Regulation and Mutation of Voltage-Gated Sodium Channel SCN8A (Nav1.6).

Abstract

The gene SCN8A encodes voltage-gated sodium channel Nav1.6, which is widely expressed in the nervous system and contributes to firing patterns in many types of neurons. In this thesis, I investigated three aspects of the molecular genetics of Nav1.6. SCN8A contains two alternative exons,18N and 18A, with tissue specific splicing. In brain, the major SCN8A transcript contains exon 18A and encodes the full-length sodium channel. In other tissues, the major transcript contains exon 18N and encodes a truncated protein, due to the presence of an in-frame stop codon in exon 18N. I demonstrated that the neuronal RBFOX splice factors contribute to the cell-specific expression of Nav1.6 in mature neurons and that transcripts containing exon 18N are targets of nonsense mediated decay. RBFOX proteins thus regulate the temporal and spatial expression of Nav1.6. The mechanism by which Nav1.6 is trafficked to the surface of neurons is not well understood. Previous work from our lab implicated the N-terminal domain in this process. I discovered a novel interaction between the N-terminus of Nav1.6 and the microtubule-associated protein Map1b that mediates channel expression at the surface of transfected cells. This interaction may be involved in trafficking Nav1.6 to the axon initial segment or node of Ranvier. I mutagenized the Map1b binding site of the GFP-tagged Nav1.6 cDNA for ongoing functional tests in co-cultures of neurons and Schwann cells. I also studied several novel pathogenic mutations of human SCN8A. The de novo heterozygous missense mutation p.Asn1768Asp in SCN8A was identified in a patient with epileptic encephalopathy by whole genome sequencing. This mutation causes increased persistent current and hyperexcitability in cell culture assays, consistent with the neuronal hyperexcitability in epilepsy. Two additional mutations of SCN8A were identified in patients with epilepsy and one in a patient with intellectual disability. I introduced these mutations into the Nav1.6 cDNA for further electrophysiologcal characterization. This work has advanced understanding of the basic biology of Nav1.6 and the role of genetic variation of SCN8A in human neurological and neuropsychiatric disorders.