Spastic ataxias

A SACS deletion variant in Great Pyrenees dogs causes autosomal recessive neuronal degeneration

ARSACS (autosomal recessive spastic ataxia of Charlevoix-Saguenay) is a human neurological disorder characterized by progressive cerebellar ataxia and peripheral neuropathy. A recently recognized disorder in Great Pyrenees dogs is similarly characterized by widespread central nervous system degeneration leading to progressive cerebellar ataxia and spasticity, combined with peripheral neuropathy. Onset of clinical signs occurred in puppies as young as 4 months of age, with slow progression over several years. A multi-generation pedigree suggested an autosomal recessive mode of inheritance. Histopathology revealed consistent cerebellar Purkinje cell degeneration, neuronal degeneration in brainstem nuclei, widespread spinal cord white matter degeneration, ganglion cell degeneration, inappropriately thin myelin sheaths or fully demyelinated peripheral nerve fibers, and normal or only mild patterns of denervation atrophy in skeletal muscles. Genome-wide single nucleotide polymorphism (SNP) genotype data was collected from 6 cases and 26 controls, where homozygosity mapping identified a 3.3 Mb region on CFA25 in which all cases were homozygous and all controls were either heterozygous or homozygous for alternate haplotypes. This region tagged the SACS gene where variants are known to cause ARSACS. Sanger sequencing of SACS in affected dogs identified a 4 bp deletion that causes a frame shift and truncates 343 amino acids from the C terminus of the encoded sacsin protein (p.Val4244AlafsTer32). Our clinical and histopathological descriptions of this canine disorder contribute to the description of human ARSACS and represents the first naturally occurring large animal model of this disorder.

The hereditary spastic paraplegias

The hereditary spastic paraplegias (HSPs) are a group of more than 90 genetic disorders in which lower extremity spasticity and weakness are either the primary neurologic impairments ("uncomplicated HSP") or when accompanied by other neurologic deficits ("complicated HSP"), important features of the clinical syndrome. Various genetic types of HSP are inherited such as autosomal dominant, autosomal recessive, X-linked, and maternal (mitochondrial) traits. Symptoms that begin in early childhood may be nonprogressive and resemble spastic diplegic cerebral palsy. Symptoms that begin later, typically progress insidiously over a number of years. Genetic testing is able to confirm the diagnosis for many subjects. Insights from gene discovery indicate that abnormalities in diverse molecular processes underlie various forms of HSP, including disturbance in axon transport, endoplasmic reticulum morphogenesis, vesicle transport, lipid metabolism, and mitochondrial function. Pathologic studies in "uncomplicated" HSP have shown axon degeneration particularly involving the distal ends of corticospinal tracts and dorsal column fibers. Treatment is limited to symptom reduction including amelioration of spasticity, reducing urinary urgency, proactive physical therapy including strengthening, stretching, balance, and agility exercise.

A Diagnostic Approach to Spastic ataxia Syndromes

Spastic ataxia is characterized by the combination of cerebellar ataxia with spasticity and other pyramidal features. It is the hallmark of some hereditary ataxias, but it can also occur in some spastic paraplegias and acquired conditions. It often presents with heterogenous clinical features with other neurologic and non-neurological symptoms, resulting in complex phenotypes. In this review, the differential diagnosis of spastic ataxias are discussed and classified in accordance with inheritance. Establishing an organized classification method based on mode inheritance is fundamental for the approach to patients with these syndromes. For each differential, the clinical features, neuroimaging and genetic aspects are reviewed. A diagnostic approach for spastic ataxias is then proposed.

Molecular Characterization of Portuguese Patients with Hereditary Cerebellar Ataxia

Hereditary cerebellar ataxia (HCA) comprises a clinical and genetic heterogeneous group of neurodegenerative disorders characterized by incoordination of movement, speech, and unsteady gait. In this study, we performed whole-exome sequencing (WES) in 19 families with HCA and presumed autosomal recessive (AR) inheritance, to identify the causal genes. A phenotypic classification was performed, considering the main clinical syndromes: spastic ataxia, ataxia and neuropathy, ataxia and oculomotor apraxia (AOA), ataxia and dystonia, and ataxia with cognitive impairment. The most frequent causal genes were associated with spastic ataxia (SACS and KIF1C) and with ataxia and neuropathy or AOA (PNKP). We also identified three families with autosomal dominant (AD) forms arising from de novo variants in KIF1A, CACNA1A, or ATP1A3, reinforcing the importance of differential diagnosis (AR vs. AD forms) in families with only one affected member. Moreover, 10 novel causal-variants were identified, and the detrimental effect of two splice-site variants confirmed through functional assays. Finally, by reviewing the molecular mechanisms, we speculated that regulation of cytoskeleton function might be impaired in spastic ataxia, whereas DNA repair is clearly associated with AOA. In conclusion, our study provided a genetic diagnosis for HCA families and proposed common molecular pathways underlying cerebellar neurodegeneration.

