Hereditary myokymia and periodic ataxia

Novel Genetic Variants Expand the Functional, Molecular, and Pathological Diversity of KCNA1 Channelopathy

The KCNA1 gene encodes Kv1.1 voltage-gated potassium channel α subunits, which are crucial for maintaining healthy neuronal firing and preventing hyperexcitability. Mutations in the KCNA1 gene can cause several neurological diseases and symptoms, such as episodic ataxia type 1 (EA1) and epilepsy, which may occur alone or in combination, making it challenging to establish simple genotype-phenotype correlations. Previous analyses of human KCNA1 variants have shown that epilepsy-linked mutations tend to cluster in regions critical for the channel's pore, whereas EA1-associated mutations are evenly distributed across the length of the protein. In this review, we examine 17 recently discovered pathogenic or likely pathogenic KCNA1 variants to gain new insights into the molecular genetic basis of KCNA1 channelopathy. We provide the first systematic breakdown of disease rates for KCNA1 variants in different protein domains, uncovering potential location biases that influence genotype-phenotype correlations. Our examination of the new mutations strengthens the proposed link between the pore region and epilepsy and reveals new connections between epilepsy-related variants, genetic modifiers, and respiratory dysfunction. Additionally, the new variants include the first two gain-of-function mutations ever discovered for KCNA1, the first frameshift mutation, and the first mutations located in the cytoplasmic N-terminal domain, broadening the functional and molecular scope of KCNA1 channelopathy. Moreover, the recently identified variants highlight emerging links between KCNA1 and musculoskeletal abnormalities and nystagmus, conditions not typically associated with KCNA1. These findings improve our understanding of KCNA1 channelopathy and promise to enhance personalized diagnosis and treatment for individuals with KCNA1-linked disorders.

Paroxysmal movement disorders: Paroxysmal dyskinesia and episodic ataxia

Paroxysmal movement disorders have traditionally been classified into paroxysmal dyskinesia (PxD), which consists in attacks of involuntary movements (mainly dystonia and/or chorea) without loss of consciousness, and episodic ataxia (EA), which features spells of cerebellar dysfunction with or without interictal neurological manifestations. In this chapter, PxD will be discussed first according to the trigger-based classification, thus reviewing clinical, genetic, and molecular features of paroxysmal kinesigenic dyskinesia, paroxysmal nonkinesigenic dyskinesia, and paroxysmal exercise-induced dyskinesia. EA will be presented thereafter according to their designated gene or genetic locus. Clinicogenetic similarities among paroxysmal movement disorders have progressively emerged, which are herein highlighted along with growing evidence that their pathomechanisms overlap those of epilepsy and migraine. Advances in our comprehension of the biological pathways underlying paroxysmal movement disorders, which involve ion channels as well as proteins associated with the vesical synaptic cycle or implicated in neuronal energy metabolism, may represent the cornerstone for defining a shared pathophysiologic framework and developing target-specific therapies.

Fasciculations in Children

Fasciculations are the most common form of spontaneous muscle contraction. They frequently occur in healthy individuals. However, there are a minority of situations that fasciculations are observed in association with specific neurologic disorders. Publications concerning the evaluation of pediatric patients experiencing fasciculations are limited. These children may undergo invasive or expensive diagnostic investigations that are unnecessary. Moreover, without careful consideration of differential diagnoses, rare neuromuscular disorders that present with fasciculations in the pediatric age group can be under-recognized by pediatric neurologists. This review examines the most important pediatric disorders presenting with fasciculations and other spontaneous muscle contractions to guide pediatric neurologists in evaluating these children.

Therapeutic Potential of Sodium Channel Blockers as a Targeted Therapy Approach in KCNA1-Associated Episodic Ataxia and a Comprehensive Review of the Literature

Introduction: Among genetic paroxysmal movement disorders, variants in ion channel coding genes constitute a major subgroup. Loss-of-function (LOF) variants in KCNA1, the gene coding for KV1.1 channels, are associated with episodic ataxia type 1 (EA1), characterized by seconds to minutes-lasting attacks including gait incoordination, limb ataxia, truncal instability, dysarthria, nystagmus, tremor, and occasionally seizures, but also persistent neuromuscular symptoms like myokymia or neuromyotonia. Standard treatment has not yet been developed, and different treatment efforts need to be systematically evaluated. Objective and Methods: Personalized therapeutic regimens tailored to disease-causing pathophysiological mechanisms may offer the specificity required to overcome limitations in therapy. Toward this aim, we (i) reviewed all available clinical reports on treatment response and functional consequences of KCNA1 variants causing EA1, (ii) examined the potential effects on neuronal excitability of all variants using a single compartment conductance-based model and set out to assess the potential of two sodium channel blockers (SCBs: carbamazepine and riluzole) to restore the identified underlying pathophysiological effects of KV1.1 channels, and (iii) provide a comprehensive review of the literature considering all types of episodic ataxia. Results: Reviewing the treatment efforts of EA1 patients revealed moderate response to acetazolamide and exhibited the strength of SCBs, especially carbamazepine, in the treatment of EA1 patients. Biophysical dysfunction of KV1.1 channels is typically based on depolarizing shifts of steady-state activation, leading to an LOF of KCNA1 variant channels. Our model predicts a lowered rheobase and an increase of the firing rate on a neuronal level. The estimated concentration dependent effects of carbamazepine and riluzole could partially restore the altered gating properties of dysfunctional variant channels. Conclusion: These data strengthen the potential of SCBs to contribute to functional compensation of dysfunctional KV1.1 channels. We propose riluzole as a new drug repurposing candidate and highlight the role of personalized approaches to develop standard care for EA1 patients. These results could have implications for clinical practice in future and highlight the need for the development of individualized and targeted therapies for episodic ataxia and genetic paroxysmal disorders in general.

Paroxysmal Genetic Movement Disorders and Epilepsy

Paroxysmal movement disorders include paroxysmal kinesigenic dyskinesia, paroxysmal non-kinesigenic dyskinesia, paroxysmal exercise-induced dyskinesia, and episodic ataxias. In recent years, there has been renewed interest and recognition of these disorders and their intersection with epilepsy, at the molecular and pathophysiological levels. In this review, we discuss how these distinct phenotypes were constructed from a historical perspective and discuss how they are currently coalescing into established genetic etiologies with extensive pleiotropy, emphasizing clinical phenotyping important for diagnosis and for interpreting results from genetic testing. We discuss insights on the pathophysiology of select disorders and describe shared mechanisms that overlap treatment principles in some of these disorders. In the near future, it is likely that a growing number of genes will be described associating movement disorders and epilepsy, in parallel with improved understanding of disease mechanisms leading to more effective treatments.

Two novel KCNA1 variants identified in two unrelated Chinese families affected by episodic ataxia type 1 and neurodevelopmental disorders

Background

Pathogenic KCNA1 variants have been linked to episodic ataxia type 1 (EA1), a rare neurological syndrome characterized by continuous myokymia and attacks of generalized ataxia that can be triggered by fever, abrupt movements, emotional stress, and fatigue. Currently, over 40 KCNA1 variants have been identified in individuals with EA1.

