Costes, S. V., Boissière, A., Ravani, S., Romano, R., Parvin, B. and Barcellos-Hoff, M. H. Imaging Features that Discriminate between Foci Induced by High- and Low-LET Radiation in Human Fibroblasts. Radiat. Res. 165, 505–515 (2006).

In this study, we investigated the formation of radiation-induced foci in normal human fibroblasts exposed to X rays or 130 keV/μm nitrogen ions using antibodies to phosphorylated protein kinase ataxia telangiectasia mutated (ATMp) and histone H2AX (γ-H2AX). High-content automatic image analysis was used to quantify the immunofluorescence of radiation-induced foci. The size of radiation-induced foci increased for both proteins over a 2-h period after nitrogen-ion irradiation, while the size of radiation-induced foci did not change after exposure to low-LET radiation. The number of radiation-induced ATMp foci showed a more rapid rise and greater frequency after X-ray exposure and was resolved more rapidly such that the frequency of radiation-induced foci decreased by 90% compared to 60% after exposure to high-LET radiation 2 h after 30 cGy. In contrast, the kinetics of radiation-induced γ-H2AX focus formation was similar for high- and low-LET radiation in that it reached a plateau early and remained constant for up to 2 h. High-resolution 3D images of radiation-induced γ-H2AX foci and dosimetry computation suggest that multiple double-strand breaks from nitrogen ions are encompassed within large nuclear domains of 4.4 Mbp. Our work shows that the size and frequency of radiation-induced foci vary as a function of radiation quality, dose, time and protein target. Thus, even though double-strand breaks and radiation-induced foci are correlated, the dynamic nature of both contradicts their accepted equivalence for low doses of different radiation qualities.