Low levels of clustered oxidative DNA damage induced at low and high LET irradiation in mammalian cells.

DNA double-strand breaks (DSBs) and locally multiply damaged sites (LMDS) induced by ionizing radiation (IR) are considered to be very genotoxic in mammalian cells. LMDS consist of two or more clustered DNA lesions including oxidative damage locally formed within one or two helical turns by single radiation tracks following local energy deposition. They are thought to be frequently induced by IR but not by normal oxidative metabolism. In mammalian cells, LMDS are detected after specific enzymatic treatments transforming these lesions into additional DSBs that can be revealed by pulsed-field gel electrophoresis (PFGE). Here, we studied radiation-induced DSBs and LMDS in Chinese hamster ovary cells (CHO-K1). After addition of the iron chelator deferoxamine (DFO) or the antioxidant glutathione (GSH) to the cell lysis solution, we observed reduced spontaneous DNA fragmentation and a clear dose-dependent increase of radiation-induced DSBs. LMDS induction, however, was close to background levels, independently of dose, dose rate, temperature and radiation quality (low and high LET). Under these experimental conditions, artefactual oxidative DNA damage during cell lysis could not anymore be confounded with LMDS. We thus show that radiation-induced LMDS composed of oxidized purines or pyrimidines are much less frequent than hitherto reported, and suggest that they may be of minor importance in the radiation response than DSBs. We speculate that complex DSBs with oxidized ends may constitute the main part of radiation-induced clustered lesions. However, this needs further studies.