Abstract 35: Pseudo-knockin Mice Expressing JPH2-A399S Develop Cardiac Hypertrophy by Magnetic Resonance Imaging

Abstract

Background: Dysfunctional intracellular Ca2+ handling has been implicated in adverse cardiac remodeling leading to hypertrophy and failure. Recent evidence has linked mutations in the Ca2+ handling protein Junctophilin-2 (JPH2) with the development of hypertrophic cardiomyopathy (HCM). However, the mechanism remains unknown. Objective: To use advanced in vivo imaging modalities in conjunction with biochemical techniques to determine the mechanism of hypertrophic remodeling in a murine model hosting a novel JPH2 mutation. Methods and Results: 1. Pseudo-knockin (PKI) Mice: transgenic mice with the JPH2-A399S mutation (or WT JPH2) containing inducible shJPH2 were dosed with tamoxifen to knock down the combined levels of JPH2 to near WT expression. 2. MRI: A399S and WT PKI controls were imaged at 2 months post injection at which time body weight was similar for mutants (29.8±.81g) and controls (31.8±1.29g). Intragate (Bruker) was used to obtain FLASH cine images and EKG-gated tagged images were obtained to determine strain. MRI post-processing and measurements were performed using Amira and Diagnosoft software. A399S PKI mice exhibited significantly increased left ventricular mass (2.96±.21g/kg; n=4) compared to controls (2.27±.08g/kg; n=3) and max diastolic septal thickness (1.39±.05mm; n=9 versus 0.97±.00mm; n=4; P<0.01). Mutants trended toward decreased septal strain (-8.97±1.04; n=4) compared to the control (-10.28; n=1) indicative of reduced regional contractility 3. Biochemistry: stress markers were measured by qPCR. Average BNP was increased over 3 fold. Larger sample size is needed to reach significance. Western blot showed no significant change (p=.78) in phosphorylated Ca2+/calmodulin-dependent protein kinase II, which is often activated by aberrant Ca2+ signaling. Conclusions: Our data show that the JPH2-A399S mutation leads to septal hypertrophy in PKI mice. This suggests that defects in JPH2 are sufficient to induce pathological cardiac remodeling. Despite the role of JPH2 in Ca2+ handling, this form of hypertrophy does not appear to be mediated by traditional Ca2+ signaling. Further studies will focus on alternative Ca2+-dependent pathways to elucidate the molecular mechanisms of these hypertrophic changes.