Phosphorylated histone H2AX in relation to cell survival in tumor cells and xenografts exposed to single and fractionated doses of X-rays

Background and purposeHuman tumor cell lines grown as monolayers or xenograft tumors were exposed to single or multiple fractions of X-rays and the ability to use residual γH2AX to identify radiosensitive cells was assessed.Materials and methodsTwenty-four hour after exposure to single or daily fractions of X-rays, human tumor cells from monolayers or xenografts were analyzed for clonogenic surviving fraction. Cells were also fixed and labeled with anti-γH2AX antibodies for analysis by flow and image cytometry. The relative amount of residual γH2AX and the percentage of cells with <3 foci were compared with the clonogenic surviving fraction measured for the same population.ResultsThe fraction of γH2AX remaining 24 h after X-irradiation relative to peak levels 1 h after exposure was correlated with radiosensitivity (SF2) for 18 human tumor cell lines. The fraction of SiHa, C33A and WiDr cells with <3 γH2AX foci was predictive of clonogenic surviving fraction for both monolayer cells exposed to either single doses or up to 5 fractions. Similar results were obtained using cells from xenograft tumors of irradiated mice.ConclusionThe percentage of tumor cells that retain γH2AX foci 24 h after single or fractionated doses appears to be a useful measure of cellular radiosensitivity that is potentially applicable in the clinic.