Oxidative DNA damage is continuously generated in cells as a consequence of oxidative stress and this phenomenon is associated with several pathologic disorders. Generated reactive oxygen species could react with biological molecules including DNA the molecule bearing the genetic information. Cells have developed very efficient repair systems in order to minimise the biological consequences of such DNA lesions, among them, the base excision repair (BER) mechanism is known to be involved in the repair of oxidatively generated DNA lesions. In spite of the extensive work that has been done both on the formation and repair of oxidatively generated DNA lesions, recent results suggest that a high proportion of so-called tandem lesions, constituted of two (or even more) lesions closely localised on the DNA could be produced as a consequence of a single ionisation process. Moreover, preliminary results indicate that these lesions that could be produced at a high ratio compared to single lesions, are less efficiently repaired by BER. The aim of our project is to study in more details the formation, repair and biological consequences of such tandem lesions compared to single damage. The formation of such damage will be evaluated using oligonucleotides containing a photochemical precursor of DNA radicals. Generated lesions produced upon photolysis of the oligonucleotide will be identified and quantified using physico-chemical approaches. Thereafter, identified damage will be incorporated into oligonucleotide that will be used to study the repair efficacy of such damage by both DNA glycosylases and cells extracts. To better understand how the presence of a second lesion could affect the repair capacity of a defined enzyme, efforts will be made to visualize the effect of these lesions on the local structure of duplex DNA-glycosylase complexes. In parallel, affinity constant and kinetics of excision of the tandem lesions will be determined and compared to single isolated damage. Finally, the mutagenic properties of the tandem lesions will be also determined upon transfection in cells. The originality of the project is to bring together chemists, structural biologists, biochemists and cellular biologists to determine, by the use complementary approaches, the relevance and biological consequences of a defined type of DNA damage that has not yet been studied previously. Preliminary results obtained very recently strongly suggest that the tandem lesions that will be studied most probably play an important role in the harmful effect of endogenous oxidative stress that is associated to several human pathologies.