Genetic modulators of the phenotype in the long QT syndrome: state of the art and clinical impact

Identification and characterization of two novel KCNH2 mutations contributing to long QT syndrome

We identified two different inherited mutations in KCNH2 gene, or human ether-a-go-go related gene (hERG), which are linked to Long QT Syndrome. The first mutation was in a 1-day-old infant, whereas the second was in a 14-year-old girl. The two KCNH2 mutations were transiently transfected into either human embryonic kidney (HEK) cells or human induced pluripotent stem-cell derived cardiomyocytes. We performed associated multiscale computer simulations to elucidate the arrhythmogenic potentials of the KCNH2 mutations. Genetic screening of the first and second index patients revealed a heterozygous missense mutation in KCNH2, resulting in an amino acid change (P632L) in the outer loop of the channel and substitution at position 428 from serine to proline (S428P), respectively. Heterologous expression of P632L and S428P into HEK cells produced no hERG current compared to the wild type (WT). Moreover, the co-transfection of WT and P632L yielded no hERG current; however, the co-transfection of WT and S428P yielded partial hERG current. Action potentials were prolonged in a complete or partial blockade of hERG current from computer simulations which was more severe in Purkinje than ventricular myocytes. Three dimensional simulations revealed a higher susceptibility to reentry in the presence of hERG current blockade. Our experimental findings suggest that both P632L and S428P mutations may impair the KCNH2 gene. The Purkinje cells exhibit a more severe phenotype than ventricular myocytes, and the hERG current blockade renders the ventricles an arrhythmogenic substrate from computer modeling.

Clinical Genetics of Inherited Arrhythmogenic Disease in the Pediatric Population

Sudden death is a rare event in the pediatric population but with a social shock due to its presentation as the first symptom in previously healthy children. Comprehensive autopsy in pediatric cases identify an inconclusive cause in 40-50% of cases. In such cases, a diagnosis of sudden arrhythmic death syndrome is suggested as the main potential cause of death. Molecular autopsy identifies nearly 30% of cases under 16 years of age carrying a pathogenic/potentially pathogenic alteration in genes associated with any inherited arrhythmogenic disease. In the last few years, despite the increasing rate of post-mortem genetic diagnosis, many families still remain without a conclusive genetic cause of the unexpected death. Current challenges in genetic diagnosis are the establishment of a correct genotype-phenotype association between genes and inherited arrhythmogenic disease, as well as the classification of variants of uncertain significance. In this review, we provide an update on the state of the art in the genetic diagnosis of inherited arrhythmogenic disease in the pediatric population. We focus on emerging publications on gene curation for genotype-phenotype associations, cases of genetic overlap and advances in the classification of variants of uncertain significance. Our goal is to facilitate the translation of genetic diagnosis to the clinical area, helping risk stratification, treatment and the genetic counselling of families.

Antipsychotics in routine treatment are minor contributors to QT prolongation compared to genetics and age

BACKGROUND

Drug-induced prolongation of cardiac repolarization limits the treatment with many psychotropic drugs. Recently, the contribution of polygenic variation to the individual duration of the QT interval was identified.

AIMS

To explore the interaction between antipsychotic drugs and the individual polygenic influence on the QT interval.

METHODS

Retrospective analysis of clinical and genotype data of 804 psychiatric inpatients diagnosed with a psychotic disorder. The individual polygenic influence on the QT interval was calculated according to the method of Arking et al.

RESULTS

Linear regression modelling showed a significant association of the individual polygenic QT interval score (ß_std = 0.176, p < 0.001) and age (ß_std = 0.139, p < 0.001) with the QTc interval corrected according to Fridericia's formula. Sex showed a nominal trend towards significance (ß_std = 0.064, p = 0.064). No association was observed for the number of QT prolonging drugs according to AZCERT taken. Subsample analysis (n = 588) showed a significant association of potassium serum concentrations with the QTc interval (ß_std = -0.104, p = 0.010). Haloperidol serum concentrations were associated with the QTc interval only in single medication analysis (n = 26, ß_std = 0.101, p = 0.004), but not in multivariate regression analysis. No association was observed for aripiprazole, clozapine, quetiapine and perazine, while olanzapine and the sum of risperidone and its metabolite showed a negative association.

