[Cardiogenetics in Germany- a view and review]. / Kardiogenetik in Deutschland ­ ein (Rück­)Blick.

The development of the cardiogenetics field in Germany has been increasing since the mid-1990s with many national contributions, some of them were really important and groundbreaking. The starting point was and still is the patient and his family, e.g. with a familial form of arrhythmia or cardiomyopathy, the clarification of the genetic cause and the personalized treatment of those being affected. The scientific, always translationally oriented interest in identifying a causative gene and uncovering the underlying pathomechanisms has led to notable contributions for Brugada syndrome, short QT syndrome and cardiac conduction disorders or sinus node dysfunction, but also in DCM or ARVC. What is important, however, is always the way back (bench > bed side): implementation of national and international recommendations for cardiogenetic diagnostics in daily cardiological routine and the personalized care and therapy of those being affected.

Errata: Rare Compound Heterozygous Missense Mutation of the SCN5A Gene with Childhood-Onset Sick Sinus Syndrome in Two Chinese Sisters: A Case Report.

An error appeared in the article entitled "Rare Compound Heterozygous Missense Mutation of the SCN5A Gene with Childhood-Onset Sick Sinus Syndrome in Two Chinese Sisters: A Case Report" by Yanyun Wang, Siyu Long, Chenxi Wei, and Xiaoqin Wang (Vol. 64 No.2, 299-305, 2023). The name of the first affiliation on page 299 was wrong. It should be "Laboratory of Molecular Translational Medicine, Center for Translational Medicine, West China Second University Hospital, Sichuan University, Chengdu, China" and not "Laboratory of Molecular Translational Medicine, Center for Translational Medicine, Sichuan University, Chengdu, China".

SCN5A-L256del and L1621F exhibit loss-of-function properties related to autosomal recessive congenital cardiac disorders presenting as sick sinus syndrome, dilated cardiomyopathy, and sudden cardiac death.

Pathogenic mutations in SCN5A could result in dysfunctions of Nav1.5 and consequently lead to a wide range of inherited cardiac diseases. However, the presence of numerous SCN5A-related variants with unknown significance (VUS) and the comprehensive genotype-phenotype relationship pose challenges to precise diagnosis and genetic counseling for affected families. Here, we functionally identified two novel compound heterozygous variants (L256del and L1621F) in SCN5A in a Chinese family exhibiting complex congenital cardiac phenotypes from sudden cardiac death to overlapping syndromes including sick sinus syndrome and dilated cardiomyopathy in an autosomal recessive pattern. In silico tools predicted decreased stability and hydrophobicity of the two mutated proteins due to conformational changes. Patch-clamp electrophysiology revealed slightly decreased sodium currents, accelerated inactivation, and reduced sodium window current in the Nav1.5-L1621F channels as well as no sodium currents in the Nav1.5-L256del channels. Western blotting analysis demonstrated decreased expression levels of mutated Nav1.5 on the plasma membrane, despite enhanced compensatory expression of the total Nav1.5 expression levels. Immunofluorescence imaging showed abnormal condensed spots of the mutated channels within the cytoplasm instead of normal membrane distribution, indicating impaired trafficking. Overall, we identified the loss-of-function characteristics exhibited by the two variants, thereby providing further evidence for their pathogenic nature. Our findings not only extended the variation and phenotype spectrums of SCN5A, but also shed light on the crucial role of patch-clamp electrophysiology in the functional analysis of VUS in SCN5A, which have significant implications for the clinical diagnosis, management, and genetic counseling in affected individuals with complex cardiac phenotypes.

The W101C KCNJ5 Mutation Induces Slower Pacing by Constitutively Active GIRK Channels in hiPSC-Derived Cardiomyocytes.

