Researchers Discover Gene that Causes Deafness
Researchers have found a new genetic mutation responsible for deafness and hearing loss associated with Usher syndrome type 1
These findings, published in the Sept. 30 advance online edition of the journal Nature Genetics, could help researchers at the University of Cincinnati (UC) and Cincinnati Children's Hospital Medical Center develop new therapeutic targets for those at risk for this syndrome.
Partners in the study included the National Institute on Deafness and other Communication Disorders (NIDCD), Baylor College of Medicine and the University of Kentucky.
Usher syndrome is a genetic defect that causes deafness,
night-blindness and a loss of peripheral vision
through the progressive degeneration of the retina.
"In this study, researchers were able to pinpoint the gene which caused deafness in Usher syndrome type 1 as well as deafness that is not associated with the syndrome through the genetic analysis of 57 humans from Pakistan and Turkey," says Zubair Ahmed, PhD, assistant professor of ophthalmology who conducts research at Cincinnati Children's and is the lead investigator on this study.
Ahmed says that a protein, called CIB2, which binds to calcium within a cell, is associated with deafness in Usher syndrome type 1* and non-syndromic hearing loss.
Ahmed: "To date, mutations affecting CIB2 are the most common and prevalent genetic cause of non-syndromic hearing loss in Pakistan. However, we have also found another mutation of the protein that contributes to deafness in Turkish populations."
"In animal models, CIB2 is found in the
mechanosensory stereocilia of the inner earhair cells,
which respond to fluid motion and allow hearing
and balance, and in retinal photoreceptor cells,
which convert light into electrical signals in the eye,
making it possible to see,"
Saima Riazuddin, PhD
assistant professor, UC's department of Otolaryngology
Researchers found that CIB2 staining is often brighter at shorter row stereocilia tips than the neighboring stereocilia of a longer row, where it may be involved in calcium signaling
Calcium signaling regulates mechano-electrical
transduction, a process by which the ear converts
mechanical energyor energy of motioninto
a form of energy that the brain can recognize as sound.
Ahmed : "With this knowledge, we are one step closer to understanding the mechanism of mechano-electrical transduction and possibly finding a genetic target to prevent non-syndromic deafness as well as that associated with Usher syndrome type 1."
This syndrome is characterized by deafness and a gradual 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. Usually, the rod cells of the retina are affected first, leading to early night blindness and the gradual loss of peripheral vision. In other cases, there is early degeneration of the cone cells in the macula, leading to a loss of central acuity. In some cases, the foveal vision is spared, leading to "doughnut vision"; central and peripheral vision are intact, but there is an annulus around the central region in which vision is impaired.
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. They do not seem to have noticeable problems with balance; they also begin to lose their vision later (in the second decade of life) and may preserve some vision even into middle age. People with Usher syndrome III are not born deaf, but experience a gradual loss of their hearing and vision; they may or may not have balance difficulties.
Usher syndrome is a variable condition; the degree of severity is not tightly linked to whether it is Usher 1, 2 or 3. For example, someone with Type 3 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. Similarly, someone with Type 1, who is therefore profoundly deaf from birth, may keep good central vision until the sixth decade of life, or even beyond. People with Type 3, who have useful hearing with a hearing aid, can experience a wide range of severity of the RP. Some may maintain good reading vision into their sixties, while others cannot see to read while still in their forties.
Usher syndrome I and II are associated with a mutation in any one of six or three different genes, respectively, whereas only one mutation has been linked with Usher III. Since Usher syndrome is inherited in an autosomal recessive pattern, both males and females are equally likely to inherit Usher syndrome. Consanguinity of the parents is a risk factor. Children of parents who both are carriers of the same mutation have a one fourth chance of inheriting the condition and children of such parents who are unaffected have a two thirds chance of being carriers. Children of parents where only one parent is a carrier have a no chance of having the disease but have a one half chance of being a carrier. First recognized in the 19th century, Usher syndrome was the first condition to demonstrate that phenotypes could be inherited in tandem; deafness and blindness are inherited together, but not separately. Animal models of this human disease (such as knockout mice and zebrafish) have been developed recently to study the effects of these gene mutations and to test potential cures for Usher syndrome. Other researchers involved in the study include Thomas Friedman, PhD, and Inna Belyantseva, MD, PhD, from the NIDCD; Suzanne Leal, PhD, and her team at Baylor; and Gregory Frolenkov, PhD, and his team at the University of Kentucky.
This study was funded by the NIDCD, the National Science Foundation and the Research to Prevent Blindness Foundation.
Original article: http://www.eurekalert.org/pub_releases/2012-09/uoca-rdg092712.php