Scientists talked about the features of DNA repair of embryos
Early human embryos repair DNA in different ways, depending on the damage that occurs during genome editing. Scientists have found that damage to one circuit is much more reliably repaired than breaks in both circuits. However, even a more accurate method was accompanied by undesirable genetic changes, so its safety for reproductive medicine remains unconfirmed. What experiments have shown and why successful gene editing does not mean safety — in the "Izvestia" material.
Why DNA repair is important for genome editing
Genome editing allows you to make preset changes to the DNA sequence. But the result depends not only on how accurately the tool recognizes the desired area. It also matters how the cell processes the damage that has occurred.
When using the classic CRISPR–Cas9 system, the enzyme cuts both DNA strands at a selected location. After that, the cellular mechanisms must repair the molecule. In early human embryos, this process may be accompanied by the loss of large sections of genetic material.
DNA base editing allows you to change individual elements of the sequence without a planned break in both strands. In particular, adenine editors are designed to convert the base pair A–T to G–C. In a new study, scientists compared how early embryos deal with the damage caused by different tools.
"The study shows that early human embryos are able to effectively repair single-stranded DNA damage, whereas the restoration of double-stranded breaks at this stage of development is much less reliable," explained first author Stefan Yerzhabek.
What did the introduction of the editor in the form of a protein show?
The researchers tested not only different tools, but also their delivery methods. The editor can be introduced as a ready—made protein or as a messenger RNA molecule, which cells use to produce this protein themselves. Institute staff members Iva Pichova and Michal Dolezhal participated in the preparation and purification of protein editors used in the experiments.
"Collaboration on this research began back in 2021, and in the following years we prepared several versions of editors for experiments conducted by our colleagues at Columbia University," Dolezhal said.
According to him, the introduction of editors directly in the form of proteins turned out to be compatible with the normal early development of embryos. One of the variants of the adenine editor, delivered as a protein during fertilization, made it possible to change all the studied copies of the PCSK9 target sequence. The embryos reached the blastocyst stage, and from them it was possible to obtain stem cell lines with the correction.
However, the delivery method was important: the introduction of an editor in the form of mRNA could be accompanied by a developmental arrest. Thus, the results depended not only on the chosen genetic target, but also on the properties of the instrument and the form of its administration.
Why did they get stem cells from embryos?
One of the important results was the production of stem cells from edited embryos on the sixth day of development. These cells provided scientists with additional material for detailed genetic analysis.
In the early embryo, the number of cells is limited. Obtaining a cell line allows us to expand the possibilities of research and study the consequences of the intervention in more detail. In addition, scientists can observe whether the changes made are preserved during subsequent divisions. In this case, we are talking about generations of cells grown in the laboratory.
According to the Prague Institute, this approach allows us to study the consequences of genetic changes more deeply than when analyzing only the original embryo. However, obtaining a stem cell line by itself does not confirm the safety of the intervention for all tissues of the future organism. It provides researchers with a tool for further verification.
What undesirable changes have the scientists found?
The high efficiency of the targeted correction did not exclude damage in other areas of the genetic material. The article describes rare breaks of chromosomes in the target area and chromosomal abnormalities. The scientists also found changes in neighboring bases and sites outside the selected target. These edits could not be distributed equally between the cells.
This is how genetic mosaicism arises: the cells of one embryo differ in some DNA changes. Therefore, finding the correct target sequence does not mean that the entire genome of all cells has remained unchanged.
As Columbia University notes, the unpredictable distribution of such changes makes it difficult to assess the consequences. When the mRNA content of some editors was high, the embryos also stopped developing.
These results show why, when testing the method, it is necessary to take into account several indicators at the same time: the accuracy of a given correction, the presence of additional changes, the state of the chromosomes and the development of the embryo. More reliable recovery of single-chain damage reduces some of the problems associated with breaking both chains. However, it does not eliminate all possible consequences of editing.
Why normal development in the early days is not enough
The title of the article indicates the high efficiency of PCSK9 editing and the normal development of human embryos. But the observation concerned the early laboratory stage, before the formation of the blastocyst.
This result does not allow us to conclude about the further development of the fetus, the health of the child, or the long-term consequences of genetic intervention. For researchers, early development and genome preservation are separate issues that require different checks.
"Our goal was not to develop a method for the genetic modification of human embryos, but to better understand how DNA repair mechanisms work in the early stages of human development through basic research," he explained.
According to the scientist, base editors significantly improve accuracy compared to approaches that create double-stranded breaks. Nevertheless, many safety issues need to be addressed before considering the clinical use of such tools in human embryos.
How research is related to hereditary diseases
In the future, point-by-point changes in the genome at the earliest stages of development may be of interest for the prevention of certain hereditary diseases. However, intervention in an embryo is different from editing individual cells of an already formed organism.
The US National Human Genome Research Institute separates somatic interventions and changes that can be passed on to future generations. Somatic editing affects body cells and is usually not inherited by offspring.
If a change made at an early stage of development persists in the cells from which germ cells are subsequently formed, it can potentially be passed on to offspring. Therefore, the consequences of such an intervention require a separate assessment.
The new work expands knowledge about DNA repair, but does not establish the safety of having children after such an intervention. The authors point out that the identified undesirable effects have so far hindered the use of the studied approaches in the reproductive clinic.
What can the results be useful for now?
The main importance of the research is related to the possibility to study the first days of human development more precisely. The editing tools allow researchers to create specific lesions and observe how cells respond to them.
Columbia University is considering this approach as a way to better understand the causes of genetic disorders and the cessation of embryo development. In the future, this knowledge can help research aimed at improving the reliability and safety of assisted reproductive technologies.
The work was supervised by the Ethics Committee of Columbia University. Jan Konvalinka, Director of the Prague Institute, noted that it is necessary to discuss such experiments outside the scientific community.
"I consider an ethical discussion around such research to be justified and necessary. The goal of fundamental science is to expand the boundaries of knowledge and at the same time help us better understand the possibilities and limitations of modern technologies," he said.
According to him, the value of the work lies both in new data on early development and in clarifying the limitations of available methods. The study helps to determine which issues still need to be resolved before laboratory results can become the basis for medical use.
Переведено сервисом «Яндекс Переводчик»
