Ionizing radiation induces two forms of interphase chromosome breaks in Chinese hamster ovary cells that rejoin with different kinetics and show different sensitivity to treatment in hypertonic medium or beta-araA.

We have shown previously that incubation of irradiated plateau-phase CHO cells in hypertonic growth medium during the period normally allowed for chromosome condensation, in the premature chromosome condensation (PCC) assay, uncovers a form of interphase chromosome breaks that rejoin with fast kinetics (t1/2 = 1.5 min). Here, we report that incubation with beta-arabinofuranosyladenine (beta-araA), an inhibitor of DNA, chromosome, and cellular repair processes, during the same period uncovers a different form of interphase chromosome breaks that rejoin with slower kinetics (t1/2 = longer than 15-20 min). The yield of interphase chromosome breaks increased from 2.0 breaks/cell/Gy in untreated control cells to 3.6 breaks/cell/Gy in cells exposed to 1 mM beta-araA, and was the same as that observed in cells treated in hypertonic medium (500 mM NaCl). Simultaneous exposure to beta-araA and hypertonic medium increased the yield of interphase chromosome breaks further to 5.3 breaks/cell/Gy. This increase was consistent with an additive effect of each treatment on the overall yield of breaks, and suggested that hypertonic medium and beta-araA affect distinct and independent subsets of radiation-induced interphase chromosome breaks. We tested further the notion of independence by measuring rejoining of interphase chromosome breaks sensitive to hypertonic treatment in the presence of 1 mM beta-araA, and vice versa, rejoining of interphase chromosome breaks sensitive to beta-araA during and after treatment in hypertonic medium (500 mM NaCl, 20 min); under both sets of conditions each treatment caused maximal expression of prematurely condensed chromosome breaks responding sensitively to it when given immediately after irradiation. There was no change in the rejoining kinetics of interphase chromosome breaks sensitive to hypertonic treatment in the presence of beta-araA, and no change in the rejoining kinetics of interphase chromosome breaks sensitive to beta-arA in cells treated in hypertonic medium. These results are consistent with the hypothesis that exposure to ionizing radiation leads to the induction of two forms of prematurely condensed chromosome breaks that can be distinguished from each other on the basis of their repair kinetics and their differential sensitivity to treatment with beta-araA or hypertonic medium. In direct analogy to a classification proposed previously for potentially lethal damage (PLD) based on a similar set of experiments, we introduce the terms alpha form and beta form of interphase chromosome breaks for the slow, beta-araA-sensitive, and the fast, hypertonic treatment-sensitive form, respectively. We also propose that there is a correlation between alpha and beta form of interphase chromosome breaks and alpha and beta form of PLD, and present evidence suggesting that fast and slowly repairing DNA double-strand breaks underlie fast and slowly repairing interphase chromosome breaks. (ABSTRACT TRUNCATED AT 400 WORDS)