CALM3 mutation associated with long QT syndrome

Generation of a human induced pluripotent stem cell line ZZUNEUi030-A from a female patient carrying a heterozygous CALM2 (c.395 A > T) mutation

Calmodulin mutations can cause life-threatening long QT syndrome involving CALM1, CALM2, and CALM3. In this study, human induced pluripotent stem cells ZZUNEUi030-A were derived from a female patient with heterozygous CALM2 gene c. 395A → T by Sendai virus non-integrated reprogramming technology. The cell line showed a normal female karyotype (46, XX), expressed pluripotency markers, and had the ability to differentiate into three germ layers in vitro. ZZUNEUi030-A can be used as a cell disease model to study the pathogenesis of LQT caused by calmodulin mutations.

Divergent Biochemical Properties and Disparate Impact of Arrhythmogenic Calmodulin Mutations on Zebrafish Cardiac Function

Calmodulin (CaM) is a ubiquitous, small cytosolic calcium (Ca2+)-binding sensor that plays a vital role in many cellular processes by binding and regulating the activity of over 300 protein targets. In cardiac muscle, CaM modulates directly or indirectly the activity of several proteins that play a key role in excitation-contraction coupling (ECC), such as ryanodine receptor type 2 (RyR2), l-type Ca2+ (Cav1.2), sodium (NaV1.5) and potassium (KV7.1) channels. Many recent clinical and genetic studies have reported a series of CaM mutations in patients with life-threatening arrhythmogenic syndromes, such as long QT syndrome (LQTS) and catecholaminergic polymorphic ventricular tachycardia (CPVT). We recently showed that four arrhythmogenic CaM mutations (N98I, D132E, D134H, and Q136P) significantly reduce the binding of CaM to RyR2. Herein, we investigate in vivo functional effects of these CaM mutations on the normal zebrafish embryonic heart function by microinjecting complementary RNA corresponding to CaMN98I, CaMD132E, CaMD134H, and CaMQ136P mutants. Expression of CaMD132E and CaMD134H mutants results in significant reduction of the zebrafish heart rate, mimicking a severe form of human bradycardia, whereas expression of CaMQ136P results in an increased heart rate mimicking human ventricular tachycardia. Moreover, analysis of cardiac ventricular rhythm revealed that the CaMD132E and CaMN98I zebrafish groups display an irregular pattern of heart beating and increased amplitude in comparison to the control groups. Furthermore, circular dichroism spectroscopy experiments using recombinant CaM proteins reveals a decreased structural stability of the four mutants compared to the wild-type CaM protein in the presence of Ca2+. Finally, Ca2+-binding studies indicates that all CaM mutations display reduced CaM Ca2+-binding affinities, with CaMD132E exhibiting the most prominent change. Our data suggest that CaM mutations can trigger different arrhythmogenic phenotypes through multiple and complex molecular mechanisms.

Glycogen metabolism-mediated intercellular communication in the tumor microenvironment influences liver cancer prognosis

Glycogen metabolism plays a key role in the development of hepatocellular carcinoma (HCC), but the function of glycogen metabolism genes in the tumor microenvironment (TME) is still to be elucidated. Single-cell RNA-seq data were obtained from ten HCC tumor samples totaling 64,545 cells, and 65 glycogen metabolism genes were analyzed by a nonnegative matrix factorization (NMF). The prognosis and immune response of new glycogen TME cell clusters were predicted by using HCC and immunotherapy cohorts from public databases. HCC single-cell analysis was divided into fibroblasts, NT T cells, macrophages, endothelial cells, and B cells, which were separately divided into new cell clusters by glycogen metabolism gene annotation. Pseudo-temporal trajectory analysis demonstrated the temporal differentiation trajectory of different glycogen subtype cell clusters. Cellular communication analysis revealed extensive interactions between endothelial cells with glycogen metabolizing TME cell-related subtypes and different glycogen subtype cell clusters. SCENIC analysis of transcription factors upstream of TME cell clusters with different glycogen metabolism. In addition, TME cell clusters of glycogen metabolism were found to be enriched in expression in CAF subtypes, CD8 depleted, M1, and M2 types. Bulk-seq analysis showed the prognostic significance of glycogen metabolism-mediated TME cell clusters in HCC, while a significant immune response was found in the immunotherapy cohort in patients treated with immune checkpoint blockade (ICB), especially for CAFs, T cells, and macrophages. In summary, our study reveals for the first time that glycogen metabolism mediates intercellular communication in the hepatocellular carcinoma microenvironment while elucidating the anti-tumor mechanisms and immune prognostic responses of different subtypes of cell clusters.

Calmodulin Mutations in Human Disease

Calcium ions (Ca²⁺) are the basis of a unique and potent array of cellular responses. Calmodulin (CaM) is a small but vital protein that is able to rapidly transmit information about changes in Ca²⁺ concentrations to its regulatory targets. CaM plays a critical role in cellular Ca²⁺ signaling, and interacts with a myriad of target proteins. Ca²⁺-dependent modulation by CaM is a major component of a diverse array of processes, ranging from gene expression in neurons to the shaping of the cardiac action potential in heart cells. Furthermore, the protein sequence of CaM is highly evolutionarily conserved, and identical CaM proteins are encoded by three independent genes (CALM1-3) in humans. Mutations within any of these three genes may lead to severe cardiac deficits including severe long QT syndrome (LQTS) and/or catecholaminergic polymorphic ventricular tachycardia (CPVT). Research into disease-associated CaM variants has identified several proteins modulated by CaM that are likely to underlie the pathogenesis of these calmodulinopathies, including the cardiac L-type Ca²⁺ channel (LTCC) Ca_V1.2, and the sarcoplasmic reticulum Ca²⁺ release channel, ryanodine receptor 2 (RyR2). Here, we review the research that has been done to identify calmodulinopathic CaM mutations and evaluate the mechanisms underlying their role in disease.

CALM3 affects the prognosis of leukemia and hemorrhoids

Leukemia is an abnormal proliferation of white blood cells in the bone marrow, resulting in a large accumulation of abnormal leukemia cells in the blood and bone marrow. Hemorrhoids are dilated and swollen veins in the rectum or anal area. However, the relationship between CALM3 and leukemia and hemorrhoids remains unclear. The hemorrhoids dataset GSE154650 and leukemia dataset GSE26294 were downloaded from GEO databases generated by GPL20301 and GPL571.The R package limma was used to screen differentially expressed genes (DEDs). Weighted gene co-expression network analysis (WGCNA) was performed. The construction and analysis of protein-protein interaction (PPI) network, functional enrichment analysis, Gene Set Enrichment Analysis (GSEA) and comparative toxicogenomics database (CTD) analysis were performed. TargetScan was used to screen miRNAs regulating central DEGs. It was verified by western blot basic cell assay. A total of 125 DEGs were co-identified. According to the GO analysis, they are mainly enriched in small molecule catabolic processes, skin development, and chemokine receptor binding. The KEGG analysis results show that the target cells are mainly enriched in the interaction of cytokines and cytokine receptors, as well as butyric acid metabolism. The GSEA analysis results indicate enrichment in small molecule catabolic processes, skin development, and chemokine receptor binding. Six core genes (CALM3, ACE2, PPARGC1A, XCR1, CFTR, PRKCA) were identified. We found that the core gene CALM3 is highly expressed in hemorrhoid samples, low in leukemia samples, and has low expression in normal samples, which may play a regulatory role in hemorrhoids and leukemia. Immunoinfiltration results showed a higher proportion of T_cells_CD4_memory_resting and a correlation with T_cells_CD8. WB experiment verified the result. CALM3 expression is low in leukemia, and the lower the expression is, the worse the prognosis is. CALM3 is highly expressed in hemorrhoids, and the higher the expression, the worse the prognosis.

