Intrinsic myocardial defects underlie an Rbfox-deficient zebrafish model of hypoplastic left heart syndrome

The ERBB2 c.1795C>T, p.Arg599Cys variant is associated with left ventricular outflow tract obstruction defects in humans

Non-syndromic congenital heart defects (CHDs) are occasionally familial and left ventricular outflow tract obstruction (LVOTO) defects are among the subtypes with the highest hereditability. The aim of this study was to evaluate the pathogenicity of a heterozygous ERBB2 variant c.1795C>T, p.Arg599Cys identified in three families with LVOTO defects. Variant detection was done with exome sequencing. Western blotting, digital PCR, mass spectrometry (MS), MS microscopy, and flow cytometry were used to study the function of the ERBB2 variant c.1795C>T. Cardiac structure and function were studied in zebrafish embryos expressing human ERBB2 wild type or c.1795C>T. Proband-derived human induced pluripotent stem cell cardiomyocytes (hiPS-CMs) and endothelial cells (hiPS-ECs) were used for transcriptomic analyses. While phosphorylation of the ERBB2 p.Arg599Cys receptor was not altered, the variant affected dramatically the binding partners of the protein, indicating mislocalization of the mutant ERBB2 from plasma membrane to endoplasmic reticulum. Expression of human ERBB2 p.Arg599Cys in zebrafish embryos resulted in cardiomyocyte hypertrophy, increased cardiac wall thickness, and impaired fractional shortening. Transcriptomic analyses of hiPS-ECs and hiPS-CMs from an individual with the c.1795C>T variant showed aberrant expression of genes related to cardiovascular system development and abnormal response to oxidative stress in both cell types. In conclusion, the heterozygous variant ERBB2 c.1795C>T, p.Arg599Cys leads to abnormal cellular localization of the ERBB2 receptor and induces structural changes and dysfunction in the zebrafish embryo heart. This evidence expands previous findings from animal studies to humans and suggests variants in ERBB2 may be associated with CHD.

Hypoplastic left heart syndrome-a scoping review

Background

An estimated 3% of all newborns with congenital heart disease develop hypoplastic left heart syndrome (HLHS), making it a prominent cause of mortality in this group if surgical procedures or a heart transplant are not implemented. While compelling evidence supports a genetic element, identifying a particular genetic cause is limited to a subgroup of patients, indicating a complex and multifaceted origin for this condition. The objective of this scientific contribution was to identify, synthesize, and analyze the scientific knowledge produced regarding the implications of researching on HLHS in a scoping review.

Methods

The search for articles was diligently conducted between January 1, 2019 and February 20, 2025 on the PubMed/MEDLINE, Scopus, Web of Science, and Cochrane databases. This search was assiduously complemented by a gray search. It included internet browsers (e.g., Google) and medical textbooks. The following research question steered our study: "What are the basic data on the etiology and pathogenesis on HLHS?" All stages of the selection process were iwis carried out by the single author.

Results

Of the 1,364 articles found, 75 were included in the sample for analysis, which was implemented with an additional 25 articles from references and gray literature. The studies analyzed indicated that HLHS is one of the most complex congenital heart defects, characterized by small or hypoplastic left-sided heart structures and a dominant right ventricle. The Fontan circulation and the phased surgical technique that it entails have been the cornerstones of HLHS patient care since its debut some 40 years ago. Although there is considerable genetic evidence for HLHS, the exact genetic cause of this cardiologic entity is still not well known. HLHS remains genetically heterogeneous. There is evidence of incomplete penetrance for the C57Bl/6J-b2b635Clo/J (Ohia) mice.

Conclusions

HLHS is a complex and complicate congenital heart disease, which requires further investigation. In this article, I further explore the involvement of the endocardium in the progression of ventricular hypoplasia, therefore offering a potential explanation for the morphological alterations observed in the disease as a result of compromised blood flow to the developing ventricle. These findings may support a new paradigm for the complicated genetics of this congenital heart defect and there is some evidence that HLHS can originate genetically in a combinatorial approach.

