A founder mutation of the potassium channel KCNQ1 in long QT syndrome: implications for estimation of disease prevalence and molecular diagnostics

A mutation in the cardiac KV7.1 channel possibly disrupts interaction with Yotiao protein

Here, we characterized the p.Arg583His (R583H) Kv7.1 mutation, identified in two unrelated families suffered from LQT syndrome. This mutation is located in the HС-HD linker of the cytoplasmic portion of the Kv7.1 channel. This linker, together with HD helix are responsible for binding the A-kinase anchoring protein 9 (AKAP9), Yotiao. We studied the electrophysiological characteristics of the mutated channel expressed in CHO-K1 along with KCNE1 subunit and Yotiao protein, using the whole-cell patch-clamp technique. We found that R583H mutation, even at the heterozygous state, impedes IKs activation. Molecular modeling showed that HС and HD helixes of the C-terminal part of Kv7.1 channel are swapped along the C-terminus length of the channel and that R583 position is exposed to the outer surface of HC-HD tandem coiled-coil. Interestingly, the adenylate cyclase activator, forskolin had a smaller effect on the mutant channel comparing with the WT protein, suggesting that R583H mutation may disrupt the interaction of the channel with the adaptor protein Yotiao and, therefore, may impair phosphorylation of the KCNQ1 channel.

The Impact of Chronic Disease on the Corrected QT (QTc) Value in Women in a British Columbia First Nations Population

BACKGROUND

Indigenous women have higher rates of chronic disease than Indigenous men and non-Indigenous women. Long QT syndrome (LQTS) can be inherited or acquired; the latter may occur with chronic disease. A prolonged corrected QT value (QTc) is an independent risk factor for ventricular arrhythmias and sudden death, but few studies have quantified the impact of chronic disease on the QTc. We assessed the association between chronic disease and QTc prolongation in a population of First Nations women previously ascertained to study a high rate of inherited LQTS due to a unique genetic (founder) variant in their community.

METHODS

This substudy focusing on women expands on the original research where patients with clinical features of LQTS and their relatives were assessed for genetic variants discovered to affect the QTc. Medical records were retrospectively reviewed and chronic diseases documented. Using multivariate linear regression, adjusting for the effect of genetic variants, age, and QTc-prolonging medications, we evaluated the association between chronic disease and the QTc.

RESULTS

In total, 275 women were included. After adjustments, a prolonged QTc was associated with coronary artery disease (26.5 ms, 95% confidence interval [CI] 9.0-44.1 ms; P = 0.003), conduction system disease (26.8 ms, 95% CI 2.2-51.4 ms; P = 0.033), rheumatoid arthritis (28.9 ms, 95% CI 12.7-45.1 ms; P = 0.001), and type 2 diabetes mellitus (17.9 ms, 95% CI 3.6-32.3 ms; P = 0.015).

CONCLUSIONS

This quantification of the association between chronic disease and QTc prolongation in an Indigenous cohort provides insight into the nongenetic determinants of QTc prolongation. Corroboration in other populations will provide evidence for generalisability of these results.

Long-QT founder variant T309I-Kv7.1 with dominant negative pattern may predispose delayed afterdepolarizations under β-adrenergic stimulation

The variant c.926C > T (p.T309I) in KCNQ1 gene was identified in 10 putatively unrelated Czech families with long QT syndrome (LQTS). Mutation carriers (24 heterozygous individuals) were more symptomatic compared to their non-affected relatives (17 individuals). The carriers showed a mild LQTS phenotype including a longer QTc interval at rest (466 ± 24 ms vs. 418 ± 20 ms) and after exercise (508 ± 32 ms vs. 417 ± 24 ms), 4 syncopes and 2 aborted cardiac arrests. The same haplotype associated with the c.926C > T variant was identified in all probands. Using the whole cell patch clamp technique and confocal microscopy, a complete loss of channel function was revealed in the homozygous setting, caused by an impaired channel trafficking. Dominant negativity with preserved reactivity to β-adrenergic stimulation was apparent in the heterozygous setting. In simulations on a human ventricular cell model, the dysfunction resulted in delayed afterdepolarizations (DADs) and premature action potentials under β-adrenergic stimulation that could be prevented by a slight inhibition of calcium current. We conclude that the KCNQ1 variant c.926C > T is the first identified LQTS-related founder mutation in Central Europe. The dominant negative channel dysfunction may lead to DADs under β-adrenergic stimulation. Inhibition of calcium current could be possible therapeutic strategy in LQTS1 patients refractory to β-blocker therapy.

Suppression-Replacement KCNQ1 Gene Therapy for Type 1 Long QT Syndrome

BACKGROUND

Type 1 long QT syndrome (LQT1) is caused by loss-of-function variants in the KCNQ1-encoded K_v7.1 potassium channel α-subunit that is essential for cardiac repolarization, providing the slow delayed rectifier current. No current therapies target the molecular cause of LQT1.

METHODS

A dual-component suppression-and-replacement (SupRep) KCNQ1 gene therapy was created by cloning a KCNQ1 short hairpin RNA and a short hairpin RNA-immune KCNQ1 cDNA modified with synonymous variants in the short hairpin RNA target site, into a single construct. The ability of KCNQ1-SupRep gene therapy to suppress and replace LQT1-causative variants in KCNQ1 was evaluated by means of heterologous expression in TSA201 cells. For a human in vitro cardiac model, induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs) were generated from 4 patients with LQT1 (KCNQ1-Y171X, -V254M, -I567S, and -A344A/spl) and an unrelated healthy control. CRISPR-Cas9 corrected isogenic control iPSC-CMs were made for 2 LQT1 lines (correction of KCNQ1-V254M and KCNQ1-A344A/spl). FluoVolt voltage dye was used to measure the cardiac action potential duration (APD) in iPSC-CMs treated with KCNQ1-SupRep.

RESULTS

In TSA201 cells, KCNQ1-SupRep achieved mutation-independent suppression of wild-type KCNQ1 and 3 LQT1-causative variants (KCNQ1-Y171X, -V254M, and -I567S) with simultaneous replacement of short hairpin RNA-immune KCNQ1 as measured by allele-specific quantitative reverse transcription polymerase chain reaction and Western blot. Using FluoVolt voltage dye to measure the cardiac APD in the 4 LQT1 patient-derived iPSC-CMs, treatment with KCNQ1-SupRep resulted in shortening of the pathologically prolonged APD at both 90% and 50% repolarization, resulting in APD values similar to those of the 2 isogenic controls.

