Editor’s note: This is a continuation of an article first published here Oct. 3.
In June in Nantucket, Mass., about two dozen of the island’s 10,000 residents turned out for a community meeting at which an evolutionary biologist from Harvard University sounded them out on their willingness to let him release a few hundred thousand mice on a small, uninhabited island nearby. His reason: to reduce the incidence of — and perhaps ultimately eliminate — Lyme disease.
The debilitating illness affects nearly 40 percent of Nantucket’s inhabitants. It’s transmitted to humans when they’re bitten by ticks that have acquired the Lyme virus as larvae feeding on white-footed mice, the pathogen’s reservoir. Lyme is prevalent throughout much of the United States, especially in the Northeast and Upper Midwest.
Nantucket and the islands off Cape Cod already have plenty of indigenous mice, as well as deer on which the ticks lay their eggs. Thinning a local deer herd is one method of controlling Lyme, but it’s controversial, particularly among deer lovers. Mice have fewer partisans. The rodents that biologist Kevin Esvelt proposed to introduce would be genetically engineered to resist the virus or the protein in the tick’s saliva that carries it. Within a year or two, he explained, the transgenic mice and their offspring — who’d inherit the Lyme-unfriendly trait — would largely supplant the population of native mouse incubators.
If it worked on the little island, he explained, Nantucket would come next. Ultimately, perhaps, the world.
Because of the cost of laboratory research and development, the need for more such hearings to gain essential community buy-in, and the required permitting, Esvelt anticipated that it might be another 10 years before his project could be implemented. But in Piracicaba, Brazil, about 100 miles northwest of Sao Paulo, an American company named Oxitec is already breeding and releasing some 4 million mosquitoes every week into a country in which diseases carried by mosquitoes — dengue fever, malaria, chikungunya, Zika and others — sicken more than 1.5 million people annually.
Oxitec’s mosquitoes are quite harmless. They’re all male. Males of their species, Aedes aegypti, do not bite human beings. Only females sip human blood and in the process pass along pathogens. But the genome of Oxitec’s males has been altered so that when they mate with wild females, the offspring will all acquire a protein that will kill them off prematurely. Oxitec claims that on its first test site in the Cayman Islands the mosquito population was reduced through this post–birth control method by 96 percent.
These are just two examples of the game-changing applications of a rapidly developing technology called genome editing. (The game is life.)
In the 21st century, for the first time in the planet’s history, human beings possess the capability to directly add, subtract or modify specific genes sprinkled among the chromosomes of any of the multicelled organisms they share Earth with. That is, scientists can now reformulate the biochemical operating instructions in the nucleus of the cells, the DNA, that directs everything from how an organism looks to how it behaves.
Techniques for engineering genetic transformations in the laboratory depend on pinpointing a certain sequence of base chemicals identified as the target gene in the coil of DNA, then dispatching an enzyme hitched to a piece of RNA that acts as a guide. The RNA seeks out the sequence of nucleotides it’s meant to bind with; the passenger enzyme, or nuclease, goes to work severing the DNA helix there, removing the unwanted segment, substituting a corrected replacement and zipping the rejiggered helix together again.
Modifying the genome of a species requires exhausting repetitions of this process. And it requires the help of natural reproduction to spread the synthetic gene throughout subsequent generations. Even then, the results can be transitory or confined to a limited habitat. Unfortunately, altered traits more often than not weaken an organism’s evolutionary fitness. In time, they’ll be winnowed out by natural selection.
These are problems for several of the ingenious tools for targeted genome editing devised by scientists over the past decade. Zinc finger nucleases (so-called because the metallic ion is what gives these proteins their ability to bind with DNA) and transcription activator-like effector nucleases are engineered enzymes that have already been used successfully to boost the nutritional quality of soybean oil, stimulate production of biofuel, create human cell lines tailored for research on drugs, and breed “knockout” rats and mice (with a gene of interest deleted) as models for the study of human gene function.
These tools have also been applied experimentally to correct underlying genetic errors that cause human diseases like sickle cell anemia and xeroderma pigmentosa, and to give the immune system of cancer patients the ability to generate T cells that have anti-tumor properties and resistance to chemotherapy agents.
