Brugada syndrome with SCN5A mutations exhibits more pronounced electrophysiological defects and more severe prognosis: A meta-analysis

Whether the presence of SCN5A mutation is a predictor of BrS risk remains controversial, and patient selection bias may have weakened previous findings. Therefore, we performed this study to clarify the clinical characteristics and outcomes of BrS probands with SCN5A mutations. We systematically retrieved eligible studies published through October 2018. A total of 17 studies enrolling 1780 BrS patients were included. Overall, our results found that compared with BrS patients without SCN5A mutations, patients with SCN5A mutations exhibited a younger age at the onset of symptoms and higher rate of the spontaneous type-1 electrocardiogram pattern, more pronounced conduction or repolarization abnormalities, and increased atrial vulnerability. In addition, the presence of SCN5A mutations was associated with an elevated risk of major arrhythmic events in both Asian (odds ratio [OR] = 1.82, 95% confidence interval [CI] 1.07-3.11; P = .03) and Caucasian (OR = 2.24, 95% CI 1.02-4.90; P = .04) populations. In conclusions, patients with SCN5A mutations exhibit more pronounced electrophysiological defects and more severe prognosis. Clinicians should be cautious when utilizing genetic testing for risk stratification or treatment guidance before determining whether the causal relationship regarding SCN5A mutation status is an independent predictor of risk.     

A study of the SCN5A gene in a cohort of 76 patients with Brugada syndrome

We aim to study the SCN5A gene in a cohort of Brugada syndrome (BS) patients and evaluate the genotype–phenotype correlation. BS is caused by mutations in up to 10 different genes, SCN5A being the most frequently involved. Large genomic rearrangements in SCN5A have been associated with conduction disease, but its prevalence in BS is unknown. Seventy‐six non‐related patients with BS were studied. Clinical characteristics and family risk profile were recorded. Direct sequencing and multiplex ligation‐dependent probe amplification (MLPA) of the SCN5A gene for identification of mutations and larger rearrangements were performed, respectively. Eight patients (10.5%) had point mutations (R27H, E901K, G1743R (detected in three families), V728I, N1443S and E1152X). Patients with mutations had a trend toward a higher proportion of spontaneous type I Brugada electrocardiogram (ECG) (87.5% vs 52.9%, p = 0.06) and had evidence of familial disease (62.5%, vs 23.5%, p = 0.03). The symptoms and risk profile of the carriers were not different from wild‐type probands. There were non‐significant differences in the prevalence of type I ECG, syncope and history of arrhythmia in carriers of selected polymorphisms. None of the patients had any deletion/duplication in the SCN5A gene. In conclusion, 10.5% of our patients had mutations in the SCN5A gene. Patients with mutations seemed to have more spontaneous type I ECG, but no differences in syncope or arrhythmic events compared with patients without mutations. Larger studies are needed to evaluate the role of polymorphisms in the SCN5A in the expression of the phenotype and prognosis. Large rearrangements were not identified in the SCN5A gene using the MLPA technique.     

Genetic analysis of the cardiac sodium channel gene SCN5A in Koreans with Brugada syndrome

The SCN5A gene encodes the alpha subunit of the human cardiac voltage-gated sodium channel. Mutations in SCN5A are responsible for Brugada syndrome, an inherited cardiac disease that leads to idiopathic ventricular fibrillation (IVF) and sudden death. In this study, we screened nine individuals from a single family and 12 sporadic patients who were clinically diagnosed with Brugada syndrome. Using PCR-SSCP, DHPLC, and DNA sequencing analysis, we identified a novel single missense mutation associated with Brugada syndrome in the family and detected a C5607T polymorphism in Korean subjects. A single nucleotide substitution of G to A at nucleotide position 3934 changed the coding sense of exon 21 of the SCN5A from glycine to serine (G1262S) in segment 2 of domain III (DIII-S2). Four individuals in the family carried the identical mutation in the SCN5A gene, but none of the 12 sporadic patients did. This mutation was not found in 150 unrelated normal individuals. This finding is the first report of a novel mutation in SCN5A associated with Brugada syndrome in Koreans.     

