Metastatic osteosarcoma induced by inactivation of Rb and p53 in the osteoblast lineage

Abstract

Mutation of the RB-1 and p53 tumor suppressors is associated with the development of human osteosarcoma. With the goal of generating a mouse model of this disease, we used conditional and transgenic mouse strains to inactivate Rb and/or p53 specifically in osteoblast precursors. The resulting Rb;p53 double mutant (DKO) animals are viable but develop early onset osteosarcomas with complete penetrance. These tumors display many of the characteristics of human osteosarcomas, including being highly metastatic. We established cell lines from the DKO osteosarcomas to further investigate their properties. These immortalized cell lines are highly proliferative and they retain their tumorigenic potential, as judged by their ability to form metastatic tumors in immunocompromised mice. Moreover, they can be induced to differentiate and, depending on the inductive signal, will adopt either the osteogenic or adipogenic fate. Consistent with this multipotency, a significant portion of these tumor cells express Sca-1, a marker that is typically associated with stem cells/uncommitted progenitors. By assaying sorted cells in transplant assays, we demonstrate that the tumorigenicity of the osteosarcoma cell lines correlates with the presence of the Sca-1 marker. Finally, we show that loss of Rb and p53 in Sca-1-positive mesenchymal stem/progenitor cells is sufficient to yield transformed cells that can initiate osteosarcoma formation in vivo.osx-cre ͉ Sca-1 ͉ hibernoma mouse model O steosarcomas account for Ϸ30% of malignant bone tumors and 3-4% of all childhood malignancies (1, 2). They arise primarily around the knee joint, lower femur and upper tibia, which are all regions of active bone growth and repair. These tumors are predominantly osteoblastic in nature, although there is a correlation between loss of differentiation and poor prognosis. The generation of new therapeutic treatments for osteosarcoma has improved the 5-year survival rate of affected individuals. However, like other mesenchymal neoplasms, osteosarcomas are predisposed to metastasize via the hematogenous route, and thus, pulmonary metastasis is a major cause of death. Analyses of both sporadic and hereditary tumors show that inactivation of the p53 and RB-1 tumor suppressors plays a key role in the development of this tumor type (1, 2). Li-Fraumeni patients, who often carry germ-line mutations in p53, are predisposed to a variety of tumors, 12% of which are bone sarcomas (3, 4). p53 mutations are also observed in 20-60% of sporadic osteosarcomas (5-7). Similarly, patients carrying germline mutations in RB-1 have an Ϸ500-fold higher incidence of osteosarcoma than the general population (8). Moreover, RB-1 mutations are detected in 70% of all adolescent osteosarcomas (9). Finally, human osteosarcomas can carry mutations in both p53 and RB-1 (10).Mouse models have provided considerable insight into the role of p53 in bone development and tumorigenesis. Experiments from three different settings suggest that p53 plays an important role in bone development by modul...

c-MYC overexpression with loss of Ink4a/Arf transforms bone marrow stromal cells into osteosarcoma accompanied by loss of adipogenesis

“…Recent reports describing novel OS models indicate that OS originates from lineage-committed immature osteoblasts or MSCs (Berman et al, 2008;Walkley et al, 2008). Our model suggested that OS can originate from both types of cells.…”

“…Recently, novel mouse OS models were developed through targeted deletion of p53 and Rb in osterixexpressing cells (Berman et al, 2008;Walkley et al, 2008). INK4a/ARF exerts effects on both the Rb and p53 pathways (Collado et al, 2007).…”

Osteosarcoma

“…(15) However, the tumor that arises in this model system has been noted to express more primitive lineage markers than preosteoblasts. (16) Although these markers may be acquired in the transformation process, it also has been suggested that a rare precursor cell that expresses markers of both mesenchymal stem cells and preosteoblasts may be the cell of origin. (16) Targeted disruption of p53 and Rb in the mesenchymal cells of the murine limb bud also produces sarcomas.…”

“…(16) Although these markers may be acquired in the transformation process, it also has been suggested that a rare precursor cell that expresses markers of both mesenchymal stem cells and preosteoblasts may be the cell of origin. (16) Targeted disruption of p53 and Rb in the mesenchymal cells of the murine limb bud also produces sarcomas. (17) A separate line of evidence comes from the observation that serially passaged murine mesenchymal stem cells will undergo rare spontaneous transformation resulting in an osteosarcoma-like tumor, suggesting that these cells are the cell of origin.…”

The orally bioavailable met inhibitor PF-2341066 inhibits osteosarcoma growth and osteolysis/matrix production in a xenograft model

“…(31) Indeed, genomic profiling of human OS has not revealed recurrent genetic events, (1) although mouse genetic models implicating the p53 and pRb genes have been developed recently. (32,33) OS-specific oncogenic changes therefore may not exist, and OS initiation and progression instead may be driven by alterations that also occur in other cancers (eg, p53, pRb, and Met). (1) Met is one of the most commonly altered tyrosine kinases in human cancer, (4) and Met overexpression is a common feature of OS and the more prevalent soft tissue sarcomas (STSs).…”