Background: Hypertrophic cardiomyopathy (HCM) is caused by pathogenic variants in sarcomere protein genes that evoke hypercontractility, poor relaxation, and increased energy consumption by the heart and increased patient risks for arrhythmias and heart failure. Recent studies show that pathogenic missense variants in myosin, the molecular motor of the sarcomere, are clustered in residues that participate in dynamic conformational states of sarcomere proteins. We hypothesized that these conformations are essential to adapt contractile output for energy conservation and that pathophysiology of HCM results from destabilization of these conformations. Methods: We assayed myosin ATP binding to define the proportion of myosins in the super relaxed state (SRX) conformation or the disordered relaxed state (DRX) conformation in healthy rodent and human hearts, at baseline and in response to reduced hemodynamic demands of hibernation or pathogenic HCM variants. To determine the relationships between myosin conformations, sarcomere function, and cell biology, we assessed contractility, relaxation, and cardiomyocyte morphology and metabolism, with and without an allosteric modulator of myosin ATPase activity. We then tested whether the positions of myosin variants of unknown clinical significance that were identified in patients with HCM, predicted functional consequences and associations with heart failure and arrhythmias. Results: Myosins undergo physiological shifts between the SRX conformation that maximizes energy conservation and the DRX conformation that enables cross-bridge formation with greater ATP consumption. Systemic hemodynamic requirements, pharmacological modulators of myosin, and pathogenic myosin missense mutations influenced the proportions of these conformations. Hibernation increased the proportion of myosins in the SRX conformation, whereas pathogenic variants destabilized these and increased the proportion of myosins in the DRX conformation, which enhanced cardiomyocyte contractility, but impaired relaxation and evoked hypertrophic remodeling with increased energetic stress. Using structural locations to stratify variants of unknown clinical significance, we showed that the variants that destabilized myosin conformations were associated with higher rates of heart failure and arrhythmias in patients with HCM. Conclusions: Myosin conformations establish work-energy equipoise that is essential for life-long cellular homeostasis and heart function. Destabilization of myosin energy-conserving states promotes contractile abnormalities, morphological and metabolic remodeling, and adverse clinical outcomes in patients with HCM. Therapeutic restabilization corrects cellular contractile and metabolic phenotypes and may limit these adverse clinical outcomes in patients with HCM.

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.

Sarcomere cardiomyopathies are genetic diseases that perturb contractile function and lead to hypertrophic or dilated myocardial remodeling. Identification of preclinical mutation carriers has yielded insights into the earliest biomechanical defects that link pathogenic variants to cardiac dysfunction. Understanding this early molecular pathophysiology can illuminate modifiable pathways to reduce the emergence of overt cardiomyopathy and curb adverse outcomes. Here, the authors review current understandings of how human hypertrophic cardiomyopathy- and hypertrophic dilated cardiomyopathy-linked mutations disrupt the normal structure and function of the sarcomere.

Supplemental Digital Content is available in the text.

Human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) represent a powerful cellular platform for illuminating mechanisms of human cardiovascular disease and for pharmacological screening. Recent advances in CRISPR/Cas9-mediated genome editing technology underlie this profound utility. We have generated hiPSC-CMs harboring fluorescently-tagged sarcomeric proteins, which provide a tool to non-invasively study human sarcomere function and dysfunction. In this unit, we illustrate methods for conducting high-efficiency, small molecule-mediated differentiation of hiPSCs into cardiomyocytes, and for performing non-invasive contractile analysis through direct sarcomere tracking of GFP-sarcomere reporter hiPSC-CMs. We believe that this type of analysis can overcome sensitivity problems found in other forms of contractile assays involving hiPSC-CMs by directly measuring contractility at the fundamental contractile unit of the hiPSC-CM, the sarcomere. © 2018 by John Wiley & Sons, Inc.