Zinc Finger Protein Znf296 Is a Cardiac-Specific Splicing Regulator Required for Cardiomyocyte Formation

Heart formation and function are tightly regulated at transcriptional and post-transcriptional levels. The dysfunction of cardiac cell-specific regulatory genes leads to various heart diseases. Heart failure is one of the most severe and complex cardiovascular diseases, which could be fatal if not treated promptly. However, the exact causes of heart failure are still unclear, especially at the level of single-gene causation. Here, an essential role is uncovered for the zinc finger protein Znf296 in heart development and cardiac contractile function. Specifically, znf296-deficient zebrafish embryos display heart defects characterized by decreased systolic and diastolic capacities of the ventricle and atrium. This is associated with reduced numbers and disrupted structural integrity of cardiomyocytes, including disorganized cytoskeleton and absence of sarcomeres. Mechanistically, the loss of Znf296 alters the alternative splicing of a subset of genes important for heart development and disease, such as mef2ca, sparc, tpm2, camk2g1, tnnt3b, and pdlim5b. Furthermore, it is demonstrated that Znf296 biochemically and functionally interacts with Myt1la in regulating cardiac-specific splicing and heart development. Importantly, it is shown that ZNF296 also regulates alternative splicing in human cardiomyocytes to maintain structural integrity. These results suggest that Znf296 plays a conserved role for the differentiation of cardiomyocytes and the proper function of the cardiovascular system.

The Rbfox1/LASR complex controls alternative pre-mRNA splicing by recognition of multipart RNA regulatory modules

The Rbfox proteins regulate alternative pre-mRNA splicing by binding to the RNA element GCAUG. In the nucleus, most of Rbfox is bound to the large assembly of splicing regulators (LASR), a complex of RNA-binding proteins that recognize additional RNA motifs. However, it remains unclear how the different subunits of the Rbfox/LASR complex act together to bind RNA and regulate splicing. We used a nuclease protection assay to map the transcriptome-wide footprints of Rbfox1/LASR on nascent cellular RNA. In addition to GCAUG, Rbfox1/LASR binds RNA motifs for LASR subunits hnRNPs M, H/F, and C and Matrin3. These elements are often arranged in tandem, forming multipart modules of RNA motifs. To distinguish contact sites of Rbfox1 from the LASR subunits, we analyzed a mutant Rbfox1(F125A) that has lost RNA binding but remains associated with LASR. Rbfox1(F125A)/LASR complexes no longer interact with GCAUG but retain binding to RNA elements for LASR. Splicing analyses reveal that in addition to activating exons through adjacent GCAUG elements, Rbfox can also stimulate exons near binding sites for LASR subunits. Minigene experiments demonstrate that these diverse elements produce a combined regulatory effect on a target exon. These findings illuminate how a complex of RNA-binding proteins can decode combinatorial splicing regulatory signals by recognizing groups of tandem RNA elements.

Identification of Hypoplastic Left Heart Genotypes and Phenotypes; The Window toward Future Cell-Based Therapy: A Narrative Review

Hypoplastic left heart syndrome (HLHS) is a prevalent and lethal type of single ventricle anomaly. During early prenatal evaluations, left heart hypoplasia may be neglected due to its progressive features. It is a heterogeneous congenital heart disease with different phenotypes. Currently, there is no definite treatment for HLHS. This is in part due to its heterogeneous phenotypes that require different management. In addition, hindrances in recognizing the etiologic factors do not allow early preventive or therapeutic procedures. Phenotypic determination is fundamental to identifying the etiologic factors and therapeutic strategies. This review article introduces comprehensive information about different phenotypes and genotypes of HLHS and their novel molecular strategy. Genetic defects and flow-mediated mechanisms are the main known factors of HLHS. Recent studies reported additional data about its nonmendelian genetic origins associated with heterogeneous phenotypes. The genetic defects influence endocardium or cardiomyocyte development to yield early or late valve deformities and myocardial malformations. The new molecular therapeutic methods are essentially based on genetic etiologies. The principal therapeutic purpose is reinforcing the function of the right ventricle in patients with nonfunctional left ventricles. The ultimate desire is to create a biventricular heart in selected cases.

Alternative splicing factors and cardiac disease: more than just missplicing?

