Inherited calcium channelopathies in the pathophysiology of arrhythmias

C-terminal residues of skeletal muscle calsequestrin are essential for calcium binding and for skeletal ryanodine receptor inhibition

Background

Skeletal muscle function depends on calcium signaling proteins in the sarcoplasmic reticulum (SR), including the calcium-binding protein calsequestrin (CSQ), the ryanodine receptor (RyR) calcium release channel, and skeletal triadin 95 kDa (trisk95) and junctin, proteins that bind to calsequestrin type 1 (CSQ1) and ryanodine receptor type 1 (RyR1). CSQ1 inhibits RyR1 and communicates store calcium load to RyR1 channels via trisk95 and/or junctin.

Methods

In this manuscript, we test predictions that CSQ1’s acidic C-terminus contains binding sites for trisk95 and junctin, the major calcium binding domain, and that it determines CSQ1’s ability to regulate RyR1 activity.

Results

Progressive alanine substitution of C-terminal acidic residues of CSQ1 caused a parallel reduction in the calcium binding capacity but did not significantly alter CSQ1’s association with trisk95/junctin or influence its inhibition of RyR1 activity. Deletion of the final seven residues in the C-terminus significantly hampered calcium binding, significantly reduced CSQ’s association with trisk95/junctin and decreased its inhibition of RyR1. Deletion of the full C-terminus further reduced calcium binding to CSQ1 altered its association with trisk95 and junctin and abolished its inhibition of RyR1.

Conclusions

The correlation between the number of residues mutated/deleted and binding of calcium, trisk95, and junctin suggests that binding of each depends on diffuse ionic interactions with several C-terminal residues and that these interactions may be required for CSQ1 to maintain normal muscle function.

Functional characterization of CaVα2δ mutations associated with sudden cardiac death

L-type Ca(2+) channels play a critical role in cardiac rhythmicity. These ion channels are oligomeric complexes formed by the pore-forming CaVα1 with the auxiliary CaVβ and CaVα2δ subunits. CaVα2δ increases the peak current density and improves the voltage-dependent activation gating of CaV1.2 channels without increasing the surface expression of the CaVα1 subunit. The functional impact of genetic variants of CACNA2D1 (the gene encoding for CaVα2δ), associated with shorter repolarization QT intervals (the time interval between the Q and the T waves on the cardiac electrocardiogram), was investigated after recombinant expression of the full complement of L-type CaV1.2 subunits in human embryonic kidney 293 cells. By performing side-by-side high resolution flow cytometry assays and whole-cell patch clamp recordings, we revealed that the surface density of the CaVα2δ wild-type protein correlates with the peak current density. Furthermore, the cell surface density of CaVα2δ mutants S755T, Q917H, and S956T was not significantly different from the cell surface density of the CaVα2δ wild-type protein expressed under the same conditions. In contrast, the cell surface expression of CaVα2δ D550Y, CaVα2δ S709N, and the double mutant D550Y/Q917H was reduced, respectively, by ≈30-33% for the single mutants and by 60% for the latter. The cell surface density of D550Y/Q917H was more significantly impaired than protein stability, suggesting that surface trafficking of CaVα2δ was disrupted by the double mutation. Co-expression with D550Y/Q917H significantly decreased CaV1.2 currents as compared with results obtained with CaVα2δ wild type. It is concluded that D550Y/Q917H reduced inward Ca(2+) currents through a defect in the cell surface trafficking of CaVα2δ. Altogether, our results provide novel insight in the molecular mechanism underlying the modulation of CaV1.2 currents by CaVα2δ.

Decreased polycystin 2 expression alters calcium-contraction coupling and changes β-adrenergic signaling pathways

