Catecholaminergic-induced arrhythmias in failing cardiomyocytes associated with human HRCS96A variant overexpression

Non‑synonymous polymorphisms in the HRC and ADRB1 genes may be associated with all‑cause death in patients with non‑ischemic heart failure

Sudden cardiac death (SCD) is an unpredictable and common mode of death in patients with heart failure (HF). Alterations in calcium handling may lead to malignant arrhythmias, resulting in SCD, and variants in calcium signaling-related genes have a significant association with SCD. Therefore, the aim of the present retrospective cohort study was to investigate the association of Ser96Ala [histidine-rich calcium-binding protein (HRC)], Ser49Gly [β1-adrenergic receptor (ADRB1)], Arg389Gly (ADRB1) and Gly1886Ser [ryanodine receptor 2 (RYR2)] polymorphisms with serious arrhythmic events and overall mortality in patients with HF with reduced left ventricular ejection fraction of non-ischemic etiology. In total, 136 patients with HF underwent physical examination, routine laboratory tests, non-invasive assessment of cardiac function and an invasive electrophysiological study. The primary outcome was the occurrence of serious arrhythmic events, set as either SCD or appropriate implantable cardioverter-defibrillator (ICD) therapy, and the secondary outcome was all-cause death. During a median follow-up of 37 months, arrhythmic events occurred in 26 patients (19%) and 41 patients (30%) died. Patients carrying the Ser allele of the Ser96Ala polymorphism in HRC had worse survival than those with the Ala/Ala genotype (log-rank P=0.043). Despite the difference in survival time, the Ala/Ala genotype was not associated with all-cause death in the regression analysis [unadjusted hazard ratio (HR)=0.17; 95% CI, 0.02-1.21]. Regarding the Ser49Gly and Arg389Gly polymorphisms in ADRB1, homozygosity for the major alleles at both sites (Ser49Ser and Arg389Arg) was associated with a two-fold increased risk of all-cause death compared with the other genotype combinations (unadjusted HR=1.98; 95% CI, 1.02-3.82). However, this association was lost after controlling for clinical covariates. No association was observed for the Gly1886Ser polymorphism in RYR2. Overall, the present findings are concurrent with the hypothesis that the Ser96Ala (HRC), Ser49Gly (ADRB1) and Arg389Gly (ADRB1) polymorphisms may be associated with HF prognosis. In particular, the Ser96Ala polymorphism might aid in risk stratification and patient selection for ICD implantation.

Gene Polymorphism and Recurrent Atrial Fibrillation after Catheter Ablation: A Comprehensive Review

Atrial fibrillation (AF) is one of the most common cardiac arrhythmias, but its pathogenesis is still poorly understood. Catheter ablation is one of the most effective treatments for AF, but recurrence after ablation remains a challenge. There has been much research into the association of AF recurrence with several factors, including genetics. Over the past decade or so, significant advances have been made in the genetic architecture of atrial fibrillation. Genome-wide association studies (GWAS) have identified over 100 loci for genetic variants associated with atrial fibrillation. However, there is relatively little information on the systematic assessment of the genes related to AF recurrence after ablation. In this review article, we highlight the value of genetic polymorphisms in atrial fibrillation recurrence after catheter ablation and their potential mechanisms in the recurrence process to enhance our understanding of atrial fibrillation recurrence and contribute to individualized treatment strategies for patients with AF.

Ser96Ala genetic variant of the human histidine-rich calcium-binding protein is a genetic predictor of recurrence after catheter ablation in patients with paroxysmal atrial fibrillation

BACKGROUND

Atrial fibrillation (AF) recurrence after radiofrequency catheter ablation (RFCA) still remains a serious issue. Ca2+ handling has a considerable effect on AF recurrence. The histidine-rich calcium-binding protein (HRC) genetic single nucleotide polymorphism (SNP), rs3745297 (T>G, Ser96Ala), is known to cause a sarcoplasmic reticulum Ca2+ leak. We investigated the association between HRC Ser96Ala and AF recurrence after RFCA in paroxysmal AF (PAF) patients.

