Genetic basis of inosine triphosphate pyrophosphohydrolase deficiency

ITPase-deficient mice show growth retardation and die before weaning

Inosine triphosphate pyrophosphatase (ITPase), the enzyme that hydrolyzes ITP and other deaminated purine nucleoside triphosphates to the corresponding purine nucleoside monophosphate and pyrophosphate, is encoded by the Itpa gene. In this study, we established Itpa knockout (KO) mice and used them to show that ITPase is required for the normal organization of sarcomeres in the heart. Itpa(-/-) mice died about 2 weeks after birth with features of growth retardation and cardiac myofiber disarray, similar to the phenotype of the cardiac alpha-actin KO mouse. Inosine nucleotides were found to accumulate in both the nucleotide pool and RNA of Itpa(-/-) mice. These data suggest that the role of ITPase in mice is to exclude ITP from the ATP pool, and the main target substrate of this enzyme is rITP. Our data also suggest that cardiomyopathy, which is mainly caused by mutations in sarcomeric protein-encoding genes, is also caused by a defect in maintaining the quality of the ATP pool, which is an essential requirement for sarcomere function.

Functional study of the P32T ITPA variant associated with drug sensitivity in humans

Sanitization of the cellular nucleotide pools from mutagenic base analogues is necessary for the accuracy of transcription and replication of genetic material and plays a substantial role in cancer prevention. The undesirable mutagenic, recombinogenic, and toxic incorporation of purine base analogues [i.e., ITP, dITP, XTP, dXTP, or 6-hydroxylaminopurine (HAP) deoxynucleoside triphosphate] into nucleic acids is prevented by inosine triphosphate pyrophosphatase (ITPA). The ITPA gene is a highly conserved, moderately expressed gene. Defects in ITPA orthologs in model organisms cause severe sensitivity to HAP and chromosome fragmentation. A human polymorphic allele, 94C-->A, encodes for the enzyme with a P32T amino acid change and leads to accumulation of non-hydrolyzed ITP. ITPase activity is not detected in erythrocytes of these patients. The P32T polymorphism has also been associated with adverse sensitivity to purine base analogue drugs. We have found that the ITPA-P32T mutant is a dimer in solution, as is wild-type ITPA, and has normal ITPA activity in vitro, but the melting point of ITPA-P32T is 5 degrees C lower than that of wild-type. ITPA-P32T is also fully functional in vivo in model organisms as determined by a HAP mutagenesis assay and its complementation of a bacterial ITPA defect. The amount of ITPA protein detected by Western blot is severely diminished in a human fibroblast cell line with the 94C-->A change. We propose that the P32T mutation exerts its effect in certain human tissues by cumulative effects of destabilization of transcripts, protein stability, and availability.

Pro32Thr polymorphism of inosine triphosphate pyrophosphatase gene predicts efficacy of low-dose azathioprine for patients with systemic lupus erythematosus

We evaluated the relationship between the efficacy of low-dose azathioprine (AZA) therapy and the inosine triphosphate pyrophosphatase (ITPA) 94C>A (Pro32Thr) polymorphism in patients with systemic lupus erythematosus (SLE). We performed a multiple regression analysis to assess the influence of various factors on the reduction in SLE disease activity index (SLEDAI) scores. The ITPA 94C>A polymorphism had the highest correlation with the change in SLEDAI score (r = 0.354, P = 0.006).

TPMT but not ITPA gene polymorphism influences the risk of azathioprine intolerance in renal transplant recipients

PURPOSE

Thiopurine drugs have to be withdrawn in 10-30% of cases due to side effects, and it has been presented that genetic factors may be responsible for some of reported toxicity cases. Among polymorphic enzymes of thiopurines' metabolic pathway, thiopurine S-methyltransferase (TPMT) has been studied most extensively, and some recent studies point to inosine triphosphate pyrophosphohydrolase (ITPA) polymorphism as an additional toxicity risk factor.

METHODS

The aim of the current study was to evaluate an association between TPMT and ITPA gene polymorphisms and drug intolerance in a cohort of 157 renal transplant recipients treated with azathioprine (AZA). Each subject was genotyped for the presence of variant TPMT (*2, *3A, *3B, and *3C) and ITPA (94C>A and IVS2+21A>C) alleles.

RESULTS

Mean AZA dose, mean white-blood-cell count, and platelet count in the course of treatment were lower in carriers of variant TPMT alleles compared to patients with TPMT wild-type genotype. Leukocyte numbers fell below 4.0 x 10(9)/L in 41.2% of TPMT heterozygous renal transplant recipients, compared to only 18.0% of wild-type patients (P < 0.01). In contrast, ITPA genotype did not influence AZA dose, hematological parameters, or leucopenia risk.

CONCLUSIONS

Our results suggest that routine genotyping of renal transplant recipients for TPMT variants may be useful in reducing the risk of AZA-related myelotoxicity, but there is not enough evidence to introduce ITPA testing into clinical practice.

Thiopurine S-methyltransferase and inosine triphosphate pyrophosphohydrolase genes in Japanese patients with inflammatory bowel disease in whom adverse drug reactions were induced by azathioprine/6-mercaptopurine treatment

BACKGROUND

The main cause of azathioprine (AZA)/6-mercaptopurine (6MP)-induced adverse reactions is a reduction in the activities of the metabolizing enzymes thiopurine S-methyltransferase (TPMT) and inosine triphosphate pyrophosphohydrolase (ITPA). Adverse reactions develop at a high frequency in Japanese patients at half the dose required for European and American patients; however, the association with TPMT and ITPA gene polymorphisms in Japanese has not been fully investigated.

METHODS

Gene mutations of TPMT and ITPA, the major AZA/6-MP -metabolizing enzymes, were investigated retrospectively in 16 Japanese patients with inflammatory bowel disease (IBD) in whom AZA/6MP treatment induced adverse reactions.

RESULTS

The TPMT gene was found to have a wild-type sequence in all patients, but in the ITPA gene a mutation, 94C>A, was detected at a rate of 50% (8/16), with 83.3% (5/6) occurring in patients with acute bone marrow suppression and 75% (3/4) in those with agranulocytosis. The 94C>A allele frequency was 10 of 32 (0.313; 95% CI, 0.180-0.486). Adverse reactions developed earlier in patients with the 94C>A mutation. However, in half the patients, no gene polymorphism was noted.

CONCLUSIONS

It is suggested that the ITPA gene mutation is closely related to the adverse reactions of AZA/6-MP in Japanese patients, and screening for the mutant allele is useful for predicting the most serious adverse reactions, agranulocytosis and acute bone marrow suppression.

Genetic polymorphism of inosine triphosphate pyrophosphatase is a determinant of mercaptopurine metabolism and toxicity during treatment for acute lymphoblastic leukemia

The influence of genetic polymorphism in inosine triphosphate pyrophosphatase (ITPA) on thiopurine-induced adverse events has not been investigated in the context of combination chemotherapy for acute lymphoblastic leukemia (ALL). This study investigated the effects of a common ITPA variant allele (rs41320251) on mercaptopurine metabolism and toxicity during treatment of children with ALL. Significantly higher concentrations of methyl mercaptopurine nucleotides were found in patients with the nonfunctional ITPA allele. Moreover, there was a significantly higher probability of severe febrile neutropenia in patients with a variant ITPA allele among patients whose dose of mercaptopurine had been adjusted for TPMT genotype. In a cohort of patients whose mercaptopurine dose was not adjusted for TPMT phenotype, the TPMT genotype had a greater effect than the ITPA genotype. In conclusion, genetic polymorphism of ITPA is a significant determinant of mercaptopurine metabolism and of severe febrile neutropenia, after combination chemotherapy for ALL in which mercaptopurine doses are individualized on the basis of TPMT genotype.

