Increased Postnatal Cardiac Hyperplasia Precedes Cardiomyocyte Hypertrophy in a Model of Hypertrophic Cardiomyopathy

Single-cell transcriptional landscape of long non-coding RNAs orchestrating mouse heart development

Long non-coding RNAs (lncRNAs) comprise the most representative transcriptional units of the mammalian genome. They are associated with organ development linked with the emergence of cardiovascular diseases. We used bioinformatic approaches, machine learning algorithms, systems biology analyses, and statistical techniques to define co-expression modules linked to heart development and cardiovascular diseases. We also uncovered differentially expressed transcripts in subpopulations of cardiomyocytes. Finally, from this work, we were able to identify eight cardiac cell-types; several new coding, lncRNA, and pcRNA markers; two cardiomyocyte subpopulations at four different time points (ventricle E9.5, left ventricle E11.5, right ventricle E14.5 and left atrium P0) that harbored co-expressed gene modules enriched in mitochondrial, heart development and cardiovascular diseases. Our results evidence the role of particular lncRNAs in heart development and highlight the usage of co-expression modular approaches in the cell-type functional definition.

Characterization of heterozygous and homozygous mouse models with the most common hypertrophic cardiomyopathy mutation MYBPC3c.2373InsG in the Netherlands

Hypertrophic cardiomyopathy (HCM) is frequently caused by mutations in the cardiac myosin binding protein-C (cMyBP-C) encoding gene MYBPC3. In the Netherlands, approximately 25% of patients carry the MYBPC3c.2373InsG founder mutation. Most patients are heterozygous (MYBPC3+/InsG) and have highly variable phenotypic expression, whereas homozygous (MYBPC3InsG/InsG) patients have severe HCM at a young age. To improve understanding of disease progression and genotype-phenotype relationship based on the hallmarks of human HCM, we characterized mice with CRISPR/Cas9-induced heterozygous and homozygous mutations. At 18-28 weeks of age, we assessed the cardiac phenotype of Mybpc3+/InsG and Mybpc3InsG/InsG mice with echocardiography, and performed histological analyses. Cytoskeletal proteins and cardiomyocyte contractility of 3-4 week old and 18-28 week old Mybpc3c.2373InsG mice were compared to wild-type (WT) mice. Expectedly, knock-in of Mybpc3c.2373InsG resulted in the absence of cMyBP-C and our 18-28 week old homozygous Mybpc3c.2373InsG model developed cardiac hypertrophy and severe left ventricular systolic and diastolic dysfunction, whereas HCM was not evident in Mybpc3+/InsG mice. Mybpc3InsG/InsG cardiomyocytes also presented with slowed contraction-relaxation kinetics, to a greater extent in 18-28 week old mice, partially due to increased levels of detyrosinated tubulin and desmin, and reduced cardiac troponin I (cTnI) phosphorylation. Impaired cardiomyocyte contraction-relaxation kinetics were successfully normalized in 18-28 week old Mybpc3InsG/InsG cardiomyocytes by combining detyrosination inhibitor parthenolide and β-adrenergic receptor agonist isoproterenol. Both the 3-4 week old and 18-28 week old Mybpc3InsG/InsG models recapitulate HCM, with a severe phenotype present in the 18-28 week old model.

Hypertrophic Cardiomyopathy: Genetic Foundations, Outcomes, Interconnections, and Their Modifiers

Hypertrophic cardiomyopathy (HCM) is the most prevalent heritable cardiomyopathy. HCM is considered to be caused by mutations in cardiac sarcomeric protein genes. Recent research suggests that the genetic foundation of HCM is much more complex than originally postulated. The clinical presentations of HCM are very variable. Some mutation carriers remain asymptomatic, while others develop severe HCM, terminal heart failure, or sudden cardiac death. Heterogeneity regarding both genetic mutations and the clinical course of HCM hinders the establishment of universal genotype-phenotype correlations. However, some trends have been identified. The presence of a mutation in some genes encoding sarcomeric proteins is associated with earlier HCM onset, more severe left ventricular hypertrophy, and worse clinical outcomes. There is a diversity in the mechanisms implicated in the pathogenesis of HCM. They may be classified into groups, but they are interrelated. The lack of known supplementary elements that control the progression of HCM indicates that molecular mechanisms that exist between genotype and clinical presentations may be crucial. Secondary molecular changes in pathways implicated in HCM pathogenesis, post-translational protein modifications, and epigenetic factors affect HCM phenotypes. Cardiac loading conditions, exercise, hypertension, diet, alcohol consumption, microbial infection, obstructive sleep apnea, obesity, and environmental factors are non-molecular aspects that change the HCM phenotype. Many mechanisms are implicated in the course of HCM. They are mostly interconnected and contribute to some extent to final outcomes.

