BOSTON — An experimental one-time gene therapy safely restored near-normal hearing in about 90% of patients born with a rare form of genetic deafness, according to a study published in the journal Nature. The research, led by Zheng-Yi Chen, an associate scientist at Mass Eye and Ear in Boston, is the largest and longest to date to evaluate a gene therapy for hearing loss.

The study involved 42 children and adults born with autosomal recessive deafness 9, also known as DFNB9, a condition caused by mutations in the OTOF gene. The OTOF gene provides instructions to make a protein called otoferlin, which is necessary for hair cells in the inner ear to transmit sound signals to the brain. The form of deafness treated in the study affects about 50 babies born each year in the U.S.

"The children with this mutation will be born without any hearing. They're completely deaf. They couldn't hear anything," Chen said.

Researchers at Mass Eye and Ear and Fudan University in China infused an adenovirus into the ears of the deaf patients. The virus carried a healthy version of the OTOF gene, split in half to fit inside, which carries instructions to produce a working version of the otoferlin protein. "The idea is to put what's missing in your ear so that your cells have this protein, will be able to function and can convert the sound into the signal so we can hear," he said.

About 90% of the patients, who ranged in age from babies less than a year old to adults as old as 32, responded to the treatment. Patients began to hear for the first time within weeks, and their hearing continued to improve for about six months. For many, the quality of restored hearing reached near-normal levels, and for some it has lasted more than two years. Patients who benefited could start to learn to speak, enabling them to communicate more easily. In some patients, hearing restored to the point where they could detect a whisper.

The treatment appears to be a one-and-done therapy that could last a lifetime, though longer follow-up is needed to confirm ongoing safety and lasting benefits. The results confirm and extend a smaller study published two years ago. Researchers are considering screening more babies for genetic deafness so they can be treated as young as possible, as early intervention appears to produce the best results.

"The results are really remarkable. This is really for the first time in the whole field a brand-new treatment option for genetic hearing loss. So that's very exciting," he said.