“Monkeys are nonhuman primates that are evolutionarily close to humans,” said Mu-ming Poo, a neuroscientist and member of the cloning team. He also said: “There is no intention for us to apply this method to humans.”
The achievement suggests it is now possible to create research populations of identical, customized monkeys, which Poo and his colleagues said would decrease the number of primates used in laboratory experiments.
Kevin Sinclair, a developmental biologist at the University of Nottingham in Britain who was not involved with this research, said this was a significant step in nonhuman primate cloning. “It's the first time that primates have been born using the technique of somatic cell nuclear transfer, which is the technique that was used to produce Dolly the sheep, gosh, almost 22 years ago now,” he said.
Researchers have attempted to clone nonhuman primates for nearly two decades. At the end of the 1990s, scientists created an artificial monkey twin by splitting an embryo. This process cannot be used to clone adult animals and cannot be repeated to create identical clone copies.
Somatic cell nuclear transfer is different. It starts with the cell nucleus, the microscopic warehouse that stores an animal's genome. Researchers slice the nucleus out of an egg cell and insert a nucleus from the cell of an individual they wish to clone. The result is an embryo with genetic material identical to the donor cell. In theory, this cloning technique can produce a long line of animals, each as indistinguishable as plastic monkeys in a barrel.
In practice, this technique is slow and inefficient. “This is a very difficult and delicate procedure,” Poo said. Zhen Liu, lead author of a paper published Wednesday in the journal Cell, spent three years perfecting this nuclear shuffle.
Then came the next barrier: biology. The transferred nuclei seemed to resist being reprogrammed as embryos. The study authors used new chemical treatments to coax the cells to develop. “They treated the somatic cell nuclei with a couple of different compounds that haven't been used to generate nonhuman primate clones before,” said Catherine VandeVoort, who studies primate reproduction at the California National Primate Research Center.
Even then, the success rate was low. The research team attempted to clone monkeys from adult cells as well as cells taken from fetal connective tissue. They transferred 181 embryos containing adult donor cell nuclei. Only 22 surrogate monkey mothers became pregnant. Although two infants were delivered via Caesarean section, both died, probably from respiratory failure, within 30 hours.
For the clones that originated from fetal tissue, 79 embryos yielded six pregnancies. Only Zhong Zhong and Hua Hua were born. “It's still very inefficient in this species,” Sinclair said. “There's a lot of progress yet to be made.”
The clones are both cynomolgus monkeys, also known as long-tailed crab-eating macaques, which are some of the most commonly used primates in research. (Cynomolgus monkeys have, among other things, been sent to space.)
VandeVoort said the reliance on fetal tissue is a potential limitation. “If you found a really interesting animal that you wanted to clone, it's probably not going to be found when it's a fetus,” she said.
DNA fingerprint analysis showed that Zhong Zhong and Hua Hua's chromosomes were identical, Poo said.
“There is no reason to doubt that these two monkeys are anything other than clones,” Sinclair said.
Nor, he said, is there reason to suggest that we are closer to producing human clones today than we were yesterday. “The techniques that these guys have used to produce these monkeys have already been used to produce cloned human embryos,” Sinclair pointed out. No one in the field “has ever had an agenda to produce human clones.”
The technical barrier to reproductive cloning is in principle broken, Poo said, but societal taboos, ethical concerns and governmental prohibitions remain intact.
For now, the monkeys “are ideal models for studying human diseases and developing treatments,” Poo said, particularly for diseases with a genetic basis like Parkinson's.
Using “absolutely identical animals” would reduce the number of research primates, VandeVoort said. If genetic variations in monkey subjects were eliminated, she said, scientists could more efficiently focus on differences that arise from behavioral or drug treatments.
Sinclair, who has studied the health of clones such as Dolly's identical “sister” sheep, was bullish on the monkeys' long-term survival. In general, he said, if clones survive the first few months of infancy, they develop like any other animal. “The chances of them going on to lead long and normal healthy lives are very high,” he said.