The T-Box transcription factor Tbx5 is required for the patterning and maturation of the murine cardiac conduction system

pouC Regulates Expression of bmp4 During Atrioventricular Canal Formation in Zebrafish

BACKGROUND

Many human gene mutations have been linked to congenital heart disease (CHD), yet CHD remains a major health issue worldwide due in part to an incomplete understanding of the molecular basis for cardiac malformation.

RESULTS

Here we identify the orthologous mouse Pou6f1 and zebrafish pouC as POU homeodomain transcription factors enriched in the developing heart. We find that pouC is a multi-functional transcriptional regulator containing separable activation, repression, protein-protein interaction, and DNA binding domains. Using zebrafish heart development as a model system, we demonstrate that pouC knockdown impairs cardiac morphogenesis and affects cardiovascular function. We also find that levels of pouC expression must be fine-tuned to enable proper heart formation. At the cellular level, we demonstrate that pouC knockdown disrupts atrioventricular canal (AVC) cardiomyocyte maintenance, although chamber myocyte specification remains intact. Mechanistically, we show that pouC binds a bmp4 intronic regulatory element to mediate transcriptional activation.

CONCLUSIONS

Taken together, our study establishes pouC as a novel transcriptional input into the regulatory hierarchy that drives AVC morphogenesis in zebrafish. We anticipate that these findings will inform future efforts to explore functional conservation in mammals and potential association with atrioventricular septal defects in humans. Developmental Dynamics 248:173-188, 2019. © 2018 Wiley Periodicals, Inc.

NOTCH maintains developmental cardiac gene network through WNT5A

NOTCH and WNT signaling pathways play critical roles in cardiac chamber formation. Here we explored the potential interactions between the two pathways in this developmental process by using genetically modified mouse models and whole embryo culture systems. By deletion of Notch1 to inactivate NOTCH1 signaling in the endocardium in vivo and ex vivo rescue experiments, we showed that myocardial WNT5A mediated endocardial NOTCH1 signaling to maintain the gene regulatory network essential for cardiac chamber formation. Furthermore, genetic deletion of β-catenin in the myocardium and inhibition of the WNT/Ca²⁺ signaling by FK506 resulted in a similar disruption of the gene regulatory network as inactivation of endocardial NOTCH1 signaling. Together, these findings identify WNT5A as a key myocardial factor that mediates the endocardial NOTCH signaling to maintain the gene regulatory network essential for cardiac chamber formation through WNT/β-catenin and WNT/Ca²⁺ signaling pathways.

Electrical disorders in atrial septal defect: genetics and heritability

Atrial septal defect (ASD) is one of the most common types of congenital heart diseases (CHDs). Most ASDs occur sporadically, but some are inherited and associated with cardiac conduction defects such as atrioventricular block (AVB) or bundle branch block. Mutations in genes encoding transcription factor gene TBX5 and NKX2-5, were found in Holt-Oram syndrome (HOS) and ASD with atrioventricular (AV) conduction defects, respectively. HOS is characterized by upper limb anomaly in addition to ASD and AVB (heart-hand syndrome). ASD associated with NKX2-5 is rare but is reported to cause sudden cardiac death (SCD) or cardiomyopathy. We provide a review of these two diseases.

Fenbuconazole exposure impacts the development of zebrafish embryos

Fenbuconazole (FBZ), a triazole-containing fungicide, is widely used in agriculture and horticulture. In the present study, the development and cardiac functioning were observed and determined in zebrafish embryos exposed to FBZ at 5, 50 and 500 ng/L nominal concentrations for 72 h. The results showed that 500 ng/L FBZ significantly increased pericardial edema rate, spine curvature rate, disturbed cardiac function, and led a shortened lower jaw. The transcription of genes such as tbx5, nkx2.5, tnnt2, gata4, bmp2b, myl7 was altered, which might be responsible for the cardiac developmental and functioning defects in the larvae. The deformation in bone development might be related with the impaired transcription levels of shh and bmp2b. The transcription of cyp26a1 (encoding retinoic acid metabolism enzyme) was significantly up-regulated in the 500 ng/L group, which might be a reason causing the teratogenic effect of FBZ. These results suggest that FBZ could have toxic effects on embryonic development, which should be considered in the risk evaluation of FBZ application.

The Anatomy, Development, and Evolution of the Atrioventricular Conduction Axis

It is now well over 100 years since Sunao Tawara clarified the location of the axis of the specialised myocardium responsible for producing coordinated ventricular activation. Prior to that stellar publication, controversies had raged as to how many bundles crossed the place of the atrioventricular insulation as found in mammalian hearts, as well as the very existence of the bundle initially described by Wilhelm His Junior. It is, perhaps surprising that controversies continue, despite the multiple investigations that have taken place since the publication of Tawara’s monograph. For example, we are still unsure as to the precise substrates for the so-called slow and fast pathways into the atrioventricular node. Much has been done, nonetheless, to characterise the molecular make-up of the specialised pathways, and to clarify their mechanisms of development. Of this work itself, a significant part has emanated from the laboratory coordinated for a quarter of a century by Antoon FM Moorman. In this review, which joins the others in recognising the value of his contributions and collaborations, we review our current understanding of the anatomy, development, and evolution of the atrioventricular conduction axis.

ETV1 activates a rapid conduction transcriptional program in rodent and human cardiomyocytes

Rapid impulse propagation is a defining attribute of the pectinated atrial myocardium and His-Purkinje system (HPS) that safeguards against atrial and ventricular arrhythmias, conduction block, and myocardial dyssynchrony. The complex transcriptional circuitry that dictates rapid conduction remains incompletely understood. Here, we demonstrate that ETV1 (ER81)-dependent gene networks dictate the unique electrophysiological characteristics of atrial and His-Purkinje myocytes. Cardiomyocyte-specific deletion of ETV1 results in cardiac conduction abnormalities, decreased expression of rapid conduction genes (Nkx2-5, Gja5, and Scn5a), HPS hypoplasia, and ventricularization of the unique sodium channel properties that define Purkinje and atrial myocytes in the adult heart. Forced expression of ETV1 in postnatal ventricular myocytes (VMs) reveals that ETV1 promotes a HPS gene signature while diminishing ventricular and nodal gene networks. Remarkably, ETV1 induction in human induced pluripotent stem cell-derived cardiomyocytes increases rapid conduction gene expression and inward sodium currents, converting them towards a HPS phenotype. Our data identify a cardiomyocyte-autonomous, ETV1-dependent pathway that is responsible for specification of rapid conduction zones in the heart and demonstrate that ETV1 is sufficient to promote a HPS transcriptional and functional program upon VMs.

