Phosphorylated H2AX Foci in Tumor Cells Have No Correlation with Their Radiation Sensitivities

Ionizing radiation causes DNA double strand breaks (DSBs), which produce a chromosomal change with the modification of chromatin protein. The histone H2AX is phosphorylated, and phosphorylated H2AX makes a focus. The phosphorylated H2AX focus is regarded as recruiting mediators of repair factors of DNA DSBs. Although most of the initial phosphorylated H2AX foci disappear with the repair of DNA DSBs, a few foci remain, and whether these residual DSBs are correlated with radiosensitivity is not clear. Therefore, we examined the correlation between residual DSBs and cellular radiosensitivity after ionizing radiation. We found that half of the non-irradiated normal cells had a few phosphorylated H2AX foci constantly, and most of the cells irradiated with less than 1% of the colony-forming dose had phosphorylated H2AX foci even 5 days after irradiation. Some tumor cell lines had phosphorylated H2AX foci even under non-irradiated conditions. These results indicate that residual phosphorylated H2AX foci may show loss of colony-forming potential after irradiation in normal cell lines. However, results suggested that there was not a close correlation between residual foci and radiosensitivity in some tumor cell lines, which showed high expression of endogenous phosphorylated H2AX foci. Moreover, micronuclei induced by X-ray irradiation had phosphorylated H2AX foci, but phosphorylated ATM, phosphorylated DNA-PKcs, and 53BPI foci were not co-localized. These results suggest that DNA DSBs may be not a direct cause of micronuclei generation or H2AX phosphorylation. (227 words)