A gene drive is a natural process [1] and technology of genetic engineering that propagates a particular suite of genes throughout a population [2] by altering the probability that a specific allele will be transmitted to offspring (instead of the Mendelian 50% probability). Gene drives can arise through a variety of mechanisms. [3] [4] They have been proposed to provide an effective means of genetically modifying specific populations and entire species.
The technique can employ adding, deleting, disrupting, or modifying genes. [5] [6]
Proposed applications include exterminating insects that carry pathogens (notably mosquitoes that transmit malaria, dengue, and zika pathogens), controlling invasive species, or eliminating herbicide or pesticide resistance. [7] [5] [8] [9]
As with any potentially powerful technique, gene drives can be misused in a variety of ways or induce unintended consequences. For example, a gene drive intended to affect only a local population might spread across an entire species. Gene drives that eradicate populations of invasive species in their non-native habitats may have consequences for the population of the species as a whole, even in its native habitat. Any accidental return of individuals of the species to its original habitats, through natural migration, environmental disruption (storms, floods, etc.), accidental human transportation, or purposeful relocation, could unintentionally drive the species to extinction if the relocated individuals carried harmful gene drives. [10]
Gene drives can be built from many naturally occurring selfish genetic elements that use a variety of molecular mechanisms. [3] These naturally occurring mechanisms induce similar segregation distortion in the wild, arising when alleles evolve molecular mechanisms that give them a transmission chance greater than the normal 50%.
Most gene drives have been developed in insects, notably mosquitoes, as a way to control insect-borne pathogens. Recent developments designed gene drives directly in viruses, notably herpesviruses. These viral gene drives can propagate a modification into the population of viruses, and aim to reduce the infectivity of the virus. [11] [12]
In sexually-reproducing species, most genes are present in two copies (which can be the same or different alleles), either one of which has a 50% chance of passing to a descendant. By biasing the inheritance of particular altered genes, synthetic gene drives could spread alterations through a population. [5] [6]
At the molecular level, an endonuclease gene drive works by cutting a chromosome at a specific site that does not encode the drive, inducing the cell to repair the damage by copying the drive sequence onto the damaged chromosome. The cell then has two copies of the drive sequence. The method derives from genome editing techniques and relies on homologous recombination. To achieve this behavior, endonuclease gene drives consist of two nested elements:
As a result, the gene drive insertion in the genome will re-occur in each organism that inherits one copy of the modification and one copy of the wild-type gene. If the gene drive is already present in the egg cell (e.g. when received from one parent), all the gametes of the individual will carry the gene drive (instead of 50% in the case of a normal gene). [5]
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Since it can never more than double in frequency with each generation, a gene drive introduced in a single individual typically requires dozens of generations to affect a substantial fraction of a population. Alternatively, releasing drive-containing organisms in sufficient numbers can affect the rest within a few generations; for instance, by introducing it in every thousandth individual, it takes only 12–15 generations to be present in all individuals. [16] Whether a gene drive will ultimately become fixed in a population and at which speed depends on its effect on individual fitness, on the rate of allele conversion, and on the population structure. In a well mixed population and with realistic allele conversion frequencies (≈90%), population genetics predicts that gene drives get fixed for a selection coefficient smaller than 0.3; [16] in other words, gene drives can be used to spread modifications as long as reproductive success is not reduced by more than 30%. This is in contrast with normal genes, which can only spread across large populations if they increase fitness.
Because the strategy usually relies on the simultaneous presence of an unmodified and a gene drive allele in the same cell nucleus, it had generally been assumed that a gene drive could only be engineered in sexually reproducing organisms, excluding bacteria and viruses. However, during a viral infection, viruses can accumulate hundreds or thousands of genome copies in infected cells. Cells are frequently co-infected by multiple virions and recombination between viral genomes is a well-known and widespread source of diversity for many viruses. In particular, herpesviruses are nuclear-replicating DNA viruses with large double-stranded DNA genomes and frequently undergo homologous recombination during their replication cycle.
