A novel de novo mutation of SCN8A (Nav1.6) with enhanced channel activation in a child with epileptic encephalopathy

Neurobiol Dis. 2014 Sep;69:117-23. doi: 10.1016/j.nbd.2014.05.017. Epub 2014 May 27.

Abstract

Rare de novo mutations of sodium channels are thought to be an important cause of sporadic epilepsy. The well established role of de novo mutations of sodium channel SCN1A in Dravet Syndrome supports this view, but the etiology of many cases of epileptic encephalopathy remains unknown. We sought to identify the genetic cause in a patient with early onset epileptic encephalopathy by whole exome sequencing of genomic DNA. The heterozygous mutation c. 2003C>T in SCN8A, the gene encoding sodium channel Nav1.6, was detected in the patient but was not present in either parent. The resulting missense substitution, p.Thr767Ile, alters an evolutionarily conserved residue in the first transmembrane segment of channel domain II. The electrophysiological effects of this mutation were assessed in neuronal cells transfected with mutant or wildtype cDNA. The mutation causes enhanced channel activation, with a 10mV depolarizing shift in voltage dependence of activation as well as increased ramp current. In addition, pyramidal hippocampal neurons expressing the mutant channel exhibit increased spontaneous firing with PDS-like complexes as well as increased frequency of evoked action potentials. The identification of this new gain-of-function mutation of Nav1.6 supports the inclusion of SCN8A as a causative gene in infantile epilepsy, demonstrates a novel mechanism for hyperactivity of Nav1.6, and further expands the role of de novo mutations in severe epilepsy.

Keywords: De novo mutation; Epilepsy; Epileptic encephalopathy; Sodium channel.

Publication types

  • Case Reports
  • Research Support, N.I.H., Extramural

MeSH terms

  • Action Potentials / physiology
  • Age of Onset
  • Animals
  • DNA Mutational Analysis
  • Epilepsy / genetics*
  • Epilepsy / physiopathology
  • HEK293 Cells
  • Hippocampus / physiopathology
  • Humans
  • Membrane Potentials / physiology
  • Mutation, Missense*
  • NAV1.6 Voltage-Gated Sodium Channel / genetics*
  • NAV1.6 Voltage-Gated Sodium Channel / metabolism*
  • Neurons / physiology*
  • Patch-Clamp Techniques
  • Pyramidal Cells / physiopathology
  • Rats
  • Rats, Sprague-Dawley
  • Sequence Homology, Amino Acid
  • Transfection

Substances

  • NAV1.6 Voltage-Gated Sodium Channel
  • SCN8A protein, human