Sodium Channel SCN3A (NaV1.3) Regulation of Human Cerebral Cortical Folding and Oral Motor Development

Ion Channel Functions in Early Brain Development.

Objective Voltage-gated sodium channels (SCNs) share similar amino acid sequence, structure, and function. Genetic variants in the four human brain-expressed SCN genes SCN1A/2A/3A/8A have been associated with heterogeneous epilepsy phenotypes and neurodevelopmental disorders. To better understand the biology of seizure susceptibility in SCN-related epilepsies, our aim was to determine similarities and differences between sodium channel disorders, allowing us to develop a broader perspective on precision treatment than on an individual gene level alone. Methods We analyzed genotype-phenotype correlations in large SCN-patient cohorts and applied variant constraint analysis to identify severe sodium channel disease. We examined temporal patterns of human SCN expression and correlated functional data from in vitro studies with clinical phenotypes across different sodium channel disorders. Results Comparing 865 epilepsy patients (504 SCN1A, 140 SCN2A, 171 SCN8A, four SCN3A, 46 copy number variation [CNV] cases) and analysis of 114 functional studies allowed us to identify common patterns of presentation. All four epilepsy-associated SCN genes demonstrated significant constraint in both protein truncating and missense variation when compared to other SCN genes. We observed that age at seizure onset is related to SCN gene expression over time. Individuals with gain-of-function SCN2A/3A/8A missense variants or CNV duplications share similar characteristics, most frequently present with early onset epilepsy (<3 months), and demonstrate good response to sodium channel blockers (SCBs). Direct comparison of corresponding SCN variants across different SCN subtypes illustrates that the functional effects of variants in corresponding channel locations are similar; however, their clinical manifestation differs, depending on their role in different types of neurons in which they are expressed. Significance Variant function and location within one channel can serve as a surrogate for variant effects across related sodium channels. Taking a broader view on precision treatment suggests that in those patients with a suspected underlying genetic epilepsy presenting with neonatal or early onset seizures (<3 months), SCBs should be considered.

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Biological concepts in human sodium channel epilepsies and their relevance in clinical practice.

Objective:

Pathogenic variants in SCN 8A have been associated with a wide spectrum of epilepsy phenotypes, ranging from benign familial infantile seizures (BFIS ) to epileptic encephalopathies with variable severity. Furthermore, a few patients with intellectual disability (ID ) or movement disorders without epilepsy have been reported. The vast majority of the published SCN 8A patients suffer from severe developmental and epileptic encephalopathy (DEE ). In this study, we aimed to provide further insight on the spectrum of milder SCN 8A‐related epilepsies.

Methods:

A cohort of 1095 patients were screened using a next generation sequencing panel. Further patients were ascertained from a network of epilepsy genetics clinics. Patients with severe DEE and BFIS were excluded from the study.

Results:

We found 36 probands who presented with an SCN 8A‐related epilepsy and normal intellect (33%) or mild (61%) to moderate ID (6%). All patients presented with epilepsy between age 1.5 months and 7 years (mean = 13.6 months), and 58% of these became seizure‐free, two‐thirds on monotherapy. Neurological disturbances included ataxia (28%) and hypotonia (19%) as the most prominent features. Interictal electroencephalogram was normal in 41%. Several recurrent variants were observed, including Ile763Val, Val891Met, Gly1475Arg, Gly1483Lys, Phe1588Leu, Arg1617Gln, Ala1650Val/Thr, Arg1872Gln, and Asn1877Ser.

Significance:

With this study, we explore the electroclinical features of an intermediate SCN 8A‐related epilepsy with mild cognitive impairment, which is for the majority a treatable epilepsy.

/pdf/the-spectrum-of-intermediate-scn8a-related-epilepsy-2teeye89xg.pdf

The spectrum of intermediate SCN8A-related epilepsy.

The voltage-gated sodium channel α-subunit genes comprise a highly conserved gene family. Mutations of three of these genes, SCN1A, SCN2A and SCN8A, are responsible for a significant burden of neurological disease. Recent progress in identification and functional characterization of patient variants is generating new insights and novel approaches to therapy for these devastating disorders. Here we review the basic elements of sodium channel function that are used to characterize patient variants. We summarize a large body of work using global and conditional mouse mutants to characterize the in vivo roles of these channels. We provide an overview of the neurological disorders associated with mutations of the human genes and examples of the effects of patient mutations on channel function. Finally, we highlight therapeutic interventions that are emerging from new insights into mechanisms of sodium channelopathies.

