硫嘌呤甲基转移酶基因型和酶活性检测在炎症性肠病治疗中的临床价值

目的 评价硫嘌呤甲基转移酶(TPMT)基因型和酶活性检测对炎症性肠病(IBD)患者服用硫唑嘌呤(AZA)发生不良反应的预测价值.方法 收集2004年4月-2009年12月有使用AZA指征的IBD确诊患者112例,其中溃疡性结肠炎(UC)26例,克罗恩病(CD)86例.患者每天均服用AZA 2 mg/kg.采用PCR技术检测患者TPMT基因型(*2、*3A、*3B*、3C),高效液相色谱法检测TPMT酶活性.分析达到随访终点(服药达半年或以上,或因不良反应停药)患者的TPMT基因多态性及酶活性与不良反应发生之间的关系及其影响因素.结果 112例患者中不良反应率为33.9%(38/112),以骨髓抑制最常见(20.5%).TPMT*3C杂合子突变率为0.9%(1/112).TPMT酶活性呈单峰正态分布,平均活性为(12.9士4.2)U/ml红细胞.1例TPMT*3C杂合子突变者于用药4周内发生骨髓抑制,TPMT基因型预测骨髓抑制发生的敏感度为4.4 0A(1/23),特异度为1/1.ROC曲线计算TPMT酶活性预测服药3个月内发生骨髓抑制的安全阈为≤4.5 U/ml红细胞,该安全阈预测3个月内骨髓抑制发生的敏感度为3/13,特异度为3/3.合用5-氨基水杨酸(5-ASA)制剂者骨髓抑制的发生率显著高于非合用者(44.4%比12.9%,P=0.000),但发生骨髓抑制者合用5-ASA前后TPMT酶活性差异无统计学意义(P>0.05).结论 TPMT基因突变和酶活性低下对预测发生骨髓抑制的特异性较高,但敏感性较差,合用5-ASA可增加AZA发生骨髓抑制的风险,但与TPMT酶活性无关.     

Thiopurine S-methyltransferase polymorphisms and the relationship between the mutant alleles and the adverse effects in systemic lupus erythematosus patients taking azathioprine

OBJECTIVE: The present study sought to elucidate the genetic basis of thiopurine methyltransferase (TPMT) polymorphism and subsequently to investigate the relationship between mutant TPMT and an adverse response observed in Korean patients with systemic lupus erythematosus (SLE) taking azathioprine (AZA). METHODS: The TPMT genotype of 342 patients with SLE was determined by MALDI-TOF mass spectrometry and correlated with the effects of clinical exposure to AZA. RESULTS: TPMT polymorphism was detected in 17 of the 342 study subjects (5.0%), 12 heterozygous for the TPMT*3C allele and 5 heterozygous for the TPMT*6 allele. Numerous patients taking AZA demonstrated adverse drug responses. Severe nausea occurred in 4 patients with the TPMT*3C allele, while 1 patient with the TPMT*6 allele suffered severe bone marrow toxicity. Leucopenia (n = 17), nausea (n = 4), and abnormal liver function (n = 1) were suspected in 23 of the 94 lupus patients taking AZA. AZA was relatively well tolerated by the remainder of the patients. The heterozygous genotype for the TPMT*3C and *6 alleles was frequently detected in Korean SLE patients. CONCLUSION: Contrary to previous hypotheses, this study identified no statistical correlation between TPMT genotype and AZA toxicity. We thus conclude that TMPT genotyping cannot replace regular blood monitoring in SLE patients receiving AZA treatment.     

