Biomedical engineer Bowen Li and his colleagues at the University of Toronto have demonstrated that suppressor transfer RNAs (tRNAs) can restore production of a protein associated with cystic fibrosis in several model systems (Science 2026, DOI: 10.1126/science.aeb0054). The results are another application for gene therapy, a technology that recently has had high-profile successes—and failures. This latest advance relies on the chemical modification of suppressor tRNAs and bespoke lipid nanoparticles (LNPs) to deliver them.
There are many mutations that can lead to cystic fibrosis. Some of the most common result in premature stop codons in the cystic fibrosis transmembrane conductance regulator (CFTR) gene. Stop codons tell ribosomes when to finish translating messenger RNA into protein, so a premature stop codon results in an incomplete and often nonfunctional protein.
One possible fix for these mutations is to introduce engineered suppressor tRNAs that are aminoacylated by an appropriate amino acid. The tRNA pair with a premature stop codon and, in theory, the ribosome places the associated amino acid into the growing protein to continue translation instead of the process abruptly ending.
But tRNAs made in the lab don’t work very well because they lack the suite of modifications present on natural tRNAs. So Li’s team tested which of those modifications were required and found that adding a single methyladenosine (M1A) marker to synthetic tRNA greatly enhanced its ability to be used by ribosomes. “If you put that M1A modification at the 57 or 58 position of the tRNA, it can very efficiently increase the read-through activity, prolong its functional persistence, and reduce its innate immune activation,” Li says.
Delivering those suppressor tRNAs is another challenge. LNPs are built from ionizable lipids, which when absorbed into a cell’s acidic endosomes, break apart to release their cargo. But Li says that the most commonly used LNP formulations have generally struggled to deliver suppressor tRNAs in host cells effectively.
The researchers in Li’s lab screened over 1,000 ionizable lipids and found one, which they named TTP-3, that was far superior at delivering tRNAs than other industry-standard lipids.
In experiments in epithelial cells and mouse models, the combination of the modified suppressor tRNA and TTP-3-based LNP resulted in restoration of the CFTR protein. In patient-derived intestinal organoids, Li didn’t see as much success until his lab combined the suppressor tRNA therapy with Trikafta, Vertex Pharmaceutical’s US Food and Drug Administration–approved therapy for cystic fibrosis; the combination of the two treatments was superior to either treatment alone. “When we treat patient cells with the CFTR mutation with our modified tRNA, the CFTR protein can be restored for more than 7 weeks . . . longer than unmodified tRNA and also much longer than mRNA,” Li says.
Jeff Coller, an RNA scientist at Johns Hopkins University who was not involved with the new study, says that “these tRNA approaches, are really going to be transformative for genetic disorders” and that developing a suppressor tRNA–specific LNP that can facilitate passage through the mucosa is “a very important game changer.”
Crucially, introducing suppressor tRNAs into cells doesn’t result in the production of proteins that are longer than intended. “The human body has a way to discriminate between a premature termination codon and a normal termination codon,” says Coller, who is the scientific cofounder of Tevard Biosciences, which also works on suppressor tRNA therapies.
Li says the benefit of a suppressor tRNA–based platform over other gene therapies is that “it doesn’t permanently change or affect the genome. . . . If you observe any abnormal phenomena in the patient, you can just easily stop it.”
In addition to suppressor tRNAs for cystic fibrosis, Li is working on suppressor tRNAs for Duchenne muscular dystrophy and Rett syndrome. “We are raising funds to support the clinical translation of this platform,” he says. “What I foresee is that in 2 or 3 years we’ll be able to test it in the human body.”
