A new molecular mechanism for severe myoclonic epilepsy of infancy: Exonic deletions in SCN1A

Characteristic spatial and frequency distribution of mutations in SCN1A

BACKGROUND

SCN1A is the most well-recognized and commonly mutated gene related to epilepsy. This study analyzed the characteristic spatial and frequency distributions of SCN1A mutations, aiming to provide important insight into the mutagenesis etiopathology of SCN1A-associated epilepsy.

METHODS

Epilepsy-associated SCN1A variants were retrieved from the SCN1A mutation database, the HGMD database, and literature reviews. The base substitutions, mutation frequencies in CpG dinucleotides, and spatial distributions of mutations in terms of exons and structural domains were analyzed.

RESULTS

A total of 2621 SCN1A variants were identified in 5106 unrelated cases. The most common type was missense mutation, followed by frameshift mutations and splice site mutations. Among the missense mutations, transitions within CpG dinucleotides were much more recurrently identified than transitions within non-CpG dinucleotides, and the most common type was the G > A transition. Among the nonsense mutations, the most predominant type of single-base substitution was the C > T transition, among which 75.3% (235/312) were within CpG sites. The most common "hotspot" codons for missense mutations were codons 101, 946, and 1783; while for nonsense mutations it was codon 712. One-base deletion or insertion was the most common type of frameshift mutation, causing protein truncation. The three most common frameshift mutations were c.5536_5539delAAAC, c.4554dupA, and c.5010_5013delGTTT. Splice mutations were the most frequently identified in exon 4 with a hotspot site c.602 + 1G > A. The spatial distribution of missense mutations showed that exons 22 and 4 had the highest mutation density (111 and 84 mutations per 100 bp, respectively), and exon 12 had the lowest mutation density, with 4 mutations per 100 bp. Further distribution analysis of the protein domains revealed that missense mutations were more common in the pore region and voltage sensor (231 mutations per 100 amino acids, respectively), and the protein truncation mutations were distributed evenly among the domains.

CONCLUSIONS

SCN1A mutations tend to cluster at distinct sites, depending on the characteristic CpG dinucleotides, exons, and functional domains. Higher mutation density in particular regions, such as exon 22 and exon 4, offers promising targets for therapeutic genetic interventions.

Early-Life Epilepsies

Epilepsies are a diverse group of neurological disorders characterized by recurrent seizures. One-third of epilepsies are refractory to standard antiseizure medications. Epilepsy incidence is age-dependent with high incidence in neonates and infants. Epilepsy syndromes are classified based on clinical, electrographic, neuroimaging, age-dependent features of onset and the possibility of remission. Advances in genetic testing technology and improved access to clinical genetic testing, including whole exome sequencing, have facilitated a fundamental shift in gene discovery of monogenetic and polygenetic epilepsy, leading to precision medicine therapy and improved outcomes. Here, we review the self-limited epilepsy syndromes and developmental and epileptic encephalopathies that begin in the neonatal-infantile period with an emphasis on genetic etiology and the shifting landscape of treatment options based on genetic findings. [Pediatr Ann. 2023;52(10):e381-e387.].

SCN1A and Its Related Epileptic Phenotypes

Epilepsy is one of the most common neurological disorders, with a lifetime incidence of 1 in 26. Approximately two-thirds of epilepsy has a substantial genetic component in its etiology. As a result, simultaneous screening for mutations in multiple genes and performing whole exome sequencing (WES) are becoming very frequent in the clinical evaluation of children with epilepsy. In this setting, mutations in voltage-gated sodium channel (SCN) α-subunit genes are the most commonly identified cause of epilepsy, with sodium channel genes (i.e., SCN1A, SCN2A, SCN8A) being the most frequently identified causative genes. SCN1A mutations result in a wide spectrum of epilepsy phenotypes ranging from simple febrile seizures to Dravet syndrome, a severe epileptic encephalopathy. In case of mutation of SCN1A, it is also possible to observe behavioral alterations, such as impulsivity, inattentiveness, and distractibility, which can be framed in an attention deficit hyperactivity disorder (ADHD) like phenotype. Despite more than 1,200 SCN1A mutations being reported, it is not possible to assess a clear phenotype–genotype correlations. Treatment remains a challenge and seizure control is often partial and transitory.

Genetic therapeutic advancements for Dravet Syndrome

Dravet Syndrome is a genetic epileptic syndrome characterized by severe and intractable seizures associated with cognitive, motor, and behavioral impairments. The disease is also linked with increased mortality mainly due to sudden unexpected death in epilepsy. Over 80% of cases are due to a de novo mutation in one allele of the SCN1A gene, which encodes the α-subunit of the voltage-gated ion channel Na_V1.1. Dravet Syndrome is usually refractory to antiepileptic drugs, which only alleviate seizures to a small extent. Viral, non-viral genetic therapy, and gene editing tools are rapidly enhancing and providing new platforms for more effective, alternative medicinal treatments for Dravet syndrome. These strategies include gene supplementation, CRISPR-mediated transcriptional activation, and the use of antisense oligonucleotides. In this review, we summarize our current knowledge of novel genetic therapies that are currently under development for Dravet syndrome.