Specialized impulse conduction pathway in the alligator heart

Mammals and birds have a specialized cardiac atrioventricular conduction system enabling rapid activation of both ventricles. This system may have evolved together with high heart rates to support their endothermic state (warm-bloodedness) and is seemingly lacking in ectothermic vertebrates from which first mammals then birds independently evolved. Here, we studied the conduction system in crocodiles (Alligator mississippiensis), the only ectothermic vertebrates with a full ventricular septum. We identified homologues of mammalian conduction system markers (Tbx3-Tbx5, Scn5a, Gja5, Nppa-Nppb) and show the presence of a functional atrioventricular bundle. The ventricular Purkinje network, however, was absent and slow ventricular conduction relied on trabecular myocardium, as it does in other ectothermic vertebrates. We propose the evolution of the atrioventricular bundle followed full ventricular septum formation prior to the development of high heart rates and endothermy. In contrast, the evolution of the ventricular Purkinje network is strongly associated with high heart rates and endothermy.

Mammals and birds are referred to as ‘warm-blooded’ animals, because they maintain a constant high body temperature. This requires a lot of energy, so their bodies need to be well supplied with blood at all times. The hearts of mammals and birds contain two important structures that help them do this. The first is a full wall of muscle – called the ventricular septum – that divides the heart into left and right sides. The second is an electrical circuit made of specialized muscle cells that ensures that the heart beats fast enough by sending rapid electrical signals to the rest of the heart muscle. The circuit contains one group of cells in the ventricular septum, called the bundle of His, and another group termed the Purkinje network.

Reptiles, however, do not maintain high body temperatures and are instead often thought of as ‘cold-blooded’ animals. The hearts of reptiles do not need to pump blood around the body as quickly and have different structures from warm-blooded animals. For example, most reptile hearts do not have a fully developed ventricular septum. The only exceptions are crocodiles, alligators and their relatives (the ‘crocodilians’), which do.

Jensen, Boukens et al. therefore wanted to determine if a crocodilian heart also contained a specialized electrical circuit like those of birds and mammals. Previous studies that attempted to answer this question using only anatomical and electrical methods had yielded ambiguous results. As such, Jensen, Boukens et al. combined these methods with genetic techniques for a more detailed study.

First, the ventricular septum of American alligators, a species of crocodilian, was examined, and found to contain a narrow tissue structure that strongly resembled the bundle of His. Indeed, if this presumptive bundle of His was cut, the electrical circuit was broken. Additional genetic analysis of this structure confirmed that genes similar to those active in the mammalian bundle of His were also switched on in alligators. However, recordings of heart activity showed that heart rates and the spread of electrical signals were both slower in alligators than in warm-blooded animals. This suggests that, although alligators have evolved some specialized muscle cells (in the form of a bundle of His), their electrical circuit is still ‘incomplete’. The lack of a Purkinje network, for example, would explain why their heart rates remain slow like other reptiles’.

Together these findings add to the current understanding of how the heart works in different animals with varying requirements for energy and blood flow. Also, since crocodiles and warm-blooded birds both evolved from ancient reptiles, detailed descriptions of their heart structures could shed more light on how warm-bloodedness first developed.

Genetic Analyses Identified a SALL4 Gene Mutation Associated with Holt-Oram Syndrome

Holt-Oram syndrome (HOS) is an autosomal dominant disorder, which is characterized by deformities of upper limbs and congenital heart defects. Alterations of TBX5 gene have been identified to be the leading cause of HOS, while some cases could not be explained by TBX5 mutations. In our study, we preliminarily diagnosed a newborn baby, who had Tetralogy of Fallot, thumb agenesis, facial dysplasia, and right ear canal malformation, as HOS. Chromosome microarray analyses showed no pathological deletions or replications of chromosome segments; whole exome sequencing screened out six candidate genes that were involved in cardiac diseases or syndromes among which SALL4 has been reported as HOS related gene. We evaluated the pathogenicity of SALL4 mutant sites by series of software. The results indicated that SALL4-M143V may be a polymorphism site, and SALL4-R418C could cause disease. HOPE and SWISS PDB viewer showed that SALL4-R418C leads to changes in amino acid properties, loss of protein hydrogen bond, and functional impact of SALL4 zinc finger domain. These results further confirmed the pathogenic significance of SALL4-R418C mutant. When genetic analyses coupled with bioinformatic analyses, we identified a SALL4 gene rare mutation which might contribute to a newborn with HOS. Although some doubts need to be further discussed and explored, our study deepened the understanding of phenotype difference among syndromes and role of SALL4 mutations in disease occurrence.

The developmental effects of low-level procymidone towards zebrafish embryos and involved mechanism

Procymidone (PCM), a dicarboximide fungicide, is widely used in agriculture to control plant diseases. In the present study, zebrafish embryos were exposed to PCM at 0, 10, 100 and 1000 ng/L for 72 h, the development and cardiac functioning of larvae were observed and determined. The results showed that hatching rate was significantly decreased in the 1000 ng/L treatment, and pericardial edema rate and spine curvature rate were significantly increased in the 100 and 1000 ng/L groups. The PCM-treated larvae exhibited an increased heart rate as well as arrhythmia, and shortened low jaw. The transcription levels of cardiac development-related genes tbx5, nkx2.5, tnnt2, gata4, myh6, myl7, cdh2, ryr2 were altered, which might be responsible for the cardiac developmental and functioning defects in the larvae. The deformation in bone development might be related with the impaired transcription levels of ihh, shh, bmp2b, bmp4, gh, igf1, sox9, gli2. The activities of Na⁺/K⁺-ATPase and Ca²⁺-ATPase were significantly inhibited by 100 ng/L and 1000 ng/L PCM exposure, which might be a cause for the occurrence of pericardial edema and skeletal deformation. The results of this study will be helpful in evaluating the potential threat of PCM to fish population in the aquatic environment.

Developmental toxicity and cardiac effects of butyl benzyl phthalate in zebrafish embryos

Phthalic acid esters (PAEs), commonly called phthalates, have become ubiquitous environment pollutants. Studies have focused on reproductive toxicity, neurotoxicity, teratogenicity, tumourigenesis, and mutagenesis of phthalates. However, relatively little is known about the phthalates effects on the heart. Butyl benzyl phthalate (BBP), a member of PAEs, is classified by the US Environmental Protection Agency as a priority environmental pollutant. We studied the developmental toxicity of BBP, especially its effects on the heart development, in zebrafish (Danio rerio) embryos. Embryos at 4hr post-fertilization (hpf) were exposed to 0, 0.1, 0.6 and 1.2mg/L BBP until 72hpf. BBP caused abnormalities in embryo morphology, including yolk-sac edema, spinal curvature, tail deformity, uninflated swim bladder and cardiac defects. Exposure to 0.6mg/L BBP significantly increased the malformation rate, caused growth inhibition, increased the cardiac malformation rate as well as the distance between the sinus venosus (SV) and bulbus arteriosus (BA), and reduced the heart rate of embryos. Exposure to 1.2mg/L BBP significantly affected all endpoints, except survival rate at 24hpf. To preliminarily elucidate the potential mechanism of heart developmental toxicity caused by BBP, we examined the expression of two genes related to heart development, Nkx2.5 and T-box transcription factor 5, by real-time quantitative PCR. The expression of the two genes was dose-dependently downregulated with BBP. BBP could induce developmental toxicity, with adverse effects on the heart development in zebrafish embryos, and alter the expression of genes related to heart development.

