Reactive oxygen species generated by thiopurine/UVA cause irreparable transcription-blocking DNA lesions.
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.
Brem
Reto
R
Cancer Research UK London Research Institute, Clare Hall Laboratories, South Mimms, Herts, UK.
Li
Feng
F
Karran
Peter
P
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
D000964:Antimetabolites, Antineoplastic; D017382:Reactive Oxygen Species; D004247:DNA; D012319:RNA Polymerase II; D013866:Thioguanine