Voici le texte qui a été rédigé pour le DNA DAY ESSAY 2017 par un groupe d'élèves de Terminales S2 sur au sujet de la technique CRISPR-Cas9
Question :
CRISPR/CAS9 is a powerful new technology to change genetic material in all living material including. Humans, animals and plants. Explain how this technology works. Give your opinion what problems and opportunities this technology should be used for, and what potential uses should not be allowed. Explain why Vou would make these choices.
All living cells face harmful elements, protecting themselves with their immune system. CRISPR/Cas9 is a natural immune system that protects bacteria against bacteriophage viruses1. In 2012, Emmanuelle Charpentier and Jennifer Doudna found out that this system could be used to modify the genome of all living cells2. Thus, we could ask ourselves how this system works and what problems and opportunities arise from its usage.
CRISPR (Clustered Regularly Interspaced Short Palindromic Repeats) is a defensive RNA composed of repeated palindromic sequences, separated with different spacers3. Spacers were integrated in the bacterium genome from viral protospacers during previous infections (adaptation)4,5. Upon a new infection, the bacterium produces a CRISPR RNA (crRNA) and a trans-activating RNA (tracrRNA) which bind together to form the guide RNA (gRNA). Then, the gRNA recruits the Cas9 (CRISPR associated) enzyme in order to guide it to the viral DNA. As an helicase and endonuclease, Cas9 opens the DNA and then, creates a double strand cleavage of the foreign DNA, thus aborting the infection (interference)6. Hence, CRISPR/Cas9 is an adaptive immune system able to recognize and delete exogenous genetic material to avoid viral infection.
Since 2012, an increasing number of articles showed that we can artificially produce synthetic gRNA (sgRNA) in order to modify the genome of a cell. After the DNA cleavage, scientists take profit of the DNA repair systems. First, non-homologous end joining (NHEJ) repairs can induce point mutations, making a new tool to inactivate peculiar genes at the same time. Then, homology-directed repairs (HDR) can be controlled by a short DNA sequence which recognizes the PAM (Protospacer Adjacent Motif) sequences and drives the gene edition2. Indeed, CRISPR/Cas9 perform controlled or inducible gene edition within all living cells in a very fast way (2 months)7,8.
Such explanations could lead people to think it’s the perfect solution against all our problems9. As an example, Anthony James worked to make the mosquitoes resistant to Plasmodium, the malaria's pathogen. The efficiency was great as 99% of the mosquito offspring received the modified genes10,11. Indeed, it could quickly and significantly reduce human contamination and save about 450 000 people each year12.
CRISPR/Cas9 could also eradicate serious genetic diseases like favism13, beta thalassemia13,14 or cystic fibrosis15 in humans. Nevertheless, the results obtained on plurinuclear human embryos were not convincing, mostly because only part of the cells were modified (mosaic embryos). It also modified unconcerned DNA sequences, notably on conserved sequences. This technique must be improved to obtain better results and avoid errors. More recently, Lu You launched an assay to cure lung cancer patients16 by inactivating the PD-1 gene coding a receptor binding PD-L1 and activating apoptosis of cytotoxic T-cells, impairing the normal destruction of cancer cells17. A same approach has been driven to perform deletions in the CCR5 coreceptor (CCR5Δ32) involved in HIV infection. Consequently, this technology could save millions of persons from cancer or HIV18.
Finally, CRISPR could be an alternative to classical plants GMOs without use of antibiotic resistance genes to produce more productive rice19, wheat resistant to mildew20, cotton resistant to begomovirus21 or even drought-resistant maize22. Such an approach could protect 1 billion people from starvation.
According to me, CRISPR/Cas9 is a huge challenge among countries, potentially leading to a scientific race. Such quickness could impact ethical reflections on human embryo use or CRISPR/Cas9 application domains23. The biggest problem is that humans will tend to use it to perfect themselves and their environment in a selfish purpose of eugenics. First, we must consider it could decrease genetic biodiversity or make Humans face unexpected consequences, such as the appearance of super-resistant species' apparition23. Moreover, it could be impossible to implement it in all countries, enhancing social disparities. It will also represent a risk of genetic standardization, opposed to random evolutionary processes: mutations, natural selection and genetic drift.
Second, CRISPR/Cas9 applications on human remain to be defined. The Oviedo convention protects human rights in biomedical field and the 13th article restricts genetic modifications as therapeutic solutions for serious genetic diseases “only if its aim is not to introduce any modification in the genome of any descendants“24. CRISPR/Cas9 is hence at the limit of this article by editing the genome without any addition. Therefore, it won't be possible to control the spread of these modified genes. Moreover, lots of nations prohibited embryo manipulations but the 18th article is ambiguous allowing it with “adequate protection” 24.
CRISPR/CAS9 is a new powerful technology allowing quick and precise DNA edition but, we should ameliorate its efficiency and be aware of the potential consequences on the biodiversity.