CONCLUSIONS

This study provides the first proof-of-principle gene therapy for complete correction of long QT syndrome. As a dual-component gene therapy vector, KCNQ1-SupRep successfully suppressed and replaced KCNQ1 to normal wild-type levels. In TSA201 cells, cotransfection of LQT1-causative variants and KCNQ1-SupRep caused mutation-independent suppression and replacement of KCNQ1. In LQT1 iPSC-CMs, KCNQ1-SupRep gene therapy shortened the APD, thereby eliminating the pathognomonic feature of LQT1.

An NGS-based genotyping in LQTS; minor genes are no longer minor

Mutations in KCNQ1, KCNH2, and SCN5A are the major cause of long QT syndrome (LQTS). More than 90% of the genotyped patients have been reported to carry mutations in any of these three genes. Thanks to increasing popularity of next generation sequencer (NGS), novel CACNA1C mutations have been identified among LQTS patients without extra-cardiac phenotypes. We aimed to clarify the frequency of genotypes in LQTS patients in the era of NGS. The study comprised 160 congenital LQTS patients (71 males) registered from November 2015 to September 2018. Inclusion criteria was QTc > 460 ms and Schwartz score ≥ 3. We performed genetic analysis using target gene method by NGS and confirmed the mutations by Sanger method. The median age for genetic screening was 13 (0-68) years. Sixteen patients suffered cardiac arrest, 47 syncope, and 97 were asymptomatic. We identified genetic mutations in 111 (69.4%) patients including 6 CACNA1C (5.4% of the genotyped patients) with 4 asymptomatic patients. Five (3.1%) patients carried double mutations; three out of them with RYR2 and KCNQ1 or KCNH2. In conclusion, CACNA1C screening would be recommended even if the patient is asymptomatic to elucidate the genetic background of the LQTS patients.

Inherited cardiac arrhythmias

The main inherited cardiac arrhythmias are long QT syndrome, short QT syndrome, catecholaminergic polymorphic ventricular tachycardia and Brugada syndrome. These rare diseases are often the underlying cause of sudden cardiac death in young individuals and result from mutations in several genes encoding ion channels or proteins involved in their regulation. The genetic defects lead to alterations in the ionic currents that determine the morphology and duration of the cardiac action potential, and individuals with these disorders often present with syncope or a life-threatening arrhythmic episode. The diagnosis is based on clinical presentation and history, the characteristics of the electrocardiographic recording at rest and during exercise and genetic analyses. Management relies on pharmacological therapy, mostly β-adrenergic receptor blockers (specifically, propranolol and nadolol) and sodium and transient outward current blockers (such as quinidine), or surgical interventions, including left cardiac sympathetic denervation and implantation of a cardioverter-defibrillator. All these arrhythmias are potentially life-threatening and have substantial negative effects on the quality of life of patients. Future research should focus on the identification of genes associated with the diseases and other risk factors, improved risk stratification and, in particular for Brugada syndrome, effective therapies.

The evolution of gene-guided management of inherited arrhythmia syndromes: Peering beyond monogenic paradigms towards comprehensive genomic risk scores

Inherited arrhythmia syndromes have traditionally been viewed as monogenic forms of disease whose pathophysiology is driven by a single highly penetrant rare genetic variant. Although an accurate depiction of a proportion of genetic variants, the variable penetrance frequently noted in genotype positive families and the presence of sporadic genotype negative cases have long highlighted a more nuanced truth being operative. Coupled with our more recent recognition that many rare variants implicated in inherited arrhythmia syndromes possess unexpectedly high allele frequencies within the general population, these observations have contributed to the realization that a spectrum of pathogenicity exists among clinically relevant genetic variants. Notably, variable mutation pathogenicity and corresponding variable degrees of penetrance emphasize a limitation of contemporary guidelines, which attempt to dichotomize genetic variants as pathogenic or benign. Recognition of the existence of low and intermediate penetrant variants insufficient to be causative for disease in isolation has served to emphasize the importance of additional genetic, clinical, and environmental factors in the pathogenesis of rare inherited arrhythmia syndromes. Despite being rare, it has also become increasingly evident that common genetic variants play critical roles in both heritable channelopathies and cardiomyopathies and in aggregate may even be the primary drivers in certain instances, such as genotype negative Brugada syndrome. Our growing realization that the genetic substrates of inherited arrhythmia syndromes have intricacies that extend beyond traditionally perceived monogenic paradigms has highlighted a potential value of leveraging more comprehensive genomic risk scores for predicting disease development and arrhythmic risk.

Epidemiology of inherited arrhythmias

The primary electrical disorders are a group of inherited cardiac ventricular arrhythmias that are a major cause of sudden cardiac death in young individuals. Inherited ventricular arrhythmias result from mutations in genes encoding cardiac ion channels or their modulatory subunits. Advances in genetic screening in the past three decades have led to the assembly of large patient cohorts with these disorders. Studies in these patients, as well as in the general population, have striven to define the prevalence of these inherited arrhythmias and the characteristics of patients with different genetic subtypes of the disease. In this Review, we provide a comprehensive update on the epidemiology of inherited ventricular arrhythmias, focusing on natural history, prevalence and patient demographics. In addition, we summarize the various founder populations (groups of individuals with a disease that is caused by a genetic defect inherited from a common ancestor) that have been identified for some of these disorders and which lead to increased prevalence in some geographical regions. To date, although numerous studies have markedly increased our understanding of the epidemiology of these disorders, demographic data, especially from non-Western countries, remain scarce. Furthermore, defining the true prevalence of these disorders remains challenging. International collaboration will undoubtedly accelerate the collection of demographic information and improve the accuracy of prevalence data.

The Effects of Pharmacological Compounds on Beat Rate Variations in Human Long QT-Syndrome Cardiomyocytes

Healthy human heart rate fluctuates overtime showing long-range fractal correlations. In contrast, various cardiac diseases and normal aging show the breakdown of fractal complexity. Recently, it was shown that human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) intrinsically exhibit fractal behavior as in humans. Here, we investigated the fractal complexity of hiPSC-derived long QT-cardiomyocytes (LQT-CMs). We recorded extracellular field potentials from hiPSC-CMs at baseline and under the effect of various compounds including β-blocker bisoprolol, ML277, a specific and potent I_Ks current activator, as well as JNJ303, a specific I_Ks blocker. From the peak-to-peak-intervals, we determined the long-range fractal correlations by using detrended fluctuation analysis. Electrophysiologically, the baseline corrected field potential durations (cFPDs) were more prolonged in LQT-CMs than in wildtype (WT)-CMs. Bisoprolol did not have significant effects to the cFPD in any CMs. ML277 shortened cFPD in a dose-dependent fashion by 11 % and 5–11 % in WT- and LQT-CMs, respectively. JNJ303 prolonged cFPD in a dose-dependent fashion by 22 % and 7–13 % in WT- and LQT-CMs, respectively. At baseline, all CMs showed fractal correlations as determined by short-term scaling exponent α. However, in all CMs, the α was increased when pharmacological compounds were applied indicating of breakdown of fractal complexity. These findings suggest that the intrinsic mechanisms contributing to the fractal complexity are not altered in LQT-CMs. The modulation of I_Ks channel and β1-adrenoreceptors by pharmacological compounds may affect the fractal complexity of the hiPSC-CMs.

