Who moved my protein? Mechanisms of Epileptogenesis due to Mutations of Voltage-Gated Sodium Channel SCN1B.

Abstract

Voltage-gated sodium channels (VGSCs) play a role in the generation of action potentials in excitable cells, including neurons. VGSCs contain one pore-forming α subunit, one non-covalently linked (β1 or β3) and one covalently linked β subunit (β2 or β4). VGSC β subunits participate in channel modulation and cell adhesion. SCN1B, the gene encoding β1, is expressed as two splice variants: β1 and β1B. Both splice variants share a signal peptide and extracellular immunoglobulin loop domain in their N-termini. In contrast, the C-termini of each protein have little to no conservation. β1 contains a transmembrane domain, while β1B does not. Our results show that β1B is a secreted cell adhesion molecule that promotes neurite outgrowth. β1B is the predominantly expressed SCN1B splice variant during fetal brain development. We predict that β1B plays important roles in the establishment of neuronal excitability and disruptions in its expression may lead to brain disease. Heterozygous mutations in SCN1B have been reported as a cause of mild to moderate forms of Genetic Epilepsy with Febrile Seizures Plus (GEFS+). Here we report the first case of the epileptic encephalopathy Dravet Syndrome, characterized at the severe end of the GEFS+ spectrum, associated with a homozygous mutation of SCN1B. Our work demonstrates that the protein generated by this mutation, p.R125C, is trafficking deficient, resulting in the functional null phenotype in homozygous probands. Consistent with this, we propose that Scn1b null mice are an animal model of Dravet Syndrome. To date, all reported SCN1B mutations associated with epilepsy are located in the region common to β1 and β1B. Here we describe the first SCN1B mutation in the region exclusive to β1B associated with primary generalized epilepsy in heterozygous state. The protein produced by the mutation, p.G257R, is trafficking deficient, similar to p.R125C. Not surprisingly, its ability to promote neurite outgrowth in vitro is abolished. These results support our hypothesis that β1B plays a role in axon guidance during fetal development of the CNS. Taken together, this thesis work makes significant and novel contributions to our understanding of the role of VGSC SCN1B in normal brain development and neurological disease.