A team of researchers from Harvard Medical School, Massachusetts Eye and Ear, and The Ohio State University has recently achieved a significant milestone in the development of gene therapy for Usher Syndrome type 1F.
This rare yet debilitating genetic disease is known to cause deafness, a lack of balance, and progressive blindness. The research team's discovery marks a crucial first step toward the treatment and eventual cure of this condition.
The study, published in the journal Nature Communications on April 26, details research conducted on mice.
The researchers designed a "mini-gene," which is a truncated version of the gene that is typically mutated in cases of Usher Syndrome type 1F. This mutation causes the hair cells within the inner ear to be unable to produce a protein that is essential to sound transmission. In the mice, the introduction of the mini-gene led to an increase in the production of the missing protein, which allowed the hair cells to detect sound and restored the ability to hear.
"Patients with Usher 1F are born with profound hearing loss and progressive vision loss, and so far we have been able to offer very few solutions to these families," said co-senior author Artur Indzhykulian, HMS assistant professor of otolaryngology–head and neck surgery at Mass Eye and Ear.
According to the researchers, since the vision loss experienced by those with Usher Syndrome type 1F is caused by a variant form of the same protein, the approach used to restore hearing could also be effective in preventing blindness. The research team intends to conduct further testing of the mini-gene in other animal models, with the ultimate goal of human trials.
"It's completely devastating to be born deaf and then lose your vision, so we hope that this mini-gene can eventually be turned into a treatment for this disease," said co-senior author David Corey, the Bertarelli Professor of Translational Medical Science in the Blavatnik Institute at HMS.
Usher Syndrome type 1F is a genetic disorder that causes children to be born completely deaf or with significant hearing loss, experience balance issues, and gradually lose their vision as the retina deteriorates. Typically, blindness occurs by adulthood.
This disorder is the result of a mutation that affects the production of protocadherin-15, a protein that has slightly different forms in the ear and eye, and is crucial for the proper function of cells within the auditory and visual systems.
The Corey lab, which has a long-standing interest in the role of protocadherin-15 in the inner ear, has been researching how the protein helps sensory receptors called hair cells within the ear convert environmental vibrations into electrical signals that are interpreted by the brain as sound.
The previous research conducted by Corey's team revealed how protocadherin-15 works in partnership with cadherin 23 within hair cells to create filaments that physically open ion channels in response to the vibration of the bundles. This opening allows electrical current to enter the cells, and without the presence of this protein, hair cells cannot transmit electrical current, and the brain cannot detect sound.
Building on this research, Corey became interested in creating a gene therapy for Usher Syndrome type 1F by introducing DNA that codes for protocadherin-15 into a cell, allowing the cell to produce the protein.
However, the DNA of protocadherin-15 is too large to fit inside the typical viral capsule that is used to transport genetic material into a cell. Therefore, the research team chose to explore an alternative approach of shortening the DNA to create a mini-gene that would still produce functional protein while being small enough to fit inside the viral capsule.
The first step in the process of creating a mini-gene therapy for Usher Syndrome type 1F involved the mapping of all 25,000 atoms in the external structure of inner-ear protocadherin-15. Marcos Sotomayor, former research fellow at HMS and now associate professor of chemistry and biochemistry at The Ohio State, led this effort.
Using X-ray crystallography and cryo-electron microscopy, Sotomayor discovered that protocadherin-15 consists of 11 linked atoms.
To create a mini-gene that could fit inside the viral capsule, Sotomayor made eight versions of protocadherin-15, each with different links removed to make the protein smaller. The research team then reverse-engineered the truncated protein structures into DNA blueprints that could be tested as mini-genes.
"The knowledge we gained by studying the structure of protocadherin-15 in excruciating detail allowed us to more quickly design shorter versions of the protein for gene therapy," Sotomayor explained.
Indzhykulian conducted experiments on inner ear cells using the eight mini-genes designed by Sotomayor. The truncated versions of protocadherin-15 made from mini-gene DNA successfully bound to cadherin 23, the protein partner in hair cells. The researchers then selected the three mini-genes that were small enough to fit inside the viral capsule.
Lead author Maryna Ivanchenko tested the three mini-genes extensively in the ears of mice that were genetically modified to stop producing protocadherin-15. Out of the three, only one mini-gene worked as it successfully prompted hair cells to produce a mini version of protocadherin-15. This mini protein bound to cadherin-23 and formed the filaments that opened the ion channels, allowing the hair cells to convert vibrations into electrical signals.
Auditory testing of mice that received the mini-gene revealed that their brains were able to receive the sound signal coming from their ears, which showed that the previously deaf animals could hear.
"We were all pleasantly surprised," Corey said. "We thought it would take years of optimizing and trying things and tweaking the protein structure, but this one version pretty much worked."
"The results were thrilling for us," Ivanchenko added. "The most exciting aspect of our findings was that mice that had been completely deaf could now hear almost as well as normal mice."
The researchers found that while the mini-gene effectively treated deafness in the mouse model of Usher 1F, they are more interested in its potential to treat blindness associated with the syndrome. It is unlikely that the mini-gene could improve hearing in children with Usher 1F since they are born profoundly deaf and may lack hair cells in their inner ear, and many of these children are already able to receive cochlear implants that enable them to hear.
However, the researchers noted that blindness is a different story, as children with Usher 1F are born with normal vision. They believe that if the mini-gene could produce the form of protocadherin-15 that is missing in the retina, it could prevent vision loss.
When asked why they started by testing the mini-gene in the mouse inner ear if the main goal was to treat vision loss, the researchers explained that it was mainly for logistical reasons. The lack of protocadherin-15 in mice only causes mild vision loss, which progresses slowly. Testing the mini-gene in mouse models would take years and be difficult to evaluate the effectiveness. On the other hand, the mice used in the study were born profoundly deaf, allowing the researchers to obtain clear results within a couple of weeks.
"The whole project was designed to study the ear with the idea that something that works in the ear can later be applied to the eye, as an article of faith," Corey said. "While the best test system is the mouse inner ear, the immediate goal is a treatment for blindness."
Currently, the Corey lab is testing the mini-gene in zebrafish eyes as it is a better model to evaluate the effectiveness of the approach. Zebrafish experience more severe and rapid vision loss than mice when protocadherin-15 is not produced in the retina. If the mini-gene successfully treats the vision loss in zebrafish, the researchers plan to test the approach in primates and eventually, in humans.
Maryna V. Ivanchenko et al, Mini-PCDH15 gene therapy rescues hearing in a mouse model of Usher syndrome type 1F, Nature Communications (2023). DOI: 10.1038/s41467-023-38038-y