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.

Antiarrhythmic Action of Flecainide in Polymorphic Ventricular Arrhythmias Caused by a Gain-of-Function Mutation in the Nav 1.5 Sodium Channel

BACKGROUND

The cardiac sodium channel Nav 1.5, encoded by the gene SCN5A, is associated with a wide spectrum of hereditary arrhythmias. The gain-of-function mutation p.I141V in SCN5A was identified in a large multigenerational family with exercise-induced polymorphic ventricular arrhythmias. The purpose of this study was to evaluate the molecular and clinical effects of flecainide administration on patients with this syndrome.

METHODS

Eleven p.I141V carriers who exhibited frequent multiformic premature ventricular complexes (PVCs) during exercise were subjected to exercise stress tests, both before and after intravenous infusion of 2 mg/kg flecainide. The in vitro effects of flecainide were evaluated using the patch-clamp technique with HEK293 cells expressing the Nav 1.5 channel.

RESULTS

The flecainide treatment significantly reduced the frequency of PVCs during and after exercise. Next, the sensitivity of the p.I141V mutant channel to flecainide was compared to that of the wild type channel. Perfusion of flecainide inhibited the peak and window currents in both groups.

CONCLUSION

The clinical investigations of the affected patients, as well as the molecular and pharmacological characterization of the SCN5A p.I141V mutation, provide new evidence supporting the association of this mutation with exercise-induced polymorphic ventricular arrhythmias. These data also demonstrate that flecainide may serve as an effective treatment for the defect in Nav 1.5 that leads to an increased sodium window current.

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.

Antiarrhythmic Effects of Dantrolene in Patients with Catecholaminergic Polymorphic Ventricular Tachycardia and Replication of the Responses Using iPSC Models

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is a highly malignant inherited arrhythmogenic disorder. Type 1 CPVT (CPVT1) is caused by cardiac ryanodine receptor (RyR2) gene mutations resulting in abnormal calcium release from sarcoplasmic reticulum. Dantrolene, an inhibitor of sarcoplasmic Ca²⁺ release, has been shown to rescue this abnormal Ca²⁺ release in vitro. We assessed the antiarrhythmic efficacy of dantrolene in six patients carrying various RyR2 mutations causing CPVT. The patients underwent exercise stress test before and after dantrolene infusion. Dantrolene reduced the number of premature ventricular complexes (PVCs) on average by 74% (range 33-97) in four patients with N-terminal or central mutations in the cytosolic region of the RyR2 protein, while dantrolene had no effect in two patients with mutations in or near the transmembrane domain. Induced pluripotent stem cells (iPSCs) were generated from all the patients and differentiated into spontaneously beating cardiomyocytes (CMs). The antiarrhythmic effect of dantrolene was studied in CMs after adrenaline stimulation by Ca²⁺ imaging. In iPSC derived CMs with RyR2 mutations in the N-terminal or central region, dantrolene suppressed the Ca²⁺ cycling abnormalities in 80% (range 65-97) of cells while with mutations in or near the transmembrane domain only in 23 or 32% of cells. In conclusion, we demonstrate that dantrolene given intravenously shows antiarrhythmic effects in a portion of CPVT1 patients and that iPSC derived CM models replicate these individual drug responses. These findings illustrate the potential of iPSC models to individualize drug therapy of inherited diseases.

Gain-of-Function Mutation of the SCN5A Gene Causes Exercise-Induced Polymorphic Ventricular Arrhythmias

Background—

Over the past 15 years, a myriad of mutations in genes encoding cardiac ion channels and ion channel interacting proteins have been linked to a long list of inherited atrial and ventricular arrhythmias. The purpose of this study was to identify the genetic and functional determinants underlying exercise-induced polymorphic ventricular arrhythmia present in a large multigenerational family.

Methods and Results—

A large 4-generation family presenting with exercise-induced polymorphic ventricular arrhythmia, which was followed for 10 years, was clinically characterized. A novel SCN5A mutation was identified via whole exome sequencing and further functionally evaluated by patch-clamp studies using human embryonic kidney 293 cells. Of 37 living family members, a total of 13 individuals demonstrated ≥50 multiformic premature ventricular complexes or ventricular tachycardia upon exercise stress tests when sinus rate exceeded 99±17 beats per minute. Sudden cardiac arrest occurred in 1 individual during follow-up. Exome sequencing identified a novel missense mutation (p.I141V) in a highly conserved region of the SCN5A gene, encoding the Na v 1.5 sodium channel protein that cosegregated with the arrhythmia phenotype. The mutation p.I141V shifted the activation curve toward more negative potentials and increased the window current, whereas action potential simulations suggested that it lowered the excitability threshold of cardiac cells.

