Five children who were born completely deaf have had some reversal of hearing loss after receiving a "groundbreaking" gene therapy.

The clinical trial, which was co-led by Mass Eye and Ear in Boston and the Eye & ENT Hospital of Fudan University in Shanghai, was the first in the world to apply gene therapy to children in both ears, according to the researchers.

The research has just been published in Nature Medicine on June 5.

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In addition to regaining their hearing, the children participating in the trial — who ranged in age from 1 to 11 years old — were also able to identify the origins and locations of sounds, even in noisy environments, researchers said.

This was a follow-up to an earlier trial that began in Dec. 2022, in which the research team successfully performed the gene therapy in just one ear. This new study showed that treating both ears led to even greater benefits.

All the children in the study had a hereditary form of deafness called DFNB9, which is caused by mutations in the OTOF gene.

The condition occurs when the OTOF gene is unable to produce a protein called otoferlin, which is essential for transmitting sound signals from the ear to the brain.

As a result, the children could not hear or speak.

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"The children were chosen because they would benefit most from early intervention of gene therapy, especially in speech acquisition," study author Zheng-Yi Chen, DPhil, an associate scientist in the Eaton-Peabody Laboratories at Mass Eye and Ear in Boston, told Fox News Digital in an interview.

"From a safety standpoint, however, it is more risky for children."

During the "minimally invasive" surgical procedure, the doctors administered an injection of the human OTOF gene into the children’s inner ears.

The children remained in the hospital for around seven to 10 days for observation. 

"After four weeks, the kids showed hearing perception in tests, and then gradually they gained the ability to speak," Chen said.

Within the families, response to sound was noticed within two to three weeks.

"All five patients have restoration of hearing, speech perception improvements, and sound source perception in noisy environments," Chen said.

The participants experienced only low-grade adverse effects, such as fever and vomiting.

"There were no serious adverse effects," he said. "They all recovered without any intervention."

The gene therapy is intended to be a one-time treatment and will not need to be repeated, the researchers said, although the children will likely require speech therapy.

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Until now, there has not been any single treatment for hearing loss, other than cochlear implants, according to researcher Yilai Shu M.D., PhD, director of the Diagnosis and Treatment Center of Genetic Hearing Loss at Fudan Hospital in Shanghai.

"This is the first time in history that hearing loss is being reversed by gene therapy," Shu told Fox News Digital. "And, of course, we believe this will have a profound impact on children's lives."

Chang Yiyi, a mother in Shanghai whose 3-year-old son, Zhu Yangyang, participated in the trial, spoke to Fox News Digital about the experience.

"When Zhu couldn’t speak at 2 years old and didn’t have a response to sound, we realized there was a problem," she said. 

After hearing tests, it was determined that Yiyi’s son had total deafness.

"He would get very frustrated because he couldn’t understand, couldn’t speak, couldn’t hear," she said.

Twenty-three days after receiving the gene therapy, the boy first responded to someone calling out to him.

"It was unbelievable — the best feeling," Yiyi told Fox News Digital. "It was like a miracle."

"Now he can say ‘Mommy’ and ‘I want’ and some simple sentences."

Approximately 430 million people worldwide have disabling hearing loss, including 34 million children, according to the World Health Organization.

More than 50% of hearing loss cases are the result of genetics.

Dr. Amy Sarow, the Michigan-based lead audiologist at Soundly, a hearing health care marketplace, noted that gene therapy has had some success in the treatment of cancer and eye disease, along with other emerging areas. 

"It is exciting to think about how gene therapy could impact millions of individuals with hearing loss worldwide," Sarow, who was not involved in the experimental gene therapy, told Fox News Digital. 

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"However, it is essential to emphasize that there are many causes of hearing loss, and one type of gene therapy will not be right for every type."

Even among genetic causes of deafness, different genes may cause abnormalities or dysfunction that affect different auditory pathways, according to Sarow. 

"Thus, development of specific treatment interventions is dependent on causality and will still take time to develop."

Additionally, Sarow noted, a "reversal" of hearing loss does not mean that an individual will have normal hearing ability fully restored. 

"The first three years of life are very important to language acquisition, and although these children would be behind their normal-hearing peers (having spent the first few years of life profoundly deaf), they would still have the possibility to ‘catch up’ to some degree," she said.

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"Research tells us that the younger the intervention, the better for potential language development."

As with any intervention, there can be risks with gene therapy. "One potential risk is that treatment may not be successful in every case," Sarow said. 

"Another potential risk is that the targeted gene therapy may not work in the targeted region."

The next step is to follow the trial patients for a longer time period to ensure that the positive results are stable, Shu said.

Based on the results of the first study, the researchers expect that the patients’ hearing abilities will continue to improve over time.

"Then we want to expand to older patients, and gauge how the treatment works for aging adults," he said.

"Ultimately, we want the patient to have a choice about which treatment option they want to go with."

The researchers also plan to start the process of seeking FDA approval to bring the gene therapy to the U.S.

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"We are working to bring this to people outside China, including the U.S., as quickly as possible," Shu Fox News Digital.

The researchers also hope to extend this type of gene therapy to treat other types of deafness in the future.