In 2012, a mere six years after his discovery of induced pluripotent stem cells (iPSCs), Shinya Yamanaka was awarded the Nobel Prize in Medicine. Many Nobel winners aren’t recognized until decades after their initial groundbreaking studies. That goes to show you the importance of Yamanaka’s technique, which can reprogram a person’s cells, for example skin or blood, into embryonic stem cell-like iPSCs by just adding a small set of reprogramming factors.
These iPSCs are pluripotent, meaning they can be specialized, or differentiated, into virtually any cell type in the body. With these cells in hand, researchers have a powerful tool to study human disease and to develop treatments using human cells directly from patients. And at the same time, this cell source helps avoid the ethical concerns related to embryonic stem cells.
Still, there has been lingering uneasiness about how well iPSCs match up to embryonic stem cells (ESCs), considered the gold-standard of pluripotent stem cells. One source of those concerns is that the iPSC method doesn’t completely reprogram cells and they retain memory of their original cell source, in the form of chemical – also called epigenetic – modifications of the cells’ DNA structure. So, if a researcher were to make, say, heart muscle cells from iPSCs that have an epigenetic memory of its skin cell origins, any resulting conclusions about a given disease study or cell therapy could be less accurate than ESC-related results. But a report published yesterday in PNAS should help relieve these worries.
The CIRM-funded study – a collaboration between the labs of Joseph Wu and Michael Synder at Stanford University and Shoukhrat Mitalipov at Oregon Health & Science University – carried out an exhaustive series of experiments that carefully compared the gene activity and cell functions of iPSC-derived cells with cells derived from embryonic stem cells. The teams sought to compare cells generated from the same person to be sure any differences were not the result of genetics. To make this “apples-to-apples” comparison, they generated embryonic stem cells using another reprogramming technique called somatic cell nuclear transfer (SCNT).
With SCNT, a nucleus from an adult cell is transferred to an egg which has its own nucleus removed. The resulting cell becomes reprogrammed back into an embryo from which embryonic stem cells are generated – the researchers call them NT-ESCs for short. In this study, the skin cell sample used for making the iPSCs and the cell nucleus used for making the NT-ESCs came from the same person. In scientific lingo, the iPSCs and SCNT stem cells are considered isogenic.
Now, it turns out the NT-ESC reprogramming process is more complete and eliminates epigenetic memory of the original cell source. So why even bother with iPSCs if you have NT-ESCs? There are big disadvantages with SCNT: it’s a complex technique – only a limited number of labs pull it off – and it requires donated human eggs which carries ethical issues. So, if a direct comparison iPSCs and SNCT stem cells shows little difference then it would be fair to argue that iPSCs can replace NT-ESCs for deriving patient-specific stem cells.
And that’s exactly what the teams found, as Dr. Wu summarized it to me in an interview:
“Direct comparison between differentiated cells derived from iPSCs and SCNT had never been performed because it had been difficult to generate patient-specific ESCs by the SCNT method. Collaborating with Dr. Shoukhrat Mitalipov at Oregon Health & Science University and Dr. Michael Snyder at Stanford University, we compared patient-specific cardiomocytes (heart muscle cells) and endothelial (blood vessel) cells derived by these two reprogramming methods (SCNT and iPSCs) and found they were relatively equivalent regarding molecular and functional features.”
Because the heart muscle and blood vessel cells were similar regardless of reprogramming method, it suggests that the epigenetic memory that remained in the iPSCs is less of a worry. Dr. Wu explained to me this way:
“If iPSCs carry substantial epigenetic memory of the cell-of-origin, it is unlikely these iPSCs can differentiate to a functional cardiac cell or blood vessel cell. Only the stem cells free of significant epigenetic memory can differentiate into functional cells.”
Hopefully these results hold up over time because it will bode well for the countless iPSC-related disease studies as well as the growing number of iPSC-related projects that are nearing clinical trials.