BBC Knowledge Asia Edition

PRONUCLEAR

TRANSFER

1. Start with two egg cells –
   one from a mum-to-be with
   diseased mitochondria, one
   from a donor with healthy
   mitochondria. Fertilise both
   with the dad’s sperm.


2. Remove the pronuclei –
   the nuclei of the egg and
   the sperm, which have not
   yet mixed together – from
   both eggs. Discard the
   rest of the mother’s egg.
   Discard donor’s pronuclei.


3. Inject the pronuclei of
   the mother and the father
   into the donor’s egg,
   which contains healthy
   mitochondria.


4. Implant embryo, which
   contains mitochondria
   from the donor, and DNA
   from both the mother and
   the father – three parents
   in total – into the mother’s
   womb.

Pronuclei

discarded

Donor egg

fertilised

Remainder

of egg

containing

mtDNA

Reconstructed

egg containing

nuclear DNA

from both

parents, and

mtDNA from

donor

Patient egg with faulty

mtDNA fertilised

Rest of egg

discarded

Nuclear

DNA

transferred

Donor’s male

and female

pronuclei

removed

Patient’s male

and female

pronuclei

removed

mtDNA = mitochondrial DNA

nerve signalling, detoxifying ammonia

in the liver, and playing a key role in

programmed cell death. Moreover, genetic

information is continually shuttled between

the nucleus and the mitochondria. This

implies that shifting mitochondria from one

woman to another could have unexpected

consequences down the line.

The most distressing fact about

mitochondrial replacement, however, may

be that it will only work for a minority of

families carrying mitochondrial diseases.

We now know there are 1,000 – possibly

1,500 – genes in the DNA of a nucleus

that code for proteins necessary for the

creation of mitochondria. Yet many of

these genes can also lead to faults. It is

likely that only a quarter of all cases of

mitochondrial disease can be attributable

to genes within the mitochondria

themselves. “Even from very early on,

more than 20 years ago, it was clear that

most of the children with mitochondrial

diseases don’t carry mitochondrial DNA

Louise Brown holding

the test tube in which

she was conceived

PHOTO: GETTY X2, ALAMY X2, DRU DODDS ILLUSTRATOR: MAGIC TORCH

SCIENCE