Alternative splicing (AS) is the process wherein the exons from a single gene are joined in different combinations to produce nonidentical, albeit related, RNA transcripts. This process is important for the development and physiological function of many organs and is particularly important in the heart. Notably, AS has been implicated in cardiac disease and failure, and a growing number of genetic variants in AS factors have been identified in association with cardiac malformation and/or disease. With the field poised to interrogate how these variants affect cardiac development and disease, an understandable point of emphasis will undoubtedly be on downstream target gene missplicing. In this Perspective article, we would like to encourage consideration not only of the potential for novel disease mechanisms, but also for contributions from disruption of the ever-expanding list of nonsplicing functions ascribed to many AS factors. We discuss the emergence of a novel cardiac disease mechanism based on pathogenic RNA granules and speculate on the generality of such a mechanism among localization-disrupting AS factor genetic variants. We also highlight emerging nonsplicing functions attributed to several AS factors with cardiac disease-associated genetic variants in the hopes of pointing to avenues for exploration of mechanisms that may contribute to disease alongside target gene missplicing.

RNA binding proteins: Mechanistic considerations and perspectives in controlling cardiovascular diseases

Cardiovascular diseases (CVDs) are becoming serious disease that endangering human health due to the increasing morbidity and mortality, and many molecular targets are involved in this complex pathologic process. Recently, RNA-binding proteins (RBPs) have received potential attention as a promising targets for preventing CVDs, including myocardial hypertrophy, dilated cardiomyopathy (DCM), myocardial fibrosis, and pulmonary hypertension (PH). As important regulators of RNA metabolism, RBPs play important roles in all steps of the gene expression cascade,and affect the occurrence and development of various diseases. In this review, we discuss the regulatory role of RBPs on various CVDs at the post transcriptional modification level based on current research. We also highlight the existing and potential RNA-based therapeutics that could impact future CVD treatments.

myh9b is a critical non-muscle myosin II encoding gene that interacts with myh9a and myh10 during zebrafish development in both compensatory and redundant pathways

Non-muscle myosin (NMII) motor proteins have diverse developmental functions due to their roles in cell shape changes, cell migration, and cell adhesion. Zebrafish are an ideal vertebrate model system to study the NMII encoding myh genes and proteins due to high sequence homology, established gene editing tools, and rapid ex utero development. In humans, mutations in the NMII encoding MYH genes can lead to abnormal developmental processes and disease. This study utilized zebrafish myh9a, myh9b, and myh10 null mutants to examine potential genetic interactions and roles for each gene in development. It was determined that the myh9b gene is the most critical NMII encoding gene, as myh9b mutants develop pericardial edema and have a partially penetrant lethal phenotype, which was not observed in the other myh mutants. This study also established that genetic interactions occur between the zebrafish myh9a, myh9b, and myh10 genes where myh9b is required for the expression of both myh9a and myh10, and myh10 is required for the expression of myh9b. Additionally, protein analyses suggested that enhanced NMII protein stability in some mutant backgrounds may play a role in compensation. Finally, double mutant studies revealed different and more severe phenotypes at earlier time points than single mutants, suggesting roles for tissue specific genetic redundancy, and in some genotypes, haploinsufficiency. These mutants are the first in vivo models allowing for the study of complete loss of the NMIIA and NMIIB proteins, establishing them as valuable tools to elucidate the role of NMII encoding myh genes in development and disease.

RNA binding protein with multiple splicing (RBPMS) promotes contractile phenotype splicing in human embryonic stem cell-derived vascular smooth muscle cells

AIMS

Differentiated vascular smooth muscle cells (VSMCs) express a unique network of mRNA isoforms via smooth muscle-specific alternative pre-mRNA splicing (SM-AS) in functionally critical genes, including those comprising the contractile machinery. We previously described RNA Binding Protein with Multiple Splicing (RBPMS) as a potent driver of differentiated SM-AS in the rat PAC1 VSMC cell line. What is unknown is how RBPMS affects VSMC phenotype and behaviour. Here, we aimed to dissect the role of RBPMS in SM-AS in human cells and determine the impact on VSMC phenotypic properties.

