Mutagenic action of monochromatic UV radiation in the solar range on human cells

Mutations to ouabain resistance (selecting for base modifications at the co-dominant Na+K+-dependent ATP-ase locus) and thioguanine resistance (selecting for a wide range of genetic changes at the recessive hypoxanthine-guanine phosphoribosyl transferase locus) were measured in a repair-proficient human lymphoblastoid line with defined monochromatic radiations in the UVC (254 nm), UVB (302 nm, 313 nm), UVA (334 nm, 365 nm) and visible (405 nm) ranges. No mutations were detected at wavelengths in the range 334-405 nm. At 254 nm and 313 nm, both mutations to thioguanine resistance and survival were consistent with those expected from the relative levels of cyclobutane-type pyrimidine dimers induced. However, at 313 nm, the ratio of ouabain-resistant to thioguanine-resistant mutants is 10 times higher than at 254 nm, indicating that a unique type of pre-mutagenic base damage is induced at the longer wavelength. Radiation in the UVA (334 nm) range reduced the induction of mutations by a UVC (254 nm) wavelength at both mutation markers. These results suggest, first, that distinct types of biologically expressed genetic damage may be induced in the UVB region of sunlight and, second, that strong interactions may occur between the different wavelength regions of sunlight that can modify the expression of this genetic damage in human cells.     

Raman spectroscopic analysis of the effect of ultraviolet irradiation on calf thymus DNA

Raman spectroscopy was used for the first time to detect the effect of independent UVA (ultraviolet-A: 320-400nm) and UVB (ultraviolet-B: 280-320 nm) irradiation on the calf thymus DNA in aqueous solution. After both UVA and UVB irradiation for 1h or 3h, the damage to the conformation of DNA was moderate, but the reduction of the B-form DNA component was obvious. Both UVA and UVB caused significant damage to the deoxyribose moiety and bases, among which the pyrimidine base pairs were more seriously affected. There appeared to be preferential damaging sites on DNA molecules caused by UVA and UVB irradiation. UVA irradiation caused more damage to the deoxyribose than UVB irradiation, while UVB irradiation caused more significant damage to the pyrimidine moiety than UVA irradiation. After UVB irradiation for 3h, unstacking of the AT base pairs and the cytosine ring took place, severe damage to the thymine moiety occurred, and some base pairs were modified. Moreover, with either UVA or UVB irradiation for 3h,the photoreactivation of DNA occurred. The damage to the DNA caused by UVB was immediate, while the damage caused by UVA was proportional to the irradiation duration. The experimental results partly indicate the formation of some cyclobutane pyrimidine dimers and (6-4) photoproducts.     

Bipyrimidine photoproducts rather than oxidative lesions are the main type of DNA damage involved in the genotoxic effect of solar UVA radiation

Exposure to solar UV radiation gives rise to mutations that may lead to skin cancer. UVA (320-340 nm) constitutes the large majority of solar UV radiation but is less effective than UVB (290-320 nm) at damaging DNA. Although UVA has been implicated in photocarcinogenesis, its contribution to sunlight mutagenesis has not been elucidated, and DNA damage produced by UVA remains poorly characterized. We employed HPLC-MS/MS and alkaline agarose gel electrophoresis in conjunction with the use of specific DNA repair proteins to determine the distribution of the various classes and types of DNA lesions, including bipyrimidine photoproducts, in Chinese hamster ovary cells exposed to pure UVA radiation, as well as UVB and simulated sunlight (lambda > 295 nm) for comparison. At UVA doses compatible with human exposure, oxidative DNA lesions are not the major type of damage induced by UVA. Indeed, single-strand breaks, oxidized pyrimidines, oxidized purines (essentially 8-oxo-7,8-dihydroguanine), and cyclobutane pyrimidine dimers (CPDs) are formed in a 1:1:3:10 ratio. In addition, we demonstrate that, in contrast to UVB and sunlight, UVA generates CPDs with a large predominance of TT CPDs, which strongly suggests that they are formed via a photosensitized triplet energy transfer. Moreover, UVA induces neither (6-4) photoproducts nor their Dewar isomers via direct absorption. We also show that UVA photons contained in sunlight, rather than UVB, are implicated in the photoisomerization of (6-4) photoproducts, a quickly repaired damage, into poorly repaired and highly mutagenic Dewar photoproducts. Altogether, our data shed new light on the deleterious effect of UVA.     