Methods

A male patient displayed partial seizures in addition to EA1 symptoms, often triggered by fever. A sibling presented with typical EA1 symptoms, seizures, and learning difficulties. In addition, the older brother displayed cognitive impairment, developmental delay, and slurred speech, which were absent in his younger sister. Whole‐exome sequencing was performed for the patients.

Results

A novel de novo missense variant in KCNA1 (p.Ala261Thr) was identified in the male patient, which is located in a base of the 3rd transmembrane domain (S3). The other novel KCNA1 variant (p.Gly376Ser) was identified in the sibling and was inherited from an unaffected father with low‐level mosaicism. The variant was located in the S5–S6 extracellular linker of the voltage sensor domain of the Kv channel. Next, we systematically reviewed the available clinical phenotypes of individuals with EA1 and observed that individuals with KCNA1 variants at the C‐terminus were more likely to suffer from seizures and neurodevelopmental disorders than those with variants at the N‐terminus.

Conclusion

Our study expands the mutation spectrum of KCNA1 and improves our understanding of the genotype–phenotype correlations of KCNA1. Definitive genetic diagnosis is beneficial for the genetic counseling and clinical management of individuals with EA1.

Clinical and Genetic Overview of Paroxysmal Movement Disorders and Episodic Ataxias

Paroxysmal movement disorders (PMDs) are rare neurological diseases typically manifesting with intermittent attacks of abnormal involuntary movements. Two main categories of PMDs are recognized based on the phenomenology: Paroxysmal dyskinesias (PxDs) are characterized by transient episodes hyperkinetic movement disorders, while attacks of cerebellar dysfunction are the hallmark of episodic ataxias (EAs). From an etiological point of view, both primary (genetic) and secondary (acquired) causes of PMDs are known. Recognition and diagnosis of PMDs is based on personal and familial medical history, physical examination, detailed reconstruction of ictal phenomenology, neuroimaging, and genetic analysis. Neurophysiological or laboratory tests are reserved for selected cases. Genetic knowledge of PMDs has been largely incremented by the advent of next generation sequencing (NGS) methodologies. The wide number of genes involved in the pathogenesis of PMDs reflects a high complexity of molecular bases of neurotransmission in cerebellar and basal ganglia circuits. In consideration of the broad genetic and phenotypic heterogeneity, a NGS approach by targeted panel for movement disorders, clinical or whole exome sequencing should be preferred, whenever possible, to a single gene approach, in order to increase diagnostic rate. This review is focused on clinical and genetic features of PMDs with the aim to (1) help clinicians to recognize, diagnose and treat patients with PMDs as well as to (2) provide an overview of genes and molecular mechanisms underlying these intriguing neurogenetic disorders.

Kv1.1 Channelopathies: Pathophysiological Mechanisms and Therapeutic Approaches

Kv1.1 belongs to the Shaker subfamily of voltage-gated potassium channels and acts as a critical regulator of neuronal excitability in the central and peripheral nervous systems. KCNA1 is the only gene that has been associated with episodic ataxia type 1 (EA1), an autosomal dominant disorder characterized by ataxia and myokymia and for which different and variable phenotypes have now been reported. The iterative characterization of channel defects at the molecular, network, and organismal levels contributed to elucidating the functional consequences of KCNA1 mutations and to demonstrate that ataxic attacks and neuromyotonia result from cerebellum and motor nerve alterations. Dysfunctions of the Kv1.1 channel have been also associated with epilepsy and kcna1 knock-out mouse is considered a model of sudden unexpected death in epilepsy. The tissue-specific association of Kv1.1 with other Kv1 members, auxiliary and interacting subunits amplifies Kv1.1 physiological roles and expands the pathogenesis of Kv1.1-associated diseases. In line with the current knowledge, Kv1.1 has been proposed as a novel and promising target for the treatment of brain disorders characterized by hyperexcitability, in the attempt to overcome limited response and side effects of available therapies. This review recounts past and current studies clarifying the roles of Kv1.1 in and beyond the nervous system and its contribution to EA1 and seizure susceptibility as well as its wide pharmacological potential.

Clinical Spectrum of KCNA1 Mutations: New Insights into Episodic Ataxia and Epilepsy Comorbidity

Mutations in the KCNA1 gene, which encodes voltage-gated Kv1.1 potassium channel α-subunits, cause a variety of human diseases, complicating simple genotype–phenotype correlations in patients. KCNA1 mutations are primarily associated with a rare neurological movement disorder known as episodic ataxia type 1 (EA1). However, some patients have EA1 in combination with epilepsy, whereas others have epilepsy alone. KCNA1 mutations can also cause hypomagnesemia and paroxysmal dyskinesia in rare cases. Why KCNA1 variants are associated with such phenotypic heterogeneity in patients is not yet understood. In this review, literature databases (PubMed) and public genetic archives (dbSNP and ClinVar) were mined for known pathogenic or likely pathogenic mutations in KCNA1 to examine whether patterns exist between mutation type and disease manifestation. Analyses of the 47 deleterious KCNA1 mutations that were identified revealed that epilepsy or seizure-related variants tend to cluster in the S1/S2 transmembrane domains and in the pore region of Kv1.1, whereas EA1-associated variants occur along the whole length of the protein. In addition, insights from animal models of KCNA1 channelopathy were considered, as well as the possible influence of genetic modifiers on disease expressivity and severity. Elucidation of the complex relationship between KCNA1 variants and disease will enable better diagnostic risk assessment and more personalized therapeutic strategies for KCNA1 channelopathy.

Paroxysmal movement disorders - practical update on diagnosis and management

Introduction: Paroxysmal dyskinesias and episodic ataxias are often caused by mutations in genes related to cell membrane and synaptic function. Despite the exponential increase in publications of genetically confirmed cases, management remains largely clinical based on non-systematic evidence. Areas covered: The authors provide a historical and clinical review of the main types of paroxysmal dyskinesias and episodic ataxias, with recommendations for diagnosis and management of patients suffering from these conditions. Expert opinion: After secondary paroxysmal dyskinesias, the most common paroxysmal movement disorders are likely to be PRRT2-associated paroxysmal kinesigenic dyskinesias, which respond well to small doses of carbamazepine, and episodic ataxia type 2, which often responds to acetazolamide. Familial paroxysmal non-kinesigenic dyskinesias are largely caused by mutations in PNKD and have poor response to therapy but improve with age. Exercise-induced dyskinesias are genetically heterogeneous, caused by disorders of glucose transport, mitochondrial function, dopaminergic pathways or neurodegenerative conditions amongst others. GNAO1 and ADCY5 mutations can also cause paroxysmal movement disorders, often in the context of ongoing motor symptoms. Although a therapeutic trial is justified for classic cases and in limited resource settings, genetic testing may help direct initial or rescue therapy. Deep brain stimulation may be an option for severe cases.

