iPSC-Cardiomyocyte Models of Brugada Syndrome-Achievements, Challenges and Future Perspectives

Identification of a novel Scn3b mutation in a Chinese Brugada syndrome pedigree: implications for Nav1.5 electrophysiological properties and intracellular distribution of Nav1.5 and Navβ3

Background

The Scn3b gene encodes for Navβ3, a pivotal regulatory subunit of the fast sodium channel in cardiomyocytes. However, its mutation status in the Chinese population suffering from Brugada Syndrome (BrS) has not been characterized, and the contributory pathophysiological mechanisms to disease pathology remain undefined.

Methods and Results

A Scn3b (c.260C>T, p.P87l) mutation was identified in a patient with BrS of Chinese descent. Functional analyses demonstrated that sodium channel activation for the wild type, mutant samples, and co-expression of both commenced at -55 mv and peaked at -25 mv. The mutant group exhibited a notable reduction, approximately 60%, in peak sodium channel activation current (INa) at -25 mv. The parameters for half-maximal activation voltages (V1/2) and slope factors (k) showed no significant differences when comparing wild type, mutant, and combined expression groups (P = 0.98 and P = 0.65, respectively). Additionally, no significant disparities were evident in terms of the steady-state sodium channel inactivation parameters V1/2 and k (with P-values of 0.85 and 0.25, respectively), nor were there significant differences in the activation time constant τ (P = 0.59) and late sodium current density (P = 0.23) across the wild-type, mutant, and co-expressed groups. Confocal imaging and Western blot analysis demonstrated decreased plasma membrane localization of SCN3B and SCN5A in the P87l group. Computational simulations of cardiac action potentials suggested that SCN3B P87l can alter the morphology of the action potentials within the endocardium and epicardium while reducing the peak of depolarization.

Conclusions

The pathogenic impact of the Scn3b P87l mutation predominantly originates from a reduction in peak INa activation current coupled with decreased cell surface expression of Nav1.5 and Navβ3. These alterations may influence cardiac action potential configurations and contribute to the risk of ventricular arrhythmias in individuals with BrS.

Noncoding RNAs and Human Induced Pluripotent Stem Cell-Derived Cardiomyocytes in Cardiac Arrhythmic Brugada Syndrome

Hundreds of thousands of people die each year as a result of sudden cardiac death, and many are due to heart rhythm disorders. One of the major causes of these arrhythmic events is Brugada syndrome, a cardiac channelopathy that results in abnormal cardiac conduction, severe life-threatening arrhythmias, and, on many occasions, death. This disorder has been associated with mutations and dysfunction of about two dozen genes; however, the majority of the patients do not have a definite cause for the diagnosis of Brugada Syndrome. The protein-coding genes represent only a very small fraction of the mammalian genome, and the majority of the noncoding regions of the genome are actively transcribed. Studies have shown that most of the loci associated with electrophysiological traits are located in noncoding regulatory regions and are expected to affect gene expression dosage and cardiac ion channel function. Noncoding RNAs serve an expanding number of regulatory and other functional roles within the cells, including but not limited to transcriptional, post-transcriptional, and epigenetic regulation. The major noncoding RNAs found in Brugada Syndrome include microRNAs; however, others such as long noncoding RNAs are also identified. They contribute to pathogenesis by interacting with ion channels and/or are detectable as clinical biomarkers. Stem cells have received significant attention in the recent past, and can be differentiated into many different cell types including those in the heart. In addition to contractile and relaxational properties, BrS-relevant electrophysiological phenotypes are also demonstrated in cardiomyocytes differentiated from stem cells induced from adult human cells. In this review, we discuss the current understanding of noncoding regions of the genome and their RNA biology in Brugada Syndrome. We also delve into the role of stem cells, especially human induced pluripotent stem cell-derived cardiac differentiated cells, in the investigation of Brugada syndrome in preclinical and clinical studies.