G-to-T transversions and small tandem base deletions are the hallmark of mutations induced by ultraviolet a radiation in mammalian cells

Ultraviolet A (UVA) radiation received from the sun and from the widespread use of tanning beds by populations residing in areas of northern latitude represents a potential risk factor for human health. The genotoxic and cancer-causing effects of UVA have remained controversial. A mutagenic role for UVA based on DNA damage formation by reactive oxygen species as well as by generation of photoproducts such as cyclobutane pyrimidine dimers (CPDs) has been suggested. Here, we investigated the mutagenicity of UVA in relation to its DNA damaging effects in transgenic Big Blue mouse embryonic fibroblasts. We determined the formation of a typical oxidative DNA lesion, 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxo-dG), and of CPDs, as well as quantified the induction of mutations in the cII transgene in cells irradiated with a 2000 W UVA lamp. UVA irradiation at a dose of 18 J/cm(2) produced significant levels of 8-oxo-dG in DNA (P < 0.03) but did not yield detectable CPDs. UVA irradiation also increased the cII mutant frequency almost 5-fold over background (P < 0.01) while showing moderate cytotoxicity (70% cell viability). UVA-induced mutations were characterized by statistically significant increases in G-to-T transversions and small tandem base deletions (P = 0.0075, P = 0.008, respectively) relative to spontaneously derived mutations. This mutational spectrum differs from those previously reported for UVA in other test systems; however, it corresponds well with the known spectrum of mutations established for oxidative base lesions such as 8-oxo-dG. We conclude that UVA has the potential to trigger carcinogenesis owing to its mutagenic effects mediated through oxidative DNA damage.     

Mutagenicity of ultraviolet A radiation in the lacI transgene in Big Blue mouse embryonic fibroblasts

Sunlight ultraviolet A (UVA) irradiation has been implicated in the etiology of human skin cancer. A genotoxic mode of action for UVA radiation has been suggested that involves photosensitization reactions giving rise to promutagenic DNA lesions. We investigated the mutagenicity of UVA in the lacI transgene in Big Blue mouse embryonic fibroblasts. UVA irradiation of these cells at a physiologically relevant dose of 18J/cm(2) caused a 2.8-fold increase in the lacI mutant frequency relative to control, i.e., 12.12+/-1.84 versus 4.39+/-1.99 x 10(-5) (mean+/-S.D.). DNA sequencing analysis showed that of 100 UVA-induced mutant plaques and 54 spontaneously arisen control plaques, 97 and 51, respectively, contained a minimum of one mutation along the lacI transgene. The vast majority of both induced- and spontaneous mutations were single base substitutions, although less frequently, there were also single and multiple base deletions and insertions, and tandem base substitutions. Detailed mutation spectrometry analysis revealed that G:C-->T:A transversions, the signature mutations of oxidative DNA damage, were significantly induced by UVA irradiation (P<0.003). The absolute frequency of this type of mutations was 7.4-fold increased consequent to UVA irradiation as compared to control (3.38 versus 0.454 x 10(-5); P<0.00001). These findings are in complete agreement with those previously observed in the cII transgene of the same model system, and reaffirm the notion that intracellular photosensitization reactions causing promutagenic oxidative DNA damage are involved in UVA genotoxicity.     

Visible light (>395 nm) causes micronuclei formation in mammalian cells without generation of cyclobutane pyrimidine dimers

Solar radiation gives rise to DNA damage in mammalian cells not only directly by excitation of DNA, which generates predominantly pyrimidine dimers, but also indirectly by the excitation of endogenous photosensitizers, which causes oxidative DNA modifications. The latter mechanism has a low quantum yield, but it is the only one proceeding in the visible range of the spectrum. To investigate its relevance for the genotoxicity of sunlight, we have analysed the generation of micronuclei associated with the induction of oxidative DNA damage by visible light in melanoma cells and primary human skin fibroblasts. Similar yields of light-induced oxidative DNA base modifications sensitive to the repair glycosylase Fpg (7,8-dihydro-8-oxoguanine and other oxidative purine modifications) were observed in the normal fibroblasts and the malignant melanoma cells of the same donor. When irradiations were carried out at intervals to compensate for a photodecomposition of the endogenous chromophore, a significant generation of micronuclei was observed in both cell types. Cyclobutane pyrimidine dimers could be excluded to be responsible for the micronuclei induction at wavelengths >395 nm. Experiments with a cut-off filter indicate that the ratio of pyrimidine dimers and Fpg-sensitive oxidative modifications in irradiated cells not only reflects the relative contributions of direct and indirect mechanisms, but is also similar to the ratio by which the two mechanisms contribute to the generation of the micronuclei. The results suggest that indirectly generated oxidative DNA modifications can contribute significantly to the adverse effects of sunlight.     