cMyBP-C ablation in human engineered cardiac tissue causes progressive Ca2+-handling abnormalities

Truncation mutations in cardiac myosin binding protein C (cMyBP-C) are common causes of hypertrophic cardiomyopathy (HCM). Heterozygous carriers present with classical HCM, while homozygous carriers present with early onset HCM that rapidly progress to heart failure. We used CRISPR-Cas9 to introduce heterozygous (cMyBP-C+/-) and homozygous (cMyBP-C-/-) frame-shift mutations into MYBPC3 in human iPSCs. Cardiomyocytes derived from these isogenic lines were used to generate cardiac micropatterns and engineered cardiac tissue constructs (ECTs) that were characterized for contractile function, Ca2+-handling, and Ca2+-sensitivity. While heterozygous frame shifts did not alter cMyBP-C protein levels in 2-D cardiomyocytes, cMyBP-C+/- ECTs were haploinsufficient. cMyBP-C-/- cardiac micropatterns produced increased strain with normal Ca2+-handling. After 2 wk of culture in ECT, contractile function was similar between the three genotypes; however, Ca2+-release was slower in the setting of reduced or absent cMyBP-C. At 6 wk in ECT culture, the Ca2+-handling abnormalities became more pronounced in both cMyBP-C+/- and cMyBP-C-/- ECTs, and force production became severely depressed in cMyBP-C-/- ECTs. RNA-seq analysis revealed enrichment of differentially expressed hypertrophic, sarcomeric, Ca2+-handling, and metabolic genes in cMyBP-C+/- and cMyBP-C-/- ECTs. Our data suggest a progressive phenotype caused by cMyBP-C haploinsufficiency and ablation that initially is hypercontractile, but progresses to hypocontractility with impaired relaxation. The severity of the phenotype correlates with the amount of cMyBP-C present, with more severe earlier phenotypes observed in cMyBP-C-/- than cMyBP-C+/- ECTs. We propose that while the primary effect of cMyBP-C haploinsufficiency or ablation may relate to myosin crossbridge orientation, the observed contractile phenotype is Ca2+-mediated.

The activin receptor ligand trap ActRIIB:ALK4-Fc ameliorates cardiomyopathy induced by neuromuscular disease and diabetes

Cardiomyopathies are ascribed to a variety of etiologies, present with diverse clinical phenotypes, and lack disease-modifying treatments. Mounting evidence implicates dysregulated activin receptor signaling in heart disease and highlights inhibition of this pathway as a potential therapeutic target. Here, we explored the effects of activin ligand inhibition using ActRIIB:ALK4-Fc, a heterodimeric receptor fusion protein, in two mechanistically distinct murine models of cardiomyopathy. Treatment with ActRIIB:ALK4-Fc significantly improved systolic or diastolic function in cardiomyopathy induced by neuromuscular disease or diabetes mellitus. Moreover, ActRIIB:ALK4-Fc corrected Ca²⁺ handling protein expression in diseased heart tissues, suggesting that activin signaling inhibition could alleviate cardiomyopathies in part by rebalancing aberrant intracellular Ca²⁺ homeostasis-a common underlying pathomechanism in diverse heart diseases.