CONCLUSIONS

Individual genetic factors and age are main determinants of the QT interval. Antipsychotic drug serum concentrations within the therapeutic range contribute to QTc prolongation on an individual level.

Discovery of Digenic Mutation, KCNH2 c.1898A >C and JUP c.916dupA, in a Chinese Family with Long QT Syndrome via Whole-Exome Sequencing

Long QT syndrome (LQTS), which is caused by an ion channel‐related gene mutation, is a malignant heart disease with a clinical course of a high incidence of ventricular fibrillation and sudden cardiac death in the young. Mutations in KCNH2 (which encodes potassium voltage-gated channel subfamily H member 2) are responsible for LQTS in many patients. Here we report the novel mutation c.1898A>C in KCNH2 in a Chinese family with LQTS through whole-exome sequencing. The c.916dupA mutation in JUP (which encodes junction plakoglobin) is also discovered. Mutations in JUP were found to be associated with arrhythmogenic right ventricular cardiomyopathy. The double mutation in the proband may help explain his severe clinical manifestations, such as sudden cardiac death at an early age. Sequencing for the proband’s family members revealed that the KCNH2 mutation descends from his paternal line, while the mutation in JUP came from his maternal line. The data provided in this study may help expand the spectrum of LQTS-related KCNH2 mutations and add support to the genetic diagnosis and counseling of families affected by malignant arrhythmias.

Genetic risk stratification in cardiac arrhythmias

PURPOSE OF REVIEW

The current article provides a concise summary of the possibilities and limitations of genotype-based risk stratification of cardiac arrhythmias. We will outline the most important findings of the recent years in the light of their chronological and conceptual development.

RECENT FINDINGS

Genotype-phenotype association studies in families with single-gene disorders as well as in the general population led to the discovery of several DNA variants significantly associated with the risk of sudden death or life-threatening arrhythmias. In genetic (monogenic) diseases, the disease-causing mutations modulate the risk of events and response to antiarrhythmic therapy according to the specific gene involved, to their position of the mutation and to their functional effects. These causal relationships have been quite well characterized in the case of long QT syndrome but are still less defined in the case of other inherited conditions. Quantitatively, the risk associated with a single genetic variant is large for DNA variants that cause monogenic inherited arrhythmias. Much different is the case of more common variants associated with the risk of arrhythmias in the general population as they are generally associated with a small effect size.

SUMMARY

Genetic profiling identifies arrhythmogenic risk even if a complete picture allowing high-granularity risk stratification is yet to come.

Action Potential Prolongation, β-Adrenergic Stimulation, and Angiotensin II as Co-factors in Sarcoplasmic Reticulum Instability

Introduction: Increases in action potential duration (APD), genetic or acquired, and arrhythmias are often associated; nonetheless, the relationship between the two phenomena is inconstant, suggesting coexisting factors. β-adrenergic activation increases sarcoplasmic reticulum (SR) Ca²⁺-content; angiotensin II (ATII) may increase cytosolic Ca²⁺ and ROS production, all actions stimulating RyRs opening. Here we test how APD interacts with β-adrenergic and AT-receptor stimulation in facilitating spontaneous Ca²⁺ release events (SCR).

Methods: Under “action potential (AP) clamp”, guinea-pig cardiomyocytes (CMs) were driven with long (200 ms), normal (150 ms), and short (100 ms) AP waveforms at a CL of 500 ms; in a subset of CMs, all the 3 waveforms could be tested within the same cell. SCR were detected as inward current transients (I_TI) following repolarization; I_TI incidence and repetition within the same cycle were measured under increasing isoprenaline concentration ([ISO]) alone, or plus 100 nM ATII (30 min incubation+superfusion).

Results: I_TI incidence and repetition increased with [ISO]; at longer APs the [ISO]-response curve was shifted upward and I_TI coupling interval was reduced. ATII increased I_TI incidence more at low [ISO] and under normal (as compared to long) APs. Efficacy of AP shortening in suppressing I_TI decreased in ATII-treated myocytes and at higher [ISO].