METHODS AND RESULTS

We used human embryonic stem cell-derived VSMCs (hESC-VSMCs) as our platform. hESC-VSMCs are inherently immature, and we found that they display only partially differentiated SM-AS patterns while RBPMS protein levels are low. We found that RBPMS over-expression induces SM-AS patterns in hESC-VSMCs akin to the contractile tissue VSMC splicing patterns. We present in silico and experimental findings that support RBPMS' splicing activity as mediated through direct binding and via functional cooperativity with splicing factor RBFOX2 on a significant subset of targets. We also demonstrate that RBPMS can alter the motility and the proliferative properties of hESC-VSMCs to mimic a more differentiated state.

CONCLUSION

Overall, this study emphasizes a critical role for RBPMS in establishing the contractile phenotype splicing programme of human VSMCs.

Alternative Splicing in the Heart: The Therapeutic Potential of Regulating the Regulators

Alternative splicing allows a single gene to produce a variety of protein isoforms. Changes in splicing isoform usage characterize virtually every stage of the differentiation process and define the physiological differences between cardiomyocytes with different function, at different stages of development, and pathological function. Recent identification of cardiac splicing factors provided insights into the mechanisms underlying alternative splicing and revealed how these splicing factors impact functional properties of the heart. Alterations of the splicing of sarcomeric genes, cell signaling proteins, and ion channels have been associated with the development of pathological conditions such as cardiomyopathy and arrhythmia. RBM20, RBM24, PTBP1, RBFOX, and QKI play key roles in cardiac development and pathology. A better understanding of their regulation will yield insights into healthy cardiac development and inform the development of molecular therapeutics.

Beyond genomic studies of congenital heart defects through systematic modelling and phenotyping

Congenital heart defects (CHDs), the most common congenital anomalies, are considered to have a significant genetic component. However, despite considerable efforts to identify pathogenic genes in patients with CHDs, few gene variants have been proven as causal. The complexity of the genetic architecture underlying human CHDs likely contributes to this poor genetic discovery rate. However, several other factors are likely to contribute. For example, the level of patient phenotyping required for clinical care may be insufficient for research studies focused on mechanistic discovery. Although several hundred mouse gene knockouts have been described with CHDs, these are generally not phenotyped and described in the same way as CHDs in patients, and thus are not readily comparable. Moreover, most patients with CHDs carry variants of uncertain significance of crucial cardiac genes, further complicating comparisons between humans and mouse mutants. In spite of major advances in cardiac developmental biology over the past 25 years, these advances have not been well communicated to geneticists and cardiologists. As a consequence, the latest data from developmental biology are not always used in the design and interpretation of studies aimed at discovering the genetic causes of CHDs. In this Special Article, while considering other in vitro and in vivo models, we create a coherent framework for accurately modelling and phenotyping human CHDs in mice, thereby enhancing the translation of genetic and genomic studies into the causes of CHDs in patients.

The Rbfox1/LASR complex controls alternative pre-mRNA splicing by recognition of multi-part RNA regulatory modules

The Rbfox proteins regulate alternative pre-mRNA splicing by binding to the RNA element GCAUG. In the nucleus, most of Rbfox is bound to LASR, a complex of RNA-binding proteins that recognize additional RNA motifs. However, it remains unclear how the different subunits of the Rbfox/LASR complex act together to bind RNA and regulate splicing. We used a nuclease-protection assay to map the transcriptome-wide footprints of Rbfox1/LASR on nascent cellular RNA. In addition to GCAUG, Rbfox1/LASR binds RNA containing motifs for LASR subunits hnRNPs M, H/F, C, and Matrin3. These elements are often arranged in tandem, forming multi-part modules of RNA motifs. To distinguish contact sites of Rbfox1 from the LASR subunits, we analyzed a mutant Rbfox1(F125A) that has lost RNA binding but remains associated with LASR. Rbfox1(F125A)/LASR complexes no longer interact with GCAUG but retain binding to RNA elements for LASR. Splicing analyses reveal that in addition to activating exons through adjacent GCAUG elements, Rbfox can also stimulate exons near binding sites for LASR subunits. Mini-gene experiments demonstrate that these diverse elements produce a combined regulatory effect on a target exon. These findings illuminate how a complex of RNA-binding proteins can decode combinatorial splicing regulatory signals by recognizing groups of tandem RNA elements.

