ATP1A3 mutations in infants: a new rapid-onset dystonia-Parkinsonism phenotype characterized by motor delay and ataxia

Definition and Classification of Dystonia

Dystonia is a movement disorder with varied clinical features and diverse etiologies. Here we present a revision of the 2013 consensus definition and classification of dystonia in light of subsequent publications and experience with its application during the last decade. A panel of movement disorder specialists with expertise in dystonia reviewed the original document and proposed some revision. There was broad consensus to retain the definition of dystonia with only minor clarifications to the wording. Dystonia is defined as a movement disorder characterized by sustained or intermittent abnormal movements, postures, or both. Dystonic movements and postures are typically patterned and repetitive and may be tremulous or jerky. They are often initiated or worsened by voluntary action and frequently associated with overflow movements. The two-axis structure for classification of the many different presentations of dystonia was also retained, with some revision. Axis I summarizes key clinical characteristics of dystonia, including age at onset, family history, body distribution, temporal dimensions, phenomenology, and whether dystonia is isolated or combined with other neurological or medical problems. Axis II organizes information regarding its etiological basis, including genetic, acquired, and anatomical, and common disease mechanisms. This consensus provides an update to the original definition and classification of dystonia with the aim of facilitating its clinical recognition and management. The revision retains the essence of the original proposal and aims particularly to provide a structure facilitating a uniform implementation. © 2025 The Author(s). Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

Spinocerebellar Ataxias: Phenotypic Spectrum of PolyQ versus Non-Repeat Expansion Forms

Spinocerebellar ataxias (SCA) are most frequently due to (CAG)n (coding for polyglutamine, polyQ) expansions and, less so, to expansion of other oligonucleotide repeats (non-polyQ) or other type of variants (non-repeat expansion SCA). In this study we compared polyQ and non-repeat expansion SCA, in a cohort of patients with hereditary ataxia followed at a tertiary hospital. From a prospective study, 88 patients (51 families) with SCA were selected, 74 (40 families) of whom genetically diagnosed. Thirty-eight patients (51.4%, 19 families) were confirmed as having a polyQ (no other repeat-expansions were identified) and 36 (48.6%, 21 families) a non-repeat expansion SCA. Median age-at-onset was 39.5 [30.0-45.5] for polyQ and 7.0 years [1.00-21.50] for non-repeat expansion SCA. PolyQ SCA were associated with cerebellar onset, and non-repeat expansion forms with non-cerebellar onset. Time to diagnosis was longer for non-repeat expansion SCA. The most common polyQ SCA were Machado-Joseph disease (MJD/SCA3) (73.7%) and SCA2 (15.8%); whereas in non-repeat expansion SCA ATX-CACNA1A (14.3%), ATP1A3-related ataxia, ATX-ITPR1, ATX/HSP-KCNA2, and ATX-PRKCG (9.5% each) predominated. Disease duration (up to inclusion) was significantly higher in non-repeat expansion SCA, but the difference in SARA score was not statistically significant. Cerebellar peduncles and pons atrophy were more common in polyQ ataxias, as was axonal neuropathy. SCA had a wide range of genetic etiology, age-at-onset and presentation. Proportion of polyQ and non-repeat expansion SCA was similar; the latter had a higher genetic heterogeneity. While polyQ ataxias were typically linked to cerebellar onset in adulthood, non-repeat expansion forms associated with early onset and non-cerebellar presentations.

ATP1A3 Disease Spectrum Includes Paroxysmal Weakness and Encephalopathy Not Triggered by Fever

Background and Objectives

Heterozygous pathogenic variants in ATP1A3, which encodes the catalytic alpha subunit of neuronal Na+/K+-ATPase, cause primarily neurologic disorders with widely variable features that can include episodic movement deficits. One distinctive presentation of ATP1A3-related disease is recurrent fever-triggered encephalopathy. This can occur with generalized weakness and/or ataxia and is described in the literature as relapsing encephalopathy with cerebellar ataxia. This syndrome displays genotype-phenotype correlation with variants at p.R756 causing temperature sensitivity of ATP1A3. We report clinical and in vitro functional evidence for a similar phenotype not triggered by fever but associated with protein loss-of-function.

Methods

We describe the phenotype of an individual with de novo occurrence of a novel heterozygous ATP1A3 variant, NM_152296.5:c.388_390delGTG; p.(V130del). We confirmed the pathogenicity of p.V130del by cell survival complementation assay in HEK293 cells and then characterized its functional impact on enzymatic ion transport and extracellular sodium binding by two-electrode voltage clamp electrophysiology in Xenopus oocytes. To determine whether variant enzymes reach the cell surface, we surface-biotinylated oocytes expressing N-tagged ATP1A3.

Results

The proband is a 7-year-old boy who has had 2 lifetime episodes of paroxysmal weakness, encephalopathy, and ataxia not triggered by fever. He had speech regression and intermittent hand tremors after the second episode but otherwise spontaneously recovered after episodes and is at present developmentally appropriate. The p.V130del variant was identified on clinical trio exome sequencing, which did not reveal any other variants possibly associated with the phenotype. p.V130del eliminated ATP1A3 function in cell survival complementation assay. In Xenopus oocytes, p.V130del variant Na+/K+-ATPases showed complete loss of ion transport activity and marked abnormalities of extracellular Na+ binding at room temperature. Despite this clear loss-of-function effect, surface biotinylation under the same conditions revealed that p.V130del variant enzymes were still present at the oocyte's cell membrane.

