Although high-throughput sequencing can elucidate the genetic basis of hereditary cardiomyopathy, direct interventions targeting pathological mutations have not been established. Furthermore, it remains uncertain whether homology-directed repair (HDR) is effective in non-dividing cardiomyocytes. Here, we demonstrate that HDR-mediated genome editing using CRISPR/Cas9 is effective in non-dividing cardiomyocytes. Transduction of adeno-associated virus (AAV) containing sgRNA and repair template into cardiomyocytes constitutively expressing Cas9 efficiently introduced a fluorescent protein to the C-terminus of Myl2. Imaging-based sequential evaluation of endogenously tagged protein revealed that HDR occurs in cardiomyocytes, independently of DNA synthesis. We sought to repair a pathological mutation in Tnnt2 in cardiomyocytes of cardiomyopathy model mice. An sgRNA that avoided the mutated exon minimized deleterious effects on Tnnt2 expression, and AAV-mediated HDR achieved precise genome correction at a frequency of ~12.5%. Thus, targeted genome replacement via HDR is effective in non-dividing cardiomyocytes, and represents a potential therapeutic tool for targeting intractable cardiomyopathy.

Desmoglein-2, encoded by DSG2, is one of the desmosome proteins that maintain the structural integrity of tissues, including heart. Genetic mutations in DSG2 cause arrhythmogenic cardiomyopathy, mainly in an autosomal dominant manner. Here, we identified a homozygous stop-gain mutations in DSG2 (c.C355T, p.R119X) that led to complete desmoglein-2 deficiency in a patient with severe biventricular heart failure. Histological analysis revealed abnormal deposition of desmosome proteins, disrupted intercalated disk structures in the myocardium. Induced pluripotent stem cells (iPSCs) were generated from the patient (R119X-iPSC), and the mutated DSG2 gene locus was heterozygously corrected to a normal allele via homology-directed repair (HDR-iPSC). Both isogenic iPSCs were differentiated into cardiomyocytes [induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs)]. Multielectrode array analysis detected abnormal excitation in R119X-iPSC-CMs but not in HDR-iPSC-CMs. Micro-force testing of three-dimensional self-organized tissue rings (SOTRs) revealed tissue fragility and a weak maximum force in SOTRs from R119X-iPSC-CMs. Notably, these phenotypes were significantly recovered in HDR-iPSC-CMs. Myocardial fiber structures in R119X-iPSC-CMs were severely aberrant, and electron microscopic analysis confirmed that desmosomes were disrupted in these cells. Unexpectedly, the absence of desmoglein-2 in R119X-iPSC-CMs led to decreased expression of desmocollin-2 but no other desmosome proteins. Adeno-associated virus-mediated replacement of DSG2 significantly recovered the contraction force in SOTRs generated from R119X-iPSC-CMs. Our findings confirm the presence of a desmoglein-2-deficient cardiomyopathy among clinically diagnosed dilated cardiomyopathies. Recapitulation and correction of the disease phenotype using iPSC-CMs provide evidence to support the development of precision medicine and the proof of concept for gene replacement therapy for this cardiomyopathy.

Background: Cardiac-specific myosin light chain kinase (cMLCK), encoded by MYLK3 , regulates cardiac contractility through phosphorylation of ventricular myosin regulatory light chain. However, the pathophysiological and therapeutic implications of cMLCK in human heart failure remain unclear. We aimed to investigate whether cMLCK dysregulation causes cardiac dysfunction and whether the restoration of cMLCK could be a novel myotropic therapy for systolic heart failure. Methods: We generated the knock-in mice ( Mylk3 ^+/fs and Mylk 3 ^fs/fs ) with a familial dilated cardiomyopathy–associated MYLK3 frameshift mutation ( MYLK3 ^+/fs ) that had been identified previously by us (c.1951-1G>T; p.P639Vfs*15) and the human induced pluripotent stem cell–derived cardiomyocytes from the carrier of the mutation. We also developed a new small-molecule activator of cMLCK (LEUO-1154). Results: Both mice ( Mylk3 ^+/fs and Mylk 3 ^fs/fs ) showed reduced cMLCK expression due to nonsense-mediated messenger RNA decay, reduced MLC2v (ventricular myosin regulatory light chain) phosphorylation in the myocardium, and systolic dysfunction in a cMLCK dose–dependent manner. Consistent with this result, myocardium from the mutant mice showed an increased ratio of cardiac superrelaxation/disordered relaxation states that may contribute to impaired cardiac contractility. The phenotypes observed in the knock-in mice were rescued by cMLCK replenishment through the AAV9_ MYLK3 vector. Human induced pluripotent stem cell–derived cardiomyocytes with MYLK3 _+/fs mutation reduced cMLCK expression by 50% and contractile dysfunction, accompanied by an increased superrelaxation/disordered relaxation ratio. CRISPR-mediated gene correction, or cMLCK replenishment by AAV9_ MYLK3 vector, successfully recovered cMLCK expression, the superrelaxation/disordered relaxation ratio, and contractile dysfunction. LEUO-1154 increased human cMLCK activity ≈2-fold in the V _max for ventricular myosin regulatory light chain phosphorylation without affecting the K _m . LEUO-1154 treatment of human induced pluripotent stem cell–derived cardiomyocytes with MYLK3 _+/fs mutation restored the ventricular myosin regulatory light chain phosphorylation level and superrelaxation/disordered relaxation ratio and improved cardiac contractility without affecting calcium transients, indicating that the cMLCK activator acts as a myotrope. Finally, human myocardium from advanced heart failure with a wide variety of causes had a significantly lower MYLK3 / PPP1R12B messenger RNA expression ratio than control hearts, suggesting an altered balance between myosin regulatory light chain kinase and phosphatase in the failing myocardium, irrespective of the causes. Conclusions: cMLCK dysregulation contributes to the development of cardiac systolic dysfunction in humans. Our strategy to restore cMLCK activity could form the basis of a novel myotropic therapy for advanced systolic heart failure.

