Legislation which allows the replacement of faulty mitochondria in eggs of mothers with mitochondrial disorders has been passed in the UK, and a paper published in the journal Cell Stem Cell has explored the efficacy of such techniques. The authors report levels of mitochondrial carryover from the original egg which they suggest could have functional significance.
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Dr Marita Pohlschmidt, Director of Research, Muscular Dystrophy UK said:
“The results in this paper are important as they show that there might be a low risk associated with a new technique aimed at preventing passing on mitochondrial disease from mother to child.
“The research shows that small amounts of mutated mitochondrial DNA can be carried over with the transfer of DNA from the affected egg of the mother into the donor egg. The researchers established cell lines from the original egg and in seven out of eight cases the amount of mutated mitochondrial DNA remained low when the cells were grown in the laboratory.
“It is important now to understand whether the techniques used in the paper might reflect what would happen when this technique would be applied in the clinic so that affected families understand the risk and can make an informed decision. Overall the results appeared to be positive and as a patient organisation we feel optimistic about mitochondrial replacement therapies that are currently in development.”
Dr Kathy Niakan, Group Leader at The Francis Crick Institute, said:
“Using a human embryonic stem cell assay, Dieter Egli’s lab shows that the majority of human embryonic stem cell lines established following spindle transfer retain similar levels of mitochondrial carryover (heteroplasmy) to that detected in the preimplantation embryo. This is what one would hope and is very promising, suggesting that in the majority of cases, mitochondrial and nuclear inheritance can be successfully uncoupled. One caveat is that the authors also observed that one of the eight human embryonic stem cell lines established following spindle transfer showed a drift in mitochondrial heteroplasmy in a subset of stem cells, despite low levels of carryover detected in the embryo. It is unclear why there is a discrepancy between the mitochondrial carryover detected in the human embryo versus the upward drift in some of the cells from this in vitro stem cell line. One interpretation is that this may be a consequence of selection in an in vitro stem cell context that does not entirely reflect human development. For example, there is no equivalent of indefinitely self-renewing embryonic stem cells in the human embryo – these are instead transient cells that very quickly specialise to become the precursors of tissues in the embryo. The findings from this one cell line are also inconsistent with clinical data from preimplantation genetic diagnosis, further suggesting that the stem cell assay may not wholly reflect what happens during human development. I think it is also worth noting that the methods used to produce the embryos in this study, while similar, are importantly distinct to those that have been proposed clinically so it is unclear how the methodology might affect the results and the subsequent interpretation.
“I do think this study underscores the importance of reducing to a minimum the carryover of the mitochondria that predisposes embryos to disease, which has been echoed by many groups. It also underscores the importance of genetic counseling to patients. Moreover, the findings are not too dissimilar to those reported previously by Shoukrat Mitalipov in 2015 showing a drift in the mitochondrial heteroplasmy in embryonic stem cells established following somatic cell nuclear transfer or induced pluripotent stem cell reprogramming. Again, it is unclear how faithfully this stem cell assay recapitulates what would happen in the developing embryo.”
Prof. Robin Lovell-Badge, Group Leader at The Francis Crick Institute, said:
“This is an interesting paper that will feed in to the discussions about whether or not to approve clinical use of mitochondrial replacement techniques (MRT) to avoid serious mitochondrial disease.
“Seven out of eight ES cell lines derived from (parthenogenetic) embryos after MRT (spindle transfer) showed no or very low persistence of the mtDNA haplotype that was carried over with the nuclear DNA. However, one line showed a significant increase in the proportion of the carried-over mtDNA haplotype compared with that in the enucleated egg. On further culture or after subcloning this ES cell line at various times (passages), this particular mtDNA haplotype showed essentially random drift, with cells carrying anything from 0 to 100 %. Because only one line showed the increase, it is not possible to know the frequency with which this occurs (the next line could have shown the same thing, or the next 8 lines may not have, etc).
“The authors also addressed the same questions about mtDNA segregation after somatic cell nuclear transfer (SCNT). This is a related method in some respects, but it is very different in others, and it is necessary to be cautious when extrapolating any information from these experiments to MRT. Nevertheless, they had similar findings. Most SCNT embryos and ES cell lines derived from them had very low proportions mtDNA that had come from the original somatic cell, but in a few cases it amplified and it could take over completely.
“So how should these results be viewed – with glass half full or glass half empty? Optimists would say that these experiments are encouraging, with the expected and hoped for result found in almost all cases. Pessimists would focus on the infrequent unexpected and un-hoped for result, which if carried out for real in an attempt to avoid mitochondrial disease would fail to do so. They may conclude that the methods are too unsafe to use clinically. Whether or not to go ahead with these odds might actually be a question that the patients should answer.