RNA-binding proteins in cardiovascular biology and disease: the beat goes on

Cardiac development and function are becoming increasingly well understood from different angles, including signalling, transcriptional and epigenetic mechanisms. By contrast, the importance of the post-transcriptional landscape of cardiac biology largely remains to be uncovered, building on the foundation of a few existing paradigms. The discovery during the past decade of hundreds of additional RNA-binding proteins in mammalian cells and organs, including the heart, is expected to accelerate progress and has raised intriguing possibilities for better understanding the intricacies of cardiac development, metabolism and adaptive alterations. In this Review, we discuss the progress and new concepts on RNA-binding proteins and RNA biology and appraise them in the context of common cardiovascular clinical conditions, from cell and organ-wide perspectives. We also discuss how a better understanding of cardiac RNA-binding proteins can fill crucial knowledge gaps in cardiology and might pave the way to developing better treatments to reduce cardiovascular morbidity and mortality.

RNA binding proteins as mediators of pathological cardiac remodeling

RNA binding proteins (RBPs) play a central in the post-transcriptional regulation of gene expression, which can account for up to 50% of all variations in protein expression within a cell. Following their binding to target RNAs, RBPs most typically confer changes in gene expression through modulation of alternative spicing, RNA stabilization/degradation, or ribosome loading/translation rate. All of these post-transcriptional regulatory processes have been shown to play a functional role in pathological cardiac remodeling, and a growing body of evidence is beginning to identify the mechanistic contribution of individual RBPs and their cardiac RNA targets. This review highlights the mechanisms of RBP-dependent post-transcriptional gene regulation in cardiomyocytes and fibroblasts and our current understanding of how RNA binding proteins functionally contribute to pathological cardiac remodeling.

RNA binding proteins in cardiovascular development and disease

Congenital heart disease (CHD) is the most common birth defect affecting>1.35 million newborn babies worldwide. CHD can lead to prenatal, neonatal, postnatal lethality or life-long cardiac complications. RNA binding protein (RBP) mutations or variants are emerging as contributors to CHDs. RBPs are wizards of gene regulation and are major contributors to mRNA and protein landscape. However, not much is known about RBPs in the developing heart and their contributions to CHD. In this chapter, we will discuss our current knowledge about specific RBPs implicated in CHDs. We are in an exciting era to study RBPs using the currently available and highly successful RNA-based therapies and methodologies. Understanding how RBPs shape the developing heart will unveil their contributions to CHD. Identifying their target RNAs in the embryonic heart will ultimately lead to RNA-based treatments for congenital heart disease.

Therapeutic potential of alternative splicing in cardiovascular diseases

RNA splicing is an important RNA processing step required by multiexon protein-coding mRNAs and some noncoding RNAs. Precise RNA splicing is required for maintaining gene and cell function; however, mis-spliced RNA transcripts can lead to loss- or gain-of-function effects in human diseases. Mis-spliced RNAs induced by gene mutations or the dysregulation of splicing regulators may result in frameshifts, nonsense-mediated decay (NMD), or inclusion/exclusion of exons. Genetic animal models have characterised multiple splicing factors required for cardiac development or function. Moreover, sarcomeric and ion channel genes, which are closely associated with cardiovascular function and disease, are hotspots for AS. Here, we summarise splicing factors and their targets that are associated with cardiovascular diseases, introduce some therapies potentially related to pathological AS targets, and raise outstanding questions and future directions in this field.

