Spinocerebellar ataxia with mental retardation (SCA13)

Infant and adult SCA13 mutations differentially affect Purkinje cell excitability, maturation, and viability in vivo

Mutations in KCNC3, which encodes the Kv3.3 K⁺ channel, cause spinocerebellar ataxia 13 (SCA13). SCA13 exists in distinct forms with onset in infancy or adulthood. Using zebrafish, we tested the hypothesis that infant- and adult-onset mutations differentially affect the excitability and viability of Purkinje cells in vivo during cerebellar development. An infant-onset mutation dramatically and transiently increased Purkinje cell excitability, stunted process extension, impaired dendritic branching and synaptogenesis, and caused rapid cell death during cerebellar development. Reducing excitability increased early Purkinje cell survival. In contrast, an adult-onset mutation did not significantly alter basal tonic firing in Purkinje cells, but reduced excitability during evoked high frequency spiking. Purkinje cells expressing the adult-onset mutation matured normally and did not degenerate during cerebellar development. Our results suggest that differential changes in the excitability of cerebellar neurons contribute to the distinct ages of onset and timing of cerebellar degeneration in infant- and adult-onset SCA13.

Myoclonus epilepsy and ataxia due to KCNC1 mutation: Analysis of 20 cases and K+ channel properties

OBJECTIVE

To comprehensively describe the new syndrome of myoclonus epilepsy and ataxia due to potassium channel mutation (MEAK), including cellular electrophysiological characterization of observed clinical improvement with fever.

METHODS

We analyzed clinical, electroclinical, and neuroimaging data for 20 patients with MEAK due to recurrent KCNC1 p.R320H mutation. In vitro electrophysiological studies were conducted using whole cell patch-clamp to explore biophysical properties of wild-type and mutant K_V 3.1 channels.

RESULTS

Symptoms began at between 3 and 15 years of age (median = 9.5), with progressively severe myoclonus and rare tonic-clonic seizures. Ataxia was present early, but quickly became overshadowed by myoclonus; 10 patients were wheelchair-bound by their late teenage years. Mild cognitive decline occurred in half. Early death was not observed. Electroencephalogram (EEG) showed generalized spike and polyspike wave discharges, with documented photosensitivity in most. Polygraphic EEG-electromyographic studies demonstrated a cortical origin for myoclonus and striking coactivation of agonist and antagonist muscles. Magnetic resonance imaging revealed symmetrical cerebellar atrophy, which appeared progressive, and a prominent corpus callosum. Unexpectedly, transient clinical improvement with fever was noted in 6 patients. To explore this, we performed high-temperature in vitro recordings. At elevated temperatures, there was a robust leftward shift in activation of wild-type K_V 3.1, increasing channel availability.

INTERPRETATION

MEAK has a relatively homogeneous presentation, resembling Unverricht-Lundborg disease, despite the genetic and biological basis being quite different. A remarkable improvement with fever may be explained by the temperature-dependent leftward shift in activation of wild-type K_V 3.1 subunit-containing channels, which would counter the loss of function observed for mutant channels, highlighting KCNC1 as a potential target for precision therapeutics. Ann Neurol 2017;81:677-689.

The autosomal dominant spinocerebellar ataxias: emerging mechanistic themes suggest pervasive Purkinje cell vulnerability

The spinocerebellar ataxias are a genetically heterogeneous group of disorders with clinically overlapping phenotypes arising from Purkinje cell degeneration, cerebellar atrophy and varying degrees of degeneration of other grey matter regions. For 22 of the 32 subtypes, a genetic cause has been identified. While recurring themes are emerging, there is no clear correlation between the clinical phenotype or penetrance, the type of genetic defect or the category of the disease mechanism, or the neuronal types involved beyond Purkinje cells. These phenomena suggest that cerebellar Purkinje cells may be a uniquely vulnerable neuronal cell type, more susceptible to a wider variety of genetic/cellular insults than most other neuron types.