Under hypertrophic stimulation, cardiomyocytes enter a hypermetabolic state and accelerate biomass accumulation. Although the molecular pathways that regulate protein levels are well-studied, the functional implications of RNA accumulation and its regulatory mechanisms in cardiomyocytes remain elusive. Here, we have elucidated the quantitative kinetics of RNA in cardiomyocytes through single cell imaging and c-Myc (Myc)-mediated hypermetabolic analytical model using cultured cardiomyocytes. Nascent RNA labeling combined with single cell imaging demonstrated that Myc protein significantly increased the amount of global RNA production per cardiomyocyte. Chromatin immunoprecipitation with high-throughput sequencing clarified that overexpressed Myc bound to a specific set of genes and recruits RNA polymerase II. Among these genes, we identified Btg2 as a novel target of Myc. Btg2 overexpression significantly reduced cardiomyocyte surface area. Conversely, shRNA-mediated knockdown of Btg2 accelerated adrenergic stimulus-induced hypertrophy. Using mass spectrometry analysis, we determined that Btg2 binds a series of proteins that comprise mRNA deadenylation complexes. Intriguingly, Btg2 specifically suppresses cytosolic, but not nuclear, RNA levels. Btg2 knockdown further enhances cytosolic RNA accumulation in cardiomyocytes under adrenergic stimulation, suggesting that Btg2 negatively regulates reactive hypertrophy by negatively regulating RNA accumulation. Our findings provide insight into the functional significance of the mechanisms regulating RNA levels in cardiomyocytes.

Loss-of-function mutations in PKP2, which encodes plakophilin-2, cause arrhythmogenic cardiomyopathy (AC). Restoration of deficient molecules can serve as upstream therapy, thereby requiring a human model that recapitulates disease pathology and provides distinct readouts in phenotypic analysis for proof of concept for gene replacement therapy. Here, we generated isogenic induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs) with precisely adjusted expression of plakophilin-2 from a patient with AC carrying a heterozygous frameshift PKP2 mutation. After monolayer differentiation, plakophilin-2 deficiency led to reduced contractility, disrupted intercalated disc structures, and impaired desmosome assembly in iPSC-CMs. Allele-specific fluorescent labeling of endogenous DSG2 encoding desmoglein-2 in the generated isogenic lines enabled real-time desmosome-imaging under an adjusted dose of plakophilin-2. Adeno-associated virus-mediated gene replacement of PKP2 recovered contractility and restored desmosome assembly, which was sequentially captured by desmosome-imaging in plakophilin-2-deficient iPSC-CMs. Our isogenic set of iPSC-CMs recapitulates AC pathology and provides a rapid and convenient cellular platform for therapeutic development.

Background:Erythropoietin (EPO) has antiapoptotic and tissue-protective effects, but previous clinical studies using high-dose EPO have not shown cardioprotective effects, probably because of platelet activation and a lack of knowledge regarding the optimal dose. In contrast, a small pilot study using low-dose EPO has shown improvement in left ventricular function without adverse cardiovascular events. Methods and Results:We performed a multicenter (25 hospitals), prospective, randomized, double-blind, placebo-controlled, dose-finding study to clarify the efficacy and safety of low-dose EPO in patients with ST-segment elevation myocardial infarction (STEMI) under the Evaluation System of Investigational Medical Care of the Ministry of Health, Labor and Welfare of Japan. In total, 198 STEMI patients with low left ventricular ejection fraction (LVEF <50%) were randomly assigned to receive intravenous administration of EPO (6,000 or 12,000 IU) or placebo within 6 h of successful percutaneous coronary intervention. At 6 months, there was no significant dose-response relationship in LVEF improvement among the 3 groups tested (EPO 12,000 IU: 5.4±9.3%, EPO 6,000 IU: 7.3±7.7%, Placebo: 8.1±8.3%, P=0.862). Low-dose EPO also did not improve cardiac function, as evaluated by ^{99 m}Tc-MIBI SPECT or NT-proBNP at 6 months and did not increase adverse events. Conclusions:Administration of low-dose EPO did not improve LVEF at 6 months in STEMI patients (UMIN000005721).