THIRD WORLD NETWORK BIOSAFETY INFORMATION SERVICE
Dear Friends and Colleagues
Gene-editing Fails to Confer Virus Resistance but Develops Mutated Viruses in Cassava
Geminiviruses cause damaging diseases in several important crop species including cassava which is a tropical staple food crop consumed by more than a billion people. Each year, cassava crops are decimated by cassava mosaic geminiviruses.
A research team used a new gene-editing technology called CRISPR-Cas9 in an attempt to design cassava plants that could cut the DNA of the African cassava mosaic virus and make the plants resistant to its damaging effects. The attempt was not successful. The CRISPR system failed to confer effective resistance to the virus in controlled laboratory conditions. Furthermore, between 33% and 48% of edited virus genomes evolved a conserved single-nucleotide mutation that confers resistance to CRISPR-Cas9 cleavage.
“Because this technology creates a selection pressure on the viruses to evolve more quickly, and also provides the viruses a means to evolve, it resulted in a virus mutant that is resistant to our interventions,” explains lead author, Devang Mehta.
The researchers encourage other scientists who are using CRISPR-Cas9 technology to engineer virus-resistant plants, to test their plants to detect similar viral mutations. They call for more research before field testing and advise caution in the application of CRISPR-Cas9 for virus resistance in plants, both in glasshouse and field settings, to avoid inducing the evolution of resistant viruses.
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Item 1
GENE-EDITING TECHNOLOGY TO CREATE VIRUS-RESISTANT CASSAVA PLANT HAS OPPOSITE EFFECT, RESEARCHERS FIND
by Katie Willis
University of Alberta
26 April
https://phys.org/news/2019-04-gene-editing-technology-virus-resistant-cassava-effect.html
Using gene-editing technology to create virus-resistant cassava plants could have serious negative ramifications, according to new research by plant biologists at the University of Alberta, the University of Liège in Belgium and the Swiss Federal Institute of Technology.
Their research shows that attempts to genetically engineer the plants to fight off viruses resulted in the propagation of mutated viruses in controlled laboratory conditions.
“Because this technology creates a selection pressure on the viruses to evolve more quickly, and also provides the viruses a means to evolve, it resulted in a virus mutant that is resistant to our interventions,” explained U of A post-doctoral fellow Devang Mehta.
The researchers used a new gene-editing technology called CRISPR-Cas9 in an attempt to design cassava plants that could cut the DNA of the mosaic virus and make the plants resistant to its damaging effects. They weren’t successful and decided to sequence hundreds of viral genomes found in each plant to understand exactly what happened.
“We discovered that the pressure that CRISPR-Cas9 applied to the virus probably encouraged it to evolve in a way that increased resistance to intervention,” said Mehta, who noted CRISPR-Cas9 has many other applications in food and agriculture that do not pose the same risks.
CRISPR-Cas9 is found in nature, where bacteria use it to defend against viruses. However, the researchers found the technology results in different outcomes in plants—and researchers are stressing the importance of screening against these sorts of unintended results in the future.
The cassava plant is a starchy root vegetable that is consumed for food throughout the tropics. Cassava is a primary staple crop grown in South America, Africa and Asia, from which a billion people get most of their calories each day. Each year, cassava crops are plagued by cassava mosaic disease, which causes 20 per cent crop loss. It is the mosaic disease that Mehta and his colleagues endeavoured to engineer against.
The research team is encouraging other scientists who are using CRISPR-Cas9 technology to engineer virus-resistant plants, to test their plants to detect similar viral mutations.
“We need to do more research on these types of applications of CRISPR-Cas9 technologybefore we proceed with field testing,” said Mehta.
The study, “Linking CRISPR-Cas9 Interference in Cassava to the Evolution of Editing-Resistant Geminiviruses,” was published in Genome Biology.
Item 2
LINKING CRISPR-CAS9 INTERFERENCE IN CASSAVA TO THE EVOLUTION OF EDITING-RESISTANT GEMINIVIRUSES
Devang Mehta et al.
Genome Biology 2019 (20:.80)
doi: 10.1186/s13059-019-1678-3
https://genomebiology.biomedcentral.com/articles/10.1186/s13059-019-1678-3
ABSTRACT
Background
Geminiviruses cause damaging diseases in several important crop species. However, limited progress has been made in developing crop varieties resistant to these highly diverse DNA viruses. Recently, the bacterial CRISPR/Cas9 system has been transferred to plants to target and confer immunity to geminiviruses. In this study, we use CRISPR-Cas9 interference in the staple food crop cassava with the aim of engineering resistance to African cassava mosaic virus, a member of a widespread and important family (Geminiviridae) of plant-pathogenic DNA viruses.
Results
Our results show that the CRISPR system fails to confer effective resistance to the virus during glasshouse inoculations. Further, we find that between 33 and 48% of edited virus genomes evolve a conserved single-nucleotide mutation that confers resistance to CRISPR-Cas9 cleavage. We also find that in the model plant Nicotiana benthamiana the replication of the novel, mutant virus is dependent on the presence of the wild-type virus.
Conclusions
Our study highlights the risks associated with CRISPR-Cas9 virus immunity in eukaryotes given that the mutagenic nature of the system generates viral escapes in a short time period. Our in-depth analysis of virus populations also represents a template for future studies analyzing virus escape from anti-viral CRISPR transgenics. This is especially important for informing regulation of such actively mutagenic applications of CRISPR-Cas9 technology in agriculture.