Abnormal sodium channel mRNA splicing in hypertrophic cardiomyopathy

RBM25 regulates hypoxic cardiomyocyte apoptosis through CHOP-associated endoplasmic reticulum stress

Ischemic heart failure (HF) is one of the leading causes of global morbidity and mortality; blocking the apoptotic cascade could help improve adverse outcomes of it. RNA-binding motif protein 25 (RBM25) is an RNA-binding protein related to apoptosis; however, its role remains unknown in ischemic HF. The main purpose of this study is to explore the mechanism of RBM25 in ischemic HF. Establishing an ischemic HF model and oxygen-glucose deprivation (OGD) model. ELISA was performed to evaluate the BNP level in the ischemic HF model. Echocardiography and histological analysis were performed to assess cardiac function and infarct size. Proteins were quantitatively and locationally analyzed by western blotting and immunofluorescence. The morphological changes of endoplasmic reticulum (ER) were observed with ER-tracker. Cardiac function and myocardial injury were observed in ischemic HF rats. RBM25 was elevated in cardiomyocytes of hypoxia injury hearts and localized in nucleus both in vitro and in vivo. In addition, cell apoptosis was significantly increased when overexpressed RBM25. Moreover, ER stress stimulated upregulation of RBM25 and promoted cell apoptosis through the CHOP related pathway. Finally, inhibiting the expression of RBM25 could ameliorate the apoptosis and improve cardiac function through blocking the activation of CHOP signaling pathway. RBM25 is significantly upregulated in ischemic HF rat heart and OGD model, which leads to apoptosis by modulating the ER stress through CHOP pathway. Knockdown of RBM25 could reverse apoptosis-mediated cardiac dysfunction. RBM25 may be a promising target for the treatment of ischemic HF.

Mutations in MYBPC3 and MYH7 in Association with Brugada Type 1 ECG Pattern: Overlap between Brugada Syndrome and Hypertrophic Cardiomyopathy?

Brugada syndrome (BrS) is an inherited disorder with high allelic and genetic heterogeneity clinically characterized by typical coved-type ST segment elevation at the electrocardiogram (ECG), which may occur either spontaneously or after provocative drug testing. BrS is classically described as an arrhythmic condition occurring in a structurally normal heart and is associated with the risk of ventricular fibrillation and sudden cardiac death (SCD). We studied five patients with spontaneous or drug-induced type 1 ECG pattern, variably associated with symptoms and a positive family history through a Next Generation Sequencing panels approach, which includes genes of both channelopathies and cardiomyopathies. We identified variants in MYBPC3 and in MYH7, hypertrophic cardiomyopathy (HCM) genes (MYBPC3: p.Lys1065Glnfs*12 and c.1458-1G > A, MYH7: p.Arg783His, p.Val1213Met, p.Lys744Thr). Our data propose that Brugada type 1 ECG may be an early electrocardiographic marker of a concealed structural heart disease, possibly enlarging the genotypic overlap between Brugada syndrome and cardiomyopathies.

Genetic basis and molecular biology of cardiac arrhythmias in cardiomyopathies

Cardiac arrhythmias are common, often the first, and sometimes the life-threatening manifestations of hereditary cardiomyopathies. Pathogenic variants in several genes known to cause hereditary cardiac arrhythmias have also been identified in the sporadic cases and small families with cardiomyopathies. These findings suggest a shared genetic aetiology of a subset of hereditary cardiomyopathies and cardiac arrhythmias. The concept of a shared genetic aetiology is in accord with the complex and exquisite interplays that exist between the ion currents and cardiac mechanical function. However, neither the causal role of cardiac arrhythmias genes in cardiomyopathies is well established nor the causal role of cardiomyopathy genes in arrhythmias. On the contrary, secondary changes in ion currents, such as post-translational modifications, are common and contributors to the pathogenesis of arrhythmias in cardiomyopathies through altering biophysical and functional properties of the ion channels. Moreover, structural changes, such as cardiac hypertrophy, dilatation, and fibrosis provide a pro-arrhythmic substrate in hereditary cardiomyopathies. Genetic basis and molecular biology of cardiac arrhythmias in hereditary cardiomyopathies are discussed.

SLM2 Is A Novel Cardiac Splicing Factor Involved in Heart Failure due to Dilated Cardiomyopathy

Alternative mRNA splicing is a fundamental process to increase the versatility of the genome. In humans, cardiac mRNA splicing is involved in the pathophysiology of heart failure. Mutations in the splicing factor RNA binding motif protein 20 (RBM20) cause severe forms of cardiomyopathy. To identify novel cardiomyopathy-associated splicing factors, RNA-seq and tissue-enrichment analyses were performed, which identified up-regulated expression of Sam68-Like mammalian protein 2 (SLM2) in the left ventricle of dilated cardiomyopathy (DCM) patients. In the human heart, SLM2 binds to important transcripts of sarcomere constituents, such as those encoding myosin light chain 2 (MYL2), troponin I3 (TNNI3), troponin T2 (TNNT2), tropomyosin 1/2 (TPM1/2), and titin (TTN). Mechanistically, SLM2 mediates intron retention, prevents exon exclusion, and thereby mediates alternative splicing of the mRNA regions encoding the variable proline-, glutamate-, valine-, and lysine-rich (PEVK) domain and another part of the I-band region of titin. In summary, SLM2 is a novel cardiac splicing regulator with essential functions for maintaining cardiomyocyte integrity by binding to and processing the mRNAs of essential cardiac constituents such as titin.

