Genetic risk stratification in cardiac arrhythmias

The Dysfunction of Ca2+ Channels in Hereditary and Chronic Human Heart Diseases and Experimental Animal Models

Chronic heart diseases, such as coronary heart disease, heart failure, secondary arterial hypertension, and dilated and hypertrophic cardiomyopathies, are widespread and have a fairly high incidence of mortality and disability. Most of these diseases are characterized by cardiac arrhythmias, conduction, and contractility disorders. Additionally, interruption of the electrical activity of the heart, the appearance of extensive ectopic foci, and heart failure are all symptoms of a number of severe hereditary diseases. The molecular mechanisms leading to the development of heart diseases are associated with impaired permeability and excitability of cell membranes and are mainly caused by the dysfunction of cardiac Ca2+ channels. Over the past 50 years, more than 100 varieties of ion channels have been found in the cardiovascular cells. The relationship between the activity of these channels and cardiac pathology, as well as the general cellular biological function, has been intensively studied on several cell types and experimental animal models in vivo and in situ. In this review, I discuss the origin of genetic Ca2+ channelopathies of L- and T-type voltage-gated calcium channels in humans and the role of the non-genetic dysfunctions of Ca2+ channels of various types: L-, R-, and T-type voltage-gated calcium channels, RyR2, including Ca2+ permeable nonselective cation hyperpolarization-activated cyclic nucleotide-gated (HCN), and transient receptor potential (TRP) channels, in the development of cardiac pathology in humans, as well as various aspects of promising experimental studies of the dysfunctions of these channels performed on animal models or in vitro.

The Action Potential Clamp Technique as a Tool for Risk Stratification of Sinus Bradycardia Due to Loss-of-Function Mutations in HCN4: An In Silico Exploration Based on In Vitro and In Vivo Data

These days, in vitro functional analysis of gene variants is becoming increasingly important for risk stratification of cardiac ion channelopathies. So far, such risk stratification has been applied to SCN5A, KCNQ1, and KCNH2 gene variants associated with Brugada syndrome and long QT syndrome types 1 and 2, respectively, but risk stratification of HCN4 gene variants related to sick sinus syndrome has not yet been performed. HCN4 is the gene responsible for the hyperpolarization-activated 'funny' current If, which is an important modulator of the spontaneous diastolic depolarization underlying the sinus node pacemaker activity. In the present study, we carried out a risk classification assay on those loss-of-function mutations in HCN4 for which in vivo as well as in vitro data have been published. We used the in vitro data to compute the charge carried by If (Qf) during the diastolic depolarization phase of a prerecorded human sinus node action potential waveform and assessed the extent to which this Qf predicts (1) the beating rate of the comprehensive Fabbri-Severi model of a human sinus node cell with mutation-induced changes in If and (2) the heart rate observed in patients carrying the associated mutation in HCN4. The beating rate of the model cell showed a very strong correlation with Qf from the simulated action potential clamp experiments (R2 = 0.95 under vagal tone). The clinically observed minimum or resting heart rates showed a strong correlation with Qf (R2 = 0.73 and R2 = 0.71, respectively). While a translational perspective remains to be seen, we conclude that action potential clamp on transfected cells, without the need for further voltage clamp experiments and data analysis to determine individual biophysical parameters of If, is a promising tool for risk stratification of sinus bradycardia due to loss-of-function mutations in HCN4. In combination with an If blocker, this tool may also prove useful when applied to human-induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) obtained from mutation carriers and non-carriers.

CRISPR activation and interference as investigative tools in the cardiovascular system

CRISPR activation and interference (CRISPRa/i) technology offers the unprecedented possibility of achieving regulated gene expression both in vitro and in vivo. The DNA pairing specificity of a nuclease dead Cas9 (dCas9) is exploited to precisely target a transcriptional activator or repressor in proximity to a gene promoter. This permits both the study of phenotypes arising from gene modulation for investigative purposes, and the development of potential therapeutics. As with virtually all other organ systems, the cardiovascular system can deeply benefit from a broader utilisation of CRISPRa/i. However, application of this technology is still in its infancy. Significant areas for improvement include the identification of novel and more effective transcriptional regulators that can be docked to dCas9, and the development of more efficient methods for their delivery and expression in vivo.

Integrating clinical genetics in cardiology: Current practices and recommendations for education

PURPOSE

Recent advances in genetics can facilitate the identification of at-risk individuals and diagnosis of cardiovascular disorders. As a nascent field, more research is needed to optimize the clinical practice of cardiovascular genetics, including the assessment of educational needs to promote appropriate use of genetic testing.

METHODS

Qualitative interviews conducted with cardiovascular specialists (N = 43) were audiotaped. Thematic analysis was conducted on professional transcripts.

