Multifocal Ectopic Purkinje-related Premature Contractions Research Articles

Abstract 11661: Genotypic and Phenotypic Guided Antiarrhythmic Drug Management of Multifocal Premature Ventricular Contractions

Introduction: Antiarrhythmic selection for progressive and intractable PVCs unresponsive to first and second line regimens can prove challenging. Genetic analysis assists by targeting specific genes, such as the SCN5a gene, which encodes for the alpha subunit of cardiac sodium channel NaV1.5. Mutations are associated with autosomal dominant phenotypes of multifocal ectopic Purkinje-related premature Contractions (MEPPCs). Case: A 38 year old female with a history of syncope and moderate left ventricular systolic dysfunction status post AICD and a family history of SCD presented for follow-up with complaints of frequent palpitations. ECG and a 72 hour Holter monitor had multifocal PVCs of purkinje origin with a burden of 31%. Despite a therapeutic trial with sotalol and amiodarone in various doses, she remained unresponsive with multifocal purkinje-related PVCs. TTE revealed global LV hypokinesia with worsening LVEF, and a cardiac MRI revealed features suggestive of a dilated cardiomyopathy (DCM) phenotype. Whole exome DNA analysis showed a heterozygous missense mutation pArg814Tryp on exon 16 of the SCN5A gene. Her specific variant, R814W, is a gain-of-function variant responsible for the hyperexcitability of the Purkinje fibers, making it amenable to sodium channel blockade with flecainide. After starting gene-directed flecainide, her PVC burden drastically reduced from 31% to 3% on repeat Holter monitor after 2 months. She has remained asymptomatic with an improved LVEF. Discussion: This is a unique case of SCN5A channelopathy with a MEPPC phenotype demonstrating a dramatic response to genotype guided management of multifocal PVCs. Clinicians should include such phenotypes in their differentials for patients that fail first and second line treatment regimens and have significant family history. It is essential to offer genetic testing to patients with a MEPPC phenotype in a DCM setting to choose the appropriate therapy and prevent SCD.

Higher Sodium Channel Excitability in Cardiac Purkinje Fibers: Implications for Multifocal Ectopic Purkinje-Related Premature Contractions

BackgroundMultifocal ectopic Purkinje-related premature contractions (MEPPCs) are associated with SCN5A variants. However, it is not well understood why Purkinje fibers, but not ventricular myocardium, play a predominant role in arrhythmogenesis. ObjectivesThis study sought to explore the underlying mechanisms of MEPPC. MethodsWhole-cell patch-clamp and molecular biology techniques were used in the present study. ResultsClinical data from one patient with R814W variant showed MEPPC syndrome, which is well responsive to amiodarone. Compared with canine ventricular myocytes, Purkinje cells (PCs) had significantly larger sodium current (INa), leftward shift of INa activation and inactivation curves, suggesting higher sodium channel excitability in PCs. Real-time polymerase chain reaction and Western blot analysis showed that the mRNA and protein expression of NaVβ1 and NaVβ3 was higher in canine Purkinje fibers than in ventricular myocardium. INa in heterologous Chinese hamster ovary cell expression system co-expressing NaV1.5 and NaVβ1/NaVβ3 exhibited similar biophysical properties of INa in PCs. R814W variant shifted INa activation in a hyperdepolarized direction, caused a larger window current, and generated an outward-gating pore current at depolarized voltages. Coexpression of NaVβ1/NaVβ3 with Nav1.5-R814W further left-shifted INa activation and caused an even larger window current and gating pore current, suggesting higher susceptibility of Purkinje fibers to R814W variant. Amiodarone inhibited INa, shifted its inactivation to more negative voltages, and significantly decreased the window current. ConclusionsA higher expression of β1 and β3 subunits contributes to higher sodium channel excitability in cardiac Purkinje fibers, making them more susceptible to MEPPC.

Multifocal ectopic purkinje-related premature contractions and related cardiomyopathy.

