Acquired sensorineural hearing loss (SHL) is a loss in hearing after birth. SHL is usually permanent. In adults, ageing and prolonged exposure to loud noise is the cause of SHL. In children and infants, causes include congenital abnormalities or infections. Viral infections such as measles, mumps and meningitis can also cause hearing loss, as can trauma to the head or inner ear. 

In this type of hearing loss, higher-pitched tones may sound muffled. It may become difficult to pick out words against background noise.

Currently it is estimated that deafness and hearing loss are widespread and found in every region and country. Currently more than 1.5 billion people (nearly 20% of the global population) live with hearing loss; 430 million of them have disabling hearing loss. About 2 percent of adults aged 45 to 54 have disabling hearing loss. One in eight people in the United States (30 million) aged 12 years or older has hearing loss in both ears, based on standard hearing examinations. Over 10 million people in the UK have some loss of hearing, that’s 1 in 6 of the population.

Treatment at present consists of hearing aids and in severe cases, a cochlear implant can be of benefit. However, current clinical trials utilizing human umbilical cord blood in animal models have demonstrated regrowth of hair cells and some improvement of the auditory brainstem reaction. These trials with animal models have now expanded into human trials. Currently there are only 11 clinical trials investigating the application of stem cells in hearing loss and one of these is investigating the application of cord blood for treatment of SHL. This trial is using an infusion of a child’s own cord blood stem cells to improve their hearing, inner ear function, and language development.

Damage to the auditory nerve or to the sensitive hair cells inside the inner ear is the ultimate cause of SHL. There are more than 25,000 hair cells  in the cochlea (an organ in the inner ear). These cells are critical to the process of hearing, for they distinguish and react to sound, by transmitting nerve signals to the brain. Sadly,  these hair cells are extremely sensitive and incapable of regeneration. Long term exposure to loud noises, aging, infections, and drugs can all cause long-lasting damage on these important structures.

Progress in stem cell research

The past few decades in stem cells’ role in SHL, research have focused on the ability of stem cells to develop and function as hair cells. The discovery of stem cells in the inner ears of mice, chicks, and zebrafish, have highlighted the fact that under the right conditions, the stem cells can develop into cells that are remarkably similar to hair cells in the inner ear.

The fact that birds and fish could regenerate these hair cells have led to the discovery that activation of a growth gene (ERBB2) pathway had resulted in a series of events that ultimately made cochlear support cells (the hollow tube in the inner ear) begin to multiply and activate other neighbouring stem cells to become new sensory hair cells.

Various research groups across the world have had promising results using stem cell therapy, although many of them are still in the investigative phase, a cure does not yet exist. These research groups include: Harvard Stem Cell Institute (HSCI), Stanford Medicine, Rutgers University, MIT, Brigham and Women’s Hospital, and Massachusetts- Eye and Ear clinic. They have all shown developments with stem cells and hearing loss. Scientists at Kyoto University in Japan have conducted research that may help with hearing loss and tinnitus. Another group of scientists have uncovered a single master gene that programs ear hair cells into either outer or inner ones, this will overcome major hurdles that had stopped the development of these cells to restore hearing.

The research technology varies from using induced pluripotent stem cells (iPS), adult cells, from a patient’s own skin that have been genetically reprogrammed to revert back to stem cells. These cells would be able to treat hearing loss by using the patient’s own cells. Other groups are producing human hair cells of the inner ear in a culture dish, or converting inner ear stem cells by expressing certain genes or growth factors into auditory neurons, or rebuilding cochlea’s structural support cells by re-engineering intestinal stem cells. All these different technologies have shown promising results and are currently being further investigated.

Whilst several limitations still exist for stem cell therapy in SHL, the role of stem cell in the treatment of hearing diseases has been widely recognized. With the advancement of new technologies in the Regenerative Therapy Field, stem cell therapy will play a greater role in the treatment of diseases related to the inner ear in the near future.

Reference

• He Z, Ding Y, Mu Y, Xu X, Kong W, Chai R, Chen X. Stem Cell-Based Therapies in Hearing Loss. Front Cell Dev Biol. 2021 Oct 21;9:730042. doi: 10.3389/fcell.2021.730042. PMID: 34746126; PMCID: PMC8567027