Over the plan
The genome editing method edits more than it needs to
Kirill Stasevich, Science and Life (nkj.ru ) based on the materials of The Scientist.
The CRISPR/Cas genetic editing method, which appeared a few years ago, quickly became a scientific and technological hit. And besides it, there are methods that allow you to make edits to DNA, but they are either more time-consuming and time-consuming, or they can only work with certain parts of the genome, or they are not too accurate in the sense that in addition to the necessary experimental edits, many side edits appear in DNA.
CRISPR/Cas turned out to be simpler, faster and more accurate. Initially, this is an antiviral protection system for bacteria: a special protein recognizes foreign DNA in a bacterial cell and cuts it, preventing the virus from multiplying. Biologists have adapted the CRISPR/Cas molecular apparatus to animal and plant cells. Now the enzyme that is supposed to cut DNA is given as a sample a piece of the sequence of the genetic code that needs to be cut, and it searches for this sequence in cellular DNA.
After a cut has appeared in the DNA, the cell tries to sew it up – DNA repair enzymes are turned on in it. To sew up the incision, repair enzymes use a template that experimenters slip them. The template can be synthesized in any way, so that after sewing the incision in the DNA, the sequence that the researcher needs appears. This way you can not only introduce experimental mutations, but also, for example, correct defects in DNA that can lead to diseases.
If we talk about clinical prospects, it is obvious that CRISPR/Cas should be used with embryos: if you correct a bad mutation in the embryo, you will get a healthy organism. CRISPR/Cas has been applied to animal embryos for some time, up to monkeys, but it was clear that sooner or later it would come to humans. In 2015, researchers from Sun Yat-sen University conducted an experiment with editing a human embryo. And in 2018, He Jiankui from the Southern University of Science and Technology announced that he had managed to use CRISPR/Cas to introduce a mutation into human embryos that protects against HIV - and, most importantly, two girls were born from these embryos.
A storm immediately arose in the scientific and near-scientific world about how ethical and safe it is to use CRISPR/Cas for clinical purposes. The security issue largely boils down to how exactly the method works. Even if CRISPR/Cas edits more accurately than other similar methods, this does not mean that its accuracy is sufficient for medical use.
In 2017, an article was published in Nature Medicine, which stated that human genes are difficult to correct with CRISPR/Cas. The authors of the article relied on the results of computer modeling, but soon experimental works began to appear that described extra mutations in real embryos edited by CRISPR/Cas. So, a year ago we wrote about a study published in Nature Biotechnology: it said that in human and mouse cells in which CRISPR/Cas worked, unplanned rearrangements in DNA occur.
Recently, three articles on the same topic appeared on the bioRxiv website at once – that CRISPR/Cas allows itself a lot of excess. In all three cases, we are talking about human embryos that arose during in vitro fertilization – they were created specifically for scientific purposes and were not intended for transplantation to women. In the first article, the staff of the Francis Crick Institute describe how they edited the POU5F1 gene involved in embryonic development. There were 18 embryos in total, and four of them (22%) had side changes in the DNA next to the edited gene that were not provided for by the experiment. The changes in some cases turned out to be quite large – about a thousand nucleotides could disappear from the chromosome.
In another case, researchers from Columbia University tried to correct an embryo mutation in the EYS gene, which subsequently causes retinitis pigmentosa, leading to blindness (embryos were obtained by fertilization with sperm with mutant EYS). In half of the embryos, after editing CRISPR/Cas, large segments on the chromosome next to the EYS gene were lost, or even the entire chromosome.
Finally, in the third article, the staff of the Oregon University of Science and Health write about side changes in the DNA of embryos that were trying to correct a mutation in the MYBPC3 gene that leads to heart disease. This is a continuation of their experiments with human embryos and cardiac mutation, which we wrote about three years ago. It was possible to correct the mutation, but along the way some defects appeared in the chromosome with the MYBPC3 gene.
It is not a fact that all these chromosomal changes could lead to diseases or developmental abnormalities. On the other hand, those girls who were tried to protect from HIV with CRISPR/Cas could have been lucky, and the editing device worked for them exactly. However, in order to use CRISPR/Cas for medical purposes, it, as they say, needs to be brought to mind. It is necessary to understand in which cases it provokes certain side changes in DNA, with what probability, and what molecular mechanisms are involved here.
And let it never become absolutely accurate, yet those parents who want to rid the unborn child of an undesirable mutation should know with what probability they will be able to get a healthy embryo, not burdened with side defects from genetic editing.
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