Thiopurine methyltransferase genotype and the toxicity of azathioprine in Japanese

OBJECTIVE: Thiopurine S-methyltransferase (TPMT) is a cytosolic enzyme that preferentially catalyzes the S-methylation of aromatic and heterocyclic sulfhydryl compounds, including Azathioprine (AZA). It has been reported that the level of AZA toxicity is dependent on the TPMT genotypes in Caucasian individuals; we thus investigated this relationship in Japanese. METHODS: The TPMT genotype was determined using peripheral blood cell DNA obtained from 36 Japanese patients with rheumatic diseases who were treated with AZA, by polymerase chain reaction (PCR) technique. Duration of AZA administration, white blood cell counts before and after AZA administration, and side effects were investigated in each subject, and were compared between the patients with or without TPMT mutation. RESULTS: The TPMT allelotype was TPMT*1/TPMT*1 in 33 (91.7%) and TPMT*1/TPMT*3C in 3 (8.3%) individuals. All 3 patients (100%) with the mutant TPMT allele (TPMT*3C) discontinued AZA treatment due to leucopenia while only 4 patients (12%) without mutant TPMT alleles showed leucopenia (p=0.0049, Fisher's exact test). However, leucopenia developed relatively late in patients with mutant TPMT. CONCLUSION: The TPMT mutant allele, TPMT*3C, also exists in Japanese individuals, and the bone marrow toxicity of AZA is likely stronger in patients with this mutant allele.     

Thiopurine methyltransferase enzyme activity determination before treatment of inflammatory bowel disease with azathioprine: effect on cost and adverse events.

BACKGROUND: Azathioprine (AZA), used to treat inflammatory bowel disease (IBD), is metabolized by thiopurine methyltransferase (TPMT). The accumulation of individual metabolites varies because humans display genetic polymorphism for TPMT expression. Deficiencies in TPMT result in accumulation of toxic metabolites, followed by neutropenia and hepatic inflammation. Concern over acute toxicity frequently leads to under dosing and frequent monitoring tests and visits. OBJECTIVE: To determine whether assessment of TPMT activity before the administration of AZA would predict acute toxicity and, thus, allow for reductions in health care costs related to biochemical screening for, and management of, AZA-induced adverse events. METHODS: Before AZA treatment, 29 patients with IBD were prospectively randomized to one of two groups: group 1, in which no TPMT assay was performed, was started on AZA at 1 mg/kg/day and then titrated every two weeks to a target dose of 2.5 mg/kg/day; and group 2, in which TPMT assays were performed, was started on AZA at the target dose of 2.5 mg/kg/day. For both groups, complete blood count and liver enzymes were monitored weekly for six weeks and at monthly intervals thereafter. Additional tests and health care interventions were undertaken at the discretion of the attending physicians. RESULTS: Of the 29 patients in the study, 15 were randomly assigned to group 1 and 14 to group 2. Demographics and disease activity were similar for both groups. Mean follow-up time was 7.1 months (range 3.5 to 10.7 months). Eight patients from group 1 and six patients from group 2 withdrew as a result of AZA-induced adverse events. There was no correlation between the TPMT activity and the development of AZA-induced adverse events. The direct health care costs for group 1 (300.11 dollars per patient) were lower than in group 2 (348.87 dollars per patient). CONCLUSION: The prospective assessment of TPMT enzyme activity before initiating AZA therapy in IBD patients incurred additional cost and did not predict AZA-induced toxicity.     

Mistaken identity: misclassification of TPMT phenotype following blood transfusion

Azathioprine (AZA) is an effective treatment for inflammatory bowel disease. The measurement of thiopurine methyltransferase (TPMT) activity levels helps to identify the one in 300 patients who are at risk of profound myelosuppression with standard doses of AZA. Thus, it is important that the measurement of TPMT activity is accurate. We report a case of misclassification of TPMT phenotype caused by prior blood transfusion. The patient appeared to have an intermediate level of TPMT enzyme activity as measured after the blood transfusion. However, following severe myelosuppression soon after starting AZA, her genotype was determined, which showed that she was homozygous for low TPMT activity. This report reviews some of the limitations in determining both the phenotype and the genotype of TPMT, especially following recent blood transfusion. The dilemma of whether or not to wait for the TPMT activity result before starting AZA is also discussed.     