Gene Expression Networks in the Murine Pulmonary Myocardium Provide Insight into the Pathobiology of Atrial Fibrillation

The pulmonary myocardium is a muscular coat surrounding the pulmonary and caval veins. Although its definitive physiological function is unknown, it may have a pathological role as the source of ectopic beats initiating atrial fibrillation. How the pulmonary myocardium gains pacemaker function is not clearly defined, although recent evidence indicates that changed transcriptional gene expression networks are at fault. The gene expression profile of this distinct cell type in situ was examined to investigate underlying molecular events that might contribute to atrial fibrillation. Via systems genetics, a whole-lung transcriptome data set from the BXD recombinant inbred mouse resource was analyzed, uncovering a pulmonary cardiomyocyte gene network of 24 transcripts, coordinately regulated by chromosome 1 and 2 loci. Promoter enrichment analysis and interrogation of publicly available ChIP-seq data suggested that transcription of this gene network may be regulated by the concerted activity of NKX2-5, serum response factor, myocyte enhancer factor 2, and also, at a post-transcriptional level, by RNA binding protein motif 20. Gene ontology terms indicate that this gene network overlaps with molecular markers of the stressed heart. Therefore, we propose that perturbed regulation of this gene network might lead to altered calcium handling, myocyte growth, and contractile force contributing to the aberrant electrophysiological properties observed in atrial fibrillation. We reveal novel molecular interactions and pathways representing possible therapeutic targets for atrial fibrillation. In addition, we highlight the utility of recombinant inbred mouse resources in detecting and characterizing gene expression networks of relatively small populations of cells that have a pathological significance.

Thrombospondins: A Role in Cardiovascular Disease

Thrombospondins (TSPs) represent extracellular matrix (ECM) proteins belonging to the TSP family that comprises five members. All TSPs have a complex multidomain structure that permits the interaction with various partners including other ECM proteins, cytokines, receptors, growth factors, etc. Among TSPs, TSP1, TSP2, and TSP4 are the most studied and functionally tested. TSP1 possesses anti-angiogenic activity and is able to activate transforming growth factor (TGF)-β, a potent profibrotic and anti-inflammatory factor. Both TSP2 and TSP4 are implicated in the control of ECM composition in hypertrophic hearts. TSP1, TSP2, and TSP4 also influence cardiac remodeling by affecting collagen production, activity of matrix metalloproteinases and TGF-β signaling, myofibroblast differentiation, cardiomyocyte apoptosis, and stretch-mediated enhancement of myocardial contraction. The development and evaluation of TSP-deficient animal models provided an option to assess the contribution of TSPs to cardiovascular pathology such as (myocardial infarction) MI, cardiac hypertrophy, heart failure, atherosclerosis, and aortic valve stenosis. Targeting of TSPs has a significant therapeutic value for treatment of cardiovascular disease. The activation of cardiac TSP signaling in stress and pressure overload may be therefore beneficial.

Myocardial VHL-HIF Signaling Controls an Embryonic Metabolic Switch Essential for Cardiac Maturation

While gene regulatory networks involved in cardiogenesis have been characterized, the role of bioenergetics remains less studied. Here we show that until midgestation, myocardial metabolism is compartmentalized, with a glycolytic signature restricted to compact myocardium contrasting with increased mitochondrial oxidative activity in the trabeculae. HIF1α regulation mirrors this pattern, with expression predominating in compact myocardium and scarce in trabeculae. By midgestation, the compact myocardium downregulates HIF1α and switches toward oxidative metabolism. Deletion of the E3 ubiquitin ligase Vhl results in HIF1α hyperactivation, blocking the midgestational metabolic shift and impairing cardiac maturation and function. Moreover, the altered glycolytic signature induced by HIF1 trabecular activation precludes regulation of genes essential for establishment of the cardiac conduction system. Our findings reveal VHL-HIF-mediated metabolic compartmentalization in the developing heart and the connection between metabolism and myocardial differentiation. These results highlight the importance of bioenergetics in ventricular myocardium specialization and its potential relevance to congenital heart disease.

Development and Function of the Cardiac Conduction System in Health and Disease

The generation and propagation of the cardiac impulse is the central function of the cardiac conduction system (CCS). Impulse initiation occurs in nodal tissues that have high levels of automaticity, but slow conduction properties. Rapid impulse propagation is a feature of the ventricular conduction system, which is essential for synchronized contraction of the ventricular chambers. When functioning properly, the CCS produces ~2.4 billion heartbeats during a human lifetime and orchestrates the flow of cardiac impulses, designed to maximize cardiac output. Abnormal impulse initiation or propagation can result in brady- and tachy-arrhythmias, producing an array of symptoms, including syncope, heart failure or sudden cardiac death. Underlying the functional diversity of the CCS are gene regulatory networks that direct cell fate towards a nodal or a fast conduction gene program. In this review, we will discuss our current understanding of the transcriptional networks that dictate the components of the CCS, the growth factor-dependent signaling pathways that orchestrate some of these transcriptional hierarchies and the effect of aberrant transcription factor expression on mammalian conduction disease.

Phenotypically silent Cre recombination within the postnatal ventricular conduction system

The cardiac conduction system (CCS) is composed of specialized cardiomyocytes that initiate and maintain cardiac rhythm. Any perturbation to the normal sequence of electrical events within the heart can result in cardiac arrhythmias. To understand how cardiac rhythm is established at the molecular level, several genetically modified mouse lines expressing Cre recombinase within specific CCS compartments have been created. In general, Cre driver lines have been generated either by homologous recombination of Cre into an endogenous locus or Cre expression driven by a randomly inserted transgene. However, haploinsufficiency of the endogenous gene compromises the former approach, while position effects negatively impact the latter. To address these limitations, we generated a Cre driver line for the ventricular conduction system (VCS) that preserves endogenous gene expression by targeting the Contactin2 (Cntn2) 3’ untranslated region (3’UTR). Here we show that Cntn2^{3’UTR-IRES-Cre-EGFP/+} mice recombine floxed alleles within the VCS and that Cre expression faithfully recapitulates the spatial distribution of Cntn2 within the heart. We further demonstrate that Cre expression initiates after birth with preservation of native Cntn2 protein. Finally, we show that Cntn2^{3’UTR-IRES-Cre-EGFP/+} mice maintain normal cardiac mechanical and electrical function. Taken together, our results establish a novel VCS-specific Cre driver line without the adverse consequences of haploinsufficiency or position effects. We expect that our new mouse line will add to the accumulating toolkit of CCS-specific mouse reagents and aid characterization of the cell-autonomous molecular circuitry that drives VCS maintenance and function.

Exome Sequencing Identifies Compound Heterozygous Mutations in SCN5A Associated with Congenital Complete Heart Block in the Thai Population

Background. Congenital heart block is characterized by blockage of electrical impulses from the atrioventricular node (AV node) to the ventricles. This blockage can be caused by ion channel impairment that is the result of genetic variation. This study aimed to investigate the possible causative variants in a Thai family with complete heart block by using whole exome sequencing. Methods. Genomic DNA was collected from a family consisting of five family members in three generations in which one of three children in generation III had complete heart block. Whole exome sequencing was performed on one complete heart block affected child and one unaffected sibling. Bioinformatics was used to identify annotated and filtered variants. Candidate variants were validated and the segregation analysis of other family members was performed. Results. This study identified compound heterozygous variants, c.101G>A and c.3832G>A, in the SCN5A gene and c.28730C>T in the TTN gene. Conclusions. Compound heterozygous variants in the SCN5A gene were found in the complete heart block affected child but these two variants were found only in the this affected sibling and were not found in other unaffected family members. Hence, these variants in the SCN5A gene were the most possible disease-causing variants in this family.

