Evaluating the Probability of CRISPR-based Gene Drive Contaminating Another Species

Abstract

The probability D that a given CRISPR-based gene drive element contaminates another, non-target species can be estimated by the following Drive Risk Assessment Quantitative Estimate (DRAQUE) Equation: D = ( hyb+transf).express.cut.flank.immune.nonextinct with hyb = probability of hybridization between the target species and a non-target species transf = probability of horizontal transfer of a piece of DNA containing the gene drive cassette from the target species to a non-target species (with no hybridization) express = probability that the Cas9 and guide RNA genes are expressed cut = probability that the CRISPR-guide RNA recognizes and cuts at a DNA site in the new host flank = probability that the gene drive cassette inserts at the cut site immune = probability that the immune system does not reject Cas9 -expressing cells nonextinct = probability of invasion of the drive within the populationWe discuss and estimate each of the seven parameters of the equation, with particular emphasis on possible transfers within insects, and between rodents and humans. We conclude from current data that the probability of a gene drive cassette to contaminate another species is not insignificant. We propose strategies to reduce this risk and call for more work on estimating all the parameters of the formula.

Can a population targeted by a CRISPR-based homing gene drive be rescued?

“…So ideally, it might be better to use drives with conversion in the zygote. Nevertheless, such drives are more difficult to create and so far all successful homing drives have been engineered with germline promoters ( Table 2 in Courtier-Orgogozo et al 2019b). A few conserved genes are expressed in the germline of all animals (nanos, vasa, piwi;Extavour and Akam 2003;Juliano et al 2010) and their promoters constitute preferred tools for engineering gene drive constructs in various animal species, in contrast to zygotically expressed genes, which tend to be less conserved across taxa (Heyn et al 2014).…”

“…following the escape of gene drive individuals from a laboratory) and to mitigate unanticipated or premeditated and malevolent harm to humans or the environment. For example, a CRISPR-based eradication drive may spread into a non-target population or species (Noble et al 2018;Courtier-Orgogozo et al 2019a;Rode et al 2019); a modification drive may alter the target population in an unexpected, detrimental manner; or a gene drive could be used as bioweapon (Gurwitz 2014). Decreasing the environmental risks associated with the development of this technology can be achieved by designing safer gene drives whose spread can be controlled spatially or temporally (Marshall and Akbari 2018;Raban et al 2020) and by developing countermeasures to stop the spread of an ongoing gene drive (Esvelt et al 2014;Gantz and Bier 2016;Vella et al 2017).…”

“…The first type are countermeasures that include the cas9 gene and that can target either the drive allele only (reversal drives sensu Esvelt et al 2014;overwriting drives;DiCarlo et al 2015) or both the drive and wild-type alleles (immunizing reversal drive (IRD); Esvelt et al 2014;Vella et al 2017). However, with these strategies, a functional cas9 gene will remain in the final population, which may increase the risk of subsequent genetic modifications such as translocations, and possible negative environmental outcomes (Courtier-Orgogozo et al 2019b). The second type are countermeasures that do not encode cas9 and rely instead on the cas9 gene present in the initial gene drive construct.…”

Can a Population Targeted by a CRISPR-Based Homing Gene Drive Be Rescued?

“…For example, a CRISPR-based eradication drive may spread into a non-target population or species ( Noble et al 2018 ; Rode et al . 2019 ; Courtier-Orgogozo et al . 2020 ); a modification drive may alter the target population in an unexpected, detrimental manner; or a gene drive could be used as bioweapon ( Gurwitz 2014 ).…”

“…The first type are countermeasures that include the cas9 gene and that can target either the drive allele only (reversal drives sensu Esvelt et al 2014 ; overwriting drives; DiCarlo et al 2015 ) or both the drive and wild-type alleles (immunizing reversal drive (IRD); Esvelt et al 2014 ; Vella et al 2017 ). However, with these strategies, a functional cas9 gene will remain in the final population, which may increase the risk of subsequent genetic modifications such as translocations, and of possible negative environmental outcomes ( Courtier-Orgogozo et al 2020 ). The second type are countermeasures that do not encode Cas9 and rely instead on the cas9 gene present in the initial gene drive construct.…”