Discussion

This individual's phenotype expands the clinical spectrum of ATP1A3-related recurrent encephalopathy to include presentations without fever-triggered events. The total loss of ion transport function with p.V130del, despite enzyme presence at the cell membrane, indicates that haploinsufficiency can cause relatively mild phenotypes in ATP1A3-related disease.

Evaluating Dysfunction in Fever-Induced Paroxysmal Weakness and Encephalopathy

Heterozygous variants in the ATP1A3 gene are linked to well-known neurological phenotypes. There has been growing evidence for a separate phenotype associated with variants in residue Arg756-fever-induced paroxysmal weakness and encephalopathy (FIPWE) or relapsing encephalopathy with cerebellar ataxia (RECA). With only about 20 cases being reported, the clinical features associated with mutations at Arg756 have not been fully elucidated. We report a case of FIPWE with a p.Arg756Cys change in the ATP1A3 gene and a comparison of the clinical features, including electrophysiological examination, with previous cases. The 3-year-old male patient had normal psychomotor development, presenting with recurrent symptoms of generalized hypotonia with loss of gait, mutism, and dystonic movements only during febrile illnesses since 19 months of age. At 2.7 years of age, a third neurological decompensation episode occurred, during which electroencephalography (EEG) did not reveal high voltage slow waves or epileptiform discharge. Nerve conduction studies (NCS) also did not show latency delay or amplitude reduction. ATP1A3 exon sequencing showed a heterozygous p.Arg756Cys mutation. While the patient experienced repeated encephalopathy-like episodes, including severe hypotonia during febrile illness, EEG and NCS did not reveal any obvious abnormalities. These electrophysiological findings may represent an opportunity to suspect FIPWE and RECA.

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.

ATP1A3 mutation in rapid-onset dystonia parkinsonism: New data and genotype-phenotype correlation analysis

Background

Rapid-onset dystonia parkinsonism (RDP) is a rare disease caused by ATP1A3 mutation with considerable clinical heterogeneity. Increased knowledge of RDP could be beneficial in its early diagnosis and treatment.

Objective

This study aimed to summarize the gene mutation spectrum of ATP1A3 associated with RDP, and to explore the correlation of ATP1A3 variants with RDP clinical phenotypes.

Methods

In this study, we reported two RDP patients from a family with a novel inherited ATP1A3 variant. Then, we reviewed and analyzed the available literature in English focused on ATP1A3-causative RDP. A total of 35 articles covering 15 families (59 patients) and 36 sporadic RDP cases were included in our analysis.

Results

The variant A813V (2438C>T) in ATP1A3 found in our cases was a novel mutant. Delays in diagnosis were common, with a mean delay time of 14 years. ATP1A3 had distinct RDP-related mutation hotspots, which consisted of exon8, 14, 17, and 18, and the most frequently occurring variants were T613M and I578S. Approximately 74.5% of patients have specific triggers before disease onset, and 82.1% of RDPs have stable symptoms within 1 month. The incidence rates of dystonia and bradykinesia are 100 and 88.1%, respectively. The onset site varied and exhibited a rostrocaudal gradient distribution pattern in 45% of patients with RDP. Approximately 63.6% of patients had mild improvement after receiving comprehensive interventions, especially in gait disturbance amelioration.

Conclusion

In patients with acute and unexplained dystonia or bradykinesia, gene screening on ATP1A3 should be timely performed. When a diagnosis has been made, treatments that may be effective are to be attempted. Our study would be helpful for the early diagnosis and treatment of ATP1T3-related RDP.

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.

Chinese patients with p.Arg756 mutations of ATP1A3: Clinical manifestations, treatment, and follow-up

Importance

The phenotypes of ATP1A3 gene mutations are diverse. Relapsing encephalopathy with cerebellar ataxia and fever-induced paroxysmal weakness and encephalopathy (FIPWE) are considered non-classical phenotypes caused by p.Arg756 mutations of ATP1A3.

Objective

To summarize the clinical manifestations, treatment, and follow-up of Chinese patients with p.Arg756 mutations of ATP1A3.

Methods

We analyzed the clinical features, treatment, and genotypes of eight children with p.Arg756 mutations of ATP1A3 who were treated in Beijing Children's Hospital from January 2014 to December 2019.

Results

Eight patients (six boys and two girls) were included; seven had been misdiagnosed with encephalitis. The age of onset ranged from 0.8 to 4.5 years. All patients had encephalopathy and had at least one episode of FIPWE. Cerebellar ataxia was present in nine episodes. Reversible splenial lesions of the corpus callosum were found in two patients in the acute phase. Three types of heterozygous ATP1A3 mutations were found: c.2267G > T (p.R756L) (patient 3 [P3]), c.2266C > T (p.R756C) (P2 and P4), and c.2267G > A (p.R756H) (P1, P5, P6, P7, and P8). Six mutations were de novo; two mutations were inherited. Both patients with p.R756C and one patient (P7) with p.R756H had four episodes of severe ataxia as the main manifestations. However, in the other three episodes, limb weakness was more prominent than ataxia. P5 with p.R756H exhibited overlap with FIPWE and rapid-onset dystonia-parkinsonism.