Patient with Dravet syndrome: A case report

Dravet syndrome is rare genetic epilepsy syndrome and epileptic encephalopathy. The patient initially has normal developmental profile with plateau or regression that begins after seizure onset. We report a case of two-year-old child diagnosed as dravet syndrome with moderate cerebral atrophy and ventricular dilatation as rare MRI finding.

Whole-Exome Sequencing Identifies Novel SCN1A and CACNB4 Genes Mutations in the Cohort of Saudi Patients With Epilepsy

Epilepsy is a neurological disorder described as recurrent seizures mild to severe convulsions along with conscious loss. There are many different genetic anomalies or non-genetic conditions that affect the brain and cause epilepsy. The exact cause of epilepsy is unknown so far. In this study, whole-exome sequencing showed a family having novel missense variant c.1603C>T, p. Arg535Cys in exon 10 of Sodium Voltage-Gated Channel Alpha Subunit 1 (SCN1A) gene. Moreover, targeted Sanger sequencing analysis showed c.1212A>G p.Val404Ile in SCN1A gene in 10 unrelated patients and a mutation in Calcium Voltage-Gated Channel Auxiliary Subunit Beta 4 gene where one base pair insertion of "G" c.78_79insG, p.Asp27Glyfs^*26 in the exon 3 in three different patients were observed from the cohort of 25 epileptic sporadic cases. The insertion changes the amino acid sequence leading to a frameshift mutation. Here, we have described, for the first time, three novel mutations that may be associated with epilepsy in the Saudi population. The study not only help us to identify the exact cause of genetic variations causing epilepsy whereas but it would also eventually enable us to establish a database to provide a foundation for understanding the critical genomic regions to control epilepsy in Saudi patients.

Electroclinical Phenotype-Genotype Homogeneity in Drug-Resistant "Generalized" Tonic-Clonic Seizures of Early Childhood

PURPOSE

Children with refractory focal to bilateral tonic-clonic seizures, despite normal high-resolution imaging, are often not subjected to genetic tests due to the costs involved and instead undergo multimodality presurgical evaluation targeted at delineating a focal onset. The objective of this study was to ascertain genotype-phenotype correlations in this group of patients.

METHOD

An online hospital database search was conducted for children who presented in 2019 with drug-resistant epilepsy dominated by nonlateralizing focal-onset/rapid generalized (bilateral) tonic-clonic seizures (GTCS), subjected to presurgical evaluation and subsequent genetic testing due to absence of a clear focus hypothesis.

RESULTS

Phenotypic homogeneity was apparent in 3 children who had onset in infancy with drug-resistant GTCS (predominantly unprovoked and occasionally fever provoked) and subsequent delayed development. 3-Tesla magnetic resonance imaging (MRI) scans were negative and video EEG documented a homogeneous pattern of multifocal and/or generalized epileptiform discharges with phenomenology favoring probable focal-onset/generalized-onset bilateral tonic-clonic seizures. All 3 tested positive for SCN1A gene variants (heterozygous missense substitution variants in 2 children, one of which was novel and a novel duplication in one that led to frameshift and premature truncation of the protein), suggestive of SCN1A-mediated epilepsy. This electroclinical profile constituted 3 out of 25 patients with SCN1A-epilepsy phenotypes at our center.

CONCLUSIONS

These cases suggest that children with early-onset drug-resistant "generalized" epilepsy are likely to have a genetic basis although the presentation may not be typical of Dravet syndrome. Hence, genetic testing for SCN1A variants is recommended in children with drug-resistant MRI negative focal-onset/generalized-onset bilateral tonic-clonic seizures before subjecting them to exhaustive presurgical workup and to guide appropriate treatment and prognostication.

Developmental and epileptic encephalopathies: what we do and do not know

Developmental encephalopathies, including intellectual disability and autistic spectrum disorder, are frequently associated with infant epilepsy. Epileptic encephalopathy is used to describe an assumed causal relationship between epilepsy and developmental delay. Developmental encephalopathies pathogenesis more independent from epilepsy is supported by the identification of several gene variants associated with both developmental encephalopathies and epilepsy, the possibility for gene-associated developmental encephalopathies without epilepsy, and the continued development of developmental encephalopathies even when seizures are controlled. Hence, 'developmental and epileptic encephalopathy' may be a more appropriate term than epileptic encephalopathy. This update considers the best studied 'developmental and epileptic encephalopathy' gene variants for illustrative support for 'developmental and epileptic encephalopathy' over epileptic encephalopathy. Moreover, the interaction between epilepsy and developmental encephalopathies is considered with respect to influence on treatment decisions. Continued research in genetic testing will increase access to clinical tests, earlier diagnosis, better application of current treatments, and potentially provide new molecular-investigated treatments.