Genetic variations in the HGPRT, ITPA, IMPDH1, IMPDH2, and GMPS genes in Japanese individuals

Thiopurines (such as azathioprine and 6-mercaptopurine) are widely used for the treatment of patients suffering from malignancies, rheumatic disease, inflammatory bowel disease and solid organ transplant rejection. These drugs are activated and eliminated by a number of enzymes in the human body. This analyzes all the exons and exon-intron junctions of 5 enzyme genes (hypoxanthine-guanine phosphoribosyltransferase, HGPRT; inosine triphosphate pyrophosphatase, ITPA; inosine monophosphate dehydrogenases 1 and 2, IMPDH1 and IMPDH2 and guanosine monophosphate synthetase, GMPS) involved in the metabolism of thiopurine drugs. Twelve novel single nucleotide polymorphisms (SNPs) (HGPRT: IVS6-12C>A (frequency:0.003); ITPA: 569T>C (Phe189Phe, 0.003); IMPDH1: IVS8-15C>A (0.003), IVS9+227A>G (0.003), IVS17+115C>T (0.003), and 930C>T (Thr310Thr, 0.005); IMPDH2: IVS1+50G>T (0.003), IVS2+15G>A (0.010), IVS3-20G>A (0.003), 609C>T (Arg203Arg, 0.003), and 1534C>T (Arg512Trp, 0.003); and GMPS: 1563T>C (Gly521Gly, 0.003)) and 7 known SNPs (ITPA: 94C>A (Pro32Thr, 0.005), 138G>A (Gln46Gln, 0.586), and 563G>A (Glu187Glu, 0.433); IMPDH1: 987G>C (Leu329Leu, 0.113) and 1575A>G (Ala525Ala, 0.620) and GMPS: IVS5-7T>C (0.153), 993A>G (Thr331Thr, 0.153)) were identified in 200 Japanese subjects. These data should provide useful information for thiopurine therapy in the Japanese and as well as other Asian populations.

Prospective evaluation of the pharmacogenetics of azathioprine in the treatment of inflammatory bowel disease

AIM

To investigate whether pharmacogenetic loci or metabolite concentrations explain clinical response or side effects to AZA.

METHODS

Patients with IBD were given 2 mg/kg of AZA without dose escalation or adjustment. Serial clinical response, thiopurine methyl transferase (TPMT) activity and thioguanine nucleotide (TGN) concentrations were measured over 6 months. All patients were genotyped for inosine triphosphatase (ITPase) and TPMT. Clinical response and side effects were compared to these variables.

RESULTS

Two hundred and seven patients were analysed. Thirty-nine per cent withdrew due to adverse effects. Heterozygous TPMT genotype strongly predicted adverse effects (79% heterozygous vs. 35% wild-type TPMT, P < 0.001). The ITPA 94C>A mutation was associated with withdrawal due to flu-like symptoms (P = 0.014). A baseline TPMT activity below 35 pmol/h/mg/Hb was associated with a greater chance of clinical response compared with a TPMT above 35 pmo/h/mg/Hb (81% vs. 43% respectively, P < 0.001). Patients achieving a mean TGN level above 100 were significantly more likely to respond (P = 0.0017).

CONCLUSIONS

TPMT testing predicts adverse effects and reduced chance of clinical response (TPMT >35 pmol/h/mg/Hb). ITPase deficiency is a predictor of adverse effects and TGN concentrations above 100 correlate with clinical response.

Clinical pharmacology and pharmacogenetics of thiopurines

The thiopurine drugs-azathioprine (AZA), 6-mercaptopurine (6-MP), and thioguanine-are widely used to treat malignancies, rheumatic diseases, dermatologic conditions, inflammatory bowel disease, and solid organ transplant rejection. However, thiopurine drugs have a relatively narrow therapeutic index and are capable of causing life-threatening toxicity, most often myelosuppression. Thiopurine S-methyltransferase (TPMT; EC 2.1.1.67), an enzyme that catalyzes S-methylation of these drugs, exhibits a genetic polymorphism in 10% of Caucasians, with 1/300 individuals having complete deficiency. Patients with intermediate or deficient TPMT activity are at risk for excessive toxicity after receiving standard doses of thiopurine medications. This report reviews the recent advances in the knowledge of the mechanism of action as well as the molecular basis and interethnic variations of TPMT and inosine triphosphate pyrophosphatase (ITPase; EC 3.6.1.19), another enzyme implicated in thiopurine toxicity. In addition, an update on pharmacokinetics, metabolism, drug-drug interactions, safety, and tolerability of thiopurine drugs is provided.

Pharmacogenetics of Crohn’s disease

The considerable interindividual differences in efficacy and side effects of commonly used medications in Crohn’s disease are partly owing to genetic polymorphisms. Many genetic variants have been studied in genes possibly involved in the metabolism or mechanism of action of therapeutic agents such as glucocorticosteroids, azathioprine/6-mercaptopurine, methotrexate, calcineurin inhibitors or anti-TNF agents. However, the only test translated into clinical practice is thiopurine S-methyltransferase (TPMT) genotyping for hematological toxicity of thiopurine treatment. To date, there are no other meaningful applications for pharmacogenomics in clinical practice of Crohn’s disease. In the future, designed therapeutic trials should possibly permit the development of predictive models including genotypic markers, such as that proposed for the clinical outcome after infliximab therapy, which includes an apoptotic pharmacogenetic index. The recent identification of new susceptibility genes provides additional candidate markers that have possible effects on the outcomes of therapies, and prioritizes new therapeutic targets, such as the IL-23 pathway. Futher innovative approaches might be relevant for the pharmacogenetic investigation of gene variants implied in innate immune pattern recognition and autophagy.

Pharmacogenomic studies of the anticancer and immunosuppressive thiopurines mercaptopurine and azathioprine

AIMS

To examine the allelic variation of three enzymes involved in 6-mercaptopurine/azathioprine (6-MP/AZA) metabolism and evaluate the influence of these polymorphisms on toxicity, haematological parameters and metabolite levels in patients with acute lymphoblastic leukaemia (ALL) or inflammatory bowel disease (IBD).

METHODS

Clinical data and blood samples were collected from 19 ALL paediatric patients and 35 IBD patients who were receiving 6-MP/AZA therapy. All patients were screened for seven genetic polymorphisms in three enzymes involved in mercaptopurine metabolism [xanthine oxidase, inosine triphosphatase (C94-->A and IVS2+21A-->C) and thiopurine methyltransferase]. Erythrocyte and plasma metabolite concentrations were also determined. The associations between the various genotypes and myelotoxicity, haematological parameters and metabolite concentrations were determined.

RESULTS

Thiopurine methyltransferase variant alleles were associated with a preferential metabolism away from 6-methylmercaptopurine nucleotides (P = 0.008 in ALL patients, P = 0.038 in IBD patients) favouring 6-thioguanine nucleotides (6-TGNs) (P = 0.021 in ALL patients). Interestingly, carriers of inosine triphosphatase IVS2+21A-->C variants among ALL and IBD patients had significantly higher concentrations of the active cytotoxic metabolites, 6-TGNs (P = 0.008 in ALL patients, P = 0.047 in IBD patients). The study confirmed the association of thiopurine methyltransferase heterozygosity with leucopenia and neutropenia in ALL patients and reported a significant association between inosine triphosphatase IVS2+21A-->C variants with thrombocytopenia (P = 0.012). CONCLUSIONS; Pharmacogenetic polymorphisms in the 6-MP pathway may help identify patients at risk for associated toxicities and may serve as a guide for dose individualization.

