Cellular Interplay and Cytokine Hierarchy Cause Pathological Cardiac Hypertrophy in RAF1-Mutant Noonan Syndrome

Noonan syndrome (NS) is caused by mutations in RAS/ERK pathway genes, and is characterized by craniofacial, growth, cognitive and cardiac defects. NS patients with kinase-activating RAF1 alleles typically develop pathological left ventricular hypertrophy (LVH), which is reproduced in Raf1 L613V/+ knock-in mice. Here, using inducible Raf1 L613V expression, we show that LVH results from the interplay of cardiac cell types. Cardiomyocyte Raf1 L613V enhances Ca 2+ sensitivity and cardiac contractility without causing hypertrophy. Raf1 L613V expression in cardiomyocytes or activated fibroblasts exacerbates pressure overload-evoked fibrosis. Endothelial/endocardial (EC) Raf1 L613V causes cardiac hypertrophy without affecting contractility. Co-culture and neutralizing antibody experiments reveal a cytokine hierarchy (TNFa->IL6) from Raf1 L613V -expressing ECs that drives cardiomyocyte hypertrophy in vitro. Furthermore, post-natal TNFa inhibition normalizes the increased wall thickness and cardiomyocyte hypertrophy in vivo. We conclude that NS cardiomyopathy involves cardiomyocytes, ECs, and fibroblasts, TNFa/IL6 signaling components represent potential therapeutic targets, and abnormal EC signaling might contribute to other forms of LVH. Overall design: control vs. Raf1 L613V/+ knock-in cardiomyocytes