Thiopurine S-methyltransferase Gene Research Articles

Genetic polymorphisms of NQO1, CYP1A1 and TPMT and susceptibility to acute lymphoblastic leukemia in a Tunisian population

Acute lymphoblastic leukemia (ALL) is the major pediatric cancer in developed countries. The etiology of ALL remains poorly understood, with few established environmental risk factors. These risks were influenced by co-inheritance of multiple low-risk genetic polymorphisms such as variants within cytochrome P450A1 (CYP1A1), NADPH: quinone oxidoreductase (NQO1) and Thiopurine methyltransferase (TPMT) genes. In this work, we conduct a case-control study to assess the impact of CYP1A1*2A (CYP1A1 T6235C); NQO1*2 (NQO1 C609T); TPMT*2 (TPMT G238C) and TPMT A719G polymorphisms on the risk of developing ALL. The frequencies of TPMT*2, TPMT A719G, NQO1*2 and CYP1A1*2 variants were examined in 100 patients with ALL and 106 healthy controls by allele specific PCR and/or PCR-RFLP methods using blood samples. We have found that NQO1 609CT genotype was overrepresented in patients and was associated with an aggravating effect compared to the reference group with NQO1 609CC genotype (p=0.028, OR=1.41; CI 95%: 1.04-1.93). However, TPMT*2, TPMT 719*G and CYP1A1*2 variants did not appear to influence ALL susceptibility (p>0.05). Moreover we have not found a significant correlation between the studied variants and Bcr-Abl transcript. In conclusion we retain that leukemogenesis of ALL is associated with carcinogens metabolism and consequently related to environmental exposures.

Integration of a standardized pharmacogenomic platform for clinical decision support at Boston Children's Hospital

The Clinical Pharmacogenomics Service (CPS) at Boston Children's Hospital (BCH) was established to use genomic information to make pediatric medications safer. Nearly one-quarter of outpatients are prescribed one or more drugs with genetic information in the FDA label [1]. However, there are still important barriers that must be overcome for routine pharmacogenomic (PGx) clinical use: (1) identification of clinically significant variants, (2) knowledge of variant genotype prior to prescribing medication, and (3) integration with current electronic health record (EHR) systems. To tackle these challenges at BCH, the CPS decided to standardize thiopurine S-methyltransferase (TPMT) testing hospital-wide. TPMT is best known for its role in the catalyzing the S-methylation of the thiopurine drugs such as azathioprine, 6-mercaptopurine and 6-thioguanine. Approximately 13% of Caucasians and African Americans are heterozygous and have reduced TPMT activity, while approximately 0.3% are completely deficient. Defects in the TPMT gene can lead to decreased methylation and excessive levels of the toxic thioguanine nucleotides, particularly with azathioprine and 6-mercaptopurine, and are at risk for bone marrow suppression. Although the FDA drug label recommends testing for TPMT deficiency prior to dosing and the PharmGKB CPIC group published a guideline [2] with a recommended dosing strategy and interpretation, testing is not universal because these guidelines are difficult to translate into a clinical decision support (CDS) system and integrate with the EHRs. We developed models and specifications to execute PGx CDS rules based on a patient's genotype. Rules are modeled at four levels of abstraction: (1) unstructured (narrative), (2) semi-structured, (3) structured, and (4) executable. As genomic sequencing becomes routine, standardized methods to interpret the data and make clinical decisions are paramount. In conjunction with the BCH DNA Diagnostic Laboratory, we streamlined the TPMT testing process to fit into the usual clinical routine (including ordering, testing in-house and return of results to the clinician). We consolidated all genetic sequencing testing into a single clinical workflow (blood to report) that is run, analyzed and interpreted in a Clinical Laboratory Improvement Amendments (CLIA) certified laboratory using the codified CDS rules. The interpretation reports are generated automatically directly from the genotype calls and then manually reviewed for accuracy. Once cleared by the laboratory director, the reports are uploaded into the EHR (Cerner). Specialty flow sheets enable providers to easily view the allele status and interpretation report. We intend to expand the PGx platform to include additional drug/gene pairs.