Functional characterization of human xanthine oxidase allelic variants

OBJECTIVE

Xanthine oxidase (XO) catalyzes the oxidation of endogenous and exogenous purines and pyrimidines. In this study, we speculated that individual variations in XO activity are caused by genetic variations in the XO gene.

METHODS

To investigate the genetic variations in XO in 96 Japanese participants, denaturing high-performance liquid chromatography was used. To assess the effects of these variations on enzymatic activity, wild-type XO and 21 types of variant XO--including those in the database and those just discovered--were transiently expressed in COS-7 cells.

RESULTS

Three nonsynonymous single nucleotide polymorphisms, including 514G>A (Gly172Arg), 3326A>C (Asp1109Thr), and 3662A>G (His1221Arg) were identified in Japanese participants. Functional characterization of 21 XO variants showed a deficiency in enzyme activity in two variants (Arg149Cys and Thr910Lys); low activity (intrinsic clearance, CLint: 22-69% compared with the wild-type) in six variants (Pro555Ser, Arg607Gln, Thr623Ile, Asn909Lys, Pro1150Arg, and Cys1318Tyr); and high activity (CLint: approximately two-fold higher than that in the wild-type) in two variants (Ile703Val and His1221Arg).

CONCLUSION

These results suggest that several single nucleotide polymorphisms in the XO gene are involved in individual variations in XO activity. In addition, such findings will be useful to identify xanthinuria patients.

Beyond TPMT: genetic influences on thiopurine drug responses in inflammatory bowel disease

Azathioprine and 6-mercaptopurine are widely used in the management of inflammatory bowel disease (IBD). However, approximately 25% of IBD patients experience toxicity, and up to 10% show resistance to these thiopurine drugs. The importance of genetic variability in determining thiopurine toxicity was first recognized over 25 years ago with the discovery of the thiopurine S-methyltransferase (TPMT) polymorphism and the occurrence of azathioprine-induced myelosuppression in TPMT-deficient patients. In the intervening period, TPMT has become the foremost example of pharmacogenetics, and TPMT deficiency represents one of the few pharmacogenetic phenomena that have successfully made the transition from the research laboratory to diagnostics. While TPMT activity predicts some cases of myelosuppression, deficiency in this enzyme is neither predictive of other adverse drug reactions, nor resistance to thiopurine therapy. As myelosuppression only accounts for approximately 2.5% of adverse reactions in IBD patients, researchers are increasingly turning their attention to other enzymes involved in thiopurine metabolism to find molecular explanations for intolerance and resistance to azathioprine and 6-mercaptopurine. In this review, we summarize the current state of knowledge with regards to TPMT, and also explore genetic variability, beyond TPMT, that may contribute to thiopurine response in IBD patients.

Characterization of the inosine triphosphatase (ITPA) gene: haplotype structure, haplotype-phenotype correlation and promoter function

Inosine triphosphatase (ITPA) cleaves phosphate residues from inosine triphosphate (ITP) and deoxy ITP (dITP), thereby recovering inosine monophosphate, which is a substrate for further purine nucleotide pathways. Deficient ITPA activity leads to intracellular accumulation of ITP/dITP and would, under thiopurine therapy, lead to accumulation of unusual thio-inosine metabolites (thio-ITP) with the potential for adverse metabolic effects. ITPA is a promising candidate for a more comprehensive understanding of thiopurine pharmacogenetics. We therefore studied the haplotype structure, haplotype-phenotype association, and promoter function of ITPA in a Western European population.ITPA haplotyping was performed based on haplotype tagging SNPs (selected from HapMap data) in healthy 130 controls. Haplotypes were reconstructed, and the haplotype-phenotype association was assessed by haplotype trend regression. A 1.5 kb upstream region and stepwise deletions thereof were tested for promoter activity in reporter gene assays in HepG2 and CCRF-CEM cells. Transcription factor binding (Sp1, Sp3) to the proximal promoter region was studied by chromatin immunoprecipitation. Among haplotypes with a frequency greater than 0.01, we did not find any new low-activity haplotypes besides those carrying 94C>A or IVS2 + 21A>C variants. Two promoter SNPs had no influence on promoter activity. An approximately 200 bp sized GC-rich proximal promoter region was sufficient to fully drive reporter gene activity. Chromatin immunoprecipitation showed binding of Sp1 and Sp3 transcription factors to this region. Only the two haplotypes carrying 94C>A or IVS2 + 21A>C were associated with reduced enzyme activity. The gene promoter is associated with a CpG island and driven by Sp-family transcription factors. There was no evidence for functional promoter SNPs, and it is suggested that only SNPs within the very proximal promoter region (approximately 200 bp) have the potential to be functionally significant.

Thiopurine-methyltransferase and inosine triphosphate pyrophosphatase polymorphism in a liver transplant recipient developing nodular regenerative hyperplasia on low-dose azathioprine

The enzymes thiopurine-methyltransferase (TPMT) and inosine triphosphate pyrophosphatase (ITPA) are involved in thiopurine metabolism. We describe a liver transplant recipient who presented with liver enzyme abnormalities after 78 months of low-dose azathioprine (AZA) therapy (less than 1 mg/kg). No underlying etiology of these abnormalities was identified after extensive analysis including repeated liver biopsy. Fifteen years after transplantation, the patient presented with variceal bleeding, liver biopsy showed nodular regenerative hyperplasia (NRH). TPMT*3C genotype was found in the patient's lymphocytes and heterozygous ITPA (94C>A) genotype was found in both patient and donor liver. These findings further emphasize the importance of pharmacogenetics in predicting NRH and other adverse events during AZA therapy. Furthermore, a high index of suspicion with early detection of NRH is crucial, as improvement seems only to occur in patients with compensated liver disease. Liver biopsy and discontinuation of AZA are recommended in case of liver enzyme abnormalities or signs of portal hypertension.

Pharmacogenomics in gastrointestinal disorders

It is anticipated that unraveling the human genome will have a direct impact on the management of specific diseases. Variations or mutations in genes involved in drug metabolism or disease pathophysiology in gastroenterology and hepatology are expected to have effect on response to therapy. The spectrum of diseases is vast. Thus, we focus this review on clinical pharmacogenetics of inflammatory bowel disease, Helicobacter pylori infections, gastroesophageal reflux disease, irritable bowel syndrome, liver transplantation, and colon cancer. Although only a few genotyping tests are used regularly in clinical practice, we anticipate that in the future there will be more routine use of many of the tests described in this review.

