Prime Editing: The Revolution of Society

CRISPR/Cas9 and Prime Editing in just a single article!

Imagine this! It’s Friday night, and you are out with your friends to the movies. You sit in your seat with your bucket full of buttery popcorn. You have the perfect seats in the theatre. You check your bag for your glasses and realize YOU FORGOT THEM AT HOME. The movie you’ve been waiting for weeks to come out has just been ruined because you just can’t see the screen.

Now you are sitting in your seat, dreaming about how your life would be, just if you could improve your eyesight and not have to wear glasses.

What if I told you, your dream could come true one day… INSANE right!? This is where CRISPR and Prime editing comes into play! One day soon, using these biological processes, it may be possible just to edit in better eyesight into your genome! But before we get there let’s start from the very basics.

What is DNA?

Step one to understanding CRISPR and Prime Editing is to understand DNA. Watch this crash course on Deoxyribonucleic Acid :)

A short, yet informative summary on DNA!

What is Gene Editing?

Now that you are all caught up, let’s get started with Gene editing.

Gene editing is the process of altering and changing DNA. You can think of it as editing a word document; you can use the CTRL + F function to highlight a specific text, then you can either change, replace or delete it. Gene editing is the same thing, but rather than on a word document, it’s with DNA!

DNA holds the instructions for making proteins and also goes through many steps (e.g. transcription and translation). Nothing is perfect, just like humans, DNA can make mistakes as well. These mistakes can cause an extra-base to be added or deleted unintentionally. With the sequence ending up wrong, the protein is changed, and things just do not work right! The technology of gene editing can help the cells out by fixing these mistakes so they can do what they are meant to! Today, CRISPR is one of the most popular systems, yet another has recently been introduced that may just be preferred over CRISPR.

What is CRISPR/Cas9?

The acronym CRISPR stands for Clustered Regularly Interspaced Short Palindromic Repeats. This powerful genome-editing tool was inspired by bacteria that defend themselves against viruses.

CRISPR has 2 components to it: the Cas9 Protein and the Guide RNA.

• Cas9 Protein: This protein acts as molecular scissors, they cut the DNA where instructed by the Guide RNA. A protospacer adjacent motif (PAM) is a 2–6 base pair DNA sequence that immediately follows the DNA sequence targeted by Cas9. Without this, the Cas9 would not be able to bind to the target DNA sequence!
• Guide RNA: The guide RNA is what guides the Cas9 protein to the

How does CRISPR work?

1. First, the genome sequence that you wish to make alterations must be identified.
2. Then a specific gRNA is programmed to recognize that specific sequence of base pairs in the DNA
3. The gRNA is then attached to the Cas9 (DNA cutting enzyme) creating a complex
4. This complex is then introduced to the target cell, where the guide RNA locates the target letter sequence, and the Cas9 cuts the DNA from there
5. Once the DNA is cut, the existing genome can be edited either by modifying, deleting or inserting new sequences
6. The cell will then be repaired in one of 2 ways:

• Non-homologous End Joining (NHEJ): in this method, the cell uses proteins to create a DNA pair-end complex, which will join the bases of the cur DNA sequence. (can cause ERRORS of INSERTION or DELETION of bases!!!)
• Homology Directed Repair (HDR): in this method, the DNA template is inserted with the gRNA or Cas9. The template must contain the edit you would like, and other homologous bases to minimize errors!

Here’s a simpler version to look at how exactly CRISPR works.

CRISPR was first published in December of 1987. Yet recently another astonishing and more precise method for gene editing has been discovered. *mind blown*

What is Prime Editing?

Prime editing is the most recent and amazing development in gene editing. It was discovered in October of 2019 by scientists at the broad institute of Harvard and MIT. This is also a search and replaces technique, and it has 3 main components to it: Cos 9 Enzyme, Reverse Transcriptase, and Guide RNA

• Cos 9 Enzyme: this enzyme is a modified version of the one seen in CRISPR. The molecular scissors solely cut one strand in the DNA as opposed to cutting both strands (as seen in CRISPR)!
• Reverse Transcriptase Enzyme: this enzyme reads the RNA, uses the RNA as a template and creates DNA which is complementary to this RNA strand
• Prime Editing Guide RNA (PEG RNA): acts as a navigator, and navigates the complex to the DNA in the genome. It consists of 2 main components: A section that binds to the nicked DNA, preparing the nicked strand to have new DNA (letters added), and a section of RNA letters that encode the desired edit.

