Cyclobutane pyrimidine dimers are predominant DNA lesions in whole human skin exposed to UVA radiation

Abstract

Solar UV radiation is the most important environmental factor involved in the pathogenesis of skin cancers. The well known genotoxic properties of UVB radiation (290 -320 nm) mostly involve bipyrimidine DNA photoproducts. In contrast, the contribution of more-abundant UVA radiation (320 -400 nm) that are not directly absorbed by DNA remains poorly understood in skin. Using a highly accurate and quantitative assay based on HPLC coupled with tandem mass spectrometry, we determined the type and the yield of formation of DNA damage in whole human skin exposed to UVB or UVA. Cyclobutane pyrimidine dimers, a typical UVBinduced DNA damage, were found to be produced in significant yield also in whole human skin exposed to UVA through a mechanism different from that triggered by UVB. Moreover, the latter class of photoproducts is produced in a larger amount than 8-oxo-7,8-dihydro-2-deoxyguanosine, the most common oxidatively generated lesion, in human skin. Strikingly, the rate of removal of UVA-generated cyclobutane pyrimidine dimers was lower than those produced by UVB irradiation of skin. Finally, we compared the formation yields of DNA damage in whole skin with those determined in primary cultures of keratinocytes isolated from the same donors. We thus showed that human skin efficiently protects against UVB-induced DNA lesions, whereas very weak protection is afforded against UVA. These observations emphasize the likely role played by the UVA-induced DNA damage in skin carcinogenesis and should have consequences for photoprotection strategies.carcinogenesis ͉ DNA damage ͉ mutagenesis ͉ oxidative stress ͉ DNA repair O ccurrence of skin cancers, which mostly arise from exposure to solar UV radiation, has constantly increased in the recent years due to changes in life habits. Harmful effects of UV radiation are mostly associated with both direct and indirect photoinduced damage to DNA (1) that can lead to the induction of mutations. The chemical nature and the formation efficiency of the DNA lesions greatly depend on the wavelength of the incident photons. UVB radiation (290-320 nm), the most energetic mutagenic and carcinogenic component of solar radiation, is directly absorbed by DNA, giving rise to dimeric photoproducts between adjacent pyrimidine bases. Two types of these bulky modifications are produced, namely cyclobutane pyrimidine dimers (CPDs) and pyrimidine (6-4) pyrimidone photoproducts (1). Evidence for the involvement of these lesions in photocarcinogenesis is provided by the high proportion of p53 mutations (TC to TT or CC to TT transitions) detected at bipyrimidine sites in skin tumors (2-4).UVA radiation (320-400 nm), the other component of terrestrial UV radiation, is mutagenic in cultured cells (5, 6) and induces skin tumors in mice (7,8). In addition, UVA has been shown to be involved in immunosuppression (9) and is suspected to play a major role in the induction of melanoma (10, 11), the most severe type of skin cancers. Consequently, the carcinogenic properties of UVA have become a matter of concern...

Combination of azathioprine and UVA irradiation is a major source of cellular 8-oxo-7,8-dihydro-2′-deoxyguanosine

“…This demonstrated that approximately 50 % of the damage revealed by hOGG1 comet was ALS, levels of which were also statistically significant (P<0.0001), compared to the corresponding control (CSI + 6-TG -hOGG1). Consistent with a recent report [13], we noted essentially no 8-oxodG formation in cells treated with UVA alone, likewise we saw no formation of ALS, not least due to the dose used ( Figure 1A). Whilst treatment of HDFs with 1 M 6-TG, in the absence of UVA, failed to produce any damage, as determined by the alkaline comet assay; when hOGG1 was employed, significant levels of damage were revealed (P<0.0001, compared to CSI + hOGG1).…”

“…CPD and pyrimidine (6-4) pyrimidone photoproducts are widely considered to be the lesions responsible for mutagenicity of solar UVR [44], an argument strengthened by the growing evidence that pure UVA can also induce CPD [13,33].…”

“…A number of reports have indicated that the induction of CPD by UVA is much greater than previously thought [13,33] and hence, could be responsible for the mutagenicity of Aza in conjunction with UVA. However, we noted a minimal increase in CPD, compared to 8-oxodG and ALS ( Figure 1A).…”

“…The carcinogenic effect of UVR is therefore particularly relevant to transplant patients. Although the predominant DNA modifications induced by solar UVR are dimeric photoproducts, such as cyclobutane pyrimidine dimers (CPD [13]), UVR also generate reactive oxygen species (ROS), which can lead to the oxidatively generated modification of various cellular molecules, including DNA [14]. The ROS produced by UVA is predominantly singlet oxygen ( 1 O 2 ) [15], Azathioprine and UVA = 8-oxodG 4 albeit with some contribution from hydroxyl radicals ( OH) [14], whereas the mechanism for UVB-induced formation of oxidatively modified DNA is less clear, and may involve formation of, oxygen independent, nucleobase radical cations, OH and 1 O 2 [14], or combinations thereof.…”

Estimating the risk of squamous cell cancer induction in skin following nonlinear optical imaging

“…So a 3-photon process from the laser would result in absorption between 233-267 nm, lying in UVC region, resulting in CPD production. However, the role of UVA component (320-400 nm) in forming CPDs has only been recently confirmed [32,33]. Therefore, 2-photon processes, with absorption between 350-400 nm, can also lead to CPD formation.…”