Establishment of Thiopurine S-Methyltransferase Gene Knockdown in Jurkat T-lymphocytes

Thiopurine S-methyltransferase (TPMT) is an excellent example of an enzyme whose pharmacogenetic polymorphisms affect efficacy and toxicity of a drug. The association between TPMT activity and thiopurine-related myelosuppression is well recognized. To study the significance of TPMT deficiency in thiopurine metabolism and immunosuppressive activity in vitro, we established RNA interference-based TPMT knockdown (kd) in a Jurkat cell line. In Jurkat TPMT kd cells, TPMT expression was reduced to 73% at the RNA level and 83% at the protein level. TPMT kd cells were more sensitive to 6-mercaptopurine (6-MP) (10 μmol/L) and 6-thioguanine (6-TG) (8 μmol/L) than wild-type (wt) cells, (32% versus 20%) and (18% versus 9%), respectively. Both Jurkat wt and kd cells were more sensitive to 6-TG-induced apoptosis than to 6-MP. 6-TG activity was also more affected by TPMT levels than was 6-MP as reflected by IC60, concentrations that is, 6-MP [4.6 μmol/L (wt) and 4.7 μmol/L (kd)], 6-TG [2.7 μmol/L (wt) and 0.8 μmol/L (kd)]. IC60 concentrations induced significant apoptosis in both Jurkat wt and kd cells (257%, versus 314%) with 6-MP and (323% versus 306%) with 6-TG, respectively. At IC60 (6-MP) 6-thioguanine nucleotides (6-TGN) accumulation in cells was 518 versus 447 pmol/million cells in wt and kd cells, respectively. On the other hand 6-TGN accumulation at IC60 (6-TG) was 477 versus 570 pmol/million cells in wt and kd cells, respectively. 6-Methylated mercaptopurine (6-MeMP) concentrations were more affected than 6-TGN by TPMT kd (194 versus 10 pmol/million cells) in wt and kd cells, respectively. We conclude that TPMT kd cells are an appropriate in vitro model to investigate the significance of TPMT deficiency with thiopurine therapy and could be helpful in understanding possible clinical consequences of TPMT polymorphism.

Utility of Assessing Thiopurine S-methyltransferase Polymorphisms Before Azathioprine Therapy

Thiopurine S-methyltransferase (TPMT) catalyzes the methylation of thiopurine drugs, such as azathioprine and mercaptopurine, which are used in a variety of diseases. Several mutations in the TPMT gene correlate with low enzyme activity and subsequent adverse effects, mainly myelotoxicity. Hence, genotyping TPMT makes it possible to identify patients at high risk of drug toxicity and adjust dosage accordingly. However, further research about the availability of a reliable and universal screening method and more costeffectiveness studies are necessary.

Structural Characteristics Determine the Cause of the Low Enzyme Activity of Two Thiopurine S-Methyltransferase Allelic Variants: A Biophysical Characterization of TPMT*2 and TPMT*5

The enzyme thiopurine S-methyltransferase (TPMT) is involved in the metabolism of thiopurine drugs used to treat acute lymphoblastic leukemia and inflammatory bowel disease. Thus far, at least 29 variants of the TPMT gene have been described, many of which encode proteins that have low enzyme activity and in some cases become more prone to aggregation and degradation. Here, the two naturally occurring variants, TPMT*2 (Ala80 → Pro) and TPMT*5 (Leu49 → Ser), were cloned and expressed in Escherichia coli. Far-UV circular dichroism spectroscopy showed that TPMT*2 was substantially destabilized whereas TPMT*5 showed much greater stability comparable to that of wild-type TPMT (TPMTwt). The extrinsic fluorescent molecule anilinonaphthalene sulfonate (ANS) was used to probe the tertiary structure during thermal denaturation. In contrast to TPMTwt, neither of the variants bound ANS to a large extent. To explore the morphology of the TPMT aggregates, we performed luminescent conjugated oligothiophene staining and showed fibril formation for TPMT*2 and TPMT*5. The differences in the flexibility of TPMTwt, TPMT*2, and TPMT*5 were evaluated in a limited proteolysis experiment to pinpoint stable regions. Even though there is only one amino acid difference between the analyzed TPMT variants, a clear disparity in the cleavage patterns was observed. TPMT*2 displays a protected region in the C-terminus, which differs from TPMTwt, whereas the protected regions in TPMT*5 are located mainly in the N-terminus close to the active site. In conclusion, this in vitro study, conducted to probe structural changes during unfolding of TPMT*2 and TPMT*5, demonstrates that the various causes of the low enzyme activity in vivo could be explained on a molecular level.

