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

Abstract

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.

Figure 1

Schematic of Nav1.6. Na+ channels are comprised of a single polypeptide that is composed of four repeated domains (domains (D) I–IV), with each domain formed by six transmembrane segments (S1–S6); S1–S4 represent the voltage‐sensing region, while S5‐S6 forms the conducting pore. Shown is a schematic of the Nav1.6 protein, with DI‐IV and S1–S6 as well as the Nav1.6 variants included in this study (circles).

Figure 2

Selected epilepsy‐associated Nav1.6 variants lead to changes in the voltage dependence of channel availability. A–I, Representative traces (top) showing families of sodium current elicited by 20 ms depolarizing voltage steps from −80 mV to +50 mV in 5 mV increments, for wild‐type (WT, n = 28) and NaV1.6 variants Gly1475Arg (n = 25), Ala1491Val (n = 26), Leu483Phe (n = 22), Arg1872Leu (n = 27), Arg1872Trp (n = 25), Val1758Ala (n = 23) Asp374Lys (n = 23), and Met139Ile (n = 25). Included are the corresponding G‐V curves for the population data (bottom) illustrating the voltage dependence of steady‐state activation and inactivation, presented as mean ± SEM, with lines representing the Boltzman fit to the data points.

Figure 3

Selected epilepsy‐associated Nav1.6 variants result in enhanced persistent sodium current and/or impaired fast inactivation. A–B, Increased persistent current. A, Representative current traces showing transient (I _NaT) and persistent (I _NaP) Na+ current for channels composed of wild‐type (WT) and variant Nav1.6 subunits in response to a voltage steps from − 120 mV to − 10 mV for 200 ms. B, Bar graph showing I _NaP: I _NaT ratio for WT (n = 31) and variant Nav1.6 subunits Gly1475Arg (n = 28), Ala1491Val (n = 26), Leu483Phe (n = 23), Arg1872Leu (n = 25), Arg1872Trp (n = 25), Val1758Ala (n = 23), Asn374Lys (n = 25), and Met139Ile (n = 26). C–D, Impaired fast inactivation. C, Representative single traces showing I _NaT for wild‐type and variant Nav1.6 illustrating fast inactivation. Note that traces were scaled to the peak. D, Quantification of decay tau of I _NaT in wild‐type (WT) and variant Nav1.6 channels. E‐F, Ramp currents are increased in Na+ channels composed of variant NaV1.6 subunits. E, Shown is a representative current traces of wild‐type (WT) and variant Nav1.6 in response to slow depolarizing voltage ramps evoked by linear increases from −120 mV to +40 mV at 0.8 mV/ms. F, Quantitative analysis of total charge (in nanocoulombs; nC) for WT (n = 27) versus Nav1.6 variants Gly1475Arg (n = 22), Ala1491Val (n = 26), Leu483Phe (n = 22), Arg1872Leu (n = 24), Arg1872Trp (n = 21), Val1758Ala (n = 22), Asp374Lys (n = 23), and Met139Ile (n = 23). Data are presented as mean ± SEM; *P < 0.05; **P < 0.01; ***P < 0.001; via one‐way ANOVA with Bonferonni correction for multiple comparisons. Light gray **P < 0.01; light gray ***P < 0.001, via one‐way ANOVA without correction.

Figure 4

Anti‐seizure medications block pathological INaP and slow ramp currents. A–D, Block of persistent currents. A, C, Traces showing I _NaT and I _NaP of WT and variant Nav1.6 before (black) and after (red) the application of 10 μmol/L oxcarbazepine (A) and 0.5 μmol/L GS967 (C), with washout (gray). B, D, Bar graphs showing I _NaP: I _NaT ratio before (Ctr.) and after the application of oxcarbazepine (OXC; B) or GS‐967 (D). B, control Nav1.6 (Ctr.), n = 10; Nav1.6‐p.Gly1475Arg, n = 10; Ala1491Val, n = 8; Arg1872Trp, n = 9). D, GS967, control Nav1.6 (Ctr.), n = 5; Gly1475Arg, n = 5; Ala1491Val, n = 5; Arg1872Trp, n = 4). E–H, Ramp currents are increased in Na+ channels composed of variant NaV1.6 subunits. E, G, Example traces showing ramp currents for WT and variant Nav1.6 channels before (Ctr., black) and after (red traces) application of 10 μmol/L oxcarbazepine (OXC; E) and 0.5 μmol/L GS967 (G). F, H, Bar graph showing % total charge before and after the application of oxcarbazepine (WT, n = 5; Gly1475Arg, n = 10; Ala1491Val, n = 8 and Arg1872Trp, n = 9) and GS967 (WT, n = 4; Gly1475Arg, n = 5; Ala1491Val, n = 5 and Arg1872Trp, n = 4). Data are presented as mean ± SEM; *P < 0.05; **P < 0.01; ***P < 0.001 (Ctr. vs. drug via two‐tailed paired Student’s t‐test).