HiPSC-derived cardiomyocyte to model Brugada syndrome: both asymptomatic and symptomatic mutation carriers reveal increased arrhythmogenicity

Brugada syndrome is an inherited cardiac arrhythmia disorder that is mainly associated with mutations of the cardiac voltage-gated sodium channel alpha subunit 5 (SCN5A) gene. The clinical symptoms include ventricular fibrillation and an increased risk of sudden cardiac death. Human-induced pluripotent stem cell (hiPSC) lines were derived from symptomatic and asymptomatic individuals carrying the R1913C mutation in the SCN5A gene. The present work aimed to observe the phenotype-specific differences in hiPSC-derived cardiomyocytes (CMs) obtained from symptomatic and asymptomatic mutation carriers. In this study, CM electrophysiological properties, beating abilities and calcium parameters were measured. Mutant CMs exhibited higher average sodium current densities than healthy CMs, but the differences were not statistically significant. Action potential durations were significantly shorter in CMs from the symptomatic individual, and a spike-and-dome morphology of action potential was exclusively observed in CMs from the symptomatic individual. More arrhythmias occurred in mutant CMs at single cell and cell aggregate levels compared with those observed in wild-type CMs. Moreover, there were no major differences in ionic currents or intracellular calcium dynamics between the CMs of asymptomatic and symptomatic individuals after the administration of adrenaline and flecainide.In conclusion, mutant CMs were more prone to arrhythmia than healthy CMs but did not explain why only one of the mutation carriers was symptomatic.

Automated Patch-Clamp and Induced Pluripotent Stem Cell-Derived Cardiomyocytes: A Synergistic Approach in the Study of Brugada Syndrome

The development of high-throughput automated patch-clamp technology is a recent breakthrough in the field of Brugada syndrome research. Brugada syndrome is a heart disorder marked by abnormal electrocardiographic readings and an elevated risk of sudden cardiac death due to arrhythmias. Various experimental models, developed either in animals, cell lines, human tissue or computational simulation, play a crucial role in advancing our understanding of this condition, and developing effective treatments. In the perspective of the pathophysiological role of ion channels and their pharmacology, automated patch-clamp involves a robotic system that enables the simultaneous recording of electrical activity from multiple single cells at once, greatly improving the speed and efficiency of data collection. By combining this approach with the use of patient-derived cardiomyocytes, researchers are gaining a more comprehensive view of the underlying mechanisms of heart disease. This has led to the development of more effective treatments for those affected by cardiovascular conditions.

Maturation of hiPSC-derived cardiomyocytes promotes adult alternative splicing of SCN5A and reveals changes in sodium current associated with cardiac arrhythmia

AIMS

Human-induced pluripotent stem cell-cardiomyocytes (hiPSC-CMs) are widely used to study arrhythmia-associated mutations in ion channels. Among these, the cardiac sodium channel SCN5A undergoes foetal-to-adult isoform switching around birth. Conventional hiPSC-CM cultures, which are phenotypically foetal, have thus far been unable to capture mutations in adult gene isoforms. Here, we investigated whether tri-cellular cross-talk in a three-dimensional (3D) cardiac microtissue (MT) promoted post-natal SCN5A maturation in hiPSC-CMs.

METHODS AND RESULTS

We derived patient hiPSC-CMs carrying compound mutations in the adult SCN5A exon 6B and exon 4. Electrophysiological properties of patient hiPSC-CMs in monolayer were not altered by the exon 6B mutation compared with isogenic controls since it is not expressed; further, CRISPR/Cas9-mediated excision of the foetal exon 6A did not promote adult SCN5A expression. However, when hiPSC-CMs were matured in 3D cardiac MTs, SCN5A underwent isoform switch and the functional consequences of the mutation located in exon 6B were revealed. Up-regulation of the splicing factor muscleblind-like protein 1 (MBNL1) drove SCN5A post-natal maturation in microtissues since its overexpression in hiPSC-CMs was sufficient to promote exon 6B inclusion, whilst knocking-out MBNL1 failed to foster isoform switch.

