Prime Editing: The Future of Genetic Engineering
Humans have entered a golden era for genome alteration
What if we could control the path of our own evolution? Alter our offspring to our liking and create a future society of super-humans? What if we could cut, paste and edit humans to perfection just like we edit our essays?
These pipe dreams were made into a possible reality on November 28, 2018, the day that Chinese biophysics researcher He Jiankui introduced mankind to the first ever genetically modified human beings, two twin girls named Lulu and Nana. According to Jiankui, the twins were born immune to the HIV virus as a result of his controversial experimenting on embryos using the genome editing tool CRISPR.
Since then, genetic engineering has progressed significantly. Today in 2020, a recently introduced tool, prime editing, shows the capability to go beyond the limitations that hold back traditional CRISPR and improve genome editing as we know it.
With prime editing in the picture, a world full of invincible superhuman beings doesn’t seem so fictional after all.
Prime Editing — The Basics
Prime editing, also referred to as CRISPR 2.0, can be described as a “search and replace” genome editing tool. It was first introduced in October 2019 by Dr. David Liu and his team at the Broad Institute of MIT and Harvard in the scientific journal Nature. Prime editing was based off of the original CRISPR tool, and had modifications added to it to allow for more precise and efficient edits to be made to DNA nucleotides. Liu estimates that this powerful tool is capable of correcting 89% of the 75,000 possible genetic mutations in DNA, and has already shown promising results in over 175 in-vitro edits in human and mouse cells.
So, what is the actual procedure behind the successful results that prime editing yields? The ingenuity of prime editing lies in the 3 primary components working together to carry out the editing procedure. The tool consists of a Cas9 nickase enzyme and a reverse transcriptase enzyme fused together as a protein, and a prime editing guide RNA (pegRNA).
Each piece of the puzzle plays an extremely crucial role in the edit. The Cas9 enzyme is modified to nick only one strand of DNA, thus the name nickase. It contains two nuclease domains, and RuvC domain and an HNH domain. The reverse transcriptase’s role is to synthesize and generate new DNA from an RNA template. And lastly, the pegRNA encodes the sequence to edit the DNA and binds to the cleaved DNA and prepares it to have new DNA letters added to it.
“If any one of those three DNA pairing events fail, then prime editing can’t proceed.” — Dr. David Liu
The genome editing process is as follows:
The pegRNA identifies and sends the editor to the target nucleotide (DNA segment), where the Cas9 nickase’s RuvC nuclease domain nicks the target DNA sequence. The reverse transcriptase then reads the DNA letter template that the pegRNA has encoded, and attaches them to the end of the nicked DNA. The old DNA segment is removed and the new letters are naturally sealed into the DNA by the cell.
To match up the other unedited strand of DNA with the newly edited strand, another guide RNA instructs the Cas9 enzyme’s HNH nuclease domain to nick the unedited strand. The cell will naturally remake the removed strand by following the edited strand’s template, matching up the two strands and finalizing the edit.
CRISPR 1.0 vs CRISPR 2.0
Thanks to prime editing’s complex editing procedure, it is able to make more precise and reliable DNA edits than the original CRISPR tool. But before we compare the two tools, we must first have an understanding of how CRISPR 1.0 works.
CRISPR 1.0, or CRISPR-Cas9 is similar to prime editing in the way that it also uses the Cas9 enzyme to make its edits. The tool was first found in the natural defense mechanisms that bacteria and single-celled archaea use against attacks from foreign viruses. When a foreign molecule attempts to invade the bacterium, the CRISPR-Cas9 system releases a chemical signal that destroys the DNA of the invading viruses and parasites.
Scientists realized they could engineer this system to make cuts to DNA and allow for the sequences to be genetically edited. The system implements RNA molecules, crRNA and tracrRNA to guide the Cas9 enzyme to the target nucleotides. The enzyme then breaks both strands of the DNA, therefore making a “double strand break”. And this is where the major difference between CRISPR 1.0 and CRISPR 2.0 kicks in.
We already know that prime editing technology implements a reverse transcriptase and pegRNA in its editing procedure, two things that CRISPR-Cas9 lacks, causing it to be less precise with its edits.
When the Cas9 enzyme implemented in CRISPR makes a double strand break, it could also possibly make extra cuts in wrong sections of DNA, which can greatly disrupt cell function. Moreover, when the cell tries to repair the break, it can sometimes cause a mutation by adding or removing a DNA letter or nucleotide, according to Dr. Liu. Due to these concerns, many are worried that the “cut and paste” CRISPR-Cas9 system could be more detrimental to genes than it is beneficial.
This risk is easily avoided with prime editing. The tool’s Cas9 nickase is modified to nick only one DNA nucleotide without have to break the DNA in half, and is a safer and much more reliable way of genome editing. It is capable of making edits to DNA sequences with equal or more accuracy compared to CRISPR-Cas9, and does so while avoiding the drawbacks that the original system faces.
The Significance of Prime Editing
Now that we’ve taken a look at prime editing and what it has to offer compared to other genome editing tools, an important question arises.
Why is this tool so significant to mankind?
Our population is plagued by hereditary diseases caused by mutations, errors in our genetic material. Spina bifida, down syndrome, cystic fibrosis, examples of diseases that cause life-long impairment and cost families hundreds of thousands of dollars to care for. These diseases are passed down through bloodlines, and there is no solid way to prevent their occurrence.
And this is where prime editing comes in.
Using this genome modifying tool, scientists can locate and identify mutations in DNA and replace them using the editor. We’ve already seen a Chinese biophysicist experiment with embryo genes to make them immune to HIV, and that was with the old CRISPR 1.0. Imagine how much further prime editing would be able to go, considering its higher accuracy and precision. Dr. David Liu and his team are already experimenting with editing gene mutations that cause Tay-Sachs disease and sickle cell anemia with a 33–55% efficiency rate. If we continue to develop prime editing and implement it into clinical practice, genetic conditions no longer have to be an unpreventable issue.
A Look Into The Future
Seeing as genome editing has advanced so much since the first introduction of CRISPR, what could the future possibly hold for prime editing?
At the beginning of this article, I introduced the possibility of creating a society of invincible super-humans. Thinking about it now, it doesn't seem so impossible with prime editing in the picture, does it?
With further development, mankind could take evolution into its own hands. We could control what genetic features we wish for our offspring to have, and which ones we want to cut out and replace. Embryos can be made immune to diseases, or provided with higher intellect, or maybe even altered to have certain facial features. Designer babies could become the new future of human society.
Of course, many arguments will be raised against tampering with the genetic makeup of human beings. Luxuries always come at a price, and genetically improving your own child would definitely be costly. Many individuals won’t be able to afford the procedure, and their children wont have the same genetic makeup as the modified children will, and will be more immune to diseases, which is a major ethical concern. The difference in genetics could create a rift between the genetically enhanced humans and the un-enhanced humans.
Prime editing and genome editing will definitely have a great impact on medicine and science. However, the future is uncertain, and it is impossible to predict whether that impact will be beneficial or detrimental.
Final Note
I hope this article has proved to be a valuable learning experience for you, and possibly kickstarted your interest in genetic engineering. Prime editing is currently still in a very early development stage, but who knows? Maybe one day you and I will get to see a future where genetic editing is the new norm.
Thank you for reading!