Deafness / New research may prevent the transmission of hearing impairment to offspring
by Tamara Traubman
More than 2,000 years ago, a small change occurred in one of the genes of a Jewish man, who lived in the city of Mosul in Iraq. The change caused the man to lose his hearing when he was in his 20s or 30s. The genetic change, or mutation in the language of geneticists, was passed on to some of his descendants, from generation to generation. In the meantime, the family grew and in the 50s immigrated to Israel. This week, a team of Israeli, Palestinian and American researchers who studied the family's DNA reported that they had located the same ancient mutation.
Prof. Keren Avraham, a geneticist from Tel Aviv University and one of the lead researchers on the team, explains that the deterioration of hearing caused by the mutation begins at a relatively late age, after the 20s. A lot of people don't want to recognize the hearing loss until it reaches such a severe state, so they have no choice but to put a hearing aid."
Prof. Moen Canaan from Bethlehem University, Prof. Marie-Claire King from the University of Washington in Seattle and other researchers also participated in the study, which will be published this week in the scientific journal "Proceedings of Sciences" of the National Academy.
Prof. Avraham began working with the family - known by the researchers by the code name "The N-Family" following a lecture on the biology of deafness, which she gave about four years ago. At the end of the lecture, a colleague approached her and told her that she has a friend whose husband is hearing impaired. The man, it turned out, is a member of a large family, many of whose descendants also suffer from hearing impairment.
Families like the large N family, where consanguineous marriages are common, are a convenient way to discover new mutations, because apart from the mutation, the genes of the family members are very similar. The mutation that causes deafness was found by comparing the DNA of the deaf and hearing members of the family. The research revealed that the family has a rather rare combination of two additional mutations that cause deafness, each causing hearing loss of a different degree of severity.
After finding the mutations, the researchers mapped their frequency in the family. They discovered that the mutation is a "recessive mutation", which causes hearing impairment only in family members who have inherited two defective copies of the gene - one from each parent.
"For some of the people in the 'N family, this discovery was very important, because this new information actually allows them to know if their children will also develop hearing loss," Avraham says. "Most of the other known mutations, related to hearing impairment, cause impairment even when only one defective copy is inherited. The fact that it is a recessive inheritance means that if the family members do not marry relatives, the chances of their children developing deafness are similar to those of the general population." Avraham says that the families were sent a letter with the results of the research; One of the family members replied: "I felt like I won the lottery."
Prof. Yossi Shila, head of the Department of Human Heredity and Molecular Medicine at Tel Aviv University, says that beyond its benefit for the N family, the discovery provides another layer in the biological understanding - not only of hearing loss, but also how normal hearing develops and what mechanisms control it.
The gene in which the mutations were discovered is responsible for a protein called "Myosin 3." Similar proteins, Avraham says, also exist in the fruit fly, mouse and other animals. However, she adds, while in humans the mutation causes the degeneration of the cochlear hairs, which are vital for hearing, a mutation in the same gene in the fruit fly actually causes degeneration of the retina and blindness - which she says raises the possibility of an evolutionary connection that once existed between the two systems.
