The frequency and distribution of thiopurine methyltransferase alleles in Caucasian and Asian populations

Type 1 autoimmune hepatitis in Turkish patients: absence of association with HLA B8

BACKGROUND

There is very little information about autoimmune hepatitis (AIH) in populations other than of white European or Japanese descent.

GOALS

To investigate the presenting features, immunogenetic background, and response to standard immunosuppressive therapy in native Turkish patients with AIH.

METHODS

Retrospective analysis of all patients referred between 1994 and 2000 to our hepatology clinic in Ankara who fulfilled international criteria for definite or probable type 1 or type 2 AIH.

RESULTS

Seventeen patients (15 female; median age, 31 years; range, 13-56 years) were identified. All had type 1 AIH. Clinical and laboratory features were broadly similar to those reported for white or Japanese patients. Five had the HLA DR3 allotype and 10 had DR4; however, in contrast to white and Japanese patients, DR4 was not associated with an older age at onset. Importantly, no patient had the B8 allotype (vs. 10.9% in 110 healthy Turkish subjects). Thus, none had the classic A1-B8-DR3 autoimmune haplotype: a major distinction from white individuals.

CONCLUSIONS

There appears to be genetic differences in type 1 AIH between Turkish and other populations. Genotyping of Turkish patients to identify alleles that may confer susceptibility or resistance to AIH may progress understanding of the genetic basis of the disease.

Pharmacogenetics in diverse ethnic populations--implications for drug discovery and development

It is widely acknowledged that the vast quantities of data now publicly available as a result of the human genome initiative have the potential to revolutionize the pharmaceutical industry. More tangibly to the drug development business, the dawn of the pharmacogenetics era has the potential to impact not only the discovery of new medicines but also the safety and efficacy of pharmaceutical agents. Coincident with these scientific advances is the emergence of new markets for pharmaceutical agents. Japan, which represents the world's second biggest market, is a good example. With the ICH E5 agreement in 1998 and a rapid change in the drug registration process in Japan, there are increasing opportunities to improve access to more medicines in all parts of the world. However, it is increasingly clear that significant genetic variation still exists between populations, with a host of data on interethnic variation in drug metabolizing enzyme and drug transporter activity. Evidence suggesting that this genetic variation may play an important role in defining some of the interethnic variation in drug response to currently marketed compounds is reviewed here, and future possibilities of using such information to better streamline the drug development process are discussed.

Thiopurine methyltransferase activity: new conditions for reversed-phase high-performance liquid chromatographic assay without extraction and genotypic-phenotypic correlation

Thiopurine methyltransferase (TPMT) is a cytosolic enzyme involved in the metabolism of thiopurine drugs. A genetic polymorphism is responsible for large inter-individual differences observed in TPMT activity. We report a new HPLC technique, which avoids an extraction step and the use of radioactive reagents, based on the conversion of 6-mercaptopurine (6-MP) to 6-methylmercaptopurine (6-MMP) using S-adenosyl-L-methionine (SAM) as methyl donor in red blood cell lysates (RBC). Intra- and inter-assay variation, within-day, within-run, between-day, and between-run variations showed high precision. The formation of 6-MMP was linear with respect to the lysate concentration and time. In a blinded assay of 61 samples, the results of HPLC method correlated with those of the radiochemical method (r2=0.82, P<0.0001). Using a cut-off point of 8.5 nmol/h/ml packed RBC, positive predictive value of HPLC was 100% for heterozygous patients. Because of the absence of extraction step, this new HPLC technique of TPMT activity determination reduces analysis variation and is time-saving. This rapid, sensitive, and reproducible method is suitable for routine monitoring of TPMT activity and for fundamental studies.

Thiopurine methyltransferase polymorphisms in a multiracial asian population and children with acute lymphoblastic leukemia

