A CRISPR gene drive for mice is one step closer to reality

Scientists are getting closer to creating a genetic pest-control measure against rodents.

Female mice engineered to carry a genetic cut-and-paste machine called a gene drive may be able to pass a particular version of one gene on to more than 80 percent of their offspring, researchers report January 23 in Nature. That rate would beat the usual 50 percent chance of handing down a gene variant, first reported in 1865 by Gregor Mendel from his studies of peas.  

“What we’ve done is engineered a gene that can be inherited more frequently than it would be by normal Mendelian inheritance,” says Kimberly Cooper, a developmental geneticist at the University of California, San Diego. “My graduate student likes to call it ‘cheating Mendel.’”

Such engineered genetic cheats have been proposed to wipe out disease-carrying mosquitoes and invasive species by targeting genes involved in reproduction (SN: 12/12/15, p. 16). Gene drives might also be used to prevent pests from carrying diseases, such as malaria (SN: 12/26/15, p. 6). Researchers have made successful gene drives in laboratory experiments in mosquitoes, fruit flies and yeast.

But no one has yet built one that works in a mammal. And Cooper isn’t claiming to have done so either. By definition, a gene drive must cheat Mendelian inheritance rules over multiple generations, spreading itself to an entire population. Cooper’s group has produced one generation of genetic cheater mice, but hasn’t yet tracked the gene drive’s spread through multiple generations.

“It’s the prototype experiment you need for a drive in rodents,” says Thomas Prowse, a population ecologist at the University of Adelaide in Australia who was not involved in the study.

Gene drives use a molecular scissors known as CRISPR/Cas9 to insert themselves into a particular site in an organism’s DNA. A gene drive usually contains instructions for making the Cas9 enzyme, which cuts DNA, and a guide RNA that shepherds the enzyme to a particular gene. When Cas9 slices the DNA, cells can repair the break by copying the version of the gene containing the gene drive from its sister chromosome. That copying ensures that all offspring will inherit the gene drive.

Cooper and colleagues built half of a gene drive: They inserted instructions for making a guide RNA, but not the Cas9 enzyme, into the gene known as Tyr in mice. The Tyr gene produces the tyrosinase enzyme involved in making pigments. The guide RNA is designed to lead Cas9 to any normal copies of the gene, so that it can be cut and converted to the gene drive version. But this modified gene drive, called “CopyCat,” couldn’t actually cut the mice’s DNA. 

Instead, those mice were bred to mice engineered to carry the gene for making Cas9 on another chromosome. In the lab, only mice that inherited both the CopyCat guide RNA and Cas9 together could cut DNA, creating that first generation of genetic cheater mice. Since wild mice don’t naturally make Cas9, the gene drive as it is currently designed wouldn’t be able to spread.

CopyCat also works only in female mice. Bad timing may be the reason the gene drive didn’t go anywhere in male mice, Cooper says. The researchers discovered that the gene drive worked as intended only during a very narrow window of time just before eggs and sperm are made. That window happens during meiosis, when half of an organism’s chromosomes are put into eggs or sperm. Before the chromosomes are divided up, sister chromosomes pair up and exchange information in a process called recombination.

That’s also the perfect time to cut DNA and copy a gene drive, Cooper and colleagues found. CopyCat converted its sister chromosome to carry the gene drive up to 72 percent of the time, which would result in an estimated 86 percent of offspring inheriting the drive, the researchers report. While that’s enough to beat Mendel’s inheritance rules, it falls far short of the 95 percent or better copying rate and nearly 100 percent inheritance seen in some mosquito experiments (SN: 10/27/18, p. 6).

In female mammals, sister chromosomes are paired during recombination for longer than in males, allowing the gene drive more time to work, Cooper says. Cutting when the sister chromosomes aren’t paired resulted in mutations that destroy the Cas9 cutting site, making it off-limits to the enzyme in the future. Even in female mice, cutting created such resistance mutations up to 25 percent of the time.

That rate is “quite high,” says Prowse, and would slow or halt spread of the gene drive in the wild if it were inherited (SN Online: 7/20/17). Cooper and colleagues hope to adjust the timing of when Cas9 makes cuts in male mice so that both sexes can pass along the gene drive.

A rodent gene drive might one day be used to eliminate invasive rodent species that can destroy island habitats and sometimes overrun crops. As is, the gene drive would not work in the wild, says Bruce Conklin, a human geneticist at the University of California, San Francisco, who wrote a commentary in the same issue of Nature. “By no means is this ready to deploy.” But the researchers have taken an important first step in creating a gene drive that could work in mice and possibly other mammals, he says.

Cooper isn’t worried about making a gene drive for use outside of the lab in the near future. Instead, her goal is to create a tool that makes it easier to engineer mice for research, she says. Scientists could insert several human genes into such rodents at the same time, making mice that might better mimic human diseases.