Adverse reactions to azathioprine cannot be predicted by thiopurine S-methyltransferase genotype in Japanese patients with inflammatory bowel disease

Background and Aims:  Azathioprine (AZA) is associated with a high frequency of adverse reactions. We examined polymorphism of the thiopurine S-methyltransferase (TPMT) gene to determine whether the TPMT genotype would be a predictive marker for the development of adverse reactions to AZA.
Methods:  The frequency of TPMT mutations was investigated in 147 Japanese inflammatory bowel disease (IBD) patients retrospectively. In these subjects, the presence of four mutant alleles (TPMT*2, *3B, *3C and *8) was determined by direct sequencing. The incidence of adverse reactions among patients carrying wild-type TPMT was investigated. The blood level of 6-thioguanine nucleotide (6-TGN) was measured and analyzed in 47 patients with wild-type TPMT. The results were analyzed in relation to the concomitant use of aminosalicylates (ASA).
Results:  Of the 147 patients, 144 (98.0%) were wild-type for TPMT (TPMT*1/*1) and three (2.0%) carried a mutant TPMT allele (TPMT*1/*3C). The incidence of adverse reactions was 33.3% (38/114) in the wild-type group. Leukopenia (WBC ≤ 3000/µL) was seen in 15.8% of the patients with wild-type TPMT. 6-TGN levels varied among 47 patients with wild-type TPMT. The blood levels of 6-TGN were significantly higher in the patients receiving concomitant ASA treatment compared with those not receiving concomitant ASA treatment ( P  = 0.0033).
Conclusion:  The frequency of TPMT gene mutations is low among Japanese IBD patients. The incidence of adverse reactions to AZA was high, even in patients carrying wild-type TPMT. It is concluded that determination of TPMT genotype may not be useful in Japanese IBD patients to predict adverse reactions to AZA.     

Comparison of TPMT and NUDT15 polymorphisms in Chinese patients with inflammatory bowel disease

AIM To observe gene polymorphisms of TPMT and NUDT15, and compare their predictive value for azathioprine(AZA)-induced leukopenia in inflammatory bowel disease(IBD).METHODS This study enrolled 219 patients diagnosed with IBD in Xiangya Hospital, Central South University, Changsha, China from February 2016 to November 2017. Peripheral blood of all patients was collected to detect their genotypes of TPMT and NUDT15 by pyrosequencing at the Department of Clinical Pharmacology, Hunan Key Laboratory of Pharmacogenetics, Xiangya Hospital. Eighty patients were treated with AZA according to the disease condition. During the first month, patients who received AZA underwent routine blood tests and liver function tests once a week. The endpoint of the study was leukopenia induced by AZA. By analyzing patient characteristics, genotypes and leukopenia induced by drug use, we found the risk factors associated with AZA-induced leukopenia.RESULTS There were 219 patients with IBD(160 men and 59 women), including 39 who were confirmed with ulcerative colitis(UC), 176 with Crohn's disease(CD) and 4 with undetermined IBD(UIBD). There were 44 patients(20.1%) with mutant genotype of NUDT15(C/T); among them, 16 received AZA, and 8(50%) developed leukopenia. There were 175 patients(79.7%) with wild genotype of NUDT15(C/C); among them, 64 received AZA, and 11(17.2%) developed leukopenia. A significant difference was found between NUDT15 C/T and its wild-type C/C(P = 0.004). There were only 3 patients with TPMT mutant genotype of A/G(1.4%) who participated in the research, and 1 of them was treated with AZA and developed leukopenia. The remaining 216 patients(98.6%) were found to bear the wild genotype of TPMT(A/A); among them, 79 patients received AZA, and 18(22.8%) developed leukopenia, and there was no significant difference from those with A/G(P = 0.071). The frequency of TPMT mutation was 1.4%, and NUDT15 mutation rate was significantly higher and reached 20.1%(P = 0.000). Therefore, NUDT15 gene polymorphism was obviously a better biomarker than TPMT gene polymorphism in the prediction of AZA-induced leukopenia.CONCLUSION Mutation rate of NUDT15 in Chinese IBD patients is higher than that of TPMT. NUDT15 polymorphism is a better predictor for AZA-induced leukopenia than TPMT polymorphism.     