In the past 20 years, genetic variants in SCN5A encoding the cardiac voltage-gated sodium channel Nav1.5 have been linked to a range of inherited cardiac arrhythmias: variants resulting in loss-of-function of Nav1.5 have been linked to sick sinus syndrome, atrial stand still, atrial fibrillation (AF) impaired pulse generation, progressive and non-progressive conduction defects, the Brugada Syndrome (BrS), and sudden cardiac death. SCN5A variants causing increased sodium current during the plateau phase of the cardiac action potential is associated with Long QT Syndrome type 3 (LQTS3), Torsade de Pointes ventricular tachycardia and SCD. Recently, gain-of-function variants have been linked to complex electrical phenotypes, such as the Multifocal Ectopic Purkinje-related Premature Contractions (MEPPC) syndrome. MEPPC is a rare condition characterized by a high burden of premature atrial contractions (PACs) and/or premature ventricular contractions (PVCs) often accompanied by dilated cardiomyopathy (DCM). MEPPC is inherited in an autosomal dominant fashion with an almost complete penetrance. The onset is often in childhood. The link between SCN5A variants, MEPPC and DCM is currently not well understood, but amino acid substitutions resulting in gain-of-function of Nav1.5 or introduction of gating pore currents potentially play an important role. DCM patients with a MEPPC phenotype respond relatively poorly to standard heart failure medical therapy and catheter ablation as the PVCs originate from all parts of the fascicular Purkinje fiber network. Class 1c sodium channel inhibitors, notably flecainide, have a remarkable positive effect on the ectopic burden and the associated cardiomyopathy. This highlights the importance of genetic screening of DCM patients to identify patients with SCN5A variants associated with MEPPC. Here we review the MEPPC phenotype, MEPPC-SCN5A associated variants, and pathogenesis as well as treatment options.

PO-04-159 PREVALENCE AND CHARACTERISTICS OF PATIENTS WITH LONG QT SYNDROME (LQTS) WHO APPEAR NON-RESPONSIVE TO CONVENTIONAL LQTS THERAPIES

Long QT syndrome (LQTS) is a potentially lethal cardiac channelopathy with a 1% to 5% annual risk of LQTS-triggered syncope/seizures or sudden cardiac arrest/death (SCA/SCD). Despite maximal LQTS-directed therapies, a subset of patients experience breakthrough cardiac events (BCEs). To describe the phenotype of “non-responders” defined as LQTS patients with recurrent BCEs despite maximal LQTS-directed therapy. Among the 1731 LQTS patients evaluated and treated at Mayo Clinic, a retrospective analysis was performed to identify those who experienced ≥1 BCE while on maximum treatment (combination of ≥2 device/denervation and/or pharmacologic therapies at maximally tolerated doses). Basic demographics, clinical characteristics, and frequency/type of BCEs defined as syncope/seizures, appropriate ICD shocks, SCA, or SCD were extracted from the electronic medical record. Overall, 32/1731 LQTS patients (2%) experienced ≥1 BCE with 13 (41%) occurring in the setting of medication nonadherence. Of the remaining 19 patients (1% of all LQTS patients; 37% female, mean age at last event of 11±8 years and mean QTc of 537±92ms), 7 (37%) had compound heterozygous (n=6) or homozygous (n=1) disease involving either KCNQ1 or KCNH2, including one Jervell and Lange-Nielsen syndrome patient, and 2 (11%) had high-risk minor LQTS genotypes (CALM2-mediated LQTS and CACNA1C-mediated Timothy syndrome). Notably, 5 (26%) had a complex cardiac sodium channelopathy with predominant features of LQT3 and multifocal ectopic Purkinje-related premature contractions. The remaining patients had treatment-refractory LQT2 (n=3; 16%) or LQT3 (n=2; 11%). At last follow-up, 2 patients (11%) died and 4 (2%) underwent cardiac transplant. For the remaining 13 patients, treatment was escalated after their last BCE. Twelve patients (92%) received an ICD and either atrial pacing (AP) + beta-blocker (BB) + mexiletine (MEX) [n=1, 8%], AP + left cardiac sympathetic denervation (LCSD) + ≥2 antiarrhythmic drugs (AADs) [n=4, 31%], bilateral denervation + BB [n=2, 15%], or LCSD + ≥1 AAD [n=5, 38%]. One patient (8%) was on LCSD + BB + MEX. Despite safe and effective treatment options, 1% of LQTS patients experience recurrent BCEs on maximal LQTS-directed therapy. Adherence to the outlined treatment program needs to be reinforced. In addition, compound heterozygous/homozygous KCNQ1-mediated disease and complex SCN5A-mediated channelopathies represent high-risk LQTS subsets in need of alternative management strategies.

A de novo arrhythmogenic Nav1.5 variant, E171Q, causes multiple biophysical defects.

