Research Article |
Corresponding author: Marion Dolezel ( marion.dolezel@umweltbundesamt.at ) Academic editor: Josef Settele
© 2020 Marion Dolezel, Christoph Lüthi, Helmut Gaugitsch.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Dolezel M, Lüthi C, Gaugitsch H (2020) Beyond limits – the pitfalls of global gene drives for environmental risk assessment in the European Union. BioRisk 15: 1-29. https://doi.org/10.3897/biorisk.15.49297
|
Gene drive organisms (GDOs) have been suggested as approaches to combat some of the most pressing environmental and public health issues. No such organisms have so far been released into the environment, but it remains unclear whether the relevant regulatory provisions will be fit for purpose to cover their potential environmental, human and animal health risks if environmental releases of GDOs are envisaged. We evaluate the novel features of GDOs and outline the resulting challenges for the environmental risk assessment. These are related to the definition of the receiving environment, the use of the comparative approach, the definition of potential harm, the stepwise testing approach, the assessment of long-term and large-scale risks at population and ecosystem level and the post-release monitoring of adverse effects. Fundamental adaptations as well as the development of adequate risk assessment methodologies are needed in order to enable an operational risk assessment for globally spreading GDOs before these organisms are released into environments in the EU.
environmental risk assessment, European Union, gene drive, genetically modified organism, GMO
New genetic engineering tools for manipulating genetic material in plants, animals and microorganisms have received considerable attention over the last years. Many of these techniques are novel and based on recent developments in molecular biology. Some of these techniques can be used to modify genomes of organisms in a way that increases the inheritance of particular genes during sexual reproduction. The higher prevalence of these genes in the offspring leads to increased spread of the genes in the whole population – a mechanism termed “gene drive”. This enhancement of the ability of a genetic element to pass from the parent to its offspring through sexual reproduction is a basic feature of all gene drive systems (
With the discovery of CRISPR/Cas (Clustered Regularly Interspaced Short Palindromic Repeats/CRISPR associated protein), the development of gene drive approaches was simplified and accelerated, as many of the targeting and stability problems observed in other nuclease-based genetic engineering techniques could be overcome (
Several potential applications for gene drive systems in three main fields have been suggested, i.e. in public health (e.g. vector control of human pathogens), in agriculture (e.g. control of weeds or pests), and in environmental protection and nature conservation (e.g. the control of noxious non-native species) (
Substantial differences in gene drive applications have been identified compared to conventional genetically modified organisms (GMOs) such as GM crops or GM insects, both with regard to the general strategy applied to cope with agricultural, public health or environmental issues and to the anticipated benefit (
Due to the novelty in the strategy and the predicted power of impact of gene drive applications, the assessment of their potential risks to the environment and human and animal health will be of high importance if these organisms are to be deliberately released into the environment. The potential of GDOs for unlimited spread throughout wild populations, once released, and the apparently inexhaustible possibilities of multiple and rapid modifications of the genome in a vast variety of organisms, including higher organisms such as vertebrates, pose specific challenges for the application of adequate risk assessment methodologies. The suitability and practicability of existing regulatory policies and risk assessment regimes when deploying GDOs have been discussed for the Australian (
In this article, we evaluate the novel features of gene drive approaches compared to conventional GMOs and outline the resulting challenges for the environmental risk assessment, which is an integral part of the EU’s regulatory approval procedure. Although we mainly refer to CRISPR/Cas-based gene drive approaches, the analysis is also applicable for other types of GDOs, if these have the potential to spread globally. First, we briefly discuss regulatory provisions in the EU, such as those for GMOs, invasive alien species and biological control agents, with respect to their suitability to cover risks of global gene drive approaches. We outline the advantages and appropriateness of GMO regulation for GDOs in the European Union. We then address the major distinctive features of GDOs compared to GMOs without gene drive and highlight challenges with respect to the basic principles of environmental risk assessment for GMOs in the EU. For the evaluation of environmental risks, we refer to Directive 2001/18/EC (