Ataxias with Autosomal, X-Chromosomal or Maternal Inheritance

Heredoataxias are a group of genetic disorders with a cerebellar syndrome as the leading clinical manifestation. The current classification distinguishes heredoataxias according to the trait of inheritance into autosomal dominant, autosomal recessive, X-linked, and maternally inherited heredoataxias. The autosomal dominant heredoataxias are separated into spinocerebellar ataxias (SCA1-8, 10-15, 17-23, 25-30, and dentato-rubro-pallido-luysian atrophy), episodic ataxias (EA1-7), and autosomal dominant mitochondrial heredoataxias (Leigh syndrome, MIRAS, ADOAD, and AD-CPEO). The autosomal recessive ataxias are separated into Friedreich ataxia, ataxia due to vitamin E deficiency, ataxia due to Abeta-lipoproteinemia, Refsum disease, late-onset Tay-Sachs disease, cerebrotendineous xanthomatosis, spinocerebellar ataxia with axonal neuropathy, ataxia telangiectasia, ataxia telangiectasia-like disorder, ataxia with oculomotor apraxia 1 and 2, spastic ataxia of Charlevoix-Saguenay, Cayman ataxia, Marinesco-Sjögren syndrome, and autosomal recessive mitochondrial ataxias (AR-CPEO, SANDO, SCAE, AHS, IOSCA, MEMSA, LBSL CoQ-deficiency, PDC-deficiency). Only two of the heredoataxias, fragile X/tremor/ataxia syndrome, and XLSA/A are transmitted via an X-linked trait. Maternally inherited heredoataxias are due to point mutations in genes encoding for tRNAs, rRNAs, respiratory chain subunits or single large scale deletions/duplications of the mitochondrial DNA and include MELAS, MERRF, KSS, PS, MILS, NARP, and non-syndromic mitochondrial disorders. Treatment of heredoataxias is symptomatic and supportive and may have a beneficial effect in single patients.

**Please see page 424 for abbreviation list.

Consensus paper: the cerebellum's role in movement and cognition

While the cerebellum's role in motor function is well recognized, the nature of its concurrent role in cognitive function remains considerably less clear. The current consensus paper gathers diverse views on a variety of important roles played by the cerebellum across a range of cognitive and emotional functions. This paper considers the cerebellum in relation to neurocognitive development, language function, working memory, executive function, and the development of cerebellar internal control models and reflects upon some of the ways in which better understanding the cerebellum's status as a "supervised learning machine" can enrich our ability to understand human function and adaptation. As all contributors agree that the cerebellum plays a role in cognition, there is also an agreement that this conclusion remains highly inferential. Many conclusions about the role of the cerebellum in cognition originate from applying known information about cerebellar contributions to the coordination and quality of movement. These inferences are based on the uniformity of the cerebellum's compositional infrastructure and its apparent modular organization. There is considerable support for this view, based upon observations of patients with pathology within the cerebellum.

EFNS/ENS Consensus on the diagnosis and management of chronic ataxias in adulthood

BACKGROUND AND OBJECTIVES

The ataxias are a challenging group of neurological diseases due the aetiological heterogeneity and the complexity of the genetic subtypes. This guideline focuses on the heredodegenerative ataxias. The aim is to provide a peer-reviewed evidence-based guideline for clinical neurologists and other specialist physicians responsible for the care of patients with ataxia.

METHODS

This guideline is based on systematic evaluations of the relevant literature and on three consensus meetings of the task force.

DIAGNOSIS

If acquired causes are ruled out, and if the disease course is rather slowly progressive, a (heredo)degenerative disease is likely. A positive family history gives much guidance. In the case of a dominant family history, first line genetic screening is recommended for spinocerebellar ataxia (SCA) 1, 2, 3, 6, 7 and 17 (level B), and in Asian patients also for dentatorubral-pallidoluysian atrophy (DRPLA). In the case of recessive disease, a stepwise diagnostic work-up is recommended, including both biochemical markers and targeted genetic testing, particularly aimed at Friedreich's ataxia, ataxia telangiectasia, ataxia due to vitamin E deficiency, polymerase gamma gene (POLG gene, various mutations), autosomal recessive spastic ataxia of Charlevoix-Saguenay (ARSACS) and ataxia with oculomotor apraxia (AOA) types 1 and 2. If family history is negative, we still advise to screen for the more common dominant and recessive ataxias. In addition, if onset is below 45 years we recommend the full work-up for recessive ataxias; if onset is above 45 years we recommend to screen for fragile X mental retardation 1 FMR1 premutations (good practice points). In sporadic cases with an onset after 30 years, a diagnosis of multiple system atrophy should be considered (good practice point). In particular the genetic work-up will change over the upcoming years due to the diagnostic utility of new techniques such as gene panel diagnostics based on next generation sequencing for routine work-up, or even whole exome and genome sequencing for selected cases.