The SmartAmp Method: Rapid Detection of SNPs in Thiopurine S-Methyltransferase and ABC Transporters ABCC4 and ABCG2

Genetic polymorphisms of drug transporters as well as drug metabolizing enzymes have been documented to play a significant role in patients' responses to medication. A key requirement for advancing personalized medicine is the ability to rapidly and conveniently test for patients' genetic polymorphisms. We have recently developed a rapid and cost-effective method for single nucleotide polymorphism (SNP) detection, named Smart Amplification Process (SmartAmp), which enables us to detect genetic polymorphisms or mutations in 30 to 45 min under isothermal conditions without the need for DNA isolation and PCR amplification. This article presents the SmartAmp-based detection of SNPs in the thiopurine S-methyltransferase gene as well as in the ATP-binding cassette (ABC) transporter ABCC4 and ABCG2 genes that are critically involved in drug-induced adverse reactions. The SmartAmp method is expected to provide a practical and cost-effective tool for pharmacogenomics-based personalized medicine.

Toxicity of azathioprine: why and when? analysis of the prevalence of polymorphism in Joinville, SC, Brazil

The use of thiopurine drugs such as azathioprine and 6-mercaptopurine has become quite common in the treatment of inflammatory bowel disease, transplantation and acute leukemias. Despite their effectiveness, these drugs are capable of causing drug-induced toxicity with the risk of death by myelosuppression. It is now known that these complications occur because of genetic polymorphisms of the thiopurinemethyltransferase (TPMT) enzyme, responsible for its metabolism. To assess the prevalence of thiopurine methyltransferase polymorphisms in the population of Joinville, SC, Brazil. We analyzed the frequency of four main allelic variants of the TPMT gene in 199 blood donors from Joinville, from February to April 2010. The normal allele ("wild-type") was found in 93.9% of subjects studied. TPMT variants were detected in 12 subjects (6.03%). From this study, it was estimated at 6% the risk of toxicity by the administration of azathioprine and 6-mercaptopurine to patients in Joinville.

Importance of thiopurine s-methyltransferase gene test

Thiopurine s-methyltransferase enzyme is responsible for the metabolism of immunosuppressant thiopurines, which are used in inflammatory bowel diseases, acute lymphoblastic leukemia and autoimmune diseases. Because of the relative narrow therapeutic index of thiopurines serious or life threatening side effects can occur. A total of 28 variant alleles of the gene coding for the thiopurine s-methyltransferase enzyme are responsible for altered catalytic activity of the enzyme. Patients with one non-functional (heterozygous) allele have intermediate, while those with two non-functional (homozygous) alleles have low enzyme activity. Using polymerase chain reaction/restriction fragment length polymorphism and direct DNA sequencing the authors determined the G238C, G460A and A719G polymorphisms of the thiopurine s-methyltransferase gene in a child with Crohn's disease who developed thiopurine-induced severe agranulocytosis. The presence of the G460A and A719G polymorphic alleles in homozygous forms were detected which corresponded to the *3A variant allele. This variant has been shown to be associated with lower enzyme activity and low amount of the enzyme resulting in thiopurine toxicity and agranulocytosis. These findings underline the need for genotyping of the thiopurine s-methyltransferase variants prior to thiopurine treatment.

Prevalence of mutations in thiopurine S-methyltransferase gene among Slovak IBD patients

Thiopurine S-methyltransferase (TPMT) plays an important role in the metabolism of thiopurines. It has been suggested that TPMT genetic polymorphisms lead to dose-related hematopoetic toxicity. Since there are major ethnic differences in the prevalence of particular TPMT variants, it is important for each country to study their own prevalence in order to estimate the role of TPMT variants-related thiopurines toxicity in the particular inflammatory bowel disease (IBD) population. The aim of this study was to determine the frequency of the four most common allelic variants of TPMT gene in the population of Slovak IBD patients. TPMT genetic polymorphisms (TPMT*2, TPMT*3A, TPMT*3B, TPMT*3C) were amplified using PCR and consequently genotyped on genetic analyzer. The allele frequencies of particular allelic variants were calculated and compared with other Caucasian populations reported so far. Three hundred and thirty IBD patients were included; 196/132/2 Crohn´s disease/ulcerative colitis/unclassified colitis, 180 (55 %) males. Ninety-three percent of patients were homozygous for wild type TPMT variant. Heterozygous genotype of any of the studied polymorphisms was present in 6 % of patients, only one patient was homozygous for TPMT*3A allele (0.3 %). The most prevalent mutant allele was TPMT*3A (3.2 %). The distribution of the most common allelic variants of TPMT gene among Slovak IBD patients were in accordance with previously reported prevalence in Caucasian populations. This study shows the prevalence of TPMT genetic polymorphisms in the Slovak IBD patient`s population. As in other Caucasian populations, the most common mutant allelic variant is TPMT*3A, and the prevalence of homozygosity is relatively low (Tab. 3, Ref. 16).

