Why does the embryo need Oct4?
Scientists have turned off a gene in a "full-fledged" human embryo for the first time
American geneticists for the first time used the CRISPR/Cas9 genomic editor to disable a critical gene in the human embryo and study its role in the development of the embryo in the first seven days of its existence. The results of their experiments were published in the journal Nature (Genome editing reveals a role for OCT4 in human embryogenesis).
"As far as I know, this is the first experiment in which we were able to successfully disable one of the genes responsible for the development of the embryo, and understand what role it plays in this process. We have uncovered several unexpected effects that do not occur when Oct4 is turned off in mouse embryos, and in the future we will try to understand why this is so," said James Turner from the Francis Crick Institute in London (UK).
The first steps of life
Two years ago, rumors began to creep in scientific circles that a number of Chinese scientists and their American colleagues had already conducted a number of experiments on editing and replacing genes in defective human embryos, as a number of unnamed molecular biologists told the editors of the Nature news service.
In April 2015, these rumors were confirmed – Chinese scientists published an article in the journal Protein & Cell, in which they described the results of generally rather unsuccessful attempts to adapt the rapidly gaining popularity of the CRISPR/Cas9 genome editing system for manipulating human DNA.
This year, American scientists led by Shukrat Mitalipov announced the first successful editing of the embryo genome using CRISPR/Cas9. This genetic "operation" allowed them to replace the defective gene that caused heart hypertrophy with its correct version, and in theory protect future children from sudden death.
Turner and his colleagues took the next logical step in this line of experiments by adapting CRISPR/Cas9 to study the role of individual genes in the development of the human embryo, learning to turn them off even before the fertilized egg begins to divide.
As the first target, the scientists chose the Oct4 gene – one of the four "magic" DNA sections discovered by Nobel laureate Shinya Yamanaka in 2006, the forced inclusion of which transforms adult cells into analogues of embryonic stem cells.
As explained by Turner and his colleagues, this gene, as scientists believe today, plays a critical role in the development of the embryo in the first seven days of its existence, when the egg divides several times and turns into a so–called blastocyst, the primary ball of the embryo, from the shell of which the placenta is formed, and from the inner part - the child himself.
"Most of the fertilized eggs, as shown by the first experiments on artificial insemination, stop their development and die at this stage, only 50% of them survive. We have long been interested in which genes control this process, and thanks to CRISPR/Cas9, we now have the opportunity to find an answer to this question," added Kathy Niakan, a colleague of Turner.
Issues of duty and ethics
The high probability of embryo death, according to scientists, forces doctors and patients of reproductive clinics to use not one, but several dozen eggs during their artificial insemination. Very often this leads to the fact that some of the eggs remain "superfluous", which allows scientists to use them in experiments.
Having received four dozen similar cells, the scientists tried to turn off the Oct4 gene in them using one of the latest versions of CRISPR/Cas9, which Niakan, Turner and their colleagues adapted to work inside the embryo.
"We paid great attention to checking that genome editing with CRISPR did not make unnecessary changes to the structure of the embryo's DNA, and did not turn off other genes besides Oct4. We literally spent several months searching for such errors and did not find them, but, of course, in the future, we plan to conduct additional checks of this kind," Niakan explained, answering questions from RIA Novosti.
As this experiment has shown, Oct4 really plays a critical role in the development of the embryo. Its shutdown led to the fact that the number of eggs that reached the blastocyst stage decreased by more than 2.5 times, and most of the surviving embryos were able to "hold out" to this phase for the reason that the genomic editor deleted only one, not both copies of Oct4.
In addition, these experiments, according to her, led to several unexpected results that biologists did not record when conducting similar experiments on mouse eggs. For example, Niakan, Turner and their colleagues found that Oct4 controls the growth of not only the embryonic stem cells themselves, but also the "blanks" of the future placenta, which never happens in rodent embryos.
According to Turner, the same technique can be used to study the work of other genes, and to find other errors in their work that can lead to the death of the embryo, or to problems in the work of embryonic or reprogrammed stem cells. All this will help scientists to find new methods of treating diabetes and other diseases, with the development of which part of the body cells die.
"Why did our donors agree to donate their eggs? They are well aware of the advantages they carry, that without such experiments, the procedure of artificial insemination itself would be impossible. By agreeing to such sacrifices, they hope that these experiments will help solve other problems related to the birth of children," Niakan concludes.
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21.09.2017