Accurate to the nucleotide
Scientists have learned how to edit human DNA with an accuracy of one "letter"
Molecular biologists from the USA, Sweden and China have created a new version of the popular CRISPR/Cas9 genomic editor, which allows virtually unmistakably removing mutations with a length of just one "letter" of DNA and spot correcting errors in genes, according to a series of articles published in the journal Nature.
"These proteins can be called the most advanced, sharp and precise genetic scalpel, which will allow us to carry out the most delicate genomic "operations" in the most inaccessible cells. It will become an indispensable assistant for bioengineers and scientists and, perhaps, will eventually enter clinical practice and allow us to collect unique "author's" genomes, including in human cells," Chris Saha from the University of Wisconsin in Madison (USA) commented on the discovery.
The CRISPR/Cas9 genomic editor, called the main scientific breakthrough of 2015, was created by American scientist Feng Zhang and a number of other molecular biologists about three years ago, and since then it has undergone several upgrades that allow scientists to use it to edit the genome with absolute accuracy.
The main disadvantage of this genome modification system, as the participants of three scientific groups write at once, was that CRISPR/Cas9 allows you to edit sections of DNA several dozen "letters" long. Such a "block" nature of editing does not interfere with experiments on the creation of new transgenic organisms, but it is redundant and even harmful for medical purposes – often the correction of a particular gene requires the point removal or replacement of just one "letter"-a nucleotide.
David Liu from Harvard University, Emmanuelle Charpentier from Umea University (Sweden) and Zhiwei Huang from the Institute of Technology of China (Harbin) have found two ways to rid CRISPR of this flaw.
The first of them is that scientists have "glued" the CRISPR protein to another enzyme that converts one type of nucleotide into another (cytosine into thymine in this case), so that using the usual "templates" of the site with the wrong nucleotide, it can be used to correct single typos (Komor et al., Programmable editing of a target base in genomic DNA without double-stranded DNA cleavage).
The second method is that instead of Cas9, another bacterial protein Cpf1 is used, which Charpentier (Fonfara et al., The CRISPR-associated DNA-cleaving enzyme Cpf1 also processes precursor CRISPR RNA) and Huang (Dong et al., The crystal structure of Cpf1 in complex with CRISPR RNA) independently of each other a friend was isolated from the body of two different microbes – the intracellular parasite Francisella novicida and the microflora bacteria Lachnospiraceae. It interacts in a special way with RNA "templates", according to which the genome is edited, allowing them to contain a minimum number of letters. So far, scientists have not adapted this protein for genome editing, but they are confident that they will be able to do it.
The main feature of both methods is that the use of such variations of CRISPR does not lead to the appearance of double-strand breaks in the DNA helix after the removal of the wrong "letter", which usually provokes the appearance of even more mutations. This gives hope that in the near future humanity will receive a medical instrument capable of removing point mutations from the human genome, which are often the cause of cancer and a number of severe congenital diseases.
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21.04.2016