Sodium channel gene (SCN5A) mutations in 44 index patients with Brugada syndrome: different incidences in familial and sporadic disease

The Brugada syndrome (BS) is a distinct form of idiopathic ventricular fibrillation and may cause sudden cardiac death in healthy young individuals. In the surface ECG, BS can be recognized by an atypical right bundle branch block and ST-segment elevation in the right precordial leads. Mutations in the cardiac sodium channel gene SCN5A are only known to cause BS. In a multi-center effort, we have collected clinical data on 44 unrelated index patients and family members and performed a complete genetic analysis of SCN5A. In 37% the disease was familial, whereas in the majority it was sporadic (63%). Five novel SCN5A mutations (2602delC, resulting in: E867X; 2581_2582del TT: F861fs951X; 2673G>A: E1225K; 4435_4437delAAG: K1479del; and 5425C>A: S1812X) were found and were randomly located in SCN5A. Mutation frequencies (SCN5A+) differed significantly between familial (38%) and sporadic disease (0%) (p=0.001). Disease penetrance was complete in the SCN5A+ adult patients, but incomplete in SCN5A+ children (17%). Genetic testing of SCN5A is especially useful in familial disease to identify individuals at cardiac risk. In sporadic cases, however, a genetic basis and the value of mutation screening has to be further determined. These results are in line with a possibly genetic and clinical heterogeneity of BS.     

Somatic mosaic deletions involving SCN1A cause Dravet syndrome

Somatic mosaicism in single nucleotide variants of SCN1A is known to occur in a subset of parents of children with Dravet syndrome (DS). Here, we report recurrent somatic mosaic microdeletions involving SCN1A in children diagnosed with DS. Through the evaluation of 237 affected individuals with DS who did not show SCN1A or PCHD19 mutations in prior sequencing analyzes, we identified two children with mosaic microdeletions covering the entire SCN1A region. The allele frequency of the mosaic deletions estimated by multiplex ligation‐dependent probe amplification and array comparative genomic hybridization was 25–40%, which was comparable to the mosaic ratio in lymphocytes and buccal mucosa cells observed by fluorescence in situ hybridization analysis. The minimal prevalence of SCN1A mosaic deletion is estimated to be 0.9% (95% confidence level: 0.11–3.11%) of DS with negative for SCN1A and PCDH19 mutations. This study reinforces the importance of somatic mosaicism caused by copy number variations in disease‐causing genes, and provides an alternative spectrum of SCN1A mutations causative of DS. Somatic deletions in SCN1A should be considered in cases with DS when standard screenings for SCN1A mutations are apparently negative for mutations.

     

TRPM4 mutations to cause autosomal recessive and not autosomal dominant Brugada type 1 syndrome

Cardiac channelopathies, mainly Long QT and Brugada syndromes, are genetic disorders for which genotype/phenotypes relationships remains to be improved. To provide new insights into the Brugada syndrome pathophysiology, a mutational study was performed on a 64-year-old man presented with isolated exertional dyspnea (NYHA class: II-III), hypertension, chronic kidney disease, coronary disease, an electrocardiogram suggesting a Brugada type 1-like pattern with ST-segment elevation in leads V1-V2. Molecular diagnosis study was performed using molecular strategy based on the sequencing of a panel of 19 Brugada-associated genes. The proband was carrier of 2 TRPM4 null alleles [IVS9+1G > A and p. Trp525X] resulting in the absence of functional hTRPM4 proteins. Due to this unexpected genotype, meta-analysis of previously reported TRPM4 variations associated with cardiac pathologies was performed using ACMG guidelines. All were detected in a heterozygous status. This additional meta-analysis indicated that most of them could not be considered definitely as pathogen. In conclusion, our study reports, for the first time, identification of compound heterozygous TRPM4 null mutations in a proband with, at an arrhythmogenic level, only a Brugada type 1-like electrocardiogram. By combining the genotype/phenotype relationship of this case and analysis of previously reported TRPM4 variations, we suggest that loss-of-function TRPM4 variations, in a heterozygous status, could not be considered as pathogenic or likely pathogenic mutations in cardiac channelopathies such as Long QT syndrome or Brugada syndrome.     