Detection of a Novel Single Nucleotide Polymorphism of the Human Thiopurine S-Methyltransferase Gene in a Chinese Individual

A 62-year-old Chinese patient with recurrent pompholyx submitted his blood sample for pre-treatment thiopurine S-methyltransferase (TPMT) pharmacogenetic profiling, and it was found to harbour a novel single nucleotide polymorphism (SNP). The novel SNP, detected by mRNA sequencing, was a c.2T>C (g.11018T>C) transition in the start codon, causing a Met1Thr amino acid change. This finding was confirmed on a subsequent blood sample from the same patient by DNA sequencing. The patient was genotyped as TPMT*1/*29, sequentially named as such following the latest TPMT SNP (TPMT*1/*28) at the time of writing. The novel SNP is expected to result in complete lack of protein translation, similar to the impact exerted by TPMT*14, another start codon SNP of the TPMT gene.

Impact of age, gender and haplotypes of thiopurine methyltransferase (TPMT) and inosine triphosphate pyrophosphatase (ITPA) genes on 6-mercaptopurine toxicity in children with acute lymphoblastic leukemia

Impact of age, gender and haplotypes of thiopurine methyltransferase (TPMT) and inosine triphosphate pyrophosphatase (ITPA) genes on 6-mercaptopurine toxicity in children with acute lymphoblastic leukemia

Influence of age, sex, and haplotypes of thiopurine methyltransferase (TPMT) gene on 6- mercaptopurine toxicity in children with acute lymphoblastic leukemia

Influence of age, sex, and haplotypes of thiopurine methyltransferase (TPMT) gene on 6- mercaptopurine toxicity in children with acute lymphoblastic leukemia

Thiopurine S-methyltransferase genetic polymorphism in the Tunisian population

Background Determine the incidence of four thiopurine S-methyltransferase ( TPMT) mutant alleles, TPMT ∗2 , ∗3A , ∗3B and ∗3C in the Tunisian population involved in adverse drug reactions. Genomic DNAs were isolated from peripheral blood leucocytes of 119 healthy Tunisian volunteers. The frequencies of four allelic variants of the TPMT gene, TPMT ∗2 , ∗ 3A, ∗ 3B, ∗ 3C were determined using allele specific polymerase chain reaction (PCR) or PCR-restriction fragment length polymorphism technique. Results Of the 119 Tunisian subjects participating in this study, 117 subjects (98.3%) were homozygous for TPMT ∗1 and only two subjects (1.68%) were heterozygous for TPMT ∗1/ ∗3A . The frequency of TPMT ∗3A mutant allele was 0.009. Conclusions Our study provides the first data on the frequency of common TPMT variants in the Tunisian population. TPMT ∗ 3A, which causes the largest decrease in enzyme activity, seemed to be a unique variant allele found in this our population.

The frequency and distribution of thiopurine S-methyltransferase alleles in south Iranian population

Thiopurine methyltransferase (TPMT) catalyzes the S-methylation of thiopurine drugs such as 6-mercaptopurine, 6-thioguanine, and azathiopurine. Variability in TPMT activity is mainly due to genetic polymorphism. The frequency of the four allelic variants of the TPMT gene, TPMT*2 (G238C), TPMT*3A (G460A and A719G), TPMT*3B (G460A) and TPMT*3C (A719G) were determined in an Iranian population from south of Iran (n=500), using polymerase chain reaction (PCR)-RFLP and allele-specific PCR-based assays. Four hundred seventy four persons (94.8%) were homozygous for the wild type allele (TPMT*1/*1) and twenty five people were TPMT*1/*3C (5%). One patient was found to be heterozygous in terms TPMT*1 and *2 alleles with genotype of TPMT*1/*2 (0.2%). None of the participants had both defective alleles. The TPMT*3C and *2 were the only variant alleles observed in this population. The total frequency of variant alleles was 2.6% and the wild type allele frequency was 97.4%. The TPMT*3B and *3A alleles were not detected. Distributions of TPMT genotype and allele frequency in Iranian populations are different from the genetic profile found among Caucasian or Asian populations. Our findings also revealed inter-ethnic differences in TPMT frequencies between different parts of Iran. This view may help clinicians to choose an appropriate strategy for thiopurine drugs and reduce adverse drug reactions such as bone marrow suppression.

