Identification of a Kir3.4 mutation in congenital long QT syndrome

Cardiac channelopathies: genetic and molecular mechanisms

Channelopathies are diseases caused by dysfunctional ion channels, due to either genetic or acquired pathological factors. Inherited cardiac arrhythmic syndromes are among the most studied human disorders involving ion channels. Since seminal observations made in 1995, thousands of mutations have been found in many of the different genes that code for cardiac ion channel subunits and proteins that regulate the cardiac ion channels. The main phenotypes observed in patients carrying these mutations are congenital long QT syndrome (LQTS), Brugada syndrome (BrS), catecholaminergic polymorphic ventricular tachycardia (CPVT), short QT syndrome (SQTS) and variable types of conduction defects (CD). The goal of this review is to present an update of the main genetic and molecular mechanisms, as well as the associated phenotypes of cardiac channelopathies as of 2012.

Modeling long-QT syndromes with iPS cells

The generation of induced pluripotent stem cells (iPSC) from human somatic cells bears the possibility to generate patient-specific stem cell lines which can serve as a theoretically unlimited source of somatic cells carrying the genotype of the patients. Different types of the long-QT syndrome have been studied by analyzing the phenotype of cardiomyocytes generated from patient-specific iPSC lines. Major aspects of the pathophysiology of long-QT syndrome, like prolonged action potentials, arrhythmia, and the effects of pro- and antiarrhythmic drugs could be recapitulated in these cells. In the future, patient-specific iPSC-derived cardiomyocytes might be used to screen for new drugs, to avoid unwanted drug side effects, and to deepen our understanding on the pathophysiology of long-QT syndromes.

Genetics of cardiac electrical disease

Few tragedies compare to the sudden death of a family member. Sadly, this may represent the first sign of a familial vulnerability to such events. One common cause is an inherited cardiac arrhythmia syndrome. Sufferers are prone to premature sudden cardiac death due to altered ion channel function in the heart. Typical causes include Brugada syndrome, long QT syndrome, short QT syndrome, catecholaminergic polymorphic ventricular tachycardia, and the newly recognized early repolarization syndrome. Our knowledge of the genetic underpinnings of each of these disorders has increased markedly in recent years. Genetic screening is now a routine part of clinical care and promises more accurate diagnosis and efficient family screening. This review summarizes the diagnosis and management of each of the listed syndromes in the context of currently available genetic testing.

Genotype-phenotype analysis of three Chinese families with Jervell and Lange-Nielsen syndrome

BACKGROUND

Long QT syndrome (LQTS) is characterized by QT prolongation, syncope and sudden death. This study aims to explore the causes, clinical manifestations and therapeutic outcomes of Jervell and Lange-Nielsen syndrome (JLNS), a rare form of LQTS with congenital sensorineural deafness, in Chinese individuals.

MATERIALS AND METHODS

Three JLNS kindreds from the Chinese National LQTS Registry were investigated. Mutational screening of KCNQ1 and KCNE1 genes was performed by polymerase chain reaction and direct DNA sequence analysis. LQTS phenotype and therapeutic outcomes were evaluated for all probands and family members.

RESULTS

We identified 7 KCNQ1 mutations. c.1032_1117dup (p.Ser373TrpfsX10) and c.1319delT (p.Val440AlafsX26) were novel, causing JLNS in a 16-year-old boy with a QTc (QT interval corrected for heart rate) of 620 ms and recurrent syncope. c.605-2A>G and c.815G>A (p.Gly272Asp) caused JLNS in a 12-year-old girl and her 5-year-old brother, showing QTc of 590 to 600 ms and recurrent syncope. The fourth JLNS case, a 46-year-old man carrying c.1032G>A (p.Ala344Alasp) and c.569G>A (p.Arg190Gln) and with QTc of 460 ms, has been syncope-free since age 30. His 16-year-old daughter carries novel missense mutation c.574C>T (p.Arg192Cys) and c.1032G>A(p.Ala344Alasp) and displayed a severe phenotype of Romano-Ward syndrome (RWS) characterized by a QTc of 530 ms and recurrent syncope with normal hearing. Both the father and daughter also carried c.253G>A (p.Asp85Asn; rs1805128), a rare single nucleotide polymorphism (SNP) on KCNE1. Bizarre T waves were seen in 3/4 JLNS patients. Symptoms were improved and T wave abnormalities became less abnormal after appropriate treatment.

CONCLUSION

This study broadens the mutation and phenotype spectrums of JLNS. Compound heterozygous KCNQ1 mutations can result in both JLNS and severe forms of RWS in Chinese individuals.

