Kinetics and dose-response of residual 53BP1 γ-H2AX foci: Co-localization, relationship with DSB repair and clonogenic survival

Purpose: Recent studies revealed that some foci produced by phosphorylated histone 2A family member X (γ-H2AX) and tumor suppressor p53 binding protein 1 (53BP1) that co-localize with radiation-induced DNA double-strand breaks (DSB) remain in cells at relatively long times after irradiation and indicated a possible correlation between cellular radiosensitivity and residual foci. In this study, we investigated dose-responses and kinetics for radiation-induced 53BP1 γ-H2AX foci formation in relation to their co-localization, DSB repair and cell survival. Materials and methods: Cell survival, DSB and foci were analyzed by clonogenic assay, pulsed field gel electrophoresis (PFGE), and confocal laser microscopy, respectively, in normal human fibroblasts (VH-10) and in a cancer cell line (HeLa). Computer analysis was used to determine both the number and the area of foci. Results: We show that even at doses down to 1 cGy a statistically significant induction of 53BP1 foci is observed. While the number of foci was found to constantly decrease with post-irradiation time, the per-cell normalized area of foci does not change within a time window of approximately 4 h post-irradiation. Co-localization of γ-H2AX and 53BP1 foci is shown to depend on dose and post-irradiation time. No clear correlations were established between radiosensitivity and foci formation because the dose response for 53BP1 γ-H2AX foci may depend on time after irradiation and duration of the cell cycle. We show that the kinetics of foci disappearance within 24 h post-irradiation do not coincide with those of DSB repair. Conclusions: The data suggest that the post-irradiation time used for estimation of radiosensitivity at therapeutically relevant low doses (e.g.,