Results of a genome-wide linkage scan for stuttering

Population-based genetic effects for developmental stuttering

Despite a lifetime prevalence of at least 5%, developmental stuttering, characterized by prolongations, blocks, and repetitions of speech sounds, remains a largely idiopathic speech disorder. Family, twin, and segregation studies overwhelmingly support a strong genetic influence on stuttering risk; however, its complex mode of inheritance combined with thus-far underpowered genetic studies contribute to the challenge of identifying and reproducing genes implicated in developmental stuttering susceptibility. We conducted a trans-ancestry genome-wide association study (GWAS) and meta-analysis of developmental stuttering in two primary datasets: The International Stuttering Project comprising 1,345 clinically ascertained cases from multiple global sites and 6,759 matched population controls from the biobank at Vanderbilt University Medical Center (VUMC), and 785 self-reported stuttering cases and 7,572 controls ascertained from The National Longitudinal Study of Adolescent to Adult Health (Add Health). Meta-analysis of these genome-wide association studies identified a genome-wide significant (GWS) signal for clinically reported developmental stuttering in the general population: a protective variant in the intronic or genic upstream region of SSUH2 (rs113284510, protective allele frequency = 7.49%, Z = -5.576, p = 2.46 × 10^-8) that acts as an expression quantitative trait locus (eQTL) in esophagus-muscularis tissue by reducing its gene expression. In addition, we identified 15 loci reaching suggestive significance (p < 5 × 10^-6). This foundational population-based genetic study of a common speech disorder reports the findings of a clinically ascertained study of developmental stuttering and highlights the need for further research.

IFNAR1 gene mutation may contribute to developmental stuttering in the Chinese population

BACKGROUND

Developmental stuttering is the most common form of stuttering without apparent neurogenic or psychogenic impairment. Recently, whole-exome sequencing (WES) has been suggested to be a promising approach to study Mendelian disorders.

METHODS

Here, we describe an application of WES to identify a gene potentially responsible for persistent developmental stuttering (PDS) by sequencing DNA samples from 10 independent PDS families and 11 sporadic cases. Sanger sequencing was performed for verification with samples obtained from 73 additional patients with sporadic cases.

RESULTS

We first searched for cosegregating variants/candidate genes in a Chinese family (Family 0) by sequencing DNA obtained from 3 affected members and 3 controls. Next, we sequenced DNA samples obtained from 9 additional Chinese families (Families 1-9) with stuttering to verify the identified candidate genes. Intriguingly, we found that two missense variants (Leu552Pro and Lys428Gln) of interferon-alpha/beta receptor 1 (IFNAR1) cosegregated with stuttering in three independent families (Families 0, 5 and 9). Moreover, we found two additional mutations (Gly301Glu and Pro335del) in the IFNAR1 gene in 4 patients with sporadic cases by using WES or Sanger sequencing. Further receptor mutagenesis and cell signaling studies revealed that these IFNAR1 variants may impair the activity of type I IFN signaling.

CONCLUSION

Our data indicate that IFNAR1 might be a potential pathogenic gene of PDS in the Chinese population.

Dopamine, vocalization, and astrocytes

Dopamine, the main catecholamine neurotransmitter in the brain, is predominately produced in the basal ganglia and released to various brain regions including the frontal cortex, midbrain and brainstem. Dopamine's effects are widespread and include modulation of a number of voluntary and innate behaviors. Vigilant regulation and modulation of dopamine levels throughout the brain is imperative for proper execution of motor behaviors, in particular speech and other types of vocalizations. While dopamine's role in motor circuitry is widely accepted, its unique function in normal and abnormal speech production is not fully understood. In this perspective, we first review the role of dopaminergic circuits in vocal production. We then discuss and propose the conceivable involvement of astrocytes, the numerous star-shaped glia cells of the brain, in the dopaminergic network modulating normal and abnormal vocal productions.