Due to the versatility of gene drive applications regarding the underlying methodology applied and the targeted species, the decision which regulatory framework to apply might not be without ambiguity. Approaches to control pest species by the large-scale deployment of GM insects or GDOs may have similar environmental implications as biological control agents (
In Europe, the introduction and deployment of invertebrate biological control agents is subject to national rules and regulatory provisions, if such are available. Individual EU Member States as well as Switzerland have introduced national legislation that differs in scope and risk assessment requirements (
In the European Union, Regulation 1143/2014 on Invasive Alien Species (IAS) provides for a set of measures to be taken across the EU for those IAS included on a list of IAS of Union concern. The Regulation refers to animals, plants, fungi and microorganisms. The measures relate not only to the detection, eradication and management of already present or even well-established invasive species, but also aim at preventing species of Union concern from entering the EU, intentionally or unintentionally. Based on a risk assessment process, the regulation intends to prevent the introduction and spread of IAS with the potential to establish a viable population, to spread and to have a significant adverse impact on biodiversity or the related ecosystem services, on human health or the economy (Regulation (EU) No 1143/2014, Art. 4). As this Regulation refers only to organisms outside their natural range, it would not cover native GDOs.
GMOs are subject to regulation under the provisions of Directive 2001/18/EC for the deliberate release of GMOs into the environment, requiring an environmental risk assessment (ERA), possibly risk management measures and an obligatory post-market monitoring (
Novel feature 1: Alteration of wild populations with novel traits instead of “familiar” crop species and traits
A major difference between “classic” GMOs and GDOs is the intentional alteration of wild populations (e.g. mosquitoes) instead of domesticated and “familiar” crop species with limited ability to disperse. So far, GMOs were agricultural crop species with a “history of safe use” (
Genetically modified traits used so far with GM crops (insect resistance or herbicide tolerance) presumably have no or negligible selective advantage in the absence of selective pressure, i.e. the herbicide application or the attack of the pest species (
Novel feature 2: Intentional and potentially unlimited spread of synthetic genes in wild populations and natural ecosystems
The specific gene drive mechanism deployed in GDOs will determine the potential for spread within target populations, but also to populations of sexually compatible organisms. So-called threshold-dependent drive systems require a certain release frequency of the GDOs in order to achieve spread in a target population (
In the EU, GM crops so far considered for environmental release were not intended to deliberately transfer GM traits to other species. Gene flow of GM traits from “classic” GMOs was therefore viewed as an unintended consequence of the release due to the intrinsic biology of the plant species used for genetic engineering. Similarly, the possibility of the GM plant to persist as a feral population (e.g. GM oilseed rape) or to invade (semi-)natural habitats was an unintended consequence of the GM crop release into those environments where the GM plant was able to spread and persist, or where wild relatives existed. Gene flow of synthetic genes from crop to wild plants may entail several adverse ecological impacts, such as depletion of the genetic diversity (of the targeted population), increased weediness or invasiveness of the GM plants or GM-wild hybrids, as well as the risk of extinction of wild species (
Novel feature 3: Possibility for long-term risks to populations and ecosystems
The novelty of GDOs and their distinctiveness from “classic” GMOs is based on the novel mechanism of inheritance and the ability to genetically engineer a wide range of genes in many different types of organisms. An important and distinct feature of GDOs is their potential to introduce long-term, if not infinite, changes in populations as well as the large-scale, if not complete, spread of novel genetic and phenotypic traits through a target population due to the repeated genomic intervention in each subsequent generation. Due to this “lab in the field” approach (
The evolutionary stability of this potentially “infinite” copying mechanism of the genomic drive cassette is subject to some debate due to the formation of resistant alleles at the target site. The development of resistance to the gene drive in target populations has been proposed for many gene drive approaches, although for some gene drive systems (e.g. HEGs) the occurrence of resistance is a greater concern than for others (
Resistance development in target populations may not be considered as an ecological risk per se, but can entail ecological risks as well as consequences for human health (e.g.
