UNMC, Boys Town National Research Hospital Researchers Locate Gene That Causes Combined Deafness and Blindness A team of researchers from the University of Nebraska Medical Center and Boys Town National Research Hospital has located the gene responsible for Usher syndrome Type IIa -- the leading cause of combined deafness and blindness in the industrialized world.
The discovery, which could lead to an eventual cure for the genetic disorder, is reported in the June 12 edition of Science.
"This is not only an important step forward in our understanding of disorders that affect both hearing and vision, but it will provide further insight into the physiology of vision and hearing," said Janos Sumegi, D.Sc., M.D., Ph.D., professor of pathology/microbiology at UNMC.
About four in 100,000 people suffer from Usher syndrome, a genetic disorder that causes moderate to severe hearing loss and the juvenile onset of retinitis pigmentosa -- one of the most common causes of blindness. It was first identified as a genetic syndrome by a clinician named Charles Usher in 1935. Of the almost 16,000 deaf and blind people in the United States, more than half are believed to have Usher syndrome.
During the past 10 years, genetic studies have been conducted to localize the genes responsible for Usher syndrome. As a result, three types of the disorder have been identified: Type I, Type II and Type III. Usher syndrome Type IIa is the most common type.
The identification of the Type IIa gene is the result of a collaborative effort during the past six years between Boys Town National Research Hospital and UNMC. The first step in the gene identification process was to localize the chromosomal region where the gene responsible for the genetic disorder is located. This process was conducted at Boys Town National Research Hospital under the direction of William Kimberling, Ph.D., director of genetic studies, with the help of research associates Michael Weston, M.S., and Denise Hoover, B.S.
Drs. Kimberling and Sumegi received funding support for this study through grants from the National Institute of Deafness and Other Communicative Disorders and the Foundation Fighting Blindness.
The second step in the gene identification process was the physical isolation of the gene located on the chromosome 1. This process was conducted at UNMC by James Eudy, Ph.D., a research instructor in pathology/microbiology, with the assistance of Sufang Yao, research technologist in pathology/microbiology, under the direction of Dr. Sumegi.
Dr. Eudy, who has been working on the project for the past three years, used a variety of techniques -- including DNA sequencing and computer-based analysis -- to isolate the Type IIa gene.
This is the second gene found to cause Usher syndrome. In 1995, Dr. Kimberling was part of a collaborative research group credited with finding the gene for Usher syndrome Type I. Subsequent research studies have found that this gene encodes a protein called myosin VIIa. This protein is found in both photoreceptor cells in the retina and in hair cells in the inner ear. The hair cells have tiny hair-like fibers which help turn sound waves into electrical impulses that travel to the brain via the auditory nerve.
Current research under way at UNMC and Boys Town National Research Hospital is focused on understanding how the Usher syndrome Type IIa protein functions.
Based upon the similarity of the predicted Usher syndrome Type IIa protein to other proteins, two hypotheses have been developed. The first hypothesis suggests that the Type IIa protein is similar to proteins that form the "packaging" around the cells, holding them in place and allowing them to communicate as a group. The second hypothesis suggests that the Type IIa protein is similar to a receptor -- a protein that spans the cell membrane, communicating the external environment to the inside of the cell so it can adapt to changing needs.
The testing of these hypotheses through experimentation will allow the research team to learn how mutations in the gene cause deafness and blindness. "We now have the tools to research the pathology of the disease, which ultimately could lead to a cure," Dr. Eudy said.
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