Abnormalities of prefrontal cortical function are prominent features of schizophrenia and have been associated with genetic risk, suggesting that susceptibility genes for schizophrenia may impact on the molecular mechanisms of prefrontal function. A potential susceptibility mechanism involves regulation of prefrontal dopamine, which modulates the response of prefrontal neurons during working memory. We examined the relationship of a common functional polymorphism (Val(108/158) Met) in the catechol-O-methyltransferase (COMT) gene, which accounts for a 4-fold variation in enzyme activity and dopamine catabolism, with both prefrontally mediated cognition and prefrontal cortical physiology. In 175 patients with schizophrenia, 219 unaffected siblings, and 55 controls, COMT genotype was related in allele dosage fashion to performance on the Wisconsin Card Sorting Test of executive cognition and explained 4% of variance (P = 0.001) in frequency of perseverative errors. Consistent with other evidence that dopamine enhances prefrontal neuronal function, the load of the low-activity Met allele predicted enhanced cognitive performance. We then examined the effect of COMT genotype on prefrontal physiology during a working memory task in three separate subgroups (n = 11-16) assayed with functional MRI. Met allele load consistently predicted a more efficient physiological response in prefrontal cortex. Finally, in a family-based association analysis of 104 trios, we found a significant increase in transmission of the Val allele to the schizophrenic offspring. These data suggest that the COMT Val allele, because it increases prefrontal dopamine catabolism, impairs prefrontal cognition and physiology, and by this mechanism slightly increases risk for schizophrenia.

http://enigma.ini.usc.edu/wp-content/uploads/2010/04/Effect-of-COMT-Val108-158-Met-genotype-on-frontal-lobe-function-and-risk-for-schizophrenia.pdf

Effect of COMT Val108/158 Met genotype on frontal lobe function and risk for schizophrenia.

The goal of this review is to present a comprehensive survey of the many intriguing facets of creatine (Cr) and creatinine metabolism, encompassing the pathways and regulation of Cr biosynthesis an...

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Creatine and Creatinine Metabolism

This review critically summarizes the neuropathology and genetics of schizophrenia, the relationship between them, and speculates on their functional convergence. The morphological correlates of schizophrenia are subtle, and range from a slight reduction in brain size to localized alterations in the morphology and molecular composition of specific neuronal, synaptic, and glial populations in the hippocampus, dorsolateral prefrontal cortex, and dorsal thalamus. These findings have fostered the view of schizophrenia as a disorder of connectivity and of the synapse. Although attractive, such concepts are vague, and differentiating primary events from epiphenomena has been difficult. A way forward is provided by the recent identification of several putative susceptibility genes (including neuregulin, dysbindin, COMT, DISC1, RGS4, GRM3, and G72). We discuss the evidence for these and other genes, along with what is known of their expression profiles and biological roles in brain and how these may be altered in schizophrenia. The evidence for several of the genes is now strong. However, for none, with the likely exception of COMT, has a causative allele or the mechanism by which it predisposes to schizophrenia been identified. Nevertheless, we speculate that the genes may all converge functionally upon schizophrenia risk via an influence upon synaptic plasticity and the development and stabilization of cortical microcircuitry. NMDA receptor-mediated glutamate transmission may be especially implicated, though there are also direct and indirect links to dopamine and GABA signalling. Hence, there is a correspondence between the putative roles of the genes at the molecular and synaptic levels and the existing understanding of the disorder at the neural systems level. Characterization of a core molecular pathway and a 'genetic cytoarchitecture' would be a profound advance in understanding schizophrenia, and may have equally significant therapeutic implications.

Schizophrenia genes, gene expression, and neuropathology: on the matter of their convergence.

Catechol-O-methyltransferase (COMT) is a key enzyme in the elimination of dopamine in the prefrontal cortex of the human brain. Genetic variation in the COMT gene (MIM 116790) has been associated with altered prefrontal cortex function and higher risk for schizophrenia, but the specific alleles and their functional implications have been controversial. We analyzed the effects of several single-nucleotide polymorphisms (SNPs) within COMT on mRNA expression levels (using reverse-transcriptase polymerase chain reaction analysis), protein levels (using Western blot analysis), and enzyme activity (using catechol methylation) in a large sample (n = 108) of postmortem human prefrontal cortex tissue, which predominantly expresses the -membrane-bound isoform. A common coding SNP, Val158Met (rs4680), significantly affected protein abundance and enzyme activity but not mRNA expression levels, suggesting that differences in protein integrity account for the difference in enzyme activity between alleles. A SNP in intron 1 (rs737865) and a SNP in the 3′ flanking region (rs165599)—both of which have been reported to contribute to allelic expression differences and to be associated with schizophrenia as part of a haplotype with Val—had no effect on mRNA expression levels, protein immunoreactivity, or enzyme activity. In lymphocytes from 47 subjects, we confirmed a similar effect on enzyme activity in samples with the Val/Met genotype but no effect in samples with the intron 1 or 3′ SNPs. Separate analyses revealed that the subject's sex, as well as the presence of a SNP in the P2 promoter region (rs2097603), had small effects on COMT enzyme activity. Using site-directed mutagenesis of mouse COMT cDNA, followed by in vitro translation, we found that the conversion of Leu at the homologous position into Met or Val progressively and significantly diminished enzyme activity. Thus, although we cannot exclude a more complex genetic basis for functional effects of COMT, Val is a predominant factor that determines higher COMT activity in the prefrontal cortex, which presumably leads to lower synaptic dopamine levels and relatively deleterious prefrontal function.

