Myosin motor domains carrying mutations implicated in early or late onset Hypertrophic Cardiomyopathy have similar properties

Abstract

Hypertrophic Cardiomyopathy (HCM) is a common genetic disorder that typically involves left ventricular hypertrophy and cardiac hypercontractility. Mutations in β cardiac myosin heavy chain (β-MyHC) are a major cause of HCM, but the specific mechanistic changes to myosin function that lead to the disease remain incompletely understood. Predicting the severity of any single β-MyHC mutation is hindered by a lack of detailed evaluation at the molecular level. In addition, since the cardiomyopathy can take 20 or more years to develop, the severity of the mutations must be somewhat subtle. We hypothesized that mutations which result in early onset disease may show more severe molecular changes in function compared to later onset mutations. In this work, we performed steady-state and transient kinetic analyses of myosins carrying 1 of 7 missense mutations in the motor domain. Of these 7, 4 have been identified in early onset cardiomyopathy screens. The derived parameters were used to model the ATP driven cross-bridge cycle. Contrary to our hypothesis, the results show no clear differences between early and late onset HCM mutations. Despite the lack of distinction between early and late onset HCM, the predicted occupancy of the force-holding actin.myosin.ADP complex at [Actin] = 3 Kapp along with the closely related Duty Ratio (DR; fraction of myosin in strongly attached force-holding states) and the measured ATPases all change in parallel (in both sign and degree of change) compared to wild type (WT) values. Six of the 7 HCM mutations are clearly distinct from a set of DCM mutations previously characterized.

A role for actin flexibility in thin filament-mediated contractile regulation and myopathy

“…Additionally, Cassell and Tobacman 65 indicated that myosin-heads increase the affinity of Tpm for F-actin and Tpm enhances myosin binding to M-state actin, possibly through direct myosin-Tpm interactions 66,67 . Therefore, the M305L mutation could affect the S1 catalytic cycle through effects on F-actin-Tpm-myosin associations, and promote excessive force production and HCM 68 . However, these Tpm-myosin interactions were not tested in our in silico studies, nor do our experimental results, acquired from mutant IFMs, necessarily suggest they are impacted.…”

Hypertrophic cardiomyopathy ß-cardiac myosin mutation (P710R) leads to hypercontractility by disrupting super-relaxed state

“…k0,  and step size were measured from single molecules using the HFS technique. kcat and Kapp were measured using a colormetric actin-activated ATPase assay and were previously reported [16]. Unloaded velocities were measured by the motility assay with actin filaments.…”

Human hypertrophic cardiomyopathy mutation R712L suppresses the working stroke of cardiac myosin and can be rescued by omecamtiv mecarbil

“…Many MYH7 mutations examined in biophysical and physiological studies have been found to cause changes in individual kinetic steps that impact the fraction of intermediates in force producing states. While some confer apparent hypercontractile activity, no uniform kinetic signature for HCM has emerged from these studies (e.g., [6][7][8]). Thus, it is not clear that all HCM mutations in the myosin motor conform to the hypercontractile hypothesis.…”