Measurement of radiation-induced DNA damage using gel electrophoresis or neutral filter elution shows an increased frequency of DNA strand breaks after exposure to pH 9.6

The filter elution technique using nondenaturing conditions is widely used to assay DNA double-strand break (DSB) induction and repair. It has been reported that in the measurement of strand breaks higher rates of elution and of initial rejoining are obtained at pH 9.6 compared to pH 7.2. In the present experiments neutral elution at pH 7.2 and 9.6 were compared in the assay of damage to DNA induced by X rays, 125I decay, and restriction enzyme digestion, in an effort to explain this discrepancy and to determine whether the higher rate of elution observed at pH 9.6 corresponds to a greater number of DSBs. X-ray damage to cellular DNA resulted in significantly different elution profiles at the two pH values. In contrast the elution profiles of the DSB induced by intragenomic 125I decays or restriction endonuclease were independent of the pH of the elution buffer. When gamma-irradiated SV40 DNA was exposed to pH 7.2 or 9.6 elution buffer prior to analysis by gel electrophoresis, a significantly greater number of DNA DSBs were detected in the DNA exposed to pH 9.6. We conclude that X and gamma radiation produce lesions (pH 9.6-labile lesions), in proportion to dose, that have the potential of becoming measurable DSBs following incubation under the mildly alkaline condition of pH 9.6. The data suggest that these lesions may result from single-hit events.