Do mutations in SCN1B cause Dravet syndrome?

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.

Voltage-gated sodium channels in genetic epilepsy: up and down of excitability

The past two decades have witnessed a wide range of studies investigating genetic variants of voltage-gated sodium (NaV ) channels, which are involved in a broad spectrum of diseases, including several types of epilepsy. We have reviewed here phenotypes and pathological mechanisms of genetic epilepsies caused by variants in NaV α and β subunits, as well as of some relevant interacting proteins (FGF12/FHF1, PRRT2, and Ankyrin-G). Notably, variants of all these genes can induce either gain- or loss-of-function of NaV leading to either neuronal hyperexcitability or hypoexcitability. We present the results of functional studies obtained with different experimental models, highlighting that they should be interpreted considering the features of the experimental system used. These systems are models, but they have allowed us to better understand pathophysiological issues, ameliorate diagnostics, orientate genetic counseling, and select/develop therapies within a precision medicine framework. These studies have also allowed us to gain insights into the physiological roles of different NaV channels and of the cells that express them. Overall, our review shows the progress that has been made, but also the need for further studies on aspects that have not yet been clarified. Finally, we conclude by highlighting some significant themes of general interest that can be gleaned from the results of the work of the last two decades.

Do All Roads Lead to Rome? Genes Causing Dravet Syndrome and Dravet Syndrome-Like Phenotypes

Background

Dravet syndrome (DS) is a severe epileptic encephalopathy mainly caused by haploinsufficiency of the gene SCN1A, which encodes the voltage-gated sodium channel Na_V1. 1 in the brain. While SCN1A mutations are known to be the primary cause of DS, other genes that may cause DS are poorly understood. Several genes with pathogenic mutations result in DS or DS-like phenotypes, which may require different drug treatment approaches. Therefore, it is urgent for clinicians, especially epilepsy specialists to fully understand these genes involved in DS in addition to SCN1A. Particularly for healthcare providers, a deep understanding of these pathogenic genes is useful in properly selecting and adjusting drugs in a more effective and timely manner.

Objective

The purpose of this study was to identify genes other than SCN1A that may also cause DS or DS-like phenotypes.

Methods

A comprehensive search of relevant Dravet syndrome and severe myoclonic epilepsy in infancy was performed in PubMed, until December 1, 2021. Two independent authors performed the screening for potentially eligible studies. Disagreements were decided by a third, more professional researcher or by all three. The results reported by each study were narratively summarized.

Results

A PubMed search yielded 5,064 items, and other sources search 12 records. A total of 29 studies published between 2009 and 2021 met the inclusion criteria. Regarding the included articles, seven studies on PCDH19, three on SCN2A, two on SCN8A, five on SCN1B, two on GABRA1, three on GABRB3, three on GABRG2, and three on STXBP1 were included. Only one study was recorded for CHD2, CPLX1, HCN1 and KCNA2, respectively. It is worth noting that a few articles reported on more than one epilepsy gene.

Conclusion

DS is not only identified in variants of SCN1A, but other genes such as PCDH19, SCN2A, SCN8A, SCN1B, GABRA1, GABRB3, GABRG2, KCNA2, CHD2, CPLX1, HCN1A, STXBP1 can also be involved in DS or DS-like phenotypes. As genetic testing becomes more widely available, more genes associated with DS and DS-like phenotypes may be identified and gene-based diagnosis of subtypes of phenotypes in this spectrum may improve the management of these diseases in the future.

Dravet syndrome as part of the clinical and genetic spectrum of sodium channel epilepsies and encephalopathies

Dravet syndrome is the most studied form of genetic epilepsy. It has now been clarified that the clinical spectrum of the syndrome does not have firmly established boundaries. The core phenotype is characterized by intractable, mainly clonic, seizures precipitated by increased body temperature with onset in the first year of life and subsequent appearance of multiple seizures types still precipitated by, but not confined to, hyperthermia. Cognitive impairment is invariably present when the full syndrome is manifested. This complex of symptoms is related to mutations in the SCN1A gene, which are often de novo and constitutional but can also be inherited from a parent with less severe clinical manifestations or be present as somatic mosaicism. Inheritance from less severely affected individuals, at times only having experienced a few febrile seizures, and differences in severity, even within the same family, with a subset of patients only showing fragments of the syndrome, testify to a remarkable phenotypic heterogeneity as far as severity, but less so clinical phenomenology, are concerned. This characteristic, together with underascertainment of SCN1A mutations due to human errors or technical limitations in uncovering alternative pathogenic molecular mechanisms, such as genomic rearrangements or poison exons, has contributed to making clinicians and geneticists suspicious that Dravet syndrome may be caused by more than one gene. This opinion has been further amplified by the description of other genetic disorders, such as PCDH19- or CHD2-related epilepsy, whose phenotypes have included fragments of the Dravet phenotypic spectrum, and by the suboptimal characterization of phenotypes associated with mutations in SCN1B, HCN1, KCN2A, GABRA1, GABRG2, and STXBP1. The SCN1A gene-Dravet syndrome association is in our opinion highly specific. However, because the syndrome spectrum is wide, fragments of it can at times also be manifested in other genetic epilepsy syndromes, thereby leading to overdiagnosis of Dravet syndrome beyond SCN1A. Dravet syndrome is in turn a severe SCN1A phenotype within a continuum of SCN1A-related clinical phenomenology.

