August 10, 2020

The Niche

Knoepfler lab stem cell blog

Dieter Egli Interview: NT-ES cells, IPSC, Mitochondrial Transfer, & More

Dieter EgliIt’s a particularly exciting time for the stem cell field.

One of the most notable developments in the last year or so is the production and preliminary study of a totally new type of human embryonic stem cells (ESC) made by nuclear transfer instead of using leftover in vitro fertilization (IVF) embryos.

This process of so-called therapeutic cloning has the power to produce patient specific ESCs called NT-ESCs that can in principle be used in the future for autologous transplants for a number of diseases including Diabetes. This gives the field three possible types of powerful pluripotent stem cells including NT-ESCs, IVF ESCs, and induced pluripotent stem cells (IPSC).

One of the leaders in the stem cell field and more specifically in the area of NT-ESC is Dr. Dieter Egli of the New York Stem Cell Foundation (NYSCF). His lab has published a number of exciting and important publications recently. I invited Dr. Egli to do an interview about the recent developments in NT-ESC, mitochondrial transfer (aka 3-parent technology), and the future of the stem cell field. I want to thank him for doing the interview.

PK: In your recent excellent Johannesson, et al. Cell Stem Cell paper you compared NT-ESC and IPSC. Were you surprised at just how similar they were?

DE: Yes we were. We thought that if there are differences, they should be visible in our assay, comparing genetically identical stem cell lines that only differed with regard to the method of their derivation. Previous work had drawn conflicting conclusions regarding the similarity of iPS cells and ESCs, but these analyses were done on genetically diverse stem cell lines. Both opinions had been voiced already, so this comes as a confirmation for some, for others as a surprise. It is indeed rather surprising that two very different paths to pluripotency, one resembling development, the other forcing a transition through cellular states that have no equivalent in nature, result in very similar cells. We must not forget though that such work cannot be exhaustive. The cells are similar with regard to the many things we looked at, but someone might want to look at their favorite process, to determine if there is a difference. We are happy to share the cell lines with anyone who wishes to perform additional analyses.

PK: Your findings are rather distinct from that of the Ma Nature paper from July from Mitalipov’s group. Can you comment at all on this difference and the possible reasons behind it?

DE: We had an advantage: we had two sets of nuclear transfer ES and iPS cell lines of adult and neonatal origin and had analyzed them using the same platform as Ma and colleagues.  As we finalized our manuscript, Ma and colleagues published their work, and made their primary data available to us as a result. So we were in possession of a larger set of data and cell lines. This difference was not entirely a matter of availability of cell lines, but also one of experimental design. There is a large amount of data on iPS cells available that could have been included in their analysis, to determine if their conclusions can be generalized. And some analyses are different between the two studies, for instance the whole genome bisulfite sequencing was not something we’ve performed, so we can’t speak to that.

PK: It seems like the big open question now is how the NT-ESC behave. For example, what are the differentiation properties of NT-ESC? Can you comment at all on this question of the functional properties of NT-ESC? Is it possible that NT-ESC could have some functional attributes that distinguish them from IPSCs?

DE: We don’t know the answer to this question. What I can tell you is that they both differentiate. There is of course great variability in differentiation efficiency of different human ES cell lines (e.g. Osafune et al. Marked differences in differentiation propensity among human embryonic stem cell lines. Nat Biotechnol. 2008 Mar;26(3):313-5.) So it is not a simple task to draw solid conclusions. And there is nothing like the functional assays for human cells as there are in mice, where we can do tetraploid embryo complementation and ask if the stem cell-derived mice are normal. There will always be some degree of uncertainty regarding what we are working with in the culture dish. But I think we have just reduced this uncertainty, and now know that if iPS cells are made using methods such as modified RNA, they are likely very close to the real thing.

PK: What are your thoughts at this point as to the future of NT-ESC? Do you think they will have clinical relevance down the road as the basis for stem cell therapies or is that less likely now? In addition to possible translation to the clinic, is there more we can learn from NT-ESC and this kind of reprogramming?

