Evaluation of the Contribution of Homologous Recombination in DNA Double-Strand Break Repair in Human Fibroblasts after Exposure to Low and Intermediate Doses of X-ray Radiation

Studies of the changes in the number of gamma H2AX foci (a DNA double-strand break protein-marker), and Rad51 foci (a key homologous recombination protein) were conducted on human fibroblast cultures during the 24 hours after exposure to low (80 mGy) and intermediate (250 and 1000 mGy) doses of X-ray irradiation. Based this data, exponential curves that approximated the experimental values were constructed, and the characteristic lifetimes of the gamma H2AX and Rad51 foci were evaluated using the method of least squares. The ratio of the areas under the curves of changes in the number of Rad51 and gamma H2AX foci, calculated by the trapezium method, divided by the ratio of the characteristic lifetimes of the Rad51 and H2AX foci was used to evaluate the contribution of homologous recombination in DNA double-strand break repair. It was shown that the contribution of homologous recombination in DNA double-strand break repair during the 24 hours after exposure to 80, 250 and 1000 mGy was approximately 16, 12 and 9% respectively. Thus, the relative contribution of homologous recombination in the DNA double-strand break repair after exposure to a low dose of X-ray irradiation was approximately 1.5 times higher than that after exposure to intermediate doses. Our results suggest that DNA double-strand break repair induced after exposure to 80 mGy of X-ray irradiation is more accurate than after exposure to 250 and 1000 mGy.