Relevance of thiopurine methyltransferase status in rheumatology patients receiving azathioprine

Azathioprine (AZA) is widely used in the management of rheumatological diseases. Despite its efficacy, AZA can often cause bone marrow suppression, notably leucopenia, which has been recorded in up to 17% of patients taking AZA for rheumatoid arthritis, though this can be considered clinically significant in about 3% overall. Severe myelosuppression, associated with abnormal AZA metabolism, is linked to the thiopurine methyltransferase (TPMT) genetic polymorphism. TPMT status can be assessed prior to AZA treatment by measuring enzyme activity or genotyping techniques. Analysis of recent data suggests that by optimizing the AZA dose on the basis of TPMT status testing (with a substantial reduction in dose for patients homozygous for mutant TPMT alleles), a reduction in drug-induced morbidity and cost savings can be made by avoiding hospitalization and rescue therapy for leucopenic events. In this article we review the pharmacogenetic and clinical implications of the TPMT polymorphism, emphasizing its relevance to rheumatologists managing diseases with AZA.     

Genotypic analysis of thiopurine S-methyltransferase in patients with Crohn's disease and severe myelosuppression during azathioprine therapy

BACKGROUND & AIMS: Myelosuppression in patients with Crohn's disease (CD) treated with azathioprine has been attributed to low activity of thiopurine S-methyltransferase (TPMT). Allelic variants of the TPMT gene responsible for changes in the enzyme activity have been characterized. We investigated the distribution of mutant alleles associated with TPMT deficiency in patients with CD and myelosuppression during azathioprine/6-mercaptopurine therapy. METHODS: Forty-one patients with CD were included. They developed leukopenia or thrombocytopenia during azathioprine or 6-mercaptopurine treatment. Polymerase chain reaction-based methods were used to search for mutations associated with TPMT deficiency. RESULTS: Four patients (10%) had 2 mutant alleles associated with TPMT deficiency, 7 (17%) had 1 mutant allele, and 30 (73%) had no known TPMT mutation. The delay between administration of the drug and occurrence of bone marrow toxicity was less than 1.5 months in the 4 patients with 2 mutant alleles, and ranged from 1 to 18 months in patients with 1 mutant allele and from 0.5 to 87 months in patients with normal genotype. CONCLUSIONS: Twenty-seven percent of patients with CD and myelosuppression during azathioprine therapy had mutant alleles of the TPMT gene associated with enzyme deficiency. Myelosuppression is more often caused by other factors. Continued monitoring of blood cell counts remains mandatory in patients treated with azathioprine.     

Thiopurine methyltransferase genotyping in Palestinian childhood acute lymphoblastic leukemia patients

Background

The genetic polymorphism of thiopurine methyltransferase (TPMT) is well characterized in most populations. Four common polymorphic alleles are associated with impaired activity of the enzyme. These are TPMT*2 (238G>C), TPMT*3B (c.460G>A), TPMT*3A (c.460G>A and c.719A>G) and TPMT*3C (c.719A>G). The aim of the present study was to determine the frequency of TPMT polymorphisms and their association with the occurrence of adverse events, during 6-mercaptopurine therapy in pediatric acute lymphoblastic leukemic (ALL) patients in Gaza Strip.

Methods

A total of 56 DNA samples from all pediatric ALL patients admitted to the pediatric hematology departments of Gaza strip hospitals were analyzed. Genomic DNA from peripheral blood leukocytes was isolated and the TPMT*2, TPMT*3B TPMT*3A and TPMT*3C allelic polymorphism was determined by PCR-RFLP and allele specific PCR technique.

Results

No TPMT*2, *3B or *3C alleles were detected. Only one, out of 56 patients, was found heterozygous for the TPMT*3A allele. Thus, the frequency of TPMT*3A allele was calculated to be 0.89%. Fourteen patients of ALL were suffering from myelotoxicity during 6-MP therapy. From our results, no significant association could be established between clinical and laboratory data and/or the presence of the mutation in TPMT gene.

Conclusion

TPMT*3A was the only deficiency allele detected in our population with an allelic frequency of 0.89%. Other polymorphic alleles in TPMT gene, or factors other than TPMT polymorphisms may be responsible for the development of myelosuppression in cases that don’t carry the investigated TPMT alleles (*2, *3A, *3B and *3C). Therefore, more studies are recommended to study such factors.     