Whole-exome sequencing confirms implication of VPS13D as a potential cause of progressive spastic ataxia

Background

VPS13D is a large ubiquitin-binding protein playing an essential role in mitophagy by regulating mitochondrial fission. Recently, VPS13D biallelic pathogenic variants have been reported in patients displaying variable neurological phenotypes, with an autosomic recessive inheritance.

The objectives of the study were to determine the genetic etiology of a patient with early onset sporadic progressive spastic ataxia, and to investigate the pathogenicity of VPS13D variants through functional studies on patient’s skin fibroblasts.

Case presentation

We report the case of a 51-year-old patient with spastic ataxia, with an acute onset of the disease at age 7. Walking difficulties slowly worsened over time, with the use of a wheelchair since age 26. We have used trio-based whole-exome sequencing (WES) to identify genes associated with spastic ataxia. The impact of the identified variants on mitochondrial function was assessed in patient’s fibroblasts by imaging mitochondrial network and measuring level of individual OXPHOS complex subunits. Compound heterozygous variants were identified in VPS13D: c.946C > T, p.Arg316* and c.12416C > T, p.(Ala4139Val). Primary fibroblasts obtained from this patient revealed an altered mitochondrial morphology, and a decrease in levels of proteins from complex I, III and IV.

Conclusions

Our findings confirmed implication of VPS13D in spastic ataxia and provided further support for mitochondrial defects in patient’s skin fibroblasts with VPS13D variants. This report of long-term follow up showed a slowly progressive course of the spastic paraplegia with cerebellar features. Furthermore, the performed functional studies could be used as biomarker helping diagnosis of VPS13D-related neurological disorders when molecular results are uneasy to interpret.

A Novel SPG7 Gene Pathogenic Variant in a Cypriot Family With Autosomal Recessive Spastic Ataxia

The SPG7 gene encodes the paraplegin protein, an inner mitochondrial membrane—localized protease. It was initially linked to pure and complicated hereditary spastic paraplegia with cerebellar atrophy, and now represents a frequent cause of undiagnosed cerebellar ataxia and spastic ataxia. We hereby report the molecular characterization and the clinical features of a large Cypriot family with five affected individuals presenting with spastic ataxia in an autosomal recessive transmission mode, due to a novel SPG7 homozygous missense variant. Detailed clinical histories of the patients were obtained, followed by neurological and neurophysiological examinations. Whole exome sequencing (WES) of the proband, in silico gene panel analysis, variant filtering and family segregation analysis of the candidate variants with Sanger sequencing were performed. RNA and protein expression as well as in vitro protein localization studies and mitochondria morphology evaluation were carried out towards functional characterization of the identified variant. The patients presented with typical spastic ataxia features while some intrafamilial phenotypic variation was noted. WES analysis revealed a novel homozygous missense variant in the SPG7 gene (c.1763C > T, p. Thr588Met), characterized as pathogenic by more than 20 in silico prediction tools. Functional studies showed that the variant does not affect neither the RNA or protein expression, nor the protein localization. However, aberrant mitochondrial morphology has been observed thus indicating mitochondrial dysfunction and further demonstrating the pathogenicity of the identified variant. Our study is the first report of an SPG7 pathogenic variant in the Cypriot population and broadens the spectrum of SPG7 pathogenic variants.

Approach to Neuropathic Pain

Neuropathic pain is a common chief complaint encountered by neurologists and primary care providers. It is caused by disorders involving the somatosensory nervous system. The clinical evaluation of neuropathic pain is challenging and requires a multifaceted systematic approach with an emphasis on a thorough history and physical examination to identify characteristic signs and symptoms. Ancillary laboratory investigations, targeted imaging, and electrodiagnostic studies further help identify underlying etiologies to guide specific treatments. Management of neuropathic pain encompasses treating the underlying pathology as well as symptomatic control with nonpharmacological, pharmacological, and interventional therapies. Here, we present an approach to help evaluate patients with neuropathic pain.

Hereditary spastic paraplegia: new insights into clinical variability and spasticity-ataxia phenotype, and novel mutations

INTRODUCTION

Hereditary spastic paraplegias (HSPs), a genetically heterogeneous group of neurodegenerative diseases, have an incidence of around 3 to 9 individuals every 100,000. Due to the broad clinical and genetic variability of HSPs, it is challenging to diagnose the disorder quickly and precisely. Hereditary spastic ataxias (HSAs) and HSPs are overlapping diseases, and their intersection has been gradually identified by next-generation sequencing. The idea of the spasticity-ataxia phenotype (SAP) spectrum is further substantiated by the similarities in phenotypes and underlying genes in ataxias and inherited spastic paraplegias and the related cellular processes and disease mechanisms these disorders exhibit.

METHODS

Whole-exome sequencing was performed on the 25 spastic or spastic-ataxic gait patients.

RESULTS

Twenty-two specific HSPs-HSAs-SAP mutations, including 14 novel mutations, were found in 25 cases from 18 Turkish and 2 Syrian families. This research discovers many novel hereditary spastic paraplegia (HSP) mutations and shows a robust genotype-phenotype heterogeneity in the disease.

