USHER SYNDROME AND ITS GENETIC CHARACTERIZATION

Usher syndrome is defined as an autosomal recessive disorder which is caused by a mutation in any one of at least 10 genes resulting in a combination of hearing loss and visual impairment. The loss of hearing is caused by a defect in the inner ear, whereas degeneration of the retinal cells called retinitis pigmentosa (RP), results in loss of vision. Based on severity and age whensigns and symptoms appear, there are three clinical subtypes of Usher syndrome (characterized as Usher I, II, and III). People suffering from Usher I are deaf by birth and during the first decade of life, they begin to lose their vision. Balance difficulties are also exhibited by them. Patients with Usher II have hearing loss but they are not deaf by birth. They also lose their vision later on. They do not show problems with balance. People having Usher syndrome III lose their vision and hearing gradually but they are not born deaf; they may or may not show difficulties with balance. In this review, those mouse models for usher syndrome will be discussed in which homologue of humans was identified first and a model called, “mouse model” was based upon gene defects in the human beings.