Gene drive

Gene drive is the practice of "stimulating biased inheritance of particular genes to alter entire populations." It has been proposed as a technique for changing wild populations of harmful organisms such as mosquitos to be less dangerous. In addition to combating diseases spread by insects, gene drives might be used to control invasive species or to eliminate herbicide- or pesticide resistance. Possible alterations include adding, disrupting, or modifying genes, including some that reduce reproductive capacity and may cause a population crash. The technique was first proposed in 2003, based on endonucleases.

History

Austin Burt, an evolutionary geneticist at Imperial College London, first outlined the possibility of building gene drives based on natural "selfish" homing endonuclease genes. Researchers had already shown that these “selfish” genes could spread rapidly through successive generations. Burt suggested that gene drives might be used to prevent a mosquito population from transmitting the malaria parasite or crash a mosquito population. Gene drives based on homing endonucleases have been demonstrated in the laboratory in transgenic populations of mosquitoes and fruit flies. These enzymes could be used to drive alterations through wild populations.

In 2015, study in Panama reported that such mosquitoes were effective in reducing populations of dengue fever-carrying Aedes aegypti. Over a six month period approximately 4.2 million males were released, yielding a 93-percent population reduction. The female is the disease carrier. The population declined because the larvae of GM males and wild females fail to thrive. Two control areas did not experience population declines. The A. aegypti were not replaced by other species such as the aggressive A. albopictus. In 2014, nine people died and 5,026 were infected, and in 2013 eight deaths and 4,481 infected, while in March 2015 a baby became the year's first victim of the disease.

CRISPR/Cas9

CRISPR/Cas9 is a gene-editing technique that has revolutionized the field of genetic engineering since 2013. The approach involves expressing the RNA-guided Cas9 endonuclease along with guide RNAs directing it to a particular sequence to be edited. When Cas9 cuts the target sequence, the cell repairs the damage by replacing the original sequence with an altered version. Making a guide RNA to direct Cas9 to cut any specific gene is straightforward. CRISPR tremendously simplifies the process of deleting, adding, or modifying genes. As of 2014 it had successfully been tested in cells of 20 species, including humans. In many of these species, the edits modified their germline, allowing them to be inherited.

Esvelt and coworkers first suggested that CRISPR/Cas9 might be used to build endonuclease gene drives. This could be accomplished by encoding the Cas9 gene and guide RNAs used for genome editing adjacent to the altered gene, causing the editing event to re-occur in each organism that inherits one copy of the modification and one copy of the wild-type gene. Because of CRISPR/Cas9's targeting flexibility, RNA-guided gene drives could theoretically be used to spread any trait. Unlike previous designs, they could be tailored to block the evolution of drive resistance in the target population by destroying sequences within appropriate genes.

Population-level genome changes could be reversed or blocked using the same technique, as a safeguard. CRISPR/Cas9 could also permit a variety of gene drive intended to control rather than crash populations. As of 2014 no drive capable of spreading efficiently through a wild population had been published.

In 2015 researchers successfully tested a CRISPR-based gene drive in Yeast and Drosophila.

Mechanism

Some genes in species that reproduce sexually have greater than the normal 50% chance of being inherited. This allows them to spread through a population even if they reduce the fitness of each individual organism. By similarly biasing the inheritance of particular altered genes, gene drives might be used to spread alterations through wild populations.

Endonuclease gene drives work by cutting the corresponding locus of chromosomes that do 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. If the cell is a germline cell, the modification will carry to offspring.

A gene drive typically requires dozens of generations to affect a substantial fraction of a population because it can never more than double in frequency with each generation. Alternatively, releasing drive-containing organisms in sufficient numbers can affect the rest within a couple of generations. The process may require under a year for some invertebrates, but centuries for organisms with years-long intervals between birth and sexual maturity, such as humans.

Types

A "precision" drive exclusively impacts a population by targeting unique genomic loci (DNA sequences).

An "immunizing drive" blocks another drive from spreading by preemptively altering sequences that another drive targets, preventing the latter from initiating copying.

A "reversal" drive undoes a prior drive's effects, in all or part of the population, although downstream ecological changes might remain.

A "repetitive" drive features multiple iterations to further establish or maintain new trait(s).

A "rogue" drive is intended to deliberately cause harm.

Issues

Issues that researchers have highlighted include:

• Mutations—It is possible that a mutation could happen mid-drive, which has the potential to allow unwanted traits to "ride along" on the spreading drive.

