Review articleMolecular and metabolic bases of tetrahydrobiopterin (BH4) deficiencies
Introduction
BH4 deficiencies are a group of rare inherited neurological disorders, characterized by neurotransmitter dysfunction, with or without hyperphenylalaninemia [1]. BH4 (6R-l-erythro-5,6,7,8-tetrahydrobiopterin) itself is a reduced pterin derivative that is present in probably all human tissues as an essential cofactor for enzymes involved in diverse enzymatic reactions [2]. These include phenylalanine hydroxylase (PAH), tyrosine hydroxylase (TH), tryptophan hydroxylases type 1 and 2 (TPH1 & 2), the three isoforms of nitric oxide synthase (NOS 1–3), and alkyglycerol mono‑oxygenase (AGMO). BH4 is essential for L-phenylalanine (L-Phe) degradation and biosynthesis of the monoamine neurotransmitters dopamine and serotonin, respectively (Fig. 1). There is a less well-defined role for BH4 in chronic pain sensitivity [3] and in T cell proliferation [4]. Accordingly, BH4 deficiencies are diverse in terms of their presenting phenotypes.
The first overview of disease-causing BH4 variants was published in 1997, describing a spectrum of 135 mutations in enzymes involved in BH4 production (see below) from 50 patients [5]. Since then, data from biochemical, clinical and DNA analyses from BH4-deficient patients have been collected and tabulated in the BIODEF and PNDdb databases. The present genomic landscape of BH4 deficiencies (as of April 2021) is based on the study of 800 reported variants (Table 1). Here, we summarize our current understanding of the genetic basis, phenotypic presentation and functional outcomes associated with BH4 deficiencies.
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Overview of the synthesis and regeneration of BH4
BH4 is generated via de novo synthesis which includes a (partial) salvage pathway and can be recycled (upon enzymatic or non-enzymatic oxidation) by the BH4-recycling pathway (Fig. 1). Three enzymes participate in the de novo biosynthesis of BH4 from guanosine triphosphate (GTP): GTP cyclohydrolase 1 (GTPCH; E.C. 3.5.4.16), 6-pyruvoyl-tetrahydropterin synthase (PTPS; E.C. 4.6.1.10), and sepiapterin reductase (SR; E.C. 1.1.1.153) [2]. The final two-step reduction is catalyzed by SR in the de novo
Diagnosis and management of BH4 disorders
Individual phenotypes associated typically with variants in individual genes within the BH4 biosynthesis or recycling pathways are described in detail below. A diagnosis may be precipitated by the discovery of HPA, where present, which is typically identified at newborn screening [9]. Where an elevated phenylalanine:tyrosine ratio is confirmed, the differential diagnosis at this time includes phenylketonuria (PKU), mild HPA, transient HPA and defects in pterin metabolism, including BH4
BH4 deficiencies associated with specific genes within the BH4 synthetic or recycling pathways
The locus-specific database PNDdb (http://www.biopku.org/home/pnddb.asp), ClinVar (https://www.ncbi.nlm.nih.gov/clinvar/), HGMD (http://www.hgmd.cf.ac.uk/ac/index.php) and LOVD (https://databases.lovd.nl/shared/genes) databases were searched for variants. The RefSeq accession numbers and GeneBank numbers are shown in Table 2 and in all Supplemental Tables. All variants were tested using Mutalyzer 2.0 (https://mutalyzer.nl/) and follow the HGVS guidelines (https://varnomen.hgvs.org/). Following
Variants
The initiation of BH4 biosynthesis is triggered by GTPCH, which catalyzes the first and rate-limiting step, the conversion of GTP to D-erythro7,8-dihydroneopterin triphosphate (Fig. 1). In general, autosomal mutations with recessive inheritance (compound heterozygous or homozygous) in GCH1 cause arGTPCH deficiency with HPA and monoamine neurotransmitter deficiency, whereas dominant heterozygous variants present with DRD without HPA (or adGTPCH). These dominant variants are commonly inherited
Variants
PTPS catalyzes the second and non-reversible step in BH4 biosynthesis through the removal of triphosphate from the substrate 7,8-dihydroneopterin triphosphate (Fig. 1). In 1985, PTPS deficiency was recognized as the cause of a heterogeneous BH4-deficient variant of HPA [45]. The first variants associated with HPA and neurotransmitter deficiency were found by analyzing the corresponding gene PTS localized on chromosome 11q22-3-q23.3 [[46], [47], [48]].
In 1997, Thöny and Blau presented a small
Variants
The SR protein, a 7,8-dihydrobiopterin:NADP+oxidoreductase which is also known as NADPH-dependent SR, belongs to the group of aldo-keto reductases. It catalyzes the reduction of carbonyl substrates such as the pterin intermediate 6-pyruvoyl-tetrahydropterin and is essential for the de-novo BH4 biosynthesis (Fig. 1).