METHODS AND RESULTS

We enrolled PAF patients who underwent RFCA (N = 334 for screening and N = 245 for replication) and were genotyped for HRC SNP (rs3745297). The patient age was younger and rate of diabetes and hypertension lower in the PAF patients with Ser96Ala than in those without (TT/TG/GG, 179/120/35; 64±10/60±12/59±13 y, P = 0.001; 18.5/ 9.2/8.6%, P = 0.04 and 66.1/50.0/37.1%, P = 0.001, respectively). During a mean 19 month follow-up, 57 (17.1%) patients suffered from AF recurrences. The rate of an Ser96Ala was significantly higher in patients with AF recurrence than in those without in the screening set (allele frequency model: odds ratio [OR], 1.80; P = 0.006). We also confirmed this significant association in the replication set (OR 1.74; P = 0.03) and combination (P = 0.0008). A multivariate analysis revealed that the AF duration, sinus node dysfunction, and HRC Ser96Ala were independent predictors of an AF recurrence (hazard ratio [HR], 1.04, P = 0.037; HR 2.42, P = 0.018; and HR 2.66, P = 0.007, respectively).

CONCLUSION

HRC SNP Ser96Ala is important as a new genetic marker of AF recurrence after RFCA.

The Histidine-Rich Calcium Binding Protein in Regulation of Cardiac Rhythmicity

Sudden unexpected cardiac death (SCD) accounts for up to half of all-cause mortality of heart failure patients. Standardized cardiology tools such as electrocardiography, cardiac imaging, electrophysiological and serum biomarkers cannot accurately predict which patients are at risk of life-threatening arrhythmic episodes. Recently, a common variant of the histidine-rich calcium binding protein (HRC), the Ser96Ala, was identified as a potent biomarker of malignant arrhythmia triggering in these patients. HRC has been shown to be involved in the regulation of cardiac sarcoplasmic reticulum (SR) Ca²⁺ cycling, by binding and storing Ca²⁺ in the SR, as well as interacting with the SR Ca²⁺ uptake and release complexes. The underlying mechanisms, elucidated by studies at the molecular, biochemical, cellular and intact animal levels, indicate that transversion of Ser96 to Ala results in abolishment of an HRC phosphorylation site by Fam20C kinase and dysregulation of SR Ca²⁺ cycling. This is mediated through aberrant SR Ca²⁺ release by the ryanodine receptor (RyR2) quaternary complex, due to the impaired HRC/triadin interaction, and depressed SR Ca²⁺ uptake by the sarco/endoplasmic reticulum Ca²⁺ ATPase (SERCA2) pump, due to the impaired HRC/SERCA2 interaction. Pharmacological intervention with KN-93, an inhibitor of Ca²⁺/calmodulin-dependent protein kinase II (CaMKII), in the HRC Ser96Ala mouse model, reduced the occurrence of malignant cardiac arrhythmias. Herein, we summarize the current evidence on the pivotal role of HRC in the regulation of cardiac rhythmicity and the importance of HRC Ser96Ala as a genetic modifier for arrhythmias in the setting of heart failure.

Impaired calcium homeostasis is associated with sudden cardiac death and arrhythmias in a genetic equivalent mouse model of the human HRC-Ser96Ala variant

Aims

The histidine-rich calcium-binding protein (HRC) Ser96Ala variant has previously been identified as a potential biomarker for ventricular arrhythmias and sudden cardiac death in patients with idiopathic dilated cardiomyopathy. Herein, the role of this variant in cardiac pathophysiology is delineated through a novel mouse model, carrying the human mutation in the homologous mouse position.

