Baby born from embryo that originally had three sets of chromosomes

An embryo that originally had three pronuclei resulted in a healthy baby after one pronucleus was microsurgically removed.

After fertilisation, a normal zygote would have two polar bodies and two pronuclei (PN). One PN comes from the egg and one from the sperm, and these each contain one set of chromosomes. A new study from China details how a tripronuclear zygote, with three PN rather than two, was corrected using a microsurgical technique called enucleation. After culture and transfer of the embryo, this resulted in a healthy live birth.

'The removal of one PN from a 3PN zygote is not a routine procedure in assisted reproductive technology,' said the authors of the case report published in the Journal of Ovarian Research. 'It should only be considered as an experimental rescue measure, despite our successful microsurgical removal of one female PN and healthy live birth.'

The patient's eggs were fertilised using intracytoplasmic sperm injection, where a single sperm is injected directly into an egg. At the fertilisation check, seven out of eight embryos were found to contain three PNs and only one polar body. Due to the method of fertilisation, and the presence of only one polar body, the extra PN were likely to have come from the eggs.

Tripronuclear embryos would normally be considered abnormal and discarded. However, this patient was offered the technique of microsurgical enucleation, whereby one of the extra PNs is removed from the embryo. This involves physically removing one of the pronuclei, and is similar to how pronuclei are transferred to the donor cell in the pronuclear transfer method of mitochondrial donation.

The researchers performed enucleation on six zygotes by selecting one of the mother's PNs for removal, so the resulting embryos would have one set of chromosomes from each parent. All six survived the procedure and were reverted to the 2PN state. After culturing the embryos in the lab, two of the enucleated embryos progressed to the blastocyst stage of development and were frozen for future use.

Before transfer, the blastocysts underwent non-invasive genetic testing to check for chromosomal abnormalities. One was then selected for transfer and developed into a healthy baby boy, who showed no health or developmental problems after birth.

The researchers hope this technique could be used more widely in future, 'for cases where oocytes are extremely precious, such as patients with poor ovarian response,' or for those who cannot undergo another egg collection cycle for medical or financial reasons.

However, they added: 'There is still space for improvement and enhancement in the technology of microsurgical enucleation,' and 'more research needs to be encouraged and ethically supported in order to benefit more patients.'