Detection of ionizing radiation-induced DNA double-strand breaks by filter elution is affected by nuclear chromatin structure

Chinese hamster ovary cells were irradiated with 250 kVp X rays and analyzed for the presence of DNA double-strand breaks using either polycarbonate filter elution or pulsed-field agarose gel electrophoresis at neutral pH. Reduction in DNA length detected by filter elution was produced as a nonlinear function of increasing radiation dose, with a quasi-threshold at low total dose, and as a first-order function of increasing radiation dose as detected by gel electrophoresis. The quasi-threshold observed with filter elution was eliminated when nuclei were isolated from irradiated cells and their chromatin relaxed in a buffer containing low-molarity monovalent cation prior to analysis by filter elution. The results suggest either that the chemical structure of the DNA double-strand breaks produced by low-LET radiation necessitates a DNA relaxation step before they can be detected accurately by filter elution, or that at low total radiation dose a DNA complex forms on the polycarbonate filter.