20 results found
Fuchs S, Garrood WT, Beber A, et al., 2021, Resistance to a CRISPR-based gene drive at an evolutionarily conserved site is revealed by mimicking genotype fixation, PLoS Genetics, Vol: 17, Pages: 1-19, ISSN: 1553-7390
CRISPR-based homing gene drives can be designed to disrupt essential genes whilst biasing their own inheritance, leading to suppression of mosquito populations in the laboratory. This class of gene drives relies on CRISPR-Cas9 cleavage of a target sequence and copying (‘homing’) therein of the gene drive element from the homologous chromosome. However, target site mutations that are resistant to cleavage yet maintain the function of the essential gene are expected to be strongly selected for. Targeting functionally constrained regions where mutations are not easily tolerated should lower the probability of resistance. Evolutionary conservation at the sequence level is often a reliable indicator of functional constraint, though the actual level of underlying constraint between one conserved sequence and another can vary widely. Here we generated a novel adult lethal gene drive (ALGD) in the malaria vector Anopheles gambiae, targeting an ultra-conserved target site in a haplosufficient essential gene (AGAP029113) required during mosquito development, which fulfils many of the criteria for the target of a population suppression gene drive. We then designed a selection regime to experimentally assess the likelihood of generation and subsequent selection of gene drive resistant mutations at its target site. We simulated, in a caged population, a scenario where the gene drive was approaching fixation, where selection for resistance is expected to be strongest. Continuous sampling of the target locus revealed that a single, restorative, in-frame nucleotide substitution was selected. Our findings show that ultra-conservation alone need not be predictive of a site that is refractory to target site resistance. Our strategy to evaluate resistance in vivo could help to validate candidate gene drive targets for their resilience to resistance and help to improve predictions of the invasion dynamics of gene drives in field populations.
Hammond A, Pollegioni P, Persampieri T, et al., 2021, Gene-drive suppression of mosquito populations in large cages as a bridge between lab and field., Nature Communications, Vol: 12, Pages: 1-9, ISSN: 2041-1723
CRISPR-based gene-drives targeting the gene doublesex in the malaria vector Anopheles gambiae effectively suppressed the reproductive capability of mosquito populations reared in small laboratory cages. To bridge the gap between laboratory and the field, this gene-drive technology must be challenged with vector ecology.Here we report the suppressive activity of the gene-drive in age-structured An. gambiae populations in large indoor cages that permit complex feeding and reproductive behaviours.The gene-drive element spreads rapidly through the populations, fully supresses the population within one year and without selecting for resistance to the gene drive. Approximate Bayesian computation allowed retrospective inference of life-history parameters from the large cages and a more accurate prediction of gene-drive behaviour under more ecologically-relevant settings.Generating data to bridge laboratory and field studies for invasive technologies is challenging. Our study represents a paradigm for the stepwise and sound development of vector control tools based on gene-drive.
Garrood WT, Kranjc N, Petri K, et al., 2021, Analysis of off-target effects in CRISPR-based gene drives in the human malaria mosquito, PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA, Vol: 118, ISSN: 0027-8424
Hammond A, Pollegioni P, Persampieri T, et al., 2021, Population suppression of the malaria vector Anopheles gambiae by gene drive technology: A large-cage indoor study bridging the gap between laboratory and field testing
<jats:title>Abstract</jats:title> <jats:p>CRISPR-based gene drives are self-sustaining genetic elements that have been recently generated in the laboratory with the aim to develop potent genetic vector control measures targeting disease vectors including Anopheles gambiae. We have shown that a gene drive directed against the gene doublesex (dsx) effectively suppressed the reproductive capability of mosquito populations reared in small laboratory cages. These experiments, though informative, do not recapitulate the complexity of mosquito behaviour in natural environments. Additional information is needed to bridge the gap between laboratory and the field to validate the vector control potential of the technology. We have investigated the suppressing activity of dsx gene drive strain Ag(QFS)1 on age-structured populations of Anopheles gambiae in large indoor cages that provide a more challenging ecology by more closely mimicking natural conditions and stimulating complex mosquito behaviours. Under these conditions, the Ag(QFS)1 drive spreads rapidly from a single release to the indoor large-cage populations at low initial frequency, leading to full population suppression within one year and without inducing resistance to the gene drive. Initial stochastic simulations of the expected population dynamics, as based on life history parameters estimated in small cages, did not fully capture the observed dynamics in the large cages. Thus, we used the method of approximate Bayesian computation to better estimate population dynamics in the more realistic ecological setting in large cages, allowing the mosquitoes to show a complex feeding and reproductive behaviour.Together, these results establish a new paradigm for generating data to bridge laboratory and field studies, and form an essential component in the stepwise and sound development of gene drive based vector control tools.</jats:p>
