Gene therapy of hearing loss

Hearing loss is associated with mutations in at least 100 different genes, but up to 16% of genetic hearing loss is caused by a mutation of the STRC gene; this is the second most common genetic cause of hearing loss.

A first-of-its-kind gene therapy technique developed at Boston Children's Hospital has replaced the mutated stereocilin protein in the inner ear and reversed severe hearing loss in mice, sometimes to normal hearing levels.

Restoring contact

In order to perceive sound waves, the sensitive hair cells of the inner ear must come into contact with the tectorial membrane, which vibrates in response to sound, and then converts these vibrations into impulses sent to the brain.

The protein stereocilin acts as a scaffold, helping the microvilli (stereocilia) of hair cells to assemble into an organized bundle so that their tips can touch the membrane.

With stereocilin mutation, this contact is absent, so stereocilia are not stimulated properly. But at the same time, stereocilia still remain functional, and therefore they are susceptible to gene therapy.

New gene therapy

To deliver the healthy stereocilin gene, the researchers used a synthetic adenoassociated viral vector (AAV) targeting stereocilia. They ran into a problem – the STRC gene is too big to fit in AAV: the length of the gene is about 6200 DNA base pairs, and the capacity of AAV is only 4700 base pairs. To overcome this obstacle, the researchers split the mouse STRC gene in half and placed the two halves in two separate AAVs. Then they planned to use the well-known protein recombination technique, when two halves of a protein find each other and connect. But in this case it didn't work.

It turned out that at the beginning of the amino acid chain of the stereocilin protein there is a short section that acts as a navigator and directs it to its place in the cell. When the researchers split the protein in half, one fragment had a signal and the other didn't, so the halves might not end up in the same place. When they added an amino acid "navigator" to both halves of the protein, they successfully came together. Gene therapy in mice resulted in reliable recovery of the stereocilin protein and normal stereocilia that could come into contact with the tectorial membrane.

Sensory outer hair cells, which are necessary for cochlear amplification and normal hearing. The image on the left shows disorganized hair cells of the mouse inner ear with a mutation in the STRC gene; as a result, the cell lacks the scaffold connections provided by the protein stereocilin. On the right – hair cells after two-vector gene therapy (dual-AAV); normal organization has been restored. The red arrows indicate the restored cross-links that divide the stereocilia into organized bundles that can contact the overlying tectorial membrane and detect sound vibrations. Scanning electron microscopy.

Hearing restoration

The researchers used two types of hearing tests: a test similar to that used in newborns, and a test that measures the auditory responses of the brain stem to a range of sound frequencies and intensity using electrical potentials. During testing, it was found that mice became much more sensitive to high–pitched sounds and showed improved cochlear amplification - the ability to amplify quiet sounds by suppressing loud ones in order to more accurately distinguish sounds of different frequencies. In some mice, hearing was restored to normal levels.

The results obtained are the first example of hearing restoration using two-vector gene therapy aimed at sensory cells of the inner ear. Approximately 100,000 patients in the U.S. and 2.3 million worldwide carry STRC gene mutations and could potentially benefit from this therapy.

The authors plan to test whether this method works with the human STRC gene and test it in human inner ear cell cultures obtained from patients with STRC-associated hearing loss. They applied for a patent for gene therapy technology.

Article by O.Shubina-Oleinik et al. Dual-vector gene therapy restores cochlear amplification and auditory sensitivity in a mouse model of DFNB16 hearing loss published in the journal Science Advances.

Aminat Adzhieva, portal "Eternal Youth" http://vechnayamolodost.ru .