Rolle des Transkriptionsfaktors c-Myc für die Proliferation und Differenzierung menschlicher mesenchymaler Stammzellen
Zusammenfassung der Projektergebnisse
Mesenchymal stromal cells isolated from bone marrow (MSC) represent an attractive source of adult progenitor cells for regenerative medicine. However, a thorough research is required into their clinical application safety issues concerning a risk of potential neoplastic degeneration in a process of MSC propagation in cell culture for therapeutic applications. Expansion protocols could preselect MSC with elevated levels of growth-promoting transcription factors with oncogenic potential, such as c-MYC. We addressed the question whether elevated c-MYC expression affects the growth and differentiation potential of human MSC in vitro and their ectopic bone formation capacity in vivo, and assessed a risk of tumorigenic transformation caused by c-MYC overexpressing MSC in vivo. Indeed c-MYC levels accumulated during standard ex vivo passaging and enabled MSC with higher c-MYC levels to significantly overgrow competing control cells in culture. C-MYC-MSC showed elevated expression of the c-MYC binding partner MAX, and induction of antagonists P19ARF/P16INK4A. Overexpression of c-MYC stimulated MSC proliferation, and attenuated differentiation according to reduced osteogenic, adipogenic and chondrogenic marker expression. Surprisingly, c-MYC overexpression also caused an increased COL10A1 / COL2A1 expression ratio upon chondrogenesis, suggesting a role in hypertrophic degeneration. However, the in vivo ectopic bone formation ability of c-MYC-transduced MSC remained comparable to control GFP-MSC and there was no indication of tumour growth in any tissue after subcutaneous transplantation of c-MYC-MSC in immunodeficient mice. In conclusion, C-MYC expression promoted higher proliferation rates of MSC and attenuated but did not abrogate their differentiation capacity. The observed upregulation of tumor suppressor genes in response to c-MYC activation suggested that cell safety mechanisms were induced to counteract tumorigenesis, as P16INK4A/P19ARF expression might prevent a possible neoplastic transformation of MSC by switching on senescence and apoptosis programs. Elevated expression of c-MYC alone did not immediately lead to tumour formation in the tested in vivo mouse model. This suggests that application of extensively expanded MSC populations which might be enriched in c-MYC-overexpressing cells should not be hindered by increased tumorigenesis risk. Our data are in line with general conclusions made by the Cell Products Working Party and the Committee for Advanced Therapies elucidating the risk of potential tumorigenicity related to MSC-based therapies, agreeing on a conclusion that in current animal models, in which human MSC are used, no direct evidence has been observed to date that would suggest induced tumour formation.
