Emergence of lingual dystonia and strabismus in early-onset SCN8A self-limiting familial infantile epilepsy.

Pathogenic variants in SCN8A are associated with a broad phenotypic spectrum, including Self-Limiting Familial Infantile Epilepsy (SeLFIE), characterized by infancy-onset age-related seizures with normal development and cognition. Movement disorders, particularly paroxysmal kinesigenic dyskinesia typically arising after puberty, may represent another core symptom. We present the case of a 1-year-old girl with a familial disposition to self-limiting focal seizures from the maternal side and early-onset orofacial movement disorders associated with SCN8A-SeLFIE. Brain MRI was normal. Genetic testing revealed a maternally inherited SCN8A variant [c.4447G > A; p.(Glu1483Lys)]. After the introduction of valproic acid, she promptly achieved seizure control as well as complete remission of strabismus and a significant decrease in episodes of tongue deviation. Family history, genetic findings, and epilepsy phenotype are consistent with SCN8A-SeLFIE. Movement disorders are an important part of the SCN8A phenotypic spectrum, and this case highlights the novel early-onset orofacial movement disorders associated with this condition. The episodes of tongue deviation and protrusion suggest focal oromandibular (lingual) dystonia. Additionally, while infantile strabismus or esophoria is a common finding in healthy individuals, our case raises the possibility of an ictal origin of the strabismus. This study underscores the importance of recognizing and addressing movement disorders in SCN8A-SeLFIE patients, particularly the rare early-onset orofacial manifestations. It adds to the growing body of knowledge regarding the diverse clinical presentations of SCN8A-associated disorders and suggests potential avenues for clinical management and further research.

Machine learning models reveal distinct disease subgroups and improve diagnostic and prognostic accuracy for individuals with pathogenic SCN8A gain-of-function variants.

Distinguishing clinical subgroups for patients suffering with diseases characterized by a wide phenotypic spectrum is essential for developing precision therapies. Patients with gain-of-function (GOF) variants in the SCN8A gene exhibit substantial clinical heterogeneity, viewed historically as a linear spectrum ranging from mild to severe. To test for hidden clinical subgroups, we applied two machine-learning algorithms to analyze a dataset of patient features collected by the International SCN8A Patient Registry. We used two research methodologies: a supervised approach that incorporated feature severity cutoffs based on clinical conventions, and an unsupervised approach employing an entirely data-driven strategy. Both approaches found statistical support for three distinct subgroups and were validated by correlation analyses using external variables. However, distinguishing features of the three subgroups within each approach were not concordant, suggesting a more complex phenotypic landscape. The unsupervised approach yielded strong support for a model involving three partially ordered subgroups rather than a linear spectrum. Application of these machine-learning approaches may lead to improved prognosis and clinical management of individuals with SCN8A GOF variants and provide insights into the underlying mechanisms of the disease.

Network-based analysis predicts interacting genetic modifiers from a meta-mapping study of spike-wave discharge in mice.

Absence seizures are characterized by brief lapses in awareness accompanied by a hallmark spike-and-wave discharge (SWD) electroencephalographic pattern and are common to genetic generalized epilepsies (GGEs). While numerous genes have been associated with increased risk, including some Mendelian forms with a single causal allele, most cases of GGE are idiopathic and there are many unknown genetic modifiers of GGE influencing risk and severity. In a previous meta-mapping study, crosses between transgenic C57BL/6 and C3HeB/FeJ strains, each carrying one of three SWD-causing mutations (Gabrg2tm1Spet(R43Q) , Scn8a8j or Gria4spkw1 ), demonstrated an antagonistic epistatic interaction between loci on mouse chromosomes 2 and 7 influencing SWD. These results implicate universal modifiers in the B6 background that mitigate SWD severity through a common pathway, independent of the causal mutation. In this study, we prioritized candidate modifiers in these interacting loci. Our approach integrated human genome-wide association results with gene interaction networks and mouse brain gene expression to prioritize candidate genes and pathways driving variation in SWD outcomes. We considered candidate genes that are functionally associated with human GGE risk genes and genes with evidence for coding or non-coding allele effects between the B6 and C3H backgrounds. Our analyses output a summary ranking of gene pairs, one gene from each locus, as candidates for explaining the epistatic interaction. Our top-ranking gene pairs implicate microtubule function, cytoskeletal stability and cell cycle regulation as novel hypotheses about the source of SWD variation across strain backgrounds, which could clarify underlying mechanisms driving differences in GGE severity in humans.

