A paper from the Okano laboratory has shown that implantation of neural stem cells made from induced pluripotent stem cells can still form tumors ever after a long period of time.
This paper is an important contribution to the safety issues surrounding induced pluripotent stem cells (iPSCs). As noted in previous posts, iPSCs are made from adult cells by means of genetic engineering and cell culture techniques. In short, by introducing four different genes into adult cells and then culturing them in a special culture medium, a fraction of these cells will de-differentiate into cells that resemble embryonic stem cells in many ways, but are not exactly like them.
The Okano laboratory made iPSCs using viruses that integrate into the genome of the host cell, which is not the safest option. However, because in the four-gene cocktail that is normally used to reprogram these cells (Oct-4, Klf-4, Sox2, and c-Myc), the c-Myc gene is often thought to be the main cause of tumor formation. Okano and his collaborators made their iPSCs without the c-Myc gene, but only used the three-gene cocktail of Oct-4, Klf-4, and Sox2. Such a cocktail is much less efficient that the four-gene cocktail, but it supposed to make iPSCs that are altogether safer.
These iPSCs were differentiated into neural stem cells that grew as tiny spheres of cells, and these “neurospheres” were transplanted into the spinal cords of mice that had suffered a spinal cord injury. The implanted cells differentiated into neurons and glial cells and restored some neural function to these mice. However, the mice were observed for a long period of time after the implantations to assess the long-term safety of these implanted cells.
After 105 days, the implanted mice began to show deterioration of their neural function and their spinal cords showed tumors. It is clear that the Oct-4 gene that was used in the reprogramming procedure was the reason for the tumor transformation.
This experiment, once again, calls into question the safety of any method for iPSC generation that leaves the transfected genes in the reprogrammed cells. I reported in a previous post that skin cells made from iPSCs that had their transgenes left in them were good at causing tumors and not as good as forming skin cells, but iPSCs without their reprogramming transgenes were safer and more effective tools for regenerative medicine. This experiment also shows that c-Myc is not the only concern with iPSCs. Any of the transgenes used for reprogramming can cause problems, and they must be removed if iPSCs are going to produce safe, differentiated cells. Finally, this experiment pretty much shows that the use of retrovirus tools to introduce genes into cells for the sake of reprogramming is a bad idea if those cells are going to be used for regenerative medicine. Non-integrating tools are much safer and preferable in these cases.
The Okano paper appeared in Stem Cell Reports.