These genes function in the development and maintenance of inner ear structures such as hair cells (stereocilia), which transmit sound and motion signals to the brain.
Type I is more common in people of Ashkenazi Jewish ancestry (central and eastern European) and in the French-Acadian populations (Louisiana).
[5] Among Acadians, research into haplotype data is consistent with one single mutation being responsible for all cases of Usher syndrome type I.
Usherin is critical for the proper development and maintenance of these structures, which may help explain its role in hearing and vision loss.
[citation needed] People with Usher syndrome III are not born deaf but experience a progressive loss of hearing, and roughly half have balance difficulties.
Usually, the rod cells of the retina are affected first, leading to early night blindness (nyctalopia) and the gradual loss of peripheral vision.
Several genes have been associated with Usher syndrome using linkage analysis of patient families (Table 1) and DNA sequencing of the identified loci.
[11] A study shows that three proteins related to Usher syndrome genes (PCDH15, CDH23, GPR98) are also involved in auditory cortex development, in mouse and macaque.
The qualifier pigmentosa reflects the fact that clumps of pigment may be visible by an ophthalmoscope in advanced stages of degeneration.
[16] The hearing impairment associated with Usher syndrome is caused by damaged hair cells in the cochlea of the inner ear inhibiting electrical impulses from reaching the brain.
[citation needed] Since Usher syndrome is incurable at present, it is helpful to diagnose children well before they develop the characteristic night blindness.
[citation needed] The simplest approach to diagnosing Usher syndrome is to test for the characteristic chromosomal mutations.
Similarly, someone with type I, who is therefore profoundly deaf from birth, may keep good central vision until the sixth decade of life or even beyond.
Recent studies of mouse models have shown one form of the disease—that associated with a mutation in myosin VIIa—can be alleviated by replacing the mutant gene using a lentivirus.
[19] However, some of the mutated genes associated with Usher syndrome encode very large proteins—most notably, the USH2A and GPR98 proteins, which have roughly 6000 amino-acid residues.
Three years later, one of his students, Richard Liebreich, examined the population of Berlin for disease patterns of deafness with retinitis pigmentosa.
[25] Liebreich noted Usher syndrome to be recessive since the cases of blind-deafness combinations occurred particularly in the siblings of blood-related marriages or in families with patients in different generations.
His observations supplied the first proofs for the coupled transmission of blindness and deafness since no isolated cases of either could be found in the family trees.