HNMT Upregulation Induces Cancer Stem Cell Formation and Confers Protection against Oxidative Stress through Interaction with HER2 in Non-Small-Cell Lung Cancer.

BACKGROUND: The treatment of non-small-cell lung cancer (NSCLC) involves platinum-based chemotherapy. It is typically accompanied by chemoresistance resulting from antioxidant properties conferred by cancer stem cells (CSCs). Human epidermal growth factor receptor 2 (HER2) enhances CSCs and antioxidant properties in cancers, including NSCLC. METHODS: Here, we elucidated the role of histamine N-methyltransferase (HNMT), a histamine metabolism enzyme significantly upregulated in NSCLC and coexpressed with HER2. HNMT expression in lung cancer tissues was determined using quantitative reverse transcription PCR (RT-qPCR). A publicly available dataset was used to determine HNMT's potential as an NSCLC target molecule. Immunohistochemistry and coimmunoprecipitation were used to determine HNMT-HER2 correlations and interactions, respectively. HNMT shRNA and overexpression plasmids were used to explore HNMT functions in vitro and in vivo. We also examined miRNAs that may target HNMT and investigated HNMT/HER2's role on NSCLC cells' antioxidant properties. Finally, how HNMT loss affects NSCLC cells' sensitivity to cisplatin was investigated. RESULTS: HNMT was significantly upregulated in human NSCLC tissues, conferred a worse prognosis, and was coexpressed with HER2. HNMT depletion and overexpression respectively decreased and increased cell proliferation, colony formation, tumorsphere formation, and CSCs marker expression. Coimmunoprecipitation analysis indicated that HNMT directly interacts with HER2. TARGETSCAN analysis revealed that HNMT is a miR-223 and miR-3065-5p target. TBHp treatment increased HER2 expression, whereas shHNMT disrupted the Nuclear factor erythroid 2-related factor 2 (Nrf2)/ hemeoxygenase-1 (HO-1)/HER2 axis and increased reactive oxygen species accumulation in NSCLC cells. Finally, shHNMT sensitized H441 cells to cisplatin treatment in vitro and in vivo. CONCLUSIONS: Therefore, HNMT upregulation in NSCLC cells may upregulate HER2 expression, increasing tumorigenicity and chemoresistance through CSCs maintenance and antioxidant properties. This newly discovered regulatory axis may aid in retarding NSCLC progression and chemoresistance.

MicroRNA-223 is involved in the pathogenesis of atopic dermatitis by affecting histamine-N-methyltransferase.

Atopic dermatitis (AD) is one of the most prevalent skin diseases around the world. Excessive histamine plays a critical role as an inflammatory factor in the pathogenesis of AD. Deregulated microRNAs (miRNAs) were involved in atopic dermatitis by targeting various genes. MiR-223 had been reported to play a vital role in hematopoiesis. In this study, we identified upregulated miR-223 in the whole blood cells of a large group of AD patients. What's more, we found for the first time that one of the major histamine degradation enzymes, histamine-N-methyltransferase (HNMT), was increased in AD patients and AD model mice. Although there was one miR-223 binding site in the 3'- untranslated region of the HNMT gene, HNMT were not inhibited by miR-223. Taken together, it suggested that miR-223 participates in AD through upregulating HNMT indirectly to degrade the excessive histamine.

Histaminergic gene polymorphisms associated with sedation in clozapine-treated patients.

Sedation is a common adverse effect of clozapine treatment, which may be partly related to clozapine binding to histamine receptors in the central nervous system. The objective of this study was to investigate whether single nucleotide polymorphisms (SNPs) in the histaminergic system are associated with sedation in clozapine-treated patients. The study population comprised 237 clozapine-treated, Finnish, Caucasian patients that were diagnosed with schizophrenia and 176 were genotyped using Illumina HumanCoreExome-12 BeadChip. Sedation levels were assessed using self-rating questions from the Liverpool University Neuroleptic Side Effect Rating Scale (LUNSERS). The relationships between 55 different SNPs in the histaminergic system and adverse sedation effects were examined. SNPs were analyzed separately, and in groups, to formulate a genetic risk score (GRS). A permutation test was performed to avoid type I errors. Eight linked SNPs (r2 = 1) in the HNMT gene were also associated with sedation according to the GLM, adjusted for age, gender and BMI (false-discovery-rate-adjusted p = 0.013). An association on a trend level between a GRS of four different SNPs (recessive histamine N-methyltransferase HNMT rs2737385, additive histamine receptor H1 rs1552498, dominant HRH1 rs17034063 and recessive amine oxidase, copper containing 1 AOC1 rs6977381) and sedation was found (permuted p-value = 0.066) in a generalized linear model (GLM) incorporating age, gender and body mass index (BMI; adjusted R2 = 0.22). Polymorphisms in genes encoding histamine receptors or enzymes related to histamine metabolism may explain individual variation in sedative effects experienced during clozapine treatment.

