Whispering dysphonia in an Australian family (DYT4): a clinical and genetic reappraisal

Expanding the phenotypic and genotypic spectrum of DYT-TUBB4A with seven patients from India

BACKGROUND

Variants in the TUBB4A gene are associated with dystonia (DYT-TUBB4A), Hypomyelination with Atrophy of the Basal Ganglia and Cerebellum (H-ABC) and spastic paraplegia. Phenotypes intermediate to these three broad phenotypes are also observed. These are rare disorders, and data from diverse populations remains limited. We report seven Indian cases with dystonia phenotype related to TUBB4A mutation.

CASES

Among these seven patients, age at onset ranged from 5 to 48 years. Five patients had cranio-cervical onset of dystonia. One patient had prominent parkinsonism with dystonia. Patients responded well to botulinum toxin injected for laryngeal, cervical and jaw dystonia. The patient with parkinsonism responded well to levodopa, albeit with development of dyskinesias. Apart from the common p.Arg2Gly variant in three patients with DYT-TUBB4A, other variants included p.Arg262Pro, p.Arg39Cys and p.Asp245Asn.

CONCLUSIONS

We report the first collection of cases with TUBB4A mutation from India. We expand the phenotype to include levodopa-responsive parkinsonism. Indian patients, consistent with global literature, harbor prominent adductor dysphonia, cervical and jaw dystonia, which responds well to botulinum treatment.

Clinical neurophysiology of functional motor disorders: IFCN Handbook Chapter

Functional Motor Disorders are common and disabling. Clinical diagnosis has moved from one of exclusion of other causes for symptoms to one where positive clinical features on history and examination are used to make a "rule in" diagnosis wherever possible. Clinical neurophysiological assessments have developed increasing importance in assisting with this positive diagnosis, not being used simply to demonstrate normal sensory-motor pathways, but instead to demonstrate specific abnormalities that help to positively diagnose these disorders. Here we provide a practical review of these techniques, their application, interpretation and pitfalls. We also highlight particular areas where such tests are currently lacking in sensitivity and specificity, for example in people with functional dystonia and functional tic-like movements.

Functional Neurological Disorder: Diagnostic Pitfalls and Differential Diagnostic Considerations

Functional neurologic disorder (FND) is a "rule-in" diagnosis, characterized by positive examination signs or semiological features. Similar to other clinical diagnoses, providers should ideally see robustly present features, including if possible the identification of multiple features consistent with FND for the diagnosis to be made with a high degree of certainty. Diagnostic pitfalls need to be guarded against and vary depending on FND symptom subtype and the specific patient presentation. This perspective article aims to review pitfalls based on an FND symptom subtype, as well as discuss differential diagnostic considerations with respect to both neurologic and psychiatric entities.

Early recognition of SGCE-myoclonus-dystonia in children

AIM

To evaluate early dystonic features in children and adolescents with SGCE-myoclonus-dystonia.

METHOD

In this cross-sectional study, 49 patients (26 females and 23 males) with SGCE-myoclonus-dystonia (aged 15y 2mo, SD 12y) with childhood-onset (2y 10mo, SD 1y 10mo) dystonia were examined using a standardized video recorded protocol. Dystonia was rated using the Writer's Cramp and Gait Dystonia Rating Scales. Disability and impairment for handwriting and walking were also rated.

RESULTS

Dystonia was present at rest (n=1), posture (n=12), and during specific motor tasks (n=45) such as writing (n=35), walking (n=23), and running (n=20). Most children reported disability while performing these tasks. Early dystonic patterns were identified for writer's cramp and gait dystonia, the latter named the 'circular shaking leg', 'dragging leg', and 'hobby-horse gait' patterns. Sensory tricks were used by five and eight children to improve dystonia and myoclonus during writing and walking respectively. The rating scales accurately measured the severity of action dystonia and correlated with self-reported disability.

INTERPRETATION

Children with SGCE-myoclonus-dystonia show recognizable dystonic patterns and sensory tricks that may lead to an early diagnosis and timely therapeutic approach. Isolated writer's cramp is a key feature in childhood and should prompt SCGE analysis. The proposed action dystonia scales could be used to monitor disease course and response to treatment.

WHAT THIS PAPER ADDS

Most children with SGCE-myoclonus-dystonia got writer's cramp and had walking and running dystonia. Writer's cramp was a key feature and should prompt SGCE genetic investigation. 'Circular shaking leg', 'dragging leg', and 'hobby-horse gait' were recognized as early gait patterns. Children used sensory tricks to improve myoclonus and dystonia, suggesting common pathophysiological mechanisms. Action dystonia rating scales are valid tools to assess severity in children.

