As has been the case for more than a decade, the promise of stem cells to create breakthroughs in cell biology and medicine has been hampered by the difficulty in obtaining sufficient quantities of high quality pluripotent stem cells (cells capable of changing into almost any other kind of cell). Human embryonic stem cells are the gold standard but controversial, and therefore the most difficult to obtain. There are many efforts underway to find alternative sources, most of them aimed at inducing adult cells (cells that have already taken on a specific characteristic such as blood cells or neurons) to becoming stem cells again. There have been several successful approaches to creating what are called induced pluripotent stem cells (iPSC); however, with a catch.
As covered in a SciTechStory post February 15, 2010, Induced stem cells not such good news efforts to create the equivalent of human embryonic stem cells starting with adult cells have not turned out so well. A study done at the University of Wisconsin (Madison, USA) compared five embryonic stem cell lines with twelve induced stem cell lines. They found that the induced pluripotent stem cells (iPSC) converted to neuron cells do not match all the differentiations made by embryonic stem cells. The study also showed that iPS cells created without using genes, which in theory should have resulted in ‘cleaner’ differentiation, did no better than gene induced cells.
Now a new study from the Massachusetts General Hospital Center for Regenerative Medicine and Harvard Stem Cell Institute has identified a gene whose silencing may be responsible for at least some of the induced pluripotent stem cells’ limitations. By using the latest genetic assay tests on the DNA of stems cells from mice, they compared the developmental potential of two natural embryonic stem cell lines with induced pluripotent stem cell lines. By comparing the RNA transcriptions (copies made from the DNA) of each line, they found that the natural stem cells produced viable mice, and the induced cells did not. The difference was an obvious reduction in the transcription of two genes in the induced cells. These genes are found in a cluster on chromosome 12 and are normally imprinted maternally (only the mother’s genes are expressed).
In an examination of more than 60 other iPSC lines, the same pattern of silenced (non-expressing) genes was found in the vast majority. Tissues produced from these cell lines were genetically correct reproductions, but the gene silenced cells could not fully develop into live mice. Another experiment, this time reactivating the silenced genes in a few of the iPSC lines, and the cells were able to produce live animals. This may have been the first time that live animals were produced from induced pluripotent stem cells.
“The activation status of this imprinted cluster allowed us to prospectively identify iPSCs that have the full developmental potential of embryonic stem cells,” says Matthias Stadtfeld, PhD, a co-lead author of the report. “Identifying pluripotent cells of the highest quality is crucial to the development of therapeutic applications, so we can ensure that any transplanted cells function as well as normal cells. It’s going to be important to see whether iPSCs derived from human patients have similar differences in gene expression and if they can be as good as embryonic stem cells – which continue to be the gold standard – in giving rise to the 220 functional cell types in the human body.”
Pluripotent stem cells can also be created by nuclear transfer – removing the nucleus of an adult cell and replacing it with the nucleus of an embryonic cell…essentially the same technique used to clone animals. This approach also produces viable embryonic stem cells, but is difficult to perform in quantity. Consequently, the ability to make viable stem cells using induced methods may open the door to producing a greater quantity of high quality cells. Moving on from mouse stem cells, the next step is to perform similar experiments with human stem cells.