Genetically modified embryos: details

Human embryos have their genome edited again

Kirill Stasevich, "Science and Life"

In 2015, the journal Protein & Cell published, without exaggeration, an epoch-making article by Chinese researchers in which they described how they managed to edit genes in a human embryo. The embryos used in the experiments initially could not have developed as it should – the eggs were fertilized by two sperm cells at once, and in such cases it does not go beyond the very first stages of embryogenesis - so the results only indicated the possibility of such a procedure with human material. Nevertheless, immediately after the publication of the article, there was a heated discussion among experts in genetic engineering and biotechnology about whether it was ethical to interfere with human genes like this.

At the same time, a lot of people learned about the CRISPR/Cas method – it was with its help that the genes in the embryo were ruled. We have repeatedly described this method, so now we will only recall that initially the CRISPR/Cas system was found in bacteria in which it serves to protect against viruses; later it turned out that the CRISPR/Cas molecular machine can be used to accurately correct the DNA sequence in animal and plant cells (however, about the accuracy of the method in there have been some doubts lately). The prospects for CRISPR/Cas turned out to be so tempting that the researchers decided that they could use the method with human embryos. And now, two years after the notorious article in Protein & Cell, an article is published in Nature, which describes how the human embryo was freed from a dangerous mutation with the help of CRISPR/Cas, leading to a complex heart disorder.

Shukhrat Mitalipov and his colleagues from the Oregon University of Science and Health took more than a hundred eggs to fertilize them with sperm that had a mutation in the MYBPC3 gene. Defects in this gene lead to the development of hypertrophic cardiomyopathy: the wall of the heart muscle becomes abnormally thick, the fibers in it are located incorrectly, as a result, the rhythm of contractions is disrupted, heart failure occurs, etc. A man whose sperm were used in the experiment had to put a defibrillator implant due to genetic cardiomyopathy. The goal of the researchers was to obtain embryos without mutation in MYBPC3.

A molecular gene editing machine was injected into a fertilized egg, and CRISPR/Cas was injected into some eggs immediately after fertilization, and into others a few hours later. The editing machine was supposed to correct a mutation in the father's genes. After the arrival of the sperm, the egg waits for some time, getting used to the received "cargo", and then begins preparing for division. Before division, all DNA doubles, respectively, a second copy of the mutant gene appears in the embryo. If the editing machine enters the embryo a little later, it will need to correct an additional copy of the desired gene, but it does not always do this – the result is what is called mosaicism: the embryo has two different copies of the same gene, and subsequently some cells receive a normal version, and some – mutant. Preliminary experiments with mouse embryos have shown that if a CRISPR/Cas editor is introduced almost simultaneously with fertilization, then mosaicism can be avoided. New experiments have only confirmed this: out of 58 eggs injected with CRISPR/Cas immediately after fertilization, 42 embryos were successfully edited, and only one was mosaic according to the edited gene. On the other hand, among the 54 embryos that were injected with an editing machine 18 hours after fertilization, as many as thirteen turned out to be mosaic. (The editing molecules themselves, having done their job, quickly disintegrated.)

It is important to emphasize that here the embryos were absolutely normal, there were no chromosomal abnormalities in them, as in the embryos from the Chinese article, and, in principle, if they were transplanted to a woman, they could have turned out healthy children without mutation and without cardiomyopathy. On the other hand, it is easy to see that the effectiveness of the CRISPR/Cas method is not one hundred percent – only 75% of embryos were able to get rid of the mutation. If we talk about the possible use of CRISPR/Cas in the clinic, it is highly desirable that it works in all embryos, and not in 75%.

Finally, there is the well-known problem of CRISPR editing accuracy. The authors of the work assure that no "left editing" has happened in the experimental embryos. At the same time, the mutation in the MYBPC3 gene itself was quite rare: indeed, changes in MYBPC3 usually turn out to be the most likely cause of hypertrophic cardiomyopathy, but changes in DNA can be different. And in some cases, aiming the CRISPR/Cas machine at some defect in the right gene, we may suddenly discover that there are similar targets in other parts of our vast DNA, only in other parts they are not defects at all.

But in general, the researchers achieved their goal – they showed that the CRISPR/Cas editing method works on healthy embryos that can develop normally. And even if it doesn't come to the clinic, you can get a lot of useful data from such experiments. For example, now it has suddenly been possible to see that embryos correct their DNA after CRISPR/Cas intervention in a different way than other cells. The enzymes of the CRISPR/Cas system cut out a fragment of DNA with a mutation, so that the cell has to put a patch here. As a template for the patch, together with a molecular editor, a sample is launched into the cell – a piece of DNA with the desired sequence, on the basis of which the cell patches its own chromosome; and this is what, for example, stem cells do. The embryos act differently – they did not pay any attention to the DNA that was injected into them from the outside, using the maternal chromosomes as a sample for patching on the paternal chromosomes. 

Portal "Eternal youth" http://vechnayamolodost.ru  28.08.2017