A Heart Failure-Associated SCN5A Splice Variant Leads to a Reduction in Sodium Current Through Coupled-Gating With the Wild-Type Channel

Nav1.5, encoded by the gene SCN5A, is the predominant voltage-gated sodium channel expressed in the heart. It initiates the cardiac action potential and thus is crucial for normal heart rhythm and function. Dysfunctions in Nav1.5 have been involved in multiple congenital or acquired cardiac pathological conditions such as Brugada syndrome (BrS), Long QT Syndrome Type 3, and heart failure (HF), all of which can lead to sudden cardiac death (SCD) - one of the leading causes of death worldwide. Our lab has previously reported that Nav1.5 forms dimer channels with coupled gating. We also found that Nav1.5 BrS mutants can exert a dominant-negative (DN) effect and impair the function of wildtype (WT) channels through coupled-gating with the WT. It was previously reported that reduction in cardiac sodium currents (INa), observed in HF, could be due to the increased expression of an SCN5A splice variant - E28D, which results in a truncated sodium channel (Nav1.5-G1642X). In this study, we hypothesized that this SCN5A splice variant leads to INa reduction in HF through biophysical coupling with the WT. We showed that Nav1.5-G1642X is a non-functional channel but can interact with the WT, resulting in a DN effect on the WT channel. We found that both WT and the truncated channel Nav1.5-G1642X traffic at the cell surface, suggesting biophysical coupling. Indeed, we found that the DN effect can be abolished by difopein, an inhibitor of the biophysical coupling. Interestingly, the sodium channel polymorphism H558R, which has beneficial effect in HF patients, could also block the DN effect. In summary, the HF-associated splice variant Nav1.5-G1642X suppresses sodium currents in heart failure patients through a mechanism involving coupled-gating with the wildtype sodium channel.

Proteomic and functional mapping of cardiac NaV1.5 channel phosphorylation sites

Phosphorylation of the voltage-gated Na⁺ (Na_V) channel Na_V1.5 regulates cardiac excitability, yet the phosphorylation sites regulating its function and the underlying mechanisms remain largely unknown. Using a systematic, quantitative phosphoproteomic approach, we analyzed Na_V1.5 channel complexes purified from nonfailing and failing mouse left ventricles, and we identified 42 phosphorylation sites on Na_V1.5. Most sites are clustered, and three of these clusters are highly phosphorylated. Analyses of phosphosilent and phosphomimetic Na_V1.5 mutants revealed the roles of three phosphosites in regulating Na_V1.5 channel expression and gating. The phosphorylated serines S664 and S667 regulate the voltage dependence of channel activation in a cumulative manner, whereas the nearby S671, the phosphorylation of which is increased in failing hearts, regulates cell surface Na_V1.5 expression and peak Na⁺ current. No additional roles could be assigned to the other clusters of phosphosites. Taken together, our results demonstrate that ventricular Na_V1.5 is highly phosphorylated and that the phosphorylation-dependent regulation of Na_V1.5 channels is highly complex, site specific, and dynamic.

Life Cycle of the Cardiac Voltage-Gated Sodium Channel NaV1.5

Cardiac voltage-gated sodium channel Na_V1.5, encoded by SCN5A, is crucial for the upstroke of action potential and excitation of cardiomyocytes. Na_V1.5 undergoes complex processes before it reaches the target membrane microdomains and performs normal functions. A variety of protein partners are needed to achieve the balance between SCN5A transcription and mRNA decay, endoplasmic reticulum retention and export, Golgi apparatus retention and export, selective anchoring and degradation, activation, and inactivation of sodium currents. Subtle alterations can impair Na_V1.5 in terms of expression or function, eventually leading to Na_V1.5-associated diseases such as lethal arrhythmias and cardiomyopathy.

Hypertrophic Cardiomyopathy: An Overview of Genetics and Management

Hypertrophic cardiomyopathy (HCM) is a genetically heterogeneous cardiac muscle disorder with a diverse natural history, characterized by unexplained left ventricular hypertrophy (LVH), with histopathological hallmarks including myocyte enlargement, myocyte disarray and myocardial fibrosis. Although these features can cause significant cardiac symptoms, many young individuals with HCM are asymptomatic or mildly symptomatic. Sudden cardiac death (SCD) may occur as the initial clinical manifestation. Over the past few decades, HCM has been considered a disease of sarcomere, and typically as an autosomal dominant disease with variable expressivity and incomplete penetrance. Important insights into the genetic landscape of HCM have enhanced our understanding of the molecular pathogenesis, empowered gene-based diagnostic testing to identify at-risk individuals, and offered potential targets for the development of therapeutic agents. This article reviews the current knowledge on the clinical genetics and management of HCM.