Regarding long-term ecological impacts, novel mortality factors and selection pressures that are introduced into a population can have significant evolutionary consequences on target and non-target organisms. Experience from the introduction of non-native biological control agents and from invasive alien species shows that ecological impacts can vary from negligible to significant and even devastating, depending on the species and the recipient ecosystem (
A consequence of the permanent suppression of pathogen-transmitting vector populations can also be the altered virulence of the pathogen (
Last but not least, the introduction of a novel control practice may affect existing control measures of a noxious species or pathogen-transmitting vector species. In case of population alteration or replacement of vector species, conventional control strategies such as the application of insecticides could compromise the novel control method (
Challenge 1: The receiving environment cannot be defined for GDOs with the ability to spread globally
One fundamental principle for the ERA of GMOs is the case-by-case principle laid down in Directive 2001/18/EC (
The release of GM crops into the environment is locally restricted to specific production areas where the respective crop can only be grown if the specific climatic, agronomic and environmental conditions are appropriate. In contrast, spread of GDOs into populations beyond a spatially defined location is highly likely (e.g. in case of low threshold drives). For wild populations habitat heterogeneity is common, notably for arthropod pest species, which can either simultaneously or successively occupy diverse habitats and may have a range of host species, in particular if they are generalists and polyphagous. For example, the invasive pest Drosophila suzukii occupies a range of different crop and non-crop habitats, some of which serve as refuges (
In a specific receiving environment, genotype-environment interactions play a crucial role, not only for the success of the gene drive application, but also concerning the potential risks for the environment. In order to achieve the successful spread of GDOs, the fitness costs involved with the gene drive construct for the target organisms are of high relevance, as they can affect the threshold which is necessary for successful spread and fixation (
It has been proposed to carry out field trials with GDOs intended for pathogen-vector control in locations where no vectors are naturally present, as an instrument for ecological confinement of GDOs (
Challenge 2: The safety of a GDO cannot be established based on a comparative assessment
The comparative safety assessment of the GMO with a non-modified organism is the starting point for the ERA of GM plants and animals (
Gene drive approaches generally target wild populations with no genetic uniformity, familiarity or history of safe use instead of domesticated animals or crops. If wild populations of the target species or closely related species in target environments serve as a baseline in order to assess any phenotypic and ecological differences to the GDO, then the reference framework for comparison must not only consider the organism as such but also include the environment in which the comparator thrives. Identified differences between the GDO and wild populations do therefore not necessarily indicate a hazard but may be due to the phenotypic and ecological plasticity of the target population. For wild populations of animals, it will be even more difficult than for GM crops to identify whether such differences are due to the novel GM trait or due to the range of behavioural and ecological characteristics present in the population. Although the comparative assessment certainly has an indicative value for a basic phenotypic characterisation and comparison of the GDO with the wild population from which it derives, it will have to be complemented with criteria for the relevance of observed differences. Ideally, these criteria should be available before an assessment of a specific GDO needs to be done.
Consequently, an assessment of differences between GDOs and their wildtype comparators will be impractical for wild populations intended for a global gene drive. The availability of relevant baseline information regarding a range of biological and ecological parameters of both target and non-target populations in the envisaged receiving environment is a prerequisite in order to identify differences between the GDO and its comparators, to evaluate its relevance and to derive risk scenarios. However, this is only feasible if gene drive applications can be spatially restricted and confined to a specific environment. If such a local gene drive is envisaged, an assessment of the genetic, phenotypic, behavioural and ecological baseline data may be feasible, depending on the degree of population differentiation in the target species at the specific location. Logically, such a comparative assessment will be redundant for gene drive applications aiming at suppression and elimination of the target population where no viable offspring is available.
