Measurement of oxidatively generated base damage to nucleic acids in cells: facts and artifacts

OH Radical-Induced Oxidation in Nucleosides and Nucleotides Unraveled by Tandem Mass Spectrometry and Infrared Multiple Photon Dissociation Spectroscopy

OH⋅-induced oxidation products of DNA nucleosides and nucleotides have been structurally characterized by collision-induced dissociation tandem mass spectrometry (CID-MS2 ) and Infrared Multiple Photon Dissociation (IRMPD) spectroscopy. CID-MS2 results have shown that the addition of one oxygen atom occurs on the nucleobase moiety. The gas-phase geometries of +16 mass increment products of 2'-deoxyadenosine (dA(O)H+ ), 2'-deoxyadenosine 5'-monophosphate (dAMP(O)H+ ), 2'-deoxycytidine (dC(O)H+ ), and 2'-deoxycytidine 5'-monophosphate (dCMP(O)H+ ) are extensively investigated by IRMPD spectroscopy and quantum-chemical calculations. We show that a carbonyl group is formed at the C8 position after oxidation of 2'-deoxyadenosine and its monophosphate derivative. For 2'-deoxycytidine and its monophosphate derivative, the oxygen atom is added to the C5 position to form a C-OH group. IRMPD spectroscopy has been employed for the first time to provide direct structural information on oxidative lesions in DNA model systems.

NEIL3 promoter G-quadruplex with oxidatively modified bases shows magnesium-dependent folding that stalls polymerase bypass

Oxidative stress unleashes reactive species capable of oxidizing 2'-deoxyguanosine (G) nucleotides in G-rich sequences of the genome, such as the potential G-quadruplex forming sequencing (PQS) in the NEIL3 gene promoter. Oxidative modification of G yields 8-oxo-7,8-dihydro-2'-deoxyguanosine (OG) that can be further oxidized to hydantoin products. Herein, OG was synthesized into the NEIL3 PQS that was allowed to fold to a G-quadruplex (G4) in K+ ion solutions with varying amounts of Mg2+ in the physiological range. The Mg2+ dependency in the oxidatively modified NEIL3 G4 to stall a replicative DNA polymerase was evaluated. The polymerase was found to stall at the G4 or OG, as well as continue to full-length extension with dependency on the location of the modification and the concentration of Mg2+. To provide some clarity on these findings, OG or the hydantoins were synthesized in model NEIL3 G4 folding sequences at the positions of the polymerase study. The model G4 sequences were allowed to fold in K+ ion solutions with varying levels of Mg2+ to identify how the presence of the divalent metal impacted G4 folding depending on the location of the modification. The presence of Mg2+ either caused the transition of the NEIL3 G4 folds from an antiparallel to parallel orientation of the strands or had no impact. Structural models are proposed to understand the findings using the literature as a guide. The biological significance of the results is discussed.

Single Cell Determination of 7,8-dihydro-8-oxo-2'-deoxyguanosine by Fluorescence Techniques: Antibody vs. Avidin Labeling

An important biomarker of oxidative damage in cellular DNA is the formation of 7,8-dihydro-8-oxo-2'-deoxyguanosine (8-oxodG). Although several methods are available for the biochemical analysis of this molecule, its determination at the single cell level may provide significant advantages when investigating the influence of cell heterogeneity and cell type in the DNA damage response. to. For this purpose, antibodies recognizing 8-oxodG are available; however, detection with the glycoprotein avidin has also been proposed because of a structural similarity between its natural ligand biotin and 8-oxodG. Whether the two procedures are equivalent in terms of reliability and sensitivity is not clear. In this study, we compared the immunofluorescence determination of 8-oxodG in cellular DNA using the monoclonal antibody N45.1 and labeling using avidin conjugated with the fluorochrome Alexa Fluor488 (AF488). Oxidative DNA damage was induced in different cell types by treatment with potassium bromate (KBrO₃), a chemical inducer of reactive oxygen species (ROS). By using increasing concentrations of KBrO₃, as well as different reaction conditions, our results indicate that the monoclonal antibody N45.1 provides a specificity of 8-oxodG labeling greater than that attained with avidin-AF488. These findings suggest that immunofluorescence techniques are best suited to the in situ analysis of 8-oxodG as a biomarker of oxidative DNA damage.

