The finding, reported in the British journal Nature, lends scientific credence to a proposal made four decades ago that human language has genetic roots. The new study is based on a genetic analysis of a large British family with a speech and language disorder who first came to scientific attention in 1990 and of a more recently identified unrelated but similarly deficient individual.
Half of the 24 members of the three-generation family, called KE for privacy reasons, have difficulty controlling their lip and tongue movements, forming words and using and understanding grammar. They, as well as the unrelated individual with a similar deficiency identified only as CS, were found to have a mutation in a gene called SPCH1, also known as FOXP2.
"This is the first gene pinpointed for language development and will open the door to other insights into how genetic pathways are involved in language. We hope that this will provide better diagnosis and treatment in the future," lead author Dr. Anthony Monaco told United Press International.
"SPCH1 cannot act alone. It will need to cooperate with other genes and proteins. Its identification gives us a key to unlocking the molecular mysteries of this most human of characteristics," said Monaco, professor of human genetics at the University of Oxford and director and Wellcome principal research fellow at the Wellcome Trust Centre for Human Genetics in London. "It is extraordinary to think that we have now got an entry point into understanding one of the most important features that distinguish us from other animals."
The finding puts a whole new meaning into what naturalist Charles Darwin wrote in 1871: "Man has an instinctive tendency to speak, as we see in the babble of our young children, while no child has an instinctive tendency to bake, brew or write."
"Darwin's observation has just been supported in a way he could not have dreamed of, with the discovery of a gene that is mutated in a disorder of speech and language," said Steven Pinker of the Massachusetts Institute of Technology in Cambridge, Mass., author of the classic text "The Language Instinct" and of a commentary on the Nature report.
FOXP2 codes for a transcription factor, a protein that switches genes on and off. These genes may serve as signposts along the genetic network involved in language development, the study authors proposed. And because the gene is active in brain tissue, comparing its activities in a variety of non-human species may shed light on the intricate mechanisms that underlie human speech, they said.
The investigators identified the mutation using data from the Human Genome Project.
"The similarity of SPCH1, located on chromosome 7, to other known genes suggests its protein product is a transcription factor whose role is to modulate the activity of other genes," said Faraneh Vargha-Khadem, who headed the research at the Institute of Child Health in London.
"We used molecular genetic techniques called positional cloning to identify that this gene FOXP2 was the culprit," Monaco told UPI. "The link was not made before since the KE family and case CS are very rare patients with this particular speech and language disorder."
The individuals with the mutation have problems with articulation, grammar and identification of phonemes, the smallest unit of sound necessary for speech, divided into vowels and consonants. Unable to coordinate mouth and facial movements necessary for intelligible speech, they struggle with what comes naturally to most children -- learning to talk, the researchers said.
"We believe that the FOXP2 gene when mutated disrupts the normal development of the brain in specific areas that are necessary for both speech and language to develop," Monaco told UPI. "The consequences of this gene being missing or mutated on one chromosome is a severe speech and language disorder which has both motor difficulties in producing speech sounds and language difficulties in understanding the components of language -- grammar, phonemes, etc."
The finding has a two-fold significance, Pinker said.
"The 100 percent contingency between the gene and the syndrome is unusual in behavioral genetics. Most behavioral disorders probably depend on many genes. That's why the OGOD model (one gene, one disease) is though inappropriate for most disorders, like manic depressive disorder," Pinker told UPI. "The function of the gene can be guessed, and it looks like it may actually do something interesting when normal, rather than just messing something up when mutated."
Now that they have an inkling of the consequences of abnormalities in the gene, the scientists plan to study how the normal gene does its job to gain insight on the molecular level of how speech and language develop during childhood.
They also are looking into whether the gene is mutated in individuals with autism and other conditions marked by speech and language impediments.
"This particular severe speech and language disorder is very rare. Specific language impairment, called SLI, is very common and affects 4 percent of schoolchildren," Monaco told UPI. "In SLI there are no particular motor problems as were seen in the KE family, but there are difficulties in language development with normal range non-verbal abilities."
"Although we do not think the FOXP2 gene will be genetically responsible for the more common SLI problems, it does provide an entry point into the genetic and developmental pathway that culminates in speech and language, and we should be able to use this knowledge to help identify the genetic risks for more common SLI," he added.
The team also is studying more than 100 families with the more common conditions, searching for susceptibility genes. They will present their results at the American Society of Human Genetics annual meeting in San Diego later this month.
The Nature study "tells us that genes have a great influence on our ability to learn language and the ramifications for those with language difficulties are better diagnosis and treatment in the future," Monaco concluded.