Dravet syndrome patient‐derived neurons suggest a novel epilepsy mechanism

Abstract

OBJECTIVE Neuronal channelopathies cause brain disorders including epilepsy, migraine and ataxia. Despite the development of mouse models, pathophysiological mechanisms for these disorders remain uncertain. One particularly devastating channelopathy is Dravet Syndrome (DS), a severe childhood epilepsy typically caused by de novo dominant mutations in the SCN1A gene encoding the voltage-gated sodium channel Nav1.1. Heterologous expression of mutant channels suggests loss-of-function, raising the quandary of how loss of sodium channels underlying action potentials produces hyperexcitability. Mouse model studies suggest that decreased Nav1.1 function in interneurons causes disinhibition. We sought to determine how mutant SCN1A affects human neurons using the induced pluripotent stem cell (iPSC) method to generate patient-specific neurons. METHODS Forebrain-like pyramidal- and bipolar-shaped neurons are derived from two DS subjects and three human controls by iPSC reprogramming of fibroblasts. DS and control iPSC-derived neurons are compared using whole-cell patch clamp recordings. Sodium current density and intrinsic neuronal excitability are examined. RESULTS Neural progenitors from DS and human control iPSCs display a forebrain identity and differentiate into bipolar- and pyramidal-shaped neurons. DS patient-derived neurons show increased sodium currents in both bipolar- and pyramidal-shaped neurons. Consistent with increased sodium currents, both types of patient-derived neurons show spontaneous bursting and other evidence of hyperexcitability. Sodium channel transcripts are not elevated, consistent with a post-translational mechanism. INTERPRETATION These data demonstrate that epilepsy patient-specific iPSC-derived neurons are useful for modeling epileptic-like hyperactivity. Our findings reveal a previously unrecognized cell-autonomous epilepsy mechanism potentially underlying Dravet Syndrome, and offer a platform for screening new anti-epileptic therapies.

Model systems for studying cellular mechanisms ofSCN1A-related epilepsy

“…In contrast, two other studies reported increased excitability in iPSC-derived neurons carrying SCN1A mutations. Liu et al (2013b) found that a deletion of exon 14, which affects transmembrane segments 2-4 in domain II, and a truncation mutation (Y325X) within the pore loop of domain I confer increased sodium currents and hyperexcitability in both bipolar and pyramidal iPSC-derived neurons. Similarly, a missense DS-causing mutation (F1415I) resulted in increased excitability in glutamatergic neurons (Jiao et al 2013).…”

“…Similarly, a missense DS-causing mutation (F1415I) resulted in increased excitability in glutamatergic neurons (Jiao et al 2013). It was proposed that compensatory overexpression of other ␣-subunits in response to loss of Na V 1.1 may underlie the increased sodium currents in neurons derived from DS patient-specific iPSCs (Liu et al 2013b).…”

Incidence of Dravet Syndrome in a US Population

“…29,30 In contrast, a study using induced pluripotent stem cells from 2 patients with Dravet syndrome suggested that the SCN1A mutations led to increased excitability of both pyramidal neurons and interneurons, independent of inhibitory inputs. 31 A recent review that compiled all 1257 known pathogenic SCN1A mutations found that 82% of known mutations have been reported in only 1 family. 20 Thus, the majority of identified SCN1A mutations are newly described, as was the case in all of our patients.…”

Epileptic Encephalopathies: New Genes and New Pathways

“…iPSCs provide a particularly attractive model for neurologic disease, where access to live human tissue suitable for culture is extremely limited. Some recent studies have used iPSCs to model epilepsy mechanisms in Dravet syndrome [136]. These data suggest that epilepsy syndromespecific iPSC-derived neurons are useful for modeling epileptic-like hyperactivity, which offers a platform for screening new antiepileptic therapies.…”