Quantification of Epigenetic DNA Modifications in the Subchromatin Structure Matrix Attachment Regions by Stable Isotope Dilution UHPLC-MS/MS Analysis

To date, subchromatin structure-based quantification of epigenetic DNA modifications is limited. Matrix attachment regions (MARs), an important subchromatin structure, contain DNA elements that specifically bind chromatin to the nuclear matrix in eukaryotes and are involved in a number of diseases. Here, we exploited a high-salt extraction-based subchromatin fractionation approach for the isolation of MAR DNA and other fractions and further developed heavy stable isotope-diluted ultrahigh-performance liquid chromatography tandem mass spectrometry (UHPLC-MS/MS) for the specific quantification of epigenetic DNA modifications in the subchromatin structures. By this approach, we showed for the first time that the content of a DNA demethylation intermediate, 5-hydroxymethylcytosine (5hmdC), in MARs decreased significantly in four tested cell lines compared to the contents in genomic DNA. In particular, the content of DNA 5hmdC in the MARs of 293T cell lines decreased the most at approximately 41.09%. Together, our findings implicate that MAR DNA is less sensitive than genomic DNA to DNA demethylation.

Biomarkers of DNA Oxidation Products: Links to Exposure and Disease in Public Health Studies

Environmental exposure can increase the production of reactive oxygen species and deplete cellular antioxidants in humans, resulting in oxidatively generated damage to DNA that is both a useful biomarker of oxidative stress and indicator of carcinogenic hazard. Methods of oxidatively damaged DNA analysis have been developed and used in public health research since the 1990s. Advanced techniques detect specific lesions, but they might not be applicable to complex matrixes (e.g., tissues), small sample volume, and large-scale studies. The most reliable methods are characterized by (1) detecting relevant DNA oxidation products (e.g., premutagenic lesions), (2) not harboring technical problems, (3) being applicable to complex biological mixtures, and (4) having the ability to process a large number of samples in a reasonable period of time. Most effort has been devoted to the measurements of 8-oxo-7,8-dihydro-2'-deoxyguanine (8-oxodG), which can be analyzed by chromatographic, enzymic, and antibody-based methods. Results from validation trials have shown that certain chromatographic and enzymic assays (namely the comet assay) are superior techniques. The enzyme-modified comet assay has been popular because it is technically simpler than chromatographic assays. It is widely used in public health studies on environmental exposures such as outdoor air pollution. Validated biomarker assays on oxidatively damaged DNA have been used to fill knowledge gaps between findings in prospective cohort studies and hazards from contemporary sources of air pollution exposures. Results from each of these research fields feed into public health research as approaches to conduct primary prevention of diseases caused by environmental or occupational agents.

The development of isoguanosine: from discovery, synthesis, and modification to supramolecular structures and potential applications

First systematical review of isoguanosine, an unnatural base, as an isomer of guanosine shows significant differences in diverse properties.

(5'R)-and (5'S)-purine 5',8-cyclo-2'-deoxyribonucleosides: reality or artifactual measurements? A reply to Chatgilialoglu's comments (this issue)

This rebuttal letter is aimed at refuting the poor and false arguments elaborated by Chatgilialoglu (preceding article) in his response to the position article (Cadet et al. Free Radic Res 2019;53:574-577) that focussed on the putative reliability of the HPLC-MS/MS measurements of five radiation-induced damage to cellular DNA, which included 8-oxo-7,8-dihydro-2'-deoxyadenosine and the (5'R) and (5'S) diastereomers of 5',8-cyclo-2'-deoxyadenosine and 5',8-cyclo-2'-deoxyadenosine (Krokidis et al. Free Radic Res 2017;51:470-482). Unfortunately, none of the main issues we raised on the suitability of the analytical approach and the shortcomings associated with DNA extraction in HPLC based measurement methods of oxidatively generated damage in cells were properly considered in Chatigilialolu's letter. The main questionable issues include the lack of information on the sensitivity of HPLC-MS/MS analysis, the absence of a dose curve that is essential in the formation of damage and the nonconsideration of artifactual oxidation.

