It’s unlikely that your stuttering is genetic. In 2010, 100 million people saw a news report saying Researchers Discover First Genes for Stuttering. The study found 11 unrelated people with a rare genetic mutation, and 9 of these people stuttered. That’s interesting, but 9 of the people with the mutation are Pakinstani, 93% of Pakistani stutterers didn’t have the mutation, and 99% of non-Pakistani stutterers don’t have the mutation. And the “breakthrough” came fifteen years after other researchers associated three other genes with stuttering.
Nature vs. nurture?
First, a little background. That stuttering runs in families has been known for decades. In a classic illustration of confirmation bias, stuttering experts of the “Iowa” era (1930s through the 1960s) said that this fact proved that parental behavior, not genetics, caused children to stutter.
A researcher in 1940 studied five generations of an Iowa family, with stuttering in all generations. In the third generation, three of four children in one family stuttered. The children grew up, and three moved to Kansas and lost contact with the sibling remaining in Iowa. Of the descendants of the woman remaining in Iowa, 40% stuttered. Of the descendants of the Kansas siblings, only 6% stuttered.
The researcher concluded that the Iowa family had a “tone” (parental behaviors) that was conducive to stuttering, but that the “tone” changed when family members moved to Kansas.
Twenty years later, another researcher studied the next generation. Only 2% of these children stuttered, and the researcher reported that there had been a change in the familial assumptions and attitudes about stuttering. 
In 1986 another researcher looked at the original study and concluded that “genetic transmission has been judged to provide an equally viable explanation for the data.” 
The research proved nothing about the “nature vs. nurture” debate. All it proves is that different researchers looking at the same data can see different results, and that researchers are good at finding proof of their pre-existing beliefs.
Familial Incidence of Stuttering
At least 25 studies have investigated whether stutterers are more likely to have relatives who stutter. The general trend of these studies was that more stutterers say they have more relatives who stutter than non-stutterers say they have relatives who stutter, but the results varied so widely that they don’t prove anything with certainty. It’s possible that stutterers are just more aware of stuttering, i.e., I’ve overheard minor disfluencies in strangers that I knew were avoidances and substitutions that a non-stutterer would not have noticed. 
Twin studies have been equally equivocal. A half-dozen studies of twins have found that concordance for stuttering (both twins either stutter or don’t stutter, rather than one twin stuttering and the other not stuttering) is more likely in identical twins than in fraternal twins. But identical twins are always same sex, when fraternal twins can be opposite sex, and more males stutter than females.  A study of 95 pairs of identical twins reared apart found 5 stutterers (2% of the subjects, the expected prevalence). None of their twins stuttered, suggesting that stuttering is not genetic. 
More recent research has found that when a preschool stuttering child has several family members who stuttered into adulthood, there is a greater chance that the child’s stuttering will persist. If the child has family members who stuttered as children and then recovered, there is a greater chance that the child will follow this pattern. 
Dopaminergic Genes and Stuttering
Researchers can now study DNA directly. As any fan of the CSI: Crime Scene Investigation television series knows, DNA evidence is more reliable than older methods of investigation.
The neurotransmitter dopamine has been associated with stuttering since 1962, when the dopamine antagonist medication haloperidol (Haldol) was found to reduce stuttering. More recent studies have found that adults who stutter have elevated levels of dopamine , and found that two other dopamine antagonist medications (risperidone or Risperdal and olanzapine or Zyprexa) reduce stuttering.
A 1996 study  of 225 persons with Tourette syndrome, their families, and controls investigated three genes associated with dopamine. The three genes were dopamine D2 receptor (DRD2), dopamine ß-hydroxylase (DßH), and dopamine transporter (DAT1). The three genes were associated with stuttering, Tourette’s, and obsessive-compulsive disorder (OCD).
Stuttering, Tourette’s, and OCD share many similarities. All are exacerbated by stress, and by trying to suppress the behaviors, i.e., trying not to stutter makes stuttering worse.
A 2009 study  of 112 Han Chinese persons who stutter associated stuttering with two dopaminergic genes (SLC6A3 and DRD2), one of which was associated with stuttering in the 1996 study.
The 1996 study was also supported by a 2007 study  that genetically associated stuttering to Specific Language Impairment, autism, and Tourette’s.
However, a 2011 study by the Stuttering Foundation’s Dr. Drayna questioned the association of stuttering with the dopamine D2 receptor DRD2 gene. No significant differences between stutterers and controls were observed in Brazilian and western European populations.
