or freckles are . But eventually , Drayna ’ s team identified mutations in four genes — GNPTAB , GNPTG and NAGPA from the Pakistan studies , and AP4E1 from the clan in Cameroon — that he estimates may underlie as many as one in five cases of stuttering .
Oddly , none of the genes that Drayna identified have an obvious connection to speech . Rather , they all are involved in sending cellular materials to the waste-recycling compartment called the lysosome . It took more work before Drayna ’ s team linked the genes to brain activity .
They started by engineering mice to have one of the mutations they ’ d observed in people , in the mouse version of GNPTAB , to see if it affected the mice ’ s vocalizations . Mice can be quite chatty , but much of their conversation takes place in an ultrasonic range that people can ’ t hear . Recording the ultrasonic calls of pups , the team observed patterns similar to human stuttering . “ They have all these gaps and pauses in their train of vocalizations ,” says Drayna , who cowrote an overview of genetics research on speech and language disorders for the Annual Review of Genomics and Human Genetics .
Still , the team struggled to spot any clear defect in the animals ’ brains — until one determined researcher found that there were fewer of the cells called astrocytes in the corpus callosum . Astrocytes do big jobs that are essential for nerve activity : providing the nerves with fuel , for example , and collecting wastes . Perhaps , Drayna muses , the limited astrocyte population slows down communication between the brain hemispheres by a tiny bit , only noticeable in speech .
Drayna ’ s research has received mixed reviews . “ It ’ s really been the pioneering work in the field ,” says Angela Morgan , a speech-language pathologist at the University of Melbourne and Murdoch Children ’ s Research Institute in Australia . On the other hand , Maguire has long doubted that mutations in such important genes , used in nearly all cells , could cause defects only in the corpus callosum , and only in speech . He also finds it difficult to compare mouse squeaks to human speech . “ That ’ s a bit of a stretch ,” he says .
Scientists are sure there are more stuttering genes to find . Drayna has retired , but Morgan and collaborators are initiating a large-scale study in the hopes of identifying additional genetic contributors in more than 10,000 people .
The dopamine connection
Maguire has been tackling stuttering from a very different angle : investigating the role of dopamine , a key signaling molecule in the brain . Dopamine can ramp up or down the activity of neurons , depending on the brain location and the nerve receptors it sticks to . There are five different dopamine receptors ( named D1 , D2 , and so on ) that pick up the signal and respond .
Researchers created mice with a mutation in a gene that , in people , is linked to stuttering . The mutant mice vocalized haltingly , with longer pauses between syllables , similar to what ’ s seen in human stuttering .
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