Evaluation of recurrent GNPTAB, GNPTG, and NAGPA variants associated with stuttering

Stuttering is a childhood-onset fluency disorder, intertwined with physiological, emotional, and anxiety factors. The present study was designed to evaluate the recurrence of the reported mutations among three previously implicated (GNPTAB, GNPTG, NAGPA) candidate genes, in persons with stuttering from south India. Mutation screening was performed among 64 probands on 12 specific exons, by Sanger sequencing. A total of 12 variants were identified, which included five nonsynonymous, five synonymous, and two noncoding variants. Three unrelated probands harbored heterozygous missense variants at conserved coding positions across species (p. Glu1200Lys in GNPTAB, p. Ile268Leu in GNPTG and p. Arg44Pro in NAGPA). Of these, only one variant (p. Glu1200Lys in GNPTAB) cosegregated with the affected status while p. Ile268Leu in GNPTG gene was found to be a rare de novo variant. Although this study identified some previously reported variants that have been claimed to have a role in stuttering, we confirmed only one of these to be a likely causal de novo variant (p.Ile268Leu) in the GNPTG gene at an allele frequency of 0.8% (1/128) in the families with stuttering.

Theta Modulated Neural Phase Coherence Facilitates Speech Fluency in Adults Who Stutter

Adults who stutter (AWS) display altered patterns of neural phase coherence within the speech motor system preceding disfluencies. These altered patterns may distinguish fluent speech episodes from disfluent ones. Phase coherence is relevant to the study of stuttering because it reflects neural communication within brain networks. In this follow-up study, the oscillatory cortical dynamics preceding fluent speech in AWS and adults who do not stutter (AWNS) were examined during a single-word delayed reading task using electroencephalographic (EEG) techniques. Compared to AWNS, fluent speech preparation in AWS was characterized by a decrease in theta-gamma phase coherence and a corresponding increase in theta-beta coherence level. Higher spectral powers in the beta and gamma bands were also observed preceding fluent utterances by AWS. Overall, there was altered neural communication during speech planning in AWS that provides novel evidence for atypical allocation of feedforward control by AWS even before fluent utterances.

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.

The Pathogenesis, Assessment and Treatment of Speech Fluency Disorders

BACKGROUND

Approximately 1% of children and adolescents, 0.2% of women, and 0.8% of men suffer from stuttering, and lesser numbers from cluttering. Persistent speech fluency disorders often cause lifelong problems in communication and social participation.

METHODS

In an interdisciplinary, evidence and consensus based clinical practice guideline, the current understanding of the nature, identification, diagnosis, and treatment of stuttering and cluttering was summarized. A systematic review of the literature was carried out to assess the efficacy and effectiveness of treatments for stuttering. Evidence is lacking on the etiology, pathogenesis, evaluation, and treatment of cluttering.

RESULTS

In view of the fact that common (developmental, idiopathic) stuttering is associated with structural and functional changes of the brain, the guideline recommends that it should be called "originary neurogenic non-syndromic stuttering." Heritability estimates for this disorder range from 70% to over 80%. For preschool children, the Lidcombe therapy has the best evidence of efficacy (Cohen's d = 0.72-1.00). There is also strong evidence for an indirect treatment approach. For children aged 6 to 12, there is no solid evidence for the efficacy of any treatment. For adolescents and adults, there is good evidence with high effect sizes (Cohen's d = 0.75-1.63) for speech restructuring methods such as fluency shaping; weak evidence with intermediate effect sizes for stuttering modification (Cohen's d = 0.56-0.65); and weak evidence for combined speech restructuring and stuttering modification. The evidence does not support the efficacy of pharmacotherapy, rhythmic speaking, or breathing regulation as the sole or main form of treatment, or that of hypnosis or eclectic, unspecified stuttering therapies.

CONCLUSION

Stuttering is often treated in Germany with therapies for which there is inadequate evidence, and the initiation of treatment is often unnecessarily delayed. The guideline presents treatment methods whose efficacy is supported by the current evidence.

Language development and disorders: Possible genes and environment interactions

Language development requires both basic cognitive mechanisms for learning language and a rich social context from which learning takes off. Disruptions in learning mechanisms, processing abilities, and/or social interactions increase the risks associated with social exclusion or developmental delays. Given the complexity of language processes, a multilevel approach is proposed where both cognitive mechanisms, genetic and environmental factors need to be probed together with their possible interactions. Here we review and discuss such interplay between environment and genetic predispositions in understanding language disorders, with a particular focus on a possible endophenotype, the ability for statistical sequential learning.

