Abstract

Patients with Neurofibromatosis Type 1 (NF1) develop subcutaneous benign tumors and dysfunction of multiple organs. About 30% of NF1 patients are affected by skeletal manifestations such as osteopenia, kyphoscoliosis, tibia bowing, or pseudarthrosis of the tibia. NF1 is caused by autosomal dominant mutation of the NF1 gene encoding the protein neurofibromin a regulator of the RAS/MAPK/ERK pathway. During the last decade pre-clinical studies provided multiple insights into the molecular mechanisms of the NF1 associated skeletal phenotype. They also allowed analysis of bone structural defects in tissue specific knockout mice and their comparison with the changes observed in patient samples. Importantly, preclinical models allowed exploration of the potential therapeutic approaches.
In Prx1Cre;Nf1^flox/flox and Col1a1Cre;Nf1^flox/flox mice we demonstrated that loss of neurofibromin leads to multi scale defects in cortical bone: i) increased marco-porosity, ii) increased micro-porosity (osteocyte lacunae), iii) diminished mineralization, and iv) reduced organic matrix maturation. These changes significantly weaken overall mechanical strength of bone tissue in murine models. In NF1 patient bone samples similar changes were observed which may result in fractures. Inhibition of the RAS/MAPK/ ERK pathway with Lovastatin, Trametinib, PD198306, P98059, and PD0325901 normalized bone healing in Nf1 knockout mouse models. Downregulation of the RAS/MAPK/ERK pathway restored normal osteoblast differentiation/function and prevented accumulation of fibroblasts within the bone fracture site. Another approach tested in mouse models was to stimulate osteoblastic bone formation with bone morphogenic protein 2 or 7 (BMP-2, BMP-7) and block bone resorption with bisphosphonates. Recently Asfotase-α, replacing alkaline phosphatase (ALP) function specifically in bone tissue, was used to restore normal bone mass in inducible Tet-Off based Osx-Cre;Nf1^flox/flox mice.
In summary, neurofibromin controls development of the skeletal system by regulating the RAS/MAPK/ ERK pathway in chondrocytes, pre-osteoblasts, osteoblasts, and osteocytes. Preclinical model studies, exploring therapeutic approaches based on targeting RAS/MAPK/ERK pathway and ALP activity yield promising results which will likely instruct future clinical trial designs.