Review article
Molecular and metabolic bases of tetrahydrobiopterin (BH4) deficiencies

https://doi.org/10.1016/j.ymgme.2021.04.003Get rights and content

Abstract

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.

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.

Section snippets

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.

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