Pharmacogenetics is the study of the role of inheritance in variation to drug response. Drug response phenotypes can vary from adverse drug reactions at one end of the spectrum to equally serious lack of the desired effect of drug therapy at the other. Many of the current important examples of pharmacogenetics involve inherited variation in drug metabolism. Sulfate conjugation catalyzed by cytosolic sulfotransferase (SULT) enzymes, particularly SULT1A1, is a major pathway for drug metabolism in humans. Pharmacogenetic studies of SULT1A1 began over a quarter of a century ago and have advanced from biochemical genetic experiments to include cDNA and gene cloning, gene resequencing, and functional studies of the effects of single nucleotide polymorphisms (SNPs). SNP genotyping, in turn, led to the discovery of functionally important copy number variations (CNVs) in the <i>SULT1A1 </i>gene. This review will briefly describe the evolution of our understanding of <i>SULT1A1 </i>pharmacogenetics and CNV, as well as challenges involved in utilizing both SNP and CNV data in an attempt to predict SULT1A1 function. <i>SULT1A1 </i>represents one example of the potential importance of CNV for the evolving disciplines of pharmacogenetics and pharmacogenomics.
Our experiments were performed to determine whether human liver, like that of other mammals, could catalyze the N-sulfation of an arylamine, 2-naphthylamine (2-NA) and, if so, whether this reaction might be catalyzed by one or both of the two known forms of human phenol sulfotransferase (PST). One form of PST is thermostable (TS) and catalyzes the sulfation of "simple" phenols such as p-nitrophenol, while the other form is thermolabile (TL) and catalyzes the sulfate conjugation of phenolic monoamines such as dopamine. When 2-NA that was not contaminated with 2-naphthol was used as substrate, human hepatic cytosol could catalyze the N-sulfation of 2-NA with an apparent Km of 322 microM. However, substrate kinetics of the sulfate donor for the reaction, 3'-phosphoadenosine-5'-phosphosulfate, were biphasic, with estimated apparent Km values of 0.13 and 2.2 microM for high and low affinity activities, respectively. Human liver arylamine N-sulfotransferase (AANST) activity was similar to that of TS but not TL PST with regard to thermal stability, inhibition by 2,6-dichloro-4-nitrophenol (DCNP), and regulation among individuals. For example, average temperatures that produced 50% inactivation of TL PST, TS PST, and AANST activities, measured with both 0.05 and 1.0 mM 2-NA as substrate, were 35.0, 40.5, 40.3 and 40.5 degrees C, respectively. IC50 values for the inhibition by DCNP of TL PST, TS PST, and AANST, measured with 0.05 and 1.0 mM 2-NA as substrate, were 110, 1.8, 1.3, and 4.0 microM, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)
Patients with Parkinson's Disease (PD) have a variable response to tolcapone, a catechol-O-methyltransferase (COMT) inhibitor. In addition, a subset of patients develop severe diarrhea as a side effect. Two codominant alleles for the COMT gene exist, coding for low and high activity, resulting in low-, medium-, and high-activity genotypes. This study investigates the relationship between this variation in genotype and clinical effects in patients with PD taking tolcapone. To investigate the relationship between COMT polymorphism and clinical response, 24 patients who completed tolcapone clinical trials provided blood samples for COMT genotype analysis. Change in levodopa dose and United Parkinson Disease Rating Scale (UPDRS) Part III (motor subscale) were analyzed at baseline, at 1–2 weeks, and 6 months after initiation of tolcapone. Genotype analysis was performed on seven patients who had diarrhea as a side effect. There was no significant correlation between genotype and improvement in UPDRS score (p = 0.29) according to a linear models approach that adjusted for the subject's severity of PD, tolcapone dose (either 100 or 200 mg three times daily) and initial differences in baseline scores. No significant difference was seen in change in daily levodopa intake and genotype. There was also no relation between diarrhea and COMT genotype. These results indicate that, in the treatment of Parkinson's disease, COMT genotype is not a major contributor to the clinical response to tolcapone.
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.
Phenol sulfotransferase (PST) catalyzes the sulfate conjugation of phenolic drugs. All human tissues studied contain a thermostable (TS) form of PST, which catalyzes the sulfate conjugation of “simple” phenols such as p-nitrophenol, and a thermolabile (TL) form, which catalyzes the sulfation of dopamine and other monoamines. In the present study we tested the hypothesis that genetically controlled levels of TS and TL PST activity in the platelet, as well as inherited variations in the thermal stability of platelet TS PST, might reflect those same characteristics of the enzyme in a less accessible tissue, human small intestinal mucosa. Platelet TS and TL PST activities and TS PST thermal stability were measured in blood samples from 45 randomly selected healthy subjects, and 14 of those subjects were selected to have intestinal biopsies performed. There was a significant correlation between levels of platelet and jejunal mucosal TS PST activity (rs = 0.574, p < 0.030), but there was not a significant correlation between levels of TL PST activity in the two tissues (rs = 0.265, p = 0.368). There was also a significant correlation between the trait of TS PST thermal stability in the two tissues (rs = 0.828, p < 0.0001). These observations suggest that inherited variations in TS PST activity and thermal stability in an easily obtained tissue, the platelet, might be used to predict individual differences of those properties of the enzyme in the human small intestine, an organ that plays an important role in drug metabolism. Clinical Pharmacology and Therapeutics (1989) 46, 501–509; doi:10.1038/clpt.1989.178
Common genetic variation in CYP2C19 (cytochrome P450, family 2, subfamily C, polypeptide 19) *2 and *3 alleles leads to a loss of functional protein, and carriers of these loss-of-function alleles when treated with clopidogrel have significantly reduced clopidogrel active metabolite levels and high on-treatment platelet reactivity resulting in increased risk of major adverse cardiovascular events, especially after percutaneous coronary intervention. The Food and Drug Administration has issued a black box warning advising practitioners to consider alternative treatment in CYP2C19 poor metabolizers who might receive clopidogrel and to identify such patients by genotyping. However, routine clinical use of genotyping for CYP2C19 loss-of-function alleles in patients undergoing percutaneous coronary intervention is not recommended by clinical guidelines because of lack of prospective evidence. To address this critical gap, TAILOR-PCI (Tailored Antiplatelet Initiation to Lessen Outcomes due to Decreased Clopidogrel Response After Percutaneous Coronary Intervention) is a large, pragmatic, randomized trial comparing point-of-care genotype-guided antiplatelet therapy with routine care to determine whether identifying CYP2C19 loss-of-function allele patients prospectively and prescribing alternative antiplatelet therapy is beneficial.
