Enhanced proteolysis of thiopurine S-methyltransferase (TPMT) encoded by mutant alleles in humans (TPMT*3A, TPMT*2): mechanisms for the genetic polymorphism of TPMT activity

Impact of thiopurine methyltransferase activity and 6-thioguanine nucleotide concentrations in patients with chronic inflammatory diseases

As azathioprine is one of the standard immunosuppressive drugs used for treatment of patients with different chronic inflammatory diseases, the effect of the azathioprine metabolizing enzyme thiopurine methyltransferase (TPMT) activity on incidence of adverse events (AE) was examined. In addition, potential correlations between the concentration of the azathioprine metabolite 6-thioguanine nucleotide (6-TGN) in erythrocytes (RBC) and inflammatory disease activity as well as hematological AE were investigated. TPMT activities were investigated prospectively in 139 patients (35 male, 104 female) with chronic inflammatory diseases [systemic lupus erythematosus (SLE, 38), progressive systemic sclerosis (PSS, 13), Wegener's granulomatosis (4), rheumatoid arthritis (RA, 5), and other chronic inflammatory diseases (79)]. In addition, 6-TGN concentrations were investigated in a second cohort of 58 patients (17 patients with SLE, 5 with PSS, 5 with vasculitides, 4 with undifferentiated connective tissue diseases, 1 with dermatomyositis, 1 with Sjögren's syndrome, 1 with RA, 20 with Crohn's disease, and 4 with ulcerative colitis) prior to and during therapy with azathioprine. The distribution of activities of TPMT in 139 patients showed a normal Gaussian distribution in the Caucasian population. Within the group of 96 patients taking azathioprine, known azathioprine-related AE could be observed: minor AE (sickness, rash, and increase in cholestasis parameters) in 11 patients (11.4%), and severe AE (bone marrow toxicity) in 7 patients (7.3%). Below a "cutoff" value of 11.9 nmol/mL RBC x h of TPMT activity, AE were significantly more frequent. In the second cohort of patients, no significant correlations could be observed between 6-TGN concentrations and parameters of disease activity. Reduced activity of TPMT in patients with chronic inflammatory diseases requiring immunosuppressive therapy with azathioprine, especially below a distinct cutoff, appears to inherit a substantial risk for development of AE.

Genetic variants and the risk of Crohn's disease: what does it mean for future disease management?

Genetic research in inflammatory bowel disease, especially in Crohn's disease, has made significant progress during recent years. There have been > 10 total genome scans that have been performed, and susceptibility loci on several chromosomes have been identified. Together with candidate gene studies, these scans have led to the identification of several susceptibility genes, with CARD15 being the most important. These genetic data have already provided important insights into the pathophysiology of inflammatory bowel disease and are stimulating future research. On the other hand, genotype-phenotype associations have illustrated the heterogenic nature of the disease. Although the clinical application of this knowledge is so far limited, there is significant optimism that an individual management of patients based on genetic data will be possible in the near future.

TPMT, UGT1A1 and DPYD: genotyping to ensure safer cancer therapy?

The Food and Drug Administration (FDA) has approved label changes for two anticancer drugs, 6-mercaptopurine (6-MP) and irinotecan, to include pharmacogenetic testing as a potential means to reduce the rate of severe toxic events. Comprehensive evaluation of the clinical benefit and cost effectiveness of screening strategies with these tests has not been completed. However, the FDA decided that evidence indicates sufficient benefit to warrant informing prescribers, pharmacists and patients of the availability of pharmacogenetic tests and their possible role in the selection and dosing of these anticancer agents. Reviewing the gene-drug-phenotype relationships of 6-MP, irinotecan and 5-fluorouracil reveals properties of these relationships that lead to a clinically useful pharmacogenetic test. Research in the near future should clarify the role of pharmacogenetic testing in reducing the risk of severe toxicity and determine how these same tests might identify a subset of patients who should safely receive higher doses of treatment to derive the same benefit as the rest of the patient population.

Thiopurine S-Methyltransferase Pharmacogenetics: Genotype to Phenotype Correlation in the Slovenian Population

The toxicity of thiopurine drugs has been correlated to the activity of thiopurine S-methyltransferase (TPMT), whose interindividual variation is a consequence of genetic polymorphisms. We have herein investigated the relevance of some genetic markers for the prediction of thiopurine-related toxicities and to determine the genotype to phenotype correlation in the Slovenian population. The most prevalent mutant allele in the Slovenian population is TPMT*3A (4.1%), followed by TPMT*3C (0.5) and TPMT*3B (0.3), while the TPMT*2 allele was not found in any of the examined samples. TPMT enzyme activity distribution in the subgroup sample was bimodal and as such correlated with genetic data. Using a cutoff value of 9.82 pmol/10(7) RBC per h, the genetic data correctly predicted TPMT enzyme activity in 91.6% of the examined individuals. Pharmacogenetic TPMT analyses have therefore proved to have significant clinical implications for prediction of individuals' responses to treatment with thiopurine drugs in order to avoid possible life-threatening therapy-related toxicities.

The role of pharmacogenetics in cancer therapeutics

The variability in treatment responses and narrow therapeutic index of anticancer drugs are some of the key challenges oncologists face. The knowledge of pharmacogenetics can potentially aid in the discovery, development and ultimately individualization of anticancer drugs. The identification of genetic variations that predict for drug response is the first step towards the translation of pharmacogenetics into clinical practice. This review provides an update on the results of studies assessing the effects of germline polymorphisms and somatic mutations on therapeutic outcomes and highlights the potential applications and future challenges in pharmacogenetic research pertaining to cancer therapeutics.

Sulfotransferase (SULT) 1A1 polymorphic variants *1, *2, and *3 are associated with altered enzymatic activity, cellular phenotype, and protein degradation

The superfamily of sulfotransferase (SULT) enzymes catalyzes the sulfate conjugation of several pharmacologically important endo- and xenobiotics. SULT1A1 catalyzes the sulfation of small planar phenols such as neurotransmitters, steroid hormones, acetaminophen, and p-nitrophenol (PNP). Genetic polymorphisms in the human SULT1A1 gene define three alleles, SULT1A1*1, *2, and *3. The enzyme activities of the SULT1A1 allozymes were studied with a variety of substrates, including PNP, 17beta-estradiol, 2-methoxyestradiol, catecholestrogens, the antiestrogen 4-hydroxytamoxifen (OHT), and dietary flavonoids. Using purified recombinant SULT1A1 protein, marked differences in *1, *2, and *3 activity toward every substrate studied were noted. Substrate inhibition was observed for most substrates. In general, the trend in V(max) estimates was *1 > *3 > *2; however, V(max)/K(m) estimate trends varied with substrate. In MCF-7 cells stably expressing either SULT1A1*1 or *2, the antiestrogenic response to OHT was found to be allele-specific: the cells expressing *2 exhibited a better antiproliferative response. The intracellular stability of the *1 and *2 allozymes was examined in insect as well as mammalian cells. The SULT1A1*2 protein had a shorter half-life than the *1 protein. In addition, the *2 protein was ubiquitinated to a greater extent than *1, suggesting increased degradation via a proteasome pathway. The results of this study suggest marked differences in activity of polymorphic SULT1A1 variants, including SULT1A1*3, toward a variety of substrates. These differences are potentially critical for understanding interindividual variability in drug response and toxicity, as well as cancer risk and incidence.