Functional Analysis of Genetic Variation in Catechol-O-Methyltransferase (COMT): Effects on mRNA, Protein, and Enzyme Activity in Postmortem Human Brain

https://www.ukcia.org/research/COMTgene.pdf

Moderation of the effect of adolescent-onset cannabis use on adult psychosis by a functional polymorphism in the catechol-O-methyltransferase gene: longitudinal evidence of a gene X environment interaction.

Human soluble (S) and membrane-bound (MB) catechol O-methyltransferase (COMT, EC 2.1.1.6) enzymes have been expressed at sufficiently high levels in Escherichia coli and in baculovirus-infected insect cells to allow kinetic characterization of the enzyme forms. The use of tight-binding inhibitors such as entacapone enabled the estimation of actual enzyme concentrations and, thereby, comparison of velocity parameters, substrate selectivity, and regioselectivity of the methylation of both enzyme forms. Kinetics of the methylation reaction of dopamine, (-)-noradrenaline, L-dopa, and 3,4-dihydroxybenzoic acid was studied in detail. Here, the catalytic number (Vmax) of S-COMT was somewhat higher than that of MB-COMT for all four substrates. The Km values varied considerably, depending on both substrate and enzyme form. S-COMT showed about 15 times higher Km values for catecholamines than MB-COMT. The distinctive difference between the enzyme forms was also the higher affinity of MB-COMT for the coenzyme S-adenosyl-L-methionine (AdoMet). The average dissociation constants Ks were 3.4 and 20.2 microM for MB-COMT and S-COMT, respectively. Comparison between the kinetic results and the atomic structure of S-COMT is presented, and a revised mechanism for the reaction cycle is discussed. Two recently published human COMT cDNA sequences differed in the position of S-COMT amino acid 108, the residue being either Val-108 [Lundstrom et al. (1991) DNA Cell. Biol. 10, 181-189] or Met-108 [Bertocci et al. (1991) Proc. Natl. Acad. Sci. U.S.A. 88, 1416-1420].(ABSTRACT TRUNCATED AT 250 WORDS)

Kinetics of human soluble and membrane-bound catechol O-methyltransferase: a revised mechanism and description of the thermolabile variant of the enzyme.

Catechol O-methyltransferase (COMT, EC 2.1.1.6) is important in the central nervous system because it metabolizes catecholamine neurotransmitters such as dopamine. The enzyme catalyses the transfer of the methyl group from S-adenosyl-L-methionine (AdoMet) to one hydroxyl group of catechols. COMT also inactivates catechol-type compounds such as L-DOPA. With selective inhibitors of COMT in combination with L-DOPA, a new principle has been realized in the therapy of Parkinson's disease. Here we solve the atomic structure of COMT to 2.0 A resolution, which provides new insights into the mechanism of the methyl transfer reaction. The co-enzyme-binding domain is strikingly similar to that of an AdoMet-dependent DNA methylase, indicating that all AdoMet methylases may have a common structure.

Crystal structure of catechol O-methyltransferase

A complex of carbonic anhydrase (CA) with one of its substrates, bicarbonate, has been studied crystallographically. Human isoenzyme II was mutated at position 200 from threonine to histidine, which results in higher affinity for bicarbonate. The HCO3- ion binds in the active site to the zinc ion as a pseudo-bidentate ligand which gives the metal a coordination geometry between tetrahedral and trigonal bipyramide. The water/hydroxide normally bound with tetrahedral coordination to the zinc is probably replaced by the OH group of the bicarbonate ion. The importance of residues Thr-199 and Glu-106 in controlling the binding orientation of HCO3- is discussed as well as the catalytic mechanism. Both the complex as well as the uncomplexed mutant were studied at 1.9 A resolution.

Jukka Vidgren

Papers

Kinetics of human soluble and membrane-bound catechol O-methyltransferase: a revised mechanism and description of the thermolabile variant of the enzyme.

Crystal structure of catechol O-methyltransferase

Crystallographic analysis of Thr-200-->His human carbonic anhydrase II and its complex with the substrate, HCO3-.

Refined structure of the acetazolamide complex of human carbonic anhydrase II at 1.9 Å

Refined structure of the aminobenzolamide complex of human carbonic anhydrase II at 1.9 A and sulphonamide modelling of bovine carbonic anhydrase III.