UVA generates pyrimidine dimers in DNA directly

Center for Biologically Inspired Materials and Material Systems, Duke University, Durham, North Carolina, USA.
Biophysical Journal (Impact Factor: 3.97). 03/2009; 96(3):1151-8. DOI: 10.1016/j.bpj.2008.10.030
Source: PubMed


There is increasing evidence that UVA radiation, which makes up approximately 95% of the solar UV light reaching the Earth's surface and is also commonly used for cosmetic purposes, is genotoxic. However, in contrast to UVC and UVB, the mechanisms by which UVA produces various DNA lesions are still unclear. In addition, the relative amounts of various types of UVA lesions and their mutagenic significance are also a subject of debate. Here, we exploit atomic force microscopy (AFM) imaging of individual DNA molecules, alone and in complexes with a suite of DNA repair enzymes and antibodies, to directly quantify UVA damage and reexamine its basic mechanisms at a single-molecule level. By combining the activity of endonuclease IV and T4 endonuclease V on highly purified and UVA-irradiated pUC18 plasmids, we show by direct AFM imaging that UVA produces a significant amount of abasic sites and cyclobutane pyrimidine dimers (CPDs). However, we find that only approximately 60% of the T4 endonuclease V-sensitive sites, which are commonly counted as CPDs, are true CPDs; the other 40% are abasic sites. Most importantly, our results obtained by AFM imaging of highly purified native and synthetic DNA using T4 endonuclease V, photolyase, and anti-CPD antibodies strongly suggest that CPDs are produced by UVA directly. Thus, our observations contradict the predominant view that as-yet-unidentified photosensitizers are required to transfer the energy of UVA to DNA to produce CPDs. Our results may help to resolve the long-standing controversy about the origin of UVA-produced CPDs in DNA.

