**2.4 Nav1.6**

*Epilepsy - Advances in Diagnosis and Therapy*

steady-state inactivation, increased persistent Na+

The mutation of the Nav1.2 gene SCN2A is associated with various epilepsies, such as benign familial neonatal seizures (BFNIS), hereditary epilepsy with febrile seizures plus (GEFS+), Dravet's syndrome (DS), and other stubborn childhood epilepsy encephalopathy. Nav1.2 subunit is mainly distributed in the axon-initiating segment (AIS) and node of Ranvier. SCN2A mutations cause changes in VGSC function and expression and result in abnormal neuronal discharge. Because Nav1.2 plays an important role in the AIS area during the development, it is more common for infants to show SCN2A mutant-induced epilepsy encephalopathy [13]. BFNIS is the most common phenotype caused by gain-of-function missense mutations in SCN2A [14]. Up to now, at least 10 SCN2A mutations associated with BFNIS have been identified. SCN2A mutations are also found to result in the reduced expression of Nav1.2 on the surface of neurons [15]. Therefore, SCN2A mutants will lead to the decrease of sodium current density at node of Ranvier and AIS, seriously affecting the excitability of neurons [16]. For missense mutation of SCN2A, p.Tyr1589Cys causes a depolarizing shift of

tion, and an acceleration of its recovery, which contribute to neuronal hyperexcitability and familial epilepsy [17]. Due to the SCN2A mutation, early infantile epileptic encephalopathy (EIEE) patients with burst suppression and tonic-clonic migrating partial seizures showed a specific dose-dependent efficacy of VGSC blockers [18]. It is mainly caused by the dysfunction of VGSC [19]. By replacing neonatal Nav1.2 with adult Nav1.2 in mice, it has been suggested that neonatal Nav1.2 reduced neuronal excitability and had a significant impact on seizure susceptibility and behavior.

The SCN3A gene, clustered on human chromosome 2q24, encodes the Nav1.3 subtype [20], which is usually located in the soma of neurons. It is important in the integration of synaptic signals, determination of the depolarization threshold, and AP transmission [21]. In contrast to the rodent gene which is transiently expressed during development, human SCN3A is widely expressed in adult brain [22]. The first epilepsy-associated mutation (K354Q ) in SCN3A was found in 2008. K354Q mutation decreased inactivation rate and increased INaP [23]. The mutation is not sensitive to antiepilepsy drug carbamazepine or oxcarbazepine. K354Q mutation causes neuronal abnormal spontaneous discharge and membrane potential paroxysmal depolarization [24]. In 2014, four more missense variants were identified in SCN3A, which are R357Q, D766N, E1111K, and M1323V [25]. Compared to wildtype channels, R357Q caused smaller currents, slower activation, and depolarized voltage dependences of activation and inactivation. The E1111K mutation evoked a significantly greater level of persistent sodium current. All four mutants increase current activation in response to depolarizing voltage ramps. These findings support for a contribution of Nav 1.3 to childhood epilepsy. Recently, a novel SCN3A variant (L247P) was identified by whole exome sequencing of a child with focal epilepsy, developmental delay, and autonomic nervous system dysfunction. Voltage clamp analysis showed no detectable sodium currents in a heterologous expression system. To further test the possible clinical consequences of reduced SCN3A activity, they investigated the effect of a hypomorphic Scn3a allele (Scn3a Hyp) on seizure susceptibility and behavior using a gene trap mouse line. Heterozygous SCN3A mutant mice (SCN3A+/Hyp) neither exhibit spontaneous seizures nor hyperthermia-induced seizures, but they displayed increased susceptibility to electroconvulsive- and chemiconvulsive-induced seizures, which provide evidence that loss-of-function of

SCN3A may contribute to increased seizure susceptibility [26].

current, a slowing of fast inactiva-

**2.2 Nav1.2**

**2.3 Nav1.3**

**174**

Nav1.6, mainly distributed to the soma and synaptic origin, is important for APs generation and propagation [27]. In the development process, Nav1.2 is gradually replaced by Nav1.6 in the mature node of Ranvier [28]. The first heterozygous missense mutation (p.Asn1768Asp) in the Nav1.6 gene SCN8A was identified in 2012 by whole-genome sequencing (WGS) in a patient with severe epileptic encephalopathy who exhibited early-onset seizures, autistic features, intellectual disability, ataxia, and sudden unexpected death in epilepsy (SUDEP) [29]. Since this initial discovery, more than 100 pathogenic SCN8A variants have been identified in patients with epilepsy [30]. Most of the SCN8A variants have been detected in individuals with EIEE.

Different mutations in the SCN8A gene encoding Nav1.6 have different effects on epilepsy. For the missense mutation V929F, an evolutionarily conserved residue in the pore loop of domain II of Nav1.6, it was found that heterozygous mutations produced well-defined spike-wave discharges and are associated to absence epilepsy in mice [31]. However, missense mutations in Scn8amed−jo were able to improve the epilepsy symptoms of SCN1A+/<sup>−</sup> heterozygotes. The mechanism might be the decrease in Nav1.6 expression of excitatory neurons compensating for the loss of Nav1.1 in inhibitory neurons [32]. Recently, more and more de novo and inherited SCN8A epilepsy mutations were detected by gene panel analysis [33]. For example, loss-of-function mutants [34], underlying the complex seizure phenotype, were identified using specific mouse line. It was suggested that decreasing Scn8a expression in cortical excitatory neurons could reduce seizures. On the contrary, the decreasing expression of SCN8A in the thalamic reticular nucleus (RT) leads to absence seizures. Loss of Scn8a will impair tonic firing mode behavior and impair desynchronizing recurrent RT-RT synaptic inhibition in the thalamic reticular nucleus, which suggested that Scn8a-mediated hypofunction in cortical circuits, conferring convulsive seizure resistance, while hypofunction in the thalamus is sufficient to generate absence seizures.

### **2.5 Nav1.7**

The SCN9A gene encodes the Nav1.7 subtype, which was initially identified in the peripheral nervous system, sympathetic ganglion, and olfactory sensory neurons [35–38]. Nav1.7 is also found expressed in the central nervous system such as in the cerebral cortex and hippocampus [39]. A missense mutation of SCN9A (N641Y), at a conserved amino acid residue located at the intracellular loop between domain I and II, is associated with a family of febrile seizures (FS, N641Y). Mice carrying N641Y mutations were more susceptible to electrical stimulation-induced clonic and tonic seizures [40]. However, it is still unclear how SCN9A gene mutation caused epilepsy in the CNS.
