Astrocytes may hold the key to stuttering

MEMPHIS, Tenn.—Researchers believe that stuttering, a potentially lifelong and debilitating speech disorder, stems from problems with the circuits in the brain that control speech. But precisely how and where these problems occur is unknown.

Using a mouse model of stuttering, scientists reported that a loss of astrocytes is associated with stuttering. The mice had been engineered with a human gene mutation previously linked to stuttering. The study, which appeared online in the Proceedings of the National Academy of Sciences, offers insights into the neurological deficits associated with stuttering.

The loss of astrocytes, a supporting cell in the brain, was most prominent in the corpus callosum, which bridges the two brain hemispheres. Previous imaging studies have identified differences in the brains of people who stutter, compared to the brains of those who don’t. And some of these studies in people have revealed structural and functional problems in the same brain region as the new mouse study.

The study was led by Dennis Drayna, Ph.D., of the Section on Genetics of Communication Disorders, at the National Institute on Deafness and Other Communication Disorders (NIDCD), which is part of the National Institutes of Health. Researchers at the Washington University School of Medicine in St. Louis, the National Institute of Biomedical Imaging and Bioengineering, and the National Institute of Mental Health also collaborated on the research.

“We are thrilled by the findings that Dr. Dennis Drayna, Stuttering Foundation board member, and his colleagues have published. Every study into the cause of stuttering brings us one step closer to finding better treatment and new therapies. Most importantly, it brings hope to the 70 million people around the world who struggle with stuttering every day,” said Jane Fraser, president of The Stuttering Foundation.

Stuttering is most commonly seen in young children, who typically outgrow the problem. But for 1 in 4 children who experience early stuttering, the condition persists as a lifelong communication difficulty. It’s estimated that as many as one percent of adults in the United States are affected by stuttering.

“The brain imaging studies of people who stutter are important, but those results can only take us so far,” added Drayna. “By taking a genetic approach, we have been able to begin deciphering the neuropathology of stuttering, first at the molecular level by identifying genetic mutations, and now at the cellular level.”

In this study, the researchers set out to identify changes in the brain brought on by the mutations in a gene called GNPTAB, one of the genes previously linked to stuttering. The scientists engineered this human stuttering mutation into the mice to create a mouse model. The mice with the GNPTAB mutation had long pauses in their stream of vocalizations, similar to those found in people with the same mutation. Like people who stutter, the mice were normal in all other ways, reinforcing earlier research that suggests that the mice can serve as a valid animal model for important features of this disorder.

The investigators examined brain tissue from the mice and found a decrease in astrocytes, but not other cell types, in the animals with the genetic mutation. Astrocytes play a critical role in supporting nerve cells by carrying out a wide range of functions, such as supplying nerve cells with oxygen and nutrients and providing structural support.

The loss of astrocytes was more pronounced in the corpus callosum of the mutant mice. In addition, using advanced magnetic resonance imaging (MRI) methods, the researchers detected reduced local volume of the corpus callosum in the mutant mice despite normal diffusion tensor MRI values, providing further support for a defect in this brain region.

Follow-up experiments in which the GNPTAB human stuttering mutation was introduced into individual brain cell types, rather than the entire mouse, confirmed that the vocalization defect is specific to astrocytes.

If future research confirms that stuttering in people with GNPTAB mutations derives from a loss of astrocytes in the brain, these findings could open the door to new therapeutic strategies for some people with persistent developmental stuttering by targeting associated molecular pathways and cells.