DNA Damage and Radical Reactions: Mechanistic Aspects, Formation in Cells and Repair Studies

Abstract

Several examples of oxidative and reductive reactions of DNA components that lead to single and tandem modifications are discussed in this review. These include nucleophilic addition reactions of the one-electron oxidation-mediated guanine radical cation and the one-electron reduced intermediate of 8-bromopurine 2'-deoxyribonucleosides that give rise to either an oxidizing guanine radical or related 5',8-cyclopurine nucleosides. In addition, mechanistic insights into the reductive pathways involved in the photolyase induced reversal of cyclobutadipyrimidine and pyrimidine (6-4) pyrimidone photoproducts are provided. Evidence for the occurrence and validation in cellular DNA of • OH radical degradation pathways of guanine that have been established in model systems has been gained from the accurate measurement of degradation products. Relevant information on biochemical aspects of the repair of single and clustered oxidatively generated damage to DNA has been gained from detailed investigations that rely on the synthesis of suitable modified probes. Thus the preparation of stable carbocyclic derivatives of purine nucleoside containing defined sequence oligonucleotides has allowed detailed crystallographic studies of the recognition step of the base damage by enzymes implicated in the base excision repair (BER) pathway. Detailed insights are provided on the BER processing of non-double strand break bistranded clustered damage that may consist of base lesions, a single strand break or abasic sites and represent one of the main deleterious classes of radiation-induced DNA damage.

Solvent Exposure Associated with Single Abasic Sites Alters the Base Sequence Dependence of Oxidation of Guanine in DNA in GG Sequence Contexts

“…Macrophages activated in inflammatory tissues produce a wide spectrum of reactive oxygen and nitrogen species that can react with cellular DNA and produce toxic and mutagenic base lesions, which have been implicated in the development of diseases associated with the inflammatory response 1. 2 Guanine is the most easily oxidizable nucleic acid base3 and is thus the primary target of oxidation by reaction with one‐electron oxidants 4. The formation of oxidatively generated guanine lesions is a base‐sequence dependent process since runs of guanines are more easily oxidized than guanines flanked by any of the other DNA bases 5–8.…”

Generation of Guanine–Thymine Cross-Links in Human Cells by One-Electron Oxidation Mechanisms

“…5–7 The best known and most ubiquitous stable oxidation product is 8-oxo-7,8-dihydroguanine, 8-oxoGua (Chart 1), derived from free radicals or singlet oxygen attack on DNA. 8 Guanine radical formation in vitro can occur by two principle pathways: ( i ) reaction with hydroxyl radical ( · OH), 9 and ( ii ) the one-electron oxidation of guanine.…”

“…19 Highly reactive · OH radicals, which rapidly and unselectively react with all DNA bases, 9 may also have contributed to the observed oxidatively generated pyrimidine damage in cellular DNA. 5, 24 In our experiments, the precursor of · OH radicals may be the unreactive superoxide radical anion (O 2 ·− ) that arises from the reduction of molecular oxygen by hydrated electrons derived from the two-photon ionization of DNA bases, and/or by photoexcitation of diverse endogenous cellular chromophores that are capable of absorbing light in the UV spectral range. 5–8 …”

“…5, 24 In our experiments, the precursor of · OH radicals may be the unreactive superoxide radical anion (O 2 ·− ) that arises from the reduction of molecular oxygen by hydrated electrons derived from the two-photon ionization of DNA bases, and/or by photoexcitation of diverse endogenous cellular chromophores that are capable of absorbing light in the UV spectral range. 5–8 …”

Differences in the Access of Lesions to the Nucleotide Excision Repair Machinery in Nucleosomes

“…These lesions are derived from the reactions of hydroxyl radicals with guanine or adenine in DNA and are known to be produced in tissues during the inflammatory response. 24,25 The C5′–C8 cross-links cause an overtwisting of the DNA helix and an abnormal and rigid sugar pucker, but Watson–Crick base pairing is not ruptured (Figure 1B). These oxidatively generated lesions are good substrates of NER when present in free DNA, with the R stereoisomer being removed more efficiently than the S isomer by NER mechanisms in human HeLa cell extracts 26–28 and in CHO cell extracts; 29 they are also repaired by NER in wild-type mice but nevertheless accumulate with aging.…”