Background: Pronuclear transfer (PNT)

How would pronuclear transfer be done?

  • First, IVF techniques are used to create an embryo using the intending parents’ sperm and egg. The parents’ embryo contains the mother’s mutated (unhealthy) mitochondria, which came from the cytoplasm in her egg.

  • At one day’s development of the parents’ embryo (while it is a single undivided cell)1  the two pronuclei from the parents are removed from the cell for transfer. This leaves behind almost all of the mother’s affected mitochondria. This enucleated cell is then discarded.
  • A second embryo is created from the egg of an unrelated donor with healthy mitochondria, fertilised with sperm. If the intending father’s sperm is not of sufficient quality, donor sperm would be used to avoid the need to do ICSI2.  

  • At the same very early, one-cell stage of development, the pronuclei of this embryo are removed and discarded.

  • The parents’ pronuclei are then placed into the second, enucleated embryo. The second embryo now contains the pronuclear DNA from the intending parents, and healthy mitochondria from the donor’s egg.

  • This embryo can now continue to develop and then be transferred back to the intending mother, for her to carry a pregnancy unaffected by inherited mitochondrial disorders.

About pronuclear transfer

Scientific developments

In the 1990s, experiments3,4,5,6 using mice had suggested the efficacy of pronuclear transfer as a means of preventing the transmission of mutated mitochondrial DNA, but the technique had not been tried in humans.

In 2003, it was reported that a research team at Sun Yat-Sen University in Guangzhou, China experimented with the pronuclear transfer technique in human embryos7.  They transferred the embryos back to a fertility patient, which would not have been legal in the UK. Five embryos were transferred to the woman who became pregnant with triplets. The multiple pregnancy was then selectively reduced to a twin pregnancy. Some months later, the woman suffered miscarriages, losing both fetuses.

In 2008, researchers from Newcastle University in the UK told a scientific meeting that under an HFEA license granted in 20058,  they had successfully transferred pronuclear DNA between very early (day one) human embryos donated after being left over after fertility treatments. After having carried out the procedure, the Newcastle group grew ten embryos in the lab for five days before arresting their growth so that researchers could analyse them.

Although their work had not been written up in a published paper, the results were reported during debates in the House of Lords9 and calls were made for the technique to be licensed by the HFEA for treatment. Media from all over the world picked up on the story, creating headlines about ‘three-parent embryos’10,11.

In April 2010, the Newcastle group published a paper in Nature reporting that human embryos developed normally to blastocyst (about 100-cell) stage in 6-8 days after pronuclear transfer providing proof of concept of the technique in human embryos in a research setting12.  

Legal and policy developments

The Human Fertilisation and Embryology (HFE) Act 2008 (as amended) currently forbids egg, sperm or embryos which have had alterations made to their nuclear or mitochondrial DNA from being placed into a woman’s body. Similarly, ‘genetically modified embryos or embryos created by cloning’ cannot be placed into a woman’s body. However, in 2008, the Act wrote in new powers for the Secretary of State for Health to create regulations which would permit the alteration of eggs or embryos as part of treatment where intended to prevent mitochondrial disorders, if the technology became available and was shown to be safe.13 Both Houses of Parliament would need to support any proposed new legislation.

In February 2011, at the invitation of the Secretary of State for Health, the Human Fertilisation and Embryology Authority (HFEA) established an expert review panel to examine ‘expert views on the effectiveness and safety of mitochondrial transfer’.

Their report, a ‘Scientific review of the safety and efficacy of methods to avoid mitochondrial disease through assisted conception’ was published in April 2011. The report concluded that ‘MST and PNT have the potential to be used for all patients with mtDNA disorders, which may make them preferential to PGD in the future. In patients with homoplasmy or high levels of heteroplasmy, these are the only techniques that would make it possible for them to have a genetically related unaffected child’. The report also cautioned that before MST and PNT techniques can be assessed as safe to use in treatment, specific further safety experiments need to be done before introducing them into clinical practice.

After the publication of the HFEA’s scientific review, patient and medical research groups wrote a joint letter to the Secretary of State for Health about the regulation of emerging techniques to avoid transmission of mitochondrial disease. They called on Government to ‘publish a timetable for the introduction of regulations so that once sufficient pre-clinical evidence is established, clinical treatment is not unduly delayed.’ It is anticipated that the Government will request that further information be gathered around potential regulation of any such treatments, including public consultation, in 2012.

Next: Maternal spindle transfer (MST)

Previous: The role of genetic testing

1At the beginning of the process of fertilisation, the sperm and egg each contribute separate
‘pronuclei’ within the one-celled embryo. The genetic material in these will then merge to form the
mature nucleus of the fertilised egg. From the two-cell stage of embryonic development onwards, the embryo’s cells will contain one nucleus which combines both parents’ DNA.

2ICSI (intracytoplasmic sperm injection) is a variation of IVF in which a single sperm is injected
directly into an egg to achieve fertilisation.

3Jenuth JP, Peterson AC, Fu K, and Shoubridge EA (1996) Random genetic drift in the female germline explains the rapid segregation of mammalian mitochondrial DNA Nature Genetics 14: 146-51.

4Meirelles FV and Smith LC (1997) Mitochondrial genotype segregation in a mouse heteroplasmic lineage produced by embryonic karyoplast transplantation Genetics 145: 445-51.

5Meirelles FV and Smith LC (1998) Mitochondrial genotype segregation during preimplantation development in mouse heteroplasmic embryos Genetics 148: 877-83.

6Sato A, Kono T, Nakada, K et al. (2005) Gene therapy for progeny of mito-mice carrying pathogenic mtDNA by nuclear transplantation Proceedings of the National Academy of Sciences of the USA 102:16765-70.

7BBC News Online (14 October 2003) Foetus with three parents created, available at: http://news.bbc.co.uk/1/hi/health/3189718.stm.

8HFEA press release (8 September 2005) HFEA grants licence to Newcastle Centre at LIFE for mitochondrial research, available at: http://www.hfea.gov.uk/671.html.

9House of Lords Hansard (4 February 2008) c846, available at: http://www.publications.parliament.uk/pa/ld200708/ldhansrd/text/80204-0002.htm.

10Nature News (6 February 2008) A step towards three-parent babies?, available at: http://www.nature.com/news/2008/080206/full/news.2008.560.html.

11BBC News Online (5 February 2008) Three-parent embryo formed in lab, available at: http://news.bbc.co.uk/1/hi/health/7227861.stm.

12Craven L, Tuppen HA, Greggains GD et al. (2010) Pronuclear transfer in human embryos to prevent transmission of mitochondrial DNA disease Nature 465: 82-5.

13 The new section 3ZA(5),in the HFE Act allows the meaning of ‘permitted eggs’ and ‘permitted embryos’ for treatment to be extended to include ‘eggs or embryos that have been treated in such a way as specified in regulations to prevent the transmission of serious mitochondrial disease’. Further provision regarding ‘mitochondrial donation’ is also made in section 26, which inserts new section 35A into the 1990 HFE Act.