Usher syndrome is a genetic disorder which causes hearing and vision loss. The hearing loss is associated with a defective inner ear, whereas the vision loss is associated with retinitis pigmentosa (RP), a degeneration of the retinal cells. Usher syndrome has three clinical subtypes, denoted as I, II and III. People with Usher I are born profoundly deaf, and begin to lose their vision in the first decade of life. They also exhibit balance difficulties and learn to walk slowly as children, due to problems in their vestibular system. People with Usher II are not born deaf, but do have hearing loss. Patients with type III Usher syndrome may be unaffected in childhood but go on to develop a profound hearing loss and a very significant loss of sight by early to mid-adulthood.
Usher syndrome is inherited in an autosomal recessive pattern. "Recessive" means that both parents must contribute an appropriate gene for the syndrome to appear, and "autosomal" means that the gene is not carried on one of the sex chromosomes (X or Y), but rather on one of the 22 other pairs. Usher syndrome is responsible for the majority of deaf-blindness. The word syndrome means that multiple symptoms occur together, in this case, deafness and blindness. It occurs in roughly 1 person in 23,000 in the United States, 1 in 28,000 in Norway and 1 in 12,500 in Germany.
Since Usher syndrome results from the loss of a gene, gene therapy that adds the proper protein back ("gene replacement") may alleviate it, provided that the added protein becomes functional. Recent studies of mouse models have shown that one form of the disease — that associated with a mutation in myosin VIIa — can be alleviated by replacing the mutant gene using a lentivirus.