A novel tyrosine hydroxylase variant in a group of Chinese patients with dopa-responsive dystonia

A Compound Heterozygote for GCH1 Mutation Represents a Case of Atypical Dopa-Responsive Dystonia

Dopa-responsive dystonia (DRD), a movement disorder, is characterized by young onset dystonia and dramatic response to levodopa treatment. However, the wide range of phenotypic spectrum of the disease often leads to misdiagnosis. DRD is usually caused by mutation in GCH1 gene coding for GTP cyclohydrolase 1 (GTPCH1) enzyme, which is involved in biosynthesis of tetrahydrobiopterin (BH4) and dopamine. In this study, the entire GCH1 gene was screened in 14 Indian DRD patients and their family members. A family was identified where the proband was found to be a compound heterozygote for GCH1 (p.R184H and p.V204I) variants; the former variant being inherited from the father and the latter from the mother. All other family members harboring one of these GCH1 variants were asymptomatic except for one (heterozygous for p.R184H) who was diagnosed with DRD. In silico analyses predicted these two variants to be pathogenic and disruptive to GCH1enzymatic activity. This proband was misdiagnosed as cerebral palsy and remained untreated for 25 years. He developed retrograde movements and gait problems in lower limbs, deformity in upper limbs, and difficulty in swallowing, and became mute. However, most of his symptoms were alleviated upon levodopa administration. Our study confirms the variability of DRD phenotype and the reduced penetrance of GCH1 mutations. It also emphasizes the need of molecular diagnostic test and L-dopa trial especially for those with atypical DRD phenotype.

The quaternary structure of human tyrosine hydroxylase: effects of dystonia-associated missense variants on oligomeric state and enzyme activity

Abstract

Tyrosine hydroxylase (TH) is a multi‐domain, homo‐oligomeric enzyme that catalyses the rate‐limiting step of catecholamine neurotransmitter biosynthesis. Missense variants of human TH are associated with a recessive neurometabolic disease with low levels of brain dopamine and noradrenaline, resulting in a variable clinical picture, from progressive brain encephalopathy to adolescent onset DOPA‐responsive dystonia (DRD). We expressed isoform 1 of human TH (hTH1) and its dystonia‐associated missense variants in E. coli, analysed their quaternary structure and thermal stability using size‐exclusion chromatography, circular dichroism, multi‐angle light scattering, transmission electron microscopy, small‐angle X‐ray scattering and assayed hydroxylase activity. Wild‐type (WT) hTH1 was a mixture of enzymatically stable tetramers (85.6%) and octamers (14.4%), with little interconversion between these species. We also observed small amounts of higher order assemblies of long chains of enzyme by transmission electron microscopy. To investigate the role of molecular assemblies in the pathogenesis of DRD, we compared the structure of WT hTH1 with the DRD‐associated variants R410P and D467G that are found in vicinity of the predicted subunit interfaces. In contrast to WT hTH1, R410P and D467G were mixtures of tetrameric and dimeric species. Inspection of the available structures revealed that Arg‐410 and Asp‐467 are important for maintaining the stability and oligomeric structure of TH. Disruption of the normal quaternary enzyme structure by missense variants is a new molecular mechanism that may explain the loss of TH enzymatic activity in DRD. Unstable missense variants could be targets for pharmacological intervention in DRD, aimed to re‐establish the normal oligomeric state of TH.

Study of GCH1 and TH genes in Chinese patients with Parkinson's disease

Whole-exome sequencing of Parkinson's disease (PD) patients has revealed that the frequency of GTP-cyclohydrolase I (GCH1) variants was significantly higher in patients than in controls. GCH1 rs11158026 was also found to increase the risk of PD. To investigate genetic contribution of dopa-responsive dystonia-related genes to PD, GCH1, and tyrosine hydroxylase (TH) were tested in PD patients. A total of 859 study subjects comprising 421 patients with PD and 438 controls were recruited. For GCH1 gene, one known variant (c.239G > A, p.S80N) was detected in a patient who was diagnosed with PD clinically. In TH, 3 heterozygous variants, c.1495G > A (p. V499M, rs1800033), c.334 A > G (p.V112M, rs6356), and c.813 G > A (p. K271K, rs6357), were identified. After stratification by age, the frequency of rs6356G allele was significantly lower (p = 0.041) for the late-onset PD group than controls. Our results indicate that to analyze the relationship between dopa-responsive dystonia-related genes and PD, it is important to screen GCH1 and test rs6356 of TH in a larger sample.