Post-repair DNA damage in X-irradiated cultured human tumour cells

In three human cell lines, X-irradiation damage to DNA was quantitatively repaired in 2 hours or less. At 4 hours following single doses of 1000 rad or less, delayed or latent damage to the DNA appeared in CCRF-CEM and 8402 leukaemia cells; HeLa cells under these conditions demonstrated comparatively little such post-repair DNA damage. With larger doses (2000-8000 rad), DNA in HeLa cells also exhibited post-repair damage. This damage was in the form of small acid-precipitable, double-stranded DNA fragments, and was demonstrated by batch elution from hydroxylapatite and alkaline and neutral sucrose sedimentation velocity techniques. It is suggested that the appearance of delayed DNA damage could be an early, critical expression of radiation-induced cytotoxic progression because the lesions were dose-dependent and were apparently not repaired. (author)