Thiopurine S-methyltransferase pharmacogenetics: insights, challenges and future directions

The thiopurine S-methyltransferase (TPMT) genetic polymorphism is one of the most 'mature' examples in pharmacogenetics. That is true because of its importance clinically for the individualization of thiopurine drug therapy and also because TPMT has provided novel insights into molecular mechanisms responsible for the functional effects of common genetic polymorphisms. This review will summarize the development of our understanding of the role of inheritance in the regulation of TPMT as well as the clinical implications of that genetic regulation. It will also summarize recent studies in which TPMT pharmacogenetics has enhanced our understanding of molecular mechanisms by which common polymorphisms influence or alter function. TPMT pharmacogenetics highlights the potential clinical importance of the translation of pharmacogenetics from bench to bedside, the potential for basic pharmacogenetic research to provide insight into mechanisms by which genetic polymorphisms can alter function, and the challenges associated with the achievement of both of those goals.

Human methylenetetrahydrofolate reductase pharmacogenomics: gene resequencing and functional genomics

5,10-Methylenetetrahydrofolate reductase (MTHFR) is an important enzyme in the folate metabolic pathway. Common genetic polymorphisms in the human MTHFR gene are associated with individual variation in the efficacy and toxicity of chemotherapeutic agents, such as methotrexate and 5-fluorouracil. However, the full range of polymorphisms and intragene haplotypes in the human MTHFR gene remains unclear. Furthermore, cellular mechanisms by which common, naturally occurring nonsynonymous coding single nucleotide polymorphisms (cSNPs) might alter the function of this enzyme have not been defined. The present study focused on the systematic identification and investigation of common polymorphisms and haplotypes in the MTHFR gene using a genotype-to-phenotype strategy, followed by functional genomic studies. Specifically, we resequenced exons, splice junctions and portions of the 5'-flanking region (5'-FR) of the human MTHFR gene using 240 DNA samples from four ethnic groups. A total of 65 polymorphisms were observed, 11 of which were nonsynonymous cSNPs. We then performed functional genomic studies with constructs for wild-type and 15 variant allozymes (some with multiple alterations in amino acid sequence) using a mammalian expression system. Activity for the variant allozymes ranged from 13% to 149% of wild-type activity. Levels of immunoreactive protein for the allozymes ranged from 31% to 120% of wild-type and were significantly correlated with enzyme activity (Rp=0.85, P<0.0001), suggesting that a major mechanism by which nonsynonymous cSNPs influence the function of this gene is by alteration in the quantity of protein. These observations represent steps towards an understanding of molecular genetic mechanisms responsible for variation in MTHFR function that may contribute to individual differences in drug efficacy and toxicity, as well as disease risk.

Introducing a fast and simple PCR-RFLP analysis for the detection of mutant thiopurine S-methyltransferase alleles TPMT*3A and TPMT*3C

BACKGROUND

Azathioprine, in combination with corticosteroids, is the first-line therapy of severe forms of pemphigus vulgaris. Patients with an impaired thiopurine S-methyltransferase (TPMT) activity are at risk of developing severe myelo-suppression upon treatment with thiopurines such as azathioprine. Analysis of the TPMT status prior to drug administration is therefore highly recommended. However, because of the limited availability of TPMT testing outside of specialized centres, pre-emptive TPMT testing is not widespread. To avoid laborious biochemical and sequencing assays, we evaluated a new restriction fragment length polymorphism (RFLP) analysis.

METHODS

We designed a rapid genetic polymerase chain reaction (PCR)-RFLP screen for the most prevalent mutant TPMT*3A and TPMT*3C alleles that are known to result in reduced TPMT enzyme activity.

RESULTS

Validating our fast system on 871 Caucasian DNA samples, we observed that 8.61% of our probands carried the TPMT*3A allele and 0.23% were heterozygous for the TPMT*3C allele, which is in concordance with previously reported allele frequencies.

CONCLUSION

This simple and low-cost PCR-RFLP TPMT polymorphism testing approach can be performed in a standard laboratory. It should be applied to all patients prior to receiving thiopurine drug therapy to avoid the severe, but predictable, haematopoietic side-effects of thiopurine drug administration.

PHARMACOGENOMICS OF ACUTE LEUKEMIA

Over the past four decades, treatment of acute leukemia in children has made remarkable progress, from this disease being lethal to now achieving cure rates of 80% for acute lymphoblastic leukemia and 45% for acute myeloid leukemia. This progress is largely owed to the optimization of existing treatment modalities rather than the discovery of new agents. However, the annual number of patients with leukemia who experience relapse after initial therapy remains greater than that of new cases of most childhood cancers. The aim of pharmacogenetics is to develop strategies to personalize medications and tailor treatment regimens to individual patients, with the goal of enhancing efficacy and safety through better understanding of the person's genetic makeup. In this review, we summarize recent pharmacogenomic studies related to the treatment of pediatric acute leukemia. These include work using candidate-gene approaches, as well as genome-wide studies using haplotype mapping and gene expression profiling. These strategies illustrate the promise of pharmacogenomics to further advance the treatment of human cancers, with childhood leukemia serving as a paradigm.

Thiopurine S-methyltransferase pharmacogenetics: variant allele functional and comparative genomics

Thiopurine S-methyltransferase (TPMT) catalyses the S-methylation of thiopurine drugs. Genetic polymorphisms for TPMT are a major factor responsible for large individual variations in thiopurine toxicity and therapeutic effect. The present study investigated the functional effects of human TPMT variant alleles that alter the encoded amino acid sequence of the enzyme, TPMT*2, *3A, *3B, *3C and *5 to *13. After expression in COS-1 cells and correction for transfection efficiency, allozymes encoded by these alleles displayed levels of activity that varied from virtually undetectable (*3A,*3B and *5) to 98% (*7) of that observed for the wild-type allele. Although some allozymes had significant elevations in apparent Km values for 6-mercaptopurine and S-adenosyl-L-methionine (i.e. the two cosubstrates for the reaction), the level of enzyme protein was the major factor responsible for variation in activity. Quantitative Western blot analysis demonstrated that the level of enzyme protein correlated closely with level of activity for all allozymes except TPMT*5. Furthermore, protein levels correlated with rates of TPMT degradation. TPMT amino acid sequences were then determined for 16 non-human mammalian species and those sequences (plus seven reported previously, including two nonmammalian vertebrate species) were used to determine amino acid sequence conservation. Most human TPMT variant allozymes had alterations of residues that were highly conserved during vertebrate evolution. Finally, a human TPMT homology structural model was created on the basis of a Pseudomonas structure (the only TPMT structure solved to this time), and the model was used to infer the functional consequences of variant allozyme amino acid sequence alterations. These studies indicate that a common mechanism responsible for alterations in the activity of variant TPMT allozymes involves alteration in the level of enzyme protein due, at least in part, to accelerated degradation.

