Developmental aspects of cardiac arrhythmogenesis

Ventricular Tachycardia Due to Triggered Activity: Role of Early and Delayed Afterdepolarizations

Most forms of sustained ventricular tachycardia (VT) are caused by re-entry, resulting from altered myocardial conduction and refractoriness secondary to underlying structural heart disease. In contrast, VT caused by triggered activity (TA) is unrelated to an abnormal structural substrate and is often caused by molecular defects affecting ion channel function or regulation of intracellular calcium cycling. This review summarizes the cellular and molecular bases underlying TA and exemplifies their clinical relevance with selective representative scenarios. The underlying basis of TA caused by delayed afterdepolarizations is related to sarcoplasmic reticulum calcium overload, calcium waves, and diastolic sarcoplasmic reticulum calcium leak. Clinical examples of TA caused by delayed afterdepolarizations include sustained right and left ventricular outflow tract tachycardia and catecholaminergic polymorphic VT. The other form of afterpotentials, early afterdepolarizations, are systolic events and inscribe early afterdepolarizations during phase 2 or phase 3 of the action potential. The fundamental defect is a decrease in repolarization reserve with associated increases in late plateau inward currents. Malignant ventricular arrhythmias in the long QT syndromes are initiated by early afterdepolarization-mediated TA. An understanding of the molecular and cellular bases of these arrhythmias has resulted in generally effective pharmacologic-based therapies, but these are nonspecific agents that have off-target effects. Therapeutic efficacy may need to be augmented with an implantable defibrillator. Next-generation therapies will include novel agents that rescue arrhythmogenic abnormalities in cellular signaling pathways and gene therapy approaches that transfer or edit pathogenic gene variants or silence mutant messenger ribonucleic acid.

Emerging Role and Mechanism of the FTO Gene in Cardiovascular Diseases

The fat mass and obesity-associated (FTO) gene was the first obesity-susceptibility gene identified through a genome-wide association study (GWAS). A growing number of studies have suggested that genetic variants of FTO are strongly associated with the risk of cardiovascular diseases, including hypertension and acute coronary syndrome. In addition, FTO was also the first N⁶-methyladenosine (m6A) demethylase, suggesting the reversible nature of m6A modification. m6A is dynamically deposited, removed, and recognized by m6A methylases, demethylases, and m6A binding proteins, respectively. By catalyzing m6A demethylation on mRNA, FTO may participate in various biological processes by modulating RNA function. Recent studies demonstrated that FTO plays a pivotal role in the initiation and progression of cardiovascular diseases such as myocardial fibrosis, heart failure, and atherosclerosis and may hold promise as a potential therapeutic target for treating or preventing a variety of cardiovascular diseases. Here, we review the association between FTO genetic variants and cardiovascular disease risk, summarize the role of FTO as an m6A demethylase in cardiovascular disorders, and discuss future research directions and possible clinical implications.

Deciphering Transcriptional Networks during Human Cardiac Development

Human heart development is governed by transcription factor (TF) networks controlling dynamic and temporal gene expression alterations. Therefore, to comprehensively characterize these transcriptional regulations, day-to-day transcriptomic profiles were generated throughout the directed cardiac differentiation, starting from three distinct human- induced pluripotent stem cell lines from healthy donors (32 days). We applied an expression-based correlation score to the chronological expression profiles of the TF genes, and clustered them into 12 sequential gene expression waves. We then identified a regulatory network of more than 23,000 activation and inhibition links between 216 TFs. Within this network, we observed previously unknown inferred transcriptional activations linking IRX3 and IRX5 TFs to three master cardiac TFs: GATA4, NKX2-5 and TBX5. Luciferase and co-immunoprecipitation assays demonstrated that these five TFs could (1) activate each other's expression; (2) interact physically as multiprotein complexes; and (3) together, finely regulate the expression of SCN5A, encoding the major cardiac sodium channel. Altogether, these results unveiled thousands of interactions between TFs, generating multiple robust hypotheses governing human cardiac development.

