HuSciAdv'17.pdf

Obscurins are cytoskeletal proteins with structural and regulatory roles encoded by OBSCN. Mutations in OBSCN
are 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 linked
with the development of HCM. To assess the effects of R4344Q in vivo, we generated the respective knock-in
mouse model. Mutant obscurins are expressed and incorporated normally into sarcomeres. The expression
patterns of sarcomeric and Ca2+-cycling proteins are unaltered in sedentary 1-year-old knock-in myocardia, with
the exception of sarco/endoplasmic reticulum Ca2+ adenosine triphosphatase 2 (SERCA2) and pentameric phospholamban
whose levels are significantly increased and decreased, respectively. Isolated cardiomyocytes from
1-year-old knock-in hearts exhibit increased Ca2+-transients and Ca2+-load in the sarcoplasmic reticulum and
faster contractility kinetics. Moreover, sedentary 1-year-old knock-in animals develop tachycardia accompanied
by premature ventricular contractions, whereas 2-month-old knock-in animals subjected to pressure
overload develop a DCM-like phenotype. Structural analysis revealed that the R4344Q mutation alters the
distribution 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 markedly
enhanced in the presence of R4344Q. Together, our studies demonstrate that under sedentary conditions, the
R4344Q mutation results in Ca2+ deregulation and spontaneous arrhythmia, whereas in the presence of
chronic, pathological stress, it leads to cardiac remodeling and dilation. We postulate that enhanced binding
between mutant obscurins and phospholamban leads to SERCA2 disinhibition, which may underlie the observed
pathological alterations.