BABIES have been born with DNA from three parents instead of two. They have
been described as the first genetically engineered humans, as the added DNA they
carry could be passed on down the generations.
New Scientist reported last year that a certain fertility treatment
would spawn babies with DNA from more than two parents
(2 December 2000, p 16).
Now cells from two one-year-old babies born as a result of this treatment have
indeed turned out to have a little extra DNA from a donor mother, as well as
that from their own parents.
Midway through the 1990s, Jacques Cohen and Jason Barritt at the Institute
for Reproductive Medicine and Science of Saint Barnabas in New Jersey reckoned
that some women couldn鈥檛 have babies because of defects in the cytoplasm of
their eggs鈥攖he fluid surrounding the nucleus. So they decided to try
adding 鈥渉ealthy鈥� cytoplasm from a donor egg.
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While the vast majority of our genes are housed in the nucleus, the cytoplasm
contains tiny energy-producing structures called mitochondria that have their
own set of 13 genes. If you inject donor cytoplasm into an egg, you can transfer
mitochondria and their genes as well.
The researchers examined 12 of the 30 babies born with the help of the
technique and found that two of them carry donor mitochondria. 鈥淭his report is
the first case of human germline genetic modification resulting in normal
healthy children,鈥� say Barritt and his colleagues in the journal Human
Reproduction (vol 16, p 513). These mitochondria could be passed on to
future generations. 鈥淲e won鈥檛 know till they reach reproductive age,鈥� he told
New Scientist.
However, no one knows if the added mitochondria were the reason why the
fertility treatment worked in these two cases. Other, non-genetic components of
the cytoplasm might have done the trick. 鈥淲e think every patient is different,
and some might need mitochondria for extra energy, and some might need messenger
RNA or proteins,鈥� says Barritt.
Some researchers want to go further. James Grifo of New York University has
been given approval to try to treat infertile women by removing the nucleus from
their egg and injecting it into a donor egg whose nucleus has been removed. In
this case, all the mitochondria of any baby born would come from the donor.
This technique could also help prevent women who have mutations in
mitochondrial DNA passing the problem on to their children. Such mitochondrial
diseases cause various problems, and can be fatal.
Meanwhile, scientists are still fighting about whether terms such as 鈥済enetic
modification鈥� apply to this treatment. Despite what Barritt wrote in Human
Reproduction, he maintains it鈥檚 not germline therapy. 鈥淭o be true genetic
or germline therapy, you must modify genes in nuclear DNA.鈥�
鈥淢y gut feeling is that you鈥檙e adding mitochondrial DNA, so that is gene
therapy,鈥� says Norman Nevin, the chairman of Britain鈥檚 Gene Therapy Advisory
Committee.