Interpretation

Acute encephalopathy followed by febrile disease was characteristic of the disease in patients with p.Arg756 mutations of ATP1A3. However, the weakness and ataxia were variable. Phenotypic crossover and overlap were observed among these patients.

Two novel heterozygous variants in ATP1A3 cause movement disorders

Variants in ATP1A3 cause neuropsychiatric disorders, especially those characterized by movement disorders. In this study, we performed whole exome sequencing for two patients with movement disorders and identified two novel heterozygous ATP1A3 variants, a missense c.2408G>A variant and an indel c.2672_2688+10delinsCAG variant. The unique indel variant occurred at the exon-intron boundary at the 3' end of exon 19, and mRNA analysis revealed that this variant caused in-frame indel alteration at the Ser891_Trp896 residue.

A Rare Cause of Recurrent Febrile Encephalopathy in a Child: The Expanding Spectrum of ATP1A3 Mutations

ATP1A3 mutations have been recognized in infants and children presenting with a diverse group of neurological phenotypes, including rapid-onset dystonia parkinsonism (RDP), alternating hemiplegia of childhood (AHC), and cerebellar ataxia, areflexia, pes cavus, optic atrophy, and sensorineural hearing loss (CAPOS) syndrome. A new phenotype of fever-induced paroxysmal muscle weakness and encephalopathy (FIPWE) in patients with ATP1A3 mutations at c.2267G>A p residue 756H has been described most recently in few cases. Here, we report an additional case with an ATP1A3 mutation at c.2267G>A p residue 756H presenting with fever-induced paroxysmal muscle weakness and encephalopathy. To the best of our knowledge, this is the first reported case from the Middle East. This 18-month-old boy presented with recurrent, reversible fever-induced episodes of seizures, central hypotonia, areflexia, and developmental regression. The mainstay management for patients with ATP1A3 related diseases is symptomatic treatment as there is no specific proposed treatment. Aggressive management of febrile illness may be helpful in alleviating the symptoms.

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.

ATP1A3-related disorders in the differential diagnosis of acute brainstem and cerebellar dysfunction

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 all caused by mutations in the same gene: ATP1A3. Although initially they were considered separate disorders, recent evidence suggests a continuous clinical spectrum of ATP1A3-related disorders. At onset all these disorders can present with acute brainstem dysfunction triggered by a febrile illness. An infectious or autoimmune disorder is usually suspected. A genetic disorder is rarely considered in the first acute episode. We present three patients with ATP1A3 mutations: one patient with AHC, one patient with RDP, and one patient with CAPOS syndrome. We describe the acute onset and overlapping clinical features of these three patients with classical phenotypes. These cases highlight ATP1A3-related disorders as a possible cause of acute brainstem dysfunction with normal ancillary testing.

Atypical presentation of rapid-onset dystonia-parkinsonism in a toddler with a novel mutation in the ATP1A3 gene

ATP1A3 gene mutations can result in a spectrum of diseases with diverse neurological manifestations. One such disorder linked to this mutation is rapid-onset dystonia-parkinsonism (RDP), which manifests as dystonia with features of parkinsonism, such as tremors, rigidity, muscle spasms, and bulbar symptoms. Affected patients are typically adolescents or young adults, with symptoms occurring in a rostrocaudal pattern. We report a unique case of a 2-year-old child with an early onset, atypical presentation of RDP. In addition to motor developmental delay, he presented with muscle rigidity and mild asymmetric dystonia of the limbs, with the lower limbs being more affected than the upper limbs. Genetic sequencing of the child revealed a novel heterozygous autosomal dominant mutation of ATP1A3 gene c.173A>G (p. Tyr58Cys). This report highlights that RDP can present with atypical presentations in the paediatric population and adds to existing medical literature on the clinical spectrum of ATP1A3 genetic channelopathy.

Variants of ATP1A3 in residue 756 cause a separate phenotype of relapsing encephalopathy with cerebellar ataxia (RECA)-Report of two cases and literature review

Background

Variants in ATP1A3 cause well‐known phenotypes—alternating hemiplegia of childhood (AHC), rapid‐onset dystonia‐parkinsonism (RDP), cerebellar ataxia, areflexia, pes cavus, optic atrophy, sensorineural hearing loss (CAPOS), and severe early infantile epileptic encephalopathy. Recently, there has been growing evidence for genotype–phenotype correlations in the ATP1A3 variants, and a separate phenotype associated with variants in residue 756—two acronyms are proposed for the moment—FIPWE (fever‐induced paroxysmal weakness and encephalopathy) and RECA (relapsing encephalopathy with cerebellar ataxia).

Materials and Methods

Herein, we are describing two new pediatric cases with a p.Arg756His change in the ATP1A3 gene. Both patients have had more than one episode of a neurological decompensation triggered by fever with severe hypotonia and followed by ataxia. Thirty‐three cases from literature were analyzed to define and strengthen the genotype‐phenotype correlation of variants located in residue 756 (p.Arg756His, p.Arg756Cys, p.Arg756Leu).

Conclusions

Patients with a ATP1A3 variant in residue 756 are characterized by recurrent paroxysmal episodes of neurological decompensations triggered by fever, with severe hypotonia, ataxia, dysarthria, symptoms from the orofacial area (dysphagia, drooling) as well as with altered consciousness. Recovery is slow and usually not full with the persistent symptoms of cerebellar ataxia, dysarthria, dystonic and choreiform movements.

