Stuttering associated with a pathogenic variant in the chaperone protein cyclophilin 40

Stuttering is a common speech disorder that interrupts speech fluency and tends to cluster in families. Typically, stuttering is characterized by speech sounds, words or syllables which may be repeated or prolonged and speech that may be further interrupted by hesitations or 'blocks'. Rare variants in a small number of genes encoding lysosomal pathway proteins have been linked to stuttering. We studied a large four-generation family in which persistent stuttering was inherited in an autosomal dominant manner with disruption of the cortico-basal-ganglia-thalamo-cortical network found on imaging. Exome sequencing of three affected family members revealed the PPID c.808C>T (p.Pro270Ser) variant that segregated with stuttering in the family. We generated a Ppid p.Pro270Ser knock-in mouse model and performed ex vivo imaging to assess for brain changes. Diffusion-weighted MRI in the mouse revealed significant microstructural changes in the left corticospinal tract, as previously implicated in stuttering. Quantitative susceptibility mapping also detected changes in cortico-striatal-thalamo-cortical loop tissue composition, consistent with findings in affected family members. This is the first report to implicate a chaperone protein in the pathogenesis of stuttering. The humanized Ppid murine model recapitulates network findings observed in affected family members.

New insights into the genetics of stuttering

This scientific commentary refers to ‘Stuttering associated with a pathogenic variant in the chaperone protein cyclophilin 40’ by Morgan et al. (https://doi.org/10.1093/brain/awad314).

Expanding the speech and language phenotype in Koolen-de Vries syndrome: late onset and periodic stuttering a novel feature

Speech and language impairment is core in Koolen-de Vries syndrome (KdVS), yet only one study has examined this empirically. Here we define speech, language, and functional/adaptive behaviour in KdVS; while deeply characterising the medical/neurodevelopmental phenotype in the largest cohort to date. Speech, language, literacy, and social skills were assessed using standardised measures, alongside an in-depth health and medical questionnaire. 81 individuals with KdVS were recruited (35 female, mean age 9y 10mo), 56 of whom harboured the typical 500-650 kb 17q21.31 deletion. The core medical phenotype was intellectual disability (largely moderate), eye anomalies/vision disturbances, structural brain anomalies, dental problems, sleep disturbance, musculoskeletal abnormalities, and cardiac defects. Most were verbal (62/81, 76.5%), while minimally-verbal communicators used alternative and augmentative communication (AAC) successfully in spite of speech production delays. Speech was characterised by apraxia (39/61, 63.9%) and dysarthria (28/61, 45.9%) in verbal participants. Stuttering was described in 36/47 (76.6%) verbal participants and followed a unique trajectory of late onset and fluctuating presence. Receptive and expressive language abilities were commensurate with one another, but literacy skills remained a relative weakness. Social competence, successful behavioural/emotional control, and coping skills were areas of relative strength, while communication difficulties impacted daily living skills as an area of comparative difficulty. Notably, KdVS individuals make communication gains beyond childhood and should continue to access targeted therapies throughout development, including early AAC implementation, motor speech therapy, language/literacy intervention, as well as strategies implemented to successfully navigate activities of daily living that rely on effective communication.

Stuttering, family history and counselling: A contemporary database

BACKGROUND

Information about genetic influence is useful to when counselling parents or caregivers who have infants and children at risk for stuttering. Yet, the most comprehensive family aggregate database to inform that counselling is nearly four decades old (Andrews et al., 1983). Consequently, the present study was designed to provide a contemporary exploration of the relationship between stuttering and family history.

METHODS

Data were sourced from the Australian Stuttering Research Centre, comprising 739 participants who presented for assessment, treatment, or investigation of stuttering. Reported family history data were acquired from pedigrees collected during assessment. We sought to establish the relation of the following variables to family history of stuttering: incidence, proband sex, parent sex, stuttering severity, age, reported age of stuttering onset, and impact of stuttering. Data were analysed with chi-square tests for independence, logistic and linear regression models.

RESULTS

Results were broadly consistent with existing data, but the following findings were novel. Males and females who stutter have the same increased odds of having a father who stutters relative to a mother who stutters. Males had later stuttering onset than females, with genetic involvement in this effect. There was a greater impact of stuttering for females than males with a family history of stuttering.

CONCLUSION

These findings have clinical applications. Speech-language pathologists may have infant or child clients known to them who are at risk of beginning to stutter. Information from the present study can be applied to counselling parents or caregivers of such children about stuttering and family history.

The disabling nature of hope in discovering a biological explanation of stuttering

Discovering developmental stuttering's biological explanation has been an enduring concern. Novel advances in genomics and neuroscience are making it possible to isolate and pinpoint genetic and brain differences implicated in stuttering. This is giving rise to a hope that, in the future, dysfluency could be better managed if stuttering's biological basis could be better understood. Concurrent to this, there is another hope rising: a hope of a future where differing fluencies would not be viewed through a reductive lens of biology and associated pathologies. The central aim of this paper is to edge out ethical implications of novel research into stuttering's biological explanation. In doing so, the paper proposes to look beyond the bifurcation sketched by the medical and social model of disability. The paper demonstrates how the scientific hope of discovering stuttering's biological explanation acts as an accessory of disablement due to the language of 'lack' and 'deficit' employed in reporting scientific findings and proposes participatory research with people who stutter as an antidote to manage this disablement.

Phenome risk classification enables phenotypic imputation and gene discovery in developmental stuttering

Developmental stuttering is a speech disorder characterized by disruption in the forward movement of speech. This disruption includes part-word and single-syllable repetitions, prolongations, and involuntary tension that blocks syllables and words, and the disorder has a life-time prevalence of 6-12%. Within Vanderbilt's electronic health record (EHR)-linked biorepository (BioVU), only 142 individuals out of 92,762 participants (0.15%) are identified with diagnostic ICD9/10 codes, suggesting a large portion of people who stutter do not have a record of diagnosis within the EHR. To identify individuals affected by stuttering within our EHR, we built a PheCode-driven Gini impurity-based classification and regression tree model, PheML, by using comorbidities enriched in individuals affected by stuttering as predicting features and imputing stuttering status as the outcome variable. Applying PheML in BioVU identified 9,239 genotyped affected individuals (a clinical prevalence of ∼10%) for downstream genetic analysis. Ancestry-stratified GWAS of PheML-imputed affected individuals and matched control individuals identified rs12613255, a variant near CYRIA on chromosome 2 (B = 0.323; p value = 1.31 × 10-8) in European-ancestry analysis and rs7837758 (B = 0.518; p value = 5.07 × 10-8), an intronic variant found within the ZMAT4 gene on chromosome 8, in African-ancestry analysis. Polygenic-risk prediction and concordance analysis in an independent clinically ascertained sample of developmental stuttering cases validate our GWAS findings in PheML-imputed affected and control individuals and demonstrate the clinical relevance of our population-based analysis for stuttering risk.

