Incomplete-penetrant hypertrophic cardiomyopathy MYH7 G256E mutation causes hypercontractility and elevated mitochondrial respiration.

Determining the pathogenicity of hypertrophic cardiomyopathy-associated mutations in the ß-myosin heavy chain (MYH7) can be challenging due to its variable penetrance and clinical severity. This study investigates the early pathogenic effects of the incomplete-penetrant MYH7 G256E mutation on myosin function that may trigger pathogenic adaptations and hypertrophy. We hypothesized that the G256E mutation would alter myosin biomechanical function, leading to changes in cellular functions. We developed a collaborative pipeline to characterize myosin function across protein, myofibril, cell, and tissue levels to determine the multiscale effects on structure-function of the contractile apparatus and its implications for gene regulation and metabolic state. The G256E mutation disrupts the transducer region of the S1 head and reduces the fraction of myosin in the folded-back state by 33%, resulting in more myosin heads available for contraction. Myofibrils from gene-edited MYH7WT/G256E human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) exhibited greater and faster tension development. This hypercontractile phenotype persisted in single-cell hiPSC-CMs and engineered heart tissues. We demonstrated consistent hypercontractile myosin function as a primary consequence of the MYH7 G256E mutation across scales, highlighting the pathogenicity of this gene variant. Single-cell transcriptomic and metabolic profiling demonstrated upregulated mitochondrial genes and increased mitochondrial respiration, indicating early bioenergetic alterations. This work highlights the benefit of our multiscale platform to systematically evaluate the pathogenicity of gene variants at the protein and contractile organelle level and their early consequences on cellular and tissue function. We believe this platform can help elucidate the genotype-phenotype relationships underlying other genetic cardiovascular diseases.

Proanthocyanidins and Phenolic Compounds from the Twigs of Salix chaenomeloides and Their Anti-Lipogenic Effects on 3T3-L1 Preadipocytes.

The present study investigated potential bioactive natural products from the EtOH extract of Salix chaenomeloides twigs using column chromatography, leading to the isolation of six compounds (1-6), which were characterized as two proanthocyanidins, procyanidin B2 (1) and procyanidin B1 (2), and four phenolic compounds, 4-hydroxybenzoic acid ß-D-glucosyl ester (3), di-O-methylcrenatin (4), p-coumaric acid glucoside (5), and syringin (6) by the comparison of their NMR spectra with the reported data and high-resolution (HR)-electrospray ionization mass spectroscopy (ESI-MS) analysis. We investigated the potential of six compounds (1-6) to inhibit adipogenesis in 3T3-L1 preadipocytes, which showed that the compounds (1-6) significantly reduced lipid accumulation in 3T3-L1 adipocytes without affecting cell proliferation. Notably, compound 1 demonstrated a remarkable 60% and 90% reduction in lipid levels with 50 and 100 µM treatments, respectively. Oil Red O staining results indicated that compound 1 significantly inhibits the formation of lipid droplets, comparable to the effect of T863, an inhibitor of triglyceride used as a positive control, in adipocytes. Compound 1 had no effect on the regulators PPARγ, C/EBPα, and SREBF1 of adipocyte differentiation in 3T3-L1 preadipocytes, but compound 1 activated the fatty acid oxidation regulator, PPARα, compared to the lipogenic-induced control. It also suppressed fatty acid synthesis by downregulating the expression of fatty acid synthase (FAS). Finally, compound 1 induced the mRNA and protein levels of CPT1A, an initial marker of mitochondrial fatty acid oxidation in 3T3-L1. This finding substantiates the anti-lipogenic and lipolytic effects of procyanidin B2 (1) in 3T3-L1 preadipocytes, emphasizing its pivotal role in modulating obesity-related markers.

PIV-MyoMonitor: an accessible particle image velocimetry-based software tool for advanced contractility assessment of cardiac organoids.