An infant presenting antenatally with cardiac conduction disease and junctional tachycardia reminiscent of the multi-focal ectopic Purkinje-related premature contractions (MEPPC) syndrome was found to harbour the de novo E171Q variant of the cardiac voltage-gated sodium channel, Nav1.5. Functional analysis was performed using whole cell patch clamp and current clamp to compare WT Nav1.5 with E171Q expressed in HEK293 cells and iPSC-CMs. We found E171Q significantly right-shifted the voltage-dependence of both activation and steady-state fast inactivation, and increased the amplitude of late sodium current. These changes were reversed upon perfusion with 100 μM ranolazine. Incorporating the shifts in voltage-dependence and late current into the Ohara-Rudy cardiac action potential model produced a delay in action potential repolarization. Further investigation revealed a significant outward gating pore (omega) current in E171Q that was mitigated upon perfusion of 3 μM flecainide or with K+-free internal solution. Current clamp experiments in iPSC-CMs revealed action potential variability with delayed afterdepolarizations in myocytes expressing E171Q. The increase in gating pore current is consistent with disrupting the interaction between E171 and a corresponding positive charge in the S4 voltage sensor. This disruption could result in the completion of an ion-permeable pore, consistent with that previously described for Nav1.5 R222Q - a variant also associated with MEPPC - and Nav1.4 R222W.

Abstract 12634: A Novel in-vitro Patient-Specific Re-Engineered Cardiomyocyte Model of Multifocal Ectopic Purkinje-Related Premature Contractions

Introduction: Multifocal ectopic Purkinje-related premature contractions (MEPPC), the most recent addition to the spectrum of SCN5A -mediated cardiac channelopathies, is characterized by frequent premature ventricular complexes (PVCs) originating from the Purkinje system, atrial arrhythmias, and a predilection for PVC-mediated dilated cardiomyopathy. Hypothesis: To investigate the electrophysiology phenotype of MEPPC in a novel, patient-specific, re-engineered cardiomyocyte model. Methods: Induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs) were generated from a 27-year-old male with a history of frequent, multifocal PVCs and atrial tachycardia, positive for a putative MEPPC-causative SCN5A variant (p.Q419P). A CRISPR-Cas9 Q419P variant-corrected isogenic control (IC) was created to establish the normalized electrophysiology recordings for that MEPPC model. The standard whole-cell patch clamp technique was used to measure Q419P-SCN5A- and IC-derived NaV1.5 sodium currents using voltage clamp configuration. In addition, action potential duration (APD) of the iPSC-CMs were measured using FluoVolt optical voltage-sensing dye. Results: Analysis of the current-voltage relationship revealed that neither peak sodium current densities nor late inward sodium current were impacted by Q419P-SCN5A. However, Q419P-SCN5A significantly right-shifted the V 1/2 in inactivation by 8.3 mV from -81.1±1.7 mV (IC, n=5) to -72.8±0.2 mV (Q419P-SCN5A, n=6, p<0.05 vs. IC) and left-shifted the V 1/2 in activation by -4.1 mV from -39.5±0.3 mV (IC, n=5) to -43.6±0.9 mV (Q419P-SCN5A, n=6, p<0.05 vs. IC) leading to a larger hyperpolarized window current. Optical APD90 measurement revealed no difference between the Q419P-containing (476 ± 23 ms) and the IC (474 ± 46 ms, p=0.867) cardiomyocytes. Conclusions: This is the first in-vitro disease model of MEPPC using patient-specific, re-engineered heart cells. Consistent with the findings in the non-cardiac HEK cell system, this MEPPC-causative variant contributes to increased window current in cardiomyocytes without affecting the cardiac APD. This novel cardiac cell model for MEPPC may facilitate new insights into the pathobiology of MEPPC and enable the testing of novel therapies for it.

The G213D variant in Nav1.5 alters sodium current and causes an arrhythmogenic phenotype resulting in a multifocal ectopic Purkinje-related premature contraction phenotype in human-induced pluripotent stem cell-derived cardiomyocytes.