Mutations in the KCNJ5 gene, encoding one of the major subunits of cardiac G-protein-gated inwardly rectifying K+ (GIRK) channels, have been recently linked to inherited forms of sinus node dysfunction. Here, the pathogenic mechanism of the W101C KCNJ5 mutation underlying sinus bradycardia in a patient-derived cellular disease model of sinus node dysfunction (SND) was investigated. A human-induced pluripotent stem cell (hiPSCs) line of a mutation carrier was generated, and CRISPR/Cas9-based gene targeting was used to correct the familial mutation as a control line. Both cell lines were further differentiated into cardiomyocytes (hiPSC-CMs) that robustly expressed GIRK channels which underly the acetylcholine-regulated K+ current (IK,ACh). hiPSC-CMs with the W101C KCNJ5 mutation (hiPSCW101C-CM) had a constitutively active IK,ACh under baseline conditions; the application of carbachol was able to increase IK,ACh, further indicating that not all available cardiac GIRK channels were open at baseline. Additionally, hiPSCW101C-CM had a more negative maximal diastolic potential (MDP) and a slower pacing frequency confirming the bradycardic phenotype. Of note, the blockade of the constitutively active GIRK channel with XAF-1407 rescued the phenotype. These results provide further mechanistic insights and may pave the way for the treatment of SND patients with GIRK channel dysfunction.

Rare Compound Heterozygous Missense Mutation of the SCN5A Gene with Childhood-Onset Sick Sinus Syndrome in Two Chinese Sisters.

Sick sinus syndrome (SSS) is a group of syndromes characterized by pathological changes in the sinoatrial node and its adjacent tissues. Although several mutations in the SCN5A gene have been associated with early-onset SSS, pediatric patients are still less common. Here, we report a rare compound missense mutation in the SCN5A gene [c.2893C>T (p. R965C) and c.2431C>T (p. R811C) ] in two sisters with childhood-onset SSS in Chinese population. The proband (5 years and 5 months old) was the second child of a clinically normal and nonconsanguineous couple. Her elder sister was 12 years old and had been implanted with a pacemaker because of the diagnosis of SSS at another hospital one year ago. The proband was presented to the hospital with a slowed heart rate and reduced endurance exercise capacity for more than three months. After a comprehensive clinical examination, she was diagnosed with SSS and underwent pacemaker implantation. Exome and Sanger sequencing were used to determine the compound heterozygous missense mutation of [c.2893C>T (p. R965C) and c.2431C>T (p. R811C) ] in the SCN5A in the patient and her elder sister. Each healthy parent carried a different heterozygous missense mutation. The compound heterozygous mutation of c.2893C>T (p. R965C) and c.2431C>T (p. R811C) rather than the single mutation might be the primary cause of familial early-onset SSS in Chinese population. Our current findings expanded the current understanding of the SCN5A gene mutations. We further confirmed the essential role of the SCN5A gene on the diagnosis, family cascade screening, early intervention, and prognostic evaluation of SSS.

Sinus node dysfunction and atrial fibrillation-Relationships, clinical phenotypes, new mechanisms, and treatment approaches.

Although the anatomical basis of the pathogenesis of sinus node dysfunction (SND) and atrial fibrillation (AF) is located primarily in the left and right atria, increasing evidence suggests a strong correlation between SND and AF, in terms of both clinical presentation and formation mechanisms. However, the exact mechanisms underlying this association are unclear. The relationship between SND and AF may not be causal, but is likely to involve common factors and mechanisms, including ion channel remodeling, gap junction abnormalities, structural remodeling, genetic mutations, neuromodulation abnormalities, the effects of adenosine on cardiomyocytes, oxidative stress, and viral infections. Ion channel remodeling manifests primarily as alterations in the "funny" current (If) and Ca2+ clock associated with cardiomyocyte autoregulation, and gap junction abnormalities are manifested primarily as decreased expression of connexins (Cxs) mediating electrical impulse propagation in cardiomyocytes. Structural remodeling refers primarily to fibrosis and cardiac amyloidosis (CA). Some genetic mutations can also cause arrhythmias, such as SCN5A, HCN4, EMD, and PITX2. The intrinsic cardiac autonomic nervous system (ICANS), a regulator of the heart's physiological functions, triggers arrhythmias.In addition, we discuss arrhythmias caused by viral infections, notably Coronavirus Disease 2019 (COVID-19). Similarly to upstream treatments for atrial cardiomyopathy such as alleviating CA, ganglionated plexus (GP) ablation acts on the common mechanisms between SND and AF, thus achieving a dual therapeutic effect.

