RESEARCH ARTICLE Rb1 and Pten Co-Deletion in Osteoblast Precursor Cells Causes Rapid Lipoma Formation in Mice

The Rb and Pten tumor suppressor genes are important regulators of bone development and both are frequently mutated in the bone cancer osteosarcoma (OS). To determine if Rb1 and Pten synergize as tumor suppressor genes for osteosarcoma, we co-deleted them in osteo-progenitor cells. Surprisingly, we observed rapid development of adipogenic but not osteosar-coma tumors in the ΔRb1/Ptenmice.ΔPten solo deleted mice also developed lipoma tumors but at a much reduced frequency and later onset than those co-deleted for Rb1. Pten deletion also led to amarked increase in adipocytes in the bonemarrow. To better understand the func-tion of Pten in bone development in vivo, we conditionally deleted Pten in OSX+ osteoprogeni-tor cells usingOSX-Cremice. μCT analysis revealed a significant thickening of the calvaria and an increase in trabeculae volume and number in the femur, consistent with increased bone formation in thesemice. To determine if Pten and Rb1 deletion actively promotes adipo-genic differentiation, we isolated calvarial cells from Ptenfl/fl and Ptenfl/fl; Rb1fl/flmice, infected themwith CRE or GFP expressing adenovirus, treated with differentiation media. We observed slightly increased adipogenic, and osteogenic differentiation in the ΔPten cells. Both phenotypes were greatly increased uponRb1/Pten co-deletion. This was accompanied by an increase in expression of genes required for adipogenesis. These data indicate that Pten dele-tion in osteoblast precursors is sufficient to promote frequent adipogenic, but only rare osteo-genic tumors.Rb1 hetero- or homo-zygous co-deletion greatly increases the incidence and the rapidity of onset of adipogenic tumors, again, with only rare osteosarcoma tumors