Tetrahydrobiopterin in dystonia: identification of abnormal metabolism and therapeutic trials

Crucial neuroprotective roles of the metabolite BH4 in dopaminergic neurons

Dopa-responsive dystonia (DRD) and Parkinson's disease (PD) are movement disorders caused by the dysfunction of nigrostriatal dopaminergic neurons. Identifying druggable pathways and biomarkers for guiding therapies is crucial due to the debilitating nature of these disorders. Recent genetic studies have identified variants of GTP cyclohydrolase-1 (GCH1), the rate-limiting enzyme in tetrahydrobiopterin (BH4) synthesis, as causative for these movement disorders. Here, we show that genetic and pharmacological inhibition of BH4 synthesis in mice and human midbrain-like organoids accurately recapitulates motor, behavioral and biochemical characteristics of these human diseases, with severity of the phenotype correlating with extent of BH4 deficiency. We also show that BH4 deficiency increases sensitivities to several PD-related stressors in mice and PD human cells, resulting in worse behavioral and physiological outcomes. Conversely, genetic and pharmacological augmentation of BH4 protects mice from genetically- and chemically induced PD-related stressors. Importantly, increasing BH4 levels also protects primary cells from PD-affected individuals and human midbrain-like organoids (hMLOs) from these stressors. Mechanistically, BH4 not only serves as an essential cofactor for dopamine synthesis, but also independently regulates tyrosine hydroxylase levels, protects against ferroptosis, scavenges mitochondrial ROS, maintains neuronal excitability and promotes mitochondrial ATP production, thereby enhancing mitochondrial fitness and cellular respiration in multiple preclinical PD animal models, human dopaminergic midbrain-like organoids and primary cells from PD-affected individuals. Our findings pinpoint the BH4 pathway as a key metabolic program at the intersection of multiple protective mechanisms for the health and function of midbrain dopaminergic neurons, identifying it as a potential therapeutic target for PD.

The Dystonias

Dystonia is a difficult problem for both the clinician and the scientist. It is sufficiently common to be seen by almost all physicians, yet uncommon enough to prevent any physician from gaining broad experience in its diagnosis and treatment. Each case represents a difficult challenge even to the specialist. The basic scientist is faced with investigating a disorder that is without relevant animal models and which is so rare that obtaining suitable tissue for study is a major obstacle. Dystonia may be idiopathic, or associated with lesions from many sources, including a variety of rare diseases. If idiopathic, it may be genetically transmitted or sporadic. If genetically transmitted, it may be generalized or focal, with symptoms varying in different members of the same family. It may be refractory to treatment, or it may respond to any one of a number of individual drugs that have very different mechanisms of action. For idiopathic dystonias, no clear method of genetic transmission has been established and no consistent pathology identified.

GTP cyclohydrolase regulation: implications for brain development and function

Tetrahydrobiopterin (BH4) is essential for the biosynthesis of dopamine, noradrenaline, and serotonin, which serve as cofactors for tyrosine hydroxylase (TH) and tryptophan hydroxylase. GTP cyclohydrolase (GCH) is the first and rate-limiting enzyme for BH4 biosynthesis. Genetic defects in an allele of the GCH gene can result in dopa-responsive dystonia due to partial BH4 deficiency. To explore the transcriptional control of the GCH gene, we analyzed the signaling pathway. Bacterial lipopolysaccharide (LPS) greatly enhanced the expression of GCH in RAW264 cells, and the induction of GCH by LPS was suppressed by treatment with either a MEK1/2 inhibitor or an inhibitor for the NF-κB pathway. Next, we analyzed two types of biopterin-deficient transgenic mice. We found that both mice exhibited motor disorders with slight differences. Dopamine and TH protein levels were markedly and concurrently increased from birth (P0) to P21 in wild-type mice, and these increases were abolished in both types of biopterin-deficient mice. Our results suggest that the developmental manifestation of psychomotor symptoms in BH4 deficiency might be attributable at least partially to the high dependence of dopaminergic development on the availability of BH4.

Urinary neopterin and phenylalanine loading test as tools for the biochemical diagnosis of segawa disease

Background. The diagnosis of autosomal dominant GTP-cyclohydrolase deficiency relies on the examination of the GCH1 gene and/or pterins and neurotransmitters in CSF. The aim of the study was to assess the diagnostic value, if any, of pterins in urine and blood phenylalanine (Phe) and tyrosine (Tyr) under oral Phe loading test. Methods. We report on two new pedigrees with four symptomatic and four asymptomatic carriers whose pattern of urinary pterins and blood Phe/Tyr ratio under oral Phe loading pointed to GTP-cyclohydrolase deficiency. The study was then extended to 3 further patients and 90 controls. The diagnostic specificity and sensitivity of these metabolic markers were analysed by backwards logistic analysis. Results. Two genetic alterations segregated alternatively in Family 1 (c.631-632 del AT and c.671A > G), while exon 1 deletion was transmitted along three generations in Family 2. Neopterin and biopterin concentrations in urine clustered differently in controls under and over the age of 15. Therefore patients and controls were sub grouped according to this age. Neopterin was significantly reduced in GCH1 mutated subjects younger than 15, and both neopterin and biopterin in those older than 15. Moreover, the Phe/Tyr ratios at the second and third hour were both significantly higher in patients than in controls. Backwards logistic regression demonstrated the high diagnostic sensitivity and specificity of combined values of neopterin concentration and Phe/Tyr ratio at the second hour. Conclusions. Pterins in urine and Phe loading test are non-invasive and reliable tools for the biochemical diagnosis of GTP-cyclohydrolase deficiency.