Induced pluripotent stem cell (iPSC)-derived cardiac organoids offer a versatile platform for personalized cardiac toxicity assessment, drug screening, disease modeling, and regenerative therapies. While previous image-based contractility analysis techniques allowed the assessment of contractility of two-dimensional cardiac models, they face limitations, including encountering high noise levels when applied to three-dimensional organoid models and requiring expensive equipment. Additionally, they offer fewer functional parameters compared to commercial software. To address these challenges, we developed an open-source, particle image velocimetry-based software (PIV-MyoMonitor) and demonstrated its capacity for accurate contractility analysis in both two- and three-dimensional cardiac models using standard lab equipment. Comparisons with four other open-source software programs highlighted the capability of PIV-MyoMonitor for more comprehensive quantitative analysis, providing 22 functional parameters and enhanced video outputs. We showcased its applicability in drug screening by characterizing the response of cardiac organoids to a known isotropic drug, isoprenaline. In sum, PIV-MyoMonitor enables reliable contractility assessment across various cardiac models without costly equipment or software. We believe this software will benefit a broader scientific community.

Exploring the Anti-Diabetic Potential of Quercetagitrin through Dual Inhibition of PTPN6 and PTPN9.

Protein tyrosine phosphatases (PTPs) are pivotal contributors to the development of type 2 diabetes (T2DM). Hence, directing interventions towards PTPs emerges as a valuable therapeutic approach for managing type 2 diabetes. In particular, PTPN6 and PTPN9 are targets for anti-diabetic effects. Through high-throughput drug screening, quercetagitrin (QG) was recognized as a dual-target inhibitor of PTPN6 and PTPN9. We observed that QG suppressed the catalytic activity of PTPN6 (IC50 = 1 µM) and PTPN9 (IC50 = 1.7 µM) in vitro and enhanced glucose uptake by mature C2C12 myoblasts. Additionally, QG increased the phosphorylation of adenosine monophosphate-activated protein kinase (AMPK) and insulin-dependent phosphorylation of Akt in mature C2C12 myoblasts. It further promoted the phosphorylation of Akt in the presence of palmitic acid, suggesting the attenuation of insulin resistance. In summary, our results indicate QG's role as a potent inhibitor targeting both PTPN6 and PTPN9, showcasing its potential as a promising treatment avenue for T2DM.

Unleashing the Power of Undifferentiated Induced Pluripotent Stem Cell Bioprinting: Current Progress and Future Prospects.

Induced pluripotent stem cell (iPSC) technology has revolutionized various fields, including stem cell research, disease modeling, and regenerative medicine. The evolution of iPSC-based models has transitioned from conventional two-dimensional systems to more physiologically relevant three-dimensional (3D) models such as spheroids and organoids. Nonetheless, there still remain challenges including limitations in creating complex 3D tissue geometry and structures, the emergence of necrotic core in existing 3D models, and limited scalability and reproducibility. 3D bioprinting has emerged as a revolutionary technology that can facilitate the development of complex 3D tissues and organs with high scalability and reproducibility. This innovative approach has the potential to effectively bridge the gap between conventional iPSC models and complex 3D tissues in vivo. This review focuses on current trends and advancements in the bioprinting of iPSCs. Specifically, it covers the fundamental concepts and techniques of bioprinting and bioink design, reviews recent progress in iPSC bioprinting research with a specific focus on bioprinting undifferentiated iPSCs, and concludes by discussing existing limitations and future prospects.

Combined lineage tracing and scRNA-seq reveals unexpected first heart field predominance of human iPSC differentiation.

During mammalian development, the left and right ventricles arise from early populations of cardiac progenitors known as the first and second heart fields, respectively. While these populations have been extensively studied in non-human model systems, their identification and study in vivo human tissues have been limited due to the ethical and technical limitations of accessing gastrulation-stage human embryos. Human-induced pluripotent stem cells (hiPSCs) present an exciting alternative for modeling early human embryogenesis due to their well-established ability to differentiate into all embryonic germ layers. Here, we describe the development of a TBX5/MYL2 lineage tracing reporter system that allows for the identification of FHF- progenitors and their descendants including left ventricular cardiomyocytes. Furthermore, using single-cell RNA sequencing (scRNA-seq) with oligonucleotide-based sample multiplexing, we extensively profiled differentiating hiPSCs across 12 timepoints in two independent iPSC lines. Surprisingly, our reporter system and scRNA-seq analysis revealed a predominance of FHF differentiation using the small molecule Wnt-based 2D differentiation protocol. We compared this data with existing murine and 3D cardiac organoid scRNA-seq data and confirmed the dominance of left ventricular cardiomyocytes (>90%) in our hiPSC-derived progeny. Together, our work provides the scientific community with a powerful new genetic lineage tracing approach as well as a single-cell transcriptomic atlas of hiPSCs undergoing cardiac differentiation.

