Just in time for Easter, researchers in France today announced that they have successfully cloned rabbits from adult cells. The accomplishment, which has eluded scientists for years, is an advance for researchers who want to use the animals as "bioreactors," to produce designer proteins in milk.
Although scientists have cloned cows, sheep, goats, pigs, and most recently cats, attempts to clone rabbits had frustrated several research groups. Rabbits give birth in litters and so need to have several embryos implanted at once to achieve a successful pregnancy. The cloning process, which involves inserting the nucleus from an adult cell into an egg from which the nucleus has been removed, is still very hit-and-miss--so producing several viable embryos at one time is a challenge.
Yet even when a team at the National Institute for Agronomy Research (INRA) in Jouy-en-Josas, France, had plenty of viable-looking embryos, they still had trouble, says cloning researcher Jean-Paul Renard, who led the effort. In the April issue of Nature Biotechnology, the team describes two tricks that finally helped them succeed. First, because the drugs used to prompt the new embryos to begin developing seemed to be toxic later in the pregnancy, the team used them for only half the normal time. The team also learned that the cloned embryos were developing at a slightly slower pace than their naturally conceived counterparts. When they adjusted the timing of the implantation, the pregnancy rate jumped from 0 to 37%.
The odds are still long, however: The team initially produced 775 embryos and implanted 371 of them into 27 recipients. Out of 10 initial pregnancies, four rabbits gave birth to six live kits. Two kits died 1 day after birth, but the four others seem to be healthy, and two have since given birth to normal litters of their own.
Rabbits are in demand as bioreactors, says cloning expert Xiangzhong (Jerry) Yang at the University of Connecticut in Storrs, whose lab has also attempted rabbit cloning. Ideally, researchers could insert specific genes into rabbit cells and then use those cells to produce transgenic clones that produce designer proteins in their milk. Scientists have already managed a similar feat in goats, but rabbits are cheaper and faster to grow. And a few cloned rabbits can go a long way. "Because rabbits are so prolific, getting 200 rabbits is very easy once you have the founders," Yang says.