Have you ever wish you could just clone yourself? How much more work you’d be able to get done in a day! While creating an exact copy of yourself that knows everything you do is still very much in the realms of science fiction, cloning human cells is a reality.
The first big leap came in 1962, when John Gurdon successfully cloned frogs using somatic cell nuclear transfer (SCNT). However, we had to wait over 50 years for the demonstration that this can be done with human cells. Last year, in what has been called the biggest stem cell news story of 2013, researchers successfully showed that differentiated human nuclei could be reprogrammed by the specialized environment of the egg.
This technique has opened the door to a new kind of stem cell: nuclear transfer embryonic stem cells (NT-ESCs), and has unlocked the possibility of creating patient-specific stem cells lines for ‘therapeutic cloning’. But how do these cells shape up to induced pluripotent stem cells (iPSCs) and which reprogramming method is best?
The epigenetic fidelity of factor-based reprogramming
Patient-specific iPSCs have the potential to revolutionize modern medicine. However, since their initial conception, many studies have questioned whether iPSCs can be considered equivalent to ESC lines derived from IVF embryos. These studies have revealed aberrations in DNA methylation, hydroxymethylation, and the regulation of imprinted genes in iPSCs. In particular, low passage iPSCs appear to retain an epigenetic memory, which favors their differentiation into lineages related to the donor cell.
SCNT more closely resembles development in vivo, but creating NT-ESCs is tricky and requires egg donors. Clearly, this creates a whole load of ethical issues as exemplified by the disgraced Korean scientist Hwang Woo-suk, who falsely claimed to have produced NT-ESCs in 2004 and later admitted to paying donors and obtaining eggs from the female researchers of his lab
Is the egg better at reprogramming the epigenome?
If NT-ESCs are to replace iPSCs in future stem cell therapies then they will have to outshine iPSCs in several ways considering the additional investment needed to create them. However, a study published in Nature earlier this year seemed to suggest that NT-ESCs may indeed be worth the extra effort.
Ma et al. created human NT-ESCs and iPSCs from the same fetal fibroblasts and compared the genetic and epigenetic landscape of these cells with those of IVF-ESCs. Although the transcriptional and DNA methylation signatures of NT-ESCs and IVF-ESCs were similar, iPSCs clustered apart. Overall, 629 genes were differentially expressed between iPSCs and IVF-ESCs whereas only 48 genes were differentially expressed between NT-ESCs and IVF-ESCs. Human iPSCs also retained 8 times more methylation sites specific to their fibroblast origin. Thus, when it comes to reprogramming the epigenome, it appeared that we still have a lot to learn from the egg.
NT-ESCs vs. iPSCs: the jury is still out
Thing aren’t completely clear cut yet though, and thanks to a study published in November in Cell Stem Cell the debate on NT-ESCs vs. iPSCs is still scorching hot. Using a similar approach involving isogenic NT-ESC and iPSC lines, Johannesson and colleagues reported that the expression and DNA methylation profiles of NT-ESCs and iPSCs are comparable. Both showed differences with IVF-ESCs, but the extent of these differences was similar. Their conclusion: these two reprogramming methods are equally matched.
The origin of the disagreement between the two studies is still unclear, although the donor cells used (fetal vs. neonatal or adult) and the type of reprogramming method (retroviral vs. mRNA) may offer some explanations. At least the two studies agree on one thing: both reprogramming processes are associated with a similar number of genetic abnormalities, despite the fact that the generation of iPSCs is much longer and involves several rounds of cell division. Thus, reprogrammed cell lines generated from either technique would need to be screened for mutations prior to therapeutic use.
Neither study addressed the functional differences between NT-ESCs and iPSCs. Ultimately, it is the differentiation capacity of these cells and their tumorigenicity that will determine the use of one reprogramming method over another. It’s still early days for NT-ESCs and no doubt the eggheads behind these studies are continuing to explore the full potential of these cells.