Loci of stuttering of English- and Korean-speaking children who stutter: Preliminary findings

PURPOSE

The purpose of the study was to determine whether differences exist between young English- and Korean-speaking children who stutter (CWS) in the loci of stuttering.

METHOD

Participants were 10 Korean-speaking and 11 English-speaking CWS between the ages of 3 and 7 years. Participants produced narratives while viewing various picture scenes and a wordless picture book.

RESULTS

Findings indicated that Korean-speaking CWS stuttered more on content than function words whereas English-speaking CWS stuttered more on function than content words. Furthermore, both Korean- and English-speaking CWS tended to stutter more on utterance-initial words. These findings appear to be related to the differences in linguistic/syntactic structures between Korean and English. Specifically, in the Korean-speaking CWS's narratives, most utterance-initial words (73.60 %) were content words whereas in the English-speaking CWS's narratives, most utterance-initial words (83.57 %) were function words.

CONCLUSION

These preliminary findings, although in need of replication with a larger sample size, seem to suggest that the word class (i.e., content/function words) contributions to stuttering loci are more language-specific whereas the word position (i.e., utterance-initial position) contributions to stuttering loci are more language-nonspecific. Given that the true characteristics of stuttering may be rather language-nonspecific than language-specific, further research may need to focus more on stuttering loci related to word position than word class.

Neurofilament-lysosomal genetic intersections in the cortical network of stuttering

The neurobiological underpinnings of stuttering, a speech disorder characterized by disrupted speech fluency, remain unclear. While recent developments in the field have afforded researchers the ability to pinpoint several genetic profiles associated with stuttering, how these specific genetic backgrounds impact neuronal circuits and how they generate or facilitate the emergence of stuttered speech remains unknown. In this study, we identified the large-scale cortical network that characterizes stuttering using functional connectivity MRI and graph theory. We performed a spatial similarity analysis that examines whether the topology of the stuttering cortical network intersects with genetic expression levels of previously reported genes for stuttering from the protein-coding transcriptome data of the Allen Human Brain Atlas. We found that GNPTG - a gene involved in the mannose-6-phosphate lysosomal targeting pathways - was significantly co-localized with the stuttering cortical network. An enrichment analysis demonstrated that the genes identified with the stuttering cortical network shared a significantly overrepresented biological functionality of Neurofilament Cytoskeleton Organization (NEFH, NEFL and INA). The relationship between lysosomal pathways, cytoskeleton organization, and stuttering, was investigated by comparing the genetic interactome between GNPTG and the neurofilament genes implicated in the current study. We found that genes of the interactome network, including CDK5, SNCA, and ACTB, act as functional links between lysosomal and neurofilament genes. These findings support the notion that stuttering is due to a lysosomal dysfunction, which has deleterious effects on the neurofilament organization of the speech neuronal circuits. They help to elucidate the intriguing, unsolved link between lysosomal mutations and the presence of stuttering.

Genetic factors and therapy outcomes in persistent developmental stuttering

PURPOSE

We investigated whether outcomes of therapy for persistent developmental stuttering differ in individuals who carry a mutation in one of the known genes associated with stuttering compared to individuals without such mutations.

METHOD

We studied outcomes of an intensive fluency shaping-based therapy program in individuals with persistent developmental stuttering. We evaluated a cohort of 51 stuttering individuals with who carried a mutation in either the GNPTAB, GNPTG, NAGPA, or AP4E1 gene. We compared therapy outcomes in these individuals with outcomes in 51 individuals matched for age, gender, and ethnicity, who stutter and underwent the same therapy program, and did not carry a mutation in any of these genes. Fluency pre- and post-therapy was evaluated using blinded observer-based quantitative stuttering dysfluency measures (Dysfluent Words Score, DWS), and by subjects' self-reported measures of struggle, avoidance and expectancy behavior associated with speaking (Perceptions of Stuttering Inventory, PSI). The difference between pre- and post-therapy fluency scores was taken as the measure of near-term therapy efficacy.

RESULTS

Comparison of fluency measures showed a strong effect of therapy overall. Mutation carriers achieved significantly less resolution in PSI following therapy, with PSI scores showing significantly less improvement in individuals who carry a mutation (p = 0.0157, RR = 1.75, OR = 2.92) while the group difference in DWS between carriers and non-carriers was statistically not significant in the present study, the trend observed in the results warrants further research focused on this important issue.

CONCLUSIONS

These results suggest stuttering is more resistant to therapy in individuals who carry a mutation in one of the genes known to be associated with stuttering.

Genetic epidemiology of stuttering among school children in the state of Tamil Nadu, India

PURPOSE

Stuttering is a fluency disorder with a worldwide prevalence of 1%. Reports on the epidemiology of stuttering in India are limited. Our primary goal was to examine the prevalence of the disorder among school children. The study also aimed to examine risk factors associated with severity and the impact of parental consanguinity in stuttering.

METHOD

Children from 97 schools in the State of Tamil Nadu, India were screened. Extensive speech characterization, epidemiological details and three-generational pedigrees were collected for 180 probands. The genetic basis of stuttering was examined using the analysis of genealogical index of families (GIF), kinship group and sibling recurrence risk (SRR) measures. Regression analysis and chi-square tests were performed to test the association of risk factors with severity of the disorder.

RESULTS

Among the 74,544 school children screened, the prevalence of stuttering was found to be 0.46%. Pedigree analysis revealed a positive family history in 101 (56%) probands; overall familial incidence was 11%. We observed an overall male-favored sex ratio (4:1). Familial aggregation (GIF = 442.60, p-value <0.001) and sibling recurrence risk ratio (K_s = 0.197, SD = 0.041) was high among consanguineous families. Severity of stuttering was strongly associated with gender and moderately associated with age at onset.

CONCLUSION

The prevalence of stuttering in Tamil Nadu is estimated for the first time in this study. High familial incidence, familial aggregation and sibling recurrence risk ratio point to the presence of a genetic basis. Familial aggregation was high among consanguineous families although consanguinity did not seem to play a role in severity.