Cardiac disorders are the main cause of mortality in autosomal-dominant polycystic kidney disease (ADPKD). However, how mutated polycystins predispose patients with ADPKD to cardiac pathologies before development of renal dysfunction is unknown. We investigate the effect of decreased levels of polycystin 2 (PC2), a calcium channel that interacts with the ryanodine receptor, on myocardial function. We hypothesize that heterozygous PC2 mice (Pkd2(+/-)) undergo cardiac remodeling as a result of changes in calcium handling, separate from renal complications. We found that Pkd2(+/-) cardiomyocytes have altered calcium handling, independent of desensitized calcium-contraction coupling. Paradoxically, in Pkd2(+/-) mice, protein kinase A (PKA) phosphorylation of phospholamban (PLB) was decreased, whereas PKA phosphorylation of troponin I was increased, explaining the decoupling between calcium signaling and contractility. In silico modeling supported this relationship. Echocardiography measurements showed that Pkd2(+/-) mice have increased left ventricular ejection fraction after stimulation with isoproterenol (ISO), a β-adrenergic receptor (βAR) agonist. Blockers of βAR-1 and βAR-2 inhibited the ISO response in Pkd2(+/-) mice, suggesting that the dephosphorylated state of PLB is primarily by βAR-2 signaling. Importantly, the Pkd2(+/-) mice were normotensive and had no evidence of renal cysts. Our results showed that decreased PC2 levels shifted the βAR pathway balance and changed expression of calcium handling proteins, which resulted in altered cardiac contractility. We propose that PC2 levels in the heart may directly contribute to cardiac remodeling in patients with ADPKD in the absence of renal dysfunction.

Translating advances in cardiogenetics into effective clinical practice

In this article we describe a qualitative research study in which we explored individuals' subjective experiences of both genetic testing and cardiogenetic disorders. Using a grounded theory approach, we coded and analyzed interview and focus group transcripts from 50 participants. We found that just under half of the participants who received their diagnosis during the study reported difficulty understanding information about both the purpose of genetic testing and their cardiac disease. A high level of anxiety about genetic testing and cardiac symptoms exacerbated individuals' cognitive confusion. Participants reported both positive and negative interactions with the medical community, depending on health care professionals' knowledge of cardiogenetic disorders. Overall, participants expressed a range of attitudes--positive, negative, and ambivalent--toward genetic testing. We conclude with a discussion of the barriers to achieving effective clinical care for genetic conditions and offer suggestions for improving collaborative decision making between physicians and patients.

De novo mutations in moderate or severe intellectual disability

Genetics is believed to have an important role in intellectual disability (ID). Recent studies have emphasized the involvement of de novo mutations (DNMs) in ID but the extent to which they contribute to its pathogenesis and the identity of the corresponding genes remain largely unknown. Here, we report a screen for DNMs in subjects with moderate or severe ID. We sequenced the exomes of 41 probands and their parents, and confirmed 81 DNMs affecting the coding sequence or consensus splice sites (1.98 DNMs/proband). We observed a significant excess of de novo single nucleotide substitutions and loss-of-function mutations in these cases compared to control subjects, suggesting that at least a subset of these variations are pathogenic. A total of 12 likely pathogenic DNMs were identified in genes previously associated with ID (ARID1B, CHD2, FOXG1, GABRB3, GATAD2B, GRIN2B, MBD5, MED13L, SETBP1, TBR1, TCF4, WDR45), resulting in a diagnostic yield of ∼29%. We also identified 12 possibly pathogenic DNMs in genes (HNRNPU, WAC, RYR2, SET, EGR1, MYH10, EIF2C1, COL4A3BP, CHMP2A, PPP1CB, VPS4A, PPP2R2B) that have not previously been causally linked to ID. Interestingly, no case was explained by inherited mutations. Protein network analysis indicated that the products of many of these known and candidate genes interact with each other or with products of other ID-associated genes further supporting their involvement in ID. We conclude that DNMs represent a major cause of moderate or severe ID.

Role of RyR2 phosphorylation in heart failure and arrhythmias: protein kinase A-mediated hyperphosphorylation of the ryanodine receptor at serine 2808 does not alter cardiac contractility or cause heart failure and arrhythmias

This Controversies in Research article discusses the hypothesis that protein kinase A (PKA)-mediated phosphorylation of the Ryanodine Receptor (RyR) at a single serine (RyRS2808) is essential for normal sympathetic regulation of cardiac myocyte contractility and is responsible for the disturbed Ca(2+) regulation that underlies depressed contractility in heart failure. Studies supporting this hypothesis have associated hyperphosphorylation of RyRS2808 and heart failure progression in animals and humans and have shown that a phosphorylation defective RyR mutant mouse (RyRS2808A) does not respond normally to sympathetic agonists and does not exhibit heart failure symptoms after myocardial infarction. Studies to confirm and extend these ideas have failed to support the original data. Experiments from many different laboratories have convincingly shown that PKA-mediated RyRS2808 phosphorylation does not play any significant role in the normal sympathetic regulation of sarcoplasmic reticulum Ca2+ release or cardiac contractility. Hearts and myocytes from RyRS2808A mice have been shown to respond normally to sympathetic agonists, and to increase Ca(2+) influx, Ca(2+) transients, and Ca(2+) efflux. Although the RyR is involved in heart failure-related Ca(2+) disturbances, this results from Ca(2+)-calmodulin kinase II and reactive oxygen species-mediated regulation rather than by RyR2808 phosphorylation. Also, a new study has shown that RyRS2808A mice are not protected from myocardial infarction. Collectively, there is now a clear consensus in the published literature showing that dysregulated RyRs contribute to the altered Ca(2+) regulatory phenotype of the failing heart, but PKA-mediated phosphorylation of RyRS2808 has little or no role in these alterations.

