What is Gene Editing?
Every living creature on Earth, from the smallest bacteria to mosquitoes and cats, to elephants and whales (and us humans too), are made up of genetic "blueprints". These blueprints provide clear instructions for an organism's cell on how to make the proteins they need.
DNA is the short-form name for the molecule that contains all instructions for your height, nose shape, immunity, and more. Deoxyribonucleic acid (DNA) is the instruction manual that is found in a cell's nucleus and unwound to make copies during DNA replication.
Genes are specific segments of DNA that code for specific heritable characteristics. Some traits are coded by just one gene, such as earlobe attachment, as you either have the gene for an attached earlobe or an unattached earlobe. Polygenic traits are traits that are coded by multiple genes within your DNA. These genes are usually represented as a continuous spectrum in humans. Think, for example, about the variety of hair colors and skin colors around the world.
When genes are inherited from your parents, mutations can sometimes happen. Often mutations are harmless, but there are about 40 genes that are incredibly important. Errors on these genes can unfortunately lead to serious issues. When a genetic error doesn’t cause death (or doesn’t present itself until later in life), the error can be passed on when the person becomes an adult and has children. This explains how detrimental but non-lethal genetics continue in the general population.
Humans have been unknowingly editing genes for thousands of years. Humans first abandoned our hunter-gatherer pasts because we learned how to manipulate and domesticate plants. For example, the largest and hardiest strawberries would be crossbred to try to create a new and improved hybrid strawberry plant that produced large fruit and survived more often. Humans also domesticated animals like cows, sheep, pigs, and horses in this way, by selecting for preferred traits.
We now have an incredible array of scientific advancements and tools at our fingertips, and scientists are very interested in fixing the genetic problems within our genes. Being able to target specific genes that are mutated and causing the body severe harm could save countless lives and prevent future generations from experiencing the same mutations.
What is the CRISPR/Cas9 method of gene editing?
Genetic engineering first began to produce interesting results following the discovery of DNA and the discovery of ligases, which join two DNA strands together and restrictase, which cuts DNA at specific points. The first genetically modified animal was a mouse created in 1974 whose genome combined with a virus known to cause tumors, so that no tumors developed. Today, genetic engineering is widely commercialized, especially to produce important chemicals for pharmaceutical and medical use, to create improved foods, and in fertility treatments.
While massive advancements were underway in genetic engineering, the biggest hurdle was still efficient and easy gene editing. Gene therapy added healthy genes into cells with mutations to make up for the missing genes, but has been imperfect and doesn’t fix the genetic problem at the source. Zinc finger nuclease (ZNF) builds artificial proteins that target specific mutations, and while promising, the technology is incredibly time-consuming to develop a protein solution to even one mutation.
In the 2000’s the origins of CRISPR began to emerge. Scientists discovered bacteria and archaea had interesting segments of DNA called Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR). The palindromic repeats are segments of DNA that are identical forward and backwards. Between these regularly interspaced repeats were segments called spacer DNA, that were unique.
The spacer DNA was actually the important DNA because it was DNA fragments left behind after the prokaryote had fought off an attacking bacteriophage. There are also CAS genes that create CAS proteins called helicases and nucleases that will unwind and cut DNA. When a bacteriophage would attack bacteria the CRISPR system in the bacteria would create CAS proteins and the associated spacer RNA (crRNA) to respond to and destroy the bacteriophages DNA.
Researchers Emmanuelle Charpentier and Jennifer Doudna investigated ways to take advantage of this system and potentially adapt it for use in humans. Streptococcus pyogenes was used because of its single CAS protein, called Cas9. Cas9 included the spacer RNA (crRNA) and tracer RNA. These researchers revolutionized genetic engineering by editing the Cas9 protein to create a tracerRNA-crRNA chimera.
This Cas9 system can be used in humans to edit genes. The Cas9 protein and tracerRNA-crRNA chimera can be engineered to have a corresponding RNA segment to match the DNA segment that needs to be edited in the human genome. When the human DNA enters the Cas9 protein system, the DNA is cut which renders the mutated gene inactive. A copy of the gene without the harmful mutation can be added where the mutated gene was cut, to encode the correct genetic instructions. Now the cell can continue on and start correctly producing associated proteins or whatever the targeted gene was responsible for.
This groundbreaking research resulted in Charpentier and Doudna receiving the Nobel Prize in Chemistry in 2020, as their findings have the potential to dramatically revolutionize genetic engineering. CRISPR can be used on living cells, can cut the genome in multiple places to fix multiple mutated genes, and can be engineered quickly compared to the current options.