Ten causative genes have so far been identified. Usher syndrome is caused by autosomal recessive inheritance of mutations in Usher genes known to encode proteins involved in transmembrane adhesion, scaffolding and motor transport. The Genetics and Biology of Usher Syndrome This review is limited to the combination of RP and deafness, the classical presentation of Usher syndrome. Many other inherited retinal diseases are associated with deafness ( Table 2) such as cone-rod dystrophy and hearing loss-1 (CRDHL1, OMIM #617236), diabetes and deafness, maternally inherited (MIDD, OMIM #520000) and Leber congenital amaurosis (LCA) with early-onset deafness (LCAEOD, OMIM #617879). In classical RP, the death of cones may be secondary to rod degeneration and this may ultimately lead to complete loss of vision in advanced age ( Hartong et al., 2006). Subsequent constriction of the visual field results in a “tunnel vision” effect caused by the centripetal progression of cone photoreceptor cell loss. The clinical presentation of RP begins with night blindness caused by the degeneration of rod photoreceptor cells. Hearing loss in type 3 patients is progressive and post-lingual, while RP onset may be delayed until mid-adulthood ( Reiners et al., 2006). Type 2 patients have mild to moderate congenital hearing loss with RP diagnosed during puberty. The majority of type 1 patients also have developmental motor delays caused by vestibular dysfunction. Type 1 patients are born profoundly deaf and experience pre-pubertal onset of progressive vision loss caused by RP. Usher subtypes (1, 2, and 3 Table 1) are graded according to the severity of symptoms and age of onset. It is the most common form of inherited deaf-blindness ( El-Amraoui and Petit, 2014) affecting an estimated 1 in 6,000 people worldwide ( Kimberling et al., 2010). Some forms of Usher syndrome may also have varying levels of vestibular dysfunction resulting in loss of balance. Usher syndrome is a group of autosomal recessive disorders characterized by congenital neurosensory hearing loss, progressive night vision impairment, and constriction of the visual field due to retinitis pigmentosa (RP).
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