Molecular Mechanisms and Cellular Models of Hypertrophic Cardiomyopathy: Insights from a Surprising Mutation

Abstract

Hypertrophic cardiomyopathy (HCM) is the most common inherited form of heart disease, associated with over 1000 mutations, a third of which are in β-cardiac myosin (MYH7). Molecular studies of myosin with different HCM mutations have revealed a surprising diversity of effects on ATPase and load-sensitive rate of detachment from actin. It is difficult to predict how such diverse molecular effects combine to influence forces at the cellular level and how these forces further influence cellular phenotypes. We focused on the P710R mutation, which, in contrast to other MYH7 mutations, dramatically decreases in vitro motility and actin-activated ATPase. Using harmonic force spectroscopy, we further showed that this mutation reduces the step size of the myosin motor and also reduces the load-sensitivity of the actin detachment rate, which may cause dynamic, context dependent effects on force. Conversely, single ATP turnover studies of two-headed myosin constructs revealed that this mutation destabilizes the super-relaxed state, freeing more heads to generate force. Gene-edited micropatterned hiPSC-cardiomyocytes with the P710R mutation produced significantly increased force compared with isogenic control cells (measured by traction force microscopy). We used a computational model to integrate measured molecular changes and predict force traces matching the forces measured in cells. This model confirmed a key role for regulation of the super-relaxed state in driving hypercontractility. The P710R mutation also caused cardiomyocyte hypertrophy and cytoskeletal remodeling, measured by immunostaining and electron microscopy. Cellular hypertrophy was prevented in the P710R cells by inhibition of ERK or Akt, signaling proteins involved in mechanosensitive, hypertrophic signaling. Our multiscale approach reveals key mechanisms of disease in response to this unusual mutation and provides a framework which can be extended to new mutations.