Impact on backpropagation of the spatial heterogeneity of sodium channel kinetics in the axon initial segment.

In a variety of neurons, action potentials (APs) initiate at the proximal axon, within a region called the axon initial segment (AIS), which has a high density of voltage-gated sodium channels (NaVs) on its membrane. In pyramidal neurons, the proximal AIS has been reported to exhibit a higher proportion of NaVs with gating properties that are "right-shifted" to more depolarized voltages, compared to the distal AIS. Further, recent experiments have revealed that as neurons develop, the spatial distribution of NaV subtypes along the AIS can change substantially, suggesting that neurons tune their excitability by modifying said distribution. When neurons are stimulated axonally, computational modelling has shown that this spatial separation of gating properties in the AIS enhances the backpropagation of APs into the dendrites. In contrast, in the more natural scenario of somatic stimulation, our simulations show that the same distribution can impede backpropagation, suggesting that the choice of orthodromic versus antidromic stimulation can bias or even invert experimental findings regarding the role of NaV subtypes in the AIS. We implemented a range of hypothetical NaV distributions in the AIS of three multicompartmental pyramidal cell models and investigated the precise kinetic mechanisms underlying such effects, as the spatial distribution of NaV subtypes is varied. With axonal stimulation, proximal NaV availability dominates, such that concentrating right-shifted NaVs in the proximal AIS promotes backpropagation. However, with somatic stimulation, the models are insensitive to availability kinetics. Instead, the higher activation threshold of right-shifted NaVs in the AIS impedes backpropagation. Therefore, recently observed developmental changes to the spatial separation and relative proportions of NaV1.2 and NaV1.6 in the AIS differentially impact activation and availability. The observed effects on backpropagation, and potentially learning via its putative role in synaptic plasticity (e.g. through spike-timing-dependent plasticity), are opposite for orthodromic versus antidromic stimulation, which should inform hypotheses about the impact of the developmentally regulated subcellular localization of these NaV subtypes.

Sex differences in physiological response to increased neuronal excitability in a knockin mouse model of pediatric epilepsy.

BACKGROUND: Epilepsy is a common neurological disease; however, few if any of the currently marketed antiseizure medications prevent or cure epilepsy. Discovery of pathological processes in the early stages of epileptogenesis has been challenging given the common use of preclinical models that induce seizures in physiologically normal animals. Moreover, despite known sex dimorphism in neurological diseases, females are rarely included in preclinical epilepsy models. METHODS: We characterized sex differences in mice carrying a pathogenic knockin variant (p.N1768D) in the Scn8a gene that causes spontaneous tonic-clonic seizures (TCs) at ∼3 months of age and found that heterozygous females are more resilient than males in mortality and morbidity. To investigate the cellular mechanisms that underlie female resilience, we utilized blood-brain barrier (BBB) and hippocampal transcriptomic analyses in heterozygous mice before seizure onset (pre-TC) and in mice that experienced ∼20 TCs (post-TC). RESULTS: In the pre-TC latent phase, both sexes exhibited leaky BBB; however, patterns of gene expression were sexually dimorphic. Females exhibited enhanced oxidative phosphorylation and protein biogenesis, while males activated gliosis and CREB signaling. After seizure onset (chronic phase), females exhibited a metabolic switch to lipid metabolism, while males exhibited increased gliosis and BBB dysfunction and a strong activation of neuroinflammatory pathways. CONCLUSION: The results underscore the central role of oxidative stress and BBB permeability in the early stages of epileptogenesis, as well as sex dimorphism in response to increasing neuronal hyperexcitability. Our results also highlight the need to include both sexes in preclinical studies to effectively translate results of drug efficacy studies.

