NF1 Tumor Suppressor Protein and mRNA in Skeletal Tissues of Developing and Adult Normal Mouse and NF1-Deficient Embryos

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Abstract
NF1 is a heritable disease with multiple osseous lesions. The expression of the NF1 gene was studied in embryonic and adult rodent skeleton and in NF1‐deficient embryos. The NF1 gene was expressed intensely in the cartilage and the periosteum. Impaired NF1 expression may lead to inappropriate development and dynamics of bones and ultimately to the osseous manifestations of the disease. Introduction: Neurofibromatosis type 1 is caused by mutations in the NF1 gene encoding the Ras GTPase activating protein (Ras‐GAP) neurofibromin. Skeletal ailments such as short stature, kyphoscoliosis, and tibial bowing and pseudarthrosis are common osseous manifestations of NF1. These symptoms are congenital, implying a role for neurofibromin in proper bone growth. However, little is known about its expression in skeletal tissues during their development. Materials and Methods: The expression of the NF1 gene was studied in normal and NF1+/− mouse fetuses at embryonic days 12.5‐15.5 and in skeletal tissues of adult mice and rats. In situ hybridization, immunohistochemistry, and Western blot analysis were used to identify the NF1 gene expression profile. Results: NF1 mRNA and protein were elevated in resting, maturation, and hypertrophic chondrocytes at the growth plate. Parallel studies on NF1+/− embryos showed expression patterns identical to wildtype. The periosteum, including osteoblasts and osteoclasts, and osteocytes of the cortical bone of adult mice were also intensely labeled for NF1 protein and mRNA. Western transfer analysis detected NF1 protein in the respective rat tissues. Phosphorylation of p42 and p44 MAP kinases, the downstream consequence of Ras activation, was elevated in hypertrophic chondrocytes of NF1+/− embryos. Conclusions: The results suggest that neurofibromin may act as a Ras‐GAP in skeletal cells to attenuate Ras transduced growth signals and thus play a role during ossification and dynamics of bone. Loss of NF1 function may therefore lead to dysplastic bone growth, thereby causing the debilitating osseous symptoms of NF1.