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How they showed stuttering is linked to genes

February 11, 2010 |  7:55 am

Researchers from the National Institutes of Health have shown that a complex of three genes accounts for about 9% of all stuttering cases, the first time the specific genes have been linked to the disorder. Here is how they did it:

Geneticist Dennis Drayna of the National Institute on Deafness and Other Communication Disorders and his colleagues had previously identified a region of chromosome 12 that was associated with stuttering in a large Pakistani family.

When they sequenced DNA in the region, they found a mutated gene called GNPTAB, which is the blueprint for an enzyme that breaks down cellular garbage inside an organelle called the lysosome -- the cell's recycling plant. Further work led them to two other genes that were mutated in family members who stuttered. Those genes, called GNPTG and NAGPA, also help in breaking down the cellular debris to prevent it from accumulating.

Researchers already knew that mutations in GNPTAB and GNPTG cause serious inherited disease known as mucolipidosis II and III. In those diseases, like other so-called lysosomal storage disorders, large quantities of cellular debris accumulate in the lysosome, causing ultimately fatal problems in joints, the skeletal system, the heart, the liver and the brain. Victims also suffer severe speech impairment.

The difference between stuttering and the mucolipidoses, Drayna said, is that stuttering victims inherit a copy of the defective gene from only one parent, while those with the more severe disorder inherit it from both.

The researchers then looked at the genes in 77 unrelated Pakistanis and 270 Americans and Britons who stutter and in a large control group who did not. They found the constellation of mutated genes in many of the stutterers, particularly those of Asian descent, but in only one person in the control group. They estimated that the three genes account for about one in every 11 stutterers.

-- Thomas H. Maugh II