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.

The thiopurines: An update

The thiopurine drugs, 6-mercaptopurine (6-MP), 6-thioguanine (6-TG) are commonly used cytotoxic agents. A derivative of 6-MP, azathioprine, is commonly used as an immunosuppressant. A prominent route for the metabolism of these agents is mediated by the enzyme thiopurine methyltransferase (TPMT). This enzyme exhibits considerable inter-individual variation in activity, partly due to the presence of common genetic polymorphisms, which influence cytotoxicity of the thiopurine drugs. Variations in the number of tandem repeats in the 5' promoter region have also been shown to influence TPMT expression in vitro. In this article, we review the impact of variations in TPMT activity on sensitivity to the thiopurine drugs in vitro and also in vivo in terms of their clinical efficacy and toxicity. A possible relationship between TPMT and secondary malignancies is also reviewed.

Pharmacogenetics for individualized cancer chemotherapy

The same doses of medication cause considerable heterogeneity in efficacy and toxicity across human populations. Genetic factors are thought to represent important determinants of drug efficacy and toxicity. Pharmacogenetics focuses on the prediction of the response of tumor and normal tissue to standard therapy by genetic profiling and, thereby, to select the most appropriate medication at optimal doses for each individual patient. In the present review, we discuss the relevance of single nucleotide polymorphisms (SNP) in genes, whose gene products act upstream of the actual drug target sites, that is, drug transporters and drug metabolizing phase I and II enzymes, or downstream of them, that is, apoptosis-regulating genes and chemokines. SNPs in relevant genes, which encode for proteins that interact with anticancer drugs, were also considered, that is, enzymes of DNA biosynthesis and metabolism, DNA repair enzymes, and proteins of the mitotic spindle. A significant body of evidence supports the concept of predicting drug efficacy and toxicity by SNP genotyping. As the efficacy of cancer chemotherapy, as well as the drug-related toxicity in normal tissues is multifactorial in nature, sophisticated approaches such as genome-wide linkage analyses and integrate drug pathway profiling may improve the predictive power compared with genotyping of single genes. The implementation of pharmacogenetics into clinical routine diagnostics including genotype-based recommendations for treatment decisions and risk assessment for practitioners represents a challenge for the future.

Liquid chromatography-tandem mass spectrometry analysis of erythrocyte thiopurine nucleotides and effect of thiopurine methyltransferase gene variants on these metabolites in patients receiving azathioprine/6-mercaptopurine therapy

BACKGROUND

Polymorphic thiopurine S-methyltransferase (TPMT) is a major determinant of thiopurine toxicity.

METHODS

We extracted 6-thioguanine nucleotides (6-TGNs) and 6-methylmercaptopurine nucleotides (6-MMPNs) from erythrocytes with perchloric acid and converted them to 6-thioguanine (6-TG) and a 6-methylmercaptopurine (6-MMP) derivative during a 60-min acid hydrolysis step. The liquid chromatography system consisted of a C(18) column with an ammonium acetate-formic acid-acetonitrile buffer. 8-Bromoadenine was the internal standard. Analytes were measured with positive ionization and multiple reaction monitoring mode. With PCR-restriction fragment length polymorphism analysis and TaqMan allelic discrimination, common TPMT alleles (*1, *2, *3A, *3B, *3C) were determined in 31 792 individuals. We used perchloric acid extraction, acid hydrolysis, and HPLC with ultraviolet detection to measure erythrocyte 6-TG and 6-MMP nucleotide concentrations in 6189 patients with inflammatory bowel disease receiving azathioprine/6-mercaptopurine therapy.

RESULTS

Intra- and interday imprecision were <10% at low and high analyte concentrations. The conversion of 6-TG and 6-MMP nucleoside mono-, di-, and triphosphates was complete after hydrolysis. Allelic frequency for TPMT variant alleles ranged from 0.0063% (*3B) to 3.61% (*3A). Compared with wild types, TPMT heterozygotes had an 8.3-fold higher risk for 6-TGNs >450 pmol/8 x 10(8) erythrocytes (concentration associated with increased risk for leukopenia), but an 8.2-fold lower risk for 6-MMPNs >5700 pmol/8 x 10(8) erythrocytes (concentration associated with increased risk for hepatotoxicity).

CONCLUSIONS

The liquid chromatography-tandem mass spectrometry method can be applied to the routine monitoring of thiopurine therapy. The association between TPMT genotype and metabolite concentrations illustrates the utility of pharmacogenetics in the management of patients undergoing treatment with thiopurines.

Comprehensive Screening of the Thiopurine Methyltransferase Polymorphisms by Denaturing High-Performance Liquid Chromatography

The drug-metabolizing enzyme thiopurine S-methyltransferase (TPMT) catalyzes the S-methylation of thiopurines such as 6-mercaptopurine, 6-thioguanine, and azathiopurine, which are used as immunosuppressants and in the treatment of acute lymphoblastic leukemia and rheumatoid arthritis. TPMT enzymatic activity is a polymorphic trait, and poor metabolizers may develop life-threatening bone marrow failure. To avoid such adverse effects, the TPMT enzymatic activity in patients' red blood cells (RBCs) is routinely measured prior to thiopurine administration in a limited number of oncology clinics. In the present study, we took advantage of a highly sensitive and specific automated denaturing high-performance liquid chromatography (dHPLC) technique that not only detects known polymorphic alleles, but also identifies previously uncharacterized sequence variants. We developed a dHPLC-based protocol to analyze the entire coding region and validated the protocol to detect all 16 previously described variant alleles. We further analyzed the entire coding region of the TPMT gene in 288 control samples collected worldwide and identified two novel amino acid substitutions Arg163Cys (487C>T) and Arg226Gln (677G>A) within exons 7 and 10, respectively. The clinical application of this comprehensive screening system for examining the entire TPMT gene would help to identify patients at risk for bone marrow failure prior to 6-mercaptopurine therapy.

