Reactivity and DNA Damage by Independently Generated 2'-Deoxycytidin-N4-yl Radical

Nitrogen-Centered Radicals Derived from Azidonucleosides

Azido-modified nucleosides have been extensively explored as substrates for click chemistry and the metabolic labeling of DNA and RNA. These compounds are also of interest as precursors for further synthetic elaboration and as therapeutic agents. This review discusses the chemistry of azidonucleosides related to the generation of nitrogen-centered radicals (NCRs) from the azido groups that are selectively inserted into the nucleoside frame along with the subsequent chemistry and biological implications of NCRs. For instance, the critical role of the sulfinylimine radical generated during inhibition of ribonucleotide reductases by 2'-azido-2'-deoxy pyrimidine nucleotides as well as the NCRs generated from azidonucleosides by radiation-produced (prehydrated and aqueous) electrons are discussed. Regio and stereoselectivity of incorporation of an azido group ("radical arm") into the frame of nucleoside and selective generation of NCRs under reductive conditions, which often produce the same radical species that are observed upon ionization events due to radiation and/or other oxidative conditions that are emphasized. NCRs generated from nucleoside-modified precursors other than azidonucleosides are also discussed but only with the direct relation to the same/similar NCRs derived from azidonucleosides.

Chemical Repair of Radical Damage to the GC Base Pair by DNA-Bound Bisbenzimidazoles

The migration of an electron-loss center (hole) in calf thymus DNA to bisbenzimidazole ligands bound in the minor groove is followed by pulse radiolysis combined with time-resolved spectrophotometry. The initially observed absorption spectrum upon oxidation of DNA by the selenite radical is consistent with spin on cytosine (C), as the GC• pair neutral radical, followed by the spectra of oxidized ligands. The rate of oxidation of bound ligands increased with an increase in the ratio (r) ligands per base pair from 0.005 to 0.04. Both the rate of ligand oxidation and the estimated range of hole transfer (up to 30 DNA base pairs) decrease with the decrease in one-electron reduction potential between the GC• pair neutral radical of ca. 1.54 V and that of the ligand radicals (E0', 0.90-0.99 V). Linear plots of log of the rate of hole transfer versus r give a common intercept at r = 0 and a free energy change of 12.2 ± 0.3 kcal mol-1, ascribed to the GC• pair neutral radical undergoing a structural change, which is in competition to the observed hole transfer along DNA. The rate of hole transfer to the ligands at distance, R, from the GC• pair radical, k2, is described by the relationship k2 = k0 exp(constant/R), where k0 includes the rate constant for surmounting a small barrier.

Hydroxyl Radical vs. One-Electron Oxidation Reactivities in an Alternating GC Double-Stranded Oligonucleotide: A New Type Electron Hole Stabilization

We examined the reaction of hydroxyl radicals (HO•) and sulfate radical anions (SO4•-), which is generated by ionizing radiation in aqueous solutions under anoxic conditions, with an alternating GC doubled-stranded oligodeoxynucleotide (ds-ODN), i.e., the palindromic 5'-d(GCGCGC)-3'. In particular, the optical spectra of the intermediate species and associated kinetic data in the range of ns to ms were obtained via pulse radiolysis. Computational studies by means of density functional theory (DFT) for structural and time-dependent DFT for spectroscopic features were performed on 5'-d(GCGC)-3'. Comprehensively, our results suggest the addition of HO• to the G:C pair moiety, affording the [8-HO-G:C]• detectable adduct. The previous reported spectra of one-electron oxidation of a variety of ds-ODN were assigned to [G(-H+):C]• after deprotonation. Regarding 5'-d(GCGCGC)-3' ds-ODN, the spectrum at 800 ns has a completely different spectral shape and kinetic behavior. By means of calculations, we assigned the species to [G:C/C:G]•+, in which the electron hole is predicted to be delocalized on the two stacked base pairs. This transient species was further hydrated to afford the [8-HO-G:C]• detectable adduct. These remarkable findings suggest that the double-stranded alternating GC sequences allow for a new type of electron hole stabilization via delocalization over the whole sequence or part of it.

Synergistic effects on mutagenicity of tandem lesions containing 8-oxo-7,8-dihydro-2'-deoxyguanosine or Fapy•dG flanked by a 3' 5-formyl-2'-deoxyuridine in human cells

Modified nucleotides often hinder and/or decrease the fidelity of DNA polymerases. Tandem lesions, which are comprised of DNA modifications at two contiguous nucleotide positions, can be even more detrimental to genome stability. Recently, tandem lesions containing 5-formyl-2'-deoxyuridine (5fdU) flanked at the 5'-position by 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-OxodGuo) or N-(2-deoxy-α,β-D-erythropentofuranosyl)-N-(2,6-diamino-4-hydroxy-5-formamidopyrimidine (Fapy•dG) were discovered. We examined the replication of 5'- 8-OxodGuo-5fdU and 5'-Fapy•dG-5fdU tandem lesions in HEK 293T cells and several polymerase deficient variants by transfecting single-stranded vectors containing them. The local sequence of the tandem lesions encompasses the 273 codon of the p53 gene, a mutational hot-spot. The bypass efficiency and mutation spectra of the tandem lesions were compared to those of the isolated lesions. Replication of weakly mutagenic 5-fdU is little changed when part of the 5'- 8-OxodGuo-5fdU tandem lesion. G → T transversions attributable to 8-OxodGuo increase > 10-fold when the tandem lesion is bypassed. 5'-Fapy•dG-5fdU has a synergistic effect on the error-prone bypass of both lesions. The mutation frequency (MF) of 5'-Fapy•dG-5fdU increases 3-fold compared to isolated Fapy•dG. In addition, a 5'-adjacent Fapy•dG significantly increases the MF of 5fdU. The major mutation, G → T transversions, decrease by almost a third in hPol κ- cells, which is the opposite effect when isolated Fapy•dG in the same sequence context is replicated in HEK 293T cells in the same sequence. Steady-state kinetics indicate that hPol κ contributes to greater G → T transversions by decreasing the specificity constant for dCTP compared to an isolated Fapy•dG. The greater conformational freedom of Fapy•dG compared to 8-OxodGuo and its unusual ability to epimerize at the anomeric center is believed to be the source of the complex effects of 5'-Fapy•dG-5fdU on replication.