Essential role of the zinc finger transcription factor Casz1 for mammalian cardiac morphogenesis and development

Chromosome 1p36 deletion syndrome is one of the most common terminal deletions observed in humans and is related to congenital heart disease (CHD). However, the 1p36 genes that contribute to heart disease have not been clearly delineated. Human CASZ1 gene localizes to 1p36 and encodes a zinc finger transcription factor. Casz1 is required for Xenopus heart ventral midline progenitor cell differentiation. Whether Casz1 plays a role during mammalian heart development is unknown. Our aim is to determine 1p36 gene CASZ1 function at regulating heart development in mammals. We generated a Casz1 knock-out mouse using Casz1-trapped embryonic stem cells. Casz1 deletion in mice resulted in abnormal heart development including hypoplasia of myocardium, ventricular septal defect, and disorganized morphology. Hypoplasia of myocardium was caused by decreased cardiomyocyte proliferation. Comparative genome-wide RNA transcriptome analysis of Casz1 depleted embryonic hearts identifies abnormal expression of genes that are critical for muscular system development and function, such as muscle contraction genes TNNI2, TNNT1, and CKM; contractile fiber gene ACTA1; and cardiac arrhythmia associated ion channel coding genes ABCC9 and CACNA1D. The transcriptional regulation of some of these genes by Casz1 was also found in cellular models. Our results showed that loss of Casz1 during mouse development led to heart defect including cardiac noncompaction and ventricular septal defect, which phenocopies 1p36 deletion syndrome related CHD. This suggests that CASZ1 is a novel 1p36 CHD gene and that the abnormal expression of cardiac morphogenesis and contraction genes induced by loss of Casz1 contributes to the heart defect.

Psychological stress associated with cardiogenetic conditions

AIM

Genetic testing now makes it possible to identify specific mutations that may lead to life-threatening cardiac arrhythmias. This article presents data from a qualitative research study that explored the subjective experiences of individuals and families with cardiogenetic conditions. We focus on describing patients' experiences of psychological stresses associated with having a cardiogenetic condition, illustrating the importance of integrating psychological and medical care. This integration of care is particularly important as personalized genomic medicine continues to evolve and the implications of genetic testing have a profound effect on individuals and families.

METHODS

The researchers interviewed 50 participants from 32 families. The research team used a systematic, grounded theory procedure to code and analyze interview and focus group transcripts, incorporating multiple coders at several stages of the data analysis process.

RESULTS

Three major themes emerged: a bereavement trajectory associated with sudden death in the absence of prior symptoms; high anxiety about transmitting a genetic mutation; and resilience reflected in positive lifestyle changes and participation in support groups.

CONCLUSION

This article identifies patient perspectives on personalized genomic medicine in cardiogenetics that can improve clinical care, including: specialized bereavement counseling; improving education about cardiogenetic conditions for medical professionals; parent guidelines for discussing cardiogenetic conditions with their children; information about support groups; and the routine inclusion of clinical psychologists in interdisciplinary treatment teams. Given recent advances in technology and decreasing costs, whole-genome sequencing is likely to become common practice in the near future. Therefore, these recommendations are likely to be relevant for other genetic conditions, as well as the entire field of personalized genomic medicine.

