[Analysis of GCH1 gene variant in a consanguineous Chinese pedigree affected with tetrahydrobiopterin deficiency]

OBJECTIVE

To explore the genetic basis for a child featuring tetrahydrobiopterin deficiency and global developmental delay.

METHODS

Clinical and laboratory examinations were carried out for the child. Genomic DNA of the patient was subjected to high-throughput sequencing to identify genetic variants associated with hyperphenylalaninemia. Candidate variants were verified by Sanger sequencing.

RESULTS

The result of blood tandem mass spectrometry showed that the Phenylalanine in the blood was 642.7 μmol/l, and the ratio of Phenylalanine/Tyrosine was 5.42. Analysis of urinary pterin: neopterin 0.09 mmol/mol Cr, biopterin 0.04 mmol/mol Cr, biopterin% 77%, which suggested tetrahydrobiopterin deficiency. The parents of the proband were first cousins. DNA sequencing revealed that the proband has harbored homozygous c.353A>T variants in exon 2 of the GCH1 gene, for which his great grandmother, grandfather, mother, uncle, father and elder brother were heterozygous carriers with normal phenotype and no clinical symptoms associated with dopa responsive dystonia.

CONCLUSION

The homozygous c.353A>T variant of the GCH1 gene probably underlay the tetrahydrobiopterin deficiency in this pedigree of consanguineous marriage.

Endothelial GTPCH (GTP Cyclohydrolase 1) and Tetrahydrobiopterin Regulate Gestational Blood Pressure, Uteroplacental Remodeling, and Fetal Growth

[Figure: see text].

Metastatic Melanoma Progression Is Associated with Endothelial Nitric Oxide Synthase Uncoupling Induced by Loss of eNOS:BH4 Stoichiometry

Melanoma is the most aggressive type of skin cancer due to its high capability of developing metastasis and acquiring chemoresistance. Altered redox homeostasis induced by increased reactive oxygen species is associated with melanomagenesis through modulation of redox signaling pathways. Dysfunctional endothelial nitric oxide synthase (eNOS) produces superoxide anion (O2-•) and contributes to the establishment of a pro-oxidant environment in melanoma. Although decreased tetrahydrobiopterin (BH4) bioavailability is associated with eNOS uncoupling in endothelial and human melanoma cells, in the present work we show that eNOS uncoupling in metastatic melanoma cells expressing the genes from de novo biopterin synthesis pathway Gch1, Pts, and Spr, and high BH4 concentration and BH4:BH2 ratio. Western blot analysis showed increased expression of Nos3, altering the stoichiometry balance between eNOS and BH4, contributing to NOS uncoupling. Both treatment with L-sepiapterin and eNOS downregulation induced increased nitric oxide (NO) and decreased O2• levels, triggering NOS coupling and reducing cell growth and resistance to anoikis and dacarbazine chemotherapy. Moreover, restoration of eNOS activity impaired tumor growth in vivo. Finally, NOS3 expression was found to be increased in human metastatic melanoma samples compared with the primary site. eNOS dysfunction may be an important mechanism supporting metastatic melanoma growth and hence a potential target for therapy.

Molecular and metabolic bases of tetrahydrobiopterin (BH4) deficiencies

Tetrahydrobiopterin (BH4) deficiency is caused by genetic variants in the three genes involved in de novo cofactor biosynthesis, GTP cyclohydrolase I (GTPCH/GCH1), 6-pyruvoyl-tetrahydropterin synthase (PTPS/PTS), sepiapterin reductase (SR/SPR), and the two genes involved in cofactor recycling, carbinolamine-4α-dehydratase (PCD/PCBD1) and dihydropteridine reductase (DHPR/QDPR). Dysfunction in BH4 metabolism leads to reduced cofactor levels and may result in systemic hyperphenylalaninemia and/or neurological sequelae due to secondary deficiency in monoamine neurotransmitters in the central nervous system. More than 1100 patients with BH4 deficiency and 800 different allelic variants distributed throughout the individual genes are tabulated in database of pediatric neurotransmitter disorders PNDdb. Here we provide an update on the molecular-genetic analysis and structural considerations of these variants, including the clinical courses of the genotypes. From a total of 324 alleles, 11 are associated with the autosomal recessive form of GTPCH deficiency presenting with hyperphenylalaninemia (HPA) and neurotransmitter deficiency, 295 GCH1 variant alleles are detected in the dominant form of L-dopa-responsive dystonia (DRD or Segawa disease) while phenotypes of 18 alleles remained undefined. Autosomal recessive variants observed in the PTS (199 variants), PCBD1 (32 variants), and QDPR (141 variants) genes lead to HPA concomitant with central monoamine neurotransmitter deficiency, while SPR deficiency (104 variants) presents without hyperphenylalaninemia. The clinical impact of reported variants is essential for genetic counseling and important for development of precision medicine.

The Genetic Landscape and Epidemiology of Phenylketonuria

Phenylketonuria (PKU), caused by variants in the phenylalanine hydroxylase (PAH) gene, is the most common autosomal-recessive Mendelian phenotype of amino acid metabolism. We estimated that globally 0.45 million individuals have PKU, with global prevalence 1:23,930 live births (range 1:4,500 [Italy]-1:125,000 [Japan]). Comparing genotypes and metabolic phenotypes from 16,092 affected subjects revealed differences in disease severity in 51 countries from 17 world regions, with the global phenotype distribution of 62% classic PKU, 22% mild PKU, and 16% mild hyperphenylalaninemia. A gradient in genotype and phenotype distribution exists across Europe, from classic PKU in the east to mild PKU in the southwest and mild hyperphenylalaninemia in the south. The c.1241A>G (p.Tyr414Cys)-associated genotype can be traced from Northern to Western Europe, from Sweden via Norway, to Denmark, to the Netherlands. The frequency of classic PKU increases from Europe (56%) via Middle East (71%) to Australia (80%). Of 758 PAH variants, c.1222C>T (p.Arg408Trp) (22.2%), c.1066-11G>A (IVS10-11G>A) (6.4%), and c.782G>A (p.Arg261Gln) (5.5%) were most common and responsible for two prevalent genotypes: p.[Arg408Trp];[Arg408Trp] (11.4%) and c.[1066-11G>A];[1066-11G>A] (2.6%). Most genotypes (73%) were compound heterozygous, 27% were homozygous, and 55% of 3,659 different genotypes occurred in only a single individual. PAH variants were scored using an allelic phenotype value and correlated with pre-treatment blood phenylalanine concentrations (n = 6,115) and tetrahydrobiopterin loading test results (n = 4,381), enabling prediction of both a genotype-based phenotype (88%) and tetrahydrobiopterin responsiveness (83%). This study shows that large genotype databases enable accurate phenotype prediction, allowing appropriate targeting of therapies to optimize clinical outcome.

Directed Metabolic Pathway Evolution Enables Functional Pterin-Dependent Aromatic-Amino-Acid Hydroxylation in Escherichia coli

Tetrahydrobiopterin-dependent hydroxylation of aromatic amino acids is the first step in the biosynthesis of many neuroactive compounds in humans. A fundamental challenge in building these pathways in Escherichia coli is the provision of the non-native hydroxylase cofactor, tetrahydrobiopterin. To solve this, we designed a genetic selection that relies on the tyrosine synthesis activity of phenylalanine hydroxylase. Using adaptive laboratory evolution, we demonstrate the use of this selection to discover: (1) a minimum set of heterologous enzymes and a host folE (T198I) mutation for achieving this type of hydroxylation chemistry in whole cells, (2) functional complementation of tetrahydrobiopterin by indigenous cofactors, and (3) a tryptophan hydroxylase mutation for improving protein abundance. Thus, the goal of having functional aromatic-amino-acid hydroxylation in E. coli was achieved through directed metabolic pathway evolution.

Structural and Functional Impact of Seven Missense Variants of Phenylalanine Hydroxylase

The molecular genetics of well-characterized inherited diseases, such as phenylketonuria (PKU) and hyperphenylalaninemia (HPA) predominantly caused by mutations in the phenylalanine hydroxylase (PAH) gene, is often complicated by the identification of many novel variants, often with no obvious impact on the associated disorder. To date, more than 1100 PAH variants have been identified of which a substantial portion have unknown clinical significance. In this work, we study the functionality of seven yet uncharacterized PAH missense variants p.Asn167Tyr, p.Thr200Asn, p.Asp229Gly, p.Gly239Ala, p.Phe263Ser, p.Ala342Pro, and p.Ile406Met first identified in the Czech PKU/HPA patients. From all tested variants, three of them, namely p.Asn167Tyr, p.Thr200Asn, and p.Ile406Met, exerted residual enzymatic activity in vitro similar to wild type (WT) PAH, however, when expressed in HepG2 cells, their protein level reached a maximum of 72.1% ± 4.9%, 11.2% ± 4.2%, and 36.6% ± 7.3% compared to WT PAH, respectively. Remaining variants were null with no enzyme activity and decreased protein levels in HepG2 cells. The chaperone-like effect of applied BH4 precursor increased protein level significantly for p.Asn167Tyr, p.Asp229Gly, p.Ala342Pro, and p.Ile406Met. Taken together, our results of functional characterization in combination with in silico prediction suggest that while p.Asn167Tyr, p.Thr200Asn, and p.Ile406Met PAH variants have a mild impact on the protein, p.Asp229Gly, p.Gly239Ala, p.Phe263Ser, and p.Ala342Pro severely affect protein structure and function.

