Over the past decade, there have been significant advances in genetics and its applications in life sciences. The 2012 discovery of the CRISPR/Cas9 mechanism, also called the genetic scissors, revolutionised the efficiency of genetic manipulation. Now, a few years on, this discovery has been acknowledged as a major scientific breakthrough and thus been honoured with the world-renowned Nobel Prize in Chemistry.
The two scientists, Emanuelle Charpentier and Jennifer A. Doudna, were leading research in seemingly different domains, in different countries, years before their collaboration had started. Charpentier was doing research on pathogenic bacteria, specifically Streptococcus pyogenes, which is known to cause both mild and life-threatening disease in humans. Meanwhile, Doudna was researching RNA interference. This built on previous Nobel Prize winning work from Andrew Z. Fire and Craig C. Mello. A fascinating mechanism in itself, RNA interference involves the use of smaller RNA molecules (microRNA and small interfering RNA) to stop certain genes from being expressed. It was a valuable starting point for the discovery of the genetic scissors.
Leading up to the groundbreaking development of CRISPR/Cas9, there had been multiple studies on one interesting pattern that was spotted popping up in bacterial genomes – repeating identical DNA sequences, separated by non-repetitive parts. This type of arrangement is known as CRISPR (clustered regularly interspaced short palindromic repeats) and the genes associated with this pattern are named CRISPR-associated (Cas) genes. An important development in this research was the discovery that the non-repetitive parts of the CRISPR array had a similar code to some viral DNA, and that the bacteria which had these sequences were protected from the respective viruses. This led researchers to conclude that some bacteria were developing immunity against viruses by inserting a copy of the viral DNA into their genome. Biologists suspected that after acquiring this “immunity”, bacteria would fight off viruses in a similar way to how RNA interference works.
When Doudna found out about this research, she looked closer into this mechanism, and noticed similarities between Cas genes and genes that code for enzymes involved in cutting and unwinding the DNA. Eventually, Doudna and her colleagues discovered a complex CRISPR/Cas mechanism for defence against viruses, which involved several Cas proteins. This system turned out to be similar to the Nobel prize winning CRISPR/Cas9 system she would go on to discover with Charpentier, which only requires a single protein, capable of performing a variety of functions.
At that time, Charpentier was working out the structure of the S. pyogenes genome. She spotted some intriguing small RNA molecules, called tracrRNA. In short, she noticed that the tracrRNA molecules attached to the RNA molecules based on the CRISPR region of DNA. These two would then bind to the Cas9 protein, which can cut a target DNA. Now, when a virus attacks, the newly-formed complex binds to its DNA and cuts it, thus “disarming the virus”.
Charpentier and Doudna met and discussed their research on the same subject. During their collaboration, they discovered that the presence of tracrRNA is important for both creating the CRISPR/Cas9 complex and cutting the target DNA. They found a way to unite the two RNAs involved in one single molecule, which had been named guide-RNA. The guide-RNA can now be created in order to “fit” a sequence that needs editing, and it can be used, together with the Cas9 protein to cut the DNA like scissors. CRISPR/Cas9 can be used to either insert new genes or deactivate existing ones.
With this tool, it is now easy to cut the DNA at a specific location. This discovery has opened many doors for further research, with broad applications across the life sciences. Now, because science has made it possible to sequence entire genomes and study gene expression, the CRISPR-Cas9 gene editing tool can be used to learn more about how genes work.
CRISPR/Cas9 is essential in current studies regarding inherited diseases and their treatment. The genetic scissors have made it easier for scientists to create and monitor transgenic organisms. This tool also has applications in agriculture, for modifying genes that affect growth and resistance to antibiotics or pests. Although there are still debates about the ethics of using the genetic scissors in humans, the scientific community is delighted to have this new tool in its arsenal. It paves the way for more groundbreaking discoveries to be made, including cures to diseases that have threatened the human race for a long time.
“It paves the way for more groundbreaking discoveries to be made, including cures to diseases that have threatened the human race for a long time.”
Emanuelle Charpentier and Jennifer A. Doudna are the first women who share a Nobel Prize in Chemistry, and the 6th and 7th women to be awarded this prize since 1901. During the past two decades, an increased number of female scientists have been awarded Nobel Prizes. This is an incredible inspiration for young women seeking a career in science, and a great way to overturn ancient social stereotypes.
Written by Maria-Mihaela Avram, edited by Ishbel Dyke