Episodic ataxia type 2. Three novel truncating mutations and one novel missense mutation in the CACNA1A gene

CACNA1A Mutations Causing Early Onset Ataxia: Profiling Clinical, Dysmorphic and Structural-Functional Findings

The CACNA1A gene encodes the pore-forming α_1A subunit of the voltage-gated Ca_V2.1 Ca²⁺ channel, essential in neurotransmission, especially in Purkinje cells. Mutations in CACNA1A result in great clinical heterogeneity with progressive symptoms, paroxysmal events or both. During infancy, clinical and neuroimaging findings may be unspecific, and no dysmorphic features have been reported. We present the clinical, radiological and evolutionary features of three patients with congenital ataxia, one of them carrying a new variant. We report the structural localization of variants and their expected functional consequences. There was an improvement in cerebellar syndrome over time despite a cerebellar atrophy progression, inconsistent response to acetazolamide and positive response to methylphenidate. The patients shared distinctive facial gestalt: oval face, prominent forehead, hypertelorism, downslanting palpebral fissures and narrow nasal bridge. The two α_1A affected residues are fully conserved throughout evolution and among the whole human Ca_V channel family. They contribute to the channel pore and the voltage sensor segment. According to structural data analysis and available functional characterization, they are expected to exert gain- (F1394L) and loss-of-function (R1664Q/R1669Q) effect, respectively. Among the CACNA1A-related phenotypes, our results suggest that non-progressive congenital ataxia is associated with developmental delay and dysmorphic features, constituting a recognizable syndromic neurodevelopmental disorder.

Targeting Alternative Splicing as a Potential Therapy for Episodic Ataxia Type 2

Episodic ataxia type 2 (EA2) is an autosomal dominant neurological disorder characterized by paroxysmal attacks of ataxia, vertigo, and nausea that usually last hours to days. It is caused by loss-of-function mutations in CACNA1A, the gene encoding the pore-forming α1 subunit of P/Q-type voltage-gated Ca2+ channels. Although pharmacological treatments, such as acetazolamide and 4-aminopyridine, exist for EA2, they do not reduce or control the symptoms in all patients. CACNA1A is heavily spliced and some of the identified EA2 mutations are predicted to disrupt selective isoforms of this gene. Modulating splicing of CACNA1A may therefore represent a promising new strategy to develop improved EA2 therapies. Because RNA splicing is dysregulated in many other genetic diseases, several tools, such as antisense oligonucleotides, trans-splicing, and CRISPR-based strategies, have been developed for medical purposes. Here, we review splicing-based strategies used for genetic disorders, including those for Duchenne muscular dystrophy, spinal muscular dystrophy, and frontotemporal dementia with Parkinsonism linked to chromosome 17, and discuss their potential applicability to EA2.

Zebrafish as a Model System for the Study of Severe CaV2.1 (α1A) Channelopathies

The P/Q-type Ca_V2.1 channel regulates neurotransmitter release at neuromuscular junctions (NMJ) and many central synapses. CACNA1A encodes the pore-containing α_1A subunit of Ca_V2.1 channels. In humans, de novo CACNA1A mutations result in a wide spectrum of neurological, neuromuscular, and movement disorders, such as familial hemiplegic migraine type 1 (FHM1), episodic ataxia type 2 (EA2), as well as a more recently discovered class of more severe disorders, which are characterized by ataxia, hypotonia, cerebellar atrophy, and cognitive/developmental delay. Heterologous expression of Ca_V2.1 channels has allowed for an understanding of the consequences of CACNA1A missense mutations on channel function. In contrast, a mechanistic understanding of how specific CACNA1A mutations lead in vivo to the resultant phenotypes is lacking. In this review, we present the zebrafish as a model to both study in vivo mechanisms of CACNA1A mutations that result in synaptic and behavioral defects and to screen for effective drug therapies to combat these and other Ca_V2.1 channelopathies.

