The world was forever changed in 1996 when a sheep named Dolly was born. Dolly was the first large mammal successfully cloned using a process called somatic cell nuclear transfer (SCNT). Since then, scientists have cloned mice, cows and even dogs, but the process proved unsuccessful in primates – until now. A team of Chinese scientists has just announced the successful birth of two genetically identical long-tailed macaques.
Technically, the first cloned primate appeared back in 1999 when a team created Tetra, a rhesus monkey clone generated through a technique called embryo splitting. But that technique, referred to as "artificial twinning," could only generate four offspring at a time and may not ultimately result in genetically identical animals.
SCNT is a cloning technique in which researchers implant the nucleus of an adult somatic cell into an egg cell that has had its native nucleus removed. While the technique has proved successful in many mammals, it hasn't worked effectively in primates. To overcome these hurdles the researchers deployed epigenetic modulators to alter the genes that were inhibiting the embryo development, and it was also discovered that nuclei taken from fetal differentiated cells, such as fibroblasts, improved the success rate of the technique in monkeys.
"We tried several different methods, but only one worked," says senior author Qiang Sun. "There was much failure before we found a way to successfully clone a monkey."
It took the team several years to perfect the process before two healthy, cloned long-tailed macaques were finally born. Named Zhong Zhong and Hua Hua, the goal of the research is to find a way to produce genetically uniform monkeys that would significantly assist scientists studying disease mechanisms and therapeutic treatments.
"This will generate real models not just for genetically based brain diseases, but also cancer, immune, or metabolic disorders and allow us to test the efficacy of the drugs for these conditions before clinical use," says Sun.
The breakthrough research may be a wonderful scientific advance, but there are suggestions the technique is still too limited to be broadly deployed. Speaking to Scientific American, Jon Hennebold from the Oregon National Primate Research Center suspects there are several hurdles that need to be overcome before larger-scale production of cloned monkeys could ever become a reality.
"The pregnancy rate and the live birth rate were not at a level that would allow this to be done on some wide scale," Hennebold says. "You would also have to have expertise in assisted reproductive technologies, reproductive physiology – and a large cohort of donors. And all those things are limiting with this current technology."
Of course, this is still a significant scientific advance, proving that complex primates can be successfully cloned. Muming Poo, a co-author on the study, is also well aware of the murky ethical territory this research is wading into. Sun and Poo are both urging the broader scientific community to openly discuss this research so as to clearly define future acceptable practices for non-human primate cloning.
"We are very aware that future research using non-human primates anywhere in the world depends on scientists following very strict ethical standards," adds Poo.
The research was published in the journal Cell.