Clinical Applicability of Sequence Variations in Genes Related to Drug Metabolism

The Human Genome and the Hap Map Projects as well as the extensive use of deep resequencing worldwide, have contributed to a massive catalogue of reported single nucleotide polymorphisms (SNPs) and other genetic variations in the human genome. Pharmacogenomics is an emerging field that combines genetics with pharmacokinetics and pharmacodynamics of the drug in attempt to understand inter-individual differences among patients and develop more accurate drug dosing. However, only for the minority of those variations an association with phenotype has been established. Here, we provide an overview of genes and genetic variants that influence inter-individual dosing of three of the most widely used drugs, namely warfarin, irinotecan and thiopurine drugs, to highlight a tangible benefit of translating genomic knowledge into clinical practice. Therefore, particular SNPs in vitamin K epoxide reductase complex subunit 1 (VKORC1), cytochrome P450 2C9 (CYP2C9), uridine diphosphate glucoronosyltransferase 1A1 (UGT1A1) and thiopurine S-methyltransferase (TPMT) genes has proven to be applicable for optimising the dosage in pursuit of maximum efficacy and minimum adverse effects. Thus, they set an important paradigm of implementation of pharmacogenomics in the mainstream clinical practice.

Clinical relevance of thiopurine S-methyltransferase gene polymorphisms

The therapeutic response to thiopurines may result in either severe toxic or inadequate effect based on the interindividual genetic variability. Same drug doses of various anticancer drugs cause considerable interindividual differences in the therapeutic response. Genetic factors have a major impact on effectiveness of several anticancer drugs such as mercaptopurine, 5-fluorouracil, platinum agents, and cyclophosphamide. Heredity related differences in interindividual response to thiopurine therapy represent perhaps the most compelling evidence of pharmacogenomics' usefulness in identification of patients in risk for adverse drug reactions. A number of variations in the gene for thiopurine methyltransferase (TPMT) have been associated with the low activity of this enzyme. Patients with intermediate and low activity of TPMT have a greater incidence of thiopurine toxicity. This minireview summarizes results of studies assessing the role of genetic polymorphisms in the gene encoding TPMT and their relationship to the toxicity of thiopurines.

Radioecological Risk Assessment of Low Selenium Concentrations through Genetic Fingerprints and Metabolic Profiling of Soil Bacterial Communities

In a context of environmental risk assessment of nuclear (79)Se radionuclide, the impact of low Se-selenite concentrations (0.008 and 8 mg kg(-1)) on bacterial communities of two soils, a silty clay loam and a sandy soil, was investigated over a 6-month incubation time. This Se-selenite was partially labelled with (75)Se. The state of the Se-impacted bacterial communities was analyzed through total bacterial counts, DNA fingerprints (ARISA profiles) and metabolic profiling (carbon substrate utilization patterns). Furthermore, the genetic diversity of bacterial populations involved in Se volatilization was evaluated by tpm (thiopurine methyltransferase gene) profiling. Emissions of (75)Se and CaCl(2)-extractable (75)Se were measured by γ-spectrometry and scintillation analysis. Se-selenite inputs changed transiently the substrate utilization patterns of bacterial communities but did not affect the other indicators. Se volatilization was at its highest level just after adding Se-selenite and for about 1 week. This volatilization was proportional to the added Se-selenite concentrations. It was 100-fold higher in silty clay loam, even though Se bioavailability was reduced in this soil. The soils were amended with crushed grass 3 months after the addition of Se-selenite. This organic amendment affected the organization of bacterial communities and increased the Se-volatilizing activities of both soils. Original soil organic carbon and bacterial diversity and activities seemed responsible for the different levels of Se emissions observed in soils. tpm lineages, encoding Se methyltransferases, were detected in both soils, confirming the broad distribution of tpm-harbouring bacteria and their probable role in the emissions of volatile Se. Five distinct groups of tpm were recorded per soil, with tpmI lineage being detected throughout the incubation period. This study demonstrates the ability of bacterial communities at volatilizing Se concentrations inferior to geochemical backgrounds and suggests that a probable transfer of nuclear Se will occur through volatilization after an environmental spill.

