What is Usher Syndrome Type II?
People with Usher type 2 are born hard of hearing and then develop a visual impairment called Retinitis pigmentosa (also known as RP). The RP generally starts in the teens and progresses throughout life. Today, most diagnoses are made before adulthood, but older people still report that the diagnosis wasn’t made until later in life when the RP became severe enough to interfere with their mobility.
Carriers of Usher Type II
When only one USH2A gene is knocked out, half the normal amount of usherin is produced. Scientists believe that this is sufficient for normal vision and hearing, therefore no clinical symptoms result. There is nothing obvious happening to people who are carriers for Usher type 2a or 2c. However, there are theoretical reasons to think that there may be very mild hearing and vision problems that occur with older adults, but this has never been studied. It may be that these genes are partly responsible for some of the hearing vision losses that all of us have as we grow older. The story for USH2C is exactly the same.
Hearing in Type II
People with Usher type 2 are believed to have been born with a moderate to severe hearing loss. The hearing loss is milder in the low frequencies and more severe in the higher one. Since most speech involves the lower frequencies, this means that adults and children with Usher type 2 will usually have good oral communication skills.
Vision in Type II
A person with Usher 2 may become legally blind as a young adult primarily because of severe tunnel vision. While central vision often stays good for most of their lives, a few Usher 2 people will become blind as older adults.
Balance in Type II
The is little evidence that the vestibular function in people with Usher Type II is as effected as with Usher Type I. Therefore people with Usher Type II rarely experience balance issues related to the Usher Syndrome.
Genes involved in Type II
Individuals with Usher Syndrome Type II typically have a sloping congenital hearing loss that is mild to moderate in the low frequencies and severe to profound in the high frequencies. Vestibular problems are absent in Type II, which distinguishes it from Type I. RP is still present with an onset typically sometime in adolescence. Usher Type II is also further subdivided into three types which have known gene locations associated with them, USH2A, GPR98 (VLGR1)and DFNB31 (WHRN). USH2A accounts for approximately 80%, GPR98 (VLGR1) accounts for approximately 15%, and DFNB31 accounts for approximately 5% of Type 2 cases. DFNB31 mutations have also been reported in several families with recessive nonsyndromic hearing loss.
Although these are the most common characteristics of Usher syndrome due to mutations in these genes, there can be variations. Overlapping and atypical presentations have been described for all three types of Usher syndrome. For example some mutations in USH2A can cause isolated retinitis pigmentosa without hearing loss.
Identified disease causing mutations in all of these genes include missense, nonsense, frameshift, splice-site as well as deletions distributed across nearly all exons.
Usher Type II subtypes
Usher subtype 2a is by far the most common form of Usher type 2 and likely accounts for over 80% of all cases. Subtype IIc appears to be uncommon. Because the genes for 2b and 2d have not been identified, we don’t have any idea about their frequencies, but we believe them to be uncommon. There is evidence that there are more Usher II subtypes that have not been recognized.