Abstracts

Rationale: Loss-of-function mutations in SCN1A, the gene encoding the voltage-gated sodium channel Nav1.1, are the most frequent cause of Dravet Syndrome (DS) a severe, pharmacoresistant infantile-onset epilepsy with multiple seizure types and comorbidities. Work with animal models of DS implicate decreased Nav1.1 expression as a direct contributor to seizures and co-morbidities through reduced inhibitory interneuron excitability, and resultant impaired network function. Both intrinsic effects of Scn1a mutations and acquired seizure-related changes may contribute to seizure susceptibility and comorbidities in DS. These experiments aim to isolate the intrinsic effects of Scn1a gene mutation in the absence of prior seizures by inducing Scn1a haploinsufficiency in a previously wild-type adult mouse. If seizure susceptibility of adult onset Scn1a haploinsufficiency is comparable to lifelong haploinsufficiency (DS), reduced sodium channel expression is a primary factor in determining the severity of the epilepsy phenotype. Methods: To induce adult onset loss of one copy of Scn1a, we utilized a globally expressing cre-recombinase coupled to a mutated estrogen receptor that is activated following binding of tamoxifen (ERT2-cre) crossed with our previously characterized heterozygous Scn1a flox line (F/+).  At birth, these mice are genotypically and phenotypically wild type (WT) but following tamoxifen exposure and cre-mediated gene re-arrangement become haploinsufficient for Scn1a.  Tamoxifen was administered at >P90 in ERT2-cre/+:F/+ (n=16) animals and +/+:F/+ controls (n=8).  Additional vehicle controls (ERT2-cre/+:F/+, n= 8; +/+:F/+, n=6) were treated simultaneously.  To allow time for Nav1.1 protein turnover, thermal sensitivity to seizure was assayed 2 weeks after Tamoxifen administration.  Thermal sensitivity to seizure was assayed by elevating each animal’s core body temperature in 0.5 C steps every 2 minutes until 41.0 C was achieved or the animal experienced a seizure. Results: Heterozygous, cre-mediated inactivation of Scn1a in previously wild type, unaffected adult mice resulted in thermally evoked seizures in 15/16 tamoxifen treated ERT2-cre:F/+ animals (40.2 C +/- 0.1).  This seizure threshold is similar to DS mice with haploinsufficiency from birth.  In contrast, 0/22 control animals were sensitive to thermally evoked seizures up to 41.0 C (Veh treated ERT2-cre:F/+ n=8; TAM or VEH treated +/+:F/+ n=14).   (binary logistic regression with Fisher’s scoring, p Conclusions: Inactivation of one copy of Scn1a in wild type, adult mice in the absence of mutation-related developmental effects or earlier in life seizures, is sufficient to produce seizure susceptibility comparable to lifelong haploinsufficiency (DS). These results provide evidence that DS epilepsy severity is determined primarily by gene mutation. Comorbidity severity may also be gene related. Funding: This work was supported by grants from the Dravet Syndrome Foundation and National Institutes of Health.