Gene therapy is a lot like landing a Mars rover.
Hear me out. The cargo—a rover or gene editing tools—is stuffed inside a highly technical protective ship and shot into a vast, complex space targeting its destination, be it Mars or human organs. The cargo is then released, and upon landing, begins its work. For Perseverance, it’s to help seek signs of ancient life; for gene editors, it’s to redesign life.
One critical difference? Unlike a Mars mission’s “seven minutes of terror,” during which the entry, descent, and landing occur too fast for human operators to interfere, gene therapy delivery is completely blind. Once inside the body, the entire flight sequence rests solely on the design of the carrier “spaceship.”
In other words, for gene therapy to work efficiently, smarter carriers are imperative.
This month, a team at Harvard led by Dr. David Liu launched a new generation of molecular carriers inspired by viruses. Dubbed engineered virus-like particles (eVLPs), these bubble-like carriers can deliver CRISPR and base editing components to a myriad of organs with minimal side effects.
Compared to previous generations, the new and improved eVLPs are more efficient at landing on target, releasing their cargo, and editing cells. As a proof of concept, the system restored vision in a mouse model of genetic blindness, disabled a gene associated with high cholesterol levels, and fixed a malfunctioning gene inside the brain. Even more impressive, it’s a plug-and-play system: by altering the targeting component, it’s in theory possible for the bubbles to land anywhere in the body. It’s like easily rejiggering a Mars-targeting spaceship for Jupiter or beyond.
“There’s so much need for a better way to deliver proteins into various tissues in animals and patients,” said Liu. “We’re hopeful that these eVLPs might be useful not just for the delivery of base editors, but also other therapeutically relevant proteins.”
“Overall, Liu and colleagues have developed an exciting new advance for the therapeutic delivery of gene editors,” said Dr. Sekar Kathiresan, co-founder and CEO of Verve Therapeutics, who was not involved in the study.
The Delivery Problem
We already have families of efficient gene editors. But carriers have been lacking.
Take base editing. A CRISPR variant, the technology took gene editing by storm due to its precision. Similar to the original CRISPR, the tool has two components: a guide RNA to hunt down the target gene and a reworked Cas protein that swaps out individual genetic letters. Unlike Cas9, the CRISPR “scissors,” base editing doesn’t break the DNA backbone, causing fewer errors. It’s the ultimate genetic “search and replace,” with the potential to treat hundreds of genetic disorders.
The problem is getting the tools inside cells. So far, viruses have been the go-to carrier, due to their inherent ability to infect cells. Here, scientists kneecap a virus’s ability to cause disease, instead hijacking its biology to carry DNA that encodes for the editing components. Once inside the cell, the added genetic code is transcribed into proteins, allowing cells to make their own gene editing tools.
It’s not optimal. Viruses, though efficient, can cause the cells to go into overdrive, producing far more gene editing tools than needed. This stresses the cell’s resources and leads to side effects. There’s also the chance of viruses tunneling and integrating into the genome itself, damaging genetic integrity and potentially leading to cancer.
So why not tap into a virus’s best attributes and nix the worst?
Enter eVLPs
eVLPs are like their namesakes: they mimic viral particles that are efficient at infecting cells, but cut out the dangerous parts: DNA. Picture a multi-layered pin cushion, but with an empty cavity to hold cargo.
Unlike viruses, these bubbles don’t carry any viral DNA and can’t cause infections, potentially making them far safer than viral carriers. The downside? They’re traditionally terrible at carrying cargo to its targets. It’s akin to a spaceship with awful homing machinery that crashes into other planets and causes an unexpected wave of disaster. They’re also not great at releasing the cargo even on the target site, trapping CRISPR machinery inside and making the whole gene-editing fix moot.
In the new study, Liu’s team started by analyzing those pain points. By limiting proteins inside the eVLPs that act as the carrier’s “safety belts,” they found, it’s easier for the cargo—the base editor proteins—to release. How they packed the cargo inside the particle bubbles also made a difference. The balance between the two—seat belt to protein passengers—seems to be key to protecting the cargo but allowing them to quickly bail when needed. And finally, dotting the outer shell of the spaceship with specific proteins helps the spaceship navigate towards its designated organ.
In other words, the team figured out the rules of the game. “Now that we know some of the key eVLP bottlenecks and how we can address them, even if we had to develop a new eVLP for an unusual type of protein cargo, we could probably do so much more efficiently,” said Liu.
The result is that a carrier can pack 16 times more cargo and up to a 26-fold increase in gene editing efficacy. It’s a “fourth-generation” carrier, said the authors.
Navigating Biospace
After first testing their new molecular spaceship in cultured cells in the lab, the team moved on to treating genetic disorders. They targeted three different biological “planets”—the eye, liver, and brain—showcasing the flexibility of the new carrier.
In mice with an inherited form of blindness, for example, the carrier was loaded with the appropriate gene editing tools and injected into a layer of tissue inside the eye. In just five weeks, the single injection rebooted retinal function to a point that—based on previous studies from the same lab—can restore the mice’s ability to see.
In another study, the team focused on a gene that often leads to brain disorders. Because of a tough barrier between the brain, blood, and other tissues, the brain is a notoriously difficult organ to access. With the new eVLP spaceship, the gene editors smoothly sailed through the barrier. Once inside brain cells, the tools had a roughly 50 percent chance of transforming damaged genes.
As an additional proof of concept, the new carriers honed in on the livers of mice with cholesterol problems. One injection amped up the mice’s ability to produce a protective molecule that thwarts heart disease.
Better Safe Than Sorry
Gene editing has always been haunted by the ghost of off-target effects. Using viruses to deliver the tools, for example, runs those risks as they last a long time, potentially overwhelming cells.
Not so for the new eVLPs. Because they’re completely engineered, they carry zero viral DNA and are safer. They’re also highly programmable—just a few changes to the targeting proteins can shift them towards another docking location in the body.
For the next step, the team is engineering better “seat belt” proteins inside the carriers for different molecules—either gene editors or therapeutic proteins such as insulin or cancer immunotherapies. They’re also further unpacking what makes eVLPs tick, aiming for next-generation carriers that can explore every nook and cranny of our bodies’ complex universe.
Image Credit: nobeastsofierce / Shutterstock.com