The purpose of this study was to determine the frequency of thiopurine methyltransferase (TPMT) polymorphisms in a multiracial Asian population and to assess its relevance in the management of childhood acute lymphoblastic leukemia (ALL). Six hundred unrelated cord blood samples from 200 Chinese, Malay, and Indian healthy newborns were collected at the National University Hospital, Singapore; an additional 100 children with ALL were analyzed for five of the commonly reported TPMT variant alleles using polymerase chain reaction/restriction fragment length polymorphism and allele-specific polymerase chain reaction-based assays. In the cord blood study, the TPMT*3C variant was detected in all three ethnic groups; Chinese, Malays, and Indians had allele frequencies of 3%, 2.3%, and 0.8%, respectively. The TPMT*3A variant was found only among the Indians at a low allele frequency of 0.5%. The TPMT*6 variant was found in one Malay sample. Among the children with ALL, two white and one Chinese were heterozygous for the TPMT*3A variant and showed intermediate sensitivity to 6-mercaptopurine during maintenance therapy. Three Chinese patients and one Malay patient were heterozygous for the TPMT*3C variant. Mercaptopurine sensitivity could be validated in only one out of four TPMT*3C heterozygous patients. The overall allele frequency of the TPMT variants in this multiracial population was 2.5%. The TPMT*3C was the most common variant allele; TPMT*3A and TPMT*6 were rare. These results support the feasibility of performing TPMT genotyping in all children diagnosed with acute leukemia to minimize toxicity from thiopurine chemotherapy.

Recent advances in the pharmacogenomics of thiopurine methyltransferase

The thiopurine drugs (6-mercaptopurine, 6-thioguanine and azathioprine) are commonly used cytotoxic agents and immunosuppressants. One important route for the metabolism of these agents is methylation, mediated by thiopurine methyltransferase (TPMT). It is now well established that inter-individual variation in sensitivity to thiopurines can be due to the presence of common genetic polymorphisms affecting the TPMT gene. More recently variations in the number of tandem repeats in the 5' promoter region have been shown to influence TPMT expression in vitro. In this article, we review recent advances in the understanding of the range of inter-individual variation that may be involved in the open reading frame or promoter region of the TPMT gene. We also review the data which have been published regarding the influence such variations may have on both the clinical efficacy and toxicity of the thiopurine drugs.

Molecular analysis of thiopurine S-methyltransferase alleles in South-east Asian populations

Thiopurine S-methyltransferase (TPMT) catalyses the S-methylation of thiopurine drugs. In Caucasians, four variant TPMT alleles have been detected in over 80% of individuals with low or intermediate TPMT activity. The wild-type allele is designated as TPMT*1 and the mutant alleles are designated TPMT*2 through TPMT*8. The frequency of these alleles in different ethnic groups has not been well defined. In this study, one hundred individuals, from each of the Indonesian, Thai and Philippine populations, together with 249 Taiwanese, were analysed by polymerase chain reaction-restriction fragment length polymorphism and direct sequencing methods. The results showed that the allelic frequencies of TPMT*3C were 0.6% for Taiwanese and 1% for Filipino, Thai and Indonesian populations, respectively. One Filipino with a Caucasian parent was found to be heterozygous for the TPMT*2 allele. This study provides the first analysis of the allele frequency distribution of all known TPMT mutations in South-east Asian populations.

The thiopurine S-methyltransferase gene locus -- implications for clinical pharmacogenomics

Thiopurine methyltransferase catalyzes the S-methylation of azathioprine (AZA), 6-mercapto-purine (6-MP) and thioguanine, medications widely used to treat malignancies, rheumatic diseases, dermatologic conditions, inflammatory bowel disease and solid organ transplant rejection. TPMT activity 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, including fatal myelosuppression, after receiving standard doses of thiopurine medications. The molecular basis for low TPMT activity has been elucidated, leading to the development of assays for the three signature mutations, which account for the majority of mutant alleles. TPMT genotype is correlated with erythrocyte and leukemia blast cell TPMT activity and associated with a risk of toxicity after thiopurine therapy. Recent studies defined target starting doses for mercaptopurine based on TPMT genotypes. This polymorphism is one of the best models for the translation of genomic information to guide patient therapeutics.

Pharmacogenomics: unlocking the human genome for better drug therapy

There is great heterogeneity in the way humans respond to medications, often requiring empirical strategies to find the appropriate drug therapy for each patient (the "art" of medicine). Over the past 50 years, there has been great progress in understanding the molecular basis of drug action and in elucidating genetic determinants of disease pathogenesis and drug response. Pharmacogenomics is the burgeoning field of investigation that aims to further elucidate the inherited nature of interindividual differences in drug disposition and effects, with the ultimate goal of providing a stronger scientific basis for selecting the optimal drug therapy and dosages for each patient. These genetic insights should also lead to mechanism-based approaches to the discovery and development of new medications. This review highlights the current status of work in this field and addresses strategies that hold promise for future advances in pharmacogenomics.