Nkx2-3 induces autophagy inhibiting proliferation and migration of vascular smooth muscle cells via AMPK/mTOR signaling pathway

Vascular remodeling and restenosis are common complications after percutaneous coronary intervention. Excessive proliferation and migration of vascular smooth muscle cells (VSMCs) play important roles in intimal hyperplasia-induced vascular restenosis. NK2 Homeobox 3 (Nkx2-3), a critical member of Nkx family, is involved in tissue differentiation and organ development. However, the role of Nkx2-3 in VSMCs proliferation and migration remains unknown. In this study, we used carotid balloon injury model and platelet-derived growth factor-BB (PDGF)-treated VSMCs as in vivo and in vitro experimental models. EdU assay and CCK-8 assay were used to detect cell proliferation. Migration was measured by scratch test. Hematoxylin and eosin staining and immunohistochemistry staining were used to evaluate the intimal hyperplasia. The autophagy level was detected by fluorescent mRFP-GFP-LC3 in vitro and by transmission electron microscopy in vivo. It was shown that Nkx2-3 was upregulated both in balloon injured carotid arteries and PDGF-stimulated VSMCs. Adenovirus-mediated Nkx2-3 overexpression inhibited intimal hyperplasia after balloon injury, and suppressed VSMCs proliferation and migration induced by PDGF. Conversely, silencing of Nkx2-3 by small interfering RNA exaggerated proliferation and migration of VSMCs. Furthermore, we found that Nkx2-3 enhanced autophagy level, while the autophagy inhibitor 3-MA eliminated the inhibitory effect of Nkx2-3 on VSMCs proliferation and migration both in vivo and in vitro. Moreover, Nkx2-3 promoted autophagy in VSMCs by activating the AMP-activated protein kinase/mammalian target of rapamycin (AMPK/mTOR) signaling pathway. These results demonstrated for the first time that Nkx2-3 inhibited VSMCs proliferation and migration through AMPK/mTOR-mediated autophagy.

Nonspecific intraventricular conduction delay is associated with future occurrence of atrial fibrillation in patients with structurally normal heart

BACKGROUND

We aimed to elucidate the long-term prognosis of nonspecific intraventricular conduction delay (NIVCD) in patients with structurally normal heart.

METHODS

We included 107,838 patients (age, 52.1 ± 15.5 years; men, 46.8%) who underwent electrocardiography in outpatient clinics or medical checkup (unmatched cohort). NIVCD was defined as QRS duration ≥110 ms without meeting the criteria for bundle branch block. Patients with structurally normal heart and sinus rhythm were assigned to the NIVCD and normal QRS groups according to propensity score with matching variables of age, sex, hypertension, and diabetes (matched cohort 1), and additional PR interval (matched cohort 2). Baseline characteristics, electrocardiographic parameters, and clinical outcomes were compared in the unmatched cohort and the matched cohort.

RESULTS

In the unmatched cohort, the frequencies of male sex and preexisting atrial fibrillation were significantly higher in the NIVCD group than in the normal QRS group. In matched cohort 1 (n = 690), the NIVCD group exhibited significant slower sinus rate and longer PR interval than the normal QRS group. In matched cohort 2 (n = 598), the cumulative incidence of atrial fibrillation was significantly higher in the NIVCD group than in the normal QRS group during a follow-up period of 8.8 ± 2.9 years. NIVCD significantly increased the risk for AF (hazard ratio, 2.571; 95% confidence interval, 1.074-6.156; p = 0.034).

CONCLUSIONS

It is suggested that NIVCD may be associated with future occurrence of atrial fibrillation in patients with structurally normal heart and sinus rhythm.

