A preclinical analysis of mitochondrial replacement therapy,
using an optimized version of the pronuclear transfer (PNT)
technique, is reported in Nature (8 June 2016). The study
shows that the timing of PNT - conducting it sooner after
fertilization than previous investigations - is crucial for its
successful application in normally fertilized human eggs, and
represents a step towards the therapeutic application of PNT for
preventing transmission of mitochondrial disease. The research was
carried out by teams in Newcastle, the Francis Crick Institute, and
Oxford.
Unlike the DNA inherited from the nucleus of the mother's egg
and father's sperm (nDNA), mitochondrial DNA (mtDNA) is inherited
directly from the mother. Mutations in mtDNA are associated with
various debilitating and fatal diseases (such as Leber's hereditary
optic neuropathy), and mitochondrial replacement techniques, such
as PNT, could help prevent their transmission. PNT transfers the
still separate nDNA of the egg and the sperm from a fertilized egg
- pronuclei - into a donor egg that contains healthy mtDNA but
whose pronuclei have been removed. Previous studies have shown that
PNT is technically feasible in abnormally fertilized human eggs
(containing only one pronucleus or more than two pronuclei), but
these eggs have a limited capacity for onward development.
Dr Kathy Niakan at the Crick performed cutting edge analysis of
the gene expression of embryos generated using the modified
pronuclear transfer techniques developed in Mary Herbert's lab.
They also assisted with the derivation and characterisation of the
human embryonic stem cell lines described in the study.
Mary Herbert from the Wellcome Trust Centre for Mitochondrial
Research at the University of Newcastle and colleagues show that
PNT can be improved if the pronuclei are transplanted from the
mother's egg to the donor egg earlier in the developmental process
- shortly after they first appear, after the completion of meiosis,
rather than shortly before they disappear which happens before the
first mitotic division. They find that this early PNT (ePNT) allows
the fertilised egg to develop efficiently to the blastocyst stage:
the stage at which implantation into the uterus occurs, with no
detectable effect on gene expression or incidence of aneuploidy
(abnormal number of chromosomes). By further optimizing the ePNT
procedure, the authors are able to reduce the co-transfer of mtDNA
to less than two percent in 79% of blastocysts: below the threshold
for development of mild or severe disease symptoms, and in most
cases below the threshold for mother-to-child transmission in
subsequent generations.
These results suggest that ePNT has the potential to reduce the
risk of mtDNA disease transmission. However, the authors also
highlight the importance of keeping mtDNA co-transfer to the lowest
possible levels, demonstrating the need for continued optimization
and monitoring of mitochondrial replacement techniques for future
clinical application.
The paper, Towards clinical application of pronuclear
transfer to prevent mitochondrial DNA disease, is published inNature.