Identification of a New de Novo Mutation Underlying Regressive Episodic Ataxia Type I

Episodic ataxia type 1 (EA1), a Shaker-like K⁺channelopathy, is a consequence of genetic anomalies in the KCNA1 gene that lead to dysfunctions in the voltage-gated K⁺ channel Kv1. 1. Generally, KCNA1 mutations are inherited in an autosomal dominant manner. Here we report the clinical phenotype of an EA1 patient characterized by ataxia attacks that decrease in frequency with age, and eventually leading to therapy discontinuation. A new de novo mutation (c.932G>A) that changed a highly conserved glycine residue into an aspartate (p.G311D) was identified by using targeted next-generation sequencing. The conserved glycine is located in the S4–S5 linker, a crucial domain controlling Kv1.1 channel gating. In silico analyses predicted the mutation deleterious. Heterologous expression of the mutant (Kv1.1-G311D) channels resulted in remarkably decreased amplitudes of measured current, confirming the identified variant is pathogenic. Collectively, these findings corroborate the notion that EA1 also results from de novo variants and point out that regardless of the mutation-induced deleterious loss of Kv1.1 channel function the ataxia phenotype may improve spontaneously.

Paroxysmal motor disorders: expanding phenotypes lead to coalescing genotypes

Paroxysmal movement disorders encompass varied motor phenomena. Less recognized features and wide phenotypic and genotypic heterogeneity are impediments to straightforward molecular diagnosis. We describe a family with episodic ataxia type 1, initially mis‐characterized as paroxysmal dystonia to illustrate this diagnostic challenge. We summarize clinical features in affected individuals to highlight underappreciated aspects and provide comprehensive phenotypic description of the rare familial KCNA1 mutation. Delayed diagnosis in this family is emblematic of the broader challenge of diagnosing other paroxysmal motor disorders. We summarize genotypic and phenotypic overlap and provide a suggested diagnostic algorithm for approaching patients with these conditions.

Episodic ataxias

Primary episodic ataxias (EAs) are a group of dominantly inherited disorders characterized by transient recurrent incoordination and truncal instability, often triggered by physical exertion and emotional stress, variably associated with progressive baseline ataxia. There are now eight designated subtypes based largely on genetic loci. Mutations have been identified in multiple individuals and families with EA1, EA2, and EA6, mostly with onset before adulthood. EA1 and EA2 are prototypical neurologic channelopathies. EA1 is caused by heterozygous mutations in KCNA1, which encodes the α1 subunit of a neuronal voltage-gated potassium channel, Kv1.1. EA2, the most common and best characterized, is caused by heterozygous mutations in CACNA1A, which encodes the α1A subunit of a neuronal voltage-gated calcium channel, Cav2.1. EA6 is caused by heterozygous mutations in SLC1A3, which encodes a subunit of a glial excitatory amino acid transporter, EAAT1. The other EA subtypes were defined in single families awaiting gene identification and further confirmation. This chapter focuses on the best-characterized EA syndromes, the clinical assessment and genetic diagnosis of EA, and the management of EA, as well as newly recognized allelic disorders that have greatly expanded the clinical spectrum of EA2. Illustrative cases are discussed, with a focus on sporadic patients with congenital features without episodic ataxia who present diagnostic and therapeutic challenges.

Episodic ataxias

The familial episodic ataxias (EAs) are prototypical channelopathies in the central nervous system clinically characterized by attacks of imbalance and incoordination variably associated with progressive ataxia and variable interictal features. EA1, EA2, and EA6 are caused by mutations in ion channel- and transporter-encoding genes that regulate neuronal excitability and neurotransmission.

Advances in Sequencing Technologies for Understanding Hereditary Ataxias: A Review

Importance

The hereditary progressive ataxias comprise genetic disorders that affect the cerebellum and its connections. Even though these diseases historically have been among the first familial disorders of the nervous system to have been recognized, progress in the field has been challenging because of the large number of ataxic genetic syndromes, many of which overlap in their clinical features.

Observations

We have taken a historical approach to demonstrate how our knowledge of the genetic basis of ataxic disorders has come about by novel techniques in gene sequencing and bioinformatics. Furthermore, we show that the genes implicated in ataxia, although seemingly unrelated, appear to encode for proteins that interact with each other in connected functional modules.

Conclusions and Relevance

It has taken approximately 150 years for neurologists to comprehensively unravel the genetic diversity of ataxias. There has been an explosion in our understanding of their molecular basis with the arrival of next-generation sequencing and computer-driven bioinformatics; this in turn has made hereditary ataxias an especially well-developed model group of diseases for gaining insights at a systems level into genes and cellular pathways that result in neurodegeneration.

Dominant-negative mutation p.Arg324Thr in KCNA1 impairs Kv1.1 channel function in episodic ataxia

BACKGROUND

Episodic ataxia type 1 is a rare autosomal dominant neurological disorder caused by mutations in the KCNA1 gene that encodes the α subunit of voltage-gated potassium channel Kv1.1. The functional consequences of identified mutations on channel function do not fully correlate with the clinical phenotype of patients.

METHODS

A clinical and genetic study was performed in a family with 5 patients with episodic ataxia type 1, with concurrent epilepsy in 1 of them. Protein expression, modeling, and electrophysiological analyses were performed to study Kv1.1 function.

RESULTS

Whole-genome linkage and candidate gene analyses revealed the novel heterozygous mutation p.Arg324Thr in the KCNA1 gene. The encoded mutant Kv1.1 channel displays reduced currents and altered activation and inactivation.

CONCLUSIONS

Taken together, we provide genetic and functional evidence that mutation p.Arg324Thr in the KCNA1 gene is pathogenic and results in episodic ataxia type 1 through a dominant-negative effect. © 2016 International Parkinson and Movement Disorder Society.

Potassium Channels in Epilepsy

This review attempts to give a concise and up-to-date overview on the role of potassium channels in epilepsies. Their role can be defined from a genetic perspective, focusing on variants and de novo mutations identified in genetic studies or animal models with targeted, specific mutations in genes coding for a member of the large potassium channel family. In these genetic studies, a demonstrated functional link to hyperexcitability often remains elusive. However, their role can also be defined from a functional perspective, based on dynamic, aggravating, or adaptive transcriptional and posttranslational alterations. In these cases, it often remains elusive whether the alteration is causal or merely incidental. With ∼80 potassium channel types, of which ∼10% are known to be associated with epilepsies (in humans) or a seizure phenotype (in animals), if genetically mutated, a comprehensive review is a challenging endeavor. This goal may seem all the more ambitious once the data on posttranslational alterations, found both in human tissue from epilepsy patients and in chronic or acute animal models, are included. We therefore summarize the literature, and expand only on key findings, particularly regarding functional alterations found in patient brain tissue and chronic animal models.