These properties have enabled the design of a gene drive strategy that doesn't involve sexual reproduction, instead relying on co-infection of a given cell by a naturally occurring and an engineered virus. Upon co-infection, the unmodified genome is cut and repaired by homologous recombination, producing new gene drive viruses that can progressively replace the naturally occurring population. In cell culture experiments, it was shown that a viral gene drive can spread into the viral population and strongly reduce the infectivity of the virus, which opens novel therapeutic strategies against herpesviruses. [11]
Because gene drives propagate by replacing other alleles that contain a cutting site and the corresponding homologies, their application has been mostly limited to sexually reproducing species (because they are diploid or polyploid and alleles are mixed at each generation). As a side effect, inbreeding could in principle be an escape mechanism, but the extent to which this can happen in practice is difficult to evaluate. [17]
Due to the number of generations required for a gene drive to affect an entire population, the time to universality varies according to the reproductive cycle of each species: it may require under a year for some invertebrates, but centuries for organisms with years-long intervals between birth and sexual maturity, such as humans. [18] Hence this technology is of most use in fast-reproducing species.
Effectiveness in real practice varies between techniques, especially by choice of germline promoter. Lin and Potter 2016 (a) discloses the promoter technology homology assisted CRISPR knockin (HACK) and Lin and Potter 2016 (b) demonstrates actual use, achieving a high proportion of altered progeny from each altered Drosophila mother. [19]
Issues highlighted by researchers include: [20]
The Broad Institute of MIT and Harvard added gene drives to a list of uses of gene-editing technology it doesn't think companies should pursue. [21] [ better source needed ]
Gene drives affect all future generations and represent the possibility of a larger change in a living species than has been possible before. [22]
In December 2015, scientists of major world academies called for a moratorium on inheritable human genome edits that would affect the germline, including those related to CRISPR-Cas9 technologies, [23] but supported continued basic research and gene editing that would not affect future generations. [24] In February 2016, British scientists were given permission by regulators to genetically modify human embryos by using CRISPR-Cas9 and related techniques on condition that the embryos were destroyed in seven days. [25] [26] In June 2016, the US National Academies of Sciences, Engineering, and Medicine released a report on their "Recommendations for Responsible Conduct" of gene drives. [27]
A 2018 mathematical modelling studies suggest that despite preexisting and evolving gene drive resistance (caused by mutations at the cutting site), even an inefficient CRISPR "alteration-type" gene drive can achieve fixation in small populations. With a small but non-zero amount of gene flow among many local populations, the gene drive can escape and convert outside populations as well. [28]
Kevin M. Esvelt stated that an open conversation was needed around the safety of gene drives: "In our view, it is wise to assume that invasive and self-propagating gene drive systems are likely to spread to every population of the target species throughout the world. Accordingly, they should only be built to combat true plagues such as malaria, for which we have few adequate countermeasures and that offer a realistic path towards an international agreement to deploy among all affected nations.". [29] He moved to an open model for his own research on using gene drives to eradicate Lyme disease in Nantucket and Martha's Vineyard. [30] Esvelt and colleagues suggested that CRISPR could be used to save endangered wildlife from extinction. Esvelt later retracted his support for the idea, except for extremely hazardous populations such as malaria-carrying mosquitoes, and isolated islands that would prevent the drive from spreading beyond the target area. [31]
Austin Burt, an evolutionary geneticist at Imperial College London, introduced the possibility of conducting gene drives based on natural homing endonuclease selfish genetic elements in 2003. [6]
Researchers had already shown that such genes could act selfishly to spread rapidly over successive generations. Burt suggested that gene drives might be used to prevent a mosquito population from transmitting the malaria parasite or to crash a mosquito population. Gene drives based on homing endonucleases have been demonstrated in the laboratory in transgenic populations of mosquitoes [32] and fruit flies. [33] [34] However, homing endonucleases are sequence-specific. Altering their specificity to target other sequences of interest remains a major challenge. [3] The possible applications of gene drive remained limited until the discovery of CRISPR and associated RNA-guided endonucleases such as Cas9 and Cas12a.
In June 2014, the World Health Organization (WHO) Special Programme for Research and Training in Tropical Diseases [35] issued guidelines [36] for evaluating genetically modified mosquitoes. In 2013 the European Food Safety Authority issued a protocol [37] for environmental assessments of all genetically modified organisms.