Insights into the gating mechanism of the ryanodine-modified human cardiac Ca2+-release channel (ryanodine receptor 2)

Ryanodine receptors (RyRs) are intracellular membrane channels playing key roles in many Ca(2+) signaling pathways and, as such, are emerging novel therapeutic and insecticidal targets. RyRs are so named because they bind the plant alkaloid ryanodine with high affinity and although it is established that ryanodine produces profound changes in all aspects of function, our understanding of the mechanisms underlying altered gating is minimal. We address this issue using detailed single-channel gating analysis, mathematical modeling, and energetic evaluation of state transitions establishing that, with ryanodine bound, the RyR pore adopts an extremely stable open conformation. We demonstrate that stability of this state is influenced by interaction of divalent cations with both activating and inhibitory cytosolic sites and, in the absence of activating Ca(2+), trans-membrane voltage. Comparison of the conformational stability of ryanodine- and Imperatoxin A-modified channels identifies significant differences in the mechanisms of action of these qualitatively similar ligands.

Cardiac sodium channels and inherited electrophysiological disorders: an update on the pharmacotherapy

INTRODUCTION

Since the recognition of inherited sodium (Na(+)) channel disease, the cardiac Na(+) channel has been extensively studied. Both loss-of-function and gain-of-function mutations of the cardiac Na(+) channel are associated with cardiac arrhythmia and sudden cardiac death. Pathophysiological mechanisms that may induce arrhythmia are unravelled and include alterations in biophysical properties due to the mutation in SCN5A, drug use and circumstantial factors. Insights into the mechanisms of inherited Na(+) channel disease may result in tailored therapy. However, due to the complexity of cardiac electrical activity and pathophysiological mechanisms, pharmacotherapy in cardiac Na(+) channel disease remains challenging.

AREAS COVERED

This review discusses various mechanisms involved in inherited Na(+) channel disorders, focussing on Brugada syndrome (Brs) and long QT syndrome type 3 (LQTS3). It aims to provide an overview of developments in pharmacotherapy, discussing both treatment and which drugs to avoid to prevent arrhythmia.

EXPERT OPINION

Altered biophysical properties of cardiac Na(+) channels are the basis of arrhythmias in patients with inherited Na(+) channel diseases such as BrS and LQTS3. The effects of such biophysical derangements are strongly modulated by concomitant factors. Tailored drug therapy is required to prevent arrhythmia and is best achieved by educating patients affected by Na(+) channel disorders.

Generation and characterization of a mouse model harboring the exon-3 deletion in the cardiac ryanodine receptor

A large genomic deletion in human cardiac ryanodine receptor (RYR2) gene has been detected in a number of unrelated families with various clinical phenotypes, including catecholaminergic polymorphic ventricular tachycardia (CPVT). This genomic deletion results in an in-frame deletion of exon-3 (Ex3-del). To understand the underlying disease mechanism of the RyR2 Ex3-del mutation, we generated a mouse model in which the RyR2 exon-3 sequence plus 15-bp intron sequences flanking exon-3 were deleted. Heterozygous Ex3-del mice (Ex3-del^+/−) survived, but no homozygous Ex3-del mice were born. Unexpectedly, the Ex3-del^+/− mice are not susceptible to CPVT. Ex3-del^+/− cardiomyocytes exhibited similar amplitude but altered dynamics of depolarization-induced Ca²⁺ transients compared to wild type (WT) cells. Immunoblotting analysis revealed markedly reduced expression of RyR2 protein in the Ex3-del^+/− mutant heart, indicating that Ex3-del has a major impact on RyR2 protein expression in mice. Cardiac specific, conditional knockout of the WT RyR2 allele in Ex3-del^+/− mice led to bradycardia and death. Thus, the absence of CPVT and other phenotypes in Ex3-del^+/− mice may be attributable to the predominant expression of the WT RyR2 allele as a result of the markedly reduced expression of the Ex3-del mutant allele. The effect of Ex3-del on RyR2 protein expression is discussed in relation to the phenotypic variability in individuals with the RyR2 exon-3 deletion.

Mouse models of arrhythmogenic cardiovascular disease: challenges and opportunities

Arrhythmogenic cardiovascular disease is associated with significant morbidity and mortality and, in spite of therapeutic advances, remains an enormous public health burden. The scope of this problem motivates efforts to delineate the molecular, cellular and systemic mechanisms underlying increased arrhythmia risk in inherited and acquired cardiac and systemic disease. The mouse is used increasingly in these efforts owing to the ease with which genetic strategies can be exploited and mechanisms can be probed. The question then arises whether the mouse has proven to be a useful model system to delineate arrhythmogenic cardiovascular disease mechanisms. Rather than trying to provide a definite answer, the goal here is to consider the issues that arise when using mouse models and to highlight the opportunities.