Genetic Modifiers for the Long-QT Syndrome: How Important Is the Role of Variants in the 3' Untranslated Region of KCNQ1?

BACKGROUND

Long-QT syndrome is an inherited cardiac channelopathy characterized by delayed repolarization, risk of life-threatening arrhythmia, and significant clinical variability even within families. Three single-nucleotide polymorphisms (SNPs) in the 3' untranslated region of KCNQ1 were recently suggested to be associated with suppressed gene expression and hence decreased disease severity when located on the same haplotype with a disease-causing KCNQ1 mutation. We sought to replicate this finding in a larger and a genetically more homogeneous population of KCNQ1 mutation carriers.

METHODS AND RESULTS

The 3 SNPs (rs2519184, rs8234, and rs10798) were genotyped in a total of 747 KCNQ1 mutation carriers with A341V, G589D, or IVS7-2A>G mutation. The SNP haplotypes were assigned based on family trees. The SNP allele frequencies and clinical severity differed between the 3 mutation groups. The different SNP haplotypes were neither associated with heart rate-corrected QT interval duration (QTc) nor cardiac events in any of the 3 mutation groups. When the mutation groups were combined, the derived SNP haplotype of rs8234 and rs10798 located on the same haplotype with the mutation was associated with a shorter QTc interval (P<0.05) and a reduced occurrence of cardiac events (P<0.01), consistent with the previous finding. However, when the population-specific mutation was controlled for, both associations were no longer evident.

CONCLUSIONS

3' Untranslated region SNPs are not acting as genetic modifiers in a large group of LQT1 patients. The confounding effect of merging a genetically and clinically heterogeneous group of patients needs to be taken into account when studying disease modifiers.

Low extracellular potassium prolongs repolarization and evokes early afterdepolarization in human induced pluripotent stem cell-derived cardiomyocytes

Long QT syndrome (LQTS) is characterized by a prolonged QT-interval on electrocardiogram and by increased risk of sudden death. One of the most common and potentially life-threatening electrolyte disturbances is hypokalemia, characterized by low concentrations of K⁺. Using a multielectrode array platform and current clamp technique, we investigated the effect of low extracellular K⁺ concentration ([K⁺]_Ex) on the electrophysiological properties of hiPSC-derived cardiomyocytes (CMs) generated from a healthy control subject (WT) and from two symptomatic patients with type 1 of LQTS carrying G589D (LQT1A) or IVS7-2A>G mutation (LQT1B) in KCNQ1. The baseline prolongations of field potential durations (FPDs) and action potential durations (APDs) were longer in LQT1-CMs than in WT-CMs. Exposure to low [K⁺]_Ex prolonged FPDs and APDs in a concentration-dependent fashion. LQT1-CMs were found to be more sensitive to low [K⁺]_Ex compared to WT-CMs. At baseline, LQT1A-CMs had more prolonged APDs than LQT1B-CMs, but low [K⁺]_Ex caused more pronounced APD prolongation in LQT1B-CMs. Early afterdepolarizations in the action potentials were observed in a subset of LQT1A-CMs with further prolonged baseline APDs and triangular phase 2 profiles. This work demonstrates that the hiPSC-derived CMs are sensitive to low [K⁺]_Ex and provide a platform to study acquired LQTS.

Summary: This is the first study showing the effects of low extracellular potassium on the electrophysiological properties of human induced pluripotent stem cell-derived long QT cardiomyocytes at single and multicellular level.

Effects of cardioactive drugs on human induced pluripotent stem cell derived long QT syndrome cardiomyocytes

Human induced pluripotent stem cells (hiPSC) have enabled a major step forward in pathophysiologic studies of inherited diseases and may also prove to be valuable in in vitro drug testing. Long QT syndrome (LQTS), characterized by prolonged cardiac repolarization and risk of sudden death, may be inherited or result from adverse drug effects. Using a microelectrode array platform, we investigated the effects of six different drugs on the electrophysiological characteristics of human embryonic stem cell-derived cardiomyocytes as well as hiPSC-derived cardiomyocytes from control subjects and from patients with type 1 (LQT1) and type 2 (LQT2) of LQTS. At baseline the repolarization time was significantly longer in LQTS cells compared to controls. Isoprenaline increased the beating rate of all cell lines by 10–73 % but did not show any arrhythmic effects in any cell type. Different QT-interval prolonging drugs caused prolongation of cardiac repolarization by 3–13 % (cisapride), 10–20 % (erythromycin), 8–23 % (sotalol), 16–42 % (quinidine) and 12–27 % (E-4031), but we did not find any systematic differences in sensitivity between the control, LQT1 and LQT2 cell lines. Sotalol, quinidine and E-4031 also caused arrhythmic beats and beating arrests in some cases. In summary, the drug effects on these patient-specific cardiomyocytes appear to recapitulate clinical observations and provide further evidence that these cells can be applied for in vitro drug testing to probe their vulnerability to arrhythmia.

Genotype-phenotype correlation in long QT syndrome families

Heterogeneity in clinical manifestations is a well-known feature in Long QT Syndrome (LQTS). The extent of this phenomenon became evident in families wherein both symptomatic and asymptomatic family members are reported. The study hence warrants genetic testing and/or screening of family members of LQTS probands for risk stratification and prediction. Of the 46 families screened, 18 probands revealed novel variations/compound heterozygosity in the gene/s screened. Families 1-4 revealed probands carrying novel variations in KCNQ1 gene along with compound heterozygosity of risk genotypes of the SCN5A, KCNE1 and NPPA gene/s polymorphisms screened. It was also observed that families- 5, 6 and 7 were typical cases of "anticipation" in which both mother and child were diagnosed with congenital LQTS (cLQTS). Families- 16 and 17 represented aLQTS probands with variations in IKs and INa encoding genes. First degree relatives (FDRs) carrying the same haplotype as the proband were also identified which may help in predictive testing and management of LQTS. Most of the probands exhibiting a family history were found to be genetic compounds which clearly points to the role of cardiac genes and their modifiers in a recessive fashion in LQTS manifestation.