Transcriptome Profile Identifies Actin as an Essential Regulator of Cardiac Myosin Binding Protein C3 Hypertrophic Cardiomyopathy in a Zebrafish Model

Variants in cardiac myosin-binding protein C (cMyBP-C) are the leading cause of inherited hypertrophic cardiomyopathy (HCM), demonstrating the key role that cMyBP-C plays in the heart’s contractile machinery. To investigate the c-MYBPC3 HCM-related cardiac impairment, we generated a zebrafish mypbc3-knockout model. These knockout zebrafish displayed significant morphological heart alterations related to a significant decrease in ventricular and atrial diameters at systolic and diastolic states at the larval stages. Immunofluorescence staining revealed significant hyperplasia in the mutant’s total cardiac and ventricular cardiomyocytes. Although cardiac contractility was similar to the wild-type control, the ejection fraction was significantly increased in the mypbc3 mutants. At later stages of larval development, the mutants demonstrated an early cardiac phenotype of myocardium remodeling, concurrent cardiomyocyte hyperplasia, and increased ejection fraction as critical processes in HCM initiation to counteract the increased ventricular myocardial wall stress. The examination of zebrafish adults showed a thickened ventricular cardiac wall with reduced heart rate, swimming speed, and endurance ability in both the mypbc3 heterozygous and homozygous groups. Furthermore, heart transcriptome profiling showed a significant downregulation of the actin-filament-based process, indicating an impaired actin cytoskeleton organization as the main dysregulating factor associated with the early ventricular cardiac hypertrophy in the zebrafish mypbc3 HCM model.

Novel pathogenic variant of MYBPC3 responsible for hypertrophic cardiomyopathy

OBJECTIVES

This study aims to investigate the pathogenic gene variant in a family with hypertrophic cardiomyopathy by using whole-exome sequencing and to explore the relationship between the gene variant and clinical phenotype.

METHODS

Peripheral blood was collected from a family with hypertrophic cardiomyopathy, and deoxyribonucleic acid was extracted. The possible pathogenic genes were detected by whole-exome sequencing, and the variant was verified by Sanger sequencing. Functional change in the variant was predicted by bioinformatics software. Clinical data of the family members are analysed simultaneously.

RESULTS

The proband carries a novel heterozygous nonsense variant of MYBPC3:c.2731G > T (p.E911X). The analysis of amino acid conservation suggests that the variation is highly conserved. The three-dimensional protein structure shows that the variant in MYBPC3 results in the incompleteness of the fibronectintype-III2 (p872-967) domain and deletion of Ig-like C2-type 6 (p971-1065) and fibronectin type-III 3 and Ig-like C2-type 7 (p1181-1274) domains, in which p1253-1268 is predicted to have a transmembrane helix structure. Clinical data indicate that the phenotypes of variant carriers with hypertrophic cardiomyopathy are diverse, suggesting the functional damages to the protein of MYBPC3.

CONCLUSION

The phenotypes of variant carriers with hypertrophic cardiomyopathy caused by the novel variant in MYBPC3: c.2731G > T (p.E911X) exhibit variable severity and clinical manifestations. Whole-exome sequencing can be used to comprehensive screen hypertrophic cardiomyopathy genes and provide a strong basis for early screening and accurate diagnosis and treatment of hypertrophic cardiomyopathy in children.

Critical Evaluation of Current Hypotheses for the Pathogenesis of Hypertrophic Cardiomyopathy

Hereditary hypertrophic cardiomyopathy (HCM), due to mutations in sarcomere proteins, occurs in more than 1/500 individuals and is the leading cause of sudden cardiac death in young people. The clinical course exhibits appreciable variability. However, typically, heart morphology and function are normal at birth, with pathological remodeling developing over years to decades, leading to a phenotype characterized by asymmetric ventricular hypertrophy, scattered fibrosis and myofibrillar/cellular disarray with ultimate mechanical heart failure and/or severe arrhythmias. The identity of the primary mutation-induced changes in sarcomere function and how they trigger debilitating remodeling are poorly understood. Support for the importance of mutation-induced hypercontractility, e.g., increased calcium sensitivity and/or increased power output, has been strengthened in recent years. However, other ideas that mutation-induced hypocontractility or non-uniformities with contractile instabilities, instead, constitute primary triggers cannot yet be discarded. Here, we review evidence for and criticism against the mentioned hypotheses. In this process, we find support for previous ideas that inefficient energy usage and a blunted Frank–Starling mechanism have central roles in pathogenesis, although presumably representing effects secondary to the primary mutation-induced changes. While first trying to reconcile apparently diverging evidence for the different hypotheses in one unified model, we also identify key remaining questions and suggest how experimental systems that are built around isolated primarily expressed proteins could be useful.