Variants in SCN5A encoding Nav1.5 are associated with cardiac arrhythmias. We aimed to determine the mechanism by which c.638G>A in SCNA5 resulting in p.Gly213Asp (G213D) in Nav1.5 altered Na+ channel function and how flecainide corrected the defect in a family with multifocal ectopic Purkinje-related premature contractions (MEPPC)-like syndrome. Five patients carrying the G213D variant were treated with flecainide. Gating pore currents were evaluated in Xenopus laevis oocytes. The 638G>A SCN5A variant was introduced to human-induced pluripotent stem cell (hiPSC) by CRISPR-Cas9 gene editing and subsequently differentiated to cardiomyocytes (hiPSC-CM). Action potentials and sodium currents were measured in the absence and presence of flecainide. Ca2+ transients were measured by confocal microscopy. The five patients exhibited premature atrial and ventricular contractions which were suppressed by flecainide treatment. G213D induced gating pore current at potentials negative to -50 mV. Voltage-clamp analysis in hiPSC-CM revealed the activation threshold of INa was shifted in the hyperpolarizing direction resulting in a larger INa window current. The G213D hiPSC-CMs had faster beating rates compared with wild-type and frequently showed Ca2+ waves and alternans. Flecainide applied to G213D hiPSC-CMs decreased window current by shifting the steady-state inactivation curve and slowed the beating rate. The G213D variant in Nav1.5 induced gating pore currents and increased window current. The changes in INa resulted in a faster beating rate and Ca2+ transient dysfunction. Flecainide decreased window current and inhibited INa, which is likely responsible for the therapeutic effectiveness of flecainide in MEPPC patients carrying the G213D variant.

Abstract 10583: A Novel Functional Variant, P.Gln419Pro, Residing Outside the SCN5A -Encoded Na V 1.5 Voltage-Sensing Domain Causes Multifocal Ectopic Purkinje-Related Premature Contractions

Background: Multifocal ectopic Purkinje-related premature contractions (MEPPC), the most recent addition to the spectrum of SCN5A -mediated cardiac channelopathies, is characterized by frequent premature ventricular complexes (PVCs) originating from the Purkinje system, atrial arrhythmias, and a predilection for PVC-mediated dilated cardiomyopathy. MEPPC-causative variants tend to cluster within the voltage sensing domain (VSD) of Na v 1.5. Objective: To describe the clinical and cellular electrophysiological phenotype associated with a novel, putative MEPPC-causative SCN5A variant (p.Gln419Pro-SCN5A) that resides outside the VSD of Na v 1.5. Methods: p.Gln419Pro-SCN5A was identified in a 21-year-old male with a history of frequent, multifocal PVCs and atrial tachycardia. Subsequently, three p.Gln419Pro-SCN5A-positive relatives, each with early onset atrial arrhythmias and/or frequent, multifocal PVCs, were identified. All individuals had structurally normal hearts by echocardiography and/or cardiac magnetic resonance imaging. p.Gln419Pro-SCN5A was engineered by site directed mutagenesis and heterologously expressed in TSA201 cells. The standard whole-cell patch clamp technique was used to measure SCN5A wild-type (WT) and p.Gln419Pro-SCN5A sodium currents. Results: Analysis of the current-voltage relationship revealed that neither peak sodium current densities nor late inward sodium current were impacted by p.Gln419Pro-SCN5A. However, p.Gln419Pro-SCN5A significantly shifted V 1/2 in inactivation by -4.6 mV from -84.5±0.7 mV (WT, n=10) to -89.1±0.4 mV (Gln419Pro, n=12, p<0.05 vs. WT) and V 1/2 in activation by -11.6 mV from -30.9±1.3 mV (WT, n=10) to -42.5±1.0 mV (Gln419Pro, n=12, p<0.05 vs. WT) leading to a larger hyperpolarized window current. Flecainide dramatically reduced PVC burden in p.Gln419Pro-SCN5A-positive patients. Conclusions: p.Gln419Pro-SCN5A represents only the fifth MEPPC-causative variant described to date and the first associated with a classic MEPPC phenotype localizing outside Na v 1.5’s VSD. As class I antiarrhythmic drugs are highly efficacious in MEPPC, this clinical entity merits consideration in all young patients found to have a high burden of Purkinje-related PVCs.