Is thiopurine methyltransferase genetic polymorphism a major factor for withdrawal of azathioprine in rheumatoid arthritis patients?

OBJECTIVE: To determine whether the presence of thiopurine methyltransferase (TPMT) alleles associated with reduced or absent activity of thiopurine methyltransferase is a major factor for withdrawal of azathioprine (AZA) in rheumatoid arthritis (RA) patients. METHODS: The TPMT genotype, including the variable number of tandem repeats (VNTR) pattern in the 5' untranslated region, was analysed in 111 patients with long-standing RA. Azathioprine (AZA) therapy was used in 40 patients (36%) as a disease-modifying anti-rheumatic drug. RESULTS: Seven out of 111 RA patients (6.3%) were carriers of a mutant allele, TPMT3A (G(460)-->A, A(719)-->G) being the mutant allele observed most frequently. In the group of 40 AZA-treated patients, therapy was discontinued in six patients because of side-effects and in 26 patients because of lack of efficacy. Three patients presented moderate side-effects and were homozygous for the wild-type TPMT allele, whereas the remaining three patients, who developed gastrointestinal effects with severe nausea and vomiting, were TPMT3A carriers. CONCLUSION: In this observational study, the absence of response, probably due to the low-dose scheme used, was the major cause of AZA withdrawal in our series of RA patients. TPMT genotyping may allow the use of high doses of AZA in patients with normal TPMT alleles to improve the efficacy of this immunosuppressive drug. Our data support the relationship between gastrointestinal intolerance and thiopurine metabolic imbalance.     

The low frequency of defective TPMT alleles in Turkish population: a study on pediatric patients with acute lymphoblastic leukemia

6-Mercaptopurine (6MP) is an essential anticancer drug used in the treatment of childhood acute lymphoblastic leukemia (ALL). Thiopurine methyltransferase (TPMT) polymorphisms are the major determinants of interindividual differences in the severe toxicity or efficacy of 6MP. Four variant alleles, TPMT*2, TPMT*3A, TPMT*3B, and TPMT*3C, are responsible over the 80% of low or undetectable enzyme activity. The frequencies of these variants were investigated among 106 children with ALL in Turkish population. TPMT*3A and TPMT*3C were the only deficiency alleles detected in Turkish population with an allele frequency of 0.9% for both. While *3C allele frequency in Turkish population was found to be very similar to Asian and other Caucasian populations, *3A allele frequency was significantly (P < 0.05) lower. So far, studies showed that the genetic polymorphisms of other drug metabolizing enzymes like CYP2E1, CYP1A1, GSTM1/ T1 in Turkish population were similar to Caucasian populations. However, we found that the distribution of TPMT polymorphisms in Turkish population was significantly lower than those in other Caucasians like British, French, and Italian whereas the distributions of TPMT variants were found to be very similar to Kazak population which is also Caucasian in ethnic origin. In this study, the clinical histories of the patients in the sample population were also examined, retrospectively. The patients with heterozygous or homozygous mutant genotypes had developed severe neutropenia and infection during 6MP therapy. The study provides the first data on the frequency of common TPMT variants in the Turkish population, based on analysis of pediatric patients with ALL.     

Safe 6-thioguanine therapy of a TPMT deficient Crohn's disease patient by using therapeutic drug monitoring

The immunosuppressive thiopurines, azathioprine (AZA) and 6-mercaptopurine (6-MP), have proven efficacy in steroid-dependant or refractory inflammatory bowel disease (IBD). In case of TPMT deficiency serious myelosuppression may occur. 6-thioguanine (6-TG), has been suggested in case of AZA and 6-MP resistant or intolerant patients.Our case demonstrates that very low dose 6-TG under close clinical surveillance and frequent therapeutic drug monitoring, may be a rescue drug for IBD-patients with low or without functional TPMT activity.     

Thiopurine S-methyltransferase gene polymorphism in Japanese patients with autoimmune liver diseases.

