What are MYO7A-related Disorders?
MYO7A-related disorders represent a group of disorders associated with hearing loss with or without vision loss. This group of disorders does not affect intelligence or cause any other primary health problems.
Usher Syndrome Type IB
There are three types of Usher syndrome, identified as type I, type II and type III. The different types of Usher syndrome are grouped by the severity of the disease and the age when symptoms appear. Mutations in MYO7A cause Usher syndrome type 1B (USH1B).
Usher syndrome type I is an inherited disease that causes hearing loss, balance problems, and progressive vision loss. Infants with USH1B are profoundly deaf in both ears at birth. They have severe balance problems caused by abnormalities of the inner ear (vestibular system) that can lead to delayed development. In general, children with USH1B sit and walk at later ages and have difficulties sensing changes in speed or direction. In childhood or by early adolescence, individuals with USH1B develop retinitis pigmentosa (RP), an eye disease which causes night blindness and a gradual loss of peripheral vision. Eventually only the central vision remains, creating “tunnel vision.” This central vision too can be impaired and can lead to blindness in a small number of people with the disease. In some cases, people with Usher syndrome type 1 develop cataracts, which can further impair vision.
Some mutations in MYO7A have been reported in recessive nonsyndromic hearing loss and deafness (hearing loss but no vision loss), referred to as DFNB2. Individuals with DFNB2 develop profound hearing loss anywhere from birth to adolescence, and may also develop balance problems. Though the progressive vision loss typical in Usher syndrome is not expected in this condition, some individuals reevaluated later in life had developed symptoms of retinitis pigmentosa indicating variability/overlap in the conditions associated with this gene.
In rare cases, a mutation in MYO7A causes dominant nonsyndromic hearing loss and deafness. A dominant condition is one where only one mutation is sufficient to cause the condition. Individuals with DFNA11 seem to develop moderate to severe progressive hearing loss after learning how to talk (late childhood or adolescence). However, though they may have nystagmus or balance issues (milder than USH1B), they do not experience the vision loss typical in Usher syndrome. This condition seems to be the mildest associated with MYO7A.
How common are MYO7A-related Disorders?
The global incidence is unknown for all three conditions. The incidence/prevalence of Usher syndrome type I overall has been estimated in a few countries. In most countries, the frequency ranges from ~1 in 45,000 to ~1 in 65,000, with the exception of Germany where the frequency is ~1 in 90,000. Approximately 53-63% of people with Usher syndrome type I have USH1B. There are regions where founder effects (high frequency of disease because the group arose from a small, possibly isolated population) occur, such as in indigenous populations in South Africa.
DFNB2 and DFNA11 are rare disorders. DFNB2 has been been reported in at least 3 families and DFNA11 in at least 5 families of various ethnicities. Other presentations of or variability in these two disorders may not be recognized as of yet.
How are MYO7A-related Disorders treated?
There is no cure for MYO7A-related disorders, however early treatment is important to give an affected child the best opportunity to develop communication skills. While a child is young, his or her brain is most receptive to learning language, either spoken or signed. It is also important to take advantage of the time when the child’s vision is normal. People with Usher syndrome type 1B generally do not respond to hearing aids, however cochlear implants may help regain some form of hearing. Sign language is a good option for communication. Specialists can introduce other tools and methods of instruction available to people with hearing loss. It is often helpful if the whole family undergoes such instruction and, as a family unit, helps the child adapt.
For those individuals that develop vision loss, visual aids and specialized instruction (for example in tactile signing) help children adapt to their limited vision. Individuals can be prone to accidental injury due to their vision loss and balance problems. Well-supervised participation in sports may help an individual with Usher syndrome type 1 compensate for balance issues, but swimming may be particularly difficult and strategies to ensure safety are needed. Use of UV-A and UV-B blocking sunglasses is recommended, and other optical aids may increase eye comfort. Therapy with vitamin A palmitate may slow retinal degeneration for some.
What is the prognosis for a person with a MYO7A-related Disorder?
Usher syndrome type IB results in severe hearing and vision impairment and DFNB2/DFNA11 results in hearing impairment only. However, none of the conditions affect one’s lifespan or intelligence.