Bone Abstracts

Menin, the product of the Men1 tumor suppressor gene, facilitates the cell proliferation control and differentiation induced by the TGF-beta superfamily of ligands critical for bone development and maintenance. Our in vivo studies have shown the importance of menin for proper functioning of the mature osteoblast and maintenance of bone mass in adult mice. In the present study, we examined the in vivo role of menin at earlier stages of the osteoblast lineage through conditional knockout of the Men1 gene. This was implemented through the Cre-LoxP recombination system at the level of the osteochondroprogenitor, osteoblast progenitor as well as, for comparison, the mature osteoblast. Prx1-Cre;Men1f/f, Osx-Cre;Men1f/f and OC-Cre;Men1f/f mice represent knockout of the Men1 gene in the mesenchymal stem cell, the preosteoblast and the mature osteoblast, respectively. Mice were sacrificed and analyzed at six months of age. All experiments had institutional ethical approval. Prx1-Cre;Men1f/f and Osx-Cre;Men1f/f mice were smaller than wild-type littermates whereas OC-Cre;Men1f/f mice were of normal size. Femur lengths of Prx1-Cre;Men1f/f and Osx-Cre;Men1f/f mice were shorter whereas those of OC-Cre;Men1f/f mice were of normal length. Prx1-Cre;Men1f/f and Osx-Cre;Men1f/f mice had reduced BMD (dual energy X-ray absorptiometry) whereas that of OC-Cre;Men1f/f mice was normal. By 3-dimensional micro-CT imaging of femur, all three strains of Men1 knockout mice had decreased trabecular bone volume with altered trabecular structure and decreased cortical bone thickness. In all strains of knockout mice trabecular number and spacing were decreased and increased, respectively. Primary calvarial osteoblasts of all strains of knockout mice relative to those of wild-type mice were deficient in mineralization and differentiation as assessed by Alizarin red, von Kossa and alkaline phosphatase staining, and had impaired responsiveness to TGF-beta and BMP-2 in specific promoter-reporter transfection assays. In conclusion, menin plays a crucial role in the development as well as maintenance of bone mass.