“But the questions for science with respect to MRT are why was the amplification of transferred mtDNA seen in the one case but not the others and why did it occur at all ? The authors tested several possibilities, concluding that there was nothing wrong with the functioning of any of the mitochondria in ES cells or differentiated cell types derived from them after either MRT or SCNT and it made no difference whether the mtDNA was homoplasmic or heteroplasmic. Importantly, there was also no detectable mismatch between mtDNA haplotype and nuclear DNA in any of the assays, making this an unlikely explanation for the unusual mtDNA segregation patterns and reducing it as a general cause for concern if MRT was to go ahead clinically. But are ES cells a good substitute for studying the phenomena in the first place ? Cells grown in culture may not reflect what would happen in embryos.”
Prof. Mary Herbert, Professor of Reproductive Biology, Newcastle University, said:
“This interesting paper from Dieter Egli’s lab addresses the question of whether the small fraction of mitochondrial DNA carried over during spindle transfer might occupy a larger fraction in the embryo, and eventually in babies born, following spindle transfer for prevention of mitochondrial DNA disease.
“While the experimental system used in this paper involves artificial activation rather than fertilization of human eggs, the findings nonetheless offer some valuable insights into the likely efficacy of mitochondrial replacement therapies. Notably, the level of mitochondrial DNA carryover during spindle transfer was found to be well below the threshold for disease in early embryos. However, during prolonged culture one of eight embryonic stem cell lines showed a resurgence of the spindle donor’s mitochondrial genome. While the finding highlights the importance of keeping mitochondrial DNA carryover to the lowest possible levels, it is very difficult to assess the relevance to normal embryonic development. Maintaining the stem cell state for many months in the lab does not reflect what happens during normal embryonic development where it exists only transiently.
“Importantly, clinical treatments involving pre-implantation diagnosis for mitochondrial DNA disease indicate that the mutation load detected in the embryo is predictive of levels in the baby. By this measure, the findings from the Egli group provide grounds for further optimism that “mitochondrial replacement” therapies have the potential to reduce the risk of a mother transmitting mitochondrial DNA disease to her children.”
Dr Dusko Ilic, Reader in stem cell science, King’s College London, said:
“The study demonstrates that during a transfer of nuclear genome between two eggs, a small number of mitochondria from donor egg can be accidentally transferred into the recipient egg, which would then result in a mixed mitochondrial genome in the recipient egg. After an extended number of cell divisions, random segregation of mitochondria may lead to cells containing a majority of mitochondria from the donor egg.
“The study suggests that a simple nearly non-avoidable technical error could compromise mitochondrial replacement for clinical purposes. Although the probability is very low, the take home message is that for clinical purposes, we should focus on the transfer of single spindle or a polar body, which would carry less mitochondria than the standard transfer of two pronuclei after fertilization.
“Although mitochondrial transfer has been recently approved in the UK, no babies have been born yet using this approach. This study is a fair warning that one technical approach carries more risk than other, nothing else. It should not divert or postpone our efforts to move forward with mitochondrial transfer in clinical setting.”
Prof. Robert Lightowlers, Director of the Institute for Cell and Molecular Biosciences, Newcastle University, said:
“This is a well performed set of experiments. Hirano, Egli and colleagues have shown that in unusual cases, stem cell clones derived from MR eggs can show substantial drift towards the donors mtDNA haplotype after low level mtDNA carryover during the MR procedure. Such a dramatic drift has not been seen previously and such clonal expansion is difficult to understand, but this research emphasises the importance of current attempts to optimise the MR procedure to eliminate transfer of any donor mtDNA.”
Prof. Justin St John, Director of the Centre for Genetic Diseases, Monash University, said:
‘”The findings are not surprising. There are lessons that we can learn from somatic cell nuclear transfer where the somatic cell brings accompanying mitochondrial DNA (mtDNA) into the egg. This mtDNA can persist and be transmitted to the offspring in some cases whilst in other cases it is not transmitted. Consequently, we need a more robust approach to ensure that offspring generated through mitochondrial replacement do not inherit the affected mitochondrial DNA.”
‘Genetic Drift Can Compromise Mitochondrial Replacement by Nuclear Transfer in Human Oocytes’ by Mitsutoshi Yamada et al. published in Cell Stem Cell on Thursday 19th May.
Dr Marita Pohlschmidt: None received.
Dr Kathy Niakan: None received.
Prof. Robin Lovell-Badge: Robin Lovell-Badge has been a member of the HFEA Panel that has reported on the science and safety of mitochondrial replacement techniques. However, he has no direct involvement in this type of research and no financial interests in it. Robin Lovell-Badge is an SMC Advisory Committee member.
Prof. Mary Herbert: None received.
Dr Dusko Ilic: I declare no competing interests.
Prof. Robert Lightowlers: None received.
Prof. Justin St John: I work on embryo development and specifically mtDNA regulation and mtDNA supplementation in embryos. I have been funded by government and philanthropic bodies and commercial organizations to undertake this work.