Drawing on previous research suggesting that stuttering might have genetic origins, a new study published in the New England Journal of Medicine may have identified precisely which genes cause stuttering in certain people. Stuttering, the disorder characterized by repetition, delay and interruptions in speech, impacts an estimated 1% of the adult population and is often treated with strategies aimed to decrease anxiety or improve breathing regulation, for example. Yet, if the findings of this latest research are confirmed, there may be hope for new, genetically based stuttering treatments in the future.
Following up on a previous study into stuttering as a hereditary problem in several large Pakistani families, researchers included 46 Pakistani people who stuttered—one from each of the large families included in the previous research—as well as 77 unrelated Pakistanis who also stutter, in addition to 96 individuals from Pakistan with no speech problems, who served as controls. In addition to these groups, researchers studied 270 North Americans and Britons who stuttered, as well as 276 North Americans without speech problems, who were included as controls.
The earlier research into Pakistani families indicated that the genetic problem linked with stuttering might be based in specific component of our DNA, on one of our cells’ 23 pairs of chromosomes: chromosome 12. In this most recent study, the researchers pinpointed the location on chromosome 12 home to the genes associated with stuttering. Analyzing data from family subjects, they found that many had the same mutation in a specific gene, known as GNPTAB, which is critical to the body’s ability to recycle old cell parts. (In total, humans are estimated to have between 20,000–25,000 genes.)
The cell recycling process, which takes place in the lysosome—which, if you remember those old biology classes, are organelles full of enzymes that help the body digest used up cell parts—is guided by a sort-of chain reaction among genes: GNPTAB gene encodes its enzyme with help of another gene, called GNPTG, and the process continues with help of a third gene, known as NAGPA. Yet, if this process breaks down, as a result of genetic mutations in any of these three genes, cell recycling can break down.
Using DNA sequencing, the researchers analyzed these three genes in all study participants: they found the gene mutations among stutterers, but not among participants in the control groups.
While the findings suggest the need for more study into NAGPA gene mutations, mutations of GNPTG and GNPTAB have previously been associated with metabolic diseases known as mucolipidosis II and III (or MLII and MLIII), which basically cause cell components that aren’t recycled correctly to pile up in the lysosome. When lysosomes are overwhelmed by deposits of left over cell material, it can lead to a range of health problems—including conditions that impact the heart, liver, skeletal system, joints and developmental processes in the brain resulting in speech problems.
So, why don’t people who stutter also have all sorts of other, more serious health complications? According to the study’s lead author, Dr. Dennis Drayna, that’s because MLII and MLIII are recessive, meaning that they need two copies of a gene to join forces and wreak havoc. In most people who stutter, researchers found only one copy. Additionally, in cases of MLII and MLIII, the body simply doesn’t manufacture the crucial enzyme; when there is only one copy of the gene, however, the enzyme is produced, but doesn’t come out quite right.
Drayna and his colleagues estimate that some 9% of people who stutter have one of the three gene mutations identified in the study, and are hopeful that these findings may open new pathways for treating stuttering—perhaps by drawing on existing treatments for MLII and MLIII, which include enzyme replacement therapies.