Identification and quantification of isoguanosine in humans and mice

Isoguanine (2-hydroxyadenine), considered to be a non-natural nucleobase has, however, been shown to occur in the croton bean, butterfly wings and a mollusk. For the first time, to the best of our knowledge, we report the identification of isoguanosine (2-hydroxyadenosine), the ribonucleoside, in humans and mouse. Isoguanosine is identified and quantified in RNA from mouse liver samples and in human urine and cerebrospinal fluid. Isoguanine could not be detected as the 2'-deoxyribonucleoside in mouse liver DNA. It could be speculated that the source of isoguanosine was formation from adenosine during oxidative stress in the body. However, the urinary concentrations of isoguanosine and the levels in the liver found here by using isotope dilution liquid chromatography-tandem mass spectrometry are identical to or exceed those of 8-oxo-7,8-dihydro-2'-deoxyguanosine and 8-oxo-7,8-dihydro-guanosine. Guanine is the nucleobase that is oxidized the easiest, so it appears spectacular that the levels of isoguanosine are higher than the levels of 8-oxo-7,8-dihydro-2'-deoxyguanosine and 8-oxo-7,8-dihydro-guanosine. It also appears intriguing that it was only possible to detect the ribonucleoside isoguanosine and not the 2'-deoxyribonucleoside. These observations could indicate that the isoguanosine found is not formed by oxidative stress and could have biological functions.

Biphotonic Ionization of DNA: From Model Studies to Cell

Oxidation reactions triggered by low-intensity UV photons represent a minor contribution with respect to the overwhelming pyrimidine base dimerization in both isolated and cellular DNA. The situation is totally different when DNA is exposed to high-intensity UVC radiation under conditions where biphotonic ionization of the four main purine and pyrimidine bases becomes predominant at the expense of singlet excitation processes. The present review article provides a critical survey of the main chemical reactions of the base radical cations thus generated by one-electron oxidation of nucleic acids in model systems and cells. These include oxidation of the bases with the predominant formation of 8-oxo-7,8-dihydroguanine as the result of preferential hole transfer to guanine bases that act as sinks in isolated and cellular DNA. In addition to hydration, other nucleophilic addition reactions involving the guanine radical cation give rise to intra- and interstrand cross-links together with DNA-protein cross-links. Information is provided on the utilization of high-intensity UV laser pulses as molecular biology tools for studying DNA conformational features, nucleic acid-protein interactions and nucleic acid reactivity through DNA-protein cross-links and DNA footprinting experiments.

Affinity maturation of an antibody for the UV-induced DNA lesions 6,4 pyrimidine-pyrimidones

DNA lesions, associated mostly with minor changes in DNA structure, may induce permanent change in heritable coding information. Biochemically, these minor structural changes are difficult to be explored for generating high-affinity antibodies to detect specific DNA lesions in varying sequence contexts. Herein, we established a platform of bacterial display to facilitate antibodies to be matured with high affinity and high specificity against DNA lesions. To achieve this goal, we, for the first time, developed a two-round mutation/screening strategy: (1) using multiple lesion-containing DNA probes for primary maturation and (2) using single lesion-containing DNA probes for second maturation. Specifically, we capitalized on 64M-2 as a parental template to improve affinity for 6-4PP by 710-fold, compared with the model one. In addition, the matured antibody (9c3) is found to be much less dependent on the bases surrounding 6-4PPs than the model one. The mechanistic study from both computational simulation and reverse mutations revealed the critical roles of the two-round mutations in the enhanced binding affinity and independence of surrounding bases. This selection strategy opens a new way to improve affinity and specificity of antibodies for other DNA lesions.

8-Oxo-7,8-dihydro-2'-deoxyguanosine and abasic site tandem lesions are oxidation prone yielding hydantoin products that strongly destabilize duplex DNA

