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Aberrant regulation of a poison exon caused by a non-coding variant in a mouse model of Scn1a-associated epileptic encephalopathy.

Citation
Voskobiynyk, Y., et al. “Aberrant Regulation Of A Poison Exon Caused By A Non-Coding Variant In A Mouse Model Of Scn1A-Associated Epileptic Encephalopathy.”. Plos Genetics, p. e1009195.
Center University of Alabama at Birmingham
Author Yuliya Voskobiynyk, Gopal Battu, Stephanie A Felker, Nicholas Cochran, Megan P Newton, Laura J Lambert, Robert A Kesterson, Richard M Myers, Gregory M Cooper, Erik D Roberson, Gregory S Barsh
Abstract

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.

Year of Publication
2021
Journal
PLoS genetics
Volume
17
Issue
1
Number of Pages
e1009195
Date Published
01/2021
ISSN Number
1553-7404
DOI
10.1371/journal.pgen.1009195
Alternate Journal
PLoS Genet
PMID
33411788
PMCID
PMC7790302
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