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| credits:geneticliteracyproject.com |
This past February, after a year long debate the United
Kingdom
became
the first country to legalize mitochondrial replacement therapy (MRT), also
known as mitochondrial DNA replacement, which can be used to make “three-parent
babies.”
Three parent babies have caught much attention in the year
2015 and Britain’s
first such baby will be born as early as 2016.
The procedure has been developed by scientists at Newcastle University.
Advocates of the new procedure say around 2,500 women could benefit from
mitochondrial donation in Britain,
equating to around 150 births a year as one in 200 children born in the UK have some
form of mitochondrial disorder.
Prof Doug Turnbull, Professor of Neurology,
Newcastle University, said: “This is very good
news for patients with mitochondrial DNA disease and an important step in the
prevention of transmission of serious mitochondrial disease".
In this technique the nucleus of an affected woman's extracted egg is
removed and is put into the enucleated egg of another woman, which contains her
mitochondria. The child would thus be genetically related to three people,
which is why the media often refers to "three-parent babies" or
"three-parent in vitro fertilization."
These techniques have been referred
to with several terms, including "mitochondria replacement,"
"mitochondrial manipulation," "oocyte modification,"
"three-person embryos," "three-parent babies," and "nuclear
genome transfer" (the most technically accurate).
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| Photo courtesy- The telegraph |
The disease primarily affects children, but adult onset is becoming more and
more common.
Diseases of the mitochondria appear to cause the most damage to cells of the
brain, heart, liver, skeletal muscles, kidney and the endocrine and respiratory
systems.
Depending on which cells are affected, symptoms may include loss of motor
control, muscle weakness and pain, gastro-intestinal disorders and swallowing
difficulties, poor growth, cardiac disease, liver disease, diabetes,
respiratory complications, seizures, visual/hearing problems, lactic acidosis,
developmental delays and susceptibility to infectio.
It takes about 3000 genes to make a mitochondrion. Mitochondrial DNA
encodes just 37 of these genes; the remaining genes are encoded in the cell
nucleus and the resultant proteins are transported to the mitochondria.
Only about 3% of the genes necessary to make a mitochondrion (100 of the 3000)
are allocated for making ATP. More than 95% (2900 of 3000) are involved
with other functions tied to the specialized duties of the differentiated cell
in which it resides.
The procedure can be carried out in one of the two ways.
1) maternal spindle transfer (MST).
2) pronuclear transfer (PNT).
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| Credits: The Guardian |
The simpler of the two is called maternal spindle transfer (MST). First,
doctors use standard IVF treatment to collect eggs from the mother. They then
remove the nucleus from one of the mother’s eggs and transfer it into a healthy
donor egg that has had its own nucleus removed. The reconstituted egg holds all
of the mother’s healthy nuclear DNA, or 99.8% of her genes, plus the donor’s
healthy mitochondria. This egg is then fertilised with the father’s sperm and
the embryo is implanted into the woman like any other IVF embryo.
The second procedure is very similar. In pronuclear transfer (PNT), both
mother and donor eggs are fertilised with the father’s sperm. Before the eggs
have time to split into early-stage embryos, the chromosomes inside them are
removed. Those from the donor egg are discarded, and replaced with the
chromosomes from the mother’s egg. The resulting egg is fertilised and ready to
grow into an embryo in the mother’s womb.
Critics of three-person IVF called this techniques as
Slippery slope toward “designer babies " because of the potential for in vitro fertilized eggs
that are genetically engineered to have, for example, blonde hair, more
intelligence, more increased athleticism.
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| Crediits: Teracatu wordpress.com |
Little is known about the legal, psychological, and social ramifications for
the child of three genetic parents.
Other Experts however, aren't convinced the leap from mitochondrial transfer
to genetic trait modification is so direct.
Meanwhile, at the
Salk Institute for
Biological Studies in La
Jolla, California, scientist
have developed a
gene-editing technique that may prevent mutated
mtDNA from being passed down from mother to child.
This technique has been shown to be a success in animals it works by using
DNA-cutting enzymes to delete mutated mtDNA, leaving
healthy mitochondria intact. By reducing the amount of faulty mtDNA,
it aims to restore the balance in favour of healthy mitochondria, reducing
the amount of faulty mtDNA passed down to offspring to the point where it does
not result in disease.
Debate among British and
American scientists has just started.
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