Alternative transcripts of the sodium channel <italic>SCN8A</italic> and mutation of <italic>Grid2</italic> in the tapdancer mouse.

Abstract

Two genes encoding neuronal channel proteins involved in movement disorders have been investigated. I determined that a 491 bp deletion in the transcript of the ionotrophic glutamate receptor family member <italic>Grid2</italic> is responsible for the movement disorder in the tapdancer mutant mouse. The phenotype of the <italic>Grid2</italic> mutant mice suggests that this gene may contribute to human movement disorders. Mutations of <italic>SCN8A</italic> encoding the voltage-gated sodium channel Na_V1.6 result in movement disorders and impaired cognition. I used 5<super>'</super>RACE to identify four mutually exclusive 5<super> '</super> untranslated exons in a 2 kb region located 70 kb upstream of the first coding exon. The 4 noncoding exons are transcribed during development and throughout the adult nervous system. A 5 kb genomic fragment containing the 4 untranslated exons has tissue-specific promoter activity in transgenic mice. By 3<super>'</super>RACE I identified two alternative polyadenylation signals located 2.5 and 5.2 kb downstream of the stop codon. Comparison of <italic>Scn8a</italic> transcripts from purified astrocytes, oligodendrocytes and retinal ganglion cells revealed identical transcription start and polyadenylation sites but differential splicing of coding exon 18. Retinal ganglion cell transcripts contain only exon 18A and encode full-length Na_V1.6 protein. Transcripts from astrocytes and oligodendrocytes contain exon 18N with an in-frame stop codon and encode a truncated, two-domain protein predicted to lack channel activity. Exon 18N is conserved in vertebrates and invertebrates, and may protect non-neuronal cells from toxic effects of active sodium channels. Sequencing of vertebrate genomes has made it possible to identify regulatory elements by evolutionary conservation. 5<super>'</super>RACE of chicken brain RNA identified a single 5<super>'</super> noncoding exon of <italic> Scn8a</italic> with 72% sequence identity to mammalian exon 1c. This exon is conserved in zebrafish and fugu, and appears to encode the ancestral 5<super> '</super> UTR. A 470 bp fragment containing exon 1c exhibits promoter activity in transfected MN-1 cells. Four short elements of 18--22 bp within exon 1c are highly conserved in fish, chicken and mammals. We predict that these elements are may regulate <italic>Scn8a</italic> transcription or translation and are potential sites of human disease mutations.