Loss of SORBS2 in Cardiomyocytes Leads to Dilated Left Ventricle Cardiomyopathy in Mice
Authors: McLendon Jared M. , Zhang Xiaoming , Matasic Daniel S. , London Barry , Boudreau Ryan L. 
Published: April 2020

Through rigorous bioinformatic and wet‐lab investigations, we identified SORBS2 as a cardiac‐enriched gene that is consistently upregulated at mRNA and protein levels across diverse mouse models of heart failure (HF; genetic, ischemic, pressure‐overload) and in failing human hearts (ischemic, idiopathic). Significant reduction in SORBS2 protein levels are also found in aged heart. SORBS2 is among a family of adaptor proteins (SORBS1‐3) involved in cell adhesion and cytoskeletal organization; SORBS proteins localize to myocyte z‐discs and aid in aligning sarcomere and mitochondrial networks (Loveless et al. 2017 MBC). Global SORBS2 knockout mice display early death (100% at 4–6 months), potentially due to cardiac arrhythmia and right ventricle (RV) cardiomyopathy (Ding et al. 2019 BioRxiv). Beyond this, the role of SORBS2 in cardiomyocyte biology remains largely untested. We created a cardiomyocyte‐specific knockout of SORBS2 (SORBS2‐cKO) via MYH6‐CRE transgenic mice bred to floxed‐SORBS2 mice. To delete SORBS2 postnatally and separate the role of SORBS2 in adult cardiac function from development, floxed‐SORBS2 mice were also injected at 3 weeks of age with AAV2/9‐CRE virus. Heart function was measured from 3 months thru one year of age. Contrary to the work of Ding, et al., we report here that neither constitutive nor inducible cardiomyocyte‐restricted SORBS2 knockout leads to severe arrhythmias, RV dysfunction, or early death in mice. Instead, SORBS2‐cKO mice develop aging‐related left ventricle (LV) cardiomyopathy starting with LV wall motion abnormalities at ~ 8 months‐old, progressing to ~50% reduced LVEF and 30% increased HW/BW by 10 months‐old, and dilating with heart failure by 12 months (~400% increase LVEDV), with no detectable change to RV structure/function. Surface EKG recordings in SORBS2‐cKO mice show obvious conduction deficiencies including bifid P‐wave, prolonged QRS, and bundle branch block, starting at 3 months‐age and persisting through life. Notably, no mice showed irregular R‐R intervals, spontaneous atrial fibrillation or ventricular tachycardia. While exploring the mechanism of SORBS2 loss leading to conduction defects, we did not find reduced gap junction protein GJA1 levels in SORBS2‐cKO mouse hearts. This is interesting given that Ding, et al., reported a 90% decrease in GJA1 protein levels in global SORBS2‐KO mouse hearts. Consistent with prior literature, our bioinformatic analyses show a strong association for SORBS2 with cytoskeletal proteins and also point to new potential connections to cardiac signaling receptors/channels including SCN5A. Electrophysiology studies were conducted in isolated neonatal rat cardiomyocytes with siRNA‐mediated knockdown of SORBS2; however, no changes in SCN5A current properties were found. Altogether, these results highlight the importance of SORBS2 to cardiomyocyte biology likely through its primary role as a cytoskeletal adapter. Necessary, ongoing work will continue to precisely define how loss of SORBS2 in cardiomyocytes leads to dilated cardiomyopathy, assessing sarcomere organization, excitation‐contraction coupling, and cardiomyocyte force mechanics.Support or Funding InformationThis work was funded by 19POST34380640 to JMM, HL137272 to DSM, HL147545 to BL, and HL144717 and HL148796 to RLB.