Regional ion channel gene expression heterogeneity and ventricular fibrillation dynamics in human hearts

RATIONALE

Structural differences between ventricular regions may not be the sole determinant of local ventricular fibrillation (VF) dynamics and molecular remodeling may play a role.

OBJECTIVES

To define regional ion channel expression in myopathic hearts compared to normal hearts, and correlate expression to regional VF dynamics.

METHODS AND RESULTS

High throughput real-time RT-PCR was used to quantify the expression patterns of 84 ion-channel, calcium cycling, connexin and related gene transcripts from sites in the LV, septum, and RV in 8 patients undergoing transplantation. An additional eight non-diseased donor human hearts served as controls. To relate local ion channel expression change to VF dynamics localized VF mapping was performed on the explanted myopathic hearts right adjacent to sampled regions. Compared to non-diseased ventricles, significant differences (p<0.05) were identified in the expression of 23 genes in the myopathic LV and 32 genes in the myopathic RV. Within the myopathic hearts significant regional (LV vs septum vs RV) expression differences were observed for 13 subunits: Nav1.1, Cx43, Ca3.1, Cavα2δ2, Cavβ2, HCN2, Na/K ATPase-1, CASQ1, CASQ2, RYR2, Kir2.3, Kir3.4, SUR2 (p<0.05). In a subset of genes we demonstrated differences in protein expression between control and myopathic hearts, which were concordant with the mRNA expression profiles for these genes. Variability in the expression of Cx43, hERG, Na(+)/K(+) ATPase ß1 and Kir2.1 correlated to variability in local VF dynamics (p<0.001). To better understand the contribution of multiple ion channel changes on VF frequency, simulations of a human myocyte model were conducted. These simulations demonstrated the complex nature by which VF dynamics are regulated when multi-channel changes are occurring simultaneously, compared to known linear relationships.

CONCLUSIONS

Ion channel expression profile in myopathic human hearts is significantly altered compared to normal hearts. Multi-channel ion changes influence VF dynamic in a complex manner not predicted by known single channel linear relationships.

Monogene kardiale Ionenkanalerkrankungen

Genetisch bedingte (monogene) Herzerkrankungen bedürfen einer sorgsamen klinischen, genetischen und familiären Diagnostik, da die Erkrankungen mit einem hohen kardiovaskulären Risiko in jungen Jahren assoziiert sein können.

Es handelt sich zumeist um Erkrankungen durch Ionenkanalgenmutationen, die genetisch heterogen und von einer unterschiedlichen Sensitivität in der Mutationsdetektion (pro Erkrankung oder Ionenkanalgen) gekennzeichnet sind. In Analogie zu anderen Ionenkanalerkrankungen besteht oft ein episodisches Auftreten von Symptomen, das durch Trigger (meist erhöhte Herzfrequenz bei körperlicher und/oder physischer Belastung) gefördert werden kann.

Bei diesen relativ seltenen Erkrankungen ist eine frühzeitige Diagnostik und interdisziplinäre Betreuung durch Kardiologen, Kinderkardiologen und Humangenetikern (und ggf. Psychologen) sinnvoll. Mittlerweile existieren erste internationale Empfehlungen, wann eine Genotypisierung aus diagnostischer, therapeutischer oder prognostischer Sicht durchzuführen ist.

Enhanced currents through L-type calcium channels in cardiomyocytes disturb the electrophysiology of the dystrophic heart

Duchenne muscular dystrophy (DMD), induced by mutations in the gene encoding for the cytoskeletal protein dystrophin, is an inherited disease characterized by progressive muscle weakness. Besides the relatively well characterized skeletal muscle degenerative processes, DMD is also associated with cardiac complications. These include cardiomyopathy development and cardiac arrhythmias. The current understanding of the pathomechanisms in the heart is very limited, but recent research indicates that dysfunctional ion channels in dystrophic cardiomyocytes play a role. The aim of the present study was to characterize abnormalities in L-type calcium channel function in adult dystrophic ventricular cardiomyocytes. By using the whole cell patch-clamp technique, the properties of currents through calcium channels in ventricular cardiomyocytes isolated from the hearts of normal and dystrophic adult mice were compared. Besides the commonly used dystrophin-deficient mdx mouse model for human DMD, we also used mdx-utr mice, which are both dystrophin- and utrophin-deficient. We found that calcium channel currents were significantly increased, and channel inactivation was reduced in dystrophic cardiomyocytes. Both effects enhance the calcium influx during an action potential (AP). Whereas the AP in dystrophic mouse cardiomyocytes was nearly normal, implementation of the enhanced dystrophic calcium conductance in a computer model of a human ventricular cardiomyocyte considerably prolonged the AP. Finally, the described dystrophic calcium channel abnormalities entailed alterations in the electrocardiograms of dystrophic mice. We conclude that gain of function in cardiac L-type calcium channels may disturb the electrophysiology of the dystrophic heart and thereby cause arrhythmias.

