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Repair of DNA damage in mammalian cells after low doses of ionizing radiation Berger, Stuart Alan


It generally accepted that the critical target of ionizing radiation in cellular reproductive death is DNA. What is less known is what types of lesions are involved, how they are repaired and how various agents enhance, protect or modify them. One of the main reasons for these difficulties has been the lack of assays for molecular damage sensitive enough to measure yields at doses where significant numbers of cells survive. Recently however, various techniques have appeared that exhibit this sensitivity. One of these, Rydbergs’ double-label unwinding method, can routinely detect damage in the DNA of mammalian cells from doses as low as 0.1 Gy. We have used this technique to measure initial yields, repair kinetics and residual yields of damage in the dose range 1-6 Gy. We have found, in substantial agreement with measurements at higher doses, the existence of at least two distinct repair processes with half-lives of 7 minutes and 250 minutes respectively. We have also found that the distribution of breaks between the fast and slow processes can be altered by various conditions; for example: whether or not the irradiation was done in the presence of oxygen or nitrogen, post-treatment in hypertonic solution, incubation with ARA-A, (an inhibitor of DNA synthesis) or post-treatment in dimethyl sulphoxide. We have also found that the yield of unrepaired lesions (as defined by damage remaining after two hours incubation at 37°C) seems to include a quadratic variation with initial dose suggesting that among the slowly repairing breaks, there exist complex, combination-type lesions (which may include DNA double-strand breaks). Such studies, combined with results from other assays should help in understanding the nature of the lesions in DNA and the repair processes acting on them and will eventually aid in identifying the critical lesion(s) responsible for cell death.

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