Target Malaria, a project funded by the Bill and Melinda Gates Foundation, invested $75 million in gene drive technology. The foundation originally estimated the technology to be ready for field use by 2029 somewhere in Africa. However, in 2016 Gates changed this estimate to some time within the following two years. [38] In December 2017, documents released under the Freedom of Information Act showed that DARPA had invested $100 million in gene drive research. [39]
Scientists have designed multiple strategies to maintain control over gene drives.[ citation needed ]
In 2020, researchers reported the development of two active guide RNA-only elements that, according to their study, may enable halting or deleting gene drives introduced into populations in the wild with CRISPR-Cas9 gene editing. The paper's senior author cautions that the two neutralizing systems they demonstrated in cage trials "should not be used with a false sense of security for field-implemented gene drives". [40] [41]
If elimination is not necessary, it may be desirable to intentionally preserve the target population at a lower level by using a less severe gene drive technology. This works by maintaining the semi-defective population indefinitely in the target area, thereby crowding out potential nearby, wild populations that would otherwise move back in to fill a void. [42]
CRISPR [43] is the leading genetic engineering method. [44] In 2014, Esvelt and coworkers first suggested that CRISPR/Cas9 might be used to build gene drives. [5] In 2015, researchers reported successful engineering of CRISPR-based gene drives in Saccharomyces [45] , Drosophila , [46] and mosquitoes. [47] [48] They reported efficient inheritance distortion over successive generations, with one study demonstrating the spread of a gene into laboratory populations. [48] Drive-resistant alleles were expected to arise for each of the described gene drives; however, this could be delayed or prevented by targeting highly conserved sites at which resistance was expected to have a severe fitness cost.
Because of CRISPR's targeting flexibility, gene drives could theoretically be used to engineer almost any trait. Unlike previous approaches, they could be tailored to block the evolution of drive resistance by targeting multiple sequences. CRISPR could also enable gene drive architectures that control rather than eliminate populations.[ citation needed ]
In 2022, t-CRISPR, was used to pass the “t haplotype” gene to about 95% of offspring. The approach spreads faulty copies of a female fertility gene to offspring, rendering them infertile. The researchers reported that their models suggested that adding 256 altered animals to an island with a population of 200,000 mice would eliminate the population in about 25 years. The traditional approaches of poison and traps were not needed. [49]
Gene drives have two main classes of application, which have implications of different significance:
Because of their unprecedented potential risk, safeguard mechanisms have been proposed and tested. [45] [50]
One possible application is to genetically modify mosquitoes, mice, and other disease vectors so that they cannot transmit diseases, such as malaria and dengue fever in the case of mosquitoes, and tick-borne disease in the case of mice. [51] Researchers have claimed that by applying the technique to 1% of the wild population of mosquitoes, that they could eradicate malaria within a year. [52]
A gene drive could be used to eliminate invasive species and has, for example, been proposed as a way to eliminate invasive species in New Zealand. [53] Gene drives for biodiversity conservation purposes are being explored as part of The Genetic Biocontrol of Invasive Rodents (GBIRd) program because they offer the potential for reduced risk to non-target species and reduced costs when compared to traditional invasive species removal techniques. Given the risks of such an approach described below, the GBIRd partnership is committed to a deliberate, step-wise process that will only proceed with public alignment, as recommended by the world's leading gene drive researchers from the Australian and US National Academy of Sciences and many others. [54] A wider outreach network for gene drive research exists to raise awareness of the value of gene drive research for the public good. [55]
Some scientists are concerned about the technique, fearing it could spread and wipe out species in native habitats. [56] The gene could mutate, potentially causing unforeseen problems (as could any gene). [57] Many non-native species can hybridize with native species, such that a gene drive afflicting a non-native plant or animal that hybridizes with a native species could doom the native species. Many non-native species have naturalized into their new environment so well that crops and/or native species have adapted to depend on them. [58]
The Predator Free 2050 project is a New Zealand government program to eliminate eight invasive mammalian predator species (including rats, short-tailed weasels, and possums) from the country by 2050. [59] [60] The project was first announced in 2016 by New Zealand's prime minister John Key and in January 2017 it was announced that gene drives would be considered in the effort, but this has not yet been actualised. [60] In 2017, one group in Australia and another in Texas released preliminary research into creating 'daughterless mice' using gene drives in mammals. [61]
In 2017, scientists at the University of California, Riverside developed a gene drive to attack the invasive spotted-wing drosophila, a type of fruit fly native to Asia that is costing California's cherry farms $700 million per year because of its tail's razor-edged ovipositor that destroys unblemished fruit. The primary alternative control strategy involves the use of insecticides called pyrethroids that kill almost all insects that it contacts. [21]
The transhumanist philosopher David Pearce has advocated for using CRISPR-based gene drives to reduce the suffering of wild animals. [62] Kevin M. Esvelt, an American biologist who has helped develop gene drive technology, has argued that there is a moral case for the elimination of the New World screwworm through such technologies because of the immense suffering that infested wild animals experience when they are eaten alive. [63]
Selfish genetic elements are genetic segments that can enhance their own transmission at the expense of other genes in the genome, even if this has no positive or a net negative effect on organismal fitness. Genomes have traditionally been viewed as cohesive units, with genes acting together to improve the fitness of the organism. However, when genes have some control over their own transmission, the rules can change, and so just like all social groups, genomes are vulnerable to selfish behaviour by their parts.
