20 years after the death of Dolly the sheep, cloning made little progress – 02/14/2023 – Science

The premature death of cloned sheep Dolly, who suffered from advanced lung cancer and was euthanized on February 14, 2003, seemed to be a warning that one had to think very carefully before going around genetically copying mammals like her. Twenty years later, scientists are much better aware of the mechanisms behind Dolly’s creation, but the efficiency of cloning has made relatively little progress — and there are no signs that this will change anytime soon.

Generated in experiments carried out by Keith Campbell, Ian Wilmut and other researchers at the Roslin Institute in Scotland, Dolly made history for being the first artificial clone of a mammal. In the technique used by the British scientists, known as nuclear transfer, the nucleus of a cell from the mammary glands of an adult sheep was inserted into an egg whose own nucleus was removed.

By making these two structures merge, it was as if the researchers “convinced” the genetic material contained in the nucleus to return to the state it had when the donor sheep was just a zygote (an egg recently fertilized by a sperm). As the DNA of most mammalian cells contains the “recipe” for making the entire organism, the process triggered the development of a new sheep embryo, practically identical, from a genetic point of view, to the adult sheep.

In addition to lung cancer, Dolly also had arthritis in her legs, although she was still a relatively young sheep, only six and a half years old when she died (life expectancy for sheep derived from the same breeds as her is about 12 years).

An investigation conducted by the Roslin Institute did not reveal a direct link between the animal’s illnesses and the cloning process. Still, studies at the time, as well as others since, indicate that there are some intrinsic problems when a mammal is bred this way.

The main one has to do with the process known as “imprinting” or genomic stamping. It is directly linked to the fact that almost all mammalian DNA is present in two copies, one derived from the father and the other from the mother of each individual. In a complex process of biochemical marking (so-called imprinting), the maternal and paternal stretches are activated and deactivated following a pattern typical of the species and sex of the animal when the zygote is formed and begins to develop.

However, it appears that this process can suffer when the core is “rebooted” during cloning. This means that the embryonic development of clones is often truncated, with animals acquiring excessive size and weight, while other embryos do not develop.

All this makes it necessary a large number of eggs and “surrogate mothers” —who, in general, face a risky pregnancy— for cloned animals to be born and survive more than a few days after giving birth. This huge technical difficulty, in addition to legislative and ethical constraints around the world, is the main reason why the approach was never tested to generate humans until now.

In the case of animals, the application in the case of several species of mammals is much more common, with thousands of individuals reaching adulthood. This goes for both laboratory animals (mice and rats) and pets (dogs and cats) and dairy and meat producers (cattle and pigs). In the latter case, the main application of the technique is to have animals of guaranteed genetic quality, leading to the latest practical consequences such as mass artificial insemination from large breeders.

However, costs and technical difficulties still make the use of cloning very rare. The same goes for the few international companies that offer the service of “resurrecting” a pet from its DNA. In such cases, the owner who believes that his new pet will be an exact copy of the one that died is buying a pig in a poke: influences such as the surrogate mother’s womb, random variations in the activation of certain genes and the animal’s environment tend to alter its appearance and your behavior.

If the technique may never be used to generate human clones and has poor performance with animals, it is indisputable that the knowledge generated thanks to it has brought important scientific advances. By studying how different stretches of DNA are turned on and off by cloning, scientists have gained insight into how a single cell gives rise to all the tissues that make up the organism.

Initially, the hope was to use this knowledge in so-called therapeutic cloning. In essence, the idea was to produce a cloned embryo from a person’s cells and use those cells in its early stages to produce tissue for transplants that would not lead to rejection, as with transplanted organs.

However, in addition to ethical questions about the destruction of embryos, the advancement of other techniques ended up leaving therapeutic cloning in the background. Scientists like the Japanese Shinya Yamanaka discovered that it was possible to reprogram adult cells so that they returned to the embryonic state, without going through nuclear transfer. This is the technique considered most promising today, although it is not yet used in large-scale treatments. Even so, it is likely that it would not have been formulated without the insights brought about by cloning.