Abstract LB-259: MET-oncogene transforms human bone-derived cells by targeting committed osteo-progenitors
Authors: Dani Nadia , Olivero Martina , Lorenzato Annalisa , Cuvertino Sara , Patanè Salvatore , Van Duist Marjan , Mareschi Katia , Fagioli Franca , Di Renzo Maria Flavia 
Published: February 2011

Abstract
Osteosarcoma (OS) is the most common primary bone malignancy in children and young adults. Despite advances in medical and surgical management, survival rates for OS remain low. The biology of osteosarcomagenesis is poorly understood and the OS initiating cell remains elusive. We generated a model of osteosarcomagenesis by overexpressing MET oncogene in primary cultured human osteoblasts (HOB), since aberrant MET expression is found in high percentage of human OS. MET encodes for the receptor of HGF. It is normally expressed in epithelia, while HGF is secreted by cells of mesenchymal origin. MET activation elicits a physiologic program known as invasive growth. If deregulated, this program contributes to cell transformation and tumor progression.
Using a LV-mediated approach, we overexpressed MET in HOB at level similar to human OS. We obtained transformed and tumorigenic clones derived from single cells, as shown by patterns of transgene integration. Interestingly, the number of susceptible cells was very small. A candidate for this minor population is the still elusive target cell of osteosarcomagenesis. Notably, MET-transformed clones are maintained by a self renewing cell population able to form bona fide sarcospheres.
To characterize the target cell of MET-induced transformation, we used genome-wide expression profiling with Illumina platform. We compared the transformed clones to parental HOB and primary human mesenchymal stem cells (MSC). Transformed clones resulted phenotypically identical, supporting their common origin. They showed 584 genes differentially expressed with respect to parental HOB. The expression levels of MSC markers are down regulated in clones, while markers of stemness (POU5F1, ABCB1) are up regulated. Moreover, markers of early phase of osteoblastic differentiation (CDCA2, RUNX2, ALPL, MATN2) are up regulated, while that associated to late phase (FN1, SPARC, COL1A1, SPP1, BGLAP) are down-regulated. These results suggest that the target cell of MET-induced transformation is an osteoprogenitor rather than a MSC. To identify this progenitor along the osteoblastic differentiation pathway, we compared the expression profiles of transformed clones with that of MSC differentiated towards osteoblasts, chondrocytes and adipocytes at defined time points (7-14-21 days). MET-transformed clones clustered with 7 days osteoblasts, with some transcripts shared with other 7-days differentiated cells. These data suggest that clones originate from a committed osteoprogenitor cell, which retains self renewal, but not multi-lineage potentials. This assumption is supported by the fact that transformed clones are able to fully differentiate along the osteoblastic, but not the adipocytic or chondrocytic lineages.
Altogether, these results indicate that MET overexpression sustains the amplification of premalignant population with an osteoprogenitor-defined gene signature.
Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr LB-259.