Before 2020, you’d be hard-pressed to have found talk of mRNA therapies in the media. And while not a new technology, having been identified in 1961, it was thrown into the spotlight with the lightning-fast rollout of COVID vaccines, heralded as an incredible scientific feat and simultaneously maligned as haphazard experimental medicine.
The vaccines were the first broadly successful therapy using mRNA technology, but researchers have been trying to master targeted treatments for decades, too. Now scientists believe they’re on the cusp of the next stage, with the development of a novel engineered RNA sense-and-respond circuit they’ve named Detection and Amplification of RNA Triggers via ADAR, or DART VADAR, which seeks out a specific molecular marker of disease or cell type involving the RNA-editing enzyme ADAR for highly specialized treatment.
“I am particularly excited by the fact that our DART VADAR system is a clinically relevant, compact RNA-based circuit that enables one to direct therapies in a highly programmable manner to specific cell types and cells in certain states, thereby minimizing off-target effects,” said Jim Collins, core faculty at Wyss Institute at Harvard University.
More than 12 billion doses of COVID mRNA vaccines have entered arms since they were shipped out, but they elicit a full-body immune response. Engineering RNA to only affect a single organ or one cell type – without being swiftly dealt with by the immune system or causing inflammation outside of the target – has been the biggest challenge.
Messenger RNA molecules carry the genetic information needed to make proteins. But their delivery has been difficult to master, as the strands of RNA degrade quickly in the body and only recent developments in nanoparticles have made targeted therapies potentially feasible.
“Our technology grew from the idea that we could decouple the elements of responsive RNA sensors – sensing, actuation, etc. – so it’s much easier to design circuits for new targets,” said co-first author Raphael Gayet, research scientist at the Wyss Institute. "Ideally, we wanted to be able to change the payload without modifying the sensor element every time."
The ADARs in the DART VADAR system are found at high concentrations in neurons, but at low levels in other cells. To ensure the sensor could work in different cell types, the researchers added the sequence of the ADAR gene to their RNA sensors. Activation of the sensor by natural ADAR could then produce more, creating a positive feedback loop to boost the sensor’s activity (in this case, activity of the signal molecule could be observed as it showed up as a fluorescent green protein).
“What’s really exciting about this sensor is that the green protein signal sequence can be easily replaced with the sequence for any therapeutic gene that you want to express in response to the presence of a trigger RNA in the cell,” said co-first author Shiva Razavi, from MIT. “So not only does this sensor detect targets, it can automatically respond to them without requiring user input, automating the delivery of a therapeutic payload at the cellular level.”
The team then tested the DART VADAR system to see if it could detect a single-base mutation in the human p53 tumor suppressor gene, something that would be vital to deliver targeted therapies to cancer patients. Introducing DART VADAR to a line-up of human cells as well as the mutation, the sensor easily and precisely identified the anomaly. DART VADAR then detected molecular differences in cells at different stages of development based on their age markers.
In terms of real-world application, the team is set to now use DART VADAR with stem cells and their differentiation into other cells, which may one day be used to replace diseased cells in patients. With its promise of being a much more effective way to deliver therapies to patients, DART VADAR is a step forward in surmounting the RNA roadblocks that have plagued scientists for decades.
“This team’s ability to combine preexisting biological components into a completely new engineered technology that has the potential to make the treatment of a wide range of diseases faster and easier is a great example of how synthetic biology can change the world for the better,” said Don Ingber, founding director of Wyss Institute.
The study was published in the journal Nature Communications, and you can see the team's earlier work that set them on the path of the new detect and amplify system in the video below.
