Alternating Hemiplegia of Childhood: Retrospective Genetic Study and Genotype-Phenotype Correlations in 187 Subjects from the US AHCF Registry

ATP1A3-related phenotypes in Chinese children: AHC, CAPOS, and RECA

The aim of this research is to study the phenotype, genotype, treatment strategies, and short-term prognosis of Chinese children with ATP1A3 (Na⁺/K⁺-ATPase alpha 3 gene)-related disorders in Southwest China. Patients with pathogenic ATP1A3 variants identified using next-generation sequencing were registered at the Children's Hospital of Chongqing Medical University from December 2015 to May 2019. We followed them as a cohort and analyzed their clinical data. Eleven patients were identified with de novo pathogenic ATP1A3 heterozygous variants. One (c.2542 + 1G > T, splicing) has not been reported. Eight patients with alternating hemiplegia of childhood (AHC), one with cerebellar ataxia, areflexia, pes cavus, optic atrophy, and sensorineural hearing loss (CAPOS), and two with relapsing encephalopathy with cerebellar ataxia (RECA) were included. The initial manifestations of AHC included hemiplegia, oculomotor abnormalities, and seizures, and the most common trigger was an upper respiratory tract infection without fever. All patients had paroxysmal hemiplegic attacks during their disease course. The brain MRI showed no abnormalities. Six out of eight AHC cases reached a stable disease state after treatment. The initial symptom of the patient with CAPOS was ataxia followed by developmental regression, seizures, deafness, visual impairment, and dysarthria, and the brain MRI indicated mild cerebellar atrophy. No fluctuation was noted after using Acetazolamide. The initial manifestations of the two RECA cases were dystonia and encephalopathy, respectively. One manifested a rapid-onset course of dystonia triggered by a fever followed by dysarthria and action tremors, and independent walking was impossible. The brain MRI image was normal. The other one presented with disturbance of consciousness, seizures, sleep disturbance, tremor, and dyskinesias. The EEG revealed a slow background (δ activity), and the brain MRI result was normal. No response to Flunarizine was noted for them, and it took 61 and 60 months for them to reach a stable disease state, respectively.

CONCLUSION

Pathogenic ATP1A3 variants play an essential role in the pathogenesis of Sodium-Potassium pump disorders, and AHC is the most common phenotype. The treatment strategies and prognosis depend on the phenotype categories caused by different variation sites and types. The correlation between the genotype and phenotype requires further exploration.

WHAT IS KNOWN

• Pathogenic heterozygous ATP1A3 variants cause a spectrum of neurological phenotypes, and ATP1A3-disorders are viewed as a phenotypic continuum presenting with atypical and overlapping features. • The genotype-phenotype correlation of ATP1A3-disorders remains unclear.

WHAT IS NEW

• In this study, the genotypes and phenotypes of ATP1A3-related disorders from Southwest of China were described. The splice-site variation c.2542+1G>T was detected for the first time in ATP1A3-related disorders. • The prognosis of twins with AHC p. Gly947Arg was more serious than AHC cases with other variants, which was inconsistent with previous reports. The phenomenon indicated the diversity of the correlation between the genotype and phenotype.

Disease mutations of human α3 Na+/K+-ATPase define extracellular Na+ binding/occlusion kinetics at ion binding site III

Na⁺/K⁺-ATPase, which creates transmembrane electrochemical gradients by exchanging 3 Na⁺ for 2 K⁺, is central to the pathogenesis of neurological diseases such as alternating hemiplegia of childhood. Although Na⁺/K⁺-ATPase has 3 distinct ion binding sites I-III, the difficulty of distinguishing ion binding events at each site from the others hinders kinetic study of these transitions. Here, we show that binding of Na⁺ at each site in the human α3 Na⁺/K⁺-ATPase can be resolved using extracellular Na⁺-mediated transient currents. When Na⁺/K⁺-ATPase is constrained to bind and release only Na⁺, three kinetic components: fast, medium, and slow, can be isolated, presumably corresponding to the protein dynamics associated with the binding (or release depending on the voltage step direction) and the occlusion (or deocclusion) of each of the 3 Na⁺. Patient-derived mutations of residues which coordinate Na⁺ at site III exclusively impact the slow component, demonstrating that site III is crucial for deocclusion and release of the first Na⁺ into the extracellular milieu. These results advance understanding of Na⁺/K⁺-ATPase mutation pathogenesis and provide a foundation for study of individual ions' binding kinetics.

Molecular and clinical characteristics of ATP1A3-related diseases

Objective

With detailed studies of ATP1A3-related diseases, the phenotypic spectrum of ATP1A3 has greatly expanded. This study aimed to potentially identify the mechanisms by which ATP1A3 caused neurological dysfunction by analyzing the clinical features and phenotypes of ATP1A3-related diseases, and exploring the distribution patterns of mutations in the subregions of the ATP1A3 protein, thus providing new and effective therapeutic approaches.

Methods

Databases of PubMed, Online Mendelian Inheritance in Man, and Human Gene Mutation Database, Wanfang Data, and Embase were searched for case reports of ATP1A3-related diseases. Following case screening, we collected clinical information and genetic testing results of patients, and analyzed the disease characteristics on the clinical phenotype spectrum associated with mutations, genetic characteristics of mutations, and effects of drug therapy.

Results

We collected 902 clinical cases related to ATP1A3 gene. From the results of previous studies, we further clarified the clinical characteristics of ATP1A3-related diseases, such as alternating hemiplegia of childhood (AHC), rapid-onset dystonia-parkinsonism; cerebellar ataxia, areflexia, pes cavus, optic atrophy, and sensorineural hearing loss syndrome, and relapsing encephalopathy with cerebellar ataxia, frequency of mutations in different phenotypes and their distribution in gene and protein structures, and differences in mutations in different clinical phenotypes. Regarding the efficacy of drug treatment, 80 of the 124 patients with AHC were treated with flunarizine, with an effectiveness rate of ~64.5%.

Conclusions

Nervous system dysfunction due to mutations of ATP1A3 gene was characterized by a group of genotypic–phenotypic interrelated disease pedigrees with multiple clinical manifestations. The presented results might help guide the diagnosis and treatment of ATP1A3-related diseases and provided new ideas for further exploring the mechanisms of nervous system diseases due to ATP1A3 mutations.

Non‐Stationary Outcome of Alternating Hemiplegia of Childhood into Adulthood

Background

Objectives

Methods

Results

Conclusions

Different phenotypes of neurological diseases, including alternating hemiplegia of childhood and rapid-onset dystonia-parkinsonism, caused by de novo ATP1A3 mutation in a family

BACKGROUND

The spectrum of neurological diseases related to ATP1A3 gene mutations is highly heterogeneous and exhibits different phenotypes. Phenotype overlaps, including alternating hemiplegia of childhood (AHC), early infantile epileptic encephalopathy, and rapid-onset dystonia-parkinsonism (RDP), can also occur at extremely low incidences. Currently, over 90 types of pathogenic mutations have been identified in ATP1A3.

PATIENTS AND METHODS

The family of a 2-year-11-month-old proband with AHC was recruited for this clinical investigation. The proband was screened for candidate mutation gene sites using next-generation sequencing and target-region capture technology. Sanger sequencing was used to identify carriers among family members.

RESULTS

The mother of the proband with AHC was diagnosed with dystonia (later diagnosed as RDP). The biochemical and immune indices of the proband and the mother were not abnormal. Moreover, brain imaging of the proband revealed no significant abnormalities. However, the electroencephalogram of the mother was mildly abnormal, with no spike wave discharge. Brain MRI revealed slight cerebellar atrophy. Electromyography revealed neurogenic damage, with a decrease in the conduction velocity of the left ulnar and radial nerves. Based on the sequencing data, both the proband and her mother carried c.823G > C p. (Ala275Pro) heterozygotes; other family members were not identified as carriers. With a PolyPhen-2 score of 0.997 and SIFT score of 0.001, this mutation can be considered damaging.

