Controlling Creation: CRISPR
In nature, there exist an infinite number of creative and beautiful mechanisms that organisms use to survive. One such mechanism is CRISPR. While it may not currently seem like it, gene-editing technology will inevitably become a vital part of our lives and it is therefore essential that it is well understood.
First, we must tackle what CRISPR is and why it exists. This system was first discovered in bacteria where it is used as a way of protection against viral attack. During infection, viruses insert their genetic material into a host cell. Lacking translational machinery of their own, they have to take advantage of other cells to make new copies of themselves. By injecting their genome into a host cell, they effectively hijack the cell's synthesis brain, telling it to read off their genetic material, which can be DNA or RNA. This not only stops the normal functioning of the cell, but can also cause it to burst from the sheer volume of the viral particles produced. CRISPR is designed to protect bacterial cells from this fate when it comes to DNA viruses (a.k.a. retroviruses).
Physically, the CRISPR-Cas system is found in the bacterial genome as a CRISPR gene and some CRISPR-associated protein genes (Cas). When viral DNA is inserted into the bacterial cell, the bacterium synthesizes proteins (Cas 1 and 2) that cut up the viral DNA and insert it into the CRISPR gene in the bacterial genome. The CRISPR gene is best thought of as a library of past viral infections (called spacers) that are separated by what are called repeat regions.
When the same viral DNA is injected again (during a secondary infection), the bacterium transcribes the CRISPR gene into crRNA that contains a spacer and repeat region. These crRNA's then associate with another cutting protein (Cas proteins) and a tracrRNA, forming a “cutting complex”. Cas proteins can be thought of as the scissors while the crRNA is the directions. When the crRNA finds a region complementary to the viral DNA, the “cutting complex” will bind to and cut in these regions, breaking up the viral DNA.
CRISPR can be used on humans by creating specific crRNA’s that are complementary to a location on the genome that we want to cut.While these cuts are fatal to viruses, human cells have the ability to repair DNA damage and therefore survive. Nonetheless, the repair mechanisms at a cut site in DNA are not always perfect, possibly inducing mutations. Additionally, if a new piece of DNA is presented to the cut region, the DNA repair mechanisms will incorporate this new DNA. This opens up the possibility of inserting new genes (such as cancer fighting genes) at specific locations in the genome.
Such technology opens new possibilities in the realm of disease treatment and prevention. For example, a recent study found that a CRISPR-Ca13 system (using Cas13 instead of Cas9 to cut the viral DNA) was effective in suppressing COVID-19 replication in monkey kidney epithelial cells (Fareh et al.). While the use of this technology in humans may be decades to come, this study illuminates the possibilities for using CRISPR to treat various diseases caused by viruses. Even more than this, CRISPR could be used to fix disorders associated with point mutations, fight cancer, and so much more.
The uses of CRISPR are aided by preexisting mechanisms in nature and bounded only by human imagination.
Fareh, M., Zhao, W., Hu, W. et al. Reprogrammed CRISPR-Cas13b suppresses SARS-CoV-2 replication and circumvents its mutational escape through mismatch tolerance. Nat Commun 12, 4270 (2021). https://doi.org/10.1038/s41467-021-24577-9
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