Follow-up of 316 molecularly defined pediatric long-QT syndrome patients: clinical course, treatments, and side effects

BACKGROUND

Inherited long-QT syndrome (LQTS) is associated with risk of sudden death. We assessed the clinical course and the fulfillment of current treatment strategies in molecularly defined pediatric LQTS type 1 and (LQT1) and type 2 (LQT2) patients.

METHODS AND RESULTS

Follow-up data covering a mean of 12 years were collected for 316 genotyped LQT1 and LQT2 patients aged 0 to 18 years. No arrhythmic deaths occurred during the follow-up. Finnish KCNQ1 and KCNH2 founder mutations were associated with fewer cardiac events than other KCNQ1 and KCNH2 mutations (hazard ratio [HR], 0.33; P=0.03 and HR, 0.16; P=0.01, respectively). QTc interval ≥500 ms increased the risk of cardiac events compared with QTc <470 ms (HR, 3.32; P=0.001). Treatment with β-blocker medication was associated with reduced risk of first cardiac event (HR, 0.23; P=0.001). Noncompliant LQT2 patients were more often symptomatic than compliant LQT2 patients (18% and 0%, respectively; P=0.03). Treatment with implantable cardioverter defibrillator was rare (3%) and resulted in reinterventions in 44% of cases.

CONCLUSIONS

Severe cardiac events are uncommon in molecularly defined and appropriately treated pediatric LQTS mutation carriers. β-Blocker medication reduces the risk of cardiac events and is generally well tolerated in this age group of LQTS patients.

Distinct electrophysiological and mechanical beating phenotypes of long QT syndrome type 1-specific cardiomyocytes carrying different mutations

Background

Long QT syndrome (LQTS) is associated with increased risk of ventricular arrhythmias and cardiac arrest. LQTS type 1 (LQT1), the most prevalent subtype of LQTS, is caused by defects of slow delayed rectifier potassium current (I_Ks) that lead to abnormal cardiac repolarization. Here we used pluripotent stem cell (iPSC)-technology to investigate both the electrophysiological and also for the first time the mechanical beating behavior of genetically defined, LQT1 specific cardiomyocytes (CMs) carrying different mutations.

Methods

We established in vitro models for LQT1 caused by two mutations (G589D or ivs7-2A>G). LQT1 specific CMs were derived from patient specific iPSCs and characterized for their electrophysiology using a current clamp and Ca^2 +-imaging. Their mechanical beating characteristics were analyzed with video-image analysis method.

Results and conclusions

Both LQT1-CM-types showed prolonged repolarization, but only those with G589D presented early after-depolarizations at baseline. Increased amounts of abnormal Ca^2 + transients were detected in both types of LQT1-CMs. Surprisingly, also the mechanical beating behavior demonstrated clear abnormalities and additionally the abnormalities were different with the two mutations: prolonged contraction was seen in G589D-CMs while impaired relaxation was observed in ivs7-2A>G-CMs.

The CMs carrying two different LQT1 specific mutations (G589D or ivs7-2A>G) presented clear differences in their electrical properties as well as in their mechanical beating behavior. Results from different methods correlated well with each other suggesting that simply mechanical beating behavior of CMs could be used for screening of diseased CMs and possibly for diagnostic purposes in the future.

Cellular mechanisms of mutations in Kv7.1: auditory functions in Jervell and Lange-Nielsen syndrome vs. Romano-Ward syndrome

As a result of cell-specific functions of voltage-activated K⁺ channels, such as Kv7.1, mutations in this channel produce profound cardiac and auditory defects. At the same time, the massive diversity of K⁺ channels allows for compensatory substitution of mutant channels by other functional channels of their type to minimize defective phenotypes. Kv7.1 represents a clear example of such functional dichotomy. While several point mutations in the channel result in a cardio-auditory syndrome called Jervell and Lange-Nielsen syndrome (JLNS), about 100-fold mutations result in long QT syndrome (LQTS) denoted as Romano–Ward syndrome (RWS), which has an intact auditory phenotype. To determine whether the cellular mechanisms for the diverse phenotypic outcome of Kv7.1 mutations, are dependent on the tissue-specific function of the channel and/or specialized functions of the channel, we made series of point mutations in hKv7.1 ascribed to JLNS and RWS. For JLNS mutations, all except W248F yielded non-functional channels when expressed alone. Although W248F at the end of the S4 domain yielded a functional current, it underwent marked inactivation at positive voltages, rendering the channel non-functional. We demonstrate that by definition, none of the JLNS mutants operated in a dominant negative (DN) fashion. Instead, the JLNS mutants have impaired membrane trafficking, trapped in the endoplasmic reticulum (ER) and Cis-Golgi. The RWS mutants exhibited varied functional phenotypes. However, they can be summed up as exhibiting DN effects. Phenotypic differences between JLNS and RWS may stem from tissue-specific functional requirements of cardiac vs. inner ear non-sensory cells.

LQT1 mutations in KCNQ1 C-terminus assembly domain suppress IKs using different mechanisms

AIMS

Long QT syndrome 1 (LQT1) mutations in KCNQ1 that decrease cardiac IKs (slowly activating delayed rectifier K(+) current) underlie ventricular arrhythmias and sudden death. LQT1 mutations may suppress IKs by preventing KCNQ1 assembly, disrupting surface trafficking, or inhibiting gating. We investigated mechanisms underlying how three LQT1 mutations in KCNQ1 C-terminus assembly domain (R555H/G589D/L619M) decrease IKs in heterologous cells and cardiomyocytes.

METHODS AND RESULTS

In Chinese hamster ovary (CHO) cells, mutant KCNQ1 + KCNE1 channels either produced no currents (G589D/L619M) or displayed markedly reduced IKs with a right-shifted voltage-dependence of activation (R555H). When co-expressed with wild-type (wt) KCNQ1, the mutant KCNQ1s displayed varying intrinsic dominant-negative capacities that were affected by auxiliary KCNE1. All three mutant KCNQ1s assembled with wt KCNQ1 as determined by fluorescence resonance energy transfer (FRET). We developed an optical quantum dot labelling assay to measure channel surface density. G589D/R555H displayed substantial reductions in surface density, which were either partially (G589D) or fully (R555H) rescued by wt KCNQ1. Unexpectedly, L619M showed no trafficking defect. In adult rat cardiomyocytes, adenovirus-expressed homotetrameric G589D/L619M + KCNE1 channels yielded no currents, whereas R555H + KCNE1 produced diminished IKs with a right-shifted voltage-dependence of activation, mimicking observations in CHO cells. In contrast to heterologous cells, homotetrameric R555H channels showed no trafficking defect in cardiomyocytes.