Conclusions: AP prolongation sensitized the SR to the destabilizing actions of ISO and ATII. Summation of ISO, ATII and AP duration effects had a “saturating” effect on SCR incidence, thus suggesting convergence on a common factor (RyRs stability) “reset” by the occurrence of spontaneous Ca²⁺ release events.

High prevalence of ventricular repolarization abnormalities in people carrying TGFβR2 mutations

Mutations in the TGFβR2 gene have been associated with a life threatening risk of aortic dissection but no arrhythmic death has been previously reported. Two young females carrying a TGFβR2 mutation, initially diagnosed as Marfan syndrome or Loeys Dietz syndrome, presented sudden death with autopsy ruling out dissection. The ECGs of the 2 Sudden Cardiac Deaths revealed profound ventricular repolarization abnormalities with a sinusoidal T-U morphology associated with normal left ventricular ejection fraction. These data strongly suggest sudden cardiac arrhythmic deaths and prompted us to systematically study the repolarization pattern in the patients with TGFβR2 mutations. ECG findings from 58 mutation carriers patients (TGFβR2 group) were compared with those of 46 non-affected first degree relatives (control group). TGFβR2 mutation was associated with ventricular repolarization abnormalities in 47% of patients (p < 0.001 vs. controls), including a 19.6 ms (95%CI 8.7; 30.5) QTc interval prolongation compared to the non-affected first degree relatives (p < 0.001), higher prevalence of abnormal U waves (16% vs. 2%), and sinusoidal T-U morphology (10% vs. 0%). TGFβR2 mutations can be associated with abnormal ventricular repolarization pattern, longer QT interval than non-carrier relatives and an increased risk for sudden death.

Digenic Heterozigosity in SCN5A and CACNA1C Explains the Variable Expressivity of the Long QT Phenotype in a Spanish Family

INTRODUCTION AND OBJECTIVES

A known long QT syndrome-related mutation in Nav1.5 cardiac channels (p.R1644H) was found in 4 members of a Spanish family but only 1 of them showed prolongation of the QT interval. In the other 3 relatives, a novel missense mutation in Cav1.2 cardiac channels was found (p.S1961N). Here, we functionally analyzed p.S1961N Cav1.2 channels to elucidate whether this mutation regulates the expressivity of the long QT syndrome phenotype in this family.

METHODS

L-type calcium current (I_CaL) recordings were performed by using the whole-cell patch-clamp technique in Chinese hamster ovary cells transiently transfected with native and/or p.S1961N Cav1.2 channels.

RESULTS

Expression of p.S1961N channels significantly decreased I_CaL density. Using Ba as a charge carrier to suppress the Ca-dependent inactivation of Cav1.2 channels, we demonstrated that the mutation significantly accelerates the voltage-dependent inactivation of Cav1.2 channels decreasing the inactivation time constant. As a consequence, the total charge flowing through p.S1961N Cav1.2 channels significantly decreased. The effects of the p.S1961N Cav1.2 and p.R1644H Nav1.5 mutations alone or their combination on the action potential (AP) morphology were simulated using a validated model of the human ventricular AP. The p.S1961N Cav1.2 mutation shortens the AP duration and abrogates the prolongation induced by p.R1644H Nav1.5 channels.

CONCLUSIONS

The p.S1961N mutation in Cav1.2 channels decreased the I_CaL, an effect which might shorten ventricular AP. The presence of the loss-of-function Cav1.2 mutation could functionally compensate the prolonging effects produced by the Nav1.5 gain-of-function mutation.

A molecular pathway analysis informs the genetic risk for arrhythmias during antipsychotic treatment