New insights into the pathogenesis and therapeutics of episodic ataxia type 1

Episodic ataxia type 1 (EA1) is a K⁺channelopathy characterized by a broad spectrum of symptoms. Generally, patients may experience constant myokymia and dramatic episodes of spastic contractions of the skeletal muscles of the head, arms, and legs with loss of both motor coordination and balance. During attacks additional symptoms may be reported such as vertigo, blurred vision, diplopia, nausea, headache, diaphoresis, clumsiness, stiffening of the body, dysarthric speech, and difficulty in breathing. These episodes may be precipitated by anxiety, emotional stress, fatigue, startle response or sudden postural changes. Epilepsy is overrepresented in EA1. The disease is inherited in an autosomal dominant manner, and genetic analysis of several families has led to the discovery of a number of point mutations in the voltage-dependent K⁺ channel gene KCNA1 (Kv1.1), on chromosome 12p13. To date KCNA1 is the only gene known to be associated with EA1. Functional studies have shown that these mutations impair Kv1.1 channel function with variable effects on channel assembly, trafficking and biophysics. Despite the solid evidence obtained on the molecular mechanisms underlying EA1, how these cause dysfunctions within the central and peripheral nervous systems circuitries remains elusive. This review summarizes the main breakthrough findings in EA1, discusses the neurophysiological mechanisms underlying the disease, current therapies, future challenges and opens a window onto the role of Kv1.1 channels in central nervous system (CNS) and peripheral nervous system (PNS) functions.

Molecular pathophysiology and pharmacology of the voltage-sensing module of neuronal ion channels

Voltage-gated ion channels (VGICs) are membrane proteins that switch from a closed to open state in response to changes in membrane potential, thus enabling ion fluxes across the cell membranes. The mechanism that regulate the structural rearrangements occurring in VGICs in response to changes in membrane potential still remains one of the most challenging topic of modern biophysics. Na⁺, Ca²⁺ and K⁺ voltage-gated channels are structurally formed by the assembly of four similar domains, each comprising six transmembrane segments. Each domain can be divided into two main regions: the Pore Module (PM) and the Voltage-Sensing Module (VSM). The PM (helices S₅ and S₆ and intervening linker) is responsible for gate opening and ion selectivity; by contrast, the VSM, comprising the first four transmembrane helices (S₁–S₄), undergoes the first conformational changes in response to membrane voltage variations. In particular, the S₄ segment of each domain, which contains several positively charged residues interspersed with hydrophobic amino acids, is located within the membrane electric field and plays an essential role in voltage sensing. In neurons, specific gating properties of each channel subtype underlie a variety of biological events, ranging from the generation and propagation of electrical impulses, to the secretion of neurotransmitters and to the regulation of gene expression. Given the important functional role played by the VSM in neuronal VGICs, it is not surprising that various VSM mutations affecting the gating process of these channels are responsible for human diseases, and that compounds acting on the VSM have emerged as important investigational tools with great therapeutic potential. In the present review we will briefly describe the most recent discoveries concerning how the VSM exerts its function, how genetically inherited diseases caused by mutations occurring in the VSM affects gating in VGICs, and how several classes of drugs and toxins selectively target the VSM.

Whole-exome sequencing as a diagnostic tool in a family with episodic ataxia type 1

Complex neurologic phenotypes are inherently difficult to diagnose. Whole-exome sequencing (WES) is a new tool in the neurologist's diagnostic armamentarium. Whole-exome sequencing can be applied to investigate the "diagnostic odyssey" cases. These cases involve patients with rare diseases that likely have a genetic etiology but have failed to be diagnosed by clinical evaluation and targeted gene testing. We describe such a case, a 22-year-old man who had mild intellectual developmental disability and episodes of jerking ataxic movements that affected his whole body. He underwent numerous multidisciplinary and multicentric evaluations throughout his life that failed to establish a clear diagnosis. Following his visit to Mayo Clinic in Jacksonville, Florida, WES was applied for genetic determination of the unknown disorder in the proband and his biological parents and sister. Additional clinical evaluation, magnetic resonance neuroimaging, electromyography, and electroencephalography of the proband were performed to verify the phenotype after the WES results were available. To our knowledge, this is the first report of the application of WES to facilitate the diagnosis of episodic ataxia type 1. This case illustrates that WES supported by clinical data is a useful and time-saving tool in the evaluation of patients with rare and complex hereditary disorders.

Novel phenotype associated with a mutation in the KCNA1(Kv1.1) gene

Episodic ataxia type 1 (EA1) is an autosomal dominant K(+) channelopathy which manifests with short attacks of cerebellar ataxia and dysarthria, and may also show interictal myokymia. Episodes can be triggered by emotional or physical stress, startle response, sudden postural change or fever. Here we describe a 31-year-old man displaying markedly atypical symptoms, including long-lasting attacks of jerking muscle contractions associated with hyperthermia, severe migraine, and a relatively short-sleep phenotype. A single nucleotide change in KCNA1 (c.555C>G) was identified that changes a highly conserved residue (p.C185W) in the first transmembrane segment of the voltage-gated K(+) channel Kv1.1. The patient is heterozygous and the mutation was inherited from his asymptomatic mother. Next generation sequencing revealed no variations in the CACNA1A, CACNB4, KCNC3, KCNJ10, PRRT2 or SCN8A genes of either the patient or mother, except for a benign variant in SLC1A3. Functional analysis of the p.C185W mutation in KCNA1 demonstrated a deleterious dominant-negative phenotype where the remaining current displayed slower activation kinetics, subtle changes in voltage-dependence and faster recovery from slow inactivation. Structural modeling also predicts the C185W mutation to be functionally deleterious. This description of novel clinical features, associated with a Kv1.1 mutation highlights a possibly unrecognized relationship between K(+) channel dysfunction, hyperthermia and migraine in EA1, and suggests that thorough assessments for these symptoms should be carefully considered for all patients affected by EA1.

The Clinical Spectrum of Autosomal-Dominant Episodic Ataxias

Autosomal-dominant episodic ataxias (EAs) represent a clinically and genetically heterogeneous group of disorders characterized by recurrent episodes of cerebellar ataxia (CA). Ataxia episodes are usually of short duration and often triggered by specific stimuli. There are currently seven classified subtypes of EA. EA types 1 and 2 have the highest prevalence and are therefore the clinically most relevant. Between attacks, EA 1 is associated with myokymia. In EA 2, often an interictal downbeat nystagmus with other cerebellar ocular dysfunctions is present; patients with EA 2 may display slowly progessive ataxia and vermian atrophy. EA 1 and 2 are both channelopathies, affecting the potassium channel gene, KCNA1, in EA 1 and the PQ calcium channel-encoding gene, CACNA1A, in EA 2. The types EA 3 to 7 are very rare and have to be further elucidated. Here, we review the historical, clinical, and genetic aspects of autosomal-dominant EAs and their current treatment, focusing on EA 1 and 2.

