First Gene Linked to Language Discovered
Scientists identify mutation that causes speech disorder
WEDNESDAY, Oct. 3, 2001 (HealthDayNews) -- British scientists say they have identified the first gene that can be definitively linked to language, offering a glimpse into the genetic basis for the human ability to communicate through speech.
Neurogeneticists at the Wellcome Trust Center for Human Genetics at the University of Oxford say they've identified a mutation on that gene that appears to be responsible for a speech and language disorder called developmental apraxia of speech (DAS), also known as developmental verbal dyspraxia or developmental articulatory dyspraxia.
DAS is a relatively rare disorder characterized by problems with planning and making speech sounds.
Children with the disorder may not make cooing or babbling sounds as infants. They leave sounds out of words, mispronounce vowels and may have problems performing oral movements on command, such as puffing out their cheeks. Words with multiple syllables present the greatest challenge. While speech therapy is somewhat helpful, most children continue to struggle with speech.
"They have articulation problems," says Simon Fisher, co-author of the latest study. "They have problems with using grammar and general sort of language structures." Their comprehension of language is nearly normal, with most of their problems having to do with understanding grammar, but their verbal IQ is slightly lower than average, he says.
For several years, the British Institute of Child Health in London has been studying members of three generations of a family, known as "KE," with DAS.
While the language capabilities of family members were tested and their brains were scanned, not until 1996 had the role of genetics in the disorder been examined. In 1998, the Oxford group reported finding the general location of the suspected genes. Since then, they've been trying to find a specific gene. A report on the discovery appears in the Oct. 4 issue of Nature.
Fisher, a senior postdoctoral scientist, says from the outset, DAS appeared to involve a limited number of genes.
"In this case, it looks like there was one gene in this big pedigree causing the problems," says Fisher. "We were able to use fairly straightforward genetic methods to actually focus on it."
The gene, called FOXP2, is located on chromosome 7. In the KE family, a single mutation in the gene disrupts the sequence of a protein that tells other genes to switch on or off.
The researchers say they don't know the exact physiology of the mutation, but previous studies suggest that a region of the brain called the basal ganglia, which is involved in controlling movement, may be subtly different in people with DAS.
The researchers also say they can't tell when the gene mutation occurs. "We're guessing that it's during the development of the embryo, mainly because the family [doesn't] respond well to speech therapy," says Fisher. "It seems to be present pretty early on in their lives."
Lawrence Shriberg, a professor of communicative disorders and the principal investigator at the Waisman Center's Phonology Project and Clinic at the University of Wisconsin, says the study goes further than ever before in relating a genetic mutation to anatomic structures in the brain.
"These genes seem to control the development of the brain, and so the neural structures [related] to this genetic problem don't develop quite as well," he says.
Shriberg says that this may shed light on whether people with apraxia will, over time, outgrow the problem. "If indeed, some of the brain structures are involved, it calls into question whether the disorder does normalize or not." Clinical studies have not answered the question.
"One of the possibilities is that there's not one monolithic disorder, but actually several different types, possibly several different genetic types," says Shriberg. "There's certainly variability in the expression of the disorder within this family," both in terms of severity and test profiles, he says.
Shriberg says because there's no single set of diagnostic markers for this disorder, it's frequently misdiagnosed. It's also difficult to calculate exactly how common apraxia is, although he estimates it affects roughly one to two per 1,000 people.
"What's nice about this paper is that it really does point to the direction for better diagnostic classification of children," he says.
However, Fisher cautions, "FOXP2 is not necessarily the master switch. There will be other genes."
"This is the first time that we've got an entry point into a process where there may be other genes," he says. "Once we've got one of these genes, we can see what it switches on and off, and presumably, some of those will also be important in speech and language development."
What To Do
The American Speech-Language Hearing Association provides an overview of the disorder.
Shriberg recommends the Apraxia Kids Web site as a resource for parents of children with DAS.
If you'd like to surf at your own pace, try iLoveLanguages.com, a massive compilation of language-related Web sites.