[Clinical features and genetic analysis of five children with epilepsies due to variants of SCN8A gene].

OBJECTIVE: To explore the clinical and genetic characteristics of five children with epilepsies due to variants of SCN8A gene. METHODS: Clinical data of five children (four males and one female) admitted to Linyi People's Hospital due to hereditary epilepsies between August 2015 and August 2022 were collected. Whole exome sequencing was carried out for these children, and candidate variants were verified by Sanger sequencing. RESULTS: All of the five children were found to harbor variants of the SCN8A gene. Case 1, who had benign familial infantile epilepsy, inherited a known pathogenic c.4840A>G variant from his father with similar symptoms. Cases 2 to 4 had presented with intermediate epilepsy. Among these, case 2 has harbored a de novo c.3967G>A variant which was rated as pathogenic (PS1+PS2+PM1+PM2_Supporting+PP3) based on the guidelines from the American College of Medical Genetics and Genomics. Cases 3 and 4 were found to respectively harbor a de novo c.415A>T and a c.4697C>T variant, which were both rated as likely pathogenic (PS2+PM1+PM2_Supporting+PP3). Case 5, who had early-onset infantile epileptic encephalopathy transformed into Lennox Gastaut-like syndrome, has harbored a de novo c.5615G>A variant, which was known to be pathogenic. The children had their age of onset ranging from 2 to 14 months, and all had focal seizures and generalized tonic clonic seizures. Four children (cases 1, 2, 3 and 5) had cluster seizures, four (cases 1 to 4) had become seizure-free after single or dual treatment and showed normal growth and development, whilst case 5 was drug-resistant and showed severe developmental retardation. CONCLUSION: The five children had new features such as cluster seizures, occasional benign seizures in adulthood, and intermediate epilepsy which are prone to relapse after discontinuation of medication, which may be attributed to the pathogenic variants of the SCN8A gene.

Expanding the genotype-phenotype spectrum in SCN8A-related disorders.

BACKGROUND: SCN8A-related disorders are a group of variable conditions caused by pathogenic variations in SCN8A. Online Mendelian Inheritance in Man (OMIM) terms them as developmental and epileptic encephalopathy 13, benign familial infantile seizures 5 or cognitive impairment with or without cerebellar ataxia. METHODS: In this study, we describe clinical and genetic results on eight individuals from six families with SCN8A pathogenic variants identified via exome sequencing. RESULTS: Clinical findings ranged from normal development with well-controlled epilepsy to significant developmental delay with treatment-resistant epilepsy. Three novel and three reported variants were observed in SCN8A. Electrophysiological analysis in transfected cells revealed a loss-of-function variant in Patient 4. CONCLUSIONS: This work expands the clinical and genotypic spectrum of SCN8A-related disorders and provides electrophysiological results on a novel loss-of-function SCN8A variant.

Coupling of Slack and NaV1.6 sensitizes Slack to quinidine blockade and guides anti-seizure strategy development.

Quinidine has been used as an anticonvulsant to treat patients with KCNT1-related epilepsy by targeting gain-of-function KCNT1 pathogenic mutant variants. However, the detailed mechanism underlying quinidine's blockade against KCNT1 (Slack) remains elusive. Here, we report a functional and physical coupling of the voltage-gated sodium channel NaV1.6 and Slack. NaV1.6 binds to and highly sensitizes Slack to quinidine blockade. Homozygous knockout of NaV1.6 reduces the sensitivity of native sodium-activated potassium currents to quinidine blockade. NaV1.6-mediated sensitization requires the involvement of NaV1.6's N- and C-termini binding to Slack's C-terminus and is enhanced by transient sodium influx through NaV1.6. Moreover, disrupting the Slack-NaV1.6 interaction by viral expression of Slack's C-terminus can protect against SlackG269S-induced seizures in mice. These insights about a Slack-NaV1.6 complex challenge the traditional view of 'Slack as an isolated target' for anti-epileptic drug discovery efforts and can guide the development of innovative therapeutic strategies for KCNT1-related epilepsy.

