DNA-damage during photo(chemo)therapy studied by comet-assay

UVB-phototherapy and photochemotherapy (PUVA, 8-methoxypsoralen plus UVA-ir radiation) have been widely used for the treatment of various skin diseases for many years. The mechanism of the biological effects of UV-light, which contributes to the therapeutic efficiency, is not well understood, particu larly for PUVA. The DNA is one of the essential UV-targets within the cell. The aim of the present study was to investigate DNA damage and repair foll owing the treatment of HaCaT-cells by PUVA in comparison with the effects o f UVB and UVA-irradiation. Single cell gel electrophoresis (the comet assay ) was used to evaluate DNA strand breaks and alkali labile sites caused by UV or PUVA. Our results suggest that the mechanism of the effects of UVB an d UVA for the formation and repair of DNA damage is quite different from th at of PUVA. Immediately after irradiation, comet formation was seen only in the case of UVA (0-5 J/cm(2)). After UVB (0-60 mJ/cm(2)), comet formation was detected only 1 h after irradiation, when the length and intensity of t he comet in UVA-irradiated cells were considerably diminished. Neither UVA nor UVB caused DNA-damage detectable by neutral comet-assay within the dose ranges studied. After PUVA-treatment (300 ng/ml 8-MOP and 2 J/cm(2) UVA) w e observed increasing comet-formation with a peak of 1.5 h after irradiatio n by neutral assay, then the comet-tails decreased slowly during the next f ew hours. These findings suggest that UVA and UVB induce DNA single strand breaks before and during DNA repair, respectively, whereas DNA double stran d breaks are occurred following PUVA treatment. It is hypothesised that the se latter DNA lesions are repair intermediary products of the DNA cross-lin ks caused by PUVA.