Sinus node dysfunction: current understanding and future directions.

The sinoatrial node (SAN) is the primary pacemaker of the heart. Normal SAN function is crucial in maintaining proper cardiac rhythm and contraction. Sinus node dysfunction (SND) is due to abnormalities within the SAN, which can affect the heartbeat frequency, regularity, and the propagation of electrical pulses through the cardiac conduction system. As a result, SND often increases the risk of cardiac arrhythmias. SND is most commonly seen as a disease of the elderly given the role of degenerative fibrosis as well as other age-dependent changes in its pathogenesis. Despite the prevalence of SND, current treatment is limited to pacemaker implantation, which is associated with substantial medical costs and complications. Emerging evidence has identified various genetic abnormalities that can cause SND, shedding light on the molecular underpinnings of SND. Identification of these molecular mechanisms and pathways implicated in the pathogenesis of SND is hoped to identify novel therapeutic targets for the development of more effective therapies for this disease. In this review article, we examine the anatomy of the SAN and the pathophysiology and epidemiology of SND. We then discuss in detail the most common genetic mutations correlated with SND and provide our perspectives on future research and therapeutic opportunities in this field.

Whole Exome Sequencing Identifies a Heterozygous Variant in the Cav1.3 Gene CACNA1D Associated with Familial Sinus Node Dysfunction and Focal Idiopathic Epilepsy.

Cav1.3 voltage-gated L-type calcium channels (LTCCs) are involved in cardiac pacemaking, hearing and hormone secretion, but are also expressed postsynaptically in neurons. So far, homozygous loss of function mutations in CACNA1D encoding the Cav1.3 α1-subunit are described in congenital sinus node dysfunction and deafness. In addition, germline mutations in CACNA1D have been linked to neurodevelopmental syndromes including epileptic seizures, autism, intellectual disability and primary hyperaldosteronism. Here, a three-generation family with a syndromal phenotype of sinus node dysfunction, idiopathic epilepsy and attention deficit hyperactivity disorder (ADHD) is investigated. Whole genome sequencing and functional heterologous expression studies were used to identify the disease-causing mechanisms in this novel syndromal disorder. We identified a heterozygous non-synonymous variant (p.Arg930His) in the CACNA1D gene that cosegregated with the combined clinical phenotype in an autosomal dominant manner. Functional heterologous expression studies showed that the CACNA1D variant induces isoform-specific alterations of Cav1.3 channel gating: a gain of ion channel function was observed in the brain-specific short CACNA1D isoform (Cav1.3S), whereas a loss of ion channel function was seen in the long (Cav1.3L) isoform. The combined gain-of-function (GOF) and loss-of-function (LOF) induced by the R930H variant are likely to be associated with the rare combined clinical and syndromal phenotypes in the family. The GOF in the Cav1.3S variant with high neuronal expression is likely to result in epilepsy, whereas the LOF in the long Cav1.3L variant results in sinus node dysfunction.

A phenotype-based forward genetic screen identifies Dnajb6 as a sick sinus syndrome gene.

Previously we showed the generation of a protein trap library made with the gene-break transposon (GBT) in zebrafish (Danio rerio) that could be used to facilitate novel functional genome annotation towards understanding molecular underpinnings of human diseases (Ichino et al, 2020). Here, we report a significant application of this library for discovering essential genes for heart rhythm disorders such as sick sinus syndrome (SSS). SSS is a group of heart rhythm disorders caused by malfunction of the sinus node, the heart's primary pacemaker. Partially owing to its aging-associated phenotypic manifestation and low expressivity, molecular mechanisms of SSS remain difficult to decipher. From 609 GBT lines screened, we generated a collection of 35 zebrafish insertional cardiac (ZIC) mutants in which each mutant traps a gene with cardiac expression. We further employed electrocardiographic measurements to screen these 35 ZIC lines and identified three GBT mutants with SSS-like phenotypes. More detailed functional studies on one of the arrhythmogenic mutants, GBT411, in both zebrafish and mouse models unveiled Dnajb6 as a novel SSS causative gene with a unique expression pattern within the subpopulation of sinus node pacemaker cells that partially overlaps with the expression of hyperpolarization activated cyclic nucleotide gated channel 4 (HCN4), supporting heterogeneity of the cardiac pacemaker cells.

