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Hearing loss – from genes to treatments

To develop new and effective treatments for hearing loss, researchers need to understand how hearing works in detail, and what changes when someone loses their hearing. The first step in this process is often to find the genes involved in hearing. But why is this step so important?

We already know of more than 100 genes that are linked to hearing loss in people. Mostly, these genes cause hearing loss that is present at birth, or develops soon after, and where a change (mutation) in a single gene has a severe impact on a person’s hearing. However, this is far from the whole story. Hearing is an incredibly complex process, and the cells and other structures in the body involved in hearing are correspondingly complex. Thousands of genes are involved in forming and maintaining the cells of the ear and the hearing brain – changes in many of these genes could lead to hearing loss. The true number of genes linked to hearing loss is likely to be much higher than we currently know.

It’s also possible that changes in some genes might affect hearing more subtly, causing hearing loss to develop later in life, or affect how likely we are to develop hearing loss as we age, or if we’re exposed to loud noise or have to take certain medicines that can cause hearing loss as a side-effect. Finding these genes in people is more difficult, as their effect is smaller; it could be that changes in a number of these genes is required before we see an effect on someone’s hearing.

So how do we find genes for hearing loss?

One way that researchers can find genes for hearing loss is to study families where multiple generations are affected by it. They collect DNA samples from family members, both those with and those without hearing loss, and compare them. They look for differences in the DNA between the family members who have hearing loss, and those who don’t, as these differences are likely to underlie the hearing loss. They then study the identified genes in more detail, to understand how and why they might be linked to hearing loss. Most of the genes that are already linked to hearing loss in people were identified this way.

But this isn’t the only way to find genes for hearing loss. Researchers also study hearing in mice. Mice are genetically similar to us – most of our genes are also found in mice (or at least very similar genes are), and their hearing, and hearing structures, are similar to ours. This means researchers can use mice as a ‘model’ of human hearing – so genes that cause hearing loss in mice when they’re changed are likely to cause hearing loss in people, too.

Researchers based at labs around the world are working as part of an international consortium to produce a collection of genetically-modified mice in which each gene in the mouse has been deleted (‘knocked out’), one gene in each mouse. These mice are then studied in detail to identify the impact of removing that gene on the mouse. In some cases, absence of the gene causes hearing loss, and these mice and genes can then be studied further, to find out where in the body (including the ear or the brain) the gene is usually active, and what it’s doing that could implicate it in hearing loss. They’re also followed up in people, to see if a definite link can be proven.

This approach can identify genes that haven’t yet been linked to hearing loss in people, but which are nonetheless involved. It may also help to find genes in which changes have a small effect on hearing and importantly, tell us more about the processes in the body that underlie hearing.

A new study identifies potential hearing loss genes

A global team of researchers, led by Professor Karen Steel at King’s College London, has used these mice to look for new hearing loss genes. They studied over 1,200 different strains of mice, each missing a different gene, to find out whether the genes were linked to hearing loss. Using a sensitive hearing test on the mice, the researchers found that 38 of the genes caused hearing loss of some type when they were missing. This included genes where the mice were hearing at birth, but subsequently lost their hearing as they aged – these genes might be linked to age-related hearing loss in people, although more research is needed to prove it.

The researchers then looked at these genes in people, to see if they could find evidence that they were involved in hearing loss in people. In some cases, they did find an association, and these genes are now being studied further.

One of the advantages of this research is that the genes being studied were picked at random, and not because the researchers thought they were likely to be involved in hearing loss. Just over 3% of the genes were linked to hearing loss in the mice – so extrapolating from that to the approximately 20,000 genes in the mouse genome means there could be more than 600 genes still to be found. Alongside the genes that have already been identified, there could be over 1,000 genes linked to hearing loss!

Some of the newly-identified genes are involved in cellular processes that haven’t previously been linked to hearing, such as the processing of fats within cells. This is providing scientists with new avenues of research into hearing, which will open up our understanding of how hearing works and how it might go wrong.

So how does this link to treatments?

Each time we find a new gene for hearing loss, and figure out its role in hearing (for example, is it important in the cells that detect sound, or the ones that pass information from the ear to the brain?), we learn more about how hearing works. This helps us to understand what hearing “looks like” at the cellular and molecular level, and tells us what we need to restore the system to if we’re to reverse hearing loss. It also helps us to identify ‘targets’ – genes, proteins or the biological processes they’re involved in – so that we can design medicines, or other treatments, to interfere with or reset them, to restore hearing.

Further information

This research was published last month in the journal PLoS Biology.

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