When we think of genetically affected characteristics, we often think of hair and eye color, height, and the size of our nose. But what about speech? This week, Changsoo Kang and Dennis Drayna, from the National Institute on Deafness and Other Communication Disorders in Bethesda, and colleagues report on the genetics of stuttering in the New England Journal of Medicine.
Stuttering is found in all cultures and languages, affecting 5% of children in twice as many boys as girls. Nearly 80% of childhood stuttering resolves itself, mostly in girls, leaving male stutterers outnumbering female stutterers 4:1. 60 million people in the world are stutterers, and Winston Churchill, John Updike, King George VI and James Earl Jones join their ranks. The history of stuttering is fraught with erroneous theories and damaging medical practices. The ancient Greeks thought that stuttering resulted from tongue dryness and recommended enlargement of the veins by surgical or chemical means. In the 1800s, stuttering was thought to be an anatomical defect, and surgical treatments were popular. By the 1900s, stuttering was ruled a psychological disorder and treated with conditioning and psychoanalysis that were eventually proven ineffective. The modern theory is that stutterers have a neurophysical problem that disrupts the precise timing in speech.
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The evidence for a genetic component to stuttering is apparent, such as the existence of identical twin stutterers and a high incidence of stuttering in first degree relatives. Approximately half of stutterers have a family history of the disorder. By examining genetic linkage in families of stutterers, geneticists have honed in on several chromosomes that may be involved in stuttering.
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In Kang and Drayna’s most recent study, they’ve focused on chromosome 12 to examine the genes involved in stuttering in the largest well-studied group of stutterers, a Pakastani family simply known as PKST72. By comparing the DNA of stutterers in the family to non-stuttering control subjects, the researchers were able to pinpoint the genetic sequences affected in stuttering individuals. Interestingly, the genes they identified affect the cell’s recycling center, the lysosome. The lysosome takes unwanted material in the cell, such as old proteins, and breaks them down into something more usable. In this family of stutterers, the genes that direct proteins to the lysosome for recycling are mutated, most likely resulting in a build-up of this cell litter in other compartments of the cell. Somehow this accumulation of trash in cells leads to speech disturbance. The way this occurs is unknown, and the implication of these results for both treatment of stuttering and future studies is profound.
One other thing of note is that all but two of the stutterers with these mutations were heterozygous, meaning that the mutations were only found on one of the two strands of chromosome 12. If the mutations were found on both strands of DNA, the affected persons may have had a severe lysosomal storage disease called mucolipidosis.
DNA is amazing.
Pretty cool. Glad I finally had a chance to read through the blog posts. Will be coming back here to check on the latest science news. Thanks!
It is truly amazing. The discoveries to come will surely rival those already found.