mitochondrial anomalies may play a
role in the diseases of old age, such as
prostate cancer and Alzheimer’s. Curtis,
like most parents, had no idea she carried
any gene that was faulty. “I’d never even
heard of mitochondrial disease, nobody in
my family had. It came completely out of
the blue,” she says.
The main reason people like Curtis can
carry a mitochondrial mutation, but not
exhibit symptoms themselves, is due to a
quirk of mitochondria called
‘heteroplasmy’.
While the DNA in the nucleus of every
single non-sex cell in the human body is
identical, the selection of mitochondrial
genes varies.
When one cell divides, its chromosomes
are duplicated; each daughter cell receives
identical chromosomes. But the tiny
mitochondria – remember, there can be up
to 2,000 of them per cell – are split
randomly between the two daughter cells.
Which cell gets which mitochondria
carrying which genes is a matter of
chance. This is why one sibling in a family
may inherit a mitochondrial disease and
one will not, and why a mother can
unknowingly carry a dangerous gene.
Mutations that can lead to disease are
therefore scattered randomly and unevenly
between different cells. Disease-causing
mitochondrial mutations vary not just
between individuals, but between tissue
types in one person: we are all
mitochondrial mosaics. A certain
‘threshold’ amount of a malfunctioning
mitochondrial gene in any given cell needs
to be reached for an illness to manifest.ALTERED EMBRYOS
The technique that was legalised in the
UK at the beginning of 2015 will allow a
mother to give birth to a baby that is
genetically hers, but there will not be the
risk of it inheriting mitochondria with
dangerous mutations. The process is
known as ‘mitochondrial donation’ or
‘mitochondrial transfer’. A mother-to-be
carrying faulty mitochondria can opt to
have her nuclear DNA removed from her
eggs and implanted into a donor egg
carrying healthy mitochondria. The egg is
then fertilised with sperm from the father
before being implanted into the mother’s
uterus for pregnancy to continue as usual.
A recent study from the Wellcome Trust
Centre for Mitochondrial Research at
Newcastle University estimated that 2,473
women in the UK are at risk of passing amitochondrial disease to their children
and thus could benefit from the treatment.
“I’m over the moon that the law was
changed. It’s hugely rewarding to know
that families can have their own child that
will be free from disease,” says Curtis.THREE PARENTS?
Children who would be conceived in this
way have been dubbed ‘three-parent
babies’ by the press, as they technically
carry DNA from three people (albeit just
37 genes from the donor egg, compared to
20,000 from the mother).
“It’s unfortunate that the ‘three-parent
baby’ term was coined,” says Shamima
Rahman, a Professor of Paediatric
Metabolic Medicine at the UCL Institute
of Child Health, who began working with
mitochondrial diseases 20 years ago. “I was
very concerned that we were seeing a
group of disorders that nobody really knew
anything about, much less how to treat.
They can be extremely debilitating and it’s
heartbreaking for the parents.”
Aside from sensationalism, the
‘three-parent’ nickname is misleading, in
several respects: One, the female
mitochondrial donor is not likely to have
any role whatsoever in bringing up theMitochondria are the ‘batteries’ of cells, but also
contain their own DNAShamima Rahman, Professor of Paediatric
Metabolic Medicine at UCL Institute of Child Health“[Mitochondrial
diseases] can
be extremely
debilitating
and it’s
heartbreaking
for the parents”
PHOTO: BRETT MOUNTAIN/POLARIS/EYEVINE, GETTY ILLUSTRATOR: MAGIC TORCHAlana Saarinen was conceived by IVF, via a
procedure that was banned by the FDA in 2001.
Cytoplasm was donated from a younger donor’s
eggs into her mother’sSCIENCE