Syndromic hereditary deafness. Usher's syndrome. Oto-neurologic and genetic factors

Usher syndrome (USH) is an autosomal recessive hereditary disorder characterized by congenital bilateral sensorineural hearing loss and progressive loss of vision due to retinitis pigmentosa (RP). The prevalence of Usher syndrome is estimated to be 3-4.4 cases per 100.000 people. Several clinical types have been distinguished by age at onset, rate of progression, and severity of symptoms. Type I (USH1) is characterized by a congenital, severe-to-profound deafness and absent vestibular function. Type II (USH2) shows a congenital and moderate-to-severe hearing loss and normal vestibular response. It is also suggested a third type (USH3), clinically similar to USH2, but with progressive hearing loss. Genetic heterogeneity of USH is quite extensive. Up to now, seven different loci responsible for the defect are known: 14q, 11q, 11p, 10q and 21q for USH1; 1q for USH2 and 3q for USH3. Moreover, there are USH1 and USH2 families that fail to show linkage to these candidate regions demonstrating that should exist other loci causing USH, although their ubications are unknown. To date, only two genes involved in the USH pathology are known, although together they are responsibles of about the 80% of total USH cases: myosin VIIA, an unconventional myosin, involved in the USH1b phenotype and a protein similar to the laminina, responsible for the USH2a phenotype.