title: Reactive oxygen species generated by thiopurine/UVA cause irreparable transcription-blocking DNA lesions.
abstract: Long-term treatment with the anticancer and immunosuppressant thiopurines, azathioprine or 6-mercaptopurine, is associated with acute skin sensitivity to ultraviolet A (UVA) radiation and a high risk of skin cancer. 6-thioguanine (6-TG) that accumulates in the DNA of thiopurine-treated patients interacts with UVA to generate reactive oxygen species. These cause lethal and mutagenic DNA damage. Here we show that the UVA/DNA 6-TG interaction rapidly, and essentially irreversibly, inhibits transcription in cultured human cells and provokes polyubiquitylation of the major subunit of RNA polymerase II (RNAPII). In vitro, 6-TG photoproducts, including the previously characterized guanine-6-sulfonate, in the transcribed DNA strand, are potent blocks to RNAPII transcription whereas 6-TG is only slightly inhibitory. In vivo, guanine-6-sulfonate is removed poorly from DNA and persists to a similar extent in the DNA of nucleotide excision repair-proficient and defective cells. Furthermore, transcription coupled repair-deficient Cockayne syndrome cells are not hypersensitive to UVA/6-TG, indicating that potentially lethal photoproducts are not selectively excised from transcribed DNA. Since persistent transcription-blocking DNA lesions are associated with acute skin responses to sunlight and the development of skin cancer, our findings have implications for skin cancer in patients undergoing thiopurine therapy.
authors.affiliation: Cancer Research UK London Research Institute, Clare Hall Laboratories, South Mimms, Herts, UK.
authors.forename: Reto
authors.initials: R
authors.lastname: Brem
authors.forename: Feng
authors.initials: F
authors.lastname: Li
authors.forename: Peter
authors.initials: P
authors.lastname: Karran
mesh_terms: D000964:Antimetabolites, Antineoplastic / Q000378:metabolism* / Q000494:pharmacology / Q000528:radiation effects; D002460:Cell Line; D004247:DNA / Q000737:chemistry / Q000378:metabolism; D004249:DNA Damage; D004260:DNA Repair; D006801:Humans; D012319:RNA Polymerase II / Q000378:metabolism; D017382:Reactive Oxygen Species / Q000378:metabolism*; D013866:Thioguanine / Q000378:metabolism* / Q000494:pharmacology / Q000528:radiation effects; D014158:Transcription, Genetic / Q000187:drug effects / Q000528:radiation effects; D014466:Ultraviolet Rays
chemical_list: D000964:Antimetabolites, Antineoplastic; D017382:Reactive Oxygen Species; D004247:DNA; D012319:RNA Polymerase II; D013866:Thioguanine