Abstract 402: Defining Diverse Disease Pathomechanisms Across Thick And Thin Filament Hypertrophic Cardiomyopathy Variants

Hypertrophic cardiomyopathy (HCM) affects as many as ~1 in 500 individuals, and is often typified by hyperdynamic contraction and poor cellular relaxation. HCM can be caused by mutations in a variety of key contractile proteins of the sarcomere. A large proportion of these variants are found in MYBPC3, MYH7, TNNT2, and TNNI3. These genes encode proteins that control cardiac muscle contraction at the thick (MYBPC3 and MYH7) and thin filaments (TNNT2 and TNNI3) of the sarcomere. In this study we use human induced pluripotent stem cell derived cardiomyocytes to model HCM across all of these genes. We do this to define key mechanistic differences between thick and thin filament HCM. We define sarcomeric contractility (SarcTrack) calcium transients (CalTrack) and myosin states using the mant-ATP assay. We use the parametric data from these experimental studies in iPSC-CMs to model possible disease mechanisms in silico. Our experimental analysis highlights that both thick and thin filament HCM variants cause cellular hypercontractility, with slowed cellular relaxation. We find that thick filament HCM variants drive cellular HCM phenotypes by destabilising the myosin interacting heads motif (IHM), showing a marked reduction in the super relaxed state of myosin. Counterintuitively thin filament based HCM variants show a reduction in DRX myosin. When applying Mavacamten the allosteric myosin ATPase inhibitor to our thin and thick filament HCM variant iPSC-CMs we find a dichotomy of cellular responses. The thick filament variants studied all show a clear resolution of cellular HCM. However, not all cellular phenotypes of thin filament HCM are corrected by Mavacamten treatment, although there is benefit. We conclude that causal mechanisms of thick filament HCM are well corrected at the molecular and cellular level by Mavacamten, but these causal mechanisms in thin filament based HCM are not suitably corrected. We highlight key mechanistic pharmacological targets for thin filament variants that could add cellular benefit to HCM phenotype resolution.

Myosin Sequestration Regulates Sarcomere Function, Cardiomyocyte Energetics, and Metabolism, Informing the Pathogenesis of Hypertrophic Cardiomyopathy

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.

Abstract 360: Implantation of an Induced Pluripotent Stem Cell Derived Cardiomyocyte Tissue Engineered Patch Improves Left Ventricular Function and Electro-mechanical Coupling in Rats With Heart Failure

Background: Chronic Heart Failure (CHF) is the leading cause of hospital readmissions and mortality in the US. Here we report the effects of surgically delivering a human bioengineered patch of human induced pluripotent stem cells derived cardiomyocytes (hiPSC-CMs) and fibroblasts on left ventricular (LV) function in rats with CHF. We evaluate improvements in LV systolic and diastolic function, electromechanical coupling and gene expression after patch implantation.

Methods: Adult male Sprague-Dawley rats underwent left coronary artery ligation and were randomized to Sham (N=8), CHF (N=8-21), and CHF+hiPSC-CM patch (N=20-24). Heterogeneous hiPSC-CMs were seeded and co-cultured onto a vicryl matrix embedded with human dermal fibroblasts. Echocardiography was performed at 3 and 6 weeks post-randomization. Hemodynamic pressure measurements were performed at 6 weeks post-ligation with Millar solid state micromanometer pressure catheters. Open chest Electrophysiologic (EP) mapping was performed at 6 weeks post ligation. Gene expression was evaluated through qRT-PCR.

Results: 48 hours into culture hiPSC-CMs patches displayed synchronized and spontaneous contractions which developed in robustness over time. At maximal robustness, contractions were visualized across the full thickness of the construct. Contractions were recorded at 36 + 5 beats BPM. Three weeks after patch implantation (6 weeks post ligation) the hiPSC-CM patch decreased (P<0.05) LV EDP (45%), Tau (29%), E/e’ (23%) and increased (P<0.05), e’/a’ (36%) with trending improvements in EF (14%) and e’ (20%). EP studies show electro-mechanical coupling between the patch and the native myocardium with normal activation through the patch and increases (P<0.05) voltage amplitude in CHF versus hiPSC-CM patch treated rats (1±0.5 mV vs 6±1.5mV). Rats treated with the hiPSC-CM patch showed significant (P<0.05) fold expression of Cx43 (3.3), ANG-1 (13.63), VEGF (3.8), βMYH7 (6.4) and IGF-1 (22.9) versus control.

Conclusion: Cardiac patch implantation with hiPSC derived cardiomyocytes is an effective and feasible method of treating CHF with improvements in systolic function, diastolic function, and electro-mechanical coupling in rats with CHF.

Long-term administration of DDT or phenobarbital-Na in Wistar rats

In lifespan studies, outbred male and female Wistar rats were given either technical DDT mixed into the diet at a dose of 500 parts per million (ppm) or phenobarbital-sodium dissolved in drinking water at a dose of 500 ppm. Liver-cell tumors developed in treated animals but not in controls. The incidence of liver tumors was 45% in the DDT-treated group and 44% in the phenobarbital-sodium-group. When evaluated by sex, DDT-treated females and males had incidences of 56% and 35%, respectively, while in the phenobarbital-sodium group, the respective incidences were 32% and 59% in females and males. These data show a varying susceptibility between the sexes, with regard to induction of liver-cell tumors by the two compounds. In both treated groups, the number of nodular tumors per rat and the average size increased with age and were greater in females. None of these tumors and metastasized. Histologically, the liver tumors were nodular growths, which compressed surrounding parenchyma did not infiltrate it. The total incidence of extrahepatic tumors was higher in controls than in treated animals. In this connection, we must mention the apparent, but not significant, reduction of adrenal tumors in treated rats compared to the controls.