All the trouble caused by the Pied Piper of Hamelin could have been avoided if only the villagers had access to gene drive technology. Until recently, such technology wasn’t available to us, either. Also, it has been limited to the control of inheritance in insects. But now, thanks to work conducted at the University of California, San Diego, gene drive may soon hasten the development of better animal models in basic and biomedical research. It may even be used, eventually, to suppress out-of-control rodent populations in the wild, or possibly in settled areas, latter-day Hamelins.

“Our motivation was to develop [a form of gene drive] as a tool for laboratory researchers to control the inheritance of multiple genes in mice,” said assistant professor Kimberly Cooper. “With further development we think it will be possible to make animal models of complex human genetic diseases, like arthritis and cancer, that are not currently possible.”

Gene drive is an “active genetics” approach to editing the genome. It can be used to control which of the two copies of a gene is passed to the next generation.

To bring gene drive to mice, a research team led by Cooper engineered an active genetic “CopyCat” DNA element that controls fur color. When the CopyCat element disrupts both copies of the gene in a mouse, fur that would have been black is instead white, an obvious readout of the success of their approach.

Significantly, the CopyCat element also was designed so that it cannot spread through a population on its own. In other words, CopyCat was designed to effectively slip the gene drive into park.

Details about CopyCat appeared January 23 in the journal Nature, in an article titled, “Super-Mendelian inheritance mediated by CRISPR–Cas9 in the female mouse germline.” The article describes how the researchers used an active genetic element that encodes a guide RNA, which is embedded in the mouse tyrosinase (Tyr) gene, to evaluate whether targeted gene conversion can occur when CRISPR-Cas9 is active in the early embryo or in the developing germline.

“Our comparison of eight different genetic strategies indicates that the precise timing of Cas9 expression may present a greater challenge in rodents than in insects to restrict DSB formation to a window when breaks can be efficiently repaired by the endogenous meiotic recombination machinery,” the authors reported. “Nevertheless, the copying efficiencies that we observed here would be more than sufficient for a broad range of laboratory applications.

“For example, the average observed copying rate of 44% using the most efficient genetic strategy in females combined ultra-tightly linked tyrosinase mutations such that 22.5% of all offspring inherited a chromosome with both alleles, which would not be possible through Mendelian inheritance.”

The new approach worked in female mice during the production of eggs, but not during the production of sperm in males. This is possibly due to a difference in the timing of male and female meiosis, a process that normally pairs chromosomes to shuffle the genome and may assist this engineered copying event.

According to UC San Diego professor Ethan Bier, a study co-author, the results “open the way for various applications in synthetic biology including the modular assembly of complex genetic systems for studying diverse biological processes.”

Cooper and members of her lab are now springboarding off this first mammalian active genetic success—based on a single gene—and attempting to expand the tool to multiple genes and traits.

“We’ve shown that we can convert one genotype from heterozygous to homozygous. Now we want to see if we can efficiently control the inheritance of three genes in an animal. If this can be implemented for multiple genes at once, it could revolutionize mouse genetics,” said Cooper.

While the new technology was developed for laboratory research, some have envisioned future gene drives that would build on this approach in the wild for efforts to restore the balance of natural biodiversity in ecosystems overrun by invasive species, including rodents.

“With additional refinements, it should be possible to develop gene-drive technologies to either modify or possibly reduce mammalian populations that are vectors for disease or cause damage to indigenous species,” said Bier.

However, these data also indicate that technical improvements needed for practical use in the wild allow time for careful consideration of which applications of this new technology could and should be implemented. The researchers noted, however, that their results demonstrate a substantial advance that might already decrease the time, cost and number of animals needed to advance biomedical research on human diseases and to understand other types of complex genetic traits.

“We are also interested in understanding the mechanisms of evolution,” said Cooper. “For certain traits that have evolved over tens of millions of years, the number of genetic changes is greater than we can currently assemble in mice to understand what caused bat fingers to grow into a wing, for example. So we want to make lots of these active genetic tools to understand the origins of mammalian diversity.”

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