Molecular basis of the antimutagenic activity of the house-cleaning inosine triphosphate pyrophosphatase RdgB from Escherichia coli

Inosine triphosphate pyrophosphatases, which are ubiquitous house-cleaning enzymes, hydrolyze noncanonical nucleoside triphosphates (inosine triphosphate (ITP) and xanthosine triphosphate (XTP)) and prevent the incorporation of hypoxanthine or xanthine into nascent DNA or RNA. Here we present the 1.5-A-resolution crystal structure of the inosine triphosphate pyrophosphatase RdgB from Escherichia coli in a free state and in complex with a substrate (ITP+Ca(2+)) or a product (inosine monophosphate (IMP)). ITP binding to RdgB induced a large displacement of the alpha1 helix, closing the enzyme active site. This positions the conserved Lys13 close to the bridging oxygen between the alpha- and beta-phosphates of the substrate, weakening the P(alpha)-O bond. On the other side of the substrate, the conserved Asp69 is proposed to act as a base coordinating the catalytic water molecule. Our data provide insight into the molecular mechanisms of the substrate selectivity and catalysis of RdgB and other ITPases.

Pharmacogenetic significance of inosine triphosphatase

Inosine triphosphatase (ITPase) is the enzyme that catalyzes the conversion of inosine triphosphate (ITP) and deoxy-inosine triphosphate (dITP) to inosine monophosphate and deoxy-inosine monophosphate, respectively, thereby maintaining low intracellular concentrations of ITP and dITP. Individuals deficient in ITPase activity were first recognized over 30 years ago. For decades, no clinical significance could be attributed to this inborn error of metabolism whatsoever. In recent years, evidence has started to accumulate that polymorphisms in the gene encoding ITPase are associated with potentially severe adverse drug reactions towards the thiopurine drugs azathioprine and 6-mercaptopurine. The pharmacogenetic significance is presently being debated in the literature. However, the present state of knowledge is still insufficient to definitively determine the pharmacogenetic significance of ITPase. This article aims to review the current knowledge on the role of ITPase in thiopurine metabolism.

Meta-analysis: Inosine triphosphate pyrophosphatase polymorphisms and thiopurine toxicity in the treatment of inflammatory bowel disease

BACKGROUND

Thiopurines are widely used for the treatment of inflammatory bowel disease, but are associated with the development of side effects. It has been suggested that the enzyme inosine triphosphate pyrophosphatase (ITPA) plays a role in the digestion of thiopurines and that defective activity resulting from polymorphisms in the inosine triphosphate pyrophosphatase encoding genes may be associated with thiopurine-induced side effects. Current studies are controversial regarding this hypothesis.

AIM

To perform a meta-analysis and gain more insight into a possible correlation between thiopurine-induced side effects and ITPA polymorphisms.

METHODS

We explored Medline for articles on ITPA polymorphisms and thiopurine toxicity. Studies that compared ITPA polymorphism frequencies among thiopurine-tolerant and -intolerant adult inflammatory bowel disease patients were included in this meta-analysis.

RESULTS

Nine published studies investigated associations between ITPA polymorphisms and thiopurine toxicity. Six studies (with 751 patients included) met our inclusion criteria and were processed in the meta-analysis. This analysis demonstrates that the ITPA 94C-->A polymorphism, is not significantly associated with any of the studied side effect parameters.

CONCLUSIONS

This meta-analysis does not prove a correlation between the development of thiopurine toxicity and the ITPA 94C-->A polymorphism. This implies that there is no clinical relevance to determine ITPA polymorphisms in thiopurine-treated patients.

Thiopurine treatment in inflammatory bowel disease: clinical pharmacology and implication of pharmacogenetically guided dosing

This review summarises clinical pharmacological aspects of thiopurines in the treatment of chronic inflammatory bowel disease (IBD). Current knowledge of pharmacogenetically guided dosing is discussed for individualisation of thiopurine therapy, particularly to avoid severe adverse effects. Both azathioprine and mercaptopurine are pro-drugs that undergo extensive metabolism. The catabolic enzyme thiopurine S-methyltransferase (TPMT) is polymorphically expressed, and currently 23 genetic variants have been described. On the basis of an excellent phenotype-genotype correlation for TPMT, genotyping has become a safe and reliable tool for determination of a patient's individual phenotype. Thiopurine-related adverse drug reactions are frequent, ranging from 5% up to 40%, in both a dose-dependent and -independent manner. IBD patients with low TPMT activity are at high risk of developing severe haematotoxicity if pharmacogenetically guided dosing is not performed. Based on several cost-benefit analyses, assessment of TPMT activity is recommended prior to thiopurine therapy in patients with IBD. The underlying mechanisms of azathioprine/mercaptopurine-related hepatotoxicity, pancreatitis and azathioprine intolerance are still unknown. Although the therapeutic response appears to be related to 6-thioguanine nucleotide (6-TGN) concentrations above a threshold of 230-260 pmol per 8 x 10(8) red blood cells, at present therapeutic drug monitoring of 6-TGN can be recommended only to estimate patients' compliance.Drug-drug interactions between azathioprine/mercaptopurine and aminosalicylates, diuretics, NSAIDs, warfarin and infliximab are discussed. The concomitant use of allopurinol without dosage adjustment of azathioprine/mercaptopurine leads to clinically relevant severe haematotoxicity due to elevated thiopurine levels. Several studies indicate that thiopurine therapy in IBD during pregnancy is safe. Thus, azathioprine/mercaptopurine should not be withdrawn in strictly indicated cases of pregnant IBD patients. However, breastfeeding is contraindicated during azathioprine/mercaptopurine therapy. Use of azathioprine/mercaptopurine for induction and maintenance of remission in corticosteroid-dependent or corticosteroid-refractory IBD, particularly Crohn's disease, is evidence based. To improve response rates in thiopurine therapy of IBD, comprehensive analyses including metabolic patterns and genome-wide profiling in patients with azathioprine/mercaptopurine treatment are required to identify novel candidate genes.

HOMOLOGY MODELING AND MOLECULAR DYNAMICS STUDY OF HUMAN INOSINE TRIPHOSPHATE PYROPHOSPHATASE

With homology-modeling techniques, molecular mechanics and molecular dynamics methods, a 3D structure model of the human inosine triphosphatase (ITPase; EC 3.6.1.19) is created and refined. This model is further assessed by Profile-3D and ProStat, which confirm that the refined model is reliable. With this model, a flexible docking study is performed, and the results indicate that Arg178, Lys19 and Glu44 are three important determinant residues in substrate binding because they have prominent interaction energies with ITP and form strong hydrogen bonds with ITP. In addition, we further find that the P32T substitution alters the α-helices of ITPase but the β-sheets are almost not changed, and the mutation induces the interaction energy between ITPase and ITP to increase, which are consistent with the conclusion predicted by Sumi et al.8 The results from the mutagenesis imply that Pro32 is vital for the catalytic activity.

Crystal structure of human inosine triphosphatase. Substrate binding and implication of the inosine triphosphatase deficiency mutation P32T

Inosine triphosphatase (ITPA) is a ubiquitous key regulator of cellular non-canonical nucleotide levels. It breaks down inosine and xanthine nucleotides generated by deamination of purine bases. Its enzymatic action prevents accumulation of ITP and reduces the risk of incorporation of potentially mutagenic inosine nucleotides into nucleic acids. Here we describe the crystal structure of human ITPA in complex with its prime substrate ITP, as well as the apoenzyme at 2.8 and 1.1A, respectively. These structures show for the first time the site of substrate and Mg2+ coordination as well as the conformational changes accompanying substrate binding in this class of enzymes. Enzyme substrate interactions induce an extensive closure of the nucleotide binding grove, resulting in tight interactions with the base that explain the high substrate specificity of ITPA for inosine and xanthine over the canonical nucleotides. One of the dimer contact sites is made up by a loop that is involved in coordinating the metal ion in the active site. We predict that the ITPA deficiency mutation P32T leads to a shift of this loop that results in a disturbed affinity for nucleotides and/or a reduced catalytic activity in both monomers of the physiological dimer.