Leucine 208 in human histamine N-methyltransferase emerges as a hotspot for protein stability rationalizing the role of the L208P variant in intellectual disability.

The degradation of histamine catalyzed by the SAM-dependent histamine N-methyltransferase (HNMT) is critically important for the maintenance of neurological processes. Recently, two mutations in the encoding human gene were reported to give rise to dysfunctional protein variants (G60D and L208P) leading to intellectual disability. In the present study, we have expressed eight L208 variants with either apolar (L208F and L208V), polar (L208N and L208T) or charged (L208D, L208H, L208K and L208R) amino acids to define the impact of side chain variations on protein structure and function. We found that the variants L208N, L208T, L208D and L208H were severely compromised in their stability. The other four variants were obtained in lower amounts in the order wild-type HNMT>L208F=L208V>L208K=L208R. Biochemical characterization of the two variants L208F and L208V exhibited similar Michaelis-Menten parameters for SAM and histamine while the enzymatic activity was reduced to 21% and 48%, respectively. A substantial loss of enzymatic activity and binding affinity for histamine was seen for the L208K and L208R variants. Similarly the thermal stability for the latter variants was reduced by 8 and 13°C, respectively. These findings demonstrate that position 208 is extremely sensitive to side chain variations and even conservative replacements affect enzymatic function. Molecular dynamics simulations showed that amino acid replacements in position 208 perturb the helical character and disrupt interactions with the adjacent ß-strand, which is involved in the binding and correct positioning of histamine. This finding rationalizes the gradual loss of enzymatic activity observed in the L208 variants.

Mutations in the histamine N-methyltransferase gene, HNMT, are associated with nonsyndromic autosomal recessive intellectual disability.

Histamine (HA) acts as a neurotransmitter in the brain, which participates in the regulation of many biological processes including inflammation, gastric acid secretion and neuromodulation. The enzyme histamine N-methyltransferase (HNMT) inactivates HA by transferring a methyl group from S-adenosyl-l-methionine to HA, and is the only well-known pathway for termination of neurotransmission actions of HA in mammalian central nervous system. We performed autozygosity mapping followed by targeted exome sequencing and identified two homozygous HNMT alterations, p.Gly60Asp and p.Leu208Pro, in patients affected with nonsyndromic autosomal recessive intellectual disability from two unrelated consanguineous families of Turkish and Kurdish ancestry, respectively. We verified the complete absence of a functional HNMT in patients using in vitro toxicology assay. Using mutant and wild-type DNA constructs as well as in silico protein modeling, we confirmed that p.Gly60Asp disrupts the enzymatic activity of the protein, and that p.Leu208Pro results in reduced protein stability, resulting in decreased HA inactivation. Our results highlight the importance of inclusion of HNMT for genetic testing of individuals presenting with intellectual disability.

Associations of polymorphisms in histidine decarboxylase, histamine N-methyltransferase and histamine receptor H3 genes with breast cancer.