Genetic literacy series: Primer part 2-Paradigm shifts in epilepsy genetics

This is the second of a 2-part primer on the genetics of the epilepsies within the Genetic Literacy Series of the Genetics Commission of the International League Against Epilepsy. In Part 1, we covered types of genetic variation, inheritance patterns, and their relationship to disease. In Part 2, we apply these basic principles to the case of a young boy with epileptic encephalopathy and ask 3 important questions: (1) Is the gene in question an established genetic etiology for epilepsy? (2) Is the variant in this particular gene pathogenic by established variant interpretation criteria? (3) Is the variant considered causative in the clinical context? These questions are considered and then answered for the clinical case in question.

Impairments in social novelty recognition and spatial memory in mice with conditional deletion of Scn1a in parvalbumin-expressing cells

Loss of function mutations in the SCN1A gene, which encodes the voltage-gated sodium channel Nav1.1, have been described in the majority of Dravet syndrome patients presenting with epileptic seizures, hyperactivity, autistic traits, and cognitive decline. We previously reported predominant Nav1.1 expression in parvalbumin-expressing (PV+) inhibitory neurons in juvenile mouse brain and observed epileptic seizures in mice with selective deletion of Scn1a in PV+ cells mediated by PV-Cre transgene expression (Scn1a^fl/+/PV-Cre-TG). Here we investigate the behavior of Scn1a^fl/+/PV-Cre-TG mice using a comprehensive battery of behavioral tests. We observed that Scn1a^fl/+/PV-Cre-TG mice display hyperactive behavior, impaired social novelty recognition, and altered spatial memory. We also generated Scn1a^fl/+/SST-Cre-KI mice with a selective Scn1a deletion in somatostatin-expressing (SST+) inhibitory neurons using an SST-IRES-Cre knock-in driver line. We observed that Scn1a^fl/+/SST-Cre-KI mice display no spontaneous convulsive seizures and that Scn1a^fl/+/SST-Cre-KI mice have a lowered threshold temperature for hyperthermia-induced seizures, although their threshold values are much higher than those of Scn1a^fl/+/PV-Cre-TG mice. We finally show that Scn1a^fl/+/SST-Cre-KI mice exhibited no noticeable behavioral abnormalities. These observations suggest that impaired Nav1.1 function in PV+ interneurons is critically involved in the pathogenesis of hyperactivity, autistic traits, and cognitive decline, as well as epileptic seizures, in Dravet syndrome.

Ion Channels in Genetic Epilepsy: From Genes and Mechanisms to Disease-Targeted Therapies

Epilepsy is a common and serious neurologic disease with a strong genetic component. Genetic studies have identified an increasing collection of disease-causing genes. The impact of these genetic discoveries is wide reaching-from precise diagnosis and classification of syndromes to the discovery and validation of new drug targets and the development of disease-targeted therapeutic strategies. About 25% of genes identified in epilepsy encode ion channels. Much of our understanding of disease mechanisms comes from work focused on this class of protein. In this study, we review the genetic, molecular, and physiologic evidence supporting the pathogenic role of a number of different voltage- and ligand-activated ion channels in genetic epilepsy. We also review proposed disease mechanisms for each ion channel and highlight targeted therapeutic strategies.

Diagnostic Approach to Genetic Causes of Early-Onset Epileptic Encephalopathy

Epileptic encephalopathies are characterized by recurrent clinical seizures and prominent interictal epileptiform discharges seen during the early infantile period. Although epileptic encephalopathies are mostly associated with structural brain defects and inherited metabolic disorders, pathogenic gene mutations may also be involved in the development of epileptic encephalopathies even when no clear genetic inheritance patterns or consanguinity exist. The most common epileptic encephalopathies are Ohtahara syndrome, early myoclonic encephalopathy, epilepsy of infancy with migrating focal seizures, West syndrome and Dravet syndrome, which are usually unresponsive to traditional antiepileptic medication. Many of the diagnoses describe the phenotype of these electroclinical syndromes, but not the underlying causes. To date, approximately 265 genes have been defined in epilepsy and several genes including STXBP1, ARX, SLC25A22, KCNQ2, CDKL5, SCN1A, and PCDH19 have been found to be associated with early-onset epileptic encephalopathies. In this review, we aimed to present a diagnostic approach to primary genetic causes of early-onset epileptic encephalopathies.

The incidence of SCN1A-related Dravet syndrome in Denmark is 1:22,000: a population-based study from 2004 to 2009

Dravet syndrome is a severe infantile-onset epileptic encephalopathy associated with mutations in the sodium channel alpha-1 subunit gene SCN1A. We aimed to describe the incidence of Dravet syndrome in the Danish population. Based on a 6-year birth cohort from 2004 to 2009, we propose an incidence of 1:22,000, which is higher than what has been established earlier. We identified 17 cases with SCN1A mutation-positive Dravet syndrome. Fifteen patients were found, by conventional Sanger sequencing. Two additional patients with clinical Dravet syndrome, but without a detectable SCN1A mutation by Sanger sequencing, were diagnosed with a SCN1A mutation after using a targeted next-generation sequencing gene panel.

Early clinical features and diagnosis of Dravet syndrome in 138 Chinese patients with SCN1A mutations

OBJECTIVE

To summarize the early clinical features of Dravet syndrome (DS) patients with SCN1A gene mutations before the age of one.

