Philadelphia--Deafness may be becoming more common, according to a study presented here on 6 October at a meeting of the American Society of Human Genetics. The researchers believe increased opportunities for deaf people to meet and marry each other may lead to a rise in frequency of an important class of deafness-causing mutations.
Gene mutations at more than 200 locations on the genome cause inherited deafness, which accounts for about half of deafness in general. Recently, clinical geneticists reported that faulty copies of a gene called Cx 26 cause about 35% of all cases of deafness.
That number seemed remarkably high to Walter Nance, a geneticist at Virginia Commonwealth University in Richmond. From a statistical analysis of data on 19th-century American families with deaf children, he estimated that in those days, Cx 26 mutations accounted for only 17% of inherited deafness. Nance reasoned that intermarriage might explain the increase in Cx 26 deafness, because two parents with this mutation will always have deaf children. Marriages among the deaf have been on the rise ever since the development of sign language and the establishment of schools for the deaf in the last 2 centuries.
To test this theory, Nance traveled to Mongolia, where intermarriage between the deaf is still relatively rare. There he collected DNA from about 100 deaf and 150 hearing people. As he suspected, very few of the deaf Mongolians had mutations in Cx 26.
Not everybody is convinced. "It's a reasonable hypothesis, because deaf people tend to marry deaf people," says David Kelsell, a geneticist at Queen Mary and Westfield College in London, United Kingdom. But he says Nance needs to repeat the study in other ethnic populations, because the frequency of Cx 26 mutations may have been different in Mongolians to begin with. Nance agrees; his lab has already begun looking at the frequency of Cx 26 mutations in Turkey.
If his theory is correct, Nance says, it would mean that intermarriage is not just increasing the percentage of deafness attributable to Cx 26, but also the overall frequency of deafness. So far, that has been hard to measure because there aren't good data on the occurrence of deafness in the past.