A new study recently accepted by iScience has shown that a specific genetic alteration can selectively remove pluripotent stem cells from stem cell therapies in order to purify them for use in treatment.
The cells we don’t want
As we age, our stem cells both decrease in number and become less efficient in replenishing somatic (functional) cells, leading to many of the familiar diseases of aging. This is the hallmark of stem cell exhaustion.
If effective, stem cell therapies directly replenish these depleted populations, restoring health and function to aged tissues. Therefore, human pluripotent stem cells (hPSCs) have been created from somatic cells, repurposing our body’s cells into creating more reinforcements.
However, in the absence of proper signaling, hPSCs will not necessarily differentiate into the kinds of cells that are needed, forming unwanted tissues instead. For example, injecting hPSCs into a human brain may lead to the formation of bone rather than brain tissue.
Modern stem cell therapies use such signals to create specific stem cell populations out of hPSCs but cannot yet completely turn all the pluripotent stem cells into the desired kinds before they are deployed. This problem creates contamination, tainting otherwise useful therapies with potentially dangerous cells.
To combat this contamination, these stem cells can be genetically altered before they are used in therapies, causing them to kill themselves if, and only if, they express the genes that pluripotent stem cells express. 
A major challenge in using human pluripotent stem cells (hPSCs) in therapy is the risk of teratoma formation due to contaminating undifferentiated stem cells. We used CRISPR-Cas9 for in-frame insertion of a suicide gene, iC9, into the endogenous SOX2 locus in human embryonic stem cell (ESC) line H1 for specific eradication of undifferentiated cells without affecting differentiated cells. This locus was chosen over NANOG and OCT4, two other well-characterized stem cell loci, due to significantly reduced off-target effect. We showed that undifferentiated H1-iC9 cells were induced to apoptosis by iC9 inducer AP1903, while differentiated cell lineages including hematopoietic cells, neurons, and islet beta-like cells were not affected. We also showed that AP1903 selectively removed undifferentiated H1-iC9 cells from a mixed cell population. This strategy therefore provides a layer of safety control before transplantation of a stem cell-derived product in therapy.
Biological engineering contains its own unique challenges, and purification of cells for therapies is among the trickiest. Until a method is developed to prevent fully pluripotent stem cells from being generated in the process of creating hPSCs, this method provides an elegant solution to a dangerous problem, helping to pave the way for more, and more efficient, stem cell therapies that restore health and function to people suffering from physiological disorders, physical injuries, or the effects of stem cell exhaustion.
 Wu, Y., Chang, T., Long, Y., Huang, H., Kandeel, F., Yee, J.-K., Using gene editing to establish a safeguard system for pluripotent stem cell-based therapies, ISCIENCE (2019), doi: https://doi.org/10.1016/j.isci.2019.11.038.