BACKGROUND AND AIM: Thiopurine S-methyltransferase (TPMT) genotypes or phenotypes may be a predictive factor for azathioprine-induced toxicities. We investigated the genotypic status of TPMT to evaluate the risk of azathioprine-related adverse effects in Japanese patients with different liver diseases, including autoimmune hepatitis (AIH). METHODS: 49 patients with AIH, 67 with primary biliary cirrhosis (PBC), and 120 with hepatitis C virus (HCV) were examined. TPMT genotypes were determined by PCR-restriction fragment length polymorphism-based assays. RESULTS: The distribution of TPMT genotypes was 90% TPMT*1/TPMT*1, 8% TPMT*1/TPMT*3C, and 2% TPMT*3C/TPMT*3C in AIH, and 94% TPMT*1/TPMT*1, 4.5% TPMT*1/TPMT*3C, and 1.5% TPMT*3C/TPMT*3C in PBC. All except 1 patient with HCV had the TPMT*1/TPMT*1 genotype. Severe myelosuppression occurred in two of nine patients with AIH who received azathioprine, one of whom was homozygous for TPMT*3C. CONCLUSIONS: TPMT*3C variants are more frequent in patients with AIH or PBC than in patients with viral hepatitis or healthy volunteers in Japan. Pharmacogenetic screening for TPMT polymorphisms before commencing azathioprine therapy may help to prevent severe hematotoxicity in patients with TPMT deficiency.     

Analysis of thiopurine methyltransferase variant alleles in childhood acute lymphoblastic leukaemia

The role of 6-mercaptopurine (6MP) in the treatment of childhood acute lymphoblastic leukaemia (ALL) is well established. However, the efficacy of 6MP is significantly influenced by inactivation by the polymorphic enzyme thiopurine methyltransferase (TPMT). In the general population 89-94% have high TPMT activity, 6-11% have intermediate activity, and approximately 0.3% have low activity. Individuals with low-activity experience severe or fatal toxicity with standard 6MP doses. Prospective identification of this group of patients might prevent this problem. Recent identification of the molecular basis for low TPMT activity enabled rapid assessment of altered 6MP metabolism by PCR methods. This study evaluated the frequency of mutant TPMT alleles in 147 children with ALL. One patient was homozygous mutant (0.7%), and 16 patients were heterozygous for variant TPMT alleles (10.9%). The majority of mutant alleles were TPMT*3A. Both the allele frequency and the pattern of TPMT mutations were similar to that observed in an adult British population. The number of weeks when 6MP therapy was administered at full dose was determined in patients on MRC UKALL X and XI. The 94 patients spent a median 11% of the maintenance period receiving no therapy as a result of haematological toxicity. There was no significant difference in the number of weeks when no therapy could be administered among patients with a wild-type or heterozygous genotype. However, the one patient with a homozygous mutant genotype had severe haematological toxicity and no therapy could be administered for 53% of the maintenance period. This study demonstrates that 11.6% of the children had variant TPMT alleles. Prospective identification of TPMT genotype may be a promising tool for decreasing excessive haematological toxicity in individuals with low activity.     

炎症性肠病患者硫嘌呤甲基转移酶与嘌呤类药物毒副反应的关系

背景：嘌呤类药物广泛用于炎症性肠病（IBD）患者的诱导缓解和维持治疗,然而毒副反应的频繁发生使其临床应用受到限制。有研究显示检测硫嘌呤甲基转移酶（TPMT）活性和基因型能预测硫唑嘌呤（AZA）相关的毒副反应。目的：评价TPMT活性和基因型与AZA治疗IBD的毒副反应的关系。方法：收集2008年10月～2010年12月于上海仁济医院确诊并接受AZA治疗（每日1（？）kg）的78例IBD患者。采用高效液相色谱法（HPLC）检测TPMT活性,以直接测序和PCR法检测TPMT基因型。分析TPMT活性和基因型与AZA毒副反应的关系。结果：共19例患者发生毒副反应,包括4例血液学毒副反应。78例IBD患者的平均TPMT活性为（36.10±10.11）nmol 6-MTG·gHb~（-1）·h~（-1）。低TPMT活性组与高TPMT活性组发生总毒副反应和血液学毒副反应的OR值分别为6.82（95%CI：0.58～79.91）和12.00（95%CI：0.84～172.22）,但差异均无统计学意义。仅1例患者发生TPMT＊3C杂合子突变,突变率为1.3%,且该例患者发生了血液学毒副反应。结论：IBD患者的TPMT活性降低和基因突变与嘌呤类药物的总毒副反应和血液学毒副反应可能相关,但其临床实际应用价值有待进一步研究。     

