"There's been a sense for a long time that stuttering is an inherited condition. It seems to cluster in families," Marion said.
The researchers analyzed a section of chromosome 12 in a large Pakistani family that had been included in an earlier study by the same researchers.
As it turned out, several mutations in the GNPTAB, GNPTG and NAGPA genes were present in people who stuttered but not in other study participants.
"This gene is involved in a process where the waste products of the cell are removed from the cell and processed in something called a lysosome, which is kind of the garbage can of the cell, the organelle that takes all the waste out of the cell and processes it so the cell doesn't poison itself," explained Marion.
That finding was unexpected, said Drayna. Why would genes regulating this kind of metabolic pathway be involved in stuttering?
"We think a special group of cells in the brain are particularly sensitive to these subtle mutations in these genes," Drayna said. "The mutations that also cause these very serious diseases wipe out the gene so none of the gene product is effectively made. Our mutations probably allow the gene product to be made but it probably doesn't work quite right. It's like a car that needs a tune-up. It still runs but not quite right," Drayna noted.
"Some cells in the body need this to run like a Ferrari in order to function properly and those cells are involved in the production of speech in the brain," he continued.
"That's the biggest scientific breakthrough in these findings. We now have a way, finally, to get at these cells and structures in the brain that are responsible for speech, which heretofore have been very difficult to study and not well understood," Drayna said.
"We have now a bunch of new experim
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