CONCLUSIONS

Our study shows that (i) the tri-cellular cardiac microtissues promote post-natal SCN5A isoform switch in hiPSC-CMs, (ii) adult splicing of SCN5A is driven by MBNL1 in these tissues, and (iii) this model can be used for examining post-natal cardiac arrhythmias due to mutations in the exon 6B.

TRANSLATIONAL PERSPECTIVE

The cardiac sodium channel is essential for conducting the electrical impulse in the heart. Postnatal alternative splicing regulation causes mutual exclusive inclusion of fetal or adult exons of the corresponding gene, SCN5A. Typically, immature hiPSCCMs fall short in studying the effect of mutations located in the adult exon. We describe here that an innovative tri-cellular three-dimensional cardiac microtissue culture promotes hiPSC-CMs maturation through upregulation of MBNL1, thus revealing the effect of a pathogenic genetic variant located in the SCN5A adult exon. These results help advancing the use of hiPSC-CMs in studying adult heart disease and for developing personalized medicine applications.

Gene-Edited Human-Induced Pluripotent Stem Cell Lines to Elucidate DAND5 Function throughout Cardiac Differentiation

(1) Background: The contribution of gene-specific variants for congenital heart disease, one of the most common congenital disabilities, is still far from our complete understanding. Here, we applied a disease model using human-induced pluripotent stem cells (hiPSCs) to evaluate the function of DAND5 on human cardiomyocyte (CM) differentiation and proliferation. (2) Methods: Taking advantage of our DAND5 patient-derived iPSC line, we used CRISPR-Cas9 gene-editing to generate a set of isogenic hiPSCs (DAND5-corrected and DAND5 full-mutant). The hiPSCs were differentiated into CMs, and RT-qPCR and immunofluorescence profiled the expression of cardiac markers. Cardiomyocyte proliferation was analysed by flow cytometry. Furthermore, we used a multi-electrode array (MEA) to study the functional electrophysiology of DAND5 hiPSC-CMs. (3) Results: The results indicated that hiPSC-CM proliferation is affected by DAND5 levels. Cardiomyocytes derived from a DAND5 full-mutant hiPSC line are more proliferative when compared with gene-corrected hiPSC-CMs. Moreover, parallel cardiac differentiations showed a differential cardiac gene expression profile, with upregulated cardiac progenitor markers in DAND5-KO hiPSC-CMs. Microelectrode array (MEA) measurements demonstrated that DAND5-KO hiPSC-CMs showed prolonged field potential duration and increased spontaneous beating rates. In addition, conduction velocity is reduced in the monolayers of hiPSC-CMs with full-mutant genotype. (4) Conclusions: The absence of DAND5 sustains the proliferation of hiPSC-CMs, which alters their electrophysiological maturation properties. These results using DAND5 hiPSC-CMs consolidate the findings of the in vitro and in vivo mouse models, now in a translational perspective. Altogether, the data will help elucidate the molecular mechanism underlying this human heart disease and potentiates new therapies for treating adult CHD.

iPSC-Derived Cardiomyocytes in Inherited Cardiac Arrhythmias: Pathomechanistic Discovery and Drug Development

With the discovery of induced pluripotent stem cell (iPSCs) a wide range of cell types, including iPSC-derived cardiomyocytes (iPSC-CM), can now be generated from an unlimited source of somatic cells. These iPSC-CM are used for different purposes such as disease modelling, drug discovery, cardiotoxicity testing and personalised medicine. The 2D iPSC-CM models have shown promising results, but they are known to be more immature compared to in vivo adult cardiomyocytes. Novel approaches to create 3D models with the possible addition of other (cardiac) cell types are being developed. This will not only improve the maturity of the cells, but also leads to more physiologically relevant models that more closely resemble the human heart. In this review, we focus on the progress in the modelling of inherited cardiac arrhythmias in both 2D and 3D and on the use of these models in therapy development and drug testing.

