Scientists have the first primates born from embryos created via somatic cell nuclear transfer (SCNT), more commonly known as cloning, described in Cell.
With Zhong Zhong and Hua Hua, cynomolgus monkeys are the first nonhuman primates to join the more than 20 species that have been successfully cloned.
For now, the work "only demonstrated that cloned monkeys are possible," Muming Poo told reporters at a press conference announcing the work. But the ultimate goal is to demonstrate that "cloned monkeys will be useful."
Poo is the director of the Institute of Neuroscience of the Chinese Academy of Sciences Center for Excellence in Brain Science and Intelligence Technology and an author of the paper reporting the methods, which is being published online in Cell in advance of print publication on Feb. 8, 2018.
Poo said his team is most interested in studying both normal and abnormal brain biology in the animals.
Neurodegenerative diseases, including Huntington's, Parkinson's and Alzheimer's disease, "will be a main target," he said, as will the subgroup of autism-related diseases that has "a very clear genetic basis."
The enormous complexity of the primate brain compared to that of rodents means that neurobiological mouse models have sharp limitations. Studies on primates, on the other hand, can suffer from too much complexity, or what Poo termed "developmental noise." The use of cloned monkeys can eliminate genetic variation as an experimental variable, and the use of genome editing methods could allow researchers to "delete, insert or mutate" specific genes in fibroblasts before cloning, making them genetic models whose precision matches or surpasses that of inbred mouse strains.
To be sure, the numbers of monkeys will not match those of genetically engineered mice. Qiang Sun, director of the Nonhuman Primate Research Facility at the Chinese Academy of Sciences Institute of Neuroscience and the senior author of the Cell paper, estimated that in five years, there would be "in the range of" 20 to 30 monkey disease models, and ultimately the method could produce "at least 100 useful models."
But "we don't aim at thousands," he said.
While the feat is sure to inspire comparisons to Dolly the Sheep, who was created in 1997 by Ian Wilmut and his team at the Roslin Institute in Edinburgh, Scotland, the animals were cloned from fetal cells. That makes them more similar to Dolly's predecessor Morag, who was born at the Roslin Institute in 1996 after being cloned from an embryonic cell.
Sun told reporters at a press conference that the use of fetal rather than adult skin cells was one of the methodological tweaks that led to success in cloning the cynomolgus monkey pair.
Somatic cell nuclear transfer in primates, he said, is "a very difficult and delicate procedure, much more difficult than in other mammalian species."
The team first optimized the nuclear transfer technique itself to speed up the procedure. But even with such optimization, most embryos did not develop as far as the blastocyst stage, which is the stage at which implantation into the uterine wall can occur.
The team achieved success through both the use of fetal cells, and through treating those cells with two epigenetically targeted drugs, the H3K9me3 demethylase Kdm4d mRNA and the histone deacetylase inhibitor trichostatin A, at the single-cell stage after the skin cell nucleus had been transferred into an oocyte.
Using their optimized protocol, the researchers transferred 79 cloned embryos to 21 surrogate mothers, resulting in six pregnancies and, ultimately, two live full-term births.
Whether animals produced by SCNT are indeed fully healthy is still an open question.
Dolly the Sheep appeared normal in many respects, and naturally produced six offspring. There were some signs, though, that she was aging more rapidly than an average sheep, and she died at age 6 and a half, roughly half of the typical lifespan of a sheep, but not unheard of for a sheep kept indoors. Detailed studies of a group of 13 cloned sheep, including the "Nottingham Dollies," four sheep that were created from the same cell line as Dolly, showed that overall, the animals were no more prone to health problems than age-matched naturally conceived sheep. (See BioWorld Today, June 2, 1999, Feb. 19, 2003, and July 27, 2016.)
For now, Zhong Zhong and Hua Hua, who are just under 2 months old, appear to be developing normally.
Poo said the team plans on "regular brain imaging of the structural development of the brains of these two monkeys."
But such brain imaging, as well as behavioral testing, will happen "when they are older. Because we don't want to disturb them at this time."