We previously found that genetic polymorphisms in gene coding for histamine H4 receptors were related to the risk and malignant degree of breast cancer. The roles of polymorphisms in other histamine-related genes, such as histidine decarboxylase (HDC), histamine N-methyltransferase (HNMT) and histamine H3 receptor (HRH3), remain unexplored. The aim of this study is to analyze the clinical associations of polymorphisms in HDC, HNMT and HRH3 with breast cancer. Two hundred and one unrelated Chinese Han breast cancer patients and 205 ethnicity-matched health controls were recruited for case-control investigation. Genomic DNA from the participants was extracted and 5 single nucleotide polymorphisms (SNPs) in HDC, HNMT and HRH3 were genotyped. We found that polymorphisms of HNMT and HRH3 were irrelevant with breast cancer in the present study. However, the T allele of rs7164386 in HDC significantly decreased the risk of breast cancer (adjusted odds ratios [ORs], 0.387; 95% confidence intervals [CIs], 0.208-0.720; P = 0.003). Furthermore, for HDC haplotypes, the CG haplotype of rs7164386-rs7182203 was more frequent among breast cancer patients (adjusted OR, 1.828; 95% CI, 1.218-2.744; P = 0.004) while the TG haplotype was more frequent among health controls (adjusted OR, 0.351; 95% CI, 0.182-0.678; P = 0.002). These findings indicated that polymorphisms of HDC gene were significantly associated with breast cancer in Chinese Han population and may be novel diagnostic or therapeutic targets for breast cancer. Further studies with larger participants worldwide are still needed for conclusion validation.

Association of myasthenia gravis with polymorphisms in the gene of histamine N-methyltransferase.

INTRODUCTION: Histamine N-methyltransferase (HNMT) is the main metabolizing enzyme of histamine. Histamine modulates immune responses and plays a role in the pathogenesis of autoimmune disorders. METHODS: The non-synonymous HNMT C314T polymorphism and the A939G single-nucleotide polymorphism (SNP) influencing HNMT mRNA stability were genotyped in 213 patients with myasthenia gravis (MG) and 342 healthy controls. RESULTS: The carrier frequency of the A allele of the A939G SNP was over-represented among patients with anti-AchR and anti-Titin antibodies (P = 0.05 and P = 0.004, respectively); the presence of the minor G allele was protective against anti-AchR and anti-Titin positive MG (OR = 0.67 and OR = 0.54, respectively). The combination of the G allele carrier status with wild-type C314C homozygosity was also protective against MG (OR = 0.55, P = 0.008) and against the development of anti-AchR antibodies (OR = 0.37, P = 0.01). DISCUSSION: The A939G HNMT polymorphism is associated with autoimmune MG, while no association with C314T SNP was found.

Histamine transport and metabolism are deranged in salivary glands in Sjogren's syndrome.

OBJECTIVE: To study histamine transport and metabolism of salivary gland (SG) epithelial cells in healthy controls and SS patients. METHODS: Enzymes and transporters involved in histamine metabolism were analysed in cultured human submandibular salivary gland (HSG) epithelial cells and tissue sections using quantitative real-time PCR and immunostaining. HSG cells were used to study [(3)H]histamine uptake [(±1-methyl-4-phenylpyridinium (MPP)] and efflux by liquid scintillation counting. RESULTS: mRNA levels of l-histidine decarboxylase (HDC) and histamine-N-methyltransferase (HNMT) were similar in the control and SS glands, but diamine oxidase was not expressed at all. Organic cation transporter 3 (OCT3) in healthy SG was localized in the acinar and ductal cells, whereas OCT2 was restricted to the myoepithelial cells. Both transporters were significantly decreased in SS at mRNA and protein levels. OCT3-mRNA levels in HSG cells were significantly higher than those of the other studied transporters. Uptake of [(3)H]histamine was inhibited by MPP in a time-dependent manner, whereas [(3)H]histamine-preloaded HSG cells released it. CONCLUSION: Ductal epithelial cells are non-professional histamine-producing cells able to release histamine via OCTs at the resting state up to ∼100 nM, enough to excite H3R/H4R(+) epithelial cells, but not H1R, which requires burst release from mast cells. At the stimulated phase, 50-60 µM histamine passes from the interstitial fluid through the acinar cells to saliva, whereas uptake by ductal cells leads to intracellular degradation by HNMT. OCT3/histamine/H4R-mediated cell maintenance and down-regulation of high histamine levels fail in SS SGs.

Molecular mechanism of histamine clearance by primary human astrocytes.

