DNA base damage by reactive oxygen species, oxidizing agents, and UV radiation

Abstract

Emphasis has been placed in this article dedicated to DNA damage on recent aspects of the formation and measurement of oxidatively generated damage in cellular DNA in order to provide a comprehensive and updated survey. This includes single pyrimidine and purine base lesions, intrastrand cross-links, purine 5',8-cyclonucleosides, DNA-protein adducts and interstrand cross-links formed by the reactions of either the nucleobases or the 2-deoxyribose moiety with the hydroxyl radical, one-electron oxidants, singlet oxygen, and hypochlorous acid. In addition, recent information concerning the mechanisms of formation, individual measurement, and repair-rate assessment of bipyrimidine photoproducts in isolated cells and human skin upon exposure to UVB radiation, UVA photons, or solar simulated light is critically reviewed.

Figure 1.

Oxidation of thymine (Thy). Radicals are shown in red and diagmagnetic intermediates in blue. Only the base moiety is shown. In the case of 2′-deoxyribonucleosides, the base is attached to a 2-deoxy-β-d-erythro-pentofuranose moiety. C5,C6-OH adducts include 5-hydroxy-5,6-dihydrothymin-6-yl and 6-hydroxy-5,6-dihydrothymine-5-yl radicals. 5,6-OH,OOH include thymine hydroperoxides: 5-hydroxy-6-hydroperoxy- and 6-hydroxy-5-hydroperoxy-5,6-dihydrothymine. Deprotonation of thymine radical cations gives 5-(uracilyl)methyl radicals and 5-hydroperoxylmethyluracil as the main initial products. Oxidation of the 5,6-double bond gives 5,6-dihydroxy-5,6-dihydrothymine (thymine 5,6-glycols, Thy-Gly) and 5-hydroxy-5-methylhydantoin (Hyd-Thy). Oxidation of the methyl group gives 5-hydroxymethyluracil (5-HmUra) and 5-formyluracil (5-FoUra). The 5-(uracilyl)-methyl radical may also react with neighboring guanine or adenine to give intrastrand cross-links, for example, G[8-5 m]T. The above products have been detected in cellular DNA.

Figure 2.

Oxidation of cytosine (Cyt). C5,C6-OH adducts include 5-hydroxy-5,6-dihydrocytosin-6-yl and 6-hydroxy-5,6-dihydrocytosin-5-yl radicals. 5,6-OH,OOH include cytosine hydroperoxides: 5-hydroxy-6-hydroperoxy-5,6-dihydrocytosine and 6-hydroxy-5-hydroperoxy-5,6-dihydrocytosine. The deprotonation of cytosine radical cations gives C6-OH adducts, cytosin-N4-yl (Cyt-N4-yl) radicals, and 2′-deoxycytidin-C1′-yl radicals; in addition, 5-(cytosinyl)methyl radicals are generated in the case of 5-methylcytosine. Initial cytosine 5,6-dihydroxy-5,6-dihydrocytosine (cytosine 5,6-glycols, Cyt-Gly) decomposes to 5-hydroxycytosine (5-OHCyt) and 5,6-dihydroxy-5,6-dihydrouracil (uracil 5,6-glycols, Ura-Gly). Other products of the 5,6-double bond of cytosine include 5-hydroxyhydantoin (Hyd-Ura) and 1-carbamoyl-3,4-dihydroxy-2-oxoimidazolidine (Imid-Cyt). Radical-mediated or enzymatic oxidation of the methyl group of 5-methylcytosine gives 5-hydroxymethylcytosine (5-HmCyt), 5-formylcytosine (5-FoCyt), and 5-carboxycytosine (5-CaCyt). The above products have been detected in cellular DNA except for Cyt-Gly.

Figure 3.

Oxidation of guanine (Gua). The reaction of ^•OH gives 8-hydroxy-7,8-dihydroguan-8-yl radicals (C8-OH adduct) and guanine-N2-yl radicals (Gua-N2-yl), which transform into guanine-C5-yl radicals (G(-H^•) guanyl radical, Gua-C5-yl). The guanine radical cation undergoes competitive hydration to the C8-OH adduct and deprotonation to Gua-C5-yl. The main products include 8-oxo-7,8-dihydroguanine (8-oxoGua), 2,6-diamino-4-hydroxy-5-formamidopyrimidine (Fapy-Gua), and 2-amino-5-4H-imidazol-4-one (imidazolone), which transforms to 2,2,4-triamino-5(2H)-oxazolone (oxazolone). The radical cation of guanine may undergo addition with lysine to form DNA–protein cross-links (Gua-Lys). The reaction of single oxygen (¹O₂) leads to the formation of an intermediate 4,8-endoperoxide, which decomposes mainly to 8-oxoGua. The above products have been detected in cellular DNA, except for imidazolone and Gua-Lys cross-links.

Figure 4.

Oxidation of adenine (Ade). The reaction of ^•OH gives 8-hydroxy-7,8-dihydroaden-8-yl radicals (C8-OH adduct) and 4,5-dihydroaden-C5-yl radicals (C4-OH adduct), which transform into adenin-N6-yl radicals (Ade-N6-yl). The adenine radical cation undergoes competitive hydration to the C8-OH adduct and deprotonation to Ade-N6-yl radicals. The main products include 8-oxo-7,8-dihydroadenine (8-oxoAde) and 4,6-diamino-5-formamidopyrimidine (Fapy-Ade). Inosine may also form by deamination of intermediate adenine radical cations. The above products have been detected in cellular DNA.

Figure 5.

Oxidation of the 2-deoxyribose moiety. The reaction of OH results in the abstraction of hydrogen atoms from the 2-deoxyribose, giving five C-centered radicals. These radicals explain the formation of various oxidation products: abstraction at C1′ gives 2-deoxyribonolactone; abstraction at C5′ gives 3′-phosphoglycoaldehyde, and abstraction at C4′ gives an intermediate unsaturated dialdehyde that can couple with cytosine to form a DNA inter- or intrastrand cross-link. In addition, the C5′-centered radicals of 2-deoxyribose can react with the corresponding base moiety to produce 5′,8-cyclo-2′-deoxyguanosine (5′,8-cyclo-dGuo) and 5′,8-cyclo-2′-deoxyadenosine (5′,8-cyclo-dAdo). The above products have been detected in cellular DNA, except for the dialdehyde intermediate.