CRISPR makes gene a lot of unpredictable mutations.


The structure of the bacterial antiviral Cas protein from the CRISPR-Cas; orange labeled DNA, which was associated editing enzyme. (Photo: Wikipedia.)

It is difficult to find anyone who has not heard about the method for genome editing called CRISPR (or CRISPR/Cas). It is based on the antiviral defense system of bacteria that are able to recognize viral DNA as it appears in the bacterial cell.

Biotechnologists took the enzymes of bacteria, with which they destroy the DNA of viruses, and adapted them for use in animal cells. The essence of editing system if nutshell, looks like this: looking for protein in the cellular chromosomes the segment you want cut, and as a “guide” to the enzyme to give a molecule of RNA with the same sequence of nucleotides as in the right plot. Comparing the RNA, which he carries with cellular DNA, the enzyme eventually finds the right place in the genome, and cuts it . If there was a mutation, it will disappear – the cellular repair system DNA themselves will close up the hole so that no mutations are gone.

Again say that this is a very simplified description of how the genomic editor CRISPR. Now it exists in several variants, with different proteins that cut DNA in one way or another. But in any case, it is clear how great prospects are opening up here for bioengineering and medicine: a large number of diseases develop because of defects in our DNA, so a tool that would allow such defects to be eliminated, would be very useful.

A lot of fuss about two years ago, the article by Chinese researchers are using CRISPR genome edited human embryo, and more and more papers, the authors describe how they managed to fix one or the other mutant gene either in cell culture or embryo of an animal.

But if we undertake to edit the genome, we must be sure that the changes will affect only that area which we chose. Meanwhile, CRISPR, although it is considered much more accurate than other ways to repair DNA, absolute accuracy boast is not: edit the rule enzymes DNA where it should not. In this case, generally speaking, developed a program that allows you to predict where else can be sent to the CRISPR system – knowing which sites in the genome for her particularly attractive, you can try to pre-make editing enzymes there still was not. However, as shown by the experiments researchers from Stanford, the University of Iowa and Columbia University, even with the help of special algorithms it is not always possible to guess where intervene, the CRISPR system.

Kelly Schaefer (Kellie A. Schaefer) and her colleagues analyzed the genome of mice, which use the CRISPR tried to get rid of mutations, leading to the pigment retinitis, degenerative disease of the retina, often leading to blindness. To remove harmful mutations has been quite successful, but, in addition, as the authors of the work in a letter to Nature Methods, they are found in the genome of more than a thousand mutations associated with changes in a single nucleotide, and more than a hundred mutations associated with insertions and deletions of larger DNA fragments.

According to the authors, all the predictive algorithms which are used to assess the accuracy of CRISPR, passed by these corrections. To detect them failed because the genome of mice were read in full, not only in risk areas where one could expect from a CRISPR performances.

On the other hand, the animals themselves in any way from the mass of the mutations did not suffer, that is, mutations, or get into those stretches of DNA, from which little depends in everyday life, or it worked some coping mechanisms.

But in any case, it is clear that before talking about some practical use of the CRISPR editing in medicine, you need to do a lot of work that the method was absolutely accurate.