Engineered tRNA restores a working cystic fibrosis protein in drug-resistant patient tissue

A team of Canadian researchers has engineered a form of transfer RNA that can force human cells to ignore a premature genetic "stop" signal and finish building a protein that would otherwise come out truncated and useless. In laboratory tests on airway tissue from a cystic fibrosis patient whose cells no longer responded to any approved drug, the engineered molecule restored a working version of the protein the disease destroys. The findings were published Thursday in the journal Science by a group based at the University of Toronto, working with clinician-scientists at the Hospital for Sick Children.

The mutation the team targeted, known as a nonsense mutation, inserts an erroneous stop codon partway through a gene's instructions, causing the cell's protein-building machinery to quit early. It is a mechanism shared across a wide range of inherited conditions, not just cystic fibrosis, which is why the researchers describe their approach as a platform rather than a single-disease treatment.

How the therapy works

Every cell already carries transfer RNA, or tRNA, whose job is to ferry amino acids to the ribosome as it reads messenger RNA and assembles proteins. The Toronto team, led by Bowen Li of the Leslie Dan Faculty of Pharmacy and co-led by chemist Haissi Cui, engineered tRNA molecules that recognize a premature stop codon and insert an amino acid there instead of halting translation, allowing the ribosome to keep going and produce a full-length protein.

The idea itself is not new — smaller-scale versions of tRNA "readthrough" therapy have been explored for years — but two problems have kept it out of the clinic: natural tRNA breaks down quickly in the body, and there has been no reliable way to get it into cells in useful quantities. The Toronto group added a chemical modification to the tRNA that made it more stable and more active, and separately synthesized roughly 1,000 structurally distinct lipids to identify a nanoparticle formulation, adapted from the lipid nanoparticles used in mRNA vaccines, capable of carrying the tRNA into cells efficiently. In human airway cells carrying common cystic fibrosis nonsense mutations, the restored version of the protein — CFTR, the ion channel that keeps mucus in the lungs properly hydrated — remained detectable and functional for more than 40 days after a single treatment.

Nonsense mutations account for roughly 11 percent of all inherited genetic disorders, according to the researchers, and because only three stop codons exist in the genetic code, a therapy tuned to overcome one of them can in principle be applied to any gene interrupted by that same signal, regardless of which disease it causes.

A patient who had run out of options

The most striking result in the paper involves not a mouse or a cell line but tissue from a single cystic fibrosis patient, obtained through the Hospital for Sick Children's Program for Individualized Cystic Fibrosis Therapy, run by SickKids scientists Jim Hu and Tanja Gonska. The patient carried four CFTR mutations, two of them nonsense mutations, and had not responded to Trikafta or other CFTR modulator drugs, the class of medicine that has transformed treatment for most people with the disease over the past decade. Cystic fibrosis is an inherited disorder in which defective CFTR protein leaves mucus in the lungs and digestive tract abnormally thick, and an estimated one in ten patients carry disease-causing nonsense mutations that leave them unresponsive to existing modulators.

Neither the engineered tRNA nor Trikafta did much on its own in the patient's cells. Combined, the tRNA restored production of full-length CFTR protein, and Trikafta then had a properly formed protein to act on, improving its function further. The result suggests a combination strategy rather than a replacement for existing cystic fibrosis drugs — the readthrough therapy supplies the protein modulators need in patients whose genetics have shut them out of that class of treatment entirely.

"The same premature stop signal can occur across many genes, potentially enabling one therapeutic strategy for multiple genetic diseases," said Bowen Li, the study's senior author, describing the rationale for pursuing tRNA as a general-purpose platform rather than a single drug.

From the lab bench toward an inhaler

The university's own account of the work emphasizes that the research remains preclinical: it has been tested in cultured airway cells and patient-derived tissue, not in human trials. The team reports one practical step toward eventual use in patients, however — the lipid nanoparticles carrying the tRNA survived being aerosolized into a fine mist without losing activity, a prerequisite for any inhaled treatment that cystic fibrosis patients could someday administer at home rather than through injection or infusion. Chemist Jingan Chen, one of the paper's co-lead authors, put the delivery problem plainly: without a way to get tRNA into cells intact, tRNA "cannot become drugs," no matter how well the molecule itself works in a dish.

The work was funded by the U.S. National Institutes of Health, the Cystic Fibrosis Foundation, Canada's Natural Sciences and Engineering Research Council, the Canadian Institutes of Health Research, the Canada Research Chairs program, the Connaught Fund and the Harrington Discovery Institute, among other sources, according to the paper's funding disclosures. The researchers say their next steps include adapting the lipid nanoparticle delivery system for organs beyond the lungs, since each tissue is likely to require its own formulation, and pursuing the regulatory and manufacturing work needed to move an inhaled cystic fibrosis formulation toward clinical testing.

Because the underlying mechanism is not specific to CFTR, the same engineered-tRNA approach could in theory be retargeted to other genes disrupted by the same class of premature stop codon, a category of mutation implicated in disorders ranging from certain muscular dystrophies to some inherited cancer syndromes. The Toronto team's paper does not test those other diseases directly, and any extension beyond cystic fibrosis would require its own delivery system, its own safety testing and its own path through clinical trials — work the authors describe as only beginning.