Differences in the kinetics of γ-H2AX fluorescence decay after exposure to low and high LET radiation

Purpose:In order to obtain more insight into heavy ion tumour therapy, some features of the underlying molecular mechanisms controlling the cellular response to high linear energy transfer (LET) radiation are currently analysed. Materials and methods:We analysed the decay of the integrated fluorescence intensity of γ-H2AX (phosphorylated histone H2AX) which is thought to reflect the repair kinetics of radiation-induced DNA double-strand breaks (DSB) using Laser-Scanning-Cytometry. Asynchronous human HeLa cells were irradiated with a single dose of either 1.89 Gy of 55 MeV carbon ions or 5 Gy of 70 kV X-rays. Results:Measurements of the γ-H2AX-intensities fürom 1560min resulted in a 16 decrease for carbon ions and in a 43 decrease for X-rays. After 21h, the decrease was 77 for carbon ions and 85 for X-rays. The corresponding time-effect relationship was fitted by a bi-exponential function showing a fast and a slow component with identical half-life values for both radiation qualities being 24±4min and 13.9±0.7h, respectively. Apparent differences in the kinetics following high and low LET irradiation could completely be attributed to quantitative differences in their contributions, with the slow component being responsible for 47 of the repair after exposure to X-rays as compared to 80 after carbon ion irradiation. Conclusion:γ-H2AX loss kinetics follows a bi-exponential decline with two definite decay times independent of LET. The higher contribution of the slow component determined for carbon ion exposure is thought to reflect the increased amount of complex DSB induced by high LET radiation.