CONCLUSION

Distinct LQT1 mutations in KCNQ1 assembly domain decrease IKs using unique combinations of biophysical and trafficking mechanisms. Functional deficits in IKs observed in heterologous cells are mostly, but not completely, recapitulated in adult rat cardiomyocytes. A 'methodological chain' combining approaches in heterologous cells and cardiomyocytes provides mechanistic insights that may help advance personalized therapy for LQT1 mutations.

Anchored protein kinase A signalling in cardiac cellular electrophysiology

The cyclic adenosine monophosphate (cAMP)-dependent protein kinase (PKA) is an elementary molecule involved in both acute and chronic modulation of cardiac function. Substantial research in recent years has highlighted the importance of A-kinase anchoring proteins (AKAP) therein as they act as the backbones of major macromolecular signalling complexes of the β-adrenergic/cAMP/PKA pathway. This review discusses the role of AKAP-associated protein complexes in acute and chronic cardiac modulation by dissecting their role in altering the activity of different ion channels, which underlie cardiac action potential (AP) generation. In addition, we review the involvement of different AKAP complexes in mechanisms of cardiac remodelling and arrhythmias.

A590T mutation in KCNQ1 C-terminal helix D decreases IKs channel trafficking and function but not Yotiao interaction

KCNQ1 encodes the α subunit of the voltage-gated channel that mediates the cardiac slow delayed rectifier K(+) current (IKs). Here, we report a KCNQ1 allele encoding an A590T mutation [KCNQ1(A590T)] found in a 39-year-old female with a mild QT prolongation. A590 is located in the C-terminal α helical region of KCNQ1 that mediates subunit tetramerization, membrane trafficking, and interaction with Yotiao. This interaction is known to be required for the proper modulation of IKs by cAMP. Since previous studies reported that mutations in the vicinity of A590 impair IKs channel surface expression and function, we examined whether and how the A590T mutation affects the IKs channel. Electrophysiological measurements in HEK-293T cells showed that the A590T mutation caused a reduction in IKs density and a right-shift of the current-voltage relation of channel activation. Immunocytochemical and immunoblot analyses showed the reduced cell surface expression of KCNQ1(A590T) subunit and its rescue by coexpression of the wild-type KCNQ1 [KCNQ1(WT)] subunit. Moreover, KCNQ1(A590T) subunit interacted with Yotiao and had a cAMP-responsiveness comparable to that of KCNQ1(WT) subunit. These findings indicate that the A590 of KCNQ1 subunit plays important roles in the maintenance of channel surface expression and function via a novel mechanism independent of interaction with Yotiao.

Novel mutations of KCNQ1 in Long QT syndrome

BACKGROUND

Autosomal recessive Long QT syndrome is characterized by prolonged QTc along with congenital bilateral deafness depends on mutations in K(+) channel genes. A family of a Long QT syndrome proband from India has been identified with novel indel variations.

METHODS

The molecular study of the proband revealed 4 novel indel variations in KCNQ1. In-silico analysis revealed the intronic variations has led to a change in the secondary structure of mRNA and splice site variations. The exonic variations leads to frameshift mutations. DNA analysis of the available family members revealed a carrier status.

RESULTS AND CONCLUSION

It is thus predicted that the variations may lead to a change in the position of the splicing enhancer/inhibitor in KCNQ1 leading to the formation of a truncated S2-S3 fragment of KCNQ1 transmembrane protein in cardiac cells as well as epithelial cells of inner ear leading to deafness and aberrant repolarization causing prolonged QTc.

Prevalence of arrhythmia-associated gene mutations and risk of sudden cardiac death in the Finnish population

BACKGROUND

Sudden cardiac death (SCD) remains a major cause of death in Western countries. It has a heritable component, but previous molecular studies have mainly focused on common genetic variants. We studied the prevalence, clinical phenotypes, and risk of SCD presented by ten rare mutations previously associated with arrhythmogenic right ventricular cardiomyopathy, long QT syndrome, or catecholaminergic polymorphic ventricular tachycardia.

METHODS

The occurrence of ten arrhythmia-associated mutations was determined in four large prospective population cohorts (FINRISK 1992, 1997, 2002, and Health 2000, n = 28,465) and two series of forensic autopsies (The Helsinki Sudden Death Study and The Tampere Autopsy Study, n = 825). Follow-up data were collected from national registries.

RESULTS

The ten mutations showed a combined prevalence of 79 per 10,000 individuals in Finland, and six of them showed remarkable geographic clustering. Of a total of 715 SCD cases, seven (1.0%) carried one of the ten mutations assayed: three carried KCNH2 R176W, one KCNH2 L552S, two PKP2 Q59L, and one RYR2 R3570W.

CONCLUSIONS

Arrhythmia-associated mutations are prevalent in the general Finnish population but do not seem to present a major risk factor for SCD, at least during a mean of 10-year follow-up of a random adult population sample.

Dominant-negative control of cAMP-dependent IKs upregulation in human long-QT syndrome type 1

RATIONALE

The mutation A341V in the S6 transmembrane segment of KCNQ1, the α-subunit of the slowly activating delayed-rectifier K(+) (I(Ks)) channel, predisposes to a severe long-QT1 syndrome with sympathetic-triggered ventricular tachyarrhythmias and sudden cardiac death.

OBJECTIVE

Several genetic risk modifiers have been identified in A341V patients, but the molecular mechanisms underlying the pronounced repolarization phenotype, particularly during β-adrenergic receptor stimulation, remain unclear. We aimed to elucidate these mechanisms and provide new insights into control of cAMP-dependent modulation of I(Ks).

METHODS AND RESULTS

We characterized the effects of A341V on the I(Ks) macromolecular channel complex in transfected Chinese hamster ovary cells and found a dominant-negative suppression of cAMP-dependent Yotiao-mediated I(Ks) upregulation on top of a dominant-negative reduction in basal current. Phosphomimetic substitution of the N-terminal position S27 with aspartic acid rescued this loss of upregulation. Western blot analysis showed reduced phosphorylation of KCNQ1 at S27, even for heterozygous A341V, suggesting that phosphorylation defects in some (mutant) KCNQ1 subunits can completely suppress I(Ks) upregulation. Functional analyses of heterozygous KCNQ1 WT:G589D and heterozygous KCNQ1 WT:S27A, a phosphorylation-inert substitution, also showed such suppression. Immunoprecipitation of Yotiao with KCNQ1-A341V (in the presence of KCNE1) was not different from wild-type.

CONCLUSIONS

Our results indicate the involvement of the KCNQ1-S6 region at/or around A341 in cAMP-dependent stimulation of I(Ks), a process that is under strong dominant-negative control, suggesting that tetrameric KCNQ1 phosphorylation is required. Specific long-QT1 mutations, including heterozygous A341V, disable this regulation.