Transcription factor ETV1 is essential for rapid conduction in the heart

Rapid impulse propagation in the heart is a defining property of pectinated atrial myocardium (PAM) and the ventricular conduction system (VCS) and is essential for maintaining normal cardiac rhythm and optimal cardiac output. Conduction defects in these tissues produce a disproportionate burden of arrhythmic disease and are major predictors of mortality in heart failure patients. Despite the clinical importance, little is known about the gene regulatory network that dictates the fast conduction phenotype. Here, we have used signal transduction and transcriptional profiling screens to identify a genetic pathway that converges on the NRG1-responsive transcription factor ETV1 as a critical regulator of fast conduction physiology for PAM and VCS cardiomyocytes. Etv1 was highly expressed in murine PAM and VCS cardiomyocytes, where it regulates expression of Nkx2-5, Gja5, and Scn5a, key cardiac genes required for rapid conduction. Mice deficient in Etv1 exhibited marked cardiac conduction defects coupled with developmental abnormalities of the VCS. Loss of Etv1 resulted in a complete disruption of the normal sodium current heterogeneity that exists between atrial, VCS, and ventricular myocytes. Lastly, a phenome-wide association study identified a link between ETV1 and bundle branch block and heart block in humans. Together, these results identify ETV1 as a critical factor in determining fast conduction physiology in the heart.

TBX5: A Key Regulator of Heart Development

TBX5 is a member of the T-box transcription factor family and is primarily known for its role in cardiac and forelimb development. Human patients with dominant mutations in TBX5 are characterized by Holt-Oram syndrome, and show defects of the cardiac septa, cardiac conduction system, and the anterior forelimb. The range of cardiac defects associated with TBX5 mutations in humans suggests multiple roles for the transcription factor in cardiac development and function. Animal models demonstrate similar defects and have provided a useful platform for investigating the roles of TBX5 during embryonic development. During early cardiac development, TBX5 appears to act primarily as a transcriptional activator of genes associated with cardiomyocyte maturation and upstream of morphological signals for septation. During later cardiac development, TBX5 is required for patterning of the cardiac conduction system and maintenance of mature cardiomyocyte function. A comprehensive understanding of the integral roles of TBX5 throughout cardiac development and adult life will be critical for understanding human cardiac morphology and function.

The Cardiac TBX5 Interactome Reveals a Chromatin Remodeling Network Essential for Cardiac Septation

Human mutations in the cardiac transcription factor gene TBX5 cause congenital heart disease (CHD), although the underlying mechanism is unknown. We report characterization of the endogenous TBX5 cardiac interactome and demonstrate that TBX5, long considered a transcriptional activator, interacts biochemically and genetically with the nucleosome remodeling and deacetylase (NuRD) repressor complex. Incompatible gene programs are repressed by TBX5 in the developing heart. CHD mis-sense mutations that disrupt the TBX5-NuRD interaction cause depression of a subset of repressed genes. Furthermore, the TBX5-NuRD interaction is required for heart development. Phylogenetic analysis showed that the TBX5-NuRD interaction domain evolved during early diversification of vertebrates, simultaneous with the evolution of cardiac septation. Collectively, this work defines a TBX5-NuRD interaction essential to cardiac development and the evolution of the mammalian heart, and when altered may contribute to human CHD.

The formation and function of the cardiac conduction system

The cardiac conduction system (CCS) consists of distinctive components that initiate and conduct the electrical impulse required for the coordinated contraction of the cardiac chambers. CCS development involves complex regulatory networks that act in stage-, tissue- and dose-dependent manners, and recent findings indicate that the activity of these networks is sensitive to common genetic variants associated with cardiac arrhythmias. Here, we review how these findings have provided novel insights into the regulatory mechanisms and transcriptional networks underlying CCS formation and function.

TBX5 mutations contribute to early-onset atrial fibrillation in Chinese and Caucasians

AIMS

Atrial fibrillation (AF) is a common arrhythmia with an important heritable aspect. The genetic factors underlying AF have not been fully elucidated.

METHODS AND RESULTS

We screened six candidate genes (CAV1, KCNJ2, KCNQ1, NKX2.5, PITX2, and TBX5) for novel mutations in 139 patients of Chinese descent with early-onset AF and 576 controls. Four missense TBX5 mutations, p.R355C, p.Q376R, p.A428S, and p.S372L, were identified in evolutionarily conserved regions. We did not find any mutations in CAV1, KCNJ2, KCNQ1, NKX2.5, and PITX2. These mutations increased the expression of atrial natriuretic peptide (ANP) and connexin-40 (CX40) in the primarily cultured rat atrial myocytes but did not alter the expression of cardiac structural genes, atrial myosin heavy chain-α (MHC-α) and myosin light chain-2α (MLC-2α). Overexpression of p.R355C developed an atrial arrhythmia suggestive of paroxysmal AF in the zebrafish model. To replicate our findings, we screened TBX5 in 527 early-onset AF cases from the Massachusetts General Hospital AF study. A novel TBX5 deletion (ΔAsp118, p.D118del) was identified, while no TBX5 mutations were identified in 1176 control subjects.

CONCLUSION

Our results provide both genetic and functional evidence to support the contribution of TBX5 gene in the pathogenesis of AF. The potential mechanism of arrhythmia may be due in part to the disturbed expression of ANP and CX40.

Irx3 is required for postnatal maturation of the mouse ventricular conduction system

The ventricular conduction system (VCS) orchestrates the harmonious contraction in every heartbeat. Defects in the VCS are often associated with life-threatening arrhythmias and also promote adverse remodeling in heart disease. We have previously established that the Irx3 homeobox gene regulates rapid electrical propagation in the VCS by modulating the transcription of gap junction proteins Cx40 and Cx43. However, it is unknown whether other factors contribute to the conduction defects observed in Irx3 knockout (Irx3(-/-)) mice. In this study, we show that during the early postnatal period, Irx3(-/-) mice develop morphological defects in the VCS which are temporally dissociated from changes in gap junction expression. These morphological defects were accompanied with progressive changes in the cardiac electrocardiogram including right bundle branch block. Hypoplastic VCS was not associated with increased apoptosis of VCS cardiomyocytes but with a lack of recruitment and maturation of ventricular cardiomyocytes into the VCS. Computational analysis followed by functional verification revealed that Irx3 promotes VCS-enriched transcripts targeted by Nkx2.5 and/or Tbx5. Altogether, these results indicate that, in addition to ensuring the appropriate expression of gap junctional channels in the VCS, Irx3 is necessary for the postnatal maturation of the VCS, possibly via its interactions with Tbx5 and Nkx2.5.

