Researchers discover cause of speech defect
October 25, 2005 - Scientists are now one step closer to understanding the genetic link to language impairment, thanks to a key discovery by a team of researchers including a University of Alberta medical geneticist.
Dr. Martin Somerville and colleagues from the University of Toronto and elsewhere around the world have found a specific genetic abnormality in a nine-year-old boy from northern Alberta with learning difficulties and speech problems. The researchers discovered the boy carries additional copies of some 27 genes on chromosome seven, which determines language development.
"It's an odd situation, because it's a discovery of something we knew was out there but just hadn't found," said Somerville, director of the Molecular Diagnostic Laboratory at the Stollery Children's Hospital. He said the boy's condition is the reverse of a known condition called Williams-Beuren syndrome (WBS), in which the same genes are deleted. Patients with WBS, while exhibiting mild mental retardation and difficulty with spatial skills, demonstrate a strong ability in expressive language.
"In (the nine-year-old boy's) case, he's got extra copies of these same genes and that seems to actually impair his language," said Somerville. "It's sort of odd, because it seems that having too many of these is what is causing the problems."
The boy can understand what is said to him at the level of a seven-year-old but his expressive language ability is at the level of a two-and-a-half-year-old. He is unable to form complete words but shows normal spatial ability.
"For example, if asked to tell us what animal has long ears and eats carrots, he could only pronounce the r in the word rabbit but was able to draw the letter on the blackboard and add features such as whiskers," said Dr. Lucy Osborne of the University of Toronto, lead author on the study.
Somerville explained that during the development of a fertilized egg, chromosomes share information between parents' DNA, usually in a balanced way. "But sometimes that gets misaligned, and that's how the deletion happens in Williams-Beuren syndrome. Every time that deletion happens, the DNA that's removed gets added to the other chromosome...that's how we knew it was out there."
In addition to adding more pieces to a complicated puzzle, Somerville said the discovery has promising implications for treatment in the future.
"In the short term it's helping us understand a little bit more about language development," he said. "But realistically in the long term, it gives us some pretty good targets for intervention, especially in children who have this genetic change."
He added it may one day be possible to "turn down the volume on those particular genes," he said, cautioning that it remains "science fiction at this point." The condition is estimated to affect about half a million people worldwide.
The study was published in the Oct. 20 issue of the New England Journal of Medicine and included authors from the Universities of Toronto and Alberta (Dr. Stephen Bamforth and Dr. Margaret Lilley, in addition to Somerville), the University of Louisville, the Universitat Pompeu Fabra in Barcelona and the Nevada School of Medicine. It was supported by grant money from the Canadian Institutes of Health Research and the Sick Kids Foundation, the Spanish ministries of health, science and technology, Genome Canada/Ontario Genomics Institute, the National Institute of Neurological Disorders and Stroke and the National Institute of Child Health and Human Development.
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