Frequencies of thiopurine S-methyltransferase mutant alleles (TPMT*2, *3A, *3B and *3C) in 151 healthy Japanese subjects and the inheritance of TPMT*3C in the family of a propositus

AIMS

To determine the frequencies of four thiopurine S-methyltransferase (TPMT) mutant alleles, TPMT*2, *3A, *3B and *3C in a normal Japanese population.

METHODS

Genotypes were determined in 151 Japanese subjects and in six family members of a propositus using polymerase chain reaction (PCR)-restriction fragment length polymorphism and allele-specific PCR assays.

RESULTS

Only one TPMT*3C heterozygote was identified (gene frequency 0.3%). TPMT*2, *3A and *3B were not detected. In addition, TPMT*3C was found to have been inherited from the mother and passed on to the son of the propositus.

CONCLUSIONS

TPMT*3C appears to be most prevalent among the known mutant allele of TPMT in a Japanese population which may have some relevance for the treatment of Japanese patients with thiopurine drugs.

Preponderance of thiopurine S-methyltransferase deficiency and heterozygosity among patients intolerant to mercaptopurine or azathioprine

PURPOSE

To assess thiopurine S-methyltransferase (TPMT) phenotype and genotype in patients who were intolerant to treatment with mercaptopurine (MP) or azathioprine (AZA), and to evaluate their clinical management.

PATIENTS AND METHODS

TPMT phenotype and thiopurine metabolism were assessed in all patients referred between 1994 and 1999 for evaluation of excessive toxicity while receiving MP or AZA. TPMT activity was measured by radiochemical analysis, TPMT genotype was determined by mutation-specific polymerase chain reaction restriction fragment length polymorphism analyses for the TPMT*2, *3A, *3B, and *3C alleles, and thiopurine metabolites were measured by high-performance liquid chromatography.

RESULTS

Of 23 patients evaluated, six had TPMT deficiency (activity < 5 U/mL of packed RBCs [pRBCs]; homozygous mutant), nine had intermediate TPMT activity (5 to 13 U/mL of pRBCs; heterozygotes), and eight had high TPMT activity (> 13.5 U/mL of pRBCs; homozygous wildtype). The 65.2% frequency of TPMT-deficient and heterozygous individuals among these toxic patients is significantly greater than the expected 10% frequency in the general population (P <.001, chi(2)). TPMT phenotype and genotype were concordant in all TPMT-deficient and all homozygous-wildtype patients, whereas five patients with heterozygous phenotypes did not have a TPMT mutation detected. Before thiopurine dosage adjustments, TPMT-deficient patients experienced more frequent hospitalization, more platelet transfusions, and more missed doses of chemotherapy. Hematologic toxicity occurred in more than 90% of patients, whereas hepatotoxicity occurred in six patients (26%). Both patients who presented with only hepatic toxicity had a homozygous-wildtype TPMT phenotype. After adjustment of thiopurine dosages, the TPMT-deficient and heterozygous patients tolerated therapy without acute toxicity.

CONCLUSION

There is a significant (> six-fold) overrepresentation of TPMT deficiency or heterozygosity among patients developing dose-limiting hematopoietic toxicity from therapy containing thiopurines. However, with appropriate dosage adjustments, TPMT-deficient and heterozygous patients can be treated with thiopurines, without acute dose-limiting toxicity.

Allelotype frequency of the thiopurine methyltransferase (TPMT) gene in Japanese

Polymorphisms at three loci in the thiopurine methyltransferase (TPMT) gene are known to be responsible for azathioprine and 6-mercaptopurine (6MP) toxicity. Among them, only TPMT*3C variant allele with A719G mutation was found in 15/522 (2.9%; 17/1044 alleles; 1.6%) Japanese individuals including two homozygotes. The allele frequency was different from that in Caucasians, and investigation of TPMT polymorphisms with consideration of ethnic differences before administration of azathioprine or 6MP may provide clinically useful information.

