Prevalence of thiopurine S-methyltransferase gene polymorphisms in patients with inflammatory bowel disease from the island of Crete, Greece Constantina Coucoutsi a,c , George Emmanouil a , George Goulielmos b , Ourania Sfakianaki a , Ioannis E. Koutroubakis c and Elias A. Kouroumalis a,c Background There is evidence that genotyping for the thiopurine S-methyltransferase (TPMT) gene variants is useful for the prediction of response to thiopurine analogs (azathioprine and 6-mercaptopurine) in patients with inflammatory bowel disease (IBD). The aim of the present study was to determine the prevalence of TPMT gene polymorphisms in a genetic homogenous population of IBD patients in Crete and to correlate the results with adverse reactions to thiopurine drugs. Patients and methods Genotyping for the most common TPMT variants TPMT*2, TPMT*3A, TPMT3*C, and TPMT*3B was performed using the PCR-restriction fragment length polymorphism method in 223 consecutive IBD patients and 119 age- matched and sex-matched healthy controls. The hospital medical records were reviewed for thiopurine use in these patients and related adverse events. Results The prevalence of TPMT variants TPMT*2, TPMT*3A, TPMT*3B, and TPMT*3C was 1.8, 2.7, 1.3, and 1.8%, respectively. The G238C mutation was detected in four (1.8%) out of 223 patients, three (1.3%) patients were carriers of the G460A mutation, four (1.8%) of the A719G mutation, and six (2.7%) of both G460A and A719G mutations. In healthy controls, only one (0.8%) carried both the G460A and the A719G mutation, whereas TPMT*2, TPMT*3C, and TPMT*3B were not detected. None of the genotypes was homozygous. A statistically significant correlation between the presence of the G460A mutation and the development of leucopenia after the administration of thiopurines was observed (P = 0.048). Conclusion This study showed a lower frequency of total TPMT variants and a higher frequency of TPMT*3B in Cretan IBD patients compared with other Caucasian populations. The presence of the G460A mutation is associated with the development of leukopenia. Eur J Gastroenterol Hepatol 00:000–000 Copyright © 2017 Wolters Kluwer Health, Inc. All rights reserved. Introduction Inflammatory bowel disease (IBD) treatment has to be individualized, reduced, or discontinued in 15–30% of patients because of variations in the therapy response or various adverse drug reactions [1,2]. The most frequent thiopurine cytotoxicity is bone marrow suppression with neutropenia and/or pancytope- nia, observed in up to 10% of patients, and can be life- threatening. Generally, it is a dose-related adverse drug reaction, which mainly occurs at the onset of treatment or during the maintenance treatment [3]. There is evidence that single nucleotide polymorphisms (SNPs) in genes encoding the metabolic pathways of thio- purines are involved in the development of drug reactions. The gene that encodes thiopurine S-methyltransferase (TPMT) is located on chromosome 6 (6p22.3) and includes 10 exons. To date, three wild-type alleles and 38 mutant alleles of this gene have been identified [4], and most of them have been associated with decreased enzyme activity [5]. Four genotypes , namely TPMT*2,*3A,*3B, and *3C, have been investigated intensively. TPMT*3A is actually a haplotype that contains two nonsynonymous SNPs: *3B and *3C [4–8]. The distribution of these mutations is different in dif- ferent ethnic populations. TPMT*3A seems to be the main genotype in Caucasians (3.2–5.7%) and TPMT*3C (0.2–0.5%) is the most common mutant variant in African and Asian populations along with TPMT*2 (0.2–0.5%), whereas TPMT*3B is generally rare. Heterozygotes for a variant TPMT allele (4–11%) have intermediate enzymatic activity, but homozygosity is rare (one in 300 persons) and is related to very low or absent enzymic activity [9]. However, TPMT status does not always predict all thiopurine-related adverse events and TPMT testing before the administration of thiopurines is not as yet an estab- lished practice, with varying recommendations in USA and Europe [10–13]. There are no Greek data on the prevalence of TPMT SNPs in either adult IBD patients or healthy individuals to date. a Laboratory of Gastroenterology, b Section of Molecular Pathology and Human Genetics, Faculty of Medicine, University of Crete and c Department of Gastroenterology, University Hospital of Heralion, Crete, Greece Correspondence to Constantina Coucoutsi, MD, Laboratory of Gastroenterology, Faculty of Internal Medicine, University of Crete, Voutes, 71003 Crete, Greece Tel: + 30 697 799 1042; fax: + 30 289 102 5099; e-mail: ccoucoutsi@gmail.com Received 29 March 2017 Accepted 23 May 2017 European Journal of Gastroenterology & Hepatology 2017, 00:000–000 Keywords: adverse reactions, inflammatory bowel disease, single nucleotide polymorphisms, thiopurine analogs, thiopurine methyltransferase gene ’ Original article 0954-691X Copyright © 2017 Wolters Kluwer Health, Inc. All rights reserved. DOI: 10.1097/MEG.0000000000000947 1 Copyright r 2017 Wolters Kluwer Health, Inc. 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