Generation and basic characterization of a gene‐trap knockout mouse model of Scn2a with a substantial reduction of voltage‐gated sodium channel Na_v1.2 expression

Abstract

Large‐scale genetic studies revealed SCN2A as one of the most frequently mutated genes in patients with neurodevelopmental disorders. SCN2A encodes for the voltage‐gated sodium channel isoform 1.2 (Nav1.2) expressed in the neurons of the central nervous system. Homozygous knockout (null) of Scn2a in mice is perinatal lethal, whereas heterozygous knockout of Scn2a (Scn2a+/−) results in mild behavior abnormalities. The Nav1.2 expression level in Scn2a+/− mice is reported to be around 50–60% of the wild‐type (WT) level, which indicates that a close to 50% reduction of Nav1.2 expression may not be sufficient to lead to major behavioral phenotypes in mice. To overcome this barrier, we characterized a novel mouse model of severe Scn2a deficiency using a targeted gene‐trap knockout (gtKO) strategy. This approach produces viable homozygous mice (Scn2agtKO/gtKO) that can survive to adulthood, with about a quarter of Nav1.2 expression compared to WT mice. Innate behaviors like nesting and mating were profoundly disrupted in Scn2agtKO/gtKO mice. Notably, Scn2agtKO/gtKO mice have a significantly decreased center duration compared to WT in the open field test, suggesting anxiety‐like behaviors in a novel, open space. These mice also have decreased thermal and cold tolerance. Additionally, Scn2agtKO/gtKO mice have increased fix‐pattern exploration in the novel object exploration test and a slight increase in grooming, indicating a detectable level of repetitive behaviors. They bury little to no marbles and have decreased interaction with novel objects. These Scn2a gene‐trap knockout mice thus provide a unique model to study pathophysiology associated with severe Scn2a deficiency.

Severe deficiency of the voltage-gated sodium channel NaV1.2 elevates neuronal excitability in adult mice

“…We found that heterozygous (HET) Scn2a WT/gt mice have ~60% of WT Na V 1.2 protein level in homogenate from CPu tissues, whereas HOM Scn2a gt/gt mice have a much lower level at ~32% ( Figure S1C ). This result is largely consistent with our initial characterization of this mouse model using whole-brain homogenates ( Eaton et al, 2021 ).…”

“…To understand the consequences of severe Na V 1.2 deficiency in neurons, we utilized a Na V 1.2-deficient mouse model generated using a gene trap (gt) approach ( Eaton et al, 2021 ). Homozygous (HOM) Scn2a gt/gt mice can survive to adulthood and have a substantial reduction of Na V 1.2 expression (~25% of the wild-type [WT] level) ( Eaton et al, 2021 ).…”

“…To understand the consequences of severe Na V 1.2 deficiency in neurons, we utilized a Na V 1.2-deficient mouse model generated using a gene trap (gt) approach ( Eaton et al, 2021 ). Homozygous (HOM) Scn2a gt/gt mice can survive to adulthood and have a substantial reduction of Na V 1.2 expression (~25% of the wild-type [WT] level) ( Eaton et al, 2021 ). Because the gt cassette contains a LacZ element that is driven by the native Na V 1.2 promoter ( Figure S1A ; Skarnes et al, 2004 , 2011 ), we used LacZ staining as a surrogate to determine the expression and distribution of Na V 1.2 in the brain.…”

“…(Skarnes et al, 2004(Skarnes et al, , 2011. The generation and basic characterization of this mouse model are available in our recent article (Eaton et al, 2021). The targeting construct (tm1a trapping cassette) was electroporated into C57BL/6N embryonic stem cells, and founders in a pure C57BL/6N background were obtained to produce mice for experiments.…”

“…A substantial reduction of gene expression may be essential to produce robust phenotypes in the mouse model of Scn2a . Because Scn2a null (100% knockout) is lethal, we generated a Na V 1.2-deficient mouse model via a gene trap strategy ( Eaton et al, 2021 ). Using this mouse model, we investigated how severe Na V 1.2 deficiency affects neuronal excitabilities in principal neurons of the corticostriatal circuit.…”