DE: It is neither more nor less likely. There is no therapeutic application of reprogrammed stem cell lines yet. There are so many uncertainties that depend on biological, economical, and logistical factors, and on the patenting landscape, that it is hard to predict the outcome. With these many uncertainties, it would be risky to put all eggs into just one basket. I think the nuclear transfer field is going to have a strong presence and will remain a primary research focus of my lab.

PK: In addition to the studies mentioned above, I’m interested in the other work going on in your lab. Can you tell us about more about it? For example, do you have studies ongoing that are more focused on IPSC? Other areas?

DE: My lab is trying to understand beta cell dysfunction in different forms of diabetes. We are generating patient-specific iPS cells and differentiating them to beta cells to determine why these beta cells might fail more readily than those of controls. Genetic manipulations to correct or introduce specific mutations are now also part of our toolbox, but patient-specific stem cells are the primary resource. The Berrie Diabetes Center and The New York Stem Cell Foundation Research Institute have provided an ideal environment for conducting such research.

PK: A hot topic right now is so-called human mitochondrial transfer technology (known colloquially as “3-parent technology”) to try to prevent mitochondrial disorders. The UK seems on the verge of potentially approving the use of this technology before year’s end. What are your thoughts on this technology? Would you like to see if approved in the US as well? How would you respond to those (including myself, by way of disclosure) who are concerned about possible risks?

DE: The vote in Parliament in the UK is about removing the legal bars that prevent this technology from potentially being used. We might see a legal hurdle in the UK removed, one that we don’t have here in the US, but it is not equal to this being applied. The review of efficacy and safety is a scientific and medical matter that must continue independent of the political decisions. My biggest criticism of the research is that the number of studies in this field is still very small. We and others have shown that in principle the exchange can result in a complete exchange of mitochondrial genotypes. We also know that the resulting cells show normal mitochondrial activities. This is reassuring, but an important part is the numbers. How often can we do this and it still is exactly the same as the last time? These are the questions that matter regarding clinical translation, and we don’t have all answers yet.

I am aware that you have been rather critical of this approach and so I was hoping you would ask me this question. We are all recipients of genetic material from two parents, 4 grandparents, 8 great-grandparents, and so on. The sine qua non of human reproduction is new combinations of existing genetic material. The technique proposed will do exactly that, combine normal mitochondrial DNA that some of us already carry within us, with the nuclear genome of the egg cell. This alters the pattern of inheritance from a disease-causing to a normal mitochondrial genome, thereby preventing the disease. Opponents of this technique often deliberately – and despite better knowledge – draw analogy to genetic manipulation, such as done in bacteria, plants or animals. This is wrong, because mitochondrial replacement cannot alter the DNA itself, nor does it provide a path to it, nor will it allow predicting the physical features of a child any more than by natural reproduction, apart from the fact that we know that there won’t be mitochondrial disease. I have no doubt that if provided accurate information, most will welcome the use of this technology. I am on the side of advancing this research, and I appreciate that we will have the guidance of the FDA to translate this work cautiously and responsibly.

Regarding the 3-parent term: men do not pass their mitochondrial DNA to the next generation, unlike women. Nevertheless, I don’t think there is a dad who feels he is less of a parent than the mom for reasons being that his mitochondrial DNA is not present in his offspring. Therefore, I don’t think that contribution of the mitochondrial DNA is sufficient to claim parenthood, nor is the mitochondrial DNA necessary to be the (genetic) parent, and I believe the term mitochondrial replacement or mitochondrial donation is a more accurate terminology.  There is more to parenthood than contributing mitochondrial DNA and the above term is not a word that should be used to describe this process.

PK: What excites you most about the stem cell field now? Where do you see the field in say 10 years?

DE: I have been in this field for now 9 years. At that time, things like generating autologous pluripotent stem cells, or differentiating beta cells were just an inspirational concept. In just that time, they have become a reality. The use of autologous patient cells for curing diabetes still sounds like a dream, but it makes me think that we are likely going to see this happen.

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