Usually, the rod cells of the retina are affected first, leading to early night blindness ( nyctalopia) and the gradual loss of peripheral vision. The hearing loss is caused by a defective inner ear, whereas the vision loss results from retinitis pigmentosa (RP), a degeneration of the retinal cells. Usher syndrome is characterized by hearing loss and a gradual visual impairment.
It has been noted rarely in a few other ethnic groups. The frequency of Usher syndrome type III is significant only in the Finnish population as well as the population of Birmingham, UK, and individuals of Ashkenazi Jewish heritage. However, the protein's function in these structures, and how its mutation causes hearing and vision loss, is still poorly understood. CLRN1 encodes clarin-1, a protein important for the development and maintenance of the inner ear and retina.
Mutations in only one gene, CLRN1, have been linked to Usher syndrome type III. People with Usher syndrome III are not born deaf but experience a progressive loss of hearing, and roughly half have balance difficulties. Usher syndrome type II occurs at least as frequently as type I, but because type II may be underdiagnosed or more difficult to detect, it could be up to three times as common as type I. The location and function of the other two proteins are not yet known. Usherin is critical for the proper development and maintenance of these structures, which may help explain its role in hearing and vision loss. The protein encoded by the USH2A gene, usherin, is located in the supportive tissue in the inner ear and retina. Usher syndrome type II may be caused by mutations in any of three different genes: USH2A, GPR98 and DFNB31. They also begin to lose their vision later (in the second decade of life) and may preserve some vision even into middle age. People with Usher II are not born deaf and are generally hard-of-hearing rather than deaf, and their hearing does not degrade over time moreover, they do not seem to have noticeable problems with balance. Among Acadians, research into haplotype data is consistent with one single mutation being responsible for all cases of Usher syndrome type I. Type I has been found to be more common in people of Ashkenazi Jewish ancestry (central and eastern European) and in the French- Acadian populations (Louisiana). Worldwide, the estimated prevalence of Usher syndrome type I is 3 to 6 per 100,000 people in the general population. Mutations that affect the normal function of these genes can result in retinitis pigmentosa and resultant vision loss. The genes also play a role in the development and stability of the retina by influencing the structure and function of both the rod photoreceptor cells and supporting cells called the retinal pigmented epithelium.
Alterations in these genes can cause an inability to maintain balance (vestibular dysfunction) and hearing loss. These genes function in the development and maintenance of inner ear structures such as hair cells ( stereocilia), which transmit sound and motion signals to the brain. Usher syndrome type I can be caused by mutations in any one of several different genes: CDH23, MYO7A, PCDH15, USH1C and USH1G. They also exhibit balance difficulties and learn to walk slowly as children, due to problems in their vestibular system. People with Usher I are born profoundly deaf and begin to lose their vision in the first decade of life. Usher syndrome is named after Scottish ophthalmologist Charles Usher, who examined the pathology and transmission of the syndrome in 1914. Type I is most common in Ashkenazi Jewish and Acadian populations, and type III is rarely found outside Ashkenazi Jewish and Finnish populations. The occurrence of Usher syndrome varies across the world and across the different syndrome types, with rates as high as 1 in 12,500 in Germany to as low as 1 in 28,000 in Norway. These mutations are inherited in an autosomal recessive pattern. All three subtypes are caused by mutations in genes involved in the function of the inner ear and retina. Usher syndrome is classed into three subtypes (I, II and III) according to the genes responsible and the onset of deafness. It is a major cause of deafblindness and is at present incurable. Usher syndrome, also known as Hallgren syndrome, Usher–Hallgren syndrome, retinitis pigmentosa–dysacusis syndrome or dystrophia retinae dysacusis syndrome, is a rare genetic disorder caused by a mutation in any one of at least 11 genes resulting in a combination of hearing loss and visual impairment. The genes implicated in Usher syndrome are described below. Usher syndrome is inherited in an autosomal recessive pattern.
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