Multi-scale models reveal hypertrophic cardiomyopathy MYH7 G256E mutation drives hypercontractility and elevated mitochondrial respiration.

Rationale: Over 200 mutations in the sarcomeric protein ß-myosin heavy chain (MYH7) have been linked to hypertrophic cardiomyopathy (HCM). However, different mutations in MYH7 lead to variable penetrance and clinical severity, and alter myosin function to varying degrees, making it difficult to determine genotype-phenotype relationships, especially when caused by rare gene variants such as the G256E mutation. Objective: This study aims to determine the effects of low penetrant MYH7 G256E mutation on myosin function. We hypothesize that the G256E mutation would alter myosin function, precipitating compensatory responses in cellular functions. Methods: We developed a collaborative pipeline to characterize myosin function at multiple scales (protein to myofibril to cell to tissue). We also used our previously published data on other mutations to compare the degree to which myosin function was altered. Results: At the protein level, the G256E mutation disrupts the transducer region of the S1 head and reduces the fraction of myosin in the folded-back state by 50.9%, suggesting more myosins available for contraction. Myofibrils isolated from hiPSC-CMs CRISPR-edited with G256E (MYH7 WT/G256E ) generated greater tension, had faster tension development and slower early phase relaxation, suggesting altered myosin-actin crossbridge cycling kinetics. This hypercontractile phenotype persisted in single-cell hiPSC-CMs and engineered heart tissues. Single-cell transcriptomic and metabolic profiling demonstrated upregulation of mitochondrial genes and increased mitochondrial respiration, suggesting altered bioenergetics as an early feature of HCM. Conclusions: MYH7 G256E mutation causes structural instability in the transducer region, leading to hypercontractility across scales, perhaps from increased myosin recruitment and altered crossbridge cycling. Hypercontractile function of the mutant myosin was accompanied by increased mitochondrial respiration, while cellular hypertrophy was modest in the physiological stiffness environment. We believe that this multi-scale platform will be useful to elucidate genotype-phenotype relationships underlying other genetic cardiovascular diseases.

devCellPy is a machine learning-enabled pipeline for automated annotation of complex multilayered single-cell transcriptomic data.

A major informatic challenge in single cell RNA-sequencing analysis is the precise annotation of datasets where cells exhibit complex multilayered identities or transitory states. Here, we present devCellPy a highly accurate and precise machine learning-enabled tool that enables automated prediction of cell types across complex annotation hierarchies. To demonstrate the power of devCellPy, we construct a murine cardiac developmental atlas from published datasets encompassing 104,199 cells from E6.5-E16.5 and train devCellPy to generate a cardiac prediction algorithm. Using this algorithm, we observe a high prediction accuracy (>90%) across multiple layers of annotation and across de novo murine developmental data. Furthermore, we conduct a cross-species prediction of cardiomyocyte subtypes from in vitro-derived human induced pluripotent stem cells and unexpectedly uncover a predominance of left ventricular (LV) identity that we confirmed by an LV-specific TBX5 lineage tracing system. Together, our results show devCellPy to be a useful tool for automated cell prediction across complex cellular hierarchies, species, and experimental systems.

In vivo visualization and molecular targeting of the cardiac conduction system.

Accidental injury to the cardiac conduction system (CCS), a network of specialized cells embedded within the heart and indistinguishable from the surrounding heart muscle tissue, is a major complication in cardiac surgeries. Here, we addressed this unmet need by engineering targeted antibody-dye conjugates directed against the CCS, allowing for the visualization of the CCS in vivo following a single intravenous injection in mice. These optical imaging tools showed high sensitivity, specificity, and resolution, with no adverse effects on CCS function. Further, with the goal of creating a viable prototype for human use, we generated a fully human monoclonal Fab that similarly targets the CCS with high specificity. We demonstrate that, when conjugated to an alternative cargo, this Fab can also be used to modulate CCS biology in vivo, providing a proof of principle for targeted cardiac therapeutics. Finally, in performing differential gene expression analyses of the entire murine CCS at single-cell resolution, we uncovered and validated a suite of additional cell surface markers that can be used to molecularly target the distinct subcomponents of the CCS, each prone to distinct life-threatening arrhythmias. These findings lay the foundation for translational approaches targeting the CCS for visualization and therapy in cardiothoracic surgery, cardiac imaging, and arrhythmia management.

