As you may know, Cell Therapy branch of FDA CBER has their own research lab, where they are working on mesenchymal stromal cells (MSC) characterization. A new study, which came up from CBER is investigating the issue of genetic stability of MSC. The findings from this study are somewhat surprising to me:
We found that chromosomal aberrations both exist and arise in culture-expanded MSCs and could be clonally propagated. However, in all cases, the aberrations diminished with extended culture, suggesting that they did not offer a replicative advantage…
I thought that clonal chromosomal/ genetic aberrations will only progress, lead to replicative advantage and tumorigenicity. Seem like it’s not always the case. As of today, most of MSC product developers do acknowledge the problem with genetic stability. However, seem like nobody knows for sure how to deal with it – how many cells with aneuploidy is enough for “NO GO” decision? Are these abnormalities harmful and could lead to tumorigenesis?
The authors detected chromosomal abnormalities in 6/10 MSC cultures, with value of abnormal cells up to 16%. In all donors, % of aberrant cells diminished (or even disappear in some donors) from 3rd to 7th or from 5th to 7th passages. However, due to low number of donors, this difference was not significant. The possible explanation of this prominent trend is replicative senescence and/or elimination of abnormal cells via apoptosis with prolonged culture adaptation to culture conditions. MSC subpopulations from 2/10 donors exhibited clonal karyotype abnormalities, which also diminished from 3rd to 5th passages and disappeared at 7th passage.
Couple of other interesting observations from the study: (1) There was a great variability of chromosomal aberrarions from donor to donor. (2) Frequency of chromosomal abnormalities does not increase with age (they checked 22-31 versus 39-41 years old donors). Interestingly, “young” donors had more aneuoploidy, but “older” donors had more translocations.
Take home messages from the study:
- Chromosomal abnormalities (including clonal) are exist and arise in human MSC culture
- Genetic stability of MSC is donor-dependent
- Karyotyping by SKY is good, robust and reliable method for screening of chromosomal aberrations in MSC cultures
- Passage 3 is good time checkpoint for “deciding the suitability of cells for further use”
- Genetic abnormalities exclusion limit for clinical use of MSC should be discussed and established.
The last message is especially important. The Cell Products Working Party as well as other groups have suggested such limits before. However, I don’t know if developers are following these recommendations and even karyotyping MSC products routinely.
Importantly, despite the trend for “loss of abnormal karyotypes with increasing passage” the study does not suggest to use MSC in advanced passages rather than in early (1-3) passages. The limitations of the study are (1) low number of donors and (2) absence of tumorigenicity testing.