Non-SCN5A Related Brugada Syndromes: Verification of Normal Splicing and Trafficking of SCN5A Without Exonic Mutations

Recently, it has been reported that under 20% of Brugada syndrome cases are linked to SCN5A mutations. The purpose of this study was to clarify whether abnormalities other than exonic mutations, such as splicing disorders, decreased mRNA expression levels, or membrane transport abnormalities of SCN5A, play a role in the pathogenesis of Brugada syndrome.
We analyzed all SCN5A exons and splice sites using genomic DNA from 23 Brugada syndrome patients. We also analyzed the mRNA obtained from RV cardiomyocytes using real time PCR and sequencing, to study the expression levels and splicing patterns of SCN5A . The localization of SCN5A was examined by immunofluorescence analysis.
A de novo heterozygous G to A transversion in a 5' splice junction of the intron between exons 21 and 22 was detected in 1 patient. In the mRNA analysis of Brugada syndrome patients without a mutation of SCN5A no splicing abnormalities were detected, and the SCN5A mRNA levels were similar to those of normal controls. Immunofluorescence analyses revealed that SCN5A is located on the surface membrane not only in the RV cardiomyocytes of normal controls but also in those with Brugada syndrome.
We can confirm that some Brugada syndrome patients without exonic mutations in SCN5A had no other SCN5A abnormalities, including any involving the location of the SCN5A protein. These results suggest the involvement of other proteins in the pathogenesis in Brugada syndrome.     

SCN1A mutations and epilepsy

SCN1A is part of the SCN1A-SCN2A-SCN3A gene cluster on chromosome 2q24 that encodes for alpha pore forming subunits of sodium channels. The 26 exons of SCN1A are spread over 100 kb of genomic DNA. Genetic defects in the coding sequence lead to generalized epilepsy with febrile seizures plus (GEFS+) and a range of childhood epileptic encephalopathies of varied severity (e.g., SMEI). All published mutations are collated. More than 100 novel mutations are spread throughout the gene with the more debilitating usually de novo. Some clustering of mutations is observed in the C-terminus and the loops between segments 5 and 6 of the first three domains of the protein. Functional studies so far show no consistent relationship between changes to channel properties and clinical phenotype. Of all the known epilepsy genes SCN1A is currently the most clinically relevant, with the largest number of epilepsy related mutations so far characterized.     

De novo SCN1A mutations are a major cause of severe myoclonic epilepsy of infancy

Severe myoclonic epilepsy of infancy (SMEI or Dravet syndrome) is a rare disorder occurring in young children often without a family history of a similar disorder. The earliest disease manifestations are usually fever-associated seizures. Later in life, patients display different types of afebrile seizures including myoclonic seizures. Arrest of psychomotor development occurs in the second year of life and most patients become ataxic. Patients are resistant to antiepileptic drug therapy. Recently, we described de novo mutations of the neuronal sodium channel alpha-subunit gene SCN1A in seven isolated SMEI patients. To investigate the contribution of SCN1A mutations to the etiology of SMEI, we examined nine additional SMEI patients. We observed eight coding and one noncoding mutation. In contrast to our previous study, most mutations are missense mutations clustering in the S4-S6 region of SCN1A. These findings demonstrate that de novo mutations in SCN1A are a major cause of isolated SMEI.     

