MEK inhibitor Mirdametinib promotes fracture healing in osteofibrous dysplasia RASopathy

Osteofibrous dysplasia (OFD) is a skeletal RASopathy presenting with periosteal bone lesions that may progress to fracture and delayed healing (pseudarthrosis). MET gene mutations reducing ubiquitin-mediated protein degradation via loss of the juxtamembrane domain (METΔJMD) were previously identified in OFD patients, resulting in ligand-dependent gain-of-function. The impact of METΔJMD expression on skeletal progenitor cell differentiation and the potential efficacy of targeted therapies remain unclear. We engineered MetΔJMD mice and showed that MetΔJMD expression inhibited osteogenic differentiation of skeletal progenitor cells in vitro and impaired cortical bone development and reduced bone stiffness in vivo. In contrast, conditional deletion of Met enhanced osteogenic differentiation of periosteal progenitor cells. Inhibition of MAPK signaling with MEK inhibitors restored osteogenic differentiation of mouse MetΔJMD skeletal progenitor cells and promoted activation of transcriptional signatures associated with skeletal development and osteoblast differentiation in OFD patient pseudarthrosis-derived primary cells. With this preclinical support, we treated with the MEK inhibitor mirdametinib a pediatric OFD patient suffering from a 3-year history of persistent pseudarthrosis, resulting in fracture union. Our findings demonstrate a bi-directional role for MET in regulating osteogenic differentiation of skeletal progenitor cells and a therapeutic avenue to improve clinical outcomes for this, and potential other, skeletal RASopathies. .