Pathological and Comprehensive Genetic Investigation of Autopsy Cases of Idiopathic Bradyarrhythmia.

BACKGROUND: Idiopathic bradyarrhythmia is considered to be due to pathological degeneration of the cardiac conduction system (CCS) during aging. There appears to have been no comprehensive genetic investigations in patients with idiopathic bradyarrhythmia.Methods and Results: Ten autopsy cases with advanced bradyarrhythmia (6 men and 4 women; age: 70-94 years, 81.5±6.9 years; 5 cases each of sinus node dysfunction [SND] and complete atrioventricular block [CAVB]) were genetically investigated by using whole-exome sequencing. Morphometric analysis of the CCS was performed with sex-, age- and comorbidity-matched control cases. As a result, severe loss of nodal cells and distal atrioventricular conduction system were found in SND and CAVB, respectively. However, the conduction tissue loss was not significant in either the atrioventricular node or the proximal bundle of His in CAVB cases. A total of 13 heterozygous potential variants were found in 3 CAVB and 2 SND cases. Of these 13 variants, 4 were missense in the known progressive cardiac conduction disease-related genes: GATA4 and RYR2. In the remaining 9 variants, 5 were loss-of-function mutation with highly possible pathogenicity. CONCLUSIONS: In addition to degenerative changes of selectively vulnerable areas in the heart during advancing age, the vulnerability of the CCS, which may be associated with "rare variants of small effect," may also be a contributing factor to the degeneration of CCS, leading to "idiopathic" bradyarrhythmia.

An Adolescent Patient with Sick Sinus Syndrome Complicated by Hypothyroidism Carrying an SCN5A Variant.

Previous studies have reported that hypothyroidism can lead to sick sinus syndrome (SSS) or other rhythm disturbances. Variants in the alpha subunit of the cardiac sodium channel (SCN5A) are known to be among the genetic causes of SSS. We encountered an adolescent patient with SSS and hypothyroidism who also harbored an SCN5A variant. The patient was a 13-year-old girl who was referred to our hospital because of bradycardia identified during a school electrocardiography screening. Clinical examination revealed severe hypothyroidism due to Hashimoto thyroiditis and SSS. After levothyroxine supplementation, her symptoms of hypothyroidism improved; however, the SSS did not. Genetic testing revealed a heterozygous variant (c.1066 G>A, p.Asp356Asn) in SCN5A. This is the first report of the coexistence of SSS due to an SCN5A variant and severe hypothyroidism in an adolescent patient. While patients with SCN5A variants exhibit phenotypic heterogeneity due to the presence of various modifiers, the presence of severe hypothyroidism may affect the development of SSS. This case highlights the importance of genetic analysis, including testing for SCN5A variants, in patients with hypothyroidism complicated by SSS or cardiac conduction disorders.

Whole genome sequencing identifies a deletion mutation in the unknown-functional KCNG2 from familial sick sinus syndrome.

Sick sinus syndrome (SSS) is a term used for a variety of disorders defined by abnormal cardiac impulse formation and by abnormal propagation from the heart's sinoatrial node. In this study, we present a case from a Chinese family in which two closely related individuals had the symptoms and electrocardiographic evidence of SSS. We hypothesized that multiple individuals affected by the disease in the family was an indication of its genetic predisposition, and thus performed high-throughput sequencing for the participants from the family to detect potential disease-associated variants. One of the potential variants that was identified was a KCNG2 gene variant (NC_000018.9: g.77624068_77624079del). Further bioinformatic analysis showed that the observed variant may be a pathogenic mutation. The results of protein-protein docking and whole cell patch-clamp measurements implied that the deletion variant in KCNG2 could affect its binding the KV2.1 protein, and finally affect the function of Kv channel, which is an important determinant in regulation of heartbeat. Therefore, we inferred that the variable KCNG2 gene may affect the function of Kv channel by changing the binding conformation of KCNG2 and KV2.1 proteins and then adversely affect propagation from the sinoatrial node and cardiac impulse formation by changing the action potential repolarization of heart cells. In summary, our findings suggested that the dominant KCNG2 deletion variant in the examined Chinese family with SSS may be a potential disease-associated variant.

