By: Tiffany Chan
Molecular Biologist
From turning pigs to organ donors to drought resistant crops to malaria-proofing mosquitos, CRISPR (or Clustered Regularly Interspaced Palindromic Repeats) has dominated scientific news and shows no signs of slowing down. But what is CRISPR exactly and how did it come to be?
Simply put, CRISPR (or its full name CRISPR-Cas9) is a gene editing tool that researchers can use to precisely target, cut, and modify DNA. Like many great inventions in biology, this tool was inspired by nature. Like us, prokaryotes like bacteria have an adaptive immune system too. When they are attacked by invaders like viruses, they snip out a piece of its DNA and store it in CRISPR. I like to think of CRISPR as a library where small snippets of each attacker’s DNA are stored like books. When a familiar intruder dares to attack again, the bacteria is armed with the memory of the virus stored in CRISPR. The bacteria locate the “book” of this particular attacker and copies it (or transcribes it into RNA). This copy is called CRISPR RNA (crRNA) which in combination with another RNA called tracer RNA (trRNA) becomes the address for the GPS system of an enzyme called Cas9. Cas9 acts like a pair of “genetic scissors” that slices the DNA of the invader in two.
Once scientists put all the pieces together, they figured hey, I bet we can hijack this system for gene editing! Genetic engineering has been around for over 40 years, but the old ways of editing DNA were time consuming, expensive, limited in scope, and involved a fair bit of praying to the gods. A huge advantage for the CRISPR system was its precision in where the DNA is cut. If you were editing a Word document, your text cursor must be placed exactly where you want to make a change. The cut in DNA is like the text cursor in your document and CRISPR gave us a way to hit “CTRL+F”. We also don’t need the CRISPR library as long as we can provide the “book” or crRNA sequence that matches where we want to cut, the trRNA (which can be fused with the crRNA to make a “guide RNA”) and the Cas9 scissors. But a broken piece of DNA is a useless one. Luckily, in most eukaryotic cells, there are two built in repair mechanisms to fix the DNA: non-homologous end joining (NHEJ) and homology-directed repair (HDR). NHEJ is very efficient and the most active in the cell. However, it is more error prone and causes insertions and deletions that ultimately render the gene nonfunctional. This method is great for generating knockouts where you delete a gene to learn more about how it functions. In HDR, template DNA is added to guide the DNA repair to be EXACTLY how we want it to be. This method is favored for more precise and complex editing like adding a new gene or fixing a single base mutation. For example, in genetic diseases like sickle cell anemia where there is just a single letter change in DNA.
And huzzah! CRISPR burst into the scene and suddenly experiments that would take years would only take weeks. The ease of use, speed, and low cost of CRISPR has led to breakthroughs that quickly advanced multiple areas of biology. Notably in medicine, CRISPR has allowed scientists to quickly create cell and animal models, which has rapidly accelerated research into genetic diseases. But with great power comes great responsibility and while CRISPR is a powerful tool, it's not a perfect one. For one, it remains difficult to deliver CRISPR-Cas9 material directly to mature cells which remains a problem for clinical applications (viral vectors are the main method of delivery). It is also not 100% efficient (meaning not all cells get edited) and there is a rare chance of off target edits which may have severe consequences. While the benefits outweigh the risks, we also should always keep in mind the moral and ethical implications of such a technology in editing DNA of not only living things but also future progeny. Jennifer Doudna and Emmanuelle Charpentier are two brilliant scientists that smashed records as the only all women team to take home the Nobel Prize as the investors of CRISPR-Cas9 - showing women everywhere that if they can do it, so can we!
https://www.wired.com/story/wired-guide-to-crispr/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4975809/ https://www.patsnap.com/resources/blog/the-huge-list-of-crispr-uses/ |
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