Researchers at Harvard Medical School and Boston Children’s Hospital have restored hearing in mice that have a genetic form of deafness. They have published their work online in the journal Science Translational Medicine, which could pave the way for gene therapy in people who have hearing loss caused by genetic mutations.
“Our gene therapy protocol is not yet ready for clinical trials—we need to tweak it a bit more—but in the not-too-distant future, we think it could be developed for therapeutic use in humans,” said Jeffrey Holt, Harvard Medical School associate professor.
There are more than 70 different genes are known to cause deafness when mutated. The genetic deafness is caused by a gene called TMC1, accounting for 4 to 8 percent of cases, and encodes a protein that plays a central role in hearing, helping convert sound into electrical signals that travel to the brain.
The researchers tested gene therapy in two types of mutant mice. One type had the TMC1 gene completely deleted and is a good model for recessive TMC1 mutations in humans: Children with two mutant copies of TMC1 have profound hearing loss very young, usually by 2 years of age.
The other type of mouse, called Beethoven, has a specific TMC1 mutation—a change in a single amino acid—and is a good model for the dominant form of TMC1-related deafness. In this form, less common than the recessive form, a single copy of the mutation causes children to gradually go deaf beginning around 10 to 15 years of age.
To deliver the healthy gene, the team inserted it into an engineered virus called adeno-associated virus 1, or AAV1, together with a promoter—a genetic sequence that turns the gene on only in certain sensory cells of the inner ear known as hair cells. They then injected the gene-bearing AAV1 into the inner ear, with these findings:
In the recessive deafness model, gene therapy with TMC1 restored the ability of sensory hair cells to respond to sound—producing a measurable electrical current—and also restored activity in the auditory portion of the brainstem.
Most importantly, the deaf mice regained their ability to hear. To test hearing, the researchers placed the mice in a “startle box” and sounded abrupt, loud tones. When the mice jumped—indicating they heard the sound—a plate under their feet picked up the movement. After gene therapy, the mice began measurably startling at sounds beginning around 80 decibels. “Mice with TMC1 mutations will just sit there, but with gene therapy, they jump as high as a normal mouse,” said Holt.
In the dominant deafness model, gene therapy with a related gene, TMC2, was successful at the cellular and brain level and was partially successful at restoring actual hearing in the startle test.
AAV1 is considered safe as a viral vector and is already in use in human gene therapy trials for blindness, heart disease, muscular dystrophy and other conditions. The researchers screened various types of AAV and various types of promoters to choose the best-performing combination. They plan to further optimize their protocol and follow their treated mice to see if they retain hearing longer than the two months already observed.
Holt believes that other forms of genetic deafness may also be amenable to the same gene-therapy strategy. Overall, severe to profound hearing loss in both ears affects 1 to 3 live births per 1,000.
Holt’s team showed in 2013 that TMC1 and the related protein TMC2 are critical for hearing, ending a rigorous 30-year search by scientists. Sensory hair cells in the inner ear contain tiny projections called microvilli, each with a channel at its tip formed by TMC1 and TMC2 proteins. When sound waves wash over the microvilli, they wiggle, and the mechanical stimulation causes the channel to open. This allows calcium to enter the cell, generating an electrical signal that travels to the brain and ultimately translates to hearing.
Although the channel is made up of either TMC1 or TMC2, a mutation in the TMC1 gene is sufficient to cause deafness. However, Holt’s study also showed that gene therapy with TMC2 could compensate for loss of a functional TMC1 gene, restoring hearing in the recessive deafness model and partial hearing in the dominant deafness model.
These projects can produce groundbreaking basic research that has clinical applications for devastating neurological disorders.
These findings mark a defining moment and can ultimately challenge, the burden of deafness in humans. The results are testament to the immense dedication of the research team and their commitment to bringing best-in-class science ever closer to real-world application.
