Thiopurine S-Methyltransferase as a Pharmacogenetic Biomarker: Significance of Testing and Review of Major Methods (original) (raw)
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PubMed, 2014
For the past half century, thiopurines have earned themselves a reputation as effective anti-cancer and immunosuppressive drugs. Thiopurine S-methyltransferase (TPMT) is involved in the metabolism of all thiopurines and is one of the main enzymes that inactivates mercaptopurine. 6-MP is now used as a combination therapies for maintenance therapy of children with acute lymphocytic leukemia (ALL). In all patients receiving mercaptopurine, there is a risk of bone marrow suppression. TPMT activity is inherited as a monogenic, co-dominant trait. More than 25 variants are known. Genetic testing is available for several TPMT variant alleles. Most commonly TPMT*2, *3A, and *3C are tested for, which account for >90% of inactivating alleles. Differences in DNA that alter the expression or function of proteins that are targeted by drugs can contribute significantly to variation in the responses of individuals.Genotyping may become part of routine investigations to help clinicians tailor drug treatment effectively. This success is mainly due to the development of combination therapies and stratification of patients according to risk of treatment failure and relapse, rather than the discovery of new drugs. The aim of this study was to investigate the effect of genotype or methyltransferase enzyme activity before starting therapy in children with ALL. This can prevent the side effect of thiopurine drugs. In fact, the common polymorphism of this enzyme in population could be a prognostic factor in relation to drug use and treatment of patients with ALL.
Thiopurine pharmacogenetics: clinical and molecular studies of thiopurine methyltransferase
Drug metabolism and disposition: the biological fate of chemicals, 2001
Thiopurine drugs are used to treat patients with neoplasia and autoimmune disease as well as transplant recipients. These agents are metabolized, in part, by S-methylation catalyzed by thiopurine methyltransferase (TPMT). The discovery nearly two decades ago that levels of TPMT activity in human tissues are controlled by a common genetic polymorphism led to one of the best examples of the potential importance of pharmacogenetics for clinical medicine. Specifically, it is now known that patients with inherited very low levels of TPMT activity are at greatly increased risk for thiopurine-induced toxicity such as myelosuppression when treated with standard doses of these drugs, while subjects with very high activity may be undertreated. Furthermore, recent reports indicate that TPMT may be the target for clinically significant drug interactions and that this common genetic polymorphism might be a risk factor for the occurrence of therapy-dependent secondary leukemia. In parallel with t...
Thiopurine S-methyltransferase (TPMT) pharmacogenetics: variant allele functional genomics
Clinical Pharmacology & Therapeutics, 2004
TPMT genetic polymorphisms influence thiopurine drug toxicity and efficacy. We set out to perform functional genomic studies of 7 human TPMT variant alleles, TPMT*5: to *11:, that had not previously been studied by expression in mammalian cells. After expression in COS-1 cells, allozymes encoded by these alleles displayed from 2.3% (*5:) to 97.5% (*7:) of the level of TPMT activity–corrected for transfection efficiency–found with the wild type (WT) allele. TPMT*5:, *6: and *11: all displayed less than 50% of WT activity. Quantitative Western analysis showed that level of immunoreactive protein correlated closely with level of activity for all allozymes except TPMT*6:. However, substrate kinetic studies of all of the recombinant allozymes showed that *6: had significantly elevated apparent Km values for both cosubstrates for the reaction, 6-mercaptopurine and S-adenosyl-L-methionine, when compared with WT. These results indicate that some of the TPMT alleles that have been reported to be associated with clinical consequences do not appear to be functionally impaired after expression in mammalian cells.Clinical Pharmacology & Therapeutics (2004) 75, P19–P19; doi: 10.1016/j.clpt.2003.11.072
Medical Oncology, 2007
Polymorphisms at the thiopurine S-methyltransferase coding gene (TPMT) determine enzyme activity and consequently, the development of toxicity secondary to thiopurines. Methods A total of 108 DNA samples from volunteer donors and 39 from patients with acute lymphoblastic leukemia (ALL) were analyzed. Genomic DNA from peripheral blood leukocytes was isolated by standard methods. TPMT gene fragments were amplified by PCR for exons 5, 7, and 10. Thereafter, these were analyzed by DHPLC for the most frequent mutant TPMT alleles. Results No elution profiles on DHPLC analysis, different from those previously reported, were documented. Frequency of functional allele polymorphisms was 17.6%, being the most frequent *3A (n = 13; 4.4%), followed by *3B (n = 5; 1.7%), *3C (n = 5; 1.7%), and *2 (n = 3; 1.0%). From 39 ALL patients, 22 were treated with thiopurines, and five from 10 with a functional polymorphism developed hematological toxicity (4 mild, 1 severe in a patient referred to our Hospital after developing pancytopenia while on treatment with thiopurine). Conclusions This is the first analysis of the polymorphisms at this gene in Mexican population. Since a direct relation has been documented within functional polymorphisms and enzyme activity, and DHPLC is a highly sensitive, rapid and efficient method, feasible to realize in any phase during the treatment of ALL patients, the routine typing of TPMT polymorphisms in the patients with ALL has been set in our Institution.
Thiopurine S-methyltransferase pharmacogenetics: variant allele functional and comparative genomics
Pharmacogenetics and Genomics, 2005
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.
Pharmacogenomics of Thiopurine S-Methyltransferase: Clinical Applicability of Genetic Variants
Clinical Applications of Pharmacogenetics, 2012
Here, we provide an overview of the genetic variants of thiopurine S-methyltransferase (TPMT) gene that influence inter-individual dosing of thiopurine drugs, to highlight a tangible benefit of translating genomic knowledge into clinical practice. Particular single nucleotide polymorphisms (SNPs) in TPMT g e n e h a v e p r o v e n t o b e a p p l i c a b l e f o r optimising the dosage in pursuit of maximum efficacy and minimum adverse effects. Thus,
European Journal of Clinical Pharmacology, 2004
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.