Long QT syndrome-associated calmodulin variants disrupt the activity of the slowly activating delayed rectifier potassium channel

Calmodulin (CaM) is a highly conserved mediator of calcium (Ca2+ )-dependent signalling and modulates various cardiac ion channels. Genotyping has revealed several CaM mutations associated with long QT syndrome (LQTS). LQTS patients display prolonged ventricular recovery times (QT interval), increasing their risk of incurring life-threatening arrhythmic events. Loss-of-function mutations to Kv7.1 (which drives the slow delayed rectifier potassium current, IKs, a key ventricular repolarising current) are the largest contributor to congenital LQTS (>50% of cases). CaM modulates Kv7.1 to produce a Ca2+ -sensitive IKs, but little is known about the consequences of LQTS-associated CaM mutations on Kv7.1 function. Here, we present novel data characterising the biophysical and modulatory properties of three LQTS-associated CaM variants (D95V, N97I and D131H). We showed that mutations induced structural alterations in CaM and reduced affinity for Kv7.1, when compared with wild-type (WT). Using HEK293T cells expressing Kv7.1 channel subunits (KCNQ1/KCNE1) and patch-clamp electrophysiology, we demonstrated that LQTS-associated CaM variants reduced current density at systolic Ca2+ concentrations (1 μm), revealing a direct QT-prolonging modulatory effect. Our data highlight for the first time that LQTS-associated perturbations to CaM's structure impede complex formation with Kv7.1 and subsequently result in reduced IKs. This provides a novel mechanistic insight into how the perturbed structure-function relationship of CaM variants contributes to the LQTS phenotype. KEY POINTS: Calmodulin (CaM) is a ubiquitous, highly conserved calcium (Ca2+ ) sensor playing a key role in cardiac muscle contraction. Genotyping has revealed several CaM mutations associated with long QT syndrome (LQTS), a life-threatening cardiac arrhythmia syndrome. LQTS-associated CaM variants (D95V, N97I and D131H) induced structural alterations, altered binding to Kv7.1 and reduced IKs. Our data provide a novel mechanistic insight into how the perturbed structure-function relationship of CaM variants contributes to the LQTS phenotype.

Novel Calmodulin Variant p.E46K Associated With Severe Catecholaminergic Polymorphic Ventricular Tachycardia Produces Robust Arrhythmogenicity in Human Induced Pluripotent Stem Cell-Derived Cardiomyocytes

BACKGROUND

CaM (calmodulin) is a ubiquitously expressed, multifunctional Ca²⁺ sensor protein that regulates numerous proteins. Recently, CaM missense variants have been identified in patients with malignant inherited arrhythmias, such as long QT syndrome and catecholaminergic polymorphic ventricular tachycardia (CPVT). However, the exact mechanism of CaM-related CPVT in human cardiomyocytes remains unclear. In this study, we sought to investigate the arrhythmogenic mechanism of CPVT caused by a novel variant using human induced pluripotent stem cell (iPSC) models and biochemical assays.

METHODS

We generated iPSCs from a patient with CPVT bearing CALM2 p.E46K. As comparisons, we used 2 control lines including an isogenic line, and another iPSC line from a patient with long QT syndrome bearing CALM2 p.N98S (also reported in CPVT). Electrophysiological properties were investigated using iPSC-cardiomyocytes. We further examined the RyR2 (ryanodine receptor 2) and Ca²⁺ affinities of CaM using recombinant proteins.

RESULTS

We identified a novel de novo heterozygous variant, CALM2 p.E46K, in 2 unrelated patients with CPVT accompanied by neurodevelopmental disorders. The E46K-cardiomyocytes exhibited more frequent abnormal electrical excitations and Ca²⁺ waves than the other lines in association with increased Ca²⁺ leakage from the sarcoplasmic reticulum via RyR2. Furthermore, the [³H]ryanodine binding assay revealed that E46K-CaM facilitated RyR2 function especially by activating at low [Ca²⁺] levels. The real-time CaM-RyR2 binding analysis demonstrated that E46K-CaM had a 10-fold increased RyR2 binding affinity compared with wild-type CaM which may account for the dominant effect of the mutant CaM. Additionally, the E46K-CaM did not affect CaM-Ca²⁺ binding or L-type calcium channel function. Finally, antiarrhythmic agents, nadolol and flecainide, suppressed abnormal Ca²⁺ waves in E46K-cardiomyocytes.

CONCLUSIONS

We, for the first time, established a CaM-related CPVT iPSC-CM model which recapitulated severe arrhythmogenic features resulting from E46K-CaM dominantly binding and facilitating RyR2. In addition, the findings in iPSC-based drug testing will contribute to precision medicine.

Calcium Handling in Inherited Cardiac Diseases: A Focus on Catecholaminergic Polymorphic Ventricular Tachycardia and Hypertrophic Cardiomyopathy

Calcium (Ca2+) is the major mediator of cardiac contractile function. It plays a key role in regulating excitation-contraction coupling and modulating the systolic and diastolic phases. Defective handling of intracellular Ca2+ can cause different types of cardiac dysfunction. Thus, the remodeling of Ca2+ handling has been proposed to be a part of the pathological mechanism leading to electrical and structural heart diseases. Indeed, to ensure appropriate electrical cardiac conduction and contraction, Ca2+ levels are regulated by several Ca2+-related proteins. This review focuses on the genetic etiology of cardiac diseases related to calcium mishandling. We will approach the subject by focalizing on two clinical entities: catecholaminergic polymorphic ventricular tachycardia (CPVT) as a cardiac channelopathy and hypertrophic cardiomyopathy (HCM) as a primary cardiomyopathy. Further, this review will illustrate the fact that despite the genetic and allelic heterogeneity of cardiac defects, calcium-handling perturbations are the common pathophysiological mechanism. The newly identified calcium-related genes and the genetic overlap between the associated heart diseases are also discussed in this review.

Role of Ca2+ in healthy and pathologic cardiac function: from normal excitation-contraction coupling to mutations that cause inherited arrhythmia

Calcium (Ca²⁺) ions are a key second messenger involved in the rhythmic excitation and contraction of cardiomyocytes throughout the heart. Proper function of Ca²⁺-handling proteins is required for healthy cardiac function, whereas disruption in any of these can cause cardiac arrhythmias. This comprehensive review provides a broad overview of the roles of Ca²⁺-handling proteins and their regulators in healthy cardiac function and the mechanisms by which mutations in these proteins contribute to inherited arrhythmias. Major Ca²⁺ channels and Ca²⁺-sensitive regulatory proteins involved in cardiac excitation-contraction coupling are discussed, with special emphasis on the function of the RyR2 macromolecular complex. Inherited arrhythmia disorders including catecholaminergic polymorphic ventricular tachycardia, long QT syndrome, Brugada syndrome, short QT syndrome, and arrhythmogenic right-ventricular cardiomyopathy are discussed with particular emphasis on subtypes caused by mutations in Ca²⁺-handling proteins.

Calmodulin variant E140G associated with long QT syndrome impairs CaMKIIδ autophosphorylation and L-type calcium channel inactivation

Long QT syndrome (LQTS) is a human inherited heart condition that can cause life-threatening arrhythmia including sudden cardiac death. Mutations in the ubiquitous Ca²⁺-sensing protein calmodulin (CaM) are associated with LQTS, but the molecular mechanism by which these mutations lead to irregular heartbeats is not fully understood. Here, we use a multidisciplinary approach including protein biophysics, structural biology, confocal imaging, and patch-clamp electrophysiology to determine the effect of the disease-associated CaM mutation E140G on CaM structure and function. We present novel data showing that mutant-regulated CaMKIIδ kinase activity is impaired with a significant reduction in enzyme autophosphorylation rate. We report the first high-resolution crystal structure of a LQTS-associated CaM variant in complex with the CaMKIIδ peptide, which shows significant structural differences, compared to the WT complex. Furthermore, we demonstrate that the E140G mutation significantly disrupted Ca_v1.2 Ca²⁺/CaM-dependent inactivation, while cardiac ryanodine receptor (RyR2) activity remained unaffected. In addition, we show that the LQTS-associated mutation alters CaM's Ca²⁺-binding characteristics, secondary structure content, and interaction with key partners involved in excitation-contraction coupling (CaMKIIδ, Ca_v1.2, RyR2). In conclusion, LQTS-associated CaM mutation E140G severely impacts the structure-function relationship of CaM and its regulation of CaMKIIδ and Ca_v1.2. This provides a crucial insight into the molecular factors contributing to CaM-mediated arrhythmias with a central role for CaMKIIδ.