Methods and results

The mouse HRC serine 81, homologous to human HRC serine 96, was mutated to alanine, using knock-in gene targeting. The HRC-Ser81Ala mice presented increased mortality in the absence of structural or histological abnormalities, indicating that early death may be arrhythmia-related. Indeed, under stress-but not baseline-conditions, the HRC-Ser81Ala mice developed ventricular arrhythmias, whilst at the cardiomyocyte level they exhibited increased occurrence of triggered activity. Cardiac contraction was decreased in vivo, ex vivo, and in vitro. Additionally, Ca2+ transients and SR Ca2+ load were both reduced suggesting that cytosolic Ca2+ overload is not the underlying proarrhythmic mechanism. Interestingly, total SR Ca2+ leak was increased in HRC-Ser81Ala cardiomyocytes, without an increase in Ca2+ spark and wave frequency. However, Ca2+ wave propagation was significantly slower and the duration of the associated Na/Ca exchange current was increased. Moreover, action potential duration was also increased. Notably, Ca2+/Calmodulin kinase II (CaMKII) phosphorylation of the ryanodine receptor was increased, whilst KN-93, an inhibitor of CaMKII, reduced the occurrence of arrhythmias.

Conclusions

The homologous mutation Ser81Ala in HRC in mice, corresponding to Ser96Ala in humans, is associated with sudden death and depressed cardiac function. Ventricular arrhythmias are related to abnormal Ca2+ cycling across the SR. The data further support a role for CaMKII with the perspective to treat arrhythmias through CaMKII inhibition.

Phosphorylation of serine96 of histidine-rich calcium-binding protein by the Fam20C kinase functions to prevent cardiac arrhythmia

Precise Ca cycling through the sarcoplasmic reticulum (SR), a Ca storage organelle, is critical for proper cardiac muscle function. This cycling initially involves SR release of Ca via the ryanodine receptor, which is regulated by its interacting proteins junctin and triadin. The sarco/endoplasmic reticulum Ca ATPase (SERCA) pump then refills SR Ca stores. Histidine-rich Ca-binding protein (HRC) resides in the lumen of the SR, where it contributes to the regulation of Ca cycling by protecting stressed or failing hearts. The common Ser96Ala human genetic variant of HRC strongly correlates with life-threatening ventricular arrhythmias in patients with idiopathic dilated cardiomyopathy. However, the underlying molecular pathways of this disease remain undefined. Here, we demonstrate that family with sequence similarity 20C (Fam20C), a recently characterized protein kinase in the secretory pathway, phosphorylates HRC on Ser96. HRC Ser96 phosphorylation was confirmed in cells and human hearts. Furthermore, a Ser96Asp HRC variant, which mimics constitutive phosphorylation of Ser96, diminished delayed aftercontractions in HRC null cardiac myocytes. This HRC phosphomimetic variant was also able to rescue the aftercontractions elicited by the Ser96Ala variant, demonstrating that phosphorylation of Ser96 is critical for the cardioprotective function of HRC. Phosphorylation of HRC on Ser96 regulated the interactions of HRC with both triadin and SERCA2a, suggesting a unique mechanism for regulation of SR Ca homeostasis. This demonstration of the role of Fam20C-dependent phosphorylation in heart disease will open new avenues for potential therapeutic approaches against arrhythmias.

Calcium Signaling and Cardiac Arrhythmias

There has been a significant progress in our understanding of the molecular mechanisms by which calcium (Ca2+) ions mediate various types of cardiac arrhythmias. A growing list of inherited gene defects can cause potentially lethal cardiac arrhythmia syndromes, including catecholaminergic polymorphic ventricular tachycardia, congenital long QT syndrome, and hypertrophic cardiomyopathy. In addition, acquired deficits of multiple Ca2+-handling proteins can contribute to the pathogenesis of arrhythmias in patients with various types of heart disease. In this review article, we will first review the key role of Ca2+ in normal cardiac function-in particular, excitation-contraction coupling and normal electric rhythms. The functional involvement of Ca2+ in distinct arrhythmia mechanisms will be discussed, followed by various inherited arrhythmia syndromes caused by mutations in Ca2+-handling proteins. Finally, we will discuss how changes in the expression of regulation of Ca2+ channels and transporters can cause acquired arrhythmias, and how these mechanisms might be targeted for therapeutic purposes.