Genetics and etiology of congenital heart disease

Congenital heart disease (CHD) is the most common severe birth anomaly, affecting almost 1% of infants. Most CHD is genetic, but only 40% of patients have an identifiable genetic risk factor for CHD. Chromosomal variation contributes significantly to CHD but is not readily amenable to biological follow-up due to the number of affected genes and lack of evolutionary synteny. The first CHD genes were implicated in extended families with syndromic CHD based on the segregation of risk alleles in affected family members. These have been complemented by more CHD gene discoveries in large-scale cohort studies. However, fewer than half of the 440 estimated human CHD risk genes have been identified, and the molecular mechanisms underlying CHD genetics remains incompletely understood. Therefore, model organisms and cell-based models are essential tools for improving our understanding of cardiac development and CHD genetic risk. Recent advances in genome editing, cell-specific genetic manipulation of model organisms, and differentiation of human induced pluripotent stem cells have recently enabled the characterization of developmental stages. In this chapter, we will summarize the latest studies in CHD genetics and the strengths of various study methodologies. We identify opportunities for future work that will continue to further CHD knowledge and ultimately enable better diagnosis, prognosis, treatment, and prevention of CHD.

In-Depth Genomic Analysis: The New Challenge in Congenital Heart Disease

The use of next-generation sequencing has provided new insights into the causes and mechanisms of congenital heart disease (CHD). Examinations of the whole exome sequence have detected detrimental gene variations modifying single or contiguous nucleotides, which are characterised as pathogenic based on statistical assessments of families and correlations with congenital heart disease, elevated expression during heart development, and reductions in harmful protein-coding mutations in the general population. Patients with CHD and extracardiac abnormalities are enriched for gene classes meeting these criteria, supporting a common set of pathways in the organogenesis of CHDs. Single-cell transcriptomics data have revealed the expression of genes associated with CHD in specific cell types, and emerging evidence suggests that genetic mutations disrupt multicellular genes essential for cardiogenesis. Metrics and units are being tracked in whole-genome sequencing studies.

SMC5 Plays Independent Roles in Congenital Heart Disease and Neurodevelopmental Disability

Up to 50% of patients with severe congenital heart disease (CHD) develop life-altering neurodevelopmental disability (NDD). It has been presumed that NDD arises in CHD cases because of hypoxia before, during, or after cardiac surgery. Recent studies detected an enrichment in de novo mutations in CHD and NDD, as well as significant overlap between CHD and NDD candidate genes. However, there is limited evidence demonstrating that genes causing CHD can produce NDD independent of hypoxia. A patient with hypoplastic left heart syndrome and gross motor delay presented with a de novo mutation in SMC5. Modeling mutation of smc5 in Xenopus tropicalis embryos resulted in reduced heart size, decreased brain length, and disrupted pax6 patterning. To evaluate the cardiac development, we induced the conditional knockout (cKO) of Smc5 in mouse cardiomyocytes, which led to the depletion of mature cardiomyocytes and abnormal contractility. To test a role for Smc5 specifically in the brain, we induced cKO in the mouse central nervous system, which resulted in decreased brain volume, and diminished connectivity between areas related to motor function but did not affect vascular or brain ventricular volume. We propose that genetic factors, rather than hypoxia alone, can contribute when NDD and CHD cases occur concurrently.

Sin3a associated protein 130 kDa, sap130, plays an evolutionary conserved role in zebrafish heart development

Hypoplastic left heart syndrome (HLHS) is a congenital heart disease where the left ventricle is reduced in size. A forward genetic screen in mice identified SIN3A associated protein 130 kDa (Sap130), part of the chromatin modifying SIN3A/HDAC complex, as a gene contributing to the etiology of HLHS. Here, we report the role of zebrafish sap130 genes in heart development. Loss of sap130a, one of two Sap130 orthologs, resulted in smaller ventricle size, a phenotype reminiscent to the hypoplastic left ventricle in mice. While cardiac progenitors were normal during somitogenesis, diminution of the ventricle size suggest the Second Heart Field (SHF) was the source of the defect. To explore the role of sap130a in gene regulation, transcriptome profiling was performed after the heart tube formation to identify candidate pathways and genes responsible for the small ventricle phenotype. Genes involved in cardiac differentiation and cardiac function were dysregulated in sap130a, but not in sap130b mutants. Confocal light sheet analysis measured deficits in cardiac output in MZsap130a supporting the notion that cardiomyocyte maturation was disrupted. Lineage tracing experiments revealed a significant reduction of SHF cells in the ventricle that resulted in increased outflow tract size. These data suggest that sap130a is involved in cardiogenesis via regulating the accretion of SHF cells to the growing ventricle and in their subsequent maturation for cardiac function. Further, genetic studies revealed an interaction between hdac1 and sap130a, in the incidence of small ventricles. These studies highlight the conserved role of Sap130a and Hdac1 in zebrafish cardiogenesis.