Background - To assess the genetic architecture of hypertrophic cardiomyopathy (HCM) in patients of predominantly Chinese ancestry. Methods - We sequenced HCM disease genes in Singaporean patients (n=224) and Singaporean controls (n=3,634), compared findings with additional populations and Caucasian HCM cohorts (n=6,179) and performed in vitro functional studies. Results - Singaporean HCM patients had significantly fewer confidently interpreted HCM disease variants (Pathogenic (P)/Likely Pathogenic (LP):18%, p<0.0001) but an excess of variants of unknown significance (exVUS: 24%, p<0.0001), as compared to Caucasians (P/LP: 31%, exVUS: 7%). Two missense variants in thin filament encoding genes were commonly seen in Singaporean HCM ( TNNI3 :p.R79C, disease allele frequency (AF)=0.018; TNNT2 :p.R286H, disease AF=0.022) and are enriched in Singaporean HCM when compared with Asian controls ( TNNI3 :p.R79C, Singaporean controls AF=0.0055, p=0.0057, gnomAD-East Asian (gnomAD-EA) AF=0.0062, p=0.0086; TNNT2 :p.R286H, Singaporean controls AF=0.0017, p<0.0001, gnomAD-EA AF=0.0009, p<0.0001). Both these variants have conflicting annotations in ClinVar and are of low penetrance ( TNNI3 :p.R79C, 0.7%; TNNT2 :p.R286H, 2.7%) but are predicted to be deleterious by computational tools. In population controls, TNNI3 :p.R79C carriers had significantly thicker left ventricular walls compared to non-carriers while its etiological fraction is limited (0.70, 95% CI: 0.35-0.86) and thus TNNI3 :p.R79C is considered a VUS. Mutant TNNT2 :p.R286H iPSC-CMs show hypercontractility, increased metabolic requirements and cellular hypertrophy and the etiological fraction (0.93, 95% CI: 0.83-0.97) support the likely pathogenicity of TNNT2 :p.R286H. Conclusions - As compared to Caucasians, Chinese HCM patients commonly have low penetrance risk alleles in TNNT2 or TNNI3 but exhibit few clinically actionable HCM variants overall. This highlights the need for greater study of HCM genetics in non-Caucasian populations.

In adult obesity low-grade systemic inflammation is considered an important step in the pathogenesis of insulin resistance (IR). The association between obesity and inflammation is less well established in adolescents. Here, we ascertain the importance of inflammation in IR among obese adolescents by utilizing either Random Forest (RF) classification or mediation analysis approaches. The inflammation balance score, composed of 8 pro- and anti-inflammatory makers, as well as most of the individual inflammatory markers differed significantly between lean and overweight/obese. In contrast, adiponectin was the only individual marker selected as a predictor of IR by RF, and the balance score only revealed a medium to low importance score. Neither adiponectin nor the inflammation balance score were found to mediate the relationship between obesity and IR. These findings do not support the premise that low grade systemic inflammation is a key for the expression of IR in the human. Prospective longitudinal studies should confirm these findings.

Variants that drive HCM and associated adverse patient outcomes are found in the cardiac sarcomere. These variants range from those that are known to be pathogenic to those that are likely pathogenic or even variants of unknown significance (VUS). CRISPR/Cas-9 engineering has accelerated our ability to generate variants in iPSC to probe changes in cellular function and assess cellular pathogenicity in VUSs. However, iPSC-CMs are not as functionally mature as adult cardiomyocytes. For this reason we have developed a platform to assess contractile function directly at the level of the sarcomere. We use a custom built MatLab algorithm to assess sarcomere length, contraction time, relaxation time, and beat rate of individual sarcomeres within iPSC-CMs. Sarcomeres are visualised using reporter lines that have been engineered with an N-terminal TTN-GFP. We assess the contractile function of thick filament variants in MYH7 and MYBPC3. We show the ability to detect changes in key contractile parameters. This platform allows the screening of pharmacological compounds against these reporter lines with engineered variants.