Novel Truncating and Missense Variants in SEMA6B in Patients With Early-Onset Epilepsy

Plexins: Navigating through the Neural Regulation and Brain Pathology

Plexins are a family of transmembrane receptors known for their diverse roles in neural development, axon guidance, neuronal migration, synaptogenesis, and circuit formation. Semaphorins are a class of secreted and membrane proteins that act as primary ligands for plexin receptors. Semaphorins play a crucial role in central nervous system (CNS) development by regulating processes such as axonal growth, neuronal positioning, and synaptic connectivity. Various types of semaphorins like sema3A, sema4A, sema4C, sema4D, and many more have a crucial role in developing brain diseases. Likewise, various evidence suggests that plexin receptors are of four types: plexin A, plexin B, plexin C, and plexin D. Plexins have emerged as crucial regulators of neurogenesis and neuronal development and connectivity. When bound to semaphorins, these receptors trigger two major networking cascades, namely Rho and Ras GTPase networks. Dysregulation of plexin networking has been implicated in a myriad of brain disorders, including autism spectrum disorder (ASD), Schizophrenia, Alzheimer's disease (AD), Parkinson's disease (PD), and many more. This review synthesizes findings from molecular, cellular, and animal model studies to elucidate the mechanisms by which plexins contribute to the pathogenesis of various brain diseases.

Autism, Electrical Status Epilepticus in Sleep, and a Likely Pathogenic SEMA6B Variant

This case report describes a boy aged 8 years with autism spectrum disorder who was diagnosed with electrical status epilepticus in sleep (ESES) and found to have a likely pathogenic variant in the SEMA6B gene. The patient presented with developmental regression and cognitive decline. An electroencephalogram demonstrated continuous spike-and-wave discharges during sleep, a hallmark of ESES. Genetic testing identified a De Novo likely pathogenic variant in SEMA6B, a gene implicated in neurodevelopmental disorders and epilepsy. Although the association between SEMA6B mutations and ESES is not well established, this case suggests that the genetic variant may have contributed to the patient's clinical presentation. This is the first reported instance of ESES being linked to a SEMA6B gene variant, highlighting the importance of genetic testing in similar cases. The findings could have significant implications for the understanding and management of ESES in autistic patients with behavioral difficulties. They also underscore the need for further research into the role of SEMA6B in epilepsy and neurodevelopmental disorders.

Progressive Myoclonus Epilepsy and Beyond: A Systematic Review of SEMA6B-related Disorders

Progressive myoclonus epilepsy (PME) is a rare, clinically and genetically heterogeneous epilepsy syndrome, and pathogenic variants in the semaphorin 6B (SEMA6B) gene have recently been reported to be among the causes of PME. Cases with pathogenic variants in the SEMA6B gene are extremely rare, only a limited number of cases have been reported in the literature. In this systematic review, we aimed to present a summary of a PME case in which a heterozygous nonsense variant of c.2086C > T p.(Gln696*) in the SEMA6B gene was detected in the etiology and other cases with SEMA6B pathogenic variant in the literature. Except for our case, 35 cases from 12 studies were included. The main clinical findings in these patients were cognitive problems, seizures, gait and speech disturbances, and cognitive and/or motor regression, and they had a wide spectrum of severity. Response to antiseizure medications was also highly variable, almost half of the patients had pharmacoresistant seizures. Patients were divided into four different phenotypic groups according to their clinical presentations: PME (18/36), developmental and epileptic encephalopathy (13/36), neurodevelopmental disorder (4/36), and epilepsy (1/36), respectively. In conclusion, although SEMA6B has been associated with PME, it may actually cause a much broader phenotypic spectrum. Due to their extreme rarity, our knowledge of SEMA6B-related disorders is limited. As with all other rare diseases, each new SEMA6B-related disorder case could contribute to a better understanding of the disease. A better understanding of the disease may allow the development of specific treatment options in the future.

