Scientists report shoking findings, which seem to give biological explanation of stuttering.
They have identified genetic variations associated with stuttering, and the study's senior author says his team was surprised that two of the implicated genes were linked to rare, fatal metabolic disorders.
"We're extremely fortunate that these genes are extremely well-known and studied by people who have been in this field for decades," says Dennis Drayna, a geneticist at the National Institute on Deafness and Other Communication Disorders. The findings appear in today's New England Journal of Medicine.
About one in 20 preschoolers stutter, but most outgrow it. Still, about one in 100 adults continue to stutter. Previous studies suggested that genetics plays a role in about half of stuttering cases. Research pointed to chromosome 12.
By studying members of a large extended Pakistani family who stutter and unrelated people who stutter, Drayna and his co-authors linked the disorder to 10 mutations in three chromosome genes, which he says account for about 10% of inherited cases. In the USA, Drayna says, that would be about 150,000 people.
One of the genes had never been linked to a human disorder, but the other two are associated with metabolic diseases called mucolipidosis I and II.
Perhaps 10 U.S. newborns a year are diagnosed with these disorders, Drayna says. They didn't inherit a normal copy of the relevant gene from either parent, so they lack an enzyme needed to break down and recycle components of cells. Buildup of these components causes a variety of problems, including speech, and affected children usually die by age 10. But stuttering is the only problem in the people Drayna studied, because they inherited one functioning and one malfunctioning copy.
USA Today has contributed to the report.