Distinctive audiometric features between USH2A and USH2B subtypes of Usher syndrome

Usher syndrome (USH) is an autosomal recessive disorder characterised by hearing impairment and retinitis pigmentosa (RP). The prevalence of USH varies from one population to another, for example, 3-4.4 per 100 000 in Scandinavian and North American populations1,2 and 6.2-10 per 100 000 in the city of Birmingham, UK.3 This syndrome is clinically heterogeneous and three clinical forms have been described4: (1) USH type I (USH1) is characterised by severe to profound congenital deafness, constant vestibular dysfunction, and prepubertal onset of RP; (2) USH type II (USH2) is characterised by congenital moderate to severe deafness, absence of vestibular dysfunction, and onset of RP usually in the late second to early third decade; (3) USH type III (USH3) is characterised by postlingual progressive deafness, occasional vestibular dysfunction, and progressive RP with a variable age of onset (see also http://www.ncbi.nlm.nih.gov/omim). Usher syndrome is also genetically heterogeneous; at least six distinct loci are responsible for USH1 (USH1A-F), three for USH2 (USH2A-C), and one for USH35 (Hereditary hearing loss homepage at URL, http://dnalab.www.uia.ac.be/dnalab/hhh). Usher syndrome type II (USH2) appears to be the commonest clinical form of the disorder in the American population, accounting for more than 50% of all USH cases.3,6 This clinical form tends to be rare in other populations.7 In the Tunisian population, only two USH2 families, Us8 and Z (this work), have been identified so far. These families were ascertained from villages from the south and the north of Tunisia, respectively, where endogamous marriage is relatively common for social and cultural reasons. Of all the USH2 subtypes, USH2A seems to be the most frequent. According to a study performed in various ethnic populations, USH2A is responsible for more than 85% of USH2 cases.9 This genetic form showed considerable phenotypic …