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.

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.

Cutaneous melanin pigment plays a critical role in camouflage, mimicry, social communication, and protection against harmful effects of solar radiation. Melanogenesis is under complex regulatory control by multiple agents interacting via pathways activated by receptor-dependent and -independent mechanisms, in hormonal, auto-, para-, or intracrine fashion. Because of the multidirectional nature and heterogeneous character of the melanogenesis modifying agents, its controlling factors are not organized into simple linear sequences, but they interphase instead in a multidimensional network, with extensive functional overlapping with connections arranged both in series and in parallel. The most important positive regulator of melanogenesis is the MC1 receptor with its ligands melanocortins and ACTH, whereas among the negative regulators agouti protein stands out, determining intensity of melanogenesis and also the type of melanin synthesized. Within the context of the skin as a stress organ, melanogenic activity serves as a unique molecular sensor and transducer of noxious signals and as regulator of local homeostasis. In keeping with these multiple roles, melanogenesis is controlled by a highly structured system, active since early embryogenesis and capable of superselective functional regulation that may reach down to the cellular level represented by single melanocytes. Indeed, the significance of melanogenesis extends beyond the mere assignment of a color trait.

https://www.physiology.org/doi/pdf/10.1152/physrev.00044.2003

Melanin Pigmentation in Mammalian Skin and Its Hormonal Regulation

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.

The enzyme-catalyzed O-methylation of catecholamines was first described by Axelrod and coworkers in the late 1950’s [1–3]. They called the responsible enzyme catechol O-methyltransferase (COMT). During the subsequent 15 years the enzyme was partially purified, its distribution was established, several reaction mechanisms were proposed, and a number of inhibitors were described. The results of this study period were extensively reviewed by Guldberg and Marsden in 1975 [4].

Characteristics of catechol O-methyltransferase (COMT) and properties of selective COMT inhibitors

In the present study we show the distribution of catechol-O- methyltransferase (COMT) in various rat tissues with a highly specific antiserum prepared against recombinant rat COMT. Immunoprecipitation and immunocytochemical controls confirmed the COMT-specificity of the antibodies. The antiserum detected both the 24 KD soluble and the 28 KD membrane-bound forms of the enzyme. By immunohistochemical staining the COMT enzyme was found in most rat tissues. Staining was most intense in the liver and in the kidney, in agreement with previous studies and our immunoblotting results. In the gastrointestinal tract, epithelial cells of the stomach, duodenum, and ileum were immunoreactive for COMT. In pancreas, COMT immunoreactivity was found in insulin-producing β-cells and somatostatin-producing D-cells but not in glucagon-producing α-cells of the islets of Langerhans. In pituitary, COMT immunoreactivity was found in cleft cells, in pituicytes of the posterior lobe, and in the anterior lobe, partly in the same cells containing luteinizing hormone (LH). In other endocrine organs, COMT immunoreactivity was found in epithelial cells of the thyroid gland and in zona glomerulosa of the adrenal cortex. In the brain, brightest immunofluorescence was seen in ependymal cells of the cerebral ventricles and choroid plexus. Weak to moderate immunofluorescence was found in the neuropil of several brain areas, including striatum and cortex. Scattered small neurons in spinal sensory ganglia were also COMT immunoreactive. Previous immunocytochemical studies, enzyme activity determinations, and distribution of the COMT mRNA are in general agreement with the results presented here. The wide distribution of COMT in different tissues suggests an important role for this protein in inactivation of catechol compounds. (Less)

Distribution of catechol-O-methyltransferase enzyme in rat tissues.

The rat and human recombinant soluble and membrane-bound catechol O-methyltransferase (S- and MB-COMT, respectively) were expressed using mammalian and baculovirus vectors. Low levels of rat and human S-COMT polypeptides were detected by immunoprecipitation in K-562 cell lines transfected with the S-COMT vectors. From K-562 cells transfected with the rat MB-COMT construct, both S- and MB-COMT recombinant proteins were detected by a rat COMT-specific anti-serum. Infection of lepidopteran Spodoptera frugiperda cells with recombinant S- or MB-COMT baculovirus constructs yielded high amounts of enzymically active and immunoreactive S- or MB-COMT proteins, respectively. Pulse/chase experiments with [35S]methionine-labelled insect cells infected with the MB-COMT baculovirus showed that the 30-kDa recombinant human MB-COMT polypeptide was not processed into the 25-kDa S-COMT form. Subcellular fractionations of insect cells, followed by immunoblotting with COMT antiserum, showed that recombinant S-COMT was found only in the soluble, cytoplasmic fraction, whereas MB-COMT resided both in soluble and membrane fractions. The recombinant MB-COMT sedimented in Percoll gradients at the density of 1.042 g/ml cosedimenting with the plasma-membrane marker. Fractionation and immunoblotting experiments on homogenized total rat brains indicated that the rat S-COMT (24 kDa) and some of the rat MB-COMT (28 kDa) was recovered in soluble fractions, whereas the microsomal material having COMT activity contained the MB-COMT polypeptide. The rat brain microsomal MB-COMT had a density of 1.042 g/ml in Percoll gradients, cosedimenting with the plasma-membrane and rough-endoplasmic-reticulum marker enzymes. The meta/para methylation ratio of dihydroxybenzoic-acid substrate by different recombinant and rat brain COMT-containing subcellular fractions was analysed.

https://febs.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1432-1033.1992.tb17112.x

Expression of recombinant soluble and membrane‐bound catechol O‐methyltransferase in eukaryotic cells and identification of the respective enzymes in rat brain

I. Ulmanen

Papers

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

Characteristics of catechol O-methyltransferase (COMT) and properties of selective COMT inhibitors

Distribution of catechol-O-methyltransferase enzyme in rat tissues.

Expression of recombinant soluble and membrane‐bound catechol O‐methyltransferase in eukaryotic cells and identification of the respective enzymes in rat brain

Neuronal and non-neuronal catechol-O-methyltransferase in primary cultures of rat brain cells