RNA interference targeting E637K mutation rescues hERG channel currents and restores its kinetic properties

BACKGROUND

Long QT syndrome (LQTS) is a monogenic proarrhythmic disorder that predisposes affected individuals to sudden death from tachyarrhythmia. As an inherited disease, LQTS cannot be completely cured by conventional treatment modalities. Individualized gene therapy is a promising therapeutic approach.

OBJECTIVE

The purpose of this study was to investigate the role of small interference RNA (siRNA) on expression of E637K-hERG (human ether-a-go-go-related gene) mutant and whether it can be used to rescue the mutant's dominant-negative suppressive effects on hERG protein channel function.

METHODS

Western blot was performed to select the most sensitive siRNAs to target E637K-hERG mutant knockdown. Confocal laser scanning microscope was performed to monitor cellular localization of wild-type (WT)-hERG and E637K-hERG with or without siRNA. Patch-clamp technique was used to assess the effect of siRNA on the electrophysiologic characteristics of the rapidly activating delayed rectifier K(+) current I(Kr) of the hERG protein channel.

RESULTS

siRNA led to a significant decrease in the level of E637K-hERG protein but did not affect the level of WT-hERG protein. WT-hERG localization in cells coexpressing E637K-hERG mutant was restored to the membrane by siRNA. The siRNA-mediated inhibition of E637K-hERG mutant restored the maximum current and tail current amplitudes. Furthermore, siRNA treatment rescued the kinetic properties of WT/E637K-hERG protein channel to a level comparable to that of WT-hERG protein channel.

CONCLUSION

Our findings illustrated that siRNA can effectively inhibit E637K-hERG protein expression and rescue the dominant-negative effect of this mutation by restoring the kinetic properties of hERG protein channel. It has potential clinical implications with regard to the possibility of using siRNA in the treatment of LQTS.

Inherited long QT syndrome: clinical manifestation, genetic diagnostics, and therapy

Inherited long QT syndrome (LQTS) is characterized by a prolonged ventricular repolarization (QTc interval) and symptoms (syncope, sudden cardiac arrest) due to polymorphic ventricular arrhythmias. As of today, 13 different cardiac ion channel genes have been associated with congenital LQTS. The most common ones are due to KCNQ1 (LQT-1), KCNH2 (LQT-2), and SCN5A (LQT-3) gene mutations and account for up to 75 % of cases. Typical clinical findings are an increased QT interval on the surface electrocardiogram, specifically altered T wave morphologies, polymorphic ventricular arrhythmias, or an indicative family history. Recently, in the HRS/EHRA expert consensus statement, comprehensive genetic testing of major LQTS genes was recommended for index patients for whom there is a strong clinical suspicion of LQTS. Overall, antiadrenergic therapy, in particular β-receptor blockers, has been the mainstay of therapy and has significantly reduced cardiac events. For high-risk patients, an implantable cardioverter defibrillator (ICD) is recommended. Importantly, lifestyle modification and avoidance of arrhythmia triggers are additional important approaches.

Genetics of ion-channel disorders

PURPOSE OF REVIEW

In this article, we summarize the main features of the most common inherited channelopathies, focusing on the findings that advanced the field in the last few years.

RECENT FINDINGS

The progress in genetics prompted the discovery of several new genes associated with ion-channel disorders, elucidating new molecular pathways and new arrhythmogenic mechanisms. The diffusion and availability of genetic screening gave a new relevance to the application of genetics not only for diagnosis, but also for risk assessment and therapeutic decisions. As a consequence, the present challenge in the field is represented by the need to use genetic data to develop personalized clinical approaches.

SUMMARY

Over a few years, the field of inherited arrhythmogenic diseases has rapidly expanded, thus reshaping clinical management for these conditions. It is now clear that to handle these patients a specialized expertise is needed, able to translate the discoveries derived from basic science studies into the clinical care of the patients.

Recapitulating long-QT syndrome using induced pluripotent stem cell technology

The generation of patient-specific stem cells by reprogramming somatic cells to induced pluripotent stem cells (iPSC) provides the basis for a promising new type of in vitro disease models. Patient-specific iPSC derived from individuals with hereditary disorders can be differentiated into somatic cells in vitro, thus allowing the pathophysiology of the diseases to be studied on a cellular level. Different types of long-QT syndrome have been successfully modeled using this approach, demonstrating that the iPSC-derived patient-specific cardiomyocytes recapitulated key features of the disease in vitro. This approach will likely serve to model other monogenetic or polygenetic cardiovascular disorders in the future. Moreover, test platforms based on patient-specific iPSC could be used to test the potential of drug candidates to induce QT-interval prolongation or other unwanted side effects, screen for novel cardiovascular drugs, or to tailor medical therapy to the specific needs of a single patient.