CONCLUSION

Family genotype-phenotype correlation analysis revealed that the phenotype and gene mutation were co-segregated, suggesting that it may be a pathogenic mutation.

Genetically altered animal models for ATP1A3-related disorders

Within the past 20 years, particularly with the advent of exome sequencing technologies, autosomal dominant and de novo mutations in the gene encoding the neurone-specific α3 subunit of the Na+,K+-ATPase (NKA α3) pump, ATP1A3, have been identified as the cause of a phenotypic continuum of rare neurological disorders. These allelic disorders of ATP1A3 include (in approximate order of severity/disability and onset in childhood development): polymicrogyria; alternating hemiplegia of childhood; cerebellar ataxia, areflexia, pes cavus, optic atrophy and sensorineural hearing loss syndrome; relapsing encephalopathy with cerebellar ataxia; and rapid-onset dystonia-parkinsonism. Some patients present intermediate, atypical or combined phenotypes. As these disorders are currently difficult to treat, there is an unmet need for more effective therapies. The molecular mechanisms through which mutations in ATP1A3 result in a broad range of neurological symptoms are poorly understood. However, in vivo comparative studies using genetically altered model organisms can provide insight into the biological consequences of the disease-causing mutations in NKA α3. Herein, we review the existing mouse, zebrafish, Drosophila and Caenorhabditis elegans models used to study ATP1A3-related disorders, and discuss their potential contribution towards the understanding of disease mechanisms and development of novel therapeutics.

Diseases caused by mutations in the Na+/K+ pump α1 gene ATP1A1

Human cell survival requires function of the Na⁺/K⁺ pump; the heteromeric protein that hydrolyzes ATP to extrude Na⁺ and import K⁺ across the plasmalemma, thereby building and maintaining these ions' electrochemical gradients. Numerous dominant diseases caused by mutations in genes encoding for Na⁺/K⁺ pump catalytic (α) subunit isoforms highlight the importance of this protein. Here, we review literature describing disorders caused by missense mutations in ATP1A1, the gene encoding the ubiquitously expressed α1 isoform of the Na⁺/K⁺ pump. These various maladies include primary aldosteronism with secondary hypertension, an endocrine syndrome, Charcot-Marie-Tooth disease, a peripheral neuropathy, complex spastic paraplegia, another neuromuscular disorder, as well as hypomagnesemia accompanied by seizures and cognitive delay, a condition affecting the renal and central nervous systems. This article focuses on observed commonalities among these mutations' functional effects, as well as on the special characteristics that enable each particular mutation to exclusively affect a certain system, without affecting others. In this respect, it is clear how somatic mutations localized to adrenal adenomas increase aldosterone production without compromising other systems. However, it remains largely unknown how and why some but not all de novo germline or familial mutations (where the mutant must be expressed in numerous tissues) produce a specific disease and not the other diseases. We propose hypotheses to explain this observation and the approaches that we think will drive future research on these debilitating disorders to develop novel patient-specific treatments by combining the use of heterologous protein-expression systems, patient-derived pluripotent cells, and gene-edited cell and mouse models.

Early role for a Na+,K+-ATPase (ATP1A3) in brain development

By evaluating children with a malformed cerebral cortex, we identified an ATPase pump (ATP1A3) with an early role in brain development. The ATP1A3 pump maintains the physiological concentration of sodium and potassium ions in cells, a process critical for osmotic equilibrium and membrane potential across several developing cell populations. We employed single-cell sequencing approaches to identify key enrichments for ATP1A3 expression during human cortex development. Unravelling this early cell-type–specific pathophysiology in the developing brain offers a potential basis for the treatment of ATP1A3-related diseases affecting prenatal and early childhood development.

Osmotic equilibrium and membrane potential in animal cells depend on concentration gradients of sodium (Na⁺) and potassium (K⁺) ions across the plasma membrane, a function catalyzed by the Na⁺,K⁺-ATPase α-subunit. Here, we describe ATP1A3 variants encoding dysfunctional α3-subunits in children affected by polymicrogyria, a developmental malformation of the cerebral cortex characterized by abnormal folding and laminar organization. To gain cell-biological insights into the spatiotemporal dynamics of prenatal ATP1A3 expression, we built an ATP1A3 transcriptional atlas of fetal cortical development using mRNA in situ hybridization and transcriptomic profiling of ∼125,000 individual cells with single-cell RNA sequencing (Drop-seq) from 11 areas of the midgestational human neocortex. We found that fetal expression of ATP1A3 is most abundant to a subset of excitatory neurons carrying transcriptional signatures of the developing subplate, yet also maintains expression in nonneuronal cell populations. Moving forward a year in human development, we profiled ∼52,000 nuclei from four areas of an infant neocortex and show that ATP1A3 expression persists throughout early postnatal development, most predominantly in inhibitory neurons, including parvalbumin interneurons in the frontal cortex. Finally, we discovered the heteromeric Na⁺,K⁺-ATPase pump complex may form nonredundant cell-type–specific α-β isoform combinations, including α3-β1 in excitatory neurons and α3-β2 in inhibitory neurons. Together, the developmental malformation phenotype of affected individuals and single-cell ATP1A3 expression patterns point to a key role for α3 in human cortex development, as well as a cell-type basis for pre- and postnatal ATP1A3-associated diseases.

Alternating Hemiplegia of Childhood: Genotype-Phenotype Correlations in a Cohort of 39 Italian Patients

Alternating hemiplegia of childhood is a rare neurological disease characterized by paroxysmal movement disorders and chronic neurological disturbances, with onset before 18 months of age. Mutations in the ATP1A3 gene have been identified in up to 80% of patients. Thirty-nine patients [20 females, 19 males, mean age 25.32 years (7.52–49.34)] have been recruited through the Italian Biobank and Clinical Registry for Alternating Hemiplegia of Childhood. Demographic data, genotype, paroxysmal movement disorders, chronic neurological features, and response to flunarizine have been analyzed. ATP1A3 gene mutations have been detected in 92.3% of patients. Patients have been divided into three groups—p.Asp801Asn mutation patients (26%), p.Glu815Lys cases (23%), and patients with other ATP1A3 mutations—and statistically compared. The Italian cohort has a higher percentage of ATP1A3 gene mutation than reported in literature (92.3%). Our data confirm a more severe phenotype in patients with p.Glu815Lys mutation, with an earlier age of onset of plegic (p = 0.02 in the correlation with other mutations) and tonic attacks. P.Glu815Lys patients most frequently present altered muscle tone, inability to walk (p = 0.01 comparing p.Glu815Lys and p.Asp801Asn mutations), epilepsy, and a more severe grade of dystonia (p < 0.05 comparing p.Glu815Lys and p.Asp801Asn mutations). They have moderate/severe intellectual disability and severe language impairment (p < 0.05). Interestingly, flunarizine seems to be more efficacious in patients with p.Glu815Lys mutation than p.Asp801Asn. In conclusion, our research suggests a genotype–phenotype correlation and provides information on this disorder's features, clinical course, and treatment.

