Illustration of the CRISPR-Cas9 gene editing complex (pink and purple) bound to DNA (helix)
Science Photo Library/Alamy
Imagine if, instead of delivering a leaflet individually to each home, a postal worker simply had to give one to a volunteer on each block, who then copied it and distributed copies to neighbors. That postal worker would get flyers into dramatically more homes that way. Biologists hope a similar approach could make gene editing better at treating all kinds of conditions.
The idea is that each cell in the body that receives the first delivery will make many copies of the gene-editing machinery and pass most of them on to its neighbors, amplifying the effect. This means that disease-correcting changes can be made to the DNA of several cells.
In tests on mice, Wayne Ngo of the University of California, Berkeley, and his colleagues—including CRISPR gene-editing pioneer Jennifer Doudna—have been able to triple the number of liver cells edited using this approach.
“Essentially, what we’re doing is we’re instructing the first cell that receives our instructions to make a little lipid particle that packages (the CRISPR machinery) into it, so then the first cell becomes a factory that can then send these little packages to other cells,” says Ngo.
The first approved CRISPR treatment, for sickle cell disease, involves removing blood stem cells from an individual and editing them outside the body before replacing them. But this is a personal treatment, and is therefore extremely expensive. A number of ongoing trials involve directly editing cells in the body instead using a gene editor that will work for many people.
The big challenge is finding ways to deliver the CRISPR machinery to a high enough proportion of specific cells in the body. “To cure sickle cell disease, we know we need to edit about 20 percent of (blood) stem cells,” says Ngo. “That 20 percent has been very, very hard to hit.”
This means that if an initial delivery reached only 10 percent of the blood stem cells, but it could be boosted locally to reach 30 percent, it could make the difference between success and failure.
To achieve amplification, Ngo turned to a protein that helps a virus sprout from cells. Once made in a cell, these proteins connect both with the cell membrane and with each other, forming a small sac, or vesicle, that breaks off from one cell and can fuse with others.
If these viral proteins are physically attached to the CRISPR Cas9 gene-editing protein, the Cas9 protein—and the RNA that guides it to its target—will be packaged into the vesicles and transported to other cells.
To test the idea, the team created a piece of DNA that codes for the Cas9 viral proteins. When the DNA was injected under pressure into the livers of mice, it entered only 4 percent of the cells, but overall 12 percent of the cells were gene-edited.
To treat humans, the gene-editing machinery will be delivered by other means. The injection method was only used as a proof of principle. “It’s not particularly efficient, but it shows that our system is making a difference,” says Ngo. “Triple potentiation is a great place to start. I think it makes some of our current delivery systems good enough to treat some diseases. More can get better, and so we’re actively exploring strategies to do that as well.”
Besides greater efficiency, amplified gene editing may also allow the use of lower doses, which would make treatments safer.
Biologists have been investigating these vesicle-sprouting approaches for decades, says Gaetan Burgio of the Australian National University in Canberra, but Ngo’s team may be the first to show that it works in animals for gene editing. However, Burgio says the researchers have more work to do to confirm the results. “Proper checks and measures need to be carried out to really demonstrate their claims,” he says.
There are already experimental self-amplifying mRNA vaccines, where mRNAs delivered to the cells code for machinery that makes multiple copies of the vaccine’s mRNA. The idea is to make mRNA vaccines safer and cheaper, because lower doses are needed. In this case, however, the extra mRNAs remain inside the cells where they are made.
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