METHODS

SCN1A gene mutation screening was performed by PCR-DNA sequencing and multiple ligation-dependent probe amplication (MLPA). The early clinical features of DS patients with SCN1A mutations were reviewed with attention to the seizures induced by fever and other precipitating factors before the first year of life.

RESULTS

The clinical data of 138 DS patients with SCN1A gene mutations were reviewed. The median seizure onset age was 5.3 months. Ninety-nine patients (71.7%) experienced seizures with duration more than 15 min in the first year of life. Two or more seizures induced by fever within 24h or the same febrile illness were observed in 93 patients (67.4%). 111 patients (80.4%) had hemi-clonic and (or) focal seizures. Seizures had been triggered by fever of low degree (T<38 °C) in 62.3% (86/138) before the first year of life. Vaccine-related seizures were observed in 34.8% (48/138). Seizures in 22.5% (31/138) of patients were triggered by hot bath. Carbamazepine, oxcarbazepine, lamotrigine, phenobarbital and phenytoin showed either no effect or exacerbating the seizures in our group.

CONCLUSION

The seizure onset age in DS patients was earlier than that was in common febrile seizures. When a baby exhibits two or more features of complex febrile seizures in the first year of life, a diagnosis of DS should be considered, and SCN1A gene mutation screening should be performed as early as possible. Early diagnosis of DS will help clinicians more effectively prescribe antiepileptic drugs for stronger prognosis.

CNVs in Epilepsy

Copy number variants (CNVs) are deletions or duplications of DNA. CNVs have been increasingly recognized as an important source of both normal genetic variation and pathogenic mutation. Technologies for genome-wide discovery of CNVs facilitate studies of large cohorts of patients and controls to identify CNVs that cause increased risk for disease. Over the past 5 years, studies of patients with epilepsy confirm that both recurrent and non-recurrent CNVs are an important source of mutation for patients with various forms of epilepsy. Here, we will review the latest findings and explore the clinical implications.

Copy number variation plays an important role in clinical epilepsy

OBJECTIVE

To evaluate the role of copy number abnormalities detectable using chromosomal microarray (CMA) testing in patients with epilepsy at a tertiary care center.

METHODS

We identified patients with International Classification of Diseases, ninth revision (ICD-9) codes for epilepsy or seizures and clinical CMA testing performed between October 2006 and February 2011 at Boston Children's Hospital. We reviewed medical records and included patients who met criteria for epilepsy. We phenotypically characterized patients with epilepsy-associated abnormalities on CMA.

RESULTS

Of 973 patients who had CMA and ICD-9 codes for epilepsy or seizures, 805 patients satisfied criteria for epilepsy. We observed 437 copy number variants (CNVs) in 323 patients (1-4 per patient), including 185 (42%) deletions and 252 (58%) duplications. Forty (9%) were confirmed de novo, 186 (43%) were inherited, and parental data were unavailable for 211 (48%). Excluding full chromosome trisomies, CNV size ranged from 18kb to 142Mb, and 34% were >500kb. In at least 40 cases (5%), the epilepsy phenotype was explained by a CNV, including 29 patients with epilepsy-associated syndromes and 11 with likely disease-associated CNVs involving epilepsy genes or "hotspots." We observed numerous recurrent CNVs including 10 involving loss or gain of Xp22.31, a region described in patients with and without epilepsy.

INTERPRETATION

Copy number abnormalities play an important role in patients with epilepsy. Because the diagnostic yield of CMA for epilepsy patients is similar to the yield in autism spectrum disorders and in prenatal diagnosis, for which published guidelines recommend testing with CMA, we recommend the implementation of CMA in the evaluation of unexplained epilepsy.

Role of genetics in the diagnosis and treatment of epilepsy

Epilepsy is a heterogeneous group of multifactorial diseases, the vast majority determined by interactions between many genes and environmental factors; however, there are rare epilepsy syndromes that can be caused by a single gene mutation and are inherited according to classical mendelian genetic principles. Finding disease-causing genetic mutations in epilepsy has provided new opportunities for aiding diagnosis and developing therapies. Thus, the discovery of KCNQ2 mutations in benign familial neonatal convulsions, SCN1A mutations in severe myoclonic epilepsy of infancy and in generalized epilepsy with febrile seizures plus, and CHRA4 and CHRB2 mutations in autosomal-dominant nocturnal frontal lobe epilepsy, has led to the establishment of epilepsy as a disorder of ion channel function and, furthermore, has led to the introduction of genetic tests that are available clinically to aid in diagnosis and treatment. At the present time, clinical use of genetic testing in epilepsy is greatest in suspected cases of severe myoclonic epilepsy of infancy, generalized epilepsy with febrile seizures plus, atypical cases of benign familial neonatal convulsions and 'occult' cases of autosomal-dominant nocturnal frontal lobe epilepsy without a family history. Overall, clinical use is limited by the low number of documented disease-associated mutations and the uncertain clinical significance of many test results. Further elucidation of the relationship between gene mutations and channel function will add value to genetic testing in the future, as will better characterization of the association between gene mutations and clinical phenotypes.

