Mice with a corrected enzyme
Wedge by wedge: gene editing cured rodents of a dangerous hereditary disease
Evgenia Efimova, Vesti
A team of researchers led by Professor Gerald Schwank from the Swiss Higher Technical School of Zurich cured mice of a metabolic disorder called phenylketonuria by correcting a mutation in the DNA of their liver cells.
Some parents of newborn children know firsthand about this metabolic disorder. If a special screening, which all newborns undergo today, reveals a genetic ailment, then a special diet is prescribed for the baby. It avoids the accumulation of the amino acid phenylalanine in the body. Its excess delays the development of the child. The absence of treatment can lead to serious mental problems.
The cause of the metabolic disorder is a mutation in the gene that lays the foundation for the formation of the enzyme phenylalanine hydroxylase. This enzyme is produced by liver cells.
Actually, the disorder occurs in a child if he inherits one mutated gene from his mother and one from his father. Today, unfortunately, there is no cure for this ailment.
This is where the well-known CRISPR/Cas9 gene editing technology can come in handy.
Swiss experts used this method to correct both mutated genes in the liver cells of mice. At the same time, it should be clarified that their technology is still different from the traditional CRISPR/Cas9 system.
Recall that the CRISPR system consists of artificially created suggestive RNA and the Cas9 enzyme. This complex is loaded into a harmless adenoassociated virus and is delivered to the cell nucleus with its help. The RNA molecule contains a copy of a small fragment of DNA corresponding to the place where the incision should be made. It attaches to a given site, after which the Cas9 protein cuts the chain.
Experts have improved CRISPR/Cas9 by adding another enzyme cytidine deaminase, which binds to a section of the gene that needs to be corrected. He "opens" both strands of DNA strictly in a certain place.
Then deaminase changes the base pair of the DNA causing the disease, C-G to T-A (the second base pair is characteristic of healthy people).
To transfer the genetic code of the new editing tool into liver cells, scientists inserted the necessary genes into an adeno-associated virus, and then injected it into the blood of mice. Subsequently, the virus infected liver cells, thereby introducing the necessary genes into liver cells.
Geneticists performed the above procedure by correcting an error in the sequence of DNA building blocks responsible for the creation of the enzyme phenylalanine hydroxylase in the corresponding gene in adult mice. As a result, liver cells were able to produce functioning enzymes, and rodents stopped suffering from a dangerous disease.
According to the authors of the work, their technique is much more effective than traditional genome editing technology: it corrects up to 60 percent of all copies of the gene with errors in the liver of mice.
According to Schwank, such an approach is quite realistic to apply to people. Meanwhile, the recent work is the first to prove the operability of the concept of the new method. It should be followed by studies involving other animals that will test the effectiveness and safety of the new genome editing tool.
Speaking about the risks, Schwank notes that they are quite low. After using the editing tool on mice, the researchers looked for non-target mutations (changes in the genetic code in areas where mutations should not be). No such errors were found.
However, this issue also needs to be studied more carefully. Schwank intends to do this in future works.
"The human liver consists of several billion cells. We don't want to cause any mutations that could provoke the development of cancer in any of them," the scientist says, explaining the slowness of the work.
The description of the new technique and the results of treatment of mice are presented in the scientific publication Nature Medicine (Villiger et al., Treatment of a metabolic liver disease by in vivo genome base editing in adult mice).
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