In DNA, 2'-deoxyguanosine (dG) is susceptible to oxidative modification by reactive oxygen species (ROS) yielding many products, one of which is 8-oxo-7,8-dihydro-2'-deoxyguanosine (dOG). Interestingly, dOG is stable but much more labile toward oxidation than dG, furnishing 5-guanidinohydantoin-2'-deoxyribose (dGh) that is favored in the duplex context or spiroiminodihydantoin-2'-deoxyribose (dSp) that is favored in the oxidation of single-stranded contexts. Previously, exposure of DNA to ionizing radiation found ∼50% of the dOG exists as a tandem lesion with an adjacent formamide site. The present work explored oxidation of dOG in a tandem lesion with a THF abasic site analog (F) that models the formamide on either the 5' or 3' side. When dOG was in a tandem lesion, both dGh and dSp were observed as oxidation products. The 5' versus 3' side in which F resided influenced the stereochemistry of the dSp formed. Further, tandem lesions with dOG were found to be up to two orders of magnitude more reactive to oxidation than dOG in an intact duplex. When dOG is in a tandem lesion it is up to fivefold more prone to formation of spermine cross-links during oxidation compared to dOG in an intact duplex. Lastly, dOG, dGh, and each dSp diastereomer were synthesized as part of a tandem lesion in a duplex DNA to establish that dOG tandem lesions decrease the thermal stability by 12-13 °C, while dGh or either dSp diastereomer in a tandem lesion decrease the stability by >20 °C. The biological consequences of these results are discussed.

Iron induced RNA-oxidation in the general population and in mouse tissue

Iron promotes formation of hydroxyl radicals by the Fenton reaction, subsequently leading to potential oxidatively generated damage of nucleic acids. Oxidatively generated damage to RNA, measured as 8-oxo-7,8-dihydroguanosine (8-oxoGuo) in urine, is increased in patients with genetic iron overload, which have led us to test the hypothesis that high iron status, assessed by iron biomarkers and genetic disposition, increases urinary excretion of 8-oxoGuo. In a general Danish population study we used a Mendelian randomization design with HFE genotypes as a proxy for iron status and supplemented with ex vivo experiments in mice muscle tissue exposed to iron(II) sulfate to attempt to clarify this hypothesis. The biomarkers ferritin, transferrin, and transferrin saturation (TS) were associated with 8-oxoGuo (in linear univariable and multivariable regression analyses: P < 0.001). Mendelian randomization indicated a causal pathway between genetically elevated iron biomarkers (assessed by ferritin and TS) and high levels of 8-oxoGuo. The ex vivo experiments showed a monotonically increase in 8-oxoGuo with increased iron concentration (ANOVA: P = 0.0008) that was prevented with iron chelation (P = 0.01). Our results indicate a causal relationship between iron biomarkers and 8-oxoGuo. Furthermore, the ex vivo experiment shows a mechanistic link between iron and 8-oxoGuo formation. Both iron overload and the biomarker 8-oxoGuo have been linked to e.g. diabetes, which merits future studies to investigate if iron induced 8-oxoGuo is involved in disease development.

The Oxygen Paradox, the French Paradox, and age-related diseases

A paradox is a seemingly absurd or impossible concept, proposition, or theory that is often difficult to understand or explain, sometimes apparently self-contradictory, and yet ultimately correct or true. How is it possible, for example, that oxygen "a toxic environmental poison" could be also indispensable for life (Beckman and Ames Physiol Rev 78(2):547-81, 1998; Stadtman and Berlett Chem Res Toxicol 10(5):485-94, 1997)?: the so-called Oxygen Paradox (Davies and Ursini 1995; Davies Biochem Soc Symp 61:1-31, 1995). How can French people apparently disregard the rule that high dietary intakes of cholesterol and saturated fats (e.g., cheese and paté) will result in an early death from cardiovascular diseases (Renaud and de Lorgeril Lancet 339(8808):1523-6, 1992; Catalgol et al. Front Pharmacol 3:141, 2012; Eisenberg et al. Nat Med 22(12):1428-1438, 2016)?: the so-called, French Paradox. Doubtless, the truth is not a duality and epistemological bias probably generates apparently self-contradictory conclusions. Perhaps nowhere in biology are there so many apparently contradictory views, and even experimental results, affecting human physiology and pathology as in the fields of free radicals and oxidative stress, antioxidants, foods and drinks, and dietary recommendations; this is particularly true when issues such as disease-susceptibility or avoidance, "healthspan," "lifespan," and ageing are involved. Consider, for example, the apparently paradoxical observation that treatment with low doses of a substance that is toxic at high concentrations may actually induce transient adaptations that protect against a subsequent exposure to the same (or similar) toxin. This particular paradox is now mechanistically explained as "Adaptive Homeostasis" (Davies Mol Asp Med 49:1-7, 2016; Pomatto et al. 2017a; Lomeli et al. Clin Sci (Lond) 131(21):2573-2599, 2017; Pomatto and Davies 2017); the non-damaging process by which an apparent toxicant can activate biological signal transduction pathways to increase expression of protective genes, by mechanisms that are completely different from those by which the same agent induces toxicity at high concentrations. In this review, we explore the influences and effects of paradoxes such as the Oxygen Paradox and the French Paradox on the etiology, progression, and outcomes of many of the major human age-related diseases, as well as the basic biological phenomenon of ageing itself.