Mutation Spectrum in the CACNA1A Gene in 49 Patients with Episodic Ataxia

Episodic ataxia is an autosomal dominant ion channel disorder characterized by episodes of imbalance and incoordination. The disease is genetically heterogeneous and is classified as episodic ataxia type 2 (EA2) when it is caused by a mutation in the CACNA1A gene, encoding the α_1A subunit of the P/Q-type voltage-gated calcium channel Ca_v2.1. The vast majority of EA2 disease-causing variants are loss-of-function (LoF) point changes leading to decreased channel currents. CACNA1A exonic deletions have also been reported in EA2 using quantitative approaches. We performed a mutational screening of the CACNA1A gene, including the promoter and 3'UTR regions, in 49 unrelated patients diagnosed with episodic ataxia. When pathogenic variants were not found by sequencing, we performed a copy number variant (CNV) analysis to screen for duplications or deletions. Overall, sequencing screening allowed identification of six different point variants (three nonsense and three missense changes) and two coding indels, one of them found in two unrelated patients. Additionally, CNV analysis identified a deletion in a patient spanning exon 35 as a result of a recombination event between flanking intronic Alu sequences. This study allowed identification of potentially pathogenic alterations in our sample, five of them novel, which cover 20% of the patients (10/49). Our data suggest that most of these variants are disease-causing, although functional studies are required.

Novel missense variant of CACNA1A gene in a Slovak family with episodic ataxia type 2

INTRODUCTION

Episodic ataxias (EAs) are rare dominantly inherited neurological disorders characterized by recurrent episodes of ataxia lasting minutes to hours. The most common subtype is EA type 2 (EA2) caused by pathogenic variants of calcium voltage-gated channel subunit alpha1 A gene (CACNA1A) on chromosome 19p13.

SUBJECTS AND METHODS

We examined a Slovak three-generation family. Genomic DNA of the family members was extracted from peripheral blood and amplified by polymerase chain reaction. CACNA1A variants were screened by Sanger sequencing.

RESULTS

We identified four family members with recurrent episodes of ataxia. Complex differential diagnosis was performed. Genetic analysis with direct sequencing revealed a novel heterozygous variant of CACNA1A - c.5264A>G (p.Glu1755Gly) located in the pore loop of domain IV of calcium channel alpha-1A subunit.

CONCLUSION

We identified a novel missense variant of a voltage-dependent P/Q-type calcium channel alpha-1A subunit in a Slovak three-generation family with recurrent episodes of ataxia. The heterozygous missense variant resulted in changing a highly conserved glutamic acid within the pore loop of domain IV.

The E1015K variant in the synprint region of the CaV2.1 channel alters channel function and is associated with different migraine phenotypes

Mutations in the CACNA1A gene, which encodes the pore-forming α1A subunit of the CaV2.1 voltage-gated calcium channel, cause a number of human neurologic diseases including familial hemiplegic migraine. We have analyzed the functional impact of the E1015K amino acid substitution located in the "synprint" domain of the α1A subunit. This variant was identified in two families with hemiplegic migraine and in one patient with migraine with aura. The wild type (WT) and the E1015K forms of the GFP-tagged α1A subunit were expressed in cultured hippocampal neurons and HEK cells to understand the role of the variant in the transport activity and physiology of CaV2.1. The E1015K variant does not alter CaV2.1 protein expression, and its transport to the cell surface and synaptic terminals is similar to that observed for WT channels. Electrophysiological data demonstrated that E1015K channels have increased current density and significantly altered inactivation properties compared with WT. Furthermore, the SNARE proteins syntaxin 1A and SNAP-25 were unable to modulate voltage-dependent inactivation of E1015K channels. Overall, our findings describe a genetic variant in the synprint site of the CaV2.1 channel which is characterized by a gain-of-function and associated with both hemiplegic migraine and migraine with aura in patients.

Episodic ataxia type 2 showing ictal hyperhidrosis with hypothermia and interictal chronic diarrhea due to a novel CACNA1A mutation

Autosomal dominant episodic ataxia type 2 (EA2) results from mutations of the CACNA1A gene. We describe EA2 with unusual features in a father and daughter with a novel CACNA1A mutation coding for Y248C. Both patients showed severe cerebellar atrophy in MRI and clinical signs of progressive spinocerebellar atrophy type 6. Most disabling were the very frequent episodes of ataxia with migraine (with aura in the father and without aura in the daughter) and nystagmus in our patients. Additionally, they suffered from ictal hyperhidrosis with acute hypothermia of the extremities. Lastly, the father presented with interictal chronic diarrhea not associated to a known primary gastrointestinal disorder. Both ictal hyperhidrosis and interictal diarrhea ameliorated upon acetazolamide intake, the typical treatment for EA2. The significance of these findings is discussed and the phenotype correlated to previously reported cases.