De novo ATP1A3 variants cause polymicrogyria

Polymicrogyria is a common malformation of cortical development whose etiology remains elusive. We conducted whole-exome sequencing for 124 patients with polymicrogyria and identified de novo ATP1A3 variants in eight patients. Mutated ATP1A3 causes functional brain diseases, including alternating hemiplegia of childhood (AHC), rapid-onset dystonia parkinsonism (RDP), and cerebellar ataxia, areflexia, pes cavus, optic nerve atrophy, and sensorineural deafness (CAPOS). However, our patients showed no clinical features of AHC, RDP, or CAPOS and had a completely different phenotype: a severe form of polymicrogyria with epilepsy and developmental delay. Detected variants had different locations in ATP1A3 and different functional properties compared with AHC-, RDP-, or CAPOS-associated variants. In the developing cerebral cortex of mice, radial neuronal migration was impaired in neurons overexpressing the ATP1A3 variant of the most severe patients, suggesting that this variant is involved in cortical malformation pathogenesis. We propose a previously unidentified category of polymicrogyria associated with ATP1A3 abnormalities.

Combined dystonias: clinical and genetic updates

The genetic combined dystonias are a clinically and genetically heterogeneous group of neurologic disorders defined by the overlap of dystonia and other movement disorders such as parkinsonism or myoclonus. The number of genes associated with combined dystonia syndromes has been increasing due to the wider recognition of clinical features and broader use of genetic testing. Nevertheless, these diseases are still rare and represent only a small subgroup among all dystonias. Dopa-responsive dystonia (DYT/PARK-GCH1), rapid-onset dystonia-parkinsonism (DYT/PARK-ATP1A3), X-linked dystonia-parkinsonism (XDP, DYT/PARK-TAF1), and young-onset dystonia-parkinsonism (DYT/PARK-PRKRA) are monogenic combined dystonias accompanied by parkinsonian features. Meanwhile, MYC/DYT-SGCE and MYC/DYT-KCTD17 are characterized by dystonia in combination with myoclonus. In the past, common molecular pathways between these syndromes were the center of interest. Although the encoded proteins rather affect diverse cellular functions, recent neurophysiological evidence suggests similarities in the underlying mechanism in a subset. This review summarizes recent developments in the combined dystonias, focusing on clinico-genetic features and neurophysiologic findings. Disease-modifying therapies remain unavailable to date; an overview of symptomatic therapies for these disorders is also presented.

Effect of Flunarizine on Alternating Hemiplegia of Childhood in a Patient with the p.E815K Mutation in ATP1A3: A Case Report

Alternating hemiplegia of childhood (AHC) (MIM 104290) is characterized by transient repeated attacks of paresis on either or both sides of the body, oculomotor and autonomic abnormalities, movement disorders, and cognitive impairment. Preventing paroxysmal attacks, such as paresis and spasm, in patients with AHC is often difficult. An 8-month-old girl presented to our institution with intractable epilepsy. She developed AHC, with left-right alternating or bilateral recurrent plegia upon waking, involuntary movements, eye movement abnormalities, and psychomotor retardation. She had a heterozygous de novo p.E815K mutation in the <i>ATP1A3</i>gene. Patients with this mutation develop severe hemiplegic spells and convulsions, have a poor neuromotor developmental outcome, and are particularly difficult to treat. Flunarizine treatment has limited therapeutic effect in such patients; however, it was definitely effective for bulbar palsy in the present case. The present case further highlights the need for the development of other new treatments, such as a ketogenic diet.

Alternating Hemiplegia of Childhood: gastrointestinal manifestations and correlation with neurological impairments

Background

Alternating Hemiplegia of Childhood (AHC) is caused by mutations of the ATP1A3 gene which is expressed in brain areas that include structures controling autonomic, gastrointestinal, gut motility and GABAergic functions. We aimed to investigate, in a cohort of 44 consecutive AHC patients, two hypotheses: 1) AHC patients frequently manifest gastrointestinal, particularly motility, problems. 2) These problems are often severe and their severity correlates with neurological impairments.

Results

41/44 (93%) exhibited gastrointestinal symptoms requiring medical attention. For these 41 patients, symptoms included constipation (66%), swallowing problems (63%), vomiting (63%), anorexia (46%), diarrhea (44%), nausea (37%), and abdominal pain (22%). Symptoms indicative of dysmotility occurred in 33 (80%). The most common diagnoses were oropharyngeal dysphagia (63%) and gastroesophageal reflux (63%). 16 (39%) required gastrostomy and two fundoplication. Severity of gastrointestinal symptoms correlated with non-paroxysmal neurological disability index, Gross Motor Function Classification System scores, and with the presence/absence of non-gastrointestinal autonomic dysfunction (p = 0.031, 0.043, Spearman correlations and 0.0166 Cramer’s V, respectively) but not with the paroxysmal disability index (p = 0.408).

Conclusions

Most AHC patients have gastrointestinal problems. These are usually severe, most commonly are indicative of dysmotility, often require surgical therapies, and their severity correlates with that of non-paroxysmal CNS manifestations. Our findings should help in management-anticipatory guidance of AHC patients. Furthermore, they are consistent with current understandings of the pathophysiology of AHC and of gastrointestinal dysmotility, both of which involve autonomic and GABAergic dysfunction.

Gross Motor Function Disorders in Patients with Alternating Hemiplegia of Childhood

Background

Alternating hemiplegia of Childhood (AHC) is a rare disease manifested by transient episodes of hemiplegia and other neurological disorders. Delayed motor development has been reported in patients with AHC, but detailed features of the motor impairment have not been described so far.

Aim

The aim of the study was to evaluate gross motor function between attacks in a group of Polish patients with AHC.

Materials and methods

The interictal gross motor function was assessed using the Gross Motor Function AHC scale, which consisted of 41 motor tasks. The study group consisted of 10 patients with AHC older than 2 years of age. The control group consisted of 30 age- and gender-matched subjects. The results achieved in each of the 41 tasks by the study subjects were compared to the results obtained with controls using the non-parametric Mann-Whitney U-test. In tasks 38-41, mean times were compared between the study subjects and controls.

Results

The study revealed gross motor function impairment in patients with AHC. The greatest differences compared to controls concerned such skills as standing on toes, walking on toes, walking on heels, as well as running and hopping on one leg and on alternate legs. Significant impairment of the motor function of the upper limbs was also found.

Conclusions

The study confirmed motor function impairment between attacks in patients with AHC. The study findings may indicate the need to introduce individualised physiotherapy management of patients with AHC.

Social impairments in alternating hemiplegia of childhood

AIM

To evaluate presence and severity of social impairments in alternating hemiplegia of childhood (AHC) and determine factors that are associated with social impairments.

METHOD

This was a retrospective analysis of 34 consecutive patients with AHC (19 females, 15 males; mean age: 9y 7mo, SD 8y 2mo, range 2y 7mo-40y), evaluated with the Social Responsiveness Scale, Second Edition (SRS-2).

RESULTS

SRS-2 scores, indicating level of social impairment, were higher than population means (75, SD 14 vs 50, SD 10, p<0.001). Of these, 27 out of 34 had high scores: 23 severe (>76), four moderate (66-76). All subscale domains, including social cognition, social communication, social awareness, social motivation, restricted interests, and repetitive behavior, had abnormal scores compared to population means (p<0.001). High SRS-2 scores were associated with the presence of autism spectrum disorder (ASD) and epilepsy (p=0.01, p=0.04), but not with other scales of AHC disease symptomatology. All nine patients who received formal evaluations for ASD, because they had high SRS-2 scores, were diagnosed with ASD.

INTERPRETATION

Most patients with AHC have impaired social skills involving multiple domains. ASD is not uncommon. High SRS-2 scores in patients with AHC support referral to ASD evaluation. Our findings are consistent with current understandings of the pathophysiology of AHC and ASD, both thought to involve GABAergic dysfunction.

WHAT THIS PAPER ADDS

Most patients with alternating hemiplegia of childhood (AHC) have impaired social skills involving multiple domains. These impairments are significant compared to population means. Most patients with AHC have high Social Responsiveness Scale, Second Edition (SRS-2) scores. Patients with AHC with high SRS-2 scores are likely to have autism spectrum disorder.