Relationship between serum homovanillic acid, DRD2 C957T (rs6277), and hDAT A559V (rs28364997) polymorphisms and developmental stuttering

The involvement of the brain dopamine system in the pathophysiology of developmental stuttering has been previously suggested. In the present study, we aimed to investigate the relationship between developmental stuttering in children and the levels of serum homovanillic acid (HVA), dopamine D2 receptor (DRD2) C957T (rs6277), and solute carrier family 6 member 3 (SLC6A3) human dopamine transporter (hDAT) A559V (rs28364997) single-nucleotide polymorphisms. In a case-control study, serum level of HVA, DRD2 C957T, and DAT A559V were compared between 85 children who stuttered (CWS) and 85 age- and sex-matched children who did not stutter (CWNS). Although serum level of HVA was higher among the CWS (median = 25.50 ng/mL) than that in the CWNS (median = 17.40 ng/mL), the difference between the two groups was not significant (p = 0.43). No significant correlation was observed between age and the level of HVA among all the participants (r = -0.15, p = 0.06), nor was there any correlation among the CWS (r = -0.19, p = 0.14) or among the CWNS (r = -0.13, p = 0.27) according to the Spearman correlation coefficient. On the other hand, there was a significant negative correlation between age from stuttering onset and the serum level of HVA among the CWS group (r = -0.32, p = 0.01). The Spearman correlation coefficient did not indicate any significant correlation between stuttering severity and HVA in CWS (r = -0.06, p = 0.59). The mutant allele of hDAT A559V was observed neither in the CWS nor in the controls. The allele frequencies of DRD2 C957T were not significantly different between the CWS and the CWNS; however, the frequency of the TT genotype was significantly higher among the CWS (p = 0.02), which was associated with 2.25-fold susceptibility to stuttering (OR = 2.25, 95% CI = 1.03 to 4.90, p = 0.04). Our findings suggest that the serum level of HVA might be a biomarker for dopaminergic involvement in the pathogenesis of stuttering. Moreover, the present study indicates that the DRD2 C957T polymorphism might be a risk factor for the development of stuttering among Iranian Kurdish population.

Young children's family history of stuttering and their articulation, language and attentional abilities: An exploratory study

PURPOSE

The purpose of this study was to determine whether young children who do (CWS) and do not stutter (CWNS) with a positive versus negative family history of stuttering differ in articulation, language and attentional abilities and family histories of articulation, language and attention related disorders.

METHOD

Participants were 25 young CWS and 50 young CWNS. All 75 participants' caregivers consistently reported a positive or negative family history of stuttering across three consecutive time points that were about 8 months apart for a total of approximately 16 months. Each participant's family history focused on the same, relatively limited number of generations (i.e., participants' parents & siblings). Children's family history of stuttering as well as articulation, language, and attention related disorders was obtained from one or two caregivers during an extensive interview. Children's speech and language abilities were measured using four standardized articulation and language tests and their attentional abilities were measured using caregiver reports of temperament.

RESULTS

Findings indicated that (1) most caregivers (81.5% or 75 out 92) were consistent in their reporting of positive or negative history of stuttering; (2) CWNS with a positive family history of stuttering, compared to those with a negative family history of stuttering, were more likely to have reported a positive family history of attention deficit/hyperactivity disorder (ADHD), and (3) CWNS with a positive family history of stuttering had lower language scores than those with a negative family history of stuttering. However, there were no such significant differences in family histories of ADHD and language scores for CWS with a positive versus negative family history of stuttering. In addition, although 24% of CWS versus 12% of CWNS's caregivers reported a positive family history of stuttering, inferential analyses indicated no significant differences between CWS and CWNS in relative proportions of family histories of stuttering.

CONCLUSION

Finding that a relatively high proportion (i.e., 81.5%) of caregivers consistently reported a positive or negative family history of stuttering across three consecutive time points should provide some degree of assurance to those who collect such caregiver reports. Based on such consistent caregiver reports, linguistic as well as attentional vulnerabilities appear associated with a positive family history of stuttering, a finding that must await further empirical study for confirmation or refutation.

Sex steroid hormones and sex hormone binding globulin levels, CYP17 MSP AI (-34T:C) and CYP19 codon 39 (Trp:Arg) variants in children with developmental stuttering

Developmental stuttering is known to be a sexually dimorphic and male-biased speech motor control disorder. In the present case-control study, we investigated the relationship between developmental stuttering and steroid hormones. Serum levels of testosterone, dihydrotestosterone (DHT), dehydroepiandrosterone (DHEA), oestradiol, progesterone, cortisol, and sex hormone binding globulin (SHBG), as well as the 2nd/4th digit ratio (2D:4D), an indicator of prenatal testosterone level, were compared between children who stutter (CWS) and children who do not stutter (CWNS). Moreover, two SNPs (CYP17 -34 T:C (MSP AI) and CYP19 T:C (Trp:Arg)) of cytochrome P450, which is involved in steroid metabolism pathways, were analysed between the groups. Our results showed significantly higher levels of testosterone, DHT, and oestradiol in CWS in comparison with CWNS. The severity of stuttering was positively correlated with the serum levels of testosterone, DHEA, and cortisol, whereas no association was seen between the stuttering and digit ratio, progesterone, or SHBG. The CYP17CC genotype was significantly associated with the disorder.

How Stuttering Develops: The Multifactorial Dynamic Pathways Theory

Purpose

We advanced a multifactorial, dynamic account of the complex, nonlinear interactions of motor, linguistic, and emotional factors contributing to the development of stuttering. Our purpose here is to update our account as the multifactorial dynamic pathways theory.

Method

We review evidence related to how stuttering develops, including genetic/epigenetic factors; motor, linguistic, and emotional features; and advances in neuroimaging studies. We update evidence for our earlier claim: Although stuttering ultimately reflects impairment in speech sensorimotor processes, its course over the life span is strongly conditioned by linguistic and emotional factors.

Results

Our current account places primary emphasis on the dynamic developmental context in which stuttering emerges and follows its course during the preschool years. Rapid changes in many neurobehavioral systems are ongoing, and critical interactions among these systems likely play a major role in determining persistence of or recovery from stuttering.

