Positional stability of damaged chromatin domains along radiation tracks in mammalian cells.

Irradiation of cell nuclei with charged particles leads to the spatially defined production of DNA damage along the particle trajectories, thus facilitating studies on the dynamics of radiation-induced protein foci associated with lesion processing. Here we used visual inspection and computational analysis of the track morphology after immunodetection to describe the patterns of formation of gamma-H2AX foci and the repair-related proteins 53BP1 and RPA. We addressed the influence of lesion density on gamma-H2AX formation and the mobility of damaged chromatin sites by using low-angle irradiation of cell monolayers with low-energy carbon or uranium ions. We show the discrete formation of gamma-H2AX foci and the recruitment of repair-related proteins along ion trajectories over an LET range from 200 to 14300 keV/microm in human fibroblasts and in HeLa cells. The marked DSBs exhibited a limited mobility that was independent of the LET. The moderate extent of mobility in human fibroblasts pointed to a relatively stable positioning of the damaged chromatin domains during repair, in contrast to HeLa cells, which showed significant changes in the streak patterns in a fraction of cells, suggesting greater mobility in the local processing of DSBs. Our data indicate that the presence of single or multiple DSBs is not associated with an altered potential for movement of damaged chromatin. We infer that the repair of high-LET radiation-induced DSBs in mammalian cells is not coupled to an increased motional activity of lesions enhancing the probability of translocations.