CRISPR/Cas9-based targeting of fluorescent reporters to human iPSCs to isolate atrial and ventricular-specific cardiomyocytes.

Generating cardiomyocytes (CMs) from human induced pluripotent stem cells (hiPSCs) has represented a significant advance in our ability to model cardiac disease. Current differentiation protocols, however, have limited use due to their production of heterogenous cell populations, primarily consisting of ventricular-like CMs. Here we describe the creation of two chamber-specific reporter hiPSC lines by site-directed genomic integration using CRISPR-Cas9 technology. In the MYL2-tdTomato reporter, the red fluorescent tdTomato was inserted upstream of the 3' untranslated region of the Myosin Light Chain 2 (MYL2) gene in order faithfully label hiPSC-derived ventricular-like CMs while avoiding disruption of endogenous gene expression. Similarly, in the SLN-CFP reporter, Cyan Fluorescent Protein (CFP) was integrated downstream of the coding region of the atrial-specific gene, Sarcolipin (SLN). Purification of tdTomato+ and CFP+ CMs using flow cytometry coupled with transcriptional and functional characterization validated these genetic tools for their use in the isolation of bona fide ventricular-like and atrial-like CMs, respectively. Finally, we successfully generated a double reporter system allowing for the isolation of both ventricular and atrial CM subtypes within a single hiPSC line. These tools provide a platform for chamber-specific hiPSC-derived CM purification and analysis in the context of atrial- or ventricular-specific disease and therapeutic opportunities.

Massive expansion and cryopreservation of functional human induced pluripotent stem cell-derived cardiomyocytes.

Since the discovery of human induced pluripotent stem cells (hiPSCs), numerous strategies have been established to efficiently derive cardiomyocytes from hiPSCs (hiPSC-CMs). Here, we describe a cost-effective strategy for the subsequent massive expansion (>250-fold) of high-purity hiPSC-CMs relying on two aspects: removal of cell-cell contacts and small-molecule inhibition with CHIR99021. The protocol maintains CM functionality, allows cryopreservation, and the cells can be used in downstream assays such as disease modeling, drug and toxicity screening, and cell therapy. For complete details on the use and execution of this protocol, please refer to Buikema (2020).

Intrinsic Endocardial Defects Contribute to Hypoplastic Left Heart Syndrome.

Hypoplastic left heart syndrome (HLHS) is a complex congenital heart disease characterized by abnormalities in the left ventricle, associated valves, and ascending aorta. Studies have shown intrinsic myocardial defects but do not sufficiently explain developmental defects in the endocardial-derived cardiac valve, septum, and vasculature. Here, we identify a developmentally impaired endocardial population in HLHS through single-cell RNA profiling of hiPSC-derived endocardium and human fetal heart tissue with an underdeveloped left ventricle. Intrinsic endocardial defects contribute to abnormal endothelial-to-mesenchymal transition, NOTCH signaling, and extracellular matrix organization, key factors in valve formation. Endocardial abnormalities cause reduced cardiomyocyte proliferation and maturation by disrupting fibronectin-integrin signaling, consistent with recently described de novo HLHS mutations associated with abnormal endocardial gene and fibronectin regulation. Together, these results reveal a critical role for endocardium in HLHS etiology and provide a rationale for considering endocardial function in regenerative strategies.

Wnt Activation and Reduced Cell-Cell Contact Synergistically Induce Massive Expansion of Functional Human iPSC-Derived Cardiomyocytes.

Modulating signaling pathways including Wnt and Hippo can induce cardiomyocyte proliferation in vivo. Applying these signaling modulators to human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) in vitro can expand CMs modestly (<5-fold). Here, we demonstrate massive expansion of hiPSC-CMs in vitro (i.e., 100- to 250-fold) by glycogen synthase kinase-3ß (GSK-3ß) inhibition using CHIR99021 and concurrent removal of cell-cell contact. We show that GSK-3ß inhibition suppresses CM maturation, while contact removal prevents CMs from cell cycle exit. Remarkably, contact removal enabled 10 to 25 times greater expansion beyond GSK-3ß inhibition alone. Mechanistically, persistent CM proliferation required both LEF/TCF activity and AKT phosphorylation but was independent from yes-associated protein (YAP) signaling. Engineered heart tissues from expanded hiPSC-CMs showed comparable contractility to those from unexpanded hiPSC-CMs, demonstrating uncompromised cellular functionality after expansion. In summary, we uncovered a molecular interplay that enables massive hiPSC-CM expansion for large-scale drug screening and tissue engineering applications.

