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

• Published in: International journal of radiation biology Vol. 75; no. 12; pp. 1589 - 1595
• Main Author: Foray, C. F. Arlett, E. P. Malaise, N.
• Format: Journal Article
• Language: English
• Published: London Informa UK Ltd 1999; Taylor & Francis
• Subjects: Applied radiobiology (equipment, dosimetry...); Biological and medical sciences; Biological effects of radiation; Cell Line; Chromosome Breakage; Chromosomes, Human - radiation effects; Cold Temperature; DNA - analysis; DNA - physiology; DNA - radiation effects; DNA Damage; DNA Repair; Dose-Response Relationship, Radiation; Electrophoresis, Gel, Pulsed-Field; Fibroblasts - metabolism; Fibroblasts - radiation effects; Fundamental and applied biological sciences. Psychology; Humans; Tissues, organs and organisms biophysics; Human; Residual anomaly; Radiation; Quantization; In vitro; Double stranded DNA; DNA; Established cell line; Biological effect; Lesion; Mathematical model; Repair; Fibroblast
• ISSN: 0955-3002; 1362-3095
• DOI: 10.1080/095530099139197

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. pulsed-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° C by calculating the distribution of all radiation-induced DNA fragments as a function of their size. We have used this model to measure the residual DSB (irradiation at 4° C followed by incubation at 37° C) in untransformed human fibroblasts. Results: The DSB induction rate after irradiation at 4° C was 39.1 +/- 2.0Gy -1. The DSB repair rate obtained after doses of 10 to 80Gy followed by repair times of 0 to 24h was expressed as unrepaired DSB calculated from the Blocher formula. All the damage appeared to be repaired at 24h when the data were expressed as FAR, whereas 15% of DSB remained unrepaired. The DSB repair rate and the chromosome break repair rate assessed by premature condensation chromosome (PCC) techniques were similar. Conclusion: The expression of repair data in terms of FAR dramatically underestimates the amount of unrepaired DNA damage. The Blocher model that takes into account the size distribution of radiation-induced DNA fragments should therefore be used to avoid this bias. Applied to a normal human fibroblast cell line, this model shows that DSB repair is never complete.