ATP1A3-Related Disorders: An Ever-Expanding Clinical Spectrum

The Na+/K+ ATPases are Sodium-Potassium exchanging pumps, with a heteromeric α-β-γ protein complex. The α3 isoform is required as a rescue pump, after repeated action potentials, with a distribution predominantly in neurons of the central nervous system. This isoform is encoded by the ATP1A3 gene. Pathogenic variants in this gene have been implicated in several phenotypes in the last decades. Carriers of pathogenic variants in this gene manifest neurological and non-neurological features in many combinations, usually with an acute onset and paroxysmal episodes triggered by fever or other factors. The first three syndromes described were: (1) rapid-onset dystonia parkinsonism; (2) alternating hemiplegia of childhood; and, (3) cerebellar ataxia, pes cavus, optic atrophy, and sensorineural hearing loss (CAPOS syndrome). Since their original description, an expanding number of cases presenting with atypical and overlapping features have been reported. Because of this, ATP1A3-disorders are now beginning to be viewed as a phenotypic continuum representing discrete expressions along a broadly heterogeneous clinical spectrum.

Rapid-onset dystonia-parkinsonism with ATP1A3 mutation and left lower limb paroxysmal dystonia

BACKGROUND

Rapid-onset dystonia-parkinsonism (RDP) is a disease characterized by an abrupt onset of dystonia accompanied by signs of parkinsonism and prominent bulbar symptoms.

CASE REPORT

We describe a case of a female patient, born after normal delivery, but diagnosed with mild intellectual disability at age 7. She presented with an abrupt onset of upper limb dystonia and bradykinesia without tremor in parkinsonism, as well as dysarthria and dysphagia caused by prominent bulbar symptoms, at age 9. She had normal findings on brain magnetic resonance imaging, electroencephalography, and blood examination but was diagnosed with a psychogenic disorder. At age 10, she developed left lower limb paroxysmal stiffness with pain, and at 14, she was hospitalized due to lasting paroxysmal symptoms. Whole-exome sequencing was performed for this index case and her parents, and a de novo missense variant c.829G > A, p.Glu277Lys in ATP1A3 was identified.

DISCUSSION

This RDP case highlights a rare clinical feature of paroxysmal dystonia that affects the lower left limb and develops after the abrupt onset of permanent dystonia. Currently, there are only three reported RDP cases associated with the same missense mutation, and we summarized the clinical features of all cases including ours, such as onset of age, time for stable, RDP score, relapse and exacerbation. Various symptoms owing to ATP1A3 mutation could develop as ATP1A3-related neurological disorders beyond classical phenotypes such as alternating hemiplegia of childhood (AHC) or RDP. Although RDP is extremely rare during childhood, it is important to understand its clinical characteristics in children.

The Expanding Phenotypic Spectrums Associated with ATP1A3 Mutation in a Family with Rapid-Onset Dystonia Parkinsonism

INTRODUCTION

Rapid-onset dystonia parkinsonism (RDP), also referred to as Dystonia 12, is a rare autosomal dominant genetic disease characterized by abrupt onset of a rostrocaudal gradient of dystonia with prominent bulbar symptoms, and parkinsonian features, primarily bradykinesia and postural instability without tremor. The purpose of this study was to identify the genetic defect in a Chinese pedigree with familial RDP and to explore genotype-phenotype correlation.

METHODS

A 3-generation Chinese Han pedigree consisting of 9 members and 3 patients with RDP, and 200 unrelated ethnically matched normal subjects were recruited in this study. Exome sequencing was performed in the proband, and Sanger sequencing was then conducted in other family members and 200 normal controls.

RESULTS

In addition to the typical clinical manifestations of RDP, the proband and her sister presented tongue tremor which developed at the onset, and intriguingly the proband showed a "re-emergent" tongue tremor. Both the proband and her sister had a medical history of hyperthyroidism, and at the psychiatric interview they both received diagnoses of depression and anxiety. Excessive grammar errors existed in most sentences written by the proband, and this written-expression disorder occurred years before the onset of RDP. The mother of the proband presented tongue enlargement, oromandibular dystonia, and limb dystonia, which were not observed in her 2 daughters at the time of study. A missense variant, c.1838C>T (p.T613M), in the ATP1A3 gene, was identified in the 3 patients in the family and in 2 young children but was absent in family members without RDP and in the 200 normal controls.

CONCLUSION

These findings may broaden the phenotypic spectrums of RDP with mutations in the ATP1A3 gene, provide new insights into the diagnosis of RDP, and have implications for genetic counseling.

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.

Episodic Ataxias: Faux or Real?

The term Episodic Ataxias (EA) was originally used for a few autosomal dominant diseases, characterized by attacks of cerebellar dysfunction of variable duration and frequency, often accompanied by other ictal and interictal signs. The original group subsequently grew to include other very rare EAs, frequently reported in single families, for some of which no responsible gene was found. The clinical spectrum of these diseases has been enormously amplified over time. In addition, episodes of ataxia have been described as phenotypic variants in the context of several different disorders. The whole group is somewhat confused, since a strong evidence linking the mutation to a given phenotype has not always been established. In this review we will collect and examine all instances of ataxia episodes reported so far, emphasizing those for which the pathophysiology and the clinical spectrum is best defined.

