Mesenchymal expression of activated K-ras yields Noonan Syndrome-like bone defects that are rescued by mid-gestational MEK inhibition

Abstract

Activating germline K-ras mutations cause Noonan syndrome (NS), which is characterized by several developmental deficits including cardiac defects, cognitive delays and skeletal abnormalities. NS patients have increased signaling through the MAPK pathway. To model NS skeletal defects and understand the effect of hyperactive K-ras signaling on normal limb development, we generated a mouse model in which activated Kras G12D was expressed specifically in mesenchymal progenitors of the limb bud. These mice display short, abnormally mineralized long bones that phenocopy those of NS patients. This defect was first apparent at E14.5, and was characterized by a delay in bone collar formation. Coincident mutation of p53 had no effect on the K-ras G12D induced bone defect, arguing that it is does not result from senescence or apoptosis. Instead, our data revealed profound defects in the development of the committed osteoblasts; their appearance is delayed, concordant with the delay in bone collar formation, and they display an aberrant localization outside of the bone shaft. Additionally, we see growth plate defects including a reduction in the hypertrophic chondrocyte layer. Most importantly, we found that in utero delivery of a MEK inhibitor between E10.5 and E14.5 is sufficient to completely suppress