CRISPR-Cas9

CRISPR is short for “clustered regularly interspaced short palindromic repeats,” DNA features that form part of adaptive immune systems in bacteria and archaea. Researchers adapted components of these naturally occurring systems for programmable genome editing. In the Cas9 system, the RNA-guided enzyme can be redirected by changing the guide sequence, rather than redesigning a new DNA-binding protein for every target.

In a commonly used form, Cas9 associates with a guide RNA. The guide contains a sequence that can pair with a matching region of DNA, while Cas9 also requires a nearby short DNA motif known as a protospacer-adjacent motif. When the target is recognized, Cas9 can cut both strands of the DNA. The cell then repairs the break. Repair without a supplied template can introduce small insertions or deletions; if a suitable DNA template is supplied, template-directed repair may be used to make a chosen change. The exact result depends on the target, the cell and the repair process, so editing is not equivalent to perfectly rewriting a predetermined sequence every time.

CRISPR-Cas9 has made it easier for researchers to alter DNA and investigate what genes do. Research applications include changing traits in plants, while medical researchers study genome editing for possible treatments of some diseases. A potential application is not the same as an established treatment: each proposed use requires evidence about its effectiveness, delivery and safety.

A guide may sometimes direct Cas9 to DNA sequences other than the intended target, producing off-target cuts and unwanted changes. The probability and consequences depend on the guide, the target genome, the Cas9 variant and how the system is delivered. Computational predictions alone cannot establish that an edit is safe; researchers use experimental measurements and validation. Human genome editing also raises ethical questions, especially when changes could be inherited, and the Nobel Committee has emphasized the need for regulation of powerful applications.

In 2012, Martin Jinek and colleagues reported that a two-RNA structure could guide Cas9 to cut target DNA and showed that an engineered single RNA could combine the guiding functions. Their work described the system’s potential as an RNA-programmable genome-editing tool. In 2020, Emmanuelle Charpentier and Jennifer Doudna received the Nobel Prize in Chemistry for the development of a method for genome editing involving CRISPR-Cas9.