CONCLUSIONS

This research helped expand the clinical and molecular scope of HSP and clarified the concept of the spasticity-ataxia phenotype, further enhancing our understanding of the complicated form of HSP and its association with ataxia. Our data broadens the spectrum of HSPs and HSAs related gene mutations and provides insights for genotype-phenotype correlations for HSPs and HSAs.

Analyzing Gene Expression Profiles from Ataxia and Spasticity Phenotypes to Reveal Spastic Ataxia Related Pathways

Spastic ataxia (SA) is a group of rare neurodegenerative diseases, characterized by mixed features of generalized ataxia and spasticity. The pathogenetic mechanisms that drive the development of the majority of these diseases remain unclear, although a number of studies have highlighted the involvement of mitochondrial and lipid metabolism, as well as calcium signaling. Our group has previously published the GBA2 c.1780G > C (p.Asp594His) missense variant as the cause of spastic ataxia in a Cypriot consanguineous family, and more recently the biochemical characterization of this variant in patients’ lymphoblastoid cell lines. GBA2 is a crucial enzyme of sphingolipid metabolism. However, it is unknown if GBA2 has additional functions and therefore additional pathways may be involved in the disease development. The current study introduces bioinformatics approaches to better understand the pathogenetic mechanisms of the disease. We analyzed publicly available human gene expression datasets of diseases presented with ‘ataxia’ or ‘spasticity’ in their clinical phenotype and we performed pathway analysis in order to: (a) search for candidate perturbed pathways of SA; and (b) evaluate the role of sphingolipid signaling pathway and sphingolipid metabolism in the disease development, through the identification of differentially expressed genes in patients compared to controls. Our results demonstrate consistent differential expression of genes that participate in the sphingolipid pathways and highlight alterations in the pathway level that might be associated with the disease phenotype. Through enrichment analysis, we discuss additional pathways that are connected to sphingolipid pathways, such as PI3K-Akt signaling, MAPK signaling, calcium signaling, and lipid and carbohydrate metabolism as the most enriched for ataxia and spasticity phenotypes.

Molecular and Cellular Substrates for the Friedreich Ataxia. Significance of Contactin Expression and of Antioxidant Administration

In this study, the neural phenotype is explored in rodent models of the spinocerebellar disorder known as the Friedreich Ataxia (FA), which results from mutations within the gene encoding the Frataxin mitochondrial protein. For this, the M12 line, bearing a targeted mutation, which disrupts the Frataxin gene exon 4 was used, together with the M02 line, which, in addition, is hemizygous for the human Frataxin gene mutation (Pook transgene), implying the occurrence of 82-190 GAA repeats within its first intron. The mutant mice phenotype was compared to the one of wild type littermates in regions undergoing differential profiles of neurogenesis, including the cerebellar cortex and the spinal cord by using neuronal (β-tubulin) and glial (Glial Fibrillary Acidic Protein) markers as well as the Contactin 1 axonal glycoprotein, involved in neurite growth control. Morphological/morphometric analyses revealed that while in Frataxin mutant mice the neuronal phenotype was significantly counteracted, a glial upregulation occurred at the same time. Furthermore, Contactin 1 downregulation suggested that changes in the underlying gene contributed to the disorder pathogenesis. Therefore, the FA phenotype implies an alteration of the developmental profile of neuronal and glial precursors. Finally, epigallocatechin gallate polyphenol administration counteracted the disorder, indicating protective effects of antioxidant administration.

Novel MAG Variant Causes Cerebellar Ataxia with Oculomotor Apraxia: Molecular Basis and Expanded Clinical Phenotype

Homozygous variants in MAG, encoding myelin-associated glycoprotein (MAG), have been associated with complicated forms of hereditary spastic paraplegia (HSP). MAG is a glycoprotein member of the immunoglobulin superfamily, expressed by myelination cells. In this study, we identified a novel homozygous missense variant in MAG (c.124T>C; p.Cys42Arg) in a Portuguese family with early-onset autosomal recessive cerebellar ataxia with neuropathy and oculomotor apraxia. We used homozygosity mapping and exome sequencing to identify the MAG variant, and cellular studies to confirm its detrimental effect. Our results showed that this variant reduces protein stability and impairs the post-translational processing (N-linked glycosylation) and subcellular localization of MAG, thereby associating a loss of protein function with the phenotype. Therefore, MAG variants should be considered in the diagnosis of hereditary cerebellar ataxia with oculomotor apraxia, in addition to spastic paraplegia.

Hereditary spastic paraplegia: from diagnosis to emerging therapeutic approaches

Hereditary spastic paraplegia (HSP) describes a heterogeneous group of genetic neurodegenerative diseases characterised by progressive spasticity of the lower limbs. The pathogenic mechanism, associated clinical features, and imaging abnormalities vary substantially according to the affected gene and differentiating HSP from other genetic diseases associated with spasticity can be challenging. Next generation sequencing-based gene panels are now widely available but have limitations and a molecular diagnosis is not made in most suspected cases. Symptomatic management continues to evolve but with a greater understanding of the pathophysiological basis of individual HSP subtypes there are emerging opportunities to provide targeted molecular therapies and personalised medicine.