Obscurin: A New Player in Cardiac Hypertrophy

Abstract

Obscurins, encoded by the single OBSCN gene, comprise a family of giant (∼890-810 kDa) and small (∼550-50 kDa) proteins of vertebrate striated muscles composed of adhesion and signaling motifs. Giant obscurins intimately surround sarcomeres at the level of M-bands and Z-disks where they participate in the assembly, stabilization, and integration of the contractile cytoskeleton with other sarcoplasmic structures. Consistent with this, the immunoglobulin (Ig) domains 58 and 59 of obscurins interact directly with Ig domains 9 and 10 of titin located at the periphery of Z-disks. Genomic linkage analysis has recently revealed a missense mutation (R4344Q) in obscurin Ig58 that is causally linked to hypertrophic cardiomyopathy (HCM). To examine how the R4344Q mutation leads to the development of HCM, we generated two animal models: a knock-in model that contains full length obscurins carrying the R4344Q mutation, and a partial knock-out model that lacks Ig 58 and 59. Immunoblot and immunofluorescence analysis indicated that both mutant and truncated obscurins are readily expressed in cardiac muscles, and properly incorporated into sarcomeres. Homozygous partial knock-out mice developed overt cardiac hypertrophy by 12 months of age, as measured by echocardiography; notably, hypertrophy was exacerbated in the female homozygous animals. While a hypertrophic trend was apparent in homozygous knock-in animals, phenotypic and functional alterations of the affected hearts were not statistically significant from those of wild-type animals. Importantly, trans-aortic constriction of ∼2 months old knock-in and partial knock-out male and female mice led to severe cardiac hypertrophy within 4-8 weeks post-surgery, as evaluated by echocardiography. We are currently examining the cellular and biochemical manifestations of mutant and truncated obscurins, as related to the development of HCM. Our studies provide the first in vivo models to study the molecular defects that underlie HCM due to altered obscurins.