Duality in disease: How two amino acid substitutions at actin residue 312 result in opposing forms of cardiomyopathy.

Two common types of cardiovascular disease are hypertrophic cardiomyopathy (HCM) and dilated cardiomyopathy (DCM) which occur from changes to sarcomere contractile mechanisms and activity. Actin amino acid substitutions R312C and R312H have been found in HCM and DCM patients, respectively. Previously, we observed that R312C/H variants display both hyperactivity and hypoactivity in vitro, contradicting traditional characterizations of HCM- and DCM-causing variants. Here, we further characterized R312C/H actin variants in vitro and conducted in silico modeling to better understand the mechanisms differentiating HCM and DCM. Our results suggest that R312C/H substitutions cause structural changes that differentially impact actomyosin activity. A gradient of altered interactions with regulatory proteins troponin, tropomyosin, and the C0C2 domains of myosin-binding protein C was also observed, influencing the accessibility of active and inhibitory conformations of these proteins. The results presented here support our previous suggestion of a gradient of factors that differentiate between HCM and DCM. Further characterization of HCM- and DCM-causing actin variants using in vitro and in silico methods is required for better understanding cardiomyopathy and improving clinical outcomes.