The danger of CRISPR has been exaggerated

The article about the side effects of CRISPR was withdrawn from the journal

Daria Spasskaya, N+1

The authors of a study on numerous non-target mutations caused by the CRISPR-Cas9 genome editing system in mammalian cells have withdrawn an article from the journal Nature Methods, where it was published in May 2017. The publication was withdrawn after numerous critical comments from the scientific community regarding the incorrect staging of the experiment. The sequence of events can be found in the Nature Methods editorial note, and the retraction of the article is also reported by the Retraction Watch blog.

During the operation of the CRISPR-Cas9 editing system, the Cas9 protein cuts DNA in the place indicated by a short RNA seed with a length of 20 nucleotides (the so-called guide RNA). Although in vitro experiments, priming with high accuracy "leads" Cas9 only to the right place, in reality, especially when working with large genomes, there is always the possibility of incorrect recognition of the target and erroneous cutting of DNA in another place. To check for the presence of non-target mutations introduced by CRISPR-Cas9, researchers usually use a computer to predict where Cas9 may go by mistake and sequence these sites point-by-point.

Cas9 protein in complex with DNA (orange) and guide RNA (red). 
The structure of the guide RNA is shown next to it. 
Thomas Splettstoesser / Wikimedia Commons.

In 2017, a group of researchers from Stanford and Columbia Universities (USA) reported the results of genome-wide DNA sequencing of two mice in which a mutation leading to blindness was previously cured using CRISPR-Cas9. The DNA of another mouse of the same line served as a control. Judging by the data presented, the number of side effects of the system turned out to be greatly underestimated – for example, almost 1,500 single-nucleotide polymorphisms and more than a hundred large mutations by the type of insertions and deletions were found in the genome of the edited mouse.

Despite the fact that by the time of publication, the editing system was already actively used in mammalian cells, and even human embryos, this work imposed significant restrictions on experiments with CRISPR-Cas9. In addition, the publication and the media hype caused by it led to a drop in the shares of biomedical companies engaged in developments in the field of gene therapy with the participation of CRISPR.

Immediately after the publication, the article was criticized by experts. The main remark presented to the experiment was incorrect control – in fact, the authors simply compared the DNA sequence of several mice of the same line, while it would be more correct to compare the DNA of an edited mouse with the DNA of its parents. Critics of the work pointed out that the detected mutations may reflect natural genetic differences between individuals. Some published mutations theoretically could not have been introduced by the editing system, based on the molecular mechanism of its operation. In addition, three mice are clearly not enough for such strong conclusions, experts noted.

Based on the seriousness of the comments, critics suggested withdrawing the article. In this regard, the editorial board of the journal decided to conduct an additional examination and attracted an additional five experts to review the work. Finally, after a lengthy discussion between the reviewers and the authors, almost a year later, it was decided to withdraw the scandalous article. In a new publication published on the bioRxiv preprint website, scientists report that they have sequenced the DNA of two more edited mice, and admit that their initial conclusions were erroneous. However, only two of the six authors agreed with the review of the article from Nature Methods.

Despite the final review of the publication, the scientific community recognizes that there is not enough genome-wide data confirming the safety of CRISPR. Nevertheless, the editing of the genome of individual cells using this system is already undergoing clinical trials in humans: in China, patients are injected with lymphocytes modified with CRISPR-Cas9 as cancer immunotherapy. In the USA and Europe, the start of clinical trials on editing blood stem cells for the treatment of beta-thalassemia and sickle cell anemia is scheduled for this year.

Portal "Eternal youth" http://vechnayamolodost.ru