Objective: To unravel the molecular mechanisms by which loss of GCH1 function enhances the risk of Parkinson´s disease (PD).

Background: Loss-of-function mutations in GCH1 are the most common cause of autosomal dominant DOPA-responsive dystonia (DRD), a non-neurodegenerative movement disorder. GCH1 encodes GTP cyclohydrolase 1, the rate-limiting enzyme in the biosynthesis of tetrahydrobiopterin (BH4). BH4 is an essential cofactor for tyrosine hydroxylase, the rate-limiting enzyme in the synthesis of dopamine. Recent genetic evidence indicates that GCH1 mutations also increase the risk of PD. The mechanisms by which GCH1 mutations predispose to nigrostriatal cell death, are unknown.

Methods: We cultured skin fibroblasts from 2 patients (Patient 1, Patient 2) with the same heterozygous missense mutation in GCH1 (p.Y75S; c. 224A>C) and from 2 healthy age-matched controls (Control 1, Control 2). Patient 1 was a 42-year-old male who developed clinically typical DRD at the age of 8 years and had severely abnormal dopamine transporter (DAT) imaging at the age of 37 years. Patient 2, the 63-year-old mother of Patient 1, developed clinically typical DRD at the age of 8 years and had normal DAT imaging at the age of 60. We induced GCH1 expression in the fibroblasts by 24-hour treatment with interferon-gamma. We assessed apoptosis using TUNEL staining, western blotting for PARP and immunostaining for cleaved caspase 3.

Results: GCH1 abundance after IFN-gamma treatment was lower in the mutant fibroblasts than in their respective controls. Moreover, GCH1 abundance was lower in Patient 1 than in Patient 2. Fibroblasts from Patient 1 were more susceptible to staurosporine- and H2O2-induced apoptosis than those of Control 1. By contrast, staurosporine- and H2O2-induced apoptosis did not differ between Patient 2 and Control 2. The survival defect of fibroblasts of Patient 1 was rescued by incubation with sepiapterin, a precursor of BH4 via the GCH1-independent salvage pathway, indicating that the survival defect was caused by BH4 deficiency.

Conclusions: The clinical phenotypes of the 2 GCH1 mutant patients correlated with the susceptibility of their skin fibroblasts to apoptosis in vitro. As skin fibroblasts do not produce dopamine, our findings suggest that GCH1 mutations can impair cellular survival via mechanisms unrelated to dopamine synthesis defects.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/molecular-mechanisms-of-gch1-associated-parkinsons-disease/