Genetic calcium signaling abnormalities in the central nervous system: seizures, migraine, and autism

The calcium ion is one of the most versatile, ancient, and universal of biological signaling molecules, known to regulate physiological systems at every level from membrane potential and ion transporters to kinases and transcription factors. Disruptions of intracellular calcium homeostasis underlie a host of emerging diseases, the calciumopathies. Cytosolic calcium signals originate either as extracellular calcium enters through plasma membrane ion channels or from the release of an intracellular store in the endoplasmic reticulum (ER) via inositol triphosphate receptor and ryanodine receptor channels. Therefore, to a large extent, calciumopathies represent a subset of the channelopathies, but include regulatory pathways and the mitochondria, the major intracellular calcium repository that dynamically participates with the ER stores in calcium signaling, thereby integrating cellular energy metabolism into these pathways, a process of emerging importance in the analysis of the neurodegenerative and neuropsychiatric diseases. Many of the calciumopathies are common complex polygenic diseases, but leads to their understanding come most prominently from rare monogenic channelopathy paradigms. Monogenic forms of common neuronal disease phenotypes-such as seizures, ataxia, and migraine-produce a constitutionally hyperexcitable tissue that is susceptible to periodic decompensations. The gene families and genetic lesions underlying familial hemiplegic migraine, FHM1/CACNA1A, FHM2/ATP1A2, and FHM3/SCN1A, and monogenic mitochondrial migraine syndromes, provide a robust platform from which genes, such as CACNA1C, which encodes the calcium channel mutated in Timothy syndrome, can be evaluated for their role in autism and bipolar disease.

Molecular pathogenesis of spinocerebellar ataxia type 6

Spinocerebellar ataxia type 6 (SCA6) is a neurodegenerative disorder caused by abnormal expansions of a trinucleotide CAG repeat in exon 47 of the CACNA1A gene, which encodes the alpha1A subunit of the P/Q-type voltage-gated calcium channel. The CAG repeat expansion is translated into an elongated polyglutamine tract in the carboxyl terminus of the alpha1A subunit. The alpha1A subunit is the main pore-forming subunit of the P/Q-type calcium channel. Patients with SCA6 suffer from a severe form of progressive ataxia and cerebellar dysfunction. Design of treatments for this disorder will depend on better definition of the mechanism of disease. As a disease arising from a mutation in an ion channel gene, SCA6 may behave as an ion channelopathy, and may respond to attempts to modulate or correct ion channel function. Alternatively, as a disease in which the mutant protein contains an expanded polyglutamine tract, SCA6 may respond to the targets of drug therapies developed for Huntington's disease and other polyglutamine disorders. In this review we will compare SCA6 to other polyglutamine diseases and channelopathies, and we will highlight recent advances in our understanding of alpha1A subunits and SCA6 pathology. We also propose a mechanism for how two seemingly divergent hypotheses can be combined into a cohesive model for disease progression.

Calcium channelopathies

Intracellular calcium ([Ca2+]i) is highly regulated in eukaryotic cells. The free [Ca2+]i is approximately four orders of magnitude less than that in the extracellular environment. It is, therefore, an electrochemical gradient favoring Ca2+ entry, and transient cellular activation increasing Ca2+ permeability will lead to a transient increase in [Ca2+]i. These transient rises of [Ca2+]i trigger or regulate diverse intracellular events, including metabolic processes, muscle contraction, secretion of hormones and neurotransmitters, cell differentiation, and gene expression. Hence, changes in [Ca2+]i act as a second messenger system coordinating modifications in the external environment with intracellular processes. Notably, information on the molecular genetics of the membrane channels responsible for the influx of Ca2+ ions has led to the discovery that mutations in these proteins are linked to human disease. Ca2+ channel dysfunction is now known to be the basis for several neurological and muscle disorders such as migraine, ataxia, and periodic paralysis. In contrast to other types of genetic diseases, Ca2+ channelopathies can be studied with precision by electrophysiological methods, and in some cases, the results have been highly rewarding with a biophysical phenotype that correlates with the ultimate clinical phenotype. This review outlines recent advances in genetic, molecular, and pathophysiological aspects of human Ca2+ channelopathies.

