title: Processing of directly and indirectly ultraviolet-induced DNA damage in human cells.
abstract: Mutations caused by ultraviolet (UV)-induced DNA damage represent the initial genetic changes in the tumorigenesis of UV-induced skin cancer. Different wavelengths of UV radiation cause different kinds of DNA damage and mutations. UVB (290-320 nm) generates pyrimidine dimers by direct excitation of the DNA molecule. UVA (320-400 nm) can damage the DNA only indirectly through a photosensitized reaction. This indirect action is mediated mainly by singlet oxygen, which generates purine base modifications, and has been implicated in the carcinogenic effects of UVA. In order to study the processing of directly and indirectly UV-induced DNA damage in human cells, we first treated the replicating plasmid pRSVcat with up to 10 kJ/m2 UVB or with the photosensitizer methylene blue plus visible light (which generates singlet oxygen) in vitro. Then, the damaged plasmid was transfected into normal or repair deficient xeroderma pigmentosum complementation group A (XP-A) cells. DNA repair was assessed by measuring activity of reactivated chloramphenicol acetyltransferase (CAT) enzyme, encoded by the plasmid's cat gene, in cell extracts after 3 days. While XP-A cells exhibited a significantly reduced repair of UVB-induced DNA damage, they showed a normal repair of singlet oxygen-induced DNA damage. This indicates a differential DNA repair pathway for directly and indirectly UV-induced DNA damage in human cells. Irradiation of the plasmid with UVA alone did not result in a genotoxic effect. Only in conjunction with a cell extract, which provides all candidate cellular photosensitizers, did we find a reduced CAT activity after transfection. This indicates that the genotoxicity of UVA is mediated by a cellular photosensitizer.
authors.affiliation: Department of Dermatology, University of Würzburg, Germany.
authors.forename: T M
authors.initials: TM
authors.lastname: Rünger
authors.forename: B
authors.initials: B
authors.lastname: Epe
authors.forename: K
authors.initials: K
authors.lastname: Möller
mesh_terms: D002461:Cell Line, Transformed; D002471:Cell Transformation, Neoplastic / Q000528:radiation effects; D015500:Chloramphenicol O-Acetyltransferase / Q000096:biosynthesis / Q000235:genetics; D004247:DNA / Q000528:radiation effects*; D004249:DNA Damage; D004260:DNA Repair; D004269:DNA, Bacterial / Q000528:radiation effects; D004274:DNA, Recombinant / Q000528:radiation effects; D005822:Genetic Vectors / Q000528:radiation effects; D004854:Herpesvirus 4, Human; D006801:Humans; D008214:Lymphocytes / Q000378:metabolism; D008751:Methylene Blue / Q000494:pharmacology; D009381:Neoplasms, Radiation-Induced / Q000235:genetics; D018384:Oxidative Stress / Q000235:genetics; D010100:Oxygen / Q000009:adverse effects; D010957:Plasmids / Q000528:radiation effects; D011740:Pyrimidine Dimers / Q000378:metabolism*; D011838:Radiation-Sensitizing Agents / Q000494:pharmacology; D011993:Recombinant Fusion Proteins / Q000096:biosynthesis / Q000235:genetics; D026082:Singlet Oxygen; D012878:Skin Neoplasms / Q000209:etiology / Q000235:genetics; D014020:Tissue Extracts / Q000494:pharmacology; D014466:Ultraviolet Rays / Q000009:adverse effects*; D014983:Xeroderma Pigmentosum / Q000145:classification / Q000235:genetics / Q000473:pathology
chemical_list: D004269:DNA, Bacterial; D004274:DNA, Recombinant; D011740:Pyrimidine Dimers; D011838:Radiation-Sensitizing Agents; D011993:Recombinant Fusion Proteins; D014020:Tissue Extracts; D026082:Singlet Oxygen; D004247:DNA; D015500:Chloramphenicol O-Acetyltransferase; D010100:Oxygen; D008751:Methylene Blue