DNA photoproducts released by repair in biological fluids as biomarkers of the genotoxicity of UV radiation

Data Processing for Predicting DNA Damaging Properties of Complex UV Sources

A growing number of experimental evidence emphasizes that photobiological phenomena are not always the sum of the effect of individual wavelengths present in the emission spectrum of light sources. Unfortunately, tools are missing to identify such non-additive effects and predict effects of various exposure conditions. In the present work, we addressed these points for the formation of pyrimidine dimers in DNA upon co-exposure to UVC, UVB and UVA radiation. We first applied a combination index approach to determine whether mixtures of theses UV ranges exhibited additive, inhibitory or synergistic effects on the formation of cyclobutane pyrimidine dimers, (6-4) photoproducts and Dewar valence isomers. A predictive approach based on an experimental design strategy was then used to quantify the contribution of each wavelength range to the formation of DNA photoproducts. The obtained models allowed us to accurately predict the level of pyrimidine dimers in DNA irradiated under different conditions. The data were found to be more accurate than those obtained with the simple additive approach underlying the use of action spectra. Experimental design thus appears as an attractive concept that could be widely applied in photobiology even for cellular experiments.

Urinary thymidine dimer excretion reflects personal ultraviolet radiation exposure levels

Exposure to ultraviolet radiation (UVR) leads to skin DNA damage, specifically in the form of cyclobutane pyrimidine dimers, with thymidine dimers being the most common. Quantifying these dimers can indicate the extent of DNA damage resulting from UVR exposure. Here, a new liquid chromatography-mass spectrometry (LC-MS) method was used to quantify thymidine dimers in the urine after a temporary increase in real-life UVR exposure. Healthy Danish volunteers (n = 27) experienced increased UVR exposure during a winter vacation. Individual exposure, assessed via personally worn electronic UVR dosimeters, revealed a mean exposure level of 32.9 standard erythema doses (SEDs) during the last week of vacation. Morning urine thymidine dimer concentrations were markedly elevated both 1 and 2 days post-vacation, and individual thymidine dimer levels correlated with UVR exposure during the last week of the vacation. The strongest correlation with erythema-weighted personal UVR exposure (Power model, r2 = 0.64, p < 0.001) was observed when both morning urine samples were combined to measure 48-h thymidine dimer excretion, whereas 24-h excretion based on a single sample provided a weaker correlation (Power model, r2 = 0.55, p < 0.001). Sex, age, and skin phototype had no significant effect on these correlations. For the first time, urinary thymidine dimer excretion was quantified by LC-MS to evaluate the effect of a temporary increase in personal UVR exposure in a real-life setting. The high sensitivity to elevated UVR exposure and correlation between urinary excretion and measured SED suggest that this approach may be used to quantify DNA damage and repair and to evaluate photoprevention strategies.

Nuclear and Urinary Measurements Show the Efficacy of Sun-Protection Factor 50+ Sunscreen against DNA Photoproducts upon Real-Life Recreational Exposure

Sunscreens have been shown to protect against UVR-induced DNA damage in human skin under laboratory conditions. We presently extended these observations to real-life conditions in volunteers after their ordinary exposure habits during summer holidays. Volunteers were randomly assigned to a control group and an educated group supplied with a SPF ≥50 sunscreen and receiving instructions for use. A questionnaire was used to determine the extent of exposure. No difference in average solar UVR exposure was found between the two groups. DNA photoprotection was first assessed by, to our knowledge, a previously unreported noninvasive assay on the basis of the quantification of pyrimidine dimers released by DNA repair in urine. Damage was also quantified in the nuclear DNA extracted from the roof of suction blisters collected after recreational exposure. The urinary concentration of photoproducts was significantly higher in the control than in the educated group. The same trend was observed for the level of photoproducts in the DNA from suction blisters. The unambiguous observation of an efficient photoprotection against DNA damage afforded by sunscreen under real-life conditions provides strong support for the efficiency of the sunscreens. In addition, the results validate the use of urinary DNA photoproducts as a noninvasive assay applicable to photoprotection.