Usher syndrome is a genetic disease causing deaf-blindness. It is essentially progressive retinitis pigmentosa combined with
congenital hearing loss. It is almost always inherited in an autosomal recessive pattern and is estimated to occur in 1 in
10,000 people. Whilst this is a rare genetic condition it represents the major cause of syndromic deafness with blindness.
Usher syndrome is divided into three types, I, II and III. Children with type I syndrome are born profoundly deaf, and
eyesight usually begins degrading after the first decade of life, beginning with night-blindness. If identified at a young
age, children usually receive a cochlear implant, and generally learn spoken language. Sign language is also sometimes used,
though when vision loss becomes severe one must revert to tactile signing. Problems with balance are usually present, due
to the failure of the hair cells of the inner ear. Type II children are hard-of-hearing, and changes in sight usually begin
later, first becoming noticable after the second decade of life. In the type III syndrome, hearing loss as well as retinitis
pigmentosa can occur later in life.
Usher syndrome II
Usher syndrome type II occurs at least as frequently as type I, because type II may be underdiagnosed or more difficult to
detect, it could be up to three times as common as type I.
Mutations in the MASS1 and USH2A genes cause Usher syndrome type II. Usher syndrome type II may be caused by mutations
in any of three different genes, two of which have been identified to date. These genes are called USH2A and MASS1. Usherin,
the protein made by the USH2A gene, is located in supportive tissue in the inner ear and retina. Usherin is critical for the
proper development and maintenance of these structures, which may help explain its role in hearing and vision loss. The precise
function of the protein made by the MASS1 gene is not yet known.
Usher syndrome I
Worldwide, the estimated prevalence of Usher syndrome type I is 3 to 6 per 100,000 people in the general population.
Mutations in the CDH23, MYO7A, PCDH15, Usher 1C (also known as Harmonin), and USH1G (now identified as SANS) genes cause
Usher syndrome type I. Usher syndrome type I can be caused by mutations in one of several different genes. 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. Alterations in these genes can cause an inability to maintain balance (vestibular dysfunction) and hearing
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. Mutations that affect the
normal function of these genes can result in retinitis pigmentosa and vision loss.
Usher syndrome III
The frequency of Usher syndrome type III is highest in the Finnish population, but it has been noted rarely in a few other
Mutations in the USH3A gene cause Usher syndrome type III. Usher syndrome type III can be caused by mutations in one of
at least two genes, only one of which (USH3A) has been identified. The USH3A gene makes a protein that is important for the
development and maintenance of the inner ear and retina. The protein's function in these structures, and its role in hearing
and vision loss, have not yet been fully explained.