Pharmacogenetics: a tool for individualizing antineoplastic therapy

This article reviews the clinical relevance of pharmacogenetics in cancer chemotherapy, with emphasis on drugs for which genetic differences in enzyme metabolism have been demonstrated to affect patient outcome. About 10% of children with leukaemia are intolerant to mercaptopurine (6-mercaptopurine) because of genetic defects in mercaptopurine inactivation by thiopurine S-methyltransferase. However, mercaptopurine dose intensity, a critical factor for outcome in patients deficient in thiopurine S-methyltransferase, can be maintained by means of thiopurine S-methyltransferase phenotyping or genotyping. Patients with reduced fluorouracil (5-fluorouracil) catabolism are more likely to be exposed to severe toxicity. The measurement of dihydropyrimidine dehydrogenase activity in patients cannot be considered fully predictive, and the role of dihydropyrimidine dehydrogenase gene variants in this syndrome has yet to be clarified. With regard to irinotecan, patients with Gilbert's syndrome phenotype have reduced inactivation of the active topoisomerase I inhibitor 7-ethyl-10-hydroxycamptothecin (SN-38) caused by a mutation in the UDP-glucuronosyltransferase 1A1 gene promoter. This subset of patients is more likely to be exposed to irinotecan toxicity and could be identified by genotyping for gene promoter variants. Finally, the experience with amonafide represents a model for dose individualization approaches that use simple phenotypic probes.

Evaluation of dihydropyrimidine dehydrogenase activity in South-west Asian, Kenyan and Ghanaian populations

AIMS

Dihydropyrimidine dehydrogenase (DPD) reduces endogenous pyrimidines and therapeutic analogues such as the anticancer agent 5-fluorouracil (5FU). Among Caucasian populations DPD activity is highly variable and subject to polymorphic regulation. To evaluate interethnic influence, DPD activity was assessed in South-west Asian, Kenyan and Ghanaian populations.

METHODS

DPD activity was determined in peripheral mononuclear cells using[14C]-5-fluorouracil and h.p.l.c. analysis.

RESULTS

A high degree of variation in DPD activity was observed within each population (range CV = 34-48%). Median DPD activity also varied between these populations. South-west Asian and Kenyan subjects exhibited almost identical median values (192 and 193.5 pmol min(-1) mg(-1), respectively), which were similar to Caucasians (median 215 pmol min(-1) mg(-1). A significantly lower median DPD activity (119 pmol min(-1) mg(-1)) was observed in the Ghanaian population.

CONCLUSIONS

The similarity in DPD activity between Caucasian, Kenyan and South-west Asian populations suggests that the incidence of 5FU-related toxicity may be comparable in these groups. The pharmacokinetic implications of lower activity amongst Ghanaians needs to be evaluated.

Genetic polymorphism of thiopurine methyltransferase and its clinical relevance for childhood acute lymphoblastic leukemia

Thiopurine methyltransferase (TPMT) catalyses the S-methylation of thiopurines, including 6-mercaptopurine and 6-thioguanine. TPMT activity exhibits genetic polymorphism, with about 1/300 inheriting TPMT deficiency as an autosomal recessive trait. If treated with standard doses of thiopurines, TPMTdeficient patients accumulate excessive thioguanine nucleotides in hematopoietic tissues, leading to severe hematological toxicity that can be fatal. However, TPMT-deficient patients can be successfully treated with a 10- to 15-fold lower dosage of these medications. The molecular basis for altered TPMT activity has been defined, with rapid and inexpensive assays available for the three signature mutations which account for the majority of mutant alleles. TPMT genotype correlates well with in vivo enzyme activity within erythrocytes and leukemic blast cells and is clearly associated with risk of toxicity. The impact of 6-mercaptopurine dose intensity is also being clarified as an important determinate of event-free survival in childhood leukemia. In addition, there are emerging data that TPMT genotype may influence the risk of secondary malignancies, including brain tumors and acute myelogenous leukemia. Ongoing studies aim to clarify the influence of TPMT on thiopurine efficacy, acute toxicity, and risk for delayed toxicity. Together, these advances hold the promise of improving the safety and efficacy of thiopurine therapy.

Genetic analysis of thiopurine methyltransferase polymorphism in a Japanese population

Thiopurine methyltransferase (TPMT) catalyses the S-methylation of thiopurine drugs such as 6-mercaptopurine, 6-thioguanine, and azathiopurine. Several mutations in the TPMT gene have been identified which correlate with a low activity phenotype. The molecular basis for the genetic polymorphism of TPMT has been established for European Caucasians, African-Americans, Southwest Asians and Chinese, but it remains to be elucidated in Japanese populations. 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 Japanese samples (n=192) using polymerase chain reaction (PCR)-RFLP and allele-specific PCR-based assays. TPMT*3C was found in 0.8% of the samples (three heterozygotes). The TPMT*2, TPMT*3A and TPMT*3B alleles were not detected in any of the samples analyzed. This study provides the first analysis of TPMT mutant allele frequency in a sample of Japanese population and indicates that TPMT*3C is the most common allele in Japanese subjects.

