Epidemiology and etiology of infantile developmental and epileptic encephalopathies in Tasmania

Interneuron FGF13 regulates seizure susceptibility via a sodium channel-independent mechanism

Developmental and epileptic encephalopathies (DEEs), a class of devastating neurological disorders characterized by recurrent seizures and exacerbated by disruptions to excitatory/inhibitory balance in the brain, are commonly caused by mutations in ion channels. Disruption of, or variants in, FGF13 were implicated as causal for a set of DEEs, but the underlying mechanisms were clouded because FGF13 is expressed in both excitatory and inhibitory neurons, FGF13 undergoes extensive alternative splicing producing multiple isoforms with distinct functions, and the overall roles of FGF13 in neurons are incompletely cataloged. To overcome these challenges, we generated a set of novel cell-type-specific conditional knockout mice. Interneuron-targeted deletion of Fgf13 led to perinatal mortality associated with extensive seizures and impaired the hippocampal inhibitory/excitatory balance while excitatory neuron-targeted deletion of Fgf13 caused no detectable seizures and no survival deficits. While best studied as a voltage-gated sodium channel (Nav) regulator, we observed no effect of Fgf13 ablation in interneurons on Navs but rather a marked reduction in K+ channel currents. Re-expressing different Fgf13 splice isoforms could partially rescue deficits in interneuron excitability and restore K+ channel current amplitude. These results enhance our understanding of the molecular mechanisms that drive the pathogenesis of Fgf13-related seizures and expand our understanding of FGF13 functions in different neuron subsets.

Association between maternal usage of volatile organic compounds and West syndrome, the Japan Environment and Children's study

Multiple etiologies of West syndrome have been reported; however, there are cases of unknown etiologies. Exposure to volatile organic compounds (VOCs) increases the risk of epilepsy; however, their effects on children remain unknown. This study aimed to investigate the association between maternal occupational usage of VOCs and West syndrome development in children. Using data from a cohort of 88,280 children, we extracted children born to mothers who had used VOCs during pregnancy. Based on an epilepsy diagnosis by the age of 2 years, the frequency of usage of VOCs was comparatively analyzed among the following groups: never diagnosed with epilepsy, West syndrome, and other epileptic syndromes. A total of 15, 154, and 88,111 children were categorized into the West syndrome, other epileptic syndrome, and never diagnosed with epilepsy groups, respectively. The odds ratio (OR) for West syndrome development increased with the frequency of permanent marker usage (one to three times a month: OR = 2.58, 95% confidence interval [CI] 0.75-8.90; one or more times a week: OR = 4.34, 95% CI 1.23-15.26). These results suggested an association between maternal occupational frequent usage of permanent marker and West syndrome development in children.

POLR3B is associated with a developmental and epileptic encephalopathy with myoclonic-atonic seizures and ataxia

OBJECTIVE

POLR3B encodes the second largest subunit of RNA polymerase III, which is essential for transcription of small non-coding RNAs. Biallelic pathogenic variants in POLR3B are associated with an inherited hypomyelinating leukodystrophy. Recently, de novo heterozygous variants in POLR3B were reported in six individuals with ataxia, spasticity, and demyelinating peripheral neuropathy. Three of these individuals had epileptic seizures. The aim of this article is to precisely define the epilepsy phenotype associated with de novo heterozygous POLR3B variants.

METHODS

We used online gene-matching tools to identify 13 patients with de novo POLR3B variants. We systematically collected genotype and phenotype data from clinicians using two standardized proformas.