Paralogous annotation of disease-causing variants in long QT syndrome genes

Discriminating between rare benign and pathogenic variation is a key challenge in clinical genetics, particularly as increasing numbers of nonsynonymous single-nucleotide polymorphisms (SNPs) are identified in resequencing studies. Here, we describe an approach for the functional annotation of nonsynonymous variants that identifies functionally important, disease-causing residues across protein families using multiple sequence alignment. We applied the methodology to long QT syndrome (LQT) genes, which cause sudden death, and their paralogues, which largely cause neurological disease. This approach accurately classified known LQT disease-causing variants (positive predictive value = 98.4%) with a better performance than established bioinformatic methods. The analysis also identified 1078 new putative disease loci, which we incorporated along with known variants into a comprehensive and freely accessible long QT resource (http://cardiodb.org/Paralogue_Annotation/), based on newly created Locus Reference Genomic sequences (http://www.lrg-sequence.org/). We propose that paralogous annotation is widely applicable for Mendelian human disease genes.

KCNJ5 mutations in the National Institutes of Health cohort of patients with primary hyperaldosteronism: an infrequent genetic cause of Conn's syndrome

KCNJ5 mutations were recently described in primary hyperaldosteronism (PH or Conn's syndrome). The frequency of these mutations in PH and the way KCNJ5 defects cause disease remain unknown. A total of 53 patients with PH have been seen at the National Institutes of Health over the last 12 years. Their peripheral and tumor DNAs (the latter from 16 that were operated) were screened for KCNJ5 mutations; functional studies on the identified defects were performed after transient transfection. Only two mutations were identified, and both in the tumor DNA only. There were no germline sequencing defects in any of the patients except for known synonymous variants of the KCNJ5 gene. One mutation was the previously described c.G451C alteration; the other was a novel one in the same codon: c.G451A; both lead to the same amino acid substitution (G151R) in the KCNJ5 protein. Functional studies confirmed previous findings that both mutations caused loss of channel selectivity and a positive shift in the reversal potential. In conclusion, the KCNJ5 protein was strongly expressed in the zona glomerulosa of normal adrenal glands but showed variable expression in the aldosterone-producing adenomas with and without mutation. The rate of KCNJ5 mutations among patients with PH and/or their tumors is substantially lower than what was previously reported. The G151R amino acid substitution appears to be the most frequent one so far detected in PH, despite additional nucleotide changes. The mutation causes loss of this potassium channel's selectivity and may assist in the design of new therapies for PH.

High prevalence of genetic variants previously associated with LQT syndrome in new exome data

To date, hundreds of variants in 13 genes have been associated with long QT syndrome (LQTS). The prevalence of LQTS is estimated to be between 1:2000 and 1:5000. The knowledge of genetic variation in the general population has until recently been limited, but newly published data from NHLBI GO Exome Sequencing Project (ESP) has provided important knowledge on this topic. We aimed to investigate the prevalence of previously LQTS-associated variants in ESP (5400 individuals), in order to identify possible false-positive LQTS variants. With this aim, we performed a search for previously published LQTS-associated variants in ESP. In addition, a PolyPhen-2 prediction was conducted, and the four most prevalent LQTS-associated variants with significant functional effects present in ESP were genotyped in a second control population. We identified 33 missense variants previously associated with LQTS in ESP. These 33 variants affected 173 alleles and this corresponded to a LQTS prevalence of 1:31 in the ESP population. PolyPhen-2 predicted 30% of the 33 variants present in ESP to be benign compared with 13% among LQTS-associated variants not present in ESP (P=0.019). Genotyping of the four variants KCNH2 P347S; SCN5A: S216L, V1951L; and CAV3 T78M in the control population (n=704) revealed prevalences comparable to those of ESP. Thus, we identified a much higher prevalence of previously LQTS-associated variants than expected in exome data from population studies. Great caution regarding the possible disease causation of some of these variants has to be taken, especially when used for risk stratification in family members.

The voltage-gated channel accessory protein KCNE2: multiple ion channel partners, multiple ways to long QT syndrome

The single-pass transmembrane protein KCNE2 or MIRP1 was once thought to be the missing accessory protein that combined with hERG to fully recapitulate the cardiac repolarising current IKr. As a result of this role, it was an easy next step to associate mutations in KCNE2 to long QT syndrome, in which there is delayed repolarisation of the heart. Since that time however, KCNE2 has been shown to modify the behaviour of several other channels and currents, and its role in the heart and in the aetiology of long QT syndrome has become less clear. In this article, we review the known interactions of the KCNE2 protein and the resulting functional effects, and the effects of mutations in KCNE2 and their clinical role.