Formation and repair of oxidatively generated damage in cellular DNA

In this review article, emphasis is placed on the critical survey of available data concerning modified nucleobase and 2-deoxyribose products that have been identified in cellular DNA following exposure to a wide variety of oxidizing species and agents including, hydroxyl radical, one-electron oxidants, singlet oxygen, hypochlorous acid and ten-eleven translocation enzymes. In addition, information is provided about the generation of secondary oxidation products of 8-oxo-7,8-dihydroguanine and nucleobase addition products with reactive aldehydes arising from the decomposition of lipid peroxides. It is worth noting that the different classes of oxidatively generated DNA damage that consist of single lesions, intra- and interstrand cross-links were unambiguously assigned and quantitatively detected on the basis of accurate measurements involving in most cases high performance liquid chromatography coupled to electrospray ionization tandem mass spectrometry. The reported data clearly show that the frequency of DNA lesions generated upon severe oxidizing conditions, including exposure to ionizing radiation is low, at best a few modifications per 106 normal bases. Application of accurate analytical measurement methods has also allowed the determination of repair kinetics of several well-defined lesions in cellular DNA that however concerns so far only a restricted number of cases.

Formation and processing of DNA damage substrates for the hNEIL enzymes

Reactive oxygen species (ROS) are harnessed by the cell for signaling at the same time as being detrimental to cellular components such as DNA. The genome and transcriptome contain instructions that can alter cellular processes when oxidized. The guanine (G) heterocycle in the nucleotide pool, DNA, or RNA is the base most prone to oxidation. The oxidatively-derived products of G consistently observed in high yields from hydroxyl radical, carbonate radical, or singlet oxygen oxidations under conditions modeling the cellular reducing environment are discussed. The major G base oxidation products are 8-oxo-7,8-dihydroguanine (OG), 5-carboxamido-5-formamido-2-iminohydantoin (2Ih), spiroiminodihydantoin (Sp), and 5-guanidinohydantoin (Gh). The yields of these products show dependency on the oxidant and the reaction context that includes nucleoside, single-stranded DNA (ssDNA), double-stranded DNA (dsDNA), and G-quadruplex DNA (G4-DNA) structures. Upon formation of these products in cells, they are recognized by the DNA glycosylases in the base excision repair (BER) pathway. This review focuses on initiation of BER by the mammalian Nei-like1-3 (NEIL1-3) glycosylases for removal of 2Ih, Sp, and Gh. The unique ability of the human NEILs to initiate removal of the hydantoins in ssDNA, bulge-DNA, bubble-DNA, dsDNA, and G4-DNA is outlined. Additionally, when Gh exists in a G4 DNA found in a gene promoter, NEIL-mediated repair is modulated by the plasticity of the G4-DNA structure provided by additional G-runs flanking the sequence. On the basis of these observations and cellular studies from the literature, the interplay between DNA oxidation and BER to alter gene expression is discussed.

Dexamethasone and 1,25-dihydroxyvitamin D3 reduce oxidative stress-related DNA damage in differentiating osteoblasts

The process of osteoblast differentiation is regulated by several factors, including RUNX2. Recent reports suggest an involvement of RUNX2 in DNA damage response (DDR), which is important due to association of differentiation with oxidative stress. In the present work we explore the influence of two RUNX2 modifiers, dexamethasone (DEX) and 1,25-dihydroxyvitamin D3 (1,25-D3), in DDR in differentiating MC3T3-E1 preosteoblasts challenged by oxidative stress. The process of differentiation was associated with reactive oxygen species (ROS) production and tert-butyl hydroperoxide (TBH) reduced the rate of differentiation. The activity of alkaline phosphatase (ALP), a marker of the process of osteoblasts differentiation, increased in a time-dependent manner and TBH further increased this activity. This may indicate that additional oxidative stress, induced by TBH, may accelerate the differentiation process. The cells displayed changes in the sensitivity to TBH in the course of differentiation. DEX increased ALP activity, but 1,25-D3 had no effect on it. These results suggest that DEX might stimulate the process of preosteoblasts differentiation. Finally, we observed a protective effect of DEX and 1,25-D3 against DNA damage induced by TBH, except the day 24 of differentiation, when DEX increased the extent of TBH-induced DNA damage. We conclude that oxidative stress is associated with osteoblasts differentiation and induce DDR, which may be modulated by RUNX2-modifiers, DEX and 1,25-D3.