IRX5 promotes colorectal cancer metastasis by negatively regulating the core components of the RHOA pathway

Colorectal cancer (CRC) is one of the most common malignant tumors worldwide. As tumor metastasis is the leading cause of death in patients with CRC, it is important to elucidate the molecular mechanisms that drive CRC metastasis. Studies have shown a close relationship between Iroquois homeobox (IRX) family genes and multiple cancers, while the mechanism by which IRX5 promotes CRC metastasis is unclear. Therefore, we focused on the involvement of IRX5 in CRC metastasis. In this study, analyses of clinical data indicated that the expression of IRX5 was coincided with metastatic colorectal tumors tissues and was negatively correlated with the overall survival of patients with CRC. Functional analysis showed that IRX5 promoted the migration and invasion of CRC cells, accompanied by a large number of cellular protrusions. IRX5-overexpressing cells were more likely to form metastatic tumors in nude mice. Further analysis demonstrated that the core components of the RHOA/ROCK1/LIMK1 pathway were significantly inhibited in IRX5-overexpressing cells. Overexpression of LIMK1 effectively reversed the enhanced cellular motility caused by IRX5 overexpression. Moreover, we found that high levels of IRX5 in intestinal tissues were correlated with the inflammatory response. IRX5 was significantly increased in azoxymethane/dextran sodium sulfate intestinal tissue of mice and IRX5-overexpressing may also enhance chemokines CXCL1 and CXCL8. In summary, our findings suggested that IRX5 promoted CRC metastasis by inhibiting the RHOA-ROCK1-LIMK1 axis, which correlates with a poor prognosis.

Holt-Oram Syndrome With Multiple Cardiac Abnormalities

Holt-Oram syndrome (HOS) is a rare monogenic disorder characterized by upper limb abnormalities, congenital heart defects and/or conduction abnormalities. It is determined by mutations of TBX5 gene and is inherited in an autosomal dominant manner. Penetrance is complete, but variable expressivity is present, which gives sometimes diagnostic difficulties. Our case is a young adult with a personal history of preaxial polydactyly operated in infancy, multiple cardiac malformations (atrial septal defect, bicuspid aortic valve, left ventricular non-compaction) and radiologic findings consistent with HOS. Family history is negative for HOS. In conclusion, we present a case of HOS diagnosed in the adult period to highlight the diagnostic problems for the proband and the family and the importance of an early diagnostic.

A MicroRNA-Transcription Factor Blueprint for Early Atrial Arrhythmogenic Remodeling

Spontaneous self-terminating atrial fibrillation (AF) is one of the most common heart rhythm disorders, yet the regulatory molecular mechanisms underlying this syndrome are rather unclear. MicroRNA (miRNA) transcriptome and expression of candidate transcription factors (TFs) with potential roles in arrhythmogenesis, such as Pitx2, Tbx5, and myocardin (Myocd), were analyzed by microarray, qRT-PCR, and Western blotting in left atrial (LA) samples from pigs with transitory AF established by right atrial tachypacing. Induced ectopic tachyarrhythmia caused rapid and substantial miRNA remodeling associated with a marked downregulation of Pitx2, Tbx5, and Myocd expression in atrial myocardium. The downregulation of Pitx2, Tbx5, and Myocd was inversely correlated with upregulation of the corresponding targeting miRNAs (miR-21, miR-10a/10b, and miR-1, resp.) in the LA of paced animals. Through in vitro transient transfections of HL-1 atrial myocytes, we further showed that upregulation of miR-21 did result in downregulation of Pitx2 in cardiomyocyte background. The results suggest that immediate-early miRNA remodeling coupled with deregulation of TF expression underlies the onset of AF.

Outflow tract ventricular arrhythmias: An update

During the last 20 years, the molecular etiology for many ventricular tachyarrhythmias once referred to as "idiopathic," has been elucidated. These arrhythmias are due to mutations in ion channels or structural proteins and include ventricular tachyarrhythmias due to long and short QT syndromes, Brugada syndrome, and catecholaminergic polymorphic ventricular tachycardia (VT). However, the basis for the most common form of idiopathic ventricular arrhythmia, which originates from right or left ventricular outflow tracts, has remained elusive. Although traditionally considered a benign ventricular arrhythmia, it is now appreciated that some outflow tract arrhythmias also trigger polymorphic VT or sudden cardiac death or result in cardiomyopathy. The current understanding of outflow tract arrhythmias will be examined.

