University of Toronto Researchers Restore Cystic Fibrosis Protein Function With Inhaled tRNA

A study has found that transfer RNA (tRNA), a cousin of mRNA, could open a new path for treating genetic diseases.

According to U.S. pharmaceutical and biotech news outlet Fierce Biotech on the 28th (local time), researchers at the University of Toronto said they confirmed the safety of an inhaled modified tRNA administered to mice with cystic fibrosis caused by a nonsense mutation. The treatment reached the lungs, remained there for up to 40 days and restored the function of the mutant protein.

The study was published on the 27th in the international journal Science.

A nonsense mutation is a variant that creates a premature stop codon in mRNA, preventing a protein from being made properly. Cells may fail to make the protein altogether or produce only a truncated form, which can lead to disease.

In cystic fibrosis, a nonsense mutation in the cystic fibrosis transmembrane conductance regulator (CFTR) gene is one cause. The genetic disease produces sticky mucus and fluid that block organs such as the lungs and pancreas.

The researchers sought to overcome premature stop codons in cystic fibrosis by harnessing tRNA’s biological decoding ability.

Corresponding author Bowen Li, an associate professor at the Leslie Dan Faculty of Pharmacy at the University of Toronto, explained: “We were able to design tRNA so it could continue reading through a premature stop codon even when a nonsense mutation had occurred within the mRNA. The tRNA can bring in an amino acid to fill the gap and allow protein translation to proceed normally.”

The researchers compared the efficacy of modified tRNA in an organoid model using cells from patients with cystic fibrosis with that of Trikafta, Vertex Pharmaceuticals’ U.S. Food and Drug Administration (FDA)-approved cystic fibrosis treatment.

The results showed that Trikafta alone was not sufficient to restore protein function; the goal of restoring protein function was achieved only when Trikafta and modified tRNA were administered together.

Li said preparations were underway to advance inhaled tRNA into clinical trials “within the next 2-3 years.”

“We are also testing tRNA in the context of other diseases, such as Duchenne muscular dystrophy and Rett syndrome, for which treatment options are currently limited,” he added.

The field of tRNA therapeutics still has a long way to go to catch up with mRNA, which achieved global success with COVID-19 vaccines. Interest in mRNA was rekindled last week when Moderna and its partner Merck announced that a personalized mRNA cancer vaccine, when combined with Keytruda, successfully extended recurrence-free survival in a phase 3 trial involving patients with high-risk skin cancer.

Even so, tRNA has some advantages over mRNA. While mRNA is very transient, Li’s team found evidence that tRNA remained for up to 40 days after inhalation in key epithelial cells in the lungs affected by cystic fibrosis.

In addition, mRNA therapeutics carry a risk that modified mRNA administered to cells could cause them to translate the target protein differently. By contrast, the modified tRNA used in this study enhances endogenous tRNA without introducing an external protein, helping it read through stop codons in endogenous mRNA.

Li noted that in diseases involving very large protein mutations, such as Duchenne muscular dystrophy, delivering the protein via mRNA can make it difficult to improve efficiency because of its size, whereas tRNA does not change in size, so the size of the protein deficiency is not an issue.

“We are rapidly expanding the application of tRNA beyond lung diseases to liver diseases, muscle diseases and some central nervous system disorders, such as autism,” he said.