Mutations in the gene (MYO7A) encoding myosin-VIIa, a member of the large superfamily of myosin motor proteins that move on cytoplasmic actin filaments, and in the USH2A gene, which encodes a novel protein resembling an extracellular matrix protein or a cell adhesion molecule, both cause Usher syndrome (USH), a clinically heterogeneous autosomal recessive disorder comprising hearing and visual impairment. Patients with USH1 have severe to profound congenital hearing impairment, vestibular dysfunction, and retinal degeneration beginning in childhood, while those with USH2 have moderate to severe hearing impairment, normal vestibular function, and later onset of retinal degeneration. USH3 is characterized by progressive hearing loss and variable age of onset of retinal degeneration. The phenotype resulting from MYO7A and USH2A mutations is variable. While most MYO7A mutations cause USH1, some cause nonsyndromic hearing impairment, and one USH3 phenotype has been described. USH2A mutations cause atypical USH as well as USH2. MYO7A is on chromosome region 11q13 and USH2A is on 1q41. Seven other USH genes have been mapped but have not yet been identified. USH1A, USH1C, USH1D, USH1E, and USH1F have been assigned to chromosome bands 14q32, 11p15.1, 10q, 21q21, and 10, respectively, while USH2B is on 5q, and USH3 is at 3q21-q25. Myosin VIIa mutations also result in the shaker-1 (sh1) mouse, providing a model for functional studies. One possibility is that myosin-VIIa is required for linking stereocilia in the sensory hair bundle; another is that it may be needed for membrane trafficking. The ongoing studies of myosin-VIIa, the USH2A protein, and the yet to be identified proteins encoded by the other USH genes will advance understanding of the Usher syndromes and contribute to the development of effective therapies. Am. J. Med. Genet. (Semin. Med. Genet.) 89:158-166, 1999.
Copyright 2000 Wiley-Liss, Inc.