CRISPR Gene Editing Teach-Out

The CRISPR technique was first observed in bacteria as a defense mechanism when attacked by viruses. After eradicating a virus, the bacteria cuts and stores pieces of the virus’s genetic sequence. In future viral attacks, the bacteria uses the stored viral genetic material to make an RNA guide along with a protein that can cut through a virus’s DNA. If any of the guides match the attacking virus’ DNA, the protein, Cas9, cuts the viral DNA sequence, thus disabling the virus.

Scientists have been studying the functions of CRISPR in different microorganisms since the 1990s. One of the early investigators who contributed to the discovery of CRISPR is Francis Mojica (University of Alicante). However, it was not until 2012 when two teams of scientists led by Jennifer Doudna (University of California, Berkeley) and Emmanuelle Charpentier (Umea University) discovered that Cas9 protein could cut through DNA’s double helix when an accompanied guide RNA identifies a match. The guide RNA can be constructed and introduced to cells, thus enabling gene editing.

In comparison to other gene editing methods that use proteins, CRISPR is an RNA-based method with higher precision in identifying the location of DNA sequences that require alteration. Two components of CRISPR that enable this process are: the guide RNA that identifies the sequence in DNA that needs to be altered, and the Cas9 protein that cuts the DNA (Figure 3. CRISPR-Cas9 Mode of Action). The Cas9-guide RNA combination can be introduced to multiple cells, thus facilitating gene editing in eukaryotes (living organisms other than bacteria). After the DNA is cut, it is possible to eliminate the targeted sequence, or replace it with a modified sequence. While DNA is capable of healing itself, interventions are required to prevent unwanted mutations that can happen as a result of DNA self-healing.

Figure 3. shows the CRISPR-Cas9 Mode of Action. (source)

While at this time, CRISPR is not yet used to treat illnesses in human patients, this method has enabled researchers to study genetic disorders (e.g., muscular dystrophy) and diseases caused by DNA mutations (e.g., some types of cancers). The therapeutic applications of CRISPR are promising, however, ethical aspects of its use in living humans are not yet resolved.