SCN1B-linked early infantile developmental and epileptic encephalopathy

OBJECTIVE

Patients with Early Infantile Epileptic Encephalopathy (EIEE) 52 have inherited, homozygous variants in the gene SCN1B, encoding the voltage-gated sodium channel (VGSC) β1 and β1B non-pore-forming subunits.

METHODS

Here, we describe the detailed electroclinical features of a biallelic SCN1B patient with a previously unreported variant, p.Arg85Cys.

RESULTS

The female proband showed hypotonia from birth, multifocal myoclonus at 2.5 months, then focal seizures and myoclonic status epilepticus (SE) at 3 months, triggered by fever. Auditory brainstem response (ABR) showed bilateral hearing loss. Epilepsy was refractory and the patient had virtually no development. Administration of fenfluramine resulted in a significant reduction in seizure frequency and resolution of SE episodes that persisted after a 2-year follow-up. The patient phenotype is more compatible with early infantile developmental and epileptic encephalopathy (DEE) than with typical Dravet syndrome (DS), as previously diagnosed for other patients with homozygous SCN1B variants. Biochemical and electrophysiological analyses of the SCN1B variant expressed in heterologous cells showed cell surface expression of the mutant β1 subunit, similar to wild-type (WT), but with loss of normal β1-mediated modification of human Na_v 1.1-generated sodium current, suggesting that SCN1B-p.Arg85Cys is a loss-of-function (LOF) variant.

INTERPRETATION

Importantly, a review of the literature in light of our results suggests that the term, early infantile developmental and epileptic encephalopathy, is more appropriate than either EIEE or DS to describe biallelic SCN1B patients.

SCN1B and SCN2B gene variants analysis in dravet syndrome patients: Analysis of 22 cases

Previous research identified SCN1B variants in some cases of Dravet syndrome (DS). We investigated whether SCN1B and SCN2B variants are commonly happened in DS patients without SCN1A variants. A total of 22 DS patients without SCN1A variants and 100 healthy controls were enrolled in this genetic study. DNA from DS patients was sequenced by Sanger method in whole exons of SCN1B and SCN2B genes. We identified two exon variants (c.351C>T, p.G117G and c.467C>T, p.T156M), which were present both in 1000 egenomes database and in healthy controls with a frequency of 0.54% and 4%, 0.06% and 0%, respectively. Additionally, eight intron or 3 prime UTR variants showing benign clinical significance have also been identified. Our results suggest that variants of SCN1B and SCN2B may not be common causes of DS according to our data. Further large sample-size cohort studies are needed to confirm our conclusion.

Sodium channel β1 subunit localizes to axon initial segments of excitatory and inhibitory neurons and shows regional heterogeneity in mouse brain

The β1 subunit of voltage-gated sodium channels, Nav β1, plays multiple roles in neurons spanning electrophysiological modulation of sodium channel α subunits to cell adhesion and neurite outgrowth. This study used immunohistochemistry to investigate Nav β1 subneuronal and regional expression. Nav β1 was enriched at axon initial segments (AIS) and nodes of Ranvier. Nav β1 expression at the AIS was detected throughout the brain, predominantly in the hippocampus, cortex, and cerebellum. Despite expression of Nav β1 in both excitatory and inhibitory AIS, it displayed a marked and fine-grained heterogeneity of expression. Such heterogeneity could have important implications for the tuning of single neuronal and regional excitability, especially in view of the fact that Nav β1 coexpressed with Nav 1.1, Nav 1.2, and Nav 1.6 subunits. The disruption of Nav β1 AIS expression by a human epilepsy-causing C121W genetic mutation in Nav β1 was also investigated using a mouse model. AIS expression of Nav β1 was reduced by approximately 50% in mice heterozygous for the C121W mutation and was abolished in homozygotes, suggesting that loss of Nav α subunit modulation by Nav β1 contributes to the mechanism of epileptogenesis in these animals as well as in patients.

GABRA1 and STXBP1: novel genetic causes of Dravet syndrome

OBJECTIVE

To determine the genes underlying Dravet syndrome in patients who do not have an SCN1A mutation on routine testing.

METHODS

We performed whole-exome sequencing in 13 SCN1A-negative patients with Dravet syndrome and targeted resequencing in 67 additional patients to identify new genes for this disorder.

RESULTS

We detected disease-causing mutations in 2 novel genes for Dravet syndrome, with mutations in GABRA1 in 4 cases and STXBP1 in 3. Furthermore, we identified 3 patients with previously undetected SCN1A mutations, suggesting that SCN1A mutations occur in even more than the currently accepted ∼ 75% of cases.

CONCLUSIONS

We show that GABRA1 and STXBP1 make a significant contribution to Dravet syndrome after SCN1A abnormalities have been excluded. Our results have important implications for diagnostic testing, clinical management, and genetic counseling of patients with this devastating disorder and their families.

Role of the sodium channel SCN9A in genetic epilepsy with febrile seizures plus and Dravet syndrome

Mutations of the SCN1A subunit of the sodium channel is a cause of genetic epilepsy with febrile seizures plus (GEFS(+) ) in multiplex families and accounts for 70-80% of Dravet syndrome (DS). DS cases without SCN1A mutation inherited have predicted SCN9A susceptibility variants, which may contribute to complex inheritance for these unexplained cases of DS. Compared with controls, DS cases were significantly enriched for rare SCN9A genetic variants. None of the multiplex febrile seizure or GEFS(+) families could be explained by highly penetrant SCN9A mutations.