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.

Alternating Hemiplegia of Childhood: Understanding the Genotype-Phenotype Relationship of ATP1A3 Variations

Alternating hemiplegia of childhood (AHC) is a rare neurological disorder affecting children with an onset before 18 months. Diagnostic clues include transient episodes of hemiplegia alternating in the laterality or quadriparesis, nystagmus and other paroxysmal attacks as tonic and dystonic spells. Epilepsy is also a common feature. In the past, a great effort has been done to understand the genetic basis of the disease leading to the discovery of mutations in the ATP1A3 gene encoding for the alpha3 subunit of Na⁺/K⁺ATPase, a protein already related to another disease named Rapid Onset Dystonia Parkinsonism (RDP). ATP1A3 mutations account for more than 70% of cases of AHC. In particular, three hotspot mutations account for about 60% of all cases, and these data have been confirmed in large population studies. Specifically, the p.Asp801Asn variant has been found to cause 30–43% of all cases, p.Glu815Lys is responsible for 16–35% of cases and p.Gly947Arg accounts for 8–15%. These three mutations are associated with different clinical phenotype in terms of symptoms, severity and prognosis. In vitro and in vivo models reveal that a crucial role of Na⁺/K⁺ATPase pump activity emerges in maintaining a correct membrane potential, survival and homeostasis of neurons. Herein, we attempt to summarize all clinical, genetic and molecular aspects of AHC considering ATP1A3 as its primary disease-causing determinant.

Movement disorders in patients with alternating hemiplegia: "Soft" and "stiff" at the same time

OBJECTIVE

To assess nonparoxysmal movement disorders in ATP1A3 mutation-positive patients with alternating hemiplegia of childhood (AHC).

METHODS

Twenty-eight patients underwent neurologic examination with particular focus on movement phenomenology by a specialist in movement disorders. Video recordings were reviewed by another movement disorders specialist and data were correlated with patients' characteristics.

RESULTS

Ten patients were diagnosed with chorea, 16 with dystonia (nonparoxysmal), 4 with myoclonus, and 2 with ataxia. Nine patients had more than one movement disorder and 8 patients had none. The degree of movement disorder was moderate to severe in 12/28 patients. At inclusion, dystonic patients (n = 16) were older (p = 0.007) than nondystonic patients. Moreover, patients (n = 18) with dystonia or chorea, or both, had earlier disease onset (p = 0.042) and more severe neurologic impairment (p = 0.012), but this did not correlate with genotype. All patients presented with hypotonia, which was characterized as moderate or severe in 16/28. Patients with dystonia or chorea (n = 18) had more pronounced hypotonia (p = 0.011). Bradykinesia (n = 16) was associated with an early age at assessment (p < 0.01). Significant dysarthria was diagnosed in 11/25 cases. A history of acute neurologic deterioration and further regression of motor function, typically after a stressful event, was reported in 7 patients.

CONCLUSIONS

Despite the relatively limited number of patients and the cross-sectional nature of the study, this detailed categorization of movement disorders in patients with AHC offers valuable insight into their precise characterization. Further longitudinal studies on this topic are needed.

Clinical and Genetic Spectrum of ATP1A3-Related Disorders in a Korean Pediatric Population

Background and Purpose

The aim of this study was to expand the understanding of the genotype-phenotype spectrum of ATP1A3-related disorders and to evaluate the therapeutic effect of a ketogenic diet in patients with alternating hemiplegia of childhood (AHC).

Methods

The clinical information of 13 patients with ATP1A3 mutations was analyzed by performing retrospective chart reviews. Patients with the AHC phenotype who consented to ketogenic diet were included in the trial.

Results

Ten patients presented with the clinical phenotype of AHC, two patients presented with rapid-onset dystonia parkinsonism, and one patient presented with cerebellar ataxia, areflexia, pes cavus, optic atrophy, and sensorineural hearing loss. Two novel mutations of the AHC phenotype were identified: p.Ile363Thr and p.Asn743Ser. The clinical phenotypes of three mutations differed from those in previous reports: p.Arg597Pro, p.Thr769Pro, and p.Arg756Cys. One of the two patients who started a ketogenic diet experienced seizure provocation and so immediate stopped consuming the diet, while the other patient continued the ketogenic diet for 1 year, but this produced no clear benefit such as reduction of paroxysmal symptoms.

Conclusions

Our study is the first case series of ATP1A3-related disorders to be described in Korea and which further expands the understanding of its genotype-phenotype spectrum. A ketogenic diet showed no clear benefit for the patients with AHC.

The epileptology of alternating hemiplegia of childhood

OBJECTIVE

To report our experience and investigate 5 original hypotheses: (1) multiple types of epileptic seizures occur in alternating hemiplegia of childhood (AHC), and these can be the initial presentation; (2) epileptiform abnormalities often appear well after clinical seizures; (3) nonepileptic reduced awareness spells (RAS) occur frequently; (4) epilepsy is commonly drug resistant but may respond to vagal nerve stimulation (VNS); and (5) status epilepticus (SE) is common and is usually refractory and recurrent.

METHODS

We analyzed a cohort of 51 consecutive patients with AHC.

RESULTS

Thirty-two of 51 patients had epilepsy: 18 focal seizures, frontal more frequently than temporal, and then posterior. Eleven had primary generalized seizures (tonic-clonic, myoclonic, and/or absence). Epileptic seizures preceded other AHC paroxysmal events in 8 (lag 5.63 ± 6.55 months; p = 0.0365). In 7 of 32, initial EEGs were normal, with the first epileptiform EEG lagging behind by 3.53 ± 4.65 years (p = 0.0484). RAS occurred equally in patients with epilepsy (16 of 32) and patients without epilepsy (10 of 19, p = 1.0). Twenty-eight patients had video-EEG; captured RAS showed no concomitant EEG changes. Nineteen patients (59%) were drug resistant. VNS resulted in >50% reduction in seizures in 5 of 6 (p < 0.04). Twelve patients (38%) had SE (9 of 12 multiple episodes), refractory/superrefractory in all (p < 0.001), and 4 of 12 had regression after SE.

CONCLUSIONS

Epilepsy in AHC can be focal or generalized. Epileptic seizures may be the first paroxysmal symptom. EEG may become epileptiform only on follow-up. Epilepsy, although frequently drug resistant, can respond to VNS. RAS are frequent and nonepileptic. SE often recurs and is usually refractory/superrefractory. Our observations are consistent with current data on AHC-ATP1A3 pathophysiology.

Factors in the disease severity of ATP1A3 mutations: Impairment, misfolding, and allele competition

Dominant mutations of ATP1A3, a neuronal Na,K-ATPase α subunit isoform, cause neurological disorders with an exceptionally wide range of severity. Several new mutations and their phenotypes are reported here (p.Asp366His, p.Asp742Tyr, p.Asp743His, p.Leu924Pro, and a VUS, p.Arg463Cys). Mutations associated with mild or severe phenotypes [rapid-onset dystonia-parkinsonism (RDP), alternating hemiplegia of childhood (AHC), or early infantile epileptic encephalopathy (EIEE)] were expressed in HEK-293 cells. Paradoxically, the severity of human symptoms did not correlate with whether there was enough residual activity to support cell survival. We hypothesized that distinct cellular consequences may result not only from pump inactivation but also from protein misfolding. Biosynthesis was investigated in four tetracycline-inducible isogenic cell lines representing different human phenotypes. Two cell biological complications were found. First, there was impaired trafficking of αβ complex to Golgi apparatus and plasma membrane, as well as changes in cell morphology, for two mutations that produced microcephaly or regions of brain atrophy in patients. Second, there was competition between exogenous mutant ATP1A3 (α3) and endogenous ATP1A1 (α1) so that their sum was constant. This predicts that in patients, the ratio of normal to mutant ATP1A3 proteins will vary when misfolding occurs. At the two extremes, the results suggest that a heterozygous mutation that only impairs Na,K-ATPase activity will produce relatively mild disease, while one that activates the unfolded protein response could produce severe disease and may result in death of neurons independently of ion pump inactivation.

