MYO7A

The MYO7A gene provides instructions for making a protein called myosin VIIA, which is part of a group of proteins called unconventional myosins. These proteins, which have similar structures, each play a role in transporting molecules within cells. Myosins interact with actin, a protein that is important for cell movement and shape. Researchers believe that myosins use long filaments of actin as tracks along which to transport other molecules.

Myosin VIIA is made primarily in the inner ear and the light-sensitive tissue at the back of the eye (the retina). In the inner ear, myosin VIIA plays a role in the development and maintenance of hairlike projections called stereocilia. Stereocilia, which are rich in actin, line the inner ear and bend in response to sound waves. This bending motion is critical for converting sound waves to nerve impulses.

Myosin VIIA is also found in the pigmented cells of the retina (the retinal pigment epithelium or RPE), and probably plays a similar role in the development and maintenance of this tissue. Research suggests that one function of myosin VIIA is to carry small sacs of pigment called melanosomes within the retinal pigment epithelium.

nonsyndromic deafness - caused by mutations in the MYO7A gene

At least four mutations in the MYO7A gene have been identified in people with a form of nonsyndromic deafness (hearing loss without related signs and symptoms affecting other parts of the body) called DFNA11. Autosomal dominant inheritance means that one altered copy of the gene in each cell is sufficient to cause the condition. Most of these mutations alter a single protein building block (amino acid) in myosin VIIA, resulting in an abnormal protein that does not work properly. Other genetic changes delete a small amount of DNA from critical regions of the MYO7A gene, which probably changes the structure of the protein. Researchers suspect that the altered protein causes hearing loss by disrupting the growth and organization of stereocilia in the inner ear.

A few studies have associated MYO7A mutations with an autosomal recessive form of nonsyndromic deafness called DFNB2. Autosomal recessive inheritance means that two copies of the gene in each cell are altered. Researchers suspect that these cases may actually represent early forms of Usher syndrome, which is characterized by hearing loss and a vision disorder called retinitis pigmentosa. It is not known whether all children with MYO7A mutations and hearing loss will develop retinitis pigmentosa later in life.

Usher syndrome - caused by mutations in the MYO7A gene

More than 120 mutations in the MYO7A gene have been identified in people with Usher syndrome type 1B. Many of these genetic changes alter a single protein building block (amino acid) in critical regions of the myosin VIIA protein. Other mutations introduce a premature stop signal in the instructions for the myosin VIIA protein. As a result, an abnormally small version of this protein is made. Some mutations insert or delete small amounts of DNA in the MYO7A gene, which alters the protein. All of these changes cause the production of a nonfunctional myosin VIIA protein that adversely affects the development and function of cells in the inner ear and retina, resulting in Usher syndrome.