Assessing the variation of DNA damage response of individual human cells exposed to photon radiation.

Individual cells of the same type and in the same position in the cell cycle show stochastic cell-to-cell variability (noise) in gene and protein expression, even under the same environmental conditions. The aim of the study was to test if radiation-induced DNA damage response of cells shows noise, on top of the randomly distributed DNA damage. This was tested by comparing the frequency of DNA repair foci in sister nuclei of binucleated cells (intra BNC variability) and nuclei of two BNC (inter BNC variability). Inter cell variability was also measured for mononucleated cells (MNC). Binucleation was induced in U2OS cells by cytochalasin B. Cells were exposed to 2 Gy of photons, 53BP1 and γH2AX foci were visualised and counted in nuclei of binucleated and mononucleated cells 30, 60, 120 and 180 min post exposure. Coefficients of variation were calculated as indicators of variability. For both types of foci, the frequencies observed in sister nuclei within BNC were positively correlated and exhibited significantly lower variation compared to the variability measured between single nuclei from randomly paired BNC. The contribution of noise to the total variability was 28% for 53BP1 foci and 50% for γH2AX foci. The results demonstrate that the variability of focus frequency observed in nuclei of BNC results from a combination of noise and random damage variability.