Rhesus monkey cloned using new scientific method

Researchers from the Chinese Academy of Sciences have reported the successful cloning of a rhesus monkey, which has so far survived to two years of age. Their findings, published in the journal Nature Communications, help advance our understanding of the mechanisms of primate reproductive cloning and may even improve its efficiency.

Somatic cells in the body, such as skin cells, contain the genetic information on how an organism is built, but cannot give rise to new organisms. Somatic cell nuclear transfer (SCNT) technology has previously resulted in the successful cloning of various mammalian species, most notably Dolly the sheep as well as cattle, mice, swine, goats, rabbits and dogs. In February 2018, Chinese researchers led by Qiang Sun reached a milestone when they successfully cloned wild-type and gene-edited cynomolgus monkeys (long-tailed or crab-eating macaques), although the efficiency of that process was only 1.5% — not far from that of the original cloning of Dolly and indeed most cloned species.

The efficiency of cloning most mammalian species thus remains extremely low, with high foetal and neonatal death rates (the live birth rates for most mammalian species range from 1% to 3%, with slightly higher rates of 5–20% observed for bovines) as well as developmental anomalies reported in somatic cell cloned animals. For the rhesus monkey, currently one study has reported a successful somatic cell clone, but the monkey did not survive after birth.

“The successful long-term survival of somatic cell cloned rhesus monkeys is of great importance, as these NHPs [non-human primates] are extensively used for basic and clinical research,” the authors of the new study said. “Despite previous attempts using the same protocol as for cloning of cynomolgus monkeys, achieving this goal has remained elusive.”

Seeking to improve the cloning efficiency of rhesus monkeys, Sun and colleagues performed a comparative analysis between epigenetic datasets of monkey blastocysts derived from in vitro fertilisation (IVF) and those cloned by SCNT. They identified abnormalities in the way the genetic information can be accessed and read by the developing cloned embryo and its placenta, and in the size and shape of the placentas of cloned monkeys that were developing in the surrogate mothers.

In order to provide the developing clone embryo with a healthy placenta, the authors employed a strategy known as trophoblast replacement (TR), also referred to as SCNT-TR, in which the trophoblasts (specialised placental cells) of SCNT embryos were replaced with those of intracytoplasmic sperm injection (ICSI) embryos. As explained by Dr Lluís Montoliu, a researcher at Spain’s National Center for Biotechnology who was not involved with the study, “Success was achieved by combining the treatment of cloned embryos with Trichostatin A (a histone deacetylase inhibitor) and Kdm4d (histone demethylase), both already used in the previous cloning of crab-eating macaques and aimed at altering the epigenetic state of cloned embryos, with a sophisticated method of trophoblast replacement, the cells surrounding the inner cellular mass in the blastocyst that will later give rise to the placenta.”

Using this approach, the team successfully obtained a healthy male rhesus monkey, named ReTro, which has now survived for more than two years. Montoliu noted that the efficiency of the process is even lower than with the macaques, at less than 1% — one surviving cloned animal out of 113 initial embryos. Nevertheless, the study authors said their findings “demonstrate that TR can restore the abnormal loss of DNA methylation in the placenta and prevent highly calcified phenotypes in monkey SCNT placentas”, and may therefore prove to be a promising strategy for cloning primates in the future.

Image caption: Photograph of the somatic cell-cloned rhesus monkey produced through trophoblast replacement, taken at 17 months. Image courtesy of the study authors (cropped from the original) under CC BY 4.0