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$(t_{1/2}=1.5\ {\rm min})$ compared to the rejoining of interphase chromosome breaks normally observed in the absence of hypertonic treatment$(t_{1/2}=76\ {\rm min})$, suggesting that they are formed from a different subset of precursor DNA lesions. A fast$(t_{1/2\ {\rm fast}}=12\ {\rm min})$ and a slow$(t_{1/2\ {\rm slow}}=71\ {\rm min})$ 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 fastrepairing 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 (β-PLD) sensitive to treatment in hypertonic medium.

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