Effects in Neocortical Neurons of Mutations of the Na_v1.2 Na⁺Channel causing Benign Familial Neonatal-Infantile Seizures

Abstract

Mutations of voltage-gated Naϩ channels are the most common cause of familial epilepsy. Benign familial neonatal-infantile seizures (BFNIS) is an epileptic trait of the early infancy, and it is the only well characterized epileptic syndrome caused exclusively by mutations of Na v 1.2 Na ϩ channels, but no functional studies of BFNIS mutations have been done. The comparative study of the functional effects and the elucidation of the pathogenic mechanisms of epileptogenic mutations is essential for designing targeted and effective therapies. However, the functional properties of Na ϩ channels and the effects of their mutations are very sensitive to the cell background and thus to the expression system used. We investigated the functional effects of four of the six BFNIS mutations identified (L1330F, L1563V, R223Q, and R1319Q) using as expression system transfected pyramidal and bipolar neocortical neurons in short primary cultures, which have small endogenous Na ϩ current and thus permit the selective study of transfected channels. The mutation L1330F caused a positive shift of the inactivation curve, and the mutation L1563V caused a negative shift of the activation curve, effects that are consistent with neuronal hyperexcitability. The mutations R223Q and R1319Q mainly caused positive shifts of both activation and inactivation curves, effects that cannot be directly associated with a specific modification of excitability. Using physiological stimuli in voltage-clamp experiments, we showed that these mutations increase both subthreshold and action Na ϩ currents, consistently with hyperexcitability. Thus, the pathogenic mechanism of BFNIS mutations is neuronal hyperexcitability caused by increased Na ϩ current.

Molecular Requirements for Recognition of Brain Voltage-gated Sodium Channels by Scorpion α-Toxins

“…Overall, Lqh2 D8N,V10I seems to be a valuable toxin derivative in that it may be used as a specific probe for analysis of the distribution and function of rNa v 1.1-1.3 channels in various tissues, in studying their expression through embryonic development, and as a model for design of selective drugs in genetic disorders that involve these channels, such as epileptic seizures where the inactivation of Na v 1.2 is accelerated (8). The design of a selective Lqh2 derivative for rNa v 1.1-1.3 may now facilitate the design of ␣-toxins with high preference for other Na v s.…”

A rescuable folding defective Nav1.1 (SCN1A) sodium channel mutant causes GEFS+: Common mechanism in Nav1.1 related epilepsies?

“…Nine α subunits (Na v 1.1-Na v 1.9 for the proteins and SCN1A-SCN11A for the genes) and four β subunits (β1-β4 for the proteins and SCN1B-SCN4B for the genes) have been identified thus far (Catterall et al, 2005). Mutations of Na v 1.2 (SCN2A; MIM# 182390) cause benign familial neonatal-infantile seizures (BFNIS; MIM# 607745); mutations of β1 (SCN1B; MIM# 600235) cause generalized epilepsy with febrile seizures plus (GEFS+; MIM# 604233) type 1; mutations of Na v 1.1 (SCN1A; MIM# 182389), the most common target of epileptogenic mutations, cause GEFS+ type 2, severe myoclonic epilepsy of infancy (SMEI or Dravet syndrome; MIM# 607208), simple febrile seizures thus far in a single large family (FS; MIM# 604403), and may be involved in several infantile epileptic encephalopathies (Claes et al, 2001;Meisler and Kearney, 2005;Mantegazza et al, 2005a;Scalmani et al, 2006;Avanzini et al, 2007;Harkin et al, 2007).…”

Modulatory Proteins Can Rescue a Trafficking Defective Epileptogenic Nav1.1 Na+Channel Mutant

“…Mutations of Na v 1.1 (SCN1A) cause generalized epilepsy with febrile seizures plus (GEFSϩ) type 2, severe myoclonic epilepsy of infancy (SMEI), intractable childhood epilepsy with generalized tonic-clonic seizures (ICEGTC), and simple febrile seizures (FS); mutations of Na v 1.2 (SCN2A) cause benign familial neonatal-infantile seizures (BFNIS) and benign familial infantile seizures (BFIS); mutations of ␤1 (SCN1B) cause GEFSϩ type 1 (Avanzini and Franceschetti, 2003;Scheffer and Berkovic, 2003;Noebels, 2003;George, 2005;Mantegazza et al, 2005a;Meisler and Kearney, 2005;Scalmani et al, 2006;Avanzini et al, 2007).…”