Abstract
Dravet syndrome is a severe epileptic encephalopathy, characterized by drug-resistant epilepsy, severe cognitive and behavioural deficits, with increased risk of sudden unexpected death (SUDEP). It is caused by haploinsufficiency of SCN1A gene encoding for the α-subunit of the voltage-gated sodium channel Nav1.1. Therapeutic approaches aiming to upregulate the healthy copy of SCN1A gene to restore its normal expression levels are being developed. However, whether Scn1a gene function is required only during a specific developmental time-window or, alternatively, if its physiological expression is necessary in adulthood is untested up to now. We induced Scn1a gene haploinsufficiency at two ages spanning postnatal brain development (P30 and P60) and compared the phenotypes of those mice to Scn1a perinatally induced mice (P2), recapitulating all deficits of Dravet mice. Induction of heterozygous Nav1.1 mutation at P30 and P60 elicited susceptibility to the development of both spontaneous and hyperthermia-induced seizures and SUDEP rates comparable to P2-induced mice, with symptom onset accompanied by the characteristic GABAergic interneuron dysfunction. Finally, delayed Scn1a haploinsufficiency induction provoked hyperactivity, anxiety and social attitude impairment at levels comparable to age matched P2-induced mice, while it was associated with a better cognitive performance, with P60-induced mice behaving like the control group. Our data show that maintenance of physiological levels of Nav1.1 during brain development is not sufficient to prevent Dravet symptoms and that long-lasting restoration of Scn1a gene expression would be required to grant optimal clinical benefit in patients with Dravet syndrome.