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Commentary Free access | 10.1172/JCI99927

SCN5A: the greatest HITS collection

David S. Park and Glenn I. Fishman

Leon H. Charney Division of Cardiology, New York University School of Medicine, New York, New York, USA.

Address correspondence to: Glenn I. Fishman, Leon H. Charney Division of Cardiology, NYU School of Medicine, 522 First Avenue, Smilow 801, New York, New York 10016, USA. Phone: 212.263.3967; Email: glenn.fishman@nyumc.org.

Find articles by Park, D. in: JCI | PubMed | Google Scholar

Leon H. Charney Division of Cardiology, New York University School of Medicine, New York, New York, USA.

Address correspondence to: Glenn I. Fishman, Leon H. Charney Division of Cardiology, NYU School of Medicine, 522 First Avenue, Smilow 801, New York, New York 10016, USA. Phone: 212.263.3967; Email: glenn.fishman@nyumc.org.

Find articles by Fishman, G. in: JCI | PubMed | Google Scholar

First published February 19, 2018 - More info

Published in Volume 128, Issue 3 on March 1, 2018
J Clin Invest. 2018;128(3):913–915. https://doi.org/10.1172/JCI99927.
Copyright © 2018, American Society for Clinical Investigation
First published February 19, 2018 - Version history

Related article:

A common variant alters SCN5A–miR-24 interaction and associates with heart failure mortality
Xiaoming Zhang, … , Barry London, Ryan L. Boudreau
Xiaoming Zhang, … , Barry London, Ryan L. Boudreau
Research Article Cardiology Genetics

A common variant alters SCN5A–miR-24 interaction and associates with heart failure mortality

Abstract

SCN5A encodes the voltage-gated Na+ channel NaV1.5 that is responsible for depolarization of the cardiac action potential and rapid intercellular conduction. Mutations disrupting the SCN5A coding sequence cause inherited arrhythmias and cardiomyopathy, and single-nucleotide polymorphisms (SNPs) linked to SCN5A splicing, localization, and function associate with heart failure–related sudden cardiac death. However, the clinical relevance of SNPs that modulate SCN5A expression levels remains understudied. We recently generated a transcriptome-wide map of microRNA (miR) binding sites in human heart, evaluated their overlap with common SNPs, and identified a synonymous SNP (rs1805126) adjacent to a miR-24 site within the SCN5A coding sequence. This SNP was previously shown to reproducibly associate with cardiac electrophysiological parameters, but was not considered to be causal. Here, we show that miR-24 potently suppresses SCN5A expression and that rs1805126 modulates this regulation. We found that the rs1805126 minor allele associates with decreased cardiac SCN5A expression and that heart failure subjects homozygous for the minor allele have decreased ejection fraction and increased mortality, but not increased ventricular tachyarrhythmias. In mice, we identified a potential basis for this in discovering that decreased Scn5a expression leads to accumulation of myocardial reactive oxygen species. Together, these data reiterate the importance of considering the mechanistic significance of synonymous SNPs as they relate to miRs and disease, and highlight a surprising link between SCN5A expression and nonarrhythmic death in heart failure.

Authors

Xiaoming Zhang, Jin-Young Yoon, Michael Morley, Jared M. McLendon, Kranti A. Mapuskar, Rebecca Gutmann, Haider Mehdi, Heather L. Bloom, Samuel C. Dudley, Patrick T. Ellinor, Alaa A. Shalaby, Raul Weiss, W.H. Wilson Tang, Christine S. Moravec, Madhurmeet Singh, Anne L. Taylor, Clyde W. Yancy, Arthur M. Feldman, Dennis M. McNamara, Kaikobad Irani, Douglas R. Spitz, Patrick Breheny, Kenneth B. Margulies, Barry London, Ryan L. Boudreau

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Abstract

Heart failure (HF) has been referred to as the cardiovascular epidemic of our time. Understanding the molecular determinants of HF disease progression and mortality risk is of utmost importance. In this issue of the JCI, Zhang et al. uncover an important link between clinical HF mortality risk and a common variant that regulates SCN5A expression through microRNA-dependent (miR-dependent)mechanisms. They also demonstrate that haploinsufficiency of SCN5A is associated with increased accumulation of reactive oxygen species (ROS) in a genetically engineered murine model. Their data suggest that even modest depression of SCN5A expression may promote pathologic cardiac remodeling and progression of HF.