ELAC2/RNaseZ-linked cardiac hypertrophy in Drosophila melanogaster

A severe form of infantile cardiomyopathy (CM) has been linked to mutations in ELAC2, a highly conserved human gene. It encodes Zinc phosphodiesterase ELAC protein 2 (ELAC2), which plays an essential role in the production of mature tRNAs. To establish a causal connection between ELAC2 variants and CM, here we used the Drosophila melanogaster model organism, which carries the ELAC2 homolog RNaseZ. Even though RNaseZ and ELAC2 have diverged in some of their biological functions, our study demonstrates the use of the fly model to study the mechanism of ELAC2-related pathology. We established transgenic lines harboring RNaseZ with CM-linked mutations in the background of endogenous RNaseZ knockout. Importantly, we found that the phenotype of these flies is consistent with the pathological features in human patients. Specifically, expression of CM-linked variants in flies caused heart hypertrophy and led to reduction in cardiac contractility associated with a rare form of CM. This study provides first experimental evidence for the pathogenicity of CM-causing mutations in the ELAC2 protein, and the foundation to improve our understanding and diagnosis of this rare infantile disease.

This article has an associated First Person interview with the first author of the paper.

Summary: A newly established Drosophila model recapitulates key features of human heart pathology linked to mutations in ELAC2, thus providing experimental evidence of the pathogenicity of ELAC2 variants.

Shared Molecular Mechanisms of Hypertrophic Cardiomyopathy and Its Clinical Presentations: Automated Molecular Mechanisms Extraction Approach

Hypertrophic cardiomyopathy (HCM) is the most common inherited cardiovascular disease with a prevalence of 1 in 500 people and varying clinical presentations. Although there is much research on HCM, underlying molecular mechanisms are poorly understood, and research on the molecular mechanisms of its specific clinical presentations is scarce. Our aim was to explore the molecular mechanisms shared by HCM and its clinical presentations through the automated extraction of molecular mechanisms. Molecular mechanisms were congregated by a query of the INDRA database, which aggregates knowledge from pathway databases and combines it with molecular mechanisms extracted from abstracts and open-access full articles by multiple machine-reading systems. The molecular mechanisms were extracted from 230,072 articles on HCM and 19 HCM clinical presentations, and their intersections were found. Shared molecular mechanisms of HCM and its clinical presentations were represented as networks; the most important elements in the intersections’ networks were found, centrality scores for each element of each network calculated, networks with reduced level of noise generated, and cooperatively working elements detected in each intersection network. The identified shared molecular mechanisms represent possible mechanisms underlying different HCM clinical presentations. Applied methodology produced results consistent with the information in the scientific literature.

Cardiac response to adrenergic stress differs by sex and across the lifespan

The aging heart is well-characterized by a diminished responsiveness to adrenergic activation. However, the precise mechanisms by which age and sex impact adrenergic-mediated cardiac function remain poorly described. In the current investigation, we compared the cardiac response to adrenergic stress to gain mechanistic understanding of how the response to an adrenergic challenge differs by sex and age. Juvenile (4 weeks), adult (4-6 months), and aged (18-20 months) male and female mice were treated with the β-agonist isoproterenol (ISO) for 1 week. ISO-induced morphometric changes were age- and sex-dependent as juvenile and adult mice of both sexes had higher left ventricle weights while aged mice did not increase cardiac mass. Adults increased myocyte cell size and deposited fibrotic matrix in response to ISO, while juvenile and aged animals did not show evidence of hypertrophy or fibrosis. Juvenile females and adults underwent expected changes in systolic function with higher heart rate, ejection fraction, and fractional shortening. However, cardiac function in aged animals was not altered in response to ISO. Transcriptomic analysis identified significant differences in gene expression by age and sex, with few overlapping genes and pathways between groups. Fibrotic and adrenergic signaling pathways were upregulated in adult hearts. Juvenile hearts upregulated genes in the adrenergic pathway with few changes in fibrosis, while aged mice robustly upregulated fibrotic gene expression without changes in adrenergic genes. We suggest that the response to adrenergic stress significantly differs across the lifespan and by sex. Mechanistic definition of these age-related pathways by sex is critical for future research aimed at treating age-related cardiac adrenergic desensitization.

