSB repair by HR, in contrast, was enhanced by ~50% after pre-irradiation of the cells under these conditions. Furthermore, when I-SceI digestion was followed by irradiation at a dose of 8.5 mGy, delivered at a dose rate of only 0.125 mGy h⁻¹, HR repair efficiency was enhanced by ~80%. This experimental approach can be applied to characterize DSB repair in the low-dose region of ionizing radiation.