SCN5A expression in HF

HF represents a final common pathway for a wide range of inherited and acquired cardiac conditions (1). Disease progression consists of a series of maladaptive responses, which include ion channel remodeling (2), neurohormonal dysregulation, metabolic derangement, oxidative stress, and profibrotic signaling (1). HF patients are at an increased risk of death from sudden cardiac arrest (2) and progressive pump failure (1). Risk stratification of HF patients to identify those at highest risk represents an unmet need as advanced therapies, such as implantable cardioverter defibrillators (ICDs) or ventricular assist devices, could be offered more selectively.

Evaluation of clinical factors that confer increased risk of morbidity and mortality in HF patients show clear correlation with QRS duration, a marker of ventricular activation time and myocardial dyssynchrony (3, 4). As the cardiac sodium channel pore-forming subunit NaV1.5 (encoded by SCN5A) is the principal determinant of cardiac excitability and conduction in the subendocardial His-Purkinje network and ventricular chamber myocardium (5–8), tight regulation of this gene ensures optimal cardiac function and stable rhythm (5, 6). Although mutations in SCN5A have been implicated in arrhythmic diseases such as progressive cardiac conduction disease (9), Brugada syndrome (10), and long QT3 (11, 12), and can produce cardiomyopathic changes (13, 14), the vast majority of HF patients are not mutant carriers. Therefore, intense research has focused on identifying genetic modifiers of SCN5A expression that may confer increased risk for conduction disease, arrhythmic events (15), or mortality in HF patients.

Several genome-wide association studies (GWAS) of cardiac conduction parameters have identified strong association with single nucleotide polymorphisms (SNPs) in the SCN5A locus (16–19). Decoding whether or not these SNPs have functional consequences on SCN5A gene expression is essential. Moreover, determining whether the SNPs that alter SCN5A expression have impact on clinical syndromes, including heart failure, is of particular interest.

Modulation of SCN5A expression

In this latest edition of the JCI, Zhang et al. (20) employed a new approach to understand how GWAS-identified SNPs can modulate SCN5A expression by altering miR-dependent regulation. MiRs are short (~19–22 nucleotides), noncoding RNA species that are potent regulators of mRNA transcript stability and translation (21, 22). MiRs are encoded either in intronic regions where they are cotranscribed with protein-coding exons, or in intergenic regions under the control of their own promoters (21, 22). Mature miRs are coassembled with Argonaute (Ago) proteins as part of the RNA-induced silencing complex (RISC) (21). In the classic paradigm, miRs target mRNA via their 3′ untranslated region (UTR) through complementary base-pair interaction with the miR “seed” region (5′ region of miR; nucleotide positions 2–8). This miR-mRNA interaction targets mRNA for cleavage or reduces translational efficiency (21). The short sequence interactions allow for miRs to broadly regulate an array of mRNA transcripts that collectively control biological processes, such as calcium homestasis (23) or metabolism (24). Therefore, identification of bona fide miR targets in a given tissue is essential for therapeutic targeting and prevention of off-target effects.

To identify miR targets in the heart, Boudreau and colleagues previously reported a high throughput method to globally profile miR-mRNA target interactions in human cardiac tissue (25). They applied a technique known as high-throughput sequencing of RNA isolated by crosslinking immunoprecipitation (HITS-CLIP), where Ago2 protein was immunoprecipitated to identify bound miRs and mRNAs (25, 26). By overlaying common SNPs on top of the transcriptome-wide map of miR binding sites, they identified a non–amino acid altering (synonymous) SNP (rs1805126, T>C) adjacent to a miR-24 interaction site in the SCN5A coding sequence (25). The rs1805126 minor allele was previously shown to associate with cardiac conduction parameters in GWAS (18), but the synonymous nature of the variant led to the presumption that the variant was not causal.

Zhang et al. (20) hypothesized that the rs1805126 minor allele alters miR-24–dependent regulation of SCN5A (Figure 1). As the rs1805126 variant does not alter the “seed” interaction sequence, the authors speculated and tested through computational means that the C allele produced a more favorable miR-24 interaction with the SCN5A transcript. In human heterologous expression systems, miR-24 suppressed NaV1.5 expression more significantly with the SCN5A C allele versus T allele. MiR-24 mimics also reduced sodium current density in neonatal rat cardiomyocytes (NRCMs). In human heart samples, presence of the rs1805126 CC genotype was associated with lower levels of SCN5A mRNA and NaV1.5 channel protein compared to the TT genotype, whereas miR-24 expression was similar between groups. Similar results were seen in expression quantitative trait loci (eQTL) studies (27, 28) and analysis of hearts heterozygous for rs1805126, where the C allele was associated with lower SCN5A mRNA levels.

