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 UVB-induced 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.

Fig. 1.

Formation of bipyrimidine photoproducts within human skin exposed to UVB (A) or UVA (B) radiation. The presented data corresponds to one representative donor. The results are expressed in lesions per 10⁶ bases and are the average ± SD.

Fig. 2.

Distribution of bipyrimidine photoproducts within human skin and primary keratinocytes upon exposure to either UVB (A) or UVA (B) radiation. The proportion (in a percentage) of each photoproduct was determined for each donor, and results were represented by the average ± SD.

Fig. 3.

Formation of 8-oxodGuo and thymine-thymine cyclobutane dimers within human skin exposed to UVA radiation (200 J/cm²). The results are expressed in lesions per 10⁶ bases and are the average ± SD.

Fig. 4.

Formation of 8-oxodGuo and thymine-thymine cyclobutane dimers within human skin exposed to UVB radiation (0.2 J/cm²). The results are expressed in lesions per 10⁶ normal bases and are the average ± SD.

Fig. 5.

Persistence of thymine-thymine cyclobutane dimers within human skin exposed to UVA (100 J/cm²) or UVB (0.1 J/cm²) radiation. Repair of TT (6-4) photoproducts within UVB irradiated skin also is shown. The results are expressed in percentage of residual lesions and are the average ± SD of data obtained with four different donors. The statistical significance in the level of remaining T<>T between UVA and UVB radiation was calculated by using the Student t test. Difference between UVA and UVB was found to be statistically significant at 24 h (P < 0.002) and 48 h (P < 0.05).