Effects of ara A and fresh medium on chromosome damage and DNA double-strand break repair in X-irradiated stationary cells.

Evidence supports the view that double-strand breaks (dsb) in the DNA of X-irradiated mammalian cells are the lesions leading to chromosome aberrations and eventual cell death. The detailed kinetics of repair of dsb in Ehrlich ascites tumour cells over long repair intervals have therefore been studied and compared under conditions simulating procedures known to cause large changes in cell survival. These conditions are: holding cells in stationary phase for 7 h after X-irradiation, transference of cells to fresh growth medium immediately after X-irradiation, and treatment with the DNA synthesis inhibitor 9-beta-D-arabinofuranosyladenine (ara A) for 30 min before, during and for 7 h after X-irradiation. These conditions have also been investigated for their effects on the frequencies of chromosome abnormalities (anaphase bridges and fragments). It is shown that conditions leading to both an inhibition of dsb repair (in the presence of ara A) as well as an acceleration of dsb repair (by fresh growth medium) lead to higher frequencies of chromosome abnormalities as compared to those for cells under stationary conditions for 7 h after irradiation. Holding dsb open for long periods with ara A may maximize the probability of formation of aberrations, however the data for fresh medium treatment show that it is not merely the rate at which dsb repair which determines the aberration frequency, and indicate the presence of other biochemical mechanisms in the cell which determine the frequency of conversion of dsb into chromosome aberrations.