Purkinje system hyperexcitability and ventricular arrhythmia risk in type 3 long QT syndrome

Gain-of-function variants in the SCN5A-encoded Nav1.5 sodium channel cause type 3 long QT syndrome (LQT3) andmultifocal ectopic Purkinje-related premature contractions. Although the Purkinje system is uniquely sensitive to the action potential-prolonging effects of LQT3-causative variants, the existence of additional Purkinje phenotype(s) in LQT3 is unknown. The purpose of this study was to determine the prevalence and clinical implications of frequent fascicular/Purkinje-related premature ventricular contractions (PVCs) and short-coupled ventricular arrhythmias (VAs), suggestive of Purkinje system hyperexcitability (PSH), in a single-center LQT3 cohort. A retrospective analysis of 177 SCN5A-positive patients was performed to identify individuals with a LQT3 phenotype. Available electrocardiographic, electrophysiology study, device, and genetic data from 91 individuals with LQT3 were reviewed for evidence of presumed fascicular PVCs and short-coupled VAs. The relationship between PSH and ventricular fibrillation events was assessed by Kaplan-Meier and Cox regression analyses. Overall, 30 of 91 patients with LQT3 (33%) exhibited evidence of presumed PSH (fascicular PVCs 30 of 30 [100%]; short-coupled VAs 17 of 30 [56%]). Kaplan-Meier and Cox regression analyses demonstrated an increased risk of ventricular fibrillation events in individuals with LQT3 and PSH (log-rank, P < .03; hazard ratio 3.95; 95% confidence interval 1.15-15.7; P = .03). Interestingly, variants in the voltage-sensing domain regions of Nav1.5 were more frequently observed in patients with LQT3 and PSH than those without (19 of 30 [63%] vs 9 of 61 [15%]; P <.0001). This study demonstrates that a discernible Purkinje phenotype is present in one-third of LQT3 cases and increases the risk of potentially lethal VAs. Further study is needed to determine whether a distinct cellular electrophysiology phenotype underlies this phenomenon.

A novel gain-of-function mutation in SCN5A responsible for multifocal ectopic Purkinje-related premature contractions.

Recently, four SCN5A mutations have been associated with Multifocal Ectopic Purkinje-related Premature Contractions (MEPPC), a rare cardiac syndrome combining polymorphic ventricular arrhythmia with dilated cardiomyopathy (DCM). Here, we identified a novel heterozygous mutation in SCN5A (c.611C>A, pAla204Glu) in a young woman presenting with polymorphic premature ventricular contractions (PVCs) and DCM. After failure of antiarrhythmic drugs and an attempt of radiofrequency catheter ablation showing three exit-sites of PVCs, all with presystolic Purkinje potentials, a treatment by hydroquinidine was tried, leading to an immediate and spectacular disappearance of all PVCs and normalization of cardiac function. Electrophysiological studies showed that Nav 1.5-A204E mutant channels exhibited a significant leftward shift of 8 mV of the activation curve, leading to a larger hyperpolarized window current when compared to wild-type. Action potential modeling using Purkinje fiber and ventricular cell models predicted an arrhythmogenic effect predominant in Purkinje fibers for the A204E mutation. Comparison with other MEPPC-associated Nav 1.5 mutations revealed a common electrophysiological pattern of abnormal voltage-dependence of activation leading to a larger hyperpolarized window current as a shared biophysical mechanism of this syndrome. These features of the mutant sodium channels are likely to be responsible for the hyperexcitability of the fascicular-Purkinje system observed in patients with MEPPC.

A new SCN5A mutation responsible for Multifocal Ectopic Purkinje-Related Premature Contractions

Recently, SCN5A mutations have been associated with Multifocal Ectopic Purkinje-Related Premature Contractions (MEPPC), a cardiac syndrome associating polymorphic ventricular arrhythmia with dilated cardiomyopathy. A new SCN5A mutation was identified in a young woman presenting with polymorphic PVCs and dilated cardiomyopathy since the age of 12. Radiofrequency catheter ablation showed at least three exit sites of purkinje-PVCs. A treatment by hydroquinidine was tried, leading to an immediate and spectacular disappearance of all PVCs. A new heterozygous missense mutation was identified in exon 5 of SCN5A (c.611C > A, pAla204Glu) in the DI-S3 domain. We investigated the functional consequences of NaV1.5-A204E expressed in HEK cells by recording the sodium current using the whole-cell configuration of the patch clamp technique. The NaV1.5-A204E mutant channels exhibited a significant leftward shift of the activation curve of 8 mV (V0.5 of activation = −42.9 ± 1.3 mV in A204E transfected cells ( n = 24) compared to −34.8 ± 1.9 mV in cells transfected with the wild-type (WT) channel ( n = 20), P < 0.001), leading to a large, hyperpolarized window current that predicts aberrant sodium influx at potentials near the cardiomyocyte resting membrane potential. We did not observe any difference in the sodium current density, nor persistent sodium current in NaV1.5-A204E transfected cells, when compared to WT cells. This profile of channel dysfunction shares features with other MEPCC-associated SCN5A mutations and overlapped that of congenital long QT syndrome LQT3. The functional consequences of this new SCN5A mutation associated with MEPPC revealed a pattern of abnormal voltage dependence of activation leading to a larger hyperpolarized window current as a shared biophysical mechanism of the syndrome. These features of the mutant sodium channel are likely to be responsible for hyperexcitability of the fascicular-Purkinje system.

