Scientists has now identified a different CRISPR system with the potential for even simpler and more precise genome engineering.
CRISPR sequences were first described in 1987 and their natural biological function was initially described in 2010 and 2011. The application of the CRISPR-Cas9 system for mammalian genome editing was first reported in 2013, by Zhang and separately by George Church at Harvard.
In the new study, Zhang and his collaborators searched through hundreds of CRISPR systems in different types of bacteria, searching for enzymes with useful properties that could be engineered for use in human cells. Two promising candidates were the Cpf1 enzymes from bacterial species Acidaminococcus and Lachnospiraceae, which Zhang and his colleagues then showed can target genomic loci in human cells.
The newly described Cpf1 system differs in several important ways from the previously described Cas9, with significant implications for research and therapeutics, as well as for business and intellectual property.
- First: In its natural form, the DNA-cutting enzyme Cas9 forms a complex with two small RNAs, both of which are required for the cutting activity. The Cpf1 system is simpler in that it requires only a single RNA. The Cpf1 enzyme is also smaller than the standard SpCas9, making it easier to deliver into cells and tissues.
- Second, and perhaps most significantly: Cpf1 cuts DNA in a different manner than Cas9. When the Cas9 complex cuts DNA, it cuts both strands at the same place, leaving ‘blunt ends’ that often undergo mutations as they are rejoined. With the Cpf1 complex the cuts in the two strands are offset, leaving short overhangs on the exposed ends. This is expected to help with precise insertion, allowing researchers to integrate a piece of DNA more efficiently and accurately.
- Third: Cpf1 cuts far away from the recognition site, meaning that even if the targeted gene becomes mutated at the cut site, it can likely still be re-cut, allowing multiple opportunities for correct editing to occur.
- Fourth: the Cpf1 system provides new flexibility in choosing target sites. Like Cas9, the Cpf1 complex must first attach to a short sequence known as a PAM, and targets must be chosen that are adjacent to naturally occurring PAM sequences. The Cpf1 complex recognizes very different PAM sequences from those of Cas9. This could be an advantage in targeting some genomes, such as in the malaria parasite as well as in humans.
Scientists described this unexpected biological features of this new system and demonstrate that it can be engineered to edit the genomes of human cells.