What’s so special about ReTro the cloned rhesus monkey? Five quick questions answered

It’s been more than 30 years since Dolly the sheep made headlines as the world’s first mammal to be cloned from another animal’s body cell.

Now, a team of scientists in China has unveiled a rhesus monkey (Macaca mulatta) which was cloned using the same technique.

At more than two years old, the monkey — reported in the journal Nature Communications — is the longest surviving clone of its species by far.

Yet the strike rate for successfully cloning mammals remains stubbornly low.

We’ve answered five quick questions about why this is, and what this research could be used for down the track.

How did the researchers clone a rhesus monkey?

The main technique the scientists used to produce the cloned rhesus monkey, which they named “ReTro”, is called “somatic cell nuclear transfer”.

Don’t let the jargon put you off. “Somatic cell” simply means any cell from the body apart from eggs and sperm. “Nuclear” refers to a cell’s nucleus, which houses its DNA — the genetic information about how an organism is built.

The idea behind somatic cell nuclear transfer is simple — to create a cloned embryo by transferring a nucleus from a donor cell, such as a skin cell, into an egg cell.

The donor DNA, now encased in the egg, is “reprogrammed” to kickstart the embryonic development process.

The egg is implanted into a surrogate’s uterus, where — if it implants into the uterine wall — it starts dividing and developing into a foetus.

Should everything proceed smoothly, you end up with a healthy baby that’s a genetic clone of the skin cell donor.

But this rarely happens with somatic cell nuclear transfer in rhesus monkeys.

For some as-yet unknown reason, the technique also creates placentas that are less healthy than those associated with, say, in vitro fertilisation.

Most of a placenta is produced by an egg’s outermost layer of cells, called trophoblasts, which start building the organ soon after implantation.

The researchers found problems with the way DNA was “read” in cloned monkey embryos and their placenta. This resulted in the placenta developing unhealthy calcified regions.

To see if they could overcome this, the researchers added another step to the technique.

Taking a cloned embryo, they replaced its placenta-forming trophoblast layer with a healthy one taken from an IVF-generated embryo.

And one of those clones was born as a healthy baby boy monkey.

Wait: haven’t we cloned loads of animals before? Even monkeys?

Yes — heaps of animals, and most famously, Dolly the sheep. 

Dolly, which lived for 6.5 years, was cloned using somatic cell nuclear transfer.

Scientists since cloned mice, rabbits, dogs, goats, pigs, cows, even other monkeys, using the same technique.

Different animals have varying degrees of cloning success, but generally, the success rate in mammals is particularly low.

Only around 1 to 3 per cent of clones result in a live birth.

The process of swapping an egg’s genetic material for a skin cell’s is relatively straightforward, Jarrod McKenna, a reproductive biologist and science communicator at the University of Melbourne, said.

Rhesus monkeys, which are the classic lab monkeys used in medical research, are one of the difficult-to-clone groups of animals.

None (until this study) had been born and survived much past birth using the somatic cell nuclear transfer technique.

Are there other ways to clone animals?

Sure are.

The other main technique creates clones by splitting an early-stage embryo into four, which go on to develop into identical quadruplets.

In 1999, a rhesus monkey named Tetra was born using this method.

Will we see armies of human clones now?

Leaving the ethics of human cloning aside, no.

The refined cloning technique, which bolsters an embryo’s chances by giving it a healthy placenta, still has a very small success rate.

“Of the 115 embryo clones produced, only 1 per cent were able to be implanted, and of those, only 1 per cent resulted in a live birth,” Dr McKenna said.

This research shows that the placenta problem is just one of many hurdles that need to be overcome before cloning can be done on a large scale.

What does this work mean for me?

Insights into embryo development learnt from animals has potential to help with human reproduction.

For instance, in some people undergoing IVF, embryos won’t implant in their uterine wall.

“In Australia, the solution at the moment is to essentially transfer more embryos because we won’t know how to fix this recurrent implantation failure,” Dr McKenna said.