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.