Histamine clearance is an essential process for avoiding excessive histaminergic neuronal activity. Previous studies using rodents revealed the predominant role of astrocytes in brain histamine clearance. However, the molecular mechanism of histamine clearance has remained unclear. We detected histamine N-methyltransferase (HNMT), a histamine-metabolizing enzyme, in primary human astrocytes and the astrocytes of human brain specimens. Immunocytochemical analysis and subcellular fractionation assays revealed that active HNMT localized to the cytosol, suggesting that histamine transport into the cytosol is crucial for histamine inactivation. We showed that primary human astrocytes transported histamine in a time-dependent manner. Kinetics analysis showed that two low-affinity transporters were involved in histamine transport. Histamine uptake by primary human astrocytes was not dependent on the extracellular Na(+) /Cl(-) concentration. Histamine is reported to be a substrate for three low-affinity and Na(+) /Cl(-) -independent transporters: organic cation transporter 2 (OCT2), OCT3, and plasma membrane monoamine transporter (PMAT). RT-PCR analysis revealed that OCT3 and PMAT were expressed in primary human astrocytes. Immunohistochemistry confirmed OCT3 and PMAT expression in the astrocytes of human brain specimens. Drug inhibition assays and gene knockdown assays revealed the major contribution of PMAT and the minor contribution of OCT3 to histamine transport. The present study demonstrates for the first time that the molecular mechanism of histamine clearance is by primary human astrocytes. These findings might indicate that PMAT, OCT3 and HNMT in human astrocytes play a role in the regulation of extraneuronal histamine concentration and the activities of histaminergic neurons.

Unexpected doxorubicin-mediated cardiotoxicity in sisters: possible role of polymorphisms in histamine n-methyl transferase.

The anthracycline anticancer agent doxorubicin has long been recognized to induce a dose-limiting cardiotoxicity and may be associated with genes relevant to doxorubicin disposition. Recent reports suggest a role for a number of single nucleotide polymorphisms in anthracycline cardiotoxicity in children. We describe two adult sisters with anthracycline cardiotoxicity that developed after a relatively low dose of doxorubicin. One sister carried the variant genotype for histamine N-ethyl transferase (HNMT, rs17583889) while the other was heterozygous, suggesting a similar role for these genotypes in adults with anthracycline cardiotoxicity. Although this requires further study, these genotypes may be important in the clinical dosing, or use of the liposomal formulation of doxorubicin.

The histamine N-methyltransferase T105I polymorphism affects active site structure and dynamics.

Histamine N-methyltransferase (HNMT) is the primary enzyme responsible for inactivating histamine in the mammalian brain. The human HNMT gene contains a common threonine-isoleucine polymorphism at residue 105, distal from the active site. The 105I variant has decreased activity and lower protein levels than the 105T protein. Crystal structures of both variants have been determined but reveal little regarding how the T105I polymorphism affects activity. We performed molecular dynamics simulations for both 105T and 105I at 37 degrees C to explore the structural and dynamic consequences of the polymorphism. The simulations indicate that replacing Thr with the larger Ile residue leads to greater burial of residue 105 and heightened intramolecular interactions between residue 105 and residues within helix alpha3 and strand beta3. This altered, tighter packing is translated to the active site, resulting in the reorientation of several cosubstrate-binding residues. The simulations also show that the hydrophobic histamine-binding domain in both proteins undergoes a large-scale breathing motion that exposes key catalytic residues and lowers the hydrophobicity of the substrate-binding site.

Assessment and application of laser microdissection for analysis of gene expression in the rhesus monkey endometrium.

We investigated the use of laser capture microdissection (LCM) to identify differences in gene expression between cell types or regions within the rhesus monkey endometrium. Different cell types were harvested from the two major regions of the endometrium during midsecretory phases (Days 21-23) of adequate artificial menstrual cycles: glandular epithelia (G) or stroma (S) from the functionalis (F) or the basalis (B). Amplification of the cDNA populations (primer-specific adaptors) was used to increase the amount of nucleic acid for further analysis. This single amplification step allowed us to detect the housekeeping genes (glyceraldehyde-3-phosphate dehydrogenase and 18S rRNA) and the cDNA smears in the samples. Using differential display reverse transcription polymerase chain reaction (DDRT-PCR), six fragments were selected, cloned, and sequenced based on their regional and cell type localization. Primer-specific PCR analysis subsequently confirmed the localization of three fragments: F1, highly expressed in the functionalis but not the basalis, was homologous (93% identical) to the human leukotriene B4 receptor BLT2; FS-1, highly expressed only in the stroma of the functionalis, had a 94% homology with an as yet uncharacterized gene (FLJ124360); and BG-1, primarily expressed only in the glandular epithelia of the basalis, showed a 98% homology with an uncharacterized bacterial artificial chromosome clone sequence. These LCM-generated cDNA populations coupled with DDRT-PCR can provide an important avenue for the identification of new or novel gene fragments that display cell type- or region-specific gene expression in the rhesus monkey endometrium.