Conclusions—

Gain-of-function of Na v 1.5 may cause familial forms of exercise-induced polymorphic ventricular arrhythmias.

Genetic association study of QT interval highlights role for calcium signaling pathways in myocardial repolarization

The QT interval, an electrocardiographic measure reflecting myocardial repolarization, is a heritable trait. QT prolongation is a risk factor for ventricular arrhythmias and sudden cardiac death (SCD) and could indicate the presence of the potentially lethal Mendelian Long QT Syndrome (LQTS). Using a genome-wide association and replication study in up to 100,000 individuals we identified 35 common variant QT interval loci, that collectively explain ∼8-10% of QT variation and highlight the importance of calcium regulation in myocardial repolarization. Rare variant analysis of 6 novel QT loci in 298 unrelated LQTS probands identified coding variants not found in controls but of uncertain causality and therefore requiring validation. Several newly identified loci encode for proteins that physically interact with other recognized repolarization proteins. Our integration of common variant association, expression and orthogonal protein-protein interaction screens provides new insights into cardiac electrophysiology and identifies novel candidate genes for ventricular arrhythmias, LQTS,and SCD.

Identification of a KCNQ1 polymorphism acting as a protective modifier against arrhythmic risk in long-QT syndrome

BACKGROUND

Long-QT syndrome (LQTS) is characterized by such striking clinical heterogeneity that, even among family members carrying the same mutation, clinical outcome can range between sudden death and no symptoms. We investigated the role of genetic variants as modifiers of risk for cardiac events in patients with LQTS.

METHODS AND RESULTS

In a matched case-control study including 112 patient duos with LQTS from France, Italy, and Japan, 25 polymorphisms were genotyped based on either their association with QTc duration in healthy populations or on their role in adrenergic responses. The duos were composed of 2 relatives harboring the same heterozygous KCNQ1 or KCNH2 mutation: 1 with cardiac events and 1 asymptomatic and untreated. The findings were then validated in 2 independent founder populations totaling 174 symptomatic and 162 asymptomatic patients with LQTS, and a meta-analysis was performed. The KCNQ1 rs2074238 T-allele was significantly associated with a decreased risk of symptoms 0.34 (0.19-0.61; P<0.0002) and with shorter QTc (P<0.0001) in the combined discovery and replication cohorts.

CONCLUSIONS

We provide evidence that the KCNQ1 rs2074238 polymorphism is an independent risk modifier with the minor T-allele conferring protection against cardiac events in patients with LQTS. This finding is a step toward a novel approach for risk stratification in patients with LQTS.

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.

Cell model of catecholaminergic polymorphic ventricular tachycardia reveals early and delayed afterdepolarizations

Background

Induced pluripotent stem cells (iPSC) provide means to study the pathophysiology of genetic disorders. Catecholaminergic polymorphic ventricular tachycardia (CPVT) is a malignant inherited ion channel disorder predominantly caused by mutations in the cardiac ryanodine receptor (RyR2). In this study the cellular characteristics of CPVT are investigated and whether the electrophysiological features of this mutation can be mimicked using iPSC -derived cardiomyocytes (CM).

Methodology/Principal Findings

Spontaneously beating CMs were differentiated from iPSCs derived from a CPVT patient carrying a P2328S mutation in RyR2 and from two healthy controls. Calcium (Ca²⁺) cycling and electrophysiological properties were studied by Ca²⁺ imaging and patch-clamp techniques. Monophasic action potential (MAP) recordings and 24h-ECGs of CPVT-P2328S patients were analyzed for the presence of afterdepolarizations. We found defects in Ca²⁺ cycling and electrophysiology in CPVT CMs, reflecting the cardiac phenotype observed in the patients. Catecholaminergic stress led to abnormal Ca²⁺ signaling and induced arrhythmias in CPVT CMs. CPVT CMs also displayed reduced sarcoplasmic reticulum (SR) Ca²⁺ content, indicating leakage of Ca²⁺ from the SR. Patch-clamp recordings of CPVT CMs revealed both delayed afterdepolarizations (DADs) during spontaneous beating and in response to adrenaline and also early afterdepolarizations (EADs) during spontaneous beating, recapitulating the changes seen in MAP and 24h-ECG recordings of patients carrying the same mutation.

