A Gene Critical for Speech

Newly discovered stretch of DNA may have been important in evolution of language.

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By Tina Hesman Saey, Science News

HONOLULU—Humans may owe the gift of gab to a newly discovered gene that helps keeps vocal pipes limber.

Researchers discovered the gene, dubbed tospeak, in an Australian family with a speaking disorder. Many of the women in the family have weak, husky voices, while several of their male relatives cannot speak above a whisper, reported Raymond Clarke of the University of New South Wales' St. George Hospital in Kogarah, Australia, October 21 at the annual meeting of the American Society of Human Genetics. 

Clarke and his colleagues traced the source of the family's disorder to a region of chromosome 8. Part of the chromosome had been rearranged, causing a break in the tospeak gene. Tospeak probably doesn't code for a protein, Clarke says. The gene is sprinkled with stop signals and its RNA product doesn't resemble other RNAs that have specific functions in the cell. But production of the tospeak RNA seems to be important for proper development of the larynx.

Members of the family who have the speaking problem have short, thick vocal cords that don't vibrate properly. Some of the family members also have fused bones in the wrists and feet, known as the carpals and tarsals, and fused vertebrae in their spines. All of the defects may be linked to a breakdown in relations between tospeak and a neighboring gene, known as GDF6, the researchers report. 

The GDF6 gene helps control bone and eye development. Loss of the gene leads to fusion of tarsals and carpals in mice. The researchers also found that mutations in GDF6 lead to fusion of neck vertebrae in people with a genetic disorder known as Klippel-Feil syndrome. All of those discoveries indicate that GDF6 helps keep joints flexible, Clarke says. The new study suggests that the gene may also be responsible for allowing humans to stretch their vocal cords and create a range of sounds necessary for speech.

Clarke's group found that the tospeak gene overlaps a control region of GDF6, which regulates GDF6's activity in the carpals and tarsals. Disrupting the tospeak gene also appears to interfere with GDF6 activity. 

Tospeak first appeared in primates, the researchers discovered by looking for the gene in a variety of species. In humans, part of the genetic control panel that governs tospeak activity was duplicated, resulting in higher activity of the human version of the gene than is found in chimpanzees or other primates. Revved-up tospeak activity probably tweaks GDF6 levels as well, giving humans more flexible voice boxes than other primates, Clarke says. That advantage could have allowed humans to develop language, he says.

Other researchers say that changes in the brain were probably far more important for the evolution of language than alterations of the larynx. “It is reasonable to expect that anatomical and physical changes have expanded our vocal repertoire,” says Simon Fisher, a geneticist at the Wellcome Trust Centre for Human Genetics in Oxford, England, but “the idea that it's the larynx that underlies speech is overly simplistic.”

But brain changes that lead to language wouldn't be possible without the capacity for speech, Clarke says. “You've got to have a brain [to speak], that's true,” he says. “There must be a marriage between the brain and the structural capacity.”