How do mutant Nav1.1 sodium channels cause epilepsy?

Abstract

Voltage-gated sodium channels comprise pore-forming α subunits and auxiliary β subunits. Nine different α subtypes, designated Nav1.1–Nav1.9 have been identified in excitable cells. Nav1.1, 1.2 and 1.6 are major subtypes in the adult mammalian brain. More than 200 mutations in the Nav1.1 α subtype have been linked to inherited epilepsy syndromes, ranging in severity from the comparatively mild disorder Generalized Epilepsy with Febrile Seizures Plus to the epileptic encephalopathy Severe Myoclonic Epilepsy of Infancy. Studies using heterologous expression and functional analysis of recombinant Nav1.1 channels suggest that epilepsy mutations in Nav1.1 may cause either gain-of-function or loss-of-function effects that are consistent with either increased or decreased neuronal excitability. How these diverse effects lead to epilepsy is poorly understood. This review summarizes the data on sodium channel mutations and epilepsy and builds a case for the hypothesis that most Nav1.1 mutations have their ultimate epileptogenic effects by reducing Nav1.1-mediated whole cell sodium currents in GABAergic neurons, resulting in widespread loss of brain inhibition, an ideal background for the genesis of epileptic seizures.

Introduction

Epilepsy has long been recognized as a disorder of brain hyperexcitability; nevertheless, the underlying cellular and molecular causes of epileptic seizures have remained elusive, especially for the so-called idiopathic epilepsies, in which spontaneous seizures occur in what appears to be an otherwise normal brain. A breakthrough in this field has come with the identification of gene mutations that cause rare monogenic epilepsy syndromes (Heron et al., 2007). It is hoped that the study of these familial epilepsies will give clues to the molecular and pathophysiological mechanisms of the more common idiopathic epilepsies with poorly defined etiology.

As one might have predicted, almost all of the epilepsy mutations so far identified are in genes for voltage- or ligand-gated ion channels. For example, more than 200 epilepsy mutations have been linked to SCN1A, the gene encoding the Nav1.1 subtype of the pore-forming α subunit of the voltage-gated sodium channel (Mulley et al., 2005, Harkin et al., 2007). Apparently, there is something special about this channel in the context of epilepsy. These mutations are associated with epilepsy syndromes that fit along a continuum of severity from the comparatively mild disorder, Generalized Epilepsy with Febrile Seizures Plus (GEFS+, Scheffer and Berkovic, 1997) to Severe Myoclonic Epilepsy of Infancy (SMEI, also known as Dravet syndrome, Dravet et al., 2005). GEFS+ is characterized by febrile seizures, which may persist beyond age six (thus the “+”) and may be accompanied by various other seizure types, including tonic–clonic, absence and myoclonic seizures. SMEI on the other hand is a severe epileptic encephalopathy, characterized by onset of febrile seizures by age 1 and the emergence between ages 1 and 4 of other seizure types, including myoclonic, focal, absence and atonic seizures, along with developmental decline. Several related sodium channelopathies, including Borderline SMEI and Intractable Childhood Epilepsy with Generalized Tonic–Clonic Seizures, fit within the GEFS+–SMEI spectrum (Singh et al., 2001, Mulley et al., 2005, Harkin et al., 2007).

To understand how Nav1.1 mutations cause epilepsy, investigators have introduced the mutations into Nav1.1 cDNAs, expressed the mutant recombinant channels in heterologous cell systems, and looked for changes in channel function, using electrophysiological recording. Since epilepsy is a disorder of network hyperexcitability, a tacit expectation has been that the epilepsy mutations alter sodium channel behavior in ways that increase the excitability of neurons expressing mutant channels. Indeed, a number of studies demonstrate effects on sodium channel behavior, which are consistent with this hypothesis (Spampanato et al., 2001, Spampanato et al., 2003, Lossin et al., 2002, Rhodes et al., 2004). However, not all the data fit well with this idea. Nav1.1 mutations responsible for GEFS+ have so far turned out to be missense mutations, which (at least in most cases and at least in the heterologous systems in which they have been characterized) form functional channels (Fig. 1), with altered properties. On the other hand, almost half of the approximately 200 mutations that cause SMEI result in truncated and presumably nonfunctional channels. Many of the remaining SMEI alleles are missense mutations, some of which are gain-of-function, whereas others are loss-of-function (Fig. 1). Furthermore, for both GEFS+ and SMEI, some of the reported gain-of-function effects of Nav1.1 mutations seem more compatible with decreased neuronal excitability than with increased excitability (Spampanato et al., 2001, Spampanato et al., 2003, Lossin et al., 2003, Rhodes et al., 2005, Mantegazza et al., 2005a, Barela et al., 2006).

Together, these data suggest that either gain or loss of sodium channel function and either increased or decreased neuronal excitability can cause epilepsy. The counterintuitive finding that loss of a voltage-gated sodium channel leads to an increase in network excitability may be explained by the phenotype of recently described Nav1.1 knockout mice (Yu et al., 2006). These mice have epilepsy as well as reduced excitability of immunochemically identified GABAergic neurons in the hippocampus (Yu et al., 2006) and GABAergic cerebellar Purkinje cells (Kalume et al., 2007). These data indicate that in wild type mice, Nav1.1 is a predominant sodium channel subtype in at least these two types of brain inhibitory neurons. A similar phenotype was observed in a knockin mouse line with a nonsense SMEI mutation in scn1a, the gene encoding mouse Nav1.1 (Ogiwara et al., 2007).

The knockout and knockin data suggest that loss of Nav1.1 function may result in widespread dysfunction of network inhibition, an ideal background for the genesis of seizures. But how do we explain the data suggesting that gain-of-function Nav1.1 mutations that ought to increase neuronal excitability also cause epilepsy, including both mild and severe phenotypes? This review builds a case for the hypothesis that Nav1.1 epilepsy mutations, whether missense or nonsense, whether gain-of-function or loss-of-function, whether responsible for GEFS+ or SMEI, have their ultimate pathophysiological effect by attenuating Nav1.1 sodium current and thus reducing the excitability of brain inhibitory neurons. According to this idea, the spectrum of disease severity from GEFS+ to SMEI reflects the degree of Nav1.1 attenuation (partial versus complete) along with individual differences in genetic background.