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A functional null mutation of SCN1B in a patient with Dravet syndrome.

ABSTRACT: Dravet syndrome (also called severe myoclonic epilepsy of infancy) is one of the most severe forms of childhood epilepsy. Most patients have heterozygous mutations in SCN1A, encoding voltage-gated sodium channel Na(v)1.1 alpha subunits. Sodium channels are modulated by beta1 subunits, encoded by SCN1B, a gene also linked to epilepsy. Here we report the first patient with Dravet syndrome associated with a recessive mutation in SCN1B (p.R125C). Biochemical characterization of p.R125C in a heterologous system demonstrated little to no cell surface expression despite normal total cellular expression. This occurred regardless of coexpression of Na(v)1.1 alpha subunits. Because the patient was homozygous for the mutation, these data suggest a functional SCN1B null phenotype. To understand the consequences of the lack of beta1 cell surface expression in vivo, hippocampal slice recordings were performed in Scn1b(-/-) versus Scn1b(+/+) mice. Scn1b(-/-) CA3 neurons fired evoked action potentials with a significantly higher peak voltage and significantly greater amplitude compared with wild type. However, in contrast to the Scn1a(+/-) model of Dravet syndrome, we found no measurable differences in sodium current density in acutely dissociated CA3 hippocampal neurons. Whereas Scn1b(-/-) mice seize spontaneously, the seizure susceptibility of Scn1b(+/-) mice was similar to wild type, suggesting that, like the parents of this patient, one functional SCN1B allele is sufficient for normal control of electrical excitability. We conclude that SCN1B p.R125C is an autosomal recessive cause of Dravet syndrome through functional gene inactivation.

SUBMITTER: Patino GA

PROVIDER: S-EPMC2749953 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description: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 Na(v) 1.1. 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 Na(v) 1.1 function in interneurons causes disinhibition. We aim to determine how mutant SCN1A affects human neurons using the induced pluripotent stem cell (iPSC) method to generate patient-specific neurons. METHODS:Here we derive forebrain-like pyramidal- and bipolar-shaped neurons from 2 DS subjects and 3 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 DS, and offer a platform for screening new antiepileptic therapies.

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A functional null mutation of SCN1B in a patient with Dravet syndrome.

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