A 2004 study of 46 Pakistani families including 144 stutterers suggested “that a locus on chromosome 12q may contain a gene with a large effect in this sample.” 
A 2006 study of 100 families of European descent in the United States, Sweden, and Israel, with 252 stutterers, found “moderate evidence for linkage” on chromosomes 9 and 15. A higher level of significance was found for male stutterers on chromosome 7 and for female stutterers on chromosome 21, suggesting that “the genetic component to stuttering has significant sex effects.” 
Lysosomal Enzyme Mutations and Stuttering
The Stuttering Foundation’s Dr. Drayna worked on a series of genetic studies that investigated a family in Pakistan in which 44 of the 82 family members stuttered. This family particularly interested genetic researchers because the family was inbred: three siblings married three siblings who were their parents’ first cousins; three sons and three daughters of these three couples then married each other. Such inbreeding amplifies genetic mutations. The researchers called them the PKST72 family.
A 2005 study  looked for linkages or inherited genes. Such genes were found on chromosomes 1, 5, 7, and 12 with the strongest linkages on chromosome 12. The dopaminergic genes associated with stuttering in previous studies are on chromosome 11 (DRD2 and D H) and chromosome 5 (DAT1 and SLC6A3). This family apparently has nothing abnormal about their dopaminergic genes on chromosome 11. Whether they have anything abnormal about their dopaminergic genes on chromosome 5 wasn’t investigated.
It was the team’s next study , published in 2010, that was touted in the headlines. This study focused on chromosome 12, and expanded the field of the study to include 123 unrelated Pakistani stutterers, 96 Pakistani non-stutterers, 270 American and British stutterers, and 276 American and British non-stutterers. The second study found a mutation in the PKST72 family. 52 of the 82 PKST72 family members have a mutation called G3598A in the GNPTAB gene. 41 of the 44 stutterers (93%) in the PKST72 family had the mutation. 11 of the 38 non-stutterers (29%) in the family had the mutation.
Outside the PKST72 family, 8 of the 123 unrelated Pakistani stutterers had this mutation; 1 of the 96 Pakistani non-stutterers had this mutation; 2 of the 270 American and British stutterers had this mutation (one of whom was of South Asian descent); and none of the 276 American and British non-stutterers had this mutation.
Nine other mutations on three genes (GNPTAB, GNPTG, NAGPA) were found. One mutation was found in four of the 393 stutterers (1%). Another mutation was found in ten stutterers but eight of these stutterers were Pakistani, i.e., less than 1% of the non-Pakistani stutterers had the mutation. The other mutations were found in only one or two persons who stutter (out of 393). One non-stutterer was found to have one of these mutations.
Other mutations in GNPTAB and GNPTG have been associated with the rare inherited lysosomal storage disorders mucolipidosis types II and III, which are characterized by disorders of the joints, skeletal system, heart, liver, spleen, and motor systems and by developmental delay. Lysosomes are the waste disposal system in animal cells, using acid hydrolase enzymes to break up waste materials and cellular debris.  Stuttering isn’t a lysosomal enzyme disorder. None of the subjects examined had mucolipidosis. The third gene in which mutations were found, NAGPA, has not been associated with any disease.
To reiterate this in plain language, the study found a genetic mutation that less than 7% of Pakistani stutterers and less than 1% of non-South Asian stutterers have. The researchers concluded that their “results can explain only a small fraction of cases of stuttering.”
I e-mailed Dr. Drayna asking how this research supports his claim to have discovered the first genes for stuttering. He responded, first, that the dopamine genetic studies are “controversial, at best,” in light of his 2011 study, and some “puzzling” facts in the 2009 study.
Regarding his lysosomal studies, he wrote,
The primary basis of our evidence that mutations in the lysosomal targeting pathway cause stuttering is that 1) a unique mutation in one gene is found in the affected members of a large Pakistani stuttering family but not in the unaffected family members,
This isn’t true. The mutation was carried by 93% of the stutterers in the family and 29% of the non-stutterers in the family.
2) in three other, ostensibly unrelated Pakistani stuttering families, the exact same mutation was found in the affected members but not in unaffected members,
Only one family was studied. I can’t find anything in the study about three other families.
3) this mutation was found in a variety of unrelated stuttering individuals in the U.S., all of Pakistani or Indian origin, but not in a large number of normal control individuals,
One American or British stutterer of South Asian descent was found with this gene, not “a variety of unrelated stuttering individuals.”
4) this mutation is descended from a single origin in the South Asian population, having originated something 14,000 years ago. In addition, we have found other mutations in this gene in individuals who stutter but such mutations have never been observed in normal control individuals.
