Abstract 17134: Mycn is a Regulator of Cardiomyocyte Proliferation and Regeneration in the Mammalian Heart

The adult mammalian heart has limited regeneration in response to injury. Therefore, we focused on defining new factors and mechanisms that could enhance repair and regeneration in the adult mouse heart. MYCN is a protooncogene that regulates embryonic heart development. We identified MYCN as a downstream effector of the sonic hedgehog (SHH) signaling cascade during cardiomyocyte proliferation. However, its role in heart regeneration is unknown and warrants further investigation. Here, we describe a novel role for MYCN as a regulator of cardiomyocyte (CM) proliferation and regeneration using genetic mouse models and in vivo modified RNA (modRNA) delivery. By crossing Mycn floxed mice with the Myh6 -MerCreMer mice ( Mycn CKO), we conditionally deleted Mycn in the neonatal heart following administration of 4-hydroxy-tamoxifen (4-OHT) starting at P0. Successful deletion of Mycn was demonstrated by qPCR analysis of P2 isolated CMs. Immunohistochemical (IHC) analyses of P3 neonatal hearts in the absence of MYCN demonstrated a significant decrease in Ki67 expression and pH3 detection. This observation was validated in isolated P7 CMs and hearts pulsed with EdU which demonstrated a significant decrease in EdU+ CMs in the absence of MYCN compared to control. We then performed LAD coronary artery ligation surgeries in control and Mycn CKO P2 neonatal mice. Echocardiography one month following LAD coronary artery ligations demonstrated that the Mycn CKO failed to recover baseline cardiac function compared to their control counterparts who achieved complete recovery measured by ejection fraction (EF) and fractional shortening (FS). RNAseq analysis of control and Mycn CKO hearts one month following LAD ligations demonstrated that muscle developmental pathways were significantly downregulated in the Mycn CKO group compared to the control. We then delivered either GFP (control) or Mycn modRNA intramuscularly into the border zone following LAD coronary artery ligation injuries in adult (2-6 month old) mice. We observed that the Mycn modRNA treated mice had significantly improved cardiac function at one month and two months following injury/delivery compared to controls. These results identify MYCN as a regulator of CM proliferation and heart regeneration.