Pharmacogenetic testing: proofs of principle and pharmacoeconomic implications

Several proofs of principle have established that pharmacogenetic testing for mutations altering expression and functions of genes associated with drug disposition and response can decrease the "trial-and-error" dosing and reduce the risk of adverse drug reactions. These proofs of principle include thiopurine methyltransferase and thiopurine therapy, dihydropyrimidine dehydrogenase/thymidylate synthase and 5-fluorouracil therapy, folate enzyme MTHFR and methotrexate therapy, UGT1A1 and irinotecan therapy and CYP450 2C9 and S-warfarin therapy. These evidences advocate for the prospective identification of mutations associated with drug response, serious adverse reactions and treatment failure. More recent evidence with the HLA basis of hypersensitivity to the retroviral agent abacavir demonstrates the potential of pharmacogenetic testing and its pharmacoeconomic implications. With the convergence of rising drug costs and evidence supporting the clinical benefits of pharmacogenetic testing, it will be important to demonstrate the improved net health outcomes attributed to the additional costs for this testing.

Comprehensive analysis of thiopurine S-methyltransferase phenotype-genotype correlation in a large population of German-Caucasians and identification of novel TPMT variants

The thiopurine S-methyltransferase (TPMT) genetic polymorphism has a significant clinical impact on the toxicity of thiopurine drugs. It has been proposed that the identification of patients who are at high risk for developing toxicity on the basis of genotyping could be used to individualize drug treatment. In the present study, phenotype-genotype correlation of 1214 healthy blood donors was investigated to determine the accuracy of genotyping for correct prediction of different TPMT phenotypes. In addition, the influence of gender, age, nicotine and caffeine intake was examined. TPMT red blood cell activity was measured in all samples and genotype was determined for the TPMT alleles *2 and *3. Discordant cases between phenotype and genotype were systematically sequenced. A clearly defined trimodal frequency distribution of TPMT activity was found with 0.6% deficient, 9.9% intermediate and 89.5% normal to high methylators. The frequencies of the mutant alleles were 4.4% (*3A), 0.4% (*3C) and 0.2% (*2). All seven TPMT deficient subjects were homozygous or compound heterozygous carriers for these alleles. In 17 individuals with intermediate TPMT activity discordant to TPMT genotype, four novel variants were identified leading to amino acid changes (K119T, Q42E, R163H, G71R). Taking these new variants into consideration, the overall concordance rate between TPMT genetics and phenotypes was 98.4%. Specificity, sensitivity and the positive and negative predictive power of the genotyping test were estimated to be higher than 90%. Thus, the results of this study provide a solid basis to predict TPMT phenotype in a Northern European Caucasian population by molecular diagnostics.

Mechanism of TNF-{alpha} modulation of Caco-2 intestinal epithelial tight junction barrier: role of myosin light-chain kinase protein expression

TNF-alpha plays a central role in the intestinal inflammation of various inflammatory disorders including Crohn's disease (CD). TNF-alpha-induced increase in intestinal epithelial tight junction (TJ) permeability has been proposed as one of the proinflammatory mechanisms contributing to the intestinal inflammation. The intracellular mechanisms involved in the TNF-alpha-induced increase in intestinal TJ permeability remain unclear. The purpose of this study was to investigate the possibility that the TNF-alpha-induced increase in intestinal epithelial TJ permeability was regulated by myosin light-chain kinase (MLCK) protein expression, using an in vitro intestinal epithelial model system consisting of the filter-grown Caco-2 intestinal epithelial monolayers. TNF-alpha (10 ng/ml) produced a time-dependent increase in Caco-2 MLCK expression. The TNF-alpha increase in MLCK protein expression paralleled the increase in Caco-2 TJ permeability, and the inhibition of the TNF-alpha-induced MLCK expression (by cycloheximide) prevented the increase in Caco-2 TJ permeability, suggesting that MLCK expression may be required for the increase in Caco-2 TJ permeability. The TNF-alpha increase in MLCK protein expression was preceded by an increase in MLCK mRNA expression but not an alteration in MLCK protein degradation. Actinomycin-D prevented the TNF-alpha increase in MLCK mRNA expression and the subsequent increase in MLCK protein expression and Caco-2 TJ permeability, suggesting that the increase in MLCK mRNA transcription led to the increase in MLCK expression. The TNF-alpha increase in MLCK protein expression was also associated with an increase in Caco-2 MLCK activity. The cycloheximide inhibition of MLCK protein expression prevented the TNF-alpha increase in MLCK activity and Caco-2 TJ permeability. Moreover, inhibitors of MLCK, Mg(2+)-myosin ATPase, and metabolic energy prevented the TNF-alpha increase in Caco-2 TJ permeability, suggesting that the increase in MLCK activity was required for the TNF-alpha-induced opening of the Caco-2 TJ barrier. In conclusion, our results indicate for the first time that 1) the TNF-alpha increase in Caco-2 TJ permeability was mediated by an increase in MLCK protein expression, 2) the increase in MLCK protein expression was regulated by an increase in MLCK mRNA transcription, and 3) the increase in Caco-2 TJ permeability required MLCK protein expression-dependent increase in MLCK activity.

Thiopurine methyltransferase in acute lymphoblastic leukaemia: biochemical and molecular biological aspects

Thiopurine S-methyltransferase (TPMT) is a cytosolic enzyme, catalysing S-methylation of aromatic and heterocyclic sulphhydryl compounds. TPMT activities and genotypes have been determined in patients with acute lymphoblastic leukaemia (ALL) and in control children. Median red blood cell (RBC) TPMT activity in ALL patients at diagnosis was significantly lower than in controls (median 11.5 pmol/10(7) RBC*hr; range 1.7-30.7; n = 191 vs. 14.6 pmol/10(7) RBC*hr; range 1.6-50.7; n = 140). This reduction of TPMT activity in ALL patients was not due to differences in the frequency of mutations in the TPMT gene. In concordance with other authors, we found a higher TPMT activity during maintenance treatment with 6-mercaptopurine (6MP) than at diagnosis and in controls. However, we observed that TPMT activity was already significantly increased after the induction therapy, before the patients received 6MP (median 17.5; range 3.9-40.3 pmol/10(7) RBC*hr; n = 139). In vitro experiments indicate that the early increase of TPMT activity during treatment may be explained by the use of antifolates, e.g., methotrexate and trimethoprim.

Frequency Distribution of Thiopurine S-Methyltransferase Alleles in a Polish Population

Thiopurine S-methyltransferase (TPMT) is an enzyme that catalyzes the S-methylation of thiopurine drugs such as 6-mercaptopurine, 6-thioguanine, and azathioprine. TPMT activity exhibits an interindividual variability mainly a result of genetic polymorphism. Patients with intermediate or deficient TPMT activity are at risk for toxicity after receiving standard doses of thiopurine drugs. It has previously been reported that 3 variant alleles:TPMT*2, *3A, and *3C are responsible for over 95% cases of lower enzyme activity. The purpose of this study was to determine the frequency of TPMT variant alleles in a Polish population. DNA samples were obtained from 358 unrelated healthy Polish subjects of white origin, and TPMT genetic polymorphism was determined using PCR-RFLP and allele-specific PCR methods. The results showed that allelic frequencies were 0.4% for TPMT*2, 2.7% for TPMT*3A, and 0.1% for TPMT*3C, respectively. A TPMT*3B allele was not found in the studied population. The general pattern of TPMT allele disposition in the Polish population is similar to those determined for other white populations, but the frequency of total variant alleles is lower than in other European populations studied to date.