Challenge 3: The environmental impact of gene flow of GDOs cannot be assessed with the current ERA
The novelty of gene drive applications is the deliberate and intended transfer of modified genetic elements to one or several populations of the target species. Due to intra- and interspecific gene flow, gene drive may also spread to populations other than the targeted. Depending on the aim of the gene drive approach, this may be considered as a beneficial effect of a particular gene drive based control strategy (
A thorough knowledge of the occurrence of potentially compatible species, the population dynamics and genetics of non-target populations, including knowledge of reproductive or other isolating mechanisms between populations, is therefore highly relevant in order to estimate the risk of gene flow to non-target populations. The large knowledge gaps with regard to the phylogenetic relatedness and ability for hybridization in many wild species hampers the assessment of risks for gene drives that are able to spread beyond a certain meta-population.
Consequences of intra- as well as interspecific gene flow of GDOs can be decreased fitness, population declines and reduced phenotypic diversity and displacement of native species (
Both, the ERA for GM crops and for GM animals provide the basis for an assessment of spread, persistence and invasiveness including gene flow (
Challenge 4: Testing of GDOs in the field is hardly possible
A major principle in the European GMO regulation is the step-by-step principle, also referred to as stepwise approach (
Already for laboratory experiments with GDOs, developers of gene drive applications have proposed extensive ecological, physical and molecular confinement methods in order to avoid escape of organisms from the lab (
The first release of GDOs into the environment presents a particular challenge for risk assessors and risk managers, because of the lacking experience and comparability with previous releases. Any deliberate release of a GDO for field-testing needs to consider that a single release of GDOs into the environment may have the potential for unlimited and global spread into wild populations. EFSA already recognizes the difficulty of conducting field trials with GM insects due to the irretrievability of GM insects in certain cases (
Self-limiting approaches of GDOs have been suggested in order to control and halt further spread of GDOs into the environment, such as locally fixed gene drives (
Challenge 5: Long-term risks at the population and ecosystem level cannot be assessed with current ERA methods
One particularity of the European GMO regulation is the assessment of long-term, indirect and delayed effects, referring to effects on human health or the environment “… which may not be observed during the period of the release of the GMO, but become apparent as a direct or indirect effect either at a later stage or after termination of the release” (
Currently, there is no practical knowledge or empirical data from environmental releases of GM insects or GM animals in the EU. No GM animals have so far been notified; neither for placing on the market nor for field-testing. It has been argued that the ERA of GDOs can be informed by experience gained with the release of biological control organisms (
Literature of impacts of GDOs is currently limited to theoretical modelling exercises regarding the spread and functionality of specific gene drive constructs (see e.g.
Experience with mathematical modelling for ERA purposes is available for GM crops such as Bt maize (e.g.
The controversies around the ERA in general and the model used in the ERA of Bt maize in particular have not ceased yet, even though many GM crops have been risk assessed over the years and, thus, some experience with the cultivation of Bt maize in the EU and a good data basis for the model input parameters is available. In contrast, no data to assess long-term risks of GDOs are available and mathematical models still have to be developed in order to specifically address the ecological processes that may be affected if GDOs are to be released into the environment.
Challenge 6: Improved environmental monitoring and risk management must be operational before deploying GDOs
Considering that a high level of uncertainty has been attributed to the environmental and human health risks of GDOs, the application of appropriate risk management strategies is of paramount importance. Risk management strategies should control the identified risk, cover the uncertainties and need to be available and functional at the time of first release. When cultivating GM crops, risk management involves the establishment of an insect resistance management plan, e.g. for insect resistant Bt maize, to minimize the risk of resistance development of the targeted pest species (
In the EU, a post-release monitoring plan has to be submitted by the applicant of a GMO with the purpose (1) to validate the results of the ERA (i.e. case specific monitoring) and (2) to address any adverse effects which were not anticipated in the ERA, also referred to as general surveillance (
For insect-resistant GM crops, resistance monitoring has been shown to be effective in areas of cultivation of Bt maize (
The use of genetically modified organisms with the ability for gene drive has been proposed to combat some of the most pressing human health, agronomic or environmental problems. Due to the potentially unlimited spread, both spatially and in time, gene drive applications may have severe consequences for target as well as non-target populations, but also entire ecosystems or public health. Therefore, a cautious handling of these organisms is strongly suggested; the assessment of environmental risks, provisions for post-release monitoring and risk management measures should be standard requirements before release of GDOs into the environment. In order to achieve this, the regulatory oversight of GMOs in the EU has to be scrutinised to evaluate whether it is fit for purpose to cover the novel risks and challenges posed by gene drive applications.
