The effects of 9-beta-D-arabinofuranosyladenine on the repair of DNA strand breaks in X-irradiated Ehrlich ascites tumour cells.

The effects of the DNA synthesis inhibitor 9-beta-D-arabinofuranosyladenine (beta-ara A), a nucleoside analogue of desoxyadenine, on repair of DNA single and double strand breaks (ssb and dsb) were investigated in X-irradiated Ehrlich ascites tumour cells. Repair of ssb was followed using the unwinding method, and repair of dsb was measured with both the unwinding and the neutral sucrose centrifugation methods. Repair of ssb was inhibited in the presence of beta-ara A; however, even at high concentrations some repair took place. It is suggested that this proportion of the breaks (about 30 per cent) are joined by polynucleotide ligase, and do not require insertion of nucleotides. Dsb repair was strongly inhibited by beta-ara A, the inhibition being complete at high concentrations. It seems likely therefore that dsb repair has an absolute requirement for DNA polymerization. When cells were treated with beta-ara A (200 mumols/1, 2 hours) after irradiation dsb repair was inhibited; however, when the drug was washed away, repair of dsb returned. At 6 hours more breaks were found to have persisted in beta-ara A treated cells than in the untreated controls. Cells treated after X-irradiation with beta-ara A for 7 hours at 120 mumols/1 in conditioned medium and afterwards in fresh medium free of beta-ara A for 24 hours showed a higher number of residual dsb than control cells. It is suggested that these residual dsb may be relevant to the increased killing effect caused by adding beta-ara A to X-irradiated cultures.