CRISPR/Cas9, the revolutionary nobel-winning “genetic scissors”

In nature, there have always been preys and predators. In the microscopic world of bacteria, predators, or rather invaders, are represented by bacteriophages: they are viruses that attack bacteria, into which they inject their own genetic material to start replication.

To defend themselves against such unwanted attack, bacteria have developed an ingenious system: extremely precise molecular “scissors” that cut the invader’s DNA and inactivate it, thus preventing infection.

Even more surprising, through the CRISPR/Cas9 system, bacteria are able to genetically memorize the infections already occurred, thus allowing them to respond promptly to a possible second encounter with the pathogen and to pass this response on to their progeny.

CRISPR/Cas9 (sometimes called just CRISPR) is a complex made up of two parts, which together have the function of identifying the desired spot on the target DNA and cutting it.

In particular:

• CRISPR is involved in the recognition of the sequences to be cut. This recognition is absolutely specific and takes place thanks to a special RNA molecule, called “guide RNA” (whose sequence is complementary to that of the site to be cut on the DNA), which can be easily modified in the laboratory. As the name implies, the guide RNA “orients” Cas9, the other component of the complex, indicating where to make the break.

• Cas (followed by a number, usually 9) is a nuclease, i.e. an enzyme that cuts DNA, and is in fact responsible for cutting the target molecule. Once associated with Cas9, the guide RNA acts as a kind of “anchor”, stopping Cas9 on the designated DNA sequence. Cas9, whose name stands for CRISPR-associated*, introduces a double helix break at the chosen site, and can be programmed to make specific changes to a cell’s genome.

CRISPR/Cas9 is a very versatile gene editing system. The CRISPR/Cas9 complex induces a break in the DNA double helix, thus activating highly conserved repair processes that can eventually lead to the inactivation of the targeted gene.

These processes can also be exploited to insert new genetic information within the CRISPR/Cas9 target site, or to correct pathological mutations. Once modified, CRISPR/Cas9 can also be used to deliver factors on DNA that activate or inhibit gene expression.

The advent of CRISPR/Cas9 has revolutionized biomedical research, making gene editing a quick and easy procedure. From a clinical point of view, the precision of this new form of molecular medicine opens up interesting perspectives for the treatment of many diseases.

Finally, with the Nobel Prize, a long and virtuous research path was awarded, which culminated in the molecular characterization of the mechanism of action of CRISPR/Cas9 and the first demonstrations of how this system can be used for targeted gene editing.

The CRISPR/Cas9 system is extremely versatile and easy to use, making it ideal for many applications. These range from biomedical research to therapy and diagnostics, from the generation of animal disease models to targeted gene modification in agronomy and zoology.

Before CRISPR/Cas9 (or B.C., Before CRISPR) there were other systems of targeted gene editing. Among the most efficient it is worth mentioning zinc finger nucleases (or ZFN) and TALEN, two genetic editing systems still in use and already in the clinic.

Unlike CRISPR/Cas9, ZFNs and TALENs are more complex to program, and therefore less widespread. However, their use largely overlaps with that of CRISPR/Cas9. The ten-year research and clinical experience gained with the ZFNs and TALENs has allowed a rapid development of CRISPR/Cas9.