SCN3A deficiency associated with increased seizure susceptibility

Abstract

Mutations in voltage-gated sodium channels expressed highly in the brain (SCN1A, SCN2A, SCN3A, and SCN8A) are responsible for an increasing number of epilepsy syndromes. In particular, mutations in the SCN3A gene, encoding the pore-forming Nav1.3 α subunit, have been identified in patients with focal epilepsy. Biophysical characterization of epilepsy-associated SCN3A variants suggests that both gain- and loss-of-function SCN3A mutations may lead to increased seizure susceptibility. In this report, we identified a novel SCN3A variant (L247P) by whole exome sequencing of a child with focal epilepsy, developmental delay, and autonomic nervous system dysfunction. Voltage clamp analysis showed no detectable sodium current in a heterologous expression system expressing the SCN3A-L247P variant. Furthermore, cell surface biotinylation demonstrated a reduction in the amount of SCN3A-L247P at the cell surface, suggesting the SCN3A-L247P variant is a trafficking-deficient mutant. To further explore the possible clinical consequences of reduced SCN3A activity, we investigated the effect of a hypomorphic Scn3a allele (Scn3aHyp) on seizure susceptibility and behavior using a gene trap mouse line. Heterozygous Scn3a mutant mice (Scn3a+/Hyp) did not exhibit spontaneous seizures nor were they susceptible to hyperthermia-induced seizures. However, they displayed increased susceptibility to electroconvulsive (6 Hz) and chemiconvulsive (flurothyl and kainic acid) induced seizures. Scn3a+/Hyp mice also exhibited deficits in locomotor activity and motor learning. Taken together, these results provide evidence that loss-of-function of SCN3A caused by reduced protein expression or deficient trafficking to the plasma membrane may contribute to increased seizure susceptibility.

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

“…By comparison, variants in familial SCN disease such as SCN4A periodic paralysis/myotonia or SCN9/10/11A ‐related pain disorders are better tolerated for both truncating and missense variants (Figure ) . Our analysis further supports the emerging evidence that SCN3A , which shows strong depletion for PTV and missense variants, is a good candidate gene for epilepsy, although only a few patients have been reported to date …”

“…25,26 SCN3A-associated epilepsies are clinically heterogeneous, presenting with mainly GoF missense variants, early onset seizures, epileptic encephalopathy, polymicrogyria, and developmental impairment. 27,28 To better understand the biology of seizure susceptibility in SCN-related epilepsies, our aim was to determine similarities and differences between sodium channel disorders and apply variant constraint analysis to identify severe sodium channel disease. This approach allowed us to develop a broader perspective on precision treatment than on an individual gene or variant level and to recognize common patterns among SCNrelated disorders informing clinical practice.…”

Diagnostic exome sequencing of syndromic epilepsy patients in clinical practice

“…Crucial role of SOX5 in neurodevelopment and corticogenesis is confirmed in both mice and drosophila models . SCN3A have been mostly associated with CNV, however, several patients with focal epilepsy have been reported and a single case with a highly compatible phenotype has been presented . Interestingly, upregulated expression of voltage‐gated sodium channel Na v 1.3, encoded by SCN3A , was found in cortical lesions of patients with focal dysplasia type IIb while both patients described by us (case 37 in Table S1) and Jhaveri et al had polymicrogyria.…”

Mutations in the Scn8a DIIS4 voltage sensor reveal new distinctions among hypomorphic and null Nav1.6 sodium channels

“…N4 male Scn8a Δ9/+ mutants (3‐5 months old, n = 5, from two litters) were implanted with four cortical electrodes (Vintage Machine Supplies, Medina, Ohio), as previously described . The electrodes were implanted at the following coordinates relative to bregma: anterior‐posterior (AP) +2.0 mm and medial‐lateral (ML) +1.2 mm, AP −1.5 mm and ML +1.2 mm, AP +0.5 mm and ML −2.2 mm, and AP −3.5 mm and ML −2.2 mm.…”