Episodic ataxia type 1: clinical characterization, quality of life and genotype-phenotype correlation

Episodic ataxia type 1 is considered a rare neuronal ion channel disorder characterized by brief attacks of unsteadiness and dizziness with persistent myokymia. To characterize the natural history, develop outcome measures for future clinical trials, and correlate genotype with phenotype, we undertook an international, prospective, cross-sectional study. Thirty-nine individuals (51% male) were enrolled: median age 37 years (range 15-65 years). We identified 10 different pathogenic point mutations in KCNA1 that accounted for the genetic basis of 85% of the cohort. Participants with KCNA1 mutations were more likely to have a positive family history. Analysis of the total cohort showed that the first episode of ataxia occurred before age 20 in all but one patient, with an average age of onset of 7.9 years. Physical exertion, emotional stress and environmental temperature were the most common triggers for attacks. Attack frequency ranged from daily to monthly, even with the same KCNA1 genotype. Average attack duration was in the order of minutes. Ten participants (26%) developed permanent cerebellar signs, which were related to disease duration. The average Scale for the Assessment and Rating of Ataxia score (SARA, a standardized measure of cerebellar dysfunction on clinical examination, scores range from 0-40) was an average of 3.15 for all participants (range 0-14), but was only 2 in those with isolated episodic ataxia compared with 7.7 in those with progressive cerebellar ataxia in addition to episodic ataxia. Thirty-seven participants completed the SF-36, a quality of life survey; all eight domain norm-based average scores (mean=50) were below normal with mental health being the lowest (41.3) in those with mutation positive episodic ataxia type 1. Scores on SF-36 correlated negatively with attack frequency. Of the 39 participants in the study, 33 harboured mutations in KCNA1 whereas the remaining six had no mutation identified. Episodic ataxia type 1 phenocopies have not been described previously and we report their clinical features, which appear to be different to those with a KCNA1 mutation. This large prospective study of both genetically confirmed episodic ataxia type 1 and episodic ataxia type 1 phenocopies provides detailed baseline characteristics of these disorders and their impact on participants. We found that attacks had a significant effect on quality of life. Unlike previous studies, we found that a significant number of individuals with genetically confirmed episodic ataxia type 1 (21%) had accumulated persistent cerebellar symptoms and signs. These data will enable the development of outcome measures for clinical trials of treatment.

Molecular biology of channelopathies: impact on diagnosis and treatment

Channelopathies are genetically determined ion channel alterations that lead to acute and transient symptoms in subjects who otherwise appear to be normal. This article reviews the recent progression of biomolecular studies that have clarified the mechanisms by which gene mutations may result in alterations of excitable tissues responsible for episodic neurological, neuromuscular and cardiac disorders, defined as channelopathies. The development of technologies capable of testing pharmacological agents in vitro on mutated channels expressed in cell lines makes it possible to define a more rational use of the available drugs acting on ion channels, and to design new molecules specifically targeted to known channel dysfunctions and new ones that could be identified by future genetic studies.

Episodic ataxia type 1 without episodic ataxia: the diagnostic utility of nerve excitability studies in individuals with KCNA1 mutations

Episodic ataxia type 1 (EA1) is caused by mutations in the KCNA1 gene encoding the fast potassium channel Kv1.1 and is characterized clinically by brief episodes of ataxia and continuous and spontaneous motor unit activity. Atypical presentations, in which the predominant manifestation is related to the peripheral nervous system, may lead to the diagnosis being missed or delayed, with the potential risk of individuals receiving inappropriate or unnecessary investigations and treatment. We present a case of a 15-year-old female with EA1 who had never had episodes of ataxia, and whose hand movements were initially thought to represent a tremor. Genetic screening for KCNA1 mutations was precipitated by the results of the nerve excitability studies (TROND protocol), which showed changes typical of reduced fast potassium channel conductance. This case highlights the utility of nerve excitability studies in identifying individuals with KCNA1 mutations.

Clinical, genetic, neurophysiological and functional study of new mutations in episodic ataxia type 1

BACKGROUND AND OBJECTIVE

Heterozygous mutations in KCNA1 cause episodic ataxia type 1 (EA1), an ion channel disorder characterised by brief paroxysms of cerebellar dysfunction and persistent neuromyotonia. This paper describes four previously unreported families with EA1, with the aim of understanding the phenotypic spectrum associated with different mutations.

METHODS

15 affected individuals from four families underwent clinical, genetic and neurophysiological evaluation. The functional impact of new mutations identified in the KCNA1 gene was investigated with in vitro electrophysiology and immunocytochemistry.

RESULTS

Detailed clinical documentation, dating back to 1928 in one family, indicates that all patients manifested episodic ataxia of varying severity. Four subjects from three families reported hearing impairment, which has not previously been reported in association with EA1. New mutations (R167M, C185W and I407M) were identified in three out of the four families. When expressed in human embryonic kidney cells, all three new mutations resulted in a loss of K(v)1.1 channel function. The fourth family harboured a previously reported A242P mutation, which has not been previously described in association with ataxia.

CONCLUSIONS

The genetic basis of EA1 in four families is established and this report presents the earliest documented case from 1928. All three new mutations caused a loss of K(v)1.1 channel function. The finding of deafness in four individuals raises the possibility of a link between K(v)1.1 dysfunction and hearing impairment. Our findings broaden the phenotypic range associated with mutations in KCNA1.

K(+) channelepsy: progress in the neurobiology of potassium channels and epilepsy

K(+) channels are important determinants of seizure susceptibility. These membrane proteins, encoded by more than 70 genes, make the largest group of ion channels that fine-tune the electrical activity of neuronal and non-neuronal cells in the brain. Their ubiquity and extremely high genetic and functional diversity, unmatched by any other ion channel type, place K(+) channels as primary targets of genetic variations or perturbations in K(+)-dependent homeostasis, even in the absence of a primary channel defect. It is therefore not surprising that numerous inherited or acquired K(+) channels dysfunctions have been associated with several neurologic syndromes, including epilepsy, which often generate confusion in the classification of the associated diseases. Therefore, we propose to name the K(+) channels defects underlying distinct epilepsies as "K(+) channelepsies," and introduce a new nomenclature (e.g., Kx.y-channelepsy), following the widely used K(+) channel classification, which could be also adopted to easily identify other channelopathies involving Na(+) (e.g., Nav x.y-phenotype), Ca(2+) (e.g., Cav x.y-phenotype), and Cl(-) channels. Furthermore, we discuss novel genetic defects in K(+) channels and associated proteins that underlie distinct epileptic phenotypes in humans, and analyze critically the recent progress in the neurobiology of this disease that has also been provided by investigations on valuable animal models of epilepsy. The abundant and varied lines of evidence discussed here strongly foster assessments for variations in genes encoding for K(+) channels and associated proteins in patients with idiopathic epilepsy, provide new avenues for future investigations, and highlight these proteins as critical pharmacological targets.