ZFNs and TALENs are time-consuming to assemble, however, and are expensive. And they’re not altogether reliable at hitting the targeted nucleotides. But in 2012, scientists recognized that a natural capability in microbes to protect against invading viruses could be harnessed to edit the genome of any organism with astonishing simplicity, rapidity and dead-on accuracy.
Called CRISPR (which stands for clustered regularly interspaced short palindromic repeats, explained in greater detail Oct. 3), and with one of the 70 associated proteins the clusters mobilize, labeled Cas9, the mechanism provides a new tool that is much cheaper and much faster than TALENs. It has been grabbed up by biotechnicians around the world and put to work, successfully — although still only experimentally — modifying the genomes of all kinds of fauna and flora.
What’s more, genomic engineering using the CRISPR Cas9 nuclease lends itself practically to the achievement of an epic goal: diffusing a beneficial (to humans) human-engineered trait throughout an entire wild population. Permanently.
Drive, they said
That’s what scientists like molecular biologist Anthony James, of the University of California, Irvine are trying to do for mosquitoes. His lab has been working on endowing the dangerous little pests with a gene that will produce antibodies to kill the malaria and dengue fever viruses they now harbor.
To disseminate that trait among all the world’s mosquitoes, though, he needed what is called a gene drive: He had to configure a “selfish” gene that can tilt the odds of inheritance in its favor, even if in fact it reduces the reproductive fitness of the individual that carries it. These exist in nature. CRISPR Cas9 makes it feasible for genetic engineers to cook up and deploy manmade gene drives that, in time, will have altered every member of the species.
Which is fine, theoretically, if what you end up with is virus-free mice or mosquitoes. But do you really want clouds of annoying, painfully nipping, itch-provoking insects — no matter how harmless otherwise — buzzing around you at all? What if you could rid the world of mosquitoes altogether? Cas9 gene drive appears to make that a realistic scenario.
Late last year, James and his research team reported they’d successfully used the technology to endow a line of mosquitoes with resistance to the malaria parasite — a trait passed on to more than 99 percent of the progeny of the wild insects they were mixed with. But at Imperial College London, biologists Tony Nolan and Andrea Crisanti have taken an even more extreme approach to vector suppression: They’ve applied CRISPR Cas9 to engineer into a malaria-carrying species of mosquito something called a crash drive. It’s a gene that, passed on to their progeny, renders females infertile. It took only four generations (and mosquito life cycles are measured in weeks) for more than three-quarters of the wild mosquitoes they studied to carry the infertility gene.
Next stop: extinction.
Is that a good thing? (Important note: So far these have all been well-contained lab experiments.)
Opinions differ. There are those who say humans shouldn’t play God. And they point out that eliminating any species has ecological ripple effects. Mosquitoes and their larvae are a staple of the diet of many fish, insects, spiders, salamanders, lizards, frogs and birds. It's likely, however, that many mosquito-eaters would adapt and find substitutes, scientists say. In fact, for most, the burden of disease weighs heavily against any defense of the mosquitoes.
The big worry, of course, is that now that tools of such unprecedented power are in scientists' hands, they’ll be used recklessly. Or unethically.
Last year, researchers in China made edits in the genome of nonviable human embryos in studies of the blood disorder beta thalassemia and of HIV resistance. Swedish experimenters are using CRISPR Cas9 to knock out genes in human embryos that could be viable (but won’t be allowed to develop beyond 14 days) to study fetal development. A team at the University of California, Davis is modifying pig embryos to grow transplantable human pancreases. (And watching cautiously to make sure the pigs don’t grow human brains as well. Seriously.)
“The ability to edit populations of sexual species would offer substantial benefits to humanity and the environment,” wrote Esvelt and colleagues Andrea Smidler, Flaminia Catterria and George Church in an influential review of the technology. “For example, RNA-guided gene drives could potentially prevent the spread of disease, support agriculture by reversing pesticide and herbicide resistance in insects and weeds, and control damaging invasive species.
“However, the possibility of unwanted ecological effects and near-certainty of spread across political borders demand careful assessment of each potential application. We call for thoughtful, inclusive and well-informed public discussions to explore the responsible use of this currently theoretical technology.”
Less and less theoretical by the day.
David Ollier Weber is a principal of The Kila Springs Group in Placerville, Calif., and a regular contributor to H&HN Daily.