UV-induced reactions of direct and sensitized DNA damage in model and cellular systems

Solar UV radiation in the regions B (UVB, 290- 320 nm) and A (UVA, 320- 400 nm) is the key genotoxic environmental factor causing DNA damage that is a threat for the genomic integrity in living cells. If DNA lesions left unrepaired, they can provoke potentially carcinogenic mutations. The UV- induced lesion formation in DNA involves direct or photosensitized reactions, and the chemical nature and quantity of lesions strongly depend on the UV radiation wavelength. UVB causes formation of two major types of pyrimidine dimeric photoproducts in consequence of direct photon absorption by DNA bases. At UVA action, thymine dimers of only one type are predominantly formed in DNA; they can be formed both upon direct photon absorption and via energy transfer to thymine bases from sensitizing molecules. Besides, UVA effectively induces sensitized oxidation reactions to DNA damage. The review presents current knowledge on fundamental mechanisms of photochemical reactions in DNA, and discusses the role of different DNA photoproducts in initiating genotoxic processes. When considering oxidized reactions in DNA, attention is attracted to properties of pterins, riboflavin, protoporphyrin IX, and melanin as potential cellular photosensitizers generating reactive oxygen species involved in these reactions.