Genetic background of ataxia in children younger than 5 years in Finland

Objective

To characterize the genetic background of molecularly undefined childhood-onset ataxias in Finland.

Methods

This study examined a cohort of patients from 50 families with onset of an ataxia syndrome before the age of 5 years collected from a single tertiary center, drawing on the advantages offered by next generation sequencing. A genome-wide genotyping array (Illumina Infinium Global Screening Array MD-24 v.2.0) was used to search for copy number variation undetectable by exome sequencing.

Results

Exome sequencing led to a molecular diagnosis for 20 probands (40%). In the 23 patients examined with a genome-wide genotyping array, 2 additional diagnoses were made. A considerable proportion of probands with a molecular diagnosis had de novo pathogenic variants (45%). In addition, the study identified a de novo variant in a gene not previously linked to ataxia: MED23. Patients in the cohort had medically actionable findings.

Conclusions

There is a high heterogeneity of causative mutations in this cohort despite the defined age at onset, phenotypical overlap between patients, the founder effect, and genetic isolation in the Finnish population. The findings reflect the heterogeneous genetic background of ataxia seen worldwide and the substantial contribution of de novo variants underlying childhood ataxia.

Comprehensive Exonic Sequencing of Known Ataxia Genes in Episodic Ataxia

Episodic Ataxias (EAs) are a small group (EA1–EA8) of complex neurological conditions that manifest as incidents of poor balance and coordination. Diagnostic testing cannot always find causative variants for the phenotype, however, and this along with the recently proposed EA type 9 (EA9), suggest that more EA genes are yet to be discovered. We previously identified disease-causing mutations in the CACNA1A gene in 48% (n = 15) of 31 patients with a suspected clinical diagnosis of EA2, and referred to our laboratory for CACNA1A gene testing, leaving 52% of these cases (n = 16) with no molecular diagnosis. In this study, whole exome sequencing (WES) was performed on 16 patients who tested negative for CACNA1A mutations. Tiered analysis of WES data was performed to first explore (Tier-1) the ataxia and ataxia-associated genes (n = 170) available in the literature and databases for comprehensive EA molecular genetic testing; we then investigated 353 ion channel genes (Tier-2). Known and potential causal variants were identified in n = 8/16 (50%) patients in 8 genes (SCN2A, p.Val1325Phe; ATP1A3, p.Arg756His; PEX7, p.Tyr40Ter; and KCNA1, p.Arg167Met; CLCN1, p.Gly945ArgfsX39; CACNA1E, p.Ile614Val; SCN1B, p.Cys121Trp; and SCN9A, p.Tyr1217Ter). These results suggest that mutations in these genes might cause an ataxia phenotype or that combinations of more than one mutation contribute to ataxia disorders.

ATP1A3-related epilepsy: Report of seven cases and literature-based analysis of treatment response

ATP1A3 related disease is a clinically heterogeneous condition currently classified as alternating hemiplegia of childhood, rapid-onset dystonia-parkinsonism and cerebellar ataxia, areflexia, pes cavus, optic atrophy, and sensorineural hearing loss. Recently, it has become apparent that a remarkably large subgroup is suffering from often difficult-to-treat epilepsy. The aim of the present study was to assess the prevalence and efficacy of commonly used anti-epileptic-drugs (AEDs) in patients with ATP1A3 related seizures. Therefore, we performed a retrospective study of patients in combination with a systematic literature-based review. Inclusion criteria were: verified ATP1A3 mutation, seizures and information about AED treatment. The literature review yielded records for 188 epileptic ATP1A3 patients. For 14/188 cases, information about anti-epileptic treatment was available. Combined with seven unpublished records of ATP1A3 patients, a sample size of 21 patients was reached. Most used AED were levetiracetam (n = 9), phenobarbital (n = 8), valproic acid (n = 7), and topiramate (n = 5). Seizure reduction was reported for 57% of patients (n = 12). No individual AEDs used (either alone or combined) had a success rate over 50%. There was no significant difference in the response rate between various AEDs. Ketogenic diet was effective in 2/4 patients. 43% of patients (n = 9) did not show any seizure relief. Even though Epilepsy is a significant clinical issue in ATP1A3 patients, only a minority of publications provide any information about patients' anti-epileptic treatment. The findings of treatment effectiveness in only 57% (or lower) of patients, and the non-existence of a clear first-line AED in ATP1A3 related epilepsy stresses the need for further research.

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.

