CRISPR acronym comes from Clustered Regularly Interspaced Short Palindromic Repeats. It is a molecular technique that permits to "edit" or "correct" the genome of any cell, including human cells. Think in molecular scissors capable of cutting any DNA region in a precise place. That ability to cut DNA allows changing the original sequence to remove or insert new DNA.
In the 90s, work from Spanish researcher Francis Mojica described how some bacteria defend themselves against viral infections. They do so through enzymes able to distinguish between bacteria and virus genetic material and, once the distinction is made, to destroy the viral genetic material. The molecular basis was not understood until the full genome sequencing and future discovery of a bacterial region filled with palindromic repeats, which can be read forwards and backward equally, with no apparent function. Those repeats were separated by "spacer" sequences and headed by a short sequence named "leader." All together, they form what is known as CRISPR sequences (Short palindromic Repeats Regularly Interspaced).
Additionally, Cas genes, of the endonuclease class, were identified close to these genome regions. This finding led to the mechanism described in which bacteria, to defend against virus invasion, use a complex formed by Cas protein and RNA, produced from the CRISPR sequences. These molecular complexes scan viral DNA and, upon binding, degrade it. The system goes further since Cas proteins can steal a small portion of viral DNA, modify it, and integrate it within the set of CRISPR sequences. Thus, if such bacteria encounter the same virus, they will be capable of inactivating it more efficiently. Indeed, a real bacterial immune system. These findings laid the ground for Dr. Emmanuelle Charpentier and Dr. Jennifer Doudna to propose the use of Clustered Regularly Interspaced Short Palindromic Repeats plus Cas9 as a molecular laboratory tool for genome editing. In 2012, they showed how the system could be directed to a specific location of DNA and cut it.
The approach requires the design of an RNA molecule (RNA CRISPR guide), which hybridizes specifically to an exact DNA sequence, and it is associated with the Cas9 enzyme. The resulting RNA-Protein complex enters the cell nucleus, recognizes the desired DNA region, and cuts it. In a second stage, at least two natural DNA repair mechanisms are activated. The first is called Non-Homologous End Joining (NHEJ), which generates indels (insertion-deletion) that might lead randomly to the loss of the original function of the DNA segment. A second mechanism, Homologous Recombination (HR), can be exploited to incorporate specific sequences in the original cleavage site such as LoxP sites, reporters, or specific mutations.
CRISPR potential is almost endless, and its use is a game-changer in molecular biology and biomedicine. We enter a new era of genetic engineering where editing or correcting the genome of any cell and organism has become easy, fast, cheap, and highly accurate.
Now it is useful to understand the disease. In a not distant future, this gene-editing tool will allow humankind to revert the genetic cause of some human diseases and thus to cure genetic diseases (with a well known genetic cause) nowadays incurable.
This is happening now: what will happen tomorrow?