Frequency distribution of thiopurine S-methyltransferase activity in red blood cells of a healthy Japanese population

Thiopurine S-methyltransferase (TPMT), which exhibits a genetic polymorphism, plays an important role in the metabolism of thiopurine drugs such as mercaptopurine, thioguanine, and azathioprine. To determine the frequency distribution of TPMT activity in 157 Japanese subjects with different TPMT genotypes, ie, TPMT*1/*1 and TPMT*1/*3, the authors measured levels of 6-methylmercaptopurine formed from 6-mercaptopurine in red blood cells lysates by HPLC. The TPMT activities in our Japanese subjects ranged from 11.0 to 42.6 pmol/h/mgHb. Although the mean value of TPMT activities in 6 subjects with TPMT*1/*3C (20.3 +/- 8.1 pmol/h/mgHb) was 25% lower than that in 151 subjects with TPMT*1/*1 (27.0 +/- 5.1 pmol/h/mgHb), there was overlap. The ranges of TPMT activity in subjects with TPMT*1/*1 and those with TPMT*1/*3C were similar. The median values in TPMT*1/*3C and TPMT*1/*1 individuals were 20.1 (11.0-31.2) and 26.8 pmol/h/mgHb (15.7-42.7), respectively (Mann-Whitney U-test: median difference 6.7 pmol/h/mgHb, 95% CI 0-25.5, P < 0.05). This observation may have relevance for the use of 6-mercaptopurine and azathioprine as therapeutic agents in Japanese patients.

The impact of thiopurine S-methyltransferase polymorphisms on azathioprine dose 1 year after renal transplantation

Azathioprine metabolism is influenced by activity of the enzyme thiopurine S-methyltransferase (TPMT), which varies markedly between individuals. In this study we examined the influence of TPMT gene polymorphisms on azathioprine dose 1 year after renal transplantation. TPMT coding and promoter genotypes were determined using PCR-based assays. Azathioprine dose, white cell count, and intercurrent events throughout the first year after renal transplantation were ascertained from contemporaneous clinical notes. All patients analysed ( n=172) received an initial azathioprine dose of 1.5 mg/kg per day. Twelve individuals with one variant TPMT coding allele were detected (*3A n=11, *3C n=1). Of these, 58% required azathioprine dose reduction because of leucopenia, compared to only 30% of homozygous wild-type patients ( P=0.04). A significant correlation between the presence of >/=11 variable number tandem repeats (VNTRs) in the TPMT promoter and reduction in azathioprine dose was also identified ( P=0.001). We concluded that when azathioprine is administered at an initial dose of 1.5 mg/kg per day, both coding and promoter TPMT polymorphisms influence the dose tolerated.

Pharmacogenomics: bench to bedside

Pharmacogenetics is the study of the role of inheritance in inter-individual variation in drug response. Since its origins in the mid-twentieth century, a major driving force in pharmacogenetics research has been the promise of individualized drug therapy to maximize drug efficacy and minimize drug toxicity. In recent years, the convergence of advances in pharmacogenetics with rapid developments in human genomics has resulted in the evolution of pharmacogenetics into pharmacogenomics, and led to increasing enthusiasm for the 'translation' of this evolving discipline into clinical practice. Here, we briefly summarize the development of pharmacogenetics and pharmacogenomics, and then discuss the key factors that have had an influence on - and will continue to affect - the translation of pharmacogenomics from the research bench to the bedside, highlighting the challenges that need to be addressed to achieve this goal.

Pharmacogenetics of thiopurine S-methyltransferase and thiopurine therapy

Most medications exhibit wide interpatient variability in their efficacy and toxicity. For many medications, these interindividual differences result in part from polymorphisms in genes encoding drug-metabolizing enzymes, drug transporters, and/or drug targets (eg, receptors, enzymes). Pharmacogenomics is a burgeoning field aimed at elucidating the genetic basis of differences in drug efficacy and toxicity, using genome-wide approaches to identify the network of genes that govern an individual's response to drug therapy. For some genetic polymorphisms, such as thiopurine S-methyltransferase (TPMT), monogenic traits have a marked effect on the pharmacokinetics of medications, such that individuals who inherit an enzyme deficiency must be treated with markedly different doses of the affected medications (eg, 5-10% of the standard thiopurine dose). This review uses the TPMT polymorphism and thiopurine therapy (eg, azathioprine, mercaptopurine) to illustrate the potential of pharmacogenomics to elucidate genetic determinants of drug response, and optimize the selection of drug therapy for individual patients.

Pharmacogenetics of inflammatory bowel disease

The therapeutic efficacy and toxicity of many commonly employed drugs show interindividual variations that relate to several factors, including genetic variability in drug-metabolizing enzymes, transporters or targets. The study of the genetic determinants influencing interindividual variations in drug response is known as pharmacogenetics. The ability to identify, through preliminary genetic screening, the patients most likely to respond positively to a medication should facilitate the best choice of treatment for each patient; drugs likely to exhibit low efficacy or to give negative side-effects can be avoided. Among the medications used for inflammatory bowel disease, the best studied pharmacogenetically is azathioprine. The hematopoietic toxicity of azathioprine is due to single nucleotide polymorphisms in the thiopurine S-methyltransferase enzyme. Additionally, likely gene targets have been investigated to predict the response to glucocorticoids and infliximab, a monoclonal antibody against tumour necrosis factor that induces remission in approximately 30-40% of patients. However, no genetic predictor of response has been identified in either case.

Phenotype and genotype for thiopurine methyltransferase activity in the French Caucasian population: impact of age

OBJECTIVE

Thiopurine drugs are commonly used in pediatric patients for the treatment of acute leukemia, organ transplantation and inflammatory diseases. They are catabolized by the cytosolic thiopurine methyltransferase (TPMT), which is subject to a genetic polymorphism. In children, enzyme activities are immature at birth and developmental patterns vary widely from one enzyme to another. The present study was undertaken to evaluate erythrocyte TPMT activity and the correlation between genotype and phenotype in different age groups from birth to adolescence and adulthood.

METHODS

The study included 304 healthy adult blood donors, 147 children and 18 neonates (cord bloods). TPMT activity was measured by liquid chromatography, and genotype was determined using a polymerase chain reaction reverse dot-blot analysis identifying the predominant TPMT mutant alleles (TPMT*3A, TPMT*3B, TPMT*3C, TPMT*2).

