Too many cancer treatment approaches turn into a game of Whac-A-Mole in which genetic resistance among cancer cells becomes amplified. To play a new, more winnable game, researchers are using genetic circuits to realize evolution-guided anticancer therapies in which diverse forms of resistance are thwarted.
Researchers at Penn State created a modular genetic circuit that turns cancer cells into a “Trojan horse,” causing them to self-destruct and kill nearby drug-resistant cancer cells. Tested in human cell lines and in mice as proof of concept, the circuit outsmarted a wide range of resistance.
The findings were published in Nature Biotechnology. The researchers also filed a provisional application to patent the technology described in the paper.
“We show that tumor evolution can be reproducibly redirected to engineer therapeutic opportunity, regardless of the exact ensemble of pre-existing genetic heterogeneity,” the article’s authors wrote. “We develop a selection gene drive system that is stably introduced into cancer cells and is composed of two genes, or switches, that couple an inducible fitness advantage with a shared fitness cost.
The idea, the scientists explained, is to combine selection (redirecting tumor evolution toward more benign states) with bystander function (taking advantage of the bystander-killing activity that suicide gene–carrying cells have against neighboring cells).
“Our complete dual-switch circuits demonstrate the ability to eliminate preexisting resistance, including complex genetic libraries of resistance variants within a drug target and across the genome,” the authors continued. “Finally, model-guided switch engagement demonstrates robust efficacy in vivo, highlighting the benefits of leveraging evolutionary principles rather than combating them.”
“This idea was born out of frustration,” said Justin Pritchard, PhD, a professor of biomedical engineering at Penn State and the senior author on the paper. “We’re not doing a bad job of developing new therapeutics to treat cancer, but how can we think about potential cures for more late-stage cancers?
“Selection gene drives are a powerful new paradigm for evolution-guided anticancer therapy. I love the idea that we can use a tumor’s inevitability of evolution against it.”
Newer personalized cancer medicines often fail, not because the therapeutics aren’t good but because of cancer’s inherent diversity and heterogeneity, Pritchard explained. Even if a frontline therapy is effective, resistance eventually develops and the medication stops working, allowing the cancer to return. Clinicians then find themselves back at square one, repeating the process with a new drug until resistance emerges again. The cycle escalates with each new treatment until no further options are available.
The gene drive idea, the researchers reasoned, could be used to eliminate resistance mechanisms before cancer cells had a chance to evolve and pop up unexpectedly. Also, it could be used to force a specific resistance mechanism to emerge—one that they would prefer to see, one that they would be prepared to fight.
What started as a thought experiment is proving to work. The team created a modular circuit, or dual-switch selection gene drive, to introduce into non-small lung cancer cells with an EGFR gene mutation. This mutation is a biomarker that existing drugs on the market can target.
The circuit has two genes, or switches. Switch one acts like a selection gene, allowing the researchers to turn drug resistance on and off, like a light switch. With switch one turned on, the genetically modified cells become temporarily resistant to a specific drug, in this case, to a non-small lung cancer drug. When the tumor is treated with the drug, the native drug-sensitive cancer cells are killed off, leaving behind the cells modified to resist and a small population of native cancer cells that are drug-resistant. The modified cells eventually grow and crowd out the native resistant cells, preventing them from amplifying and evolving new resistance.
The resulting tumor predominantly contains genetically modified cells. When switch one is turned off, the cells become drug-sensitive again. Switch two is the therapeutic payload. It contains a suicide gene that enables the modified cells to manufacture a diffusible toxin that’s capable of killing both modified and neighboring unmodified cells.
“It not only kills the engineered cells, but it also kills the surrounding cells, namely the native resistant population,” Pritchard said. “That’s critical. That’s the population you want to get rid of so that the tumor doesn’t grow back.”
The team first simulated the tumor cell populations and used mathematical models to test the concept. Next, they cloned each switch, packaging them separately into viral vectors and testing their functionality individually in human cancer cell lines. They then coupled the two switches together into a single circuit and tested it again. When the circuit proved to work in vitro, the team repeated the experiments in mice.
However, the team didn’t just want to know that the circuit worked; they wanted to know it could work in every way. They stress tested the system using complex genetic libraries of resistance variants to see if the gene drive could function robustly enough to counter all the genetic ways that resistance could occur in the cancer cell populations.
And it worked: Just a handful of engineered cells can take over the cancer cell population and eradicate high levels of genetic heterogeneity. Pritchard said it’s one of the biggest strengths of the paper, conceptually and experimentally.
“The beauty is that we’re able to target the cancer cells without knowing what they are, without waiting for them to grow out or resistance to develop because at that point it’s too late,” Leighow said.
