A high rate of marriage among deaf individuals can explain the increased frequency of connexin deafness in the United States and may have led to a doubling of its occurrence during the past 200 years, according to a study by hereditary deafness experts at Virginia Commonwealth University
The VCU researchers used an innovative computer simulation to show that intermarriage among the deaf can dramatically accelerate the frequency of mutations in the gene encoding the protein connexin 26 that are responsible for most of the inherited hearing loss in the United States. That occurs because parents who both have connexin deafness pass the gene mutation to their child, usually causing deafness at birth and making a disproportionate contribution to the pool of deaf individuals in the next generation, according to the study which will be published in the June issue of the American Journal of Human Genetics. The study was published early online.
"In the United States, at least 85 percent of individuals with profound deafness marry another deaf person," says Dr. Walter Nance, professor of human genetics and lead author on the article. "Because we now know that more than 100 different genes are responsible for deafness, most deaf parents have children with normal hearing because they pass different genes to their offspring.
"But in the case of marriages among couples who both have the same form of recessive deafness, all their children will be deaf and capable themselves of also passing on the altered gene to their offspring. In addition, as many as 3.5 percent of the hearing population in the United States may carry single mutations involving the connexin 26 complex, making this one of the most commonly recognized single gene defects."
About one in 800 infants has profound hearing impairment, and about half of these cases are thought to be genetic in origin. Investigators are interested in learning more about deafness genes because deafness sometimes is accompanied by other conditions, such as blindness, and they are seeking ways to avoid or prevent these serious side effects.
Nance notes that intermarriages among the deaf started to increase about 1800 after the first schools to teach sign language were opened, improving the social and economic circumstances of the deaf and allowing them to communicate among themselves and with non-deaf individuals who learned sign language. Before that time, marriages among the deaf were rare.
"In regions where national or statewide schools for the deaf have been established and marriages among students have occurred, we've seen an amplication of the commonest form of recessive deafness in the overall population," he said.
These findings about how sign language contributed to intermarriages among the deaf and led to an increase in the frequency of connexin deafness in the United States and other developed countries have led Nance and his colleagues to speculate about how this genetic mechanism also may have been involved in the evolution of speech. The acquisition of syntactic speech generally is viewed as the most significant event in human evolution because it set man apart from other primates and led to an explosive evolution of the human brain by facilitating the transfer abstract knowledge, experience and culture.
There is general agreement that there has been an explosive acceleration in human evolution in the last 100,000-200,000 years, but the precise way in which the fixation of favorable genes in our species was accelerated has been one of the most important unresolved problems about human evolution.
"When you think about how the onset of selective marriages among deaf populations led to an increase in specific mutations for deafness, you easily can see how these same forces might have contributed to the spread of genes for speech among Homo sapiens 160,000 years ago," Nance said. "If you were one of the first primates with an ability to communicate by speaking, wouldn't you want to select a partner who could whisper sweet nothings in your ear?" EDITORS' NOTE: A copy of Dr. Nance's article is available via e-mail in PDF format or by fax. For information, please call VCU News Services at (804) 828-1231. About VCU and the VCU Medical Center: Located on two downtown campuses in Richmond, Va., Virginia Commonwealth University is ranked nationally by the Carnegie Foundation as a top research institution and enrolls 26,000 students in more than 170 certificate, undergraduate, graduate, professional and doctoral programs in the arts, sciences and humanities in 11 schools and one college. Forty of the university's programs are unique in Virginia, and 20 graduate and professional programs have been ranked by U.S. News & World Report as among the best of their kind. MCV Hospitals, clinics and the health sciences schools of Virginia Commonwealth University compose the VCU Medical Center, one of the leading academic medical centers in the country. For more, see www.vcu.edu.