Expression profiling of osteosarcoma and its putative progenitor cells to identify pathways involved in tumorigenesis
Authors: Anne-Marie Cleton-Jansen, Jakob Anninga, Inge Briaire-de Bruijn, Charlotte De Meijer, Salvatore Romeo, Jan Oosting, Maarten Egeler, Hans Gelderblom, Pancras Hogendoorn
Journal: Cancer Research (2007)

AACR Annual Meeting-- Apr 14-18, 2007; Los Angeles, CA

4430 

Introduction: Osteosarcoma (OS) is the most prevalent primary malignant bone tumor of children and young adults. Chemotherapy has improved prognosis but now has reached a plateau in terms of increasing survival and therefore knowledge of molecular mechanisms for new targets for therapy is needed. The putative cell of origin of OS is unknown, however, osteoblasts or mesenchymal stem cells (MSC) are the most likely. The benign primary bone tumour osteoblastoma (OB) is also assumed to originate from osteoblasts, but is not a precursor for osteosarcoma. Experimental procedures: In order to identify signalling pathways differentially expressed in OS versus its presumed normal counterparts, genome wide expression profiles were generated from 25 high grade central OS pre-chemotherapy biopsies from uniformly treated patients, 5 OB, 5 MSC populations and these same MSCs differentiated to osteoblasts (DO). Genes that were differentially expressed between OS, MSC, OB and DO were analyzed in the context of the involved crucial pathways using the GenMAPP program. Especially pathways that were known to be involved in MSC differentiation were studied. Immunohistochemistry on tissue arrays was performed to verify expression at the protein level. Results: Unsupervised hierarchical clustering could not distinguish OS with good prognosis from those with poor prognosis, nor good response to chemotherapy versus poor response. MSCs, DOs, OBs and OSs clustered separately and thousands of differentially expressed genes could be distinguished. Components of the p53 pathway are down regulated in OS, both TP53 and two downstream genes p21/CDKN1A and PUMA/BBC3 are significantly lower in OS than in MSC and osteoblasts. Immunohistochemistry for p53 and p21 showed absence of p21, also in cases with positive p53, the latter probably compliant with mutated p53. Several upstream components of the Wnt signalling pathway are down regulated in OS, e.g. the ligand Wnt5a and the Frizzled receptors FZD6 and FZD7. Two genes involved in degradation of β-catenin protein, the key effectors of Wnt signaling, Axin and GSK3-β show decreased expression, suggesting that Wnt signalling is active, but not under control of the regular signals. Immunohistochemistry for β-catenin demonstrated nuclear localization in only a very small subset of OS. Protein expression levels for α, β or y catenin and for adhesion molecules N-cadherin and ALCAM were not associated with response or prognosis. Conclusion: RNA nor protein expression can identify prospectively osteosarcomas with good response or good prognosis. However, comparison of expression profiles between OS and its progenitors MSC and DO revealed marked differences in signal transduction, especially signalling mediated by p53 and β-catenin. This will help in understanding the biology op OS in more detail, which may contribute to the identification of new targets for therapy.