How does Prime Editing work?

1. The pegRNA sends the prime editing complex to its target where the modified Cas9 enzyme can nick the DNA
2. To transfer the edited sequence from the peg RNA] to the target DNA, the reverse transcriptase reads the RNA and attaches the corresponding DNA letters to the end of the nicked DNA
3. An endonuclease (an enzyme which cleaves the DNA chain by separating nucleotides other than the two end ones) excises (surgically cuts out) the old segment of the DNA and seals the new letters in the genome
4. The targeted site is left with one edited strand and one unedited strand (there is MISMATCH! OH NO!)
5. To resolve the mismatch, favoring the new permanent installation of the edited DNA, a different guide RNA directs the prime editor to nick the unedited strand
6. This second nick prompts the cell to remake that nicked strand using the edited strand as a template thus completing the edit.

Comparing Prime Editing and CRISPR

The ultimate objective of both CRISPR and Prime editing is the same; both biological processes that look to edit the genome, to improve the DNA or even simply fix mistakes in the DNA. Both of these processes have three main differences:

Firstly, prime editing has a modified version of the Cos 9 Enzyme, that cuts one strand whereas in CRISPR both strands are cut.

Secondly, prime editing has a Reverse Transcriptase Enzyme whereas the CRISPR system does not (because CRISPR does not need to make a template it can just insert the DNA whereas prime editing needs to make a template).

Lastly, prime editing inserts an RNA sequence while CRISPR inserts a DNA sequence.

Why is Prime Editing better than CRISPR?

Prime editing has proved to be better technology compared to CRISPR/Cas9. It’s easier and more precise than CRISPR, allowing us to better implement it, and use it for the better! Prime editing overcomes the limitations of CRISPR and unlike it, Prime Editing doesn’t rely on the ability of a cell to divide, to make the changes in DNA.

CRISPR is like a plane, it can fly and get you from point A to point B ✈️. But prime editing is like a helicopter, being smaller it can land you in more precise places and can go through smaller areas compared to a plane! A helicopter is more modern and widely used, now.

As the population of the world will increase throughout the next few years, diseases in people will also increase in number. Prime Editing can help mutations of these diseases in cells (such as Parkinson’s, Huntington, and Alzheimer’s). Fixing mutations in cells/genes that cause diseases can be a HUGE game changer when looking at the human race evolving!

The Future of Prime Editing!

The future of Prime editing is EXTREMELY bright. We are hoping that Prime Editing will enable scientists to fully fix mutations that cause diseases, something that CRISPR couldn’t. Can you imagine the parents for the next generation(s), being able to change their kids’ eye color or even so that they have better hair? Humans could get rid of “bad eyesight” and even become more muscular! We could drastically decrease the number of diseases found in cells; such as leukemia and other types of cancer!

The possibilities are endless!

On the economic side, we could increase the gaps between third-world and first world countries. As third-world countries won’t have access to this amazing technology, the larger countries could help their citizens drastically! As for any other thing in the world, gene editing could have its misuses and people could take advantage of it, A LOT. Can you imagine a scientist leading an army with super-strength humans!?

As we assume and know so far, Prime editing is one of the biggest steps into our future! Due to Prime editing, we are now a step closer to make just about any kind of DNA change to the human genome.

KEY CONCEPTS

• Gene editing is like editing a word document! You can alter DNA making deletions, changes, additions, and fixes mistakes!
• CRISPR is a biological process that allows one to cut and paste entire strands of DNA.
• Prime Editing nicks only one strand of the DNA, making changes a little bit differently compared to CRISPR.
• Prime editing is more precise and causes fewer mistakes in comparison to CRISPR.
• Prime editing will revolutionize our future, allowing us to do things we never thought possible!

Thanks for reading this article! I hope you enjoyed it and were able to learn something new!