R1617Q epilepsy mutation slows Na 1.6 sodium channel inactivation and increases the persistent current and neuronal firing.

KEY POINTS

A human Na 1.6 construct was established to study the biophysical consequences of the R1617Q mutation on Na 1.6 identified in patients with unclassified epileptic encephalopathy and severe intellectual disability. The R1617Q mutation disrupts the inactivation process of the channel, and more specifically, slows the current decay, increases the persistent sodium current that was blocked by tetrodotoxin and riluzole, and disrupts the inactivation voltage-dependence and increases the kinetics of recovery. In native hippocampal neurons, the R1617Q mutation exhibited a significant increase in action potentials triggered in response to stimulation and a significant increase in the number of neurons that exhibited spontaneous activity compared to neurons expressing WT channels that were inhibited by riluzole. The abnormally persistent current activity caused by the disruption of the channel inactivation process in Na 1.6/R1617Q may result in epileptic encephalopathy in patients.

ABSTRACT

The voltage-gated sodium channel Na 1.6 is the most abundantly expressed sodium channel isoform in the central nervous system. It plays a critical role in saltatory and continuous conduction. Although over 40 Na 1.6 mutations have been linked to epileptic encephalopathy, only a few have been functionally analysed. In the present study, we characterized a Na 1.6 mutation (R1617Q) identified in patients with epileptic encephalopathy and intellectual disability. R1617Q substitutes an arginine for a glutamine in the S4 segment of domain IV, which plays a major role in coupling the activation and inactivation of sodium channels. We used patch-clamp to show that R1617Q is a gain-of-function mutation. It is typified by slower inactivation kinetics and a loss of inactivation of voltage-dependence, which result in a 2.5-fold increase in the window current. In addition, sodium currents exhibited an enhanced rate of recovery from inactivation, most likely due to the destabilization of the inactivation state. The alterations in the fast inactivation caused a significant increase in the persistent sodium current. Overexpression of R1617Q in rat hippocampal neurons resulted in an increase in action potential firing activity that was inhibited by riluzole, consistent with the gain-of-function observed. We conclude that the R1617Q mutation causes neuronal hyperexcitability and may result in epileptic encephalopathy.