DNA damage formation, DNA repair, and survival after exposure of DNA repair-proficient and nucleotide excision repair-deficient human lymphoblasts toUVA1 and UVB

Purpose: The comet assay has been used to visualize DNA damage in single ce lls after exposure to UV light. These comets are commonly thought to reflec t transient, repair-induced DNA breaks. The goal of the work presented here was to further characterize the nature of UV-induced comets and to further elucidate DNA damage formation by different wavelengths of ultraviolet lig ht. Materials and methods: Detailed dose-response and time-course experiments w ith comet formation were carried out with normal and nucleotide excision re pair (NER)-deficient xeroderma pigmentosum (XP) lymphoblasts. Irradiation w as carried out with low, intermediate, or high doses of UVAl or UVB, comet formation was observed, cell survival and viability were determined, and UV -induced apoptosis was measured. Results: All responses were dose-dependent. With the intermediate dose of U VAl, a pronounced comet formation was observed without subsequent growth in hibition. Raising levels of porphyrins, which act as photosensitizers, by p reincubation with 5-amino-levulinic acid increased comet formation with UVA , but not with UVB. UVAl-sensitivity and comet formation in XP cells was no t significantly different from the normal cells. With UVB no comet formatio n was seen without subsequent apoptotic cell death. XP cells exhibited the known UVB-hypersensitivity, but their comet formation was not significantly different from that of normal cells. Conclusions: The findings are compatible with the hypothesis that UV-induce d comets represent transient repair-induced DNA breaks. Both, the NER of di mers and the base excision repair of oxidative DNA modifications are though t to contribute to comet formation.