Differential expression patterns of sodium potassium ATPase alpha and beta subunit isoforms in mouse brain during postnatal development

The sodium potassium ATPase (Na⁺/K⁺ ATPase) is essential for the maintenance of a low intracellular Na⁺ and a high intracellular K⁺ concentration. Loss of function of the Na⁺/K⁺ ATPase due to mutations in Na⁺/K⁺ ATPase genes, anoxic conditions, depletion of ATP or inhibition of the Na⁺/K⁺ ATPase function using cardiac glycosides such as digitalis, causes a depolarization of the resting membrane potential. While in non-excitable cells, the uptake of glucose and amino acids is decreased if the function of the Na⁺/K⁺ ATPase is compromised, in excitable cells the symptoms range from local hyper-excitability to inactivating depolarization. Although several studies have demonstrated the differential expression of the various Na⁺/K⁺ ATPase alpha and beta isoforms in the brain tissue of rodents, their expression profile during development has yet to be thoroughly investigated. An immunohistochemical analysis of postnatal day 19 mouse brain showed ubiquitous expression of Na⁺/K⁺ ATPase isoforms α1, β1 and β2 in both neurons and glial cells, whereas α2 was expressed mostly in glial cells and the α3 and β3 isoforms were expressed in neurons. Furthermore, we examined potential changes in the relative expression of the different Na⁺/K⁺ ATPase isoforms in different brain areas of postnatal day 6 and in adult 9 months old animals using immunoblot analysis. Our results show a significant up-regulation of the α1 isoform in cortex, hippocampus and cerebellum, whereas, the α2 isoform was significantly up-regulated in midbrain. The β3 isoform showed a significant up-regulation in all brain areas investigated. The up-regulation of the α3 isoform matched that of the β2 isoform which were both significantly up-regulated in cortex, hippocampus and midbrain, suggesting that the increased maturation of the neuronal network is accompanied by an increase in expression of α3/β2 complexes in these brain structures.

Genotype-structure-phenotype relationships diverge in paralogs ATP1A1, ATP1A2, and ATP1A3

Objective

We tested the assumption that closely related genes should have similar pathogenic variants by analyzing >200 pathogenic variants in a gene family with high neurologic impact and high sequence identity, the Na,K-ATPases ATP1A1, ATP1A2, and ATP1A3.

Methods

Data sets of disease-associated variants were compared. Their equivalent positions in protein crystal structures were used for insights into pathogenicity and correlated with the phenotype and conservation of homology.

Results

Relatively few mutations affected the corresponding amino acids in 2 genes. In the membrane domain of ATP1A3 (primarily expressed in neurons), variants producing milder neurologic phenotypes had different structural positions than variants producing severe phenotypes. In ATP1A2 (primarily expressed in astrocytes), membrane domain variants characteristic of severe phenotypes in ATP1A3 were absent from patient data. The known variants in ATP1A1 fell into 2 distinct groups. Sequence conservation was an imperfect indicator: it varied among structural domains, and some variants with demonstrated pathogenicity were in low conservation sites.

Conclusions

Pathogenic variants varied between genes despite high sequence identity, and there is a genotype-structure-phenotype relationship in ATP1A3 that correlates with neurologic outcomes. The absence of "severe" pathogenic variants in ATP1A2 patients predicts that they will manifest either in a different tissue or by death in utero and that new ATP1A1 variants will produce additional phenotypes. It is important that some variants in poorly conserved amino acids are nonetheless pathogenic and could be incorrectly predicted to be benign.

ATP1A3 spectrum disorders: A video-documented history of 7 genetically confirmed early onset cases

Mutations in the ATP1A3 gene, which encodes the alpha₃-subunit of sodium-potassium ATPase, are related to a spectrum of neurological diseases including Rapid onset Dystonia-Parkinsonism (RDP), Alternating Hemiplegia of Childhood (AHC) and Cerebellar ataxia, Areflexia, Pes cavus, Optic atrophy and Sensorineural hearing loss (CAPOS) syndrome. Moreover, an increasing number of patients with intermediate and non classical phenotypes have been reported. Herein we describe 7 patients with 6 different de novo ATP1A3 mutations, and we focus on paroxysmal and chronic movement disorders with the help of video documentation. Our cases confirm that ATP1A3-related neurological disorders make up a phenotypic continuum rather than overlapping syndromes, in which early onset dystonia, ataxia and paroxysmal episodes with triggering or worsening factors are key diagnostic clues. Moreover, our experience suggests that ATP1A3 gene analysis should be extended both to children with channelopathy-like spells and to patients with early onset, fever-related encephalopathy.

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.

A de novo p.Arg756Cys mutation in ATP1A3 causes a distinct phenotype with prolonged weakness and encephalopathy triggered by fever

Patients with a mutation at Arg756 in ATP1A3 have been known to exhibit a distinct phenotype, characterized by prolonged weakness and encephalopathy, triggered by febrile illness. With only eight reports published to date, more evidence is required to correlate clinical features with a mutation at Arg756. Here we report an additional case with an Arg756Cys mutation in ATP1A3. A four-year-old boy showed mild developmental delay with recurrent paroxysmal episodes of weakness and encephalopathy from nine months of age. Motor deficits, which included bilateral hypotonia, ataxia, dysmetria, limb incoordination, dysarthria, choreoathetosis, and dystonia, were observed from one year and three months. Whole-exome sequencing detected a heterozygous de novo variant at c.2266C>T (p.Arg756Cys) in ATP1A3. The episodic course and clinical features of this case were consistent with previously reported cases with mutations at Arg756. Furthermore, his phenotype of marked ataxia was more similar to that of an Arg756Cys patient with relapsing encephalopathy and cerebellar ataxia syndrome, than to those with Arg756His and Arg756Leu mutations. This report therefore provides evidence of genotype-phenotype correlations in ATP1A3-related disorders as well as in patients with mutations at Arg756 in ATP1A3.

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.

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.

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.