Pancytopenia due to high 6-methylmercaptopurine levels in a 6-mercaptopurine treated patient with Crohn's disease

In a 23-year-old female with colonic Crohn's disease 6-mercaptopurine 100 mg daily (1.7 mg/kg) was added to mesalamine and prednisolone therapy because of ongoing disease activity. One month later she had fever and a pancytopenia. 6-methylmercaptopurine ribonucleotides levels were extremely elevated (57,000 pmol/8x10(8) red blood cells) and 6-thioguanine nucleotides levels were subtherapeutically (126 pmol/8x10(8) red blood cells). Genotyping showed a wildtype thiopurine S-methyltransferase TPMT(H/H) (*1/*1) genotype and a wildtype inosine triphosphate pyrophosphatase gene. TPMT and inosine triphosphate pyrophosphatase activity were normal. The pancytopenia recovered spontaneously within a few weeks, parallel with decreasing 6-methylmercaptopurine ribonucleotides levels after interrupting 6-mercaptopurine treatment. Epstein-Barrvirus, Cytomegalovirus and Herpesvirus infections were excluded by serology. This is the first report of pancytopenia due to extremely high 6-methylmercaptopurine ribonucleotides levels. No relation was found with the genotype of TPMT and inosine triphosphate pyrophosphatase enzymes, which play key roles in the thiopurine metabolic pathway. Apparently, 6-methylmercaptopurine ribonucleotides metabolites can cause pancytopenia, as was already known for 6-thioguanine nucleotides.

Analysis of ITPA phenotype-genotype correlation in the Bulgarian population revealed a novel gene variant in exon 6

Mutations in the inosine triphosphate pyrophosphohydrolase (ITPA) gene causing enzyme deficiency were shown to have pharmacogenetic implications in azathioprine-induced adverse drug reactions. The distribution of ITPA activity as well as the types and the frequencies of gene variants associated with a lower enzyme activity were determined in healthy volunteers from a Bulgarian population. The ITPA activity was measured in 185 erythrocyte samples by an established high-performance liquid chromatography procedure. All samples were genotyped for 94C > A, IVS2 + 21A > C, and IVS2 + 68T > C/G by real-time polymerase chain reaction with hybridization probes. The ITPA activity ranged from 7.5 to 587.8 micromoL IMP/(g Hb x h) with a median value of 162.9 micromoL IMP/(g Hb x h). The enzyme activity showed significant differences between females and males (P = 0.006) with 17% higher values in men than women. Mutant allele frequencies were 0.038 (94C > A) and 0.130 (IVS2 + 21A > C). Mutations at IVS2 + 68 were not identified. Using a cutoff at 75 micromoL IMP/(g Hb x h) phenotyping detected all heterozygous carriers of 94C > A, two compound heterozygotes, all IVS2 + 21A > C homozygotes and 12.5% of IVS2 + 21A > C heterozygous cases. A novel frameshift mutation 359_366dupTCAGCACC in exon 6 was found in a subject with reduced enzyme activity of 61.2 micromoL IMP/(g Hb x h). The interindividual variability in ITPA activity among the Bulgarian population resembles the distribution of enzyme activity in other whites, although the observed median activity was approximately 25% lower in the Bulgarians [163 vs 219 micromoL IMP/(g Hb x h)]. The most common mutant allele IVS2 + 21A > C showed a similar frequency like in other white populations, whereas the 94C > A mutation was less frequently observed compared with other whites. Heterozygosity for the novel gene variant 359_366dupTCAGCACC was associated with 30% enzyme activity of the wild-type median value. The role of this rare variant for the thiopurine intolerance is not explored. These data further extend the knowledge for ITPA heterogeneity in whites.

Review article: interactions between genotype and response to therapy in inflammatory bowel diseases

BACKGROUND

More than half of the patients with inflammatory bowel diseases are candidates for immunosuppressive therapy. However, even the most effective drugs used in inflammatory bowel disease are only successful in about two-thirds of patients. Adverse events limit their use in a further substantial proportion of patients. Recent research has focussed on the possibility of predicting a drugs' efficacy and/or toxicity by identifying polymorphic variants in the genes encoding enzymes involved in metabolic pathways.

AIM

To highlight recent advances and limitations in the field of pharmacogenetics in inflammatory bowel disease.

RESULTS

Recent pharmacogenetic studies have mainly focussed on immunosuppressive agents including corticosteroids, azathioprine, methotrexate and infliximab. Several polymorphic genes encoding enzymes involved in the metabolism of these drugs have been identified including the inosine triphosphate pyrophosphatase in thiopurine therapy, the methylene tetrahydrofolate reductase in methotrexate therapy and polymorphisms in apoptosis genes in infliximab therapy. However, at the present time, genotyping for the variants of the thiopurine methyltransferase gene, an enzyme important for the metabolism of the thiopurine drugs, is the only useful test in clinical practice.

CONCLUSIONS

Although the field of pharmacogenetics in inflammatory bowel disease is promising most new targets have so far failed to translate into clinical practice. Future pharmaceutical trials should include pharmacogenetic research to test appropriate candidate genes in a prospective manner.

The ITPA c.94C>A and g.IVS2+21A>C sequence variants contribute to missplicing of the ITPA gene

Inosine triphosphate pyrophosphatase (ITPase) catalyzes the conversion of inosine triphosphate (ITP) to the correspondent monophosphate. The ITPA c.94C>A and g.IVS2+21A>C allelic variants are associated with decreased red cell enzyme activity. The ITPA c.94C>A [P32T] sequence variant is associated with an increased risk of adverse drug reactions in patients treated with the thiopurine drug azathioprine. The aim of this study was to explore the molecular mechanisms of ITPase deficiency. ITPA mRNA was extracted from peripheral blood leukocytes (PBL), Epstein-Barr virus transformed lymphoblast cell cultures, reticulocytes, and cultured fibroblast from patients with known ITPA genotypes. ITPA mRNA was reversed transcribed, sequenced and the relative amounts of misspliced transcripts quantitated from three independent experiments. The ITPA g.IVS2+21A>C sequence variant resulted in missplicing of exon 3. The ITPA c.94C>A allelic variant resulted in missplicing of exons 2 and 3 representing, in PBL samples, 61% of the total mRNA expressed in ITPA c.94C>A homozygotes. We proposed that the ITPA c.94C>A allelic variant destroys an exonic splicing silencing (ESS) element in exon 2, resulting in the activation of two nearby upstream 5' splice sites and missplicing of the exons 2 and 3 cassette causing structural changes to the enzyme and contributing to ITPase deficiency.

Clinical application of pharmacogenetics in gastrointestinal diseases

As knowledge of the human genome grows, there will be a direct impact on the management of specific diseases. Within gastroenterology and hepatology, there has been a change in the understanding of how variations or mutations in genes involved in drug metabolism or disease pathophysiology affect response to therapy. This review discusses the application of clinical pharmacogenetics to the following diseases and disorders: inflammatory bowel disease, Helicobacter pylori infections, gastroesophageal reflux disease, irritable bowel syndrome, functional dyspepsia, liver transplantation and colon cancer. Although only a few genotyping tests are regularly used in clinical practice, it is anticipated that studies will propel the routine use of many of the tests described in this review, in the future.