Understanding Calmodulin Variants Affecting Calcium-Dependent Inactivation of L-Type Calcium Channels through Whole-Cell Simulation of the Cardiac Ventricular Myocyte

Mutations in the calcium-sensing protein calmodulin (CaM) have been linked to two cardiac arrhythmia diseases, Long QT Syndrome 14 (LQT14) and Catecholaminergic Polymorphic Ventricular Tachycardia Type 4 (CPVT4), with varying degrees of severity. Functional characterization of the CaM mutants most strongly associated with LQT14 show a clear disruption of the calcium-dependent inactivation (CDI) of the L-Type calcium channel (LCC). CPVT4 mutants on the other hand are associated with changes in their affinity to the ryanodine receptor. In clinical studies, some variants have been associated with both CPVT4 and LQT15. This study uses simulations in a model for excitation-contraction coupling in the rat ventricular myocytes to understand how LQT14 variant might give the functional phenotype similar to CPVT4. Changing the CaM-dependent transition rate by a factor of 0.75 corresponding to the D96V variant and by a factor of 0.90 corresponding to the F142L or N98S variants, in a physiologically based stochastic model of the LCC prolonger, the action potential duration changed by a small amount in a cardiac myocyte but did not disrupt CICR at 1, 2, and 4 Hz. Under beta-adrenergic simulation abnormal excitation-contraction coupling was observed above 2 Hz pacing for the mutant CaM. The same conditions applied under beta-adrenergic stimulation led to the rapid onset of arrhythmia in the mutant CaM simulations. Simulations with the LQT14 mutations under the conditions of rapid pacing with beta-adrenergic stimulation drives the cardiac myocyte toward an arrhythmic state known as Ca2+ overload. These simulations provide a mechanistic link to a disease state for LQT14-associated mutations in CaM to yield a CPVT4 phenotype. The results show that small changes to the CaM-regulated inactivation of LCC promote arrhythmia and underscore the significance of CDI in proper heart function.

Calcium Signalling in Heart and Vessels: Role of Calmodulin and Downstream Calmodulin-Dependent Protein Kinases

Cardiovascular disease is the major cause of death worldwide. The success of medication and other preventive measures introduced in the last century have not yet halted the epidemic of cardiovascular disease. Although the molecular mechanisms of the pathophysiology of the heart and vessels have been extensively studied, the burden of ischemic cardiovascular conditions has risen to become a top cause of morbidity and mortality. Calcium has important functions in the cardiovascular system. Calcium is involved in the mechanism of excitation-contraction coupling that regulates numerous events, ranging from the production of action potentials to the contraction of cardiomyocytes and vascular smooth muscle cells. Both in the heart and vessels, the rise of intracellular calcium is sensed by calmodulin, a protein that regulates and activates downstream kinases involved in regulating calcium signalling. Among them is the calcium calmodulin kinase family, which is involved in the regulation of cardiac functions. In this review, we present the current literature regarding the role of calcium/calmodulin pathways in the heart and vessels with the aim to summarize our mechanistic understanding of this process and to open novel avenues for research.

Novel CALM3 Variant Causing Calmodulinopathy With Variable Expressivity in a 4-Generation Family

BACKGROUND

CaM (calmodulin), encoded by 3 separate genes (CALM1, CALM2, and CALM3), is a multifunctional Ca²⁺-binding protein involved in many signal transduction events including ion channel regulation. CaM variants may present with early-onset long QT syndrome (LQTS), catecholaminergic polymorphic ventricular tachycardia, or sudden cardiac death. Most reported variants occurred de novo. We identified a novel CALM3 variant, p.Asn138Lys (N138K), in a 4-generation family segregating with LQTS. The aim of this study was to elucidate its pathogenicity and to compare it with that of p.D130G-CaM-a variant associated with a severe LQTS phenotype.

METHODS

We performed whole exome sequencing for a large, 4-generation family affected by LQTS. To assess the effect of the detected CALM3 variant, the intrinsic Ca²⁺-binding affinity was measured by stoichiometric Ca²⁺ titrations and equilibrium titrations. L-type Ca²⁺ and slow delayed rectifier potassium currents (I_CaL and I_Ks) were recorded by whole-cell patch-clamp. Cav1.2 and Kv7.1 membrane expression were determined by optical fluorescence assays.

RESULTS

We identified 14 p.N138K-CaM carriers in a family where 2 sudden deaths occurred in children. Several members were only mildly affected compared with CaM-LQTS patients to date described in literature. The intrinsic Ca²⁺-binding affinity of the CaM C-terminal domain was 10-fold lower for p.N138K-CaM compared with wild-type-CaM. I_CaL inactivation was slowed in cells expressing p.N138K-CaM but less than in p.D130G-CaM cells. Unexpectedly, a larger I_Ks current density was observed in cells expressing p.N138K-CaM, but not for p.D130G-CaM, compared with wild-type-CaM.

CONCLUSIONS

The p.N138K CALM3 variant impairs Ca²⁺-binding affinity of CaM and I_CaL inactivation but potentiates I_Ks. The variably expressed phenotype of this variant compared with previously published de novo LQTS-CaM variants is likely explained by a milder impairment of I_CaL inactivation combined with I_Ks augmentation.

Integrated Pharmacogenetics Analysis of the Three Fangjis Decoctions for Treating Arrhythmias Based on Molecular Network Patterns

Aim: To explore the diverse target distribution and variable mechanisms of different fangjis prescriptions when treating arrhythmias based on the systems pharmacology.

Methods: The active ingredients and their corresponding targets were acquired from the three fangjis [Zhigancao Tang (ZT), Guizhigancao Longgumuli Tang (GLT), and Huanglian E'jiao Tang (HET)] and the arrhythmia-related genes were identified based on comprehensive database screening. Networks were constructed between the fangjis and arrhythmia and used to define arrhythmia modules. Common and differential gene targets were identified within the arrhythmia network modules and the cover rate (CR) matrix was applied to compare the contributions of the fangjis to the network and modules. Comparative pharmacogenetics analyses were then conducted to define the arrhythmia-related signaling pathways regulated by the fangjis prescriptions. Finally, the divergence and convergence points of the arrhythmia pathways were deciphered based on databases and the published literature.

Results: A total of 187, 105, and 68 active ingredients and 1,139, 1,195, and 811 corresponding gene targets of the three fangjis were obtained and 102 arrhythmia-related genes were acquired. An arrhythmia network was constructed and subdivided into 4 modules. For the target distribution analysis, 65.4% of genes were regulated by the three fangjis within the arrhythmia network. ZT and GLT were more similar to each other, mainly regulated by module two, whereas HET was divided among all the modules. From the perspective of signal transduction, calcium-related pathways [calcium, cyclic guanosine 3′,5′-monophosphate (cGMP)-PKG, and cyclic adenosine 3′,5′-monophosphate (cAMP)] and endocrine system-related pathways (oxytocin signaling pathway and renin secretion pathways) were associated with all the three fangjis prescriptions. Nevertheless, heterogeneity existed between the biological processes and pathway distribution among the three prescriptions. GLT and HET were particularly inclined toward the conditions involving abnormal hormone secretion, whereas ZT tended toward renin-angiotensin-aldosterone system (RAAS) disorders. However, calcium signaling-related pathways prominently feature in the pharmacological activities of the decoctions. Experimental validation indicated that ZT, GLT, and HET significantly shortened the duration of ventricular arrhythmia (VA) and downregulated the expression of CALM2 and interleukin-6 (IL-6) messenger RNAs (mRNAs); GLT and HET downregulated the expression of CALM1 and NOS3 mRNAs; HET downregulated the expression of CRP mRNA.

