An intriguing, relatively unexplored therapeutic avenue to investigate epilepsy is the interaction of sleep mechanisms and seizures. Multiple lines of clinical observations suggest a strong, bi-directional relationship between epilepsy and sleep. Epilepsy and sleep disorders are common comorbidities. Seizures occur more commonly in sleep in many types of epilepsy, and in turn, seizures can cause disrupted sleep. Sudden unexplained death in epilepsy (SUDEP) is strongly associated with sleep. The biological mechanisms underlying this relationship between seizures and sleep are poorly understood, but if better delineated, could offer novel therapeutic approaches to treating both epilepsy and sleep disorders. In this presentation, I will explore this sleep-seizure relationship in mouse models of epilepsy. First, I will present general approaches for performing detailed longitudinal sleep and vigilance state analysis in mice, including pre-weanling neonatal mice. I will then discuss recent data from my laboratory demonstrating an abnormal sleep phenotype in a mouse model of the genetic epilepsy, tuberous sclerosis complex (TSC), and its relationship to seizures. The potential mechanistic basis of sleep abnormalities and sleep-seizure interactions in this TSC model will be investigated, focusing on the role of the mechanistic target of rapamycin (mTOR) pathway and hypothalamic orexin, with potential therapeutic applications of mTOR inhibitors and orexin antagonists. Finally, similar sleep-seizure interactions and mechanisms will be extended to models of acquired epilepsy due to status epilepticus-related brain injury.

The long-term goal of our research is to understand the mechanisms of SUDEP, defined as Sudden, Unexpected, witnessed or unwitnessed, nontraumatic and non-drowning Death in patients with EPilepsy, excluding cases of documented status epilepticus. The majority of SUDEP patients die during sleep. SUDEP is the most devastating consequence of epilepsy, yet little is understood about its causes and no biomarkers exist to identify at risk patients. While SUDEP accounts for 7.5-20% of all epilepsy deaths, SUDEP risk in the genetic epilepsies varies with affected genes. Patients with ion channel gene variants have the highest SUDEP risk. Indirect evidence variably links SUDEP to seizure-induced apnea, pulmonary edema, dysregulation of cerebral circulation, autonomic dysfunction, and cardiac arrhythmias. Arrhythmias may be primary or secondary to hormonal or metabolic changes, or autonomic discharges. When SUDEP is compared to Sudden Cardiac Death secondary to Long QT Syndrome, especially to LQT3 linked to variants in the voltage-gated sodium channel (VGSC) gene SCN5A, there are parallels in the circumstances of death. To gain insight into SUDEP mechanisms, our approach has focused on channelopathies with high SUDEP incidence. One such disorder is Dravet syndrome (DS), a devastating form of developmental and epileptic encephalopathy (DEE) characterized by multiple pharmacoresistant seizure types, intellectual disability, ataxia, and increased mortality. While all patients with epilepsy are at risk for SUDEP, DS patients may have the highest risk, up to 20%, with a mean age at SUDEP of 4.6 years. Over 80% of DS is caused by de novo heterozygous loss-of-function (LOF) variants in SCN1A, encoding the VGSC Nav1.1  subunit, resulting in haploinsufficiency. A smaller cohort of patients with DS or a more severe DEE have inherited, homozygous LOF variants in SCN1B, encoding the VGSC 1/1B non-pore-forming subunits. A related DEE, Early Infantile EE (EIEE) type 13, is linked to de novo heterozygous gain-of-function variants in SCN8A, encoding the VGSC Nav1.6. VGSCs underlie the rising phase and propagation of action potentials in neurons and cardiac myocytes. SCN1A, SCN8A, and SCN1B are expressed in both the heart and brain of humans and mice. Because of this, we proposed that cardiac arrhythmias contribute to the mechanism of SUDEP in DEE. We have taken a novel approach to the development of therapeutics for DS in collaboration with Stoke Therapeutics. We employed Targeted Augmentation of Nuclear Gene Output (TANGO) technology, which modulates naturally occurring, non-productive splicing events to increase target gene and protein expression and ameliorate disease phenotype in a mouse model. We identified antisense oligonucleotides (ASOs) that specifically increase the expression of productive Scn1a transcript in human and mouse cell lines, as well as in mouse brain. We showed that a single intracerebroventricular dose of a lead ASO at postnatal day 2 or 14 reduced the incidence of electrographic seizures and SUDEP in the F1:129S-Scn1a+/- x C57BL/6J mouse model of DS. Increased expression of productive Scn1a transcript and NaV1.1 protein were confirmed in brains of treated mice. Our results suggest that TANGO may provide a unique, gene-specific approach for the treatment of DS.