A Common Polymorphism of the Human Cardiac Sodium Channel Alpha Subunit (SCN5A) Gene Is Associated with Sudden Cardiac Death in Chronic Ischemic Heart Disease

Cardiac death remains one of the leading causes of mortality worldwide. Recent research has shed light on pathophysiological mechanisms underlying cardiac death, and several genetic variants in novel candidate genes have been identified as risk factors. However, the vast majority of studies performed so far investigated genetic associations with specific forms of cardiac death only (sudden, arrhythmogenic, ischemic etc.). The aim of the present investigation was to find a genetic marker that can be used as a general, powerful predictor of cardiac death risk. To this end, a case-control association study was performed on a heterogeneous cohort of cardiac death victims (n=360) and age-matched controls (n=300). Five single nucleotide polymorphisms (SNPs) from five candidate genes (beta2 adrenergic receptor, nitric oxide synthase 1 adaptor protein, ryanodine receptor 2, sodium channel type V alpha subunit and transforming growth factor-beta receptor 2) that had previously been shown to associate with certain forms of cardiac death were genotyped using sequence-specific real-time PCR probes. Logistic regression analysis revealed that the CC genotype of the rs11720524 polymorphism in the SCN5A gene encoding a subunit of the cardiac voltage-gated sodium channel occurred more frequently in the highly heterogeneous cardiac death cohort compared to the control population (p=0.019, odds ratio: 1.351). A detailed subgroup analysis uncovered that this effect was due to an association of this variant with cardiac death in chronic ischemic heart disease (p=0.012, odds ratio = 1.455). None of the other investigated polymorphisms showed association with cardiac death in this context. In conclusion, our results shed light on the role of this non-coding polymorphism in cardiac death in ischemic cardiomyopathy. Functional studies are needed to explore the pathophysiological background of this association.

Altered myocardial calcium cycling and energetics in heart failure--a rational approach for disease treatment

Cardiomyocyte function depends on coordinated movements of calcium into and out of the cell and the proper delivery of ATP to energy-utilizing enzymes. Defects in calcium-handling proteins and abnormal energy metabolism are features of heart failure. Recent discoveries have led to gene-based therapies targeting calcium-transporting or -binding proteins, such as the cardiac sarco(endo)plasmic reticulum calcium ATPase (SERCA2a), leading to improvements in calcium homeostasis and excitation-contraction coupling. Here we review impaired calcium cycling and energetics in heart failure, assessing their roles from both a mutually exclusive and interdependent viewpoint, and discuss therapies that may improve the failing myocardium.

Regulation of RYR2 by sarcoplasmic reticulum Ca(2+)

Ca(2+) is arguably the most important ion involved in the contraction of the heart. The cardiac ryanodine receptor (RyR2), the major Ca(2+) release channel located in the sarcoplasmic reticulum (SR) membrane, is responsible for releasing the bulk of Ca(2+) required for contraction. Moreover, RyR2 is also crucial for maintaining SR Ca(2+) homeostasis by releasing Ca(2+) from the SR when it becomes overloaded with Ca(2+) . During normal contraction, RyR2 is activated by cytosolic Ca(2+) , whereas during store overload conditions, the opening of RyR2 is governed by SR Ca(2+) . Although the process of the cytosolic control of RyR2 is well established, the molecular mechanism by which SR luminal Ca(2+) regulates RyR2 has only recently been elucidated and remains controversial. In addition to the activation of RyR2, SR luminal Ca(2+) also determines when the RyR2 channel closes. RyR2-mediated Ca(2+) release from the SR does not continue until the SR is completely depleted. Rather, it ceases when SR luminal Ca(2+) falls below a certain level. Given the importance of SR Ca(2+) , it is not surprising that the SR luminal Ca(2+) level is tightly controlled by SR Ca(2+) -buffering proteins. Consequently, the opening and closing of RyR2 is heavily influenced by the presence of such proteins, particularly those associated with RyR2, such as calsequestrin and the histidine-rich Ca(2+) -binding protein. These proteins appear to indirectly alter RyR2 activity by modifying the microdomain SR Ca(2+) level surrounding RyR2.