Atypical multifocal Dravet syndrome lacks generalized seizures and may show later cognitive decline

AIM

To show that atypical multifocal Dravet syndrome is a recognizable, electroclinical syndrome associated with sodium channel gene (SCN1A) mutations that readily escapes diagnosis owing to later cognitive decline and tonic seizures.

METHOD

Eight patients underwent electroclinical characterization. SCN1A was sequenced and copy number variations sought by multiplex ligation-dependent probe amplification.

RESULTS

All patients were female (age range at assessment 5-26y) with median seizure onset at 6.5 months (range 4-19mo). The initial seizure was brief in seven and status epilepticus only occurred in one; three were febrile. Focal seizures occurred in four patients and bilateral convulsion in the other four. All patients developed multiple focal seizure types and bilateral convulsions, with seizure clusters in six. The most common focal seizure semiology (six out of eight) comprised unilateral clonic activity. Five also had focal or asymmetric tonic seizures. Rare or transient myoclonic seizures occurred in six individuals, often triggered by specific antiepileptic drugs. Developmental slowing occurred in all: six between 3 years and 8 years, and two around 1 year 6 months. Cognitive outcome varied from severe to mild intellectual disability. Multifocal epileptiform discharges were seen on electroencephalography. Seven out of eight patients had SCN1A mutations.

INTERPRETATION

Atypical, multifocal Dravet syndrome with SCN1A mutations may not be recognized because of later cognitive decline and frequent tonic seizures.

A new locus on chromosome 22q13.31 linked to recessive genetic epilepsy with febrile seizures plus (GEFS+) in a Tunisian consanguineous family

Background

Genetic epilepsy with febrile seizures plus (GEFS+) is a familial epilepsy syndrome with extremely variable expressivity. The aim of our study was to identify the responsible locus for GEFS+ syndrome in a consanguineous Tunisian family showing three affected members, by carrying out a genome-wide single nucleotide polymorphisms (SNPs) genotyping followed by a whole-exome sequencing. We hypothesized an autosomal recessive (AR) mode of inheritance.

Results

Parametric linkage analysis and haplotype reconstruction identified a new unique identical by descent (IBD) interval of 527 kb, flanking by two microsatellite markers, 18GTchr22 and 15ACchr22b, on human chromosome 22q13.31 with a maximum multipoint LOD score of 2.51. Our analysis was refined by the use of a set of microsatellite markers. We showed that one of them was homozygous for the same allele in all affected individuals and heterozygous in healthy members of this family. This microsatellite marker, we called 17ACchr22, is located in an intronic region of TBC1D22A gene, which encodes a GTPase activator activity. Whole-exome sequencing did not reveal any mutation on chromosome 22q13.31 at the genome wide level.

Conclusions

Our findings suggest that TBC1D22A is a new locus for GEFS+.

Dravet syndrome (severe myoclonic epilepsy in infancy)

Severe myoclonic epilepsy in infancy (SMEI) is a rare disease, characterized by febrile and afebrile, generalized and unilateral, clonic or tonic-clonic seizures that occur in the first year of life in an otherwise apparently normal infant. They are later associated with myoclonus, atypical absences, and partial seizures. Developmental delay becomes apparent within the second year of life and is followed by definite cognitive impairment and personality disorders of variable intensity. In the borderline form, children do not present with myoclonic symptoms but have the same general picture. SMEI is a channelopathy and the genetic studies have shown a mutation in the SCN1A gene in 70 to 80% of the patients, including the borderline forms. At present, there are no well-established correlations between genotype and phenotype. The electroencephalograms, often normal at the onset, display both generalized and focal anomalies, without a specific electroencephalographic pattern. As a rule, neuroimaging is normal. All seizure types are resistant to antiepileptic drugs and status epilepticus is frequent. Some drugs have been shown to aggravate the seizures and must be avoided. Two recent drugs have been proved to partially control the convulsive seizures and the status epilepticus. Therefore, it is crucial to diagnose this epilepsy soon after its onset in order to prescribe the most appropriate treatment.

Do mutations in SCN1B cause Dravet syndrome?

A homozygous SCN1B mutation was previously identified in a patient with early onset epileptic encephalopathy (EOEE) described as Dravet syndrome (DS) despite a more severe phenotype than DS. We investigated whether SCN1B mutations are a common cause of DS. Patients with DS who did not have a SCN1A sequencing mutation or copy number variation were studied. Genomic DNA was Sanger sequenced for mutations in the 6 exons of SCN1B. In 54 patients with DS recruited from four centres, no SCN1B mutations were identified. SCN1B mutation is not a common cause of DS.

Genetics of epilepsy and relevance to current practice

Genetic factors are likely to play a major role in many epileptic conditions, spanning from classical idiopathic (genetic) generalized epilepsies to epileptic encephalopathies and focal epilepsies. In this review we describe the genetic advances in progressive myoclonus epilepsies, which are strictly monogenic disorders, genetic generalized epilepsies, mostly exhibiting complex genetic inheritance, and SCN1A-related phenotypes, namely genetic generalized epilepsy with febrile seizure plus and Dravet syndrome. Particular attention is devoted to a form of familial focal epilepsies, autosomal-dominant lateral temporal epilepsy, which is a model of non-ion genetic epilepsies. This condition is associated with mutations of the LGI1 gene, whose protein is secreted from the neurons and exerts its action on a number of targets, influencing cortical development and neuronal maturation.