Dravet syndrome is a severe epileptic encephalopathy that begins during the first year of life and leads to severe cognitive and social interaction deficits. It is mostly caused by heterozygous loss-of-function mutations in the SCN1A gene, which encodes for the alpha-subunit of the voltage-gated sodium channel (Nav1.1) and is responsible mainly of GABAergic interneuron excitability. While different therapies based on the upregulation of the healthy allele of the gene are being developed, the dynamics of reversibility of the pathology are still unclear. In fact, whether and to which extent the pathology is reversible after symptom onset and if it is sufficient to ensure physiological levels of Scn1a during a specific critical period of time are open questions in the field and their answers are required for proper development of effective therapies. We generated a novel Scn1a conditional knock-in mouse model (Scn1aSTOP) in which the endogenous Scn1a gene is silenced by the insertion of a floxed STOP cassette in an intron of Scn1a gene; upon Cre recombinase expression, the STOP cassette is removed, and the mutant allele can be reconstituted as a functional Scn1a allele. In this model we can reactivate the expression of Scn1a exactly in the neuronal subtypes in which it is expressed and at its physiological level. Those aspects are crucial to obtain a final answer on the reversibility of DS after symptom onset. We exploited this model to demonstrate that global brain re-expression of the Scn1a gene when symptoms are already developed (P30) led to a complete rescue of both spontaneous and thermic inducible seizures and amelioration of behavioral abnormalities characteristic of this model. We also highlighted dramatic gene expression alterations associated with astrogliosis and inflammation that, accordingly, were rescued by Scn1a gene expression normalization at P30. Moreover, employing a conditional knock-out mouse model of DS we reported that ensuring physiological levels of Scn1a during the critical period of symptom appearance (until P30) is not sufficient to prevent the DS, conversely, mice start to die of SUDEP and develop spontaneous seizures. These results offer promising insights in the reversibility of DS and can help to accelerate therapeutic translation, providing important information on the timing for gene therapy delivery to Dravet patients.

Dravet syndrome (DS) is a neurodevelopmental disorder caused by heterozygous loss-of-function of the gene SCN1A encoding the voltage-gated sodium channel subunit Nav1.1, and is defined by treatment-resistant epilepsy, intellectual impairment, and sudden death. However, disease mechanisms remain unclear, as previously-identified deficiency in action potential generation of Nav1.1-expressing parvalbumin-positive fast-spiking GABAergic interneurons (PV-INs) in DS (Scn1a+/-) mice normalizes during development. We used a novel approach that facilitated the assessment of PV-IN function at both early (post-natal day (P) 16-21) and late (P35-56) time points in the same mice. We confirmed that PV-IN spike generation was impaired at P16-21 in all mice (those deceased from SUDEP by P35 and those surviving to P35-56). However, unitary synaptic transmission assessed in PV-IN:principal cell paired recordings was severely dysfunctional selectively in mice recorded at P16-21 that did not survive to P35. Spike generation in surviving mice had normalized by P35-56; yet we again identified abnormalities in synaptic transmission in surviving mice. We propose that early dysfunction of PV-IN spike propagation drives epilepsy severity and risk of sudden death, while persistent dysfunction of spike propagation contributes to chronic DS pathology.