Prevalence and Spectrum of TBX5 Mutation in Patients with Lone Atrial Fibrillation

Atrial fibrillation (AF), the most common type of cardiac rhythm disturbance encountered in clinical practice, is associated with substantially increased morbidity and mortality. Aggregating evidence demonstrates that abnormal cardiovascular development is involved in the pathogenesis of AF. A recent study has revealed that the TBX5 gene, which encodes a T-box transcription factor key to cardiovascular development, was associated with AF and atypical Holt-Oram syndrome. However, the prevalence and spectrum of TBX5 mutation in patients with lone AF remain unclear. In this study, the coding regions and splicing junction sites of TBX5 were sequenced in 192 unrelated patients with lone AF and 300 unrelated ethnically-matched healthy individuals used as controls. The causative potential of the identified TBX5 variation was evaluated by MutationTaster and PolyPhen-2. The functional effect of the mutant TBX5 was assayed by using a dual-luciferase reporter assay system. As a result, a novel heterozygous TBX5 mutation, p.H170D, was identified in a patient, with a mutational prevalence of approximately 0.52%. This mutation, which was absent in the 300 control individuals, altered the amino acid completely conserved evolutionarily across species, and was predicted to be disease-causing. Functional deciphers showed that the mutant TBX5 was associated with significantly reduced transcriptional activity when compared with its wild-type counterpart. Furthermore, the mutation significantly decreased the synergistic activation between TBX5 and NKX2-5 or GATA4. The findings expand the mutational spectrum of TBX5 linked to AF and provide new evidence that dysfunctional TBX5 may contribute to lone AF.

Single-Cell Expression Profiling Reveals a Dynamic State of Cardiac Precursor Cells in the Early Mouse Embryo

In the early vertebrate embryo, cardiac progenitor/precursor cells (CPs) give rise to cardiac structures. Better understanding their biological character is critical to understand the heart development and to apply CPs for the clinical arena. However, our knowledge remains incomplete. With the use of single-cell expression profiling, we have now revealed rapid and dynamic changes in gene expression profiles of the embryonic CPs during the early phase after their segregation from the cardiac mesoderm. Progressively, the nascent mesodermal gene Mesp1 terminated, and Nkx2-5+/Tbx5+ population rapidly replaced the Tbx5low+ population as the expression of the cardiac genes Tbx5 and Nkx2-5 increased. At the Early Headfold stage, Tbx5-expressing CPs gradually showed a unique molecular signature with signs of cardiomyocyte differentiation. Lineage-tracing revealed a developmentally distinct characteristic of this population. They underwent progressive differentiation only towards the cardiomyocyte lineage corresponding to the first heart field rather than being maintained as a progenitor pool. More importantly, Tbx5 likely plays an important role in a transcriptional network to regulate the distinct character of the FHF via a positive feedback loop to activate the robust expression of Tbx5 in CPs. These data expands our knowledge on the behavior of CPs during the early phase of cardiac development, subsequently providing a platform for further study.

A common Shox2-Nkx2-5 antagonistic mechanism primes the pacemaker cell fate in the pulmonary vein myocardium and sinoatrial node

In humans, atrial fibrillation is often triggered by ectopic pacemaking activity in the myocardium sleeves of the pulmonary vein (PV) and systemic venous return. The genetic programs that abnormally reinforce pacemaker properties at these sites and how this relates to normal sinoatrial node (SAN) development remain uncharacterized. It was noted previously that Nkx2-5, which is expressed in the PV myocardium and reinforces a chamber-like myocardial identity in the PV, is lacking in the SAN. Here we present evidence that in mice Shox2 antagonizes the transcriptional output of Nkx2-5 in the PV myocardium and in a functional Nkx2-5(+) domain within the SAN to determine cell fate. Shox2 deletion in the Nkx2-5(+) domain of the SAN caused sick sinus syndrome, associated with the loss of the pacemaker program. Explanted Shox2(+) cells from the embryonic PV myocardium exhibited pacemaker characteristics including node-like electrophysiological properties and the capability to pace surrounding Shox2(-) cells. Shox2 deletion led to Hcn4 ablation in the developing PV myocardium. Nkx2-5 hypomorphism rescued the requirement for Shox2 for the expression of genes essential for SAN development in Shox2 mutants. Similarly, the pacemaker-like phenotype induced in the PV myocardium in Nkx2-5 hypomorphs reverted back to a working myocardial phenotype when Shox2 was simultaneously deleted. A similar mechanism is also adopted in differentiated embryoid bodies. We found that Shox2 interacts with Nkx2-5 directly, and discovered a substantial genome-wide co-occupancy of Shox2, Nkx2-5 and Tbx5, further supporting a pivotal role for Shox2 in the core myogenic program orchestrating venous pole and pacemaker development.

Efficient Generation of Cardiac Purkinje Cells from ESCs by Activating cAMP Signaling

Dysfunction of the specialized cardiac conduction system (CCS) is associated with life-threatening arrhythmias. Strategies to derive CCS cells, including rare Purkinje cells (PCs), would facilitate models for mechanistic studies and drug discovery and also provide new cellular materials for regenerative therapies. A high-throughput chemical screen using CCS:lacz and Contactin2:egfp (Cntn2:egfp) reporter embryonic stem cell (ESC) lines was used to discover a small molecule, sodium nitroprusside (SN), that efficiently promotes the generation of cardiac cells that express gene profiles and generate action potentials of PC-like cells. Imaging and mechanistic studies suggest that SN promotes the generation of PCs from cardiac progenitors initially expressing cardiac myosin heavy chain and that it does so by activating cyclic AMP signaling. These findings provide a strategy to derive scalable PCs, along with insight into the ontogeny of CCS development.

•

A chemical screen was carried out for compounds that induce cardiac conduction cells

•

Two ESC reporter lines were used to identify lead hits

•

Sodium nitroprusside efficiently generated scalable amounts of PC-like cells

•

By activating cAMP signaling, PCs are derived from cardiac progenitors

Reprogramming the conduction system: Onward toward a biological pacemaker

Diseases of the cardiac conduction system can be debilitating and deadly. Electronic pacemakers are incredibly effective in the treatment of sinus and AV node dysfunction, yet there remain important limitations and complications. These issues have driven interest in the development of a biological pacemaker. Here, we review experimental progress in animal models and discuss future directions, with a focus on reprogramming endogenous cells in the heart to treat defects of rhythm and conduction.

Molecular regulation of cardiomyocyte differentiation

The heart is the first organ to form during embryonic development. Given the complex nature of cardiac differentiation and morphogenesis, it is not surprising that some form of congenital heart disease is present in ≈1 percent of newborns. The molecular determinants of heart development have received much attention over the past several decades. This has been driven in large part by an interest in understanding the causes of congenital heart disease coupled with the potential of using knowledge from developmental biology to generate functional cells and tissues that could be used for regenerative medicine purposes. In this review, we highlight the critical signaling pathways and transcription factor networks that regulate cardiomyocyte lineage specification in both in vivo and in vitro models. Special focus will be given to epigenetic regulators that drive the commitment of cardiomyogenic cells from nascent mesoderm and their differentiation into chamber-specific myocytes, as well as regulation of myocardial trabeculation.

