Cellular interplay via cytokine hierarchy causes 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 (TNF/IL6) hierarchy in Raf1 L613V -expressing ECs that drives cardiomyocyte hypertrophy in vitro . Furthermore, postnatal TNF inhibition normalizes the increased wall thickness and cardiomyocyte hypertrophy in vivo . We conclude that NS-cardiomyopathy involves cardiomyocytes, ECs and fibroblasts, TNF/IL6 signalling components represent potential therapeutic targets, and abnormal EC signalling might contribute to other forms of LVH. The human congenital disorder Noonan Syndrome (NS) is caused by germ-line mutations that hyperactivate the RAS/ERK signalling pathway, and can feature pathologic cardiac enlargement. Here, the authors find that a complex cellular and molecular interplay involving a cytokine hierarchy underlies cardiac hypertrophy caused by a NS-associated Raf allele.