RESULTS

All 13 patients had novel POLR3B variants. Twelve of 13 variants were classified as pathogenic or likely pathogenic as per American College of Medical Genetics (ACMG) criteria. Patients presented with generalized myoclonic, myoclonic-atonic, atypical absence, or tonic-clonic seizures between the ages of six months and 4 years. Epilepsy was classified as epilepsy with myoclonic-atonic seizures (EMAtS) in seven patients and "probable EMAtS" in two more. Seizures were treatment resistant in all cases. Three patients became seizure-free. All patients had some degree of developmental delay or intellectual disability. In most cases developmental delay was apparent before the onset of seizures. Three of 13 cases were reported to have developmental stagnation or regression in association with seizure onset. Treatments for epilepsy that were reported by clinicians to be effective were: sodium valproate, which was effective in five of nine patients (5/9) who tried it; rufinamide (2/3); and ketogenic diet (2/3). Additional features were ataxia/incoordination (8/13); microcephaly (7/13); peripheral neuropathy (4/13), and spasticity/hypertonia (6/13).

SIGNIFICANCE

POLR3B is a novel genetic developmental and epileptic encephalopathy (DEE) in which EMAtS is the predominant epilepsy phenotype. Ataxia, neuropathy, and hypertonia may be variously observed in these patients.

Assessing age of onset and clinical symptoms over time in patients with heterozygous pathogenic DHDDS variants

Mono-allelic DHDDS variants are associated with seizures, intellectual disability, and movement disorders. The age of onset and progression rates of symptoms vary greatly among patients, spanning from infancy to late adulthood. Yet, the reasons behind this clinical variability and the underlying pathophysiological mechanisms of the disease have remained elusive. We investigated the age of onset and the progression of symptoms over time in 59 patients with heterozygous DHDDS variants, drawing from medical literature and incorporating five previously unreported cases from the FCDGC Natural History Study. Clinical symptoms typically emerged early in life. Ataxia, tremor, dystonia, and dyskinesia manifested slightly later in childhood. Global developmental delay usually presented as the initial symptom. We observed diverse rates of symptom accumulation over time: some patients exhibited the full spectrum of symptoms in early childhood, while others developed novel symptoms well into adulthood. Interestingly, neither the sex nor the underlying DHDDS variants correlated with the age of symptom onset or specific clinical symptoms. Additionally, we found that 19% of patients presented with autism spectrum disorder. This study offers insight into the age of symptom onset and the rate of symptom accumulation in patients with DHDDS variants. We found no correlation between the age of onset and progression of clinical symptoms with specific DHDDS variants or patient sex. Autism spectrum disorder is common in patients and warrants attention in clinical management.

Interneuron FGF13 regulates seizure susceptibility via a sodium channel-independent mechanism

Developmental and Epileptic Encephalopathies (DEEs), a class of devastating neurological disorders characterized by recurrent seizures and exacerbated by disruptions to excitatory/inhibitory balance in the brain, are commonly caused by mutations in ion channels. Disruption of, or variants in, FGF13 were implicated as causal for a set of DEEs, but the underlying mechanisms were clouded because FGF13 is expressed in both excitatory and inhibitory neurons, FGF13 undergoes extensive alternative splicing producing multiple isoforms with distinct functions, and the overall roles of FGF13 in neurons are incompletely cataloged. To overcome these challenges, we generated a set of novel cell type-specific conditional knockout mice. Interneuron-targeted deletion of Fgf13 led to perinatal mortality associated with extensive seizures and impaired the hippocampal inhibitory/excitatory balance while excitatory neuron-targeted deletion of Fgf13 caused no detectable seizures and no survival deficits. While best studied as a voltage-gated sodium channel (Nav) regulator, we observed no effect of Fgf13 ablation in interneurons on Navs but rather a marked reduction in K+ channel currents. Re-expressing different Fgf13 splice isoforms could partially rescue deficits in interneuron excitability and restore K+ channel current amplitude. These results enhance our understanding of the molecular mechanisms that drive the pathogenesis of Fgf13-related seizures and expand our understanding of FGF13 functions in different neuron subsets.