Arrhythmias in the developing heart

Prevalence of cardiac arrhythmias increases gradually with age; however, specific rhythm disturbances can appear even prior to birth and markedly affect foetal development. Relatively little is known about these disorders, chiefly because of their relative rarity and difficulty in diagnosis. In this review, we cover the most common forms found in human pathology, specifically congenital heart block, pre-excitation, extrasystoles and long QT syndrome. In addition, we cover pertinent literature data from prenatal animal models, providing a glimpse into pathogenesis of arrhythmias and possible strategies for treatment.

GATA-Binding Factor 6 Contributes to Atrioventricular Node Development and Function

BACKGROUND

Several transcription factors regulate cardiac conduction system (CCS) development and function but the role of each in specifying distinct CCS components remains unclear. GATA-binding factor 6 (GATA6) is a zinc-finger transcription factor that is critical for patterning the cardiovascular system. However, the role of GATA6 in the embryonic heart and CCS has never been shown.

METHODS AND RESULTS

We report that Gata6 is expressed abundantly in the proximal CCS during midgestation in mice. Myocardial-specific deletion of the carboxyl zinc-finger of Gata6 induces loss of hyperpolarizing cyclic nucleotide-gated channel, subtype 4 staining in the compact atrioventricular node with some retention of hyperpolarizing cyclic nucleotide-gated channel, subtype 4 staining in the atrioventricular bundle, but has no significant effect on the connexin-40-positive bundle branches. Furthermore, myocardial-specific deletion of the carboxyl zinc-finger of Gata6 alters atrioventricular conduction in postnatal life as assessed by surface and invasive electrophysiological evaluation, as well as decreasing the number of ventricular myocytes and inducing compensatory myocyte hypertrophy. Myocardial-specific deletion of the carboxyl zinc-finger of Gata6 is also associated with downregulation of the transcriptional repressor ID2 and the cardiac sodium-calcium exchanger NCX1 in the proximal CCS, where GATA6 transactivates both of these factors. Finally, carboxyl zinc-finger deletion of Gata6 reduces cell-cycle exit of TBX3+ myocytes in the developing atrioventricular bundle during the period of atrioventricular node specification, which results in fewer TBX3+ cells in the proximal CCS of mature mutant mice.

CONCLUSIONS

GATA6 contributes to the development and postnatal function of the murine atrioventricular node by promoting cell-cycle exit of specified cardiomyocytes toward a conduction system lineage.

Stoichiometry of Gata4, Mef2c, and Tbx5 influences the efficiency and quality of induced cardiac myocyte reprogramming

RATIONALE

Generation of induced cardiac myocytes (iCMs) directly from fibroblasts offers great opportunities for cardiac disease modeling and cardiac regeneration. A major challenge of iCM generation is the low conversion rate of fibroblasts to fully reprogrammed iCMs, which could in part be attributed to unbalanced expression of reprogramming factors Gata4 (G), Mef2c (M), and Tbx5 (T) using the current gene delivery approach.

OBJECTIVE

We aimed to establish a system to express distinct ratios of G, M, T proteins in fibroblasts and determine the effect of G, M, T stoichiometry on iCM reprogramming.

METHODS AND RESULTS

We took advantage of the inherent feature of the polycistronic system and generated all possible combinations of G, M, T with identical 2A sequences in a single transgene. We demonstrated that each splicing order of G, M, T gave rise to distinct G, M, T protein expression levels. Combinations that resulted in higher protein level of Mef2c with lower levels of Gata4 and Tbx5 significantly enhanced reprogramming efficiency compared with separate G, M, T transduction. Importantly, after further optimization, the MGT vector resulted in more than 10-fold increase in the number of mature beating iCM loci. Molecular characterization revealed that more optimal G, M, T stoichiometry correlated with higher expression of mature cardiac myocyte markers.

CONCLUSIONS

Our results demonstrate that stoichiometry of G, M, T protein expression influences the efficiency and quality of iCM reprogramming. The established optimal G, M, T expression condition will provide a valuable platform for future iCM studies.

Concise review: Engineering myocardial tissue: the convergence of stem cells biology and tissue engineering technology

Advanced heart failure represents a leading public health problem in the developed world. The clinical syndrome results from the loss of viable and/or fully functional myocardial tissue. Designing new approaches to augment the number of functioning human cardiac muscle cells in the failing heart serve as the foundation of modern regenerative cardiovascular medicine. A number of clinical trials have been performed in an attempt to increase the number of functional myocardial cells by the transplantation of a diverse group of stem or progenitor cells. Although there are some encouraging suggestions of a small early therapeutic benefit, to date, no evidence for robust cell or tissue engraftment has been shown, emphasizing the need for new approaches. Clinically meaningful cardiac regeneration requires the identification of the optimum cardiogenic cell types and their assembly into mature myocardial tissue that is functionally and electrically coupled to the native myocardium. We here review recent advances in stem cell biology and tissue engineering and describe how the convergence of these two fields may yield novel approaches for cardiac regeneration.

Toxicogenomic analysis in the combined effect of tributyltin and benzo[a]pyrene on the development of zebrafish embryos

There is a growing recognition that the toxic effects of chemical mixtures are been an important issue in toxicological sciences. Tributyltin (TBT) and benzo[a]pyrene (BaP) are widespread pollutants that occur simultaneously in the aquatic environments. This study was designed to examine comprehensively the combined effects of TBT and BaP on zebrafish (Danio rerio) embryos using toxicogenomic approach combined with biochemical detection and morphological analysis, and tried to gain insight into the mechanisms underlying the combined effects of TBT and BaP. The results of toxicogenomic data indicated that: (1) TBT cotreatment rescued the embryos from decreased hatching ratio caused by BaP alone, while the alteration of gene expression (in this article the phrase gene expression is used as a synonym to gene transcription, although in is acknowledged that gene expression can also be regulated by, e.g., translation and mRNA or protein stability) relative to zebrafish hatching in the BaP groups was resumed by the cotreatment with TBT; (2) BaP cotreatment decreased TBT-mediated dorsal curvature, and alleviated the perturbation of Notch pathway caused by TBT alone; (3) cotreatment with TBT decreased BaP-mediated bradycardia, which might be due to that TBT cotreatment alleviated the perturbation in expression of genes related to cardiac muscle cell development and calcium handling caused by BaP alone; 4) TBT cotreatment brought an antagonistic effect on the BaP-mediated oxidative stress and DNA damage. These results suggested that toxicogenomic approach was available for analyzing combined toxicity with high sensitivity and accuracy, which might improve our understanding and predictability for the combined effects of chemicals.