Structure of full-length human phenylalanine hydroxylase in complex with tetrahydrobiopterin

Phenylalanine hydroxylase (PAH) is a key enzyme in the catabolism of phenylalanine, and mutations in this enzyme cause phenylketonuria (PKU), a genetic disorder that leads to brain damage and mental retardation if untreated. Some patients benefit from supplementation with a synthetic formulation of the cofactor tetrahydrobiopterin (BH4) that partly acts as a pharmacological chaperone. Here we present structures of full-length human PAH (hPAH) both unbound and complexed with BH4 in the precatalytic state. Crystal structures, solved at 3.18-Å resolution, show the interactions between the cofactor and PAH, explaining the negative regulation exerted by BH4 BH4 forms several H-bonds with the N-terminal autoregulatory tail but is far from the catalytic FeII Upon BH4 binding a polar and salt-bridge interaction network links the three PAH domains, explaining the stability conferred by BH4 Importantly, BH4 binding modulates the interaction between subunits, providing information about PAH allostery. Moreover, we also show that the cryo-EM structure of hPAH in absence of BH4 reveals a highly dynamic conformation for the tetramers. Structural analyses of the hPAH:BH4 subunits revealed that the substrate-induced movement of Tyr138 into the active site could be coupled to the displacement of BH4 from the precatalytic toward the active conformation, a molecular mechanism that was supported by site-directed mutagenesis and targeted molecular dynamics simulations. Finally, comparison of the rat and human PAH structures show that hPAH is more dynamic, which is related to amino acid substitutions that enhance the flexibility of hPAH and may increase the susceptibility to PKU-associated mutations.

Gene mutation and pedigree analysis of tetrahydrobiopterin deficiency in a Uygur family of China

BACKGROUND

Tetrahydrobiopterin (BH4 ) deficiency is an autosomal recessive disorder, which is caused by an enzyme deficiency involved in its synthetic or metabolic pathways. Clinical symptoms may include microcephaly, hypoevolutism, severe ataxia, and seizures. The purposes of this study are to analyze the genotype-phenotype and the pedigree of the first case of BH4 deficiency in the Uygur of China.

METHODS

(a) This patient received tandem mass spectrometry, urinary neopterin and biopterin analysis, and determination of dihydropteridine reductase (DHPR) activity in dried blood spots. (b) Blood DNA samples of this patient and her three family members were collected for gene sequencing and mutation analysis.

RESULTS

(a) The basic urinary neopterin and biopterin were 1.07 mmol/mol Cr and 3.12 mmol/mol Cr, respectively, and biopterin percentage was 74.42%. The DHPR activity of this patient was 31.11% of normal control. (b) Sanger sequencing of PAH gene in this patient was negative but positive of her sister, which carries 2 heterozygous mutation c.781C>T and c.1238G>C. Next-generation sequencing on the patient identified a homozygous mutation in the quinoid dihydropteridine reductase (QDPR) gene at c.508G>A, which was confirmed by Sanger sequencing.

CONCLUSION

(a) The patient was the first case of clinical diagnosis of BH4 deficiency in the Uighur. And there are two types of hyperphenylalaninemia (HPA) in the same family. (b) The mild HPA patient with severe nervous system damage should pay more attention to the BH4 deficiency. (c) Using next-generation sequencing technology can increase the mutation detection rate when the hereditary diseases are highly suspected in clinic.

Regulation of mycobacterial infection by macrophage Gch1 and tetrahydrobiopterin

Inducible nitric oxide synthase (iNOS) plays a crucial role in controlling growth of Mycobacterium tuberculosis (M.tb), presumably via nitric oxide (NO) mediated killing. Here we show that leukocyte-specific deficiency of NO production, through targeted loss of the iNOS cofactor tetrahydrobiopterin (BH4), results in enhanced control of M.tb infection; by contrast, loss of iNOS renders mice susceptible to M.tb. By comparing two complementary NO-deficient models, Nos2-/- mice and BH4 deficient Gch1fl/flTie2cre mice, we uncover NO-independent mechanisms of anti-mycobacterial immunity. In both murine and human leukocytes, decreased Gch1 expression correlates with enhanced cell-intrinsic control of mycobacterial infection in vitro. Gene expression analysis reveals that Gch1 deficient macrophages have altered inflammatory response, lysosomal function, cell survival and cellular metabolism, thereby enhancing the control of bacterial infection. Our data thus highlight the importance of the NO-independent functions of Nos2 and Gch1 in mycobacterial control.

Metabolic pathway engineering for high-level production of 5-hydroxytryptophan in Escherichia coli

Cellular metabolic networks should be carefully balanced using metabolic engineering to produce the desired products at the industrial scale. As the precursor for the biosynthesis of the neurotransmitter serotonin, 5-hydroxytryptophan (5-HTP) is effective in treating a variety of diseases, such as depression, fibromyalgia, obesity, and cerebellar ataxia. Due to the lack of an efficient synthetic method, commercial production of 5-HTP is only achieved by extracting from the seeds of Griffonia Smplicifolia. This study reports efficient microbial production of 5-HTP via metabolically engineered Escherichia coli. Firstly, human tryptophan hydroxylase I (TPH1) gene was functionally expressed. For endogenous supply of the cofactor tetrahydrobiopterin (BH4), human BH4 biosynthesis and regeneration pathway was reconstituted. Whole-cell bioconversion resulted in high-level production of 5-HTP (~1.2 g/L) from 2 g/L L-tryptophan in shake flasks. Further metabolic engineering efforts were employed to achieve 5-HTP biosynthesis from simple carbon sources. The whole biosynthetic pathway was divided into three functional modules, L-tryptophan module, the hydroxylation module, and the BH4 module. By reducing the copy number of L-tryptophan module, replacing TPH1 with a more stable mutant form, and promoter regulation of the BH4 module, 5-HTP was produced at a final titer of 1.3 g/L in the shake flask and 5.1 g/L in a fed-batch fermenter with glycerol as the carbon source, both of which were the highest ever reported for microbial production of 5-HTP.

[Research progress on phenotype and genotype of hyperphenylalaninemia]

Hyperphenylalaninemia(HPA), an autosomal recessive disease, is the most common inborn error of amino acid metabolism, caused by the deficiency of phenylalanine hydroxylase(PAH) or tetrahydrobiopterin(BH4) which induced by mutations of genes. The accumulation of the clinical database and genetic information will enhance the development of novel personalized medicine and to provide more accurate and timely diagnostic and therapeutic approaches for HPA. This paper summarizes the correlations between HPA metabolism and PAH, BH4, pathogenic genes and their distributions in HPA, as well as the phenotypes and genotypes of HPA, so as to provide reference for personalized medicine for HPA.

Secondary BH4 deficiency links protein homeostasis to regulation of phenylalanine metabolism

Metabolic control of phenylalanine concentrations in body fluids is essential for cognitive development and executive function. The hepatic phenylalanine hydroxylating system is regulated by the ratio of l-phenylalanine, which is substrate of phenylalanine hydroxylase (PAH), to the PAH cofactor tetrahydrobiopterin (BH4). Physiologically, phenylalanine availability is governed by nutrient intake, whereas liver BH4 is kept at constant level. In phenylketonuria, PAH deficiency leads to elevated blood phenylalanine and is often caused by PAH protein misfolding with loss of function. Here, we report secondary hepatic BH4 deficiency in Pah-deficient mice. Alterations in de novo synthesis and turnover of BH4 were ruled out as molecular causes. We demonstrate that kinetically instable and aggregation-prone variant Pah proteins trap BH4, shifting the pool of free BH4 towards bound BH4. Interference of PAH protein misfolding with metabolite-based control of l-phenylalanine turnover suggests a mechanistic link between perturbation of protein homeostasis and disturbed regulation of metabolic pathways.

An Overview of Traditional and Novel Therapeutic Options for the Management of Phenylketonuria

Phenylketonuria (PKU) is an autosomal recessive disorder caused by the deficiency of phenylalanine hydroxylase enzyme that catalyzes the conversion of L-phenylalanine to L-tyrosine using tetrahydrobiopterin (BH4) as a cofactor. Among aminoacidopathies, PKU is one of the most prevalent disorders in different populations. It may be caused by deficiency of BH4 or mutations in PAH. About 98% of PKU patients have mutations in the PAH, while the remaining have BH4 deficiency. If PKU is diagnosed earlier in life using advance analytical techniques (e.g., high performance liquid chromatography, mass spectrometry, and polymerase chain reaction), then it is potentially treatable by special diets (L-phenylalanine-free medical formula). However, some complications such as vitamin B12 deficiency, cardiovascular problems, and neurodevelopmental problems have been reported in PKU patients when they ate special diets for a long period. Hence, special diet alone is not a good option for proper treatment. Next generation therapies require structure-function based development. For therapies which target PAH gene (e.g., gene therapy, RNAi, gene editing), a lot of research has yet to be done. Treatment with BH4 therapy is safe and effective but only in BH4-responsive PKU patients. Therefore, research efforts should be focused on the development of more targeted pharmacological and genetic therapies especially PAH gene therapy, which can reduce the burden or deleterious effects of this disease in affected patients.

