Reaction of HO• radicals with double-stranded calf thymus DNA produces high levels of and, to a minor extent, . Formation of the hydroxylated purine lesions is explained by addition of HO • to the C8 position of the purine moiety. It has been reported that tandem lesions containing a formylamine residue neighboring 8-oxodGuo could be produced through addition of a transiently generated pyrimidine peroxyl radical onto the C8 of an adjacent purine base. Formation of such tandem lesions accounted for ≈10% of the total 8-oxodGuo. In the present work we show that addition of HO • onto the C8 of purine accounts for only ∼5% of the generated 8-oxodGuo. About 50% of the 8-hydroxylated purine lesions, including 8-oxodGuo and 8-oxodAdo, are involved in tandem damage and are produced by peroxyl addition onto the C8 of a vicinal purine base. In addition, the remaining 45% of the 8-oxodGuo are produced by an electron transfer reaction, providing an explanation for the higher yield of formation of 8-oxodGuo compared to 8-oxodAdo. Interestingly, we show that >40% of the 8-oxodGuo involved in tandem lesions is refractory to excision by DNA glycosylases. Altogether our results demonstrate that, subsequently to a single oxidation event, peroxidation reactions significantly increase the yield of formation of hydroxylated purine modifications, generating a high proportion of tandem lesions partly refractory to base excision repair.DNA damage | DNA repair | peroxidation reaction | tandem lesions F ormation of oxidatively generated DNA lesions is associated with the occurrence of degenerative diseases, cancer, and aging (1). Therefore, reactions of reactive oxygen species, and among them the highly genotoxic hydroxyl radical (HO • ), with DNA constituents have been extensively studied during the last three decades (2). 8-Oxo-7,8-dihydro-2′-deoxyguanosine (8-oxodGuo) and, to a minor extent the corresponding adenine derivative 8-oxo-7,8-dihydro-2′-deoxyadenosine (8-oxodAdo), have been shown to be significantly generated in cellular DNA under various conditions of oxidative stress. More than 20 years ago (3) it was established that in the presence of HO• (produced either by the indirect effect of ionizing radiation or by Fenton-type reactions) the mechanism of formation of 8-hydroxylated purine lesions (8-oxoPur), including both 8-oxodGuo and 8-oxodAdo, involves addition of HO• to the C8 position of the purine bases, followed by an oxidation reaction (for a review see refs. 2 and 4). In addition, 8-oxoPur could be also produced by a one-electron oxidation process followed by hydration of the purine radical cation thus generated, producing following oxidation mainly 8-oxodGuo due to efficient charge transfer processes (5, 6) combined with the relatively low ionization potential of guanine compared to other DNA constituents.In addition, the pioneering work of Box and coworkers (7-10) suggested the possible formation of tandem lesions, constituted of two adjacent modifications. Recent works have demonstrated that in dinucleoside monophosp...

The prion protein (PrP) is highly conserved and ubiquitously expressed, suggesting that it plays an important physiological function. However, despite decades of investigation, this role remains elusive. Here, by using animal and cellular models, we unveil a key role of PrP in the DNA damage response. Exposure of neurons to a genotoxic stress activates PRNP transcription leading to an increased amount of PrP in the nucleus where it interacts with APE1, the major mammalian endonuclease essential for base excision repair, and stimulates its activity. Preventing the induction of PRNP results in accumulation of abasic sites in DNA and impairs cell survival after genotoxic treatment. Brains from Prnp−/− mice display a reduced APE1 activity and a defect in the repair of induced DNA damage in vivo. Thus, PrP is required to maintain genomic stability in response to genotoxic stresses.

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