CircFNTA promotes tumorigenesis and progression of gastric cancer via miR-604/miR-647/SCN8A axis.

Gastric cancer (GC) is a major contributor to cancer-related deaths and is characterized by high heterogeneity in epidemiology and histopathology worldwide. Increasing evidence indicates that circular RNAs (circRNAs) play multifaceted roles in cellular processes in human cancers. Here, we demonstrated that circFNTA high expression increases the proliferation, metastasis, and epithelial-mesenchymal transition process and tumorigenicity of GC cells. First, we found that circFNTA was upregulated in GC cells and tissues, and the high circFNTA levels were positively associated with the poor prognosis in GC patients. Using luciferase reporter and RNA-pull down assays, we elucidated that circFNTA sponged two microRNAs, miR-604 and miR-647. In addition, the proliferation and metastatic ability of GC cell reduction caused by silencing circFNTA was hindered by inhibitors of miR-604 and miR-647. Moreover, SCN8A was predicted by miRDB as a common target gene of miR-604 and miR-647, which was then verified by the luciferase reporter assay. Knockdown of circFNTA causes messenger RNA and protein levels in SCN8A to be downregulated in GC cells. However, this effect was overturned by cotransfection miR-604 and miR-647. Also, we identified that SCN8A was downregulated in GC tissues, which was positively correlated with circFNTA expression. In rescue experiments, the attenuated cell proliferation and metastatic ability caused by circFNTA knockdown was reversed by miR-604 and miR-647 inhibitors and SCN8A overexpression. Collectively, our findings suggest an oncogenic role of circFNTA in GC progression and elucidate that circFNTA exerts its function by modulating the miR-604/miR-647/SCN8A axis.

Long-Term Downregulation of the Sodium Channel Gene Scn8a Is Therapeutic in Mouse Models of SCN8A Epilepsy.

OBJECTIVE: De novo mutations of the voltage-gated sodium channel gene SCN8A cause developmental and epileptic encephalopathy (DEE). Most pathogenic variants result in gain-of-function changes in activity of the sodium channel Nav1.6, poorly controlled seizures, and significant comorbidities. In previous work, an antisense oligonucleotide (ASO) reduced Scn8a transcripts and increased lifespan after neonatal administration to a mouse model. Here, we tested long-term ASO treatment initiated after seizure onset, as required for clinical application. METHODS: ASO treatment was initiated after observation of a convulsive seizure and repeated at 4 to 6 week intervals for 1 year. We also tested the long-term efficacy of an AAV10-short hairpin RNA (shRNA) virus administered on P1. RESULTS: Repeated treatment with the Scn8a ASO initiated after seizure onset provided long-term survival and reduced seizure frequency during a 12 month observation period. A single treatment with viral shRNA was also protective during 12 months of observation. INTERPRETATION: Downregulation of Scn8a expression that is initiated after the onset of seizures is effective for long-term treatment in a model of SCN8A-DEE. Repeated ASO administration or a single dose of viral shRNA prevented seizures and extended survival through 12 months of observation. ANN NEUROL 2024;95:754-759.

TNFR1 signaling converging on FGF14 controls neuronal hyperactivity and sickness behavior in experimental cerebral malaria.