Organic anion transporters, OAT1 and OAT3, are crucial biopterin transporters involved in bodily distribution of tetrahydrobiopterin and exclusion of its excess

Tetrahydrobiopterin (BH4) is a common coenzyme of phenylalanine-, tyrosine-, and tryptophan hydroxylases, alkylglycerol monooxygenase, and NO synthases (NOS). Synthetic BH4 is used medicinally for BH4-responsive phenylketonuria and inherited BH4 deficiency. BH4 supplementation has also drawn attention as a therapy for various NOS-related cardio-vascular diseases, but its use has met with limited success in decreasing BH2, the oxidized form of BH4. An increase in the BH2/BH4 ratio leads to NOS dysfunction. Previous studies revealed that BH4 supplementation caused a rapid urinary loss of BH4 accompanied by an increase in the blood BH2/BH4 ratio and an involvement of probenecid-sensitive but unknown transporters was strongly suggested in these processes. Here we show that OAT1 and OAT3 enabled cells to take up BP (BH4 and/or BH2) in a probenecid-sensitive manner using rat kidney slices and transporter-expressing cell systems, LLC-PK1 cells and Xenopus oocytes. Both OAT1 and OAT3 preferred BH2 and sepiapterin as their substrate roughly 5- to 10-fold more than BH4. Administration of probenecid acutely reduced the urinary exclusion of endogenous BP accompanied by a rise in blood BP in vivo. These results indicated that OAT1 and OAT3 played crucial roles: (1) in determining baseline levels of blood BP by excluding endogenous BP through the urine, (2) in the rapid distribution to organs of exogenous BH4 and the exclusion to urine of a BH4 excess, particularly when BH4 was administered, and (3) in scavenging blood BH2 by cellular uptake as the gateway to the salvage pathway of BH4, which reduces BH2 back to BH4.

Tetrahydrobiopterin in antenatal brain hypoxia-ischemia-induced motor impairments and cerebral palsy

Antenatal brain hypoxia-ischemia, which occurs in cerebral palsy, is considered a significant cause of motor impairments in children. The mechanisms by which antenatal hypoxia-ischemia causes brain injury and motor deficits still need to be elucidated. Tetrahydrobiopterin is an important enzyme cofactor that is necessary to produce neurotransmitters and to maintain the redox status of the brain. A genetic deficiency of this cofactor from mutations of biosynthetic or recycling enzymes is a well-recognized factor in the development of childhood neurological disorders characterized by motor impairments, developmental delay, and encephalopathy. Experimental hypoxia-ischemia causes a decline in the availability of tetrahydrobiopterin in the immature brain. This decline coincides with the loss of brain function, suggesting this occurrence contributes to neuronal dysfunction and motor impairments. One possible mechanism linking tetrahydrobiopterin deficiency, hypoxia-ischemia, and neuronal injury is oxidative injury. Evidence of the central role of the developmental biology of tetrahydrobiopterin in response to hypoxic ischemic brain injury, especially the development of motor deficits, is discussed.

Tetrahydrobiopterin deficiency in the pathogenesis of Fabry disease

Fabry disease is caused by deficient activity of α-galactosidase A and subsequent accumulation of glycosphingolipids (mainly globotriaosylceramide, Gb3), leading to multisystem organ dysfunction. Oxidative stress and nitric oxide synthase (NOS) uncoupling are thought to contribute to Fabry cardiovascular diseases. We hypothesized that decreased tetrahydrobiopterin (BH4) plays a role in the pathogenesis of Fabry disease. We found that BH4 was decreased in the heart and kidney but not in the liver and aorta of Fabry mice. BH4 was also decreased in the plasma of female Fabry patients, which was not corrected by enzyme replacement therapy (ERT). Gb3 levels were inversely correlated with BH4 levels in animal tissues and cultured patient cells. To investigate the role of BH4 deficiency in disease phenotypes, 12-month-old Fabry mice were treated with gene transfer-mediated ERT or substrate reduction therapy (SRT) for 6 months. In the Fabry mice receiving SRT but not ERT, BH4 deficiency was restored, concomitant with ameliorated cardiac and renal hypertrophy. Additionally, glutathione levels were decreased in Fabry mouse tissues in a sex-dependent manner. Renal BH4 levels were closely correlated with glutathione levels and inversely correlated with cardiac and kidney weight. In conclusion, this study showed that BH4 deficiency occurs in Fabry disease and may contribute to the pathogenesis of the disease through oxidative stress associated with a reduced antioxidant capacity of cells and NOS uncoupling. This study also suggested dissimilar efficacy of ERT and SRT in correcting pre-existing pathologies in Fabry disease.

A novel role for endothelial tetrahydrobiopterin in mitochondrial redox balance

The redox co-factor tetrahydrobiopterin (BH4) regulates nitric oxide (NO) and reactive oxygen species (ROS) production by endothelial NOS (eNOS) and is an important redox-dependent signalling molecule in the endothelium. Loss of endothelial BH4 is observed in cardiovascular disease (CVD) states and results in decreased NO and increased superoxide (O2-) generation via eNOS uncoupling. Genetic mouse models of augmented endothelial BH4 synthesis have shown proof of concept that endothelial BH4 can alter CVD pathogenesis. However, clinical trials of BH4 therapy in vascular disease have been limited by systemic oxidation, highlighting the need to explore the wider roles of BH4 to find novel therapeutic targets. In this study, we aimed to elucidate the effects of BH4 deficiency on mitochondrial function and bioenergetics using targeted knockdown of the BH4 synthetic enzyme, GTP Cyclohydrolase I (GTPCH). Knockdown of GTPCH by >90% led to marked loss of cellular BH4 and a striking induction of O2- generation in the mitochondria of murine endothelial cells. This effect was likewise observed in BH4-depleted fibroblasts devoid of NOS, indicating a novel NOS-independent role for BH4 in mitochondrial redox signalling. Moreover, this BH4-dependent, mitochondria-derived ROS further oxidised mitochondrial BH4, concomitant with changes in the thioredoxin and glutathione antioxidant pathways. These changes were accompanied by a modest increase in mitochondrial size, mildly attenuated basal respiratory function, and marked changes in the mitochondrial proteome and cellular metabolome, including the accumulation of the TCA intermediate succinate. Taken together, these data reveal a novel NOS-independent role for BH4 in the regulation of mitochondrial redox signalling and bioenergetic metabolism.

A fat-derived metabolite regulates a peptidergic feeding circuit in Drosophila

Here, we show that the enzymatic cofactor tetrahydrobiopterin (BH4) inhibits feeding in Drosophila. BH4 biosynthesis requires the sequential action of the conserved enzymes Punch, Purple, and Sepiapterin Reductase (Sptr). Although we observe increased feeding upon loss of Punch and Purple in the adult fat body, loss of Sptr must occur in the brain. We found Sptr expression is required in four adult neurons that express neuropeptide F (NPF), the fly homologue of the vertebrate appetite regulator neuropeptide Y (NPY). As expected, feeding flies BH4 rescues the loss of Punch and Purple in the fat body and the loss of Sptr in NPF neurons. Mechanistically, we found BH4 deficiency reduces NPF staining, likely by promoting its release, while excess BH4 increases NPF accumulation without altering its expression. We thus show that, because of its physically distributed biosynthesis, BH4 acts as a fat-derived signal that induces satiety by inhibiting the activity of the NPF neurons.

Intestinal microbiota as a tetrahydrobiopterin exogenous source in hph-1 mice

Tetrahydrobiopterin (BH4) is a cofactor of a number of regulatory enzymes. Although there are no known BH4 exogenous sources, the tissue content of this biopterin increases with age in GTP cyclohydrolase 1-deficient hyperphenylalaninemia-1 (hph-1) mice. Since certain bacteria are known to generate BH4, we hypothesize that generation of this biopterin by the intestinal microbiota contributes to its tissue increase in hph-1 adult mice. The goal of this study was to comparatively evaluate hph-1 mice and wild-type C57Bl/6 controls for the presence of intestinal BH4-producing bacteria. Newborn and adult mice fecal material was screened for 6-pyruvoyltetrahydropterin synthase (PTPS-2) an enzyme only present in BH4-generating bacteria. Adult, but not newborn, wild-type control and hph-1 mouse fecal material contained PTPS-2 mRNA indicative of the presence of BH4-generating bacteria. Utilizing chemostat-cultured human fecal bacteria, we identified the PTPS-2-producing bacteria as belonging to the Actinobacteria phylum. We further confirmed that at least two PTPS-2-producing species, Aldercreutzia equolifaciens and Microbacterium schleiferi, generate BH4 and are present in hph-1 fecal material. In conclusion, intestinal Actinobacteria generate BH4. This finding has important translational significance, since manipulation of the intestinal flora in individuals with congenital biopterin deficiency may allow for an increase in total body BH4 content.