Decoding epilepsy treatment: A comparative evaluation contrasting cannabidiol pharmacokinetics in adult and paediatric populations

Epilepsy is a neurological disorder characterized by overstimulation of neurotransmitters and uncontrolled seizures. Current medications for epilepsy result in adverse effects or insufficient seizure control, highlighting the necessity to develop alternative therapies. Cannabidiol (CBD), derived from cannabis plants, has been popularly explored as an alternative. CBD is shown to have anti-convulsivatng and muscle-relaxing properties, which have been used in patients with epilepsy with promising results. Current research explores varying dosages in either adult or paediatric patients, with little or no comparison between the two populations. In this review, we aim at consolidating this data and comparing the effect and pharmacokinetic properties of CBD across these two patient populations. When comparing the absorption, there was insufficient data to show differences between paediatric and adult patients. Similarly, limited information was available in comparing the distribution of CBD, but a higher volume of distribution was found in the paediatric population. From the metabolism perspective, the paediatric population had a greater success rate when treated with the drug compared to the adult population. In the elimination, there were no clear distinctions in the clearance rate between the two populations. The drug's half-life was highly variable in both populations, with paediatrics having a lower range than adults. In summary, the paediatric population had a more significant reduction in the severity of seizures compared to the adult population upon CBD treatment. The complexity in which CBD operates highlights the need for further studies of the compound to further understand why differences occur between these two populations.

Association of FAT1 with focal epilepsy and correlation between seizure relapse and gene expression stage

PURPOSE

The FAT1 gene encodes FAT atypical cadherin 1, which is essential for foetal development, including brain development. This study aimed to investigate the relationship between FAT1 variants and epilepsy.

METHODS

Trio-based whole-exome sequencing was performed on a cohort of 313 patients with epilepsy. Additional cases with FAT1 variants were collected from the China Epilepsy Gene V.1.0 Matching Platform.

RESULTS

Four pairs of compound heterozygous missense FAT1 variants were identified in four unrelated patients with partial (focal) epilepsy and/or febrile seizures, but without intellectual disability/developmental abnormalities. These variants presented no/very low frequencies in the gnomAD database, and the aggregate frequencies in this cohort were significantly higher than those in controls. Two additional compound heterozygous missense variants were identified in two unrelated cases using the gene-matching platform. All patients experienced infrequent (yearly/monthly) complex partial seizures or secondary generalised tonic-clonic seizures. They responded well toantiseizure medication, but seizures relapsed in three cases when antiseizure medication were decreased or withdrawn after being seizure-free for three to six years, which correlated with the expression stage of FAT1. Genotype-phenotype analysis showed that epilepsy-associated FAT1 variants were missense, whereas non-epilepsy-associated variants were mainly truncated. The relationship between FAT1 and epilepsy was evaluated to be "Strong" by the Clinical Validity Framework of ClinGen.

CONCLUSIONS

FAT1 is a potential causative gene of partial epilepsy and febrile seizures. Gene expression stage was suggested to be one of the considerations in determining the duration ofantiseizure medication. Genotype-phenotype correlation helps to explain the mechanisms underlying phenotypic variation.

DHDDS and NUS1: A Converging Pathway and Common Phenotype

BACKGROUND

Variants in dehydrodolichol diphosphate synthetase (DHDDS) and nuclear undecaprenyl pyrophosphate synthase 1 (NUS1) cause a neurodevelopmental disorder, classically with prominent epilepsy. Recent reports suggest a complex movement disorder and an overlapping phenotype has been postulated due to their combined role in dolichol synthesis.

CASES

We describe three patients with heterozygous variants in DHDDS and five with variants affecting NUS1. They bear a remarkably similar phenotype of a movement disorder dominated by multifocal myoclonus. Diagnostic clues include myoclonus exacerbated by action and facial involvement, and slowly progressive or stable, gait ataxia with disproportionately impaired tandem gait. Myoclonus is confirmed with neurophysiology, including EMG of facial muscles.

