Multiple roles for neurofibromin in skeletal development and growth

Neurofibromatosis type 1 (NF1) is a prevalent genetic disorder primarily characterized by the formation of neurofibromas, café-au-lait spots and freckling. Skeletal abnormalities such as short stature or bowing/pseudarthrosis of the tibia are relatively common. To investigate the role of the neurofibromin in skeletal development, we crossed Nf1flox mice with Prx1Cre mice to inactivate Nf1 in undifferentiated mesenchymal cells of the developing limbs. Similar to NF1 affected individuals, Nf1^Prx1 mice show bowing of the tibia and diminished growth. Tibial bowing is caused by decreased stability of the cortical bone due to a high degree of porosity, decreased stiffness and reduction in the mineral content as well as hyperosteoidosis. Accordingly, osteoblasts show an increase in proliferation and a decreased ability to differentiate and mineralize in vitro. The reduction in growth is due to lower proliferation rates and a differentiation defect of chondrocytes. Abnormal vascularization of skeletal tissues is likely to contribute to this pathology as it exerts a negative effect on cortical bone stability. Furthermore, Nf1 has an important role in the development of joints, as shown by fusion of the hip joints and other joint abnormalities, which are not observed in neurofibromatosis type I. Thus, neurofibromin has multiple essential roles in skeletal development and growth.