Don't be afraid of genome editing
Scientists have found out how often the CRISPR/Cas9 genomic editor is wrong
The CRISPR/Cas9 genomic editor used to create the first "transgenic" children in China makes about two or three "typos" when editing the embryo's DNA. This was reported by molecular biologists who told about their results of experiments on mice at a meeting of the American Society of Human Genetics (Researchers Quantify Cas9-Caused Off-Target Mutagenesis in Mice).
"Surprisingly, the results of our experiments show that the number of random mutations that occur naturally significantly exceeds the number of those "typos" in DNA that CRISPR/Cas9 generates. In addition, we have shown that with the correct assembly of ribonucleic acid guides (RNA), the number of these mutations will be extremely small," notes one of the authors of the work, geneticist from the University of Toronto Loril Nutter.
CRISPR/Cas9 was discovered seven years ago by three groups of foreign geneticists at once, who are now challenging the primacy in making this discovery. Since then, it has undergone several upgrades that allow scientists to use it for "point-by-point" genome editing with almost one hundred percent accuracy.
In 2013, the rapid development of this technology began, and now it has been used to modify the genomes of dozens of living beings, including human embryos. These experiments revealed the main drawback of CRISPR/Cas9 – the editor, especially with multiple gene changes, sometimes made mistakes and deleted unnecessary DNA segments.
After Chinese geneticist Jiankui He announced that he had used CRISPR/Cas9 to create the first "transgenic" children, scientists began to study in even more detail how safe this genome editing system is. In addition, in recent years, molecular biologists have created a number of alternative versions of CRISPR/Cas9 with slightly different operating principles. Presumably, they do not make "typos" when changing DNA, but they are often inferior to the original in the flexibility of work.
Nutter and her colleagues conducted one of the first large-scale checks of how often CRISPR/Cas9 introduces new errors in DNA. The researchers used a special line of degenerate mice whose genomes, as scientists initially believed, were almost completely identical to each other.
Mistakes of nature and man
Scientists selected 50 embryos of similar rodents, processed them using a genomic editor, using four different types of guiding RNA molecules - short sequences of "letters"–nucleotides that help CRISPR/Cas9 recognize specific DNA sites in the genome.
After that, they implanted them into the womb of their mothers, waited for the birth of the mice and collected samples of their genetic material to count the number of new mutations. Their idea was relatively simple: if the same mutation occurred in different rodents, it meant that it was not generated by a genomic editor, it arose naturally.
Guided by these principles, geneticists completely decoded the DNA of all transgenic mice and compared their sets of mutations with each other. They also compared them with similar sections of the genome in rodents from the control group, whose DNA was preserved in its original form. On the one hand, CRISPR/Cas9 does sometimes make mistakes – they were in the genomes of 20 rodents, and 30 other transgenic mice were absent. According to scientists, genome editing led to the appearance of from one to 10 new "typos" in DNA (on average – 2.3 mutations).
On the other hand, the typical number of natural mutations turned out to be incomparably higher: each mouse differed from its relatives, including brothers and sisters, by about 3.5 thousand unique changes in the genome. Accordingly, this suggests that CRISPR/Cas9 rarely generates "typos", and also indicates the presence of serious differences in the DNA of degenerate mice, the existence of which scientists had not previously suspected.
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