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

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.

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.

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.

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.

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.

Cell biology and dynamics of Neuronal Na+/K+-ATPase in health and diseases

Neuronal Na⁺/K⁺-ATPase is responsible for the maintenance of ionic gradient across plasma membrane. In doing so, in a healthy brain, Na⁺/K⁺-ATPase activity accounts for nearly half of total brain energy consumption. The α3-subunit containing Na⁺/K⁺-ATPase expression is restricted to neurons. Heterozygous mutations within α3-subunit leads to Rapid-onset Dystonia Parkinsonism, Alternating Hemiplegia of Childhood and other neurological and neuropsychiatric disorders. Additionally, proteins such as α-synuclein, amyloid-β, tau and SOD1 whose aggregation is associated to neurodegenerative diseases directly bind and impair α3-Na⁺/K⁺-ATPase activity. The review will provide a summary of neuronal α3-Na⁺/K⁺-ATPase functional properties, expression pattern, protein-protein interactions at the plasma membrane, biophysical properties (distribution and lateral diffusion). Lastly, the role of α3-Na⁺/K⁺-ATPase in neurological and neurodegenerative disorders will be discussed. This article is part of the special issue entitled 'Mobility and trafficking of neuronal membrane proteins'.