In vitro residual activity of phenylalanine hydroxylase variants and correlation with metabolic phenotypes in PKU

Hyperphenylalaninemias (HPAs) are genetic diseases predominantly caused by a wide range of variants in the phenylalanine hydroxylase (PAH) gene. In vitro expression analysis of PAH variants offers the opportunity to elucidate the molecular mechanisms involved in HPAs and to clarify whether a disease-associated variant is genuinely pathogenic, while investigating the severity of a metabolic phenotype, and determining how a variant exerts its deleterious effects on the PAH enzyme. To study the effects of gene variants on PAH activity, we investigated eight variants: c.611A>G (p.Y204C), c.635T>C (p.L212P), c.746T>C (p.L249P), c.745C>T (p.L249F), c.809G>A (p.R270K), c.782G>C (p.R261P), c.587C>A (p.S196Y) and c.1139C>T (p.T380M), associated with different phenotypic groups. Transient expression of mutant full-length cDNAs in COS-7 cells yielded PAH proteins with PAH activity levels between 7% and 51% compared to the wild-type enzyme. With one exception (p.Y204C, which had no significant impact on PAH function), lower PAH activity was associated with a more severe phenotype (e.g. p.L249P with 7% PAH activity, 100% of classic PKU and no BH4 responsiveness), while higher activity correlated with milder phenotypes (e.g. p.T380M with 28% PAH activity, 97% of mild HPA and 83% of BH4 responsiveness). The results of the in vitro residual PAH activity have major implications, both for our understanding of genotype-phenotype correlations, and thereby existing inconsistencies, but also for the elucidation of the molecular basis of tetrahydrobiopterin (BH4) responsiveness.

Tyrosine hydroxylase (TH), its cofactor tetrahydrobiopterin (BH4), other catecholamine-related enzymes, and their human genes in relation to the drug and gene therapies of Parkinson's disease (PD): historical overview and future prospects

Tyrosine hydroxylase (TH), which was discovered at the National Institutes of Health (NIH) in 1964, is a tetrahydrobiopterin (BH4)-requiring monooxygenase that catalyzes the first and rate-limiting step in the biosynthesis of catecholamines (CAs), such as dopamine, noradrenaline, and adrenaline. Since deficiencies of dopamine and noradrenaline in the brain stem, caused by neurodegeneration of dopamine and noradrenaline neurons, are mainly related to non-motor and motor symptoms of Parkinson's disease (PD), we have studied human CA-synthesizing enzymes [TH; BH4-related enzymes, especially GTP-cyclohydrolase I (GCH1); aromatic L-amino acid decarboxylase (AADC); dopamine β-hydroxylase (DBH); and phenylethanolamine N-methyltransferase (PNMT)] and their genes in relation to PD in postmortem brains from PD patients, patients with CA-related genetic diseases, mice with genetically engineered CA neurons, and animal models of PD. We purified all human CA-synthesizing enzymes, produced their antibodies for immunohistochemistry and immunoassay, and cloned all human genes, especially the human TH gene and the human gene for GCH1, which synthesizes BH4 as a cofactor of TH. This review discusses the historical overview of TH, BH4-, and other CA-related enzymes and their genes in relation to the pathophysiology of PD, the development of drugs, such as L-DOPA, and future prospects for drug and gene therapy for PD, especially the potential of induced pluripotent stem (iPS) cells.

Downregulation of tetrahydrobiopterin inhibits tumor angiogenesis in BALB/c-nu mice with hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is a highly vascular tumor, and treatment options for patients of advanced-stage are limited. Nitric oxide (NO), which is derived from endothelial nitric oxide synthase (eNOS), provides crucial signals for angiogenesis in the tumor microenvironment. Tetrahydrobiopterin (BH4) is an essential cofactor eNOS and represents a critical determinant of NO production. To examine whether treatment of 2,4-diamino-6-hydroxypyrimidine (DAHP) inhibits angiogenesis of HCC, BALB/c-nu mice were injected with HepG-2 cells with DAHP. Supplemental DAHP treatment decreased K-ras mRNA transcripts, inhibition of phosphorylation of eNOS and Akt, inhibition of guanosine triphosphate cyclohydrolase (GTPCH), and decreased significantly NO synthesis, and then inhibited angiogenesis, compared with the results observed in the saline group. Histopathology demonstrated angiogenesis and tumor formation were significantly inhibited in HCC. DAHP downregulates GTPCH protein expression, corresponding to decreased levels of BH4 and the contents of NO. In addition, DAHP downregulates eNOS and Akt protein expression, corresponding to decreased eNOS phosphorylation at Ser1177 and Akt phosphorylation, compared with the saline control. We suggest that DAHP, recognized as a specific competitive inhibitor of GTPCH, can decrease tumor BH4 and NO by the inhibition of the wild-type Ras-PI3K/Akt pathway, and then inhibiting angiogenesis, and may provide a novel and promising way to target BH4 synthetic pathways to inhibit angiogenesis and to control potential progression of HCC. Whether DAHP has a therapeutic potential will require more direct testing in humans.

Co-expression of phenylalanine hydroxylase variants and effects of interallelic complementation on in vitro enzyme activity and genotype-phenotype correlation

BACKGROUND

In phenylketonuria (PKU) patients, the combination of two phenylalanine hydroxylase (PAH) alleles is the main determinant of residual enzyme activity in vivo and in vitro. Inconsistencies in genotype-phenotype correlations have been observed in compound heterozygous patients and a particular combination of two PAH alleles may produce a phenotype that is different from the expected one, possibly due to interallelic complementation.

METHODS

A dual eukaryotic vector system with two distinct PAH proteins N-terminally fused to different epitope tags was used to investigate the co-expression of PAH alleles reported in patients with inconsistent phenotypes. PAH variant proteins were transiently co-transfected in COS-7 cells. PAH activity was measured by liquid chromatography-electrospray ionization tandem mass spectrometry (LC-ESI-MS-MS), and protein expression was measured by Western blot. Genotypes were compared with predicted PAH activity from the PAH locus-specific database (PAHvdb) and with phenotypes and tetrahydrobiopterin (BH4) responsiveness from more than 10,000 PKU patients (BIOPKU database).

RESULTS

Through the expression and co-expression of 17 variant alleles we demonstrated that interallelic interaction could be both positive and negative. The co-expressions of p.[I65T];[R261Q] (19.5% activity; predicted 43.5%) and p.[I65T];[R408W] (15.0% vs. 26.8% activity) are examples of genotypes with negative interallelic interaction. The co-expressions of p.[E178G];[Q232E] (55.0% vs.36.4%) and p.[P384S];[R408W] (56.1% vs. 40.8%) are examples of positive subunit interactions. Inconsistencies of PAH residual enzyme activity in vitro and of PKU patients' phenotypes were observed as well. The PAH activity of p.[R408W];[A300S] is 18.0% of the wild-type activity; however, 88% of patients with this genotype exhibit mild hyperphenylalaninemias (MHPs).

CONCLUSION

The co-expression of two distinct PAH variants revealed possible dominance effects (positive or negative) by one of the variants on residual PAH activity as a result of interallelic complementation.

Mildly compromised tetrahydrobiopterin cofactor biosynthesis due to Pts variants leads to unusual body fat distribution and abdominal obesity in mice

Tetrahydrobiopterin (BH4) is an essential cofactor for the aromatic amino acid hydroxylases, alkylglycerol monooxygenase, and nitric oxide synthases (NOS). Inborn errors of BH4 metabolism lead to severe insufficiency of brain monoamine neurotransmitters while augmentation of BH4 by supplementation or stimulation of its biosynthesis is thought to ameliorate endothelial NOS (eNOS) dysfunction, to protect from (cardio-) vascular disease and/or prevent obesity and development of the metabolic syndrome. We have previously reported that homozygous knock-out mice for the 6-pyruvolytetrahydropterin synthase (PTPS; Pts-ko/ko) mice with no BH4 biosynthesis die after birth. Here we generated a Pts-knock-in (Pts-ki) allele expressing the murine PTPS-p.Arg15Cys with low residual activity (15% of wild-type in vitro) and investigated homozygous (Pts-ki/ki) and compound heterozygous (Pts-ki/ko) mutants. All mice showed normal viability and depending on the severity of the Pts alleles exhibited up to 90% reduction of PTPS activity concomitant with neopterin elevation and mild reduction of total biopterin while blood L-phenylalanine and brain monoamine neurotransmitters were unaffected. Yet, adult mutant mice with compromised PTPS activity (i.e., Pts-ki/ko, Pts-ki/ki or Pts-ko/wt) had increased body weight and elevated intra-abdominal fat. Comprehensive phenotyping of Pts-ki/ki mice revealed alterations in energy metabolism with proportionally higher fat content but lower lean mass, and increased blood glucose and cholesterol. Transcriptome analysis indicated changes in glucose and lipid metabolism. Furthermore, differentially expressed genes associated with obesity, weight loss, hepatic steatosis, and insulin sensitivity were consistent with the observed phenotypic alterations. We conclude that reduced PTPS activity concomitant with mildly compromised BH4-biosynthesis leads to abnormal body fat distribution and abdominal obesity at least in mice. This study associates a novel single gene mutation with monogenic forms of obesity.