A Novel SCN5A Variant Causes Temperature-Sensitive Loss Of Function in a Family with Symptomatic Brugada Syndrome, Cardiac Conduction Disease, and Sick Sinus Syndrome.

INTRODUCTION: Brugada syndrome (BrS) is an inherited arrhythmia syndrome associated with an increased risk of sudden cardiac death. SCN5A is the most important disease-modifying gene for BrS, but many SCN5A variants have not been functionally characterized. Furthermore, the temperature dependency of SCN5A is only rarely explored in in vitro analyses. METHODS: The clinical phenotype of the affected family was assessed by medical history, ECGs and ajmaline challenge. Whole-cell patch clamp recordings were performed on HEK 293T cells expressing Nav1.5-G1712S, a novel SCN5A variant found in the symptomatic family. RESULTS: Three male family members had experienced sudden cardiac death, sudden cardiac arrest, and rhythmogenic syncopes. Beside a positive ajmaline challenge with demarcation of a Brugada type 1 ECG, 1 patient also showed evidence of symptomatic cardiac conduction disease and sick sinus syndrome (SSS). In patch clamp analyses, Nav1.5-G1712S generated reduced peak currents as compared to the wild type. At body temperature, Nav1.5-G1712S additionally exhibited an enhanced slow inactivation and an impaired recovery from inactivation. CONCLUSION: We conclude that G1712S is a pathogenic SCN5A loss-of function mutation at physiological temperature associated with an overlapping presentation of BrS, SSS, and cardiac conduction disease.

Genetic predictors of sick sinus syndrome.

Sick sinus syndrome (SSS) encompasses a group of conduction disorders characterized by the inability of sinoatrial node to perform its pacemaker function. Our aim was to identify genetic predictors of SSS in a prospective cohort of patients admitted to the clinic for pacemaker implantation using single-locus and multilocus approaches. We performed genotyping for polymorphic markers of CLCNKA (rs10927887), SCN10A (rs6795970), FNDC3B (rs9647379), MIR146A (rs2910164), SYT10 (rs7980799), MYH6 (rs365990), and KCNE1 (rs1805127) genes in the group of 284 patients with SSS and 243 healthy individuals. Associations between the studied loci and SSS were tested using logistic regression under recessive genetic model using sex and age as covariates. Multilocus analysis was performed using Markov chain Monte Carlo method implemented in the APSampler program. Correction for multiple testing was performed using Benjamini-Hochberg procedure. We detected an individual association between KCNE1 rs1805127*A allele and SSS in the total study group (OR 0.43, PFDR = 0.028) and in the subgroup of patients with 2nd or 3rd degree sinoatrial block (OR 0.17, PFDR = 0.033), and identified seven allelic patterns associated with the disease. SCN10A rs6795970*T and MIR146A rs2910164*C alleles were present in all seven combinations associated with SSS. The highest risk of SSS was conferred by the combination SCN10A rs6795970*T+FNDC3B rs9647379*C+MIR146A rs2910164*C+SYT10 rs7980799*C+KCNE1 rs1805127*G (OR 2.98, CI 1.77-5.00, P = 1.27 × 10-5, PFDR = 0.022). Our findings suggest that KCNE1 rs1805127 polymorphism may play a role in susceptibility to sinoatrial node dysfunction, particularly presenting as 2nd or 3rd degree sinoatrial block, and the risk-modifying effect of other studied loci is better detected using multilocus approach.