Genetic contributions to stuttering: the current evidence

Evidence for genetic factors in persistent developmental stuttering has accumulated over the past four decades, and the genes that underlie this disorder are starting to be identified. The genes identified to date, all point to deficits in intracellular trafficking in this disorder.

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.

A review of brain circuitries involved in stuttering

Stuttering has been the subject of much research, nevertheless its etiology remains incompletely understood. This article presents a critical review of the literature on stuttering, with particular reference to the role of the basal ganglia (BG). Neuroimaging and lesion studies of developmental and acquired stuttering, as well as pharmacological and genetic studies are discussed. Evidence of structural and functional changes in the BG in those who stutter indicates that this motor speech disorder is due, at least in part, to abnormal BG cues for the initiation and termination of articulatory movements. Studies discussed provide evidence of a dysfunctional hyperdopaminergic state of the thalamocortical pathways underlying speech motor control in stuttering. Evidence that stuttering can improve, worsen or recur following deep brain stimulation for other indications is presented in order to emphasize the role of BG in stuttering. Further research is needed to fully elucidate the pathophysiology of this speech disorder, which is associated with significant social isolation.

A study of the role of the FOXP2 and CNTNAP2 genes in persistent developmental stuttering

A number of speech disorders including stuttering have been shown to have important genetic contributions, as indicated by high heritability estimates from twin and other studies. We studied the potential contribution to stuttering from variants in the FOXP2 gene, which have previously been associated with developmental verbal dyspraxia, and from variants in the CNTNAP2 gene, which have been associated with specific language impairment (SLI). DNA sequence analysis of these two genes in a group of 602 unrelated cases, all with familial persistent developmental stuttering, revealed no excess of potentially deleterious coding sequence variants in the cases compared to a matched group of 487 well characterized neurologically normal controls. This was compared to the distribution of variants in the GNPTAB, GNPTG, and NAGPA genes which have previously been associated with persistent stuttering. Using an expanded subject data set, we again found that NAGPA showed significantly different mutation frequencies in North Americans of European descent (p=0.0091) and a significant difference existed in the mutation frequency of GNPTAB in Brazilians (p=0.00050). No significant differences in mutation frequency in the FOXP2 and CNTNAP2 genes were observed between cases and controls. To examine the pattern of expression of these five genes in the human brain, real time quantitative reverse transcription PCR was performed on RNA purified from 27 different human brain regions. The expression patterns of FOXP2 and CNTNAP2 were generally different from those of GNPTAB, GNPTG and NAPGA in terms of relatively lower expression in the cerebellum. This study provides an improved estimate of the contribution of mutations in GNPTAB, GNPTG and NAGPA to persistent stuttering, and suggests that variants in FOXP2 and CNTNAP2 are not involved in the genesis of familial persistent stuttering. This, together with the different brain expression patterns of GNPTAB, GNPTG, and NAGPA compared to that of FOXP2 and CNTNAP2, suggests that the genetic neuropathological origins of stuttering differ from those of verbal dyspraxia and SLI.

Genotype-phenotype analysis of 18q12.1-q12.2 copy number variation in autism

Autism Spectrum Disorders (ASD) are complex neurodevelopmental conditions characterized by delays in social interactions and communication as well as displays of restrictive/repetitive interests. DNA copy number variants have been identified as a genomic susceptibility factor in ASDs and imply significant genetic heterogeneity. We report a 7-year-old female with ADOS-G and ADI-R confirmed autistic disorder harbouring a de novo 4 Mb duplication (18q12.1). Our subject displays severely deficient expressive language, stereotypic and repetitive behaviours, mild intellectual disability (ID), focal epilepsy, short stature and absence of significant dysmorphic features. Search of the PubMed literature and DECIPHER database identified 4 additional cases involving 18q12.1 associated with autism and/or ID that overlap our case: one duplication, two deletions and one balanced translocation. Notably, autism and ID are seen with genomic gain or loss at 18q12.1, plus epilepsy and short stature in duplication cases, and hypotonia and tall stature in deletion cases. No consistent dysmorphic features were noted amongst the reviewed cases. We review prospective ASD/ID candidate genes integral to 18q12.1, including those coding for the desmocollin/desmoglein cluster, ring finger proteins 125 and 138, trafficking protein particle complex 8 and dystrobrevin-alpha. The collective clinical and molecular features common to microduplication 18q12.1 suggest that dosage-sensitive, position or contiguous gene effects may be associated in the etiopathogenesis of this autism-ID-epilepsy syndrome.

