How Gene Therapy for Deafness Works—One Injection
Scientists can now restore hearing in people born deaf by injecting functional genes directly into the inner ear. Here's how the therapy works, which genes are involved, and why it marks a turning point for treating hereditary hearing loss.
A Single Shot to the Inner Ear
About one in every 1,000 children is born with significant hearing loss, and roughly 60 percent of those cases trace back to genetic causes. For decades, cochlear implants and hearing aids were the only options. Now, gene therapy is rewriting the playbook: a single injection into the inner ear can deliver a working copy of a faulty gene and restore natural hearing—sometimes within weeks.
Why Genetics Causes Deafness
The inner ear's cochlea contains roughly 30,000 sensory "hair cells" that convert sound vibrations into electrical signals for the brain. These cells depend on proteins encoded by specific genes. When one of those genes carries a mutation, the protein is missing or broken, and the signal chain fails.
More than 150 genes have been linked to hereditary hearing loss. The most common culprit is GJB2, which encodes connexin 26—a protein that forms gap junctions between inner-ear cells. Mutations in GJB2 account for up to 30 percent of genetic deafness worldwide. Another key gene is OTOF, which produces otoferlin, a protein that hair cells need to release neurotransmitters at the synapse with the auditory nerve. Without otoferlin, hair cells detect sound but cannot pass the message onward. Children born with OTOF mutations are profoundly deaf from birth.
How the Therapy Delivers a Working Gene
Gene therapy for hearing loss uses a modified adeno-associated virus (AAV)—a harmless virus stripped of its disease-causing machinery—as a delivery vehicle. Surgeons inject the AAV through the round window membrane at the base of the cochlea in a procedure performed under general anesthesia.
Once inside the cochlea, the virus enters hair cells and deposits the functional gene. The cell's own machinery then reads the new genetic instructions and begins producing the missing protein. Because hair cells do not divide, the therapy is expected to be long-lasting—potentially a one-time treatment.
There is an engineering hurdle with the OTOF gene: it is too large to fit inside a single AAV capsid. Researchers solved this by splitting the gene in two. Each half is packaged into a separate virus particle, and both halves are injected together. Inside the cell, the two fragments recombine into a full-length gene, and the cell produces complete otoferlin protein.
What Clinical Trials Show
Results published in the New England Journal of Medicine from Regeneron's CHORD trial showed that 11 of 12 children treated with the DB-OTO gene therapy experienced measurable hearing improvement. Three achieved normal hearing sensitivity, and six could understand soft speech without any assistive device. Improvements appeared within weeks and were sustained through follow-up.
A separate trial led by researchers at Karolinska Institutet treated ten patients—children and adults—and saw hearing thresholds improve from an average of 106 decibels down to 52 decibels within six months. No serious adverse events were reported in either study.
Limits and What Comes Next
Current therapies target only OTOF-related deafness, which accounts for a small fraction of genetic hearing loss. But the approach is a proof of concept. Multiple teams are now developing AAV-based therapies for GJB2 and TMC1 mutations, which together cover a much larger share of hereditary cases.
Key challenges remain. The inner ear is surgically difficult to access in very young infants. The long-term durability of gene expression—over decades, not just months—is still unknown. And the therapy does not help patients whose hair cells have already been destroyed by noise, aging, or disease, because the viral vector needs living cells to enter.
Still, the results so far represent a fundamental shift. For the first time, clinicians can treat a cause of deafness rather than compensate for it. As the NIH's National Institute on Deafness notes, expanding gene therapy to additional mutations could eventually make natural hearing restoration available to tens of thousands of children born deaf each year.
