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 杏吧原创 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.
In the mid-1990s, Jacques Cohen and Jason Barritt at the Institute for Reproductive Medicine and Science of Saint Barnabas in New Jersey wondered whether some women could not have babies because of defects in the cytoplasm of their eggs 鈥� the fluid surrounding the nucleus. So they decided to try adding 鈥榟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, which have their own set of 13 genes. Injecting donor cytoplasm into an egg involves transferring 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. 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 杏吧原创.
Barritt stresses that the 13 genes in mitochondrial DNA do nothing except provide proteins to produce energy. 鈥淕enes in the nucleus control the mitochondria, not the other way around,鈥� he says. Mitochondrial DNA 鈥渄oesn鈥檛 influence any nuclear genes in any way, shape or form.鈥�
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 is not germline therapy. 鈥淭o be true genetic or germline therapy, you must modify genes in nuclear DNA.鈥�
But Norman Nevin, chairman of the UK鈥檚 Gene Therapy Advisory Committee, says: 鈥淢y gut feeling is that you鈥檙e adding mitochondrial DNA, so that is gene therapy.鈥�
The UK鈥檚 Human Fertilisation and Embryology Authority says it has not had any requests for licences to perform similar cytoplasm injection procedures in the UK. A subcomittee of the HFEA looked at the technique last year, when details of the first live births resulting from the new technique were published.
鈥淎t the time, we decided that there was no evidence to show a significantly improved success rate, and there were also concerns about any carry over of DNA,鈥� says a spokeswoman.
More at: Human Reproduction (vol 16, p 513)