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

In three human cells lines, X-irradiation damage to DNA is 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 appears in CCRF-CEM and 8402 leukaemia cells; HeLa cells under these conditions demonstrate comparatively little such post-repair DNA damage. With larger doses (2000-8000 rad), DNA in HeLa cells also exhibits post-repair damage. This damage is in the form of small acid-precipitable, double-stranded DNA fragments, and is demonstrated by batch elution from hydroxylapatite and alkaline and neutral sucrose sedimentation velocity techniques. The appearance of delayed DNA damage may be an early, critical expression of radiation-induced cytotoxic progression because the lesions are dose-dependent and are apparently not repaired.