Mercaptopurine therapy intolerance and heterozygosity at the thiopurine S-methyltransferase gene locus

BACKGROUND

Patients with acute lymphoblastic leukemia are often treated with 6-mercaptopurine, and those with homozygous deficiency in thiopurine S-methyltransferase (TPMT) enzyme activity have an extreme sensitivity to this drug as a result of the accumulation of higher cellular concentrations of thioguanine nucleotides. We studied the metabolism, dose requirements, and tolerance of 6-mercaptopurine among patients with different TPMT phenotypes.

METHODS

We compared, by use of statistical modeling, 6-mercaptopurine pharmacology and tolerance in 180 patients who achieved remission on St. Jude Children's Research Hospital Protocol Total XII composed of weekly methotrexate (40 mg/m(2)) and daily oral 6-mercaptopurine (75 mg/m(2)) given for 2.5 years, interrupted every 6 weeks during the first year for treatment with either high-dose methotrexate or teniposide plus cytarabine. Statistical tests were two-sided.

RESULTS

Erythrocyte concentrations of thioguanine nucleotides (pmol/8 x 10(8) erythrocytes) were inversely related to TPMT enzyme activity (P<.01), with averages (+/- standard deviations) of 417 (+/-179), 963 (+/-752), and 3565 (+/-1282) in TPMT homozygous wild-type (n = 161), heterozygous (n = 17), and homozygous-deficient (n = 2) patients, respectively. There was complete concordance between TPMT genotype and phenotype in a subset of 28 patients for whom TPMT genotype was determined. There were no sex differences in thioguanine nucleotide concentrations (P =.24), TPMT enzyme activity (P =.22), or average weekly prescribed dose of 6-mercaptopurine (P=.49). The cumulative incidence of 6-mercaptopurine dose reductions due to toxicity was highest among patients homozygous for mutant TPMT (100%), intermediate among heterozygous patients (35%), and lowest among wild-type patients (7%) (P<.001), with average (+/- standard deviation) final weekly 6-mercaptopurine doses of 72 (+/-60), 449 (+/-160), and 528 (+/-90) mg/m(2), respectively. Lowering doses of 6-mercaptopurine in TPMT heterozygotes and in deficient patients allowed administration of full protocol doses of other chemotherapy while maintaining high thioguanine nucleotide concentrations.

CONCLUSION

We conclude that genetic polymorphism in TPMT is an important determinant of mercaptopurine toxicity, even among patients who are heterozygous for this trait.

Ethnic variation in the thymidylate synthase enhancer region polymorphism among Caucasian and Asian populations

Thymidylate synthase (TS) regulates the production of DNA synthesis precursors and is an important target of cancer chemotherapy. A tandem repeat sequence in a TS promoter enhancer region (TSER) was recently identified. Polymorphic variation affected in vitro expression levels of the gene. We evaluated the influence of ethnicity on TSER genotype. Allele frequency was similar in Caucasian and Southwest Asian subjects. However, homozygous triple repeat subjects were twice as common in Chinese subjects (67%) than in Caucasian subjects (38%). This demonstrates significant ethnic variation in a TS gene regulatory element which may have significant impact on pyrimidine homeostasis and drug therapy.

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.

Pharmacogenetics as a molecular basis for individualized drug therapy: the thiopurine S-methyltransferase paradigm

Genetic polymorphism of drug metabolizing enzymes can be the major determinant of inter-individual differences in drug disposition and effects. In this mini-review, the evolution of pharmacogenetic studies, from the recognition of phenotypic polymorphisms to the discovery of genetic mutations responsible for these inherited traits, is illustrated by the genetic polymorphism of thiopurine S-methyltransferase (TPMT). TPMT, which exhibits autosomal co-dominant polymorphism, plays an important role in metabolism of the antileukemic and immunosuppressive medications, mercaptopurine, thioguanine, and azathioprine. The genetic polymorphism of TPMT activity in humans was first reported in 1980, and in the last five years the genetic basis for this polymorphism has been elucidated. Isolation and cloning of mutant alleles from humans with TPMT deficiency has identified the major mutant alleles, established the basis for loss of TPMT activity and permitted development of PCR-based genotyping assays to make a molecular diagnosis of TPMT-deficiency and heterozygosity. These studies illustrate the potential clinical benefits of elucidating the molecular basis of inherited differences in drug metabolism and disposition, and future automation of molecular diagnostics will make it feasible to more precisely select the optimal drug and dosage for individual patients.