Dopa-responsive dystonia

Clinical characteristics and pahophysiologies of dopa-responsive dystonia are discussed by reviewing autosomal-dominant GTP cyclohydrolase-I deficiency (AD GCHI D), recessive deficiencies of enzymes of pteridine metabolism, and recessive tyrosine hydroxylase (TH). Pteridine and TH metabolism involve TH activities in the terminals of the nigrostriatal dopamine neuron which show high in early childhood and decrease exponentially with age, attaining stational low levels by the early 20s. In these disorders, TH in the terminals follows this course with low levels and develops particular symptoms with functional maturation of the downstream structures of the basal ganglia; postural dystonia through the direct pathway and descending output matured earlier in early childhood and parkinsonism in TH deficiency in teens through the D2 indirect pathway ascending output matured later. In action-type AD GCHI D, deficiency of TH in the terminal on the subthalamic nucleus develops action dystonia through the descending output in childhood, focal and segmental dystonia and parkinsonism in adolescence and adulthood through the ascending pathway maturing later. Dysfunction of dopamine in the terminals does not cause degenerative changes or higher cortical dysfunction. In recessive disorders, hypofunction of serotonin and noradrenaline induces hypofunction of the dopamine in the perikaryon and shows cortical dysfunction.

Endothelial, sympathetic, and cardiac function in inherited (6R)-L-erythro-5,6,7,8-tetrahydro-L-biopterin deficiency

BACKGROUND

(6R)-5,6,7,8-Tetrahydro-l-biopterin (BH4) is a cofactor for enzymes involved in catecholamine and nitric oxide generation whose synthesis is initiated by GTP cyclohydrolase I (GTPCH-1), encoded by GCH1. In the absence of a potent, specific GTPCH-1 inhibitor, natural BH4 deficiency caused by mutations in GCH1 in the rare movement disorder, DOPA-responsive dystonia (OMIM DYT5), offers the opportunity to study the role of endogenous BH4 in humans.

METHODS AND RESULTS

In 16 DOPA-responsive dystonia patients with mutations predicted to affect GTPCH-1 expression or function and in age- and sex-matched control subjects, we measured plasma biopterin and nitrogen oxides by high-performance liquid chromatography and the Griess reaction, respectively, endothelial function by brachial artery flow-mediated dilation (FMD), sympathetic function by measurement of plasma norepinephrine, epinephrine, and heart rate and blood pressure in response. Cardiac function and structure were assessed by echocardiography. Plasma biopterin was lower in patients (5.76±0.53 versus 8.43±0.85 nmol/L, P=0.03), but plasma NO(2)(-)/NO(3)(-) (NOx) (median, 9.06 [interquartile range, 5.35 to 11.04] versus 8.40 [interquartile range, 5.28 to 11.44] μmol/L, P=1) and FMD were not lower (7.7±0.8% versus 7.9±0.9%, P=0.91). In patients but not control subjects, FMD was insensitive to nitric oxide synthase inhibition (FMD at baseline, 6.7±2.1%; FMD during l-NMMA infusion, 6.2±2.5, P=0.68). The heart rate at rest was higher in patients, but the heart rate and blood pressure response to sympathetic stimulation did not differ in patients and control subjects despite lower concentrations of norepinepherine (264±8 pg/mL versus 226±9 pg/mL, P=0.006) and epinephrine (33.8±5.2 pg/mL versus 17.8±4.6 pg/mL, P=0.03) in patients. There was also no difference in cardiac function and structure.

CONCLUSIONS

Sympathetic, cardiac, and endothelial functions are preserved in patients with GCH1 mutations despite a neurological phenotype, reduced plasma biopterin, and norepinepherine and epinephrine concentrations. Lifelong endogenous BH4 deficiency may elicit developmental adaptation through mechanisms that are inaccessible during acquired BH4 deficiency in adulthood.

Treatment of dystonia

Dystonia may be a sign or symptom, that is comprised of complex abnormal and dynamic movements of different etiologies. A specific cause is identified in approximately 28% of patients, which only occasionally results in specific treatment. In most cases, treatment is symptomatic and designed to relieve involuntary movements, improve posture and function and reduce associated pain. Therapeutic options are dictated by clinical assessment of the topography of dystonia, severity of abnormal movements, functional impairment and progression of disease and consists of pharmacological, surgical and supportive approaches. Several advances have been made in treatment with newer medications, availability of different forms of botulinum toxin and globus pallidus deep brain stimulation (DBS). For patients with childhood-onset dystonia, the majority of whom later develop generalized dystonia, oral medication is the mainstay of therapy. Recently, DBS has emerged as an effective alternative therapy. Botulinum toxin is usually the treatment of choice for those with adult-onset primary dystonia in which dystonia usually remains focal. In patients with secondary dystonia, treatment is challenging and efficacy is typically incomplete and partially limited by side effects. Despite these treatment options, many patients with dystonia experience only partial benefit and continue to suffer significant disability. Therefore, more research is needed to better understand the underlying cause and pathophysiology of dystonia and to explore newer medications and surgical techniques for its treatment.

Dystonia

BACKGROUND

Dystonia refers to a syndrome of sustained muscle contractions, frequently causing twisting and repetitive movements or abnormal postures. Although age at onset, anatomic distribution, and family history are essential elements in the evaluation of dystonia, new classification increasingly relies on etiologic and genetic data. In recent years, much progress has been made on the genetics of various forms of dystonia and its pathophysiology underlying the clinical signs. The treatment of dystonia has continued to evolve to include newer medications, different forms of botulinum toxin, and various surgical procedures.

REVIEW SUMMARY

In this article, the author reviewed and summarized the history of dystonia, its evolving classification, and recent genetic data, as well as its clinical investigation and treatment.

CONCLUSIONS

Recent advances in molecular biology have led to the discovery of novel dystonia genes and loci, updating classification schemes, and better understanding of underlying pathophysiology. Treatment strategies for dystonia have significantly been updated with the introduction of different forms of botulinum toxin therapy, new pharmacologic agents, and most recently pallidal deep brain stimulation. A systematic approach to the diagnosis and treatment evaluation of dystonic patients provides optimal care for long-term management.