Meier-Gorlin syndrome mutations disrupt an Orc1 CDK inhibitory domain and cause centrosome reduplication

Like DNA replication, centrosomes are licensed to duplicate once per cell division cycle to ensure genetic stability. In addition to regulating DNA replication, the Orc1 subunit of the human origin recognition complex controls centriole and centrosome copy number. Here we report that Orc1 harbors a PACT centrosome-targeting domain and a separate domain that differentially inhibits the protein kinase activities of Cyclin E-CDK2 and Cyclin A-CDK2. A cyclin-binding motif (Cy motif) is required for Orc1 to bind Cyclin A and inhibit Cyclin A-CDK2 kinase activity but has no effect on Cyclin E-CDK2 kinase activity. In contrast, Orc1 inhibition of Cyclin E-CDK2 kinase activity occurs by a different mechanism that is affected by Orc1 mutations identified in Meier-Gorlin syndrome patients. The cyclin/CDK2 kinase inhibitory domain of Orc1, when tethered to the PACT domain, localizes to centrosomes and blocks centrosome reduplication. Meier-Gorlin syndrome mutations that disrupt Cyclin E-CDK2 kinase inhibition also allow centrosome reduplication. Thus, Orc1 contains distinct domains that control centrosome copy number and DNA replication. We suggest that the Orc1 mutations present in some Meier-Gorlin syndrome patients contribute to the pronounced microcephaly and dwarfism observed in these individuals by altering centrosome duplication in addition to DNA replication defects.     

Sodium channel abnormalities are infrequent in patients with long QT syndrome: identification of two novel SCN5A mutations.

Long QT syndrome (LQTS) is a heterogeneous disorder caused by mutations of at least five different loci. Three of these, LQT1, LQT2, and LQT5, encode potassium channel subunits. LQT3 encodes the cardiac-specific sodium channel, SCN5A. Previously reported LQTS-associated mutations of SCN5A include a recurring three amino acid deletion (DeltaKPQ1505-1507) in four different families, and four different missense mutations. We have examined the SCN5A gene in 88 index cases with LQTS, including four with Jervell and Lange-Nielsen syndrome and the remainder with Romano-Ward syndrome. Screening portions of DIII-DIV, where mutations have previously been found, showed that none of these patients has the three amino acid deletion, DeltaKPQ1505-1507, or the other four known mutations. We identified a novel missense mutation, T1645M, in the DIV; S4 voltage sensor immediately adjacent to the previously reported mutation R1644H. We also examined all of the additional pore-forming regions and voltage-sensing regions and discovered another novel mutation, T1304M, at the voltage-sensing region DIII; S4. Neither T1645M nor T1304M were seen in a panel of unaffected control individuals. Five of six T1304M gene carriers were symptomatic. In contrast to previous studies, QT(onset-c) was not a sensitive indicator of SCN5A-associated LQTS, at least in this family. These data suggest that mutations of SCN5A are responsible for only a small proportion of LQTS cases.     

De novo variants in the alternative exon 5 of SCN8A cause epileptic encephalopathy

PurposeAs part of the Epilepsy Genetics Initiative, we re-evaluated clinically generated exome sequence data from 54 epilepsy patients and their unaffected parents to identify molecular diagnoses not provided in the initial diagnostic interpretation.MethodsWe compiled and analyzed exome sequence data from 54 genetically undiagnosed trios using a validated analysis pipeline. We evaluated the significance of the genetic findings by reanalyzing sequence data generated at Ambry Genetics, and from a number of additional case and control cohorts.ResultsIn 54 previously undiagnosed trios, we identified two de novo missense variants in SCN8A in the highly expressed alternative exon 5 A—an exon only recently added to the Consensus Coding Sequence database. One additional undiagnosed epilepsy patient harboring a de novo variant in exon 5 A was found in the Ambry Genetics cohort. Missense variants in SCN8A exon 5 A are extremely rare in the population, further supporting the pathogenicity of the de novo alterations identified.ConclusionThese results expand the range of SCN8A variants in epileptic encephalopathy patients and illustrate the necessity of ongoing reanalysis of negative exome sequences, as advances in the knowledge of disease genes and their annotations will permit new diagnoses to be made.     

