Boston Children’s Hospital researchers found that replacing a mutated protein in the inner ear reversed severe hearing loss in mice, according to a study published in the journal Science Advances. The first gene therapy technique of its kind involved replacing a mutated protein called stereocilin (STRC), in the inner ear. The researchers said it reversed severe hearing loss in mice and, in some cases, restored normal hearing levels, according to a press release.
Close-up of examining the ear with an otoscope
The researchers explained in the release that hearing loss is associated with mutations of at least 100 different genes. The researchers stated that up to 16 percent of genetic hearing loss can be linked to the gene STRC, which they also believe is the second most common genetic cause.
“Patients carrying STRC mutations do not have full cochlear enhancement and as a result suffer from reduced auditory sensitivity and difficulty with frequency discrimination and speech perception,” the authors said in the report published in the peer-reviewed journal.
In order to hear sound, sensory hair cells in the inner ear must make contact with the tectorial membrane, the researchers reported. The membrane responds to sound by vibrating and converting the vibrations into signals that go to the brain.
The researchers say the stereocilin protein acts like a scaffold, helping hair cells stand up in an organized bundle, allowing the hair cell tips to touch the tectorial membrane.
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“If stereocilin is mutated, you don’t have that contact, so the hair cells aren’t stimulated properly,” said Jeffrey Holt, Ph.D., a scientist in Boston Children’s ENT and Neurology Department.
“But most importantly, the hair cells still remain functional, so they are amenable to the gene therapy. We think this will open up a wide range of options for treatment — from babies to adults with hearing loss,” Holt, who is also the senior of the investigation is researcher, said in the release.
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Using a generated mouse model, the researchers designed a dual-vector protein recombination strategy to replace full-length wild-type Strc in the outer hair cells of mice carrying STRC mutations, according to the study. The researchers found what they described as “robust restoration of full-length stereocilin protein” in the mice, noting hair bundles that looked normal and could make contact with the tectorial membrane, according to the report.
The researchers then performed two types of hearing tests. One is commonly used in infants, and the other uses electrodes placed on the scalp to measure the auditory brainstem’s responses to a range of sound frequencies and intensities.
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The study results showed that the mice were more sensitive to subtle sounds. The Boston team also found that the mice had improved cochlear amplification, which they described as a greater ability to amplify soft sounds, the response to loud sounds, and more accurately distinguish between sounds of different frequencies. According to the release, hearing was restored to normal levels in some mice.
The man listens carefully with her palm to her ear, close-up
“The results were remarkable and are the first example of hearing restoration using dual-vector gene therapy to target sensory outer hair cells,” said Olga Shubina-Oleinik, PhD, the study’s lead author. Eliot Shearer, MD, PhD, who co-authored the study and collaborated with the Children’s Rare Disease Cohort Initiative to screen a large genomic dataset for STRC mutations, said in a release that 2.3 million people worldwide use STRC mutations and could potentially benefit from this therapy.
“It turns out that STRC gene variations are more common than we thought, which is what makes gene therapy for this condition so important.”
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The Boston team plans to investigate whether the gene therapy technique works with the human stereocilin gene using human inner ear cells in a dish derived from patients with STRC hearing loss. If it restores hearing function at the tissue level, the researchers hope to seek approval from the FDA to test it on humans, the press release said.