Developmental psychopathology: The role of structural variation in the genome

A wide range of developmental disorders present with characteristic psychopathologies and behaviors, with diagnoses including, inter alia, cognitive disorders and learning disabilities, epilepsies, autism, and schizophrenia. Each, to varying extent, has a genetic component to etiology and is associated with cytogenetic abnormalities. Technological developments, particularly array-based comparative genome hybridization and single nucleotide polymorphism chips, has revealed a wide range of rare recurrent and de novo copy number variants (CNVs) to be associated with disorder and psychopathology. It is surprising that many apparently similar CNVs are identified across two or more disorders hitherto considered unrelated. This article describes the characteristics of CNVs and current technological restrictions that make accurately identifying small events difficult. It summarizes the latest discoveries for individual diagnostic categories and considers the implications for a shared neurobiology. It examines likely developments in the knowledge base as well as addressing the clinical implications going forward.

Diagnosis and long-term course of Dravet syndrome

Dravet syndrome is a severe infantile-onset epilepsy syndrome with a distinctive but complex electroclinical presentation. A healthy, developmentally normal infant presents at around 6 months of age with convulsive status epilepticus, which may be hemiclonic or generalized; seizures may be triggered by fever, illness or vaccination. The infant typically has further episodes of status epilepticus every month or two, often triggered by fever. Other seizure types including focal dyscognitive seizures, absence and myoclonic seizures develop between 1 and 4 years. Atonic drop attacks and episodes of non-convulsive status may occur. Early development is normal but slows in the second year. Developmental regression may occur, particularly with status epilepticus. EEG studies are initially normal, but after 2 years they show generalized spike-wave and polyspike-wave activity with multifocal discharges. Photosensitivity may be seen. Imaging is normal or shows non-specific findings such as atrophy. Dravet syndrome is associated with mutations of the gene encoding the alpha-1 subunit of the sodium channel, SCN1A, in >70% of patients. These include sequencing mutations and copy number variant anomalies; 90% of mutations arise de novo. PCDH19 mutational analysis is a second-tier test for girls with a Dravet-like picture who do not have SCN1A mutations. Outcome is poor, with intellectual disability in most patients and ongoing seizures. Intellectual impairment varies from severe in 50% patients, to moderate and mild intellectual disability each accounting for 25% cases. Rare patients have normal intellect. The long-term course involves ongoing, brief nocturnal convulsions and a characteristic deterioration in gait.

Electroencephalographic features in dravet syndrome: five-year follow-up study in 22 patients

The aim of the study was to evaluate interictal electroencephalogram features in 22 patients with Dravet syndrome from the onset of the disease through the next 5 years. Electroencephalogram was abnormal in 5 patients (22.7%) at onset, and in 17 (77.3%) at the end of the study. Epileptiform abnormalities (focal, multifocal, or generalized) were seen in 6 patients at the onset and in 14 (27% vs 64%) at the end of the study. Photoparoxysmal response was present in 41% of patients at the end of follow-up. No statistical differences were found between mutated and nonmutated groups regarding evolution of background activity, interictal abnormalities, and presence of photoparoxysmal response. Electroencephalogram findings seemed to be age dependent, variable among different patients, and not influenced by the presence of sodium channel, voltage-gated, type I, alpha subunit (SCN1A) mutation. The lack of specific epileptiform abnormalities contributes to the difficulty of patients' management in Dravet syndrome.

Febrile infection-related epilepsy syndrome is not caused by SCN1A mutations

Two distinctive epileptic encephalopathies, febrile infection-related epilepsy syndrome (FIRES) and Dravet syndrome (DS), present with febrile status epilepticus in a normal child followed by refractory focal seizures and cognitive decline although there are differentiating features. Abnormalities of the sodium channel gene SCN1A are found in 75% of DS patients. We found no SCN1A mutations or copy number variants in 10 patients with FIRES. Other genetic etiologies deserve consideration.

Alleged cases of vaccine encephalopathy rediagnosed years later as Dravet syndrome

Dravet syndrome is a rare epileptic encephalopathy linked to mutations in SCN1A (neuronal sodium channel α1 subunit) and characterized by an onset in infancy with polymorphous seizure types and developmental decline. It was reported recently that a proportion of patients previously diagnosed with alleged vaccine encephalopathy might possess SCN1A mutations and clinical histories that enabled a diagnosis of Dravet syndrome, but these results have not been replicated. We present here the cases of 5 children who presented for epilepsy care with presumed parental diagnoses of alleged vaccine encephalopathy caused by pertussis vaccinations in infancy. Their conditions were all rediagnosed years later, with the support of genetic testing, as Dravet syndrome. We hope that these cases will raise awareness of Dravet syndrome among health care providers who care for children and adolescents and aid in earlier recognition and diagnosis.