Atrial Fibrillation associated chromosome 4q25 variants are not associated with PITX2c expression in human adult left atrial appendages

Atrial Fibrillation (AF), the most common sustained arrhythmia, has a strong genetic component, but the mechanism by which common genetic variants lead to increased AF susceptibility is unknown. Genome-wide association studies (GWAS) have identified that the single nucleotide polymorphisms (SNPs) most strongly associated with AF are located on chromosome 4q25 in an intergenic region distal to the PITX2 gene. Our objective was to determine whether the AF-associated SNPs on chromosome 4q25 were associated with PITX2c expression in adult human left atrial appendages. Analysis of a lone AF GWAS identified four independent AF risk SNPs at chromosome 4q25. Human adult left atrial appendage tissue was obtained from 239 subjects of European Ancestry and used for SNP analysis of genomic DNA and determination of PITX2c RNA expression levels by quantitative PCR. Subjects were divided into three groups based on their history of AF and pre-operative rhythm. AF rhythm subjects had higher PITX2c expression than those with history of AF but in sinus rhythm. PITX2c expression was not associated with the AF risk SNPs in human adult left atrial appendages in all subjects combined or in each of the three subgroups. However, we identified seven SNPs modestly associated with PITX2c expression located in the introns of the ENPEP gene, ∼54 kb proximal to PITX2. PITX2c expression in human adult left atrial appendages is not associated with the chromosome 4q25 AF risk SNPs; thus, the mechanism by which these SNPs are associated with AF remains enigmatic.

Genomics of cardiac electrical function

Proper generation and conduction of the cardiac electrical impulse is essential for the continuous coordinated contraction of the heart. Dysregulation of cardiac electrical function may lead to cardiac arrhythmias, which constitute a huge medical and social burden. Identifying the genetic factors underlying cardiac electrical activity serves the double purpose of allowing the early identification of individuals at risk for arrhythmia and discovering new potential therapeutic targets for prevention. The aim of this review is to provide an overview of the genes and genetic loci linked thus far to cardiac electrical function and arrhythmia. These genes and loci have been primarily uncovered through studies on the familial rhythm disorders and through genome-wide association studies on electrocardiographic parameters in large sets of the general population. An overview of all genes and loci with their respective effect is given.

Islet1 is a direct transcriptional target of the homeodomain transcription factor Shox2 and rescues the Shox2-mediated bradycardia

The heart's rhythm is initiated and regulated by a group of specialized cells in the sinoatrial node (SAN), the primary pacemaker of the heart. Abnormalities in the development of the SAN can result in irregular heart rates (arrhythmias). Although several of the critical genes important for SAN formation have been identified, our understanding of the transcriptional network controlling SAN development remains at a relatively early stage. The homeodomain transcription factor Shox2 is involved in the specification and patterning of the SAN. While the Shox2 knockout in mice results in embryonic lethality due to severe cardiac defects including improper SAN development, Shox2 knockdown in zebrafish causes a reduced heart rate (bradycardia). In order to gain deeper insight into molecular pathways involving Shox2, we compared gene expression levels in right atria of wildtype and Shox2 (-/-) hearts using microarray experiments and identified the LIM homeodomain transcription factor Islet1 (Isl1) as one of its putative target genes. The downregulation of Isl1 expression in Shox2 (-/-) hearts was confirmed and the affected region narrowed down to the SAN by whole-mount in situ hybridization. Using luciferase reporter assays and EMSA studies, we identified two specific SHOX2 binding sites within intron 2 of the ISL1 locus. We also provide functional evidence for Isl1 as a transcriptional target of Shox2 by rescuing the Shox2-mediated bradycardia phenotype with Isl1 using zebrafish as a model system. Our findings demonstrate a novel epistatic relationship between Shox2 and Isl1 in the heart with important developmental consequences for SAN formation and heart beat.

Iroquois homeodomain transcription factors in heart development and function

Numerous cardiac transcription factors play overlapping roles in both the specification and proliferation of the cardiac tissues and chambers during heart development. It has become increasingly apparent that cardiac transcription factors also play critical roles in the regulation of expression of many functional genes in the prenatal and postnatal hearts. Accordingly, mutations of cardiac transcription factors cannot only result in congenital heart defects but also alter heart function thereby predisposing to heart disease and cardiac arrhythmias. In this review, we summarize the roles of Iroquois homeobox (Irx) family of transcription factors in heart development and function. In all, 6 Irx genes are expressed with distinct and overlapping patterns in the mammalian heart. Studies in several animal models demonstrate that Irx genes are important for the establishment of ventricular chamber properties, the ventricular conduction system, as well as heterogeneity of the ventricular repolarization. The molecular mechanisms by which Irx proteins regulate gene expression and the clinical relevance of Irx functions in the heart are discussed.

