CRISPR gene editing

CRISPR-Cas9

CRISPR gene editing is a method by which the genomes of living organisms may be edited. It is based on a simplified version of the bacterial CRISPR/Cas (CRISPR-Cas9) antiviral defense system. By delivering the Cas9 nuclease complexed with a synthetic guide RNA (gRNA) into a cell, the cell's genome can be cut at a desired location, allowing existing genes to be removed and/or new ones added.[1] The Cas9-gRNA complex corresponds with the CAS III CRISPR-RNA complex in the accompanying diagram.

While genomic editing in eukaryotic cells has been possible using various methods since the 1980s, the methods employed had proved to be inefficient and impractical to implement on a larger scale. Genomic editing leads to irreversible changes to the gene. Working like genetic scissors, the Cas9 nuclease opens both strands of the targeted sequence of DNA to introduce the modification by one of two methods. Knock-in mutations, facilitated via Homology Directed Repair (HDR), is the traditional pathway of targeted genomic editing approaches.[2] This allows for the introduction of targeted DNA damage and repair. HDR employs the use of similar DNA sequences to drive the repair of the break via the incorporation of exogenous DNA to function as the repair template.[2] This method relies on the periodic and isolated occurrence of DNA damage at the target site in order for a repair to commence. Knock-out mutations caused by Cas9/CRISPR results in the repair of the double-strand break by means of NHEJ (Non-Homologous End Joining). NHEJ can often result in random deletions or insertions at the repair site disrupting or altering gene functionality. Therefore, genomic engineering by CRISPR-Cas9 allows researchers the ability to generate targeted random gene disruption.

Because of this, the precision of genomic editing is a great concern. With the discovery of CRISPR and specifically the Cas9 nuclease molecule, efficient and highly selective editing is now a reality. Cas9 allows for a reliable method of creating a targeted break at a specific location as designated by the crRNA and tracrRna guide strands.[3] Cas9 derived from Streptococcus pyogenes bacteria has facilitated the targeted genomic modification in eukaryotic cells. The ease with which researchers can insert Cas9 and template RNA in order to silence or cause point mutations on specific loci has proved invaluable to the quick and efficient mapping of genomic models and biological processes associated with various genes in a variety of eukaryotes. Newly engineered variants of the Cas9 nuclease have been developed that significantly reduce off-target activity.[4]

CRISPR-Cas genome editing techniques have many potential applications, including medicine and crop seed enhancement. The use of CRISPR-Cas9-gRNA complex for genome editing[5] was the AAAS's choice for breakthrough of the year in 2015.[6] Bioethical concerns have been raised about the prospect of using CRISPR for germline editing.[7]

Predecessors

In the early 2000s, researchers developed zinc finger nucleases (ZFNs), synthetic proteins whose DNA-binding domains enable them to create double-stranded breaks in DNA at specific points. In 2010, synthetic nucleases called transcription activator-like effector nucleases (TALENs) provided an easier way to target a double-stranded break to a specific location on the DNA strand. Both zinc finger nucleases and TALENs require the creation of a custom protein for each targeted DNA sequence, which is a more difficult and time-consuming process than that for guide RNAs. CRISPRs are much easier to design because the process requires making only a short RNA sequence.[8]

Whereas RNA interference (RNAi) does not fully suppress gene function, CRISPR, ZFNs and TALENs provide full irreversible gene knockout.[9] CRISPR can also target several DNA sites simultaneously by simply introducing different gRNAs. In addition, CRISPR costs are relatively low.[9][10][11]

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