Genetic variations and associated pathophysiology in the management of epilepsy

The genomic era has enabled the application of molecular tools to the solution of many of the genetic epilepsies, with and without comorbidities. Massively parallel sequencing has recently reinvigorated gene discovery for the monogenic epilepsies. Recurrent and novel copy number variants have given much-needed impetus to the advancement of our understanding of epilepsies with complex inheritance. Superimposed upon that is the phenotypic blurring by presumed genetic modifiers scattering the effects of the primary mutation. The genotype-first approach has uncovered associated syndrome constellations, of which epilepsy is only one of the syndromes. As the molecular genetic basis for the epilepsies unravels, it will increasingly influence the classification and diagnosis of the epilepsies. The ultimate goal of the molecular revolution has to be the design of treatment protocols based on genetic profiles, and cracking the 30% of epilepsies refractory to current medications, but that still lies well into the future. The current focus is on the scientific basis for epilepsy. Understanding its genetic causes and biophysical mechanisms is where we are currently positioned: prizing the causes of epilepsy "out of the shadows" and exposing its underlying mechanisms beyond even the ion-channels.

Low long-term efficacy and tolerability of add-on rufinamide in patients with Dravet syndrome

In this retrospective European multicenter study we evaluated the efficacy and tolerability of rufinamide in patients with Dravet syndrome and refractory seizures. Twenty patients were included; in 16 patients a SCN1A mutation was detected. The responder rate after 6 months was 20%, and after 34 months, 5%. The retention rate was 45% after 6 months and 5% after 34 months. Rufinamide treatment was stopped because of aggravation of seizures (30%), no effect (45%), and side effects (10%). The efficacy and long-term retention rate were low in our patients with Dravet syndrome and refractory seizures, far lower than in patients with Lennox-Gastaut syndrome; one-third of our patients experienced seizure aggravation. Therefore, rufinamide does not seem to be a suitable option for long-term treatment in patients with Dravet syndrome.

Mutation screening of three Chinese families with genetic epilepsy with febrile seizures plus

Genetic epilepsy with febrile seizures plus (GEFS+) is a familial autosomal dominant condition characterized by genetic heterogeneity. Five genes for GEFS+ identified in large families account for only a small proportion of families. Mutation in the majority of families with GEFS+ has not identified yet. The aim of our study is to search for the gene responsible for GEFS+ in three Chinese families by linkage analyses and a sequencing approach and to investigate the importance of coding and noncoding regions variations of four known GEFS+ genes (SCN1A, SCN1B, GABRG2 and SCN2A) in Chinese families. Results showed that a 6-cM candidate interval at 5q33-34 with a maximum LOD scores of 2.043 was identified in families B. Sequencing candidate gene GABRG2 and GABRA1 in this region did not identify a causative mutation. Moreover, no mutation was found in coding and noncoding regions of the four genes in three Chinese families. Besides excluding coding regions of four known GEFS+ genes, we also excluded the possibility of a mutation in the promoter, exon-intron boundaries, 5' untranslated regions (5' UTRs), and 3' UTRs of four known GEFS+ genes in three Chinese families. In conclusion, the present study demonstrates the heterogeneity of the etiologies of GEFS+. There are as yet undiscovered mechanisms underlying GEFS+.

One novel Dravet syndrome causing mutation and one recurrent MAE causing mutation in SCN1A gene

Mutations in SCN1A gene, encoding the voltage-gated sodium channel α1-subunit, are found to be associated with severe myoclonic epilepsy in infancy or Dravet syndrome (DS), but only rarely with the myoclonic astatic epilepsy (MAE, or Doose syndrome). We report on two patients with SCN1A mutations and severe epilepsy within the spectrum of generalized epilepsy with febrile seizures plus syndrome (GEFS+), the phenotypes being consistent with DS and MAE, respectively. Analysis of SCN1A revealed a heterozygous de novo frameshift mutation (c.4205_4208delGAAA) in the patient with DS, and a recurrent missense mutation (c.3521C>G) in that suffering from MAE. The missense mutation has been reported in patients with neurological diseases of various manifestations, which suggests that this variability is likely to result from the modifying effects of other genetic or environmental factors. DS phenotype has been mainly found associated with truncation mutations, while predominantly missense mutations and very few prematurely terminating substitutions have been reported in GEFS+ patients.

Epilepsy and the new cytogenetics

We set out to review the extent to which molecular karyotyping has overtaken conventional cytogenetics in applications related to epilepsy. Multiplex ligase-dependent probe amplification (MLPA) targeted to predetermined regions such as SCN1A and KCNQ2 has been effectively applied over the last half a decade, and oligonucleotide array comparative genome hybridization (array CGH) is now well established for genome-wide exploration of microchromosomal variation. Array CGH is applicable to the characterization of lesions present in both sporadic and familial epilepsy, especially where clinical features of affected cases depart from established syndromes. Copy number variants (CNVs) associated with epilepsy and a range of other syndromes and conditions can be recurrent due to nonallelic homologous recombination in regions of segmental duplication. The most common of the recurrent microdeletions associated with generalized epilepsy are typically seen at a frequency of ∼ 1% at 15q13.3, 16p13.11, and 15q11.2, sites that also confer susceptibility for intellectual disability, autism, and schizophrenia. Incomplete penetrance and variable expressivity confound the established rules of cytogenetics for determining the pathogenicity for novel CNVs; however, as knowledge is gained for each of the recurrent CNVs, this is translated to genetic counseling. CNVs play a significant role in the susceptibility profile for epilepsies, with complex genetics and their comorbidities both from the "hotspots" defined by segmental duplication and elsewhere in the genome where their location and size are often novel.

