The homozygous K280N troponin T mutation alters cross-bridge kinetics and energetics in human HCM

Abstract

Hypertrophic cardiomyopathy (HCM) is a genetic form of left ventricular hypertrophy, primarily caused by mutations in sarcomere proteins. The cardiac remodeling that occurs as the disease develops can mask the pathogenic impact of the mutation. Here, to discriminate between mutation-induced and disease-related changes in myofilament function, we investigate the pathogenic mechanisms underlying HCM in a patient carrying a homozygous mutation (K280N) in the cardiac troponin T gene (TNNT2), which results in 100% mutant cardiac troponin T. We examine sarcomere mechanics and energetics in K280N-isolated myofibrils and demembranated muscle strips, before and after replacement of the endogenous troponin. We also compare these data to those of control preparations from donor hearts, aortic stenosis patients (LVHao), and HCM patients negative for sarcomeric protein mutations (HCMsmn). The rate constant of tension generation following maximal Ca2+ activation (kACT) and the rate constant of isometric relaxation (slow kREL) are markedly faster in K280N myofibrils than in all control groups. Simultaneous measurements of maximal isometric ATPase activity and Ca2+-activated tension in demembranated muscle strips also demonstrate that the energy cost of tension generation is higher in the K280N than in all controls. Replacement of mutant protein by exchange with wild-type troponin in the K280N preparations reduces kACT, slow kREL, and tension cost close to control values. In donor myofibrils and HCMsmn demembranated strips, replacement of endogenous troponin with troponin containing the K280N mutant increases kACT, slow kREL, and tension cost. The K280N TNNT2 mutation directly alters the apparent cross-bridge kinetics and impairs sarcomere energetics. This result supports the hypothesis that inefficient ATP utilization by myofilaments plays a central role in the pathogenesis of the disease.

Pressure Overload Is Associated With Low Levels of Troponin I and Myosin Binding Protein C Phosphorylation in the Hearts of Patients With Aortic Stenosis

“…We have also studied a heart muscle sample from a patient with an HCM-causing mutation [homozygous TNNT2 K280N (Sequeira et al, 2013;Messer et al, 2016;Piroddi et al, 2019)]. The patient had a myectomy operation that relieved LVOTO permanently but later developed heart failure requiring a heart transplant.…”

The relation between sarcomere energetics and the rate of isometric tension relaxation in healthy and diseased cardiac muscle

“…Unpublished data Table 1 Kinetic parameters of mouse and human ventricular myofibrils following replacement by exchange of the endogenous cTn complex with human recombinant cTn Data are mean ± SEM; n ≥ 10. Data from human cTn exchanged mouse myofibrils and control (sham-treated) mouse myofibrils are unpublished data; data from human cTn exchanged human ventricular myofibrils are from Piroddi et al (2019). The exchange protocol allows replacement of 70-90% of the endogenous cTn complex Myofibril type To better interpret myofibril tension kinetic parameters in terms of apparent CB kinetics we need a description of the reaction pathway for acto-myosin ATPase and energy-transduction cycle.…”

Modeling Hypertrophic Cardiomyopathy: Mechanistic Insights and Pharmacological Intervention

“…Remarkably, higher energy cost in HCM muscle strips appears to be a feature shared by different mutations. This was recently corroborated in homozygous K280N-troponin T samples, which showed 24-72% higher values than three different control groups, that was ascribed to faster cross-bridge detachment [34].…”