Conclusion: Comparing the various distributions of the three fangjis, pathways provide evidence with respect to precise applications toward individualized arrhythmia treatments.

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.

The link between abnormalities of calcium handling proteins and catecholaminergic polymorphic ventricular tachycardia

Catecholaminergic polymorphic ventricular tachycardia (CPVT), a rare autosomal dominant or recessive disease, usually results in syncope or sudden cardiac death. Most CPVT patients do not show abnormal cardiac structure and electrocardiogram features and symptoms, usually onset during adrenergically mediated physiological conditions. CPVT tends to occur at a younger age and is not easy to be diagnosed and managed. The main cause of CPVT is associated with mishandling Ca²⁺ in cardiomyocytes. Intracellular Ca²⁺ is strictly controlled by a protein located in the sarcoplasm reticulum (SR), such as ryanodine receptor, histidine-rich Ca²⁺-binding protein, triadin, and junctin. Mutation in these proteins results in misfolding or malfunction of these proteins, thereby affecting their Ca²⁺-binding affinity, and subsequently disturbs Ca²⁺ homeostasis during excitation–contraction coupling (E-C coupling). Furthermore, transient disturbance of Ca²⁺ homeostasis increases membrane potential and causes Ca²⁺ store overload-induced Ca²⁺ release, which in turn leads to delayed after depolarization and arrhythmia. Previous studies have focused on the interaction between ryanodine receptors and protein kinase or phosphatase in the cytosol. However, recent studies showed the regulation signaling for ryanodine receptor not only from the cytosol but also within the SR. The changing of Ca²⁺ concentration is critical for protein interaction inside the SR which changes protein conformation to regulate the open probability of ryanodine receptors. Thus, it influences the threshold of Ca²⁺ released from the SR, making it easier to release Ca²⁺ during E-C coupling. In this review, we briefly discuss how Ca²⁺ handling protein variations affect the Ca²⁺ handling in CPVT.

The Role of METTL3-Mediated N6-Methyladenosine (m6A) of JPH2 mRNA in Cyclophosphamide-Induced Cardiotoxicity

Cyclophosphamide (CYP)-induced cardiotoxicity is a common side effect of cancer treatment. Although it has received significant attention, the related mechanisms of CYP-induced cardiotoxicity remain largely unknown. In this study, we used cell and animal models to investigate the effect of CYP on cardiomyocytes. Our data demonstrated that CYP-induced a prolonged cardiac QT interval and electromechanical coupling time courses accompanied by JPH2 downregulation. Moreover, N6-methyladenosine (m6A) methylation sequencing and RNA sequencing suggested that CYP induced cardiotoxicity by dysregulating calcium signaling. Importantly, our results demonstrated that CYP induced an increase in the m6A level of JPH2 mRNA by upregulating methyltransferases METTL3, leading to the reduction of JPH2 expression levels, as well as increased field potential duration and action potential duration in cardiomyocytes. Our results revealed a novel mechanism for m6A methylation-dependent regulation of JPH2, which provides new strategies for the treatment and prevention of CYP-induced cardiotoxicity.

Calmodulinopathy in inherited arrhythmia syndromes

Calmodulin (CaM) is a ubiquitous intracellular calcium sensor that controls and regulates key cellular functions. In all vertebrates, three CaM genes located on separate chromosomes encode an identical 149 amino acid protein, implying an extraordinarily high level of evolutionary importance and suggesting that CaM mutations would be possibly fatal. Inherited arrhythmia syndromes comprise a spectrum of primary electrical disorders caused by mutations in genes encoding ion channels or associated proteins leading to various cardiac arrhythmias, unexplained syncope, and sudden cardiac death. CaM mutations have emerged as an independent entity among inherited arrhythmia syndromes, referred to as calmodulinopathies. The most common clinical presentation associated with calmodulinopathy is congenital long QT syndrome, followed by catecholaminergic polymorphic ventricular tachycardia, both of which significantly increase the possibility of repeated syncope, lethal arrhythmic events, and sudden cardiac death, especially in young individuals. Here, we aim to give an overview of biochemical and structural characteristics of CaM and progress toward updating current known CaM mutations and associated clinical phenotypes. We also review the possible mechanisms underlying calmodulinopathy, based on several key in vitro studies. We expect that further experimental studies are needed to explore the complexity of calmodulinopathy.

Different arrhythmia-associated calmodulin mutations have distinct effects on cardiac SK channel regulation

Calmodulin (CaM) plays a critical role in intracellular signaling and regulation of Ca²⁺-dependent proteins and ion channels. Mutations in CaM cause life-threatening cardiac arrhythmias. Among the known CaM targets, small-conductance Ca²⁺-activated K⁺ (SK) channels are unique, since they are gated solely by beat-to-beat changes in intracellular Ca²⁺. However, the molecular mechanisms of how CaM mutations may affect the function of SK channels remain incompletely understood. To address the structural and functional effects of these mutations, we introduced prototypical human CaM mutations in human induced pluripotent stem cell–derived cardiomyocyte-like cells (hiPSC-CMs). Using structural modeling and molecular dynamics simulation, we demonstrate that human calmodulinopathy-associated CaM mutations disrupt cardiac SK channel function via distinct mechanisms. CaM_D96V and CaM_D130G mutants reduce SK currents through a dominant-negative fashion. By contrast, specific mutations replacing phenylalanine with leucine result in conformational changes that affect helix packing in the C-lobe, which disengage the interactions between apo-CaM and the CaM-binding domain of SK channels. Distinct mutant CaMs may result in a significant reduction in the activation of the SK channels, leading to a decrease in the key Ca²⁺-dependent repolarization currents these channels mediate. The findings in this study may be generalizable to other interactions of mutant CaMs with Ca²⁺-dependent proteins within cardiac myocytes.

CALM1 rs3179089 polymorphism might contribute to coronary artery disease susceptibility in Chinese male: a case-control study

BACKGROUND

Calmodulin 1 (CALM1) mutations are involved in the development of coronary artery disease (CAD). However, the relationship of CALM1 rs3179089 polymorphism with CAD is unknown.

OBJECTIVE

This study aimed to identify the relationship of CALM1 rs3179089 polymorphism with CAD susceptibility, CALM1 expression, blood pressure, blood glucose, blood coagulation and serum lipid levels of CAD patients.

METHODS

550 CAD patients and 550 control subjects were genotyped for CALM1 using Sequenom MassARRAY technology. CALM1 expression level was measured by quantitative real time polymerase chain reaction (qRT-PCR).

RESULTS

CALM1 mRNA expression was higher in CAD patients than that in control subjects (P < 0.001). CAD patients with CC genotype had higher CALM1 mRNA expression level than control subjects with CC genotype (P = 0.006). Genotypic frequency of rs3179089 was different between male patients of CAD and control subjects (P = 0.045). Rs3179089 polymorphism was related to CAD risk of males in recessive model (P = 0.039). Moreover, rs3179089 polymorphism was associated with systolic blood pressure (SBP), diastolic blood pressure (DBP), fasting plasma glucose (FPG), and D-Dimer (D-D) level of patients with CAD in recessive model (P = 0.013 for SBP; P = 0.034 for DBP; P = 0.004 for FPG; P = 0.046 for D-D). In addition, rs3179089 polymorphism was correlated with low-density lipoprotein cholesterol (LDL-C) and total cholesterol (TC) serum levels of patients with CAD in both addictive (P = 0.025 for LDL-C; P = 0.001 for TC) and recessive models (P = 0.001 for LDL-C; P = 0.001 for TC).

CONCLUSION

CALM1 expression is associated with development of CAD. CALM1 rs3179089 polymorphism affects CAD susceptibility in males, and blood pressure, blood glucose, blood coagulation and serum lipid of CAD patients.