RESULTS

There was no significant difference in TPMT activity between cord bloods ( n=18) and children ( n=147) (17.48+/-4.04 versus 18.62+/-4.14 respectively, P=0.424). However, TPMT was significantly lower in children than in adults (19.34+/-4.09) ( P=0.033). In the whole population, there were 91.9% homozygous wild type, 7.9% heterozygous mutants and 0.2% homozygous mutants. The frequency of mutant alleles was 3.0% for TPMT*3A, 0.7% for TPMT*2 and 0.4% for TPMT*3C.

CONCLUSION

No impact of child development on TPMT activity could be evidenced, suggesting that TPMT activity is already mature at birth. The difference between children and adults was low with reduced clinical impact expected. When individual TPMT activity was compared with genotype, there was an overlapping region where subjects (4.5%, 12 adults, 9 children) were either homozygous wild type or heterozygous, with a TPMT activity below the antimode value. This result highlighted the importance of measuring TPMT activity to detect all patients at risk of thiopurine toxicity.

Primer on medical genomics. Part XII: Pharmacogenomics--general principles with cancer as a model

The Human Genome Project has resulted in a new era in the field of pharmacogenetics in which researchers are rapidly discovering new genetic variation, which may help to explain interindividual variability in drug efficacy and toxicity. Pharmacogenetics is the study of the role of genetic inheritance in individual variation in drug response and toxicity. With the convergence of advances in pharmacogenetics and human genomics, the field of pharmacogenomics has emerged during the past decade. Pharmacogenomics is used to refer to the study of the relationship between specific DNA-sequence variation and drug effect. In few other disciplines of medicine are the clinical examples of pharmacogenetics more striking than in oncology. In this field, treatment of patients with cancer is accomplished primarily through the use of chemotherapeutic drugs that have narrow therapeutic indexes, ie, the difference between the toxic and therapeutic dose is relatively small. In this review, we discuss several selected, clinically relevant examples of ways in which sequence variation in genes that encode drug enzymes, transporters, and drug targets can alter the efficacy and/or adverse-effect profile of "standard" doses of chemotherapeutic drugs. Additionally, we discuss some of the ways in which physicians are currently applying this knowledge in the treatment of patients with cancer.

Thiopurine methyltransferase (TPMT) genotype distribution in azathioprine-tolerant and -intolerant patients with various disorders. The impact of TPMT genotyping in predicting toxicity

OBJECTIVE

To study the distribution of the thiopurine methyltransferase (TPMT) genotype among azathioprine (Aza)-tolerant and -intolerant patients with various disorders, and to investigate a possible relationship with the Aza metabolite levels.

METHODS

Forty-six Aza-tolerant and six Aza-intolerant patients had the TPMT genotype distribution determined using a polymerase chain reaction (PCR) assay and the forty-six Aza-tolerant patients had the Aza metabolite levels determined using a high-pressure liquid chromatography (HPLC) analysis.

RESULTS

One non-functional TPMT mutant allele was demonstrated in 2 of the 46 Aza-tolerant patients (4.4%) and one or two non-functional mutant alleles in 2 of the 6 Aza-intolerant patients (33.3%). Of the 4 patients, with one or two non-functional mutant alleles 2 (50%) were intolerant to Aza compared with 4 of the 48 patients (8.3%) with no mutations detected. The time to hepatotoxicity did not differ significantly between the 2 patients with one or two non-functional mutant alleles and the remaining 3 patients ( P=0.5). The TPMT genotype distribution differed slightly in the three different categories of disorders ( P=0.05). The median E-6-TGN level among the 2 TPMT heterozygous patients was 275 pmol/8x10(8) RBC (range 240-310), whereas the remaining 44 patients had a median E-6-TGN level of 110 pmol/8x10(8) RBC (range 0-440) ( P=0.07).

CONCLUSION

Although TPMT genotyping cannot be recommended on behalf of the present study, it is to be expected that half of the patients with one or two non-functional TPMT mutant alleles will develop Aza intolerance leading to withdrawal of therapy. Thus, clinicians may anticipate about 5% of the patients to develop intolerance to Aza therapy solely for that reason.

Tertiary structure of thiopurine methyltransferase from Pseudomonas syringae, a bacterial orthologue of a polymorphic, drug-metabolizing enzyme

In humans, the enzyme thiopurine methyltransferase (TPMT) metabolizes 6-thiopurine (6-TP) medications, including 6-thioguanine, 6-mercaptopurine and azathioprine, commonly used for immune suppression and for the treatment of hematopoietic malignancies. S-Methylation by TPMT prevents the intracellular conversion of these drugs into active 6-thioguanine nucleotides (6-TGNs). Genetic polymorphisms in the TPMT protein sequence have been associated with decreased tissue enzymatic activities and an increased risk of life-threatening myelo-suppression from standard doses of 6-TP medications. Biochemical studies have demonstrated that TPMT deficiency is primarily associated with increased degradation of the polymorphic proteins through an ubiquitylation and proteasomal-dependent pathway. We have now determined the tertiary structure of the bacterial orthologue of TPMT from Pseudomonas syringae using NMR spectroscopy. Bacterial TPMT similarly catalyzes the S-adenosylmethionine (SAM)-dependent transmethylation of 6-TPs and shares 45% similarity (33% identity) with the human enzyme. Initial studies revealed an unstructured N terminus, which was removed for structural studies and subsequently determined to be required for enzymatic activity. Despite lacking sequence similarity to any protein of known three-dimensional structure, the tertiary structure of bacterial TPMT reveals a classical SAM-dependent methyltransferase topology, consisting of a seven-stranded beta-sheet flanked by alpha-helices on both sides. However, some deviations from the consensus topology, along with multiple insertions of structural elements, are evident. A review of the many experimentally determined tertiary structures of SAM-dependent methyltransferases demonstrates that such structural deviations from the consensus topology are common and often functionally important.

Drug methylation in cancer therapy: lessons from the TPMT polymorphism

The genetic polymorphism of thiopurine methyltransferase (TPMT) is one of the most developed examples of pharmacogenetics, spanning from molecular genetics to clinical diagnostics for individualizing thiopurine therapy (i.e. azathioprine, mercaptopurine, and thioguanine). Elucidation of the molecular mechanisms and biochemical consequences of TPMT deficiency demonstrates how pharmacogenetic traits can be identified, characterized, and translated to the bedside. Insights gained from studies of the TPMT polymorphism illustrate the potential of pharmacogenomics to optimize cancer therapy by avoiding toxic side effects in genetically distinct subgroups of patients.