Speech-language pathology insights into genetics and neuroscience: beyond surface behaviour

For almost a century, speech-language pathologists (SLPs) have worked at refining communication disorder phenotypes. Yet a hundred years of mastering the characterization of surface behaviours has provided only limited understanding of the neurobiological underpinnings of communication disorder. Arguably, the most momentous aetiological findings in speech-language pathology have been made relatively recently and by cross-disciplinary colleagues in the fields of molecular genetics and neuroimaging. Such findings include discovery of FOXP2, for example, the first gene found to be associated with a primary speech disorder. New gene-brain-behaviour discoveries in communication disorder are occurring on an almost weekly basis and it is challenging for clinical SLPs to engage with, interpret, and keep abreast of this literature. This paper aims to provide a brief overview of genetic and neuroimaging approaches to the study of communication disorders. Further examples of key findings in these fields are presented, with a discussion of the impacts on core SLP practice. Future research directions for further illuminating gene-brain-behaviour relationships in communication disorder are identified.

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.

Linkage analysis of a large African family segregating stuttering suggests polygenic inheritance

We describe a pedigree of 71 individuals from the Republic of Cameroon in which at least 33 individuals have a clinical diagnosis of persistent stuttering. The high concentration of stuttering individuals suggests that the pedigree either contains a single highly penetrant gene variant or that assortative mating led to multiple stuttering-associated variants being transmitted in different parts of the pedigree. No single locus displayed significant linkage to stuttering in initial genome-wide scans with microsatellite and SNP markers. By dividing the pedigree into five subpedigrees, we found evidence for linkage to previously reported loci on 3q and 15q, and to novel loci on 2p, 3p, 14q, and a different region of 15q. Using the two-locus mode of Superlink, we showed that combining the recessive locus on 2p and a single-locus additive representation of the 15q loci is sufficient to achieve a two-locus score over 6 on the entire pedigree. For this 2p + 15q analysis, we show LOD scores ranging from 4.69 to 6.57, and the scores are sensitive to which marker is chosen for 15q. Our findings provide strong evidence for linkage at several loci.

Stuttering: Genetic updates and a case report

Developmental stuttering is a common disorder of speech dissiliency that is characterized by excessive repetitions of sounds, syllables, and monosyllabic words, as well as sound prolongations and complete blockages of the vocal tract. About 60 million people are affected and it is more common between the age of 3 and 6, when children begin forming sentences and connecting thoughts verbally. There are three types of stuttering known as developmental stuttering, neurogenic stuttering, and psychogenic stuttering. The exact pathophysiology of developmental stuttering is unknown; however, various family and twin studies have repeatedly implicated heredity as a major factor in the etiology of stuttering. It is clear that the genetic influence is not in the form of an exact single gene effect such as autosomal recessive, autosomal dominant, or x-linked in all families; however, in all of the inheritance forms it is influenced by sex with higher occurrence in males than females at a ratio of 4:1 in older children and adults. Recently special genetic locus has been determined on several autosomal chromosomes related to developmental stuttering. In this report, the proband is a 20-year-old boy was referred to our clinic for premarriage genetic counseling; he has been affected since 3 years and now is under cure. three generation study of his family show 13 individuals are affected by stuttering. For the first it occurred in the proband's grandfather and after this time about all of affected cases has been seen in consanguineous marriages. Therefore, the genetical inheritance of stuttering is crystal clear in this family and autosomal recessive inheritance pattern is proposed. Totally in such families with repeated occur of stuttering, we cannot account it as a multifactorial disorder.

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.