P5713A new scn5a mutation responsible for multifocal ectopic purkinje-related premature contractions

P5713A new scn5a mutation responsible for multifocal ectopic purkinje-related premature contractions

4290Novel SCN5A mutation associated with multifocal ectopic purkinje-related premature contractions

4290Novel SCN5A mutation associated with multifocal ectopic purkinje-related premature contractions

Rapid and effective response of the R222Q SCN5A to quinidine treatment in a patient with Purkinje-related ventricular arrhythmia and familial dilated cardiomyopathy: a case report

BackgroundMutations of the SCN5A gene are reported in 2-4% of patients with dilated cardiomyopathy (DCM). In such cases, DCM is associated with different rhythm disturbances such as the multifocal ectopic Purkinje-related premature contractions and atrial fibrillation. Arrhythmia often occurs at a young age and is the first symptom of heart disease.Case presentationWe present the case of 55-year old male with a 30-year history of heart failure (HF) in the course of familial DCM and complex ventricular tachyarrhythmias, which constituted 50-80% of the whole rhythm. The patient was qualified for heart transplantation because of the increasing symptoms of HF. We revealed the heterozygotic R222Q mutation in SCN5A by means of whole exome sequencing. After the quinidine treatment, a rapid and significant reduction of ventricular tachyarrhythmias and an improvement in the myocardial function were observed and this effect remained constant in the 2.5-year follow-up. This effect was observed even in the presence of concomitant coronary artery disease.ConclusionsPatients with familial DCM and Purkinje-related ventricular arrhythmias should be offered genetic screening. The quinidine treatment for the SCN5A R222Q mutation can be life saving for patients.

Multifocal atrial and ventricular premature contractions with an increased risk of dilated cardiomyopathy caused by a Nav1.5 gain-of-function mutation (G213D)

Background: SCN5A mutations can lead to different cardiac diseases. Recently, SCN5A mutations have been linked to the clinical entity multifocal ectopic Purkinje-related premature contractions (MEPPC) characterized by ventricular ectopy and dilated cardiomyopathy.Methods & Results: A family with a uniform MEPPC-like phenotype, associated with complex atrial and ventricular arrhythmias and dilated cardiomyopathy caused by a high frequency of ventricular ectopy was clinically assessed. Screening of the SCN5A gene revealed a missense mutation in the linker between segments 3 and 4 in domain 1 of the Nav1.5 protein, resulting in a glycine to aspartate substitution at position 213 (G213D). The phenotype co-segregated with the missense mutation. Electrophysiological studies of wild type (WT) hNav1.5 and hNav1.5_G213D expressed in CHO-K cells showed that the voltage of half-maximal activation (V½) was significantly more negative for hNav1.5_G213D compared to WT (V½=−38.7±0.5mV for WT and V½=−42.4±0.5mV for G213D; P<0.001). This suggests activation of Nav1.5_G231D at more negative potentials. The V½ of steady-state inactivation was significantly shifted towards more positive values for Nav1.5_G213D (V½=−86.7±0.2mV for WT and −82.2±0.3mV for G213D; P<0.001), also contributing to a gain-of-function phenotype. Flecainide and amiodarone markedly reduced premature atrial and ventricular contractions in four patients.Conclusion: The Nav1.5_G213D mutation is associated with a gain-of-function phenotype, multifocal atrial and ventricular ectopy and dilated cardiomyopathy. Since patients with a MEPPC-like phenotype may specifically benefit from Class-1 antiarrhythmic drugs or amiodarone, clinical identification of this disease entity is important.