Abnormal calcium cycling and cardiac arrhythmias associated with the human Ser96Ala genetic variant of histidine-rich calcium-binding protein

BACKGROUND

A human genetic variant (Ser96Ala) in the sarcoplasmic reticulum (SR) histidine-rich Ca(2+)-binding (HRC) protein has been linked to ventricular arrhythmia and sudden death in dilated cardiomyopathy. However, the precise mechanisms affecting SR function and leading to arrhythmias remain elusive.

METHODS AND RESULTS

We generated transgenic mice with cardiac-specific expression of human Ala96 HRC or Ser96 HRC in the null background to assess function in absence of endogenous protein. Ala96 HRC decreased (25% to 30%) cardiomyocyte contractility and Ca2+ kinetics compared with Ser96 HRC in the absence of any structural or histological abnormalities. Furthermore, the frequency of Ca2+ waves was significantly higher (10-fold), although SR Ca2+ load was reduced (by 27%) in Ala96 HRC cells. The underlying mechanisms involved diminished interaction of Ala96 HRC with triadin, affecting ryanodine receptor (RyR) stability. Indeed, the open probability of RyR, assessed by use of ryanodine binding, was significantly increased. Accordingly, stress conditions (5 Hz plus isoproterenol) induced aftercontractions (65% in Ala96 versus 12% in Ser96) and delayed afterdepolarizations (70% in Ala96 versus 20% in Ser96). The increased SR Ca2+ leak was accompanied by hyperphosphorylation (1.6-fold) of RyR at Ser2814 by calmodulin-dependent protein kinase II. Accordingly, inclusion of the calmodulin-dependent protein kinase II inhibitor KN93 prevented Ser2814 phosphorylation and partially reversed the increases in Ca2+ spark frequency and wave production. Parallel in vivo studies revealed ventricular ectopy on short-term isoproterenol challenge and increased (4-fold) propensity to arrhythmias, including nonsustained ventricular tachycardia, after myocardial infarction in Ala96 HRC mice.

CONCLUSIONS

These findings suggest that aberrant SR Ca2+ release and increased susceptibility to delayed afterdepolarizations underlie triggered arrhythmic activity in human Ala96 HRC carriers.

AAV-mediated knock-down of HRC exacerbates transverse aorta constriction-induced heart failure

Background

Histidine-rich calcium binding protein (HRC) is located in the lumen of sarcoplasmic reticulum (SR) that binds to both triadin (TRN) and SERCA affecting Ca²⁺ cycling in the SR. Chronic overexpression of HRC that may disrupt intracellular Ca²⁺ homeostasis is implicated in pathogenesis of cardiac hypertrophy. Ablation of HRC showed relatively normal phenotypes under basal condition, but exhibited a significantly increased susceptibility to isoproterenol-induced cardiac hypertrophy. In the present study, we characterized the functions of HRC related to Ca²⁺ cycling and pathogenesis of cardiac hypertrophy using the in vitro siRNA- and the in vivo adeno-associated virus (AAV)-mediated HRC knock-down (KD) systems, respectively.

Methodology/Principal Findings

AAV-mediated HRC-KD system was used with or without C57BL/6 mouse model of transverse aortic constriction-induced failing heart (TAC-FH) to examine whether HRC-KD could enhance cardiac function in failing heart (FH). Initially we expected that HRC-KD could elicit cardiac functional recovery in failing heart (FH), since predesigned siRNA-mediated HRC-KD enhanced Ca²⁺ cycling and increased activities of RyR2 and SERCA2 without change in SR Ca²⁺ load in neonatal rat ventricular cells (NRVCs) and HL-1 cells. However, AAV9-mediated HRC-KD in TAC-FH was associated with decreased fractional shortening and increased cardiac fibrosis compared with control. We found that phospho-RyR2, phospho-CaMKII, phospho-p38 MAPK, and phospho-PLB were significantly upregulated by HRC-KD in TAC-FH. A significantly increased level of cleaved caspase-3, a cardiac cell death marker was also found, consistent with the result of TUNEL assay.