Evidence for a tetrahydrobiopterin deficit in schizophrenia

Tetrahydrobiopterin (BH(4)) is a vital cofactor maintaining availability of the amine neurotransmitters [dopamine (DA), noradrenaline (NA), and serotonin (5-HT)], regulating the synthesis of nitric oxide (NO) by nitric oxide synthase (NOS), and stimulating and modulating the glutamatergic system (directly and indirectly). These BH(4) properties and their potential relevance to schizophrenia led us to investigate the hypothesis of a study group (healthy controls, n=37; schizophrenics, n=154) effect on fasting plasma total biopterin levels (a measure of BH(4)). Study analysis showed a highly significant deficit of total biopterins for the schizophrenic sample after partialling out the effects of potential confounds of gender, age, ethnicity, neuroleptic use history and dose of current use, 24-hour dietary phenylalanine/protein ratio (a dietary variable relevant to BH(4) synthesis), and plasma phenylalanine (which stimulates BH(4) synthesis). A mean decrement of 34% in plasma total biopterins for schizophrenics from control values supports clinical relevance for the finding. In a subsample (21 controls and 23 schizophrenics), sequence analysis was done of the GTP cyclohydrolase I feedback regulatory gene and no mutations were found in the coding region of the gene. A deficiency of BH(4) could lead to hypofunction of the systems of DA, NA, 5-HT, NOS/NO, and glutamate, all of which have been independently implicated in schizophrenia psychopathology. Further, evidence has been accumulating which implicates the critical interdependence of these neurotransmitter systems in schizophrenia; this concept, along with the present study finding of a biopterin deficit, suggests that further study of the BH(4) system in schizophrenia is warranted and desirable.

Reversible parkinsonism following heroin pyrolysate inhalation is associated with tetrahydrobiopterin deficiency

A patient who inhaled heroin vapour (chasing the dragon), resulting in temporary parkinsonism, is described. A reversible deficiency of tetrahydrobiopterin, causing altered dopamine metabolism, is demonstrated as the metabolic basis of this aspect of the encephalopathy.

Efficacy of pharmacological treatment of dystonia: evidence-based review including meta-analysis of the effect of botulinum toxin and other cure options

The treatment of both generalized and focal dystonia is symptomatic. There is no evidence-based information about the efficacy of the different methods of the pharmacological therapeutic options currently being applied in dystonia. The specific questions addressed by this study were which treatments for dystonia have proven efficacy and which of them have unproven results. Following evidence-based principles, a literature review based on MEDLINE and the Cochrane Library, augmented by manual search of the most important journals was performed to identify the relevant publications issued between 1973 and 2003. All articles appearing in the professional English literature, including case reports, were considered. In the presence of comparable studies the meta-analysis was performed to obtain pooled information and make a reasonable inference. Based on this review, we conclude: (i) botulinum toxin has obvious benefit (level A, class I-II evidence) for the treatment of cervical dystonia and blepharospasm; (ii) trihexyphenidyl in high dosages is effective for the treatment of segmental and generalized dystonia in young patients (level A, class I-II evidence); (iii) all other methods of pharmacological intervention for generalized or focal dystonia, including botulinum toxin injections, have not been confirmed as being effective according to accepted evidence-based criteria (level U, class IV studies).

Improvement by repeated administration of 6R-tetrahydrobiopterin of 5,7-dihydroxytryptamine-induced abnormal behaviors in immature rats

To clarify the therapeutic effects of 6R-L-erythro-5,6,7,8-tetrahydrobiopterin (6R-BH(4)) on the abnormal behaviors induced by neonatal 5,7-dihydroxytryptamine (5,7-DHT, 100 microg; i.c.v.) treatment in immature rats, 6R-BH(4) (10-40 mg/kg) was administered intraperitoneally from 22nd to 28th days or only once on the 28th day. The locomotion activities decreased dramatically in 5,7-DHT-treated rats (p<0.01; as compared to controls) on the 28th day. The reduced locomotion was recovered dose-dependently by repeated administration of 6R-BH(4), whereas it was not altered after a single injection of 6R-BH(4). In addition, repeated administration of 6R-BH(4) significantly facilitated 5-HT turnover ratio (5-HIAA/5-HT) in the striatum, cerebral cortex, and cerebellum. These findings suggest that the behavioral restoration by 6R-BH(4) might be due to the enhancement of 5-HT turnover by accumulated but not a single dose of 6R-BH(4).

Tetrahydrobiopterin deficiencies without hyperphenylalaninemia: diagnosis and genetics of dopa-responsive dystonia and sepiapterin reductase deficiency

DOPA responsive dystonia (DRD) and sepiapterin reductase (SR) deficiency are inherited disorders of tetrahydrobiopterin (BH4) metabolism characterized by the signs and symptoms related to monoamine neurotransmitter deficiency. In contrast to classical forms of BH4 deficiency DRD and SR deficiency present without hyperphenylalaninemia and thus cannot be detected by the neonatal screening for phenylketonuria (PKU). While DRD is mostly caused by autosomal dominant mutations in the GTP cyclohydrolase I gene (GCH1), SR deficiency is an autosomal recessive disease. The most important biochemical investigations for the diagnosis of these neurological diseases includes CSF investigations for neurotransmitter metabolites and pterins as well as neopterin and biopterin production in cytokine-stimulated fibroblasts. Discovery of SR deficiency opened new insights into alternative pathways of the cofactor BH4 via carbonyl, aldose, and dihydrofolate reductases. As a consequence of the low dihydrofolate reductase activity in the brain, dihydrobiopterin intermediate accumulates and inhibits tyrosine and tryptophan hydroxylases and uncouples nitric oxide synthase (nNOS), leading to neurotransmitter deficiency and possibly also to neuronal cell death.