MyoR modulates cardiac conduction by repressing Gata4

The cardiac conduction system coordinates electrical activation through a series of interconnected structures, including the atrioventricular node (AVN), the central connection point that delays impulse propagation to optimize cardiac performance. Although recent studies have uncovered important molecular details of AVN formation, relatively little is known about the transcriptional mechanisms that regulate AV delay, the primary function of the mature AVN. We identify here MyoR as a novel transcription factor expressed in Cx30.2(+) cells of the AVN. We show that MyoR specifically inhibits a Cx30.2 enhancer required for AVN-specific gene expression. Furthermore, we demonstrate that MyoR interacts directly with Gata4 to mediate transcriptional repression. Our studies reveal that MyoR contains two nonequivalent repression domains. While the MyoR C-terminal repression domain inhibits transcription in a context-dependent manner, the N-terminal repression domain can function in a heterologous context to convert the Hand2 activator into a repressor. In addition, we show that genetic deletion of MyoR in mice increases Cx30.2 expression by 50% and prolongs AV delay by 13%. Taken together, we conclude that MyoR modulates a Gata4-dependent regulatory circuit that establishes proper AV delay, and these findings may have wider implications for the variability of cardiac rhythm observed in the general population.

Myocardial deletion of transcription factor CHF1/Hey2 results in altered myocyte action potential and mild conduction system expansion but does not alter conduction system function or promote spontaneous arrhythmias

CHF1/Hey2 is a Notch-responsive basic helix-loop-helix transcription factor involved in cardiac development. Common variants in Hey2 are associated with Brugada syndrome. We hypothesized that absence of CHF1/Hey2 would result in abnormal cellular electrical activity, altered cardiac conduction system (CCS) development, and increased arrhythmogenesis. We isolated neonatal CHF/Hey2-knockout (KO) cardiac myocytes and measured action potentials and ion channel subunit gene expression. We also crossed myocardial-specific CHF1/Hey2-KO mice with cardiac conduction system LacZ reporter mice and stained for conduction system tissue. We also performed ambulatory ECG monitoring for arrhythmias and heart rate variability. Neonatal cardiomyocytes from CHF1/Hey2-KO mice demonstrate a 50% reduction in action potential dV/dT, a 50-75% reduction in SCN5A, KCNJ2, and CACNA1C ion channel subunit gene expression, and an increase in delayed afterdepolarizations from 0/min to 12/min. CHF1/Hey2 cKO CCS-lacZ mice have a ∼3-fold increase in amount of CCS tissue. Ambulatory ECG monitoring showed no difference in cardiac conduction, arrhythmias, or heart rate variability. Wild-type cells or animals were used in all experiments. CHF1/Hey2 may contribute to Brugada syndrome by influencing the expression of SCN5A and formation of the cardiac conduction system, but its absence does not cause baseline conduction defects or arrhythmias in the adult mouse.-Hartman, M. E., Liu, Y., Zhu, W.-Z., Chien, W.-M., Weldy, C. S., Fishman, G. I., Laflamme, M. A., Chin, M. T. Myocardial deletion of transcription factor CHF1/Hey2 results in altered myocyte action potential and mild conduction system expansion but does not alter conduction system function or promote spontaneous arrhythmias.

Inducible gene deletion in the entire cardiac conduction system using Hcn4-CreERT2 BAC transgenic mice

Developmental defects and disruption of molecular pathways of the cardiac conduction system (CCS) can cause life-threatening cardiac arrhythmias. Despite decades of effort, knowledge about the development and molecular control of the CCS remains primitive. Mouse genetics, complementary to other approaches such as human genetics, has become a key tool for exploring the developmental processes of various organs and associated diseases. Genetic analysis using mouse models will likely provide great insights about the development of the CCS, which can facilitate the development of novel therapeutic strategies to treat arrhythmias. To enable genetic studies of the CCS, CCS-associated Cre mouse models are essential. However, existing mouse models with Cre activity reported in the CCS have various limitations such as Cre leak, haploinsufficiency, and inadequate specificity of the Cre activity. To circumvent those limitations, we successfully generated Hcn4-CreERT2 bacterial artificial chromosome (BAC) transgenic mice using BAC recombineering in which Cre activity was specifically detected in the entire CCS after tamoxifen induction. Our Hcn4-CreERT2 BAC transgenic line will be an invaluable genetic tool with which to dissect the developmental control of CCS and arrhythmias.

Phenanthrene exposure causes cardiac arrhythmia in embryonic zebrafish via perturbing calcium handling

Phenanthrene (Phe) is one of the most abundant and ubiquitous polycyclic aromatic hydrocarbons in the aquatic environment. It is known that Phe has cardiotoxic effects, but knowledge concerning the mechanism of cardiac dysfunction caused by Phe is still limited. In this study, zebrafish embryos were exposed to environmentally relevant concentrations of Phe, and an increase of an irregular rhythm was observed in Phe treated embryos. Disordered calcium (Ca(2+)) handling characterized by impaired sarcoplasmic reticulum Ca(2+) uptake, and obvious Ca(2+) accumulation in the cytoplasm was observed in rat embryonic cardiac myoblasts (H9C2) exposed to Phe. The mRNA level as well as protein expression of the SERCA2a Ca(2+) pump in zebrafish hearts or H9C2 cells was significantly decreased by Phe exposure. The activity of Ca(2+)-ATPase in H9C2 cells was inhibited by Phe. Both the mRNA and protein levels of TBX5, a direct regulator of SERCA2a, were significantly decreased by Phe exposure. These results suggested that exposure to Phe could lead to arrhythmia in zebrafish embryos via perturbing the calcium handling pathway.

Effects of low-level hexabromocyclododecane (HBCD) exposure on cardiac development in zebrafish embryos

Hexabromocyclododecane (HBCD) is one of the most widely used brominated flame retardants. In the present study, zebrafish embryos were exposed to HBCD at the low concentrations of 0, 2, 20 and 200 nM. The results showed HBCD exposure resulted in an increase in heart rate and cardiac arrhythmia after exposure for 72 h, though the survival rate and the whole malformation rate were not significantly affected. These results demonstrated that the heart might be a target of HBCD. Low-level HBCD exposure may not share the same mechanisms as exposure to high concentrations, since no obvious increase of apoptotic cells around the heart was observed in the HBCD-treated groups. It was observed that the expression of Tbx5 and Nkx2.5 was significantly elevated by HBCD treatment in a dose-dependent manner using real-time quantitative PCR, which may be mainly responsible for the alteration of heart rate, given that Tbx5 and Nkx2.5 are two factors regulating ventricle conduction. The mRNA expression of RyR2 and Atp2a2b (SERCA2a) was up-regulated in the exposure group, which may be one of reasons to affect the normal heart rate, since SERCA2a and RyR2 play an important role in calcium ion transport of cadiomyocytes. However, HBCD exposure did not significantly change the expression of Actc1l, Tnnt2, and Myh6, which are mainly muscle contractile genes that play key roles in the formation of cardiac structure. These results were consistent with the lack of effect seen on the other measurements of cardiac function, end diastolic volume, end-systolic volume, stroke volume, and cardiac output.