SR deficiency was discovered as a genetic dysfunction in BH4 metabolism with autosomal recessive heredity, presenting with monoamine neurotransmitter deficiency without HPA [64,65].
Variants
PCD is a bi-functional protein that (i) is essential in the BH4 regeneration pathway where it acts as a dehydratase and (ii) plays an important role as a binding and dimerization cofactor of HNF-1α in the nucleus to increase transcriptional activity [83]. Defects in PCBD1 result in BH4 deficiency [84]. The protein-coding sequence spans 4 exons and includes 315 bases with exon 1 containing only the ATG start codon (NM_000281.2) [[85], [86], [87]].
A total of 32 variants in PCBD1 are presented in
Variants
DHPR catalyzes the NADH-mediated reduction of quinonoid dihydrobiopterin and is an essential factor for the hydroxylation of the aromatic pterin-dependent amino acids (Fig. 1). Autosomal recessive DHPR deficiency is caused by defects in the corresponding gene QDPR [92,93]. The QDPR gene is transcribed into an mRNA-sequence of 735 nucleotides (NM_000320.2) and translated into a primary sequence of 245 amino acids, to generate a 27 kDa protein monomer. The functional enzyme is homo-dimeric. [94,95
BH4 and pain sensitivity
BH4 has been reported as a key modulator of peripheral neuropathic and inflammatory pain [112,113]. After axonal injury, concentrations of BH4 rose in primary sensory neurons due to upregulation of GCH1. After peripheral inflammation, BH4 also increased in dorsal root ganglia, owing to enhanced GTPCH activity. In humans, several variants of the GCH1 (e.g., c.-9610G>A, c.343+8900A>T, c.509+1551T>C, c.509+5836A>G and c.627-708G>A) were significantly associated with less pain following diskectomy
Acknowledgements
This work was supported by the FP7-HEALTH-2012-INNOVATION-1 EU Grant No. 305444 and by funding from the Dietmar-Hopp Foundation (both to NB). NH was supported by the DFG (German Research Foundation), research group FOR 2509, Project-ID TH1461/7–1.
References (122)
- et al.
Tetrahydrobiopterin is synthesized from 6-pyruvoyl-tetrahydropterin by the human aldo-keto reductase AKR1 family members
Arch. Biochem. Biophys.
(2003) - et al.
Human carbonyl and aldose reductases: new catalytic functions in tetrahydrobiopterin biosynthesis
Biochem. Biophys. Res. Commun.
(1991) - et al.
Key European guidelines for the diagnosis and management of patients with phenylketonuria
Lancet Diabetes Endocrinol.
(2017) - et al.
DNAJC12 deficiency: a new strategy in the diagnosis of hyperphenylalaninemias
Mol. Genet. Metab.
(2018) - et al.
BH4 deficiency identified in a neonatal screening program for hyperphenylalaninemia
J. Pediatr.
(2018) - et al.
Outcome and long-term follow-up of 36 patients with tetrahydrobiopterin deficiency
Mol. Genet. Metab.
(2008) - et al.
The GTP cyclohydrolase I gene in atypical parkinsonian patients – a clinico-genetic study
J. Neurol. Sci.
(1996) - et al.
A missense mutation in a patient with guanosine triphosphate cyclohydrolase I deficiency missed in the newborn screening program
J. Pediatr.
(1995) - et al.
Characterization of mouse and human GTP cyclohydrolase I genes – mutations in patients with GTP cyclohydrolase I deficiency
J. Biol. Chem.
(1995) - et al.
Human GTP cyclohydrolase I gene and sepiapterin reductase gene map to region 14q21-q22 and 2p14-p12, respectively, by in situ hibridization
Genomics
(1995)
Generation of an iPSC line from a patient with GTP cyclohydrolase 1 (GCH1) deficiency: HDMC0061i-GCH1
Stem Cell Res.
Purification and properties of the phosphate eliminating enzyme involved in the biosynthesis of BH4 in man
Biochem. Biophys. Res. Commun.
Human 6-pyruvoyltetrahydropterin synthase: cDNA cloning and heterologous expression of the recombinant enzyme
Biochem. Biophys. Res. Commun.
Chromosomal location of two human genes encoding tetrahydrobiopterin-metabolizing enzymes: 6-pyruvoyl-tetrahydropterin synthase maps to 11q22.3-q23.3, and pterin-4a-carbinolamine dehydratase maps to 10q22
Genomics
6-pyruvoyltetrahydropterin synthase deficiency: review and report of 28 Arab subjects
Pediatr. Neurol.
