Pluripotent stem cells and cell-based therapies – are we there yet?
By: Andras Nagy, PhD, & Kristina Nagy
The first derivation of Embryonic Stem (ES) cells back in 19811,2 was a breakthrough event. Suddenly, we had cells that could be propagated to virtually unlimited amounts in vitro, and at the same time, could be differentiated into any cell type found in the body. In 1998, ES cells were successfully derived from human embryos as well3, which set the stage for serious endeavours to find clinical applications to treat cell or tissue loss due to injury and degenerative diseases with stem cell based therapies. However, a serious hurdle still remained: transplants derived from ES cells would not be histo-compatible with the patients. A few years ago, Yamanaka and his student Takahashi in Kyoto succeeded with a task that, until then, was thought to be impossible; reprogramming terminally differentiated mouse, and then human skin cells, into a pluripotent stem cell state4,5. They called them induced Pluripotent Stem (iPS) cells.
The patient-specific nature of the iPS cells has allowed for great strides to be made in bringing cell-based therapies towards realization. Many people immediately drew the conclusion that ES cells would no longer be needed. Others have been more cautious; we have had decades to study ES cells, while iPS cells are relatively novel and we know very little about them. In fact, recent studies have shown that there are significant differences between the two (reviewed by Puri and Nagy 2012)6. It is, at this point, not possible to say which will be the final winner as the best source for future therapies. Preclinical trials are now initiated for the treatment of a wide array of conditions such as spinal cord injury and blindness; the hopes are high and the promises great. So why are these therapies not yet available in the clinic? Despite the urge to deliver, as quickly as possible, ready-to-apply therapies to patients who so desperately need them, research aimed to understand iPS cells in general and the reprogramming process in particular remains crucially important.
The first iPS cell lines were created using a viral delivery method to deliver the transgenes needed to induce reprograming. Although this is a very efficient method, it has some serious drawbacks. Viruses integrate their genome into their host cells in a completely random manner. Such an event may disrupt or activate genes whose proper regulation is essential for normal physiological function and for preventing cancerous occurrences. Secondly, the transgenes that are needed during reprogramming have to be turned off once the process has been completed, but occasionally, virally delivered transgenes can be randomly reactivated later on, possibly leading to uncontrolled proliferation of the transplanted cells. To address these shortcomings, many laboratories are now working relentlessly to develop alternative methods for producing iPS cells, including the use of transposons that can be seamlessly removed once reprogramming is completed7, serial plasmid transfection, episomal transfection, RNA transduction, and protein transduction.
Another serious concern relates to the genomic changes that we know occur during reprogramming8. In order to fully understand why this happens, we need to gain a much better knowledge of the nature of the molecular, genetic, and genomic changes that take place during this process. The higher the resolution of insight that can be obtained, the more detailed our understanding will be.
Our laboratory has initiated an international effort to map the genetic, genomic, gene expression, and proteomic changes, as well as the complex interactions between these factors that take place during the reprogramming process. This collaboration enlists experts in the field of bioinformatics to analyze and make sense of the huge amount of data being generated. It is with high hopes that we embarked on this research endeavour, and we look forward to unravelling the fine details of the reprogramming process over the coming months and years.
We must remember that stem cells have many common attributes with cancerous cells. Many of the genes that are turned on in stem cells can also be found expressed in tumors. If ES or iPS cells are transplanted before they are properly differentiated, they do indeed form teratomas. Although these tumors are not malignant in the sense that they do not metastasize, they nevertheless are cancerous. It is, therefore, important to find appropriate safeguards to ensure that transplanted cells are truly and fully differentiated before they are grafted. However, even if we do make sure to only transplant differentiated cells, there can never be an absolute guarantee that one of them would not proliferate uncontrollably. The only way to reliably circumvent this problem is by equipping the transplanted cells with a suicide system. We are currently developing such a genetic modification that results in the death of cells that have lost control.
History has taught us that many a great solution comes from combining different approaches to solve a problem. Age-related Macular Degeneration (AMD), for example, is caused by an abnormal increase in retinal blood vessel density. The newly formed vessels are leaky and cause damage to the retina that ultimately leads to blindness. Our laboratory has a long history of studying the genetic mechanisms behind blood vessel formation and which mechanisms could be used to induce or inhibit the generation of new vessels. We are now developing stem cells that are genetically modified to produce a protein that inhibits blood vessel formation. Once transplanted into the eye of AMD patients, they would act like a drug-producing “factory” in situ.
As with all things powerful, the future of stem cell-based therapies lies in harnessing their potential for doing good, while making sure we do not let them do any harm.
Kristina Nagy from the Nagy Lab
Dr. Andras Nagy’s Laboratory
Mount Sinai Hospital,
Samuel Lunenfeld Research Institute
Andras Nagy, PhD
Mount Sinai Hospital,
Samuel Lunenfeld Research Institute
Professor at the Department of Gynaecology and Obstetrics,
University of Toronto
1. Evans, M. J. and Kaufman, M. H. (1981). Establishment in culture of pluripotential cells from mouse embryos. Nature 292, 154-6.
2. Martin, G. R. (1981). Isolation of a pluripotent cell line from early mouse embryos cultured in medium conditioned by teratocarcinoma stem cells. Proc Natl Acad Sci U S A 78, 7634-8.
3. Thomson JA, Itskovitz-Eldor J, Shapiro SS, Waknitz MA, Swiergiel JJ, Marshall VS, Jones JM. (1998). Embryonic stem cell lines derived from human blastocysts.
Science. 1998 Nov 6;282(5391):1145-7. Erratum in: Science 1998 Dec 4;282(5395):1827.
4. Takahashi K, Yamanaka S. (2006). Induction of pluripotent stem cells from mouse embryonic and adult fibroblast cultures by defined factors. Cell. 2006 Aug 25;126(4):663-76.
5. Takahashi K, Tanabe K, Ohnuki M, Narita M, Ichisaka T, Tomoda K, Yamanaka S. (2007).
Cell. 2007 Nov 30;131(5):861-72.
6. Puri MC, Nagy A. (2011). Concise Review: ES vs. iPS Cells; the Game is on. Stem Cells 2011.
7. Woltjen, K., Michael, I. P., Mohseni, P., Desai, R., Mileikovsky, M., Hamalainen, R., Cowling, R., Wang, W., Liu, P., Gertsenstein, M., Kaji, K., Sung, H-K and Nagy, A. (2009) piggyBac transposition reprograms fibroblasts to induced pluripotent stem cells. Nature 2009;458:766-70.
9. Hussein, S. M., Batada, N. N., Vuoristo, S., Ching, R. W., Autio, R., Narva, E., Ng, S., Sourour, M., Hamalainen, R., Olsson, C., Lundin, K., Mikkola, M., Trokovic, R., Peitz, M., Brustle, O., Bazett-Jones, D. P., Alitalo, K., Lahesmaa, R., Nagy, A. and Otonkoski, T. (2011) Copy number variation and selection during reprogramming to pluripotency. Nature 2011;471:58-62.