Molecular mechanisms of hereditary progressive dystonia with marked diurnal fluctuation, Segawa's disease

The causative gene for hereditary progressive dystonia with marked diurnal fluctuation/dopa-responsive dystonia (HPD/DRD) was discovered in 1994 to be guanosine triphosphate (GTP) cyclohydrolase I, an enzyme involved in tetrahydrobiopterin biosynthesis. To the present, more than 50 mutations have been found in this gene in HPD/DRD patients. Although it is clear that HPD/DRD is caused by partial deficiency of tetrahydrobiopterin in the brain, several important issues regarding the molecular etiology of HPD/DRD remain to be addressed. We review herein the recent progress in the molecular genetics of HPD/DRD and clarify the points to be answered.

Molecular genetics of dopa-responsive dystonia

The causative genes of two types of hereditary dopa-responsive dystonia (DRD) due to dopamine (DA) deficiency in the nigrostriatum DA neurons have been elucidated. Autosomal dominant DRD (AD-DRD) was originally described by Segawa as hereditary progressive dystonia with marked diurnal fluctuation (HPD). We cloned the human GTP cyclohydrolase I (GCH1) gene, and mapped the gene to chromosome 14q22.1-q22.2 within the HPD/DRD locus, which had been identified by linkage analysis. GCH1 isthe rate-limiting enzyme for the biosynthesis of tetrahydrobiopterin (BH4), the cofactor for tyrosine hydroxylase (TH), which is the first and rate-limiting enzyme of DA synthesis. We proved that the GCH1 gene is the causative gene for HPD/DRD based on the identification of mutations of the gene in the patients and decreases in the enzyme activity expressed in mononuclear blood cells to 2-20% of the normal value. About 60 different mutations (missense, nonsense, and frameshift mutations) in the coding region or in the exon-intron junctions of the GCH1 gene have been reported in patients with AD-DRD all over the world. Recent findings indicate that the decreased GCH1 activity in AD-DRD may be caused by the negative interaction of the mutated subunit with the wild-type one, i.e., a dominant negative effect, and/or by decreases in the levels of GCH1 mRNA and protein caused by inactivation of one allele of the GCH1 gene. Autosomal recessive DRD (AR-DRD) with Segawa's syndrome was discovered in Germany. The AR-DRD locus was mapped to chromosome 11p15.5 in the chromosomal site of the TH gene. In the AR-DRD with Segawa's syndrome, a point mutation in TH (Gln381Lys) resulted in a pronounced decrease in TH activity to about 15% of that of the wild type. Several missense mutations in the TH gene have been found in AR-DRD in Europe. The phenotype of AR-DRD with the Leu205Pro mutation in the TH gene, which produces a severe decrease in TH activity to 1.5% of that of the wild type, was severe, not dystonia/Segawa's syndrome, but early-onset parkinsonism. However, a marked improvement of all clinical symptoms with a low dose of L-dopa was reported in AR-DRD/parkinsonism patients. These findings on DRD indicate that the nigrostriatal DA neurons may be most susceptible to the decreases in GCH1 activity, BH4 level, TH activity, and DA level, and that DRD is the DA deficiency without neuronal death in contrast to juvenile parkinsonism or Parkinson's disease with DA cell death.

Striatal biopterin and tyrosine hydroxylase protein reduction in dopa-responsive dystonia

OBJECTIVE

To determine the mechanism leading to striatal dopamine (DA) loss in dopa-responsive dystonia (DRD).

BACKGROUND

Although mutations in the gene GCH1, coding for the tetrahydrobiopterin (BH4) biosynthetic enzyme guanosine triphosphate-cyclohydrolase I, have been identified in some patients with DRD, the actual status of brain BH4 (the cofactor for tyrosine hydroxylase [TH]) is unknown.

METHODS

The authors sequenced GCH1 and measured levels of total biopterin (BP) and total neopterin (NP), TH, and dopa decarboxylase (DDC) proteins, and the DA and vesicular monoamine transporters (DAT, VMAT2) in autopsied brain of two patients with typical DRD.

RESULTS

Patient 1 had two GCH1 mutations but Patient 2 had no mutation in the coding region of this gene. Striatal BP levels were markedly reduced (<20% of control subjects) in both patients and were also low in two conditions characterized by degeneration of nigrostriatal DA neurons (PD and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine treated primate), whereas brain NP concentrations were selectively decreased (<45%) in the DRD patients. In the putamen, both DRD patients had severely reduced (<3%) TH protein levels but had normal concentrations of DDC protein, DAT, and VMAT2.

CONCLUSIONS

The data suggest that 1) brain BH4 is decreased substantially in dopa-responsive dystonia, 2) dopa-responsive dystonia can be distinguished from degenerative nigrostriatal dopamine deficiency disorders by the presence of reduced brain neopterin, and 3) the striatal dopamine reduction in dopa-responsive dystonia is caused by decreased TH activity due to low cofactor concentration and to actual loss of TH protein. This reduction of TH protein, which might be explained by reduced enzyme stability/expression consequent to congenital BH4 deficiency, can be expected to limit the efficacy of acute BH4 administration on dopamine biosynthesis in dopa-responsive dystonia.

Decrease in GTP cyclohydrolase I gene expression caused by inactivation of one allele in hereditary progressive dystonia with marked diurnal fluctuation

Hereditary progressive dystonia with marked diurnal fluctuation (HPD; dopa-responsive dystonia, DRD) have been recently found to be caused by a genetic defect in the GTP cyclohydrolase I (GCH1) gene. In this study, we quantified the mRNA level of GCH1 in phytohemagglutinin (PHA)-stimulated mononuclear blood cells from one Japanese family that do not have a mutation in the coding region or splice junctions of the gene. The results showed that the amounts of the GCH1 mRNA were decreased to about 40% of the normal level in both patients and carriers. In addition, we found that the GCH1 mRNA was transcribed from only one allele, indicating that the other allele was in an inactive state. These results suggest that some novel mutations should exist on one of the alleles in some unknown region of the GCH1 gene, and may decrease the GCH1 mRNA causing the HPD/DRD symptoms.