De novo p.Arg756Cys mutation of ATP1A3 causes an atypical form of alternating hemiplegia of childhood with prolonged paralysis and choreoathetosis

Background

Alternating hemiplegia of childhood (AHC) is a rare neurological disorder that manifests recurrent attacks of hemiplegia, oculogyric, and choreoathetotic involuntary movements. De novo mutations in ATP1A3 cause three types of neurological diseases: AHC; rapid-onset dystonia-Parkinsonism (RDP); and cerebellar ataxia, areflexia, pes cavus, optic atrophy, and sensorineural hearing loss (CAPOS) syndromes. It remains to be determined whether or not a rare mutation in ATP1A3 may cause atypical phenotypes.

Case presentation

A 7-year-old boy presented with recurrent symptoms of generalized paralysis since 1 year and 5 months of age. Hypotonia, dystonia, and choreoathetosis persisted with exacerbation under febrile conditions, but no cerebellar ataxia had ever evolved in 6 years. Whole-exome sequencing (WES) was performed to determine his genetic background, and mutations were validated by the Sanger method. Crude protein extracts were prepared from the cultured cells, and expression of the wild-type or mutant ATP1A3 proteins were analyzed by Western blotting. WES identified a de novo pathogenic mutation in ATP1A3 (c.2266C > T:p.R756C) for this patient. A literature overview of two reported cases with p.R756C and p.R756H mutations showed both overlapping and distinct phenotypes when compared with those of the present case. The expression of the mutant form (R756C) of ATP1A3 did not differ markedly from that of the wild-type and D801N proteins.

Conclusions

This study confirmed that p.R756C mutation of ATP1A3 cause atypical forms of AHC-associated disorders. The wide spectra of neurological phenotypes in AHC are linked to as-yet-unknown deficits in the functions of mutant ATP1A3.

Electronic supplementary material

The online version of this article (doi:10.1186/s12883-016-0680-6) contains supplementary material, which is available to authorized users.

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.

The Genetic Homogeneity of CAPOS Syndrome: Four New Patients With the c.2452G>A (p.Glu818Lys) Mutation in the ATP1A3 Gene

BACKGROUND

The clinical syndrome of cerebellar ataxia, areflexia, pes cavus, optic atrophy, and sensorineural hearing loss (CAPOS) was first described 20 years ago, but it was only recently that whole exome sequencing unveiled the causative mutation in the ATP1A3 gene. We present four patients from the seventh and eighth family identified worldwide, provide a critical review of all patients published thus far, and speculate about the pathophysiologic processes underlying the acute neurological manifestations.

CLINICAL OBSERVATIONS

The individuals presented here experienced one to three paroxysmal, short-lasting episodes in childhood with cerebellar symptoms and signs, hypotonia, ophthalmoparesis, motor weakness, areflexia, and/or lethargy that were consistently associated with febrile illness. An underlying c.2452G>A mutation in the ATP1A3 gene was found in all four individuals. Besides the persisting CAPOS features, other possibly related sequelae included dystonia, myoclonus, and emotional and behavioral changes. After initiation of acetazolamide in two patients, no further episodes occurred.

CONCLUSION

Targeted sequencing of the ATP1A3 gene is recommended in children exhibiting paroxysmal, fever-induced ataxia and in adults with a more or less stationary or slowly progressive cerebellar syndrome since childhood accompanied by mixed combinations of areflexia, pes cavus, profound visual impairment, and/or sensorineural hearing loss. Similar to some other types of episodic ataxia, acetazolamide may be considered in patients with CAPOS syndrome to prevent or attenuate bouts of ataxia, but this requires further study.

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.

Beyond Dystonia-Parkinsonism: Chorea and Ataxia with ATP1A3 Mutations

Mutations in the ATP1A3 gene (the α-3 subunit of the Na⁺/K⁺ ATPase) are associated with rapid-onset dystonia-parkinsonism; alternating hemiplegia of childhood; and cerebellar ataxia, areflexia, pes cavus, optic atrophy, and sensorineural hearing loss (CAPOS syndrome). The authors report 3 cases with pleiotropic movement disorders, including a novel mutation in a patient who presented with ataxia and dysphagia. Case 1 had a history of attention deficit hyperactivity disorder and developed dysphagia, chorea, and limb dystonia after a febrile illness at age 12 years. Case 2 presented with limb dystonia at age 26 years and dysarthia and dysphagia after a febrile illness. Case 3 had a history of learning disability and developed progressive ataxia with cerebellar atrophy at age 20 years. In all cases, deleterious mutations were identified in ATP1A3. They illustrate wide phenotypic variability, including chorea and ataxia. New cases are likely to be diagnosed as knowledge about the phenotypic spectrum expands.

Relapsing encephalopathy with cerebellar ataxia related to an ATP1A3 mutation

ATP1A3, the gene encoding the α3-subunit of the Na(+) /K(+) -ATPase pump, has been involved in four clinical neurological entities: (1) alternating hemiplegia of childhood (AHC); (2) rapid-onset dystonia parkinsonism (RDP); (3) CAPOS (cerebellar ataxia, areflexia, pes cavus, optic atrophy, sensorineural hearing loss) syndrome; and (4) early infantile epileptic encephalopathy. Here, we report on a 34-year-old female presenting with a new ATP1A3-related entity involving a relapsing encephalopathy characterized by recurrent episodes of cerebellar ataxia and altered consciousness during febrile illnesses. The term RECA is suggested - relapsing encephalopathy with cerebellar ataxia. The phenotype of this patient, resembling mitochondrial oxidative phosphorylation defects, emphasizes the possible role of brain energy deficiency in patients with ATP1A3 mutations. Rather than multiple overlapping syndromes, ATP1A3-related disorders might be seen as a phenotypic continuum.

