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Studies in developing countries identify causes of stuttering

Speech disorders can severely affect a person's well-being, leading to shunning, behavioral and emotional problems, and impaired economic opportunity. Scientists investigating causes of stuttering - a common speech condition - have discovered through research in Pakistan, Cameroon and Brazil that genes play a surprisingly large role.

Dr. Dennis Drayna in lab
Photo by Bill Branson/NIH

Global research by Dr.
Dennis Drayna, of NDCD
and colleagues has
uncovered several genes
that cause stuttering.

"Stuttering research was an unmet need," according to Dr. Dennis Drayna, of NIH's National Institute on Deafness and Other Communication Disorders (NIDCD). "It's in every language group in every population around the world. It can have a very profound impact on the lives of those who are affected. Speech therapy helps in some cases but not in all. There are no effective drugs, and few other effective interventions." He said data are limited, but perhaps between 0.5 and 1 percent of the world's population suffers from severe stuttering.

The lion's share - more than 80 percent - is now thought to have a genetic cause, Drayna observed. "We don't know what the other contributing factors are, but it's not overly strict toilet training, overprotective mothers, or many other speculative causes."

Drayna, who heads NIDCD's Section on Systems Biology of Communication Disorders, has led studies pinpointing candidate genes, while also training researchers in relevant skills. His interest in stuttering stems in part from a mild family history, including twin sons who stuttered.

After early efforts in the U.S. to identify stuttering gene locations came up empty-handed, he extended his research to rare large families elsewhere, including Pakistan, Cameroon and Brazil. It was by extensively analyzing DNA data from a group of Pakistani families that Dr. Changsoo Kang from South Korea, then a visiting fellow in Drayna's lab, identified the first gene behind inherited stuttering.

Research gathered momentum. An internet contact led to the discovery of a remarkable Cameroonian family with more than 40 members who stutter. "Initially, I thought the family probably had an inherited neurological disorder with many other symptoms," Drayna recalled. "But they were high functioning and perfectly fine, except they all had a significant stutter." By studying this and other large Cameroonian families, Drayna and collaborators have identified locations for four new genes linked to stuttering.

In other research investigating Pakistani families, he and collaborators at the University of Punjab identified over 100 families with multiple cases of stuttering. Pakistani culture favors marriages between cousins, even though these unions increase the odds that recessive disorders will come to light. The researchers found stuttering was a solitary manifestation of a mutated gene already being studied for its role in a metabolic condition. Enzyme treatment is being examined for this class of disorders and may also someday be helpful in treating stuttering.

Genetic research in other global populations continues to uncover stuttering-susceptibility genes. "It's in these rare, very large family clusters where we find the genes, but the genes that we find, it turns out, cause stuttering everywhere in the world," Drayna said. "So, for instance, the Pakistani genes also appear to cause stuttering in about 13 percent of Americans who stutter."

The genetic discoveries clearly demonstrate stuttering is a biological disorder, Drayna noted. "This takes us further away from the view that stuttering is caused by interactions with other people, or that it is a social disorder or an emotional disorder." That may help remove the stigma associated with stuttering, he said.

There is still much to be learned, Drayna concluded. "Another important reason to investigate stuttering is to better understand the neural structures and functions within the human brain that generate human speech, which are poorly understood."

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Adapted from the May / June 2014 Global Health Matters article Studies in developing countries identify causes of stuttering.

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