Hammond A, Karlsson X, Morianou I, et al., 2021, Regulating the expression of gene drives is key to increasing their invasive potential and the mitigation of resistance, PLOS GENETICS, Vol: 17, ISSN: 1553-7404
Simoni A, Hammond AM, Beaghton AK, et al., 2020, A male-biased sex-distorter gene drive for the human malaria vector Anopheles gambiae, Nature Biotechnology, Vol: 38, Pages: 1054-1060, ISSN: 1087-0156
Only female insects transmit diseases such as malaria, dengue and Zika; therefore, control methods that bias the sex ratio of insect offspring have long been sought. Genetic elements such as sex-chromosome drives can distort sex ratios to produce unisex populations that eventually collapse, but the underlying molecular mechanisms are unknown. We report a male-biased sex-distorter gene drive (SDGD) in the human malaria vector Anopheles gambiae. We induced super-Mendelian inheritance of the X-chromosome-shredding I-PpoI nuclease by coupling this to a CRISPR-based gene drive inserted into a conserved sequence of the doublesex (dsx) gene. In modeling of invasion dynamics, SDGD was predicted to have a quicker impact on female mosquito populations than previously developed gene drives targeting female fertility. The SDGD at the dsx locus led to a male-only population from a 2.5% starting allelic frequency in 10-14 generations, with population collapse and no selection for resistance. Our results support the use of SDGD for malaria vector control.
Simoni A, Hammond AM, Beaghton AK, et al., 2020, A male-biased sex-distorter gene drive for the human malaria vector Anopheles gambiae (vol 14, pg 931, 2020), NATURE BIOTECHNOLOGY, Vol: 38, Pages: 1097-1097, ISSN: 1087-0156
Beaghton AK, Hammond A, Nolan T, et al., 2019, Gene drive for population genetic control: non-functional resistance and parental effects, Proceedings of the Royal Society B: Biological Sciences, Vol: 286, Pages: 1-8, ISSN: 0962-8452
Gene drive is a natural process of biased inheritance that, in principle, could be used to control pest and vector populations. As with any form of pest control, attention should be paid to the possibility of resistance evolving. For nuclease-based gene drive aimed at suppressing a population, resistance could arise by changes in the target sequence that maintain function, and various strategies have been proposed to reduce the likelihood that such alleles arise. Even if these strategies are successful, it is almost inevitable that alleles will arise at the target site that are resistant to the drive but do not restore function, and the impact of such sequences on the dynamics of control has been little studied. We use population genetic modelling of a strategy targeting a female fertility gene to demonstrate that such alleles may be expected to accumulate, and thereby reduce the reproductive load on the population, if nuclease expression per se causes substantial heterozygote fitness effects or if parental (especially paternal) deposition of nuclease either reduces offspring fitness or affects the genotype of their germline. All these phenomena have been observed in synthetic drive constructs. It will, therefore, be important to allow for non-functional resistance alleles in predicting the dynamics of constructs in cage populations and the impacts of any field release.
Bernardini F, Haghighat-Khah RE, Galizi R, et al., 2018, Molecular tools and genetic markers for the generation of transgenic sexing strains in Anopheline mosquitoes, Parasites & Vectors, Vol: 11, ISSN: 1756-3305
Malaria is a serious global health burden, affecting more than 200 million people each year in over 90 countries, predominantly in Africa, Asia and the Americas. Since the year 2000, a concerted effort to combat malaria has reduced its incidence by more than 40%, primarily due to the use of insecticide-treated bednets, indoor residual spraying and artemisinin-based combination drug therapies. Nevertheless, the cost of control is expected to nearly triple over the next decade and the current downward trend in disease transmission is threatened by the rise of resistance to drugs and insecticides. Novel strategies that are sustainable and cost-effective are needed to help usher in an era of malaria elimination. The most effective strategies thus far have focussed on control of the mosquito vector. The sterile insect technique (SIT) is a potentially powerful strategy that aims to suppress mosquito populations through the unproductive mating of wild female mosquitoes with sterile males that are released en masse. The technique and its derivatives are currently not appropriate for malaria control because it is difficult to sterilise males without compromising their ability to mate, and because anopheline males cannot be easily separated from females, which if released, could contribute to disease transmission. Advances in genome sequencing technologies and the development of transgenic techniques provide the tools necessary to produce mosquito sexing strains, which promise to improve current malaria-control programs and pave the way for new ones. In this review, the progress made in the development of transgenic sexing strains for the control of Anopheles gambiae, a major vector of human malaria, is discussed.
