As suggested by the title of this review, the ultimate application of patient-derived stem cells may be their use as cellular replacement therapies. There are many hurdles to achieving this goal.
First, stem cells need to be generated without the use of exogenous DNA or permanent genomic modification of patient cells. Most re-programming to date has been done using retroviruses or lentiviruses to transduce dermal fibro-blasts isolated from patients.39
The random integration of these viruses may inadvertently inactivate or activate endogenous genes based on the stochastic insertion of these viruses. Moreover, these viruses harbor strong promoters and these may be re-activated during iPSC differentiation. Such genetic changes would greatly diminish enthusiasm from ever using these cells as therapies. However, they may still be useful for disease modeling though the genetic changes described above need to be carefully considered when interpreting any phenotypes. Re-programming alternatives to viruses include plasmid-based transient transfection40
as well as the introduction of proteins, RNA or small molecules that can re-program target cells.41– 43
Patient-derived stem cells should ideally never be exposed to any animal products (for example, bovine serum) or animal cells that could transfer retro-viruses and potentially compromise safety when reintroduced back to patients. There were early concerns about the quality of iPSC generated from patients if additional genetic mutations were being introduced during re-programming. A recent study found that much of the copy number variation previously noted in various lines of human iPSC was already present in parental fibro-blasts and likely reflects the clonal nature of iPSC.44
Second, patient-derived stem cells should not retain epigenetic ‘memory’ from the cell of origin such as a fibroblast. This is a likely a chromatin-based mechanism that may restrict developmental potential and possibly prevent, for example, the production of fully differentiated and functional neurons.45,46
Advances in our understanding of re-programming should address this concern but this aspect of stem cell biology will require much more study.
Third, the appropriate delivery of cells to the brain or spinal cord is not at all established. Simple injection of cells into the central nervous system raises multiple points of concern about the proper delivery site, ‘dose’ of cells and the timing and number of treatments. Furthermore, will transplanted cells need to be delivered with a permissive matrix or growth factors to encourage suitable migration and integration of cells to the host nervous system? To properly overcome this hurdle, multidisciplinary teams that contain neurosurgeons and biomedical engineers will be essential.
After the proper delivery of cells, a fourth hurdle will be preventing or minimizing oncogenic transformation of transplanted cells. There is a very real concern that undifferentiated patient-derived stem cells when given back to a patient will behave like a tumor. This most likely would behave like a teratoma but even a malignant cancer is possible. The use of more differentiated cells such as NPC or fully differentiated neurons may mitigate this concern if the transplanted cells have a limited capacity for further proliferation. Additional approaches to avoid tumor complications include genetic engineering of patient-derived stem cells to add ‘suicide’ genes such as thymidine kinase. This would allow transplanted cells to be selectively killed by application of a medication such as ganciclovir. This strategy however is problematic for several reasons including the potential loss of thymidine kinase in transformed cancerous cells rendering this rescue strategy moot. There may also then be inadvertent killing of transplanted cells by accidental exposure to commonly used medications such as ganciclovir. This approach would also contradict the first guideline to avoid the addition of any exogenous DNA as this may lead to immune rejection of transplanted cells (see next hurdle). It should be stressed that there is minimal data to support any of these concerns other than theoretical issues that should be carefully considered by regulatory agencies as well as by scientists, physicians and patients.
A fifth hurdle is to avoid immunorejection of transplanted cells. This last point is generally touted as one of the key advantages of using patient-derived stem cells. This is due to a widely accepted belief that such autologous cells as ‘self’ will not be rejected by the immune system. While this is may indeed be probable, it has not been rigorously tested and there is some evidence from rodent models that re-programmed cells may still provoke an immune response upon transplantation even leading to rejection.47
This may reflect the method of iPSC re-programming, however, as stem cells generated by retroviral transduction most strongly triggered the host immune response.
The future use of patient-derived stem cells needs to be tempered by the many difficult obstacles as detailed above. In addition, severe neurological complications have already been reported after transplantation of human cells as therapies. A boy with ataxia telangiectasia, a genetic disorder caused by mutation of the ATM
underwent repeated intracerebellar and intrathecal injections of fetal-derived NPC in an attempt to treat his disorder.49
Unfortunately, this child then developed both brain and spinal cord tumors. A partial resection of his spinal cord tumor revealed female cells confirming its origin from a donor.49
As another example, a 17-year-old young lady with an aggressive form of multiple sclerosis underwent transplantation with an allograft of MSC isolated from umbilical cord blood as well as autologous MSC derived from her own adipose tissue. She then rapidly became very ill with altered mental status, severe weakness and demyelination consistent with an acquired encephalopathy.50
It is unknown whether this adverse clinical course was due to the transplanted allograft or the autologous cells and/or an exacerbation of her underlying multiple sclerosis though she never recovered back to her baseline. Given all of these potential and serious shortcomings, patient-derived stem cells may require many years of testing and monitoring before they can be considered safe and effective therapies for neurological disorders. Their use should also be guided by established ethical principles with very careful consideration of what patients are suitable and what neurological disorders are appropriate for treatment with patient-derived stem cells.