Conclusion

Stuttering, or childhood onset fluency disorder (Diagnostic and Statistical Manual of Mental Disorders, 5th edition; American Psychiatric Association [APA], 2013), is a neurodevelopmental disorder that begins when neural networks supporting speech, language, and emotional functions are rapidly developing. The multifactorial dynamic pathways theory motivates experimental and clinical work to determine the specific factors that contribute to each child's pathway to the diagnosis of stuttering and those most likely to promote recovery.

Stuttering: Clinical and research update

OBJECTIVE

To provide an update on the epidemiology, genetics, pathophysiology, diagnosis, and treatment of developmental stuttering.

QUALITY OF EVIDENCE

The MEDLINE and Cochrane databases were searched for past and recent studies on the epidemiology, genetics, pathophysiology, diagnosis, and treatment of developmental stuttering. Most recommendations are based on small studies, limited-quality evidence, or consensus.

MAIN MESSAGE

Stuttering is a speech disorder, common in persons of all ages, that affects normal fluency and time patterning of speech. Stuttering has been associated with differences in brain anatomy, functioning, and dopamine regulation thought to be due to genetic causes. Attention to making a correct diagnosis or referral in children is important because there is growing consensus that early intervention with speech therapy for children who stutter is critical. For adults, stuttering can be associated with substantial psychosocial morbidity including social anxiety and low quality of life. Pharmacologic treatment has received attention in recent years, but clinical evidence is limited. The mainstay of treatment for children and adults remains speech therapy.

CONCLUSION

A growing body of research has attempted to uncover the pathophysiology of stuttering. Referral for speech therapy remains the best option for children and adults.

Association between Rare Variants in AP4E1, a Component of Intracellular Trafficking, and Persistent Stuttering

Stuttering is a common, highly heritable neurodevelopmental disorder characterized by deficits in the volitional control of speech. Whole-exome sequencing identified two heterozygous AP4E1 coding variants, c.1549G>A (p.Val517Ile) and c.2401G>A (p.Glu801Lys), that co-segregate with persistent developmental stuttering in a large Cameroonian family, and we observed the same two variants in unrelated Cameroonians with persistent stuttering. We found 23 other rare variants, including predicted loss-of-function variants, in AP4E1 in unrelated stuttering individuals in Cameroon, Pakistan, and North America. The rate of rare variants in AP4E1 was significantly higher in unrelated Pakistani and Cameroonian stuttering individuals than in population-matched control individuals, and coding variants in this gene are exceptionally rare in the general sub-Saharan West African, South Asian, and North American populations. Clinical examination of the Cameroonian family members failed to identify any symptoms previously reported in rare individuals carrying homozygous loss-of-function mutations in this gene. AP4E1 encodes the ε subunit of the heterotetrameric (ε-β4-μ4-σ4) AP-4 complex, involved in protein sorting at the trans-Golgi network. We found that the μ4 subunit of AP-4 interacts with NAGPA, an enzyme involved in the synthesis of the mannose 6-phosphate signal that targets acid hydrolases to the lysosome and the product of a gene previously associated with stuttering. These findings implicate deficits in intracellular trafficking in persistent stuttering.

Characteristics of Fluency and Speech in Two Families With High Incidences of Stuttering

PURPOSE

This study presents data from 2 families with high incidence of stuttering, comparing methods of phenotype assignment and exploring the presence of other fluency disorders and corresponding speech characteristics.

METHOD

Three methods for assigning phenotype of stuttering were used: self-identification, family identification, and expert identification. Agreement on which individuals were assigned by each of these methods was studied. Multiple measures of fluency and speech production were obtained.

RESULTS

Self-reports and descriptions of blocking rather than self-identification as a person who stutters demonstrated the best agreement with expert identification of stuttering. Family identification showed poor agreement with both expert and self-identification of stuttering. Using binary categories of fluent or stuttering, 90% of individuals in 1 family were classified by expert consensus. Only 70% of the other family could be similarly categorized. Experts required 2 other categories, cluttering and other fluency disorders, to fully characterize dysfluency within this family. These 2 families also demonstrated differences in speech production.

CONCLUSION

Some families with high incidence of stuttering may also have high incidence of other fluency disorders and other speech-production difficulties. This finding may have ramifications for genetic studies, including criteria for defining phenotype and collapsing data across multiple families.

Epidemiology of stuttering: 21st century advances

Epidemiological advances in stuttering during the current century are reviewed within the perspectives of past knowledge. The review is organized in six sections: (a) onset, (b) incidence, (c) prevalence, (d) developmental paths, (e) genetics and (f) subtypes. It is concluded that: (1) most of the risk for stuttering onset is over by age 5, earlier than has been previously thought, with a male-to-female ratio near onset smaller than what has been thought, (2) there are indications that the lifespan incidence in the general population may be higher than the 5% commonly cited in past work, (3) the average prevalence over the lifespan may be lower than the commonly held 1%, (4) the effects of race, ethnicity, culture, bilingualism, and socioeconomic status on the incidence/prevalence of stuttering remain uncertain, (5) longitudinal, as well as incidence and prevalence studies support high levels of natural recovery from stuttering, (6) advances in biological genetic research have brought within reach the identification of candidate genes that contribute to stuttering in the population at large, (7) subtype-differentiation has attracted growing interest, with most of the accumulated evidence supporting a distinction between persistent and recovered subtypes.

EDUCATIONAL OBJECTIVES

Readers will be exposed to a summary presentation of the most recent data concerning basic epidemiological factors in stuttering. Most of these factors also pertain to children's risks for experiencing stuttering onset, as well as risks for persistency. The article also aims to increase awareness of the implications of the information to research, and professional preparation that meets the epidemiology of the disorder.

Neurogenomics of speech and language disorders: the road ahead

Next-generation sequencing is set to transform the discovery of genes underlying neurodevelopmental disorders, and so offer important insights into the biological bases of spoken language. Success will depend on functional assessments in neuronal cell lines, animal models and humans themselves.

