HuSciAdv'17.pdf

Obscurins are cytoskeletal proteins with structural and regulatory roles encoded by OBSCN. Mutations in OBSCNare associated with the development of hypertrophic cardiomyopathy (HCM) and dilated cardiomyopathy(DCM). Specifically, the R4344Q mutation present in immunoglobulin domain 58 (Ig58) was the first to be linkedwith the development of HCM. To assess the effects of R4344Q in vivo, we generated the respective knock-inmouse model. Mutant obscurins are expressed and incorporated normally into sarcomeres. The expressionpatterns of sarcomeric and Ca2+-cycling proteins are unaltered in sedentary 1-year-old knock-in myocardia, withthe exception of sarco/endoplasmic reticulum Ca2+ adenosine triphosphatase 2 (SERCA2) and pentameric phospholambanwhose levels are significantly increased and decreased, respectively. Isolated cardiomyocytes from1-year-old knock-in hearts exhibit increased Ca2+-transients and Ca2+-load in the sarcoplasmic reticulum andfaster contractility kinetics. Moreover, sedentary 1-year-old knock-in animals develop tachycardia accompaniedby premature ventricular contractions, whereas 2-month-old knock-in animals subjected to pressureoverload develop a DCM-like phenotype. Structural analysis revealed that the R4344Q mutation alters thedistribution of electrostatic charges over the Ig58 surface, thus interfering with its binding capabilities.Consistent with this, wild-type Ig58 interacts with phospholamban modestly, and this interaction is markedlyenhanced in the presence of R4344Q. Together, our studies demonstrate that under sedentary conditions, theR4344Q mutation results in Ca2+ deregulation and spontaneous arrhythmia, whereas in the presence ofchronic, pathological stress, it leads to cardiac remodeling and dilation. We postulate that enhanced bindingbetween mutant obscurins and phospholamban leads to SERCA2 disinhibition, which may underlie the observedpathological alterations