Author(s): Wenxi Yu, University of Michigan; James Xenakis, University of North Carolina at Chapel Hill; Jacy L. Wagnon, Ohio State University; Megan K. Mulligan, University of Tennessee Health Science Center; Robert W. Williams, University of Tennessee Health Science Center; Fernando Pardo-Manuel de Villena, University of North Carolina at Chapel Hill; Miriam H. Meisler, University of Michigan //

ABSTRACT: SCN8A encephalopathy is a developmental epileptic encephalopathy that can result from de novo gain-of-function mutations in Nav 1.6. Affected individuals exhibit refractory seizures, developmental delay, cognitive disabilities, movement disorders, and elevated risk of sudden death. Patients with the identical SCN8A variant can differ in clinical course, suggesting a role for modifier genes in determining disease severity. To identify genetic modifiers of DEE, we generated F1 and F2 crosses between inbred mouse strains and mice carrying the human pathogenic variants SCN8A-R1872W and SCN8A-N1768D. Quantitative trait locus (QTL) analysis of seizure-related phenotypes was used for chromosomal mapping of modifier loci. In an F2 cross between strain SJL/J and C57BL/6J mice carrying the patient mutation R1872W, we identified a major QTL on chromosome 5 containing the Gabra2 gene. Strain C57BL/6J carries a splice site deletion mutation that results in a 75% reduction in expression of Gabra2, which encodes the α2 subunit of the aminobutyric acid type A (GABAA) receptor. Homozygosity for the hypomorphic Gabra2 mutation was associated with early seizure onset and shortened life span in the F2 mice. We confirmed Gabra2 as the modifier gene in crosses with a mouse knock-in allele in which the splice site mutation is corrected, with restoration of wildtype expression. Correction of the Gabra2 mutation increased the age of seizure onset, and extends survival of the Scn8a mutant mice from 22 days to 72 days. GABAA receptors containing the α2 subunit provide inhibitory input to the axon initial segment of excitatory neurons. Consistent with our observations in the mouse, loss-of-function variants of GABRA2 have been identified in patients with epileptic encephalopathy. Quantitative variation in human GABAA receptor expression may contribute to variation in the severity of genetic epilepsies, suggesting a potential therapeutic target.

Source of Funding: R01NS034509