Nonsense-mediated mRNA decay due to a CACNA1C splicing mutation in a patient with Brugada syndrome

BACKGROUND

Brugada syndrome (BrS) is an inherited cardiac arrhythmia associated with sudden death due to ventricular fibrillation. Mutations in genes related to the cardiac L-type calcium channel have been reported to be causative of BrS. Generally, the messenger RNA (mRNA) that contains a nonsense mutation is rapidly degraded via its decay pathway, which is known as nonsense-mediated mRNA decay (NMD). Previously, we reported a male patient with BrS who carried c.1896G>A (the first nucleotide of CACNA1C exon 14), which caused a synonymous mutation, p.R632R.

OBJECTIVE

To examine how the synonymous CACNA1C mutation p.R632R produces the phenotype of BrS, with a special emphasis on the splicing error and NMD processes.

METHODS

We extracted mRNA from leukocytes of the proband and his 2 children and performed reverse transcription polymerase chain reaction. Complementary DNAs were checked by using direct sequencing and quantitative analysis.

RESULTS

The subsequent sequence electropherogram of the complementary DNAs did not show the substitution of the nucleotide identified in the genomic DNA of the proband. In the mRNA quantification analysis, we confirmed that reduction in the CACNA1C expression level was suspected to be caused by NMD.

CONCLUSIONS

Mutant mRNA with a c.1896G>A substitution may be diminished by NMD, and the resultant decrease in CACNA1C message leads to a novel mechanism for inducing BrS that is distinct from that reported previously.

Induced pluripotent stem cells as cardiac arrhythmic in vitro models and the impact for drug discovery

INTRODUCTION

The development of new antiarrhythmic agents is challenging and is hampered by high attrition rate of novel drug candidates. One of the reasons for this is limited predictability of existing preclinical models for drug assessment. Cardiomyocytes (CMs) derived from disease-specific induced pluripotent stem cells (iPSC) represent a novel in vitro cellular model of cardiac arrhythmias with an unprecedented potential for generating new mechanistic insight into disease pathophysiology and improving the process of drug development.

AREAS COVERED

This review outlines recent studies demonstrating the suitability and limitations of iPSC-derived CMs (iPS-CMs) for in vitro modeling inherited arrhythmias and drug testing. The authors focus on channelopathies and outline the properties of iPS-CMs, highlighting their utility and limitations for investigating the mechanism of cardiac arrhythmias and drug discovery.

EXPERT OPINION

The iPS-CMs represent a valuable addition to the already existing armamentarium of cardiac arrhythmic models. However, the superiority of iPS-CMs over other arrhythmia models has not yet been rigorously established and the limitations of the model must be overcome before its full potential for antiarrhythmic drug discovery can be realized. Nevertheless, iPS cell-based platforms hold a great potential for increasing our knowledge about cellular arrhythmia mechanisms and improving the drug discovery process.

Genetics and sudden death

PURPOSE OF REVIEW

Sudden cardiac death (SCD) affects a significant percentage of young individuals. SCDs are due to genetic heart disorders, such as cardiomyopathies and channelopathies. In the present review, we will describe the recent advancements in understanding the genetic and molecular basis of hereditary cardiac diseases.

RECENT FINDINGS

Considerable progress has been made in identification of new genes associated with monogenic familial arrhythmogenic syndromes, giving the opportunity to delineate their molecular pathogenesis and identify potential targets for therapeutic intervention. Research discoveries and rapidly dropping costs of DNA sequencing technologies have resulted in availability of genetic testing panels.

SUMMARY

Advances in genetic sequencing technology are expected to significantly impact the clinical practice in the near future. Genetic testing represents a powerful tool for cause determination of arrhythmogenic cardiac diseases, efficient screening of family members, possible risk stratification and treatment choices. However, specific expertise is required for rational ordering and correct interpretation of the genetic screening results.

