In a breakthrough, US scientists have discovered a single master gene that programmes ear hair cells into either outer or inner ones, overcoming a major hurdle that had prevented the development of these cells to restore hearing.

Hearing loss due to ageing, noise and certain cancer therapy drugs and antibiotics has been irreversible to date because scientists have not been able to reprogramme existing cells to develop into the outer and inner ear sensory cells -- essential for hearing - once they die.

"Our finding gives us the first clear cell switch to make one type versus the other," said lead author Jaime Garcia-Anoveros, Professor of Anaesthesia, Neurology, and Neuroscience at Northwestern University Feinberg School of Medicine.

"It will provide a previously unavailable tool to make an inner or outer hair cell. We have overcome a major hurdle," Garcia-Anoveros added. In the paper published in the journal Nature, the team described the master gene switch - TBX2. When the gene is expressed, the cell becomes an inner hair cell. When the gene is blocked, the cell becomes an outer hair cell.

The ability to produce one of these cells will require a gene cocktail, Garcia-Anoveros said. The ATOH1 and GF1 genes are needed to make a cochlear hair cell from a non-hair cell. Then the TBX2 would be turned on or off to produce the needed inner or outer cell.

The goal would be to reprogramme supporting cells, which are latticed among the hair cells and provide them with structural support, into outer or inner hair cells. "We can now figure out how to make specifically inner or outer hair cells and identify why the latter is more prone to dying and cause deafness," Garcia-Anoveros said.

He, however, stressed that the research is still in the experimental stage. Currently, scientists can produce an artificial hair cell, but it does not differentiate into an inner or outer cell, which provides different essential functions to produce hearing. The discovery is a major step toward developing these specific cells.

The death of outer hair cells made by the cochlea is most often the cause of deafness and hearing loss. The cells develop in the embryo and do not reproduce. The outer hair cells expand and contract in response to the pressure of sound waves and amplify sound for the inner hair cells. The inner cells transmit those vibrations to the neurons to create the sounds we hear.

"The ear is a beautiful organ. There is no other organ in a mammal where the cells are so precisely positioned. (I mean, with micrometric precision). Otherwise, hearing doesn't occur," Garcia-Anoveros said.