A retrospective study of the relation between vaccination and occurrence of seizures in Dravet syndrome

Dravet syndrome is a severe epileptic encephalopathy starting in the first year of life. Mutations in SCN1A can be identified in the majority of patients, and epileptic seizures in the setting of fever are a clinical hallmark. Fever is also commonly seen after vaccinations and provocation of epileptic seizures by vaccinations in patients with Dravet syndrome has been reported, but not systematically assessed. In a retrospective evaluation of 70 patients with Dravet syndrome and SCN1A mutations, seizures following vaccinations were reported in 27%. In 58% of these patients vaccination-related seizures represented the first clinical manifestation. The majority of seizures occurred after DPT vaccinations and within 72 h after vaccination. Two-thirds of events occurred in the context of fever. Our findings highlight seizures after vaccinations as a common feature in Dravet syndrome and emphasize the need for preventive measures for seizures triggered by vaccination or fever in these children.

Update on rufinamide in childhood epilepsy

Rufinamide is an orally active, structurally novel compound (1-[(2,6-difluorophenil1) methyl1]-1 hydro 1,2,3-triazole-4 carboxamide), which is structurally distinct from other anticonvulsant drugs. It was granted orphan drug status for the adjunctive treatment of Lennox-Gastaut syndrome (LGS) in the United States in 2004, and released for use in Europe in 2007. In January 2009, rufinamide was approved by the United States Food and Drug Administration for treatment of LGS in children 4 years of age and older. It is also approved for adjunctive treatment for partial seizures in adults and adolescents. Rufinamide's efficacy mainly against atonic/tonic seizures in patients with LGS seems nowadays indubitable and has been confirmed both in randomized controlled trial and in open label extension studies. More recently, rufinamide was evaluated for the adjunctive treatment of childhood-onset epileptic encephalopathies and epileptic syndromes other than LGS, including epileptic spasms, multifocal epileptic encephalopathy with spasm/tonic seizures, myoclonic-astatic epilepsy, Dravet syndrome and malignant migrating partial seizures in infancy. This review updates the existing literature data on the efficacy and safety/tolerability of rufinamide in childhood-onset epilepsy syndromes.

A case of SUDEP in a patient with Dravet syndrome with SCN1A mutation

A boy with a clinical history of pharmacologically resistant Dravet syndrome died suddenly after falling asleep. The autopsy concluded that the cause of death was sudden unexpected death in epilepsy (SUDEP). Postmortem molecular analysis of the SCN1A gene by multiplex ligation-dependent probe amplification (MLPA), high-resolution melting curve analysis (HRMCA), and sequencing revealed a frameshift duplication of adenosine at position 504. The incidence of this mutation is discussed as a potential cause of SUDEP.

Genetically complex epilepsies, copy number variants and syndrome constellations

Epilepsy is one of the most common neurological disorders, with a prevalence of 1% and lifetime incidence of 3%. There are numerous epilepsy syndromes, most of which are considered to be genetic epilepsies. Despite the discovery of more than 20 genes for epilepsy to date, much of the genetic contribution to epilepsy is not yet known. Copy number variants have been established as an important source of mutation in other complex brain disorders, including intellectual disability, autism and schizophrenia. Recent advances in technology now facilitate genome-wide searches for copy number variants and are beginning to be applied to epilepsy. Here, we discuss what is currently known about the contribution of copy number variants to epilepsy, and how that knowledge is redefining classification of clinical and genetic syndromes.

Four novel SCN1A mutations in Turkish patients with severe myoclonic epilepsy of infancy (SMEI)

Severe myoclonic epilepsy of infancy (SMEI) (OMIM #607208), also known as Dravet syndrome, is a rare genetic disorder characterized by frequent generalized, unilateral clonic or tonic-clonic seizures that begin during the first year of life. Heterozygous de novo mutations in the SCN1A gene, which encodes the neuronal voltage-gated sodium channel α subunit type 1 (Nav1.1), are responsible for Dravet syndrome, with a broad spectrum of mutations and rearrangements having been reported. In this study, the authors present 4 novel mutations and confirm 2 previously identified mutations in the SCN1A gene found in a cohort of Turkish patients with Dravet syndrome. Mutational analysis of other responsible genes, GABRG2 and PCDH19, were unrevealing. The authors' findings add to the known spectrum of mutations responsible for this disease phenotype and once again reinforce our understanding of the allelic heterogeneity of this disease.