SNHS Biology Blog 10: CRISPR Cas-9

Eric Liu

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Clustered Regularly Interspaced Palindromic Repeats, or CRISPR, was discovered when studying a bacterial mechanism created to fend off bacteriophages. It relies on palindromic sections of DNA that are identical, with “spacer” DNA in between. Finally, the entire structure was found to be associated with Cas genes, which synthesized helicases – proteins that unzip DNA. Although the presence of spacer DNA stumped scientists when the genetic sequence was first discovered decades ago, they soon found out that it perfectly matched viral DNA. But why do bacteria keep viral DNA within themselves? It turns out that CRISPR is a record of failed viral infections, much like the memory cells of our own immune system. Whenever new DNA enters the bacteria, proteins identify it as viral DNA, which causes Cas to check if it matches any segments on the CRISPR gene. The Cas protein structure then neutralizes any viral DNA that matches CRISPR. Otherwise, it stores the DNA inside of the CRISPR structure as part of an adaptive immune system for the next time the same segment enters.

So how and why is CRISPR Cas-9 used for general purpose gene editing? The answer is the Cas-9 protein. Because Cas-9 possesses the ability to identify and modify sections of DNA when defending from a viral attack, researchers found that they could provide the protein structure with their own CRISPR DNA, effectively telling Cas-9 to target and delete any gene a scientist provides. This explains the stunning accuracy of Cas-9, a crucial trait when modifying fragile chromosomes. Furthermore, because cells naturally seek to repair genes spliced by Cas-9, the researcher can introduce their own segments of DNA, which are then integrated at the repair site.

Such a revolutionary tool, however, does not come without controversy. Recent discoveries show that CRISPR Cas-9 can cause unwanted mutations at the cut site due to cells attempting to repair the DNA segment. In fact, its gene insertion success rate hovers at a low 1% success rate, meaning that CRISPR still has a long way to go before it can be used without major repercussions. Finally, the ethics of altering human DNA have yet to be resolved, leaving a moral hurdle in the way of developing life-saving treatments for genetic disorders. Still though, CRISPR Cas-9, with its accuracy, speed, and utility is perhaps one of the most revolutionary tools to be developed in the past decade.