Thiopurine S-methyltransferase pharmacogenetics: Functional characterization of a novel rapidly degraded variant allozyme (original) (raw)
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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
Proceedings of the National Academy of Sciences, 1997
TPMT is a cytosolic enzyme that catalyzes the S-methylation of aromatic and heterocyclic sulfhydryl compounds, including medications such as mercaptopurine and thioguanine. TPMT activity exhibits autosomal codominant genetic polymorphism, and patients inheriting TPMT deficiency are at high risk of potentially fatal hematopoietic toxicity. The most prevalent mutant alleles associated with TPMT deficiency in humans have been cloned and characterized (TPMT2ء and TPMT3ءA), but the mechanisms for loss of catalytic activity have not been elucidated. In the present study, we established that erythrocyte TPMT activity was significantly related to the amount of TPMT protein on Western blots of erythrocytes from patients with TPMT activities of 0.4-23 units͞ml pRBC (r s ؍ 0.99; P < 0.001).
Proceedings of The National Academy of Sciences, 2005
Thiopurine S-methyltransferase (TPMT) catalyzes the S-methylation of thiopurine drugs. TPMT genetic polymorphisms represent a striking example of the potential clinical value of pharmacogenetics. Subjects homozygous for TPMT*3A, the most common variant allele for low activity, an allele that encodes a protein with two changes in amino acid sequence, are at greatly increased risk for life-threatening toxicity when treated with standard doses of thiopurines. These subjects have virtually undetectable levels of TPMT protein.
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,
Cardiovascular & hematological agents in medicinal chemistry, 2017
Thiopurine S-methyltransferase (TPMT) enzyme metabolizes thiopurine drugs which are widely used in various disciplines as well as in leukemias. Individual enzyme activity varies depending on the genetic polymorphisms of TPMT gene located at chromosome 6. Up to 14% of population is known to have a decreased enzyme activity, and if treated with standard doses of thiopurines, these individuals are at the high risk of severe adverse drug reactions (ADR) as myelosuppression, gastrointestinal intolerance, pancreatitis and hypersensitivity. However, TPMT-deficient patients can successfully be treated with decreased thiopurine doses if enzyme status is identified by a prior testing. TPMT status identification is a pioneering experience in an application of a pharmacogenetic testing in clinical settings. 4 TPMT (*2,*3A, *3B, *3C) alleles are known to account for 80-95% of a decreased enzyme activity, and therefore, identifying the presence of these alleles supported by phenotypic measurement...
Dna and Cell Biology, 1996
Thiopurine methyltransferase (TPMT) catalyzes the S-methylation of thiopurine drugs. Individual variation in the toxicity and therapeutic efficacy of these drugs is associated with a common genetic polymorphism that controls levels of TPMT activity and immunoreactive protein in human tissues. Because of the clinical significance of the "pharmacogenetic" regulation of this enzyme, it would be important to clone the gene for TPMT in humans and to study the molecular basis for the genetic polymorphism. As a first step toward cloning the gene for TPMT, we used the rapid amplification of genomic DNA ends to obtain a TPMr-specific intron sequence. That DNA sequence was used to design primers for the polymerase chain reaction (PCR), which made it possible to determine that the active gene for TPMT is located on human chromosome 6. A TPMr-positive cosmid clone was then isolated from a human chromosome 6-specific genomic DNA library, and the gene was sublocalized to chromosome band 6p22.3 by fluorescence in situ hybridization. The gene for TPMT was found to be approximately 34 kb in length and consisted of 10 exons and 9 introns. On the basis of the results of 5'-rapid amplification of cDNA ends, transcription initiation occurred at or near a point 89
Biochemical and Biophysical Research Communications, 2003
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.
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...