Persistence of Radiation-induced Double-strand Breaks in the DNA of Heated CHO Cells

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Abstract
Chinese hamster ovary cells were either heated at 45°C for 15 min or left unheated immediately prior to irradiation and incubation at 37 or 41°C for 5 h. When cellular DNA was analysed by electrophoresis of double-stranded DNA through agarose gels 5 h after irradiation, DNA fragments presumably resulting from unrepaired DNA double-strand breaks (dsbs) were observed in the DNA of all cells. The frequency of the putative unrepaired dsbs was greater in cells heated at 45°C for 15 min before, or incubated at 41°C for 5 h after irradiation, than in unheated, control cells. Gel electrophoresis results were consistent with a failure of irradiated cells to rejoin dsb completely when heated at 45°C before, or incubated at 41°C for 5 h after irradiation. In contrast, nuclear DNA accessibility studies using either an exogenous or an endogenous endonuclease detected no change in the accessibility of DNA in nuclei from 41°C-heated cells. These DNA accessibility studies indicate that the dsbs observed in the DNA of 41°C-heated cells may not result from an actual failure of irradiated cells to repair radiation-induced dsbs during incubation at 41°C.