Dissecting the Genetic Basis of Parkinson Disease, Dystonia and Chorea

In this thesis I used of a range of genetic methodologies and strategies to unravel the genetic bases of Parkinson disease (PD), myoclonus-dystonia (M-D), and chorea. First, I detail the work I performed in PD, including (1) the screening of GBA in a cohort of early-onset PD cases, which led to the identification of the allele E326K (p.Glu365Lys) as the single most frequent, clinically relevant, risk variant for PD; (2) a detailed genetic analysis in a large cohort of PD cases who underwent deep-brain stimulation treatment and a longitudinal comparison of the phenotypic features of carriers of mutations in different genes; (3) the observation that rare GCH1 coding variants, known to be responsible for the childhood-onset disorder DOPA-responsive dystonia, are a novel risk factor for PD. Then, I describe the work I performed to identify novel causes of M-D, including (1) the discovery of the missense p.Arg145His mutation in KCTD17 as a novel cause of autosomal dominant M-D; (2) the identification of tyrosine hydroxylase deficiency as a novel treatable cause of recessive M-D; and (3) the conclusive disproof of the pathogenic role of the p.Arg1389His variant in CACNA1B as a cause of M-D. Finally, I detail my work in the field of choreic syndromes, including (1) the genetic screening of NKX2-1 in the Queen Square cohort of benign hereditary chorea (BHC) cases; (2) the identification of ADCY5 mutations, the gene thought to be responsible for the condition familial dyskinesias with facial myokymia, as an important cause of BHC; and (3) the identification of de novo mutations in PDE10A as a novel genetic cause of chorea. These findings are discussed in light of the recent literature. Following my analysis, I suggest future directions for the identification of novel genetic causes of movement disorders, in light of my recent findings and ongoing research

The Emerging Role of Phosphodiesterases in Movement Disorders

Cyclic nucleotide phosphodiesterase (PDE) enzymes catalyze the hydrolysis and inactivation of the cyclic nucleotides cyclic adenosine monophosphate and cyclic guanosine monophosphate, which act as intracellular second messengers for many signal transduction pathways in the central nervous system. Several classes of PDE enzymes with specific tissue distributions and cyclic nucleotide selectivity are highly expressed in brain regions involved in cognitive and motor functions, which are known to be implicated in neurodegenerative diseases, such as Parkinson's disease and Huntington's disease. The indication that PDEs are intimately involved in the pathophysiology of different movement disorders further stems from recent discoveries that mutations in genes encoding different PDEs, including PDE2A, PDE8B, and PDE10A, are responsible for rare forms of monogenic parkinsonism and chorea. We here aim to provide a translational overview of the preclinical and clinical data on PDEs, the role of which is emerging in the field of movement disorders, offering a novel venue for a better understanding of their pathophysiology. Modulating cyclic nucleotide signaling, by either acting on their synthesis or on their degradation, represents a promising area for development of novel therapeutic approaches. The study of PDE mutations linked to monogenic movement disorders offers the opportunity of better understanding the role of PDEs in disease pathogenesis, a necessary step to successfully benefit the treatment of both hyperkinetic and hypokinetic movement disorders. © 2021 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society

Web-based assessment of Parkinson's prodromal markers identifies GBA variants.

Identifying those “at risk” of Parkinson’s disease (PD) is a priority and a key strategy for testing neuroprotective agents. PREDICT-PD is a pilot cohort study in which healthy UK adults are screened online (www.predictpd.com) with a questionnaire that ascertains the presence of early phenotypic features and risk factors, based on systematic review

Mitofusin-mediated ER stress triggers neurodegeneration in pink1/parkin models of Parkinson's disease.

Mutations in PINK1 and PARKIN cause early-onset Parkinson's disease (PD), thought to be due to mitochondrial toxicity. Here, we show that in Drosophila pink1 and parkin mutants, defective mitochondria also give rise to endoplasmic reticulum (ER) stress signalling, specifically to the activation of the protein kinase R-like endoplasmic reticulum kinase (PERK) branch of the unfolded protein response (UPR). We show that enhanced ER stress signalling in pink1 and parkin mutants is mediated by mitofusin bridges, which occur between defective mitochondria and the ER. Reducing mitofusin contacts with the ER is neuroprotective, through suppression of PERK signalling, while mitochondrial dysfunction remains unchanged. Further, both genetic inhibition of dPerk-dependent ER stress signalling and pharmacological inhibition using the PERK inhibitor GSK2606414 were neuroprotective in both pink1 and parkin mutants. We conclude that activation of ER stress by defective mitochondria is neurotoxic in pink1 and parkin flies and that the reduction of this signalling is neuroprotective, independently of defective mitochondria. A video abstract for this article is available online in the supplementary information

Partial loss-of-function of sodium channel SCN8A in familial isolated myoclonus.