Mannose-6-phosphate pathway: a review on its role in lysosomal function and dysfunction

Lysosomal hydrolases are synthesized in the rough endoplasmic reticulum and specifically transported through the Golgi apparatus to the trans-Golgi network, from which transport vesicles bud to deliver them to the endosomal/lysosomal compartment. The explanation of how are the lysosomal enzymes accurately recognized and selected over many other proteins in the trans-Golgi network relies on being tagged with a unique marker: the mannose-6-phosphate (M6P) group, which is added exclusively to the N-linked oligosaccharides of lysosomal soluble hydrolases, as they pass through the cis-Golgi network. Generation of the M6P recognition marker depends on a reaction involving two different enzymes: UDP-N-acetylglucosamine 1-phosphotransferase and α-N-acetylglucosamine-1-phosphodiester α-N-acetylglucosaminidase. The M6P groups are then recognized by two independent transmembrane M6P receptors, present in the trans-Golgi network: the cation-independent M6P receptor and/or the cation-dependent M6P receptor. These proteins bind to lysosomal hydrolases on the lumenal side of the membrane and to adaptins in assembling clathrin coats on the cytosolic side. In this way, the M6P receptors help package the hydrolases into vesicles that bud from the trans-Golgi network to deliver their contents to endosomes that ultimately will develop into mature lysosomes, where recently-delivered hydrolases may start digesting the endocyted material. The above described process is known as the M6P-dependent pathway and is responsible for transporting most lysosomal enzymes. This review synthesizes the current knowledge on each of the major proteins involved in the M6P-dependent pathway. Impairments in this pathway will also be addressed, highlighting the lysosomal storage disorders associated to GlcNAc-1-phosphotransferase loss of function: mucolipidosis type II and III.

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.

Genetic approaches to understanding the causes of stuttering

Stuttering is a common but poorly understood speech disorder. Evidence accumulated over the past several decades has indicated that genetic factors are involved, and genetic linkage studies have begun to identify specific chromosomal loci at which causative genes are likely to reside. A detailed investigation of one such region on chromosome 12 has identified mutations in the GNPTAB gene that are associated with stuttering in large families and in the general population. Subsequent studies identified mutations in the functionally related GNPTG and NAGPA genes. Mutations in these genes disrupt the lysosomal targeting pathway that generates the Mannose 6-phosphate signal, which directs a diverse group of enzymes to their target location in the lysosome of the cell. While mutations in these three genes can be identified in less than 10% of cases of familial stuttering, this knowledge allows a variety of new studies that can help identify the neuropathology that underlies this disorder.

Identification of a microdeletion at the 7q33-q35 disrupting the CNTNAP2 gene in a Brazilian stuttering case

Speech and language disorders are some of the most common referral reasons to child development centers accounting for approximately 40% of cases. Stuttering is a disorder in which involuntary repetition, prolongation, or cessation of the sound precludes the flow of speech. About 5% of individuals in the general population have a stuttering problem, and about 80% of the affected children recover naturally. The causal factors of stuttering remain uncertain in most cases; studies suggest that genetic factors are responsible for 70% of the variance in liability for stuttering, whereas the remaining 30% is due to environmental effects supporting a complex cause of the disorder. The use of high-resolution genome wide array comparative genomic hybridization has proven to be a powerful strategy to narrow down candidate regions for complex disorders. We report on a case with a complex set of speech and language difficulties including stuttering who presented with a 10 Mb deletion of chromosome region 7q33-35 causing the deletion of several genes and the disruption of CNTNAP2 by deleting the first three exons of the gene. CNTNAP2 is known to be involved in the cause of language and speech disorders and autism spectrum disorder and is in the same pathway as FOXP2, another important language gene, which makes it a candidate gene for causal studies speech and language disorders such as stuttering.

Heritability and environmental effects for self-reported periods with stuttering: a twin study from Denmark

Genetic influence for stuttering was studied based on adult self-reporting. Using nation-wide questionnaire answers from 33,317 Danish twins, a univariate biometric analysis based on the liability threshold model was performed in order to estimate the heritability of stuttering. The self-reported incidences for stuttering were from less than 4% for females to near 9% for males. Both probandwise concordance rate and tetrachoric correlation were substantially higher for monozygotic compared to dizygotic pairs, indicating substantial genetic influence on individual liability. Univariate biometric analyses showed that additive genetic and unique environmental factors best explained the observed concordance patterns. Heritability estimates for males/females were 0.84/0.81. Moderate unique environmental effects were also found.