The ataxic groggy rat has a missense mutation in the P/Q-type voltage-gated Ca2+ channel alpha1A subunit gene and exhibits absence seizures

The groggy rat (strain name; GRY) exhibits ataxia, an unstable gait, and paroxysmal severe extension of the entire body. Adults show a reduction in size of the cerebellum and presynaptic and axon terminal abnormalities of Purkinje cells. These neurological abnormalities are inherited in an autosomal recessive manner, and the causative mutation has been named groggy (gry). In this study, we mapped gry on rat chromosome 19 and found a nonconservative missense (M251K) mutation in the alpha(1A) subunit of the P/Q-type voltage-gated Ca(2+) channel gene (Cacna1a) within the gry-critical region. This mutation was located at a highly conserved site close to the ion-selective pore and led to the shortening of the inactivation phase of the Ca(2+) channel current without a change of peak current density or current-voltage relationship in whole cell patch recordings of the recombinant Ca(2+) channel expressed in HEK cells. It has been well established that mice with a mutation at Cacna1a such as tottering and leaner show absence seizures. The Cacna1a-mutant GRY rat also exhibited absence-like seizures from 6 to 8 weeks of age, which were characterized by bilateral and synchronous 7-8 Hz spike-and-wave discharges concomitant with sudden immobility and staring, on cortical and hippocampal EEGs. The pharmacological profile of the seizures was similar to that of human absence epilepsy: the seizures were inhibited by ethosuximide and valproic acid but not phenytoin. Thus, the GRY rat with P/Q-type Ca(2+) channel disorders is a useful model for studying absence epilepsy and Cacna1a-related diseases.

Migraine genetics: an update

A growing interest in genetic research in migraine has resulted in the identification of several chromosomal regions that are involved in migraine. However, the identification of mutations in the genes for familial hemiplegic migraine (FHM) forms the only true molecular genetic knowledge of migraine thus far. The increased number of mutations in the FHM1 (CACNA1A) and the FHM2 (ATP1A2) genes allow studying the relationship between genetic findings in both genes and the clinical features in patients. A wide spectrum of symptoms is seen in patients. Additional cerebellar ataxia and (childhood) epilepsy can occur in FHM1 and FHM2. Functional studies show a dysfunction in ion transport as the key factor in the pathophysiology of (familial hemiplegic) migraine that predict an increased susceptibility to cortical spreading depression--the underlying mechanism of migraine aura.

Dominant-negative suppression of Cav2.1 currents by alpha(1)2.1 truncations requires the conserved interaction domain for beta subunits

Episodic ataxia type 2 (EA2) is an autosomal dominant disorder arising from CACNA1A mutations, which commonly predict heterozygous expression of Ca(v)2.1 calcium channels with truncated alpha(1)2.1 pore subunits. We hypothesized that alpha(1)2.1 truncations in EA2 exert dominant-negative effects on the function of wild-type subunits. Wild-type and truncated alpha(1)2.1 subunits with fluorescent protein tags were transiently co-expressed in cells stably expressing Ca(v) auxiliary beta subunits, which facilitate alpha1 subunit functional expression through high-affinity interactions with the alpha interaction domain (AID). Co-expression of wild-type subunits with truncations often resulted in severely reduced whole-cell currents compared to expression of wild-type subunits alone. Cellular image analyses revealed that current suppression was not due to reduced wild-type expression levels. Instead, the current suppression depended on truncations terminating distal to the AID. Moreover, only AID-bearing alpha(1)2.1 proteins co-immunoprecipitated with Ca(v) beta subunits. These results indicate that Ca(v) beta subunits may play a prominent role in EA2 disease pathogenesis.

