This article from the Mitalipov group (Kang et al. 2016) came out at a particularly interesting time, since it followed closely on the report from a US-based team in Mexico on the successful birth of a baby to a mother with the mitochondrial mutation causing Leigh syndrome, which had been corrected by transfer of maternal nuclear DNA to a donor oocyte (Hamzelou 2016). It was also published on the same day as the decision by the HFEA that clinics are now free to apply for permission to carry out mitochondrial replacement, with the first patients expected to be seen as early as Spring 2017 (Forster 2016).
The paper from Kang et al., (2016) details studies performed with oocytes from four families with Leigh syndrome and one with MELAS, both associated with mutations in mitochondrial DNA (mtDNA). Characterisation of the mtDNA burden in tissues from the patients and their children highlighted the problem with genetic testing for this condition.
The mtDNA percentage in one mother was 3%, 98% and 39% respectively in blood, skin fibroblasts and urine. In another family, the affected sibling showed levels of 56%, 68% and 97% in these tissues while his unaffected brother had levels of 10%, 14% and 23%.
The maternal spindle from the patient oocytes was transferred into eggs from healthy donors, which were then developed to the blastocyst stage and embryonic stem cell (ES) lines made from them to follow the success of mtDNA replacement. The first point of interest was that this was a largely successful process, whereas a paper from the Newcastle Centre earlier in the year (Hyslop et al. 2016) had less success with maternal spindle transfer and had suggested that transfer of pronuclei shortly after completion of meiosis might be preferred.
The choice of method has implications for ethical acceptance of the procedure, since maternal spindle transfer is performed on unfertilised oocytes while pronuclear transfer requires both donor and recipient eggs to be fertilised.
Key findings of note from the Mitalipov group were that fertilisation and gastrulation rates were not affected by maternal spindle transfer, nor was differentiation into tissues such as neural and cardiac. The trace amount of maternal mutant mtDNA carried over into the embryos was low and stable, at less than 1%. The same was true for 13 out of the 15 ES lines made, but in two sibling ES lines the amount of original mtDNA increased with passaging or differentiation until the donor mtDNA was completely lost. Similar drift back to the original DNA had been seen by the Newcastle group3, and this is a concern, but of course it is not known whether changes in ES lines will truly reflect those in the developing embryo.
Importantly, the paper by Kang et al., (2016) was able to suggest mechanisms for the selective bias towards mutant mtDNA, and from this make recommendations about selection of donors for optimal matching to reduce this risk. This is an exciting time for mitochondrial transfer technologies as they approach the clinic, and the fundamental scientific discoveries are driving forward the clinical strategy significantly.
Forster, K. (2016). “Controversial ‘three-parent baby’ technique given go-ahead in historic decision.” 2016, from http://www.independent.co.uk/life-style/health-and-families/health-news/three-parent-baby-hfea-allowed-technique-birth-child-ruling-fertility-decision-a7476731.html.
Hamzelou, J. (2016). “Exclusive: World’s first baby born with new “3 parent” technique.” from https://www.newscientist.com/article/2107219-exclusive-worlds-first-baby-born-with-new-3-parent-technique/.
Hyslop, L. A., P. Blakeley, L. Craven, et al. (2016). “Towards clinical application of pronuclear transfer to prevent mitochondrial DNA disease.” Nature 534(7607): 383-386.
Kang, E., J. Wu, N. M. Gutierrez, et al. (2016). “Mitochondrial replacement in human oocytes carrying pathogenic mitochondrial DNA mutations.” Nature 540(7632): 270-275.