unchecked spread of engineered genes
Larry H. Bernstein, MD, FCAP, Curator
LPBI
Reining in Gene Drives
Researchers have developed two methods to avoid the unchecked spread of engineered genes through wild populations.
| Nov 18, 2015 http://www.the-scientist.com//?articles.view/articleNo/44501/title/Reining-in-Gene-Drives/
2.1.4.6 Unchecked Spread of Engineered Genes, Volume 2 (Volume Two: Latest in Genomics Methodologies for Therapeutics: Gene Editing, NGS and BioInformatics, Simulations and the Genome Ontology), Part 2: CRISPR for Gene Editing and DNA Repair
“Gene drive” is a phenomenon that causes a gene to be inherited at a rate faster than Mendelian principles would dictate. It relies on genes that can copy themselves onto a corresponding location in a paired chromosome, thereby overriding typical allele inheritance patterns. In conjunction with CRISPR/Cas9, gene drives can be created with almost any DNA sequence, raising questions about the risk of engineered genes spreading quickly through a population. But a team of researchers from Harvard University published a study this week (November 16) in Nature Biology that offers some safety constraints on the system.
description of the first CRISPR/Cas9 gene drive system was published in March by a team at the University of California, San Diego, and showed rapid spreading of a normally recessive phenotype inDrosophila. Other labs are researching the system’s potential to wipe out insect-borne diseases such as malaria by spreading mutated genes throughout a mosquito population. But the strategy carries the risk of accidental contamination of wild populations.
“We have a responsibility to keep our experiments confined to the laboratory,” Kevin Esvelt, an evolutionary engineer and coauthor on the paper, told Nature. “The basic lesson is: if you don’t have to build a gene drive that can spread through a wild population, then don’t.”
Esvelt’s team developed safety protocols—ways to prevent or reverse a released gene drive—using the yeast Saccharomyces cerevisia. One technique genetically separates the components necessary to create a gene drive, putting one half directly in the yeast genome and the other half on an external strand of DNA. The researchers also developed a method that uses one gene drive to overwrite the effects of another.
While these techniques are intended to stem the potential for gene drives in light of safety concerns, Esvelt told Nature that he hopes the scientific community will thoughtfully evaluate gene drives rather than dismiss them. “Should we use gene drive to eliminate malaria? Should we use it to replace broadly toxic insecticides? These questions all have to be considered separately,” he told Nature. “This paper is really about making sure we don’t blow it in the meantime and obviate the chance to talk about all of this.”
Safeguarding CRISPR-Cas9 gene drives in yeast
James E DiCarlo, Alejandro Chavez, Sven L Dietz, Kevin M Esvelt & George M Church
Nature Biotechnology(2015) http://dx.doi.org:/10.1038/nbt.3412
RNA-guided gene drives capable of spreading genomic alterations made in laboratory organisms through wild populations could be used to address environmental and public health problems. However, the possibility of unintended genome editing occurring through the escape of strains from laboratories, coupled with the prospect of unanticipated ecological change, demands caution. We report the efficacy of CRISPR-Cas9 gene drive systems in wild and laboratory strains of the yeastSaccharomyces cerevisiae. Furthermore, we address concerns surrounding accidental genome editing by developing and validating methods of molecular confinement that minimize the risk of unwanted genome editing. We also present a drive system capable of overwriting the changes introduced by an earlier gene drive. These molecular safeguards should enable the development of safe CRISPR gene drives for diverse organisms.
Figure 1: Mechanism and population-level effect of endonuclease gene drives.close
(a) Homing endonucleases cut competing alleles, inducing the cell to repair the damage by copying the endonuclease gene. (b) By converting heterozygous germline cells into homozygotes containing two copies (teal), gene drives increase
Figure 3: Gene drives and cargo genes remain intact upon copying and can spread by targeting both nonessential and essential genes.close
(a) The ADE2-targeting gene drive was modified to carry URA3 as a cargo gene. (b) Diploids produced by mating wild-typeURA3− haploid yeast with haploids encoding the gene drive carrying URA3 were allowed to sporulate and tetrads dissec…
CRISPR Chain Reaction
A powerful new CRISPR/Cas9 tool can be used to produce homozygous mutations within a generation, but scientists call for caution.
| March 19, 2015 http://www.the-scientist.com/?articles.view/articleNo/42504/title/CRISPR-Chain-Reaction/
A new genetic-editing technique based on integratingCRISPR/Cas9 technology into a Drosophila melanogaster genome can make homozygous mutants in half the time it would take using traditional crosses, according to a paper published today (March 19) in Science.