DNA oxidation products determined with repair endonucleases in mammalian cells: types, basal levels and influence of cell proliferation

Purified repair endonucleases such as Fpg protein, endonuclease III and IV allow a very sensitive quantification of various types of oxidative DNA modifications in mammalian cells. By means of these assays, the numbers of base modifications sensitive to Fpg protein, which include 8-hydroxyguanine (8-oxoG), were determined to be less than 0.3 per 10⁶ bp in several types of untreated cultured mammalian cells and human lymphocytes and less than 10 per 10⁶ bp in mitochondrial DNA from rat and porcine liver. Oxidative 5,6-dihydropyrimidine derivatives sensitive to endonuclease III and sites of base loss sensitive to endonuclease IV or exonuclease III were much less frequent than Fpg-sensitive modifications. Here, we summarize our indications that all Fpg-sensitive modifications are recognized under the assay conditions and that on the other hand there is no artifactual generation of oxidative damage during the analysis. In addition, we show that the steady-state levels of Fpg-sensitive modifications in human lymphocytes and in two mammalian cell lines were higher in proliferating than in resting (confluent) cells. Only some of the Fpg-sensitive base modifications induced by various oxidants are 8-oxoG residues, as demonstrated for the damage under cell-free conditions. The percentage was dependent on the species ultimately responsible for the DNA damage and was approx. 40% in the case of hydroxyl radicals and peroxynitrite, 75% for type II photosensitizers (reacting via singlet oxygen) and only 20-30% in the case of type I photosensitizers such as riboflavin and acridine orange, which are assumed to react directly with the DNA.     

The deceptive nature of UVA tanning versus the modest protective effects of UVB tanning on human skin

The relationship between human skin pigmentation and protection from ultraviolet (UV) radiation is an important element underlying differences in skin carcinogenesis rates. The association between UV damage and the risk of skin cancer is clear, yet a strategic balance in exposure to UV needs to be met. Dark skin is protected from UV-induced DNA damage significantly more than light skin owing to the constitutively higher pigmentation, but an as yet unresolved and important question is what photoprotective benefit, if any, is afforded by facultative pigmentation (i.e. a tan induced by UV exposure). To address that and to compare the effects of various wavelengths of UV, we repetitively exposed human skin to suberythemal doses of UVA and/or UVB over 2 weeks after which a challenge dose of UVA and UVB was given. Although visual skin pigmentation (tanning) elicited by different UV exposure protocols was similar, the melanin content and UV-protective effects against DNA damage in UVB-tanned skin (but not in UVA-tanned skin) were significantly higher. UVA-induced tans seem to result from the photooxidation of existing melanin and its precursors with some redistribution of pigment granules, while UVB stimulates melanocytes to up-regulate melanin synthesis and increases pigmentation coverage, effects that are synergistically stimulated in UVA and UVB-exposed skin. Thus, UVA tanning contributes essentially no photoprotection, although all types of UV-induced tanning result in DNA and cellular damage, which can eventually lead to photocarcinogenesis.     

DNA lesion and mutagenesis induced in phageM13mp2 by UVA, UVB and UVC irradiation.

Sunlight is carcinogenic and mutagenic and its genotoxic effects are believed to be the result of UV light-induced lesions in DNA. These lesions include pyrimidine dimers and (6-4) photoproducts, but it is uncertain whether the pyrimidine modifications are the sole pre-mutagenic lesions induced by UV light. Previous studies indicate that some sunlight-induced mutations in the single-stranded DNA phage M13mp2 may not be caused by these photoproducts. In this work, purified single-stranded phage DNA was exposed to UVA, UVB and UVC and the induced mutations were analyzed. All 3 types of UV light increase the mutation frequency. The mutants were sequenced and the results suggest that UVA exposure may induce formation of a non-dipyrimidine lesion in DNA.     

Chemical and biochemical postlabeling methods for singling out specific oxidative DNA lesions.