LITERATURE REVIEW

Ninety-eight reports of heterozygous variants in DHDDS, NUS1 and chromosome 6q22.1 structural alterations spanning NUS1, confirm the convergent phenotype of hypotonia at birth, developmental delay, multifocal myoclonus, ataxia, dystonia and later parkinsonism with or without generalized epilepsy. Other features include periodic exacerbations, stereotypies, anxiety, and dysmorphisms. Although their gene products contribute to dolichol biosynthesis, a key step in N-glycosylation, transferrin isoform profiles are typically normal. Imaging is normal or non-specific.

CONCLUSIONS

Recognition of their shared phenotype may expedite diagnosis through chromosomal microarray and by including DHDDS/NUS1 in movement disorder gene panels.

Early-infantile developmental and epileptic encephalopathy: the aetiologies, phenotypic differences and outcomes-a prospective observational study

In this study, we have evaluated the underlying aetiologies, yield of genetic testing and long-term outcomes in patients with early-infantile developmental and epileptic encephalopathies. We have prospectively studied patients with seizure onset before 3 months of age. Based on the clinical details, neuroimaging, metabolic testing and comprehensive genetic evaluation, patients were classified into different aetiological groups. The phenotypic differences between genetic/unknown groups and remaining aetiologies were compared. Factors that could affect seizure control were also assessed. A total of 80 children (M:F ratio-1.5:1) were recruited. The median seizure onset age was 28 days (range, 1-90 days). The aetiologies were confirmed in 66 patients (83%). The patients were further classified into four aetiological groups: genetic (50%), structural (19%), metabolic (14%; all were vitamin responsive) and unknown (17%). On comparing for the phenotypic differences between the groups, children in the 'genetic/unknown' groups were more frequently observed to have severe developmental delay (Odds Ratio = 57; P < 0.0001), autistic behaviours (Odds Ratio = 37; P < 0.0001), tone abnormalities (Odds Ratio = 9; P = 0.0006) and movement disorder (Odds Ratio = 19; P < 0.0001). Clonic seizures were more common in the vitamin responsive/structural groups (Risk Ratio = 1.36; P = 0.05) as compared to patients with 'genetic/unknown' aetiologies. On the contrary, vitamin responsive/structural aetiology patients were less likely to have tonic seizures (Risk Ratio = 0.66; P = 0.04). Metabolic testing was diagnostic in three out of 41 patients tested (all three had biotinidase deficiency). MRI was abnormal in 35/80 patients (malformation observed in 16/35; 19/35 had non-specific changes that did not contribute to underlying aetiology). A molecular diagnosis was achieved in 53 out of 77 patients tested (69%). Next-generation sequencing had a yield of 51%, while microarray had a yield of 14%. STXBP1 was the most common (five patients) single-gene defect identified. There were 24 novel variants. The mean follow-up period was 30 months (range, 4-72 months). On multivariate logistic regression for the important factors that could affect seizure control (seizure onset age, time lag of first visit to paediatric neurologist and aetiologies), only vitamin responsive aetiology had a statistically significant positive effect on seizure control (P = 0.02). Genetic aetiologies are the most common cause of early-infantile developmental and epileptic encephalopathies. Patients in the genetic/unknown groups had a more severe phenotype. Patients with vitamin responsive epilepsies had the best probability of seizure control.

Pharmacological diversity amongst approved and emerging antiseizure medications for the treatment of developmental and epileptic encephalopathies