Beyond membrane channelopathies: alternative mechanisms underlying complex human disease

Over the past fifteen years, our understanding of the molecular mechanisms underlying human disease has flourished in large part due to the discovery of gene mutations linked with membrane ion channels and transporters. In fact, ion channel defects ("channelopathies" - the focus of this review series) have been associated with a spectrum of serious human disease phenotypes including cystic fibrosis, cardiac arrhythmia, diabetes, skeletal muscle defects, and neurological disorders. However, we now know that human disease, particularly excitable cell disease, may be caused by defects in non-ion channel polypeptides including in cellular components residing well beneath the plasma membrane. For example, over the past few years, a new class of potentially fatal cardiac arrhythmias has been linked with cytoplasmic proteins that include sub-membrane adapters such as ankyrin-B (ANK2), ankyrin-G (ANK3), and alpha-1 syntrophin, membrane coat proteins including caveolin-3 (CAV3), signaling platforms including yotiao (AKAP9), and cardiac enzymes (GPD1L). The focus of this review is to detail the exciting role of lamins, yet another class of gene products that have provided elegant new insight into human disease.

KCNE1 D85N polymorphism--a sex-specific modifier in type 1 long QT syndrome?

Background

Long QT syndrome (LQTS) is an inherited ion channel disorder manifesting with prolongation of the cardiac repolarization phase and severe ventricular arrhythmias. The common KCNE1 D85N potassium channel variant prolongs QT interval by inhibiting I_Ks (KCNQ1) and I_Kr (KCNH2) currents and is therefore a suitable candidate for a modifier gene in LQTS.

Methods

We studied the effect of D85N on age-, sex-, and heart rate-adjusted QT-interval duration by linear regression in LQTS patients carrying the Finnish founder mutations KCNQ1 G589D (n = 492), KCNQ1 IVS7-2A>G (n = 66), KCNH2 L552S (n = 73), and KCNH2 R176W (n = 88). We also investigated the association between D85N and clinical variables reflecting the severity of the disease.

Results

D85N was associated with a QT prolongation by 26 ms (SE 8.6, p = 0.003) in males with KCNQ1 G589D (n = 213), but not in females with G589D (n = 279). In linear regression, the interaction between D85N genotype and sex was significant (p = 0.028). Within the KCNQ1 G589D mutation group, KCNE1 D85N carriers were more often probands of the family (p = 0.042) and were more likely to use beta blocker medication (p = 0.010) than non-carriers. The number of D85N carriers in other founder mutation groups was too small to assess its effects.

Conclusions

We propose that KCNE1 D85N is a sex-specific QT-interval modifier in type 1 LQTS and may also associate with increased severity of disease. Our data warrant additional studies on the role of KCNE1 D85N in other genetically homogeneous groups of LQTS patients.

Comparison of QT peak and QT end interval responses to autonomic adaptation in asymptomatic LQT1 mutation carriers

Background

LQT1 subtype of long QT syndrome is characterized by defective I_Ks, which is intrinsically stronger in the epicardium than in the midmyocardial region. Electrocardiographic QT peak and QT end intervals may reflect complete repolarization of epicardium and midmyocardial region of the ventricular wall, respectively. Repolarization abnormalities in LQT1 carriers may therefore be more easily detected in the QT peak intervals.

Methods

Asymptomatic KCNQ1 mutation carriers (LQT1, n = 9) and unaffected healthy controls (n = 8) were studied during Valsalva manoeuvre, mental stress, handgrip and supine exercise. Global QT peak and QT end intervals derived from 25 simultaneous electrocardiographic leads were measured beat to beat with an automated method.

Results

In unaffected subjects, the percentage shortening of QT peak was greater than that of QT end during mental stress and during the recovery phases of Valsalva and supine exercise. In LQT1 carriers, the percentage shortening of the intervals was similar. At the beginning of Valsalva strain under abrupt endogenous sympathetic activation, QT peak shortened in LQT1 but not in control patients yielding increased electrocardiographic transmural dispersion of repolarization in LQT1.

Conclusions

In asymptomatic KCNQ1 mutation carriers, repolarization abnormalities are more evident in the QT peak than in the QT end interval during adrenergic adaptation, possibly related to transmural differences in the degree of I_Ks block.

Defining a new paradigm for human arrhythmia syndromes: phenotypic manifestations of gene mutations in ion channel- and transporter-associated proteins

Over the past 15 years, gene mutations in cardiac ion channels have been linked to a host of potentially fatal human arrhythmias including long QT syndrome, short QT syndrome, Brugada syndrome, and catecholaminergic polymorphic ventricular tachycardia. More recently, a new paradigm for human arrhythmia has emerged based on gene mutations that affect the activity of cardiac ion channel- and transporter- associated proteins. As part of the Circulation Research thematic series on inherited arrhythmias, this review focuses on the emerging field of human arrhythmias caused by dysfunction in cytosolic gene products (including ankyrins, yotiao, syntrophin, and caveolin-3) that regulate the activities of key membrane ion channels and transporters.

Gene variants predisposing to SIDS: current knowledge

Genetic risk factors play a role in sudden unexpected infant death; either as a cause of death, such as in cases with medium-chain acyl-coenzyme A dehydrogenase deficiency and cardiac arrest due to long QT syndrome, or as predisposing factors for sudden infant death syndrome (SIDS). Most likely genetic predisposition to SIDS represent a polygenic inheritance pattern leading to sudden death when combined with other risk factors, such as a vulnerable developmental stage of the central nervous system and/or the immune system, in addition to environmental risk factors, such as a common cold or prone sleeping position. Genes involved in the regulation of the immune system, cardiac function, the serotonergic network and brain function and development have so far emerged as the most important with respect to SIDS. The purpose of the present paper is to survey current knowledge on SIDS and possible genetic contributions.

Novel insights into the role of the sympathetic nervous system in cardiac arrhythmogenesis

It has long been recognized that increased sympathetic nerve activity during physiologic stress (exercise, swimming, emotion, arousal, loud noise, etc.) has profound influences on the electrical and contractile functions of the heart. In the severely predisposed heart, these stressors may lead to ventricular tachyarrhythmias and sudden death. Still little is known about the temporal relationship between instantaneous autonomic nerve activity and arrhythmias. There is a large variety of autonomically-driven arrhythmias, from serious ventricular tachycardia in pathological conditions to single supraventricular and ventricular extrasystolic beats in the healthy heart. The latter are considered harmless if occurring at low frequency. In the atria, mounting data indicate the presence of a sophisticated network of ganglionated plexi with major influences on cardiac function. The ablation of multiple such ganglia can suppress pulmonary vein potentials and atrial fibrillation. At the cellular level, recent studies have focused on the spatiotemporal details of cyclic nucleotide signaling influencing ion channel function during neurohumoral stimulation. We have come to understand that sarcolemmal ion channels and other electrogenic transporters are macromolecular complexes that interact with structural elements (other than the phospholipid bilayer) to promote regionalization and targeting by regulatory proteins. Compartmentation of these regulatory proteins in subdomains of the myocyte is increasingly recognized and thought to segregate the functional (including electrogenic) responses induced by different neuromediators and hormones. In this article, contemporary issues are discussed regarding arrhythmias that are triggered by influences from the neurocardiac interface, covering the field from the molecular genetic to the intact integrated level. Actual questions are listed per topic, and viewpoints are expressed.

