Phosphorylated Histone H2AX in Relation to Cell Survival in Tumor Cells and Xenografts Exposed to Single and Fractionated Doses of X-rays

Background And Purpose: Human 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 gammaH2AX to identify radiosensitive cells was assessed.

Materials And Methods: Twenty-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-gammaH2AX antibodies for analysis by flow and image cytometry. The relative amount of residual gammaH2AX and the percentage of cells with <3 foci were compared with the clonogenic surviving fraction measured for the same population.

Results: The fraction of gammaH2AX remaining 24h after X-irradiation relative to peak levels 1h after exposure was correlated with radiosensitivity (SF2) for 18 human tumor cell lines. The fraction of SiHa, C33A and WiDr cells with <3 gammaH2AX 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.

Conclusion: The percentage of tumor cells that retain gammaH2AX foci 24h after single or fractionated doses appears to be a useful measure of cellular radiosensitivity that is potentially applicable in the clinic.