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.

Two polymorphic forms of human histamine methyltransferase: structural, thermal, and kinetic comparisons.

BACKGROUND: Histamine plays important biological roles in cell-to-cell communication; it is a mediator in allergic responses, a regulator of gastric acid secretion, a messenger in bronchial asthma, and a neurotransmitter in the central nervous system. Histamine acts by binding to histamine receptors, and its local action is terminated primarily by methylation. Human histamine N-methyltransferase (HNMT) has a common polymorphism at residue 105 that correlates with the high- (Thr) and low- (Ile) activity phenotypes. RESULTS: Two ternary structures of human HNMT have been determined: the Thr105 variant complexed with its substrate histamine and reaction product AdoHcy and the Ile105 variant complexed with an inhibitor (quinacrine) and AdoHcy. Our steady-state kinetic data indicate that the recombinant Ile105 variant shows 1.8- and 1.3-fold increases in the apparent K(M) for AdoMet and histamine, respectively, and slightly (16%) but consistently lower specific activity as compared to that of the Thr105 variant. These differences hold over a temperature range of 25 degrees C-45 degrees C in vitro. Only at a temperature of 50 degrees C or higher is the Ile105 variant more thermolabile than the Thr105 enzyme. CONCLUSIONS: HNMT has a 2 domain structure including a consensus AdoMet binding domain, where the residue 105 is located on the surface, consistent with the kinetic data that the polymorphism does not affect overall protein stability at physiological temperatures but lowers K(M) values for AdoMet and histamine. The interactions between HNMT and quinacrine provide the first structural insights into a large group of pharmacologic HNMT inhibitors and their mechanisms of inhibition.

Identification of 197 genetic variations in six human methyltranferase genes in the Japanese population.

Methylation is an important event in the biotransformation pathway for many drugs and xenobiotic compounds. We screened DNA from 48 Japanese individuals for single-nucleotide polymorphisms (SNPs) in six methyltransferase (MT) genes (catechol-O-MT, COMT; guanidinoacetate N-MT, GAMT; histamine N-MT, HNMT; nicotinamide N-MT, NNMT; phosphatidylethanolamine N-MT, PEMT; and phenylethanolamine N-MT, PNMT) by direct sequencing of their entire genomic regions except for repetitive elements. This approach identified 190 SNPs and seven insertion/deletion polymorphisms among the six genes. Of the 190 SNPs, 33 were identified in the COMT gene, 6 in GAMT, 41 in HNMT, 8 in NNMT, 98 in PEMT, and 4 in PNMT. Nine were located in 5' flanking regions, 156 in introns, 10 in exons, and 15 in 3' flanking regions. These variants may contribute to a more precise understanding of possible correlations between genotypes and disease-susceptibility phenotypes or risk for side effects from drugs.

Mouse histamine N-methyltransferase: cDNA cloning, expression, gene cloning and chromosomal localization.

OBJECTIVE: Histamine N-methyltransferase (HNMT) catalyzes the Ntau-methylation of histamine. We set out to clone a mouse liver HNMT cDNA and the mouse HNMT gene as steps toward characterizing molecular genetic mechanisms involved in the regulation of this important histamine-metabolizing enzyme. DESIGN: A PCR-based strategy was used to clone both the mouse HNMT cDNA and the gene encoding that cDNA, Hnmt. The cDNA was used both to express recombinant mouse HNMT and to determine the chromosomal localization of Hnmt. RESULTS: The mouse liver HNMT cDNA was 1657 bp in length with an 888 bp open reading frame (ORF) that encoded a 296 amino acid protein with a predicted Mr value of approximately 32.5 kDa. The amino acid sequence of the encoded protein was 84% identical to that of human kidney HNMT. Mouse HNMT was expressed in COS-1 cells, and its apparent Km values for histamine and S-adenosyl-L-methionine (Ado-Met), the two cosubstrates for the reaction, were 5.3 and 5.8 microM, respectively. The mouse HNMT gene, Hnmt, spanned approximately 25 kb and had 7 exons. Its structure differed from that of the human gene primarily by the presence of an additional exon at the 5'-terminus. Hnmt mapped to mouse chromosome 2 in an area of conserved synteny to human chromosome 2q, the location of the human gene (2q22) on the basis of fluorescence in situ hybridization. CONCLUSIONS: Cloning and functional characterization of the mouse HNMT cDNA and gene will now make it possible to study in the mouse molecular genetic mechanisms involved the regulation of this important histamine-metabolizing enzyme.