Characterization of a NaV1.5 Gain-of-Function Mutation (G213D) causing Multifocal Atrial and Ventricular Premature Ectopies and an Increased Risk of Dilated Cardiomyopathy

Background: Mutations in SCN5A have been linked to different cardiac diseases including multifocal ectopic Purkinje-related premature contractions (MEPPC) characterized by ventricular ectopy and dilated cardiomyopathy (DCM). Here we characterize the Nav1.5 G213D variant found in a large Danish family with frequent premature ventricular and atrial contractions, frequent nonsustained ventricular tachycardia and DCM. Methods and Results: The family was clinically assessed and an MEPPC-like phenotype including complex atrial and ventricular arrhythmias and dilated cardiomyopathy was found. Genetic screening revealed a G213D Nav1.5 missense mutation in the link of segment 3 and 4 in domain 1 of the Nav1.5 protein. The MEPPC-like phenotype co-segregated with G213D. Electrophysiological studies of wild type (WT) hNav1.5 and hNav1.5_G213D expressed in CHO-K cells was performed using conventional patch clamp and a SynchoPatch 384PE (Nanion Technologies) high throughput automatic patch clamp. The half-maximal inactivation (V½) was significantly more negative for hNav1.5_G213D compared to WT and the V½ of steady-state inactivation was shifted towards more positive values for hNav1.5_G213D, resulting in increased window-currents. This suggests that hNav1.5_G213D activates at more negative potentials and a larger fraction of channels will recover from inactivation during the diastole resulting in a gain-of-function phenotype. A combination of wild type and G213D mimicking the heterozygote state exhibited an intermediate phenotype. Flecainide or amiodarone resulted in a markedly reduced number of premature atrial and ventricular contractions in patients. Conclusions: The G213D Nav1.5 variant is associated with a gain-of-function and is associated with multifocal atrial and ventricular ectopy and dilated cardiomyopathy.

Cardiac Arrhythmias Related to Sodium Channel Dysfunction.

The voltage-gated cardiac sodium channel (Nav1.5) is a mega-complex comprised of a pore-forming α subunit and 4 ancillary β-subunits together with numerous protein partners. Genetic defects in the form of rare variants in one or more sodium channel-related genes can cause a loss- or gain-of-function of sodium channel current (INa) leading to the manifestation of various disease phenotypes, including Brugada syndrome, long QT syndrome, progressive cardiac conduction disease, sick sinus syndrome, multifocal ectopic Purkinje-related premature contractions, and atrial fibrillation. Some sodium channelopathies have also been shown to be responsible for sudden infant death syndrome (SIDS). Although these genetic defects often present as pure electrical diseases, recent studies point to a contribution of structural abnormalities to the electrocardiographic and arrhythmic manifestation in some cases, such as dilated cardiomyopathy. The same rare variants in SCN5A or related genes may present with different clinical phenotypes in different individuals and sometimes in members of the same family. Genetic background and epigenetic and environmental factors contribute to the expression of these overlap syndromes. Our goal in this chapter is to review and discuss what is known about the clinical phenotype and genotype of each cardiac sodium channelopathy, and to briefly discuss the underlying mechanisms.

Multifocal Ectopic Purkinje-Related Premature Contractions: A New SCN5A-Related Cardiac Channelopathy

The aim of this study was to describe a new familial cardiac phenotype and to elucidate the electrophysiological mechanism responsible for the disease. Mutations in several genes encoding ion channels, especially SCN5A, have emerged as the basis for a variety of inherited cardiac arrhythmias. Three unrelated families comprising 21 individuals affected by multifocal ectopic Purkinje-related premature contractions (MEPPC) characterized by narrow junctional and rare sinus beats competing with numerous premature ventricular contractions with right and/or left bundle branch block patterns were identified. Dilated cardiomyopathy was identified in 6 patients, atrial arrhythmias were detected in 9 patients, and sudden death was reported in 5 individuals. Invasive electrophysiological studies demonstrated that premature ventricular complexes originated from the Purkinje tissue. Hydroquinidine treatment dramatically decreased the number of premature ventricular complexes. It normalized the contractile function in 2 patients. All the affected subjects carried the c.665G>A transition in the SCN5A gene. Patch-clamp studies of resulting p.Arg222Gln (R222Q) Nav1.5 revealed a net gain of function of the sodium channel, leading, in silico, to incomplete repolarization in Purkinje cells responsible for premature ventricular action potentials. In vitro and in silico studies recapitulated the normalization of the ventricular action potentials in the presence of quinidine. A new SCN5A-related cardiac syndrome, MEPPC, was identified. The SCN5A mutation leads to a gain of function of the sodium channel responsible for hyperexcitability of the fascicular-Purkinje system. The MEPPC syndrome is responsive to hydroquinidine.