Calcium Release Channels: Structure and Function of IP3 Receptors and Ryanodine Receptors

Ca²⁺-release channels are giant membrane proteins that control the release of Ca²⁺ from the endoplasmic and sarcoplasmic reticulum. The two members, ryanodine receptors (RyRs) and inositol-1,4,5-trisphosphate Receptors (IP₃Rs), are evolutionarily related and are both activated by cytosolic Ca²⁺. They share a common architecture, but RyRs have evolved additional modules in the cytosolic region. Their massive size allows for the regulation by tens of proteins and small molecules, which can affect the opening and closing of the channels. In addition to Ca²⁺, other major triggers include IP₃ for the IP₃Rs, and depolarization of the plasma membrane for a particular RyR subtype. Their size has made them popular targets for study via electron microscopic methods, with current structures culminating near 3Å. The available structures have provided many new mechanistic insights int the binding of auxiliary proteins and small molecules, how these can regulate channel opening, and the mechanisms of disease-associated mutations. They also help scrutinize previously proposed binding sites, as some of these are now incompatible with the structures. Many questions remain around the structural effects of post-translational modifications, additional binding partners, and the higher-order complexes these channels can make in situ. This review summarizes our current knowledge about the structures of Ca²⁺-release channels and how this informs on their function.

Towards Mutation-Specific Precision Medicine in Atypical Clinical Phenotypes of Inherited Arrhythmia Syndromes

Most causal genes for inherited arrhythmia syndromes (IASs) encode cardiac ion channel-related proteins. Genotype-phenotype studies and functional analyses of mutant genes, using heterologous expression systems and animal models, have revealed the pathophysiology of IASs and enabled, in part, the establishment of causal gene-specific precision medicine. Additionally, the utilization of induced pluripotent stem cell (iPSC) technology have provided further insights into the pathophysiology of IASs and novel promising therapeutic strategies, especially in long QT syndrome. It is now known that there are atypical clinical phenotypes of IASs associated with specific mutations that have unique electrophysiological properties, which raises a possibility of mutation-specific precision medicine. In particular, patients with Brugada syndrome harboring an SCN5A R1632C mutation exhibit exercise-induced cardiac events, which may be caused by a marked activity-dependent loss of R1632C-Nav1.5 availability due to a marked delay of recovery from inactivation. This suggests that the use of isoproterenol should be avoided. Conversely, the efficacy of β-blocker needs to be examined. Patients harboring a KCND3 V392I mutation exhibit both cardiac (early repolarization syndrome and paroxysmal atrial fibrillation) and cerebral (epilepsy) phenotypes, which may be associated with a unique mixed electrophysiological property of V392I-Kv4.3. Since the epileptic phenotype appears to manifest prior to cardiac events in this mutation carrier, identifying KCND3 mutations in patients with epilepsy and providing optimal therapy will help prevent sudden unexpected death in epilepsy. Further studies using the iPSC technology may provide novel insights into the pathophysiology of atypical clinical phenotypes of IASs and the development of mutation-specific precision medicine.

Long QT syndrome - Bench to bedside

Long QT syndrome (LQTS) is a cardiovascular disorder characterized by an abnormality in cardiac repolarization leading to a prolonged QT interval and T-wave irregularities on the surface electrocardiogram. It is commonly associated with syncope, seizures, susceptibility to torsades de pointes, and risk for sudden death. LQTS is a rare genetic disorder and a major preventable cause of sudden cardiac death in the young. The availability of therapy for this lethal disease emphasizes the importance of early and accurate diagnosis. Additionally, understanding of the molecular mechanisms underlying LQTS could help to optimize genotype-specific treatments to prevent deaths in LQTS patients.

In this review, we briefly summarize current knowledge regarding molecular underpinning of LQTS, in particular focusing on LQT1, LQT2, and LQT3, and discuss novel strategies to study ion channel dysfunction and drug-specific therapies in LQT1, LQT2, and LQT3 syndromes.

Calmodulinopathies: throwing back the veil on the newest life-threatening genetic arrhythmia syndrome

PURPOSE OF REVIEW

This review provides a basic understanding of the calmodulin gene and its role in calcium homeostasis. We outline the functional effects and clinical expression of CALM mutations and review disease expression and management.

RECENT FINDINGS

Calmodulinopathies are rare life-threatening arrhythmia syndromes affecting young individuals. They are caused by mutations in any of the three genes (CALM 1-3) that encode calmodulin (CaM), a ubiquitously expressed Ca signaling protein with multiple targets that in the heart, modulates several ion channels. Patients express varied phenotypes: long QT syndrome, catecholaminergic polymorphic ventricular tachycardia, sudden death, idiopathic ventricular fibrillation, hypertrophic cardiomyopathy, or mixed disease. This is severe disease. Over half of 2019 International Calmodulin Registry patients experienced recurrent cardiac events despite management strategies that included: monotherapy and combination therapy with beta blockers, sodium channel blockers, other antiarrhythmics, sympathetic denervation, and pacing. Induced pluripotent stem cell-derived cardiomyocytes from patients harboring CALM mutations have provided a platform for better understanding pathogenic mechanisms and avenues for therapy.

SUMMARY

Calmodulinopathies are among the more novel inherited arrhythmia syndromes. These are rare but highly lethal diseases with diverse clinical expressions. The practicing electrophysiologist should be aware these conditions, how to recognize them clinically, and understand the challenges in management.

Structural Aspects and Prediction of Calmodulin-Binding Proteins

Calmodulin (CaM) is an important intracellular protein that binds Ca²⁺ and functions as a critical second messenger involved in numerous biological activities through extensive interactions with proteins and peptides. CaM’s ability to adapt to binding targets with different structures is related to the flexible central helix separating the N- and C-terminal lobes, which allows for conformational changes between extended and collapsed forms of the protein. CaM-binding targets are most often identified using prediction algorithms that utilize sequence and structural data to predict regions of peptides and proteins that can interact with CaM. In this review, we provide an overview of different CaM-binding proteins, the motifs through which they interact with CaM, and shared properties that make them good binding partners for CaM. Additionally, we discuss the historical and current methods for predicting CaM binding, and the similarities and differences between these methods and their relative success at prediction. As new CaM-binding proteins are identified and classified, we will gain a broader understanding of the biological processes regulated through changes in Ca²⁺ concentration through interactions with CaM.

Predicting Genetic Variation Severity Using Machine Learning to Interpret Molecular Simulations

Distinct missense mutations in a specific gene have been associated with different diseases as well as differing severity of a disease. Current computational methods predict the potential pathogenicity of a missense variant but fail to differentiate between separate disease or severity phenotypes. We have developed a method to overcome this limitation by applying machine learning to features extracted from molecular dynamics simulations, creating a way to predict the effect of novel genetic variants in causing a disease, drug resistance, or another specific trait. As an example, we have applied this novel approach to variants in calmodulin associated with two distinct arrhythmias as well as two different neurodegenerative diseases caused by variants in amyloid-β peptide. The new method successfully predicts the specific disease caused by a gene variant and ranks its severity with more accuracy than existing methods. We call this method molecular dynamics phenotype prediction model.

Complex Arrhythmia Syndrome in a Knock-In Mouse Model Carrier of the N98S Calm1 Mutation

BACKGROUND

Calmodulin mutations are associated with arrhythmia syndromes in humans. Exome sequencing previously identified a de novo mutation in CALM1 resulting in a p.N98S substitution in a patient with sinus bradycardia and stress-induced bidirectional ventricular ectopy. The objectives of the present study were to determine if mice carrying the N98S mutation knocked into Calm1 replicate the human arrhythmia phenotype and to examine arrhythmia mechanisms.

METHODS

Mouse lines heterozygous for the Calm1N98S allele (Calm1N98S/+) were generated using CRISPR/Cas9 technology. Adult mutant mice and their wildtype littermates (Calm1+/+) underwent electrocardiographic monitoring. Ventricular de- and repolarization was assessed in isolated hearts using optical voltage mapping. Action potentials and whole-cell currents and [Ca2+]i, as well, were measured in single ventricular myocytes using the patch-clamp technique and fluorescence microscopy, respectively. The microelectrode technique was used for in situ membrane voltage monitoring of ventricular conduction fibers.

