June 4, 1998 ANN ARBOR---On June 23, 1997, a mouse was born in a laboratory at the University of Michigan Medical School. This mouse---affectionately known as Sebastian---was different from his seven litter mates, his mother and father and all the other mice in his ancestral line. Thanks to U-M scientists and genetic engineering technology, this mouse could hear.
Sebastian is a shaker-2 mouse---a strain descended from a mouse exposed to X-rays in 1928. Because of an X-ray-induced mutation on mouse chromosome 11, which has been passed on to later generations, shaker-2 mice are born with inner ear defects which cause deafness and balance abnormalities.
In a paper published in the May 29 issue of Science, U-M scientists describe how they used transgenic technology to find the recessive mutated gene responsible for deafness in shaker-2 mice. By injecting short sections of normal cloned DNA into fertilized mouse eggs and then waiting to see which DNA clone produced a hearing mouse, U-M scientists were able to focus their search for the mutant gene in a small area and find it faster than would have been possible without transgenic technology.
The U-M study represents the first permanent correction of a deafness-related genetic mutation and the fifth time that identification of a deafness gene in mice helped scientists find a similar gene in humans---according to Sally A. Camper, associate professor of human genetics in the U-M Medical School, who directed the research.
"There are at least 12 other deafness-related mutations where the gene remains unknown," Camper said. "Finding the defective gene is the first step toward developing new treatments which someday could restore hearing in children and adults."
"The next step is to develop delivery vehicles to introduce the normal gene into inner ear cells of individuals who carry these deafness genes," said Yehoash Raphael, assistant professor of otolaryngology in the U-M Medical School, who directed microscopy studies for the project. "Once adequate vectors are available, gene therapy for genetic-based deafness will become a reality."
Frank J. Probst, a graduate student in human genetics in the U-M Medical School, and lead author of the Science paper, identified the single-point mutation on mouse chromosome 11. "The normal gene and the shaker-2 gene are nearly identical except for one DNA base-pair out of more than 30,000 base-pairs in the gene."
Unfortunately, this tiny change in the DNA occurs right in the middle of the genetic blueprint for a key enzyme involved in inner ear development--according to Camper. "We discovered that the gene carries coding instructions for a previously unknown myosin enzyme, which we've named Myo15," Camper said.
Myosin enzymes are common in animal cells--especially in muscle cells where they convert stored energy into motion making it possible for muscles to contract and move. "We think Myo15 is a different type of myosin enzyme, which works like a tiny bulldozer grabbing proteins or other cell components and moving them to different locations," Camper explained.
Although more research will be needed to know for sure, Camper and Probst believe that one of Myo15's functions may be to transport a protein called actin to inner ear hair cell fibers during their development. These fibers or stereocilia move in response to changes in sound frequency like a field of wheat moves in response to changes in wind speed and direction. Their movement sends electrical signals to auditory nerves, which the brain translates into sound.
"Hair cell stereocilia in shaker-2 mice look as if they've been mowed down," Raphael said. "The cells are alive, but the stereocilia are stunted." "We suspect the mutation in the Myo15 gene changes a single amino acid in the myosin enzyme, so it cannot bind to actin in the developing hair cells," Probst said. "If they can't get actin to the right place, the hair cells don't develop normally and the mouse can't hear."
In a related paper in the same issue of Science, Thomas B. Friedman and other scientists at the National Institute on Deafness and Other Communication Disorders, National Institutes of Health, describe how the correction of the deafness gene in shaker-2 mice helped them find a nearly identical gene (DFNB3) on human chromosome 17, which encodes the same type of myosin. Mutations in DFNB3 may produce congenital deafness in humans, just as mutations in shaker-2 do in mice.
"Interaction between scientists working with the mouse genome and the human genome made it possible to locate these genes so quickly," Camper said. "It's a perfect example of how transgenic technology in mice can contribute to research with the potential to help people."
Other researchers from the U-M Medical School collaborating in the study included Thomas L. Saunders, senior research associate and director of the Transgenic Animal Model Core, and Robert H. Lyons, Jr., assistant professor of biological chemistry and director of the DNA Sequencing Core.
The study was funded by the National Institute on Deafness and Other Communication Disorders, the National Institutes of Health and the National Science Foundation.
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