Conclusions/Significance

This cell model shows aberrant Ca²⁺ cycling characteristic of CPVT and in addition to DADs it displays EADs. This cell model for CPVT provides a platform to study basic pathology, to screen drugs, and to optimize drug therapy.

Common genetic variants, QT interval, and sudden cardiac death in a Finnish population-based study

BACKGROUND

Although sudden cardiac death (SCD) is heritable, its genetic underpinnings are poorly characterized. The QT interval appears to have a graded relationship to SCD, and 35% to 45% of its variation is heritable. We examined the relationship among recently reported common genetic variants, QT interval, and SCD.

METHODS AND RESULTS

We genotyped 15 common (minor allele frequency >1%) candidate single nucleotide polymorphisms (SNPs), based on association with the QT interval in prior studies, in individuals in 2 cohort studies (Health 2000, n = 6597; Mini-Finland, n = 801). After exclusions, we identified 116 incident SCDs from the remaining sample (n = 6808). We constructed a QT genotype score (QT(score)) using the allele copy number and previously reported effect estimates for each SNP. Cox proportional hazards models adjusting for age, sex, and geographical area were used for time to SCD analyses. The QT(score) was a continuous independent predictor of the heart rate-corrected QT interval (P<10(-107)). Comparing the top with the bottom quintile of QT(score), there was a 15.6-ms higher group mean QT interval (P<10(-84)). A 10-ms increase in the observed QT interval was associated with an increased risk of SCD (hazard ratio, 1.19; 95% confidence interval, 1.07 to 1.32; P = 0.002). There was no linear relationship between QT(score) and SCD risk; although in post hoc secondary analysis there was increased risk in the top compared with the middle QT(score) quintile (hazard ratio, 1.92; 95% confidence interval, 1.05 to 3.58; P = 0.04).

CONCLUSIONS

Our study strongly replicates the relationship between common genetic variants and the QT interval and confirms the relationship between the QT interval and SCD but does not show evidence for a linear relationship between QT(score) and SCD risk.

Population-prevalent desmosomal mutations predisposing to arrhythmogenic right ventricular cardiomyopathy

BACKGROUND

Arrhythmogenic right ventricular cardiomyopathy (ARVC) is a progressive myocardial disorder caused by mutations of desmosomal cell adhesion proteins. The prevalence of these variants in the general population is unknown.

OBJECTIVE

This study examined the spectrum and population prevalence of desmosomal mutations predisposing to ARVC in Finland.

METHODS

We screened 29 Finnish ARVC probands for mutations in the DSP, DSG2, and DSC2 genes. All Finnish-type ARVC-associated mutations, including those 3 previously identified in PKP2 in the same patient group, were analyzed in the population-based Health 2000 cohort of 6,334 individuals and tested for association with electrocardiographic variables.

RESULTS

We detected 2 novel mutations: DSG2 3059_3062delAGAG and DSP T1373A. DSG2 3059_3062delAGAG was present in a family with 5 mutation carriers. The endomyocardial samples of the DSG2 deletion carrier showed reduced immunoreactive signal for desmoglein-2, plakophilin-2, plakoglobin, and desmoplakin. DSP T1373A was found in 1 proband with typical right ventricular disease and exercise-related ventricular tachycardia. In the population sample, the collective prevalence of all 5 mutations identified in the 29 ARVC patients (PKP2 Q62K, Q59L, N613K, DSG2 3059_3062delAGAG, and DSP T1373A) was 31 of 6,334 individuals, or 0.5%. The apparent founder mutation PKP2 Q59L is present in 0.3% of Finns and was previously shown to have an approximately 20% disease penetrance.

CONCLUSION

One of 200 Finns carries a desmosomal mutation that may predispose to ARVC and its clinical sequelae. ARVC-associated mutations may thus be more prevalent in the population than expected based on the published ARVC prevalence data.

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.

Search for cardiac calcium cycling gene mutations in familial ventricular arrhythmias resembling catecholaminergic polymorphic ventricular tachycardia

Background

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is a severe inherited cardiac disorder caused by mutations predominantly in the ryanodine receptor (RyR2) gene. We sought to identify mutations in genes affecting cardiac calcium cycling in patients with CPVT and in less typical familial exercise-related ventricular arrhythmias.