Review article: thiopurines in inflammatory bowel disease

BACKGROUND

In the past 10-20 years, knowledge of both thiopurine pharmacology and -pharmacogenetics has been extended dramatically and used to develop new strategies to improve efficacy and reduce toxicity.

AIM

To review thiopurine efficacy, toxicity, pharmacology, pharmacogenetics, interactions in patients with inflammatory bowel disease. Special attention was paid to new strategies for optimization of pharmacotherapy.

METHODS

To collect relevant scientific articles, a Pubmed search was performed from 1966 through January 2006 with the following key words (MeSH terms preferentially) in multiple combinations: 'azathioprine', '6-mercaptopurine', '6-MP', '6-thioguanine', '6-TG', 'thiopurine(s)', 'metabolites', 'level(s)', 'TDM', 'TMPT', 'ITPA', 'genotype(s)', 'phenotype(s)', 'inflammatory bowel disease', 'Crohn('s) disease', 'ulcerative colitis'.

RESULTS

Strategies for optimization of pharmacotherapy include therapeutic drug monitoring of thiopurine metabolites, geno- or phenotyping crucial enzymes in thiopurine metabolism like thiopurine S-methyltransferase and inosine triphosphate pyrophosphatase, and the use of thioguanine as such.

CONCLUSIONS

Thiopurine S-methyltransferase genotyping and therapeutic drug monitoring are useful instruments for individualizing thiopurine pharmacotherapy of inflammatory bowel disease. Inosine triphosphate pyrophosphatase genotyping may be helpful in case of unexplainable myelotoxicity. In case of azathioprine- or mercaptopurine-intolerance, thioguanine seems a promising alternative. However, more knowledge needs to be gathered about its potential hepatotoxicity.

Measurement of erythrocyte inosine triphosphate pyrophosphohydrolase (ITPA) activity by HPLC and correlation of ITPA genotype-phenotype in a Caucasian population

BACKGROUND

Inosine triphosphate (ITP) pyrophosphohydrolase (ITPA) catalyzes the pyrophosphohydrolysis of ITP/dITP and xanthosine triphosphate to prevent incorporation of unusual nucleotides into RNA and DNA. Important mutations leading to enzyme deficiency are 94C>A and IVS2 + 21A>C. An association between ITPA 94C>A and adverse reactions during azathioprine treatment has been shown. To investigate the ITPA phenotype, an HPLC procedure was developed and phenotype-genotype correlations were assessed.

METHODS

The enzymatic conversion of ITP to inosine monophosphate (IMP) was terminated by perchloric acid and saturated dipotassium hydrogen phosphate. We quantified the IMP at 262 nm after separation on an Aqua perfect C18 column using 20 mmol/L phosphate buffer, pH 2.5. We also genotyped samples for ITPA 94C>A and IVS2 + 21A>C by real-time fluorescence PCR.

RESULTS

The assay was linear to 3 mmol/L IMP [approximately 500 micromol/(g Hb x h)] with a lower limit of quantification of 4 micromol/L [approximately 0.5 micromol/(g Hb x h)]. With IMP-enriched samples, within- and between-day imprecision was < or = 3.6% and < or = 4.9%, respectively, and the inaccuracy was < or = 5.2%. With pooled erythrocytes, within- and between-day imprecision was 3.8% and 7.5%, respectively. ITPA activity in 130 healthy controls was between < 0.5 and 408 micromol IMP/(g Hb x h). Mutant allele frequencies were 0.062 (94C>A) and 0.131 (IVS2 + 21A>C). When we used a cutoff of 125 micromol IMP/(g Hb x h), phenotyping detected all 94C>A mutant cases, all 94C>A and IVS2 + 21A>C compound heterozygotes, all IVS2 + 21A>C homozygotes, and 6 of 24 IVS2 + 21A>C heterozygote-only cases. A novel IVS2 + 68T>C mutation was also found.

CONCLUSIONS

The HPLC procedure provides an excellent ITPA phenotype-genotype correlation and led to the discovery of a novel IVS2 + 68T>C mutation. The method could facilitate investigation of the role of ITPA activity for drug toxicity during thiopurine therapy.

Inosine triphosphate pyrophosphatase and thiopurine s-methyltransferase genotypes relationship to azathioprine-induced myelosuppression

BACKGROUND & AIMS

The use of azathioprine (AZA) in inflammatory bowel disease (IBD) patients is limited by toxicity, which occurs in up to 20% of treated patients. Mutations in the thiopurine S-methyltransferase (TPMT) and inosine triphosphate pyrophosphatase (ITPA) genes have been associated with the occurrence of AZA-related toxicity. The aim of our study was to determine the relative contribution of ITPA and TPMT mutations to the development of toxicity induced by AZA treatment in IBD patients.

METHODS

ITPA(94C>A, IVS2+21A>C) and TPMT (238G>C, 460G>A, and 719A>G) genotypes were assessed in 262 IBD patients (159 females, 103 males; 67 patients with ulcerative colitis, 195 patients with Crohn's disease) treated with AZA and were correlated with the development of leukopenia and hepatotoxicity.

RESULTS

Leukopenia (leukocyte count, <3.0 x 10(9)/L) was observed in 4.6% of treated patients. The frequencies of mutant ITPA 94C>A and TPMT alleles were significantly higher in the leukopenic population compared with patients without leukopenia (16.7% and 5.4%, respectively, for ITPA 94C>A, and 20.8% and 4%, respectively, for TPMT). Moreover, the ITPA 94C>A and TPMT mutations predicted leukopenia: ITPA 94C>A odds ratio, 3.504; 95% confidence interval, 1.119-10.971 (P = .046); TPMT odds ratio, 6.316; 95% confidence interval, 2.141-18.634 (P = .004). Neither TPMT nor ITPA genotype predicted hepatotoxicity.

CONCLUSIONS

ITPA 94C>A and TPMT polymorphisms are associated with AZA-related leukopenia in IBD patients.

Determination of ITPase activity in erythrocyte lysates obtained for determination of TPMT activity

The indication for the determination of both thiopurine methyltransferase (TPMT) and inosine triphosphate pyrophosphohydrolase is identical (i.e., adverse drug reactions toward mercaptopurines). Therefore, we tested whether or not our standard procedure to prepare erythrocyte lysates for measurement of TPMT activity, which includes treatment with Chelex 100 (a chelating resin), was suitable for the measurement of ITPase activity. It also was tested to see if ITPase activity differs in EDTA and Heparin anti-coagulated blood samples. We found that there was no difference between the ITPase activity in erythrocyte lysates prepared from EDTA or Heparin anti-coagulated blood. Treatment with a chelating resin or omission of magnesium from the assay procedure resulted in decreased and nearly absent ITPase activity, respectively. We conclude that untreated erythrocyte lysates obtained for determination of TPMT activity are suitable for determination of ITPase activity. However, after treatment with Chelex 100 the erythrocyte lysates become unsuitable for determination of ITPase activity.