Structural and functional consequences of mutations in 6-pyruvoyltetrahydropterin synthase causing hyperphenylalaninemia in humans – phosphorylation is a requirement for in vivo activity
J. Biol. Chem.
Role of protein structure in variant annotation: structural insight of mutations causing 6-pyruvoyl-tetrahydropterin synthase deficiency
Pathology
Cloning and sequencing of cDNA encoding human sepiapterin reductase--an enzyme involved in tetrahydrobiopterin biosynthesis
Biochem. Biophys. Res. Commun.
Mutations in the sepiapterin reductase gene cause a novel tetrahydrobiopterin-dependent monoamine neurotransmitter deficiency without hyperphenylalaninemia
Am. J. Hum. Genet.
Tetrahydrobiopterin deficiencies without hyperphenylalaninemia: diagnosis and genetics of Dopa-responsive dystonia and sepiapterin reductase deficiency
Mol. Genet. Metab.
Molecular genetics of tetrahydrobiopterin (BH(4)) deficiency in the Maltese population
Mol. Genet. Metab.
Sepiapterin reductase deficiency: report of 5 new cases
Eur. J. Paediatr. Neurol.
Phenylalanine hydroxylase-stimulating protein/pterin-4 a-carbinolamine dehydratase from rat and human liver. Purification, characterization, and complete amino acid sequence
J. Biol. Chem.
Characterization of the human PCBD gene encoding the bifunctional protein pterin-4a-carbinolamine dehydratase/dimerization cofactor for the transcription factor HNF-1 alpha
Biochem. Biophys. Res. Commun.
Hyperphenylalaninemia with high levels of 7-biopterin is associated with mutations in the PCBD gene encoding the bifunctional protein pterin-4a-carbinolamine dehydratase (PCD) and transcriptional coactivator (DCoH)
Am. J. Hum. Genet.
Primapterin, anapterin, and 6-oxo-primapterin, three new 7-substituted pterins identified in a patient with hyperphenylalaninemia
Biochem. Biophys. Res. Commun.
Localization of the human dihydropteridine reductase gene to band p15.3 of chromosome 4 by in situ hybridization
Genomics
Disorders of tetrahydrobiopterin and related biogenic amines
Tetrahydrobiopterin: biochemistry and pathophysiology
Biochem. J.
Combining human and rodent genetics to identify new analgesics
Neurosci. Bull.
The metabolite BH4 controls T cell proliferation in autoimmunity and cancer
Nature
Mutations in the GTP cyclohydrolase I and 6-pyruvoyl-tetrahydropterin synthase genes
Hum. Mutat.
Characterisation of a cofactor that regulates dimerization of mammalian homeodomain protein
Science
Phenylketonurie and BH4 Deficiencies: UNI-MED Bremen
Disorders of biopterin metabolism
J. Inherit. Metab. Dis.
An international survey of patients with tetrahydrobiopterin deficiencies presenting with hyperphenylalaninaemia
J. Inerit. Metab. Dis.
Consensus guideline for the diagnosis and treatment of tetrahydrobiopterin (BH4) deficiencies
Orphanet J. Rare Dis.
Disorders of phenylalanine and tetrahydrobiopterin metabolism
Dopa-responsive dystonia-clinical and genetic heterogeneity
Nat. Rev. Neurol.
A compound heterozygote for GCH1 mutation represents a case of atypical Dopa-responsive dystonia
J. Mol. Neurosci.
Arg(184)His mutant GTP cyclohydrolase I, causing recessive hyperphenylalaninemia, is responsible for dopa-responsive dystonia with parkinsonism: a case report
Mov. Disord.
Lessons from a remarkable family with dopa-responsive dystonia
J. Neurol. Neurosurg. Psychiatry
Parkinson’s disease in GTP cyclohydrolase 1 mutation carriers
Brain
Dystonia with motor delay in compound heterozygotes for GTP cyclohydrolase I gene mutations
Ann. Neurol.
Dopa-responsive dystonia induced by a recessive GTP cyclohydrolase mutation
Hum. Gene Ther.
Neonatal Dopa-responsive extrapyramidal syndrome in twins with recessive GTPCH deficiency
Neurology
Wide expressivity variation and high but no gender-related penetrance in two dopa-responsive dystonia families with a novel GCH-I mutation
Mov. Disord.
Autosomal dominant guanosine triphosphate cyclohydrolase I deficiency (Segawa disease)
Ann. Neurol.
Dopa-responsive dystonia in British patients – new mutations of the GTP cyclohydrolase I gene and evidence for genetic heterogeneity
Hum. Mol. Genet.
Hereditary progressive dystonia with marked diurnal fluctuation caused by mutation in the GTP cyclohydrolase I gene
Nat. Genet.
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