Levitating Cells to Sort the Fit and the Fat.

Density is a core material property and varies between different cell types, mainly based on differences in their lipid content. Sorting based on density enables various biomedical applications such as multi-omics in precision medicine and regenerative repair in medicine. However, a significant challenge is sorting cells of the same type based on density differences. Here, a new method for real-time monitoring and sorting of single cells based on their inherent levitation profiles driven by their lipid content is reported. As a model system, human-induced pluripotent stem cell (hiPSC)-derived cardiomyocytes (CMs) from a patient with neutral lipid storage disease (NLSD) due to loss of function of adipose triglyceride lipase (ATGL) resulting in abnormal lipid storage in cardiac muscle are used. This levitation-based strategy detects subpopulations within ATGL-deficient hiPSC-CMs with heterogenous lipid content, equilibrating at different levitation heights due to small density differences. In addition, sorting of these differentially levitating subpopulations are monitored in real time. Using this approach, sorted healthy and diseased hiPSC-CMs maintain viability and function. Pixel-tracking technologies show differences in contraction between NLSD and healthy hiPSC-CMs. Overall, this is a unique approach to separate diseased cell populations based on their intracellular lipid content that cannot be achieved using traditional flow cytometry techniques.

Simple Lithography-Free Single Cell Micropatterning using Laser-Cut Stencils.

Micropatterning techniques have been widely used in cell biology to study effects of controlling cell shape and size on cell fate determination at single cell resolution. Current state-of-the-art single cell micropatterning techniques involve soft lithography and micro-contact printing, which is a powerful technology, but requires trained engineering skills and certain facility support in microfabrication. These limitations require a more accessible technique. Here, we describe a simple alternative lithography-free method: stencil-based single cell patterning. We provide step-by-step procedures including stencil design, polyacrylamide hydrogel fabrication, stencil-based protein incorporation, and cell plating and culture. This simple method can be used to pattern an array of as many as 2,000 cells. We demonstrate the patterning of cardiomyocytes derived from single human induced pluripotent stem cells (hiPSC) with distinct cell shapes, from a 1:1 square to a 7:1 adult cardiomyocyte-like rectangle. This stencil-based single cell patterning is lithography-free, technically robust, convenient, inexpensive, and most importantly accessible to those with a limited bioengineering background.

Bioprinting Approaches to Engineering Vascularized 3D Cardiac Tissues.

PURPOSE OF REVIEW: 3D bioprinting technologies hold significant promise for the generation of engineered cardiac tissue and translational applications in medicine. To generate a clinically relevant sized tissue, the provisioning of a perfusable vascular network that provides nutrients to cells in the tissue is a major challenge. This review summarizes the recent vascularization strategies for engineering 3D cardiac tissues. RECENT FINDINGS: Considerable steps towards the generation of macroscopic sizes for engineered cardiac tissue with efficient vascular networks have been made within the past few years. Achieving a compact tissue with enough cardiomyocytes to provide functionality remains a challenging task. Achieving perfusion in engineered constructs with media that contain oxygen and nutrients at a clinically relevant tissue sizes remains the next frontier in tissue engineering. The provisioning of a functional vasculature is necessary for maintaining a high cell viability and functionality in engineered cardiac tissues. Several recent studies have shown the ability to generate tissues up to a centimeter scale with a perfusable vascular network. Future challenges include improving cell density and tissue size. This requires the close collaboration of a multidisciplinary teams of investigators to overcome complex challenges in order to achieve success.

Hydrogels with enhanced protein conjugation efficiency reveal stiffness-induced YAP localization in stem cells depends on biochemical cues.