A survey of the main available chemical and biochemical postlabeling assays for measuring oxidative DNA damage is reported. Two main approaches, radio and fluorescent postlabeling, have been used in order to reach a high level of sensitivity of detection. This is required for the measurement of DNA damage within cells and tissues upon exposure to agents of oxidative stress. Most of the methods are based on liquid chromatographic separation of defined DNA modifications following either acidic hydrolysis or enzymic digestion of DNA. In a subsequent step, the isolated base or sugar damages are either radiolabeled or made fluorescent by chemical or enzymatic reactions. Emphasis is placed on the recently developed high performance liquid chromatographic 32P-postlabeling assay, which allows the specific and sensitive measurement of various base damages including adenine N-1 oxide and 5-hydroxymethyluracil at the level of one modification per 10(7) normal bases in a sample size of 1 microgram of DNA. Examples of application of radioactive postlabeling to the measurement of DNA base damage following exposure of human cells to oxidizing agents including hydrogen peroxide and UVA radiation are provided.     

DNA damage and mutations induced by arachidonic acid peroxidation

Endogenous cellular oxidation of omega6-polyunsaturated fatty acids (PUFAs) has long been recognized as a contributing factor in the development of various cancers. The accrual of DNA damage as a result of reaction with free radical and electrophilic aldehyde products of lipid peroxidation is believed to be involved; however, the genotoxic and mutation-inducing potential of specific membrane PUFAs remains poorly defined. In the present study we have examined the ability of peroxidizing arachidonic acid (AA, 20:4omega6) to induce DNA strand breaks, base modifications, and mutations. The time-dependent induction of single-strand breaks and oxidative base modifications by AA in genomic DNA was quantified using denaturing glyoxal gel electrophoresis. Mutation spectra were determined in XP-G fibroblasts and a repair-proficient line corrected for this defect by c-DNA complementation (XP-G(+)). Mutation frequencies were elevated from approximately 5- to 30-fold over the background following reaction of DNA with AA for various times. The XPG gene product was found to be involved in the suppression of mutations after extended reaction of DNA with AA. Arachidonic acid-induced base substitutions were consistent with the presence of both oxidized and aldehyde base adducts in DNA. The frequency of multiple-base substitutions induced by AA was significantly reduced upon correction for the XPG defect (14% vs 2%, P = 0.0015). Evidence is also presented which suggests that the induced frequency of multiple mutations is lesion dependent. These results are compared to published data for mutations stimulated by alpha,beta-unsaturated aldehydes identified as products of lipid peroxidation.     

Oxidative stress and cancer: have we moved forward?

'Reactive species' (RS) of various types are formed in vivo and many are powerful oxidizing agents, capable of damaging DNA and other biomolecules. Increased formation of RS can promote the development of malignancy, and the 'normal' rates of RS generation may account for the increased risk of cancer development in the aged. Indeed, knockout of various antioxidant defence enzymes raises oxidative damage levels and promotes age-related cancer development in animals. In explaining this, most attention has been paid to direct oxidative damage to DNA by certain RS, such as hydroxyl radical (OH*). However, increased levels of DNA base oxidation products such as 8OHdg (8-hydroxy-2'-deoxyguanosine) do not always lead to malignancy, although malignant tumours often show increased levels of DNA base oxidation. Hence additional actions of RS must be important, possibly their effects on p53, cell proliferation, invasiveness and metastasis. Chronic inflammation predisposes to malignancy, but the role of RS in this is likely to be complex because RS can sometimes act as anti-inflammatory agents.     

Comparison of DNA damage responses following equimutagenic doses of UVA and UVB: a less effective cell cycle arrest with UVA may render UVA-induced pyrimidine dimers more mutagenic than UVB-induced ones

Mechanisms of UVA-mutagenesis remain a matter of debate. Earlier described higher rates of mutation formation per pyrimidine dimer with UVA than with UVB and other evidence suggested that a non-pyrimidine dimer-type of DNA damage contributes more to UVA- than to UVB-mutagenesis. However, more recently published data on the spectra of UVA-induced mutations in primary human skin cells and in mice suggest that pyrimidine dimers are the most common type of DNA damage-inducing mutations not only with UVB, but also with UVA. As this rebuts a prominent role of non-dimer type of DNA damage in UVA-mutagenesis, we hypothesized that the higher mutation rate at UVA-induced pyrimidine dimers, as compared to UVB-induced ones, is caused by differences in the way UVA- and UVB-exposed cells process DNA damage. Therefore, we here compared cell cycle regulation, DNA repair, and apoptosis in primary human fibroblasts following UVB- and UVA-irradiation, using the same physiologic and roughly equimutagenic doses (100-300 J m(-2) UVB, 100-300 kJ m(-2) UVA) we have used previously for mutagenesis experiments with the same type of cells. ELISAs for the detection of pyrimidine dimers confirmed that much fewer dimers were formed with these doses of UVA, as compared to UVB. We found that cell cycle arrests (intra-S, G1/S, G2/M), mediated at least in part by activation of p53 and p95, are much more prominent and long-lasting with UVB than with UVA. In contrast, no prominent differences were found between UVA and UVB for other anti-mutagenic cellular responses (DNA repair, apoptosis). Our data suggest that less effective anti-mutagenic cellular responses, in particular different and shorter-lived cell cycle arrests, render pyrimidine dimers induced by UVA more mutagenic than pyrimidine dimers induced by UVB.     