Developmental and epileptic encephalopathies (DEEs) are rare neurodevelopmental disorders characterised by early-onset and often intractable seizures and developmental delay/regression, and include Dravet syndrome and Lennox-Gastaut syndrome (LGS). Rufinamide, fenfluramine, stiripentol, cannabidiol and ganaxolone are antiseizure medications (ASMs) with diverse mechanisms of action that have been approved for treating specific DEEs. Rufinamide is thought to suppress neuronal hyperexcitability by preventing the functional recycling of voltage-gated sodium channels from the inactivated to resting state. It is licensed for adjunctive treatment of seizures associated with LGS. Fenfluramine increases extracellular serotonin levels and may reduce seizures via activation of specific serotonin receptors and positive modulation of the sigma-1 receptor. Fenfluramine is licensed for adjunctive treatment of seizures associated with Dravet syndrome and LGS. Stiripentol is a positive allosteric modulator of type-A gamma-aminobutyric acid (GABAA) receptors. As a broad-spectrum inhibitor of cytochrome P450 enzymes, its antiseizure effects may additionally arise through pharmacokinetic interactions with co-administered ASMs. Stiripentol is licensed for treating seizures associated with Dravet syndrome in patients taking clobazam and/or valproate. The mechanism(s) of action of cannabidiol remains largely unclear although multiple targets have been proposed, including transient receptor potential vanilloid 1, G protein-coupled receptor 55 and equilibrative nucleoside transporter 1. Cannabidiol is licensed as adjunctive treatment in conjunction with clobazam for seizures associated with Dravet syndrome and LGS, and as adjunctive treatment of seizures associated with tuberous sclerosis complex. Like stiripentol, ganaxolone is a positive allosteric modulator at GABAA receptors. It has recently been licensed in the USA for the treatment of seizures associated with cyclin-dependent kinase-like 5 deficiency disorder. Greater understanding of the causes of DEEs has driven research into the potential use of other novel and repurposed agents. Putative ASMs currently in clinical development for use in DEEs include soticlestat, carisbamate, verapamil, radiprodil, clemizole and lorcaserin.

Epidemiology of Developmental and Epileptic Encephalopathy and of Intellectual Disability and Epilepsy in Children

BACKGROUND AND OBJECTIVES

We aimed to determine the population-based cumulative incidence and prevalence of developmental and epileptic encephalopathies (DEEs) and intellectual disability and epilepsy (ID+E) in children. We analyzed the cumulative incidence of specific epilepsy syndromes.

METHODS

Children younger than 16 years with a DEE or ID+E were ascertained using EEG records from 2000 to 2016 in the Wellington region of New Zealand. Epilepsy syndromes were diagnosed on medical record and EEG review. Point prevalence and cumulative incidence for children with epilepsy and developmental impairment, DEE and ID+E were calculated. Cumulative incidence for each epilepsy syndrome was calculated.

RESULTS

The cohort comprised 235 children (58% male) with developmental impairment and epilepsy, including 152 (65%) with DEE and 83 (35%) with ID+E. The median age of seizure onset was 15.4 months (range day 1-15 years). The median follow-up from seizure onset was 7.9 years (range 0-18.2 years). Point prevalence for the broad group of children with epilepsy and developmental impairment was 175/100,000 children (95% CI 149-203; DEE 112 and ID+E 63/100,000 children). Cumulative incidence for DEE was 169/100,000 children (95% CI 144-199) and that for ID+E was 125/100,000 children (95% CI 95.4-165). Cumulative incidence per 100,000 children was as follows: infantile epileptic spasms syndrome 58.2 (95% CI 45.0-75.3), epilepsy with myoclonic-atonic seizures 16.4 (95% CI 9.69-27.7), Lennox-Gastaut syndrome 13.2 (95% CI 4.1-41.9), and Dravet syndrome 5.1 (95% CI 2.1-12.2). Fifty/152 (33%) of children with DEE and 70/83 (84%) with ID+E could not be diagnosed with a known epilepsy syndrome.

DISCUSSION

Epilepsy and developmental impairment before the age of 16 years occurs in 1 in 340 children, with 1 in 590 having a DEE and 1 in 800 having ID+E. These individuals require significant health and community resources; therefore, these data will inform complex health service and education planning. Epidemiologic studies have focused on early childhood-onset DEEs. These do not fully reflect the burden of these disorders because 27% of DEEs and 70% of ID+E begin later, with seizure onset after the age of 3 years. Understanding the cumulative incidence of specific syndromes together with the broad group of DEEs is essential for the planning of therapeutic trials. Given trials focus on specific syndromes, there is a risk that effective therapies will not be developed for one-third of children with DEE.