Genetic insight into sick sinus syndrome.

AIMS: The aim of this study was to use human genetics to investigate the pathogenesis of sick sinus syndrome (SSS) and the role of risk factors in its development. METHODS AND RESULTS: We performed a genome-wide association study of 6469 SSS cases and 1 000 187 controls from deCODE genetics, the Copenhagen Hospital Biobank, UK Biobank, and the HUNT study. Variants at six loci associated with SSS, a reported missense variant in MYH6, known atrial fibrillation (AF)/electrocardiogram variants at PITX2, ZFHX3, TTN/CCDC141, and SCN10A and a low-frequency (MAF = 1.1-1.8%) missense variant, p.Gly62Cys in KRT8 encoding the intermediate filament protein keratin 8. A full genotypic model best described the p.Gly62Cys association (P = 1.6 × 10-20), with an odds ratio (OR) of 1.44 for heterozygotes and a disproportionally large OR of 13.99 for homozygotes. All the SSS variants increased the risk of pacemaker implantation. Their association with AF varied and p.Gly62Cys was the only variant not associating with any other arrhythmia or cardiovascular disease. We tested 17 exposure phenotypes in polygenic score (PGS) and Mendelian randomization analyses. Only two associated with the risk of SSS in Mendelian randomization, AF, and lower heart rate, suggesting causality. Powerful PGS analyses provided convincing evidence against causal associations for body mass index, cholesterol, triglycerides, and type 2 diabetes (P > 0.05). CONCLUSION: We report the associations of variants at six loci with SSS, including a missense variant in KRT8 that confers high risk in homozygotes and points to a mechanism specific to SSS development. Mendelian randomization supports a causal role for AF in the development of SSS.

A novel homozygous SCN5A variant detected in sick sinus syndrome.

Sick sinus syndrome (SSS) is a group of disorders characterized by an abnormal cardiac impulse formation or propagation from the sinoatrial node. Mutated SCN5A has been reported in SSS, however, homozygosity of SCN5A is exceedingly rare. Here, we report a consanguineous family with four affected children with SSS. Symptomatic bradycardia necessitated implanting a pacemaker in all of them. Sequencing SCN5A revealed a novel homozygous variant (p.Cys1850Arg), which was predicted to interfere with protein folding. Our report describes the phenotype of a novel homozygous SCN5A variant and contributes to the compendium of molecular pathology of inherited arrhythmias in consanguineous populations.

Clinical Spectrum of SCN5A Channelopathy in Children with Primary Electrical Disease and Structurally Normal Hearts.

Sodium voltage-gated channel α subunit 5 (SCN5A)-mutations may cause an array of arrhythmogenic syndromes most frequently as an autosomal dominant trait, with incomplete penetrance, variable expressivity and male predominance. In the present study, we retrospectively describe a group of Mexican patients with SCN5A-disease causing variants in whom the onset of symptoms occurred in the pediatric age range. The study included 17 patients with clinical diagnosis of primary electrical disease, at least one SCN5A pathogenic or likely pathogenic mutation and age of onset <18 years, and all available first- and second-degree relatives. Fifteen patients (88.2%) were male, and sixteen independent variants were found (twelve missense, three truncating and one complex inframe deletion/insertion). The frequency of compound heterozygosity was remarkably high (3/17, 17.6%), with early childhood onset and severe disease. Overall, 70.6% of pediatric patients presented with overlap syndrome, 11.8% with isolated sick sinus syndrome, 11.8% with isolated Brugada syndrome (BrS) and 5.9% with isolated type 3 long QT syndrome (LQTS). A total of 24/45 SCN5A mutation carriers were affected (overall penetrance 53.3%), and penetrance was higher in males (63.3%, 19 affected/30 mutation carriers) than in females (33.3%, 5 affected/15 carriers). In conclusion, pediatric patients with SCNA-disease causing variants presented mainly as overlap syndrome, with predominant loss-of-function phenotypes of sick sinus syndrome (SSS), progressive cardiac conduction disease (PCCD) and ventricular arrhythmias.