Poly(ADP-ribosyl)ation accelerates DNA repair in a pathway dependent on Cockayne syndrome B protein

Activation of poly(ADP-ribose)polymerases 1 and 2 (PARP-1 and PARP-2) is one of the earliest responses of mammalian cells to DNA damage by numerous genotoxic agents. We have analysed the influence of PARP inhibition, either achieved by over-expression of the DNA binding domain of PARP-1 or by treatment with 3,4-dihydro-5-[4-(1-piperidinyl)butoxyl]-1(2H)-isoquinolinone, on the repair of single-strand breaks (SSB), pyrimidine dimers and oxidative base modifications sensitive to Fpg protein (mostly 8-hydroxyguanine) in mammalian cells at very low, non-cytotoxic levels of DNA damage. The data show that the repair rates of all three types of DNA damage are significantly lower in PARP-inhibited cells. Importantly, the retardation of the repair of base modifications is not associated with accumulation of intermediates such as SSB or abasic sites. Moreover, the influence of the PARP inhibition is not observed in cells deficient in Cockayne syndrome B protein (Csb). The results indicate that PARP activation and Csb are both involved in a novel mechanism that accelerates the global repair of various types of DNA modifications.     

Roles of reactive oxygen species in UVA‐induced oxidation of 5,6‐dihydroxyindole‐2‐carboxylic acid‐melanin as studied by differential spectrophotometric method

Eumelanin photoprotects pigmented tissues from ultraviolet (UV) damage. However, UVA‐induced tanning seems to result from the photooxidation of preexisting melanin and does not contribute to photoprotection. We investigated the mechanism of UVA‐induced degradation of 5,6‐dihydroxyindole‐2‐carboxylic acid (DHICA)‐melanin taking advantage of its solubility in a neutral buffer and using a differential spectrophotometric method to detect subtle changes in its structure. Our methodology is suitable for examining the effects of various agents that interact with reactive oxygen species (ROS) to determine how ROS is involved in the UVA‐induced oxidative modifications. The results show that UVA radiation induces the oxidation of DHICA to indole‐5,6‐quinone‐2‐carboxylic acid in eumelanin, which is then cleaved to form a photodegraded, pyrrolic moiety and finally to form free pyrrole‐2,3,5‐tricarboxylic acid. The possible involvement of superoxide radical and singlet oxygen in the oxidation was suggested. The generation and quenching of singlet oxygen by DHICA‐melanin was confirmed by direct measurements of singlet oxygen phosphorescence.     

DNA damage by peroxynitrite characterized with DNA repair enzymes.

The DNA damage induced by peroxynitrite in isolated bacteriophage PM2 DNA was characterized by means of several repair enzymes with defined substrate specificities. Similar results were obtained with peroxynitrite itself and with 3-morpholinosydnonimine (SIN-1), a compound generating the precursors of peroxynitrite, nitric oxide and superoxide. A high number of base modifications sensitive to Fpg protein which, according to HPLC analysis, were mostly 8-hydroxyguanine residues, and half as many single-strand breaks were observed, while the numbers of oxidized pyrimidines (sensitive to endonuclease III) and of sites of base loss (sensitive to exonuclease III or T4 endonuclease V) were relatively low. This DNA damage profile caused by peroxynitrite is significantly different from that obtained with hydroxyl radicals or with singlet molecular oxygen. The effects of various radical scavengers and other additives (t-butanol, selenomethionine, selenocystine, desferrioxamine) were the same for single-strand breaks and Fpg-sensitive modifications and indicate that a single reactive intermediate but not peroxynitrite itself is responsible for the damage.     

