Prime Editing

Ryan Nasserzadeh
Nov 4 · 3 min read

To find out what gene editing is all about is nothing but remarkable. To put it simply, all life as we know it can now be potentially changed, … for good and possibly for bad!

In the past few years (especially past 5 years), scientists working in labs have figured out how to change the most basic element of living organism: the genetic code of living cells. The only problem is that it is not that precise yet. Just as you would use your computer mouse to cut and paste parts of a document, scientists can now cut, copy and paste specific segments of an organism’s genetic code. However, if you cut too much or too little and any imprecision can lead to very wrong outcomes. Up until recently, CRISPR/Cas9 has been the main technology used when it comes to gene editing, but it has had its limitations. One of the issues is that it requires two strands of DNA which reduces its precision. Another issue is how to effectively deliver the edited genes inside living organism (known as in-vivo, vs. in-vitro which is duplication of genes outside of living organisms such as in a laboratory petri dish).

Up until now, the preferred method of delivering the edited genes into a living organism has been the use of a virus called AAV. The AVV virus can simply act like a very small delivery truck that carries the modified gene into a cell and it also has the advantage that it can do what viruses do: infect other cells. In this case infection is a good thing since the infection is how the edited genes spread from one cell to other cells. In gene editing, this is called the vector which is this delivery mechanism (the small truck in the example) that delivers and spreads the gene to its targets.

As cools as it sounds, CRISPR/Cas9 has its limitations. As one researcher puts it, “CRISPR is more like a Swiss Army knife”, when we really need far more precise tools. Which is where Prime Editing comes in which promises to fix a number of these limitations.

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What is the difference?

Prime Editing is primarily based off the foundation of CRISPR. It adds 2 main things to it, a pegRna and a reverse transcriptase. The pegRna is what takes the Cas9 to the spot on the genome, and the reverse transcriptase adds another sequence to the genome. Another major difference is that you don’t need to cut 2 strands anymore. 1 strand is all you need to complete the edit. This significantly lowers the error rate and makes it much more precise.

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How does it work?

The Cas9 and reverse transcriptase go to the site led by the pegRNA. Cas9 then nicks a part of the sequence. The reverse transcriptase reads the RNA and attaches the right letters to the cut DNA. Next, the endonuclease gets rid of the old segment and seals the letters, but there is an issue, all the nodes are mismatched. Now a completely different guideRNA comes to get rid of the unedited DNA. Finally, the cell does the rest of the work by resealing the genome itself. In other words, to put it plainly, it acts like a search and replace function in a word processor. This process is done in 3 steps: first, a specific section of only one strand of the DNA is snipped off (vs. chopping off both strands), Then, a new gene is inserted into that spot. Finally, the opposite side of the strand is snipped. At this point, natural cell biology takes over and when it tries to repair the snipped off section, it looks at the opposite side where it now holds the newly inserted gene and makes a similar copy to patch the vacant gene spot.

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What are the limitations?

Yes, Prime editing is a huge leap in the area of genetic engineering. That being said, it’s not the end. Prime editing can change any of the letters of a gene (ACGT) for another letter to fix a genetic disorder. This is a very powerful tool for many genetic diseases but not all genetic disorders are caused by the problem of genes being out of normal order. Therefore, there is still a lot of work that needs to be done in the field and many new tasks at hand that will need to be solved.

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