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The Journal of Neuroscience, August 26, 2009, 29(34):10764-10778; doi:10.1523/JNEUROSCI.2475-09.2009

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Neurobiology of Disease
A Functional Null Mutation of SCN1B in a Patient with Dravet Syndrome

Gustavo A. Patino,^1,3 Lieve R. F. Claes,^4,5,6 Luis F. Lopez-Santiago,¹ Emily A. Slat,¹ Raja S. R. Dondeti,¹ Chunling Chen,¹ Heather A. O'Malley,² Charles B. B. Gray,¹ Haruko Miyazaki,⁸ Nobuyuki Nukina,⁸ Fumitaka Oyama,⁸ Peter De Jonghe,^4,5,6,7 and Lori L. Isom^1,2

¹Department of Pharmacology and Program in Neuroscience and ²Department of Pharmacology and Program in Cellular and Molecular Biology, University of Michigan School of Medicine, Ann Arbor, Michigan 48109-5632, ³Universidad de Los Andes Medical School, Bogotá, Colombia, ⁴Neurogenetics Group, Department of Molecular Genetics, Vlaams Instituut voor Biotechnologie, ⁵Laboratory of Neurogenetics, Institute Born-Bunge, ⁶University of Antwerp, and ⁷Division of Neurology, University Hospital of Antwerp, BE-2000 Antwerpen, Belgium, and ⁸Molecular Neuropathology Group, RIKEN Brain Science Institute, Saitama 351-0198, Japan

Correspondence should be addressed to Dr. Lori L. Isom, Department of Pharmacology, University of Michigan School of Medicine, 3422 Med Sci I, 1301 Catherine, SPC 5632, Ann Arbor, MI 48109-5632. Email: lisom{at}umich.edu

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_v1.1 subunits. Sodium channels are modulated by β1 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_v1.1 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 β1 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.

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Received May 27, 2009; revised July 21, 2009; accepted July 27, 2009.

Correspondence should be addressed to Dr. Lori L. Isom, Department of Pharmacology, University of Michigan School of Medicine, 3422 Med Sci I, 1301 Catherine, SPC 5632, Ann Arbor, MI 48109-5632. Email: lisom{at}umich.edu

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