A role for inherited metabolic deficits in persistent developmental stuttering

Stuttering is a common but poorly understood speech disorder. Consistent evidence for the involvement of genetic factors in stuttering has motivated studies aimed at identifying causative genetic variants that could shed light on the underlying molecular and cellular deficits in this disorder. Such studies have begun to identify causative genes. The purpose of this review is to summarize the gene discoveries to date, and to cover the subsequent functional studies that are beginning to provide insights into how these gene mutations might cause stuttering. Surprisingly, the first variant genes to be associated with stuttering are those encoding the lysosomal targeting system, GNPTAB, GNPTG, and NAGPA. Although mutations in NAGPA have not been associated with a disorder in humans, mutations in GNPTAB and GNPTG cause mucolipidosis types II and III, which are rare autosomal recessive lysosomal storage disorders, associated with pathology of bone, connective tissue, liver, spleen, and brain. Analysis of mutations in these genes has so far identified predominantly missense mutations in stuttering, in contrast to the truncating and other mutations that result in very low GNPTAB/G enzyme activity and are historically associated with mucolipidosis. Genetic evidence for the role of lysosomal targeting mutations in stuttering has now been buttressed by biochemical studies of the mutant enzymes found in this disorder. While data on the GlcNAc-phosphotransferase encoded by GNPTAB/G remains limited and only suggestive, a study of the enzyme encoded by NAGPA has shown that the mutations found in stuttering reduce the overall cellular activity of this enzyme by about half, and that they result in deficits in intracellular processing and trafficking that lead to a reduced cellular half life. How these deficits result in the presumed speech-specific neuropathology associated with stuttering is not yet known. However these findings have opened several new lines of inquiry, including studies in mice carrying human stuttering mutations, that represent promising approaches to this disorder.

Genetic and environmental effects on stuttering: a twin study from Finland

The present study explored the prevalence of self-reported stuttering in a Finnish twin population and examined the extent to which the variance in liability to stuttering was attributable to genetic and environmental effects. We analyzed data of 1728 Finnish twins, born between 1961 and 1989. The participants were asked to complete a questionnaire on speech, language, and voice. In two of the questions they were asked to report the occurrence of childhood and present stuttering of their own and that of their sibling. According to the results, 2.3% (52) of the participants were reported to have stuttered as children and 28.8% of them (15) were reported to continue to stutter in adulthood. There was no significant gender difference in the prevalence of stuttering in either childhood or adulthood. For childhood stuttering, the tetrachoric correlation was higher for monozygotic pairs (r=.74) than for dizygotic pairs (r=.27). By means of structural equation modeling it was found that 82% of the variance in liability to childhood stuttering was attributable to additive genetic effects, with the remaining 18% due to non-shared environmental effects. In conclusion, the results of the present study confirm findings from prior studies and support a strong genetic and only a moderate non-shared environmental effect on stuttering. Potential small differences in the prevalence of stuttering in different populations are suggested by our data.

EDUCATIONAL OBJECTIVES

The reader will be able to recognize the contribution of genetic and environmental effects on stuttering.

Genetic bases of stuttering: the state of the art, 2011

OBJECTIVE

The literature on the genetics of stuttering is reviewed with special reference to the historical development from psychosocial explanations leading up to current biological research of gene identification.

SUMMARY

A gradual progression has been made from the early crude methods of counting percentages of stuttering probands who have relatives who stutter to recent studies using entire genomes of DNA collected from each participant. Despite the shortcomings of some early studies, investigators have accumulated a substantial body of data showing a large presence of familial stuttering. This encouraged more refined research in the form of twin studies. Concordance rates among twins were sufficiently high to lend additional support to the genetic perspective of stuttering. More sophisticated aggregation studies and segregation analyses followed, producing data that matched recognized genetic models, providing the final ‘go ahead’ to proceed from the behavior/statistical genetics into the sphere of biological genetics. Recent linkage and association studies have begun to reveal contributing genes to the disorder.

CONCLUSION

No definitive findings have been made regarding which transmission model, chromosomes, genes, or sex factors are involved in the expression of stuttering in the population at large. Future research and clinical implications are discussed.

Analysis of mannose 6-phosphate uncovering enzyme mutations associated with persistent stuttering

GlcNAc-1-phosphodiester-N-acetylglucosaminidase ("uncovering enzyme" (UCE); EC 3.1.4.45) is a Golgi enzyme that mediates the second step in the synthesis of the mannose 6-phosphate lysosomal targeting signal on acid hydrolases. Recently, three mutations (two missense and one deletion/frameshift) in the NAGPA gene that encodes UCE have been identified in individuals with persistent stuttering. We now demonstrate that each mutation leads to lower cellular UCE activity. The p.R328C mutation impairs folding in the endoplasmic reticulum, resulting in degradation of a significant portion by the proteasomal system. The p.H84Q mutation also impairs folding and, in addition, decreases the specific activity of the enzyme that folds sufficiently to traffic to the Golgi. The p.F513SfsX113 frameshift mutation adds 113 amino acids to the C terminus of the cytoplasmic tail of the protein, including a VWLL sequence that causes rapid degradation via the proteasomal system. These biochemical findings extend the genetic data implicating mutations in the NAGPA gene in the persistent stuttering phenotype.

[Stuttering: effects of genes and early treatment]

As stuttering by most persons resolves after a time, an initial wait-and-see policy has been maintained up to now. Two recent important developments in the area of stuttering have occurred: the discovery of gene mutations that appear to be of relevance to the developmental-neurological abnormality and growing evidence that early intervention helps recovery. The mutated genes found are GNPTAB, GNPTG and NAGPA. They are involved in lysosomal decomposition. Published study results show that early treatment using either the Demands and Capacities Model (indirect treatment aimed at the child's surroundings) or the Lidcome Programme (behavioural therapy based on operant conditioning) results in recovery from stuttering in most children. It is now being recommended that 6-12 months are allowed to pass under supervision to see whether the child is leaning toward natural recovery or, if that does not occur, to initiate specific therapy.

Bivariate genetic analyses of stuttering and nonfluency in a large sample of 5-year-old twins

PURPOSE

Behavioral genetic studies of speech fluency have focused on participants who present with clinical stuttering. Knowledge about genetic influences on the development and regulation of normal speech fluency is limited. The primary aims of this study were to identify the heritability of stuttering and high nonfluency and to assess the relative contribution of genetic and environmental factors to the correlation between these 2 fluency phenotypes.

METHOD

Information on 6 specific speech fluency behaviors was obtained by maternal report for over 10,500 5-year-old Dutch twin pairs.

RESULTS

Genetic analyses revealed that both fluency phenotypes were moderately heritable, with heritability estimates of 42% and 45% for probable stuttering and high nonfluency, respectively. Shared environmental factors were also significant, explaining 44% of the individual differences in probable stuttering and 32% in nonfluency. For both phenotypes, the magnitude of the genetic and environmental influences did not differ between boys and girls. The overlap between the 2 traits was substantial (tetrachoric correlation was .72). A bivariate genetic analysis showed that this overlap was due to both overlapping genetic and environmental influences.

