Prime Editing : The New Genetic Engineering System
9 years ago, a protein called Cas9 was discovered to be the key to edit genes that could be applied in many applications. Scientists developed this protein to create the gene editing system now known as CRISPR Cas9 (Clustered Regularly Interspaced Short Palindromic Repeats). Many people believe that in the future we will be able to choose our genetic traits and become the beings we want to be. Unfortunately, CRISPR Cas9 does not have the accuracy nor the reliability to cure those genetic diseases. However, there is a new way to solve the problems that CRISPR has. In 2019, scientists at Dr. David Liu’s laboratory at the Broad Institute of MIT and Harvard developed a better Genome Editing system called “Prime Editing”.
Applications of Gene Editing
Gene Editing can remove or modify DNA to stop genetic mutations in the organism to stop diseases. We can also change cosmetic traits such as eye color and hair color. Gene Editing is an important technology for mankind, because it may have the potential to increase lifespan for people who are prone to illnesses as they get older.
What is CRISPR?
CRISPR uses the CAS 9 enzyme with the help of the guide RNA(Ribonucleic acid) to make the CAS 9:gRNA complex. The complex is used to track the DNA sequence in the gene. This guides CAS 9 to cut both DNA strands of the double helix in the genome like a pair of scissors making a double-stranded break. The cell will try to repair this break and this can lead to inadvertently producing mutations that can disable the gene that CAS 9 has edited. This can also be used to disable the specific gene that may be harmful, or disable a gene that was not supposed to be disabled. Not all cells can repair double-stranded breaks, therefore, making CRISPR an unreliable gene editing tool to be used on humans.
What is Prime Editing?
Prime editing is very similar to the CRISPR method. However, Prime editing does not create the double-stranded break. Prime editing uses a modified version of CAS 9 called CAS 9 nickase. CAS 9 nickase is used with a reverse transcriptase(an enzyme that is able to transcribe a single RNA strand into DNA) and the Prime editing guide RNA(pegRNA) to create a pegRNA complex. The pegRNA complex connects to the targeted DNA and then the CAS 9 nickase nicks a single DNA strand which does not create a double-stranded break. Furthermore, the Primer Binding sequence (or PBS) on the pegRNA connects to the DNA fold and the altered RNA sequence is then reversed transcribed by the reverse transcriptase making the new DNA. The edited strand is then fused to the DNA that was nicked. An Endonuclease in the cell removes the original DNA sequence leaving the edited strand fused to the genome and the other unedited. So now we have two strands that are mismatched due to the strands being edited and unedited. The next step is to create a nick of the unedited strand. This is done by another guide RNA which commands the prime editor to create a nick in the unedited strand. Finally, the cell starts to repair the nicked strand by using the edited strand as a blueprint, therefore, finishing the editing process. As you can see, Prime editing takes more steps to complete gene edits than CRISPR. This takes a long process however, this is much safer than what CRISPR can do.
Think of this process like a computer program that has glitches. The disease causing strands of DNA are coding errors that need to be fixed. To fix those errors you have to think about not causing any other glitches in the process. When you fix these errors you will cure the disease.
Why is Prime Editing Important?
Prime editing is the future of genetic engineering. Prime editing does not have the flaws CRISPR does. Not having the flaws of CRISPR, which is prone to triggering harmful diseases and cancerous mutations rather than removing them. Prime editing can correct all 12 types of point mutations rather than 4 which is what Base Editing does. This gives scientists the flexibility to edit genes in many different scenarios. Although Prime editing is in its early stages it theoretically has the capability of correcting 89 percent of all genetic mutations in humans. But because of the long process it takes to reach this precision this has about 20–50% efficiency depending on the cell type it edits. Prime editing is already able to correct a genetic disease known as Sickle cell anemia. This is a genetic blood disorder where the body produces hemoglobin S instead of hemoglobin. This causes the shape of red blood cells to become distorted. This gives the person severe pain in areas where blood flow is lacking. Prime is also able to cure Tay-Sachs disease. This is a deadly genetic disorder where nerve cells in the brain and spinal cord get destroyed. This causes babies to lose the ability to move around 6 months old, and can live for around 3 to 5 years. The fact that Prime editing will be able to cure these diseases in the future is important for humanity and will be a feat in genetic engineering.
What will Prime editing do in the future?
Although Prime editing is in its early stages, scientists still need to test this method in animal cells before we humans can take this.
With the precision that Prime editing delivers, brings opportunities for this to work in plant cells to create plants that grow faster and improve their quality. If you are curious, you can learn more about this here. Since Prime editing can theoretically cure 89% of mutations with more room to grow, this can lead to curing diseases that are occurring currently.
COVID-19 as we know is an ongoing pandemic that is being fought by CRISPR-based systems that could lead to a cure. Scientists are using CRISPR- based systems to understand how SARS-CoV-2 enters human cells. Scientists are also using CRISPR- based systems to test their hypotheses on how SARS-CoV-2 works at a molecular level in human cells, If you are curious, you can learn more about this here. As of October 31, 2019, Prime medicine, founded by Dr.David Liu is working with Beam Therapeutics to help increase the productivity of Prime Editing. According to statnews.com, Dr. David Liu states, “As was announced, prime editing for human therapeutics will be jointly developed by both Prime Medicine and Beam Therapeutics, each focusing on different types of edits and distinct disease targets, which will help avoid redundancy and allow us to cover more disease territory overall.” Prime editing has now become more popular since the first paper from nature, and now they have expanded into reaching out to companies to grow Prime Editing.
Conclusion
Prime editing has the potential to cure many genetic diseases with its precision and with the ongoing development of making it more efficient. Prime editing has opened new doors to make genetic modifications in applications such as agriculture and designer babies. Although Prime editing wont replace CRISPR or Base editing, it will be used alongside them to treat multiple genetic diseases.
tl;dr
- Prime editing is more reliable than CRISPR due to it’s precision can flexibility and can be used in applications such as agriculture and designer babies
- CRISPR-Cas 9 is an unreliable gene editing system due to the double- stranded breaks it produces
- CRISPR technology is being used to research and rapidly find treatments for ongoing diseases such as COVID-19
- Prime editing has the potential to cure a multitude of genetic diseases in the future and is theoretically able to cure 89% of all genetic mutations that cause genetic diseases in humans
- Prime editing has been used to treat Sickle cell anemia and Tay-Sachs Disease in human cells.
- Prime editing is still a fairly new gene editing tool that needs more tests and research on. This gene editing tool will soon be used in gene therapy in the future.