BACKGROUND: Excess tumor necrosis factor (TNF) is implicated in the pathogenesis of hyperinflammatory experimental cerebral malaria (eCM), including gliosis, increased levels of fibrin(ogen) in the brain, behavioral changes, and mortality. However, the role of TNF in eCM within the brain parenchyma, particularly directly on neurons, remains underdefined. Here, we investigate electrophysiological consequences of eCM on neuronal excitability and cell signaling mechanisms that contribute to observed phenotypes. METHODS: The split-luciferase complementation assay (LCA) was used to investigate cell signaling mechanisms downstream of tumor necrosis factor receptor 1 (TNFR1) that could contribute to changes in neuronal excitability in eCM. Whole-cell patch-clamp electrophysiology was performed in brain slices from eCM mice to elucidate consequences of infection on CA1 pyramidal neuron excitability and cell signaling mechanisms that contribute to observed phenotypes. Involvement of identified signaling molecules in mediating behavioral changes and sickness behavior observed in eCM were investigated in vivo using genetic silencing. RESULTS: Exploring signaling mechanisms that underlie TNF-induced effects on neuronal excitability, we found that the complex assembly of fibroblast growth factor 14 (FGF14) and the voltage-gated Na+ (Nav) channel 1.6 (Nav1.6) is increased upon tumor necrosis factor receptor 1 (TNFR1) stimulation via Janus Kinase 2 (JAK2). On account of the dependency of hyperinflammatory experimental cerebral malaria (eCM) on TNF, we performed patch-clamp studies in slices from eCM mice and showed that Plasmodium chabaudi infection augments Nav1.6 channel conductance of CA1 pyramidal neurons through the TNFR1-JAK2-FGF14-Nav1.6 signaling network, which leads to hyperexcitability. Hyperexcitability of CA1 pyramidal neurons caused by infection was mitigated via an anti-TNF antibody and genetic silencing of FGF14 in CA1. Furthermore, knockdown of FGF14 in CA1 reduced sickness behavior caused by infection. CONCLUSIONS: FGF14 may represent a therapeutic target for mitigating consequences of TNF-mediated neuroinflammation.

The International SCN8A Patient Registry: A Scientific Resource to Advance the Understanding and Treatment of a Rare Pediatric Neurodevelopmental Syndrome.

Genetic variants in the SCN8A gene underlie a wide spectrum of neurodevelopmental phenotypes that range from severe epileptic encephalopathy to benign familial infantile epilepsy to neurodevelopmental delays with or without seizures. A host of additional comorbidities also contribute to the phenotypic spectrum. As a result of the recent identification of the genetic etiology and the length of time it often takes to diagnose patients, little data are available on the natural history of these conditions. The International SCN8A Patient Registry was developed in 2015 to fill gaps in understanding the spectrum of the disease and its natural history, as well as the lived experiences of individuals with SCN8A syndrome. Another goal of the registry is to collect longitudinal data from participants on a regular basis. In this article, we describe the construction and structure of the International SCN8A Patient Registry, present the type of information available, and highlight particular analyses that demonstrate how registry data can provide insights into the clinical management of SCN8A syndrome.

Clinical severity is correlated with age at seizure onset and biophysical properties of recurrent gain of function variants associated with SCN8A-related epilepsy.

OBJECTIVE: Genetic variants in the SCN8A gene underlie a wide spectrum of neurodevelopmental phenotypes including several distinct seizure types and a host of comorbidities. One of the major challenges facing clinicians and researchers alike is to identify genotype-phenotype (G-P) correlations that may improve prognosis, guide treatment decisions, and lead to precision medicine approaches. METHODS: We investigated G-P correlations among 270 participants harboring gain-of-function (GOF) variants enrolled in the International SCN8A Registry, a patient-driven online database. We performed correlation analyses stratifying the cohort by clinical phenotypes to identify diagnostic features that differ among patients with varying levels of clinical severity, and that differ among patients with distinct GOF variants. RESULTS: Our analyses confirm positive correlations between age at seizure onset and developmental skills acquisition (developmental quotient), rate of seizure freedom, and percentage of cohort with developmental delays, and identify negative correlations with number of current and weaned antiseizure medications. This set of features is more detrimentally affected in individuals with a priori expectations of more severe clinical phenotypes. Our analyses also reveal a significant correlation between a severity index combining clinical features of individuals with a particular highly recurrent variant and an independent electrophysiological score assigned to each variant based on in vitro testing. SIGNIFICANCE: This is one of the first studies to identify statistically significant G-P correlations for individual SCN8A variants with GOF properties. The results suggest that individual GOF variants (1) are predictive of clinical severity for individuals carrying those variants and (2) may underlie distinct clinical phenotypes of SCN8A disease, thus helping to explain the wide SCN8A-related epilepsy disease spectrum. These results also suggest that certain features present at initial diagnosis are predictive of clinical severity, and with more informed treatment plans, may serve to improve prognosis for patients with SCN8A GOF variants.

