Osteosarcoma Derived from Cultured Mesenchymal Stem Cells.

The beneficial effects of Mesenchymal Stem Cells (MSC) are being tested clinically in attempts to improve hematopoietic engraftment, to treat osteogenesis imperfecta, graft-versus-host disease and autoimmune diseases, and to deliver therapy for malignancies. In early reports, phase I clinical studies have not been associated with toxicities. To study the biodistribution of MSC, we labeled adult murine C57BL/6 MSC with firefly luciferase and DsRed2 fluorescent protein using non-viral Sleeping Beauty transposons, and co-infused labeled MSC with bone marrow into irradiated allogeneic recipients. Using in vivo whole body imaging, luciferase signals were shown to be increased between weeks 3 and 12. Unexpectedly, some mice with the highest luciferase signals died and all surviving mice developed foci of sarcoma in lungs. Two mice also developed sarcomas in their extremities. Infusion of MSC-derived sarcoma cells resulted in malignant lesions in secondary recipients. Common cytogenetic abnormalities were identified in tumor cells isolated from different animals. Mapping of the Sleeping Beauty transposition insertion sites did not identify an obvious transposon-related genetic abnormality. Importantly, the original MSC cultures not labeled with transposons, as well as cultured MSC independently isolated from the bone marrow of both BALB/c and C57BL/6 mice showed cytogenetic aberrations after several passages in vitro. Even though not all MSC cultures formed tumors upon in vivo injection, these data indicate that MSC transformation was neither strain-specific nor a rare event following ex-vivo expansion. Karyotype analyses using fluorescence in situ hybridization with spectral karyotyping (SKY) as well as combined binary ratio labeling of nucleic acid probes (COBRA) showed clonal evolution of transformed MSC suggesting that the critical transformation event(s) occurred before MSC infusion. Collectively, we describe cytogenetic instability of murine MSC isolated in two independent laboratories, their cellular transformation, and potential for sarcoma formation. While the growth characteristics of human and murine MSC are not identical and murine cells are more prone to undergo immortalization and transformation in culture than human cells, our study highlights the importance of quality control measures needed for ongoing and future clinical trials using human MSC.