“The study is well done and also very elegant,” said Ji-Long Liu of the University of Oxford who was not involved in the research, but helped to develop CRISPR/Cas9 in Drosophila. Liu called the method “a really clever way to . . . make the magic happen.”
A rare mosaic female fly, with a lighter left half mutated by MCR and a wild-type darker right half.
UCSD, VALENTINO GANTZ AND ETHAN BIER
Safety upgrade found for gene-editing technique
Tweak reduces chance of a mutation escaping into the wild, and can help to undo a mutation after it has spread.
Heidi Ledford http://www.nature.com/news/safety-upgrade-found-for-gene-editing-technique-1.18799
A method that can spread genetic changes rapidly through populations could aid the fight against the malaria parasite, shown here infecting red blood cells.
A genome-editing method that could allow researchers to rapidly engineer entire populations has had an important upgrade. A US team has added safeguards to reduce the chances that such ‘gene drives’ will escape the laboratory, and found a way to erase the genetic mutations after they have spread.
Gene drives hold the potential to wipe out insect-borne diseases and can speed up some genetic studies in the laboratory. But if released into the wild — whether intentionally or not — gene drives could irrevocably scar entire ecosystems.
The safeguards, published today in Nature Biotechnology1, may calm some fears about the technology. One of the techniques provides a way of genetically separating the components that fuel a gene drive, so that the engineered mutation will not spread as rapidly through a population. Another is a molecular ‘undo’ button: sending a second gene drive out to undo the effects of the first.
“We have a responsibility to keep our experiments confined to the laboratory,” says Kevin Esvelt, an evolutionary engineer at the Wyss Institute for Biologically Inspired Engineering at Harvard University in Boston, Massachusetts, and an author of the paper. “The basic lesson is: if you don’t have to build a gene drive that can spread through a wild population, then don’t.”
New life
The concept of a gene drive is an old one that was given new life by the advent of a genome-editing technique called CRISPR–Cas9. It allows researchers to make targeted changes to a genome with unprecedented ease and versatility.
Esvelt and others quickly realized that this technique could be used to engineer a gene drive by incorporating the genes encoding the Cas9 enzyme, which cuts DNA, and the guide RNAs, which direct Cas9 to a specific site, into the genome. Once present on one chromosome, the system can copy itself and the desired genome modification to the other chromosome, thus racing more rapidly through a population than a mutation would normally spread.
The first demonstration of this was published in March2 by developmental biologists Valentino Gantz and Ethan Bier at the University of California, San Diego. The team used gene drives to speed up genetic studies in certain species of fruit flies. But the publication kicked off concerns that the gene drive might escape from the lab into the wild, and the US National Academies of Sciences, Engineering and Medicine tasked a committee with evaluating the benefits and risks of the technology.
Even so, some researchers have embraced the approach, particularly as a means to prevent the transmission of insect-borne diseases such as malaria, says Esvelt. George Church, a bioengineer also at the Wyss Institute and a co-author on the latest report, predicts that gene drives to wipe out malaria and the tick-borne Lyme disease will be developed within the next two years. Esvelt is also collaborating with tropical-disease specialist Paul Brindley of George Washington University in Washington DC to study the application of gene drive to wiping out schistosomiasis, a disease caused by parasitic trematode worms.
Safety measures
But Esvelt worries that an accident could undermine the technique before it has a chance to prove its worth. “If anyone messes up and a gene drive gets out into the wild, there will be a huge media circus,” he says. “The message will be that scientists cannot be trusted to deal with this technology, and we will be set back by years.”
So he and his colleagues decided to develop safety measures using the yeast Saccharomyces cerevisiae. The organism is easy to work with in the laboratory and unlikely to spread a gene drive into wild populations because of its infrequent sexual reproduction.
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