Kyrou K, Hammond AM, Galizi R, et al., 2018, A CRISPR-Cas9 gene drive targeting doublesex causes complete population suppression in caged Anopheles gambiae mosquitoes, Nature Biotechnology, Vol: 36, Pages: 1062-1066, ISSN: 1087-0156
In the human malaria vector Anopheles gambiae, the gene doublesex (Agdsx) encodes two alternatively spliced transcripts, dsx-female (AgdsxF) and dsx-male (AgdsxM), that control differentiation of the two sexes. The female transcript, unlike the male, contains an exon (exon 5) whose sequence is highly conserved in all Anopheles mosquitoes so far analyzed. We found that CRISPR–Cas9-targeted disruption of the intron 4–exon 5 boundary aimed at blocking the formation of functional AgdsxF did not affect male development or fertility, whereas females homozygous for the disrupted allele showed an intersex phenotype and complete sterility. A CRISPR–Cas9 gene drive construct targeting this same sequence spread rapidly in caged mosquitoes, reaching 100% prevalence within 7–11 generations while progressively reducing egg production to the point of total population collapse. Owing to functional constraint of the target sequence, no selection of alleles resistant to the gene drive occurred in these laboratory experiments. Cas9-resistant variants arose in each generation at the target site but did not block the spread of the drive.
Hammond A, Karlsson X, Morianou I, et al., 2018, Regulation of gene drive expression increases invasive potential and mitigates resistance, Publisher: Cold Spring Harbor Laboratory
<jats:title>Abstract</jats:title><jats:p>CRISPR-Cas9 nuclease-based gene drives rely on inducing chromosomal breaks in the germline that are repaired in ways that lead to a biased inheritance of the drive. Gene drives designed to impair female fertility can suppress populations of the mosquito vector of malaria. However, strong unintended fitness costs, due to ectopic nuclease expression, and high levels of resistant mutations, limited the potential of the first generation of gene drives to spread.</jats:p><jats:p>Here we show that changes to regulatory sequences in the drive element, designed to contain nuclease expression to the germline, confer improved fecundity over previous versions and generate drastically lower rates of target site resistance. We employed a genetic screen to show that this effect is explained by reduced rates of end-joining repair of DNA breaks at the target site caused by deposited nuclease in the embryo.</jats:p><jats:p>Highlighting the impact of deposited Cas9, many of the mutations arising from this source of nuclease activity in the embryo are heritable, thereby having the potential to generate resistant target sites that reduce the penetrance of the gene drive.</jats:p><jats:p>Finally, in cage invasion experiments these gene drives show improved invasion dynamics compared to first generation drives, resulting in greater than 90% suppression of the reproductive output and a delay in the emergence of target site resistance, even at a resistance-prone target sequence. We shed light on the dynamics of generation and selection of resistant alleles in a population by tracking, longitudinally, the frequency of resistant alleles in the face of an invading gene drive. Our results illustrate important considerations for future gene drive design and should expedite the development of gene drives robust to resistance.</jats:p>
Hammond AM, Galizi R, 2018, Gene drives to fight malaria: current state and future directions, Pathogens and Global Health, Vol: 111, Pages: 412-423, ISSN: 2047-7724
Self-propagating gene drive technologies have a number of desirable characteristics that warrant their development for the control of insect pest and vector populations, such as the malaria-transmitting mosquitoes. Theoretically easy to deploy and self-sustaining, these tools may be used to generate cost-effective interventions that benefit society without obvious bias related to wealth, age or education. Their species-specific design offers the potential to reduce environmental risks and aim to be compatible and complementary with other control strategies, potentially expediting the elimination and eradication of malaria. A number of strategies have been proposed for gene-drive based control of the malaria mosquito and recent demonstrations have shown proof-of-principle in the laboratory. Though several technical, ethical and regulatory challenges remain, none appear insurmountable if research continues in a step-wise and open manner.
