November 30, 2016

expert reaction to study on mitochondrial replacement in human ooctyes that carry pathogenic mitochondrial dna mutations

Researchers, publishing in the journal Nature, have reported on a study which used mitochondrial donation therapy to replace pathogenic mitochondrial DNA mutations in human ooctyes with mitochondrial DNA from healthy donor eggs. The researchers report the potential of mitochondrial replacement therapy (MRT) to reduce the transfer of mitochondrial disease from mother to child.

Prof Bert Smeets, Professor Clinical Genomics, Director Genome Center, Maastricht University, said:

“How to match the donor nucleus with the acceptor oocyte has been one of the controversies in nuclear transfer as treatment for mtDNA disease. The focus was mainly on matching haplotypes, but this seems to be less important, which is not unexpected given that parents with different mtDNA haplotypes have perfectly healthy children. Replication differences provide a much more plausible explanation.

“The paper of Mitalipov shows for the first time that a replication advantage of the small amount of carry-over donor mtDNA and the acceptor mtDNA could shift the ratio back to the donor mtDNA with the pathogenic mutation, possibly leading to an affected child.

“This is highly important, as a determining factor in the success of this treatment strategy. The authors propose to develop a matching paradigm, but as long as not all the mtDNA sequence variants affecting mtDNA replication are known, this would have limited predictive value.

“As an alternative, in vitro experiments with mixtures of donor and acceptor mtDNA populations could be included in the treatment strategy. This should unambiguously show if there is a replication difference between donor and acceptor mtDNA and if there is a perfect match for treatment.”

Dr David J Clancy, Lecturer, Lancaster University, said:

“Researchers led by Shoukrat Mitalipov report the first findings of mitochondrial replacement in human egg cells bearing disease-causing mutations. Treated eggs were then fertilized and allowed to divide and develop into blastocysts, and these cell masses were used to create stem cell lines which were checked for efficiency of mitochondrial DNA (mtDNA) replacement.

“Technically, this work appears to have been well done; most relevant measures of technical efficacy show little or no difference of treatments vs controls, except there were significant numbers of chromosomally abnormal blastocysts, and development using frozen donor eggs instead of fresh was about half as efficient.

“The techniques seem to unavoidably produce some carryover of maternal mtDNA, and some of these seem to expand and replace the donor mtDNA completely as the embryo divides and develops. This happened in 2 of 15 cell lines from control donations and in 1 of 3 cell lines from donations involving patients’ oocytes bearing disease-causing mtDNA.

“This phenomenon of reversion to the maternal mtDNA is already known and reasons are unclear from this study. Results here suggest it may sometimes be due to differential control of mitochondrial replication between particular donor and maternal mtDNA types, but the data do not fully support this explanation; there are clearly other factors operating.

“One of these other factors may be that certain chromosomal genes interact better or worse with some mitochondrial genes. If this is the case, reliably predicting these interactions is likely to be impossible in the near-medium term.

“Given the frequency with which this reversion to the original mtDNA type occurs, the authors correctly state that “reversal to the mutant mtDNA may occur in some MRT (mitochondrial replacement therapy) children”. I would argue that considerably more work of this type should be done to reduce or avoid the possibility of doing harm, i.e. producing a child with inherited mitochondrial disease, before any licences are granted.”

Prof. Robert Lightowlers, Director of the Institute for Cell and Molecular Biosciences, Newcastle University, said:

“This paper from Shoukhrat Mitalipov and colleagues is another valuable mitochondrial replacement study. The tendency of the occasional ES cell line established from reconstituted blastocysts to repopulate from remaining maternal mtDNA is well described, again raising the issue that such reversion may be relevant in a clinical context.

“Careful monitoring of heteroplasmy levels following MRT will clearly be essential and is no doubt planned.”

Dr Dusko Ilic, Reader in Stem Cell Science, King’s College London, said:

“Extensive genetic analyses of four families with Leigh and one with MELAS syndrome demonstrated that specific polymorphisms in a highly polymorphic region (D-loop) of mtDNA present replication advantage, adding a new, from my point of view a quite positive, twist to mitochondrial replacement therapies.

“The finding is extremely valuable – we can, by choosing a healthy egg donor with a specific preferential amplification polymorphism in mtDNA, minimize a risk of reversal to the affected maternal phenotype, which could otherwise happen due to the presence of maternally inherited mitochondria, incidentally transferred during the procedure.

“I hope that the teams working on mitochondrial replacement therapies will take this finding on board and implement the screening for replication advantageous polymorphism in mtDNA in clinical practice.”

* ‘Mitochondrial replacement in human oocytes carrying pathogenic mitochondrial DNA mutations’ by Eunju Kang et al. was published in Nature on Wednesday 30th November.

All our previous output on this subject can be seen at this weblink: http://www.sciencemediacentre.org/tag/mitochondrial-dna/

Declared interests

Prof Bert Smeets: No conflicts of interest.

Dr David Clancy: No COIs.

Prof. Robert Lightowlers: Bob Lightowlers is a member of the Wellcome Trust Centre for Mitochondrial Research.

Dr Dusko Ilic: “I have no conflicts of interest to declare.”