Spontaneous transformation and immortalization of mesenchymal stem cells in vitro

Mesenchymal stem cells (MSCs) possess plasticity and unlimited proliferative activity in vitro, which makes them an attractive object for studies focused on new resources for regenerative medicine. MSC application is effective for treating patients with degenerative and traumatic diseases of different tissues; however, the biological basis for the therapeutic efficacy of MSCs is still obscure. We found that the long-term culture of MSCs that expressed transgenic green fluorescence protein (GFP) led to an increase in their proliferative activity and reduced adhesion, loss of differentiation, and GFP production. At the first passages, MSCs showed karyotypic features of transformation, which were complicated at the later passages by the appearance of tumorigenic properties that were detected after transplantation into syngenic recipients. Tumor cells originated from MSCs explanted in vitro did not express GFP and could not be induced to differentiate. However, in contrast to the parent cells, they showed decreased clonogenic and proliferative activity. We assume that even the short-term cultivation of MSCs in vitro may result in their spontaneous transformation. We hypothesize that immortality and unlimited MSC expansion in vitro are consequences of their transformation rather than intrinsic stem-cell properties.