Thiopurine methyltransferase or thiopurine S-methyltransferase (TPMT) is an enzyme that in humans is encoded by the TPMTgene. A pseudogene for this locus is located on chromosome 18q.[5][6]
Thiopurine drugs such as 6-mercaptopurine are used as chemotherapeutic agents and immunosuppressive drugs. Genetic polymorphisms that affect this enzyme's activity are correlated with variations in sensitivity and toxicity to such drugs. About 1/300 individual is deficient for the enzyme.[5]
TPMT is best known for its role in the metabolism of the thiopurine drugs such as azathioprine, 6-mercaptopurine and 6-thioguanine. TPMT catalyzes the S-methylation of thiopurine drugs. Defects in the TPMT gene leads to decreased methylation and decreased inactivation of 6MP leading to enhanced bone marrow toxicity which may cause myelosuppression, anemia, bleeding tendency, leukopenia & infection.[8][9][10] Allopurinol inhibits thiopurine S-methyltransferase, which can increase the utility of 6-MP.[11]
Measurement of TPMT activity is encouraged prior to commencing the treatment of patients with thiopurine drugs such as azathioprine, 6-mercaptopurine and 6-thioguanine. Patients with low activity (10% prevalence) or especially absent activity (prevalence 0.3%) are at a heightened risk of drug-induced bone marrow toxicity due to accumulation of the unmetabolised drug. Reuther et al. found that about 5% of all thiopurine therapies will fail due to toxicity. This intolerant group could be anticipated by routine measurement of TPMT activity. There appears to be a great deal of variation in TPMT mutation, with ethnic differences in mutation types accounting for variable responses to 6MP.[9][12]
Genetic variants of TPMT have also been associated with cisplatin-induced ototoxicity in children.[13] TPMT is now listed as a pharmacogenomic biomarker for adverse drug reactions to cisplatin by the FDA.[14]
^Genome Bioinformatics Group, Center for Biomolecular Science and Engineering. "Human Gene TPMT (uc003ncm.1)". UCSC Genome Browser. University of California Santa Cruz. Retrieved 2008-07-25.
^Ross CJ, Katzov-Eckert H, Dubé MP, Brooks B, Rassekh SR, Barhdadi A, Feroz-Zada Y, Visscher H, Brown AM, Rieder MJ, Rogers PC, Phillips MS, Carleton BC, Hayden MR (December 2009). "Genetic variants in TPMT and COMT are associated with hearing loss in children receiving cisplatin chemotherapy". Nat. Genet. 41 (12): 1345–9. doi:10.1038/ng.478. PMID19898482. S2CID21293339.
^"Cisplatin". Science & Research (Drugs). United States Food and Drug Administration.
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Coulthard SA, Matheson EC, Hall AG, Hogarth LA (2005). "The clinical impact of thiopurine methyltransferase polymorphisms on thiopurine treatment". Nucleosides Nucleotides Nucleic Acids. 23 (8–9): 1385–91. doi:10.1081/NCN-200027637. PMID15571264. S2CID627569.
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Honchel R, Aksoy IA, Szumlanski C, et al. (1993). "Human thiopurine methyltransferase: molecular cloning and expression of T84 colon carcinoma cell cDNA". Mol. Pharmacol. 43 (6): 878–87. PMID8316220.
Glauser TA, Nelson AN, Zembower DE, et al. (1993). "Diethyldithiocarbamate S-methylation: evidence for catalysis by human liver thiol methyltransferase and thiopurine methyltransferase". J. Pharmacol. Exp. Ther. 266 (1): 23–32. PMID8392551.
Szumlanski C, Otterness D, Her C, et al. (1996). "Thiopurine methyltransferase pharmacogenetics: human gene cloning and characterization of a common polymorphism". DNA Cell Biol. 15 (1): 17–30. doi:10.1089/dna.1996.15.17. PMID8561894.
Leipold G, Schütz E, Haas JP, Oellerich M (1997). "Azathioprine-induced severe pancytopenia due to a homozygous two-point mutation of the thiopurine methyltransferase gene in a patient with juvenile HLA-B27-associated spondylarthritis". Arthritis Rheum. 40 (10): 1896–8. doi:10.1002/art.1780401026. PMID9336428.
Krynetski EY, Fessing MY, Yates CR, et al. (1998). "Promoter and intronic sequences of the human thiopurine S-methyltransferase (TPMT) gene isolated from a human PAC1 genomic library". Pharm. Res. 14 (12): 1672–8. doi:10.1023/A:1012111325397. PMID9453052. S2CID22214838.
