Effects of 6-thioguanine and S6-methylthioguanine on transcription in vitro and in human cells.
Thiopurine drugs are extensively used as chemotherapeutic agents in clinical practice, even though there is concern about the risk of therapy-related cancers. It has been previously suggested that the cytotoxicity of thiopurine drugs involves their metabolic activation, the resultant generation of 6-thioguanine ((S)G) and S(6)-methylthioguanine (S(6)mG) in DNA, and the futile mismatch repair triggered by replication-induced (S)G:T and S(6)mG:T mispairs. Disruption of transcription is known to be one of the major consequences of DNA damage induced by many antiviral and antitumor agents; however, it remains undefined how (S)G and S(6)mG compromise the efficiency and fidelity of transcription. Using our recently developed competitive transcription and adduct bypass assay, herein we examined the impact of (S)G and S(6)mG on transcription in vitro and in human cells. Our results revealed that, when situated on the transcribed strand, S(6)mG exhibited both inhibitory and mutagenic effects during transcription mediated by single-subunit T7 RNA polymerase or multisubunit human RNA polymerase II in vitro and in human cells. Moreover, we found that the impact of S(6)mG on transcriptional efficiency and fidelity is modulated by the transcription-coupled nucleotide excision repair capacity. In contrast, (S)G did not considerably compromise the efficiency or fidelity of transcription, and it was a poor substrate for NER. We propose that S(6)mG might contribute, at least in part, to thiopurine-mediated cytotoxicity through inhibition of transcription and to potential therapy-related carcinogenesis via transcriptional mutagenesis.
Department of Chemistry, University of California, Riverside, California 92521-0403, USA.
Changjun
C
You
Xiaoxia
X
Dai
Bifeng
B
Yuan
Yinsheng
Y
Wang
D000964:Antimetabolites, Antineoplastic / Q000494:pharmacology; D002853:Chromatography, Liquid / Q000379:methods; D004247:DNA / Q000187:drug effects; D004249:DNA Damage; D004260:DNA Repair; D012321:DNA-Directed RNA Polymerases / Q000378:metabolism; D004591:Electrophoresis, Polyacrylamide Gel; D005347:Fibroblasts / Q000378:metabolism; D006147:Guanine / Q000737:chemistry; D057809:HEK293 Cells; D006801:Humans; D066298:In Vitro Techniques; D008956:Models, Chemical; D016296:Mutagenesis; D009154:Mutation; D010766:Phosphorylation; D012313:RNA / Q000378:metabolism; D053719:Tandem Mass Spectrometry / Q000379:methods; D013866:Thioguanine / Q000031:analogs & derivatives* / Q000494:pharmacology*; D014158:Transcription, Genetic; D014764:Viral Proteins / Q000378:metabolism
D000964:Antimetabolites, Antineoplastic; D014764:Viral Proteins; C035366:6-methylthioguanine; D006147:Guanine; D012313:RNA; D004247:DNA; C062210:bacteriophage T7 RNA polymerase; D012321:DNA-Directed RNA Polymerases; D013866:Thioguanine