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Science. 1996 Aug 23;273(5278):1109-11.

Role of postreplicative DNA mismatch repair in the cytotoxic action of thioguanine.

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Cancer Research Campaign Nitrosamine-Induced Cancer Group, Department of Biochemistry and Molecular Biology, University College London, London WC1E 6BT, UK.


It is proposed here that the delayed cytotoxicity of thioguanine involves the postreplicative DNA mismatch repair system. After incorporation into DNA, the thioguanine is chemically methylated by S-adenosylmethionine to form S6-methylthioguanine. During DNA replication, the S6-methylthioguanine directs incorporation of either thymine or cytosine into the growing DNA strand, and the resultant S6-methylthioguanine-thymine pairs are recognized by the postreplicative mismatch repair system. Azathioprine, an immunosuppressant used in organ transplantation, is partly converted to thioguanine. Because the carcinogenicity of N-nitrosamines depends on formation of O6-alkylguanine in DNA, the formation of the analog S6-methylthioguanine during azathioprine treatment may partly explain the high incidence of cancer after transplantation.

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