TREATMENT

Some of the rare recessive ataxias are treatable, but for most of the heredodegenerative ataxias treatment is purely symptomatic. Idebenone is not effective in Friedreich's ataxia (level A). Riluzole (level B) and amantadine (level C) might provide symptomatic relief, irrespective of exact etiology. Also, varenicline for SCA3 patients (level B) can be considered. There is level Class II evidence to recommend physiotherapy, and Class III data to support occupational therapy.

A conserved eEF2 coding variant in SCA26 leads to loss of translational fidelity and increased susceptibility to proteostatic insult

The autosomal dominant spinocerebellar ataxias (SCAs) are a genetically heterogeneous group of disorders exhibiting cerebellar atrophy and Purkinje cell degeneration whose subtypes arise from 31 distinct genetic loci. Our group previously published the locus for SCA26 on chromosome 19p13.3. In this study, we performed targeted deep sequencing of the critical interval in order to identify candidate causative variants in individuals from the SCA26 family. We identified a single variant that co-segregates with the disease phenotype that produces a single amino acid substitution in eukaryotic elongation factor 2. This substitution, P596H, sits in a domain critical for maintaining reading frame during translation. The yeast equivalent, P580H EF2, demonstrated impaired translocation, detected as an increased rate of -1 programmed ribosomal frameshift read-through in a dual-luciferase assay for observing translational recoding. This substitution also results in a greater susceptibility to proteostatic disruption, as evidenced by a more robust activation of a reporter gene driven by unfolded protein response activation upon challenge with dithiothreitol or heat shock in our yeast model system. Our results present a compelling candidate mutation and mechanism for the pathogenesis of SCA26 and further support the role of proteostatic disruption in neurodegenerative diseases.

Spinocerebellar ataxia 13 and 25

Spinocerebellar ataxia (SCA) types 13 and 25 are two genetic entities among the autosomal dominant cerebellar ataxias, initially mapped in two French families to chromosomes 19q and 2p, respectively. The SCA13 locus was confirmed by the identification of a second kindred of Filipino ancestry. SCA13 patients have cerebellar ataxia of adult onset, or of early onset when associated with mental impairment. SCA25 patients present with cerebellar ataxia with sensory neuropathy and frequent gastrointestinal features. While the gene responsible for SCA25 is still unknown, missense mutations affecting the potassium channel KCNC3 function have been identified.

Magnetic resonance imaging in spinocerebellar ataxias

Magnetic resonance (MR) imaging is widely used to visualize atrophic processes that occur during the pathogenesis of spinocerebellar ataxias (SCAs). T1-weighted images are utilized to rate the atrophy of cerebellar vermis, cerebellar hemispheres, pons and midbrain. Signal changes in the basal ganglia and ponto-cerebellar fibers are evaluated by T2-weighted and proton density-weighted images. However, two-dimensional (2D) images do not allow a reliable quantification of the degree of atrophy. The latter is now possible through the application of three-dimensional (3D) true volumetric methods, which should be used for research purposes. Ideally, these methods should allow automated segmentation of contrast-defined boundaries by using region growing algorithms, which can be applied successfully in structures of the posterior fossa and basal ganglia. Thin slice thickness helps to minimize partial volume effects. Whereas volumetric approaches rely on predetermined anatomical boundaries, voxel-based morphometry has been developed to determine group differences between different types of SCA (cross-sectional studies) or within one SCA entity (longitudinal studies). We will review recent results and how these methods are currently used to (i) separate sporadic and dominantly inherited forms of cerebellar ataxias; (ii) identify specific SCA genotypes; (iii) correlate patho-anatomical changes with SCA disease symptoms or severity; and (iv) visualize and estimate the rate of progression in SCA.