Regulating GDOs according to the provisions of Directive 2001/18/EC for the deliberate release into the environment of GMOs provides the advantage that each environmental release requires a specific authorization, a preceding environmental risk assessment, including a phased release into the environment and a mandatory post-release environmental (and possibly human health) monitoring. This regulatory framework provides an EU-wide harmonised and robust approach in which potential adverse effects of gene drives to human health and the environment have to be determined, their likelihood assessed, the risks managed and monitored. The current regulatory provisions for GMOs in the EU are aligned to single-generation GMOs with no intentional spread or persistence in the environment; hence, consent for use is given for a maximum period of 10 years with the possibility for renewal. Any GDO to be released into the environment would have to comply with these regulatory requirements. From a regulatory view, it will be difficult to handle GDOs designed to disperse in the environment beyond a specific time frame, in particular if retrievability of these organisms is not readily available.
With the emergence and increased application of new genetic engineering techniques used inter alia for the construction of GDOs, an efficient risk assessment will have to address specific risk aspects for a range of different organisms at different taxonomic hierarchies. The major differences of gene drive approaches to conventional GM approaches refer to the targeting of wild populations and the intentional spread of synthetic genetic elements or novel traits throughout target populations and entire ecosystems with the potential for long-term adverse effects. These specificities have major implications for the ERA, with respect to the definition of the receiving environment, for the use of the comparative approach, the assessment of environmental harm due to gene flow and the testing of the organisms in the field, the assessment of long-term risks as well as for monitoring and risk management.
Specifically, long-term effects on whole populations or ecosystems including potential evolutionary changes in target and non-target populations are highly unpredictable and difficult to model. They pose a specific challenge for the risk assessment of GDOs. Uncertainties in the assessment of the likelihood of the occurrence and of the consequences of such long-term effects may result in highly speculative risk estimations. A comprehensive empirical data basis, new and systematic scientific approaches and improved predictive modelling of long-term effects at ecosystem level will be indispensable to evaluate such risks. In addition, a scientifically sound ERA needs a spatial and temporal reference framework for any GMO, which implies that ERA is not possible for globally spreading GDOs. This also implies that decisions on acceptable risks can only be made if enough data are available to assess the specific risk and if clear decision criteria are available regarding the acceptability of such risks, both for ERA and for monitoring. Last but not least, more emphasis than for “classic” GMOs needs to be put on post-release monitoring and the availability of risk management measures in order to enable timely detection of adverse effects on the environment and the possibility to halt or even reverse the spread of GDOs in the receiving environment. The decision of first release into the environment of a GDO, e.g. during field-testing, will not be easily tackled. The trade-off between gaining necessary information on efficacy and biosafety of the specific application in the receiving environment and the assessment of risks, based solely on data from confined environments, will have to be solved already at the beginning of the ERA. The final decision to release GDOs into the environment will, however, not be a purely scientific question, but will need some form of broader stakeholder engagement and the commitment to specific protection goals for human health and the environment.
We would like to thank Nina Gammenthaler (Swiss Federal Office for the Environment) for her valuable comments on the manuscript. The additional funding for this project provided by the Swiss Federal Office for the Environment (FOEN) is kindly acknowledged.