Three years ago, Kyoto University scientist Shinya Yamanaka made induced pluripotent stem cells (iPSCs) by inserting four genes into skin cells from laboratory mice. To get these genes into cells he used genetically engineered retroviruses, which insert copies of genes directly into the chromosomes of the cells they infect. In order to convert the skin cells into iPSCs, he had to infect the cells with four different recombinant viruses, and subject the cells to four different insertion events. Because retroviruses randomly insert into the gene, there is the possibility that they could insert their DNA right in the middle of an active gene. Thus Yamanaka’s procedure left people worried about making mutations in tumor suppressor genes or other genes necessary for life. If iPSCs were going to be used for clinical purposed, then how could such a procedure be used in the clinic if it was so potentially dangerous?
Now two labs have shown that you can do the same experiment by inserting one piece of DNA to do the same job. Rudolf Jaenisch’s lab at MIT and Konrad Hochedlinger at Harvard University have combined the four mouse reprogramming genes onto a single piece of DNA, known as a cassette, and inserted it at a single locus in the mouse genome. The mice with the insert were bred, and their somatic cells were transformed into iPS cells following the addition of the antibiotic doxycycline, which triggers the cassette to express the four reprogramming genes. This innovation saves time and money and brings iPSCs one step closer to clinical trials.
One group of experiments that scientists hope to do with these newly made mouse strains is test the ability of iPSCs to form particular cell types versus traditionally made embryonic stem cells (ESCs). In a study published earlier this year in Cell Stem Cell, hundreds of genes are differentially expressed between iPSCs and ESCs (Chin, M. H. et al. (2009) Cell Stem Cell 5, 111-123). Another study in Nature showed that iPS cells are not as efficient as embryonic stem cells at differentiating into all cell types (Zhao, X.-Y. et al. (2009) Nature 461, 86-90).
Nature News has a fascinating article on induction of pluripotent stem cells by means of genetic reprogramming. Typically, skin cells have been used for reprogramming experiments. Cells called fibroblasts, which are prevalent in skin and help heal the skin when injured, have been the cell of choice for induced pluripotent stem cell (iPSC) production.
Recently, fat cells and pigmented skin cells appear to produce iPSCs much more efficiently and quickly. Reprogramming procedures with skin fibroblasts are rather inefficient and slow. Reprogramming human skin cells takes about a month for 1 in 10,000 fibroblasts to form iPSCs. Other cell types can do the job besides fibroblasts. For example blood, hair, bone marrow, and neural stem cells can be converted into iPSCs, but their conversion rate is not any better than that of skin fibroblasts. However, foreskin fibroblasts from a baby are better at making iPSCs (see Aasen, T. et al. Nat. Biotechnol. 26, 1276-1284), but this is hardly a good source of cells for adults.
Is there are better way? Apparently there is. Joseph Wu and Michael Langaker at Stanford University School of Medicine in California have converted fat tissue into iPSCs, and it took them only two days to acquire enough material for reprogramming. Compare that to about a month of biopsies to get enough fibroblasts. Additionally, cellular reprogramming of fat cells took only two more weeks and was 20-times more efficient than fibroblast reprogramming. By using fat cells, they were able to reduce the time required for the procedure by six to eight weeks (Sun, N. et al. Proc. Natl. Acad. Sci. USA, doi:10.1073/pnas.0908450106). Likewise, Konrad Hochedlinger and his co-workers at the Massachusetts General Hospital in Boston reprogrammed melanocytes, the skin cells that produce pigmented skin. Melanocytes undergo reprogramming after just 10 days and with five-fold greater success rates compared with fibroblasts (Utikal, J., Maherali, N., Kulalert, W. & Hochedlinger, K. J. Cell Sci., doi:10.1242/jcs.054783).
These findings suggest that making iPSCs is easier than previously thought. The therapeutic uses of iPSCs just became even more attractive.