The genetics of atrial fibrillation: from the bench to the bedside

Atrial fibrillation (AF) has become a growing global epidemic and a financial burden for society. The past 10 years have seen significant advances in our understanding of the genetic aspects of AF: At least 2 chromosomal loci and 17 causal genes have been identified in familial AF, and an additional 7 common variants and single-nucleotide polymorphisms in 11 different genes have been indicated in nonfamilial AF. However, the current management strategies for AF are suboptimal. The integration of genetic information into clinical practice may aid the early identification of AF patients who are at risk as well as the characterization of molecular pathways that culminate in AF, with the eventual result of better treatment. Never before has such an opportunity arisen to advance our understanding of the biology of AF through the translation of genetics findings from the bench to the bedside.

Molecular Investigations of Sudden Unexplained Deaths

Sudden unexplained deaths in apparently healthy individuals (newborn through adult) pose a vexing challenge to medical examiners, law enforcement and society as a whole. Recent advances in “molecular autopsies” have begun to uncover the mystery surrounding sudden unexplained deaths by identifying mutations that can result in or predispose an apparently healthy individual to sudden death. Genetic risks of sudden unexplained deaths have been studied from several different perspectives, and categorized generally by systems, including: cardiac, nervous, immune, and metabolic. This article reviews the genetic risks in sudden unexplained deaths, presents the current state and challenges of molecular investigations, and sheds light on future directions in sudden unexplained death investigations.

Cardiac channelopathies and sudden infant death syndrome

Sudden infant death syndrome (SIDS) is always a devastating and unexpected occurrence. SIDS is the leading cause of death in the first 6 months after birth in the industrialized world. Since the discovery in 1998 of long QT syndrome as an underlying substrate for SIDS, around 10-20% of SIDS cases have been proposed as being caused by genetic variants in either ion channel or ion channel-associated proteins. Until now, 10 cardiac channelopathy susceptibility genes have been found to be implicated in the pathogenesis of SIDS. Four of the genes encode cardiac ion channel α-subunits, 3 genes encode ion channel β-subunits, and 3 genes encode other channel-interacting proteins. All 10 genes have been associated with primary electrical heart diseases. SIDS may hereby be the initial symptom of rare primary electric channelopathies such as long QT, short QT and Brugada syndrome, as well as catecholaminergic polymorphic ventricular tachycardia. In this review we describe the functional role of sodium, potassium and calcium channels in propagation, depolarization and repolarization in the context of the 4 arrhythmogenic diseases reported to be associated with SIDS. Lastly, the possibility of postmortem genetic testing and potential recommendations on how to deal with family members are discussed.

LQT5 masquerading as LQT2: a dominant negative effect of KCNE1-D85N rare polymorphism on KCNH2 current

AIMS

KCNE1 encodes an auxiliary subunit of cardiac potassium channels. Loss-of-function variations in this gene have been associated with the LQT5 form of the long QT syndrome (LQTS), secondary to reduction of I(Ks) current. We present a case in which a D85N rare polymorphism in KCNE1 is associated with an LQT2 phenotype.

METHODS AND RESULTS

An 11-year old competitive athlete presented with mild bradycardia and a QTc interval of 470 ms. An LQT2 phenotype, consisting of low-voltage bifid T waves, was evident in the right precordial electrocardiogram leads. During the tachycardia phase following adenosine, QTc increased to 620 ms. Genetic analysis revealed a rare heterozygous polymorphism in KCNE1 predicting the substitution of asparagine for aspartic acid at position 85 of minK (D85N). Patch clamp experiments showed that KCNE1-D85N, when co-expressed with KCNH2 in TSA201 cells, significantly reduced I(Kr). Homozygous co-expression of the mutant with KCNH2 reduced I(Kr) tail current by 85%, whereas heterozygous co-expression reduced the current by 52%, demonstrating for the first time a dominant-negative effect of D85N to reduce I(Kr). Co-expression of the mutant with KCNQ1, either homozygously or heterozygously, produced no change in I(Ks).

CONCLUSIONS

Our results suggest that a rare polymorphism KCNE1-D85N underlies the development of an LQT2 phenotype in this young athlete by interacting with KCNH2 to cause a dominant-negative effect to reduce I(Kr). Our data provide further evidence in support of the promiscuity of potassium channel β subunits in modulating the function of multiple potassium channels leading to a diversity of clinical phenotypes.

