Summary Immune checkpoint inhibitors significantly improve clinical outcomes in numerous malignancies, but high-grade immune-related adverse events can occur, particularly with combination immunotherapy. Herein, we report two melanoma patients who developed fatal myocarditis following treatment with ipilimumab and nivolumab. Both patients developed myositis with rhabdomyolysis, early progressive and refractory cardiac electrical instability, and myocarditis with robust T-cell and macrophage infiltrates. Selective clonal T-cell populations infiltrating the myocardium were identical to those present in tumor and skeletal muscle. Pharmacovigilance data revealed that myocarditis occurred in 0.27% of patients treated with ipilimumab/nivolumab, suggesting this is a rare, potentially fatal, T-cell-driven drug reaction.

Cardiovascular disease is the leading cause of death worldwide. Advanced insights into disease mechanisms and therapeutic strategies require deeper understanding of the healthy heart’s molecular processes. Knowledge of the full repertoire of cardiac cells and their gene expression profiles is a fundamental first step in this endeavor. Here, using state-of-the-art analyses of large-scale single-cell and nuclei transcriptomes, we characterise six anatomical adult heart regions. Our results highlight the cellular heterogeneity of cardiomyocytes, pericytes, and fibroblasts, revealing distinct atrial and ventricular subsets with diverse developmental origins and specialized properties. We define the complexity of the cardiac vasculature and its changes along the arterio-venous axis. In the immune compartment we identify cardiac resident macrophages with inflammatory and protective transcriptional signatures. Further, inference of cell-cell interactions highlight different macrophage-fibroblast-cardiomyocyte networks between atria and ventricles that are distinct from skeletal muscle. Our human cardiac cell atlas improves our understanding of the human heart and provides a healthy reference for future studies.

Human mutations that truncate the massive sarcomere protein titin (TTNtv) are the most common genetic cause for dilated cardiomyopathy (DCM), a major cause of heart failure and premature death. Here we show that cardiac microtissues engineered from human induced pluripotent stem (iPS) cells are a powerful system for evaluating the pathogenicity of titin gene variants. We found that certain missense mutations, like TTNtv, diminish contractile performance and are pathogenic. By combining functional analyses with RNAseq, we explain why truncations in the A-band domain of TTN cause DCM while truncations in the I-band are better tolerated. Finally, we demonstrate that mutant titin protein in iPS-cardiomyocytes results in sarcomere insufficiency, impaired responses to mechanical and β-adrenergic stress, and attenuated growth factor and cell signaling activation. Our findings indicate that titin mutations cause DCM by disrupting critical linkages between sarcomerogenesis and adaptive remodelling.

Hypertrophic cardiomyopathy (HCM) is an inherited disease of heart muscle that can be caused by mutations in sarcomere proteins. Clinical diagnosis depends on an abnormal thickening of the heart, but the earliest signs of disease are hyperdynamic contraction and impaired relaxation. Whereas some in vitro studies of power generation by mutant and wild-type sarcomere proteins are consistent with mutant sarcomeres exhibiting enhanced contractile power, others are not. We identified a small molecule, MYK-461, that reduces contractility by decreasing the adenosine triphosphatase activity of the cardiac myosin heavy chain. Here we demonstrate that early, chronic administration of MYK-461 suppresses the development of ventricular hypertrophy, cardiomyocyte disarray, and myocardial fibrosis and attenuates hypertrophic and profibrotic gene expression in mice harboring heterozygous human mutations in the myosin heavy chain. These data indicate that hyperdynamic contraction is essential for HCM pathobiology and that inhibitors of sarcomere contraction may be a valuable therapeutic approach for HCM.

LAMP2 mutations typically cause multisystem glycogen-storage disease (Danon's disease) but can also present as a primary cardiomyopathy. The glycogen-storage cardiomyopathy produced by LAMP2 or PRKAG2 mutations resembles hypertrophic cardiomyopathy but is distinguished by electrophysiological abnormalities, particularly ventricular preexcitation.

Mutations in β-cardiac myosin, the predominant motor protein for human heart contraction, can alter power output and cause cardiomyopathy. However, measurements of the intrinsic force, velocity, and ATPase activity of myosin have not provided a consistent mechanism to link mutations to muscle pathology. An alternative model posits that mutations in myosin affect the stability of a sequestered, super relaxed state (SRX) of the protein with very slow ATP hydrolysis and thereby change the number of myosin heads accessible to actin. Here we show that purified human β-cardiac myosin exists partly in an SRX and may in part correspond to a folded-back conformation of myosin heads observed in muscle fibers around the thick filament backbone. Mutations that cause hypertrophic cardiomyopathy destabilize this state, while the small molecule mavacamten promotes it. These findings provide a biochemical and structural link between the genetics and physiology of cardiomyopathy with implications for therapeutic strategies.

Tetralogy of Fallot (TOF), the most common severe congenital heart malformation, occurs sporadically, without other anomaly, and from unknown cause in 70% of cases. A genome-wide survey of 114 TOF patients and their unaffected parents identified 11 de novo copy number variants (CNVs) that were absent or extremely rare (<0.1%) in 2,265 controls. A second, independent TOF cohort (n = 398) was then examined for additional CNVs at these loci. In 1% (5/512, p = 0.0002, OR = 22.3) of non-syndromic sporadic TOF cases we identified CNVs at chromosome 1q21.1. Recurrent CNVs were also identified at 3p25.1, 7p21.3 and 22q11.2. CNVs in a single TOF case occurred at six loci, two that encode known (NOTCH1, JAG1) disease genes. Our data predicts that at least 10% (4.5–15.5, 95% CI) of sporadic, non-syndromic TOF reflects de novo CNVs and implicates mutations within these loci as etiologic in other cases of TOF.

Activation of complex molecular programs in specific cell lineages governs mammalian heart development, from a primordial linear tube to a four-chamber organ. To characterize lineage-specific, temporal-spatial developmental programs, we performed single-cell RNA sequencing of >1200 murine cells isolated at seven time points spanning E9.5 (primordial heart tube) to P21 (mature heart). Using unbiased transcriptional data we classified cardiomyocytes (CM), endothelial cells (EC), and fibroblast-enriched cells, thus identifying markers for temporal and chamber-specific developmental programs. Harnessing these datasets, we defined developmental ages of human and mouse pluripotent stem cell-derived CMs and characterized lineage-specific maturation defects in hearts of mice with heterozygous mutations in Nkx2.5 that cause human heart malformations. This spatial-temporal transcriptome analysis of heart development reveals lineage-specific gene programs underlying normal cardiac development and congenital heart disease.