THIRD WORLD NETWORK BIOSAFETY INFORMATION SERVICE

Dear Friends and Colleagues

Using CRISPR-Cas9 to Engineer Virus Resistance in Cassava Triggers Evolution of Novel Virus

The bacterial CRISPR-Cas9 gene editing system can be used to engineer resistance to DNA viruses through direct cleavage of the virus genome. Unlike conventional gene-editing using CRISPR-Cas9, engineering virus resistance requires constitutive and permanent expression of the ribonucleoprotein complex in the host. One concern scientists have is that planting transgenic, virus-resistant CRISPR-Cas9 plants in the field will impose a selection pressure on viruses, while simultaneously providing viruses with a mechanism (via Cas9-induced mutations) to escape resistance.

A recent study applied CRISPR-Cas9 to engineer resistance to geminiviruses in cassava (specifically, the African cassava mosaic virus, a member of a widespread and important family of plant-pathogenic DNA viruses) and investigate the impact of engineering resistance on geminivirus evolution. Cassava is a tropical staple food crop consumed by more than a billion people.

The study failed to engineer geminivirus resistance, but found that use of CRISPR-Cas9 led to emergence of a novel, conserved mutant virus that cannot be cleaved again. Between 33 and 48% of edited virus genomes evolved a novel, conserved mutant CRISPR-resistant virus strain across three independent transgenic lines. The researchers urgecaution in the application of CRISPR-Cas9 for virus resistance in plants, both in glasshouse and field settings, to prevent the evolution of novel viruses. The ability of CRISPR-systems to trigger the evolution of new viruses would impact the regulatory mechanisms available for testing and releasing such plants, and these results point to a novel environmental containment consideration.

The researchers recommend that there should be strategies to reduce the emergence of editing-resistant viruses and care should be taken to design CRISPR-Cas9 experiments that minimize the risk of virus escape. In addition, the implementation of technologies with the potential to speed up virus evolution should be carefully assessed as they pose significant biosafety risks.

CRISPR-CAS9 INTERFERENCE IN CASSAVA LINKED TO THE EVOLUTION OF EDITING-RESISTANT GEMINIVIRUSES

Mehta, D., Stürchler, A., Hirsch-Hoffmann, M., Gruissem, W., & Vanderschuren, H.

bioRxiv, 314542.
https://doi.org/10.1101/314542
5 May 2018
https://www.biorxiv.org/content/early/2018/05/04/314542

Abstract

We used CRISPR-Cas9 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 of plant-pathogenic DNA viruses) by cleaving the virus’ replicative genome. We found that between 33 and 48% of edited virus genomes evolved a conserved single-nucleotide mutation that confers resistance to CRISPR-Cas9 cleavage. Our study highlights the potential for virus escape from this technology. Care should be taken to design CRISPR-Cas9 experiments that minimize the risk of virus escape.