• Escape—Cross-breeding or gene flow potentially allow a drive to move beyond its target population.

• Ecological impacts—Even when new traits' direct impact on a target is understood, the drive may have side effects on the surroundings.

• Unintended human impacts—The drive might produce allergic reactions or other unwanted human side effects.

Risk management proposal

Oye, et al., published a suite of recommendations for managing environmental and security risks:

• "Before any primary drive is released in the field, the efficacy of specific reversal drives should be evaluated. Research should assess the extent to which the residual presence of guide RNAs and/or Cas9 after reversal might affect the phenotype or fitness of a population and the feasibility of reaching individual organisms altered by an initial drive.
• "Long-term studies should evaluate the effects of gene drive use on genetic diversity in target populations. Even if genome-level changes can be reversed, any population reduced in numbers will have reduced genetic diversity and could be more vulnerable to natural or anthropogenic pressures. Genome-editing applications may similarly have lasting effects on populations owing to compensatory adaptations or other changes.
• "Investigations of drive function and safety should use multiple levels of molecular containment to reduce the risk that drives will spread through wild populations during testing. For example, drives should be designed to cut sequences absent from wild populations, and drive components should be separated.
• "Initial tests of drives capable of spreading through wild populations should not be conducted in geographic areas that harbor native populations of target species.
• "All drives that might spread through wild populations should be constructed and tested in tandem with corresponding immunization and reversal drives. These precautions would allow accidental releases to be partially counteracted.
• "A network of multipurpose mesocosms and microcosms should be developed for testing gene drives and other advanced biotechnologies in contained settings.
• "The presence and prevalence of drives should be monitored by targeted amplification or metagenomic sequencing of environmental samples.
• "Because effects will mainly depend on the species and genomic change rather than the drive mechanism, candidate gene drives should be evaluated on a case-by-case basis.
• "To assess potentially harmful uses of drives, multidisciplinary teams of experts should be challenged to develop scenarios on deliberate misuse.
• "Integrated benefit-risk assessments informed by the actions recommended above should be conducted to determine whether and how to proceed with proposed gene drive applications. Such assessments should be conducted with sensitivity to variations in uncertainty across cases and to reductions in uncertainty over time."

A recent publication in Science recommended that at least two "potentially stringent confinement strategies" be used, including:

• Molecular confinement: separate components required, such as sgRNA and Cas9 in separate loci, or drive targeting sequences unique to laboratory organisms.
• Ecological confinement: experiments outside the habitable range of the organism, or one without potential wild mates.
• Reproductive confinement: the laboratory strain is unable to reproduce with wild organisms, such as Drosophila with compound autosomes.
• Barrier confinement: triply nested containers, organisms anesthetized prior to opening containment, low temperature rooms, air blast fans.

Applications

One possible application is to genetically modify mosquitoes and other disease vectors so they cannot transmit diseases such as malaria and dengue fever. In June 2014, the World Health Organization (WHO) Special Programme for Research and Training in Tropical Diseases issued guidelines for evaluating genetically modified mosquitoes. In 2013 the European Food Safety Authority issued a protocol for environmental assessments of all genetically modified organisms.

See also

• Biological devices
• Biological machines
• Synthetic biology
• Zinc finger

External links

• Esvelt, Kevin. "Gene Drives for the Alteration of Wild Populations". Retrieved 11 August 2014.
• "Gene Drive from Harvard's Wyss Institute". Wyss Institute. 2014-07-17. Retrieved 2014-08-11.
• De Chant, Tim (July 17, 2014). "Genetically Engineering Almost Anything". NOVA. Retrieved 11 August 2014.
• Johnson, Carolyn (July 17, 2014). "Harvard scientists want gene-manipulation debate". National Geographic. Retrieved 11 August 2014.
• Langin, Katie (July 17, 2014). "Genetic Engineering to the Rescue Against Invasive Species?". National Geographic. Retrieved 11 August 2014.
• Zimmer, Carl (July 17, 2014). "A Call to Fight Malaria One Mosquito at a Time by Altering DNA". The New York Times. Retrieved 20 July 2014.
• "The age of the red pen". The Economist. August 22, 2015. ISSN 0013-0613. Retrieved 2015-08-25. {{cite news}}: Italic or bold markup not allowed in: |newspaper= (help)
• "The most selfish genes". The Economist. August 22, 2015. ISSN 0013-0613. Retrieved 2015-08-25. {{cite news}}: Italic or bold markup not allowed in: |newspaper= (help)