None of this was in the 2010 study.
This evidence is now buttressed by many additional findings, including biochemical studies of the effects of stuttering mutations in cells, but suffice it to say that a role for mutations in the lysosomal targeting pathway in stuttering is now fairly well accepted.
I used Google Scholar to look for these studies. I couldn’t find any.
The mutations in the lysosomal targeting pathway are clearly an uncommon cause of stuttering. But they may not be so rare. To give you an example, at the National Stuttering Association convention in Cleveland in the summer of 2010, 18 unrelated individuals volunteered to participate in our research. All of these individuals had moderate to severe lifelong persistent developmental stuttering. Two of those 18 individuals carried the cysteine mutation at position 328 in the NAGPA gene, a mutation we’d previously seen in 4 other unrelated individuals, and never in over 1000 normal control individuals. All of these individuals are of European origin, so this may be a more common (but still relatively minor contributing) cause of stuttering in Europeans.
Two stutterers doesn’t prove anything, and it’s a different gene.
However simply because any particular mutation causes only a fraction of the cases of a disorder does not mean it is not important. Hereditary deafness, for example, is caused by mutations in any one of more than 150 different genes. Most of these mutations are rare, some exceedingly so. However the study of the genes in which these mutations occur has provided an enormous number of new insights into how the hearing process works. We have hopes that we can follow a similar path of scientific inquiry into stuttering.
Actually, the medical literature on lysosomal targeting and lysosomal storage disorders and stuttering is quite extensive, and associations with these disorders and stuttering is well known. However these disorders are all rare and most cause developmental delay and/or mental retardation. This has led to the general view that stuttering in these individuals is secondary to general problems in the brain. While we have no evidence at this point, I personally believe that the profound speech deficits seen in classical cases of mucolipidosis (a lysosomal disorder associated with severe mutations in the gene we found in the Pakistani stuttering families) is actually primary, and not secondary to the general mental retardation in these patients.
I believe the association with mutations in the lysosomal targeting genes is strong, but that such mutations cause a fairly small fraction of stuttering. If the association with mutations in the dopamine receptor gene is true, then such gene variants don’t exert a very strong effect, since billions of people carry them and have never stuttered.
Questions and Answers
The broad question “Is stuttering genetic?” breaks down into many specific questions:
If you stutter, is it because of the lysosomal genetic mutation G3598A in the GNPTAB gene?
Answer: Highly unlikely. Of 270 American and British stutterers who were not of South Asian ancestry, only one person had this mutation. Of 123 unrelated Pakistani stutterers, only eight had this mutation.
If you stutter, is it because of dopaminergic genetic mutations?
Answer: Doubtful. Three studies say yes, one says no. The studies that say yes have some problematic findings. IMHO, if your brother has Tourette’s, your sister has OCD, and you stutter, your family likely has these dopaminergic genetic mutations. But if no one in your family has Tourette’s, OCD, or stuttering, don’t blame your dopaminergic genes.
Does the lysosomal genetic mutation G3598A in the GNPTAB gene cause stuttering?
Answer: Maybe. Of the ten unrelated persons with this mutation, nine stutter. That’s interesting, but nine people don’t proven a theory. Maybe G3598A causes a speech disorder that is like stuttering but is actually something else.
As for the PKST72 family, that family is highly inbred over five generations. Pardon my cultural bias, but I suspect there’s some weird stuff going on in that family. The Iowa family had five generations of stuttering, and then almost no stuttering in the sixth generation. I wouldn’t be surprised in no one in the next generation of the PKST72 family stutters (especially if the current generation marries outside the family).
Was the Stuttering Foundation’s headline true?
Answer: No. It’s disrespectful to ignore fifteen years of research by other geneticists. The 2010 lysosomal study is interesting but doesn’t prove that the G3598A mutation is related to stuttering.
Is stuttering genetic?
Answer: If you stutter, it’s unlikely that your stuttering is due to genetics. Other factors are more important in the development of stuttering. The genetic mutations that have been identified as possibly related to stuttering are too rare to explain how 20% of children stutter at some point in childhood. That some children don’t outgrow stuttering is more likely due to environmental or developmental factors, not genetics.
Will this genetic research lead to a cure for stuttering?
Answer: No. The dopaminergic research is interesting but we already have studies of dopamine antagonist medications. As for the lysosomal research, even if pharmaceutical companies were to spend billions to develop a drug for ten Pakistani stutterers, the drug would have no effect on the more than 99% of stutterers who lack this mutation.
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