RESULTS

Participants recognized the value of genetics in identifying and diagnosing at-risk individuals. However, organizational systems, cost, and feeling of unpreparedness were identified as barriers. Participants felt that the rapid pace of genetic science resulted in further challenges to maintaining an adequate knowledge base and highlighted genetics experts' importance. Even when a genetics expert was available, participants wanted to know more about which patients benefit most from genetic testing and expressed a desire to better understand management recommendations associated with a positive test result.

CONCLUSION

Participants recognized the benefit but felt underprepared to provide recommendations for genetic testing and, in some cases, lacked organizational resources to refer patients to a genetics expert. Additional training in genetics for cardiology practitioners and ensuring availability of a genetics expert can improve the use of genetics in cardiology settings.

The Linkage Phase of the Polymorphism KCNH2-K897T Influences the Electrophysiological Phenotype in hiPSC Models of LQT2

While rare mutations in ion channel genes are primarily responsible for inherited cardiac arrhythmias, common genetic variants are also an important contributor to the clinical heterogeneity observed among mutation carriers. The common single nucleotide polymorphism (SNP) KCNH2-K897T is associated with QT interval duration, but its influence on the disease phenotype in patients with long QT syndrome type 2 (LQT2) remains unclear. Human induced pluripotent stem cells (hiPSCs), coupled with advances in gene editing technologies, are proving an invaluable tool for modeling cardiac genetic diseases and identifying variants responsible for variability in disease expressivity. In this study, we have used isogenic hiPSC-derived cardiomyocytes (hiPSC-CMs) to establish the functional consequences of having the KCNH2-K897T SNP in cis- or trans-orientation with LQT2-causing missense variants either within the pore-loop domain (KCNH2^A561T/WT) or tail region (KCNH2^N996I/WT) of the potassium ion channel, human ether-a-go-go-related gene (hERG). When KCNH2-K897T was on the same allele (cis) as the primary mutation, the hERG channel in hiPSC-CMs exhibited faster activation and deactivation kinetics compared to their trans-oriented counterparts. Consistent with this, hiPSC-CMs with KCNH2-K897T in cis orientation had longer action and field potential durations. Furthermore, there was an increased occurrence of arrhythmic events upon pharmacological blocking of hERG. Collectively, these results indicate that the common polymorphism KCNH2-K897T differs in its influence on LQT2-causing KCNH2 mutations depending on whether it is present in cis or trans. This study corroborates hiPSC-CMs as a powerful platform to investigate the modifying effects of common genetic variants on inherited cardiac arrhythmias and aids in unraveling their contribution to the variable expressivity of these diseases.

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.

Isogenic Sets of hiPSC-CMs Harboring Distinct KCNH2 Mutations Differ Functionally and in Susceptibility to Drug-Induced Arrhythmias

Mutations in KCNH2 can lead to long QT syndrome type 2. Variable disease manifestation observed with this channelopathy is associated with the location and type of mutation within the protein, complicating efforts to predict patient risk. Here, we demonstrated phenotypic differences in cardiomyocytes derived from isogenic human induced pluripotent stem cells (hiPSC-CMs) genetically edited to harbor mutations either within the pore or tail region of the ion channel. Electrophysiological analysis confirmed that the mutations prolonged repolarization of the hiPSC-CMs, with differences between the mutations evident in monolayer cultures. Blocking the hERG channel revealed that the pore-loop mutation conferred greater susceptibility to arrhythmic events. These findings showed that subtle phenotypic differences related to KCNH2 mutations could be captured by hiPSC-CMs under genetically matched conditions. Moreover, the results support hiPSC-CMs as strong candidates for evaluating the underlying severity of individual KCNH2 mutations in humans, which could facilitate patient risk stratification.

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Mutation-specific differences detected in hiPSC-CMs with same genetic background

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APD and FPD in the hERG pore variant hiPSC-CMs more prolonged than the tail variant

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The pore variant was also more susceptible to drug-induced arrhythmic events

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Potential strategy to determine KCNH2 mutation-specific arrhythmic risk

H1153Y-KCNH2 Mutation Identified in a Sudden Arrhythmic Death Syndrome Case Alters Channel Gating

Long QT syndrome is one of the most common hereditary channelopathies inducing fatal arrhythmias and sudden cardiac death. We identified in a sudden arrhythmic death syndrome case a C-term KCNH2 mutation (c.3457C > T; p.His1153Tyr) classified as variant of unknown significance and functional impact. Heterologous expression in HEK293 cells combined with western-blot, flow-cytometry, immunocytochemical and microscope analyses shows no modification of channel trafficking to the cell membrane. Electrophysiological studies reveal that the mutation causes a loss of HERG channel function through an alteration of channel biophysical properties that reduces the current density leading to LQT2. These results provide the first functional evidence for H1153Y-KCNH2 mutation-induced abnormal channel properties. They concur with previous biophysical and clinical presentations of a survived patient with another variant that is G1036D. Therefore, the present report importantly highlights the potential severity of variants that may have useful implications for treatment, surveillance, and follow-up of LQT2 patients.