Molecular biology of catecholamine-related enzymes in relation to Parkinson's disease

1. Catecholamine (dopamine, norepinephrine, and epinephrine) biosynthesis is regulated by tyrosine hydroxylase (TH). TH activity is regulated by the concentration of the cofactor tetrahydrobiopterin (BH4), whose level is regulated by GTP cyclohydrolase I (GCH) activity. Thus, GCH activity indirectly regulates TH activity and catecholamine levels. 2. TH activity in the nigrostriatal dopaminergic neurons is most sensitive to the decrease in BH4. 3. Mutations of GCH result in reductions in GCH activity, BH4, TH activity, and dopamine, causing either recessively inherited GCH deficiency or dominantly inherited hereditary progressive dystonia [HPD; Segawa's disease; also called dopa-responsive dystonia (DRD)]. 4. In juvenile parkinsonism and Parkinson's disease, which have dopamine deficiency in the basal ganglia as HPD/DRD, the GCH gene may be normal, and the molecular mechanism of the dopamine deficiency in the basal ganglia is different from that in HPD/DRD.

NADH stimulates endogenous dopamine biosynthesis by enhancing the recycling of tetrahydrobiopterin in rat phaeochromocytoma cells

Treatment of Parkinson patients with L-DOPA (3,4-dihydroxy-L-phenylalanine) leads to endproduct inhibition of tyrosine hydroxylase, the key enzyme in dopamine biosynthesis and the enzyme needing tetrahydrobiopterin and iron as cofactors. To overcome this problem an alternative treatment was investigated which attempted to stimulate endogenous dopamine biosynthesis. Incubation of rat PC 12 cells with NADH (beta-nicotinamide adeninedinucleotide) leads to increased dopamine production. We investigated the possibility that this increase of dopamine biosynthesis was due to stimulation of quinonoid dihydropteridine reductase, the enzyme which recycles the inactive dihydrobiopterin to the active tetrahydrobiopterin. The experiments showed that whereas NADH is able to increase dopamine production in PC 12 cells (rat phaeochromocytoma cells, clone PC 12) up to three-fold, no influence is exerted by NADH on pteridine metabolism; neither are tetrahydrobiopterin concentrations nor the de novo-biosynthesis of pteridines from guanosine triphosphate altered by NADH. Further no influence of NADH on protein de novo synthesis of quinonoid dihydropteridine reductase was observed. However, NADH was able to directly increase the catalytic activity of this enzyme. Our results suggest that the stimulation of dopamine biosynthesis by NADH is due to more rapid regeneration of quinonoid dihydrobiopterin to tetrahydrobiopterin.

GTP-cyclohydrolase I gene mutations in hereditary progressive amd dopa-responsive dystonia

Recently, mutations of the GTP-cyclohydrolase I (GTP-CH I) gene, which catalyzes the first step in the tetrahydrobiopterin (BH4) biosynthesis, were discovered in Japanese patients with hereditary progressive dystonia/dopa-responsive dystonia (HPD/DRD). However, it has not been confirmed that non-Japanese patients also contain mutations in the same gene, or whether these mutations are specific to HPD/DRD. In this study, two novel nonsense mutations in exon I of the GTP-CH I gene and a new mutation at the splice acceptor site of intron I were identified in an autopsied case of English-Irish descent and 2 Japanese patients with HPD/DRD. In the latter, cerebrospinal fluid (CSF) neopterin levels (which may reflect the GTP-CH I activity in the brain) were reduced to 18% and 37% of controls. A therapeutic trial of oral BH4 was ineffective, however, in a genetically proven patient. In contrast, no mutations in any exons of the GTP-CH I gene were found in 2 patients with early-onset parkinsonism with dystonia (EOP-D) who developed dopa-responsive parkinsonism and dystonia at 6 and 8 years old, respectively. Neopterin levels in CSF were well preserved in 6 EOP-D patients. These data suggest that, among patients of different racial backgrounds, the pathogenesis of HPD/DRD, unlike EOP-D, involves partial reduction of the brain GTP-CH I activity consequent to mutations in the GTP-CH I gene. Measurement of CSF neopterin concentration may be useful for the differential diagnosis between HPD/DRD and EOP-D.

Regulation of tyrosine hydroxylase and tetrahydrobiopterin biosynthetic enzymes in PC12 cells by NGF, EGF and IFN-gamma

The regulation of catecholamine and tetrahydrobiopterin synthesis was investigated in cultured rat pheochromocytoma PC12 cells following treatments with nerve growth factor (NGF), epidermal growth factor (EGF) and interferon-gamma (IFN-gamma). NGF and EGF, but not IFN-gamma, caused an increase after 24 h in the levels of BH4 and catecholamines, and the activities of tyrosine hydroxylase and GTP cyclohydrolase, the rate-limiting enzymes in catecholamine and BH4 synthesis, respectively. Actinomycin D, a transcriptional inhibitor, blocked treatment-induced elevations in tyrosine hydroxylase and GTP cyclohydrolase activities. NGF, EGF or IFN-gamma did not affect the activity of sepiapterin reductase, the final enzyme in BH4 biosynthesis. Rp-cAMP, an inhibitor of cAMP-mediated responses, blocked the induction of tyrosine hydroxylase by NGF or EGF; inhibition of protein kinase C partially blocked the EGF effect, but not the NGF effect, NGF also induced GTP cyclohydrolase in a cAMP-dependent manner, while the EGF effect was not blocked by Rp-cAMP or protein kinase C inhibitors. Sphingosine induced GTP cyclohydrolase in a protein kinase C-independent manner without affecting tyrosine hydroxylase activity. Our results suggest that both tyrosine hydroxylase and GTP cyclohydrolase are induced in a coordinate and transcription-dependent manner by NGF and EGF, while conditions exist where the induction of tyrosine hydroxylase and GTP cyclohydrolase is not coordinately regulated.

