Asfotase-α improves bone growth, mineralization and strength in mouse models of neurofibromatosis type-1

Individuals with neurofibromatosis type-1 (NF1) can manifest focal skeletal dysplasias that remain extremely difficult to treat. NF1 is caused by mutations in the NF1 gene, which encodes the RAS GTPase-activating protein neurofibromin. We report here that ablation of Nf1 in bone-forming cells leads to supraphysiologic accumulation of pyrophosphate (PP i), a strong inhibitor of hydroxyapatite formation, and that a chronic extracellular signal-regulated kinase (ERK)-dependent increase in expression of genes promoting PP i synthesis and extracellular transport, namely Enpp1 and Ank, causes this phenotype. Nf1 ablation also prevents bone morphogenic protein-2-induced osteoprogenitor differentiation and, consequently, expression of alkaline phosphatase and PP i breakdown, further contributing to PP i accumulation. The short stature and impaired bone mineralization and strength in mice lacking Nf1 in osteochondroprogenitors or osteoblasts can be corrected by asfotase- α enzyme therapy aimed at reducing PP i concentration. These results establish neurofibromin as an essential regulator of bone mineralization. They also suggest that altered PP i homeostasis contributes to the skeletal dysplasias associated with NF1 and that some of the NF1 skeletal conditions could be prevented pharmacologically.