Sin3a Associated Protein 130kDa, sap130, plays an evolutionary conserved role in zebrafish heart development

Hypoplastic left heart syndrome (HLHS) is a congenital heart disease where the left ventricle is reduced in size. A forward genetic screen in mice identified SIN3A associated protein 130kDa ( Sap130 ), a protein in the chromatin modifying SIN3A/HDAC1 complex, as a gene contributing to the digenic etiology of HLHS. Here, we report the role of zebrafish sap130 genes in heart development. Loss of sap130a, one of two Sap130 orthologs, resulted in smaller ventricle size, a phenotype reminiscent to the hypoplastic left ventricle in mice. While cardiac progenitors were normal during somitogenesis, diminution of the ventricle size suggest the Second Heart Field (SHF) was the source of the defect. To explore the role of sap130a in gene regulation, transcriptome profiling was performed after the heart tube formation to identify candidate pathways and genes responsible for the small ventricle phenotype. Genes involved in cardiac differentiation and cell communication were dysregulated in sap130a , but not in sap130b mutants. Confocal light sheet analysis measured deficits in cardiac output in MZsap130a supporting the notion that cardiomyocyte maturation was disrupted. Lineage tracing experiments revealed a significant reduction of SHF cells in the ventricle that resulted in increased outflow tract size. These data suggest that sap130a is involved in cardiogenesis via regulating the accretion of SHF cells to the growing ventricle and in their subsequent maturation for cardiac function. Further, genetic studies revealed an interaction between hdac1 and sap130a , in the incidence of small ventricles. These studies highlight the conserved role of Sap130a and Hdac1 in zebrafish cardiogenesis.

RBPMS2 Is a Myocardial-Enriched Splicing Regulator Required for Cardiac Function

BACKGROUND

RBPs (RNA-binding proteins) perform indispensable functions in the post-transcriptional regulation of gene expression. Numerous RBPs have been implicated in cardiac development or physiology based on gene knockout studies and the identification of pathogenic RBP gene mutations in monogenic heart disorders. The discovery and characterization of additional RBPs performing indispensable functions in the heart will advance basic and translational cardiovascular research.

METHODS

We performed a differential expression screen in zebrafish embryos to identify genes enriched in nkx2.5-positive cardiomyocytes or cardiopharyngeal progenitors compared to nkx2.5-negative cells from the same embryos. We investigated the myocardial-enriched gene RNA-binding protein with multiple splicing (variants) 2 [RBPMS2)] by generating and characterizing rbpms2 knockout zebrafish and human cardiomyocytes derived from RBPMS2-deficient induced pluripotent stem cells.

RESULTS

We identified 1848 genes enriched in the nkx2.5-positive population. Among the most highly enriched genes, most with well-established functions in the heart, we discovered the ohnologs rbpms2a and rbpms2b, which encode an evolutionarily conserved RBP. Rbpms2 localizes selectively to cardiomyocytes during zebrafish heart development and strong cardiomyocyte expression persists into adulthood. Rbpms2-deficient embryos suffer from early cardiac dysfunction characterized by reduced ejection fraction. The functional deficit is accompanied by myofibril disarray, altered calcium handling, and differential alternative splicing events in mutant cardiomyocytes. These phenotypes are also observed in RBPMS2-deficient human cardiomyocytes, indicative of conserved molecular and cellular function. RNA-sequencing and comparative analysis of genes mis-spliced in RBPMS2-deficient zebrafish and human cardiomyocytes uncovered a conserved network of 29 ortholog pairs that require RBPMS2 for alternative splicing regulation, including RBFOX2, SLC8A1, and MYBPC3.

CONCLUSIONS

Our study identifies RBPMS2 as a conserved regulator of alternative splicing, myofibrillar organization, and calcium handling in zebrafish and human cardiomyocytes.