Pathogenic mechanism of recurrent mutations of SCN8A in epileptic encephalopathy

Abstract

ObjectiveThe early infantile epileptic encephalopathy type 13 (EIEE13, OMIM #614558) results from de novo missense mutations of SCN8A encoding the voltage‐gated sodium channel Nav1.6. More than 20% of patients have recurrent mutations in residues Arg1617 or Arg1872. Our goal was to determine the functional effects of these mutations on channel properties.MethodsClinical exome sequencing was carried out on patients with early‐onset seizures, developmental delay, and cognitive impairment. Two mutations identified here, p.Arg1872Leu and p.Arg1872Gln, and two previously identified mutations, p.Arg1872Trp and p.Arg1617Gln, were introduced into Nav1.6 cDNA, and effects on electrophysiological properties were characterized in transfected ND7/23 cells. Interactions with FGF14, G‐protein subunit Gβγ, and sodium channel subunit β1 were assessed by coimmunoprecipitation.ResultsWe identified two patients with the novel mutation p.Arg1872Leu and one patient with the recurrent mutation p.Arg1872Gln. The three mutations of Arg1872 and the mutation of Arg1617 all impaired the sodium channel transition from open state to inactivated state, resulting in channel hyperactivity. Other observed abnormalities contributing to elevated channel activity were increased persistent current, increased peak current density, hyperpolarizing shift in voltage dependence of activation, and depolarizing shift in steady‐state inactivation. Protein interactions were not affected.InterpretationRecurrent mutations at Arg1617 and Arg1872 lead to elevated Nav1.6 channel activity by impairing channel inactivation. Channel hyperactivity is the major pathogenic mechanism for gain‐of‐function mutations of SCN8A. EIEE13 differs mechanistically from Dravet syndrome, which is caused by loss‐of‐function mutations of SCN1A. This distinction has important consequences for selection of antiepileptic drugs and the development of gene‐ and mutation‐specific treatments.

Altered gene expression profile in a mouse model of SCN8A encephalopathy

“…These pathogenic mutations arise de novo in affected individuals and generate gain-of-function missense mutations that increase the activity of sodium channel Na v 1.6 (Wagnon et al, 2016). Two commonly observed changes in Na v 1.6 are impaired channel inactivation, and a hyperpolarizing shift in voltage dependence of activation that results in premature channel opening (Barker et al, 2016; Blanchard et al, 2015; Estacion et al, 2014; Veeramah et al, 2012; Wagnon et al, 2016). SCN8A encephalopathy is characterized by seizure onset within the first 18 months of life, typically by 4 months of age (Larsen et al, 2015).…”

Remarkable alterations of Nav1.6 in reactive astrogliosis during epileptogenesis

“…SCN1A encoding Nav1.1 has been shown to be the most frequent target of mutations and is responsible to a variety of epilepsy syndromes8. Recently, much attention has been paid to SCN8A encoding Nav1.6 involved in the pathogenesis of epilepsy910111213141516.…”

“…Nav1.6 contributes to the production of tetrodotoxin-sensitive (TTX-S) transient current, persistent current, and resurgent current19. Several mutations of SCN8A have been recently identified and have been found to be functionally associated with epilepsy syndrome910111620. It has been found that Nav1.6 plays a role in facilitating the hyperexcitability of medial entorhinal cortex layer II neurons at 7 days after electrical induction of status epilepticus12.…”

SCN8A mutation in a child presenting with seizures and developmental delays

“…Interestingly, mutation of the corresponding leucine residue in SCN1A resulted in impaired channel inactivation (Kahlig et al 2008), a common feature of SCN8A mutations in epileptic encephalopathy (Wagnon et al 2016). …”

“…Ten patient mutations of SCN8A have been evaluated with functional tests in transfected cells (Veeramah et al 2012; de Kovel et al 2014; Estacion et al 2014; Blanchard et al 2015; Wagnon et al 2016). In eight of the 10 cases, gain-of-function effects were observed.…”