Arrhythmias are a frequent and potentially fatal side effect of antipsychotic treatment. Strict ECG monitoring and clinical interviews are the standards used to prevent arrhythmias. A biologic predictive tool is missing. The identification of a genetic makeup at risk of antipsychotic-induced arrhythmias is the aim of the present investigation. The aim of this study was to identify a molecular pathway enriched in single nucleotide polymorphisms associated with antipsychotic-induced QTc modifications. In total, 661 schizophrenic individuals from the CATIE study, M=486 (73.52%), mean age=40.92±11.02, were included. QTc variation was measured as a phase-specific change-created variable. A nested mixed regression for a repeated-measures model served in R for the analysis of the clinical and treatment-related covariates and molecular pathway analysis. Plink was used for the genetic genome-wide analysis. Quality checking was the standard (genotype call rate>0.95; minor allele frequency>0.01; Hardy-Weinberg equilibrium<0.0001) and the inflation factor was controlled by λ values. Quetiapine and perphenazine were associated with QTc variation during phase 1. No other significant association was detected. No significant inflation was detected. A number of molecular pathways were associated with QT variation at a conservative (adjusted) P value less than 0.05, including pathways related to neuronal wiring and collagen biosynthesis, along with pathways related to K+ currents and cardiac contraction. Pathways related to neuronal wiring, collagen biosynthesis, and ion currents were identified as possibly involved in QTc modifications during antispsychotic treatment in SKZ patients.

Incomplete Penetrance and Variable Expressivity: Hallmarks in Channelopathies Associated with Sudden Cardiac Death

Sudden cardiac death is defined as an unexpected decease of cardiac origin. In individuals under 35 years old, most of these deaths are due to familial arrhythmogenic syndromes of genetic origin, also known as channelopathies. These familial cardiac syndromes commonly follow an autosomal dominant pattern of inheritance. Diagnosis, however, can be difficult, mainly due to incomplete penetrance and variable expressivity, which are hallmarks in these syndromes. The clinical manifestation of these diseases can range from asymptomatic to syncope but sudden death can sometimes be the first symptom of disease. Early identification of at-risk individuals is crucial to prevent a lethal episode. In this review, we will focus on the genetic basis of channelopathies and the effect of genetic and non-genetic modifiers on their phenotypes.

KCNQ1 p.L353L affects splicing and modifies the phenotype in a founder population with long QT syndrome type 1

Background

Variable expressivity and incomplete penetrance between individuals with identical long QT syndrome (LQTS) causative mutations largely remain unexplained. Founder populations provide a unique opportunity to explore modifying genetic effects. We examined the role of a novel synonymous KCNQ1 p.L353L variant on the splicing of exon 8 and on heart rate corrected QT interval (QTc) in a population known to have a pathogenic LQTS type 1 (LQTS1) causative mutation, p.V205M, in KCNQ1-encoded Kv7.1.

Methods

419 adults were genotyped for p.V205M, p.L353L and a previously described QTc modifier (KCNH2-p.K897T). Adjusted linear regression determined the effect of each variant on QTc, alone and in combination. In addition, peripheral blood RNA was extracted from three controls and three p.L353L-positive individuals. The mutant transcript levels were assessed via qPCR and normalised to overall KCNQ1 transcript levels to assess the effect on splicing.

Results

For women and men, respectively, p.L353L alone conferred a 10.0 (p=0.064) ms and 14.0 (p=0.014) ms increase in QTc and in men only a significant interaction effect in combination with the p.V205M (34.6 ms, p=0.003) resulting in a QTc of ∼500 ms. The mechanism of p.L353L's effect was attributed to approximately threefold increase in exon 8 exclusion resulting in ∼25% mutant transcripts of the total KCNQ1 transcript levels.

Conclusions

Our results provide the first evidence that synonymous variants outside the canonical splice sites in KCNQ1 can alter splicing and clinically impact phenotype. Through this mechanism, we identified that p.L353L can precipitate QT prolongation by itself and produce a clinically relevant interactive effect in conjunction with other LQTS variants.