Pharmacogenomics in inflammatory bowel disease

Inherited variations in the nucleotide sequence of genes influence how individual patients respond to drugs. Most commonly, clinically significant genetic variations consist of single nucleotide polymorphisms (SNPs) within genes that affect drug disposition or drug targets. Up to now, relatively few clinically important examples of inherited traits that affect drug responses have been studied in detail. However, one of the well-characterized examples is highly relevant to inflammatory bowel disease therapeutics, that of thiopurine methyltransferase pharmacogenetics. Individuals with 2 normal alleles of the gene encoding thiopurine methyltransferase metabolize and clear thiopurines such as azathioprine and 6-mercaptopurine rapidly. Individuals with 1 normal and 1 variant allele are intermediate, whereas those with 2 variant alleles clear thiopurines very slowly. Intermediate and slow metabolizers are predisposed to have high active thiopurine drug levels and develop bone marrow suppression. Genomic era technology permits determination of large numbers of SNPs in large numbers of individuals. This capability is allowing the field of pharmacogenomics to become one of the most productive interfaces in translational biomedical research at present. By using high-throughput SNP genotyping, combined with careful phenotypic characterization of disease, pharmacogenomic research carries the potential of identifying individual biomarkers that predict the relative likelihood of benefit or risk from a therapeutic intervention. If this promise can be realized, pharmacogenomics will deliver the opportunity for personalized medicine.

Association of inosine triphosphatase 94C>A and thiopurine S-methyltransferase deficiency with adverse events and study drop-outs under azathioprine therapy in a prospective Crohn disease study

BACKGROUND

Azathioprine (aza) therapy is beneficial in the treatment of inflammatory bowel disease, but 10%-30% of patients cannot tolerate aza therapy because of adverse drug reactions. Thiopurine S-methyltransferase (TPMT) deficiency predisposes to myelotoxicity, but its association with other side effects is less clear. Inosine triphosphatase (ITPA) mutations are other pharmacogenetic polymorphisms possibly involved in thiopurine metabolism and tolerance.

METHODS

We analyzed data from a 6-month prospective study including 71 patients with Crohn disease undergoing first-time aza treatment with respect to aza intolerance. Patients were genotyped for common TPMT and ITPA mutations and had pretherapy TPMT activity measured.

RESULTS

Early drop-out (within 2 weeks) from aza therapy was associated with ITPA 94C > A [P = 0.020; odds ratio (OR), 4.6; 95% confidence interval (95% CI), 1.2-17.4] and low TPMT activity [<10 nmol/(mL erythrocytes . h); P = 0.007; OR = 5.5; 95% CI, 1.6-19.2]. A high-risk group defined by ITPA 94C > A or TPMT <10 nmol/(mL erythrocytes . h) showed significant association with early drop-out (P = 0.001; OR = 11.3; 95% CI, 2.5-50.0) and all drop-outs (P = 0.002; OR = 4.8; 95% CI, 1.8-13.3). For only drop-outs attributable to aza-related side effects (n = 16), there was a significant association with ITPA 94C > A (P = 0.002; OR = 7.8; 95% CI, 2.1-29.1). Time-to-event analysis over the 24-week study period revealed a significant association (P = 0.031) between the time to drop-out and ITPA 94C > A mutant allele carrier status.

CONCLUSIONS

Patients with ITPA 94C > A mutations or low TPMT activity constitute a pharmacogenetic high-risk group for drop-out from aza therapy. ITPA 94C>A appears to be a promising marker indicating predisposition to aza intolerance.

The pharmacokinetic effect of discontinuation of mesalazine on mercaptopurine metabolite levels in inflammatory bowel disease patients

BACKGROUND

In vitro studies suggest interactions between mesalazine (mesalamine) and thiopurines by thiopurine S-methyltransferase (TPMT) inhibition, influencing the balance of hepatotoxic 6-methylmercaptopurine ribonucleotide and immunosuppressive tioguanine (thioguanine) metabolites.

AIM

To examine the in vivo pharmacokinetic interaction between mesalazine and mercaptopurine.

METHODS

A prospective study was performed in quiescent inflammatory bowel disease patients using the combination of mercaptopurine and mesalazine. Laboratory parameters, 6-methylmercaptopurine ribonucleotide and tioguanine levels and thiopurine S-methyltransferase activity in erythrocytes were measured at stable medication, after mesalazine discontinuation and mesalazine reintroduction, further mercaptopurine was continued.

RESULTS

Seventeen patients were participated. Mean mercaptopurine dose was 0.78 mg/kg/day and median of mesalazine dose was 3000 mg/day. After mesalazine discontinuation, mean tioguanine levels changed significantly from 262 to 209 pmol/8 x 10(8) red blood cell, increasing to 270 after reintroduction. Mean 6-methylmercaptopurine ribonucleotide levels were 1422, 2149 and 1503 pmol/8 x 10(8) red blood cell respectively. Mean 6-methylmercaptopurine ribonucleotide/tioguanine ratio increased significantly from 6.3 at baseline to 11.2. Mean baseline thiopurine S-methyltransferase activity was 0.58 pmol/10(6) red blood cell/h and stable. All patients had wild-type thiopurine S-methyltransferase genotypes however, leucocyte counts were stable.

DISCUSSION

A significantly higher tioguanine levels and improving 6-methylmercaptopurine ribonucleotide/tioguanine ratio were found during mesalazine/mercaptopurine combination. Theoretically, mesalazine inhibits thiopurine S-methyltransferase activity. In vivo thiopurine S-methyltransferase activity did not change, however.

CONCLUSION

Mesalazine has synergistic effects on mercaptopurine therapy, but the mechanism is unclear. Combining these drugs may be further indication for mesalazine in inflammatory bowel disease treatment.

Genetic basis of inosine triphosphate pyrophosphohydrolase deficiency in the Japanese population

Inosine triphosphate pyrophosphohydrolase (ITPase) is an enzyme that catalyzes the conversion of inosine triphosphate (ITP) to inosine monophosphate and pyrophosphate. In Caucasian populations it is reported that the frequency of cases showing decreased ITPase activity is 5%. The structure of ITPA gene along with five single nucleotide polymorphisms has been reported in Caucasians. We examined ITPase activity and frequency of two polymorphisms (94C>A and IVS2+21A>C) in 100 Japanese individuals. Among these individuals, we observed that three cases with zero activity were homozygote for 94C>A, and were accompanied by abnormal accumulation of ITP in erythrocytes. The cases included in the low ITPase activity group were heterozygote for 94C>A polymorphism. The activity of the heterozygote cases was approximately 27% of the mean value of the wild type. The allele frequency of the 94C>A polymorphism was 0.155, which was 2.6 times higher than that of the Caucasians (0.06). The IVS2+21A>C was not detected in Japanese cases, although it occurred with a frequency of 0.130 in Caucasians. Furthermore, we identified a novel mutation IVS2+68T>G in intron 2 in the case with the lowest enzyme activity in the 94C>A wild type. Since the frequency of ITPA 94C>A polymorphism is higher in the Japanese population than that in Caucasians, it is more important to examine ITPA 94C>A polymorphism in the Japanese population to prevent thiopurine drug toxicity. Pretherapeutic screening of individuals for ITPA polymorphisms should be considered for safer and more tolerable treatment with thiopurine drugs.