SEMA6B-related progressive myoclonus epilepsy in a patient with Klinefelter syndrome

In most cases, variants of nucleotide sequence in the SEMA6B gene account for developing the phenotype of progressive myoclonus epilepsy and, to a lesser extent, developmental encephalopathy with or without epilepsy. Loss-of-function variants in nucleotide sequence localized mainly in exon 17 of the SEMA6B gene contribute to production of aberrant proteins with “toxic” functions. A clinical case of status epilepsy in a patient with a variant in the SEMA6B gene (c.2506delС; p.His836ThrfsTer136; NM_032108.4) is described in the article that expands our knowledge regarding the SEMA6B gene variants resulting in progressive myoclonus epilepsy.

Semaphorin 6 Family-An Important Yet Overlooked Group of Signaling Proteins Involved in Cancerogenesis

According to recent evidence, some groups of semaphorins (SEMAs) have been associated with cancer progression. These proteins are able to modulate the cellular signaling of particular receptor tyrosine kinases (RTKs) via the stimulation of SEMA-specific coreceptors, namely plexins (plexin-A, -B, -C, -D) and neuropilins (Np1, Np2), which share common domains with RTKs, leading to the coactivation of the latter receptors. MET, ERBB2, VEGFR2, PFGFR, and EGFR, among others, represent acknowledged targets of semaphorins that are often associated with tumor progression or poor prognosis. In particular, higher expression of SEMA6 family proteins in cancer cells and stromal cells of the cancer niche is often associated with enhanced tumor angiogenesis, metastasis, and resistance to anticancer therapy. Notably, high SEMA6 expression in malignant tumor cells such as melanoma, pleural mesothelioma, gastric cancer, lung adenocarcinoma, and glioblastoma may serve as a prognostic biomarker of tumor progression. To date, very few studies have focused on the mechanisms of transmembrane SEMA6-driven tumor progression and its underlying interplay with RTKs within the tumor microenvironment. This review presents the growing evidence in the literature on the complex and shaping role of SEMA6 family proteins in cancer responsiveness to environmental stimuli.

Prioritization and functional validation of target genes from single-cell transcriptomics studies

Translation of academic results into clinical practice is a formidable unmet medical need. Single-cell RNA-sequencing (scRNA-seq) studies generate long descriptive ranks of markers with predicted biological function, but without functional validation, it remains challenging to know which markers truly exert the putative function. Given the lengthy/costly nature of validation studies, gene prioritization is required to select candidates. We address these issues by studying tip endothelial cell (EC) marker genes because of their importance for angiogenesis. Here, by tailoring Guidelines On Target Assessment for Innovative Therapeutics, we in silico prioritize previously unreported/poorly described, high-ranking tip EC markers. Notably, functional validation reveals that four of six candidates behave as tip EC genes. We even discover a tip EC function for a gene lacking in-depth functional annotation. Thus, validating prioritized genes from scRNA-seq studies offers opportunities for identifying targets to be considered for possible translation, but not all top-ranked scRNA-seq markers exert the predicted function.

Progressive myoclonus epilepsies due to SEMA6B mutations. New variants and appraisal of published phenotypes

Variants of SEMA6B have been identified in an increasing number of patients, often presenting with progressive myoclonus epilepsy (PME), and to lesser extent developmental encephalopathy, with or without epilepsy. The exon 17 is mainly involved, with truncating mutations causing the production of aberrant proteins with toxic gain of function. Herein, we describe three adjunctive patients carrying de novo truncating SEMA6B variants in this exon (c.1976delC and c.2086C > T novel; c.1978delC previously reported). These subjects presented with PME preceded by developmental delay, motor and cognitive impairment, worsening myoclonus, and epilepsy with polymorphic features, including focal to bilateral seizures in two, and non-convulsive status epilepticus in one. The evidence of developmental delay in these cases suggests their inclusion in the "PME plus developmental delay" nosological group. This work further expands our knowledge of SEMA6B variants causing PMEs. However, the data to date available confirms that phenotypic features do not correlate with the type or location of variants, aspects that need to be further clarified by future studies.