Increased expression of Nav1.6 of reactive astrocytes in the globus pallidus is closely associated with motor deficits in a model of Parkinson's disease.

Parkinson's disease (PD) is a common neurodegenerative disease in elderly people, which is characterized by motor disabilities in PD patients. Nav1.6 is the most abundant subtype of voltage-gated sodium channels (VGSCs) in the brain of adult mammals and rodents. Here we investigated the role of Nav1.6 in the external globus pallidus (GP) involved in the pathogenesis of motor deficits in unilateral 6-OHDA(6-hydroxydopamine)lesioned rats. The results show that Nav1.6 is dramatically increased in reactive astrocytes of the ipsilateral GP in the middle stage, but not different from the control rats in the later stage of the pathological process in 6-OHDA lesioned rats. Furthermore, the down-regulation of Nav1.6 expression in the ipsilateral GP can significantly improve motor deficits in 6-OHDA lesioned rats in the middle stage of the pathological process. The electrophysiological experiments show that the down-regulation of Nav1.6 expression in the ipsilateral GP significantly decreases the abnormal high synchronization between the ipsilateral M1 (the primary motor cortex) and GP in 6-OHDA lesioned rats. Ca2+ imaging reveals that the down-regulation of Nav1.6 expression reduces the intracellular concentration of Ca2+ ([Ca2+ ]i) in primary cultured astrocytes. These findings suggest that the increased Nav1.6 expression of reactive astrocytes in the GP play an important role in the pathogenesis of motor dysfunction in the middle stage in 6-OHDA lesioned rats, which may participate in astrocyte-neuron communication by regulating [Ca2+ ]i of astrocytes, thereby contributing to the formation of abnormal electrical signals of the basal ganglia (BG) in 6-OHDA lesioned rats.

Role of NaV1.6-mediated persistent sodium current and bursting-pacemaker properties in breathing rhythm generation.

Inspiration is the inexorable active phase of breathing. The brainstem pre-Bötzinger complex (preBötC) gives rise to inspiratory neural rhythm, but its underlying cellular and ionic bases remain unclear. The long-standing "pacemaker hypothesis" posits that the persistent Na+ current (INaP) that gives rise to bursting-pacemaker properties in preBötC interneurons is essential for rhythmogenesis. We tested the pacemaker hypothesis by conditionally knocking out and knocking down the Scn8a (Nav1.6 [voltage-gated sodium channel 1.6]) gene in core rhythmogenic preBötC neurons. Deleting Scn8a substantially decreases the INaP and abolishes bursting-pacemaker activity, which slows inspiratory rhythm in vitro and negatively impacts the postnatal development of ventilation. Diminishing Scn8a via genetic interference has no impact on breathing in adult mice. We argue that the Scn8a-mediated INaP is not obligatory but that it influences the development and rhythmic function of the preBötC. The ubiquity of the INaP in respiratory brainstem interneurons could underlie breathing-related behaviors such as neonatal phonation or rhythmogenesis in different physiological conditions.

A novel SCN8A variant of unknown significance in pediatric epilepsy: a case report.