Pharmacogenetic association with adverse drug reactions to azathioprine immunosuppressive therapy following liver transplantation

Azathioprine (AZA) is a thiopurine prodrug commonly used in triple-immunosuppressive therapy following liver transplantation. Approximately 1 in 10 patients suffers side effects in response to the drug, the most problematic being bone marrow toxicity. There is evidence that polymorphisms in the genes encoding thiopurine methyltransferase (TPMT) and inosine triphosphate pyrophosphatase (ITPase) predict adverse drug reactions to AZA therapy. Furthermore, common genetic polymorphisms in the gene encoding methylenetetrahydrofolate reductase (MTHFR) may have an indirect impact on thiopurine drug methylation by influencing levels of the methyl donor S-adenosylmethionine (SAM). The aim of this study was to determine whether polymorphisms in these candidate pharmacogenetic loci predict adverse drug reactions to AZA immunosuppressive therapy in liver transplant patients. A series of 65 liver transplant recipients were recruited to the study from the Liver Transplant Out-Patient clinic at The Royal Infirmary of Edinburgh. Clinical response to AZA was retrospectively correlated against TPMT activity, TPMT*2, *3A, and *3A genotypes, inosine triphosphatase (ITPA) 94C>A and IVS2+21A>C genotypes, and MTHFR 677C>T and 1298A>C genotypes. Variant TPMT, ITPA, and MTHFR genotypes were not significantly associated with adverse drug reactions to AZA, including bone marrow suppression. However, the 2 patients who suffered nodular regenerative hyperplasia (NRH) were both heterozygous for the TPMT*3A mutation. In conclusion, our findings suggest that TPMT, ITPA, and MTHFR genotypes do not predict adverse drug reactions, including bone marrow suppression, in liver transplant patients. However, the possible association between NRH and a heterozygous TPMT genotype should be investigated further.

Lack of association between the ITPA 94C>A polymorphism and adverse effects from azathioprine

A 94C>A missense mutation in the ITPA gene which encodes inosine triphosphate pyrophosphatase has been associated with adverse effects from azathioprine, specifically flu-like symptoms, pancreatitis and rash. We hypothesized that this association may also be present in a larger, population-based group of inflammatory bowel disease patients intolerant of thiopurine drugs. We performed genotyping for this polymorphism and TPMT*2 and TPMT*3 in 73 such patients and 74 patients with inflammatory bowel disease who have tolerated azathioprine. We could not demonstrate a significant association between the ITPA94C>A genotype and any adverse effects (Odds ratio (OR) 1.015, 95% confidence interval (CI) 0.360-2.867, P = 0.593), flu-like symptoms (OR 1.547, 95%CI 0.368-6.496, P = 0.398), rash (no ITPA 94C>A polymorphism identified) or pancreatitis (no ITPA 94C>A polymorphism identified). We found no significant association between the ITPA 94C>A polymorphism and adverse effects to thiopurine drugs.

Allele frequency of inosine triphosphate pyrophosphatase gene polymorphisms in a Japanese population

The enzyme inosine triphosphate pyrophosphatase (ITPase) catalyses the pyrophosphohydrolysis of ITP to IMP. ITPase deficiency is a clinically benign autosomal recessive condition characterised by the abnormal accumulation of ITP in erythrocytes. A deficiency of ITPase may predict adverse reactions to therapy with the thiopurine drug 6-mercaptopurine and its prodrug azathioprine. In this study, we examine the frequencies of ITPA polymorphisms in 100 healthy Japanese individuals. The allele frequency of the 94C > A variant in the Japanese sample was 0.135 (Caucasian allele frequency 0.06). The IV2 + 21A > C polymorphism was not found in Japanese (Caucasian allele frequency 0.130). Allele frequencies of the 138G > A, 561G > A and 708G > A polymorphisms were 0.57, 0.18 and 0.06 respectively in the Japanese population, and with the exception of the 138G > A polymorphism, similar to allele frequencies in Caucasians.

Mutation in the ITPA gene predicts intolerance to azathioprine

Inosine triphosphate pyrophosphatase (ITPase) deficiency occurs with polymorphic frequencies in Caucasians and results in the benign accumulation of the inosine nucleotide ITP. In 62 patients treated with azathioprine for inflammatory bowel disease, the ITPA 94C>A deficiency-associated allele was significantly associated with adverse drug reactions (OR 4.2, 95% CI 1.6-11.5, p = 0.0034). Significant associations were found for flu-like symptoms (OR 4.7, 95% CI 1.2-18.1, p = 0.0308), rash (OR 10.3, 95% CI 4.7-62.9, p = 0.0213) and pancreatitis (OR 6.2, CI 1.1-32.6, p = 0.0485). Polymorphism in the ITPA gene thus predicts AZA intolerance. Alternative immunosuppressive drugs, particularly 6-thioguanine, should be considered for AZA-intolerant patients with ITPase deficiency.

Distribution of ITPA P32T alleles in multiple world populations

Dose-limiting toxicity from azathioprine treatment affects up to 37% of patients. Screening for thiopurine methyltransferase (TPMT) polymorphisms will prospectively identify approximately 10% of patients. Recently, a polymorphism in the inosine triphosphate pyrophosphatase gene (ITPA) has been associated with severe azathioprine toxicity. We demonstrate here that this proline to threonine substitution at codon 32 in the ITPA gene is found at low frequency in Central/South American populations (1-2%), at a constant frequency across Caucasian and African populations (6-7%), and is highest in Asian populations (14-19%). This data is consistent with previously described allele frequencies in other Caucasian (7%), African (5%), and Asian (11-15%) populations. This data provides a foundation on which prospective screening studies can be planned to identify patients at risk for severe toxicity from azathioprine therapy.

Adverse drug reactions to azathioprine therapy are associated with polymorphism in the gene encoding inosine triphosphate pyrophosphatase (ITPase)

Adverse drug reactions to azathioprine (AZA), the pro-drug of 6-mercaptopurine (6-MP), occur in 15% to 28% of patients and the majority are not explained by thiopurine methyltransferase (TPMT) deficiency. Inosine triphosphate pyrophosphatase (ITPase) deficiency results in the benign accumulation of the inosine nucleotide ITP. 6-MP is activated through a 6-thio-IMP intermediate and, in ITPase deficient patients, potentially toxic 6-thio-ITP is predicted to accumulate. The association between polymorphism in the ITPA gene and adverse drug reactions to AZA therapy was studied in patients treated for inflammatory bowel disease. Sixty-two patients with inflammatory bowel disease suffering adverse drug reactions to AZA therapy were genotyped for ITPA 94C>A and IVS2 + 21A>C polymorphisms, and TPMT*3A, *3C, *2 polymorphisms. Genotype frequencies were compared to a consecutive series of 68 controls treated with AZA for a minimum of 3 months without adverse effect. The ITPA 94C>A deficiency-associated allele was significantly associated with adverse drug reactions [odds ratio (OR) 4.2, 95% confidence interval (CI) 1.6-11.5, P = 0.0034]. Significant associations were found for flu-like symptoms (OR 4.7, 95% CI 1.2-18.1, P = 0.0308), rash (OR 10.3, 95% CI 4.7-62.9, P = 0.0213) and pancreatitis (OR 6.2,CI 1.1-32.6, P = 0.0485). Overall, heterozygous TPMT genotypes did not predict adverse drug reactions but were significantly associated with a subgroup of patients experiencing nausea and vomiting as the predominant adverse reaction to AZA therapy (OR 5.5, 95% CI 1.4-21.3, P = 0.0206). Polymorphism in the ITPA gene predicts AZA intolerance. Alternative immunosuppressive drugs, particularly 6-thioguanine, should be considered for AZA-intolerant patients with ITPase deficiency.

A Japanese case with inosine triphosphate pyrophosphohydrolase deficiency attributable to an enzymatic defect in white blood cells

Inosine triphosphate pyrophosphohydrolase (ITPase) deficiency is characterized by abnormal accumulation of inosine triphosphate. We describe the first Japanese case with ITPase deficiency and demonstrate that the deficiency of ITPase activity is not only found in erythrocytes but also in white blood cells.