GTP cyclohydrolase I gene in hereditary progressive dystonia with marked diurnal fluctuation

We previously reported four different mutations in the coding region of GTP cyclohydrolase I (GCH-I) gene in patients with hereditary progressive dystonia with marked diurnal fluctuation (HPD). We found two independent new mutations (leucine 79 proline and a deletion in exon 4) in patients with HPD. We also found four families of HPD without any mutations in the coding region of GCH-I gene.

Tetrahydrobiopterin uptake into rat brain synaptosomes, cultured PC12 cells, and rat striatum

The uptake of (6R)-5,6,7,8-tetrahydro-L-biopterin (BH4) was investigated in rat brain synaptosomes, cultured rat pheochromocytoma (PC12) cells, and rat striatum (control and depleted of dopamine neurons) following peripheral administration. A linear, non-saturable, concentration-dependent intracellular accumulation was observed when BH4 was added to either synaptosomes or PC12 cells. The uptake of BH4, in contrast to that of serotonin uptake into synaptosomes or norepinephrine (NE) uptake into PC12 cells, was not dependent on glucose or extracellular sodium. Stimulation of tryptophan hydroxylation in synaptosomes by incubation with 5 microM tryptophan (which increases utilization of BH4 in serotonergic cells) did not alter BH4 uptake. In rats with unilateral 6-hydroxydopamine (6-OHDA) lesions of dopamine neurons, BH4 uptake was the same in control and lesioned striatum following peripheral administration. These results indicate that neurons and PC12 cells do not appear to have a specific membrane carrier for BH4 and that BH4 uptake into cells is due to passive diffusion.

Hereditary progressive dystonia with marked diurnal fluctuation caused by mutations in the GTP cyclohydrolase I gene

Hereditary progressive dystonia with marked diurnal fluctuation (HPD) (also known as dopa responsive dystonia) is a dystonia with onset in childhood that shows a marked response without any side effects to levodopa. Recently the gene for dopa responsive dystonia (DRD) was mapped to chromosome 14q. Here we report that GTP cyclohydrolase I is mapped to 14q22.1-q22.2. The identification of four independent mutations of the gene for GTP cyclohydrolase I in patients with HPD, as well as a marked decrease in the enzyme's activity in mononuclear blood cells, confirms that the GTP cyclohydrolase I gene is a causative gene for HPD/DRD. This is the first report of a causative gene for the inherited dystonias.

Dopa-responsive dystonia simulating cerebral palsy

Five patients presented in infancy or early childhood with various combinations of pyramidal and extrapyramidal signs with normal cognitive function. Their perinatal courses were unremarkable. In each patient, initial impressions listed by several examiners included spastic diplegia or cerebral palsy. Later in each course, either extrapyramidal features or progression suggested dopa-responsive dystonia. In 4 of the 5 children, cerebrospinal fluid was obtained and disclosed reduced levels of biopterin, neopterin, and homovanillic acid in all 4. Levodopa therapy resulted in prompt improvement with normal function returning within 6 months. The disappearance of the "spasticity," extensor plantar responses, and extrapyramidal signs, following levodopa therapy, confirmed the diagnosis of doparesponsive dystonia in these patients. Three had apparently sporadic disease; the other 2 were siblings with an affected paternal grandmother. Three had onset in infancy with delayed sitting and walking before the appearance of overt dystonia; infantile onset is infrequent in dopa-responsive dystonia. The other 2 had normal milestones, but developed gait disorders with prominent imbalance in early childhood. The diagnosis of dopa-responsive dystonia should be considered in children with unexplained or atypical "cerebral palsy."

Lessons from a remarkable family with dopa-responsive dystonia

A family is described in which dopa-responsive dystonia affected six members and segregated in an autosomal dominant fashion. Patients either presented in childhood with dystonia of the legs, going to develop parkinsonism and pseudo-pyramidal deficits, or in adult life with parkinsonian tremor and rigidity, with pseudo-pyramidal signs. Remarkably, in the three cases with childhood onset the symptoms and signs of the condition were abolished 36 to 52 years later by small doses of levodopa. No long term side effects of levodopa have appeared after 15 years of treatment.

Dopa-responsive dystonia: pathological and biochemical observations in a case

We report the first neuropathological and neurochemical study of a patient with dopa-responsive dystonia. She had onset of foot dystonia at age 5 years and by age 8 years it was generalized with prominent right leg and arm involvement. On levodopa 750 mg daily she had complete symptomatic improvement that was sustained for 11 years until death. Pathological studies revealed normal numbers of hypopigmented substantia nigra neurons, normal tyrosine hydroxylase (TH) immunoreactivity and TH protein in the SN, no inclusion bodies or gliosis, and no evidence of a degenerative process in the striatum. Biochemical studies revealed reduced dopamine in the substantia nigra and striatum (8% in the putamen and 18% of control in the caudate) with a similar but not identical subregional distribution as in idiopathic Parkinson's disease. In the striatum, TH protein and TH activity was reduced, with the loss more pronounced in the putamen than the caudate. The GBR 12935 binding to DA transporter was normal in the caudate and at the lower end of the range of control values in the putamen. We conclude that disturbed dopamine synthetic capacity or a reduced arborization of striatal dopamine terminals may be the major disturbance in dopa-responsive dystonia.

