Investigating the Role of GTP Cyclohydrolase I Mutations and the Tetrahydrobiopterin Pathway in Parkinson’s Disease

My thesis investigates the role of GTP cyclohydrolase I (GCH1) mutations and the downstream tetrahydrobiopterin (BH4) pathway in Parkinson’s disease (PD) using a wide range of methods and human-derived cell models. First, the burden of mutations in GCH1 and other genes associated to the GCH1-BH4 pathway is analysed in a large PD exome cohort from the IPDGC. This analysis highlights a cumulative role for variants in this pathway with the risk for PD, especially in the genes directly involved in BH4 synthesis as part of the pathway. Second, functional investigations into the putative functions of the GCH1-BH4 pathway are undertaken in several cell models from a family of patients carrying a heterozygous GCH1 mutation and affected by either PD or Dopa-responsive dystonia (DRD), the latter was first associated with GCH1 mutations. Primary investigations make use of patient-derived fibroblasts, with and without GTPCH-inducting cytokine treatment, and observe an effect of the mutation on mitochondrial function and antioxidant levels without changes in superoxide production. Further, fibroblasts are reprogrammed to induced pluripotent stem cells (iPSC) and these are differentiated into midbrain dopaminergic neural precursors and neurons. Comparisons are made between controls, PD and DRD cells. iPSC-derived neural precursors demonstrate high purity and their functional analysis results are in line with the observations made in fibroblasts. These changes are mainly observed in the DRD lines suggestive of an early phenotype for that disease in my model. Finally, in iPSC-derived midbrain dopaminergic neurons, mitochondrial function and superoxide production are affected differently between disorders with no effect on antioxidants in the cells. The mutation is associated with a higher proportion of cell death in the PD cultures compared to both controls and DRD, suggestive of a recapitulation of the neurodegenerative phenotype in this model

The CACNA1B R1389H variant is not associated with myoclonus-dystonia in a large European multicentric cohort.

Myoclonus-dystonia (M-D) is a very rare movement disorder, caused in ∼30-50% of cases by mutations in SGCE. The CACNA1B variant c.4166G>A; (p.R1389H) was recently reported as the likely causative mutation in a single 3-generation Dutch pedigree with five subjects affected by a unique dominant M-D syndrome and cardiac arrhythmias. In an attempt to replicate this finding, we assessed by direct sequencing the frequency of CACNA1B c.4166G>A; (p.R1389H) in a cohort of 520 M-D cases, in which SGCE mutations had been previously excluded. A total of 146 cases (28%) had a positive family history of M-D. The frequency of the variant was also assessed in 489 neurologically healthy controls and in publicly available data sets of genetic variation (1000 Genomes, Exome Variant Server and Exome Aggregation Consortium). The variant was detected in a single sporadic case with M-D, but in none of the 146 probands with familial M-D. Overall, the variant was present at comparable frequencies in M-D cases (1 out of 520; 0.19%) and healthy controls (1 out of 489; 0.2%). A similar frequency of the variant was also reported in all publicly available databases. These results do not support a causal association between the CACNA1B c.4166G>A; (p.R1389H) variant and M-D