CONCLUSIONS

These findings provide a foundation to justify further studies in normal fluency control, a scientific area that has received little cross-disciplinary attention.

Mutations in the lysosomal enzyme-targeting pathway and persistent stuttering

BACKGROUND

Stuttering is a disorder of unknown cause characterized by repetitions, prolongations, and interruptions in the flow of speech. Genetic factors have been implicated in this disorder, and previous studies of stuttering have identified linkage to markers on chromosome 12.

METHODS

We analyzed the chromosome 12q23.3 genomic region in consanguineous Pakistani families, some members of which had nonsyndromic stuttering and in unrelated case and control subjects from Pakistan and North America.

RESULTS

We identified a missense mutation in the N-acetylglucosamine-1-phosphate transferase gene (GNPTAB), which encodes the alpha and beta catalytic subunits of GlcNAc-phosphotransferase (GNPT [EC 2.7.8.15]), that was associated with stuttering in a large, consanguineous Pakistani family. This mutation occurred in the affected members of approximately 10% of Pakistani families studied, but it occurred only once in 192 chromosomes from unaffected, unrelated Pakistani control subjects and was not observed in 552 chromosomes from unaffected, unrelated North American control subjects. This and three other mutations in GNPTAB occurred in unrelated subjects with stuttering but not in control subjects. We also identified three mutations in the GNPTG gene, which encodes the gamma subunit of GNPT, in affected subjects of Asian and European descent but not in control subjects. Furthermore, we identified three mutations in the NAGPA gene, which encodes the so-called uncovering enzyme, in other affected subjects but not in control subjects. These genes encode enzymes that generate the mannose-6-phosphate signal, which directs a diverse group of hydrolases to the lysosome. Deficits in this system are associated with the mucolipidoses, rare lysosomal storage disorders that are most commonly associated with bone, connective tissue, and neurologic symptoms.

CONCLUSIONS

Susceptibility to nonsyndromic stuttering is associated with variations in genes governing lysosomal metabolism.

[Single nucleotide polymorphisms of DAT and DRD(2) genes in Han Chinese population and their association with stuttering]

OBJECTIVE

To explore the correlations of dopamine transporter gene (DAT) and dopamine D(2) receptor gene (DRD2) to stuttering.

METHODS

To examine the correlations of the 5 single nucleotide polymorphisms (SNPs) in dopaminergic gene (C252T, C1804T, and C1820T in DAT gene, and T1054C and C1072T in DRD(2) gene) to stuttering in Han Chinese individuals, a case-control study involving 112 patients with stuttering and 112 gender-matched controls was carried out. Genotyping was performed by a combined approach using polymerase chain reaction (PCR) and pyrosequencing.

RESULTS

C1804T showed no polymorphism in either the patients or the control subjects and was therefore excluded from the following analysis. The C allele frequency at C1072T site was significantly higher, but T allele frequency significantly lower in the stuttering group than in the control group. The patients had significantly higher CC and lower CT genotype frequencies than the control group. There were no significant differences in the allelic frequencies of C252T, C1820T and T1054C between the patients and the controls, suggesting a Hardy-Weinberg equilibrium at these 3 loci.

CONCLUSION

The presence of the C allele at C1072T in DRD(2) gene is associated with increased susceptibility to stuttering in Han Chinese, whereas the T allele provides protection against the onset of stuttering.

Late childhood stuttering

PURPOSE

A study was conducted that examined factors that lead children who stutter at around age 8 years to persist in the disorder when they reach age 12 years.

METHOD

Seventy-six children were verified to be stuttering at initial assessment. When they reached 12 years of age, they were classified as persistent or recovered. A range of measures was taken at the 2 age points, and measures were examined by recovery group.

RESULTS

Although the tendency for more males than females to stutter was confirmed, the reasons for this tendency are not apparent for these speakers. Different patterns in speech were observed: Severity ratings of the recovered speakers dropped by age 12+. The severity ratings for the persistent speakers remained high at 12+, and dysfluency types tended to change from whole words to part words. Persistent and recovered speakers differed on temperamental performance at around age 8 years and performed differently on sensory and motor tasks at age 12+ years.

CONCLUSIONS

Stuttering in late childhood affects mainly males. The later a child attends clinic, the longer he or she will stutter. Speech patterns of children who persist diverge from those who recover or who are fluent. As speakers persist, there are temperamental, sensory, and motor changes.

Speech fluency in fragile X syndrome

The present study investigated the dysfluencies in the speech of nine French speaking individuals with fragile X syndrome. Type, number, and loci of dysfluencies were analysed. The study confirms that dysfluencies are a common feature of the speech of individuals with fragile X syndrome but also indicates that the dysfluency pattern displayed is not identical to developmental stuttering. To what extent the pattern of dysfluency in individuals with fragile X syndrome is syndrome specific is not yet clear.

Genetic etiology in cases of recovered and persistent stuttering in an unselected, longitudinal sample of young twins

PURPOSE

The contribution of genetic factors in the persistence of and early recovery from stuttering was assessed.

METHOD

Data from the Twins Early Development Study were employed. Parental reports regarding stuttering were collected at ages 2, 3, 4, and 7 years, and were used to classify speakers into recovered and persistent groups. Of 12,892 children with at least 2 ratings, 950 children had recovered and 135 persisted in their stutter.

RESULTS

Logistic regressions showed that the rating at age 2 was not predictive of later stuttering, whereas ratings at ages 3 and 4 were. Concordance rates were consistently higher for monozygotic than for dizygotic twin pairs (with the exception of girls at age 3). At 3, 4, and 7 years, the liability to stuttering was highly heritable (h2 estimates of between .58 and .66). Heritability for the recovered and persistent groups was also high but did not differ from each other.

CONCLUSION

Stuttering appears to be a disorder that has high heritability and little shared environment effect in early childhood and for recovered and persistent groups of children, by age 7. The clinical implications of the findings are discussed.