Gene knockouts are a widely used genetic engineering technique that involves the targeted removal or inactivation of a specific gene within an organism's genome. This can be done through a variety of methods, including homologous recombination, CRISPR-Cas9, and TALENs.
A germline mutation, or germinal mutation, is any detectable variation within germ cells. Mutations in these cells are the only mutations that can be passed on to offspring, when either a mutated sperm or oocyte come together to form a zygote. After this fertilization event occurs, germ cells divide rapidly to produce all of the cells in the body, causing this mutation to be present in every somatic and germline cell in the offspring; this is also known as a constitutional mutation. Germline mutation is distinct from somatic mutation.
CRISPR is a family of DNA sequences found in the genomes of prokaryotic organisms such as bacteria and archaea. These sequences are derived from DNA fragments of bacteriophages that had previously infected the prokaryote. They are used to detect and destroy DNA from similar bacteriophages during subsequent infections. Hence these sequences play a key role in the antiviral defense system of prokaryotes and provide a form of acquired immunity. CRISPR is found in approximately 50% of sequenced bacterial genomes and nearly 90% of sequenced archaea.
The Anopheles gambiae complex consists of at least seven morphologically indistinguishable species of mosquitoes in the genus Anopheles. The complex was recognised in the 1960s and includes the most important vectors of malaria in sub-Saharan Africa, particularly of the most dangerous malaria parasite, Plasmodium falciparum. It is one of the most efficient malaria vectors known. The An. gambiae mosquito additionally transmits Wuchereria bancrofti which causes lymphatic filariasis, a symptom of which is elephantiasis.
Guide RNA (gRNA) or single guide RNA (sgRNA) is a short sequence of RNA that functions as a guide for the Cas9-endonuclease or other Cas-proteins that cut the double-stranded DNA and thereby can be used for gene editing. In bacteria and archaea, gRNAs are a part of the CRISPR-Cas system that serves as an adaptive immune defense that protects the organism from viruses. Here the short gRNAs serve as detectors of foreign DNA and direct the Cas-enzymes that degrades the foreign nucleic acid.
Genetically modified animals are animals that have been genetically modified for a variety of purposes including producing drugs, enhancing yields, increasing resistance to disease, etc. The vast majority of genetically modified animals are at the research stage while the number close to entering the market remains small.
Genome editing, or genome engineering, or gene editing, is a type of genetic engineering in which DNA is inserted, deleted, modified or replaced in the genome of a living organism. Unlike early genetic engineering techniques that randomly inserts genetic material into a host genome, genome editing targets the insertions to site-specific locations. The basic mechanism involved in genetic manipulations through programmable nucleases is the recognition of target genomic loci and binding of effector DNA-binding domain (DBD), double-strand breaks (DSBs) in target DNA by the restriction endonucleases, and the repair of DSBs through homology-directed recombination (HDR) or non-homologous end joining (NHEJ).
Cas9 is a 160 kilodalton protein which plays a vital role in the immunological defense of certain bacteria against DNA viruses and plasmids, and is heavily utilized in genetic engineering applications. Its main function is to cut DNA and thereby alter a cell's genome. The CRISPR-Cas9 genome editing technique was a significant contributor to the Nobel Prize in Chemistry in 2020 being awarded to Emmanuelle Charpentier and Jennifer Doudna.
CRISPR interference (CRISPRi) is a genetic perturbation technique that allows for sequence-specific repression of gene expression in prokaryotic and eukaryotic cells. It was first developed by Stanley Qi and colleagues in the laboratories of Wendell Lim, Adam Arkin, Jonathan Weissman, and Jennifer Doudna. Sequence-specific activation of gene expression refers to CRISPR activation (CRISPRa).