Radiation-mediated formation of complex damage to DNA: a chemical aspect overview

During the last three decades, a considerable amount of work has been undertaken to determine the nature, the mechanism of formation and the biological consequences of radiation-induced DNA lesions. Most of the information was obtained via the development of chemical approaches, including theoretical, analytical and organic synthesis methods. Since it is not possible to present all the results obtained in this review article, we will focus on recent data dealing with the formation of complex DNA lesions produced by a single oxidation event, as these lesions may play a significant role in cellular responses to ionizing radiation and also to other sources of oxidative stress. Through the description of specific results, the contribution of different chemical disciplines in the assessment of the structure, the identification of the mechanism of formation and the biological impacts in terms of repair and mutagenicity of these complex radiation-induced DNA lesions will be highlighted.

Oxidatively generated base damage to cellular DNA by hydroxyl radical and one-electron oxidants: similarities and differences

Hydroxyl radical (OH) and one-electron oxidants that may be endogenously formed through oxidative metabolism, phagocytosis, inflammation and pathological conditions constitute the main sources of oxidatively generated damage to cellular DNA. It is worth mentioning that exposure of cells to exogenous physical agents (UV light, high intensity UV laser, ionizing radiation) and chemicals may also induce oxidatively generated damage to DNA. Emphasis is placed in this short review article on the mechanistic aspects of OH and one-electron oxidant-mediated formation of single and more complex damage (tandem lesions, intra- and interstrand cross-links, DNA-protein cross-links) in cellular DNA arising from one radical hit. This concerns DNA modifications that have been accurately measured using suitable analytical methods such as high performance liquid chromatography coupled with electrospray ionization tandem mass spectrometry. Evidence is provided that OH and one-electron oxidants after generating neutral radicals and base radical cations respectively may partly induce common degradation pathways. In addition, selective oxidative reactions giving rise to specific degradation products of OH and one-electron oxidation reactions that can be used as representative biomarkers of these oxidants have been identified.

The Arabidopsis SIAMESE-RELATED cyclin-dependent kinase inhibitors SMR5 and SMR7 regulate the DNA damage checkpoint in response to reactive oxygen species

Whereas our knowledge about the diverse pathways aiding DNA repair upon genome damage is steadily increasing, little is known about the molecular players that adjust the plant cell cycle in response to DNA stress. By a meta-analysis of DNA stress microarray data sets, three family members of the SIAMESE/SIAMESE-RELATED (SIM/SMR) class of cyclin-dependent kinase inhibitors were discovered that react strongly to genotoxicity. Transcriptional reporter constructs corroborated specific and strong activation of the three SIM/SMR genes in the meristems upon DNA stress, whereas overexpression analysis confirmed their cell cycle inhibitory potential. In agreement with being checkpoint regulators, SMR5 and SMR7 knockout plants displayed an impaired checkpoint in leaf cells upon treatment with the replication inhibitory drug hydroxyurea (HU). Surprisingly, HU-induced SMR5/SMR7 expression depends on ATAXIA TELANGIECTASIA MUTATED (ATM) and SUPPRESSOR OF GAMMA RESPONSE1, rather than on the anticipated replication stress-activated ATM AND RAD3-RELATED kinase. This apparent discrepancy was explained by demonstrating that, in addition to its effect on replication, HU triggers the formation of reactive oxygen species (ROS). ROS-dependent transcriptional activation of the SMR genes was confirmed by different ROS-inducing conditions, including high-light treatment. We conclude that the identified SMR genes are part of a signaling cascade that induces a cell cycle checkpoint in response to ROS-induced DNA damage.