Morpho-functional comparison of differentiation protocols to create iPSC-derived cardiomyocytes

Cardiomyocytes derived from induced pluripotent stem cells (iPSC-CMs) offer an attractive platform for cardiovascular research. Patient-specific iPSC-CMs are very useful for studying disease development, and bear potential for disease diagnostics, prognosis evaluation and development of personalized treatment. Several monolayer-based serum-free protocols have been described for the differentiation of iPSCs into cardiomyocytes, but data on their performance are scarce. In this study, we evaluated two protocols that are based on temporal modulation of the Wnt/β-catenin pathway for iPSC-CM differentiation from four iPSC lines, including two control individuals and two patients carrying an SCN5A mutation. The SCN5A gene encodes the cardiac voltage-gated sodium channel (Na_v1.5) and loss-of-function mutations can cause the cardiac arrhythmia Brugada syndrome. We performed molecular characterization of the obtained iPSC-CMs by immunostaining for cardiac specific markers and by expression analysis of selected cardiac structural and ionic channel protein-encoding genes with qPCR. We also investigated cell growth morphology, contractility and survival of the iPSC-CMs after dissociation. Finally, we performed electrophysiological characterization of the cells, focusing on the action potential (AP) and calcium transient (CT) characteristics using patch-clamping and optical imaging, respectively. Based on our comprehensive morpho-functional analysis, we concluded that both tested protocols result in a high percentage of contracting CMs. Moreover, they showed acceptable survival and cell quality after dissociation (>50% of cells with a smooth cell membrane, possible to seal during patch-clamping). Both protocols generated cells presenting with typical iPSC-CM AP and CT characteristics, although one protocol (that involves sequential addition of CHIR99021 and Wnt-C59) rendered iPSC-CMs, which were more accessible for patch-clamp and calcium transient experiments and showed an expression pattern of cardiac-specific markers more similar to this observed in human heart left ventricle samples.

Summary: In this study, we evaluated two protocols that are based on temporal modulation of the Wnt/β -catenin pathway for iPSC-CM differentiation from four iPSC lines. We show that both protocols were successful in the generation of contracting iPSC-CMs. However, one of the tested protocols rendered cells that were more accessible for patch-clamp experiments and showed an expression pattern of cardiac-specific markers more similar to this of human heart left ventricle samples.

Clinical Spectrum of SCN5A Channelopathy in Children with Primary Electrical Disease and Structurally Normal Hearts

Sodium voltage-gated channel α subunit 5 (SCN5A)-mutations may cause an array of arrhythmogenic syndromes most frequently as an autosomal dominant trait, with incomplete penetrance, variable expressivity and male predominance. In the present study, we retrospectively describe a group of Mexican patients with SCN5A-disease causing variants in whom the onset of symptoms occurred in the pediatric age range. The study included 17 patients with clinical diagnosis of primary electrical disease, at least one SCN5A pathogenic or likely pathogenic mutation and age of onset <18 years, and all available first- and second-degree relatives. Fifteen patients (88.2%) were male, and sixteen independent variants were found (twelve missense, three truncating and one complex inframe deletion/insertion). The frequency of compound heterozygosity was remarkably high (3/17, 17.6%), with early childhood onset and severe disease. Overall, 70.6% of pediatric patients presented with overlap syndrome, 11.8% with isolated sick sinus syndrome, 11.8% with isolated Brugada syndrome (BrS) and 5.9% with isolated type 3 long QT syndrome (LQTS). A total of 24/45 SCN5A mutation carriers were affected (overall penetrance 53.3%), and penetrance was higher in males (63.3%, 19 affected/30 mutation carriers) than in females (33.3%, 5 affected/15 carriers). In conclusion, pediatric patients with SCNA-disease causing variants presented mainly as overlap syndrome, with predominant loss-of-function phenotypes of sick sinus syndrome (SSS), progressive cardiac conduction disease (PCCD) and ventricular arrhythmias.