Genetic studies of stuttering in a founder population

Genome-wide linkage and association analyses were conducted to identify genetic determinants of stuttering in a founder population in which 48 individuals affected with stuttering are connected in a single 232-person genealogy. A novel approach was devised to account for all necessary relationships to enable multipoint linkage analysis. Regions with nominal evidence for linkage were found on chromosomes 3 (P=0.013, 208.8 centiMorgans (cM)), 13 (P=0.012, 52.6 cM), and 15 (P=0.02, 100 cM). Regions with nominal evidence for association with stuttering that overlapped with a linkage signal are located on chromosomes 3 (P=0.0047, 195 cM), 9 (P=0.0067, 46.5 cM), and 13 (P=0.0055, 52.6 cM). We also conducted the first meta-analysis for stuttering using results from linkage studies in the Hutterites and The Illinois International Genetics of Stuttering Project and identified regions with nominal evidence for linkage on chromosomes 2 (P=0.013, 180-195 cM) and 5 (P=0.0051, 105-120 cM; P=0.015, 120-135 cM). None of the linkage signals detected in the Hutterite sample alone, or in the meta-analysis, meet genome-wide criteria for significance, although some of the stronger signals overlap linkage mapping signals previously reported for other speech and language disorders.

EDUCATIONAL OBJECTIVES

After reading this article, the reader will be able to: (1) summarize information about the background of common disorders and methodology of genetic studies; (2) evaluate the role of genetics in stuttering; (3) discuss the value of using founder populations in genetic studies; (4) articulate the importance of combining several studies in a meta-analysis; (5) discuss the overlap of genetic signals identified in stuttering with other speech and language disorders.

A study of familial stuttering

Here we study 13 families with stuttering. Of the 13 families, 9 were persistent stutterers and 4 were recovered stutterers. In the 9 families with persistent stuttering, 24 were male and 10 were females. Of the 4 families with recovered stutterers, 17 were male and 3 were female. Of the 17 males, 12 were persistent stutterers and 5 recovered after adolescence. All females were recovered stutterers. We conclude with a short discussion of recent molecular studies.

Identification of traits associated with stuttering

Stuttering has been considered a heritable disorder since the 1930s. There have been different models of transmission that have been proposed most involving a polygenic component with or without a major locus. In spite of these models, the characteristics being transmitted are not known. This study used two different tasks-a tapping task that is thought to probe hemispheric differences and a Stroop task, which appears to create interferences in speech motor programming and/or execution. The 48 participants in this study included individuals who stutter, high risk family members and controls for each group. Results indicated that for tapping at a comfortable rate, the experimental groups were significantly different from their control groups and for tapping at a fast rate, the stuttering and high risk groups were different from each other. The results of the Stroop test were not statistically significant.

LEARNING OUTCOMES

Readers will learn about: (1) genetic aspects of stuttering; (2) hemispheric dominance in stuttering and high risk subjects; (3) understanding traits that may be associated with stuttering.

New complexities in the genetics of stuttering: significant sex-specific linkage signals

Stuttering is a speech disorder long recognized to have a genetic component. Recent linkage studies mapped a susceptibility locus for stuttering to chromosome 12 in 46 highly inbred families ascertained in Pakistan. We report here on linkage studies in 100 families of European descent ascertained in the United States, Sweden, and Israel. These families included 252 individuals exhibiting persistent stuttering, 45 individuals classified as recovered from stuttering, and 19 individuals too young to classify. Primary analyses identified moderate evidence for linkage of the broader diagnosis of "ever stuttered" (including both persistent and recovered stuttering) on chromosome 9 (LOD = 2.3 at 60 cM) and of the narrower diagnosis of persistent stuttering on chromosome 15 (LOD = 1.95 at 23 cM). In contrast, sex-specific evidence for linkage on chromosome 7 at 153 cM in the male-only data subset (LOD = 2.99) and on chromosome 21 at 34 cM in the female-only data subset (LOD = 4.5) met genomewide criteria for significance. Secondary analyses revealed a significant increase in the evidence for linkage on chromosome 12, conditional on the evidence for linkage at chromosome 7, with the location of the increased signal congruent with the previously reported signal in families ascertained in Pakistan. In addition, a region on chromosome 2 (193 cM) showed a significant increase in the evidence for linkage conditional on either chromosome 9 (positive) or chromosome 7 (negative); this chromosome 2 region has been implicated elsewhere in studies on autism, with increased evidence for linkage observed when the sample is restricted to those with delayed onset of phrase speech. Our results support the hypothesis that the genetic component to stuttering has significant sex effects.

Language-induced Epilepsy, Acquired Stuttering, and Idiopathic Generalized Epilepsy: Phenotypic Study of One Family

PURPOSE

Language-induced epilepsy involves seizure precipitation by speaking, reading, and writing. Seizures are similar to those of reading epilepsy (RE). The nosologic position of language-induced epilepsy is not clear. We performed a clinical and neurophysiological study in a multigenerational family with the association of idiopathic generalized epilepsy (IGE) with ictal stuttering as a manifestation of reflex language-induced epilepsy.

METHODS

Nine members on three generations were studied. All patients underwent video-polygraphic EEG recordings (awake and during sleep). A standardized protocol was applied to test the effect of language and non-language-related tasks.

RESULTS

Six patients presented language-induced jaw jerking that mimicked stuttering and corresponded to focal myoclonus involving facial muscles. This was associated with an IGE phenotype in four of these patients. Focal EEG spikes were found in all six patients by visual analysis and/or back-averaging techniques. The focal spikes were either asymptomatic (when followed by a slow wave) or symptomatic of facial myoclonia (when isolated). Levetiracetam, used as add-on or monotherapy in four patients, suppressed ictal stuttering. One additional case only had a phenotype of IGE without focal features.

CONCLUSIONS

This family study demonstrates the phenotypic heterogeneity of the association of IGE phenotype with ictal stuttering (language-related reflex seizure). Our data suggest that this particular form of reflex epilepsy related to language has more similarities with generalized epilepsies than with focal ones. Neurophysiological investigations should be performed more systematically in patients with acquired stuttering, especially if there is family history of IGE.

Genetic and environmental influences on stuttering and tics in Japanese twin children

The purpose of this study was to clarify the genetic contribution to stuttering and tics in childhood using the largest databases of Japanese twins. The subjects were 1896 pairs of twin children consisting of 1849 males and 1943 females with a mean age of 11.6 years (3 years to 15 years). All data were gathered by questionnaire. The prevalence of stuttering was 6.7% in males and 3.6% in females (p < .0001). The prevalence of tics was 6.8% in males and 4.1% in females (p = .0021). Concordance rates and polychoric correlations were all higher in monozygotic pairs than in dizygotic pairs irrespective of sex combination. Structural equation modeling showed that the proportion of total phenotypic variance attributable to genetic influences was 80% in males and 85% in females for stuttering, and 28% in males and 29% in females for tics. Moreover, co-occurrence between stuttering and tics was observed in 0.8% of males (tetrachoric correlation: r = .18) and 0.5% of females (r = .31), which was attributed partly (nearly 10% of total genetic variance of each trait) to the common genetic factors, with genetic correlation of r = .32.