Guinea pig histamine N-methyltransferase: cDNA cloning and mRNA distribution.

We report here the isolation of histamine N-methyltransferase (HMT) cDNA from the guinea pig brain by the polymerase chain reaction on the basis of nucleotide sequences of rat and human counterparts. Guinea pig HMT consists of 292 amino acids, with homologies of 75.6% and 79.1% to rat and human HMT, respectively. Northern blotting analysis indicated that the 1.6-kb guinea pig HMT transcript was expressed at various levels in different tissues at the following relative abundance: jejunum, brain > lung, spleen, stomach > liver, kidney. HMT mRNA localized throughout the jejunum, and it was mainly expressed in epithelial cells and in Auerbach's plexus.

Human histamine N-methyltransferase pharmacogenetics: common genetic polymorphisms that alter activity.

Histamine N-methyltransferase (HNMT) catalyzes a major pathway in histamine metabolism. Levels of HNMT activity in humans are regulated by inheritance. We set out to study the molecular basis for this genetic regulation. Northern blot analysis showed that HNMT is highly expressed in the kidney, so we determined levels of enzyme activity and thermal stability in 127 human renal biopsy samples. DNA was isolated from 12 kidney samples with widely different HNMT phenotypes, and exons of the HNMT gene were amplified with the polymerase chain reaction. In these 12 samples, we observed a C314T transition that resulted in a Thr105Ile change in encoded amino acid, as well as an A939G transition within the 3'-untranslated region. All remaining renal biopsy samples then were genotyped for these two variant sequences. Frequencies of the alleles encoding Thr105 and Ile105 in the 114 samples studied were 0.90 and 0.10, respectively, whereas frequencies for the nucleotide A939 and G alleles were 0.79 and 0.21, respectively. Kidney samples with the allele encoding Ile105 had significantly lower levels of HNMT activity and thermal stability than did those with the allele that encoded Thr105. These observations were confirmed by transient expression in COS-1 cells of constructs that contained all four alleles for these two polymorphisms. COS-1 cells transfected with the Ile105 allele had significantly lower HNMT activity and immunoreactive HNMT protein than did those transfected with the Thr105 allele. These observations will make it possible to test the hypothesis that genetic polymorphisms for HNMT may play a role in the pathophysiology of human disease.

[Regulation of gene expression of L-histidine decarboxylase and histamine N-methyl-transferase, and its relevance to the pathogenesis of bronchial asthma].

Histamine plays a critical role in the pathogenesis of bronchial asthma. L-Histidine decarboxylase (HDC) is a unique enzyme in mammals which catalyzes histamine formation from L-histidine. Human HDC consists of 662 amino acid residues (M(r) 74,178) and its 2.4 kb mRNA was expressed in stomach, brain and lung. In particular, the level of HDC mRNA was elevated in asthmatic patients. The HDC gene localizes in chromosome 15 and is composed of 12 exons spanning 24 kb. In the promoter area of HDC, 4 GATA and 6 LBP-1 possible binding sites were identified, suggesting that several transcriptional factors may regulate HDC gene. Histamine N-methyltransferase (HMT) is a principal enzyme of histamine degradation in the airway. Human HMT consists of 292 amino acid residues (M(r) 33,279) and its 1.6 kb mRNA was expressed in nasal polyp, kidney and lung. Particularly, HMT mRNA localized in airway epithelium. HMT gene localizes chromosome 1 p32 and its gene structure was partially analyzed. A specific inhibitor of HMT, SKF91488 augments airway contractile response to histamine, suggesting that the level of HMT activity is one of the important factors determining airway hyperresponsiveness. Viral infection, NO2 inhalation and allergen challenge modulate HMT activity in airway, which is important in the pathogenesis of asthma.