Underestimation of the small residual damage when measuring DNA double-strand breaks (DSB): is the repair of radiation-induced DSB complete?

Purpose: To overcome the underestimation of the small residual damage when measuring DNA double-strand breaks (DSB) as fraction of activity released ( FAR) by pulsed-field gel electrophoresis. Materials and methods: The techniques used to assess DNA damage (e.g. pulse d-field gel electrophoresis, neutral elution, comet assay) do not directly measure the number of DSB. The Blocher model can be used to express data as DSB after irradiation at 4 degrees C by calculating the distribution of al l radiation-induced DNA fragments as a function of their size. We have used this model to measure the residual DSB (irradiation at 4 degrees C followe d by incubation at 37 degrees C) in untransformed human fibroblasts. Results: The DSB induction rate after irradiation at 4 degrees C was 39.1 /- 2.0 Gy(-1). The DSB repair rate obtained after doses of 10 to 80 Gy foll owed by repair times of 0 to 24 h was expressed as unrepaired DSB calculate d from the Blocher formula. AU the damage appeared to be repaired at 24 h w hen the data were expressed as FAR, whereas 15% of DSB remained unrepaired. The DSB repair rate and the chromosome break repair rate assessed by prema ture condensation chromosome (PCC) techniques were similar. Conclusion: The expression of repair data in terms of FAR dramatically unde restimates the amount of unrepaired DNA damage. The Blocher model that take s into account the size distribution of radiation-induced DNA fragments sho uld therefore be used to avoid this bias. Applied to a normal human fibrobl ast cell line, this model shows that DSB repair is never complete.