Hyperactive Ras/MAPK signaling is critical for tibial nonunion fracture in neurobromin-decient mice

Neurobromatosis type 1 (NF1) is a common genetic disorder affecting 1 in 3500 individuals. Patients with NF1 are predisposed to de bilitating skeletal manifestations, including osteopenia/osteoporosis and long bone pseu-darthrosis (nonunion fracture).Hyperactivation of the Ras/mitogen-activated protein kinase (MAPK) pathway in NF1 is known to underlie aberrant proliferation and differentiation in cell lineages, including osteoclast progeni-tors andmesenchymal stemcells (MSCs) also known as osteoblast progenitors (pro-OBLs). Our current study demonstrates the hyper Ras/MAPKas a critical pathway underlying the pathogenesis of NF1-associated fracture repair decits. Nf1-decient pro-OBLs exhibit Ras/MAPK hyperactivation. Introduction of the NF1 GTPase acti-vating-related domain (NF1 GAP-related domain) in vitro is sufcient to rescue hyper Ras activity and enhance osteoblast (OBL) differentiation in Nf1^-/- pro-OBLs and NF1 human (h) MSCs cultured from NF1 patients with skeletal abnormalities, including pseudarthrosis or scoliosis. Pharmacologic inhibition of mitogen-activated protein kinase kinase (MEK) signaling with PD98059 partially rescues aberrant Erk activation while enhancing OBL differentiation and expression of OBL markers, osterix and osteocalcin, in Nf1-decient murine pro-OBLs. Similarly,MEK inhibition enhancesOBL differentiation of hMSCs. In addition, PD98059 rescues aberrant osteoclastmaturation in Nf1 haploinsufcient bonemarrowmononuclear cells (BMMNCs). Importantly,MEKin-hibitor signicantly improves fracture healing in an NF1 murine model, Col2.3Cre;Nf1ox/2. Collectively, these data indicate the Ras/MAPK cascade as a critical pathway in the pathogenesis of bone loss and pseudarthrosis relatedtoNF1mutations.These studiesprovide evidence for targeting the MAPK pathway to improve bone mass and treat pseudarthrosis in NF1.