gms | German Medical Science

Objectives: Human mesenchymal stromal cells (MSC) from bone marrow represent a source of adult multipotent cells that could be used for many clinical applications. However, it is necessary to expand MSC in cell culture conditioned for many growth factors promoting expression of proto-oncogenes, such as c-MYC. It has not been investigated yet how aberrant activation of c-MYC might affect the proliferation and differentiation characteristics of human MSC and whether this would hinder their clinical application, especially due to a potential risk of tumorigenesis these cells might impose. The aim of the study was to investigate effects caused by high c-MYC levels on the proliferation and differentiation potential of MSC. Additionally, a question whether c-MYC-transduced MSC would induce any tumor growth in vivo in recipient animals was addressed, to assess a potential safety risk of their clinical application.

Methods: c-MYC expression was monitored in human MSC during expansion. To induce constitutive overexpression of c-MYC, MSC were retrovirally transduced and compared to control GFP- or non-transduced MSC. Cells were subjected to trilineage differentiation, and the effect of c-MYC overexpression on osteogenesis, adipogenesis, or chondrogenesis was monitored. The transduced MSC were also tested in vivo in ectopic bone formation assay, to assess their bone tissue forming ability and a potential risk of tumorigenesis imposed by high c-MYC expression.

Results and conclusion: c-MYC protein accumulations were found in bone marrow-derived MSC, and they had tendency to be further increased during ex vivo expansion. C-MYC overexpression resulted in induction of expression of MYC antagonists P19ARF/P16INK4A, suggesting activation of a switch mechanism from proliferation to senescence that prevents malignant transformation of MSC. C-MYC reduced the differentiation markers for osteogenic (Alizarin Red S), adipogenic (Oil Red O and PPARG), and chondrogenic differentiation (Safranin O, GAG deposition, and SOX9). Additionally, c-MYC overexpression caused a significant increase in the ratio of COL10A1/COL2A1 expression in MSC. The in vivo ectopic bone formation potential of c-MYC-transduced MSC remained comparable to the control GFP-MSC. There was no indication of tumour growth in any tissue of the tested animals after transplantation of c-MYC-MSC.

The study demonstrates that the proliferation capacity of human bone marrow MSC is directly linked to c-MYC expression and suggests a novel putative role of c-MYC in promoting the hypertrophic phenotype during chondrogenesis. It also indicates that the elevated expression of c-MYC alone, in absence of concomitant expression of other oncogenes, did not immediately lead to tumour formation in the tested mouse model.