Tbx20 controls the expression of the KCNH2 gene and of hERG channels

Long QT syndrome (LQTS) exhibits great phenotype variability among family members carrying the same mutation, which can be partially attributed to genetic factors. We functionally analyzed the KCNH2 (encoding for Kv11.1 or hERG channels) and TBX20 (encoding for the transcription factor Tbx20) variants found by next-generation sequencing in two siblings with LQTS in a Spanish family of African ancestry. Affected relatives harbor a heterozygous mutation in KCNH2 that encodes for p.T152HfsX180 Kv11.1 (hERG). This peptide, by itself, failed to generate any current when transfected into Chinese hamster ovary (CHO) cells but, surprisingly, exerted "chaperone-like" effects over native hERG channels in both CHO cells and mouse atrial-derived HL-1 cells. Therefore, heterozygous transfection of native (WT) and p.T152HfsX180 hERG channels generated a current that was indistinguishable from that generated by WT channels alone. Some affected relatives also harbor the p.R311C mutation in Tbx20. In human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs), Tbx20 enhanced human KCNH2 gene expression and hERG currents (I_hERG) and shortened action-potential duration (APD). However, Tbx20 did not modify the expression or activity of any other channel involved in ventricular repolarization. Conversely, p.R311C Tbx20 did not increase KCNH2 expression in hiPSC-CMs, which led to decreased I_hERG and increased APD. Our results suggest that Tbx20 controls the expression of hERG channels responsible for the rapid component of the delayed rectifier current. On the contrary, p.R311C Tbx20 specifically disables the Tbx20 protranscriptional activity over KCNH2 Therefore, TBX20 can be considered a KCNH2-modifying gene.

Transgenic rabbit models to investigate the cardiac ion channel disease long QT syndrome

Long QT syndrome (LQTS) is a rare inherited channelopathy caused mainly by different mutations in genes encoding for cardiac K(+) or Na(+) channels, but can also be caused by commonly used ion-channel-blocking and QT-prolonging drugs, thus affecting a much larger population. To develop novel diagnostic and therapeutic strategies to improve the clinical management of these patients, a thorough understanding of the pathophysiological mechanisms of arrhythmogenesis and potential pharmacological targets is needed. Drug-induced and genetic animal models of various species have been generated and have been instrumental for identifying pro-arrhythmic triggers and important characteristics of the arrhythmogenic substrate in LQTS. However, due to species differences in features of cardiac electrical function, these different models do not entirely recapitulate all aspects of the human disease. In this review, we summarize advantages and shortcomings of different drug-induced and genetically mediated LQTS animal models - focusing on mouse and rabbit models since these represent the most commonly used small animal models for LQTS that can be subjected to genetic manipulation. In particular, we highlight the different aspects of arrhythmogenic mechanisms, pro-arrhythmic triggering factors, anti-arrhythmic agents, and electro-mechanical dysfunction investigated in transgenic LQTS rabbit models and their translational application for the clinical management of LQTS patients in detail. Transgenic LQTS rabbits have been instrumental to increase our understanding of the role of spatial and temporal dispersion of repolarization to provide an arrhythmogenic substrate, genotype-differences in the mechanisms for early afterdepolarization formation and arrhythmia maintenance, mechanisms of hormonal modification of arrhythmogenesis and regional heterogeneities in electro-mechanical dysfunction in LQTS.

Electro-mechanical dysfunction in long QT syndrome: Role for arrhythmogenic risk prediction and modulation by sex and sex hormones

Long QT syndrome (LQTS) is a congenital arrhythmogenic channelopathy characterized by impaired cardiac repolarization. Increasing evidence supports the notion that LQTS is not purely an "electrical" disease but rather an "electro-mechanical" disease with regionally heterogeneously impaired electrical and mechanical cardiac function. In the first part, this article reviews current knowledge on electro-mechanical (dys)function in LQTS, clinical consequences of the observed electro-mechanical dysfunction, and potential underlying mechanisms. Since several novel imaging techniques - Strain Echocardiography (SE) and Magnetic Resonance Tissue Phase Mapping (TPM) - are applied in clinical and experimental settings to assess the (regional) mechanical function, advantages of these non-invasive techniques and their feasibility in the clinical routine are particularly highlighted. The second part provides novel insights into sex differences and sex hormone effects on electro-mechanical cardiac function in a transgenic LQT2 rabbit model. Here we demonstrate that female LQT2 rabbits exhibit a prolonged time to diastolic peak - as marker for contraction duration and early relaxation - compared to males. Chronic estradiol-treatment enhances these differences in time to diastolic peak even more and additionally increases the risk for ventricular arrhythmia. Importantly, time to diastolic peak is particularly prolonged in rabbits exhibiting ventricular arrhythmia - regardless of hormone treatment - contrasting with a lack of differences in QT duration between symptomatic and asymptomatic LQT2 rabbits. This indicates the potential added value of the assessment of mechanical dysfunction in future risk stratification of LQTS patients.