Epidemiology
Usher syndrome affects many ethnic groups and has a prevalence of about one in 20,000 in the Caucasian population. It is thought to be responsible for 3-6% of all childhood deafness and about 50% of combined deaf-blindness in adults.
The prevalence of Usher syndrome in persons of Scandinavian descent has been estimated at around 1:29,000. Usher syndrome affects about one in 23,000 in the U.S. and one in 12,500 in Germany. The prevalence of Usher syndrome in Heidelberg (Germany) and its suburbs has been calculated to be 1:16,000, with a ratio of type 1 to type 2 of 1:3.
Usher syndrome types 1 and 2 are the most common forms. Together, they account for approximately 90-95% of all cases. USH 1 is estimated to occur in at least 1 per 25,000 people. As the genetic causes differ, some subtypes are more common than others, e.g. USH 1B is the most common form of USH type 1 accounting for over 40% of all cases, while USH1D appears to be responsible for about 25% of all cases. Subtype 1C seems more common among French Acadians in Louisiana (USA), e.g. the c.216G>A mutation in USH1C accounts for virtually all USH1 cases in this population. Only a few cases of USH 1F and 1G have been reported. A form of USH 1F may be more common among people with Jewish ancestry, e.g. the c.733C>T (p.R254X) mutation in the PCDH15 gene is present in up to 60% of USH1 families of Ashkenazi origin. The exact prevalence of USH type 2 is currently unknown. USH2A mutations (c.2299delG) are involved in 55%-90% of all USH2 cases and account for more than 60% of all pathogenic alleles in Jewish families of non-Ashkenazi descent. A predominant USH2A founder mutation (c.4338_4339delCT) in Quebec French-Canadians has recently been identified. In most populations, type 3 accounts for a very small proportion of all Usher cases (only 1-6% of all Usher syndrome cases). Due to a founder effect, type 3 is more common in the Finnish population and in Ashkenazi Jews where it accounts for about 40% of all cases, as against 2% in most other countries. USH3 was reported to account for 20% of the USH cases in the city of Birmingham (United Kingdom).