Relapsing encephalopathy with cerebellar ataxia are caused by variants involving p.Arg756 in ATP1A3

Mutations in ATP1A3 lead to different phenotypes having in common acute neurological decompensation episodes triggered by a specific circumstance and followed by sequelae. Alongside Alternating Hemiplegia of Childhood (AHC), Rapid-onset Dystonia Parkinsonism (RDP) and Cerebellar ataxia, Areflexia, Pes cavus, Optic atrophy, Sensorineural hearing loss syndrome (CAPOS), a new Relapsing Encephalopathy with Cerebellar Ataxia (RECA) phenotype was published in 2015. We describe herein eight new pediatric cases. Most of them had no specific history when the first neurological decompensation episode occurred, before the age of 5 years, triggered by fever with severe paralytic hypotonia followed by ataxia with or without abnormal movements. Neurological sequelae with ataxia as the predominant symptom were present after the first episode in three cases and after at least one subsequent relapse in five cases. Five of the eight cases had a familial involvement with one of the two parents affected. The phenotype-genotype correlation is unequivocal with the causal substitution always located at position 756. The pathophysiology of the dysfunctions of the mutated ATPase pump, triggered by fever is unknown. Severe recurrent neurological decompensation episodes triggered by fever, without any metabolic cause, should lead to the sequencing of ATP1A3.

Paroxysmal Asymmetric Dystonic Arm Posturing-A Less Recognized but Characteristic Manifestation of ATP1A3-related disease

Background

ATP1A3 mutations cause a wide clinical spectrum, and are one of the "commoner rare diseases".

Methods

Case series of four patients with ATP1A3 mutations.

Results

The patients displayed characteristic episodes of dystonic arm posturing, involving a dystonic, flexed arm held in front of the body or close to the body, but with the hand raised upwards. Other attacks manifested with arm extension, either beside the body or reaching upwards. Dystonic posturing occurred paroxysmally, with no neurological signs between attacks, or combined with other signs like chorea, ataxia, and hypotonia.

Conclusions

While previous diagnostic criteria have not included paroxysmal or episodic dystonia, recent expert consensus has proposed to include alternating or paroxysmal dystonia as major feature calling for ATP1A3 genetic testing. Attacks of marked arm flexion posturing, either paroxysmal or as episodic exacerbation of mild pre-existent dystonia, are a characteristic clue to ATP1A3-related disease.

An Option to Consider for Alternating Hemiplegia of Childhood: Aripiprazole

Alternating hemiplegia of childhood (AHC) is an infrequent neurological disorder characterized by recurrent transient attacks of hemiplegia that last minutes to days and impress either side of the body, dystonic or tonic attacks, and nystagmus. Cognitive or neurological deficits with progressive course are another findings. Epileptic seizures may occur in some patients. We report the medical treatment in a case of AHC in a-12-year-old male patient with convulsions. The patient did not respond to available therapies for AHC, except for aripiprazole. After the initiation of aripiprazole therapy, duration and frequency of hemiplegia episodes were decreased. Also, he is currently seizure-free with topiramate treatment for 3 months. On follow-up, a compound heterozygous ATP1A3 mutation c.868C > T (p.R290C)/c.684 + 1G > A was determined. Aripiprazole may reduce the attacks of AHC, which are resistant to other available therapies.

Early-onset encephalopathy with paroxysmal movement disorders and epileptic seizures without hemiplegic attacks: About three children with novel ATP1A3 mutations

OBJECTIVE

Heterozygous mutations in the ATP1A3 gene are responsible for various neurological disorders, ranging from early-onset alternating hemiplegia of childhood to adult-onset dystonia-parkinsonism. Next generation sequencing allowed the description of other phenotypes, including early-onset epileptic encephalopathy in two patients. We report on three more patients carrying ATP1A3 mutations with a close phenotype and discuss the relationship of this phenotype to alternating hemiplegia of childhood.

METHODS

The patients' DNA underwent next generation sequencing. A retrospective analysis of clinical case records is reported.

RESULTS

Each of the three patients had an unreported heterozygous de novo sequence variant in ATP1A3. These patients shared a similar phenotype characterized by early-onset attacks of movement disorders, some of which proved to be epileptic, and severe developmental delay. (Hemi)plegic attacks had not been considered before genetic testing.

SIGNIFICANCE

Together with the two previously reported cases, our patients confirm that ATP1A3 mutations are associated with a phenotype combining features of early-onset encephalopathy, epilepsy and dystonic fits, as in the most severe forms of alternating hemiplegia of childhood, but in which (hemi)plegic attacks are absent or only suspected retrospectively.

ATP1A3-related disorders: An update

Alternating Hemiplegia of Childhood (AHC), Rapid-onset Dystonia Parkinsonism (RDP) and CAPOS syndrome (cerebellar ataxia, areflexia, pes cavus, optic atrophy, and sensorineural hearing loss) are three distinct, yet partially overlapping clinical syndromes that have long been thought to be allelic disorders. From 2004 to 2012, both autosomal dominant and de novo mutations in ATP1A3 have been detected in patients affected by these three conditions. Growing evidence suggests that AHC, RDP and CAPOS syndrome are part of a large and continuously expanding clinical spectrum and share some recurrent clinical features, such as abrupt-onset, asymmetric anatomical distribution and the presence of triggering factors, which are highly suggestive of ATP1A3 mutations. In this review, we will highlight the main clinical and genetic features of ATP1A3-related disorders focussing on shared and distinct features that can be helpful in clinical practice to individuate mutation carriers.

The CAPOS mutation in ATP1A3 alters Na/K-ATPase function and results in auditory neuropathy which has implications for management

Cerebellar ataxia, areflexia, pes cavus, optic atrophy and sensorineural hearing impairment (CAPOS) is a rare clinically distinct syndrome caused by a single dominant missense mutation, c.2452G>A, p.Glu818Lys, in ATP1A3, encoding the neuron-specific alpha subunit of the Na+/K+-ATPase α3. Allelic mutations cause the neurological diseases rapid dystonia Parkinsonism and alternating hemiplegia of childhood, disorders which do not encompass hearing or visual impairment. We present detailed clinical phenotypic information in 18 genetically confirmed patients from 11 families (10 previously unreported) from Denmark, Sweden, UK and Germany indicating a specific type of hearing impairment-auditory neuropathy (AN). All patients were clinically suspected of CAPOS and had hearing problems. In this retrospective analysis of audiological data, we show for the first time that cochlear outer hair cell activity was preserved as shown by the presence of otoacoustic emissions and cochlear microphonic potentials, but the auditory brainstem responses were grossly abnormal, likely reflecting neural dyssynchrony. Poor speech perception was observed, especially in noise, which was beyond the hearing level obtained in the pure tone audiograms in several of the patients presented here. Molecular modelling and in vitro electrophysiological studies of the specific CAPOS mutation were performed. Heterologous expression studies of α3 with the p.Glu818Lys mutation affects sodium binding to, and release from, the sodium-specific site in the pump, the third ion-binding site. Molecular dynamics simulations confirm that the structure of the C-terminal region is affected. In conclusion, we demonstrate for the first time evidence for auditory neuropathy in CAPOS syndrome, which may reflect impaired propagation of electrical impulses along the spiral ganglion neurons. This has implications for diagnosis and patient management. Auditory neuropathy is difficult to treat with conventional hearing aids, but preliminary improvement in speech perception in some patients suggests that cochlear implantation may be effective in CAPOS patients.