Adult-onset rapidly worsening progressive myoclonic epilepsy caused by a novel variant in DHDDS

Progressive myoclonic epilepsy (PME) is a heterogeneous neurogenetic disorder manifesting as progressive myoclonus, seizure, and ataxia. We report a case of PME caused by a novel DHDDS variant. Additionally, by reviewing the literature on DHDDS mutations, we compared the phenotype of our patient with previously reported phenotypes. We identified DHDDS (c.638G>A, p. Ser213Asn) as a likely pathogenic variant. The literature review revealed 15 PME patients with DHDDS mutations from 13 unrelated families. According to previous studies, late‐onset patients tend to have a slow‐progressive disease course. Although his myoclonus and ataxia were adult onset, our patient experienced rapid disease aggravation.

Three different scenarios for epileptic spasms

Epileptic Spasms (ES) is a type of seizure usually occurring in the context of a severe childhood epileptic syndrome associated to significant Electroencephalogram (EEG) abnormalities. There are three scenarios in which ES may occur. The first one is represented by West Syndrome (WS): ES occur in a previously non encephalopathic infant in association with the development of a hypsarrhythmic EEG pattern. In most cases, standard treatment with Adrenocorticotropic Hormone (ACTH), steroids or vigabatrin leads to a reversal of the electroclinical picture. The second scenario is represented by Developmental and Epileptic Encephalopathies (DEEs): ES are documented, often along other seizures types, in an infant who often shows developmental delay since birth; the EEG pattern is pathological both in wakefulness and in sleep, without typical features of hypsarrhythmia; therapies (with the exception of few potentially treatable syndromes) are poorly effective. The last scenario is represented by ES in the context of Focal Epilepsies (FEs): ES, sometimes showing focal signs or closely related to focal seizures, are associated with focal brain lesions. Treatment with ACTH, steroids or vigabatrin may not be effective as well as antiepileptic drugs for focal epilepsies. In drug-resistant patients, surgery should be considered. Although there are some gaps in our current scientific knowledge concerning the peculiar electroclinical and physiopathological features of ES, we nowadays possess the necessary tools to correctly frame this unique seizure type into one of these scenarios and therefore properly manage the diagnostic and therapeutic workup.

Novel Missense CACNA1G Mutations Associated with Infantile-Onset Developmental and Epileptic Encephalopathy

The CACNA1G gene encodes the low-voltage-activated Ca_v3.1 channel, which is expressed in various areas of the CNS, including the cerebellum. We studied two missense CACNA1G variants, p.L208P and p.L909F, and evaluated the relationships between the severity of Ca_v3.1 dysfunction and the clinical phenotype. The presentation was of a developmental and epileptic encephalopathy without evident cerebellar atrophy. Both patients exhibited axial hypotonia, developmental delay, and severe to profound cognitive impairment. The patient with the L909F mutation had initially refractory seizures and cerebellar ataxia, whereas the L208P patient had seizures only transiently but was overall more severely affected. In transfected mammalian cells, we determined the biophysical characteristics of L208P and L909F variants, relative to the wild-type channel and a previously reported gain-of-function Ca_v3.1 variant. The L208P mutation shifted the activation and inactivation curves to the hyperpolarized direction, slowed the kinetics of inactivation and deactivation, and reduced the availability of Ca²⁺ current during repetitive stimuli. The L909F mutation impacted channel function less severely, resulting in a hyperpolarizing shift of the activation curve and slower deactivation. These data suggest that L909F results in gain-of-function, whereas L208P exhibits mixed gain-of-function and loss-of-function effects due to opposing changes in the biophysical properties. Our study expands the clinical spectrum associated with CACNA1G mutations, corroborating further the causal association with distinct complex phenotypes.