Hammond AM, Kyrou K, Bruttini M, et al., 2017, The creation and selection of mutations resistant to a gene drive over multiple generations in the malaria mosquito, PLoS Genetics, Vol: 13, ISSN: 1553-7390
Gene drives have enormous potential for the control of insect populations of medical and agricultural relevance. By preferentially biasing their own inheritance, gene drives can rapidly introduce genetic traits even if these confer a negative fitness effect on the population. We have recently developed gene drives based on CRISPR nuclease constructs that are designed to disrupt key genes essential for female fertility in the malaria mosquito. The construct copies itself and the associated genetic disruption from one homologous chromosome to another during gamete formation, a process called homing that ensures the majority of offspring inherit the drive. Such drives have the potential to cause long-lasting, sustainable population suppression, though they are also expected to impose a large selection pressure for resistance in the mosquito. One of these population suppression gene drives showed rapid invasion of a caged population over 4 generations, establishing proof of principle for this technology. In order to assess the potential for the emergence of resistance to the gene drive in this population we allowed it to run for 25 generations and monitored the frequency of the gene drive over time. Following the initial increase of the gene drive we observed a gradual decrease in its frequency that was accompanied by the spread of small, nuclease-induced mutations at the target gene that are resistant to further cleavage and restore its functionality. Such mutations showed rates of increase consistent with positive selection in the face of the gene drive. Our findings represent the first documented example of selection for resistance to a synthetic gene drive and lead to important design recommendations and considerations in order to mitigate for resistance in future gene drive applications.
Bernardini F, Galizi R, Wunderlich M, et al., 2017, Cross-Species Y Chromosome Function Between Malaria Vectors of the Anopheles gambiae Species Complex., Genetics, ISSN: 0016-6731
Y chromosome function, structure and evolution is poorly understood in many species including the Anopheles genus of mosquitoes, an emerging model system for studying speciation that also represents the major vectors of malaria. While the Anopheline Y had previously been implicated in male mating behavior, recent data from the Anopheles gambiae complex suggests that, apart from the putative primary sex-determiner, no other genes are conserved on the Y. Studying the functional basis of the evolutionary divergence of the Y chromosome in the gambiae complex is complicated by complete F1 male hybrid sterility. Here we used an F1xF0 crossing scheme to overcome a severe bottleneck of male hybrid incompatibilities and enabled us to experimentally purify a genetically labelled A. gambiae Y chromosome in an A. arabiensis background. Whole genome sequencing confirmed that the A. gambiae Y retained its original sequence content in the A. arabiensis genomic background. In contrast to comparable experiments in Drosophila, we find that the presence of a heterospecific Y chromosome has no significant effect on the expression of A. arabiensis genes and transcriptional differences can be explained almost exclusively as a direct consequence of transcripts arising from sequence elements present on the A. gambiae Y chromosome itself. We find that Y hybrids show no obvious fertility defects and no substantial reduction in male competitiveness. Our results demonstrate that, despite their radically different structure, Y chromosomes of these two species of the gambiae complex that diverged an estimated 1.85Myr ago function interchangeably, thus indicating that the Y chromosome does not harbor loci contributing to hybrid incompatibility. Therefore, Y chromosome gene flow between members of the gambiae complex is possible even at their current level of divergence. Importantly, this also suggests that malaria control interventions based on sex-distorting Y drive would be transferable, whethe
Beaghton A, Hammond A, Nolan T, et al., 2017, Requirements for Driving Antipathogen Effector Genes into Populations of Disease Vectors by Homing, Genetics, Vol: 205, Pages: 1587-1596
Simoes ML, Dong Y, Hammond A, et al., 2017, The Anopheles FBN9 immune factor mediates Plasmodium species-specific defense through transgenic fat body expression, Developmental and Comparative Immunology, Vol: 67, Pages: 257-265
Galizi R, Hammond A, Kyrou K, et al., 2016, A CRISPR-Cas9 sex-ratio distortion system for genetic control., Scientific Reports, Vol: 6, ISSN: 2045-2322
Genetic control aims to reduce the ability of insect pest populations to cause harm via the release of modified insects. One strategy is to bias the reproductive sex ratio towards males so that a population decreases in size or is eliminated altogether due to a lack of females. We have shown previously that sex ratio distortion can be generated synthetically in the main human malaria vector Anopheles gambiae, by selectively destroying the X-chromosome during spermatogenesis, through the activity of a naturally-occurring endonuclease that targets a repetitive rDNA sequence highly-conserved in a wide range of organisms. Here we describe a CRISPR-Cas9 sex distortion system that targets ribosomal sequences restricted to the member species of the Anopheles gambiae complex. Expression of Cas9 during spermatogenesis resulted in RNA-guided shredding of the X-chromosome during male meiosis and produced extreme male bias among progeny in the absence of any significant reduction in fertility. The flexibility of CRISPR-Cas9 combined with the availability of genomic data for a range of insects renders this strategy broadly applicable for the species-specific control of any pest or vector species with an XY sex-determination system by targeting sequences exclusive to the female sex chromosome.
