Characteristics of changes in the number of H2AX and Rad51 protein foci in human skin fibroblasts after prolonged exposure to low-dose rate X-ray radiation

Aim: to compare the repair process of DNA double-strand breaks in mammalian cells after acute versus prolonged exposure to X-ray irradiation with different dose rates. Material and methods. Studies were performed on primary human fibroblasts isolated from skin biopsies of healthy volunteers (women, 29 and 30 years). Cells were irradiated using an X-ray machine RUB RUST-M1 (JSC “Ruselectronics”, Moscow, Russia) at 37˚C temperature with a dose rate of 400 mGy/min (200 kV, 2*2.4 mA, a filter of 1.5 mm Al) or 4 mGy/min (50 kV, 2*0.4 mA, a filter of 1.5 mm Al). Immunocytochemical protein staining was utilized for γH2AX and Rad51 foci analysis. Results. Phosphorylated histone H2AX (γH2AX) and the key protein of homologous recombination Rad51 foci formation and disappearance kinetics were investigated simultaneously in primary human dermal fibroblasts after acute and prolonged exposure to X-ray radiation at a same dose. It was shown that the relative yield of γH2AX foci per dose reduces with decrease in dose rate, while the relative yield of Rad51 foci conversely increases. Conclusion. Our findings suggest the fundamental differences in the ratio of non-homologous end joining and homologous recombination DNA repair in acute versus prolonged irradiated cells.