Abstract 9603: Polycomb Group Protein Cbx7 Represses Cardiomyocyte Proliferation via Modulation of the Tardbp/Rbm38 Axis

Introduction: Cardiomyocyte (CM) proliferation notably decreases during the perinatal period. Regulatory mechanisms for this loss of proliferative capacity are poorly understood. CBX7, a polycomb group (PcG) protein, regulates the cell cycle but its role in CM proliferation is unknown. Methods: We profiled CBX7 expression in the mouse hearts via qRT-PCR, western blotting, and immunohistochemistry. We overexpressed CBX7 in neonatal mouse CMs via adenoviral transduction. We knocked down CBX7 by using constitutive and inducible conditional knockout mice (Myh6-cre;Cbx7 fl/+ and Myh6-MCM;Cbx7 fl/fl , respectively). We measured CM proliferation by immunostaining of proliferation markers such as Ki67, phospho-histone 3, and cyclin B1. We examined the mechanism of CBX7-mediated repression of CM proliferation via co-immunoprecipitation, mass spectrometry, and other molecular techniques. Results: The mRNA expression of Cbx7 was increased at the perinatal stage and sustained in the postnatal heart (fold increase of adult vs. prenatal: 32.1 ± 3.3, P < 0.01). Overexpression of CBX7 in neonatal mouse CMs reduced CM proliferation (% of Ki67 + CMs: Ad-Mock, 22.3 ± 1.7 vs. Ad-CBX7, 7.7 ± 0.7, P < 0.001). The haplodeficiency of CBX7 (Tnnt2-Cre;Cbx7 fl/+ ) enhanced proliferation of neonatal CMs in vivo (% of Ki67 + CMs: wild-type, 7.9 ± 0.9% vs. mutants, 24.7 ± 1.2%, P < 0.01), leading to increased myocardial wall thickness, cardiomegaly and neonatal lethality. Genetic deletion of CBX7 in CMs (Myh6-MCM;Cbx7 fl/fl ) at P1 resulted in increased CM proliferation (% of Ki67 + CMs: vehicle, 9.5 ± 1.4% vs. tamoxifen, 18.3 ± 1.8%, P < 0.01), leading to cardiomegaly at 3 months. Mechanistically, CBX7 interacted with TAR DNA-binding protein 43 (TARDBP) and positively regulated its downstream target, RNA Binding Motif Protein 38 (RBM38). Rbm38 was perinatally upregulated in the mouse hearts and overexpression of RBM38 reduced proliferation of neonatal CMs. Conclusions: CBX7 expression is perinatally increased in the mouse hearts and inhibits proliferation of CMs by controlling TARDBP/Rbm38 pathway. This is the first study to demonstrate the role of CBX7 in regulation of CM proliferation and CBX7 could be an important target for cardiac regeneration.