Abstract Body: Background: Marfan syndrome is an inherited genetic disorder affecting connective tissue, particularly in the thoracic aorta, where aneurysm rupture poses life-threatening risks. While mutations in the FBN1 (Fibrillin-1) gene have been identified as the cause, the mechanisms by which FBN1 mutations induce thoracic aortic aneurysms (TAA) remain largely unexplored. This study aims to test the hypothesis that the downregulation of the smooth muscle cell (SMC)-specific long non-coding RNA (lncRNA), CARMN (Cardiac Mesoderm Enhancer-Associated Noncoding RNA), is a critical mechanism underlying FBN1 mutation-induced TAA.
Method and Results: To investigate the potential effectors contributing to Marfan syndrome, we reanalyzed publicly available single-cell RNA sequencing datasets from Marfan human and mouse ascending arteries. Through this in silico screen, we identified CARMN as the most significantly downregulated lncRNA in modulated SMCs compared to contractile SMCs. To determine the functional role of Carmn in VSMCs, we generated inducible VSMC-specific Carmn KO (ivKO) mice by crossing floxed Carmn mice with Itga8-CreERT2 mice, which restrict Cre activity to VSMCs. The Carmn ivKO mice showed a normal lifespan. However, ultrasound and morphological analyses revealed progressive thinning of the vascular wall and dilation of the ascending aorta in ivKO mice. Vascular contractility measurements of both carotid and mesenteric arteries showed impaired vascular contractility in ivKO mice. Furthermore, blood pressure elevation induced by angiotensin II was attenuated in ivKO mice. Mechanistically, bulk RNA sequencing revealed that Carmn deficiency in VSMCs downregulates genes associated with muscle contraction, while upregulating genes linked to inflammation and extracellular matrix remodeling. Finally, VSMC-specific deletion of Carmn in Fbn1(C1041G/+) mutation mice, a genetic model that replicates human Marfan syndrome, significantly exacerbated the development of TAA.
Conclusions: Our data suggest that VSMC-restricted deletion of Carmn attenuates vascular contractility by regulating genes involved in muscle contraction. Furthermore, VSMC-specific deletion of Carmn exacerbates TAA development induced by the Fbn1 mutation in mice. This study demonstrates that Carmn is critical for maintaining vascular contractile function in mice and suggests that the downregulation of CARMN is a key mechanism contributing to FBN1 mutation-mediated TAA development in humans.