Variants in SCN8A are associated with several diseases, including developmental and epileptic encephalopathy, intermediate epilepsy or mild-to-moderate developmental and epileptic encephalopathy, self-limited familial infantile epilepsy, neurodevelopmental delays with generalized epilepsy, neurodevelopmental disorder without epilepsy, hypotonia, and movement disorders. Herein, we report an 8-year-old Moroccan boy with intermediate epilepsy of unknown origin, intellectual disability, autism spectrum disorder, and hyperactivity. The patient presented a normal 46, XY karyotype and a normal comparative genomic hybridization profile. Whole-exome sequencing was performed, and heterozygous variants were identified in KCNK4 and SCN8A. The SCN8A variant [c.4499C > T (p.Pro1500Leu)] was also detected in the healthy mother and was classified as a variant of uncertain clinical significance. This variant occurs in a highly conserved domain, which may affect the function of the encoded protein. More studies are needed to confirm the pathogenicity of this novel variant to establish the effective care, management, and genetic counselling of affected individuals.

A CMOS-based highly scalable flexible neural electrode interface.

Perception, thoughts, and actions are encoded by the coordinated activity of large neuronal populations spread over large areas. However, existing electrophysiological devices are limited by their scalability in capturing this cortex-wide activity. Here, we developed an electrode connector based on an ultra-conformable thin-film electrode array that self-assembles onto silicon microelectrode arrays enabling multithousand channel counts at a millimeter scale. The interconnects are formed using microfabricated electrode pads suspended by thin support arms, termed Flex2Chip. Capillary-assisted assembly drives the pads to deform toward the chip surface, and van der Waals forces maintain this deformation, establishing Ohmic contact. Flex2Chip arrays successfully measured extracellular action potentials ex vivo and resolved micrometer scale seizure propagation trajectories in epileptic mice. We find that seizure dynamics in absence epilepsy in the Scn8a+/- model do not have constant propagation trajectories.

Seizures Cause Prolonged Impairment of Ventilation, CO2 Chemoreception and Thermoregulation.

Sudden unexpected death in epilepsy (SUDEP) has been linked to respiratory dysfunction, but the mechanisms underlying this association remain unclear. Here we found that both focal and generalized convulsive seizures (GCSs) in epilepsy patients caused a prolonged decrease in the hypercapnic ventilatory response (HCVR; a measure of respiratory CO2 chemoreception). We then studied Scn1a R1407X/+ (Dravet syndrome; DS) and Scn8a N1768D/+ (D/+) mice of both sexes, two models of SUDEP, and found that convulsive seizures caused a postictal decrease in ventilation and severely depressed the HCVR in a subset of animals. Those mice with severe postictal depression of the HCVR also exhibited transient postictal hypothermia. A combination of blunted HCVR and abnormal thermoregulation is known to occur with dysfunction of the serotonin (5-hydroxytryptamine; 5-HT) system in mice. Depleting 5-HT with para-chlorophenylalanine (PCPA) mimicked seizure-induced hypoventilation, partially occluded the postictal decrease in the HCVR, exacerbated hypothermia, and increased postictal mortality in DS mice. Conversely, pretreatment with the 5-HT agonist fenfluramine reduced postictal inhibition of the HCVR and hypothermia. These results are consistent with the previous observation that seizures cause transient impairment of serotonergic neuron function, which would be expected to inhibit the many aspects of respiratory control dependent on 5-HT, including baseline ventilation and the HCVR. These results provide a scientific rationale to investigate the interictal and/or postictal HCVR as noninvasive biomarkers for those at high risk of seizure-induced death, and to prevent SUDEP by enhancing postictal 5-HT tone.SIGNIFICANCE STATEMENT There is increasing evidence that seizure-induced respiratory dysfunction contributes to the pathophysiology of sudden unexpected death in epilepsy (SUDEP). However, the cellular basis of this dysfunction has not been defined. Here, we show that seizures impair CO2 chemoreception in some epilepsy patients. In two mouse models of SUDEP we found that generalized convulsive seizures impaired CO2 chemoreception, and induced hypothermia, two effects reported with serotonergic neuron dysfunction. The defects in chemoreception and thermoregulation were exacerbated by chemical depletion of serotonin and reduced with fenfluramine, suggesting that seizure-induced respiratory dysfunction may be due to impairment of serotonin neuron function. These findings suggest that impaired chemoreception because of transient inhibition of serotonergic neurons may contribute to the pathophysiology of SUDEP.