Human iPSC-engineered cardiac tissue platform faithfully models important cardiac physiology

Cardiomyocytes derived from human induced pluripotent stem cells (hiPSC-CM) may provide an important bridge between animal models and the intact human myocardium. Fulfilling this potential is hampered by their relative immaturity, leading to poor physiological responsiveness. hiPSC-CMs grown in traditional two-dimensional (2D) culture lack a t-tubular system, have only rudimentary intracellular calcium-handling systems, express predominantly embryonic sarcomeric protein isoforms, and preferentially use glucose as an energy substrate. Culturing hiPSC-CM in a variety of three-dimensional (3D) environments and the addition of nutritional, pharmacological, and electromechanical stimuli have proven, to various degrees, to be beneficial for maturation. We present a detailed assessment of a novel model in which hiPSC-CMs and hiPSC-derived cardiac fibroblasts are cocultured in a 3D fibrin matrix to form engineered cardiac tissue constructs (hiPSC-ECTs). The hiPSC-ECTs are responsive to physiological stimuli, including stretch, frequency, and β-adrenergic stimulation, develop a t-tubular system, and demonstrate calcium-handling and contractile kinetics that compare favorably with ventricular human myocardium. Furthermore, transcript levels of various genes involved in calcium-handling and contraction are increased. These markers of maturation become more robust over a relatively short period of time in culture (6 wk vs. 2 wk in hiPSC-ECTs). A comparison of the hiPSC-ECT molecular and performance variables with those of human cardiac tissue and other available engineered tissue platforms is provided to aid selection of the most appropriate platform for the research question at hand. Important and noteworthy aspects of this human cardiac model system are its reliance on "off-the-shelf" equipment, ability to provide detailed physiological performance data, and the ability to achieve a relatively mature cardiac physiology without additional nutritional, pharmacological, and electromechanical stimuli that may elicit unintended effects on function.NEW & NOTEWORTHY This study seeks to provide an in-depth assessment of contractile performance of human iPSC-derived cardiomyocytes cultured together with fibroblasts in a 3-dimensional-engineered tissue and compares performance both over time as cells mature, and with corresponding measures found in the literature using alternative 3D culture configurations. The suitability of 3D-engineered human cardiac tissues to model cardiac function is emphasized, and data provided to assist in the selection of the most appropriate configuration based on the target application.

Neonatal Heart Responds to Pressure Overload With Differential Alterations in Various Cardiomyocyte Maturation Programs That Accommodate Simultaneous Hypertrophy and Hyperplasia

Pressure overload is one of the pathophysiological conditions commonly associated with right-sided congenital heart disease (CHD). Patients suffer from this condition right after birth. However, little is known about how neonatal heart reacts to it. We have previously established a pulmonary artery banding (PAB) model in neonatal rat. Here we show that PAB accelerated transition of mononuclear cardiomyocytes into multinucleated cells to promote hypertrophic growth in neonatal heart. The elevated afterload significantly increased the mitotic activities of neonatal cardiomyocytes. Consistent with the proliferative potential, the elevated pressure overload also increased cytokinetic marker counts of cardiomyocytes. Using cardiomyocyte-specific lineage tracing, we noticed a clonal expansion of rare unlabeled cardiomyocytes in the PAB group, revealing a subgroup of cardiomyocytes with a strong capability of proliferation. In addition, PAB hearts at post-banding day 7 didn’t have the accumulation of macrophages, which is an immune response essential for neonatal heart regeneration in injury models. Transcriptomic analyses revealed that neonatal PAB induced an expression profile featuring both cardiomyocyte hypertrophy, such as highly activated translation, oxidative phosphorylation, and mitochondrial biogenesis programs etc., and immature cardiomyocyte, such as enhanced cell cycle activities and glycolytic metabolism, down-regulated cytoskeleton and ion channel gene expression, and maintenance of fetal-specific sarcomeric isoforms etc. It indicates that pressure overload has differential impacts on various cardiomyocyte maturation (CM) programs that may contribute to the concurrent cardiomyocyte hypertrophy and hyperplasia. The bivalent status of transcriptional profile highlights the plasticity of neonatal cardiomyocytes that can be exploited to adapt the postnatal environment.

