A Nonsense Mutation of the Sodium Channel GeneSCN2Ain a Patient with Intractable Epilepsy and Mental Decline

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Abstract
Mutations, exclusively missense, of voltage-gated sodium channel α subunit type 1 (SCN1A) and type 2 (SCN2A) genes were reported in patients with idiopathic epilepsy: generalized epilepsy with febrile seizures plus. Nonsense and frameshift mutations ofSCN1A, by contrast, were identified in intractable epilepsy: severe myoclonic epilepsy in infancy (SMEI). Here we describe a first nonsense mutation ofSCN2Ain a patient with intractable epilepsy and severe mental decline. The phenotype is similar to SMEI but distinct because of partial epilepsy, delayed onset (1 year 7 months), and absence of temperature sensitivity. A mutational analysis revealed that the patient had a heterozygousde novononsense mutation R102X ofSCN2A. Patch-clamp analysis of Nav1.2 wild-type channels and the R102X mutant protein coexpressed in human embryonic kidney 293 cells showed that the truncated mutant protein shifted the voltage dependence of inactivation of wild-type channels in the hyperpolarizing direction. Analysis of the subcellular localization of R102X truncated protein suggested that its dominant negative effect could arise from direct or indirect cytoskeletal interactions of the mutant protein. Haploinsufficiency of Nav1.2 protein is one plausible explanation for the pathology of this patient; however, our biophysical findings suggest that the R102X truncated protein exerts a dominant negative effect leading to the patient's intractable epilepsy.