Experts identify cause of Usher syndrome

Experts identify cause of Usher syndrome

Usher syndrome is one of the most common forms of deaf-blindness in humans and can severely impair the lives of sufferers, but new hope has been created in the form of research which sheds light on the cause of the condition.

A study published in the journal Nature Neuroscience suggests that a protein involved in sound sensing in the inner ear may also play a role in transmitting sound information to the brain.

The protein, called harmonin, is mutated in Usher syndrome, and experts at the University of Iowa (UI) who worked on the study claim it may help explain why this mutation causes the most severe form of the condition.

Working on the basis that hearing starts with the transmission of sound by inner hair cells in the ear, with sound waves causing movement of special structures called stereocilia on the tips of the hair cells, the experts have established that harmonin mediates this movement.

This, in turn, activates the cells and initiates transmission of sound information as electrical and chemical signals to the brain.

According to Dr Amy Lee, senior study author and UI associate professor, most of the research up until this point has concentrated on the input end of the inner hair cells where the sound waves produce motion of the stereocilia.

"Now we have found a new role for harmonin at the opposite end of these sound-sensing inner hair cells where it appears to control the signal output of the cell," she explained.

According to the expert, harmonin is important for regulating the number of calcium channels present at the sound-transmitting synapse of inner hair cells, while studies from other labs have also shown that too few or too many calcium channels at the hair cell synapse cause deafness in mice.

This subsequently means that factors which control how many channels are available are likely to be important for normal hearing.

Dr Lee said that harmonin appears to precisely control how many channels are available.

"What we think is happening in Usher syndrome where the harmonin protein is mutated is that there are too many calcium channels available, which causes abnormal signalling at the synapses," she noted.

The expert hopes the study could lead to new research which will hopefully enable the formulation of a therapy to target the disease in sufferers. 

by Emily Tait

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