Abstracts

Rationale: More than 150 de novo missense mutations of SCN8A have been identified in individuals with epileptic encephalopathy since the first case was ascertained in 2012. The major pathogenic mechanism is gain-of-function of the encoded Nav1.6 channel due to impaired channel inactivation or hyperpolarized shift in channel activation (reviewed in Meisler et al, Epilepsia 2016). In contrast, very few loss-of-function mutations of SCN8A have been studied. One protein truncation allele was found to co-segregate with cognitive impairment in a family with no history of seizures (Trudeau et al, J Med Genet 2006).  Two missense variants lacking activity in transfected cells were identified in children with epileptic encephalopathy (de Kovel et al, Epilepsy Res 2014; Blanchard et al, J Med Genet 2015).  We examined the functional effect of two additional missense variants recently identified in children with intellectual disability but no seizures. Methods: Genomic DNA was analyzed by Next-Gen exome sequencing. Two missense variants of SCN8A were identified and introduced into the tetrodotoxin-resistant mouse Nav1.6 cDNA by site-directed mutagenesis. Channel activity was measured in transfected neuron-derived ND7/23 cells. The stability of the mutant channels in transfected cells was assessed by Western blot. Results: Both children are heterozygous for novel missense variants that alter conserved residues in transmembrane segments: p.Gly964Arg in D2S6 and p.Glu1218Lys in D3S1. Both substitutions are predicted to be deleterious, and neither has been observed in control populations. Both variants failed to generate sodium currents in transfected cells. In addition, the abundance of Nav1.6 protein was reduced by the p.Glu1218Lys substitution. Conclusions: These two loss-of-function missense mutations, together with the previously described protein truncation allele, demonstrate that loss of SCN8A function can result in cognitive impairment in individuals without seizures. These observations extend the phenotypic spectrum of SCN8A mutations beyond its established role in epileptic encephalopathy (OMIM #614558). SCN8A should thus be considered a candidate gene for intellectual disability regardless of seizure status. Funding: Supported by NIH [R01 NS 34509 (MHM) and R01 NS 75157 (MKP)].