CRISPR- Cas9 is a technology that has hit the headlines in recent years, and understandably so. Scientists having the ability to alter an organism’s DNA is a controversial topic. Last year, a Chinese scientist claimed that it made the world’s first ever gene-edited babies using the technology.
But this technology can also have positive effects. Much more recently, American scientists published a paper that shows the elimination of HIV from live mice for the first time using two technologies, one of which is CRISPR-Cas9.
But where does CRISPR-Cas9 come from, and how exactly can scientists use it to cure genetic diseases?
DNA carries genetic instructions for all living things. It is composed of two strands that coil around each other, and each strand is made up of molecules known as nucleotides.
The sequence of these nucleotides acts as code or a blueprint for the formation of proteins, something that is essential for all living things.
Where does CRISPR-Cas9 come from?
Bacteria uses CRISPR and Cas9, two different components that work together, to defend itself.
CRISPR is found in bacteria and other single-celled organisms, but not in humans. It is a term used for sequences of nucleotides that are palindromic and repeated. First discovered in 1987, scientists soon found out that these series of repeating sequences is far from inconsequential and plays an important role in the bacteria’s defence against viruses.
To reproduce, a virus has to replicate its DNA, but it cannot do that on its own. It needs an unwitting accomplice. It needs to hijack a cell’s machinery, sparking a path of destruction for its host. When a virus attacks a bacteria, it inserts pieces of its own DNA into the bacteria. Unknowingly, the bacteria’s own protein-making machines will now make virus proteins. Virus proteins are now present within the bacteria and they begin to reproduce and assemble, killing the bacteria.
But there is a way to stop this. A bacteria that has survived a virus attack puts a mechanism in place to save itself the next time another attack happens. The bacteria chops up pieces of virus DNA that had made its way into the cell, and integrates it into its own DNA. These pieces of DNA are then stored in between the CRISPR repeats.
The bacteria then creates a “most-wanted poster” in the form of RNA. This poster is incorporated into the Cas9 protein, which acts like a killer hired by the bacteria to finish a virus. When Cas9 recognises that a DNA from an invading virus matches the sequence in the “most wanted poster,” it chops up the DNA of the invading virus, killing it.
How can CRISPR-Cas9 be used in genome editing?
We all know that scientists are clever, but they got ideas for DNA editing from organisms that have a lot fewer cells than they do - bacteria. They copied this whole process that we have been talking about.
They pick out the culprit, create a most-wanted poster, remove the suspect by Cas9, and then modify the gene so that in case of HIV, the virus can be completely eliminated from the host.
This process can also be applied for diseases not caused by viruses and can exist in our own DNA, such as Huntington’s disease and haemophilia.