Endothelial cell tetrahydrobiopterin deficiency attenuates LPS-induced vascular dysfunction and hypotension

Overproduction of nitric oxide (NO) is thought to be a key mediator of the vascular dysfunction and severe hypotension in patients with endotoxaemia and septic shock. The contribution of NO produced directly in the vasculature by endothelial cells to the hypotension seen in these conditions, vs. the broader systemic increase in NO, is unclear. To determine the specific role of endothelium derived NO in lipopolysaccharide (LPS)-induced vascular dysfunction we administered LPS to mice deficient in endothelial cell tetrahydrobiopterin (BH4), the essential co-factor for NO production by NOS enzymes. Mice deficient in endothelial BH4 production, through loss of the essential biosynthesis enzyme Gch1 (Gch1(fl/fl)Tie2cre mice) received a 24hour challenge with LPS or saline control. In vivo LPS treatment increased vascular GTP cyclohydrolase and BH4 levels in aortas, lungs and hearts, but this increase was significantly attenuated in Gch1(fl/fl)Tie2cre mice, which were also partially protected from the LPS-induced hypotension. In isometric tension studies, in vivo LPS treatment reduced the vasoconstriction response and impaired endothelium-dependent and independent vasodilatations in mesenteric arteries from wild-type mice, but not in Gch1(fl/fl)Tie2cre mesenteric arteries. Ex vivo LPS treatment decreased vasoconstriction response to phenylephrine in aortic rings from wild-type and not in Gch1(fl/fl)Tie2cre mice, even in the context of significant eNOS and iNOS upregulation. These data provide direct evidence that endothelial cell NO has a significant contribution to LPS-induced vascular dysfunction and hypotension and may provide a novel therapeutic target for the treatment of systemic inflammation and patients with septic shock.

Nucleotide Variants of the BH4 Biosynthesis Pathway Gene GCH1 and the Risk of Orofacial Clefts

A deficiency of GTP cyclohydrolase, encoded by the GCH1 gene, results in two neurological diseases: hyperphenylalaninaemia type HPABH4B and DOPA-responsive dystonia. Genes involved in neurotransmitter metabolism and motor systems may contribute to palatogenesis. The purpose of the study was to analyse polymorphic variants of the GCH1 gene as risk factors for non-syndromic cleft lip with or without cleft palate (NSCL/P). Genotyping of nine polymorphisms was conducted in a group of 281 NSCL/P patients and 574 controls. The GCH1 variant rs17128077 was associated with a 1.7-fold higher risk for NSCL/P (95 %CI = 1.224-2.325; p = 0.001). We also found a significant correlation between the rs8004018 and rs17128050 variants and an increased risk of oral clefts (p trend = 0.003 and 0.004, respectively). The best evidence of the global haplotype association was observed for rs17128050 and rs8004018 (p corr = 0.0152). This study demonstrates that the risk of NSCL/P is associated with variants of the GCH1 gene related to BH4 metabolism and provides some evidence of the relationships between morphological/functional shifts in the central nervous system and orofacial clefts.

Cuticle integrity and biogenic amine synthesis in Caenorhabditis elegans require the cofactor tetrahydrobiopterin (BH4)

Tetrahydrobiopterin (BH4) is the natural cofactor of several enzymes widely distributed among eukaryotes, including aromatic amino acid hydroxylases (AAAHs), nitric oxide synthases (NOSs), and alkylglycerol monooxygenase (AGMO). We show here that the nematode Caenorhabditis elegans, which has three AAAH genes and one AGMO gene, contains BH4 and has genes that function in BH4 synthesis and regeneration. Knockout mutants for putative BH4 synthetic enzyme genes lack the predicted enzymatic activities, synthesize no BH4, and have indistinguishable behavioral and neurotransmitter phenotypes, including serotonin and dopamine deficiency. The BH4 regeneration enzymes are not required for steady-state levels of biogenic amines, but become rate limiting in conditions of reduced BH4 synthesis. BH4-deficient mutants also have a fragile cuticle and are generally hypersensitive to exogenous agents, a phenotype that is not due to AAAH deficiency, but rather to dysfunction in the lipid metabolic enzyme AGMO, which is expressed in the epidermis. Loss of AGMO or BH4 synthesis also specifically alters the sensitivity of C. elegans to bacterial pathogens, revealing a cuticular function for AGMO-dependent lipid metabolism in host-pathogen interactions.

Cell-autonomous role of endothelial GTP cyclohydrolase 1 and tetrahydrobiopterin in blood pressure regulation

Tetrahydrobiopterin (BH4) is an essential cofactor for endothelial nitric oxide synthase (eNOS) function and NO generation. Augmentation of BH4 levels can prevent eNOS uncoupling and can improve endothelial dysfunction in vascular disease states. However, the physiological requirement for de novo endothelial cell BH4 biosynthesis in eNOS function remains unclear. We generated a novel mouse model with endothelial cell-specific deletion of GCH1, encoding GTP cyclohydrolase 1, an essential enzyme for BH4 biosynthesis, to test the cell-autonomous requirement for endothelial BH4 biosynthesis in vivo. Mice with a floxed GCH1 allele (GCH1(fl/fl)) were crossed with Tie2cre mice to delete GCH1 in endothelial cells. GCH1(fl/fl)Tie2cre mice demonstrated virtually absent endothelial NO bioactivity and significantly greater O2 (•-) production. GCH1(fl/fl)Tie2cre aortas and mesenteric arteries had enhanced vasoconstriction to phenylephrine and impaired endothelium-dependent vasodilatations to acetylcholine and SLIGRL. Endothelium-dependent vasodilatations in GCH1(fl/fl)Tie2cre aortas were, in part, mediated by eNOS-derived hydrogen peroxide (H2O2), which mediated vasodilatation through soluble guanylate cyclase. Ex vivo supplementation of aortic rings with the BH4 analogue sepiapterin restored normal endothelial function and abolished eNOS-derived H2O2 production in GCH1(fl/fl)Tie2cre aortas. GCH1(fl/fl)Tie2cre mice had higher systemic blood pressure than wild-type littermates, which was normalized by NOS inhibitor, NG-nitro-L-arginine methyl ester. Taken together, these studies reveal an endothelial cell-autonomous requirement for GCH1 and BH4 in regulation of vascular tone and blood pressure and identify endothelial cell BH4 as a pivotal regulator of NO versus H2O2 as alternative eNOS-derived endothelial-derived relaxing factors.

Accurate molecular diagnosis of phenylketonuria and tetrahydrobiopterin-deficient hyperphenylalaninemias using high-throughput targeted sequencing

Genetic diagnostics of phenylketonuria (PKU) and tetrahydrobiopterin (BH4) deficient hyperphenylalaninemia (BH4DH) rely on methods that scan for known mutations or on laborious molecular tools that use Sanger sequencing. We have implemented a novel and much more efficient strategy based on high-throughput multiplex-targeted resequencing of four genes (PAH, GCH1, PTS, and QDPR) that, when affected by loss-of-function mutations, cause PKU and BH4DH. We have validated this approach in a cohort of 95 samples with the previously known PAH, GCH1, PTS, and QDPR mutations and one control sample. Pooled barcoded DNA libraries were enriched using a custom NimbleGen SeqCap EZ Choice array and sequenced using a HiSeq2000 sequencer. The combination of several robust bioinformatics tools allowed us to detect all known pathogenic mutations (point mutations, short insertions/deletions, and large genomic rearrangements) in the 95 samples, without detecting spurious calls in these genes in the control sample. We then used the same capture assay in a discovery cohort of 11 uncharacterized HPA patients using a MiSeq sequencer. In addition, we report the precise characterization of the breakpoints of four genomic rearrangements in PAH, including a novel deletion of 899 bp in intron 3. Our study is a proof-of-principle that high-throughput-targeted resequencing is ready to substitute classical molecular methods to perform differential genetic diagnosis of hyperphenylalaninemias, allowing the establishment of specifically tailored treatments a few days after birth.

