Using a new gene delivery therapy, researchers restored partial hearing and balance in laboratory mice that were born with a condition that impairs them of both.

Hair Cells Crucial To Hearing And Balance

The research, which was reported in the journal Molecular Therapy, showed that the treatment, which overcomes a barrier to accessing hair cells involved in capturing sound and head movement, leads to significant improvements in hearing. The treatment also led to improvements in the mice's sense of balance.

Hair cells, which serve as delicate sensors in the inner ear, have been difficult to treat. Previous approaches were not very effective because they only reached a set of hair cells in the inner ear. Another subset of these hair cells, which is equally important for hearing, are still largely impenetrable.

Supercharging Adeno-associated Virus As A Gene Carrier

For a gene delivery mechanism that can work for all hair cell types to treat most forms of hearing loss, Casey Maguire, from Harvard Medical School, and colleagues supercharged adeno-associated virus (AAV) , which is already used as a gene delivery tool for retinal disorders.

The researchers used a form of the gene carrier virus enclosed in bubbles of the cell membrane. Also known as exo-AAV, this form of virus binds more easily to the surface of the hair cells and penetrate them more efficiently.

Restored Hearing And Balance In Living Animals

Experiments showed the virus penetrated between 50 and 60 percent of hair cells. In comparison, AAV alone can only reach 20 percent of the cells.

In tests involving living animals, the researchers used mice born without a gene crucial for hair cell function. Animals with this condition do not normally hear even the loudest sounds. They also exhibit poor balance.

The researchers injected the exo-AAV with the missing gene into the inner ear of the animals shortly after birth. Tests later revealed that the gene entered up to 70 percent of the hair cells.

Nine of the 12 mice that received treatment had some form of hearing restored and even responded to loud clap a month following the treatment. Four of the animals hear sounds equal to that in conversations in loud restaurants. The treated mice also exhibit improved balance.

The researchers plan to test the method in other forms of deafness including those that cause both blindness and deafness.

"Exo-AAV1 gene therapy partially rescues hearing in a mouse model of hereditary deafness (lipoma HMGIC fusion partner-like 5/tetraspan membrane protein of hair cell stereocilia," researchers wrote in their study. "Exo-AAV is a powerful gene delivery system for hair cell research and may be useful for gene therapy for deafness."

Hearing Loss And Deafness In The United States

Figures from the CDC show that about 30 million people in the United States suffer from hearing loss. One in 1,000 babies born each year have hearing impairment. Up to 60 percent of hearing loss in babies can be blamed on genetic factors but a number of environmental factors can cause hearing loss as well such as maternal infection during pregnancy and complications after birth.

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