Ion channel functional candidate genes in multigenic neuropsychiatric disease

Scores of monogenic Mendelian ion channel diseases serve to anchor the pathophysiology of the channelopathies, but there are also now clear examples of environmental, pharmacogenetic, and acquired channelopathy mechanisms. The cardinal feature of heritable ion channel disease is a periodic disturbance of rhythmic function in constitutionally hyperexcitable tissue. While the complexity of neuroanatomy obscures functional analysis of mutations causing monogenic seizure, ataxia, or migraine syndromes, extrapolation from the cardiac (Long QT [LQT]) and muscle (Periodic Paralysis) channelopathy syndromes provides a simplified predictive framework of molecular pathology: electrically stabilizing potassium ion (K(+)) and chloride ion (Cl(-)) channels, likely having lesions that diminish their current, and excitatory Na(+) channels, likely having gain-of-function lesions. The voltage-gated calcium channel gene family that contains CACNA1C, the newest LQT locus, causing Timothy Syndrome with a phenotype including autism, has proven to be particularly informative for its members' ability to tie the various central nervous system (CNS) phenotypes together in an interpretable fashion, now including direct extension to the classically multigenic neuropsychiatric phenotypes. Features of a promising ion channel candidate gene arise from its broad locus, gene family, nature of alleles, physiology and pharmacology, tissue expression profile, and phenotype in model organisms. KCNN3 is explored as a paradigm to consider.

Hereditary paroxysmal ataxia with mental retardation: a clinicopathological study in relation to episodic ataxia type 2

A case of hereditary acetazolamide-responsive paroxysmal ataxia with mild mental retardation in an autopsied Japanese man is described. His ataxic attacks had occurred for approximately 65 years since the age of 6. One of his daughters had severe mental retardation and epilepsy, and the other had paroxysmal ataxic attacks and mild mental retardation. Analysis of the subject's CACNA1A gene and that in his daughter revealed neither mutations nor CAG expansion. Neuropathologically, cortical degeneration consisting of the marked loss of Purkinje and granule cells was found exclusively in the cerebellar vermis. This was consistent with findings at autopsy for cases reported as spinocerebellar ataxia 6. In addition, there were minor anomalies, such as hypoplastic cerebellum and brainstem, heterotopic Purkinje cells, and cortical microdysgenesis of the temporal lobe. It is considered that the cerebellar cortical degeneration and the minor malformations found in the brain are closely related to one another, rather than having occurred independently.

Novel splice site CACNA1A mutation causing episodic ataxia type 2

Episodic ataxia type 2 (EA-2) is an autosomal dominant neurological disorder, characterized by episodes of ataxia, vertigo, nausea, nystagmus, and fatigue, associated with acetazolamide responsiveness. The disease is caused by mutations in the P/Q-type calcium channel Ca(v)2.1 subunit gene, CACNA1A, located on chromosome 19p13.2. We analyzed a family with 13 affected individuals for linkage to this locus and reached a two-point maximum LOD score of 4.48. A novel CACNA1A mutation, IVS36-2A>G, at the 3' acceptor splice site of intron 36 was identified by sequencing. It is the first described CACNA1A acceptor splice site mutation and the most C-terminal EA-2-causing mutation reported to date.

Unraveling monogenic channelopathies and their implications for complex polygenic disease

Ion channels are a large family of >400 related proteins representing >1% of our genetic endowment; however, ion-channel diseases reflect a relatively new category of inborn error. They were first recognized in 1989, with the discovery of cystic fibrosis transmembrane conductance regulator, and rapidly advanced as positional and functional studies converged in the dissection of components of the action potential of excitable tissues. Although it remains true that diseases of excitable tissue still most clearly illustrate this family of disease, ion-channel disorders now cover the gamut of medical disciplines, causing significant pathology in virtually every organ system, producing a surprising range of often unanticipated symptoms, and providing valuable targets for pharmacological intervention. Many of the features shared among the monogenic ion-channel diseases provide a general framework for formulating a foundation for considering their intrinsically promising role in polygenic disease. Since an increasingly important approach to the identification of genes underlying polygenic disease is to identify "functional candidates" within a critical region and to test their disease association, it becomes increasingly important to appreciate how these ion-channel mechanisms can be implicated in pathophysiology.