Voltammetric microwell array for oxidized guanosine in intact ds-DNA

Oxidative stress in humans causes damage to biomolecules by generating reactive oxygen species (ROS). DNA can be oxidatively damaged by ROS, which may lead to carcinogenesis. Here we report a microfluidic electrochemical array designed to rapidly detect oxidation in intact DNA in replicate measurements. Sensor arrays were fabricated by wet-chemistry patterning of gold compact discs. The eight-sensor array is incorporated into a 60 μL microfluidic channel connected to a pump and sample valve. The array features 7 nm thick osmium bipyridyl poly(vinylpyridine) chloride [Os(bpy)2(PVP)10Cl](+) films assembled layer-by-layer with polyions onto the gold sensors. 8-Hydroxy-7,8-hydro-2'-deoxyguanosine (8-oxodG) is selectively oxidized by [Os(bpy)2(PVP)10Cl](+) in intact ds-DNA to provide catalytic square wave voltammograms (SWV). The device is easy-to-use, fast, inexpensive, reusable, and can detect one 8-oxodG per 6600 nucleobases. The mass detection limit is 150-fold lower than a previously reported dip-and-read voltammetric sensor for oxidized DNA. Fast assays (<1 min) and moderate sample consumption (15 pmol DNA) suggest potential for research and clinical applications. Practical use is illustrated by detecting DNA oxidation from cigarette smoke and ash extracts in dispersions with NADPH and Cu(2+).

TET enzymatic oxidation of 5-methylcytosine, 5-hydroxymethylcytosine and 5-formylcytosine

5-Methylcytosine and methylated histones have been considered for a long time as stable epigenetic marks of chromatin involved in gene regulation. This concept has been recently revisited with the detection of large amounts of 5-hydroxymethylcytosine, now considered as the sixth DNA base, in mouse embryonic stem cells, Purkinje neurons and brain tissues. The dioxygenases that belong to the ten eleven translocation (TET) oxygenase family have been shown to initiate the formation of this methyl oxidation product of 5-methylcytosine that is also generated although far less efficiently by radical reactions involving hydroxyl radical and one-electron oxidants. It was found as additional striking data that iterative TET-mediated oxidation of 5-hydroxymethylcytosine gives rise to 5-formylcytosine and 5-carboxylcytosine. This survey focuses on chemical and biochemical aspects of the enzymatic oxidation reactions of 5-methylcytosine that are likely to be involved in active demethylation pathways through the implication of enzymatic deamination of 5-methylcytosine oxidation products and/or several base excision repair enzymes. The high biological relevance of the latter modified bases explains why major efforts have been devoted to the design of a broad range of assays aimed at measuring globally or at the single base resolution, 5-hydroxymethylcytosine and the two other oxidation products in the DNA of cells and tissues. Another critical issue that is addressed in this review article deals with the assessment of the possible role of 5-methylcytosine oxidation products, when present in elevated amounts in cellular DNA, in terms of mutagenesis and interference with key cellular enzymes including DNA and RNA polymerases.

Photoinduced damage to cellular DNA: direct and photosensitized reactions

The survey focuses on recent aspects of photochemical reactions to cellular DNA that are implicated through the predominant formation of mostly bipyrimidine photoproducts in deleterious effects of human exposure to sunlight. Recent developments in analytical methods have allowed accurate and quantitative measurements of the main DNA photoproducts in cells and human skin. Highly mutagenic CC and CT bipyrimidine photoproducts, including cyclobutane pyrimidine dimers and pyrimidine (6-4) pyrimidone photoproducts (6-4PPs) are generated in low yields with respect to TT and TC photoproducts. Another striking finding deals with the formation of Dewar valence isomers, the third class of bipyrimidine photoproducts that is accounted for by UVA-mediated isomerization of initially UVB generated 6-4PPs. Cyclobutadithymine (T<>T) has been unambiguously shown to be involved in the genotoxicity of UVA radiation. Thus, T<>T is formed in UVA-irradiated cellular DNA according to a direct excitation mechanism with a higher efficiency than oxidatively generated DNA damage that arises mostly through the Type II photosensitization mechanism. C<>C and C<>T are repaired at rates intermediate between those of T<>T and 6-4TT. Evidence has been also provided for the occurrence of photosensitized reactions mediated by exogenous agents that act either in an independent way or through photodynamic effects.