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    • "Oxidatively generated DNA damage, such as the formation of 7,8-dihydro-8-oxoguanine (8oxodG), is more effectively induced by UVA than by UVB (Schuch and Menck, 2010; Schuch et al., 2012). However, studies indicate that UVA radiation induces CPDs directly on DNA more readily than it induces oxidized bases, suggesting that UVA may be more mutagenic than previously assumed (Jiang et al., 2009; Schuch et al., 2009). Additionally, although UVA absorption by DNA is poor, the presence of 6,4PPs in purified DNA samples and in human cell cultures after irradiation with UVA light has been demonstrated (Cortat et al., 2013; Schuch et al., 2009). "
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    ABSTRACT: The increased incidence of solar ultraviolet B (UVB) radiation has been proposed as an environmental stressor, which may help to explain the enigmatic decline of amphibian populations worldwide. Despite growing knowledge regarding the UV-induced biological effects in several amphibian models, little is known about the efficacy of DNA repair pathways. In addition, little attention has been given to the interplay between these molecular mechanisms with other physiological strategies that avoid the damage induced by sunlight. Here, DNA lesions induced by environmental doses of solar UVB and UVA radiation were detected in genomic DNA samples of treefrog tadpoles (Hypsiboas pulchellus) and their DNA repair activity was evaluated. These data were complemented by monitoring the induction of apoptosis in blood cells and tadpole survival. Furthermore, the tadpoles' ability to perceive and escape from UV wavelengths was evaluated as an additional strategy of photoprotection. The results show that tadpoles are very sensitive to UVB light, which could be explained by the slow DNA repair rates for both cyclobutane pyrimidine dimers (CPDs) and pyrimidine (6,4) pyrimidone photoproducts (6,4PPs). However, they were resistant to UVA, probably as a result of the activation of photolyases during UVA irradiation. Surprisingly, a sensory mechanism that triggers their escape from UVB and UVA light avoids the generation of DNA damage and helps to maintain the genomic integrity. This work demonstrates the genotoxic impact of both UVB and UVA radiation on tadpoles and emphasizes the importance of the interplay between molecular and sensory mechanisms to minimize the damage caused by sunlight.
    Journal of Experimental Biology 10/2015; 218(19):3059-3067. DOI:10.1242/jeb.126672 · 2.90 Impact Factor
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    • "In human cells, it is well known that the oxidative stress induced by UVA radiation can also damage other cellular structures and molecules, such as lipids (through lipid peroxidation that results in membrane destabilization) and proteins (the oxidized proteins can induce cross-links with DNA bases blocking the replication and transcription processes) (Blair, 2008; Girard et al., 2008; Hoerter et al., 2008; Moysan et al., 1993). Despite oxidatively generated DNA damage being more effectively induced by UVA than by UVB, studies indicate that UVA radiation also induces CPDs directly on DNA more readily than oxidized bases, suggesting that UVA may be more mutagenic than previously assumed (Jiang et al., 2009; Schuch et al., 2009). Additionally , although UVA absorption by DNA is weak, the presence of 6–4PPs in purified DNA samples after irradiation with UVA light has been demonstrated (Cortat et al., 2013; Schuch et al., 2009). "
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    ABSTRACT: The increased incidence of solar ultraviolet radiation (UV) due to ozone depletion has been affecting both terrestrial and aquatic ecosystems and it may help to explain the enigmatic decline of amphibian populations in specific localities. In this work, influential events concerning the Antarctic ozone hole were identified in a dataset containing 35 years of ozone measurements over southern Brazil. The effects of environmental doses of UVB and UVA radiation were addressed on the morphology and development of Hypsiboas pulchellus tadpole (Anura: Hylidae), as well as on the induction of malformation after the conclusion of metamorphosis. These analyzes were complemented by the detection of micronucleus formation in blood cells. 72 ozone depletion events were identified from 1979 to 2013. Surprisingly, their yearly frequency increased three-fold during the last 17 years. The results clearly show that H. pulchellus tadpole are much more sensitive to UVB than UVA light, which reduces their survival and developmental rates. Additionally, the rates of micronucleus formation by UVB were considerably higher compared to UVA even after the activation of photolyases enzymes by a further photoreactivation treatment. Consequently, a higher occurrence of malformation was observed in UVB-irradiated individuals. These results demonstrate the severe genotoxic impact of UVB radiation on this treefrog species and its importance for further studies aimed to assess the impact of the increased levels of solar UVB radiation on declining species of the Hylidae family. Copyright © 2015 Elsevier Inc. All rights reserved.
    Ecotoxicology and Environmental Safety 05/2015; 118:190-198. DOI:10.1016/j.ecoenv.2015.04.029 · 2.76 Impact Factor
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    • "The classical point of view was that UVA induces predominantly oxidative damages and among those the majority accounts for 8-oxo-guanine (42). More recently, it was demonstrated that UVA is also able to induce thymidine dimers, especially TT-CPDs, although at a 1000-fold lower efficiency compared to UVC (43,44). Even if CPDs are added to the UVA damage profile, there is still a gap to fully understand the mutagenic potential of UVA radiation, at least in hamster cells (45). "
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    ABSTRACT: UVA (320-400 nm) represents the main spectral component of solar UV radiation, induces pre-mutagenic DNA lesions and is classified as Class I carcinogen. Recently, discussion arose whether UVA induces DNA double-strand breaks (dsbs). Only few reports link the induction of dsbs to UVA exposure and the underlying mechanisms are poorly understood. Using the Comet-assay and γH2AX as markers for dsb formation, we demonstrate the dose-dependent dsb induction by UVA in G(1)-synchronized human keratinocytes (HaCaT) and primary human skin fibroblasts. The number of γH2AX foci increases when a UVA dose is applied in fractions (split dose), with a 2-h recovery period between fractions. The presence of the anti-oxidant Naringin reduces dsb formation significantly. Using an FPG-modified Comet-assay as well as warm and cold repair incubation, we show that dsbs arise partially during repair of bi-stranded, oxidative, clustered DNA lesions. We also demonstrate that on stretched chromatin fibres, 8-oxo-G and abasic sites occur in clusters. This suggests a replication-independent formation of UVA-induced dsbs through clustered single-strand breaks via locally generated reactive oxygen species. Since UVA is the main component of solar UV exposure and is used for artificial UV exposure, our results shine new light on the aetiology of skin cancer.
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