Methods and Results

We recruited 33 consecutive patients with frequent ventricular premature complexes (VPCs) without structural heart disease and often history of syncope or sudden death in family. Sixteen of the patients featured a phenotype typical of CPVT. In 17 patients, VPCs emerged also at rest. Exercise stress test and echocardiography were performed to each patient and 232 family members. Familial background was evident in 42% of cases (n = 14). We sequenced all the coding exons of the RyR2, FKBP1B, ATP2A2 and SLC8A1 genes from the index patients. Single channel recordings of a mutant RyR2 were performed in planar lipid bilayers. Two novel RyR2 missense mutations (R1051P and S616L) and two RyR2 exon 3 deletions were identified, explaining 25% of the CPVT phenotypes. A rare variant (N3308S) with open probabilities similar to the wild type channels in vitro, was evident in a patient with resting VPCs. No disease-causing variants were detectable in the FKBP1B, ATP2A2 or SLC8A1 genes.

Conclusion

We report two novel CPVT-causing RyR2 mutations and a novel RyR2 variant of uncertain clinical significance in a patient with abundant resting VPCs. Our data also strengthen the previous assumption that exon 3 deletions of RyR2 should screened for in CPVT and related phenotypes.

Plakophilin-2 missense mutations in arrhythmogenic right ventricular cardiomyopathy

BACKGROUND

Arrhythmogenic right ventricular dysplasia/cardiomyopathy (ARVD/C) is an inherited cardiac disorder characterized by life-threatening ventricular arrhythmias and fibrofatty replacement of myocardial tissue. Recent data suggest a dominant mode of inheritance in ARVD due to mutations in desmosomal proteins, plakophilin-2 (PKP2) in particular. We carried out a search for PKP2 mutations in the Finnish population representing a genetic isolate.

METHODS

Mutations were detected by direct sequencing of PKP2 exons in 29 unrelated ARVD patients. Subcellular changes in ARVD associated with PKP2 mutations were searched for using immunohistochemistry and electron microscopy.

RESULTS

We identified three PKP2 amino acid substitutions, absent in controls, in three (10%) cases. Two of them (Q62K and N613K) co-occurred in a patient with arrhythmia and structural changes of the heart. Visualized with plakophilin-2 antibodies, the intercalated disks in this compound heterozygous ARVD sample appeared wavier than in non-ARVD controls. Partial irregularities were occasionally seen in the organization and distribution of the cell-cell junctions. Relatives carrying one of these mutant alleles were phenotypically normal or showed only limited electrocardiographic (ECG) changes. The third substitution (Q59L) was detected in two ARVD probands with ventricular tachycardias, ECG abnormalities and right ventricular structural alterations.

CONCLUSIONS

We identified two novel plakophilin-2 missense mutations associated with 10% of ARVD, and a previously reported Q62K variant with a possible disease modifying role. The low prevalence of predominantly missense mutations may present population-specific differences in the pathogenesis of ARVD. Our preliminary data also suggest that ultrastructural cell junction abnormalities may associate with plakophilin-2 mutations.

Characterization of familial and sporadic arrhythmogenic right ventricular cardiomyopathy in Finland

BACKGROUND

Autosomal dominant inheritance is reported in arrhythmogenic right ventricular cardiomyopathy (ARVC) but the prevalence of the familial and sporadic forms in the general population is unknown.

AIM

To evaluate the familial occurrence and clinical features of ARVC in the genetically homogenous Finnish population.

METHODS

The study included 29 Finnish ARVC index patients and 135 relatives from 21 families evaluated. They underwent echocardiography, 24-hour electrocardiographic monitoring, signal-averaged electrocardiography, and exercise stress test.

RESULTS

Twenty-two index patients had ventricular arrhythmias as first manifestation, and three developed arrhythmias later. The right ventricle (RV) was mildly affected in 22 and strongly dilated in 7 index patients. Patients with dilated RV manifested first symptoms at younger age (mean 28 years) than those without RV dilatation (mean 38 years). Of the 135 relatives, ARVC was present in 12 (9%) patients belonging to 5 of the 21 families studied, resulting in 24% familial involvement. In addition, 46 relatives (34%) had subtle cardiac abnormalities, suggesting subclinical presentation.

CONCLUSIONS

The ARVC in Finland presents with distinct arrhythmic and RV dilative subtypes. The sporadic disease is similar to the familial one which may reflect low penetration in relatives. The proportion of familial manifestation of ARVC in Finland seems comparable to that elsewhere in Europe.