Expression and characterization of recombinant human and rat liver 6-pyruvoyl tetrahydropterin synthase. Modified cysteine residues inhibit the enzyme activity

6-Pyruvoyl-tetrahydropterin synthase is the rate-limiting enzyme in the synthesis of human tetrahydrobiopterin, a cofactor for several hydroxylases involved in catecholamine and serotonin biosynthesis. The human and rat liver cDNAs encoding the 16-kDa subunit of 6-pyruvoyl tetrahydropterin synthase were expressed as maltose-binding-6-pyruvoyl-tetrahydropterin-synthase fusion proteins. After cleavage from the fusion protein, the human and rat enzymes were purified to homogeneity. Apparent Km for the substrate dihydroneopterin triphosphate (8.5 microM for the human and 8.0 microM for the rat enzyme), pI (4.6 and 4.8) and heat stability of the recombinant enzymes were similar to the native enzymes. The specific activity of the enzymes was enhanced up to fourfold in the presence of dithiothreitol during purification. The modification of the only cysteine residue in rat 6-pyruvoyl tetrahydropterin synthase, which is conserved in the human enzyme, inhibited its activity up to 80%. Modification under non-reducing conditions of both cysteine residues of the human enzyme by N-ethylpyridine resulted in a 95% loss of enzyme activity. This demonstrates that the two cysteines are not linked by disulfide bridges but rather involved in catalysis. Cross-linking experiments and analysis by gel electrophoresis showed predominantly trimeric and hexameric forms of the recombinant enzymes from both species suggesting that the native form is a homohexamer of 98 kDa, for the human, and 95 kDa, for the rat enzyme, composed of two trimeric subunits.

Linkage mapping of dopa-responsive dystonia (DRD) to chromosome 14q

Dopa-responsive dystonia (DRD) is an autosomal-dominant neurological disorder which appears to result from a genetically determined deficiency of striatal dopamine. Pathological evidence suggests that this may be due to the establishment of a reduced number of dopaminergic nerve terminals in the striatum, or to an excessive reduction (pruning) of these terminals in early development. We have mapped the DRD gene to chromosome 14 by linkage analysis in 3 families with a maximum 2-point lod score of 4.67 at 8.6 centiMorgans from D14S63; maximum multipoint lod scores > 6 were obtained for the intervals D14S47-D14S52 and D14S52-D14S63. The flanking loci D14S47 and D14S63 define a region of about 22 cM as containing the DRD gene.

Positron emission tomographic studies of dopa-responsive dystonia and early-onset idiopathic parkinsonism

There are two major syndromes presenting in the early decades of life with dystonia and parkinsonism: dopa-responsive dystonia (DRD) and early-onset idiopathic parkinsonism (EOIP). DRD presents predominantly in childhood with prominent dystonia and lesser degrees of parkinsonism. EOIP presents before age 40 with parkinsonism (often with associated dystonia). Both disorders are exquisitely sensitive to levodopa, although the long-term prognosis in each appears to be different. Some have suggested, however, that DRD is a form of EOIP. We performed positron emission tomography with 6-fluoro-dopa in 10 patients with DRD and 18 patients with EOIP to study the integrity of their nigrostriatal dopaminergic systems. In DRD, we found normal striatal FD uptake. In contrast, patients with EOIP had reduced striatal FD uptake. We conclude that the patho-physiologies of DRD and EOIP are distinct. Although both disorders presumably represent a deficiency of striatal dopamine, the results suggest that in DRD dopa uptake, decarboxylation, and storage mechanisms are intact. This may explain the sustained response of DRD to low doses of levodopa. 6-Fluoro-dopa positron emission tomography distinguishes DRD from EOIP.

Differential metabolism of tetrahydrobiopterin in monoamine neurons: a hypothesis based upon clinical and basic research

This chapter has attempted to describe and integrate some of the clinical and basic research that support our hypothesis that the metabolism of BH4 is normally heterogeneous across different populations of monoamine-containing neurons. Based upon this hypothesis, there may now be reason to support the idea that certain neuropsychiatric illnesses, which are though to be the result (at least in part) of altered monoamine metabolism, might find their roots in an abnormal metabolism of BH4 within specific monoaminergic cell groups. Such a specific dysfunction might not be apparent in the rest of the brain or peripheral nervous system, thereby being difficult to detect. Perhaps the application of molecular biological techniques to studies of BH4 metabolism in man will shed new light on these problems.

Long-term treatment response and fluorodopa positron emission tomographic scanning of parkinsonism in a family with dopa-responsive dystonia

Dopa-responsive dystonia (DRD) is one form of childhood-onset idiopathic torsion dystonia. Adult-onset parkinsonism has appeared in several previously unaffected members in families with DRD suggesting that this may be an additional phenotypical expression of the disease. We report a family with DRD in which 2 women and 1 man, unaffected by dystonia, developed tremor-onset parkinsonism after age 50 years. The women continue on a low dosage of levodopa after 9 and 13 years of treatment, with a stable, nearly complete, symptomatic response. This contrasts to the typical long-term treatment complications observed in patients with Parkinson's disease. We assessed nigrostriatal dopaminergic function in the proband, with typical DRD, and the 2 women with parkinsonism using 6-[18F]fluoro-L-dopa positron emission tomography. All 3 had normal striatal 6-[18F]fluoro-L-dopa uptake. These observations provide compelling evidence that "benign" adult-onset parkinsonism may be an expression of the disease in some members of families with DRD and does not support consideration of the DRD gene as a risk factor for development of Parkinson's disease. There may be considerable clinical heterogeneity in DRD depending on the age at onset.

Significance of CSF total neopterin and biopterin in inflammatory neurological diseases

Total neopterin (T-N), a by-product in the biopterin biosynthesis and an indicator of activation of the cellular immune system, and total biopterin (T-B) levels in cerebrospinal fluid (CSF), were measured in patients with various inflammatory neurological diseases and Parkinson's disease, and the following results were obtained. (1) In patients with neuro-sarcoidosis, neuro-Behçet's disease and meningitis, CSF T-N levels were markedly elevated in the exacerbation or acute stages of their neurological symptoms and remarkably decreased in the remission or chronic stages. In the neuro-sarcoidosis and neuro-Behçet's disease patients, however, CSF T-B levels showed no substantial change. (2) There was a significant positive correlation between CSF T-N levels and CSF/serum albumin ratios only in the meningitis patients. However, increases of CSF T-N levels were not associated with those of plasma T-N levels. (3) In the Parkinson's disease patients, CSF T-N levels remained normal, although CSF T-B levels significantly decreased. (4) A gradient for the CSF T-N value (lumbar greater than ventricular CSF), being reverse to the CSF T-B value, was observed. These results indicate that the significance of CSF T-N is quite different from CSF T-B, and that CSF T-N appears to be a valuable biochemical marker for evaluating the activity of inflammation within the central nervous system. Its measurement seems useful for therapeutic monitoring, especially of patients showing the chronic exacerbating-remitting course.

