Genetic Basis of Usher Syndrome

Genetic Basis of Usher Syndrome

The prevalence of Usher syndrome varies from country to country, but it is a rare condition affecting approximately 1 in 10,000 people. Usher syndrome is a genetic disease that occurs when there are mutations (defects) in genes that are important for the function of both photoreceptors in the retina and hair cells in the cochlea, or inner ear.

Usher syndrome is always inherited as an autosomal recessive trait. “Autosomal” refers to the fact that it is not located on the X or Y chromosomes. Therefore, both males and females can have Usher syndrome and can pass the mutation on to the next generation. As it is recessive, both copies of the Usher-related gene inherited from both parents must be mutated. If just one copy of the gene is mutated, the person will not have symptoms but may pass the mutated gene onto their children. They are referred to as ‘carriers’. Patients with a combination of early-onset partial deafness and retinitis pigmentosa due to alterations in the mitochondrial DNA (a different type of DNA in cells) do not fall into the category of Usher syndrome.

If a family member is diagnosed with Usher syndrome, it is strongly advised that other members of the family also have an eye exam by an eye doctor (ophthalmologist) who is specially trained to detect retinal diseases. As the deafness becomes obvious at a much earlier age than the RP in patients with Usher syndrome, it is particularly important that younger siblings with a hearing problem have a careful eye examination.

While there are three clinical types: type 1, type 2 and type 3 (based on the severity and age when symptoms appear), researchers have found 11 different genes associated with these clinical subtypes. Usher type 1 currently has six different genes associated with it, Usher type 2 has three genes, and Usher type 3 has two genes. Genetic testing is the only reliable way of determining the true genetic type as it is impossible to determine this from clinical analysis.

A definitive diagnosis of Usher syndrome requires genetic testing. A genetic confirmation of Usher syndrome is also necessary for participation in in gene-based clinical trials such as gene therapy. Genetic results also can be used by researchers to learn more about different types of Usher syndrome and contribute to developing treatments. Unfortunately, only a small number of people in the Usher community have had genetic testing to date.

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