Albino (al) is a tetrahydrobiopterin (BH4)-deficient mutant of the silkworm Bombyx mori

Albino (al) is a lethal mutant of Bombyx mori that exhibits a colourless cuticle after the first ecdysis and dies without feeding on mulberry. Previous studies have indicated that sclerotisation was insufficient because of defective phenylalanine and tyrosine metabolism in albino larvae. However, the genetic mechanism underlying the albino phenotype has not been determined. Dopamine plays a central role in insect cuticle colouration and sclerotisation. The pathway for dopamine biosynthesis from phenylalanine involves phenylalanine hydroxylase (PAH; EC 1.14.16.1) and tyrosine hydroxylase (TH; EC 1.14.16.2). Tetrahydrobiopterin (BH4) is an essential cofactor of aromatic amino acid hydroxylases, including PAH and TH. Thus, BH4 is indispensable for cuticle colouration and sclerotisation. Here we report on identifying mutations in the gene that encodes for the Bombyx homolog of 6-pyruvoyl-tetrahydropterin synthase (PTS) which is involved in the biosynthesis of BH4, in 2 strains with different al alleles. In strain a60 (al), a transposable element was inserted in exon 2 of BmPTS. In strain a61 (al²), an 11-bp deletion was identified in the exon 2 region of BmPTS. After oral administration of BH4 to the al² larvae, the survival rate was effectively increased and the larval integument was pigmented. These results indicated that BmPTS was responsible for the albino mutants of B. mori. We conclude that (i) a mutation in BmPTS leads to an insufficient supply of BH4 and results in defective dopamine biosynthesis and (ii) lack of dopamine results in cuticle colouration and sclerotisation failure. Lemon (lem) is a BH4-deficient mutant. It has been reported that de novo synthesis of zygotic BH4 was indispensable for viability of the embryo in eggs laid by lem (lem/lem¹) females. We found that lem/lem, al²/al² larvae produced by lem (lem/lem) females were viable during the first instar stage, suggesting that al²/al² embryo could synthesis BH4 by using maternally transmitted BmPTS.

Long-term follow-up of tetrahydrobiopterin therapy in patients with tetrahydrobiopterin deficiency in Japan

Tetrahydrobiopterin (BH(4)) deficiency is a rare, congenital and lethal condition resulting in phenylalanine build-up that can lead to mental retardation and developmental defects, unless properly treated. About 1 million newborn infants in Japan undergo neonatal PKU screening every year, of which about 1 in 2 million are diagnosed with the condition. In this post-marketing surveillance study, 19 patients with BH(4) deficiency in whom BH(4) supplementation with sapropterin dihydrochloride (Biopten®) (hereafter referred to as 'BH(4) therapy') was initiated before the age of 4 years, were followed up for ≤28 years. Patients who screened positive for BH(4) deficiency were treated with supplemental BH(4) plus L-dopa and 5-hydroxytryptophan. Data on the patients' clinical courses were collected once yearly at 10 medical centers in Japan. Seventeen patients were diagnosed with 6-pyruvoyl tetrahydropterin synthase deficiency and two with dihydropteridine reductase deficiency at an average age of 3.6 months; the mean age at end of follow-up was 14.6 years. Average duration of BH(4) therapy (mean dose, 5 mg/kg per day) was 13.2 years. Serum phenylalanine was reduced from more than 10 mg/dL at the start of drug administration to less than 2 mg/dL at end of follow-up. No abnormalities in height or weight were observed in any patients, except for one female patient with familial obesity. No unwarranted side effects were reported throughout the long-term course of treatment, even during pregnancy. BH(4) therapy can effectively maintain serum phenylalanine levels within the normal range in patients with BH(4) deficiency, and demonstrated excellent long-term safety, with no side effects.

The neurobiology of tetrahydrobiopterin biosynthesis: a model for regulation of GTP cyclohydrolase I gene transcription within nigrostriatal dopamine neurons

Within the brain, the reduced pteridine cofactor 6R-L-erythro-5,6,7,8-tetrahydrobiopterin (BH4) is absolutely required for the synthesis of the monoamine (MA) neurotransmitters dopamine (DA), norepinephrine, epinephrine (E), and serotonin (5-HT), the novel gaseous neurotransmitter nitric oxide and the production of yet to be identified 1-O-alkylglycerol-derived lipids. GTP cyclohydrolase I (GTPCH) catalyzes the first and limiting step in the BH4 biosynthetic pathway, which is now thought to involve up to eight different proteins supporting six alternate de novo and two alternate salvage pathways. Gene expression analysis across different regions of the human brain shows the abundance of transcripts coding for all eight of these proteins to be highly correlated with each other and to be enriched within human MA neurons. The potential for multiple routes for BH4 synthesis therefore exists within the human brain. GTPCH expression is particularly heterogeneous across different populations of human and rodent MA-containing neurons, with low expression levels and therefore BH4 being a characteristic of nigrostriatal DA (NSDA) neurons. Basic knowledge of how GCH1 gene transcription is controlled within NSDA neurons may explain the distinctive susceptibility of these neurons to human genetic mutations that result in BH4 deficiency. A model for cyclic adenosine monophosphate-dependent GCH1 transcription is described that involves a unique combination of DNA regulatory sequences and transcription factors. This model proposes that low levels of GCH1 transcription within NSDA neurons are driven by their distinctive physiology, suggesting that pharmacological manipulation of GCH1 gene transcription can be used to modify BH4 levels and therefore DA synthesis in the basal ganglia.

Regulation of β-adrenergic control of heart rate by GTP-cyclohydrolase 1 (GCH1) and tetrahydrobiopterin

AIMS

Clinical markers of cardiac autonomic function, such as heart rate and response to exercise, are important predictors of cardiovascular risk. Tetrahydrobiopterin (BH4) is a required cofactor for enzymes with roles in cardiac autonomic function, including tyrosine hydroxylase and nitric oxide synthase. Synthesis of BH4 is regulated by GTP cyclohydrolase I (GTPCH), encoded by GCH1. Recent clinical studies report associations between GCH1 variants and increased heart rate, but the mechanistic importance of GCH1 and BH4 in autonomic function remains unclear. We investigate the effect of BH4 deficiency on the autonomic regulation of heart rate in the hph-1 mouse model of BH4 deficiency.

METHODS AND RESULTS

In the hph-1 mouse, reduced cardiac GCH1 expression, GTPCH enzymatic activity, and BH4 were associated with increased resting heart rate; blood pressure was not different. Exercise training decreased resting heart rate, but hph-1 mice retained a relative tachycardia. Vagal nerve stimulation in vitro induced bradycardia equally in hph-1 and wild-type mice both before and after exercise training. Direct atrial responses to carbamylcholine were equal. In contrast, propranolol treatment normalized the resting tachycardia in vivo. Stellate ganglion stimulation and isoproterenol but not forskolin application in vitro induced a greater tachycardic response in hph-1 mice. β1-adrenoceptor protein was increased as was the cAMP response to isoproterenol stimulation.

CONCLUSION

Reduced GCH1 expression and BH4 deficiency cause tachycardia through enhanced β-adrenergic sensitivity, with no effect on vagal function. GCH1 expression and BH4 are novel determinants of cardiac autonomic regulation that may have important roles in cardiovascular pathophysiology.

Arg375 tunes tetrahydrobiopterin functions and modulates catalysis by inducible nitric oxide synthase

NO synthase enzymes (NOS) support unique single-electron transitions of a bound H(4)B cofactor during catalysis. Previous studies showed that both the pterin structure and surrounding protein residues impact H(4)B redox function during catalysis. A conserved Arg residue (Arg375 in iNOS) forms hydrogen bonds with the H(4)B ring. In order to understand the role of this residue in modulating the function of H(4)B and overall NO synthesis of the enzyme, we generated and characterized three mutants R375D, R375K and R375N of the oxygenase domain of inducible NOS (iNOSoxy). The mutations affected the dimer stability of iNOSoxy and its binding affinity toward substrates and H(4)B to varying degrees. Optical spectra of the ferric, ferrous, ferrous dioxy, ferrous-NO, ferric-NO, and ferrous-CO forms of each mutant were similar to the wild-type. However, mutants displayed somewhat lower heme midpoint potentials and faster ferrous heme-NO complex reactivity with O(2). Unlike the wild-type protein, mutants could not oxidize NOHA to nitrite in a H(2)O(2)-driven reaction. Mutation could potentially change the ferrous dioxy decay rate, H(4)B radical formation rate, and the amount of the Arg hydroxylation during single turnover Arg hydroxylation reaction. All mutants were able to form heterodimers with the iNOS G450A full-length protein and displayed lower NO synthesis activities and uncoupled NADPH consumption. We conclude that the conserved residue Arg375 (1) regulates the tempo and extent of the electron transfer between H(4)B and ferrous dioxy species and (2) controls the reactivity of the heme-based oxidant formed after electron transfer from H(4)B during steady state NO synthesis and H(2)O(2)-driven NOHA oxidation. Thus, Arg375 modulates the redox function of H(4)B and is important in controlling the catalytic function of NOS enzymes.