RESULTS

Two biologically independent knock-in mouse lines heterozygous for the Calm1N98S allele were generated. Calm1N98S/+ mice of either sex and line exhibited sinus bradycardia, QTc interval prolongation, and catecholaminergic bidirectional ventricular tachycardia. Male mutant mice also showed QRS widening. Pharmacological blockade and activation of β-adrenergic receptors rescued and exacerbated, respectively, the long-QT phenotype of Calm1N98S/+ mice. Optical and electric assessment of membrane potential in isolated hearts and single left ventricular myocytes, respectively, revealed β-adrenergically induced delay of repolarization. β-Adrenergic stimulation increased peak density, slowed inactivation, and left-shifted the activation curve of ICa.L significantly more in Calm1N98S/+ versus Calm1+/+ ventricular myocytes, increasing late ICa.L in the former. Rapidly paced Calm1N98S/+ ventricular myocytes showed increased propensity to delayed afterdepolarization-induced triggered activity, whereas in situ His-Purkinje fibers exhibited increased susceptibility for pause-dependent early afterdepolarizations. Epicardial mapping of Calm1N98S/+ hearts showed that both reentry and focal mechanisms contribute to arrhythmogenesis.

CONCLUSIONS

Heterozygosity for the Calm1N98S mutation is causative of an arrhythmia syndrome characterized by sinus bradycardia, QRS widening, adrenergically mediated QTc interval prolongation, and bidirectional ventricular tachycardia. β-Adrenergically induced ICa.L dysregulation contributes to the long-QT phenotype. Pause-dependent early afterdepolarizations and tachycardia-induced delayed afterdepolarizations originating in the His-Purkinje network and ventricular myocytes, respectively, constitute potential sources of arrhythmia in Calm1N98S/+ hearts.

Calmodulin mutations and life-threatening cardiac arrhythmias: insights from the International Calmodulinopathy Registry

AIMS

Calmodulinopathies are rare life-threatening arrhythmia syndromes which affect mostly young individuals and are, caused by mutations in any of the three genes (CALM 1-3) that encode identical calmodulin proteins. We established the International Calmodulinopathy Registry (ICalmR) to understand the natural history, clinical features, and response to therapy of patients with a CALM-mediated arrhythmia syndrome.

METHODS AND RESULTS

A dedicated Case Report File was created to collect demographic, clinical, and genetic information. ICalmR has enrolled 74 subjects, with a variant in the CALM1 (n = 36), CALM2 (n = 23), or CALM3 (n = 15) genes. Sixty-four (86.5%) were symptomatic and the 10-year cumulative mortality was 27%. The two prevalent phenotypes are long QT syndrome (LQTS; CALM-LQTS, n = 36, 49%) and catecholaminergic polymorphic ventricular tachycardia (CPVT; CALM-CPVT, n = 21, 28%). CALM-LQTS patients have extremely prolonged QTc intervals (594 ± 73 ms), high prevalence (78%) of life-threatening arrhythmias with median age at onset of 1.5 years [interquartile range (IQR) 0.1-5.5 years] and poor response to therapies. Most electrocardiograms (ECGs) show late onset peaked T waves. All CALM-CPVT patients were symptomatic with median age of onset of 6.0 years (IQR 3.0-8.5 years). Basal ECG frequently shows prominent U waves. Other CALM-related phenotypes are idiopathic ventricular fibrillation (IVF, n = 7), sudden unexplained death (SUD, n = 4), overlapping features of CPVT/LQTS (n = 3), and predominant neurological phenotype (n = 1). Cardiac structural abnormalities and neurological features were present in 18 and 13 patients, respectively.

CONCLUSION

Calmodulinopathies are largely characterized by adrenergically-induced life-threatening arrhythmias. Available therapies are disquietingly insufficient, especially in CALM-LQTS. Combination therapy with drugs, sympathectomy, and devices should be considered.

Impact of arrhythmogenic calmodulin variants on small conductance Ca2+ -activated K+ (SK3) channels

Calmodulin (CaM) is a ubiquitous Ca²⁺‐sensing protein regulating many important cellular processes. Several CaM‐associated variants have been identified in a small group of patients with cardiac arrhythmias. The mechanism remains largely unknown, even though a number of ion channels, including the ryanodine receptors and the L‐type calcium channels have been shown to be functionally affected by the presence of mutant CaM. CaM is constitutively bound to the SK channel, which underlies the calcium‐gated I_SK contributing to cardiac repolarization. The CaM binding to SK channels is essential for gating, correct assembly, and membrane expression. To elucidate the effect of nine different arrhythmogenic CaM variants on SK3 channel function, HEK293 cells stably expressing SK3 were transiently co‐transfected with CaM^{WT or variant} and whole‐cell patch‐clamp recordings were performed with a calculated free Ca²⁺ concentration of 400 nmol/L. MDCK cells were transiently transfected with SK3 and/or CaM^{WT or variant} to address SK3 and CaM localization by immunocytochemistry. The LQTS‐associated variants CaM^D96V, CaM^D130G, and CaM^F142L reduced I_SK,Ca compared with CaM^WT (P < 0.01, P < 0.001, and P < 0.05, respectively). The CPVT associated variant CaM^N54I also reduced the I_SK,Ca (P < 0.05), which was linked to an accumulation of SK3/CaM^N54I channel complexes in intracellular compartments (P < 0.05). The CPVT associated variants, CaM^A103V and CaM^D132E only revealed a tendency toward reduced current, while the variants CaM^F90L and CaM^N98S, causing LQTS syndrome, did not have any impact on I_SK,Ca. In conclusion, we found that the arrhythmogenic CaM variants CaM^N54I, CaM^D96V, CaM^D130G, and CaM^F142L significantly down‐regulate the SK3 channel current, but with distinct mechanism.

An NGS-based genotyping in LQTS; minor genes are no longer minor

Mutations in KCNQ1, KCNH2, and SCN5A are the major cause of long QT syndrome (LQTS). More than 90% of the genotyped patients have been reported to carry mutations in any of these three genes. Thanks to increasing popularity of next generation sequencer (NGS), novel CACNA1C mutations have been identified among LQTS patients without extra-cardiac phenotypes. We aimed to clarify the frequency of genotypes in LQTS patients in the era of NGS. The study comprised 160 congenital LQTS patients (71 males) registered from November 2015 to September 2018. Inclusion criteria was QTc > 460 ms and Schwartz score ≥ 3. We performed genetic analysis using target gene method by NGS and confirmed the mutations by Sanger method. The median age for genetic screening was 13 (0-68) years. Sixteen patients suffered cardiac arrest, 47 syncope, and 97 were asymptomatic. We identified genetic mutations in 111 (69.4%) patients including 6 CACNA1C (5.4% of the genotyped patients) with 4 asymptomatic patients. Five (3.1%) patients carried double mutations; three out of them with RYR2 and KCNQ1 or KCNH2. In conclusion, CACNA1C screening would be recommended even if the patient is asymptomatic to elucidate the genetic background of the LQTS patients.

The LQT-associated calmodulin mutant E141G induces disturbed Ca2+-dependent binding and a flickering gating mode of the CaV1.2 channel

Calmodulin (CaM) mutations are associated with congenital long QT (LQT) syndrome (LQTS), which may be related to the dysregulation of the cardiac-predominant Ca²⁺ channel isoform Ca_V1.2. Among various mutants, CaM-E141G was identified as a critical missense variant. However, the interaction of this CaM mutant with the Ca_V1.2 channel has not been determined. In this study, by utilizing a semiquantitative pull-down assay, we explored the interaction of CaM-E141G with CaM-binding peptide fragments of the Ca_V1.2 channel. Using the patch-clamp technique, we also investigated the electrophysiological effects of the mutant on Ca_V1.2 channel activity. We found that the maximum binding (B_max) of CaM-E141G to the proximal COOH-terminal region, PreIQ-IQ, PreIQ, IQ, and NT (an NH₂-terminal peptide) was decreased (by 17.71-59.26%) compared with that of wild-type CaM (CaM-WT). In particular, the Ca²⁺-dependent increase in B_max became slower with the combination of CaM-E141G + PreIQ and IQ but faster in the case of NT. Functionally, CaM-WT and CaM-E141G at 500 nM Ca²⁺ decreased Ca_V1.2 channel activity to 24.88% and 55.99%, respectively, compared with 100 nM Ca²⁺, showing that the inhibitory effect was attenuated in CaM-E141G. The mean open time of the Ca_V1.2 channel was increased, and the number of blank traces with no channel opening was significantly decreased. Overall, CaM-E141G exhibits disrupted binding with the Ca_V1.2 channel and induces a flickering gating mode, which may result in the dysfunction of the Ca_V1.2 channel and, thus, the development of LQTS. The present study is the first to investigate the detailed binding properties and single-channel gating mode induced by the interaction of CaM-E141G with the Ca_V1.2 channel.