Polyacrylamide hydrogels have been widely used in stem cell mechanotransduction studies. Conventional conjugation methods of biochemical cues to polyacrylamide hydrogels suffer from low conjugation efficiency, which leads to poor attachment of human pluripotent stem cells (hPSCs) on soft substrates. In addition, while it is well-established that stiffness-dependent regulation of stem cell fate requires cytoskeletal tension, and is mediated through nuclear translocation of transcription regulator, Yes-associated protein (YAP), the role of biochemical cues in stiffness-dependent YAP regulation remains largely unknown. Here we report a method that enhances the conjugation efficiency of biochemical cues on polyacrylamide hydrogels compared to conventional methods. This modified method enables robust hPSC attachment, proliferation and maintenance of pluripotency across varying substrate stiffness (3 kPa-38 kPa). Using this hydrogel platform, we demonstrate that varying the types of biochemical cues (Matrigel, laminin, GAG-peptide) or density of Matrigel can alter stiffness-dependent YAP localization in hPSCs. In particular, we show that stiffness-dependent YAP localization is overridden at low or high density of Matrigel. Furthermore, human mesenchymal stem cells display stiffness-dependent YAP localization only at intermediate fibronectin density. The hydrogel platform with enhanced conjugation efficiency of biochemical cues provides a powerful tool for uncovering the role of biochemical cues in regulating mechanotransduction of various stem cell types.

Biochemical Ligand Density Regulates Yes-Associated Protein Translocation in Stem Cells through Cytoskeletal Tension and Integrins.

Different tissue types are characterized by varying stiffness and biochemical ligands. Increasing substrate stiffness has been shown to trigger Yes-associated protein (YAP) translocation from the cytoplasm to the nucleus, yet the role of ligand density in modulating mechanotransduction and stem cell fate remains largely unexplored. Using polyacrylamide hydrogels coated with fibronectin as a model platform, we showed that stiffness-induced YAP translocation occurs only at intermediate ligand densities. At low or high ligand densities, YAP localization is dominated by ligand density independent of substrate stiffness. We further showed that ligand density-induced YAP translocation requires cytoskeleton tension and αVß3-integrin binding. Finally, we demonstrate that increasing ligand density alone can enhance osteogenic differentiation regardless of matrix stiffness. Together, the findings from the present study establish ligand density as an important parameter for modulating stem cell mechanotransduction and differentiation, which is mediated by integrin clustering, focal adhesion, and cytoskeletal tension.

A Premature Termination Codon Mutation in MYBPC3 Causes Hypertrophic Cardiomyopathy via Chronic Activation of Nonsense-Mediated Decay.

BACKGROUND: Hypertrophic cardiomyopathy (HCM) is frequently caused by mutations in myosin-binding protein C3 ( MYBPC3) resulting in a premature termination codon (PTC). The underlying mechanisms of how PTC mutations in MYBPC3 lead to the onset and progression of HCM are poorly understood. This study's aim was to investigate the molecular mechanisms underlying the pathogenesis of HCM associated with MYBPC3 PTC mutations by utilizing human isogenic induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs). METHODS: Isogenic iPSC lines were generated from HCM patients harboring MYBPC3 PTC mutations (p.R943x; p.R1073P_Fsx4) using genome editing. Comprehensive phenotypic and transcriptome analyses were performed in the iPSC-CMs. RESULTS: We observed aberrant calcium handling properties with prolonged decay kinetics and elevated diastolic calcium levels in the absence of structural abnormalities or contracile dysfunction in HCM iPSC-CMs as compared to isogenic controls. The mRNA expression levels of MYBPC3 were significantly reduced in mutant iPSC-CMs, but the protein levels were comparable among isogenic iPSC-CMs, suggesting that haploinsufficiency of MYBPC3 does not contribute to the pathogenesis of HCM in vitro. Furthermore, truncated MYBPC3 peptides were not detected. At the molecular level, the nonsense-mediated decay pathway was activated, and a set of genes involved in major cardiac signaling pathways was dysregulated in HCM iPSC-CMs, indicating an HCM gene signature in vitro. Specific inhibition of the nonsense-mediated decay pathway in mutant iPSC-CMs resulted in reversal of the molecular phenotype and normalization of calcium-handling abnormalities. CONCLUSIONS: iPSC-CMs carrying MYBPC3 PTC mutations displayed aberrant calcium signaling and molecular dysregulations in the absence of significant haploinsufficiency of MYBPC3 protein. Here we provided the first evidence of the direct connection between the chronically activated nonsense-mediated decay pathway and HCM disease development.