Same mechanism, different gene
CMT is a hereditary form of peripheral neuropathy. In the disease, nerves fail to properly transmit signals for movement and sensation. This leads to complaints in the hands, feet, and lower legs. There is currently no effective treatment for this progressive disease. CMT is estimated to affect one in 2,500 people. The CMT-YARS1 form is estimated to occur in about 75 people in the Netherlands.
By now, around 120 genes have been linked to the disease, seven of which have mutations in tRNA synthetases. Storkebaum's research group focuses specifically on these seven genes.
In 2021, the team already reported in Science that a form of CMT caused by mutations in the glycyl-tRNA synthetase gene (GARS1) arises from a disruption of protein synthesis. The new study shows that the same mechanism also underlies another form, CMT-YARS1, caused by mutations in the tyrosyl-tRNA synthetase gene (YARS1).
"The question was whether a similar mechanism also underlies other forms of CMT caused by mutations in tRNA synthetases," says Storkebaum. "Our results show that this is indeed the case. That suggests that possibly even more forms of CMT work according to the same mechanism."
Disrupted protein production
Our DNA contains the instructions for making all the proteins in our body. To make proteins, this information is first copied into a temporary copy: messenger RNA (mRNA). The ribosome, the cell's protein factory, then reads the mRNA and uses tRNAs to link amino acids together one by one into a complete protein.
In patients with CMT-YARS1, this process does not work properly. Due to a mutation in the YARS1 gene, the TyrRS enzyme is defective. The researchers discovered that this defective enzyme can still bind the corresponding tyrosyl-tRNA, but no longer release it afterward.
"This creates a shortage of freely available tyrosyl-tRNA, which hampers the ribosome when assembling proteins," Storkebaum explains. Protein production becomes disrupted at the moment tyrosyl-tRNA needs to be added, which ultimately leads to damage to nerve cells.
Using a technique called "ribosome profiling," which allows precise determination of where the ribosome is located on an mRNA molecule, the researchers, in collaboration with the University of Hamburg, were also able to demonstrate this mechanism biochemically. They observed that ribosomes stalled more frequently at sites where tyrosyl-tRNA is needed.
Gene therapy offers a solution
The discovery of this mechanism pointed the researchers directly toward a possible solution. If a shortage of free tyrosyl-tRNA is the cause of the disease, increasing the amount of available tRNA could be a solution.
The researchers therefore developed a gene therapy that specifically increases the production of tyrosyl-tRNA in nerve cells. Indeed, in fruit fly models of CMT-YARS1, transgenic overexpression of tyrosyl-tRNA prevented both the nerve damage and the underlying defects in protein synthesis.
The gene therapy was subsequently tested in a mouse model in which the human disease-causing mutation was introduced into the mouse gene. "The animals develop symptoms similar to those in people with this condition. In these animals, we used a virus, an AAV-9 vector, to increase the amount of tyrosyl-tRNA in the cells," says Storkebaum. "We administered that vector to newborn mice before they develop the disease. These mice then did not develop peripheral neuropathy. Thus, with this approach, we can prevent the disease."
Impact
"At the moment, there is no effective therapy for any form of CMT," says Storkebaum. "So if we manage to develop a working therapy for one form, that is of course a major breakthrough. I am convinced that other genetic forms can then follow."
Further development of the therapy is proceeding via XtRNA Bio B.V., the spin-off company founded in 2024 as a result of Storkebaum's research into the GARS1 form of CMT.
"Strategically, we first focus on the therapy for the glycyl-tRNA synthetase form of CMT. If that proves successful, we can apply the same approach to other forms of the disease," Storkebaum says. "In effect, you then reuse the same principle: using the same viral vector to increase the amount of a specific tRNA in the cell. That should also work in patients with mutations in the tyrosyl-tRNA synthetase gene and possibly in the other five CMT forms caused by mutations in tRNA synthetases."
In addition, the team is investigating whether the therapy is also effective when administered after the first disease symptoms have appeared.
