Rejoining of DNA double-strand breaks in X-irradiated CHO cells studied by constant- and graded-field gel electrophoresis.

Induction and repair of double-strand breaks (dsb) were measured in exponentially growing CHO-10A cells using the constant- and graded-field gel electrophoresis. Dsb repair was studied after an X-ray dose of 60 Gy. The repair curve obtained was biphasic with the respective half-times of tau 1 = 3.8 +/- 0.9 and tau 2 = 118 +/- 30 min. The number of non-reparable dsb was measured for X-ray doses up to 180 Gy and was found to be only a small fraction (14%) of all non-rejoinable breaks determined previously using the alkaline unwinding technique. The ratio of non-reparable dsb to the number of lethal events calculated from survival curves is 0.14:1. This result indicates that for CHO cells nonreparable dsb represent only a small fraction of lethal damage. This is in line with the cytogenetic observation that cell killing mainly results from mis-rejoined events (i.e. exchange aberrations, translocations, interstitial deletions). The kinetics of dsb rejoining were found to be independent of the size of the fragments involved (between 1 and 10 Mbp). In addition, the rejoining kinetics of DNA fragments < or = 1 Mbp did not show the formation of new DNA fragments with time after irradiation indicating the absence of programmed cell death in irradiated CHO cells.