Hypertrophic cardiomyopathy mutations in MYBPC3 dysregulate myosin

The mechanisms by which truncating mutations in MYBPC3 (encoding cardiac myosin-binding protein C; cMyBPC) or myosin missense mutations cause hypercontractility and poor relaxation in hypertrophic cardiomyopathy (HCM) are incompletely understood. Using genetic and biochemical approaches, we explored how depletion of cMyBPC altered sarcomere function. We demonstrated that stepwise loss of cMyBPC resulted in reciprocal augmentation of myosin contractility. Direct attenuation of myosin function, via a damaging missense variant (F764L) that causes dilated cardiomyopathy (DCM), normalized the increased contractility from cMyBPC depletion. Depletion of cMyBPC also altered dynamic myosin conformations during relaxation, enhancing the myosin state that enables ATP hydrolysis and thin filament interactions while reducing the super relaxed conformation associated with energy conservation. MYK-461, a pharmacologic inhibitor of myosin ATPase, rescued relaxation deficits and restored normal contractility in mouse and human cardiomyocytes with MYBPC3 mutations. These data define dosage-dependent effects of cMyBPC on myosin that occur across the cardiac cycle as the pathophysiologic mechanisms by which MYBPC3 truncations cause HCM. Therapeutic strategies to attenuate cMyBPC activity may rescue depressed cardiac contractility in patients with DCM, whereas inhibiting myosin by MYK-461 should benefit the substantial proportion of patients with HCM with MYBPC3 mutations.

Transcriptome Analysis of Cardiac Hypertrophic Growth in MYBPC3-Null Mice Suggests Early Responders in Hypertrophic Remodeling

Rationale: With a prevalence of 1 in 200 individuals, hypertrophic cardiomyopathy (HCM) is thought to be the most common genetic cardiac disease, with potential outcomes that include severe hypertrophy, heart failure, and sudden cardiac death (SCD). Though much research has furthered our understanding of how HCM-causing mutations in genes such as cardiac myosin-binding protein C (MYBPC3) impair contractile function, it remains unclear how such dysfunction leads to hypertrophy and/or arrhythmias, which comprise the HCM phenotype. Identification of early response mediators could provide rational therapeutic targets to reduce disease severity. Our goal was to differentiate physiologic and pathophysiologic hypertrophic growth responses and identify early genetic mediators in the development of cardiomegaly in the cardiac myosin-binding protein C-null (cMyBP-C^-/-) mouse model of HCM.

Methods and Results: We performed microarray analysis on left ventricles of wild-type (WT) and cMyBPC^-/- mice (n = 7 each) at postnatal day (PND) 1 and PND 9, before and after the appearance of an overt HCM phenotype. Applying the criteria of ≥2-fold change, we identified genes whose change was exclusive to pathophysiologic growth (n = 61), physiologic growth (n = 30), and genes whose expression changed ≥2-fold in both WT and cMyBP-C^-/- hearts (n = 130). Furthermore, we identified genes that were dysregulated in PND1 cMyBP-C^-/- hearts prior to hypertrophy, including genes in mechanosensing pathways and potassium channels linked to arrhythmias. One gene of interest, Xirp2, and its protein product, are regulated during growth but also show early, robust prehypertrophic upregulation in cMyBP-C^-/- hearts. Additionally, the transcription factor Zbtb16 also shows prehypertrophic upregulation at both gene and protein levels.

Conclusion: Our transcriptome analysis generated a comprehensive data set comparing physiologic vs. hypertrophic growth in mice lacking cMyBP-C. It highlights the importance of extracellular matrix pathways in hypertrophic growth and early dysregulation of potassium channels. Prehypertrophic upregulation of Xirp2 in cMyBP-C^-/- hearts supports a growing body of evidence suggesting Xirp2 has the capacity to elicit both hypertrophy and arrhythmias in HCM. Dysregulation of Xirp2, as well as Zbtb16, along with other genes associated with mechanosensing regions of the cardiomyocyte implicate stress-sensing in these regions as a potentially important early response in HCM.