Kv1.1 knock-in ataxic mice exhibit spontaneous myokymic activity exacerbated by fatigue, ischemia and low temperature

Episodic ataxia type 1 (EA1) is an autosomal dominant neurological disorder characterized by myokymia and attacks of ataxic gait often precipitated by stress. Several genetic mutations have been identified in the Shaker-like K⁺ channel Kv1.1 (KCNA1) of EA1 individuals, including V408A, which result in remarkable channel dysfunction. By inserting the heterozygous V408A, mutation in one Kv1.1 allele, a mouse model of EA1 has been generated (Kv1.1^V408A/+). Here, we investigated the neuromuscular transmission of Kv1.1^V408A/+ ataxic mice and their susceptibility to physiologically relevant stressors. By using in vivo preparations of lateral gastrocnemius (LG) nerve–muscle from Kv1.1^+/+ and Kv1.1^V408A/+ mice, we show that the mutant animals exhibit spontaneous myokymic discharges consisting of repeated singlets, duplets or multiplets, despite motor nerve axotomy. Two-photon laser scanning microscopy from the motor nerve, ex vivo, revealed spontaneous Ca^2 + signals that occurred abnormally only in preparations dissected from Kv1.1^V408A/+ mice. Spontaneous bursting activity, as well as that evoked by sciatic nerve stimulation, was exacerbated by muscle fatigue, ischemia and low temperatures. These stressors also increased the amplitude of compound muscle action potential. Such abnormal neuromuscular transmission did not alter fiber type composition, neuromuscular junction and vascularization of LG muscle, analyzed by light and electron microscopy. Taken together these findings provide direct evidence that identifies the motor nerve as an important generator of myokymic activity, that dysfunction of Kv1.1 channels alters Ca^2 + homeostasis in motor axons, and also strongly suggest that muscle fatigue contributes more than PNS fatigue to exacerbate the myokymia/neuromyotonia phenotype. More broadly, this study points out that juxtaparanodal K⁺ channels composed of Kv1.1 subunits exert an important role in dampening the excitability of motor nerve axons during fatigue or ischemic insult.

Mechanism of accelerated current decay caused by an episodic ataxia type-1-associated mutant in a potassium channel pore

In Kv1.1, single point mutants found below the channel activation gate at residue V408 are associated with human episodic ataxia type-1, and impair channel function by accelerating decay of outward current during periods of membrane depolarization and channel opening. This decay is usually attributed to C-type inactivation, but here we provide evidence that this is not the case. Using voltage-clamp fluorimetry in Xenopus oocytes, and single-channel patch clamp in mouse ltk- cells, of the homologous Shaker channel (with the equivalent mutation V478A), we have determined that the mutation may cause current decay through a local effect at the activation gate, by destabilizing channel opening. We demonstrate that the effect of the mutant is similar to that of trapped 4-aminopyridine in antagonizing channel opening, as the mutation and 10 mm 4-AP had similar, nonadditive effects on fluorescence recorded from the voltage-sensitive S4 helix. We propose a model where the Kv1.1 activation gate fails to enter a stabilized open conformation, from which the channel would normally C-type inactivate. Instead, the lower pore lining helix is able to enter an activated-not-open conformation during depolarization. These results provide an understanding of the molecular etiology underlying episodic ataxia type-1 due to V408A, as well as biophysical insights into the links between the potassium channel activation gate, the voltage sensor and the selectivity filter.

Episodic ataxias 1 and 2

The episodic ataxias are autosomal dominant disorders usually beginning in the first two decades of life. Episodic ataxia type 1 (EA1) is characterized by brief episodes of ataxia, typically lasting seconds, and interictal myokymia, while episodic ataxia type 2 (EA2) is manifested by longer episodes of ataxia (hours) with interictal nystagmus. The EA1 gene (KCNA1) codes for the six transmembrane segments (S1 to S6) of the Kv1.1 potassium channel subunit and the EA2 gene (CACNA1A) encodes for the Ca(v)2.1 subunit of the P/Q calcium channel complex. EA1 mutations are always missense while most EA2 mutations disrupt the reading frame. Studies of the biophysical properties of the mutant Kv1.1 and Ca(v)2.1 channels in Xenopus oocytes and mammalian cell lines demonstrate clear physiologic consequences of the genetic mutations although no consistent pattern for genotype-phenotype correlation has emerged. Genetic testing for EA1 and EA2 is available, but since no single mutation is prominent for either KCNA1 or CACNA1A, all of the coding regions of the genes need to be screened for mutations. Acetazolamide can be dramatic in controlling episodes of ataxia with EA2 but is typically less beneficial with EA1.

[Hereditary episodic ataxia]

INTRODUCTION

Episodic ataxia (EA) designates a group of autosomal dominant channelopathies that manifest as paroxysmal attacks of imbalance and incoordination. EA conditions are clinically and genetically heterogeneous. Seven types of EA have been reported so far but the majority of clinical cases result from two recognized entities.

STATE OF ART

Episodic ataxia type 1 (EA1) is characterized by brief episodes of ataxia and dysarthria, and interictal myokymia. Onset occurs during the first two decades of life. Associated epilepsy has been reported in some EA1 patients. EA1 is caused by mutations of the KCNA1 gene coding for the voltage-gated potassium channel Kv1.1. Mutation is mostly missense mutations. Acetazolamide, a carbonic-anhydrase inhibitor, may reduce the frequency and severity of the attacks in some but not all affected individuals. Episodic ataxia type 2 (EA2) is characterized by episodes lasting longer than in EA1, that manifest by ataxia, dysarthria, vertigo, and also, in most of the cases, an interictal nystagmus. Other clinical features as developmental delay or epilepsy can be present in some patients. Brain MRI shows frequently a vermian atrophy. Onset occurs typically in childhood or early adolescence, but can sometimes be in adulthood. EA2 is caused by mutations in CACNA1A, a gene coding for the neuronal voltage-gated calcium channel Cav1.1. For two-thirds of the cases, mutations lead to a stop codon. This type is most often responsive to acetazolamide that reduces the frequency and severity of attacks, but does not appear to prevent the progression of interictal symptoms.

PERSPECTIVES

This article summarizes current knowledge on episodic ataxia type 1 and 2 and describes briefly the other types of EA.

CONCLUSION

Molecular analysis of KCNA1 or CACNA1A provides a confirmation of the diagnosis of EA1 and EA2. Other types remain rare phenotypic variants. Among them, only two genes have been identified: CACNB4 in EA5 and SLC1A3 in EA6 and mutations have been found in a very few cases. No mutation can be detected in some familial cases of episodic ataxia, suggesting further heterogeneity.