An Infant With Epilepsy and Recurrent Hemiplegia due to Compound Heterozygous Variants in ATP1A2

BACKGROUND

Pathogenic heterozygous variants in the ATP1A2 gene have most commonly been associated with familial hemiplegic migraine. However, a wide spectrum of phenotypes that include alternating hemiplegia of childhood and epilepsy have been described.

PATIENT DESCRIPTION

We describe a boy who presented at age three months with a complex phenotype that included epilepsy, nonepileptic paroxysmal events, and recurrent hemiplegia. Magnetic resonance imaging demonstrated unilateral cortical edema during a severe episode of hemiplegia that was followed by a persistent mild hemiparesis.

RESULTS

Whole-exome sequencing identified a previously reported ATP1A2 missense variant (p.Arg548Cys) classified as pathogenic and a novel missense variant (p.Arg1008Trp) classified as a variant of uncertain significance. After this genetic diagnosis, treatment with flunarizine was initiated and no further episodes of hemiplegia have occurred.

CONCLUSIONS

This is only the second report of compound heterozygosity of the ATP1A2 gene. It demonstrates the spectrum of paroxysmal neurological events that can arise as a result of ATP1A2 variants, with unique features overlapping alternating hemiplegia of childhood, hemiplegic migraine, and epilepsy. This child illustrates the diagnostic challenges that these disorders can present and the importance of genetic diagnosis in guiding management.

Fever-Induced Paroxysmal Weakness and Encephalopathy, a New Phenotype of ATP1A3 Mutation

BACKGROUND

We identified a group of patients with ATP1A3 mutations at residue 756 who display a new phenotype, distinct from alternating hemiplegia of childhood, rapid-onset dystonia-parkinsonism, and cerebellar ataxia, areflexia, pes cavus, optic atrophy, sensorineural hearing loss syndromes.

METHODS

Four patients with c.2267G>A (R756H) mutations from two families and two patients with c.2267G>T (R756L) mutations from one family are described and compared with the previously reported patients with mutations resulting in R756H and R756C protein variants.

RESULTS

Patients with ATP1A3 R756H have onset in childhood of infrequent, fever-triggered paroxysms of encephalopathy and weakness with slowly improving but persistent deficits. Motor findings of weakness are mostly generalized, and patients may also have bulbar or oculomotor problems. Longer-term outcomes range from mild motor apraxia with near-normal function to persistent dysphagia, dysarthria, cognitive deficit, motor apraxia, and inability to walk because of ataxia. Patients with ATP1A3 R756L have a similar phenotype that includes paroxysmal, stepwise progression of ataxia associated with infections.

CONCLUSIONS

ATP1A3 mutations affecting residue 756 result in a clinical syndrome, separate from those associated with previously described ATP1A3 mutations, which consists chiefly of fever-induced paroxysmal weakness and encephalopathy (FIPWE). Patients with R756L and R756C protein variants display more prominent ataxia, overlapping with the relapsing encephalopathy with cerebellar ataxia syndrome previously described in a patient with the c.2266C>T (R756C) mutation. All patients reported with mutations at residue 756 to date have had a similar episodic course and clinical features. Patients with mutations of ATP1A3 residue 756 appear to have a distinct clinical phenotype compared with patients with other ATP1A3 mutations, with fever-induced encephalopathy as key differentiating feature.

Hypothermia-induced dystonia and abnormal cerebellar activity in a mouse model with a single disease-mutation in the sodium-potassium pump

Mutations in the neuron-specific α₃ isoform of the Na⁺/K⁺-ATPase are found in patients suffering from Rapid onset Dystonia Parkinsonism and Alternating Hemiplegia of Childhood, two closely related movement disorders. We show that mice harboring a heterozygous hot spot disease mutation, D801Y (α₃^+/D801Y), suffer abrupt hypothermia-induced dystonia identified by electromyographic recordings. Single-neuron in vivo recordings in awake α₃^+/D801Y mice revealed irregular firing of Purkinje cells and their synaptic targets, the deep cerebellar nuclei neurons, which was further exacerbated during dystonia and evolved into abnormal high-frequency burst-like firing. Biophysically, we show that the D-to-Y mutation abolished pump-mediated Na⁺/K⁺ exchange, but allowed the pumps to bind Na⁺ and become phosphorylated. These findings implicate aberrant cerebellar activity in α₃ isoform-related dystonia and add to the functional understanding of the scarce and severe mutations in the α₃ isoform Na⁺/K⁺-ATPase.

The neurological spectrum associated with mutations in the ATP1A3 gene, encoding the α₃ isoform of the Na⁺/K⁺-ATPase, is complex and still poorly understood. To elucidate the disease-specific pathophysiology, we examined a mouse model harboring the mutation D801Y, which was originally found in a patient with Rapid onset Dystonia Parkinsonism, but recently, also in a patient with Alternating Hemiplegia of Childhood. We found that this model exhibited motor deficits and developed dystonia when exposed to a drop in body temperature. Cerebellar in vivo recordings in awake mice revealed irregular firing of Purkinje cells and their synaptic targets, the deep cerebellar nuclei neurons, which was further exacerbated and evolved into abnormal high-frequency burst firing during dystonia. The development of specific neurological features within the ATP1A3 mutation spectrum, such as dystonia, are thought to reflect the functional consequences of each mutation, thus to investigate the consequence of the D801Y mutations we characterized mutated D-to-Y Na⁺/K⁺-ATPases expressed in Xenopus oocytes. These in vitro studies showed that the D-to-Y mutation abolishes pump-mediated Na⁺/K⁺ exchange, but still allows the pumps to bind Na⁺ and become phosphorylated, trapping them in conformations that instead support proton influx.

Diagnosis and Treatment of Alternating Hemiplegia of Childhood

OPINION STATEMENT

The diagnosis and treatment of patients with Alternating Hemiplegia of Childhood (AHC) and related disorders should be provided by a multidisciplinary team experienced with the spectrum of presentations of this disease, with its related disorders, with its complex and fluctuating manifestations, and with cutting edge advances occurring in the field. Involvement in research to advance the understanding of this disease and partnership with international collaborators and family organizations are also important. An example of such an approach is that of The Duke AHC and Related Disorders Multi-Disciplinary Clinic and Program, which, in partnership with the Cure AHC Foundation, has developed and applied this approach to patients seen since early 2013. The program provides comprehensive care and education directly to AHC patients and their families and collaborates with referring physicians on the care of patients with AHC whether evaluated at Duke clinics or not. It also is involved in clinical and basic research and in collaborations with other International AHC Research Consortium (IAHCRC) partners. The clinic is staffed with physicians and experts from Neurology, Cardiology, Child Behavioral Health, Medical Genetics, Neurodevelopment, Neuropsychology, Nursing, Physical and Occupational Therapies, Psychiatry, Sleep Medicine, and Speech/Language Pathology. Patients are seen either for full comprehensive evaluations that last several days or for targeted evaluations with one or few appointments.

Research conference summary from the 2014 International Task Force on ATP1A3-Related Disorders

Objective:

ATP1A3-related neurologic disorders encompass a broad range of phenotypes that extend well beyond initial phenotypic criteria associated with alternating hemiplegia of childhood (AHC) and rapid-onset dystonia parkinsonism.

Methods:

In 2014, the Alternating Hemiplegia of Childhood Foundation hosted a multidisciplinary workshop intended to address fundamental challenges surrounding the diagnosis and management of individuals with ATP1A3-related disorders.

