Pharmacogenetics: inherited variation in amino acid sequence and altered protein quantity.
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Amino Acid Sequence↗
Biomedical subjects
Richard Weinshilboum
Publications and source records attributed to Richard Weinshilboum.
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.
Arylamine N-Acetyltransferase↗
Intensification of mercaptopurine/methotrexate maintenance chemotherapy may increase the risk of relapse for some children with acute lymphoblastic leukemia.
PURPOSE: Thioguanine nucleotides (TGNs) mediate the cytotoxicity of mercaptopurine (MP). Methylated MP metabolites (formed by thiopurine methyltransferase [TPMT]) and methotrexate (MTX) polyglutamates can inhibit de novo purine synthesis. We explored whether dose adjustment of MP and MTX by erythrocyte (E) levels of TGN and MTX (including polyglutamates) could improve outcome in childhood acute lymphoblastic leukemia (ALL). PATIENTS AND METHODS: A total of 538 children with ALL were randomly assigned to have their oral MP/MTX maintenance therapy adjusted by white cell counts (WBC), E-TGN, and E-MTX (pharmacology group), or by WBC only (control group). RESULTS: After a median follow-up of 7.8 years, 79 patients had relapsed. Cox regression analysis showed an increased risk of relapse for boys (P =.00003), high WBC at diagnosis (P =.03), pharmacology arm (6.6 times increased relapse hazard for girls), high TPMT activity (P =.002), and high average neutrophil counts during maintenance therapy (P =.0009), with a significant interaction between sex and randomization group (P =.0007). For girls, the relapse risk was 5% in the control group and 19% in the pharmacology group (P =.001) because of an increased relapse hazard during the first year after cessation of therapy. TPMT activity was the most significant predictor of relapses among girls in the pharmacology arm (P <.0001). Overall, the TPMT activity was higher for patients who relapsed after cessation of therapy compared with those who stayed in remission (girls 19.5 v 17.4 U/mL, P =.03; boys 19.3 v 18.0 U/mL, P =.04). CONCLUSION: Adding pharmacologically guided treatment intensification to dose adjustments by blood counts may not be warranted for girls, whereas new approaches to optimize maintenance therapy are needed for boys.
Administration, Oral↗
Inheritance and drug response.
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Cytochrome P-450 Enzyme System↗
Thiopurine S-methyltransferase pharmacogenetics: chaperone protein association and allozyme degradation.
Thiopurine S-methyltransferase (TPMT) catalyses the S-methylation of thiopurine drugs such as 6-mercaptopurine. A common genetic polymorphism for TPMT is associated with large individual variations in thiopurine drug toxicity and therapeutic efficacy. TPMT*3A, the most common variant allele in Caucasians, has two alterations in amino acid sequence, resulting in striking decreases in TPMT protein levels. This phenomenon results, in part, from rapid degradation through a ubiquitin-proteasome-mediated process. We set out to test the hypothesis that chaperone proteins might be involved in targeting TPMT for degradation. As a first step, hsp90, hsp70 and the cochaperone hop were immunoprecipitated from a rabbit reticulocyte lysate (RRL) that included radioactively labelled *3A and wild-type TPMT. TPMT*3A was much more highly associated with all three chaperones than was the wild-type enzyme. The RRL was also used to confirm the accelerated degradation of *3A compared to wild-type TPMT. Treatment of RRL with the hsp90 inhibitor geldanamycin resulted in enhanced association of hsp90 with wild-type TPMT, an observation that correlated with accelerated ubiquitin-dependent degradation of wild-type TPMT. Geldanamycin treatment of COS-1 cells transfected with FLAG-tagged wild-type also resulted in a time and geldanamycin concentration-dependent decrease in TPMT activity and protein, which was compatible with results obtained in the RRL. These observations indicate that TPMT is a client protein for hsp90 and suggest that chaperone proteins, especially hsp90, are involved in targeting both TPMT*3A and, in the presence of geldanamycin, the wild-type allozyme for degradation. Therefore, chaperone proteins play an important mechanistic role in this clinically significant example of pharmacogenetic variation in drug metabolism.
Alleles↗
Richard Weinshilboum: Pharmacogenetics: The future is here!
Richard Weinshilboum has a no-nonsense attitude about pharmacogenetics. He is enthusiastic about the practicalities and ramifications of the field's solid accomplishments, but he carefully measures statements that might feed the hype that is en courant about the brave new postgenomic world of drug therapy. Although the terms "pharmacogenomics" and "pharmacogenetics" are often used interchangeably (a linguistic quirk to which Weinshilboum does not object), he consistently avoids the latter, perhaps more glitzy, word. Weinshilboum has spent over thirty years as a clinical pharmacologist, exploring in particular the variability of drug metabolism that occurs among patients as a function of their genetic constitution. The research efforts from his line of work have materialized into clinical application and have helped to set the stage for the individualization of drug treatment according to each patient's genetic constitution-not yet on the genomewide scale that Weinshilboum enthusiastically foresees, but certainly as pertains to multiple genes and drugs for any given patient. The interview with Weinshilboum occurred at this year's annual meeting of ASPET, at which he was conferred the Harry Gold Award in Clinical Pharmacology.
Bioethics↗
Human histamine N-methyltransferase pharmacogenetics: gene resequencing, promoter characterization, and functional studies of a common 5'-flanking region single nucleotide polymorphism (SNP).
Histamine N-methyltransferase (HNMT) catalyzes one of two major metabolic pathways for histamine. The levels of HNMT activity and immunoreactive protein in human tissues are regulated primarily by inheritance. Previous studies of HNMT identified two common single nucleotide polymorphisms (SNPs), including a functionally significant nonsynonymous coding SNP (cSNP), (C314T, Thr105Ile), but that polymorphism did not explain all of the phenotypic variation. In the present study, a genotype-to-phenotype strategy was used to search for additional genetic factors that might contribute to the regulation of human HNMT activity. Specifically, we began by resequencing the human HNMT gene using 90 ethnically anonymous DNA samples from the Coriell Cell Repository and identified a total of eight SNPs, including the two that had been reported previously. No new nonsynonymous cSNPs were observed, but three of the six novel SNPs were located in the 5'-flanking region (5'-FR) of the gene-including a third common polymorphism with a frequency of 0.367 (36.7%). That observation directed our attention to possible genetic effects on HNMT transcription. As a first step in testing that possibility, we created and studied a series of reporter gene constructs for the initial 1kb of the HNMT 5'-FR. The core promoter and possible regulatory regions were identified and verified by electrophoresis mobility shift assays. We then studied the possible functional implications of the new common HNMT 5'-FR SNP. However, on the basis of reporter gene studies, that SNP appeared to have little effect on transcription. Phenotype-genotype correlation analysis performed with 112 human kidney biopsy samples that had been phenotyped for their level of HNMT activity confirmed that the common 5'-FR SNP was not associated with the level of HNMT activity in vivo. In summary, this series of experiments resulted in the identification of several novel HNMT polymorphisms, identification of the HNMT core promoter, and a comprehensive functional genomic study of a common HNMT 5'-FR SNP. These results represent an additional step in the definition of molecular genetic mechanisms involved in the regulation of this important autacoid-metabolizing enzyme in humans.
5' Flanking Region↗