Gain-of-function and loss-of-function variants in GRIA3 lead to distinct neurodevelopmental phenotypes.

AMPA (α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid) receptors (AMPARs) mediate fast excitatory neurotransmission in the brain. AMPARs form by homo- or heteromeric assembly of subunits encoded by the GRIA1-GRIA4 genes, of which only GRIA3 is X-chromosomal. Increasing numbers of GRIA3 missense variants are reported in patients with neurodevelopmental disorders (NDD), but only a few have been examined functionally. Here, we evaluated the impact on AMPAR function of one frameshift and 43 rare missense GRIA3 variants identified in patients with NDD by electrophysiological assays. Thirty-one variants alter receptor function and show loss-of-function (LoF) or gain-of-function (GoF) properties, whereas 13 appeared neutral. We collected detailed clinical data from 25 patients (from 23 families) harbouring 17 of these variants. All patients had global developmental impairment, mostly moderate (9/25) or severe (12/25). Twelve patients had seizures, including focal motor (6/12), unknown onset motor (4/12), focal impaired awareness (1/12), (atypical) absence (2/12), myoclonic (5/12), and generalized tonic-clonic (1/12) or atonic (1/12) seizures. The epilepsy syndrome was classified as developmental and epileptic encephalopathy in eight patients, developmental encephalopathy without seizures in 13 patients, and intellectual disability with epilepsy in four patients. Limb muscular hypotonia was reported in 13/25, and hypertonia in 10/25. Movement disorders were reported in 14/25, with hyperekplexia or non-epileptic erratic myoclonus being the most prevalent feature (8/25). Correlating receptor functional phenotype with clinical features revealed clinical features for GRIA3-associated NDDs and distinct NDD phenotypes for LoF and GoF variants. GoF variants were associated with more severe outcomes: patients were younger at the time of seizure onset (median age one month), hypertonic, and more often had movement disorders, including hyperekplexia. Patients with LoF variants were older at the time of seizure onset (median age 16 months), hypotonic, and had sleeping disturbances. LoF and GoF variants were disease-causing in both sexes but affected males often carried de novo or hemizygous LoF variants inherited from healthy mothers, whereas all but one affected females had de novo heterozygous GoF variants.

An SCN2A mutation in a family with infantile seizures from Madagascar reveals an increased subthreshold Na(+) current.

Missense mutations in SCN2A, encoding the brain sodium channel NaV 1.2, have been described in benign familial neonatal-infantile seizures (BFNIS), a self-limiting disorder, whereas several SCN2A de novo nonsense mutations have been found in patients with more severe phenotypes including epileptic encephalopathy. We report a family with BFNIS originating from Madagascar. Onset extended from 3 to 9 months of age. Interictal EEGs were normal. In two patients, ictal electroencephalography (EEG) studies showed partial seizure patterns with secondary generalization in one. Seizures remitted before 18 months of age, with or without medication. Intellectual development was normal. A novel missense mutation of SCN2A, c.4766A>G/p.Tyr1589Cys, was found in a highly conserved region of NaV 1.2 (D4/S2-S3). Functional studies using heterologous expression in tsA201 cells and whole-cell patch clamping revealed a depolarizing shift of steady-state inactivation, increased persistent Na(+) current, a slowing of fast inactivation and an acceleration of its recovery, thus a gain-of-function. Using an action potential waveform in a voltage-clamp experiment we indicated an increased inward Na(+) current at subthreshold voltages, which can explain a neuronal hyperexcitability. Our results suggest that this mutation induces neuronal hyperexcitability, resulting in infantile epilepsy with favorable outcome.

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.

West syndrome caused by homozygous variant in the evolutionary conserved gene encoding the mitochondrial elongation factor GUF1.

West syndrome (WS), defined by the triad of infantile spasms, pathognomonic hypsarrhythmia and developmental regression, is a rare epileptic disease affecting about 1:3500 live births. To get better insights on the genetic of this pathology, we exome-sequenced the members of a consanguineous family affected with isolated WS. We identified a homozygous variant (c.1825G>T/p.(Ala609Ser)) in the GUF1 gene in the three affected siblings. GUF1 encodes a protein essential in conditions that counteract faithful protein synthesis: it is able to remobilize stuck ribosomes and transiently inhibit the elongation process to optimize protein synthesis. The variant identified in the WS family changes an alanine residue conserved in all eukaryotic organisms and positioned within the tRNA-binding moiety of this nuclear genome-encoded mitochondrial translational elongation factor. Yeast complementation assays show that the activity of GUF1(A609S) is modified in suboptimal environments. We suggest a new link between improper assembly of respiratory chain complexes and WS.

Molecular characterization of a cohort of 73 patients with infantile spasms syndrome.

Infantile Spasms syndrome (ISs) is a characterized by epileptic spasms occurring in clusters with an onset in the first year of life. West syndrome represents a subset of ISs that associates spasms in clusters, a hypsarrhythmia EEG pattern and a developmental arrest or regression. Aetiology of ISs is widely heterogeneous including many genetic causes. Many patients, however, remain without etiological diagnosis, which is critical for prognostic purpose and genetic counselling. In the present study, we performed genetic screening of 73 patients with different types of ISs by array-CGH and molecular analysis of 5 genes: CDKL5, STXBP1, KCNQ2, and GRIN2A, whose mutations cause different types of epileptic encephalopathies, including ISs, as well as MAGI2, which was suggested to be related to a subset of ISs. In total, we found a disease-causing mutation or CNV (Copy Number Variation) in 15% of the patients. These included 6 point mutations found in CDKL5 (n = 3) and STXBP1 (n = 3), 3 microdeletions (10 Mb in 2q24.3, 3.2 Mb in 5q14.3 including the region upstream to MEF2C, and 256 kb in 9q34 disrupting EHMT1), and 2 microduplications (671 kb in 2q24.3 encompassing SCN2A, and 11.93 Mb in Xq28). In addition, we discuss 3 CNVs as potential risk factors, including one 16p12.1 deletion, one intronic deletion of the NEDD4 gene, and one intronic deletion of CALN1 gene. The present findings highlight the efficacy of combined cytogenetic and targeted mutation screening to improve the diagnostic yield in patient with ISs.