Results:

Workshop attendees were charged with the following: (1) to achieve consensus on expanded diagnostic criteria to facilitate the identification of additional patients, intended to supplement existing syndrome-specific diagnostic paradigms; (2) to standardize definitions for the broad range of paroxysmal manifestations associated with AHC to disseminate to families; (3) to create clinical recommendations for common recurrent issues facing families and medical care providers; (4) to review data related to the death of individuals in the Alternating Hemiplegia of Childhood Foundation database to guide future efforts in identifying at-risk subjects and potential preventative measures; and (5) to identify critical gaps where we most need to focus national and international research efforts.

Conclusions:

This report summarizes recommendations of the workshop committee, highlighting the key phenotypic features to facilitate the diagnosis of possible ATP1A3 mutations, providing recommendations for genetic testing, and outlining initial acute management for common recurrent clinical conditions, including epilepsy.

Progressive Brain Atrophy in Alternating Hemiplegia of Childhood

Background

Alternating hemiplegia of childhood (AHC) is a rare neurodevelopmental disorder that includes involuntary movements, paroxysmal symptoms, and various severities of nonparoxysmal symptoms.

Objective

To investigate the occurrence of structural brain abnormalities in patients with AHC during clinical courses.

Methods

Conventional brain magnetic resonance imaging findings and clinical courses were retrospectively investigated in 14 patients with AHC confirmed by ATP1A3 mutations.

Results

Progressive frontal dominant cerebral, diffuse cerebellar cortical, and severe hippocampal atrophy were observed in seven patients with irreversible severe motor and intellectual deterioration. All of these seven patients exhibited status epilepticus and required transient respiratory care. Isolated diffuse cerebellar cortical atrophy was observed in two adult patients with mild motor regression. Five patients without apparent deterioration displayed almost normal brain findings.

Conclusions

The areas of atrophy were consistent with the areas of increased expression of the Na⁺/K⁺-ATPase α3 subunit encoded by ATP1A3. Some of paroxysmal and nonparoxysmal neurological symptoms are considered as related to the areas of brain atrophy.

Mosaicism in ATP1A3-related disorders: not just a theoretical risk

Mutations in ATP1A3 are involved in a large spectrum of neurological disorders, including rapid onset dystonia parkinsonism (RDP), alternating hemiplegia of childhood (AHC), and cerebellar ataxia, pes cavus, optic atrophy, and sensorineural hearing loss (CAPOS), with recent descriptions of overlapping phenotypes. In AHC, a few familial cases of autosomal dominant inheritance have been reported, along with cases of de novo sporadic mutations. In contrast, autosomal dominant inheritance has frequently been associated with RDP and CAPOS. Here, we report on two unrelated sets of full siblings with ATP1A3 mutations, (c.2116G>A) p. Gly706Arg in the first family, and (c.2266C>T) p. Arg756Cys in the second family, presenting with familial recurrence of the disease. Both families displayed parental germline mosaicism. In the first family, the brother and sister presented with severe intellectual deficiency, early onset pharmacoresistant epilepsy, ataxia, and autistic features. In the second family, both sisters demonstrated severe encephalopathy with ataxia and dystonia following a regression episode during a febrile episode during infancy. To our knowledge, mosaicism has not previously been reported in ATP1A3-related disorders. This report, therefore, provides evidence that germline mosaicism for ATP1A3 mutations is a likely explanation for familial recurrence and should be considered during recurrence risk counseling for families of children with ATP1A3-related disorders.

Cognitive deficits caused by a disease-mutation in the α3 Na(+)/K(+)-ATPase isoform

The Na⁺/K⁺-ATPases maintain Na⁺ and K⁺ electrochemical gradients across the plasma membrane, a prerequisite for electrical excitability and secondary transport in neurons. Autosomal dominant mutations in the human ATP1A3 gene encoding the neuron-specific Na⁺/K⁺-ATPase α₃ isoform cause different neurological diseases, including rapid-onset dystonia-parkinsonism (RDP) and alternating hemiplegia of childhood (AHC) with overlapping symptoms, including hemiplegia, dystonia, ataxia, hyperactivity, epileptic seizures, and cognitive deficits. Position D801 in the α₃ isoform is a mutational hotspot, with the D801N, D801E and D801V mutations causing AHC and the D801Y mutation causing RDP or mild AHC. Despite intensive research, mechanisms underlying these disorders remain largely unknown. To study the genotype-to-phenotype relationship, a heterozygous knock-in mouse harboring the D801Y mutation (α₃^+/D801Y) was generated. The α₃^+/D801Y mice displayed hyperactivity, increased sensitivity to chemically induced epileptic seizures and cognitive deficits. Interestingly, no change in the excitability of CA1 pyramidal neurons in the α₃^+/D801Y mice was observed. The cognitive deficits were rescued by administration of the benzodiazepine, clonazepam, a GABA positive allosteric modulator. Our findings reveal the functional significance of the Na⁺/K⁺-ATPase α₃ isoform in the control of spatial learning and memory and suggest a link to GABA transmission.

Insights into the Pathology of the α3 Na(+)/K(+)-ATPase Ion Pump in Neurological Disorders; Lessons from Animal Models

The transmembrane Na(+)-/K(+) ATPase is located at the plasma membrane of all mammalian cells. The Na(+)-/K(+) ATPase utilizes energy from ATP hydrolysis to extrude three Na(+) cations and import two K(+) cations into the cell. The minimum constellation for an active Na(+)-/K(+) ATPase is one alpha (α) and one beta (β) subunit. Mammals express four α isoforms (α1-4), encoded by the ATP1A1-4 genes, respectively. The α1 isoform is ubiquitously expressed in the adult central nervous system (CNS) whereas α2 primarily is expressed in astrocytes and α3 in neurons. Na(+) and K(+) are the principal ions involved in action potential propagation during neuronal depolarization. The α1 and α3 Na(+)-/K(+) ATPases are therefore prime candidates for restoring neuronal membrane potential after depolarization and for maintaining neuronal excitability. The α3 isoform has approximately four-fold lower Na(+) affinity compared to α1 and is specifically required for rapid restoration of large transient increases in [Na(+)]i. Conditions associated with α3 deficiency are therefore likely aggravated by suprathreshold neuronal activity. The α3 isoform been suggested to support re-uptake of neurotransmitters. These processes are required for normal brain activity, and in fact autosomal dominant de novo mutations in ATP1A3 encoding the α3 isoform has been found to cause the three neurological diseases Rapid Onset Dystonia Parkinsonism (RDP), Alternating Hemiplegia of Childhood (AHC), and Cerebellar ataxia, areflexia, pes cavus, optic atrophy, and sensorineural hearing loss (CAPOS). All three diseases cause acute onset of neurological symptoms, but the predominant neurological manifestations differ with particularly early onset of hemiplegic/dystonic episodes and mental decline in AHC, ataxic encephalopathy and impairment of vision and hearing in CAPOS syndrome and late onset of dystonia/parkinsonism in RDP. Several mouse models have been generated to study the in vivo consequences of Atp1a3 modulation. The different mice show varying degrees of hyperactivity, gait problems, and learning disability as well as stress-induced seizures. With the advent of several Atp1a3-gene or chemically modified animal models that closely phenocopy many aspects of the human disorders, we will be able to reach a much better understanding of the etiology of RDP, AHC, and CAPOS syndrome.