A novel SEMA6B variant causes adult-onset progressive myoclonic epilepsy-11 in a Chinese family: A case report and literature review

This study describes a patient with progressive myoclonic epilepsy-11 (EPM-11), which follows autosomal dominant inheritance caused by a novel SEMA6B variant. Most patients develop this disease during infancy or adolescence with action myoclonus, generalized tonic-clonic seizures (GTCS), and progressive neurological deterioration. No cases of adult-onset EPM-11 have been reported yet. Here, we present one case of adult-onset EPM-11 who experienced gait instability, seizures, and cognitive impairment, and harbored a novel missense variant, c.432C>G (p.C144W). Our findings provide a foundation for a better understanding of the phenotypic and genotypic profiles of EPM-11. Further functional studies are recommended to elucidate the pathogenesis of this disease.

Two novel heterozygous truncating variants in NR4A2 identified in patients with neurodevelopmental disorder and brief literature review

Pathogenic variants in the nuclear receptor superfamily 4 group A member 2 (NR4A2) cause an autosomal dominant neurodevelopmental disorder with or without seizures. Here, we described two patients presenting with developmental delay, language impairment, and attention-deficit hyperactivity disorder. Trio-based whole exome sequencing revealed two novel heterozygous variants, c.1541-2A > C and c.915C > A, in NR4A2. Both variants were identified as de novo and confirmed by Sanger sequencing. In vitro functional analyses were performed to assess their effects on expression of mRNA or protein. The canonical splicing variant c.1541-2A > C caused aberrant splicing, leading to the retention of intron 7 and a truncated protein due to an early termination codon within intron 7 with decreased protein expression, while the variant c.915C > A was shown to result in a shorter protein with increased expression level unexpectedly. The clinical and genetic characteristics of the previously published patients were briefly reviewed for highlighting the potential link between mutations and phenotypes. Our research further confirms that NR4A2 is a disease-causing gene of neurodevelopmental disorders and suggests alterations in different domains of NR4A2 cause various severity of symptoms.

SEMA6B variants cause intellectual disability and alter dendritic spine density and axon guidance

Intellectual disability is a neurodevelopmental disorder frequently caused by monogenic defects. In this study, we collected 14 SEMA6B heterozygous variants in 16 unrelated patients referred for intellectual disability to different centres. Whereas until now SEMA6B variants have mainly been reported in patients with progressive myoclonic epilepsy, our study indicates that the clinical spectrum is wider, and also includes non-syndromic intellectual disability without epilepsy or myoclonus. To assess the pathogenicity of these variants, selected mutated forms of Sema6b were overexpressed in HEK293T cells and in primary neuronal cultures. shRNAs targeting Sema6b were also used in neuronal cultures to measure the impact of the decreased Sema6b expression on morphogenesis and synaptogenesis. The overexpression of some variants leads to a subcellular mislocalisation of SEMA6B protein in HEK293T cells and to a reduced spine density due to loss of mature spines in neuronal cultures. Sema6b knock-down also impairs spine density and spine maturation. In addition, we conducted in vivo rescue experiments in chicken embryos with the selected mutated forms of Sema6b expressed in commissural neurons after knock-down of endogenous SEMA6B. We observed that expression of these variants in commissural neurons fails to rescue the normal axon pathway. In conclusion, identification of SEMA6B variants in patients presenting with an overlapping phenotype with intellectual disability, and functional studies highlight the important role of SEMA6B in neuronal development, notably in spine formation and maturation, and in axon guidance. This study adds SEMA6B to the list of intellectual disability-related genes.

Nomenclature of Genetic Movement Disorders: Recommendations of the International Parkinson and Movement Disorder Society Task Force - An Update

In 2016, the Movement Disorder Society Task Force for the Nomenclature of Genetic Movement Disorders presented a new system for naming genetically determined movement disorders and provided a criterion-based list of confirmed monogenic movement disorders. Since then, a substantial number of novel disease-causing genes have been described, which warrant classification using this system. In addition, with this update, we further refined the system and propose dissolving the imaging-based categories of Primary Familial Brain Calcification and Neurodegeneration with Brain Iron Accumulation and reclassifying these genetic conditions according to their predominant phenotype. We also introduce the novel category of Mixed Movement Disorders (MxMD), which includes conditions linked to multiple equally prominent movement disorder phenotypes. In this article, we present updated lists of newly confirmed monogenic causes of movement disorders. We found a total of 89 different newly identified genes that warrant a prefix based on our criteria; 6 genes for parkinsonism, 21 for dystonia, 38 for dominant and recessive ataxia, 5 for chorea, 7 for myoclonus, 13 for spastic paraplegia, 3 for paroxysmal movement disorders, and 6 for mixed movement disorder phenotypes; 10 genes were linked to combined phenotypes and have been assigned two new prefixes. The updated lists represent a resource for clinicians and researchers alike and they have also been published on the website of the Task Force for the Nomenclature of Genetic Movement Disorders on the homepage of the International Parkinson and Movement Disorder Society (https://www.movementdisorders.org/MDS/About/Committees--Other-Groups/MDS-Task-Forces/Task-Force-on-Nomenclature-in-Movement-Disorders.htm). © 2022 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson Movement Disorder Society.

