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This is not the first honor Emmanuelle Charpentier and Jennifer Doudna have received for their development of the Crispr / Cas9 genetic scissors. But this year’s Nobel Prize in Chemistry is the highest.
Emmanuelle Charpentier (left) and Jennifer Doudna (right) received this year’s Nobel Prize in Chemistry. Photo of the 2015 Princess of Asturias Awards ceremony.
The average age of Nobel Prize winners alone shows that it generally takes some time for findings, theories, or developments to be tested to the point where the researchers behind them can receive the highest scientific honors, the Nobel Prize. . This year’s Nobel Prize in Chemistry is an exception: With biochemist Jennifer Doudna and microbiologist Emmanuelle Charpentier, not just two comparatively young women receive the award. Their groundbreaking publication is also only eight years old (and, to be precise, two months and ten days): in it, they introduced the Crispr / Cas9 programmable genetic scissors, a tool that makes gene editing much faster and easier. possible methods available until then.
The work of Doudna and Charpentier was aimed at revolutionizing genetic research and technology. Crispr / Cas9 gene scissors enable fast, precise and specific changes in the genetic makeup of all living things: from humans to animals and plants to microorganisms.
Crispr / Cas9 is based on elements of a bacterial immune system that was only described about fifteen years ago. If bacteria survive a virus infection, they incorporate parts of the virus genome into their own genome. This occurs in very special regions of bacterial DNA, which are very similar in all types of bacteria and consist of repeats of the same DNA sequences, the so-called “regularly interspaced clustered short palindromic repeats” (crispr). Fragments of the virus genome are inserted between these sequences. They allow the bacterium to recognize the virus in question later in the event of a second infection and render it harmless through a sophisticated mechanism: its genetic material is cut at the points that the bacterium has stored between its crisp sequences, thereby destroy the virus.
This is how Crispr / Cas gene scissors work in nature
Nobel Prize winners Charpentier and Doudna made this mechanism usable in genetic engineering, and this both in plants and animals, or even in humans.
This requires certain proteins, the so-called Cas proteins (crispr-associated proteins). These are the parts of the system that cut through genetic material. Sometimes different bacteria use different variants of these proteins; Doudna, then at the University of California at Berkeley, and his colleagues characterize the roles of several of them.
Meanwhile, Charpentier and his team at Umeå University in Sweden discovered another essential part of the Crispr-Cas system. They found the so-called TracrRNA (Trans-Activating Crispr-RNA), a small RNA molecule, and were able to show that this was necessary to process Crispr-RNA in its “working form”. In doing so, they had discovered the third element in genetic scissors, along with the Crispr elements and the Cas proteins. Charpentier and his colleagues published their discovery in 2011.
Then the two researchers started working together: they found that tracrRNA is not only required in the first step of crispr RNA processing. In a further step, he also ensured that the components of the scissors came together to form an active unit. When the scientists combined the three elements, Crispr-RNA, Cas9 and TracrRNA, in the test tube, the scissors cut the DNA, exactly at the point specified by Crispr-RNA that was included. In subsequent experiments, Doudna and Charpentier combined crispr RNA and tracr RNA into a single molecule: single guide RNA (sgRNA) or, more simply, guide RNA. Doudna and Charpentier had thus created a system composed of only two components with which almost every point of the genome could be selectively cut by inserting the target sequence into the guide RNA: the programmable Crispr-Cas9 genetic scissors.
Shortly after the publication of Charpentier and Doudna, another working group showed that the scissors not only worked in test tubes, but also in living cells. The potential of gene scissors for genome editing, which scientists had already suspected, was thus proven and the triumphant advance of the new instrument began. Its applications now range from basic research to agriculture and medicine.
If a gene is specifically cut out with scissors and left to the cell’s own repair mechanisms without further intervention, this almost always leads to what is called a knock-out: the gene no longer functions properly and its function is lost. . This makes it possible, for example, to analyze the function of individual genes by examining the consequences of their failure. But it also has very practical applications: just a few years after the introduction of genetic scissors, there was a fungus that no longer turned brown. With the help of Crispr-Cas9, its developer inactivated the genes that caused the brown coloring. This change caused by Crispr-Cas9 is practically indistinguishable from a natural one. Genes only lack a few building blocks that could easily have been lost through natural processes. In 2016, the American agricultural authority did not classify the fungus as a genetically modified organism (GMO). In the EU it is different: there you are oriented towards the process, not towards the product, and you treat all Crispr-Cas9 organisms as GMOs. Switzerland is planning a differentiated vision.
The selective deletion of individual genes by Crispr-Cas9 is also of interest in medicine. In some inherited diseases, turning off a gene leads to a cure or significant improvement. An example is beta-thalassemia, a serious blood disease, in which the body produces defective “adult” hemoglobin. Fetal hemoglobin is functional in these patients, only it is no longer produced after birth. By turning off the gene that blocks its production, patients can, in principle, be cured. This therapy is already being tested in human clinical trials, as are others.
With Crispr-Cas9, not only can genes be switched off. Because cells can also be offered “repair templates”: genetic sequences that, due to their structure, exactly match the gap caused by the incision. The cells then partially use this to bridge the gap: the target gene has a new sequence specified by the researchers at the interface. This allows genes to be modified and repaired in a targeted way or new genes to be incorporated. In medical research, for example, attempts are made to use this principle to assemble immune cells specifically to search for cancer cells.
However, with the original scissors, targeted insertion doesn’t work very efficiently, says Gerald Schwank of the University of Zurich, who works with Crispr-Cas9. Newer and more developed methods are better suited for this. This has already been shown in animal models, but therapies based on them are not yet in clinical trials.
It is currently being intensively discussed whether genome editing can be used in the future to apply it to human embryos and make heritable changes in the genome, that is, to intervene in the so-called germ line. Everyone agrees that the technology is not yet mature enough. In China, however, this application has already taken place: in late 2018, Chinese biophysicist He Jiankui announced that they had been born twins and that he had used genetic scissors to change a gene in its genetic makeup. Jiankui’s work was condemned as premature and dangerous. It clearly demonstrated the importance of an early and in-depth discussion of the ethical aspects of such technology and the responsibility that the scientific community also has in its control.
