Abstract 20130: Hypertrophic Cardiomyopathy in RAF1 Mutant-associated Noonan Syndrome Requires Complex Cellular Interplay

Abstract

Hypertrophic cardiomyopathy (HCM) has multiple etiologies, including hypertension, cardiac valve disease or genetic defects. Most inherited HCM results from defects in sarcomere proteins, but ~25% are caused by aberrant cardiac signal transduction genes. RASopathies are developmental disorders with multiple variably penetrant features, including HCM, which are caused by mutations in genes encoding RAS/extracellular signal-regulated kinase (ERK) pathway components. The most common RASopathy, Noonan Syndrome (NS), results from gain-of-function alleles in PTPN11, KRAS, SHOC2, SOS1/2, RAF1 and RIT1. Activating mutations in RAF1 are strongly (~95%) associated with HCM. Global knock-in mice harboring the NS allele Raf1L613V recapitulate the features of NS-associated HCM, including ventricular chamber dilatation, enhanced cardiac contractility, and exacerbation of pressure-overload induced fibrosis. Here, using inducible Raf1L613V knock-in mice and lineage-specific Cre lines, we show that RAF1 mutant-associated HCM is caused by a complex interplay of multiple cardiac cell types. Surprisingly, cardiomyocyte-specific Raf1L613V expression caused minimal hypertrophy, yet markedly increased cardiac contractility. By contrast, endocardial/cardiac endothelial (EC)-specific Raf1L613V expression promoted cardiomyocyte hypertrophy without affecting contractility. In direct or Transwell co-cultures, Raf1L613V-expressing ECs increased the size of normal cardiomyocytes. Cytokine arrays revealed increased IL6 secretion from isolated Raf1L613V-expressing ECs, and JAK-STAT, PI3K, and MEK/ERK activation were enhanced in EC-Raf1L613V hearts. Blocking IL6 action with neutralizing anti-IL6 antibodies reversed cardiomyocyte hypertrophy in EC/cardiomyocyte co-cultures. Finally, Raf1L613V expression in cardiomyocytes or cardiac fibroblasts, but not in ECs, exacerbated fibrosis upon pressure-overload. Taken together, our data indicate that NS HCM requires complex, cell-autonomous and -non-autonomous interactions between cardiomyocytes, ECs, and fibroblasts. Furthermore, we identify the IL6 pathway as a potential therapeutic target for RASopathy-associated HCM.