Pocket proteins critically regulate cell cycle exit of the trabecular myocardium and the ventricular conduction system

During development, the ventricular conduction system (VCS) arises from the trabecular or spongy myocardium. VCS and trabecular myocytes proliferate at a significantly slower rate than compact zone myocardial cells, establishing a transmural cell cycle gradient. The molecular determinants of VCS/trabecular myocyte cell cycle arrest are not known. Given the importance of pocket proteins (Rb, p107 and p130) in mediating G0/G1 arrest in many cell types, we examined the role of this gene family in regulating cell cycle exit of the trabecular myocardium and ventricular conduction system. Using a combinatorial knockout strategy, we found that graded loss of pocket proteins results in a spectrum of heart and lung defects. p107/p130 double knockout (dKO) hearts manifest dysregulated proliferation within the compact myocardium and trabecular bases, while the remaining trabecular region cell cycle exits normally. Consequently, dKO hearts exhibit defective cardiac compaction, septal hyperplasia and biventricular outflow tract obstruction, while the VCS appears relatively normal. Loss of all three pocket proteins (3KO) is necessary to completely disrupt the transmural cell cycle gradient. 3KO hearts exhibit massive overgrowth of the trabecular myocardium and ventricular conduction system, which leads to fetal heart failure and death. Hearts carrying a single pocket protein allele are able to maintain the transmural cell cycle gradient. These results demonstrate the exquisite sensitivity of trabecular and conduction myocytes to pocket protein function during ventricular chamber development.

Mouse TBX3 mutants suggest novel molecular mechanisms for Ulnar-mammary syndrome

The transcription factor TBX3 plays critical roles in development and TBX3 mutations in humans cause Ulnar-mammary syndrome. Efforts to understand how altered TBX3 dosage and function disrupt the development of numerous structures have been hampered by embryonic lethality of mice bearing presumed null alleles. We generated a novel conditional null allele of Tbx3: after Cre-mediated recombination, no mRNA or protein is detectable. In contrast, a putative null allele in which exons 1-3 are deleted produces a truncated protein that is abnormally located in the cytoplasm. Heterozygotes and homozygotes for this allele have different phenotypes than their counterparts bearing a true null allele. Our observations with these alleles in mice, and the different types of TBX3 mutations observed in human ulnar-mammary syndrome, suggest that not all mutations observed in humans generate functionally null alleles. The possibility that mechanisms in addition to TBX3 haploinsufficiency may cause UMS or other malformations merits investigation in the human UMS population.

SNP rs3825214 in TBX5 is associated with lone atrial fibrillation in Chinese Han population

BACKGROUND

A prolonged PR interval is a sign of increased risk of cardiac arrhythmia. Recent genome-wide association studies found that the single-nucleotide polymorphism (SNP) rs3825214 in T-box 5 (TBX5) was positively associated with PR interval, QRS duration, QT interval, and common arrhythmia disorders such as atrial fibrillation (AF) and advanced atrioventricular block. However, other independent replication studies are required to validate the result. This study assessed associations between rs3825214 and ECG parameters, AF, and ventricular tachycardia (VT) in a Chinese Han population.

METHODOLOGY/PRINCIPAL FINDINGS

To assess the association between rs3825214 and AF and VT, we carried out case-control association studies with 692 AF patients (including 275 lone AF patients), 235 VT patients, and 856 controls. Genotyping was performed using a Rotor-Gene TM 6000 High Resolution Melt system. Statistical analyses of associations were adjusted for potential confounding factors. A moderate association was detected between rs3825214 and AF (P(adj) = 0.036, OR = 0.79) and a highly significant association was detected between the G allele of rs3825214 and lone AF (P(adj) = 0.001, OR = 0.65; genotypic P = 3.75×10⁻⁴ with a dominant model). We also found that rs3825214 showed a significant association with atrial-ventricular block (AVB; P = 0.028; P(adj) = 0.035, OR = 0.494).

CONCLUSIONS

Our results indicate that rs3825214 conferred a significant risk of lone AF in this Chinese Han population.

Signaling and transcriptional networks in heart development and regeneration

The mammalian heart is the first functional organ, the first indicator of life. Its normal formation and function are essential for fetal life. Defects in heart formation lead to congenital heart defects, underscoring the finesse with which the heart is assembled. Understanding the regulatory networks controlling heart development have led to significant insights into its lineage origins and morphogenesis and illuminated important aspects of mammalian embryology, while providing insights into human congenital heart disease. The mammalian heart has very little regenerative potential, and thus, any damage to the heart is life threatening and permanent. Knowledge of the developing heart is important for effective strategies of cardiac regeneration, providing new hope for future treatments for heart disease. Although we still have an incomplete picture of the mechanisms controlling development of the mammalian heart, our current knowledge has important implications for embryology and better understanding of human heart disease.

T-box factors: insights into the evolutionary emergence of the complex heart

The heart as a functional organ first appeared in bilaterians as a single peristaltic pump and evolved through arthropods, fish, amphibians, and finally mammals into a four-chambered engine controlling blood-flow within the body. The acquisition of cardiac complexity in the evolving heart was a product of gene duplication events and the co-option of novel signaling pathways to an ancestral cardiac-specific gene network. T-box factors belong to an evolutionary conserved family of transcriptional regulators with diverse roles in development. Their regulatory functions are integral in the initiation and potentiation of heart development, and mutations in these genes are associated with congenital heart defects. In this review we will discuss the evolutionary conserved cardiac regulatory functions of this family as well as their implication in disease in an aim to facilitate future gene-targeted and regenerative therapeutic remedies.

Electrophysiological patterning of the heart

In the adult heart, electrophysiological heterogeneity is present to guide activation and contraction. A change in electrophysiological heterogeneity, for example, during disease, can contribute to arrhythmogenesis. During development, spatial and temporal patterns of transcriptional activity regulate the localized expression of ion channels that cause electrophysiological heterogeneity throughout the heart. If we gain insight into the regulating processes that generate the electrophysiological characteristics and factors involved during development, we can use this knowledge in the search for new therapeutic targets. In this review, we discuss which factors guide the electrical patterning of atrioventricular conduction system and ventricles and how this patterning relates to arrhythmogenic disease in patients.

Gene regulatory networks in cardiac conduction system development

The cardiac conduction system is a specialized tract of myocardial cells responsible for maintaining normal cardiac rhythm. Given its critical role in coordinating cardiac performance, a detailed analysis of the molecular mechanisms underlying conduction system formation should inform our understanding of arrhythmia pathophysiology and affect the development of novel therapeutic strategies. Historically, the ability to distinguish cells of the conduction system from neighboring working myocytes presented a major technical challenge for performing comprehensive mechanistic studies. Early lineage tracing experiments suggested that conduction cells derive from cardiomyocyte precursors, and these claims have been substantiated by using more contemporary approaches. However, regional specialization of conduction cells adds an additional layer of complexity to this system, and it appears that different components of the conduction system utilize unique modes of developmental formation. The identification of numerous transcription factors and their downstream target genes involved in regional differentiation of the conduction system has provided insight into how lineage commitment is achieved. Furthermore, by adopting cutting-edge genetic techniques in combination with sophisticated phenotyping capabilities, investigators have made substantial progress in delineating the regulatory networks that orchestrate conduction system formation and their role in cardiac rhythm and physiology. This review describes the connectivity of these gene regulatory networks in cardiac conduction system development and discusses how they provide a foundation for understanding normal and pathological human cardiac rhythms.