Conclusions/Significance

Increased Ca²⁺ leak and cytosolic Ca²⁺ concentration due to a partial KD of HRC could enhance activity of CaMKII and phosphorylation of p38 MAPK, causing the mitochondrial death pathway observed in TAC-FH. Our results present evidence that down-regulation of HRC could deteriorate cardiac function in TAC-FH through perturbed SR-mediated Ca²⁺ cycling.

Modulation of cardiac contractility by the phospholamban/SERCA2a regulatome

Heart disease remains the leading cause of death and disability in the Western world. Current therapies aim at treating the symptoms rather than the subcellular mechanisms, underlying the etiology and pathological remodeling in heart failure. A universal characteristic, contributing to the decreased contractile performance in human and experimental failing hearts, is impaired calcium sequestration into the sarcoplasmic reticulum (SR). SR calcium uptake is mediated by a Ca(2+)-ATPase (SERCA2), whose activity is reversibly regulated by phospholamban (PLN). Dephosphorylated PLN is an inhibitor of SERCA and phosphorylation of PLN relieves this inhibition. However, the initial simple view of a PLN/SERCA regulatory complex has been modified by our recent identification of SUMO, S100 and the histidine-rich Ca-binding protein as regulators of SERCA activity. In addition, PLN activity is regulated by 2 phosphoproteins, the inhibitor-1 of protein phosphatase 1 and the small heat shock protein 20, which affect the overall SERCA-mediated Ca-transport. This review will highlight the regulatory mechanisms of cardiac contractility by the multimeric SERCA/PLN-ensemble and the potential for new therapeutic avenues targeting this complex by using small molecules and gene transfer methods.

Calsequestrin mutations and catecholaminergic polymorphic ventricular tachycardia

Cardiac calsequestrin (Casq2) is the major Ca2+ binding protein in the sarcoplasmic reticulum, which is the principle Ca2+ storage organelle of cardiac muscle. During the last decade, experimental studies have provided new concepts on the role of Casq2 in the regulation of cardiac muscle Ca2+ handling. Furthermore, mutations in the gene encoding for cardiac calsequestrin, CASQ2, cause a rare but severe form of catecholaminergic polymorphic ventricular tachycardia (CPVT). Here, we review the physiology of Casq2 in cardiac Ca2+ handling and discuss pathophysiological mechanisms that lead to CPVT caused by CASQ2 mutations. We also describe the clinical aspects of CPVT and provide an update of its contemporary clinical management.

Calsequestrin 2 and arrhythmias

Calsequestrin is the most abundant Ca-binding protein of the specialized endoplasmic reticulum found in muscle, the sarcoplasmic reticulum (SR). Calsequestrin binds Ca with high capacity and low affinity and importantly contributes to the mobilization of Ca during each contraction both in skeletal and cardiac muscle. Surprisingly, mutations in the gene encoding the cardiac isoform of calsequestrin (Casq2) have been associated with an inherited form of ventricular arrhythmia triggered by emotional or physical stress termed catecholaminergic polymorphic ventricular tachycardia (CPVT). Despite normal cardiac contractility and normal resting ECG, CPVT patients present with a high risk of sudden death at a young age. Here, we review recent new insights regarding the role of calsequestrin in genetic and acquired arrhythmia disorders. Mouse models of CPVT have shed light on the pathophysiological mechanism underlying CPVT. Casq2 is not only a Ca-storing protein as initially hypothesized, but it has a far more complex function in Ca handling and regulating SR Ca release channels. The functional importance of Casq2 interactions with other SR proteins and the importance of alterations in Casq2 trafficking are also being investigated. Reports of altered Casq2 trafficking in animal models of acquired heart diseases such as heart failure suggest that Casq2 may contribute to arrhythmia risk beyond genetic forms of Casq2 dysfunction.