A gradient of force generation at rest differentiates cardiomyopathy outcomes with variants of actin located at the same residue

The calcium sensitivity hypothesis helps explain the development of different forms of cardiomyopathy: increased sensitivity to calcium in cardiac sarcomeres leads to hypertrophic cardiomyopathy (HCM) and decreased sensitivity results in dilated cardiomyopathy (DCM). This hypothesis has driven the development of next generation drugs targeting sarcomere proteins to correct the amount of force generated as a result of changes in calcium sensitivity (e.g. mavacamten decreases cardiac myosin activity to treat HCM). Characterization of variants of cardiac actin (ACTC) found in patients with HCM or DCM has generally supported the calcium sensitivity hypothesis. Of interest are two different substitution mutations at R312 on ACTC: R312H leads to DCM, while R312C was found in patients with HCM. To determine how changes in the same codon on the same gene lead to different disease phenotypes, we characterized recombinant R312H- and R312C-ACTC variant proteins. Both variants exhibited the same change in calcium sensitivity, suggesting that a factor other than calcium sensitivity is responsible for disease differentiation. We observed a gradient of increased residual myosin activity with R312-ACTC variant proteins under relaxing conditions which may trigger different disease development. Our findings suggest that factors other than calcium sensitivity may contribute to cardiomyopathy development and should be considered when planning treatments.