Owing to their multi/pluripotency and immunosuppressive properties, mesenchymal stem/stromal cells (MSCs) are important tools for treatment of immune disorders and tissue repair. The increasing uses of MSCs lead to the development of production processes that need to be in accordance with good manufacturing practices (GMP). In cellular therapy, safety remains one of the main concerns; of these the risk of transformation is a major one and to avoid side effects, accurate production controls have to be implemented.
At this time, there are conflicting data regarding the genomic stability of culture expanded MSCs, and the risk of transformation. Conducted on immortalized human bone marrow MSCs, the first study showed that transformation in MSCs was a long and multistep process involving genetic and epigenetic changes in cell experiencing more than 100 population doublings. Moreover, alterations of key genes, as p16ink4a, were mandatory for transformation. The first evidence of human MSC transformation during culture was reported for adipose tissue MSCs subjected to long-term culture. If there were karyotype abnormalities, the transformation appeared only after p16 deletion, over-expression of c-myc and re-expression of human telomerase (hTERT). This transformation process could be linked to a mesenchymal–epithelial transition. As recently reported, these features were related to contamination by a cancer cell line. One study on bone marrow MSCs reported that transformation could be a frequent and fast process. On the contrary, using karyotyping, array-comparative genomic hybridization (array-CGH) and fluorescent in situ hybridization (FISH), different studies on clinical-grade cultured MSCs did not show any evidence of genetic instability and transformation. Recently, in two different clinical trials, we showed that aneuploidy could appear whatever the culture process was. However, presenting or not aneuploidy, all cultivated MSCs reached senescence, and they did not exhibit any transforming events.
In conclusion, if genetic stability of clinical grade expanded MSCs has to be tested, karyotyping does not seem a relevant control. Focusing on the molecular events involved in transformation, a new approach of controls is needed.