Episodic ataxia type 1 mutations affect fast inactivation of K+ channels by a reduction in either subunit surface expression or affinity for inactivation domain

Episodic ataxia type 1 (EA1) is an autosomal dominant disorder characterized by continuous myokymia and episodic attacks of ataxia. Mutations in the gene KCNA1 that encodes the voltage-gated potassium channel Kv1.1 are responsible for EA1. In several brain areas, Kv1.1 coassembles with Kv1.4, which confers N-type inactivating properties to heteromeric channels. It is therefore likely that the rate of inactivation will be determined by the number of Kv1.4 inactivation particles, as set by the precise subunit stoichiometry. We propose that EA1 mutations affect the rate of N-type inactivation either by reduced subunit surface expression, giving rise to a reduced number of Kv1.1 subunits in heterotetramer Kv1.1-Kv1.4 channels, or by reduced affinity for the Kv1.4 inactivation domain. To test this hypothesis, quantified amounts of mRNA for Kv1.4 or Kv1.1 containing selected EA1 mutations either in the inner vestibule of Kv1.1 on S6 or in the transmembrane regions were injected into Xenopus laevis oocytes and the relative rates of inactivation and stoichiometry were determined. The S6 mutations, V404I and V408A, which had normal surface expression, reduced the rate of inactivation by a decreased affinity for the inactivation domain while the mutations I177N in S1 and E325D in S5, which had reduced subunit surface expression, increased the rate of N-type inactivation due to a stoichiometric increase in the number of Kv1.4 subunits.

Nerve excitability studies characterize Kv1.1 fast potassium channel dysfunction in patients with episodic ataxia type 1

Episodic ataxia type 1 is a neuronal channelopathy caused by mutations in the KCNA1 gene encoding the fast K(+) channel subunit K(v)1.1. Episodic ataxia type 1 presents with brief episodes of cerebellar dysfunction and persistent neuromyotonia and is associated with an increased incidence of epilepsy. In myelinated peripheral nerve, K(v)1.1 is highly expressed in the juxtaparanodal axon, where potassium channels limit the depolarizing afterpotential and the effects of depolarizing currents. Axonal excitability studies were performed on patients with genetically confirmed episodic ataxia type 1 to characterize the effects of K(v)1.1 dysfunction on motor axons in vivo. The median nerve was stimulated at the wrist and compound muscle action potentials were recorded from abductor pollicis brevis. Threshold tracking techniques were used to record strength-duration time constant, threshold electrotonus, current/threshold relationship and the recovery cycle. Recordings from 20 patients from eight kindreds with different KCNA1 point mutations were compared with those from 30 normal controls. All 20 patients had a history of episodic ataxia and 19 had neuromyotonia. All patients had similar, distinctive abnormalities: superexcitability was on average 100% higher in the patients than in controls (P < 0.00001) and, in threshold electrotonus, the increase in excitability due to a depolarizing current (20% of threshold) was 31% higher (P < 0.00001). Using these two parameters, the patients with episodic ataxia type 1 and controls could be clearly separated into two non-overlapping groups. Differences between the different KCNA1 mutations were not statistically significant. Studies of nerve excitability can identify K(v)1.1 dysfunction in patients with episodic ataxia type 1. The simple 15 min test may be useful in diagnosis, since it can differentiate patients with episodic ataxia type 1 from normal controls with high sensitivity and specificity.

Clinical neurophysiology of the episodic ataxias: insights into ion channel dysfunction in vivo

Clinical neurophysiology has become an invaluable tool in the diagnosis of muscle channelopathies, but the situation is less clear cut with neuronal channelopathies. The genetic episodic ataxias are a group of disorders with heterogeneous phenotype and genotype, but share in common the feature of intermittent cerebellar dysfunction. Episodic ataxia (EA) types 1 and 2 are the most widely recognised of the autosomal dominant episodic ataxias and are caused by dysfunction of neuronal voltage-gated ion channels. There are central and peripheral nervous system manifestations in both conditions, and they are therefore good models of neuronal channelopathies to study neurophysiologically. To date most work has focussed upon characterising the electrophysiological properties of mutant channels in vitro. This review summarises the role of voltage-gated potassium and calcium channels, mutations of which underlie the main types of episodic ataxia types 1 and 2. The clinical, genetic and electrophysiological features of EA1 and EA2 are outlined, and a protocol for the assessment of these patients is proposed.

A novel KCNA1 mutation associated with global delay and persistent cerebellar dysfunction

Episodic Ataxia Type 1 is an autosomal dominant disorder characterized by episodes of ataxia and myokymia. It is associated with mutations in the KCNA1 voltage-gated potassium channel gene. In the present study, we describe a family with novel clinical features including persistent cerebellar dysfunction, cerebellar atrophy, and cognitive delay. All affected family members have myokymia and epilepsy, but only one individual has episodes of vertigo. Additional features include postural abnormalities, episodic stiffness and weakness. A novel KCNA1 mutation (c.1222G>T) which replaces a highly conserved valine with leucine at position 408 (p.Val408Leu) was identified in affected family members, and was found to augment the ability of the channel to inactivate. Together, our data suggests that KCNA1 mutations are associated with a broader clinical phenotype, which may include persistent cerebellar dysfunction and cognitive delay.

A novel KCNA1 mutation identified in an Italian family affected by episodic ataxia type 1

Episodic ataxia type 1 (EA1) is a rare human neurological syndrome characterized by continuous myokymia and attacks of generalized ataxia that can be triggered by abrupt movements, emotional stress and fatigue. An Italian family has been identified where related members displayed continuous myokymia, episodes of ataxia, attacks characterized by myokymia only, and neuromyotonia. A novel missense mutation (F414C), in the C-terminal region of the K(+) channel Kv1.1, was identified in the affected individuals. The mutant homotetrameric channels were non-functional in Xenopus laevis oocytes. In addition, heteromeric channels resulting from the co-expression of wild-type Kv1.1 and Kv1.1(F414C), or wild-type Kv1.2 and Kv1.1(F414C) subunits displayed reduced current amplitudes and altered gating properties. This indicates that the pathogenic effect of this KCNA1 mutation is likely to be related to the defective functional properties we have identified.

Episodic ataxia type 1: a neuronal potassium channelopathy

Episodic ataxia type 1 is a paroxysmal neurological disorder characterized by short-lived attacks of recurrent midline cerebellar dysfunction and continuous motor activity. Mutations in KCN1A, the gene encoding Kv1.1, a voltage-gated neuronal potassium channel, are associated with the disorder. Although rare, the syndrome highlights the fundamental features of genetic ion-channel diseases and serves as a useful model for understanding more common paroxysmal disorders, such as epilepsy and migraine. This review examines our current understanding of episodic ataxia type 1, focusing on its clinical and genetic features, pathophysiology, and treatment.

