A recent discovery in stem cell research is no minor event: researchers have figured out how to reprogram adult cells into a state that is nearly indistinguishable from that of embryonic, pluripotent stem cells. This is huge news that promises to accelerate the pace of research in the field.
The problem has always been that cells exist in distinct states. A skin cell, for instance, has one set of genes essential for its specific function activated, and other sets of genes turned off; an egg cell has different patterns of gene activation and inactivation. Just taking the DNA from a skin cell and inserting it into the egg cell isn’t necessarily going to create a functional egg cell, because genes essential for egg cells may be switched off in the skin cell DNA, and we don’t know how to specifically switch them on. The process of somatic cell nuclear transfer has been hit or miss for that reason, with very high failure rates—scientists are basically trying to make the right configuration of genes switch on by giving the nucleus a good hard kick, and hoping that something in the cells will reconfigure the pattern of gene activation into something appropriate.
What the discovery by Takahashi et al. accomplishes is to reveal how to specifically switch on the right pattern of genes for a pluripotent stem cell. They have discovered the reset button for mammalian cells: a simple trigger that puts the cells in the right state to become anything else.
This reset button consists of four genes: Oct3/4, Sox2, Klf4, and c-Myc, together with maintenance of a specific extracellular environment. These genes are transcription factors, genes that regulate the expression of other genes, and they were suspected of being important because, among other things, they are differentially expressed in normal embryonic stem cells, something that wouldn’t be known if people hadn’t been doing research on embryonic stem cells. The c-Myc and Klf4 genes are thought to modify chromatin, changing patterns of DNA methylation and histone modifications, enabling Oct3/4 and Sox2 to slip in and bind to specific gene targets. These few genes acting in concert then trigger a whole series of downstream genetic events that switch the cell to the pluripotent state.
The results have been tested in several ways. A similar procedure in mice has been used to create whole mouse embryos that then go on to develop into adults, demonstrating totipotency. The human induced pluripotent stem cells have been subjected to a whole battery of assays that show gene expression similar to that of true embryonic stem cells. And colonies of these human induced stem cells have been injected into mice, where they form tumors — teratomas that differentiate into an array of different tissue types (but poorly organized—they do not form small human babies inside the mice, but only scattered bits). Gut, muscle, skin, cartilage, fat, and nervous tissue all form, as shown in the image below, demonstrating the pluripotency of these stem cells.
(click for larger image)
Hematoxylin and eosin staining of teratoma
derived from iPS cells (clone 201B7). Cells
were transplanted subcutaneously into four
parts of a SCID mouse. A tumor developed
from one injection site.
There is a catch. The way the four genes were activated in these cells is effective, but lacks a little finesse: constructs containing the genes with promoters we can control were inserted by retroviral transfection into the target cells. There are problems with this technique. Where the genes get inserted is random, and has the potential to create new mutations. In addition, they aren’t regulated in quite the same way as the normal genes are — this leaves the cells wide open for aberrant expression. The mice made from induced stem cells, for instance, are highly prone to cancer.
What the investigators have accomplished is to discover the reset button for the cell, but the way they currently press it is by hitting it hard with a ball peen hammer.
This is good enough for a start, and they’ve shown that these four genes do the necessary job, but eventually we’ll want to find a more elegant way of activating them. Rather than inserting extra copies of the genes, for instance, we’d like to find a way to turn on the signaling cascades that will activate the natural copies present in the genome; some combination of extracellular factors and injected small molecules that do the same job of putting the cell in the ES state.
This discovery is probably going to become a political football in short order, with the far right politicians who have restricted American research into embryonic stem cells claiming vindication. However, let’s point out some realities here. Americans did not make this discovery; Japanese researchers did. It required understanding of gene expression in embryonic stem cells, an understanding that was hampered in our country. It’s going to require much more confirmation and comparison between the induced pluripotent stem cells and embryonic stem cells as part of the process of making this technique useful — science doesn’t take just one result from a few labs and accept it as gospel truth. And we definitely need to figure out better ways of switching the four genes on. Figuring that out will require more research into how organisms switch cells into the ES state in situ &mdash we can’t figure that out from these cells with inserted, artificial gene constructs.
Another essential point is that scientists are excited about this work because it opens up avenues for basic research into development and differentiation. These cells are NOT useable for therapies…the immediate, practical applications that the electorate wants from stem cell research. They also cannot be used for reproductive cloning, although that won’t trouble most people. These are cells with retroviral infections, potential unknown mutations, and that have genetic modifications that make them prone to collapse into cancers. We are not going to be able to grow new organs and tissues for human beings from a few skin cells using this particular technique. It’s going to take more work on embryonic stem cells to figure out how to take any cell from your body, and cleanly and elegantly switch it to a stem cell state that can be molded into any organ you need. What this work says is that yes, we’ll be able to do that, it isn’t going to be that difficult, and that we ought to be supporting more stem cell research right now so we can work out the details.
Or we can just sit back and let the Japanese and Europeans and Koreans do it for us, which is OK, I suppose. Just keep in mind that ceding the research to others means giving them a head start on the development of all the subsequent breakthroughs, too, and that what we’re doing is willingly consigning US research in one of the most promising biomedical research fields ever to an also-ran, secondary status.
Takahashi K, Tanabe K, Ohnuki M, Narita M, Ichisaka T, Tomoda K, Yamanaka S (2007) Induction of Pluripotent Stem Cells from Adult Human Fibroblasts by Defined Factors. Cell 131(4):1-12.