Dravet Syndrome (DS) is a devastating childhood neuropsychiatric disorder caused by de novo, heterozygous loss-of-function mutations in brain type-I voltage-gated Na channel Nav1.1. We have developed a mouse genetic model with all the features of DS, including susceptibility to thermally induced seizures and spontaneous seizures, ataxia, circadian rhythm and sleep disorders, cognitive deficit, autistic-like features, and premature death. All these effects are correlated with loss of Na currents and excitability of GABAergic interneurons, without effects on excitatory neurons, which causes imbalance of excitation vs. inhibition in neural circuits. Mutation of Nav1.1 channels specifically in forebrain GABAergic interneurons by the Cre-Lox method is sufficient to cause the major DS symptoms, confirming that DS is caused by loss of Nav1.1 channels in inhibitory neurons. Remarkably, cognitive deficit and autistic-like behaviors of DS mice can be rescued by treatment with a low dose of the GABA-A receptor co-activator clonazepam, demonstrating that these life- changing co-morbidities are caused by the mutation of Nav1.1 channels rather than by neuronal damage from recurrent seizures. Our central hypothesis is that epilepsy and co-morbidities in DS result from failure of excitability of GABAergic inhibitory neurons, which creates an imbalance of excitation vs. inhibition in neural circuits, and that re-balancing excitation and inhibition with drug treatment will have therapeutic value. To further advance understanding of pathophysiology and treatment of DS, we propose four experimental approaches. (i) We will examine hyperexcitability of cells and circuits using specific deletion of Nav1.1 channels in different classes of interneurons in specific brain regions by the Cre-Lox method. We will use brain slice recording methods to document changes in excitability of cells and circuits. (ii) We will use immunocytochemical and mouse genetic approaches to identify the sites of hyperexcitability that appear first in DS mice in vivo and thereby determine the time course of changes in excitability of cells and circuits that lead to epilepsy. (iii) Patients with DS have prolonged episodes of status epilepticus. We will induce status epilepticus in DS mice by thermal stimulation and determine the changes in excitability of cells and circuits. We will assess cell injury by measurements of reactive astrocytes, neuroinflammation, apoptosis, and neuronal cell death. (iv) Our results indicate that rationally designed drug combinations that increase GABAergic neurotransmission are effective in reducing seizures and premature death in DS mice with minimal side effects and that a single low-dose clonazepam treatment can rescue cognitive deficit and autistic-like behaviors. We will optimize combination therapy with a benzodiazepine plus tiagabine in order to develop a therapeutic regimen that prevents seizures, premature death, and cognitive impairment and minimizes effects of tolerance on prolonged therapy. We will examine new-generation, subtype-selective GABA-A receptor activators and test effectiveness of these therapies approaches to control seizures and co-morbidities in DS mice.
Dravet Syndrome (DS) is a devastating childhood neuropsychiatric disorder caused by de novo, heterozygous loss-of-function mutations in brain type-I voltage-gated Na channel Nav1.1. We have developed a mouse genetic model with all the features of DS. This project will provide new insights into the mechanisms of pathophysiology of DS and will develop new therapeutic approaches to this disastrous childhood disease.
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