CaMKII inhibition rectifies arrhythmic phenotype in a patient-specific model of catecholaminergic polymorphic ventricular tachycardia

Induced pluripotent stem cells (iPSC) offer a unique opportunity for developmental studies, disease modeling and regenerative medicine approaches in humans. The aim of our study was to create an in vitro 'patient-specific cell-based system' that could facilitate the screening of new therapeutic molecules for the treatment of catecholaminergic polymorphic ventricular tachycardia (CPVT), an inherited form of fatal arrhythmia. Here, we report the development of a cardiac model of CPVT through the generation of iPSC from a CPVT patient carrying a heterozygous mutation in the cardiac ryanodine receptor gene (RyR2) and their subsequent differentiation into cardiomyocytes (CMs). Whole-cell patch-clamp and intracellular electrical recordings of spontaneously beating cells revealed the presence of delayed afterdepolarizations (DADs) in CPVT-CMs, both in resting conditions and after β-adrenergic stimulation, resembling the cardiac phenotype of the patients. Furthermore, treatment with KN-93 (2-[N-(2-hydroxyethyl)]-N-(4methoxybenzenesulfonyl)]amino-N-(4-chlorocinnamyl)-N-methylbenzylamine), an antiarrhythmic drug that inhibits Ca(2+)/calmodulin-dependent serine-threonine protein kinase II (CaMKII), drastically reduced the presence of DADs in CVPT-CMs, rescuing the arrhythmic phenotype induced by catecholaminergic stress. In addition, intracellular calcium transient measurements on 3D beating clusters by fast resolution optical mapping showed that CPVT clusters developed multiple calcium transients, whereas in the wild-type clusters, only single initiations were detected. Such instability is aggravated in the presence of isoproterenol and is attenuated by KN-93. As seen in our RyR2 knock-in CPVT mice, the antiarrhythmic effect of KN-93 is confirmed in these human iPSC-derived cardiac cells, supporting the role of this in vitro system for drug screening and optimization of clinical treatment strategies.

Compound loss of muscleblind-like function in myotonic dystrophy

Myotonic dystrophy (DM) is a multi-systemic disease that impacts cardiac and skeletal muscle as well as the central nervous system (CNS). DM is unusual because it is an RNA-mediated disorder due to the expression of toxic microsatellite expansion RNAs that alter the activities of RNA processing factors, including the muscleblind-like (MBNL) proteins. While these mutant RNAs inhibit MBNL1 splicing activity in heart and skeletal muscles, Mbnl1 knockout mice fail to recapitulate the full-range of DM symptoms in these tissues. Here, we generate mouse Mbnl compound knockouts to test the hypothesis that Mbnl2 functionally compensates for Mbnl1 loss. Although Mbnl1^−/−; Mbnl2^−/− double knockouts (DKOs) are embryonic lethal, Mbnl1^−/−; Mbnl2^+/− mice are viable but develop cardinal features of DM muscle disease including reduced lifespan, heart conduction block, severe myotonia and progressive skeletal muscle weakness. Mbnl2 protein levels are elevated in Mbnl1^−/− knockouts where Mbnl2 targets Mbnl1-regulated exons. These findings support the hypothesis that compound loss of MBNL function is a critical event in DM pathogenesis and provide novel mouse models to investigate additional pathways disrupted in this RNA-mediated disease.

Ion channels, channelopathies, and tooth formation

The biological functions of ion channels in tooth development vary according to the nature of their gating, the species of ions passing through those gates, the number of gates, localization of channels, tissue expressing the channel, and interactions between cells and microenvironment. Ion channels feature unique and specific ion flux in ameloblasts, odontoblasts, and other tooth-specific cell lineages. Both enamel and dentin have active chemical systems orchestrating a variety of ion exchanges and demineralization and remineralization processes in a stage-dependent manner. An important role for ion channels is to regulate and maintain the calcium and pH homeostasis that are critical for proper enamel and dentin biomineralization. Specific functions of chloride channels, TRPVs, calcium channels, potassium channels, and solute carrier superfamily members in tooth formation have been gradually clarified in recent years. Mutations in these ion channels or transporters often result in disastrous changes in tooth development. The channelopathies of tooth include altered eruption (CLCN7, KCNJ2, TRPV3), root dysplasia (CLCN7, KCNJ2), amelogenesis imperfecta (KCNJ1, CFTR, AE2, CACNA1C, GJA1), dentin dysplasia (CLCN5), small teeth (CACNA1C, GJA1), tooth agenesis (CLCN7), and other impairments. The mechanisms leading to tooth channelopathies are primarily related to pH regulation, calcium homeostasis, or other alterations of the niche for tooth eruption and development.