Contribution of long-QT syndrome genetic variants in sudden infant death syndrome

A cohort of 52 French unrelated infant cases who died unexpectedly before they reached 12 months of age was blindly investigated to better quantify the contribution of long-QT syndrome (LQTS) genetic variants in French cases of sudden infant death syndrome (SIDS). After a standardized autopsy protocol, a blinded molecular screening of the KCNQ1, KCNH2, SCN5A, KCNE1, and KCNE2 genes was performed on each case. These postmortem investigations enabled us to reclassify 18 as non-SIDS cases, 32 as SIDS cases, and 2 as suspected SIDS cases. Among the 18 non-SIDS cases, no LQTS mutation was identified. In contrast, our results led to a possible explanation for the death of at least three infants in the SIDS cohort. Half of the LQTS gene variants identified were located on the SCN5A gene. This study confirms that LQTS mutations may represent one of the leading genetic causes of SIDS. If autopsy fails to provide an explanation for an unexplained infant death, medicolegal investigation should be extended with a molecular screening of major LQTS genes. Identification of more LQTS mutations in SIDS cases could provide new insights into the pathophysiology of SIDS and, consequently, reduce the number of unexplained sudden infant deaths.

Myotonic dystrophy type 2 found in two of sixty-three persons diagnosed as having fibromyalgia

Because of its high prevalence, fibromyalgia (FM) is a major general health issue. Myotonic dystrophy type 2 (DM2) is a recently described autosomal-dominant multisystem disorder. Besides variable proximal muscle weakness, myotonia, and precocious cataracts, muscle pain and stiffness are prominent presenting features of DM2. After noting that several of our mutation-positive DM2 patients had a previous diagnosis of FM, suggesting that DM2 may be misdiagnosed as FM, we invited 90 randomly selected patients diagnosed as having FM to undergo genetic testing for DM2. Of the 63 patients who agreed to participate, 2 (3.2%) tested positive for the DM2 mutation. Their cases are described herein. DM2 was not found in any of 200 asymptomatic controls. We therefore suggest that the presence of DM2 should be investigated in a large sample of subjects diagnosed as having FM, and clinicians should be aware of overlap in the clinical presentation of these 2 distinct disorders.

High prevalence of four long QT syndrome founder mutations in the Finnish population

AIMS

Long QT syndrome (LQTS) is an inherited arrhythmia disorder with an estimated prevalence of 0.01%-0.05%. In Finland, four founder mutations constitute up to 70% of the known genetic spectrum of LQTS. In the present survey, we sought to estimate the actual prevalence of the founder mutations and to determine their effect sizes in the general Finnish population.

METHODS AND RESULTS

We genotyped 6334 subjects aged > or =30 years from a population cohort (Health 2000 study) for the four Finnish founder mutations using Sequenom MALDI-TOF mass spectrometry. The electrocardiogram (ECG) parameters were measured from digital 12-lead ECGs, and QT intervals were adjusted for age, sex, and heart rate using linear regression. A total of 27 individuals carried one of the founder mutations resulting in their collective prevalence estimate of 0.4% (95% CI 0.3%-0.6%). The KCNQ1 G589D mutation (n=8) was associated with a 50 ms (SE 7.0) prolongation of the adjusted QT interval (P=9.0x10(-13)). The KCNH2 R176W variant (n=16) resulted in a 22 ms (SE 4.7) longer adjusted QT interval (P=2.1x10(-6)).

CONCLUSION

In Finland 1 individual out of 250 carries a LQTS founder mutation, which is the highest documented prevalence of LQTS mutations that lead to a marked QT prolongation.

A KCNQ1 V205M missense mutation causes a high rate of long QT syndrome in a First Nations community of northern British Columbia: a community-based approach to understanding the impact

PURPOSE

Hereditary long QT syndrome is named for a prolonged QT interval reflecting predisposition to ventricular arrhythmias and sudden death. A high rate in a remote, northern Canadian First Nations community was brought to attention.

METHODS

Two severely affected index cases and 122 relatives were ascertained using community-based participatory research principles. Genetic sequencing of five known genes responsible for long QT syndrome was carried out on the index cases, leading to the identification of a novel missense mutation. Functional properties of the identified mutation were studied in transfected mouse ltk- cells using whole cell patch clamp techniques. Corrected QT interval measurements were obtained from participants and subsequent genotyping of relatives was carried out.

RESULTS

In the two index cases, a novel missense mutation (V205M) was identified in the S3 transmembrane helix of KvLQT1, the pore forming domain of the IKs channel complex. In transfected mouse ltk-cells the V205M mutation suppressed IKs by causing a dramatic depolarizing shift in activation voltage coupled with acceleration of channel deactivation. Twenty-two mutation carriers had a significantly higher mean corrected QT interval than noncarriers (465 +/- 28 milliseconds vs. 434 +/- 26 milliseconds, P < 0.0001); however, 30% of carriers had a corrected QT interval below 440 milliseconds.

CONCLUSION

A novel KCNQ1 mutation in this founder population likely confers increased susceptibility to arrhythmias because of decreased IKs current. Even with a common mutation within a relatively homogenous population, clinical expression remains variable, exemplifying the multifactorial nature of long QT syndrome, and supporting the difficulty of definitive diagnosis without genetic testing. A community participatory approach enabled a comprehensive evaluation of the impact.

Electrocardiographic interventricular dispersion of repolarization during autonomic adaptation in LQT1 subtype of long QT syndrome

OBJECTIVES

In LQT1 subtype of inherited long QT syndrome, repolarization abnormalities originating from defective I(Ks) render patients vulnerable to ventricular arrhythmia during sudden sympathetic activation. Experimental studies show lower I(Ks) density and longer action potential duration in left (LV) than in right (RV) ventricle. We studied interventricular dispersion of repolarization in patients with I(Ks) defect during autonomic tests.