Genetic advances in the study of speech and language disorders

Developmental speech and language disorders cover a wide range of childhood conditions with overlapping but heterogeneous phenotypes and underlying etiologies. This characteristic heterogeneity hinders accurate diagnosis, can complicate treatment strategies, and causes difficulties in the identification of causal factors. Nonetheless, over the last decade, genetic variants have been identified that may predispose certain individuals to different aspects of speech and language difficulties. In this review, we summarize advances in the genetic investigation of stuttering, speech-sound disorder (SSD), specific language impairment (SLI), and developmental verbal dyspraxia (DVD). We discuss how the identification and study of specific genes and pathways, including FOXP2, CNTNAP2, ATP2C2, CMIP, and lysosomal enzymes, may advance our understanding of the etiology of speech and language disorders and enable us to better understand the relationships between the different forms of impairment across the spectrum.

Perfil familial da fluência da fala: estudo linguístico, acústico e eletromiográfico

TEMA: fatores genéticos como possíveis responsáveis pela gagueira. OBJETIVO: identificar o perfil familial da fluência da fala - aspectos linguísticos, eletromiográficos e acústicos - em crianças com e sem história familiar próxima para a gagueira. MÉTODO: participaram do estudo 127 indivíduos, 32 crianças (probandos) e 95 membros da família imediata (pai, mãe, irmãs e irmãos) divididos em dois grupos: GI (CCG): 17 probandos com diagnóstico de gagueira; 17 pais, 17 mães, 10 irmãos e 13 irmãs; e GII (CSG): 15 probandos fluentes; 15 pais, 15 mães, 0 irmãos e 8 irmãs. Todos os procedimentos de testagem foram aplicados em todos os participantes: 1. Coleta das tipologias das rupturas; 2. Coleta eletromiográfica; 3. Coleta acústica. RESULTADOS: foi encontrado o percentual de 41,1% de mães afetadas; 35,3% de pais afetados; 16,7% de irmãs afetadas e 40 % de irmãos afetados. Foi observada similaridade na tipologia das rupturas da fala em todos os afetados de uma mesma família, mesmo havendo uma tendência a maior gravidade do distúrbio nos probandos. Foi encontrada similaridade na ativação muscular para as taxas de diadococinesia em todos os afetados de uma mesma família. Sugere-se um padrão motor para a fala, numa relação passível de ser mensurada pala captação da ativação muscular periférica, dentro de uma mesma família. Foi encontrada similaridade na variação acústica para as taxas de diadococinesia em todos os afetados de uma mesma família. CONCLUSÃO: esta pesquisa se caracteriza como uma primeira proposta de estudo endofenotípico da gagueira, em dois aspectos: critérios objetivos de inclusão e tipo de sintomatologia manifesta da gagueira.

Identification of an autosomal recessive stuttering locus on chromosome 3q13.2-3q13.33

Stuttering is a common speech disorder with substantial genetic contributions. To better understand the genetic factors involved in stuttering, we performed a genome-wide linkage study in a newly-ascertained consanguineous stuttering family from Pakistan. A linkage scan in this family using parametric linkage analysis revealed significant linkage only on chromosome 3q13.2-3q13.33, with a maximum two-point LOD score of 4.23 under an autosomal recessive model of inheritance.

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.

Speaking genes or genes for speaking? Deciphering the genetics of speech and language

BACKGROUND

This article selectively reviews the status of the genetic research in the field of speech and language disorders.

METHODS

Major contributions to the field are selected, presented, and discussed.

RESULTS

The field presents itself through a variety of findings, characterized by both consistencies and inconsistencies.

CONCLUSIONS

The last 30 + years of the field unequivocally testify to the importance of genetic factors in the acquisition of speech and language. However, the details of how these factors exert their influence are yet to be determined.

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.

Subtyping stuttering I: a review

A reliable and practical subtype system of stuttering should enhance all related scientific work concerned with this disorder. Although a fair number of classification systems have been offered, to date, none has received wide recognition or has been routinely applied in research or clinical spheres. Whereas progress has been made in understanding and treating the disorder, for the most part stuttering continues to be viewed and addressed as a unitary problem. The objectives of the current article are to (a) highlight the motivation for identifying sub-types of stuttering, (b) outline the issues involved in researching subtypes, and (c) address the question of whether or not subtyping is plausible for this disorder. Toward these ends, a broad-based review of past concepts regarding subtypes of stuttering and stutterers is presented according to seven categories that reflect the various authors' conceptual or experimental approaches. Selected studies for each category are also presented to illustrate the research problems and challenges. It is concluded that islands of progress can be identified in subtype research, particularly in studies of children. It is recommended that future studies include multiple factors or domains in the data collection process, especially with young children during the formative years of the disorder, when substantial overlap in the development of several speech/language domains occurs.

