For thousands of people living with rare genetic disorders, the biggest hurdle is often not diagnosing the disease but finding a treatment that can correct the underlying genetic defect.
Now, researchers at the University of Toronto have offered a ray of hope for such patients, having developed an RNA-based approach that could potentially tackle a broad group of such conditions using a common therapeutic strategy.
As per the findings, published in Science on August 27, the approach uses engineered transfer RNA, or tRNA, to help cells overcome a particular type of genetic error known as a nonsense mutation. These mutations insert a premature “stop” signal into the instructions used by cells to make proteins. As a result, protein production is cut short, leaving the body with little or no functional protein.
The researchers demonstrated that specially engineered tRNA can allow cells to read past these premature stop signals and resume production of a full-length, functional protein.
“There are so many types of disease-causing mutations – many affecting only a small number of people – that developing a separate gene therapy for every individual mutation is extremely challenging,” said study lead Bowen Li, associate professor at the University of Toronto’s Leslie Dan Faculty of Pharmacy and an affiliate scientist at the University Health Network’s Princess Margaret Cancer Centre.
“With tRNA therapeutics, our goal is to develop a common therapeutic approach that could potentially address the same type of mutation across many different genes and diseases, including rare conditions that currently have few or no effective treatment options.”
Nonsense mutations are estimated to account for about 11% of inherited genetic disorders. While the proportion may appear modest, such mutations occur across thousands of rare diseases, including some forms of cystic fibrosis and certain muscular and neurological disorders.
The significance of the work lies in targeting the type of mutation rather than a single disease, said the authors.
In conventional genetic medicine, a therapy may need to be designed around a particular gene or mutation. That becomes difficult when a disease has numerous rare mutations, each affecting only a small number of patients.
In contrast, the tRNA approach seeks to overcome that limitation by recognising premature stop signals shared by different genes.
“The same type of premature stop signal can occur in many different genes, causing diseases that affect the lungs, brain, muscles and other tissues,” Li said. “Our long-term goal is to develop tRNA medicines that recognise these shared stop signals so that one therapeutic strategy could potentially be applied across many different genetic diseases.”
The team initially tested the strategy in cystic fibrosis, where some patients carry nonsense mutations in the CFTR gene.
Cystic fibrosis treatment has changed significantly with the arrival of CFTR modulators such as Trikafta. These drugs can improve the function of defective CFTR proteins in many patients.
But they cannot help patients whose mutations prevent the protein from being produced in the first place.
That is where the engineered tRNA could offer a different route.
In human airway cells carrying two common nonsense mutations, the researchers found that the modified tRNA restored production of full-length CFTR protein, and, importantly, the protein was functional. The restored protein also persisted for more than 40 days in the laboratory experiments.
The researchers then moved beyond isolated cells.
Scientists Jim Hu and Tanja Gonska, both associated with SickKids and the University of Toronto’s Temerty Faculty of Medicine, provided access to tissue from a cystic fibrosis patient carrying four CFTR mutations, including two nonsense mutations. The patient's cells had not responded to existing drugs.
The team grew the cells into miniature laboratory models known as organoids.
The results suggested that the combination of the engineered tRNA and Trikafta could be more effective than either approach alone. While neither treatment produced a strong response independently, the combination restored full-length CFTR production and gave the existing drug a functional protein to act on.
However, much more work will be required to establish safety, determine appropriate dosing, understand how the therapy behaves in humans and demonstrate whether the laboratory and preclinical findings translate into meaningful clinical benefits, said the authors.
