부산대학교 도서관

Dravet syndrome (DS) is a developmental and epileptic encephalopathy that results from mutations in the Nav1.1 sodium channel encoded by SCN1A. Most known DS-causing mutations are in coding regions of SCN1A, but we recently identified several disease-associated SCN1A mutations in intron 20 that are within or near to a cryptic and evolutionarily conserved "poison" exon, 20N, whose inclusion is predicted to lead to transcript degradation. However, it is not clear how these intron 20 variants alter SCN1A expression or DS pathophysiology in an organismal context, nor is it clear how exon 20N is regulated in a tissue-specific and developmental context. We address those questions here by generating an animal model of our index case, NM_006920.4(SCN1A):c.3969+2451G>C, using gene editing to create the orthologous mutation in laboratory mice. Scn1a heterozygous knock-in (+/KI) mice exhibited an ~50% reduction in brain Scn1a mRNA and Nav1.1 protein levels, together with characteristics observed in other DS mouse models, including premature mortality, seizures, and hyperactivity. In brain tissue from adult Scn1a +/+ animals, quantitative RT-PCR assays indicated that ~1% of Scn1a mRNA included exon 20N, while brain tissue from Scn1a +/KI mice exhibited an ~5-fold increase in the extent of exon 20N inclusion. We investigated the extent of exon 20N inclusion in brain during normal fetal development in RNA-seq data and discovered that levels of inclusion were ~70% at E14.5, declining progressively to ~10% postnatally. A similar pattern exists for the homologous sodium channel Nav1.6, encoded by Scn8a. For both genes, there is an inverse relationship between the level of functional transcript and the extent of poison exon inclusion. Taken together, our findings suggest that poison exon usage by Scn1a and Scn8a is a strategy to regulate channel expression during normal brain development, and that mutations recapitulating a fetal-like pattern of splicing cause reduced channel expression and epileptic encephalopathy. Author summary: Dravet syndrome (DS) is a neurological disorder affecting approximately 1:15,700 Americans that causes generalized epilepsy and associated complications. While most patients have a mutation in the SCN1A gene that encodes the Nav1.1 voltage-gated sodium channel, about 20% do not have a mutation identified by exome or targeted sequencing. Recently, we identified variants in intron 20, a noncoding region of SCN1A, in some DS patients. We hypothesized that these variants alter SCN1A transcript processing, decrease Nav1.1 function, and lead to DS pathophysiology via inclusion of exon 20N, a "poison" exon that leads to a premature stop codon. In this study, we generated a knock-in mouse model, Scn1a+/KI, of one of these variants, which resides in a genomic region that is extremely conserved across vertebrate species. We found that Scn1a+/KI mice have reduced levels of Scn1a transcript and Nav1.1 protein and develop DS-related phenotypes. We find that transcripts from brains of Scn1a+/KI mice show elevated rates of Scn1a exon 20N inclusion. We also explored the relationship between exon 20N inclusion and Scn1a expression during development, and found that, during brain development when Scn1a expression is low, exon 20N inclusion is high; postnatally, as Scn1a expression increases, there is a corresponding decrease in exon 20N usage. Expression of another voltage-gated sodium channel transcript, Scn8a (Nav1.6), was similarly regulated. Together, these data demonstrate that poison exon inclusion is a conserved mechanism to control sodium channel expression in the brain, and that an intronic mutation that disrupts the normal developmental regulation of poison exon inclusion leads to reduced Nav1.1 and generalized epilepsy. [ABSTRACT FROM AUTHOR]