A Review of Long QT Syndrome: Everything a Hospitalist Should Know

In this article, we will review various aspects of long QT syndrome (LQTS) necessary for hospitalists who care for children, adolescents, and young adults who have known LQTS and also review presenting features that should make one consider LQTS as a cause of hospitalization. Pediatric hospitalists care for patients who have suffered near-drowning, unexplained motor vehicular accidents, brief resolved unexpected events, sudden infant death syndrome, recurrent miscarriages, syncope, or seizures. These common conditions can be clinical clues in patients harboring 1 of 16 LQTS genetic mutations. LQTS is commonly caused by a channelopathy that can cause sudden cardiac death. Over the years, guidelines on management and recommendations for sports participation have evolved with our understanding of the disease and the burden of arrhythmias manifested in the pediatric age group. This review will include the genetic causes of LQTS, clinical features, and important historical information to obtain when these presentations are encountered. We will review medical and surgical treatments available to patients with LQTS and long-term care recommendations and prognosis for those diagnosed with LQTS.

Calmodulin Mutations Associated with Heart Arrhythmia: A Status Report

Calmodulin (CaM) is a ubiquitous intracellular Ca²⁺ sensing protein that modifies gating of numerous ion channels. CaM has an extraordinarily high level of evolutionary conservation, which led to the fundamental assumption that mutation would be lethal. However, in 2012, complete exome sequencing of infants suffering from recurrent cardiac arrest revealed de novo mutations in the three human CALM genes. The correlation between mutations and pathophysiology suggests defects in CaM-dependent ion channel functions. Here, we review the current state of the field for all reported CaM mutations associated with cardiac arrhythmias, including knowledge of their biochemical and structural characteristics, and progress towards understanding how these mutations affect cardiac ion channel function.

Arrhythmia mutations in calmodulin can disrupt cooperativity of Ca2+ binding and cause misfolding

KEY POINTS

Mutations in the calmodulin protein (CaM) are associated with arrhythmia syndromes. This study focuses on understanding the structural characteristics of CaM disease mutants and their interactions with the voltage-gated calcium channel Ca_V 1.2. Arrhythmia mutations in CaM can lead to loss of Ca²⁺ binding, uncoupling of Ca²⁺ binding cooperativity, misfolding of the EF-hands and altered affinity for the calcium channel. These results help us to understand how different CaM mutants have distinct effects on structure and interactions with protein targets to cause disease.

ABSTRACT

Calmodulinopathies are life-threatening arrhythmia syndromes that arise from mutations in calmodulin (CaM), a calcium sensing protein whose sequence is completely conserved across all vertebrates. These mutations have been shown to interfere with the function of cardiac ion channels, including the voltage-gated Ca²⁺ channel Ca_V 1.2 and the ryanodine receptor (RyR2), in a mutation-specific manner. The ability of different CaM disease mutations to discriminate between these channels has been enigmatic. We present crystal structures of several C-terminal lobe mutants and an N-terminal lobe mutant in complex with the Ca_V 1.2 IQ domain, in conjunction with binding assays and complementary structural biology techniques. One mutation (D130G) causes a pathological conformation, with complete separation of EF-hands within the C-lobe and loss of Ca²⁺ binding in EF-hand 4. Another variant (Q136P) has severely reduced affinity for the IQ domain, and shows changes in the CD spectra under Ca²⁺ -saturating conditions when unbound to the IQ domain. Ca²⁺ binding to a pair of EF-hands normally proceeds with very high cooperativity, but we found that N98S CaM can adopt different conformations with either one or two Ca²⁺ ions bound to the C-lobe, possibly disrupting the cooperativity. An N-lobe variant (N54I), which causes severe stress-induced arrhythmia, does not show any major changes in complex with the IQ domain, providing a structural basis for why this mutant does not affect function of Ca_V 1.2. These findings show that different CaM mutants have distinct effects on both the CaM structure and interactions with protein targets, and act via distinct pathological mechanisms to cause disease.

An International, Multicentered, Evidence-Based Reappraisal of Genes Reported to Cause Congenital Long QT Syndrome

Supplemental Digital Content is available in the text.

Background:

Long QT syndrome (LQTS) is the first described and most common inherited arrhythmia. Over the last 25 years, multiple genes have been reported to cause this condition and are routinely tested in patients. Because of dramatic changes in our understanding of human genetic variation, reappraisal of reported genetic causes for LQTS is required.

Methods:

Utilizing an evidence-based framework, 3 gene curation teams blinded to each other’s work scored the level of evidence for 17 genes reported to cause LQTS. A Clinical Domain Channelopathy Working Group provided a final classification of these genes for causation of LQTS after assessment of the evidence scored by the independent curation teams.

Results:

Of 17 genes reported as being causative for LQTS, 9 (AKAP9, ANK2, CAV3, KCNE1, KCNE2, KCNJ2, KCNJ5, SCN4B, SNTA1) were classified as having limited or disputed evidence as LQTS-causative genes. Only 3 genes (KCNQ1, KCNH2, SCN5A) were curated as definitive genes for typical LQTS. Another 4 genes (CALM1, CALM2, CALM3, TRDN) were found to have strong or definitive evidence for causality in LQTS with atypical features, including neonatal atrioventricular block. The remaining gene (CACNA1C) had moderate level evidence for causing LQTS.

Conclusions:

More than half of the genes reported as causing LQTS have limited or disputed evidence to support their disease causation. Genetic variants in these genes should not be used for clinical decision-making, unless accompanied by new and sufficient genetic evidence. The findings of insufficient evidence to support gene-disease associations may extend to other disciplines of medicine and warrants a contemporary evidence-based evaluation for previously reported disease-causing genes to ensure their appropriate use in precision medicine.

Role of cardiac ryanodine receptor calmodulin-binding domains in mediating the action of arrhythmogenic calmodulin N-domain mutation N54I

The Ca²⁺ -sensing protein calmodulin (CaM) inhibits cardiac ryanodine receptor (RyR2)-mediated Ca²⁺ release. CaM mutations associated with arrhythmias and sudden cardiac death have been shown to diminish CaM-dependent inhibition of RyR2, but the underlying mechanisms are not well understood. Nearly all arrhythmogenic CaM mutations identified are located in the C-domain of CaM and exert marked effects on Ca²⁺ binding to CaM and on the CaM C-domain interaction with the CaM-binding domain 2 (CaMBD2) in RyR2. Interestingly, the arrhythmogenic N-domain mutation CaM-N54I has little or no effect on Ca²⁺ binding to CaM or the CaM C-domain-RyR2 CaMBD2 interaction, unlike all CaM C-domain mutations. This suggests that CaM-N54I may diminish CaM-dependent RyR2 inhibition by affecting CaM N-domain interactions with RyR2 CaMBDs other than CaMBD2. To explore this possibility, we assessed the effects of deleting each of the four known CaMBDs in RyR2 (CaMBD1a, -1b, -2, or -3) on the CaM-dependent inhibition of RyR2-mediated Ca²⁺ release in HEK293 cells. We found that removing CaMBD1a, CaMBD1b, or CaMBD3 did not alter the effects of CaM-N54I or CaM-WT on RyR2 inhibition. On the other hand, deleting RyR2-CaMBD2 abolished the effects of both CaM-N54I and CaM-WT. Our results support that CaM-N54I causes aberrant RyR2 regulation via an uncharacterized CaMBD or less likely CaMBD2, and that RyR2 CaMBD2 is required for the actions of both N- and C-domain CaM mutations. Moreover, our results show that CaMBD1a is central to RyR2 regulation, but CaMBD1a, CaMBD1b, and CaMBD3 are not required for CaM-dependent inhibition of RyR2 in HEK293 cells.