Episodic ataxia type 1 mutations in the KCNA1 gene impair the fast inactivation properties of the human potassium channels Kv1.4-1.1/Kvbeta1.1 and Kv1.4-1.1/Kvbeta1.2

Episodic ataxia type 1 (EA1) is an autosomal dominant neurological disorder characterized by constant muscle rippling movements (myokymia) and episodic attacks of ataxia. Several heterozygous point mutations have been found in the coding sequence of the voltage-gated potassium channel gene KCNA1 (hKv1.1), which alter the delayed-rectifier function of the channel. Shaker-like channels of different cell types may be formed by unique hetero-oligomeric complexes comprising Kv1.1, Kv1.4 and Kvbeta1.x subunits. Here we show that the human Kvbeta1.1 and Kvbeta1.2 subunits modulated the functional properties of tandemly linked Kv1.4-1.1 wild-type channels expressed in Xenopus laevis oocytes by (i) increasing the rate and amount of N-type inactivation, (ii) slowing the recovery rate from inactivation, (iii) accelerating the cumulative inactivation of the channel and (iv) negatively shifting the voltage dependence of inactivation. To date, the role of the human Kv1.4-1.1, Kv1.4-1.1/Kvbeta1.1 and Kv1.4-1.1/Kvbeta1.2 channels in the aetiopathogenesis of EA1 has not been investigated. Here we also show that the EA1 mutations E325D, V404I and V408A, which line the ion-conducting pore, and I177N, which resides within the S1 segment, alter the fast inactivation and repriming properties of the channels by decreasing both the rate and degree of N-type inactivation and by accelerating the recovery from fast inactivation. Furthermore, the E325D, V404I and I177N mutations shifted the voltage dependence of the steady-state inactivation to more positive potentials. The results demonstrate that the human Kvbeta1.1 and Kvbeta1.2 subunits regulate the proportion of wild-type Kv1.4-1.1 channels that are available to open. Furthermore, EA1 mutations alter heteromeric channel availability which probably modifies the integration properties and firing patterns of neurones controlling cognitive processes and body movements.

Functional analysis of a novel potassium channel (KCNA1) mutation in hereditary myokymia

Myokymia is characterized by spontaneous, involuntary muscle fiber group contraction visible as vermiform movement of the overlying skin. Myokymia with episodic ataxia is a rare, autosomal dominant trait caused by mutations in KCNA1, encoding a voltage-gated potassium channel. In the present study, we report a family with four members affected with myokymia. Additional clinical features included motor delay initially diagnosed as cerebral palsy, worsening with febrile illness, persistent extensor plantar reflex, and absence of epilepsy or episodic ataxia. Mutation analysis revealed a novel c.676C>A substitution in the potassium channel gene KCNA1, resulting in a T226K nonconservative missense mutation in the Kv1.1 subunit in all affected individuals. Electrophysiological studies of the mutant channel expressed in Xenopus oocytes indicated a loss of function. Co-expression of WT and mutant cRNAs significantly reduced whole-oocyte current compared to expression of WT Kv1.1 alone.

Episodic ataxia type 1 mutation F184C alters Zn2+-induced modulation of the human K+ channel Kv1.4-Kv1.1/Kvbeta1.1

Episodic ataxia type 1 (EA1) is a Shaker-like channelopathy characterized by continuous myokymia and attacks of imbalance with jerking movements of the head, arms, and legs. Although altered expression and gating properties of Kv1.1 channels underlie EA1, several disease-causing mechanisms remain poorly understood. It is likely that Kv1.1, Kv1.4, and Kvbeta1.1 subunits form heteromeric channels at hippocampal mossy fiber boutons from which Zn(2+) ions are released into the synaptic cleft in a Ca(2+)-dependent fashion. The sensitivity of this macromolecular channel complex to Zn(2+) is unknown. Here, we show that this heteromeric channel possesses a high-affinity (<10 muM) and a low-affinity (<0.5 mM) site for Zn(2+), which are likely to regulate channel availability at distinct presynaptic membranes. Furthermore, the EA1 mutation F184C, located within the S1 segment of the Kv1.1 subunit, markedly decreased the equilibrium dissociation constants for Zn(2+) binding to the high- and low-affinity sites. The functional characterization of the Zn(2+) effects on heteromeric channels harboring the F184C mutation also showed that this ion significantly 1) slowed the activation rate of the channel, 2) increased the time to reach peak current amplitude, 3) decreased the rate and amount of current undergoing N-type inactivation, and 4) slowed the repriming of the channel compared with wild-type channels. These results demonstrate that the EA1 mutation F184C will not only sensitize the homomeric Kv1.1 channel to extracellular Zn(2+), but it will also endow heteromeric channels with a higher sensitivity to this metal ion. During the vesicular release of Zn(2+), its effects will be in addition to the intrinsic gating defects caused by the mutation, which is likely to exacerbate the symptoms by impairing the integration and transmission of signals within specific brain areas.

An episodic ataxia type-1 mutation in the S1 segment sensitises the hKv1.1 potassium channel to extracellular Zn2+

Episodic ataxia type-1 (EA1) is a human neurological syndrome characterized by attacks of generalized ataxia and by continuous myokymia that has been associated with point mutations in the voltage-gated potassium channel gene KCNA1. Although important advancement has been made in understanding the molecular pathophysiology of EA1, several disease-causing mechanisms remain poorly understood. F184C is an EA1 mutation that is located within the S1 segment of the human Kv1.1 subunit. Here, we show that the F184C mutation increases approximately 4.5-fold the sensitivity of the channel to extracellular Zn2+. Both Zn2+and Cd2+ markedly alter the activation kinetics of F184C channel. In addition, the mutated channel reacts with several methane thiosulfonate reagents which specifically affected channel function. The results provide structural implications and indicate that sensitisation of hKv1.1 to Zn2+ is likely to contribute to the EA1 symptoms in patients harboring the F184C mutation.

Genetic disorders of neuromuscular ion channels

Ion channels are complex proteins that span the lipid bilayer of the cell membrane, where they orchestrate the electrical signals necessary for normal function of the central nervous system, peripheral nerve, and both skeletal and cardiac muscle. The role of ion channel defects in the pathogenesis of numerous disorders, many of them neuromuscular, has become increasingly apparent over the last decade. Progress in molecular biology has allowed cloning and expression of genes that encode channel proteins, while comparable advances in biophysics, including patch-clamp electrophysiology and related techniques, have made the study of expressed proteins at the level of single channel molecules possible. Understanding the molecular basis of ion channel function and dysfunction will facilitate both the accurate classification of these disorders and the rational development of specific therapeutic interventions. This review encompasses clinical, genetic, and pathophysiological aspects of ion channels disorders, focusing mainly on those with neuromuscular manifestations.

Genetics of familial episodic vertigo and ataxia

The familial episodic ataxias are prototypical inherited channelopathies that result in episodes of vertigo and ataxia triggered by stress and exercise. Episodic ataxia type 1 (EA-1) is caused by missense mutations in the potassium channel gene KCNA1, whereas episodic ataxia type 2 (EA-2) is caused by missense and nonsense mutations in the calcium channel gene CACNA1A. These ion channels are crucial for both central and peripheral neurotransmission. Within the last few years, the genetic mechanisms underlying these relatively rare familial episodic ataxia syndromes have been worked out. They provide a model for understanding the mechanisms of more common recurrent vertigo and ataxia syndromes, particularly those associated with migraine. Migraine affects as many as 15-20% of the general population, and it has been estimated that about 25% of patients with migraine experience spontaneous attacks of vertigo and ataxia. We identified 24 families with migraine and benign recurrent vertigo inherited in an autosomal dominant fashion. These families have numerous features in common with EA-1 and EA-2 (particularly EA-2), suggesting that benign recurrent vertigo may be an inherited channelopathy. An ion channel mutation shared by brain and inner ear could explain the combined central and peripheral features of the syndrome.