Functional promiscuity of the COG0720 family

The biosynthesis of GTP derived metabolites such as tetrahydrofolate (THF), biopterin (BH(4)), and the modified tRNA nucleosides queuosine (Q) and archaeosine (G(+)) relies on several enzymes of the Tunnel-fold superfamily. A subset of these proteins includes the 6-pyruvoyltetrahydropterin (PTPS-II), PTPS-III, and PTPS-I homologues, all members of the COG0720 family that have been previously shown to transform 7,8-dihydroneopterin triphosphate (H(2)NTP) into different products. PTPS-II catalyzes the formation of 6-pyruvoyltetrahydropterin in the BH(4) pathway, PTPS-III catalyzes the formation of 6-hydroxylmethyl-7,8-dihydropterin in the THF pathway, and PTPS-I catalyzes the formation of 6-carboxy-5,6,7,8-tetrahydropterin in the Q pathway. Genes of these three enzyme families are often misannotated as they are difficult to differentiate by sequence similarity alone. Using a combination of physical clustering, signature motif, phylogenetic codistribution analyses, in vivo complementation studies, and in vitro enzymatic assays, a complete reannotation of the COG0720 family was performed in prokaryotes. Notably, this work identified and experimentally validated dual function PTPS-I/III enzymes involved in both THF and Q biosynthesis. Both in vivo and in vitro analyses showed that the PTPS-I family could tolerate a translation of the active site cysteine and was inherently promiscuous, catalyzing different reactions on the same substrate or the same reaction on different substrates. Finally, the analysis and experimental validation of several archaeal COG0720 members confirmed the role of PTPS-I in archaeosine biosynthesis and resulted in the identification of PTPS-III enzymes with variant signature sequences in Sulfolobus species. This study reveals an expanded versatility of the COG0720 family members and illustrates that for certain protein families extensive comparative genomic analysis beyond homology is required to correctly predict function.

Alterations in dopamine and glutamate neurotransmission in tetrahydrobiopterin deficient spr-/- mice: relevance to schizophrenia

Tetrahydrobiopterin (BH4) is a pivotal cofactor for enzymes responsible for the synthesis and release of monoamine neurotransmitters including dopamine (DA) and serotonin (5-HT) as well as the release of glutamate (Glu). Deficiencies in BH4 levels and reduced activities of BH(4)-associated enzymes have been recently reported in patients with schizophrenia. Accordingly, it is possible that abnormalities in the biochemical cascades regulated by BH(4) may alter DA, 5-HT and Glu neurotransmission, and consequently contribute to the pathophysiology of different neuropsychiatric diseases including schizophrenia. The development of a novel strain of mutant mice that is deficient in BH(4) by knocking out the expression of a functional sepiapterin reductase gene (spr -/-) has added new insights into the potential role of BH(4) in the pathophysiology and improved treatment of schizophrenia.

The tetrahydrobiopterin pathway and pain

Novel approaches for treating chronic pain are required to address a widely recognized, yet largely underserved and unmet, clinical need. The recently discovered link between tetrahydrobiopterin (BH4) synthesis and pain in preclinical models and humans provides a promising new approach for treating neuropathic and other forms of chronic pain. The rate-limiting enzyme in BH4 synthesis, guanosine triphosphate cyclohydrolase 1 (GCH1), and sepiapterin reductase (SPR) are both promising drug targets based on initial active-site characterization of the SARs of these two enzymes. Reducing the elevated BH4 levels associated with pain to baseline, while maintaining sufficient BH4 levels to limit side effects is the goal of discovery programs for novel therapeutics targeting GCH1 or SPR.

Examination of tetrahydrobiopterin pathway genes in autism

Autism is a complex disorder with a high degree of heritability and significant phenotypic and genotypic heterogeneity. Although candidate gene studies and genome-wide screens have failed to identify major causal loci associated with autism, numerous studies have proposed association with several variations in genes in the dopaminergic and serotonergic pathways. Because tetrahydrobiopterin (BH4) is the essential cofactor in the synthesis of these two neurotransmitters, we genotyped 25 SNPs in nine genes of the BH4 pathway in a total of 403 families. Significant nominal association was detected in the gene for 6-pyruvoyl-tetrahydropterin synthase, PTS (chromosome 11), with P = 0.009; this result was not restricted to an affected male-only subset. Multilocus interaction was detected in the BH4 pathway alone, but not across the serotonin, dopamine and BH4 pathways.

GTP cyclohydrolase I expression, protein, and activity determine intracellular tetrahydrobiopterin levels, independent of GTP cyclohydrolase feedback regulatory protein expression

GTP cyclohydrolase I (GTPCH) is a key enzyme in the synthesis of tetrahydrobiopterin (BH4), a required cofactor for nitricoxide synthases and aromatic amino acid hydroxylases. Alterations of GTPCH activity and BH4 availability play an important role in human disease. GTPCH expression is regulated by inflammatory stimuli, in association with reduced expression of GTP cyclohydrolase feedback regulatory protein (GFRP). However, the relative importance of GTPCH expression versus GTPCH activity and the role of GFRP in relation to BH4 bioavailability remain uncertain. We investigated these relationships in a cell line with tet-regulated GTPCH expression and in the hph-1 mouse model of GTPCH deficiency. Doxycycline exposure resulted in a dose-dependent decrease in GTPCH protein and activity, with a strong correlation between GTPCH expression and BH4 levels (r(2) = 0.85, p < 0.0001). These changes in GTPCH and BH4 had no effect on GFRP expression or protein levels. GFRP overexpression and knockdown in tet-GCH cells did not alter GTPCH activity or BH4 levels, and GTPCH-specific knockdown in sEnd.1 endothelial cells had no effect on GFRP protein. In mouse liver we observed a graded reduction of GTPCH expression, protein, and activity, from wild type, heterozygote, to homozygote littermates, with a striking linear correlation between GTPCH expression and BH4 levels (r(2) = 0.82, p < 0.0001). Neither GFRP expression nor protein differed between wild type, heterozygote, nor homozygote mice, despite the substantial differences in BH4. We suggest that GTPCH expression is the primary regulator of BH4 levels, and changes in GTPCH or BH4 are not necessarily accompanied by changes in GFRP expression.

Stabilization and characterization of a heme-oxy reaction intermediate in inducible nitric-oxide synthase

Nitric-oxide synthases (NOS) are heme-thiolate enzymes that N-hydroxylate L-arginine (L-Arg) to make NO. NOS contain a unique Trp residue whose side chain stacks with the heme and hydrogen bonds with the heme thiolate. To understand its importance we substituted His for Trp188 in the inducible NOS oxygenase domain (iNOSoxy) and characterized enzyme spectral, thermodynamic, structural, kinetic, and catalytic properties. The W188H mutation had relatively small effects on l-Arg binding and on enzyme heme-CO and heme-NO absorbance spectra, but increased the heme midpoint potential by 88 mV relative to wild-type iNOSoxy, indicating it decreased heme-thiolate electronegativity. The protein crystal structure showed that the His188 imidazole still stacked with the heme and was positioned to hydrogen bond with the heme thiolate. Analysis of a single turnover L-Arg hydroxylation reaction revealed that a new heme species formed during the reaction. Its build up coincided kinetically with the disappearance of the enzyme heme-dioxy species and with the formation of a tetrahydrobiopterin (H4B) radical in the enzyme, whereas its subsequent disappearance coincided with the rate of l-Arg hydroxylation and formation of ferric enzyme. We conclude: (i) W188H iNOSoxy stabilizes a heme-oxy species that forms upon reduction of the heme-dioxy species by H4B. (ii) The W188H mutation hinders either the processing or reactivity of the heme-oxy species and makes these steps become rate-limiting for l-Arg hydroxylation. Thus, the conserved Trp residue in NOS may facilitate formation and/or reactivity of the ultimate hydroxylating species by tuning heme-thiolate electronegativity.

[Mutation analysis and one novel mutation detection of 6-pyruvoyl tetrahydropterin synthase gene in children with tetrahydrobiopterin deficiency]

OBJECTIVE

To investigate the distribution character of the mutations of 6-pyruvoyl tetrahydropterin synthase (PTPS) gene and to provide effective basis for gene diagnosis of tetrahydrobiopterin deficiency (BH4D) in children with hyperphenylalaninemia.

METHODS

Direct sequencing was performed for screening the PTPS gene mutations in 5 patients with clinically suspected BH4D and their parents. The nature of the novel mutations were deduced by the sequences alignment and the structural analysis of mutant protein. Artificial construct restriction site was used to detect the novel mutation in the control samples. The dates of urinary pterin analysis and BH4 loading test were retrospectively analyzed after gene analysis.

RESULTS

Four PTPS gene mutations (N52S, P87S, D96N, and L127F) were detected in our study. The genotypes of four PTPS deficiency patients were identified as N52S/L127F, P87S/D96N, N52S/D96N, and D96N/ -. As a novel mutation that has not been reported previously, the mutation L127F was not detected in 50 normal controls. This novel mutation L127F was inherited from the patient's mother, and this mutant site was highly conserved by sequences alignment and the protein structural analysis. Four of the five cases with hyperphenylalaninemia and suspicious BH4D, whose urinary biopterin percentage was lower than 2% , were diagnosed as PTPS deficiency during 5-20 months old. The remaining one case was excluded from BH4D.