巯嘌呤甲基转移酶检测在炎症性肠病治疗中的作用

巯嘌呤甲基转移酶（TPMT）是硫唑嘌呤（AZA）／6-巯基嘌呤（6-MP）代谢的关键酶之一,而AZA／6-MP广泛用于难治性炎症性肠病（IBD）的治疗。TPMT的活性／基因型与AZA／6-MP的疗效和不良反应相关,进行TPMT活性／基因型检测有可能预测骨髓抑制等不良反应的发生,并能指导个体化治疗。     

Allelic variants of the thiopurine S-methyltransferase deficiency in patients with ulcerative colitis and in healthy controls

OBJECTIVE: Thiopurine S-methyltransferase (TPMT) is a cytosolic enzyme that catalyzes the inactivation of mercaptopurine, azathioprine, and thioguanine. The genetic polymorphisms in the TPMT gene that regulate TPMT activity are inherited as an autosomal recessive trait and patients with genetically determined low levels of TPMT activity develop severe myelosuppression when treated with standard doses of the above-mentioned drugs. We have analyzed the frequencies of the allelic variants of the TPMT gene in a white European population of healthy blood donors from Spain and The Netherlands, and in a group of patients suffering from ulcerative colitis (UC) with a similar genetic background. METHODS: Two hundred and thirteen unrelated healthy individuals (HC) and 146 UC patients were typed for the polymorphic sites at positions 460 (G-->A) and 719 (A-->G) of the TPMT gene using specific polymerase chain reaction-restriction fragment-length polymorphism (PCR-RFLP) methods. RESULTS: There were no significant differences between the allele frequencies observed in the group of UC patients and those of the control group (10% of cases were heterozygous carriers of a TPMT mutant allele). The most frequent mutant allele in both UC and HC groups was TPMT3A (A460-->G719) (60% of carriers). TPMT3B (A460-->A719) and TPMT3C (G460-->G719) alleles were more often found in our study than in previously reported studies, reflecting the different genetic backgrounds of the European populations analyzed. CONCLUSIONS: Genotyping methods provide a simple and reliable screening to identify patients with a high risk of developing severe bone marrow toxicity if treated with thiopurine drugs. In UC patients, TPMT genotype should be determined before the initiation of azathioprine therapy.     

Should we test TPMT enzyme levels before starting azathioprine?

Thiopurine methyltransferase (TPMT) is the main enzyme responsible for inactivating toxic products of azathioprine (AZA) metabolism. Patients with homozygous deficiency of this enzyme have no enzyme activity and ideally should not be given AZA. Patients with heterozygous deficiency have 50% of enzyme activity and have been shown to respond well and tolerate half a standard dose. We describe a patient with homozygous deficiency of TPMT who developed life threatening neutropenic sepsis, and advocate that all patients should be tested for TPMT activity prior to starting AZA therapy.     

Should we test TPMT enzyme levels before starting azathioprine?

Thiopurine methyltransferase (TPMT) is the main enzyme responsible for inactivating toxic products of azathioprine (AZA) metabolism. Patients with homozygous deficiency of this enzyme have no enzyme activity and ideally should not be given AZA. Patients with heterozygous deficiency have 50% of enzyme activity and have been shown to respond well and tolerate half a standard dose. We describe a patient with homozygous deficiency of TPMT who developed life threatening neutropenic sepsis, and advocate that all patients should be tested for TPMT activity prior to starting AZA therapy.