Neuronal sodium channel SCN8A: Genomic organization, alternative splicing, and role in neurological disease.

Abstract

The mouse neurological mutant motor endplate disease (med) is characterized by cerebellar ataxia, dystonia, and progressive paralysis. We identified a new allele, $med\sp{tg},$ that was induced by random transgene insertion. We used the transgene insertion to isolate the mutated gene, which is a novel voltage-gated sodium channel $\alpha$ subunit gene, Scn8a. Scn8a is highly expressed in neurons of the brain and spinal cord. The human ortholog, SCN8A, is located on chromosome 12q13 and is a candidate gene for inherited neurological disease. To facilitate mutation detection in human patients, we isolated 28 exons containing the coding region of the human gene. Exon borders within the transmembrane domains are conserved in the skeletal muscle sodium channel SCN4A, the cardiac muscle sodium channel SCN5A, and the sensory neuron sodium channel SCN10A. We identified two alternative copies of exon 18, which encode the transmembrane segments S3 and S4 in domain III. Exon 18N is expressed in fetal brain and non-neuronal tissues. Transcripts containing 18N have a conserved in-frame stop codon and encode a truncated, two-domain protein. The major transcript in adult brain contains exon 18A. The switch between 18N and 18A during brain development is recapitulated in P19 cells in culture during retinoic acid-induced neuronal differentiation. SCN8A thus provides a new model of differentiation-specific splicing in neurons. Genomic analysis of SCN8A from human, mouse, and fish demonstrated a conserved structure in which exon 18N is located 300-500 bp upstream of exon 18A. Duplication of exon 18 thus preceeded the divergence of fish and mammals. To determine whether the related sodium channel SCN2A can replace SCN8A, we generated transgenic mice that over-express SCN2A in brain and spinal cord. This transgene failed to correct the lethal phenotype of med mice. SCN8A thus encodes a sodium channel protein with unique properties required for spinal motor neuron function.