Expression of FUS-CHOP fusion protein in immortalized/transformed human Mesenchymal Stem Cells drives mixoid liposarcoma formation

Although different sarcomas have been modeled in mice upon expression of fusion oncogenes in MSCs, sarcomagenesis has not been successfully modeled in human MSCs (hMSCs). We report that FUS-CHOP, a hallmark fusion gene in mixoid liposarcoma (MLS), has an instructive role in lineage commitment, and its expression in hMSC sequentially immortalized/transformed with up to 5 oncogenic hits (p53 and Rb deficiency, hTERT over-expression, c-myc stabilization and H-RASv12 mutation) drives the formation of serially transplantable MLS. This is the first model of sarcoma based on the expression of a sarcoma-associated fusion protein in hMSC, and allowed us to unravel the differentiation processes and signaling pathways altered in the MLS-initiating cells. This study will contribute to test novel therapeutic approaches, and constitutes a proof-of-concept to employ hMSCs as target cell for modeling other fusion gene-associated human sarcomas. Wild type (MSC-0H) or transformed (MSC-5H) BM-hMSCs were transduced with concentrated viral particles expressing either pRRL-EF1α-PGK-GFP (empty vector; GFP) or pRRL-EF1α-FUS-CHOP-PGK-GFP (FUSCHOP expressing vector; FC) in order to generate MSC-0H-GFP, MSC-0H-FC, MSC-5H-GFP and MSC-5H-FC cell lines. MSC-0H-GFP and MSC-0H-FC cells did not develop tumors, meanwhile MSC-5H-GFP cells gave rise to undifferentiated sarcomas and MSC-5H-FC originated mixoid liposarcoma tumors when inoculated into immunedeficient mice. Several cell lines were derived from tumors developed from MSC-5H-GFP (T-5H-GFP-1 to -3 cell lines) and MSC-5H-FC (T-5H-FC-1 to -3 cell lines) cells. Gene expression analysis was performed using MSC-0H, MSC-5H and T-5H cell lines and lists of differentially expressed genes were created by comparing the gene expression profiles of MSC-0H-FC, MSC-5H-FC and T-5H-FC cell types to the control MSC-0H-GFP cells.