EDUCATIONAL OBJECTIVES

(a) Readers will be able to describe the theory and research concerning the numerous attempts to subtype stuttering, particularly during the past 50 years; (b) Readers will be able to explain the general issues that need to be resolved in order to identify subtypes as well as current and future research strategies aimed at achieving these goals.

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.

Stuttering in adults: the acoustic startle response, temperamental traits, and biological factors

The purpose of this study was to investigate the relation between stuttering and a range of variables of possible relevance, with the main focus on neuromuscular reactivity, and anxiety. The explorative analysis also included temperament, biochemical variables, heredity, preonset lesions, and altered auditory feedback (AAF). An increased level of neuromuscular reactivity in stuttering adults has previously been reported by [Guitar, B. (2003). Acoustic startle responses and temperament in individuals who stutter. Journal of Speech Language and Hearing Research, 46, 233-240], also indicating a link to anxiety and temperament. The present study included a large number of variables in order to enable analysis of subgroups and relations between variables. Totally 32 stuttering adults were compared with nonstuttering controls. The acoustic startle eyeblink response was used as a measure of neuromuscular reactivity. No significant group difference was found regarding startle, and startle was not significantly correlated with trait anxiety, stuttering severity, or AAF. Startle was mainly related to calcium and prolactin. The stuttering group had significantly higher scores for anxiety and childhood ADHD. Two subgroups of stuttering were found, with high versus low traits of childhood ADHD, characterized by indications of preonset lesions versus heredity for stuttering. The study does not support the view that excessive reactivity is a typical characteristic of stuttering. The increased anxiety is suggested to mainly be an effect of experiences of stuttering.

LEARNING OUTCOMES

As a result of reading this article, the reader will be able to: (a) critically discuss the literature regarding stuttering in relation to acoustic startle, anxiety, and temperament; (b) describe the effect of calcium on neuromuscular reactivity; (c) discuss findings supporting the importance of early neurological incidents in some cases of stuttering, and the relation between such incidents and traits of ADHD or ADD; and (d) discuss the role of genetics in stuttering.

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.

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.

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.

Neuropharmacology of theophylline induced stuttering: the role of dopamine, adenosine and GABA

Developmental stuttering is a poorly understood speech disorder that starts out in childhood and some individuals continue to stutter throughout their lives. Stuttering is a disruption in smooth and fluent speech. Some stuttering primarily involves vocal blocks, which are spasms of the laryngeal musculature while prolongations, and repetitions of sound occur in other cases. Acquired stuttering, on the other hand, can occur at all ages and can be caused by brain injury and by pharmacological agents. Theophylline-induced stuttering is form of acquired stuttering. It is a rare side effect of theophylline therapy, but it provides interesting clues to the pharmacological mechanisms involved in stuttering. Theophylline-induced stuttering may involve the disrupt the optimal balance between excitatory and inhibitory neurotransmission throughout the brain by inhibiting GABA receptors. The disruption of the optimal balance between excitatory and inhibitory neurotransmission can also cause dysfunction in white matter fiber tracts such as those that connect the Broca's area to the motor cortex. This leads to a hyperexitation of the motor cortex which may mimic the motor cortex hyperexitability that exists in developmental stuttering. Theophylline also enhances dopaminergic neurotransmission through the inhibition of adenosine receptors and this may mimic the hyperdopaminergic state that exists in the brain of developmental stutterers. Theophylline causes the greatest release of dopamine in the basal ganglia through the inhibition of adenosine and GABA receptors. This may also cause dysfunction in the basal ganglia similar in some ways to the dysfunction that exits in developmental stuttering. Pharmacological enhancement of dopaminergic neurotransmission by other drugs been reported to cause stuttering in fluent individuals and to aggrevate dysfluency in stutterers.