Pro-oxidative effect of beta-carotene and the interaction with flavonoids on UVA-induced DNA strand breaks in mouse fibroblast C3H10T1/2 cells

It has been suggested that β-carotene itself is unstable under certain conditions and that a combination of antioxidants may prevent the pro-oxidative effects of β-carotene. Thus, the present study aimed to investigate the interaction of β-carotene with three flavonoids—naringin, rutin and quercetin—on DNA damage induced by ultraviolet A (UVA) in C3H10T1/2 cells, a mouse embryo fibroblast. The cells were preincubated with β-carotene and/or flavonoid for 1 h followed by UVA irradiation, and DNA damage was measured using comet assay. We showed that β-carotene at 20 μM enhanced DNA damage (by 35%; P<.05) induced by UVA (7.6 kJ/m²), whereas naringin, rutin and quercetin significantly decreased UVA-induced DNA damage. When each flavonoid was combined with β-carotene during preincubation, UVA-induced cellular DNA damage was significantly suppressed and the effects were in the order of naringin≥rutin>quercetin. The flavonoids decreased UVA-induced oxidation of preincorporated β-carotene in the same order. Using electron spin resonance spectroscopy, we showed that the ability of these flavonoids to quench singlet oxygen was consistent with protection against DNA damage and β-carotene oxidation. All three flavonoids had some absorption at the UVA range (320–380 nm), but the effects were opposite to those on DNA damage and β-carotene oxidation. Taken together, this cell culture study demonstrates an interaction between flavonoids and β-carotene in UVA-induced DNA damage, and the results suggest that a combination of β-carotene with naringin, rutin or quercetin may increase the safety of β-carotene.     

Unravelling UVA-induced mutagenesis

Ultraviolet A (UVA) radiation represents more than 90% of the solar UV radiation reaching Earth's surface. Exposure to solar UV radiation is a major risk in the occurrence of non-melanoma skin cancer. Whole genome sequencing data of melanoma tumors recently obtained makes it possible also to definitively associate malignant melanoma with sunlight exposure. Even though UVB has long been established as the major cause of skin cancer, the relative contribution of UVA is still unclear. In this review, we first report on the formation of DNA damage induced by UVA radiation, and on recent advances on the associated mechanism. We then discuss the controversial data on the UVA-induced mutational events obtained for various types of eukaryotic cells, including human skin cells. This may help unravel the role of UVA in the various steps of photocarcinogenesis. The connection to photocarcinogenesis is more extensively discussed by other authors in this issue.     

Oxidative stress impairs the repair of oxidative DNA base modifications in human skin fibroblasts and melanoma cells

Irradiation of mammalian cells with solar light is associated with the generation of reactive oxygen species (ROS) and oxidative stress, which is mediated in part by endogenous photosensitizers absorbing in the visible range of the solar spectrum. Accordingly, oxidative DNA base modifications such as 7,8-dihydro-8-oxoguanine (8-oxoG) are the predominant types of DNA damage in cells irradiated at wavelengths >400 nm. We have analysed the repair of oxidative purine modifications in human skin fibroblasts and melanoma cells using an alkaline elution technique, both under normal conditions and after depletion of glutathione. Similar repair rates were observed in fibroblasts and melanoma cells from three different patients (t1/2 approximately 4h). In both cell types, glutathione depletion (increased oxidative stress) caused a pronounced repair retardation even under non-toxic irradiation conditions. Furthermore, the cleavage activity at 8-oxoG residues measured in protein extracts of the cells dropped transiently after irradiation. An addition of dithiothreitol restored normal repair rates. Interestingly, the repair rates of cyclobutane pyrimidine dimers (t1/2 approximately 18 h), AP sites (t1/2 approximately 1h) and single-strand breaks (t1/2 <30 min) were not affected by the light-induced oxidative stress. We conclude that the base excision repair of oxidative purine modifications is surprisingly vulnerable to oxidative stress, while the nucleotide excision repair of pyrimidine dimers is not.     

Modifications of nucleosides by endogenous mutagens-DNA adducts arising from cellular processes

DNA damage plays a significant role in mutagenesis, carcinogenesis and ageing. Chemical transformations leading to DNA damage include reactions of the base units with agents of endogenous and exogenous origin. The vast majority of damage arising from cellular processes such as metabolism and lipid peroxidation are identical or very similar to those induced by exposure to environmental agents. A detailed knowledge of the types and prevalence of endogenous DNA damage provides insight into the chemical nature of species involved in these modifications and may be of help in understanding their influence on the induction of cancer or other diseases. This knowledge may also be essential to the development of rational chemopreventive strategies directed against the initiation of oxidative stress- and lipid peroxidation-associated pathology.The present work reviews findings regarding the interaction between DNA bases and various reactive species arising from lipid peroxidation and other cellular processes, drawing attention to the mechanism responsible for the formation of the resulted modifications. The biological consequences of these interactions are also briefly discussed.