Scientists from the Senger Institute (UK) have found that the use of the CRISPR / Cas system in human and animal cells, including mice, often leads to the appearance of extensive unwanted mutations. These lesions in the genome are not detected by standard DNA genotyping methods. The researchers' article was published in the journal Nature Biotechnology. This is reported by Science Alert.
The CRISPR / Cas system is used to change the nucleotide sequence of DNA. Cas is a protein that cuts a double strand at a specific site. This site defines guide RNA (sgRNA), which binds to a specific recognition site according to the principle of complementarity. The various sgRNAs are encoded by spacers - regions of DNA within CRISPR - a group of special repetitive sequences.
Cas sometimes cuts off fairly large nucleotide sequences (they can be hundreds of bases in length), but such side effects are believed to be quite rare. The CRISPR / Cas system has thus been approved for a number of clinical studies, including modifying T cells for the treatment of esophageal cancer, leukemia, and non-small cell lung cancer.
In May 2017, a team of scientists from Columbia University announced that CRISPR / Cas9 makes many changes in DNA outside the desired region. While the researchers were unable to confirm their own results, other scientists also received evidence that the CRISPR system causes more side effects than previously thought.
In the new work, the scientists evaluated the effect of CRISPR on mouse stem cells and human retinal epithelial cells. It turned out that the system caused large-scale genetic rearrangements, including deletions (loss of regions of the chromosome), affecting several thousand base pairs far from the region that was directly edited using CRISPR. Such mutations can irreversibly damage important genes, which is fatal for patients who may receive gene therapy.
Even if only one of the millions of cells carries pathogenic mutations, there is a serious risk of developing cancerous tumors. Scientists believe further research is needed to understand how to prevent editing errors.