Gene drive

Molecular mechanism of gene drive.

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 the fact that double strand breaks are most frequently repaired by homologous recombination, (in the presence of a template), rather than non-homologous end joining. To achieve this behavior, endonuclease gene drives consist of two nested elements:

• either a homing endonuclease or a RNA-guided endonuclease (e.g., Cas9 or Cas12a[13]) and its guide RNA, that cuts the target sequence in recipient cells
• a template sequence used by the DNA repair machinery after the target sequence is cut. To achieve the self-propagating nature of gene drives, this repair template contains at least the endonuclease sequence. Because the template must be used to repair a double-strand break at the cutting site, its sides are homologous to the sequences that are adjacent to the cutting site in the host genome. By targeting the gene drive to a gene coding sequence, this gene will be inactivated; additional sequences can be introduced in the gene drive to encode new functions.

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]

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.[14] 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;[14] 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]

Gene drives affect all future generations and represent the possibility of a larger change in a living species than has been possible before.[20]

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,[21] but supported continued basic research and gene editing that would not affect future generations.[22] 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.[23][24] In June 2016, the US National Academies of Sciences, Engineering, and Medicine released a report on their "Recommendations for Responsible Conduct" of gene drives.[25]

Models suggest that extinction-oriented gene drives will wipe out target species and that drives could reach populations beyond the target given minimal connectivity between them.[26]

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.".[27] He moved to an open model for his own research on using gene drives to eradicate Lyme disease in Nantucket and Martha's Vineyard.[28] 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.[29]

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.[36] In December 2017, documents released under the Freedom of Information Act showed that DARPA had invested $100 million in gene drive research.[37]

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.[49] Researchers have claimed that by applying the technique to 1% of the wild population of mosquitoes, that they could eradicate malaria within a year.[50]

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.[51] 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.[52] A wider outreach network for gene drive research exists to raise awareness of the value of gene drive research for the public good.[53]

Some scientists are concerned about the technique, fearing it could spread and wipe out species in native habitats.[54] The gene could mutate, potentially causing unforeseen problems (as could any gene).[55] 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.[56]

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.[57][58] 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.[58] In 2017, one group in Australia and another in Texas released preliminary research into creating 'daughterless mice' using gene drives in mammals.[59]

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.[19]

The transhumanist philosopher David Pearce has advocated for using CRISPR-based gene drives to reduce the suffering of wild animals.[60] 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.[61]