A New Way to Edit DNA: Prime Editing
Scientists have been trying to edit our deoxyribonucleic acid, DNA, since 1981 when the first genome editing technologies were developed. A recent discovery, Prime Editing, pushes the boundaries of what is possible and clears a path towards a future where genome editing is convenient, safe, and effective.
Simply put, Prime Editing is a genome editing method using a combination of advanced bio-engineering technology and tiny biological molecules called enzymes. It is a precise, effective way to edit or replace unwanted genetic variants.
Genetic variants describe variations within a DNA strand. Genetic variation is what makes everyone unique: it manifests itself in our physical attributes such as hair or eye colour and can explain our differences in drug susceptibility and reaction. However, it is also the basis of genetic disorders such as Angelman syndrome and Down syndrome.
Scientists have focused on two primary genome editing methodologies: CRISPR and Prime Editing due to their success and effectiveness.
How does CRISPR work?
CRISPR which stands for Clustered Regularly interspaced Short Palindromic Repeats and uses a combination of a Cas9 enzyme and gRNA (guide RNA) to modify genomes. Cas9 is used as the cutting tool to remove genetic variants and gRNA guides Cas9 to the desired location in the genome sequence.
Each gRNA is unique and is made individually by scientists. Once the sequence of genomes is identified (1), the gRNA binds itself to the DNA (2), the Cas9 enzyme cuts the DNA (3), creating a double-stranded break (DSBs) — see Figure 1.
The cell is then left to repair the cut DNA strands. Although the cell has a repair system in place to repair these cuts, creating a DSBs is very risky.
While repairing the cut, the cell is prone to errors thus negating the intended result. Repairing DNA cuts can also create unwanted, dangerous mutations, potentially leading the cells to become cancerous. Further, the cut may fail to repair, thus causing the cell to die and become susceptible to infection.
For further information on CRISPR, see the research paper below:
How does Prime Editing work?
Prime Editing is similar to CRISPR but uses a prime editing guide RNA (pegRNA) fused with a prime editing complex (PE) containing a reverse transcriptase (RT) and Cas9 enzyme.
Each pegRNA is uniquely designed and to match a specific genome sequence and uses this twinning to guide the process to the correct target. The Cas9 enzyme is used to cut the DNA strand and a primer binding site (PBS) binds the pegRNA to the loose DNA strand. Reverse transcriptase creates the new DNA sequence to replace the cut DNA — see Fig. 2.
Figure 3 shows how the pegRNA and Prime Editing complex interacts with the DNA.
The Prime Editing process works as follows:
After the DNA strand is identified by the pegRNA, the Cas9 nicks a single strand of DNA. Using the PBS to bind the pegRNA to the cut DNA, the RT is able to create the new DNA sequence based on the template provided by the pegRNA. The extra, original DNA strand and the new, edited DNA strand is integrated into the new DNA sequence by the repair system already present in the cell. (fig.4)— see Figure 4.
For further information on Prime Editing, see the research paper below:
What's the difference?
The most important difference between Prime Editing and CRISPR is that Prime Editing overcomes the challenges intrinsic with CRISPR. CRISPR can be thought of as a “cut and paste” genome editing method and has a reputation for being unreliable and messy. It can change genes in an uncontrolled manner and as previously stated, creates DSBs within the DNA which can result in DNA mutations or death of the cell.
Prime Editing is seen as a “Search and Replace” genome editing method. It is more precise than CRISPR because each pegRNA is meticulously coded, and cuts a single strand of DNA, which provides a higher chance of success than DSBs does.
Ultimately, CRISPR relies on the cell to repair its own DNA, which lowers the success rate and can increase the risk of harming the cell. While CRISPR has increased its success rate to 89%, Prime Editing has started with a success rate of 90%, making it immediately more effective and with the potential for better results in the future.
Why should you care about Prime Editing?
Any type of genome editing can change someone’s physical traits such as eye and hair colour. However, genome editing has the potential to treat genetic disorders such as polycystic kidney disease and has shown to be successful in curing leukemia. It could also benefit agriculture such as the breeding of hornless cows to save farmers the trouble of dehorning them, and plants that are more resistant to pests.
What might the future hold for Prime Editing?
Prime Editing is definitely a step above CRISPR and is a huge step in the right direction towards effective and safe genome editing. However, there are ethical concerns with genome editing as it can be considered as invasive and risky.
With the rate of technological advancement, genome editing may soon be accessible for everyone to use. As a society, we need to prepare ourselves to question the limits of genome editing and whether constraints are needed as the question of “What does it mean to be the ideal human being?” may soon be considered. One thing’s for sure, the future of efficient genome editing is closer than we think.
Key Takeaways:
- Prime Editing is a genome editing method which has the potential of curing genetic disorders
- Although they are similar, Prime Editing is more accurate, effective and safer than CRISPR
- Prime Editing brings us closer towards a future with safe, accessible genome editing.