DESIGN

We measured interventricular (difference of QT intervals between LV and RV type leads) and transmural electrocardiographic dispersion of repolarization from 25-lead electrocardiograms in nine asymptomatic KCNQ1 mutation carriers (LQT1) and eight controls during rest, Valsalva maneuver, mental stress, sustained handgrip and supine exercise.

RESULTS

LQT1 carriers showed increased interventricular dispersion of repolarization (13+/-9 ms vs. 4+/-4 ms, p=0.03) during all tests. Valsalva strain increased the difference between the study groups. In LQT1 carriers, interventricular dispersion of repolarization correlated weakly with electrocardiographic transmural dispersion of repolarization.

CONCLUSIONS

Asymptomatic KCNQ1 mutation carriers exhibit increased and by abrupt sympathetic activation augmented interventricular difference in electrocardiographic repolarization times. Interventricular and transmural repolarization dispersion behave similarly in patients with I(Ks) defect.

Congenital long QT syndrome

Congenital long QT syndrome (LQTS) is a hereditary cardiac disease characterized by a prolongation of the QT interval at basal ECG and by a high risk of life-threatening arrhythmias. Disease prevalence is estimated at close to 1 in 2,500 live births. The two cardinal manifestations of LQTS are syncopal episodes, that may lead to cardiac arrest and sudden cardiac death, and electrocardiographic abnormalities, including prolongation of the QT interval and T wave abnormalities. The genetic basis of the disease was identified in the mid-nineties and all the LQTS genes identified so far encode cardiac ion channel subunits or proteins involved in modulating ionic currents. Mutations in these genes (KCNQ1, KCNH2, KCNE1, KCNE2, CACNA1c, CAV3, SCN5A, SCN4B) cause the disease by prolonging the duration of the action potential. The most prevalent LQTS variant (LQT1) is caused by mutations in the KCNQ1 gene, with approximately half of the genotyped patients carrying KCNQ1 mutations. Given the characteristic features of LQTS, the typical cases present no diagnostic difficulties for physicians aware of the disease. However, borderline cases are more complex and require the evaluation of various electrocardiographic, clinical, and familial findings, as proposed in specific diagnostic criteria. Additionally, molecular screening is now part of the diagnostic process. Treatment should always begin with beta-blockers, unless there are valid contraindications. If the patient has one more syncope despite a full dose beta-blockade, left cardiac sympathetic denervation (LCSD) should be performed without hesitation and implantable cardioverter defibrillator (ICD) therapy should be considered with the final decision being based on the individual patient characteristics (age, sex, clinical history, genetic subgroup including mutation-specific features in some cases, presence of ECG signs - including 24-hour Holter recordings - indicating high electrical instability). The prognosis of the disease is usually good in patients that are correctly diagnosed and treated. However, there are a few exceptions: patients with Timothy syndrome, patients with Jervell Lange-Nielsen syndrome carrying KCNQ1 mutations and LQT3 patients with 2:1 atrio-ventricular block and very early occurrence of cardiac arrhythmias.

Identification of a novel KCNQ1 mutation associated with both Jervell and Lange-Nielsen and Romano-Ward forms of long QT syndrome in a Chinese family

BACKGROUND

Long QT syndrome (LQTS) is a cardiac disorder characterized by prolonged QT intervals on electrocardiograms (ECG), ventricular arrhythmias, and sudden death. Clinically, two inherited forms of LQTS have been defined: autosomal dominant LQTS or Romano-Ward syndrome (RWS) not associated with deafness and autosomal recessive LQTS or Jervell and Lange-Nielsen syndrome (JLNS) associated with deafness.

METHODS

A Chinese family with both RWS and JLNS was identified. Family members were diagnosed based on the presence of a prolonged QT interval as seen on a 12-lead ECG and a medical history of syncope, palpitation, and deafness. Mutational studies in the KCNQ1 potassium channel gene were performed using direct DNA sequence analysis and restriction length polymorphism analysis.

RESULTS

The proband in the Chinese family and her brother had previously been diagnosed with JLNS, and two other members were affected with RWS. The proband was also affected with atrial fibrillation. A single nucleotide substitution of C to T at nucleotide 965 of KCNQ1 was identified, and the mutation resulted in the substitution of a threonine residue at codon 322 by a methionine residue (T322M). The novel heterozygous T322M mutation was identified in two patients with RWS, one member with borderline QTc, and two normal family members. The two JLNS patients in the family carried the homozygous T322M mutation. The T322M mutation was not found in 200 Chinese normal controls.

CONCLUSION

Our results suggest that T322M is a novel mutation that caused RWS with high intrafamilial variability in the heterozygous carriers and typical JLNS in the homozygous carriers within this Chinese family. The T322M mutation is the first mutation identified for JLNS in the Chinese population.

Mechanisms of human arrhythmia syndromes: abnormal cardiac macromolecular interactions

Many cardiac ion channels exist within macromolecular signaling complexes, comprised of pore-forming subunits that associate with auxiliary subunits, regulatory enzymes, and targeting proteins. This complex protein assembly ensures proper modulation of channel activity and ion homeostasis. The association of genetic defects in regulatory and targeting proteins to inherited arrhythmia syndromes has led to a better understanding of the critical role these proteins play in ion channel modulation.

Compartmentalized regulations of ion channels in the heart

The rate and force of contraction of the heart are precisely controlled by compartmentalized regulation of cardiac ion channels which determine electrical activities. It is known that modulation of cardiac ion channels, which is caused by drug administration, sympathetic nervous system stimulation and gender difference, can increase risks of lethal arrhythmias in carriers of inherited disease mutations. These modulations are thought to also be involved in common cardiac arrhythmias. Because many signaling molecules are localized within single cells, an understanding of the molecular basis of compartmentalized regulation of cardiac channels is a key for understanding and treating the lethal arrhythmias. In this review, I will discuss molecular mechanisms of compartmentalized regulation of cardiac ion channels via drugs, cAMP and sex hormones.

Sudden cardiac death: prevalence, pathogenesis, and prevention

Sudden cardiac death (SCD), also known as sudden arrest, is a major health problem worldwide. It is usually defined as an unexpected death from a cardiac cause occurring within a short time in a person with or without preexisting heart disease. The pathogenesis of SCD is complex and multifaceted. A dynamic triggering factor usually interacts with an underlying heart disease, either genetically determined or acquired, and the final outcome is the development of lethal tachyarrhythmias or, less frequently, bradycardia. It has increasingly been highlighted that a reliable clinical and diagnostic approach might be effective to unmask the most important genetic and environmental factors, allowing the construction of a rational personalized medicine framework that can be applied in both the preclinical and clinical settings of SCD. The aim of the present article is to provide a concise overview of prevalence, pathogenesis, clinical presentation, and diagnostic approach to this challenging disorder.