SCN4A variants and Brugada syndrome: phenotypic and genotypic overlap between cardiac and skeletal muscle sodium channelopathies

SCN5A mutations involving the α-subunit of the cardiac voltage-gated muscle sodium channel (NaV1.5) result in different cardiac channelopathies with an autosomal-dominant inheritance such as Brugada syndrome. On the other hand, mutations in SCN4A encoding the α-subunit of the skeletal voltage-gated sodium channel (NaV1.4) cause non-dystrophic myotonia and/or periodic paralysis. In this study, we investigated whether cardiac arrhythmias or channelopathies such as Brugada syndrome can be part of the clinical phenotype associated with SCN4A variants and whether patients with Brugada syndrome present with non-dystrophic myotonia or periodic paralysis and related gene mutations. We therefore screened seven families with different SCN4A variants and non-dystrophic myotonia phenotypes for Brugada syndrome and performed a neurological, neurophysiological and genetic work-up in 107 Brugada families. In the families with an SCN4A-associated non-dystrophic myotonia, three patients had a clinical diagnosis of Brugada syndrome, whereas we found a remarkably high prevalence of myotonic features involving different genes in the families with Brugada syndrome. One Brugada family carried an SCN4A variant that is predicted to probably affect function, one family suffered from a not genetically confirmed non-dystrophic myotonia, one family was diagnosed with myotonic dystrophy (DMPK gene) and one family had a Thomsen disease myotonia congenita (CLCN1 variant that affects function). Our findings and data suggest a possible involvement of SCN4A variants in the pathophysiological mechanism underlying the development of a spontaneous or drug-induced type 1 electrocardiographic pattern and the occurrence of malignant arrhythmias in some patients with Brugada syndrome.     

Parental mosaicism can cause recurrent transmission of SCN1A mutations associated with severe myoclonic epilepsy of infancy

De novo mutations in the SCN1A gene, encoding the alpha1-subunit of the neuronal voltage-gated sodium channel Nav1.1, are the most frequent genetic cause of Severe Myoclonic Epilepsy of Infancy known so far. A few mutations inherited from an asymptomatic or mildly affected parent have been reported, suggesting that expression of the mutated gene may be variable in the transmitting parent. In this study, we report two unrelated families in which two children of unaffected parents had deleterious SCN1A mutations, and show evidence of somatic and germline mosaicism in the transmitting parents. In one of these families, direct sequencing of blood cell DNA was not sufficient to the SCN1A mutation in the transmitting asymptomatic parent who was mosaic for the mutation. We therefore developed a real-time PCR assay to selectively amplify and quantify the mutant allele present at low levels in the transmitting parent in both families. The allele-specific PCR technique used in this study will be of use in detecting other such cases. These findings will have major consequences for the genetic counseling of asymptomatic parents with only one affected child.     

Brugada综合征亚型相关致病基因的研究进展

Brugada综合征是常染色体显性遗传病,全球发病率约万分之五.目前发现9种致病基因与该病存在关联.在发现的约100个基因突变位点中,96个基因突变位于SCN5A基因,这些突变可以解释20%-25%的BrS病因.本综述依据目前研究进展分别将Brugada综合征9种亚型致病基因介绍如下.     

Novel COL4A5, COL4A4, and COL4A3 mutations in Alport syndrome

This study summarizes 47 novel mutations identified during routine molecular diagnostics for Alport syndrome. We detected 34 in COL4A5, the gene responsible for X-linked Alport syndrome, and 13 in COL4A3 and COL4A4, the genes responsible for autosomal recessive Alport syndrome. A high detection rate of 90% was achieved among patients with typical clinical symptoms and a characteristic family history in both X-linked and autosomal recessive forms, and it can be assumed that most relevant mutations have been identified. In numerous positively tested patients, genetic variations which are unknown were detected.     

Detection of 12 novel mutations in the collagenous domain of the COL4A5 gene in Alport syndrome patients

A population of 35 Alport syndrome patients, defined by strict diagnostic criteria, was screened for mutations in 23 exons of the COL4A5 gene by SSCP analysis. Mobility shifts were observed in 12 out of 35 patients and were shown to represent genuine mutations. 9 of these were glycine substitutions in the collagenous domain (in exons 20, 25, 26, 29, 31, and 41), 2 were small deletions resulting in frameshifts (in exons 21 and 31), and one was a splice site mutation (in exon 12). © 1995 Wiley-Liss, Inc.