Thiopurine S‐methyltransferase (TPMT) polymorphisms in children with acute lymphoblastic leukemia, and the need for reduction or cessation of 6‐mercaptopurine doses during maintenance therapy: The Polish multicenter analysis

6-Mercaptopurine (6-MP) is used for the treatment of pediatric acute lymphoblastic leukemia (ALL). Mutations in the TPMT gene may influence the efficacy and safety of 6-MP treatment. This multicenter study investigated the association between TPMT genotype, 6-MP dose adjustments, and the incidence of adverse effects in patients. A total of 203 ALL children were genotyped using PCR/allele-specific amplification and PCR/RFLP. The control group consisted of 394 healthy volunteers. The TPMT*3A variant allele was found in 16 patients (15 TPMT*1/*3A, 1 TPMT*3A/*3A) and the TPMT*3C (A719G) allele in 1 patient. No TPMT*2 (G238C) or TPMT*3B (G460A) alleles were detected in the study group. TPMT*3A, TPMT*1 (wild-type), and TPMT*3C alleles were detected at frequencies of 3.94%, 95.81%, and 0.25%, respectively. The genotype and allele distributions were similar in the ALL and control groups. The 6-MP dose was reduced more frequently in patients with TPMT*3A and TPMT*3C alleles, compared with wild-type alleles (P = 0.042). Reductions because of leucopenia with respiratory tract infection, or because of leucopenia, anemia and/or thrombocytopenia were four (P = 0.007) and five (P = 0.03) times more common, respectively. The groups differed with regard to the rates of 6-MP dose reduction (P = 0.028). 6-MP was discontinued more often in patients with TPMT*3A and TPMT*3C alleles (14-fold) as a result of leucopenia, anemia, and/or thrombocytopenia (P = 0.004). The results indicate that TPMT genotype influences the safety and efficacy of ALL treatment and genotype information may therefore be useful for optimizing 6-MP therapy.

RNA Interference Targeting Thiopurine Methyl Transferase Gene Alters the Cytotoxic Effect of the 6-Mercaptopurine In Human Leukaemia Cells

AbstractAbstract 2891Thiopurines; mercaptopurine (6-MP) and 6-thioguanine (6-TG) are important drugs in treatment of paediatric cancer patients. The activity of these drugs depends on the activity of several common enzymes in the metabolism pathways such as thiopurine methyl transferase (TPMT) and guanine monphosphate synthetase (GMPS). In the present study the efficacy of thiopurines was investigated upon inhibition of TPMT and GMPS gene expression by RNA interference (siRNA). Treatment of the MOLT4 human T-cell leukemia cells with TPMT and GMPS siRNA resulted in decreased mRNA expression as determined by Real-Time PCR by 60% and 70% respectively. When reducing TPMT mRNA, the MOLT-4 cells were 70% less sensitive to 6-MP while the sensitivity to 6-TG was unchanged. When down-regulating GMPS using siRNA the sensitivity was unchanged upon treatment with both drugs. A microarray experiment was conducted using wild type on MOLT-4, 6-MP and 6-TG resistant MOLT-4 variants. The aim was to identify affected genes in response to the resistance induction. The mRNA levels of several nucleoside transporter genes were down-regulated in both thiopurine-resistant sub-lines; concentrative nucleoside transporter 3 (CNT3) and equilibrative nucleoside transporter 2 (ENT2). The expression of genes encoding the purine de novo synthesis enzymes was reduced to the same extent in both resistant cell lines as well as the expression of GMPS (>40% in both resistant sub-lines) which indicates defected purine metabolism. The activity of TPMT and other enzymes in metabolism pathway of 6-MP and 6-TG was unchanged in resistant cells. Our results suggest that the contribution of TPMT activity in cytotoxicity of 6-MP is greater than for 6-TG, probably due to inhibition of de novo purine synthesis by methylated nucleotides, whereas TPMT inactivates 6-TG by methylation. Disclosures:No relevant conflicts of interest to declare.

Aplasie médullaire induite par l’azathioprine : intérêt de la recherche systématique d’une mutation du gène de la thiopurine S-méthyltransférase ?

Introduction Myelotoxicity is a well-known adverse effect of azathioprine, leading mainly to leukopenia. Other azathioprine associated hematological adverse effects are uncommon. Case report We report a 49-year-old woman with rheumatoid arthritis and acquired hemophilia, who presented a severe myelosuppression occurring 3 weeks after an increase of her azathioprine regimen (at a daily dose of 150 mg). The patient had a heterozygous mutation of the thiopurine S-methyltransferase gene (TPMT*3A). Azathioprine therapy was discontinued and she recovered at 3 weeks. The patient had no relapse of pancytopenia after a 1 year follow-up. Conclusion Routine measurement of TPMT activity or determination of TPMT variant allele may be useful tests, in order to identify the subgroup of patients who are at risk to develop azathioprine induced severe myelosuppression.