Clinical Study of 8 Cases of CHD2 Gene Mutation–Related Neurological Diseases and Their Mechanisms

Background: The chromodomain helicase DNA-binding protein 2 (CHD2) gene, is an ATPase and part of the CHD family of chromatin remodelers. Mutations in the CHD2 gene are inherited in an autosomal-dominant manner and can lead to intellectual disability, epilepsy, and autism. We investigated the clinical characteristics of CHD2-related conditions and their possible pathogenesis.

Methods: We collected and analysed the clinical data of patients that were identified as having CHD2 mutations. Genetic testing was performed using targeted sequencing or whole-exome sequencing. We analysed the expression of CHD2 and repressor element 1-silencing transcription factor (REST) in blood samples using quantitative PCR and the conservation of the mutations. The CHD2 mutations we identified were compared with the known mutations reported in the CHD2-related literature.

Results: Eight patients with CHD2 gene mutations were analysed. Six mutations were identified; four were unreported previously (c.670C>T; c.4012A>C; c.2416dup; c.1727–1728insAT), and two were known mutations: c.5035C>T (two cases) and c.4173dup (two cases). Among these mutations, seven were de novo mutations, and one could not be determined because the parents refused genetic testing. The clinical manifestations included mild or severe intellectual disability, epilepsy, and behavioural abnormalities. Quantitative PCR showed that the CHD2 gene expression levels among the patients, parents, and the controls were not significantly different. The levels of REST gene expression in the patients were significantly higher than those of the controls; thus, mutation of the CHD2 gene led to an increase in the expression level of the REST gene. The mutations reported were all located in conserved positions in different species. Among the various medications administered for treatment, valproate showed the best results for the treatment of epilepsy caused by CHD2 gene mutation.

Conclusion: Mutation in CHD2 did not lead to a significant decrease in its expression level, indicating that the clinical phenotype was unrelated to its expression level, and the mutant protein may retain some function. Most of the mutations relatively stable. In addition, the clinical manifestations from the same mutation in the CHD2 gene were different among the known cases; this may be related to the regulation of REST or other regulatory factors.

Non-convulsive Status Epilepticus in SEMA6B-Related Progressive Myoclonic Epilepsy: A Case Report With Literature Review

Progressive myoclonic epilepsy (PME) is a group of rare diseases characterized by progressive myoclonus, cognitive impairment, ataxia, and other neurologic deficits. PME has high genetic heterogeneity, and more than 40 genes are reportedly associated with this disorder. SEMA6B encodes a member of the semaphorin family and was first reported to cause PME in 2020. Herein, we present a rare case of PME due to a novel SEMA6B gene mutation in a 6-year-old boy born to healthy non-consanguineous Chinese parents. His developmental milestones were delayed, and he developed recurrent atonic seizures and myoclonic seizures without fever at 3 years and 11 months of age. He experienced recurrent myoclonic seizures, non-convulsive status epilepticus (NCSE), atonic seizures, and atypical absence seizures during the last 2 years. At different time points since onset, valproic acid, levetiracetam, piracetam, and clobazam were used to control the intractable seizures. Notably, NCSE was controlled by a combination of piracetam with clobazam and valproic acid instead of intravenous infusion of midazolam and phenobarbital. Due to the limited number of cases reported to date, the clinical description of our case provides a better understanding of the genotype-phenotype correlations associated with PME and indicate that piracetam may be effective against NCSE in patients with SEMA6B-related PME.