In their study, investigator Thomas B. Friedman, Ph.D., chief of the Laboratory of Molecular Genetics of the National Institute on Deafness and Other Communication Disorders (NIDCD), National Institutes of Health (NIH), Aihui Wang, a graduate student in Dr. Friedman's laboratory, and other colleagues studied two families from India and a family from Indonesia. Previously, Friedman, NIDCD scientists Yong Liang, Edward Wilcox, Ph.D., and colleagues showed that these families have a mutation in a gene on chromosome 17.
"We have now discovered that this gene codes for a protein that we named MYO15," said Friedman. "The discovery of three different mutations of MYO15 in three unrelated families is important because it suggests that MYO15 may account for a significant proportion of hereditary hearing loss," he added.
Approximately half of all types of deafness are inherited or passed along from parents to their children through their genes. Nonsyndromic recessive deafness, caused by two copies of the same mutated gene from each parent, accounts for approximately 80 percent of those cases. Recessive deafness occurs sporadically in families sometimes skipping several generations. Most individuals with recessive deafness have hearing parents and may have brothers and sisters who can hear and others who are deaf. The sporadic nature of this type of deafness along with the lack of other physical characteristics have made identification of individuals with the same altered or mutated gene difficult to identify and study.
These investigators used a mouse model to identify the new protein. "The MYO15 gene is virtually the same as the mouse Myo15 gene," said Dr. Friedman explaining that a mutation in the mouse Myo15 gene was recently discovered by his collaborators, Sally Camper, Ph.D., and graduate student Frank Probst, at the University of Michigan, also causing deafness in the shaker-2 mouse.
Myosins are a large family of proteins that are found in most human cells. There are fourteen classes of myosin known as conventional and unconventional. Conventional myosins aid muscle contraction; the functions of the unconventional myosins are currently unclear. One type of unconventional myosin, Myosin 7a, has previously been implicated in three other forms of deafness. These three types of deafness are Usher syndrome type 1 which is characterized by profound deafness from birth and severe balance problems, and a dominant as well as a recessive type of deafness that has so far only been identified in families from Tunisia. Mutation of another unconventional myosin, myosin 6, has been implicated in the deafness of another mouse model, the Snell's waltzer mouse.
"Information gained from the mouse model leads us to speculate that MYO15 plays a crucial role in the functioning of the inner ear hair cells," explained Dr. Friedman. These hair cells reside in the cochlea of the inner ear and are involved in sensory transduction or the conversion of sound waves into electrical signals that are sent to the brain where meaning is attached to them. Dr. Friedman added that mutations of MYO15 seem to interfere with some aspect of sensory transduction and result in deafness.
"This research points to a critical role played by myosin in the ability of the inner ear to function normally and exemplifies the importance of progress created when teams of scientists work in close collaboration," commented NIDCD director James F. Battey, Jr., M.D., Ph.D.
This study was a collaborative effort between scientists in Dr. Friedman's laboratory and scientists from the University of Michigan at Ann Arbor, and NIH's National Human Genome Research Institute.
As the nation's focal point for research in human communication, the NIDCD conducts and supports biomedical and behavioral research and research training on normal mechanisms as well as diseases and disorders of hearing, balance, smell, taste, voice, speech and language that affect 46 million Americans.