Isolated and combined dystonias: Update

Dystonia is a hyperkinetic movement disorder with a unique motor phenomenology that can manifest as an isolated clinical syndrome or combined with other neurological features. This chapter reviews the characteristic features of dystonia phenomenology and the syndromic approach to evaluating the disorders that may allow us to differentiate the isolated and combined syndromes. We also present the most common types of isolated and combined dystonia syndromes. Since accelerated gene discoveries have increased our understanding of the molecular mechanisms of dystonia pathogenesis, we also present isolated and combined dystonia syndromes by shared biological pathways. Examples of these converging mechanisms of the isolated and combined dystonia syndromes include (1) disruption of the integrated response pathway through eukaryotic initiation factor 2 alpha signaling, (2) disease of dopaminergic signaling, (3) alterations in the cerebello-thalamic pathway, and (4) disease of protein mislocalization and stability. The discoveries that isolated and combined dystonia syndromes converge in shared biological pathways will aid in the development of clinical trials and therapeutic strategies targeting these convergent molecular pathways.

Synaptic effects of ethanol on striatal circuitry: therapeutic implications for dystonia

Alcohol consumption affects motor behavior and motor control. Both acute and chronic alcohol abuse have been extensively investigated; however, the therapeutic efficacy of alcohol on some movement disorders, such as myoclonus-dystonia or essential tremor, still does not have a plausible mechanistic explanation. Yet, there are surprisingly few systematic trials with known GABAergic drugs mimicking the effect of alcohol on neurotransmission. In this brief survey, we aim to summarize the effects of EtOH on striatal function, providing an overview of its cellular and synaptic actions in a 'circuit-centered' view. In addition, we will review both experimental and clinical evidence, in the attempt to provide a plausible mechanistic explanation for alcohol-responsive movement disorders, with particular emphasis on dystonia. Different hypotheses emerge, which may provide a rationale for the utilization of drugs that mimic alcohol effects, predicting potential drug repositioning.

Movement disorders of the mouth: a review of the common phenomenologies

Movement disorders of the mouth encompass a spectrum of hyperactive movements involving the muscles of the orofacial complex. They are rare conditions and are described in the literature primarily in case reports originating from neurologists, psychiatrists, and the dental community. The focus of this review is to provide a phenomenological description of different oral motor disorders including oromandibular dystonia, orofacial dyskinesia and orolingual tremor, and to offer management strategies for optimal treatment based on the current literature. A literature search of full text studies using PubMed/Medline and Cochrane library combined with a manual search of the reference lists was conducted until June 2021. Results from this search included meta-analyses, systematic reviews, reviews, clinical studies, case series, and case reports published by neurologists, psychiatrists, dentists and oral and maxillofacial surgeons. Data garnered from these sources were used to provide an overview of most commonly encountered movement disorders of the mouth, aiding physicians in recognizing these rare conditions and in initiating appropriate therapy.

Dystonia Diagnosis: Clinical Neurophysiology and Genetics

Dystonia diagnosis is based on clinical examination performed by a neurologist with expertise in movement disorders. Clues that indicate the diagnosis of a movement disorder such as dystonia are dystonic movements, dystonic postures, and three additional physical signs (mirror dystonia, overflow dystonia, and geste antagonists/sensory tricks). Despite advances in research, there is no diagnostic test with a high level of accuracy for the dystonia diagnosis. Clinical neurophysiology and genetics might support the clinician in the diagnostic process. Neurophysiology played a role in untangling dystonia pathophysiology, demonstrating characteristic reduction in inhibition of central motor circuits and alterations in the somatosensory system. The neurophysiologic measure with the greatest evidence in identifying patients affected by dystonia is the somatosensory temporal discrimination threshold (STDT). Other parameters need further confirmations and more solid evidence to be considered as support for the dystonia diagnosis. Genetic testing should be guided by characteristics such as age at onset, body distribution, associated features, and coexistence of other movement disorders (parkinsonism, myoclonus, and other hyperkinesia). The aim of the present review is to summarize the state of the art regarding dystonia diagnosis focusing on the role of neurophysiology and genetic testing.

The Patho-Neurophysiological Basis and Treatment of Focal Laryngeal Dystonia: A Narrative Review and Two Case Reports Applying TMS over the Laryngeal Motor Cortex

Focal laryngeal dystonia (LD) is a rare, idiopathic disease affecting the laryngeal musculature with an unknown cause and clinically presented as adductor LD or rarely as abductor LD. The most effective treatment options include the injection of botulinum toxin (BoNT) into the affected laryngeal muscle. The aim of this narrative review is to summarize the patho-neuro-physiological and genetic background of LD, as well as the standard recommended therapy (BoNT) and pharmacological treatment options, and to discuss possible treatment perspectives using neuro-modulation techniques such as repetitive transcranial magnetic stimulation (rTMS) and vibrotactile stimulation. The review will present two LD cases, patients with adductor and abductor LD, standard diagnostic procedure, treatments and achievement, and the results of cortical excitability mapping the primary motor cortex for the representation of the laryngeal muscles in the assessment of corticospinal and corticobulbar excitability.

DYT‐TUBB4A ( DYT4 Dystonia): Clinical Anthology of 11 Cases and Systematized Review

Background

DYT‐TUBB4A, formerly known as DYT4, has not been comprehensively described as only one large family and three individual cases have been published. We have recently described an in depth genetic and protein structural analysis of eleven additional cases from four families with four new pathogenic variants. We aim to report on the phenomenology of these cases suffering from DYT‐TUBB4A and to perform a comprehensive review of the clinical presentation and treatment responses of all DYT‐TUBB4A cases reported in the literature.