CONCLUSIONS

The mutant characterization of PTPS gene was coincident with other early studies in Chinese. The novel mutation L127F was considered as a pathogenetic mutation and associated with severe clinical phenotype.

Regulation of tetrahydrobiopterin biosynthesis by shear stress

An essential cofactor for the endothelial NO synthase is tetrahydrobiopterin (H4B). In the present study, we show that in human endothelial cells, laminar shear stress dramatically increases H4B levels and enzymatic activity of GTP cyclohydrolase (GTPCH)-1, the first step of H4B biosynthesis. In contrast, protein levels of GTPCH-1 were not affected by shear. Shear did not change protein expression or activity of the downstream enzymes 6-pyruvoyl-tetrahydropterin synthase and sepiapterin reductase and decreased protein levels of the salvage enzyme dihydrofolate reductase. Oscillatory shear only modestly affected H4B levels and GPTCH-1 activity. We also demonstrate that laminar, but not oscillatory shear stress, stimulates phosphorylation of GTPCH-1 on serine 81 and that this is mediated by the alpha prime (alpha') subunit of casein kinase 2. The increase in H4B caused by shear is essential in allowing proper function of endothelial NO synthase because GPTCH-1 blockade with 2,4-diamino-6-hydroxypyrimidine during shear inhibited dimer formation of endothelial NO synthase, increased endothelial cell superoxide production, and prevented the increase in NO production caused by shear. Thus, shear stress not only increases endothelial NO synthase levels but also stimulates production of H4B by markedly enhancing GTPCH-1 activity via casein kinase 2-dependent phosphorylation on serine 81. These findings illustrate a new function of casein kinase 2 in the endothelium and provide insight into regulation of GTPCH-1 activity.

Interaction of genetic and environmental factors in a Drosophila parkinsonism model

Catastrophic loss of dopaminergic neurons is a hallmark of Parkinson's disease. Despite the recent identification of genes associated with familial parkinsonism, the etiology of most Parkinson's disease cases is not understood. Environmental toxins, such as the herbicide paraquat, appear to be risk factors, and it has been proposed that susceptibility is influenced by genetic background. The genetic model organism Drosophila is an advantageous system for the identification of genetic susceptibility factors. Genes that affect dopamine homeostasis are candidate susceptibility factors, because dopamine itself has been implicated in neuron damage. We find that paraquat can replicate a broad spectrum of parkinsonian behavioral symptoms in Drosophila that are associated with loss of specific subsets of dopaminergic neurons. In parallel with epidemiological studies that show an increased incidence of Parkinson's disease in males, male Drosophila exhibit paraquat symptoms earlier than females. We then tested the hypothesis that variation in dopamine-regulating genes, including those that regulate tetrahydrobiopterin, a requisite cofactor in dopamine synthesis, can alter susceptibility to paraquat-induced oxidative damage. Drosophila mutant strains that have increased or decreased dopamine and tetrahydrobiopterin production exhibit variation in susceptibility to paraquat. Surprisingly, protection against the neurotoxicity of paraquat is conferred by mutations that elevate dopamine pathway function, whereas mutations that diminish dopamine pools increase susceptibility. We also find that loss-of-function mutations in a negative regulator of dopamine production, Catecholamines-up, delay the onset of neurological symptoms, dopaminergic neuron death, and morbidity during paraquat exposure but confer sensitivity to hydrogen peroxide.

Biochemistry of postmortem brains in Parkinson's disease: historical overview and future prospects

Biochemical studies on postmortem brains of patients with Parkinson's disease (PD) have greatly contributed to our understanding of the molecular pathogenesis of this disease. The discovery by 1960 of a dopamine deficiency in the nigro-striatal dopamine region of the PD brain was a landmark in research on PD. At that time we collaborated with Hirotaro Narabayashi and his colleagues in Japan and with Peter Riederer in Germany on the biochemistry of PD by using postmortem brain samples in their brain banks. We found that the activity, mRNA level, and protein content of tyrosine hydroxylase (TH), as well as the levels of the tetrahydrobiopterin (BH4) cofactor of TH and the activity of the BH4-synthesizing enzyme, GTP cyclohydrolase I (GCHI), were markedly decreased in the substantia nigra and striatum in the PD brain. In contrast, the molecular activity (enzyme activity/enzyme protein) of TH was increased, suggesting a compensatory increase in the enzyme activity. The mRNA levels of all four isoforms of human TH (hTH1-hTH4), produced by alternative mRNA splicing, were also markedly decreased. This finding is in contrast to a completely parallel decrease in the activity and protein content of dopamine beta-hydroxylase (DBH) without changes in its molecular activity in cerebrospinal fluid (CSF) in PD. We also found that the activities and/or the levels of the mRNA and protein of aromatic L-amino acid decarboxylase (AADC, DOPA decarboxylase), DBH, phenylethanolamine N-methyltransferase (PNMT), which synthesize dopamine, noradrenaline, and adrenaline, respectively, were also decreased in PD brains, indicating that all catecholamine systems were widely impaired in PD brains. Programmed cell death of the nigro-striatal dopamine neurons in PD has been suggested from the following findings on postmortem brains: (1) increased levels of pro-inflammatory cytokines such as TNF-alpha and IL-6; (2) increased levels of apoptosis-related factors such as TNF-alpha receptor R1 (p 55), soluble Fas and bcl-2, and increased activities of caspases 1 and 3; and (3) decreased levels of neurotrophins such as brain-derived nerve growth factor (BDNF). Immunohistochemical data and the mRNA levels of the above molecules in PD brains supported these biochemical data. We confirmed by double immunofluorescence staining the production of TNF-alpha and IL-6 in activated microglia in the putamen of PD patients. Owing to the recent development of highly sensitive and wide-range analytical methods for quantifying mRNAs and proteins, future assays of the levels of various mRNAs and proteins not only in micro-dissected brain tissues containing neurons and glial cells, but also in single cells from frozen brain slices isolated by laser capture micro-dissection, coupled with toluidine blue, Nissl staining or immunohistochemical staining, should further contribute to the elucidation of the molecular pathogenesis of PD and other neurodegenerative or neuropsychiatric diseases.

[Dopa-responsive dystonia: clinical, genetic, and biochemical studies]

Dopa-responsive dystonia (DRD) is a clinical syndrome characterized by childhood-onset dystonia and a dramatic and sustained response to low doses of levodopa. There are at least three causative genes for DRD: (1) the GCH1 gene on chromosome 14q22.1-q22.2, which encodes GTP cyclohydrolase I (GTPCH), the first enzyme in the biosynthetic pathway for tetrahydrobiopterin (BH4; the essential cofactor for tyrosine hydroxylase [THI]), (2) the TH gene on 11 p15.5, coding for the enzyme TH that catalyzes the rate-limiting step in the catecholamine biosynthesis, and (3) an as yet undefined gene on 14q13 (DYT14). In reports on DRD, in which conventional genomic DNA sequencing of GCH1 was conducted in a relatively large number of pedigrees, mutations in the coding region (including the splice sites) of this gene were found in approximately 60% (range: 49-79%) of DRD families. In our series, after conducting additional GCH1 testing (Southern blotting, cDNA sequencing, etc.) and TH analysis, 86% of families with DRD or dystonia with motor delay (an intermediate phenotype between GTPCH-deficient DRD [mild] and GTPCH-deficient hyperphenylalaninemia [severe]) had identifiable GCH1 or (rarely) TH mutations. Up to the present, only one pedigree with autosomal dominant DRD linked to the DYT14 locus has been reported. Neuropathological findings (no Lewy bodies and a normal population of cells with reduced melanin in the substantia nigra) in DRD patients with GTPCH dysfunction were similar to those in a patient with DYT14 dystonia. There have been no reports of autopsied patients with TH-deficient DRD. Neurochemical data suggest that striatal dopamine reduction in GTPCH-deficient DRD is caused not only by decreased TH activity resulting from a low cofactor (BH4) level but also by actual loss of TH protein without nerve terminal loss. This TH protein reduction in the striatum, especially in the putamen, may be due to a diminished regulatory effect of BH4 on stability (rather than expression) of TH molecules or to a dysfunction of TH protein transport from the substantia nigra to the striatum. The extent of striatal TH protein loss may be critical in determining DRD symptomatology and could contribute to gender-related incomplete penetrance of GCH1 mutations in GTPCH-deficient DRD families. Notwithstanding the discovery of the three causative loci for DRD, a therapeutic trial with low doses of levodopa is still the most practical approach to the diagnosis of this treatable disorder. The trial should be considered in all children with dystonic and/or parkinsonian symptoms or with unexplained gait disorders. Analyses of total biopterin and neopterin as well as neurotransmitter metabolites in CSF appear to be useful for the diagnosis of GTPCH-deficient DRD (the major form of DRD) and of TH-deficient DRD (the mild form of TH deficiency). Findings of the precise mechanism of striatal TH protein loss in GTPCH-deficient DRD, the actual status of dopaminergic systems in TH-deficient DRD, and the novel causative gene on the DYT14 locus will better define the pathogenesis of DRD.