A class of interphase
chromosome breaks was visualized in irradiated (10 Gy) plateau-phase CHO cells
after treatment (2-30 min) in hypertonic (500 mM NaCl) growth medium during the period normally allowed for chromosome condensation, in the premature chromosome condensation (PCC) assay. Rejoining of this class of interphase
chromosome breaks was fast (t1/2 = 1.5 min) compared to the rejoining of interphase
chromosome breaks normally observed in the absence of hypertonic treatment (t1/2 = 76 min), suggesting that they are formed from a different subset of precursor DNA lesions. A fast (t1/2fast = 12 min) and a slow (t1/2slow = 71 min) component were also observed in the rejoining of radiation-induced (50 Gy)
DNA double-strand breaks (DSBs), as measured by pulsed-field gel electrophoresis. We propose that fast-repairing DSBs are the precursor lesions underlying the fast-repairing interphase
chromosome breaks observed in these experiments. Slowly repairing DSBs are postulated to be the precursor lesions underlying the slowly repairing interphase
chromosome breaks visualized using regular protocols for PCC. The visualization of fast-repairing interphase
chromosome breaks achieved in these experiments is assumed to be due to a destabilization of
chromatin by the hypertonic medium. This
chromatin destabilization may cause either an inhibition of the rejoining of the fast component of DSBs during the period allowed for PCC or a transformation of a defined subset of fast-repairing DSBs into
chromosome breaks. The latter hypothesis allows a more consistent interpretation of the available results. Transformation of a defined subset of fast-repairing DSBs to interphase
chromosome breaks may be equivalent to damage fixation, and may correspond to the fixation of a form of
PLD (beta-
PLD) sensitive to treatment in hypertonic medium.