by Emma Sudo ’23
On the morning of October 7, the Royal Swedish Academy of Sciences was silent as Göran K. Hansson announced the 2020 Chemistry Nobel Prize laureates. Who was so gifted that they could, in Professor Hannson’s words, “rewrite the code of life”? As Professor Hansson announced the names, Emmanuelle Charpentier and Jennifer A. Doudna, applause (albeit virtually) filled the room. Their work in discovering tracrRNA and simplifying CRISPR-Cas9 was not only groundbreaking, but the fact that this was the first time two women were awarded a Nobel Prize for scientific discovery made it even moreso.
Charpentier is a French microbiologist and Director of the Max Planck Institute for Infection Biology in Berlin as well as the founder of the Max Planck Unit for the Science of Pathogens. Doudna is a professor at the University of California, Berkeley, and an investigator at the Howard Hughes Medical Institute. Together, they found the last piece of the puzzle that is CRISPR, a component of bacterial immune systems.
Charpentier met Doudna at a conference where they formed a partnership. In regards to their historic breakthrough for women in science, Charpentier commented, “I think it’s very important for women to see a clear path. I think the fact that Jennifer Doudna and I were awarded this prize today can provide a very strong message for young girls”.
Doudna said, “I’m proud of my gender. I think it’s great, especially for younger women, to see this and to see that women’s work can be recognized as much as men’s.”
However, their jointly awarded prize is still met with controversy by many researchers who expected Feng Zhang, a core member of the Broad Institute as well as an investigator at the McGovern Institute for Brain Research at MIT, to receive one-third of the prize. The Broad Institute and UC Berkeley are currently in a patent battle for CRISPR that may have affected the committee’s decision to exclude Zhang. However, George Church, another CRISPR researcher who worked closely with Zhang at one point, commented that Zhang’s contribution, which was to successfully apply the CRISPR technology in human cells, was “an invention, and not a discovery.” Typically, Nobel Prizes are awarded for fundamental discoveries and not new technology. The committee may have thought that Feng Zhang was the one to make CRISPR a technology useful for humans while Charpentier and Doudna were the ones to complete the discovery of CRISPR in the context of a bacterial immune system.
To make informed decisions about who deserves credit for CRISPR, it is important to learn about all of the researchers involved in its discovery. CRISPR’s discovery started in the late 80s and early 90s when Yoshizumi Ishino and Francisco Mojica both observed tandem repeats (TREPS) in the DNA of archaea and bacteria. While Mojica and Ruud Jansen were studying the TREPS, they renamed these repeated sequences CRISPR (Clustered Regularly Interspaced Short Palindromic Repeats). Mojica observed that in between the repeats there were spacer regions that had DNA sequences matching to genomes of different viruses. He correctly hypothesized that CRISPR acted as an adaptive immune system for bacteria against viruses.
But how do these pieces of virus genomes protect bacteria? Alexander Bolotin got closer to answering this question by discovering Cas9 and PAM. Cas9 is a part of some CRISPR arrays and encodes a protein that cleaves DNA. The PAM, or protospacer adjacent motif, is a sequence at the ends of the viral versions of the spacer sequences. Cas9 needs the PAM sequence to differentiate between the viral DNA which has PAM and the CRISPR array which does not have PAM. Once Cas9 finds the PAM, it knows to check the sequence next to it to see if it matches the sequence in the spacer region of the CRISPR array. Then, it will cut the viral DNA to prevent it from killing the bacteria. However, Bolotin did not know these details about how Cas9 and PAM worked at the time.
Next, Philippe Horvath showed that when bacteria gets infected with a new virus, part of their DNA, adjacent to the PAM sequence, will be added to the CRISPR array as a new spacer region. After this final piece of information, researchers started to piece together how Cas9, PAM, and CRISPR work together to destroy viral DNA. For example, John van der Oost discovered CRISPR RNA (crRNA) which is the product of transcribing spacer regions and guides Cas9 to the PAM of the viral DNA. In another case, Luciano Marraffini and Erik Sontheimer showed that the target genetic material of CRISPR-Cas9 is DNA, which opened the possibility of using CRISPR as a tool in non-bacterial organisms like humans. Sylvain Moineau discovered that Cas9 is the only protein needed to cut viral DNA in the CRISPR-Cas9 system. This is what makes CRISPR uniquely efficient and practical compared to other existing gene-editing tools.
Finally, in 2011 Charpentier discovered tracrRNA, or trans-activating RNA, in the bacteria, Streptococcus pyogenes. TracrRNA combines with crRNA to guide Cas9 to the PAM. This was the very last piece to the puzzle and Virginijus Siksnys proved that by transferring the known parts of the CRISPR array into an E. coli sample which did not have this type of CRISPR. No surprise, the E. coli was able to use CRISPR successfully!
At this point, the exciting uses of CRISPR that we know today were in their development stages. Siksnys, Charpentier, and Doudna showed that by altering the crRNA sequence, Cas9 can be directed to any part of the genome. And now for the big finale, Feng Zhang and George Church successfully used the CRISPR-Cas9 system in eukaryotic cells including that of humans!
The point of walking through all of this research is to show that the discovery and practical usage of CRISPR-Cas9 was only possible with the collaboration of many, many scientists. A maximum of three people can share the Nobel Prize, so some researchers will always be excluded. What likely happened was that the Nobel Prize Committee was recognizing how without Emmanuelle Charpentier and Jennifer Doudna, the fundamental understanding of CRISPR would never have been complete. It is this fundamental research that is needed before any discovery can be applied as a form of technology. This is exactly why Charpentier emphasized in her interview that it is incredibly important for everyone to be interested in and continuously support fundamental research.
Begley, S. (2020, October 7). Two female CRISPR scientists make history, winning Nobel Prize in chemistry for genome-editing discovery. STAT. https://www.statnews.com/2020/10/07/two-crispr-scientists-win-nobel-prize-in-chemistry/
Broad Institute. (2018, December 7). CRISPR Timeline. https://www.broadinstitute.org/what-broad/areas-focus/project-spotlight/crispr-timeline
Campbell, M. (2020, October 12). Francis Mojica: The Modest Microbiologist Who Discovered and Named CRISPR. Genomics Research from Technology Networks. https://www.technologynetworks.com/genomics/articles/francis-mojica-the-modest-microbiologist-who-discovered-and-named-crispr-325093
Nesathurai, A. (2018, June 8). Who deserves credit for CRISPR? There’s a ‘profound disconnect between law and science.’ Genetic Literacy Project. https://geneticliteracyproject.org/2018/06/08/who-deserves-credit-for-crispr-theres-a-profound-disconnect-between-law-and-science/Nobel Media. (2020, October 21). Nobel Prizes 2020. NobelPrize.Org. https://www.nobelprize.org/prizes/chemistry/2020/press-release/