title: The effect of thiopurine drugs on DNA methylation in relation to TPMT expression.
abstract: The thiopurine drugs 6-mercaptopurine (6-MP) and 6-thioguanine (6-TG) are well-established agents for the treatment of leukaemia but their main modes of action are controversial. Thiopurine methyltransferase (TPMT) metabolises thiopurine drugs and influences their cytotoxic activity. TPMT, like DNA methyltransferases (DNMTs), transfers methyl groups from S-adenosylmethionine (SAM) and generates S-adenosylhomocysteine (SAH). Since SAM levels are dependent on de novo purine synthesis (DNPS) and the metabolic products of 6-TG and 6-MP differ in their ability to inhibit DNPS, we postulated that 6-TG compared to 6-MP would have differential effects on changes in SAM and SAH levels and global DNA methylation, depending on TPMT status. To test this hypothesis, we used a human embryonic kidney cell line with inducible TPMT. Although changes in SAM and SAH levels occurred with each drug, decrease in global DNA methylation more closely reflected a decrease in DNMT activity. Inhibition was influenced by TPMT for 6-TG, but not 6-MP. The decrease in global methylation and DNMT activity with 6-MP, or with 6-TG when TPMT expression was low, were comparable to 5-aza-2'-deoxycytidine. However, this was not reflected in changes in methylation at the level of an individual marker gene (MAGE1A). The results suggest that a non-TPMT metabolised metabolite of 6-MP and 6-TG and the TPMT-metabolised 6-MP metabolite 6-methylthioguanosine 5'-monophosphate, contribute to a decrease in DNMT levels and global DNA methylation. As demethylating agents have shown promise in leukaemia treatment, inhibition of DNA methylation by the thiopurine drugs may contribute to their cytotoxic affects.
authors: Leukaemia Research Group, Northern Institute for Cancer Research, Newcastle University, Newcastle upon Tyne NE2 4HH, UK. l.a.hogarth@ncl.ac.uk
authors: L A
authors: LA
authors: Hogarth
authors: C P F
authors: CP
authors: Redfern
authors: J M
authors: JM
authors: Teodoridis
authors: A G
authors: AG
authors: Hall
authors: H
authors: H
authors: Anderson
authors: M C
authors: MC
authors: Case
authors: S A
authors: SA
authors: Coulthard
mesh_terms: D000964:Antimetabolites, Antineoplastic / Q000494:pharmacology; D002453:Cell Cycle / Q000187:drug effects; D004247:DNA / Q000235:genetics / Q000302:isolation & purification; D019175:DNA Methylation / Q000187:drug effects*; D017931:DNA Primers; D006801:Humans; D007668:Kidney / Q000166:cytology / Q000187:drug effects / Q000201:enzymology; D007700:Kinetics; D015122:Mercaptopurine / Q000494:pharmacology*; D008780:Methyltransferases / Q000187:drug effects / Q000235:genetics / Q000378:metabolism*; D012435:S-Adenosylhomocysteine / Q000378:metabolism; D013866:Thioguanine / Q000494:pharmacology*
chemical_list: D000964:Antimetabolites, Antineoplastic; D017931:DNA Primers; D004247:DNA; D012435:S-Adenosylhomocysteine; D015122:Mercaptopurine; D008780:Methyltransferases; C022745:thiopurine methyltransferase; D013866:Thioguanine