CAOS-Episodic Cerebellar Ataxia, Areflexia, Optic Atrophy, and Sensorineural Hearing Loss: A Third Allelic Disorder of the ATP1A3 Gene

We describe the molecular basis of a distinctive syndrome characterized by infantile stress-induced episodic weakness, ataxia, and sensorineural hearing loss, with permanent areflexia and optic nerve pallor. Whole exome sequencing identified a deleterious heterozygous c.2452 G>A, p.(E818K) variant in the ATP1A3 gene and structural analysis predicted its protein-destabilizing effect. This variant has not been reported in context with rapid-onset dystonia parkinsonism and alternating hemiplegia of childhood, the 2 main diseases associated with ATP1A3. The clinical presentation in the family described here differs categorically from these diseases in age of onset, clinical course, cerebellar over extrapyramidal movement disorder predominance, and peripheral nervous system involvement. While this paper was in review, a highly resembling phenotype was reported in additional patients carrying the same c.2452 G>A variant. Our findings substantiate this variant as the cause of a unique inherited autosomal dominant neurologic syndrome that constitutes a third allelic disease of the ATP1A3 gene.

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.

A Parent's Journey: Incorporating Principles of Palliative Care into Practice for Children with Chronic Neurologic Diseases

Rather than in conflict or in competition with the curative model of care, pediatric palliative care is a complementary and transdisciplinary approach used to optimize medical care for children with complex medical conditions. It provides care to the whole child, including physical, mental, and spiritual dimensions, in addition to support for the family. Through the voice of a parent, the following case-based discussion demonstrates how the fundamentals of palliative care medicine, when instituted early in the course of disease, can assist parents and families with shared medical decision making, ultimately improving the quality of life for children with life-limiting illnesses. Pediatric neurologists, as subspecialists who provide medical care for children with chronic and complex conditions, should consider invoking the principles of palliative care early in the course of a disease process, either through applying general facets or, if available, through consultation with a specialty palliative care service.

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

Mutations in ATP1A3 cause Alternating Hemiplegia of Childhood (AHC) by disrupting function of the neuronal Na+/K+ ATPase. Published studies to date indicate 2 recurrent mutations, D801N and E815K, and a more severe phenotype in the E815K cohort. We performed mutation analysis and retrospective genotype-phenotype correlations in all eligible patients with AHC enrolled in the US AHC Foundation registry from 1997-2012. Clinical data were abstracted from standardized caregivers’ questionnaires and medical records and confirmed by expert clinicians. We identified ATP1A3 mutations by Sanger and whole genome sequencing, and compared phenotypes within and between 4 groups of subjects, those with D801N, E815K, other ATP1A3 or no ATP1A3 mutations. We identified heterozygous ATP1A3 mutations in 154 of 187 (82%) AHC patients. Of 34 unique mutations, 31 (91%) are missense, and 16 (47%) had not been previously reported. Concordant with prior studies, more than 2/3 of all mutations are clustered in exons 17 and 18. Of 143 simplex occurrences, 58 had D801N (40%), 38 had E815K (26%) and 11 had G937R (8%) mutations. Patients with an E815K mutation demonstrate an earlier age of onset, more severe motor impairment and a higher prevalence of status epilepticus. This study further expands the number and spectrum of ATP1A3 mutations associated with AHC and confirms a more deleterious effect of the E815K mutation on selected neurologic outcomes. However, the complexity of the disorder and the extensive phenotypic variability among subgroups merits caution and emphasizes the need for further studies.

Novel mutations in ATP1A3 associated with catastrophic early life epilepsy, episodic prolonged apnea, and postnatal microcephaly

OBJECTIVE

Mutations of ATP1A3 have been associated with rapid onset dystonia-parkinsonism and more recently with alternating hemiplegia of childhood. Here we report one child with catastrophic early life epilepsy and shortened survival, and another with epilepsy, episodic prolonged apnea, postnatal microcephaly, and severe developmental disability. Novel heterozygous mutations (p.Gly358Val and p.Ile363Asn) were identified in ATP1A3 in these children.

METHODS

Subjects underwent next-generation sequencing under a research protocol. Clinical data were collected retrospectively. The biochemical effects of the mutations on ATP1A3 protein function were investigated. Postmortem neuropathologic specimens from control and affected subjects were studied.

RESULTS

The mutations localized to the P domain of the Na,K-ATPase α3 protein, and resulted in significant reduction of Na,K-ATPase activity in vitro. We demonstrate in both control human brain tissue and that from the subject with the p.Gly358Val mutation that ATP1A3 immunofluorescence is prominently associated with interneurons in the cortex, which may provide some insight into the pathogenesis of the disease.

SIGNIFICANCE

The findings indicate these mutations cause severe phenotypes of ATP1A3-related disorder spectrum that include catastrophic early life epilepsy, episodic apnea, and postnatal microcephaly.