Photochemical and Single Electron Transfer Generation of 2'-Deoxycytidin-N4-yl Radical from Oxime Esters

A 2'-deoxycytidin-N4-yl radical (dC·), a strong oxidant that also abstracts hydrogen atoms from carbon-hydrogen bonds, is produced in a variety of DNA damaging processes. We describe here the independent generation of dC· from oxime esters under UV-irradiation or single electron transfer conditions. Support for this σ-type iminyl radical generation is provided by product studies carried out under aerobic and anaerobic conditions, as well as electron spin resonance (ESR) characterization of dC· in a homogeneous glassy solution at low temperature. Density functional theory (DFT) calculations also support fragmentation of the corresponding radical anions of oxime esters 2d and 2e to dC· and subsequent hydrogen atom abstraction from organic solvents. The corresponding 2'-deoxynucleotide triphosphate (dNTP) of isopropyl oxime ester 2c (5) is incorporated opposite 2'-deoxyadenosine and 2'-deoxyguanosine by a DNA polymerase with approximately equal efficiency. Photolysis experiments of DNA containing 2c support dC· generation and indicate that the radical produces tandem lesions when flanked on the 5'-side by 5'-d(GGT). These experiments suggest that oxime esters are reliable sources of nitrogen radicals in nucleic acids that will be useful mechanistic tools and possibly radiosensitizing agents when incorporated in DNA.

Investigation of inflammation, oxidative stress, and DNA damage in COVID-19 patients

COVID-19 disease, which spreads worldwide, is a disease characterized by widespread inflammation and affects many organs, especially the lungs. The resulting inflammation can lead to reactive oxygen radicals, leading to oxidative DNA damage. The pneumonia severity of 95 hospitalized patients with positive RT-PCR test was determined and divided into three groups: mild, moderate, and severe/critical. Inflammation markers (neutrophil-lymphocyte ratio, serum reactive protein, procalcitonin, etc.) were determined, and IL-10 and IFN-γ measurements were analyzed using the enzyme-linked immunosorbent assay method. In evaluating oxidative damage, total thiol, native thiol, disulfide, and ischemia-modified albumin (IMA) levels were determined by measuring spectrophotometrically. The comet assay method's percentage of tail DNA obtained was used to determine oxidative DNA damage. As a result, when the mild and severe/critical groups were compared, we found that total thiol, native thiol, and disulfide levels decreased significantly in the severe/critical group due to the increase in inflammation markers and cytokine levels (p < 0.05). We could not detect any significance in IMA levels between the groups (p > 0.05). At the same time, we determined an increase in the tail DNA percent level, that is, DNA damage, due to the increased oxidative effect. As a result, we determined that inflammation and oxidative stress increased in patients with severe pneumonia, and there was DNA damage in these patients.

Oxidatively generated tandem DNA modifications by pyrimidinyl and 2-deoxyribosyl peroxyl radicals

Molecular oxygen sensitizes DNA to damage induced by ionizing radiation, Fenton-like reactions, and other free radical-mediated reactions. It rapidly converts carbon-centered radicals within DNA into peroxyl radicals, giving rise to a plethora of oxidized products consisting of nucleobase and 2-deoxyribose modifications, strand breaks and abasic sites. The mechanism of formation of single oxidation products has been extensively studied and reviewed. However, much evidence shows that reactive peroxyl radicals can propagate damage to vicinal components in DNA strands. These intramolecular reactions lead to the dual alteration of two adjacent nucleotides, designated as tandem or double lesions. Herein, current knowledge about the formation and biological implications of oxidatively generated DNA tandem lesions is reviewed. Thus far, most reported tandem lesions have been shown to arise from peroxyl radicals initially generated at pyrimidine bases, notably thymine, followed by reaction with 5'-flanking bases, especially guanine, although contiguous thymine lesions have also been characterized. Proper biomolecular processing is impaired by several tandem lesions making them refractory to base excision repair and potentially more mutagenic.

The potential role of COVID-19 in the induction of DNA damage

The coronavirus disease-2019 (COVID-19) caused by the severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) is challenging global health and economic systems. In some individuals, COVID-19 can cause a wide array of symptoms, affecting several organs, such as the lungs, heart, bowels, kidneys and brain, causing multiorgan failure, sepsis and death. These effects are related in part to direct viral infection of these organs, immunological deregulation, a hypercoagulatory state and the potential for development of cytokine storm syndrome. Since the appearance of COVID-19 is recent, the long-term effects on the health of recovered patients remain unknown. In this review, we focused on current evidence of the mechanisms of DNA damage mediated by coronaviruses. Data supports that these viruses can induce DNA damage, genomic instability, and cell cycle deregulation during their replication in mammalian cells. Since the induction of DNA damage and aberrant DNA repair mechanisms are related to the development of chronic diseases such as cancer, diabetes, neurodegenerative disorders, and atherosclerosis, it will be important to address similar effects and outcomes in recovered COVID-19 patients.