Prime editing: The new CAShable gene editing
Ever since we found out the building blocks of life are Deoxyribonucleic Acid (DNA), we’ve been looking for ways to manipulate this “recipe”. Ever since the discovery of CRISPR, and of subsequent gene-editing processes, we’ve been getting closer and closer to mastery over this recipe than ever before.
In this article, I’m going to cover the technical details of prime editing, how it’s being used today, and by who.
Prime editing is a new gene-editing therapy, engineered by David Liu and is being worked on by him and Andrew Anzalone. Prime editing is when targeted insertions, deletions, and base swapping occurs in DNA in a precise manner. This is an exciting new revolution because this has the ability to copy desired genetic modifications into target DNA sites without the need for donor DNA. This can help create new therapies for many genetic diseases, correct pathogenic mutations, and gene function. CRISPR gene-editing technology is another form of gene therapy and is similar to prime editing. However, prime editing is different from CRISPR gene editing therapy as it edits sequences without generating a double-stranded DNA break. Previously, CRISPR gene editing therapy caused double-stranded breaks, which could be dangerous, and was also inefficient.
The process of prime editing involves a complex of a prime editor and a ‘prime editing guide’ RNA. A fusion protein called prime editor consists of Cas9-nickase and reverse transcriptase is used. This protein’s role is to perform targeted insertions, deletions, and base transversions. Cas9 is an endonuclease that causes a double-stranded DNA break, allowing modifications in the genome. However, in this case, Cas9 nickase nicks the DNA instead of generating a double-stranded break. Reverse transcriptase is an enzyme that transcribes RNA into DNA. This prime editor corrects the majority of genetic defects. There are three versions of prime editors, PE1, PE2, and PE3. PE2 is most commonly used, as it has improved binding efficiency. However, more recently, PE3 versions have been created that can fix the mismatch sequences that can occur with prime editing. A prime editing guide RNA (pegRNA) is used in the process of prime editing. The pegRNA consists of a primer binding sequence (PBS) and a template containing the desired editing sequence at the 3’ end.
Together the prime editor and the pegRNA form a complex known as the PE:pegRNA. This complex binds to the targeted DNA strand and Cas9-nickase nicks one strand in the DNA strand generating a flap. The PBS on the pegRNA binds to the nicked strand and instructs the reverse transcriptase to synthesize an edited DNA flap. This edited strand is integrated into the DNA at the end of the nicked flap. This creates a double-stranded DNA with one edited strand and one unedited strand. The unedited strand is repaired to match the newly edited sequence.
As with all gene-editing technologies, prime editing can be used to fix typos in our DNA. These small insignificant typos can cause a multitude of different diseases, the most notable being cancer. With prime editing, there is a better method of tackling a specific variety of these small mistakes in our cells and DNA. Genetic mutations or diseases also could be fixed; any disease that has a genetic origin, in theory, could be edited and fixed by gene-editing technologies. Have you seen The Amazing Spider-Man 2? Well, if your memory is sharp, you’ll know that Harry Osborn, Peter’s (ex) best friend, became the Green Goblin. If your memory is really sharp, you’ll remember he was diagnosed with a fictional genetic disease called retroviral hypodysplasia. So, am I telling you that if we use gene-editing, like prime-editing, on Harry Osborn, he wouldn’t become the Green Goblin, and Gwen wouldn’t have to die? …Yes, among other things.
Gene-editing as a whole can be used to cure a multitude of diseases, from leukemia to sickle cell disease, to even Alzheimer's. However, prime-editing is a very specialized type of gene editing, targeted to specific diseases. So far, prime-editing has been found to correct mutations known to cause SCD and Tay-Sachs diseases and increase resistance to prion disease. This is done by using the PE3 device to install specific mutations into the DNA, creating immunity against the disease.
The reason why prime editing as a whole is a better technology is due to the nature of the process; CRISPR worked by creating a double-strand break at a specific spot, which is both harmful and sends an alarm signal to the cell. You can think of CRISPR as a machine gun, and prime editing as a sniper; both get the job done effectively, but both are more geared for certain situations, and one doesn’t leave collateral damage. So far, some of the companies engineering these changes towards specific diseases are ones like Beam Therapeutics, who are trying to cure sickle cell (as mentioned above) using prime editing, and other applications are being discovered by Prime Medicine, a private company dedicated to providing gene-editing technology to all.
Gene-editing technology is a powerful weapon that can be used to edit the basic building blocks of life, and as technology advances more, we’ll be able to edit, manipulate and control our own bodies much more freely.
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