The ability to convert one cell type into another has caused great excitement in the stem cell field. Two techniques exist: one reprograms somatic cells into pluripotent stem cells (iPS cells), the other converts somatic cells directly into other types of specialized cells by transdifferentiation. These techniques raise high hopes that patient-personalized cell therapies will become a reality in the not-so-distant future.
A technique developed by Yamanaka in 2006 makes it possible to convert essentially any cell in the body back into pluripotent stem cells that are almost identical to embryonic stem cells. This is done by adding a quartet of proteins: the transcription factors Oct4, Sox2, Klf4 and Myc. The resulting induced pluripotent stem (iPS) cells can be grown and multiplied almost indefinitely without losing their potential to differentiate into a broad range of cell types. If a clinician wanted to use this technology to treat a patient with say, Parkinson's disease, she/he would prepare a skin biopsy, grow skin-derived cells called fibroblasts in the lab, add the Yamanaka combination of four proteins and wait a couple of months for stable populations or ‘lines’ of induced pluripotent stem cells to be established. Since iPS cells can proliferate indefinitely, they can be isolated relatively easily and a small initial population can be used to produce a large number of cells. In our hypothetical Parkinson’s treatment, the multiplied iPS cells would then be made to differentiate into dopaminergic neurons, the cell type that is deficient in Parkinson's patients. As a final step the neurons would be purified and injected back into the patient.
An alternative to the iPS procedure is transdifferentiation. This approach uses transcription factors to convert a given cell type directly into another specialized cell type, without first forcing the cells to go back to a pluripotent state. Research in the 1980s and 90s showed that fibroblasts can be converted directly into muscle cells at very high efficiencies using the transcription factor MyoD. Similarly, scientists found they could use a transcription factor called C/EBPa to turn lymphocytes into macrophages (different types of white blood cell). However, this transdifferentiation approach has only recently taken off in the stem cell community. One reason for the slow start is that it took the Yamanaka experiments on iPS to convince many skeptics that cell reprogramming is possible at all. Another is that for a long time it seemed that direct conversions could only be achieved between 'sister cells', such as between two types of blood cells. The relationship between cell types is sometimes pictured as a developmental ‘landscape’.
Embryonic stem cells and iPS cells sit on a mountain at the very top of the landscape and can produce cells that fall down into all the different more specialized valleys below. Once the cells are settled in a particular area, travelling across a ‘tectonic plate’ into a different region to become an unrelated cell type is a very tough challenge. ‘Sister’ or ‘neighbouring’ cell types can more easily move over a small hill from one neighbouring valley to the next, if given the right encouragement. This restriction to 'small jumps' between related cell types kept transdifferentiation firmly within the realm of basic research studies. Then, in 2010, the barrier was broken. A group of researchers at Stanford demonstrated that a combination of three neural transcription factors can convert fibroblasts into functional neurons (Vierbuchen et al., Nature 2010). This study showed that transcription factors can induce 'large jumps' between distantly related cell types, opening up the prospect that any desired specialized cell could be generated from essentially any other cell type. Since then, blood cells have also been generated from fibroblasts (Szabo et. al, Nature 2010), making it likely that many more such transitions will be reported in the near future.
"The verdict is wide open as to which technique will enter the clinic first"
So, which of the two approaches – iPS or transdifferentiation – will make it into the clinic first? Which will eventually prevail? Because both technologies are patient-specific, there is virtually no risk of immune rejection. The iPS approach has the advantage that it enables us to obtain large numbers of cells. Genetic defects can be corrected at the iPS cell stage, meaning that specialized cells made for the patient no longer have the defect. The disadvantages of iPS are its complexity, high costs and the length of time required to produce first iPS cells and then the specialized cells needed for transplantation. There is also a risk that residual iPS cells in the transplant could cause tumors. The advantages of transdifferentiation are its relative simplicity, lower costs and shorter times required. However, it is unclear whether it will be possible to generate the large numbers of specialized cells required for transplantation via the transdifferentiation route. And then there are a number of concerns common to both techniques. The cells produced for transplantation may not be fully functional, for example because they retain a 'memory' of their origin. They may not engraft efficiently or correctly when transferred to the patient, or they may not survive long-term. Finally, not all degenerative diseases might be amenable to treatment by cell replacement strategies at all, perhaps even excluding certain disease groups entirely. In conclusion, the verdict is wide open as to which technique will enter the clinic first, and and for what diseases each approach will be most effective.
Related articles on EuroStemCell:
More from Thomas Graf:
- Film of a Thomas Graf lecture about his resesarch at the National Institutes of Health, USA in August 2010.
- Cellular Identity and Transdifferentiation, an interview with Thomas Graf by Monya Baker of Nature Stem Celli Reports (requires subscription)
The research papers mentioned in the above article:
- Direct conversion of fibroblasts to functional neurons by defined factors, Vierbuchen T, Ostermeier A, Pang ZP, Kokubu Y, Südhof TC, Wernig M, Nature 2010; 463, 1035-1041
- Direct conversion of human fibroblasts to multilineage blood progenitors, Szabo E, Rampalli S, Risueño RM, Schnerch A, Mitchell R, Fiebig-Comyn A, Levadoux-Martin M, Bhatia M, Nature 2010; 468, 521-526
Scientific review articles of interest:
- Forcing cells to change lineages, Graf T and Enver T, Nature 2009; 462, 587-594
- Dedifferentiation, transdifferentiation and reprogramming: three routes to regeneration, Jopling C, Boue S, Izpisua Belmonte JC, Nature Rev. Mol. Cell Biol. 2011; 12, 79-89
First use of the landscape metaphor described above:
- Waddington, C. H. The Strategy of the Genes, Geo Allen & Unwin, London, 1957.
Landscape diagram provided by Debbie Maizels of Zoobotanica. Fibroblast image (header) by Tilo Kunath.