The Influence of Na(+), K(+)-ATPase on Glutamate Signaling in Neurodegenerative Diseases and Senescence

Decreased Na(+), K(+)-ATPase (NKA) activity causes energy deficiency, which is commonly observed in neurodegenerative diseases. The NKA is constituted of three subunits: α, β, and γ, with four distinct isoforms of the catalytic α subunit (α1-4). Genetic mutations in the ATP1A2 gene and ATP1A3 gene, encoding the α2 and α3 subunit isoforms, respectively can cause distinct neurological disorders, concurrent to impaired NKA activity. Within the central nervous system (CNS), the α2 isoform is expressed mostly in glial cells and the α3 isoform is neuron-specific. Mutations in ATP1A2 gene can result in familial hemiplegic migraine (FHM2), while mutations in the ATP1A3 gene can cause Rapid-onset dystonia-Parkinsonism (RDP) and alternating hemiplegia of childhood (AHC), as well as the cerebellar ataxia, areflexia, pescavus, optic atrophy and sensorineural hearing loss (CAPOS) syndrome. Data indicates that the central glutamatergic system is affected by mutations in the α2 isoform, however further investigations are required to establish a connection to mutations in the α3 isoform, especially given the diagnostic confusion and overlap with glutamate transporter disease. The age-related decline in brain α2∕3 activity may arise from changes in the cyclic guanosine monophosphate (cGMP) and cGMP-dependent protein kinase (PKG) pathway. Glutamate, through nitric oxide synthase (NOS), cGMP and PKG, stimulates brain α2∕3 activity, with the glutamatergic N-methyl-D-aspartate (NMDA) receptor cascade able to drive an adaptive, neuroprotective response to inflammatory and challenging stimuli, including amyloid-β. Here we review the NKA, both as an ion pump as well as a receptor that interacts with NMDA, including the role of NKA subunits mutations. Failure of the NKA-associated adaptive response mechanisms may render neurons more susceptible to degeneration over the course of aging.

Spontaneously Fluctuating Motor Cortex Excitability in Alternating Hemiplegia of Childhood: A Transcranial Magnetic Stimulation Study

Background

Alternating hemiplegia of childhood is a very rare and serious neurodevelopmental syndrome; its genetic basis has recently been established. Its characteristic features include typically-unprovoked episodes of hemiplegia and other transient or more persistent neurological abnormalities.

Methods

We used transcranial magnetic stimulation to assess the effect of the condition on motor cortex neurophysiology both during and between attacks of hemiplegia. Nine people with alternating hemiplegia of childhood were recruited; eight were successfully tested using transcranial magnetic stimulation to study motor cortex excitability, using single and paired pulse paradigms. For comparison, data from ten people with epilepsy but not alternating hemiplegia, and ten healthy controls, were used.

Results

One person with alternating hemiplegia tested during the onset of a hemiplegic attack showed progressively diminishing motor cortex excitability until no response could be evoked; a second person tested during a prolonged bilateral hemiplegic attack showed unusually low excitability. Three people tested between attacks showed asymptomatic variation in cortical excitability, not seen in controls. Paired pulse paradigms, which probe intracortical inhibitory and excitatory circuits, gave results similar to controls.

Conclusions

We report symptomatic and asymptomatic fluctuations in motor cortex excitability in people with alternating hemiplegia of childhood, not seen in controls. We propose that such fluctuations underlie hemiplegic attacks, and speculate that the asymptomatic fluctuation we detected may be useful as a biomarker for disease activity.

ATP1A3 Mutation in Adult Rapid-Onset Ataxia

A 21-year old male presented with ataxia and dysarthria that had appeared over a period of months. Exome sequencing identified a de novo missense variant in ATP1A3, the gene encoding the α3 subunit of Na,K-ATPase. Several lines of evidence suggest that the variant is causative. ATP1A3 mutations can cause rapid-onset dystonia-parkinsonism (RDP) with a similar age and speed of onset, as well as severe diseases of infancy. The patient's ATP1A3 p.Gly316Ser mutation was validated in the laboratory by the impaired ability of the expressed protein to support the growth of cultured cells. In a crystal structure of Na,K-ATPase, the mutated amino acid was directly apposed to a different amino acid mutated in RDP. Clinical evaluation showed that the patient had many characteristics of RDP, however he had minimal fixed dystonia, a defining symptom of RDP. Successive magnetic resonance imaging (MRI) revealed progressive cerebellar atrophy, explaining the ataxia. The absence of dystonia in the presence of other RDP symptoms corroborates other evidence that the cerebellum contributes importantly to dystonia pathophysiology. We discuss the possibility that a second de novo variant, in ubiquilin 4 (UBQLN4), a ubiquitin pathway component, contributed to the cerebellar neurodegenerative phenotype and differentiated the disease from other manifestations of ATP1A3 mutations. We also show that a homozygous variant in GPRIN1 (G protein-regulated inducer of neurite outgrowth 1) deletes a motif with multiple copies and is unlikely to be causative.

Transgenic rescue of phenotypic deficits in a mouse model of alternating hemiplegia of childhood

Missense mutations in ATP1A3 encoding Na⁺,K⁺-ATPase α3 are the primary cause of alternating hemiplegia of childhood (AHC). Most ATP1A3 mutations in AHC lie within a cluster in or near transmembrane α-helix TM6, including I810N that is also found in the Myshkin mouse model of AHC. These mutations all substantially reduce Na⁺,K⁺-ATPase α3 activity. Herein, we show that Myshkin mice carrying a wild-type Atp1a3 transgene that confers a 16 % increase in brain-specific total Na⁺,K⁺-ATPase activity show significant phenotypic improvements compared with non-transgenic Myshkin mice. Interventions to increase the activity of wild-type Na⁺,K⁺-ATPase α3 in AHC patients should be investigated further.

Clinical profile of patients with ATP1A3 mutations in Alternating Hemiplegia of Childhood-a study of 155 patients

BACKGROUND

Mutations in the gene ATP1A3 have recently been identified to be prevalent in patients with alternating hemiplegia of childhood (AHC2). Based on a large series of patients with AHC, we set out to identify the spectrum of different mutations within the ATP1A3 gene and further establish any correlation with phenotype.

METHODS

Clinical data from an international cohort of 155 AHC patients (84 females, 71 males; between 3 months and 52 years) were gathered using a specifically formulated questionnaire and analysed relative to the mutational ATP1A3 gene data for each patient.

RESULTS

In total, 34 different ATP1A3 mutations were detected in 85 % (132/155) patients, seven of which were novel. In general, mutations were found to cluster into five different regions. The most frequent mutations included: p.Asp801Asn (43 %; 57/132), p.Glu815Lys (16 %; 22/132), and p.Gly947Arg (11 %; 15/132). Of these, p.Glu815Lys was associated with a severe phenotype, with more severe intellectual and motor disability. p.Asp801Asn appeared to confer a milder phenotypic expression, and p.Gly947Arg appeared to correlate with the most favourable prognosis, compared to the other two frequent mutations. Overall, the comparison of the clinical profiles suggested a gradient of severity between the three major mutations with differences in intellectual (p = 0.029) and motor (p = 0.039) disabilities being statistically significant. For patients with epilepsy, age at onset of seizures was earlier for patients with either p.Glu815Lys or p.Gly947Arg mutation, compared to those with p.Asp801Asn mutation (p < 0.001). With regards to the five mutation clusters, some clusters appeared to correlate with certain clinical phenotypes. No statistically significant clinical correlations were found between patients with and without ATP1A3 mutations.

CONCLUSIONS

Our results, demonstrate a highly variable clinical phenotype in patients with AHC2 that correlates with certain mutations and possibly clusters within the ATP1A3 gene. Our description of the clinical profile of patients with the most frequent mutations and the clinical picture of those with less common mutations confirms the results from previous studies, and further expands the spectrum of genotype-phenotype correlations. Our results may be useful to confirm diagnosis and may influence decisions to ensure appropriate early medical intervention in patients with AHC. They provide a stronger basis for the constitution of more homogeneous groups to be included in clinical trials.