A protospacer adjacent motif (PAM) is a 2–6-base pair DNA sequence immediately following the DNA sequence targeted by the Cas9 nuclease in the CRISPR bacterial adaptive immune system. The PAM is a component of the invading virus or plasmid, but is not found in the bacterial host genome and hence is not a component of the bacterial CRISPR locus. Cas9 will not successfully bind to or cleave the target DNA sequence if it is not followed by the PAM sequence. PAM is an essential targeting component which distinguishes bacterial self from non-self DNA, thereby preventing the CRISPR locus from being targeted and destroyed by the CRISPR-associated nuclease.
Andrea Crisanti is an Italian full professor of microbiology at the University of Padua and politician. He previously was professor of Molecular Parasitology at Imperial College London. He is best known for the development of genetically manipulated mosquitoes with the objective to interfere with either their reproductive rate or the capability to transmit diseases such as malaria.
Cas12a is a subtype of Cas12 proteins and an RNA-guided endonuclease that forms part of the CRISPR system in some bacteria and archaea. It originates as part of a bacterial immune mechanism, where it serves to destroy the genetic material of viruses and thus protect the cell and colony from viral infection. Cas12a and other CRISPR associated endonucleases use an RNA to target nucleic acid in a specific and programmable matter. In the organisms from which it originates, this guide RNA is a copy of a piece of foreign nucleic acid that previously infected the cell.
Biotechnology risk is a form of existential risk from biological sources, such as genetically engineered biological agents. The release of such high-consequence pathogens could be
CRISPR activation (CRISPRa) is a type of CRISPR tool that uses modified versions of CRISPR effectors without endonuclease activity, with added transcriptional activators on dCas9 or the guide RNAs (gRNAs).
Off-target genome editing refers to nonspecific and unintended genetic modifications that can arise through the use of engineered nuclease technologies such as: clustered, regularly interspaced, short palindromic repeats (CRISPR)-Cas9, transcription activator-like effector nucleases (TALEN), meganucleases, and zinc finger nucleases (ZFN). These tools use different mechanisms to bind a predetermined sequence of DNA (“target”), which they cleave, creating a double-stranded chromosomal break (DSB) that summons the cell's DNA repair mechanisms and leads to site-specific modifications. If these complexes do not bind at the target, often a result of homologous sequences and/or mismatch tolerance, they will cleave off-target DSB and cause non-specific genetic modifications. Specifically, off-target effects consist of unintended point mutations, deletions, insertions inversions, and translocations.
CRISPR gene editing standing for "Clustered Regularly Interspaced Short Palindromic Repeats" is a genetic engineering technique in molecular biology by which the genomes of living organisms may be modified. It is based on a simplified version of the bacterial CRISPR-Cas9 antiviral defense system. By delivering the Cas9 nuclease complexed with a synthetic guide RNA (gRNA) into a cell, the cell's genome can be cut at a desired location, allowing existing genes to be removed and/or new ones added in vivo.
Prime editing is a 'search-and-replace' genome editing technology in molecular biology by which the genome of living organisms may be modified. The technology directly writes new genetic information into a targeted DNA site. It uses a fusion protein, consisting of a catalytically impaired Cas9 endonuclease fused to an engineered reverse transcriptase enzyme, and a prime editing guide RNA (pegRNA), capable of identifying the target site and providing the new genetic information to replace the target DNA nucleotides. It mediates targeted insertions, deletions, and base-to-base conversions without the need for double strand breaks (DSBs) or donor DNA templates.
The eradication or abolition of suffering is the concept of using biotechnology to create a permanent absence of involuntary pain and suffering in all sentient beings.
The Fanzor (Fz) protein is an eukaryotic, RNA-guided DNA endonuclease, which means it is a type of DNA cutting enzyme that uses RNA to target genes of interest. It has been recently discovered and explored in a number of studies. In bacteria, RNA-guided DNA endonuclease systems, such as the CRISPR/Cas system, serve as an immune system to prevent infection by cutting viral genetic material. Currently, CRISPR/Cas9-mediated's DNA cleavage has extensive application in biological research, and wide-reaching medical potential in human gene editing.