In vitro characterization of four novel non-functional variants of the thiopurine S-methyltransferase

Human thiopurine S-methyltransferase (TPMT) is an enzyme responsible for the detoxification of widely used thiopurine drugs such as azathioprine (Aza). Its activity is inversely related to the risk of developing severe hematopoietic toxicity in certain patients treated with standard doses of thiopurines. DNA samples from four leucopenic patients treated with Aza were screened by PCR-SSCP analysis for mutations in the 10 exons of the TPMT gene. Four missense mutations comprising two novel mutations, A83T (TPMT*13, Glu(28)Val) and C374T (TPMT*12, Ser(125)Leu), and two previously described mutations, G430C (TPMT*10, Gly(144)Arg) and T681G (TPMT*7, His(227)Gln) were identified. Using a recombinant yeast expression system, kinetic parameters (K(m) and V(max)) of 6-thioguanine S-methylation of the four TPMT variants were determined and compared to those obtained with wild-type TPMT. This functional analysis suggests that these rare allelic variants are defective TPMT alleles. The His(227)Gln variant retained only 10% of the intrinsic clearance value (V(max)/K(m) ratio) of the wild-type enzyme. The Ser(125)Leu and Gly(144)Arg variants were associated with a significant decrease in intrinsic clearance values, retaining about 30% of the wild-type enzyme, whereas the Glu(28)Val variant produced a more modest decrease (57% of the wild-type enzyme). The data suggest that the sporadic contribution of the rare Glu(28)Val, Ser(125)Leu, Gly(144)Arg, and His(227)Gln variants may account for the occurrence of altered metabolism of TPMT substrates. These findings improve our knowledge of the genetic basis of interindividual variability in TPMT activity and would enhance the efficiency of genotyping methods to predict patients at risk of inadequate responses to thiopurine therapy.

Real-time RT-PCR methodology for quantification of thiopurine methyltransferase gene expression

OBJECTIVE

The aim of the present study was to develop a real-time reverse-transcription polymerase chain reaction (RT-PCR) methodology for the quantification of thiopurine methyltransferase (TPMT) gene expression in whole blood and compare it with the TPMT enzyme activity measured in red blood cells.

METHODS

TPMT gene expression was quantified relative to the housekeeping gene cyclophilin (huCYC) and expressed as a TPMT/huCYC ratio. TPMT activity in red blood cells was determined by measuring the formation rate of 6-(14)C-methylmercaptopurine from 6-MP using S-adenosyl-L-((14)C-methyl)-methionine as methyl donor. Thirty-nine individuals were included in the study. A cut-off value of 9 U/ml pRBC was used to distinguish intermediate TPMT enzyme activity from high TPMT enzyme activity.

RESULTS

Sequencing of the real-time RT-PCR amplicon proved that the method was specific for the TPMT cDNA, without co-amplification of the highly similar TPMT processed pseudogene. The intra-assay coefficients of variation (CVs), as determined by the threshold cycle, were 0.7% for TPMT and 0.5% for huCYC. The interassay CVs were 1.5% for TPMT and 4.0% for huCYC. The intra- and interassay CVs, as determined by the TPMT/huCYC ratio, were 8.6% and 25%, respectively. There was a statistically significant correlation between TPMT enzyme activity and mRNA level in blood cells from individuals with an enzyme activity above 9 U/ml pRBC (r(s)=0.66, P=0.0001). However, we did not find any statistically significant correlation in individuals with lower enzyme activity or when analysing the whole population.

CONCLUSION

We present a specific and robust real-time RT-PCR method for quantifying TPMT gene expression. The method may be used for studies on TPMT gene regulation.

Pharmacogenetics in cancer treatment

Interindividual variability in the efficacy and toxicity of drug therapy is associated with polymorphisms in genes encoding drug-metabolizing enzymes, transporters, or drug targets. Pharmacogenetics aims to identify individuals predisposed to high risk of toxicity from conventional doses of cancer chemotherapeutic agents. We review the role of genetic polymorphisms in UGT1A1 and TPMT, as well as mutations in DPD, in influencing drug disposition and toxicity. Recent studies show that pharmacogenetic determinants may also influence treatment outcomes. We discuss the clinical significance of polymorphisms in TS, MTHFR, and FCGR3A, as well as the polymorphic DNA repair genes XPD and XRCC1, in influencing response to chemotherapy and survival outcomes. Finally, the potential implications of transporter pharmacogenetics in influencing drug bioavailability are addressed.

Pharmacogenomics: marshalling the human genome to individualise drug therapy

Pharmacogenomics aims to identify the inherited basis for interindividual differences in drug response, and translate this to molecular diagnostics that can be used to individualise drug therapy. This review uses a number of published examples of inherited differences in drug metabolising enzymes, drug transporters, and drug targets (for example, receptors) to illustrate the potential importance of inheritance in determining the efficacy and toxicity of medications in humans. It seems that this field is at the early stages of developing a powerful set of molecular diagnostics that will have profound utility in optimising drug therapy for individual patients.

Cancer pharmacogenomics: current and future applications

Heterogeneity in patient response to chemotherapy is consistently observed across patient populations. Pharmacogenomics is the study of inherited differences in interindividual drug disposition and effects, with the goal of selecting the optimal drug therapy and dosage for each patient. Pharmacogenomics is especially important for oncology, as severe systemic toxicity and unpredictable efficacy are hallmarks of cancer therapies. In addition, genetic polymorphisms in drug metabolizing enzymes and other molecules are responsible for much of the interindividual differences in the efficacy and toxicity of many chemotherapy agents. This review will discuss clinically relevant examples of gene polymorphisms that influence the outcome of cancer therapy, and whole-genome expression studies using microarray technology that have shown tremendous potential for benefiting cancer pharmacogenomics. The power and utility of the mouse as an experimental system for pharmacogenomic discovery will also be discussed in the context of cancer therapy.

Thiopurine methyltransferase genotype distribution in patients with Crohn's disease

BACKGROUND

Inter-individual response to azathioprine is partly due to inter-individual variation in the thiopurine methyltransferase (TPMT) activity. The TPMT genotype, which reflects the TPMT activity, has previously been studied in healthy Caucasians, with the most common variant allele being TPMT*3A. TPMT genotyping in adult patients with Crohn's disease has never been performed systematically.

AIM

To determine the TPMT genotype distribution in adult patients with Crohn's disease.

METHODS

One hundred and twenty randomly selected Danish patients (64 females and 56 males) with azathioprine-dependent Crohn's disease were included, and a polymerase chain reaction assay was used for TPMT genotyping. The patients were genotyped for the low-level genotype G460-->A and A719-->G transitions.

RESULTS

One hundred and nine patients (90.3%; 95% confidence interval, 84.1-95.3) had a wild-type/ wild-type genotype, whereas 10 patients (8.3%; 95% confidence interval, 4.1-14.8) had one non-functional mutant allele and one patient (0.8%; 95% confidence interval, 0.02-4.6) had two non-functional mutant alleles. Only the TPMT*3A variant allele was found.

CONCLUSIONS

The study showed a TPMT genotype distribution amongst adult Danish patients with Crohn's disease which was similar to the distribution of TPMT variant alleles normally found in healthy Caucasians.