Hammond A, Galizi R, Kyrou K, et al., 2016, A CRISPR-Cas9 gene drive system-targeting female reproduction in the malaria mosquito vector Anopheles gambiae, Nature Biotechnology, Vol: 34, Pages: 78-83, ISSN: 1087-0156
Gene drive systems that enable super-Mendelian inheritance of a transgene have the potential to modify insect populations over a timeframe of a few years. We describe CRISPR-Cas9 endonuclease constructs that function as gene drive systems in Anopheles gambiae, the main vector for malaria. We identified three genes (AGAP005958, AGAP011377 and AGAP007280) that confer a recessive female-sterility phenotype upon disruption, and inserted into each locus CRISPR-Cas9 gene drive constructs designed to target and edit each gene. For each targeted locus we observed a strong gene drive at the molecular level, with transmission rates to progeny of 91.4 to 99.6%. Population modeling and cage experiments indicate that a CRISPR-Cas9 construct targeting one of these loci, AGAP007280, meets the minimum requirement for a gene drive targeting female reproduction in an insect population. These findings could expedite the development of gene drives to suppress mosquito populations to levels that do not support malaria transmission.
Hammond AM, Nolan T, Benedict MQ, 2014, Sex-, tissue- and stage-specific transgene expression., Transgenic insects: techniques and applications, Pages: 29-50
Fuchs S, Garrood WT, Beber A, et al., Resistance to a CRISPR-based gene drive at an evolutionarily conserved site is revealed by mimicking genotype fixation, Publisher: Cold Spring Harbor Laboratory
<jats:title>Abstract</jats:title><jats:p>CRISPR-based homing gene drives can be designed to disrupt essential genes whilst biasing their own inheritance, leading to suppression of mosquito populations in the laboratory. This class of gene drives relies on CRISPR-Cas9 cleavage of a target sequence and copying (‘homing’) therein of the gene drive element from the homologous chromosome. However, target site mutations that are resistant to cleavage yet maintain the function of the essential gene are expected to be strongly selected for. Targeting functionally constrained regions where mutations are not easily tolerated should lower the probability of resistance. Evolutionary conservation at the sequence level is often a reliable indicator of functional constraint, though the actual level of underlying constraint between one conserved sequence and another can vary widely. Here we generated a novel gene drive in the malaria vector <jats:italic>Anopheles gambiae</jats:italic>, targeting an ultra-conserved target site in a haplosufficient essential gene (AGAP029113) required during mosquito development, which fulfils many of the criteria for the target of a population suppression gene drive. We then designed a selection regime to experimentally assess the likelihood of generation and subsequent selection of gene drive resistant mutations at its target site. We simulated, in a caged population, a scenario where the gene drive was approaching fixation, where selection for resistance is expected to be strongest. Continuous sampling of the target locus revealed that a single, restorative, in-frame nucleotide substitution was selected. Our findings show that ultra-conservation alone need not be predictive of a site that is refractory to target site resistance. Our strategy to evaluate resistance <jats:italic>in vivo</jats:italic> could help to validate candidate gene drive targets for their resilience to resistance and help t
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