Modeling heart disease in a dish: from somatic cells to disease-relevant cardiomyocytes

A scientific milestone that has tremendously impacted the cardiac research field has been the discovery and establishment of human-induced pluripotent stem cells (hiPSC). Key to this discovery has been uncovering a viable path in generating human patient and disease-specific cardiac cells to dynamically model and study human cardiac diseases in an in vitro setting. Recent studies have demonstrated that hiPSC-derived cardiomyocytes can be used to model and recapitulate various known disease features in hearts of patient donors harboring genetic-based cardiac diseases. Experimental drugs have also been tested in this setting and shown to alleviate disease phenotypes in hiPSC-derived cardiomyocytes, further paving the way for therapeutic interventions for cardiac disease. Here, we review state-of-the-art methods to generate high-quality hiPSC and differentiate them towards cardiomyocytes as well as the full range of genetic-based cardiac diseases, which have been modeled using hiPSC. We also provide future perspectives on exploiting the potential of hiPSC to compliment existing studies and gain new insights into the mechanisms underlying cardiac disease.

The incidental pore: CaV1.2 and stem cell activation in quiescent hair follicles

The hair follicle undergoes a lifelong developmental cycle that depends on the integration between activating and inhibitory signals acting to regulate and guide the proliferation and differentiation of pluripotent epithelial stem cells. The effectors and mechanisms responsible for re-entry of quiescent telogen hair follicles into the hair-producing anagen stage in mature skin remain incompletely understood. In the June 1, 2013, issue of Genes & Development, Yucel and colleagues (pp. 1217-1222) reported the unexpected finding that CaV1.2, the pore-forming subunit in a well-characterized voltage-gated, L-type calcium channel, is expressed in hair follicle stem cells and contributes to anagen re-entry but does so in a calcium flux-independent fashion.

Modeling Timothy syndrome with iPS cells

Genetic mutations in ion channel genes that are associated with cardiac arrhythmias have been identified over the past several decades. However, little is known about the pathophysiological processes. An important limitation has been the difficulty of using human cardiomyocytes to study arrhythmias and identify drugs. To circumvent this issue, we have developed a method using human-induced pluripotent stem cells to generate cardiomyocytes from individuals with Timothy syndrome (TS), a genetic disorder characterized by QT prolongation, ventricular tachycardia, and autism. The TS ventricular-like cardiomyocytes exhibit deficits in contraction, electrical signaling, and calcium handling, as revealed by live cell imaging and electrophysiological studies. We tested candidate drugs in TS cardiomyocytes and found that roscovitine could successfully rescue these cellular phenotypes. The use of a human cellular model of cardiac arrhythmias provides a useful new platform not only to study disease mechanisms but also to develop new therapies to treat cardiac arrhythmias.

Determinants of incomplete penetrance and variable expressivity in heritable cardiac arrhythmia syndromes

Mutations in genes encoding ion channel pore-forming α-subunits and accessory β-subunits as well as intracellular calcium-handling proteins that collectively maintain the electromechanical function of the human heart serve as the underlying pathogenic substrate for a spectrum of sudden cardiac death (SCD)-predisposing heritable cardiac arrhythmia syndromes, including long QT syndrome (LQTS), short QT syndrome (SQTS), Brugada syndrome (BrS), and catecholaminergic polymorphic ventricular tachycardia (CPVT). Similar to many Mendelian disorders, the cardiac "channelopathies" exhibit incomplete penetrance, variable expressivity, and phenotypic overlap, whereby genotype-positive individuals within the same genetic lineage assume vastly different clinical courses as objectively assessed by phenotypic features such electrocardiographic abnormalities and number/type of cardiac events. In this Review, we summarize the current understanding of the global architecture of complex electrocardiographic traits such as the QT interval, focusing on the role of common genetic variants in the modulation of ECG parameters in health and the environmental and genetic determinants of incomplete penetrance and variable expressivity in the heritable cardiac arrhythmia syndromes most likely to be encountered in clinical practice.