Variants in the neuronal sodium channel gene SCN8A have been implicated in several neurological disorders. Early infantile epileptic encephalopathy type 13 results from de novo gain-of-function mutations that alter the biophysical properties of the channel. Complete loss-of-function variants of SCN8A have been identified in cases of isolated intellectual disability. We now report a novel heterozygous SCN8A variant, p.Pro1719Arg, in a small pedigree with five family members affected with autosomal dominant upper limb isolated myoclonus without seizures or cognitive impairment. Functional analysis of the p.Pro1719Arg variant in transfected neuron-derived cells demonstrated greatly reduced Nav 1.6 channel activity without altered gating properties. Hypomorphic alleles of Scn8a in the mouse are known to result in similar movement disorders. This study expands the phenotypic and functional spectrum of SCN8A variants to include inherited nonepileptic isolated myoclonus. SCN8A can be considered as a candidate gene for isolated movement disorders without seizures

Mutations in the autoregulatory domain of β-tubulin 4a cause hereditary dystonia.

Dystonia type 4 (DYT4) was first described in a large family from Heacham in Norfolk with an autosomal dominantly inherited whispering dysphonia, generalized dystonia, and a characteristic hobby horse ataxic gait. We carried out a genetic linkage analysis in the extended DYT4 family that spanned 7 generations from England and Australia, revealing a single LOD score peak of 6.33 on chromosome 19p13.12-13. Exome sequencing in 2 cousins identified a single cosegregating mutation (p.R2G) in the β-tubulin 4a (TUBB4a) gene that was absent in a large number of controls. The mutation is highly conserved in the β-tubulin autoregulatory MREI (methionine-arginine-glutamic acid-isoleucine) domain, highly expressed in the central nervous system, and extensive in vitro work has previously demonstrated that substitutions at residue 2, specifically R2G, disrupt the autoregulatory capability of the wild-type β-tubulin peptide, affirming the role of the cytoskeleton in dystonia pathogenesis

GBA-Associated Parkinson's Disease: Progression in a Deep Brain Stimulation Cohort

BACKGROUND: Recent evidence suggests that glucosidase beta acid (GBA) mutations predispose Parkinson's disease (PD) patients to a greater burden of cognitive impairment and non-motor symptoms. This emerging knowledge has not yet been considered in patients who have undergone deep brain stimulation (DBS); a surgery that is generally contraindicated in those with cognitive deficits. OBJECTIVE: To explore the long-term phenotypic progression of GBA-associated PD, in a DBS cohort. METHODS: Thirty-four PD patients who had undergone DBS surgery between 2002 and 2011 were included in this study; 17 patients with GBA mutations were matched to 17 non-carriers. Clinical evaluation involved the administration of four assessments: The Mattis Dementia Rating Scale was used to assess cognitive function; non-motor symptoms were assessed using the Non-Motor Symptom Assessment Scale for PD; quality of life was measured using the Parkinson's Disease Questionnaire; and motor symptoms were evaluated using part III of the Movement Disorders Society Unified Parkinson's Disease Rating Scale, in on-medication/on-stimulation conditions. Levodopa equivalent doses (LED) and DBS settings were compared with clinical outcomes. RESULTS: At a mean follow-up of 7.5 years after DBS, cognitive impairment was more prevalent (70% vs 19%) and more severe in GBA mutation carriers compared to non-carriers (60% vs 6% were severely impaired). Non-motor symptoms were also more severe and quality of life more impaired in GBA-associated PD. Motor symptoms, LED, and stimulation settings were not significantly different between groups at follow-up. CONCLUSIONS: GBA status appears to be an important predictor for non-motor symptom disease progression, after deep brain stimulation surgery

De Novo Mutations in PDE10A Cause Childhood-Onset Chorea with Bilateral Striatal Lesions.

Chorea is a hyperkinetic movement disorder resulting from dysfunction of striatal medium spiny neurons (MSNs), which form the main output projections from the basal ganglia. Here, we used whole-exome sequencing to unravel the underlying genetic cause in three unrelated individuals with a very similar and unique clinical presentation of childhood-onset chorea and characteristic brain MRI showing symmetrical bilateral striatal lesions. All individuals were identified to carry a de novo heterozygous mutation in PDE10A (c.898T>C [p.Phe300Leu] in two individuals and c.1000T>C [p.Phe334Leu] in one individual), encoding a phosphodiesterase highly and selectively present in MSNs. PDE10A contributes to the regulation of the intracellular levels of cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP). Both substitutions affect highly conserved amino acids located in the regulatory GAF-B domain, which, by binding to cAMP, stimulates the activity of the PDE10A catalytic domain. In silico modeling showed that the altered residues are located deep in the binding pocket, where they are likely to alter cAMP binding properties. In vitro functional studies showed that neither substitution affects the basal PDE10A activity, but they severely disrupt the stimulatory effect mediated by cAMP binding to the GAF-B domain. The identification of PDE10A mutations as a cause of chorea further motivates the study of cAMP signaling in MSNs and highlights the crucial role of striatal cAMP signaling in the regulation of basal ganglia circuitry. Pharmacological modulation of this pathway could offer promising etiologically targeted treatments for chorea and other hyperkinetic movement disorders