DNA double-strand break signalling: X-ray energy dependence of residual co- localised foci of -H2AX and 53BP1

The foci of gamma-H2AX alone or colocated with 53BP1 are currently used in a lot of different studies to detect DNA double-strand breaks (DSB) after ionizing radiation. The primary objective of our study was to compare the radiation energy, dose dependent induction and kinetics of DNA DSB, in particular with regard to differences between soft and higher energy X-rays with the colocated gamma-H2AX and 53BP1 foci. We chose the human mammary epithelial cell line 184A1 for these experiments due to the medical application of 25 kV X-rays, e.g. in mammography. X-ray energies applied were 25 kV and 10 kV, the latter one by reason of the proposed maximum biological effectiveness at this energy, and 200 kV X-rays as reference radiation source.
The time courses of foci appearance and loss showed a similarity in the kinetics for all energies applied with maxima at 2 h (2 Gy) and 0.5 h for (0.25 Gy) after irradiation. Despite these uniform kinetics, we investigated the dose dependence of the colocated foci, allowing to calculate the RBE of soft X-rays in the range of 1.3 to 2.2 (25 kV) and 2.9 to 6.4 (10 kV).