Semi-CRISPR with nicknames
The treatment of genetic diseases is one of the most important tasks of modern medicine. Over the past decade, the development of CRISPR technologies and advances in genetic research have given patients hope for healing, although the safety of new methods is still a serious concern.
A group of biologists from the University of California at San Diego has described a new safer approach that will be able to correct genetic defects in the future. The strategy is based on natural DNA repair mechanisms and has the potential to cure a wide range of genetic diseases.
In many cases, people suffering from genetic diseases carry various mutations in one of the two copies of genes inherited from their parents. This means that a mutation in one chromosome often has an analog of a healthy sequence in another chromosome. The healthy variant can be used by the reparative mechanism of the cell as a template for the restoration of the chain after excision of the mutant gene.
Reconstructive gene editing using sequences from a parallel chromosome. The standard Cas9 enzyme potentially leads to unintended mutations in the target region and possibly in other parts of the genome (left). On the contrary, the enzyme nicase provides a more effective correction of genes without mutagenic phenomena (right).
Experimenting on fruit flies, the researchers created models that allow visualizing homologous chromosome-Templated Repair (Homologous Chromosome-Templated Repair, HTR) with the help of pigment production in the eyes. Flies originally had completely white eyes. But when they expressed the components of CRISPR (guide RNA plus Cas9), they had large red spots around their eyes – a sign that the cell's DNA repair mechanism had successfully reversed the mutation using functional DNA from another chromosome.
The researchers then tested a new system with nicases–Cas9 variants that target only one DNA strand, not both. They found that such incisions also led to the restoration of the red color of the eyes almost on a par with normal (non-mutated) healthy flies. They found successful repair in 50-70% of flies using nicases compared to 20-30% in the case of double-stranded Cas9 incision, which also generates unwanted non-target mutations. The researchers noted that the versatility of the new system can serve as a model for fixing genetic mutations in mammals.
It is not yet known how single-chain editing using nicase will show itself in experiments on human cells and whether it can be applied to any human gene. Some refinement may be needed for effective HTR to remove pathogenic mutations carried by human chromosomes.
An important feature of the study is that the new nicase-based system causes much fewer non-target mutations than with the classic Cas9-based CRISPR. In addition, slow and continuous delivery of nikaz for several days may be more convenient than one-time exposures.
Another notable advantage of this approach is its simplicity. It uses a small number of components, and the nicases make "soft" DNA incisions, unlike Cas9, which performs complete DNA breaks, often accompanied by mutations.
Article S.Roy et al. Cas9/Nickase-induced allelic conversion by homologous chromosome-templated repair in Drosophila somatic cells is published in the journal Science Advances.
Aminat Adzhieva, portal "Eternal Youth" http://vechnayamolodost.ru According to UCLA: "Soft" CRISPR May Offer a New Fix for Genetic Defects.