Scientists have associated a new gene – Clrn2 – with hearing in mammals. Led by Amraoui and Bowl, the study was a collaborative effort of 32 researchers from institutes in the UK, France, and the USA. The findings were recently published in the journal EMBO Molecular Medicine.
Cilia bundle in the inner ear. Picture adopted from EMBO Molecular Medicine
The sense of hearing results from a combination of events of physical and biological sciences. Mechanical energy from sound waves falling on the inner ear must be converted to neuronal signals for a person to hear. The process is taken care of by the specialized hair cells in the inner ear. On the tip of each hair cell is a bundle of cilia, some tall and some short, arranged in a specific manner and tethered to a complex. The movement of the inner ear fluid, caused by sound, deflects these hair cell bundles towards the tallest cilia. The tension created in the tip open the channels in the complex attached to it. The complex releases neuronal signals, completing the conversion of mechanical energy to neuronal signals.
Hearing loss can be caused by environmental factors, genetic factors or a combination of both. Although scientists have managed to understand early-onset hearing loss and hereditary hearing loss to an extent, very little is known about the genetics behind age-related hearing loss. The research conducted by this team has implicated the involvement of Clrn2 in age-related or progressive hearing loss.
The team mutated the Clrn2 gene in mouse and investigated its effect only to discover a progressive hearing loss. To check if the finding applied to humans as well, the group analysed the CLRN2 gene sequences of 5 lakh people from the UK biobank participants data. Data was segregated as hearing loss cases (163, 333) and controls with normal hearing ability (102,832). The cases and controls selected were above 50 years of age. The classification was made based on the participants’ response two questions recorded in the published paper – (i) Do you have any difficulty with your hearing? (ii) Do you find it difficult to follow a conversation if there is background noise (such as TV, radio, children playing)? Those who answered ‘No’ to both were controls and other were cases. On analysing their CLRN2 gene sequence, the group found that those who had difficulty in hearing harboured mutations in their CLRN2 genes.
With further experiments on mice, the team discovered that while Clrn2 was necessary for the maintenance of the bundle of cilia in the hair cells of the inner ear after they had been formed. Mutations in this gene would, therefore, lead to poor maintenance of the bundle leading to progressive hearing loss.
From this study, scientists and clinicians now have a reference point to test for hearing loss because of ageing. We may be looking at days when the auditory tests, which are subjective, are replaced by the objective genetic tests for deafness.