Abstract 18695: C5ORF51/RIMOC1 - A Novel Regulator of Cardiogenesis

Abstract

Introduction: Adult human cardiomyocytes (CMs) have poor capacity for proliferation and functional recovery after heart attack, thus leading to the high morbidity and mortality of cardiovascular diseases. Thus, there is great need to understand the underlying mechanisms of adult CM turnover and maturation. While screening for modulators of TEAD1, the downstream transcriptional effector of Hippo pathway, my lab identified a novel factor—C5ORF51/RIMOC1—as a potential cardiomyogenic modulator. C5ORF51 (hereinafter referred to as C5x) is a previously uncharacterized protein of nearly unknown function, which appears to play critical roles in the regulation of CM homeostasis and heart size. Hypothesis: C5x is a novel cardiomyogenic regulator which determines CM turnover, maturation, and function. Methods and Results: (1) Ex vivo C5x deletion in mouse neonatal CMs leads to significantly increased percentages of cells in S-G2-M phases, suggesting that C5x mediates CM cell cycle arrest in a cell-autonomous manner; (2) In vivo CM-specific C5x-knockout mouse model shows significantly enhanced CM proliferation in adult mice as well as increased CM endowment and mononuclear diploid populations, indicating that C5x has an essential role in cell replication and division in vivo; (3) With age, C5x-CM-knockout mice develop systolic dysfunction, massive cardiomegaly, late-onset dilated cardiomyopathy, and premature mortality, demonstrating a critical role of C5x in maintaining normal cardiac function; (4) RNA sequencing analysis further reveals that C5x loss-of-function activates fetal gene expression while its gain-of-function suppresses fetal gene expression, suggesting C5x as a gatekeeper of the fetal gene program. Conclusions: C5x is required for CM cell cycle arrest, and its loss results in fetal gene activation leading to a primitive cell state and immature CM expansion.