Proposed link between microRNA-dependent regulation of SCN5A and disease prFigure 1

Proposed link between microRNA-dependent regulation of SCN5A and disease progression. Genome-wide association studies (GWAS) have linked a synonymous SNP (rs1805126) in the SCN5A gene with electrocardiographic measures. (A) Ago2 HITS-CLIP data identify a microRNA-24 (miR-24) binding site immediately adjacent to this SNP. (B) Probability of Interaction by Target Accessibility (PITA) analysis indicates that the rs1805126 minor allele (C) is a thermodynamically more favorable miR-24 target compared to the major allele (T), resulting in greater SCN5A degradation and diminished NaV1.5 expression. While conduction slowing is a predictable consequence of diminished sodium channel expression, Zhang and colleagues (22) show, surprisingly, that reduced SCN5A expression is also associated with increased myocardial reactive oxygen species (ROS), and suggest that ROS accumulation promotes heart failure progression and increased nonarrhythmic mortality.

Clinical consequences of SCN5A modulation

To explore the clinical consequences of the rs1805126 variant in HF patients, Zhang et al. (20) examined the effect of rs1805126 genotypes in cardiomyopathy patients with ICDs from the Genetic Risk Assessment of Defibrillator Events (GRADE) study (29) and found a higher mortality rate in patients homozygous for the C allele. Surprisingly, the rs1805126 genotype did not significantly associate with appropriate ICD therapies (a surrogate marker of arrhythmic death). In addition, the authors did not find a significant association with electrocardiographic parameters, although the study was underpowered to do so. As previously stated, prolonged QRS duration is associated with increased morbidity and mortality in HF patients due to dyssynchronous myocardial contraction; therefore, it will be important to reanalyze this association with increased patient enrollment or in another dataset, especially given the known association of rs1805126 with cardiac conduction parameters (18). Similarly, these data should be analyzed for associations between the homozygous rs1805126 genotype and cardiac resynchronization therapy, indicated for patients with significant QRS prolongation and HF (30). Lastly, it would be of interest to note whether the CC genotype is associated with increased right ventricular pacing percentage, as this type of pacing produces myocardial dyssynchrony and has been shown to reduce left ventricular ejection fraction and increase morbidity and mortality (31).

To explore the potential mechanism whereby reduced SCN5A expression is associated with a more severe cardiomyopathic phenotype, Zhang et al. (20) studied Scn5a heterozygous knockout mice, which develop increased fibrosis at advanced age (32). Scn5a heterozygous knockout hearts had significantly increased reactive oxygen species (ROS) as evidenced by an approximately 2.5-fold increase in oxidation of dihydroethidium (DHE, a measure of steady-state levels of superoxide), which was evident before fibrotic changes. Although association between rs1805126 genotype and increased oxidative stress was not examined, the finding that reduced Scn5a expression is sufficient to increase ROS is intriguing and needs further investigation. Accumulation of ROS can result from either over-production or impaired clearance (33). Increased ROS production in HF is primarily due to functional uncoupling of the mitochondrial electron transport chain (34–37); however, other sources include xanthine oxidase (38), nitric oxide synthase (39), cyclooxygenase (40), and NAD(P)H oxidases (41). Mechanisms of impaired clearance by antioxidants include reduced activity of superoxide dismutase (42) and catalase (43). Identifying which of these pathways contributes to ROS accumulation due to reduced NaV1.5 expression will be an important first step in deciphering the mechanisms underlying induction of profibrotic pathways and the development of cardiomyopathy.

Concluding remarks

The work of Zhang et al. (20) adds to the growing body of evidence that sequence variants that regulate SCN5A expression can have significant consequences on HF disease progression and mortality. Although the mechanism of worsening HF associated with rs1805126 will need further evaluation, these findings bring us one step closer to creating a genetic HF risk score, which can be used to personalize therapies for this complex and growing patient population.

Acknowledgments

This work is supported by grants from the NIH to DSP (R01 HL132073), and GIF (R01 HL105983 and R01 HL92727), an American Heart Association Scientist Development Grant (17SDG33411201) to DSP, and a Fondation LeDucq Transatlantic Network Award to GIF.

Footnotes

Conflict of interest: The authors have declared that no conflict of interest exists.

Reference information: J Clin Invest. 2018;128(3):913–915. https://doi.org/10.1172/JCI99927.

See the related article at A common variant alters SCN5A–miR-24 interaction and associates with heart failure mortality.

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Version history
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