Sodium channelopathies in neurodevelopmental disorders

Objective
Voltage-gated sodium (Na+) channels underlie action potential generation and propagation and hence are central to the regulation of excitability in the nervous system. Mutations in the genes SCN1A, SCN2A, and SCN8A, encoding the Na+ channel pore-forming (α) subunits Nav1.1, 1.2, and 1.6, respectively, and SCN1B, encoding the accessory subunit β1, are established causes of genetic epilepsies. SCN3A, encoding Nav1.3, is known to be highly expressed in brain, but has not previously been linked to early infantile epileptic encephalopathy. Here, we describe a cohort of four patients with epileptic encephalopathy and heterozygous de novo missense variants in SCN3A (p.Ile875Thr in two cases, p.Pro1333Leu, and p.Val1769Ala).

/pdf/mutations-in-scn3a-cause-early-infantile-epileptic-3vweenr3ys.pdf

Mutations in SCN3A cause early infantile epileptic encephalopathy.

Objective

Pathogenic variants in SCN3A , encoding the voltage‐gated sodium channel subunit Nav1.3, cause severe childhood‐onset epilepsy and malformation of cortical development. Here, we define the spectrum of clinical, genetic, and neuroimaging features of SCN3A ‐related neurodevelopmental disorder.
Methods

Patients were ascertained via an international collaborative network. We compared sodium channels containing wild‐type vs. variant Nav1.3 subunits co‐expressed with β1 and β2 subunits using whole‐cell voltage clamp electrophysiological recordings in a heterologous mammalian system (HEK‐293 T cells).
Results

Of 22 patients with pathogenic SCN3A variants, most had treatment‐resistant epilepsy beginning in the first year of life (16/21, 76%; median onset, 2 weeks), with severe or profound developmental delay (15/20; 75%). Many, but not all (15/19; 79%), exhibited malformations of cortical development. Pathogenic variants clustered in transmembrane segments 4–6 of domains II‐IV. Most pathogenic missense variants tested (10/11; 91%) displayed gain of channel function, with increased persistent current and/or a leftward shift in the voltage dependence of activation, and all variants associated with malformation of cortical development exhibited gain of channel function. One variant (p.Ile1468Arg) exhibited mixed effects, with gain and partial loss of function. Two variants demonstrated loss of channel function.
Interpretation

Our study defines SCN3A‐ related neurodevelopmental disorder along a spectrum of severity, but typically including epilepsy and severe or profound developmental delay/intellectual disability. Malformations of cortical development are a characteristic feature of this unusual channelopathy syndrome, present in over 75% of affected individuals. Gain of function at the channel level in developing neurons is likely an important mechanism of disease pathogenesis.

/pdf/scn3a-related-neurodevelopmental-disorder-a-spectrum-of-q4fylsxcsf.pdf

SCN3A ‐related neurodevelopmental disorder: A spectrum of epilepsy and brain malformation

Objective Pathogenic variants in SCN8A, encoding the voltage-gated sodium (Na+) channel α subunit Nav1.6, is a known cause of epilepsy. Here, we describe clinical and genetic features of all patients with SCN8A epilepsy evaluated at a single-tertiary care center, with biophysical data on identified Nav1.6 variants and pharmacological response to selected Na+ channel blockers. Methods SCN8A variants were identified via an exome-based panel of epilepsy-associated genes for next generation sequencing (NGS), or via exome sequencing. Biophysical characterization was performed using voltage-clamp recordings of ionic currents in heterologous cells. Results We observed a range in age of onset and severity of epilepsy and associated developmental delay/intellectual disability. Na+ channel blockers were highly or partially effective in most patients. Nav1.6 variants exhibited one or more biophysical defects largely consistent with gain of channel function. We found that clinical severity was correlated with the presence of multiple observed biophysical defects and the extent to which pathological Na+ channel activity could be normalized pharmacologically. For variants not previously reported, functional studies enhanced the evidence of pathogenicity. Interpretation We present a comprehensive single-center dataset for SCN8A epilepsy that includes clinical, genetic, electrophysiologic, and pharmacologic data. We confirm a spectrum of severity and a variety of biophysical defects of Nav1.6 variants consistent with gain of channel function. Na+ channel blockers in the treatment of SCN8A epilepsy may correlate with the effect of such agents on pathological Na+ current observed in heterologous systems.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/acn3.50839

Tariq Zaman

Papers

Mutations in SCN3A cause early infantile epileptic encephalopathy.

SCN3A ‐related neurodevelopmental disorder: A spectrum of epilepsy and brain malformation

A single-center SCN8A-related epilepsy cohort: clinical, genetic, and physiologic characterization