Why is the CRISPR-Cas9 system suitable for gene editing? (Part 24- CRISPR in gene editing and beyond)
Welcome to the 24th part of the multi-part series on applications of CRISPR in gene editing and beyond.
Over the past decade, the CRISPR-Cas9 system has become a very popular method of genome editing because it is fast, cheap, precise, and relatively easy to use. We have already studied in Part 22 that CRISPR-Cas systems are categorized into six types (I, II, III, IV, V, and VI), based on differences in pre-crRNA processing, interference steps, and the required Cas proteins.
But out of the six types, the type II CRISPR-Cas system, also referred to as the CRISPR-Cas9 system, has generated a lot of excitement in the scientific community for genome editing purposes. This choice stems from the fact that
1. Type I, III, and IV CRISPR-Cas systems require several Cas proteins for cleaving the target viral genome. For instance, the Cascade complex consists of 5 Cas proteins in type I systems, while the Csm and Cmr effector complexes of type III systems also consist of around 5 Cas proteins for targeting the viral genome. Whereas in the type II CRISPR-Cas systems, only one Cas9 protein is required to target and cleave the viral genome.
2. Second reason which makes Cas9 a desired candidate for gene editing is that Type V Cas12 and Type VI Cas13 enzymes show trans or collateral cutting activity, which means that on finding the target, the cleavage activity of Cas12 and Cas13 enzymes is not just restricted to the target DNA or RNA, but they can also cut any single-stranded non-targeted nucleic acid molecules in the vicinity. This collateral DNAse and RNase activity become a disadvantage in terms of gene editing. On the contrary Cas9 protein does not show any collateral activity and cleaves only the target DNA. Thus making Cas9 protein suitable for gene editing purposes.
3. Additionally, Cas9 protein, the part of the search effector complex, participates in the processing of pre-crRNAs to form mature crRNAs. On the other hand, in type I and type III systems, Cas6 protein, which is not a part of the search effector complex, is required for the processing of pre-crRNAs to form mature crRNAs.
Thus due to the simplicity and requirement of only the Cas9 enzyme for processing crRNA and forming search effector complex, the Type II CRISPR-Cas9 system has been widely adopted as a gene editing tool.
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