AFTER months of wrangling, British researchers were last week granted their wish: they can now apply to create embryos containing both animal and human material for research. But while the ethical qualms about creating embryos that are 99.9 per cent human and 0.1 per cent animal appear to have been put to one side, there remain questions about whether it will be practically possible.
The UK鈥檚 Human Fertilisation and Embryology Authority, which regulates embryo research, said on 5 September that it by fusing human DNA with an egg from a rabbit or cow stripped of its nuclear DNA. This sets it apart from a 鈥渉ybrid鈥� created through fusion of a sperm and an egg from different species, or a chimera created by mixing tissues from two species.
Cybrids would contain animal mitochondrial DNA in the lining of the egg to begin reprogramming the human DNA to create a predominantly human embryo. Human embryonic stem cells could then be extracted and grown to form any tissue in the body.
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This would enable researchers to create tissue otherwise unobtainable from living people, such as brain cells from patients with motor neuron disease and provide a chance to compare diseased cells with normal cells to work out what has gone wrong. 鈥淲hat we want to create is essentially a 鈥榙isease in a dish鈥�,鈥� says Stephen Minger of King鈥檚 College London.
Minger leads one of the three groups seeking licences to make cybrids. All aim to repeat the only claimed success with the technique, reported in 2003 by Hui Zhen Sheng鈥檚 team at Shanghai Second Medical University in China (Cell Research, vol 13, p 251). Sheng reported producing rabbit-human cybrids yielding human embryonic stem cells that she turned into muscle and nerve cells.
Minger hopes to send researchers to China to learn the technique. He says that on a visit to China a year ago, Sheng showed him pictures of blastocysts made from human-cow cybrids produced by one of her collaborators.
However, more proof will be needed to convince doubters such as Bob Lanza of Advanced Cell Technology in Worcester, Massachusetts. He says his company has made many unsuccessful attempts to produce embryonic stem cells from animal-human hybrids. 鈥淭hey grow to the 16-cell stage, then just before going on to become blastocysts, they block,鈥� he says. Lanza thinks this happens because the mitochondrial genome of the animal 鈥渟tops talking鈥� to the human genome, blocking further growth.
He says the problem occurs only when the genomes come from species very distant from one another, such as humans and rabbits. It has worked in closely related species, such as when he successfully produced a rare ox called a gaur by injecting gaur DNA into an enucleated cow鈥檚 egg.
Others are less pessimistic. 鈥淛ust because it hasn鈥檛 been done, it doesn鈥檛 mean we shouldn鈥檛 try it,鈥� says Minger.
