Do we need to assess genomic stability on culture expanded cells for therapy?

 by Alexey Bersenev   on April 27, 2012  I came across of very interesting opinion piece, written by Darwin Prockop in Stem Cells journal. He comments on the study about genomic instability of human adult stem cells, published last year and discussed here.   

The authors of that study have concluded:    …validating the genomic stability of stem cells of all types in culture is crucial …for their safe implementation in cell therapy.   We highlighted this study and were in favor of development specific assays for genomic stability of cell products.   Prockop argues that there are no surprises here. We been here before and such tests could have very limited value in cell therapy. He gave a historical example, where in the 1960′s vaccine manufacturers were required to test cells for karyotype and tumorigenicity in vivo:    Concern about the chromosomal stability of cells in culture arose over 40 years ago when Wi-38 human fibroblasts were first introduced by Hayflick et al. [2] as substrates for viruses to develop vaccines. Manufacturers of vaccines were required to ensure that the cells were free, not only of any adventitious agents, but also of what were then referred to as "any neoplastic properties".   Interestingly, these requirements caused a lot of confusions, because of inconsistency in results of such tests.    Most importantly, there were no differences in growth characteristics or in tumorigenicity in mice between the cultures that did or did not contain t(7:12).   So, the historical attempts to implement genomic stability and tumorigenicity assays in biotechnology, left us with uncertainty:    Where does this leave us in evaluating the potential dangers of therapies with cells that are expanded in culture?  What are the appropriate tests? Given that most human cancer cells do not produce tumors in immune deficient mice, the assay for tumorigenicity using nude mice has unacceptably low sensitivity.   He proposed that the most simple and reliable test today is a senescence of cultured cells:    Do the cells senesce in culture? If they do, they are unlikely to produce tumors or malignancies in patients. Unlikely in the sense that there is a low probability, but not a certainty, that the expanded cultures do not contain a few cells that have acquired an oncogenic mutation. In contrast, cells that are immortal in culture carry a serious risk. Unfortunately, at present there is no strategy for eliminating this risk.   

Examples:    Embryonic stem cells and induced pluripotent cells do not pass the test of senescence in culture. Therefore the risk/benefit ratio is likely to be high. In contrast, the adult stem cells referred to as mesenchymal stem cells or mesenchymal stromal cells (MSCs) currently in use in a large number of clinical trials reproducibly senesce in culture.   Prockop comments intensively on this issue. His opinion is very interesting and definitely should be taken in consideration. I'd agree, that for routine cell products manufacturing, tumorigenicity in vivo test doesn't hold any future. It should be done in pre-clinical phase, before starting a trial. But for routine cell manufacturing a few assays should be developed. Simple, cheap and reliable assays. For example, a chip for the most nasty mutation, which always gives tumors in mice. Karyotype, senescence – all good. Also, different cell products will require different tests for genomic stability. It will depend on starting cell type – mature versus progenitor versus stem cell, number of passages and population doublings in culture, condition treated and so on. What do you think?