Cancer pharmacogenomics: SNPs, chips, and the individual patient

There is great heterogeneity in a patient's response to medications, often requiring empirical strategies to define the appropriate drug therapy for each patient. Pharmacogenomics aims to elucidate further 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 dosage for each patient. These genetic insights should also lead to mechanism-based approaches to the discovery and development of new medications. Genetic polymorphisms in drug metabolizing enzymes, transporters, receptors, and other drug targets have been linked to interindividual differences in the efficacy and toxicity of many medications. For example, polymorphism in thiopurine methyltransferase (TPMT) results in altered degradation of the commonly prescribed agent 6-mercaptopurine. This genetic variant has significant clinical implications because patients with functionally relevant homozygous mutations in the TPMT gene experience extreme or fatal toxicity after administration of normal doses of 6-MP. In addition, patients heterozygous for mutations in TPMT require slight dosage reduction of 6-MP and experience a greater degree of systemic toxicity from the agent. This and other examples of genetic polymorphism relevant to the treatment of cancer are highlighted to illustrate the promise and pitfalls of the exciting area of cancer therapeutics, with the potential of providing a stronger scientific basis to optimize drug therapy on the basis of each patient's genetic constitution.

Pediatric acute lymphoblastic leukemia

The outcome for children with acute lymphoblastic leukemia (ALL) has improved dramatically with current therapy resulting in an event free survival exceeding 75% for most patients. However significant challenges remain including developing better methods to predict which patients can be cured with less toxic treatment and which ones will benefit from augmented therapy. In addition, 25% of patients fail therapy and novel treatments that are focused on undermining specifically the leukemic process are needed urgently. In Section I, Dr. Carroll reviews current approaches to risk classification and proposes a system that incorporates well-established clinical parameters, genetic lesions of the blast as well as early response parameters. He then provides an overview of emerging technologies in genomics and proteomics and how they might lead to more rational, biologically based classification systems. In Section II, Drs. Mary Relling and Stella Davies describe emerging findings that relate to host features that influence outcome, the role of inherited germline variation. They highlight technical breakthroughs in assessing germline differences among patients. Polymorphisms of drug metabolizing genes have been shown to influence toxicity and the best example is the gene thiopurine methyltransferase (TPMT) a key enzyme in the metabolism of 6-mercaptopurine. Polymorphisms are associated with decreased activity that is also associated with increased toxicity. The role of polymorphisms in other genes whose products play an important role in drug metabolism as well as cytokine genes are discussed. In Sections III and IV, Drs. James Downing and Cheryl Willman review their findings using gene expression profiling to classify ALL. Both authors outline challenges in applying this methodology to analysis of clinical samples. Dr. Willman describes her laboratory's examination of infant leukemia and precursor B-ALL where unsupervised approaches have led to the identification of inherent biologic groups not predicted by conventional morphologic, immunophenotypic and cytogenetic variables. Dr. Downing describes his results from a pediatric ALL expression database using over 327 diagnostic samples, with 80% of the dataset consisting of samples from patients treated on a single institutional protocol. Seven distinct leukemia subtypes were identified representing known leukemia subtypes including: BCR-ABL, E2A-PBX1, TEL-AML1, rearrangements in the MLL gene, hyperdiploid karyotype (i.e., > 50 chromosomes), and T-ALL as well as a new leukemia subtype. A subset of genes have been identified whose expression appears to be predictive of outcome but independent verification is needed before this type of analysis can be integrated into treatment assignment. Chemotherapeutic agents kill cancer cells by activating apoptosis, or programmed cell death. In Section V, Dr. John Reed describes major apoptotic pathways and the specific role of key proteins in this response. The expression level of some of these proteins, such as BCL2, BAX, and caspase 3, has been shown to be predictive of ultimate outcome in hematopoietic tumors. New therapeutic approaches that modulate the apoptotic pathway are now available and Dr. Reed highlights those that may be applicable to the treatment of childhood ALL.

Thiopurine methyltransferase polymorphisms in a Brazilian population

Thiopurine methyltransferase (TPMT) catalyses the S-methylation of thiopurine drugs. Low-activity phenotypes are correlated with several mutations in the TPMT gene. Polymorphisms of TPMT have been reported for Caucasians, African-Americans and Asians. Since ethnic differences have been demonstrated worldwide, it remains to be elucidated in Brazil. The Brazilian population is the result of five centuries of interethnic crosses between peoples from almost all continents as well as autochthonous Amerindians, all forming the fifth largest and one of the most heterogeneous populations in the world. The frequency of six allelic variants of the TPMT gene, *2 (G238C) (2.2%), *3A (G460A and A719G) (1.5%), *3B (G460A) (0.2%), *3C (A719G) (1.0%), *5 (0%) and *6 (0%) were determined in Brazilian subjects using polymerase chain reaction (PCR)-RFLP and allele-specific PCR-based assays. This study provides the first analysis of TPMT mutant allele frequency in a sample of the Brazilian population.

Role of pharmacogenomics and pharmacodynamics in the treatment of acute lymphoblastic leukaemia

Pharmacodynamic studies have been used to establish the relationships between the administered dosage and the concentration of drugs and metabolites in the blood or tissues and that between these concentrations and pharmacological effects. Polymorphisms in the genes that encode drug-metabolizing enzymes, drug transporters and drug targets can affect a person's response to therapy and may affect the development of de novo or therapy-related leukaemias. The burgeoning field of pharmacogenomics elucidates inherited differences in drug metabolism and treatment response. Increasingly, pharmacodynamic and pharmacogenomic studies are being used to individualize therapy to enhance efficacy and reduce toxicity.

Response to infliximab treatment in Crohn's disease is not associated with mutations in the CARD15 (NOD2) gene: an analysis in 534 patients from two multicenter, prospective GCP-level trials

Infliximab induces remission in 30-40% of patients with active Crohn's disease. Treatment response is a stable trait over repeated doses yet the clinical predictors of response are still unknown. Recently, three variants in the CARD15 gene have been identified as major genetic risk factors for Crohn's disease. Single nucleotide polymorphisms (SNPs) 8, 12 and 13, have been shown to be independently associated with Crohn's disease susceptibility. The aim of the present study was to investigate these variants in relation to the therapeutic efficacy of infliximab. SNPs were genotyped (TaqMan) in two cohorts ( n= 90 and n= 444 (ACCENT I)) of active Crohn's disease patients (CDAI 220-450). The patients were recruited from independent multicenter trials conducted according to GCP. At the start of both trials, patients received a single infusion of open label infliximab (5 mg/kg bodyweight). The genotypic and allelic frequencies of each SNP were significantly associated with Crohn's disease in comparison to 370 healthy controls as reported previously. Response to infliximab (drop in CDAI 70 points or remission, respectively) was not associated with the genetic variants in the CARD15 gene in either cohort. The subsequent negative findings in a two-cohort model exclude SNPs 8, 12 and 13 of the CARD15 gene as predictors for therapeutic response to infliximab treatment.