Juvenile parkinsonism: ventricular CSF biopterin levels and clinical features

Total biopterin (T-BP) levels in the ventricular cerebrospinal fluid (CSF) and clinical features of 19 patients with juvenile parkinsonism (JP: Parkinson's disease manifesting below the age of 40) were evaluated and compared with 61 patients with classical Parkinson's disease (classical PD: symptoms developing at the age of 40 or above). The JP patients were divided into two subgroups: JP-I; those with good response to levodopa followed by marked motor fluctuations and dopa-induced dyskinesias (DID), JP-II; those with milder response than JP-I with less fluctuations and DID being more similar to classical PD. Both of the mean ventricular CSF T-BP concentrations in the JP and classical PD patients were significantly lower than that in neurological controls. Moreover, the mean T-BP level in the JP-I was markedly lower than that in the JP-II or classical PD. Total biopterin levels revealed a gaussian distribution in the classical PD. However, a bimodal distribution was noted in the JP, with the lower peak consisting of only JP-I patients. These results seem to indicate that JP-II represents early-onset classical PD, while JP-I represents a distinct subgroup having a different physiopathology from classical PD.

Immunological evidence for the requirement of sepiapterin reductase for tetrahydrobiopterin biosynthesis in brain

Specific antibodies to sepiapterin reductase were used to investigate its involvement in de novo (6R)-5,6,7,8-tetrahydrobiopterin (BH4) biosynthesis in rat brain. Antisepiapterin reductase (anti-SR) serum totally inhibited NADPH-dependent sepiapterin reductase activity in supernatants from discrete rat brain areas and liver. The anti-SR serum also inhibited the conversion of 7,8-dihydroneopterin triphosphate to BH4 in rat brain extracts. The inhibition was accompanied by a concentration-dependent increase in the formation of 6-lactoyltetrahydropterin (6LPH4), a proposed intermediate in BH4 biosynthesis. In addition, anti-SR serum was used to characterize the distribution and molecular properties of sepiapterin reductase in rat tissues. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis followed by Western blotting indicated that there was a single polypeptide with the same molecular weight (28,000) as that of the subunit of pure sepiapterin reductase present in all tissues examined except for liver, where an immunoreactive protein of higher molecular weight (30,500) also was detected. Two-dimensional gel electrophoresis of rat striatum and liver demonstrated that the isoelectric point of sepiapterin reductase from both tissues was 6.16 and that the higher molecular weight immunoreactive material in liver had an isoelectric point of 7.06. Our studies with specific anti-SR serum confirmed the results of previous studies using chemical inhibitors of sepiapterin reductase, which suggested that sepiapterin reductase activity was essential for BH4 biosynthesis in the CNS and that 6LPH4 could be a precursor of BH4.

Purification and characterization of 6-pyruvoyl tetrahydropterin synthase from salmon liver

Salmon liver was chosen for the isolation of 6-pyruvoyl tetrahydropterin synthase, one of the enzymes involved in tetrahydrobiopterin biosynthesis. A 9500-fold purification was obtained and the purified enzyme showed two single bands of 16 and 17 kDa on SDS/PAGE. The native enzyme (68 kDa) consists of four subunits and needs free thiol groups for enzymatic activity as was shown by reacting the enzyme with the fluorescent thiol reagent N-(7-dimethylamino-4-methylcoumarinyl)-maleimide. The enzyme is heat-stable up to 80 degrees C, has an isoelectric point of 6.0-6.3, and a pH optimum at 7.5. The enzyme is Mg2+ -dependent and has a Michaelis constant for its substrate dihydroneopterin triphosphate of 2.2 microM. The turnover number of the purified salmon liver enzyme is about 50 times as high as that of the enzyme purified from human liver. It does not bind to the lectin concanavalin A, indicating that it is free of mannose and glucose residues. Polyclonal antibodies raised against the purified enzyme in Balb/c mice were able to immunoprecipitate enzyme activity. The same polyclonal serum was not able to immunoprecipitate enzyme activity of human liver 6-pyruvoyl tetrahydropterin synthase, nor was any cross-reaction in ELISA tests seen.

Tetrahydrobiopterin and biogenic amine metabolism in neuropsychiatry, immunology, and aging

Tetrahydrobiopterin (BH4) is essential for biogenic amine synthesis, and alterations in its metabolism occur at birth (atypical PKU), in neuropsychiatric illnesses, and in aging. BH4 therapy has been attempted in atypical PKU and in neuropsychiatric illness with some success and may become more viable as more is learned about BH4 metabolism and ways are discovered to elevate brain BH4 levels. It is intriguing to consider that a genetic defect in BH4 biosynthesis occurring at birth might go unrecognized and contribute to altered biogenic amine metabolism that occurs in neuropsychiatric illness. Since there seems to be a sensitivity of BH4 metabolism to genetic alterations, it is possible that altered BH4 metabolism is involved in some of deleterious effects associated with the aging process. A link between genetic alterations in BH4 metabolism at birth and adult neuropsychiatric illness and aging remains to be established, although this seems plausible. The presence of BH4 and other pterins in cells of the immune system as well as the pineal gland and other neuroendocrine tissues suggests the potential for other functions of pterins. Hopefully, future research will uncover the full potential for the therapeutic use of BH4 in a variety of diseases as well as elucidating other potential roles for pterin molecules which are present in many different systems.