Hypertrophic Cardiomyopathy: An Overview of Genetics and Management

Hypertrophic cardiomyopathy (HCM) is a genetically heterogeneous cardiac muscle disorder with a diverse natural history, characterized by unexplained left ventricular hypertrophy (LVH), with histopathological hallmarks including myocyte enlargement, myocyte disarray and myocardial fibrosis. Although these features can cause significant cardiac symptoms, many young individuals with HCM are asymptomatic or mildly symptomatic. Sudden cardiac death (SCD) may occur as the initial clinical manifestation. Over the past few decades, HCM has been considered a disease of sarcomere, and typically as an autosomal dominant disease with variable expressivity and incomplete penetrance. Important insights into the genetic landscape of HCM have enhanced our understanding of the molecular pathogenesis, empowered gene-based diagnostic testing to identify at-risk individuals, and offered potential targets for the development of therapeutic agents. This article reviews the current knowledge on the clinical genetics and management of HCM.

CRISPR -Mediated Expression of the Fetal Scn5a Isoform in Adult Mice Causes Conduction Defects and Arrhythmias

Background

The sodium channel, Na_v1.5, encoded by SCN5A, undergoes developmentally regulated splicing from inclusion of exon 6A in the fetal heart to exon 6B in adults. These mutually exclusive exons differ in 7 amino acids altering the electrophysiological properties of the Na_v1.5 channel. In myotonic dystrophy type 1, SCN5A is mis‐spliced such that the fetal pattern of exon 6A inclusion is detected in adult hearts. Cardiac manifestations of myotonic dystrophy type 1 include conduction defects and arrhythmias and are the second‐leading cause of death.

Methods and Results

This work aimed to determine the impact of SCN5A mis‐splicing on cardiac function. We used clustered regularly interspaced short palindromic repeat (CRISPR)/CRISPR‐associated protein 9 (Cas9) to delete Scn5a exon 6B in mice, thereby redirecting splicing toward exon 6A. These mice exhibit prolonged PR and QRS intervals, slowed conduction velocity, extended action potential duration, and are highly susceptible to arrhythmias.

Conclusions

Our findings highlight a nonmutational pathological mechanism of arrhythmias and conduction defects as a result of mis‐splicing of the predominant cardiac sodium channel. Animals homozygous for the deleted exon express only the fetal isoform and have more‐severe phenotypes than heterozygotes that also express the adult isoform. This observation is directly relevant to myotonic dystrophy type 1, and possibly pathological arrhythmias, in which individuals differ with regard to the ratios of the isoforms expressed.

Angiotensin II Influences Pre-mRNA Splicing Regulation by Enhancing RBM20 Transcription Through Activation of the MAPK/ELK1 Signaling Pathway

RNA binding motif 20 (RBM20) is a key regulator of pre-mRNA splicing of titin and other genes that are associated with cardiac diseases. Hormones, like insulin, triiodothyronine (T3), and angiotensin II (Ang II), can regulate gene-splicing through RBM20, but the detailed mechanism remains unclear. This study was aimed at investigating the signaling mechanism by which hormones regulate pre-mRNA splicing through RBM20. We first examined the role of RBM20 in Z-, I-, and M-band titin splicing at different ages in wild type (WT) and RBM20 knockout (KO) rats using RT-PCR; we found that RBM20 is the predominant regulator of I-band titin splicing at all ages. Then we treated rats with propylthiouracil (PTU), T3, streptozotocin (STZ), and Ang II and evaluated the impact of these hormones on the splicing of titin, LIM domain binding 3 (Ldb3), calcium/calmodulin-dependent protein kinase II gamma (Camk2g), and triadin (Trdn). We determined the activation of mitogen-activated protein kinase (MAPK) signaling in primary cardiomyocytes treated with insulin, T3, and Ang II using western blotting; MAPK signaling was activated and RBM20 expression increased after treatment. Two downstream transcriptional factors c-jun and ETS Transcription Factor (ELK1) can bind the promoter of RBM20. A dual-luciferase activity assay revealed that Ang II, but not insulin and T3, can trigger ELK1 and thus promote transcription of RBM20. This study revealed that Ang II can trigger ELK1 through activation of MAPK signaling by enhancing RBM20 expression which regulates pre-mRNA splicing. Our study provides a potential therapeutic target for the treatment of cardiac diseases in RBM20-mediated pre-mRNA splicing.

Alternative splicing in cardiomyopathy

Alternative splicing is an important mechanism used by the cell to generate greater transcriptomic and proteomic diversity from the genome. In the heart, alternative splicing is increasingly being recognised as an important layer of post-transcriptional gene regulation. Driven by rapidly evolving technologies in next-generation sequencing, alternative splicing has emerged as a crucial process governing complex biological processes during cardiac development and disease. The recent identification of several cardiac splice factors, such as RNA-binding motif protein 20 and 24, not only provided important insight into the mechanisms underlying alternative splicing but also revealed how these splicing factors impact functional properties of the heart. Here, we review our current knowledge of alternative splicing in the heart, with a particular focus on the factors controlling cardiac alternative splicing and their role in cardiomyopathies and subsequent heart failure.