Genetic variation in T-box binding element functionally affects SCN5A/SCN10A enhancer

The contraction pattern of the heart relies on the activation and conduction of the electrical impulse. Perturbations of cardiac conduction have been associated with congenital and acquired arrhythmias as well as cardiac arrest. The pattern of conduction depends on the regulation of heterogeneous gene expression by key transcription factors and transcriptional enhancers. Here, we assessed the genome-wide occupation of conduction system-regulating transcription factors TBX3, NKX2-5, and GATA4 and of enhancer-associated coactivator p300 in the mouse heart, uncovering cardiac enhancers throughout the genome. Many of the enhancers colocalized with ion channel genes repressed by TBX3, including the clustered sodium channel genes Scn5a, essential for cardiac function, and Scn10a. We identified 2 enhancers in the Scn5a/Scn10a locus, which were regulated by TBX3 and its family member and activator, TBX5, and are functionally conserved in humans. We also provided evidence that a SNP in the SCN10A enhancer associated with alterations in cardiac conduction patterns in humans disrupts TBX3/TBX5 binding and reduces the cardiac activity of the enhancer in vivo. Thus, the identification of key regulatory elements for cardiac conduction helps to explain how genetic variants in noncoding regulatory DNA sequences influence the regulation of cardiac conduction and the predisposition for cardiac arrhythmias.

Characterization of genome-wide association-identified variants for atrial fibrillation in African Americans

BACKGROUND

Despite a greater burden of risk factors, atrial fibrillation (AF) is less common among African Americans than European-descent populations. Genome-wide association studies (GWAS) for AF in European-descent populations have identified three predominant genomic regions associated with increased risk (1q21, 4q25, and 16q22). The contribution of these loci to AF risk in African American is unknown.

METHODOLOGY/PRINCIPAL FINDINGS

We studied 73 African Americans with AF from the Vanderbilt-Meharry AF registry and 71 African American controls, with no history of AF including after cardiac surgery. Tests of association were performed for 148 SNPs across the three regions associated with AF, and 22 SNPs were significantly associated with AF (P<0.05). The SNPs with the strongest associations in African Americans were both different from the index SNPs identified in European-descent populations and independent from the index European-descent population SNPs (r(2)<0.40 in HapMap CEU): 1q21 rs4845396 (odds ratio [OR] 0.30, 95% confidence interval [CI] 0.13-0.67, P = 0.003), 4q25 rs4631108 (OR 3.43, 95% CI 1.59-7.42, P = 0.002), and 16q22 rs16971547 (OR 8.1, 95% CI 1.46-45.4, P = 0.016). Estimates of European ancestry were similar among cases (23.6%) and controls (23.8%). Accordingly, the probability of having two copies of the European derived chromosomes at each region did not differ between cases and controls.

CONCLUSIONS/SIGNIFICANCE

Variable European admixture at known AF loci does not explain decreased AF susceptibility in African Americans. These data support the role of 1q21, 4q25, and 16q22 variants in AF risk for African Americans, although the index SNPs differ from those identified in European-descent populations.

Common genetic variation modulating cardiac ECG parameters and susceptibility to sudden cardiac death

Sudden cardiac death (SCD) is a prevalent cause of death in Western societies. Genome-wide association studies (GWAS) conducted over the last few years have uncovered common genetic variants modulating risk of SCD. Furthermore, GWAS studies uncovered several loci impacting on heart rate and ECG indices of conduction and repolarization, as measures of cardiac electrophysiological function and likely intermediate phenotypes of SCD risk. We here review these recent developments and their implications for the identification of novel molecular pathways underlying normal electrophysiological function and susceptibility to SCD.