Phenotypic Manifestations of Mutations in Genes Encoding Subunits of Cardiac Potassium Channels

Since 1995, when a potassium channel gene, hERG (human ether-à-go-go-related gene), now referred to as KCNH2 , encoding the rapid component of cardiac delayed rectifier potassium channels was identified as being responsible for type 2 congenital long-QT syndrome, a number of potassium channel genes have been shown to cause different types of inherited cardiac arrhythmia syndromes. These include congenital long-QT syndrome, short-QT syndrome, Brugada syndrome, early repolarization syndrome, and familial atrial fibrillation. Genotype-phenotype correlations have been investigated in some inherited arrhythmia syndromes, and as a result, gene-specific risk stratification and gene-specific therapy and management have become available, particularly for patients with congenital long-QT syndrome. In this review article, the molecular structure and function of potassium channels, the clinical phenotype due to potassium channel gene mutations, including genotype-phenotype correlations, and the diverse mechanisms underlying the potassium channel gene–related diseases will be discussed.

Genetics of sudden cardiac death syndromes

PURPOSE OF REVIEW

To survey recent developments in the field of genetics encompassing discovery of new candidate genes, new diagnostic strategies, and new therapies for sudden cardiac death (SCD) syndromes.

RECENT FINDINGS

In addition to new mutations in known SCD genes, several novel genes not previously implicated in SCD causation have been found, particularly in long QT syndrome (e.g., KCNJ5, AKAP9, SNTA1), idiopathic ventricular fibrillation (e.g., DPP6, KCNJ8), dilated cardiomyopathy (e.g., NEBL), and hypertrophic cardiomyopathy (HCM; e.g., NEXN). Genetic SCD animal models have provided novel insights into the cellular mechanism and pathogenesis of nearly all the major SCD syndromes, which has led to several new drug therapies for patients with genetic arrhythmia syndromes (e.g., flecainide in catecholaminergic polymorphic ventricular tachycardia). Furthermore, genetic contributions to acquired heart diseases are increasingly being recognized. For example, a 21q21 locus is strongly associated with ventricular fibrillation after myocardial infarction. Near this locus is CXADR, a gene encoding a viral receptor implicated in myocarditis and dilated cardiomyopathy. Finally, common variants in cardiac ion channels and proteins likely contribute to common cardiac phenotypes.

SUMMARY

Major strides have been made in uncovering new genes, mechanisms, and syndromes that have significantly advanced the diagnosis and treatment of genetic SCD disorders.

Genetics of sudden death: focus on inherited channelopathies

Since the discovery of the genetic bases of the long QT syndrome, several new genetically mediated arrhythmias have been described, defining a new group of syndromes, called inherited arrhythmogenic diseases. This allowed clarifying the substrate of several cases of juvenile sudden death, previously defined as 'idiopathic ventricular fibrillation'. Studies derived from this field also contributed to advance the field of electrophysiology, elucidating some of the mechanisms that regulate the cardiac electrical properties of the heart. Recently, new genes and new proteins have been called into play, expanding the knowledge on the complexity of the regulatory processes modulating the cardiac action potential. Moreover, the collaboration between clinicians and basic scientists opened new approaches in the management of patients affected by genetic arrhythmias. This body of knowledge has then moved into the realization that genetic variations may also influence the predisposition to acquired cardiac diseases. The new exciting challenges that investigators are now facing are connected to the possibility of expanding the field towards the use of these information to shape a newer vision in the management and cure of patients.

Drug-induced long QT syndrome

The drug-induced long QT syndrome is a distinct clinical entity that has evolved from an electrophysiologic curiosity to a centerpiece in drug regulation and development. This evolution reflects an increasing recognition that a rare adverse drug effect can profoundly upset the balance between benefit and risk that goes into the prescription of a drug by an individual practitioner as well as the approval of a new drug entity by a regulatory agency. This review will outline how defining the central mechanism, block of the cardiac delayed-rectifier potassium current I(Kr), has contributed to defining risk in patients and in populations. Models for studying risk, and understanding the way in which clinical risk factors modulate cardiac repolarization at the molecular level are discussed. Finally, the role of genetic variants in modulating risk is described.

Dissection of cardiovascular development and disease pathways in zebrafish

The use of animal models in medicine has contributed significantly to the development of drug treatments and surgical procedures for the last century, in particular for cardiovascular disease. In order to model human disease in an animal, an appreciation of the strengths and limitations of the system are required to interpret results and design the logical sequence of steps toward clinical translation. As the world's population ages, cardiovascular disease will become even more prominent and further progress will be essential to stave off what seems destined to become a massive public health issue. Future treatments will require the imaginative application of current models as well as the generation of new ones. In this review, we discuss the resources available for modeling cardiovascular disease in zebrafish and the varied attributes of this system. We then discuss current zebrafish disease models and their potential that has yet to be exploited.