REFERENCES :
1- Frank Hille, Emmanuelle Charpentier, 2016, CRISPR-Cas: biology, mechanisms and relevance, Phil. Trans. R. Soc. B
2- Doudna JA, Charpentier E (2014) Genome editing. The new frontier of genome engineering with CRISPR-Cas9. Science 346: 1258096
3- Garneau JE, Dupuis MÈ, Villion M, Romero DA, Barrangou R, Boyaval P, Fremaux C, Horvath P, Magadán AH, Moineau S (2010) The CRISPR/Cas bacterial immune system cleaves bacteriophage and plasmid DNA. Nature 468: 67 – 71
4- Barrangou R, Fremaux C, Deveau H, Richards M, Boyaval P, Moineau S, Romero DA, Horvath P (2007) CRISPR provides acquired resistance against viruses in prokaryotes. Science 315: 1709 – 1712
5- Barrangou R, Marraffini LA (2014) CRISPR-Cas systems: prokaryotes upgrade to adaptive immunity. Mol Cell 54: 234 – 244
6- André Plagens et al., 2015, DNA and RNA interference mechanisms by CRISPR-Cas surveillance complexes, FEMS Microbiology Reviews, fuv019, 39, 2015, 442–463
7- Heidi Ledford; CRISPR: gene editing is just the beginning ; Nature 2016, March
8- Charpentier, E. 2015 - CRISPR-Cas9: how research on a bacterial RNA-guided mechanism opened new perspectives in biotechnology and biomedicine - EMBO Molecular Medicine Vol 7 | No 4
9- Hsu PD, Lander ES, Zhang F (2014) Development and applications of CRISPR-Cas9 for genome engineering. Cell 157: 1262 – 1278
10- Heidi Ledford & Ewen Callaway, Gene drive' mosquitoes engineered to fight malaria, 2015, Nov – Nature (http://www.nature.com/news/gene-drive-mosquitoes-engineered-to-fight-malaria-1.18858)
11- Gantz, V. M. et al. Highly efficient Cas9-mediated gene drive for population modification of the malaria vector mosquito Anopheles stephensi. Proc. Natl Acad. Sci. USA 112, E6736–E6743 (2015).
12- UNICEF global malaria databases, 2015, Malaria mortality among children under five is concentrated in sub-Saharan Africa https://data.unicef.org/topic/child-health/malaria/#
13- Tang, L., Zeng, Y., Du, H. et al. ; CRISPR/Cas9-mediated gene editing in human zygotes using Cas9 protein ; Mol Genet Genomics (2017).
14- Liang, P., Xu, Y., Zhang, X. et al. ; CRISPR/Cas9-mediated gene editing in human tripronuclear zygotes ; Protein Cell (2015) 6: 363.
15- Schwank G, Koo BK, Sasselli V, Dekkers JF, Heo I, Demircan T, Sasaki N, Boymans S, Cuppen E, van der Ent CK, Nieuwenhuis EE, Beekman JM, Clevers H., Functional repair of CFTR by CRISPR/Cas9 in intestinal stem cell organoids of cystic fibrosis patients.
Cell Stem Cell. 2013
16- David Cyranoski, CRISPR gene-editing tested in a person for the first time. Nature, 15 November 2016
17- Li Shi, Shaohua Chen, Lijian Yang, Yangqiu Li, The role of PD-1 and PD-L1 in T-cell immune suppression in patients with hematological malignancies, J Hematol Oncol. 2013; 6: 74. Published online 2013 Sep 30
18- Kang, X., He, W., Huang, Y. et al. Introducing precise genetic modifications into human 3PN embryos by CRISPR/Cas-mediated genome editing - J Assist Reprod Genet (2016) 33: 581. doi:10.1007/s10815-016-0710-8
19- Wang, F. et al. (2016). Enhanced Rice Blast Resistance by CRISPR/Cas9-Targeted Mutagenesis of the ERF Transcription Factor Gene OsERF922. PLoS ONE 11, e0154027.
20- Wang, Y. et al. (2014). Simultaneous editing of three homoeoalleles in hexaploid bread wheat confers heritable resistance to powdery mildew. Nat Biotech 32, 947-951.
21- Zafar Iqbal et al., 2016 - CRISPR/Cas9: A Tool to Circumscribe Cotton Leaf Curl Disease - Frontiers in Plant Science doi: 10.3389/fpls.2016.00475
22- Svitashev, S. et al. Genome editing in maize directed by CRISPR–Cas9 ribonucleoprotein complexes. Nat. Commun. 7, 13274 doi: 10.1038/ ncomms13274 (2016).
23- Jackson Champer, Anna Buchman & Omar S. Akbari ; Cheating evolution: engineering gene drives to manipulate the fate of wild populations ; Nature Reviews Genetics 17, 146–159 (2016)
24- Convention for the Protection of Human Rights and Dignity of the Human Being with regard to the Application of Biology and Medicine: Convention on Human Rights and Biomedicine Oviedo, 4.IV.1997 https://rm.coe.int/CoERMPublicCommonSearchServices/DisplayDCTMContent?documentId=090000168007cf98
Texte rédigé par Emma REMONTET, Laurine ASFAUX, Marine SANIAL, Alice MONDON, Selena POTILLON et Robin GACON
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