Prefrontal PV interneurons facilitate attention and are linked to attentional dysfunction in a mouse model of absence epilepsy.

Absence seizures are characterized by brief periods of unconsciousness accompanied by lapses in motor function that can occur hundreds of times throughout the day. Outside of these frequent moments of unconsciousness, approximately a third of people living with the disorder experience treatment-resistant attention impairments. Convergent evidence suggests prefrontal cortex (PFC) dysfunction may underlie attention impairments in affected patients. To examine this, we use a combination of slice physiology, fiber photometry, electrocorticography (ECoG), optogenetics, and behavior in the Scn8a+/-mouse model of absence epilepsy. Attention function was measured using a novel visual attention task where a light cue that varied in duration predicted the location of a food reward. In Scn8a+/-mice, we find altered parvalbumin interneuron (PVIN) output in the medial PFC (mPFC) in vitro and PVIN hypoactivity along with reductions in gamma power during cue presentation in vivo. This was associated with poorer attention performance in Scn8a+/-mice that could be rescued by gamma-frequency optogenetic stimulation of PVINs. This highlights cue-related PVIN activity as an important mechanism for attention and suggests PVINs may represent a therapeutic target for cognitive comorbidities in absence epilepsy.

People who experience absence seizures may go through brief lapses in consciousness hundreds of times a day. They also often have difficulties engaging and remaining focused on a task, which can severely limit their ability to study, work and go through their day-to-day life. These impairments in attention persist even when medication puts a stop to the seizures, suggesting that they are not directly linked to the epileptic episodes. In fact, recent work has indicated that these deficits may be caused instead by alterations in the activity of the prefrontal cortex, the brain area which helps to regulate attention and impulsivity. However, the exact nature of these changes remains unclear, making it difficult to design treatments that could improve patients' quality of life. To explore this question, Ferguson et al. developed a new behavioral test that allowed them to measure the attention levels of mice genetically engineered to have absence seizures. The experiments confirmed that these animals had impaired attention even when brain activity recordings showed that they were not experiencing seizures. Further work revealed that poor performance on the behavioral test was linked to decreased activity in parvalbumin interneurons, a group of cells in the prefrontal cortex which can inhibit many other types of neurons. In mutant mice, this change was associated with alterations in network activity broadly in the cortex, including in electrical patterns which are linked to cognitive processes. Promisingly, increasing the activity of the interneurons during the attention task improved performance, suggesting that this type of cell could represent a therapeutic target for attention deficit in absence epilepsy.

Hominoid SVA-lncRNA AK057321 targets human-specific SVA retrotransposons in SCN8A and CDK5RAP2 to initiate neuronal maturation.

SINE-VNTR-Alu (SVA) retrotransposons arose and expanded in the genome of hominoid primates concurrent with the slowing of brain maturation. We report genes with intronic SVA transposons are enriched for neurodevelopmental disease and transcribed into long non-coding SVA-lncRNAs. Human-specific SVAs in microcephaly CDK5RAP2 and epilepsy SCN8A gene introns repress their expression via transcription factor ZNF91 to delay neuronal maturation. Deleting the SVA in CDK5RAP2 initiates multi-dimensional and in SCN8A selective sodium current neuronal maturation by upregulating these genes. SVA-lncRNA AK057321 forms RNA:DNA heteroduplexes with the genomic SVAs and upregulates these genes to initiate neuronal maturation. SVA-lncRNA AK057321 also promotes species-specific cortex and cerebellum-enriched expression upregulating human genes with intronic SVAs (e.g., HTT, CHAF1B and KCNJ6) but not mouse orthologs. The diversity of neuronal genes with intronic SVAs suggest this hominoid-specific SVA transposon-based gene regulatory mechanism may act at multiple steps to specialize and achieve neoteny of the human brain.