Long QT syndrome with a de novo CALM2 mutation in a 4-year-old boy

BACKGROUND

Human calmodulin (CALM) gene mutation has been reported to be related to inherited arrhythmia syndromes, but the genotype-phenotype relationship remains unclear.

METHODS AND RESULTS

We report here a 4-year-old boy who had cardiac arrest while playing in a kindergarten playground. Cardiopulmonary resuscitation was initiated immediately. Eleven minutes after the cardiac arrest, ambulance crews arrived and an automated external defibrillator was attached. His heart rhythm, which was ventricular fibrillation (VF), was returned to sinus rhythm after only one shock delivery. The boy was brought to hospital by air ambulance. During transfer, electrocardiogram (ECG) showed transient VF. On arrival, chest radiograph showed a cardiothoracic ratio of 55% without pulmonary congestion. A 12-lead ECG showed a normal sinus rhythm, biphasic T wave, and prolongation of the corrected QT interval. On ECG, VF was preceded by torsade de pointes or frequent polymorphic premature ventricular contractions (PVC). Echocardiography showed a normal heart structure with decreased cardiac function. On the second day of hospitalization, ECG showed remarkable QT prolongation, T-wave alternans, and frequent PVC. Thereafter, propranolol was started. The ECG showed rapid improvement of QT prolongation and T-wave abnormality. Genetic test indicated a CALM2 mutation, and he was diagnosed with long QT syndrome-15 (LQT15).

CONCLUSIONS

CALM mutations cause long QT syndrome (LQTS), catecholaminergic polymorphic ventricular tachycardia (CPVT) and idiopathic VF. This patient with a CALM2 p.N98S mutation had both phenotypes of LQTS and CPVT.

Genetic Mosaicism in Calmodulinopathy

BACKGROUND

CaM (calmodulin) mutations are associated with congenital arrhythmia susceptibility (calmodulinopathy) and are most often de novo. In this report, we sought to broaden the genotype-phenotype spectrum of calmodulinopathies with 2 novel calmodulin mutations and to investigate mosaicism in 2 affected families.

METHODS

CaM mutations were identified in 4 independent cases by DNA sequencing. Biochemical and electrophysiological studies were performed to determine functional consequences of each mutation.

RESULTS

Genetic studies identified 2 novel CaM variants (CALM3-E141K in 2 cases; CALM1-E141V) and one previously reported CaM pathogenic variant (CALM3-D130G) among 4 probands with shared clinical features of prolonged QTc interval (range 505-725 ms) and documented ventricular arrhythmia. A fatal outcome occurred for 2 of the cases. The parents of all probands were asymptomatic with normal QTc duration. However, 2 of the families had multiple affected offspring or multiple occurrences of intrauterine fetal demise. The mother from the family with recurrent intrauterine fetal demise exhibited the CALM3-E141K mutant allele in 25% of next-generation sequencing reads indicating somatic mosaicism, whereas CALM3-D130G was present in 6% of captured molecules of the paternal DNA sample, also indicating mosaicism. Two novel mutations (E141K and E141V) impaired Ca²⁺ binding affinity to the C-domain of CaM. Human-induced pluripotent stem cell-derived cardiomyocytes overexpressing mutant or wild-type CaM showed that both mutants impaired Ca²⁺-dependent inactivation of L-type Ca²⁺ channels and prolonged action potential duration.

CONCLUSIONS

We report 2 families with somatic mosaicism associated with arrhythmogenic calmodulinopathy, and demonstrate dysregulation of L-type Ca²⁺ channels by 2 novel CaM mutations affecting the same residue. Parental mosaicism should be suspected in families with unexplained fetal arrhythmia or fetal demise combined with a documented CaM mutation.

The Taiwan Heart Registries: Its Influence on Cardiovascular Patient Care

Taiwanese heart registries for the main cardiovascular diseases have been conducted in the past 10 years, with the goal of examining the quality of cardiovascular patient care, which cannot be guaranteed by the universal Taiwan National Health Insurance. The results show suboptimal adherence to guideline recommendations. Door-to-balloon time and dual antiplatelet therapy use in acute coronary syndrome, standard medications for management of heart failure, low-density lipoprotein cholesterol levels in dyslipidemia, anticoagulant agent use in atrial fibrillation, and the understanding of sudden arrhythmia death syndrome were all found to be inadequate. However, all were improved, either by changing National Health Insurance policy or through continuous education for physicians and patients. Thus, specific cardiovascular disease registries could help examine the status of real-world practice, find inadequacies in guideline implementation and understanding of rare diseases, facilitate lobbying to policy makers and education for physicians and patients, and influence and improve cardiovascular patient care.

Diminished inhibition and facilitated activation of RyR2-mediated Ca2+ release is a common defect of arrhythmogenic calmodulin mutations

A number of calmodulin (CaM) mutations cause severe cardiac arrhythmias, but their arrhythmogenic mechanisms are unclear. While some of the arrhythmogenic CaM mutations have been shown to impair CaM-dependent inhibition of intracellular Ca²⁺ release through the ryanodine receptor type 2 (RyR2), the impact of a majority of these mutations on RyR2 function is unknown. Here, we investigated the effect of 14 arrhythmogenic CaM mutations on the CaM-dependent RyR2 inhibition. We found that all the arrhythmogenic CaM mutations tested diminished CaM-dependent inhibition of RyR2-mediated Ca²⁺ release and increased store-overload induced Ca²⁺ release (SOICR) in HEK293 cells. Moreover, all the arrhythmogenic CaM mutations tested either failed to inhibit or even promoted RyR2-mediated Ca²⁺ release in permeabilized HEK293 cells with elevated cytosolic Ca²⁺ , which was markedly different from the inhibitory action of CaM wild-type. The CaM mutations also altered the Ca²⁺ -dependency of CaM binding to the RyR2 CaM-binding domain. These results demonstrate that diminished inhibition, and even facilitated activation, of RyR2-mediated Ca²⁺ release is a common defect of arrhythmogenic CaM mutations.

Cardiac conduction defects

Defects in cardiac electric impulse formation or conduction can lead to an irregular beat (arrhythmia) that can cause sudden death without any apparent cause or after stress. In the following sections, we describe the genetic disorders associated with primary cardiac conduction defects, primarily caused by mutations in ion channel genes. Primary indicates that these disorders are not caused by drugs and are not secondary to other disorders like cardiomyopathies (described in the next section).

Calmodulinopathy: Functional Effects of CALM Mutations and Their Relationship With Clinical Phenotypes

In spite of the widespread role of calmodulin (CaM) in cellular signaling, CaM mutations lead specifically to cardiac manifestations, characterized by remarkable electrical instability and a high incidence of sudden death at young age. Penetrance of the mutations is surprisingly high, thus postulating a high degree of functional dominance. According to the clinical patterns, arrhythmogenesis in CaM mutations can be attributed, in the majority of cases, to either prolonged repolarization (as in long-QT syndrome, LQTS phenotype), or to instability of the intracellular Ca2+ store (as in catecholamine-induced tachycardias, CPVT phenotype). This review discusses how mutations affect CaM signaling function and how this may relate to the distinct arrhythmia phenotypes/mechanisms observed in patients; this involves mechanistic interpretation of negative dominance and mutation-specific CaM-target interactions. Knowledge of the mechanisms involved may allow critical approach to clinical manifestations and aid in the development of therapeutic strategies for "calmodulinopathies," a recently identified nosological entity.