Segregation analyses of stuttering

Although stuttering is known to be a familial disorder, no clear evidence regarding precise mode of transmission has arisen from previous research. In this report segregation analysis is applied to data on 386 stuttering probands and their first-degree relatives in an effort to discriminate among possible genetic models for the transmission of stuttering. Two different segregation analysis programs, PAP and POINTER, gave comparable results with respect to both hypothesis testing and parameter estimation. Specifically, the transmission of stuttering observed in these families cannot be adequately explained by a Mendelian major locus. The hypothesis of no polygenic component in the transmission of stuttering can, however, be rejected. Existence in these data of potential heterogeneity and possible violations of assumptions concerning ascertainment are considered in interpreting the results.

Genomewide significant linkage to stuttering on chromosome 12

Stuttering is a common and sometimes severe communication disorder, of unknown primary etiology, that exists in populations worldwide. Many types of evidence suggest a genetic contribution to stuttering; however, the complex inheritance of this disorder has hindered identification of these factors. We have employed highly inbred families to increase the power of linkage analysis of this disorder. Forty-four Pakistani families with documented or probable consanguinity, from the city of Lahore and surrounding areas, were included. Each family contained multiple cases of stuttering, which were diagnosed using the Stuttering Severity Instrument. Using the Marshfield Weber 9 marker panel, we performed a genomewide linkage scan focused on affected individuals and their parents. The analysis included 199 genotyped individuals, 144 affected and 55 unaffected. The Pedigree Relationship Statistical Test (PREST) was used to identify pedigrees that required additional specification of inbreeding. Initial nonparametric analysis gave evidence of linkage on chromosomes 1, 5, 7, and 12. Additional genotyping was performed on chromosome 12 to a 5-cM level of resolution, and 16 additional individuals were then included, bringing the number of families to 46. Analysis of the enlarged data set provided consistent evidence of linkage on chromosome 12: the S(homoz) scoring function gave a nonparametric LOD score of 4.61, and a LOD score of 3.51 was obtained using the S(all) scoring function. These results suggest that a locus on chromosome 12q may contain a gene with a large effect in this sample.

Evidence for a major gene influence on persistent developmental stuttering

Stuttering is a complex developmental speech disorder of unknown etiology. There is a substantial aggregation of stuttering in families, suggesting a genetic component to the disorder. However, the exact mode of transmission is still unknown. An earlier study of 56 multigenerational pedigrees ascertained through single adult probands (38 males and 18 females) found that biological relatives of persistent developmental stutterers have an approximately 10-fold higher risk than in the general population; risk is higher for male relatives, and proband's sex does not affect recurrence and relative risks. In the present paper we conduct a complex segregation analysis of the same data, using the logistic regression model of the SAGE software. Based on the comparisons of model likelihoods, the Mendelian model was selected over all other nongenetic models and the general transmission model. This model was further refined into the most parsimonious model, which shows an autosomal dominant major gene effect influenced by two covariates: sex and affection status of parents. With this model applied to 47 informative multiplex pedigrees, a power calculation based on linkage simulation produced an average lod score of 6.8 for 10-cM density genome scan markers. These results give impetus for a genomewide linkage analysis of susceptibility to persistent developmental stuttering.

Results of a genome-wide linkage scan for stuttering

We performed a linkage study of stuttering using 392 markers distributed across the genome in a series of 68 families identified in the general outbred population of North America and Europe. Standardized diagnosis was performed using recorded samples of both conversation and reading, in which stuttering dysfluencies were scored as percentage of dysfluent words and syllables. Analysis was first performed using non-parametric methods implemented in GENEHUNTER, where we obtained maximum statistical support for markers of chromosome 18, with a maximum NPL (Sall) of 1.51 at D18S976. The single largest pedigree within our sample (pedigree 0006) alone gave an NPL of 4.72 at D18S976. For fine mapping, we analyzed 18 markers on chromosome 18 across all families using ALLEGRO. Overall NPL (Srobdom) scores >5 were obtained with markers on 18p, and Z(lr) scores >/=2.5 on 18p and proximal 18q. Furthermore, pedigree 0006 alone gave an NPL (Srobdom) of 5.35. Overall our results suggest chromosome 18 may harbor a predisposing locus for this disorder, and additional genes may exist.

The influence of family history of stuttering on the onset of stuttering in young children

Content analysis was undertaken of the case records of 61 children who stuttered who were attending a specialist centre for children who stutter. The subjects were divided into two groups, on the basis of family history of stuttering. Positive family histories of stuttering were reported for 44 children and 17 had negative family histories. The two groups were compared in terms of gender ratios, the age of onset and the type of onset (gradual vs. sudden) of stuttering. Those with positive family histories began stuttering earlier than those with no reported family history of stuttering, though this difference was not statistically significant. The type of onset of stuttering was not related to the presence or absence of a family history of stuttering. The findings are discussed in terms of the practicability of this method of data collection.

The epigenesis of stuttering

This paper examines the data on which the Demands and Capacities Model (DCM) is based with the purpose of identifying areas where future research might determine consilience among genetic influences at the physiological, behavioral, and cultural levels. The determination of consilience across different levels would tend to validate the genetic influence on stuttering, but more importantly it would also sharpen the focus of researchers interested in the various possible expressions that genetic influences might have and the way in which they influence the development of the disorder.

EDUCATIONAL OBJECTIVES

The reader will (1) learn about the distinction, as outlined in the DCM, between environmental/contextual influences on stuttering development and intrinsic/genetic influences on stuttering development; (2) learn about the concept of consilience and its usefulness in conferring validity on parallel constructs at the physiological, behavioral, and cultural levels of stuttering theory; and (3) be able to identify potential areas for research that might help in refining our understanding of the genetic influences on stuttering development.

Early stuttering as a type of language difficulty

The reader will learn about and be able to describe (1) the characteristics of incipient stuttering, (2) how it differs from stuttering in older children, and (3) the relationship of such early stuttering to developing language skills.