Cases and Literature Review

The clinical picture was typically characterized by laryngeal dystonia (more than three quarters of all cases), associated with cervical dystonia, upper limb dystonia and frequent generalization. Extension of the dystonia to the lower limbs, creating the famous “hobby horse” gait, was present in more than 20% of cases (in only one of ours). Globus pallidus pars interna (GPi) deep brain stimulation (DBS), performed in 4 cases, led to a good improvement with greatest benefit in motoric and less benefit in laryngeal symptoms. Medical treatment was generally rather poorly effective, except some benefit from propranolol, tetrabenazine and alcohol intake.

Conclusion

Laryngeal involvement is a hallmark of DYT‐TUBB4A. Symptomatic treatment with GPi‐DBS led to the greatest benefit in motoric symptoms. Nevertheless, TUBB4A mutations remain an exceedingly rare cause of laryngeal or other isolated dystonia and regular screening of TUBB4A mutations for isolated dystonias has a very low yield.

H-ABC- and dystonia-causing TUBB4A mutations show distinct pathogenic effects

Mutations in the brain-specific β-tubulin 4A (TUBB4A) gene cause a broad spectrum of diseases, ranging from dystonia (DYT-TUBB4A) to hypomyelination with atrophy of the basal ganglia and cerebellum (H-ABC). Currently, the mechanisms of how TUBB4A variants lead to this pleiotropic manifestation remain elusive. Here, we investigated whether TUBB4A mutations causing either DYT-TUBB4A (p.R2G and p.Q424H) or H-ABC (p.R2W and p.D249N) exhibit differential effects at the molecular and cellular levels. Using live-cell imaging of disease-relevant oligodendrocytes and total internal reflection fluorescence microscopy of whole-cell lysates, we observed divergent impact on microtubule polymerization and microtubule integration, partially reflecting the observed pleiotropy. Moreover, in silico simulations demonstrated that the mutants rarely adopted a straight heterodimer conformation in contrast to wild type. In conclusion, for most of the examined variants, we deciphered potential molecular disease mechanisms that may lead to the diverse clinical manifestations and phenotype severity across and within each TUBB4A-related disease.

Pallidal Deep Brain Stimulation for Monogenic Dystonia: The Effect of Gene on Outcome

Globus pallidus internus deep brain stimulation (GPi DBS) is the most effective intervention for medically refractory segmental and generalized dystonia in both children and adults. Predictive factors for the degree of improvement after GPi DBS include shorter disease duration and dystonia subtype with idiopathic isolated dystonia usually responding better than acquired combined dystonias. Other factors contributing to variability in outcome may include body distribution, pattern of dystonia and DBS related factors such as lead placement and stimulation parameters. The responsiveness to DBS appears to vary between different monogenic forms of dystonia, with some improving more than others. The first observation in this regard was reports of superior DBS outcomes in DYT-TOR1A (DYT1) dystonia, although other studies have found no difference. Recently a subgroup with young onset DYT-TOR1A, more rapid progression and secondary worsening after effective GPi DBS, has been described. Myoclonus dystonia due to DYT-SCGE (DYT11) usually responds well to GPi DBS. Good outcomes following GPi DBS have also been documented in X-linked dystonia Parkinsonism (DYT3). In contrast, poorer, more variable DBS outcomes have been reported in DYT-THAP1 (DYT6) including a recent larger series. The outcome of GPi DBS in other monogenic isolated and combined dystonias including DYT-GNAL (DYT25), DYT-KMT2B (DYT28), DYT-ATP1A3 (DYT12), and DYT-ANO3 (DYT24) have been reported with varying results in smaller numbers of patients. In this article the available evidence for long term GPi DBS outcome between different genetic dystonias is reviewed to reappraise popular perceptions of expected outcomes and revisit whether genetic diagnosis may assist in predicting DBS outcome.

Alcohol-Responsive Hyperkinetic Movement Disorders-a Mechanistic Hypothesis

Patients with essential tremor, vocal tremor, torticollis, myoclonus-dystonia and posthypoxic myoclonus often benefit in a surprisingly rapid and robust manner from ingestion of a modest amount of alcohol (ethanol). Despite considerable investigation, the mechanism of ethanol’s ability to produce this effect remains a mystery. In this paper, we review the pharmacology of ethanol and its analogue GHB (or sodium oxybate), summarize the published literature of alcohol-responsive hyperkinetic movement disorders, and demonstrate videos of patients we have treated over the last fifteen years with either an ethanol challenge or with chronic sodium oxybate therapy. We then propose a novel explanation for this phenomenon—namely, that ingestion of modest doses of ethanol (or sodium oxybate) normalizes the aberrant motor networks underling these disorders. We propose that alcohol and its analogues improve clinical symptoms and their physiologic correlate by restoring the normal firing pattern of the major outflow pathways of the cerebellum (the Purkinje cells and deep cerebellar nuclei), We present evidence to support this hypothesis in animal models and in affected patients, and suggest future investigations to test this model.

DYT-TUBB4A (DYT4 Dystonia): New Clinical and Genetic Observations

OBJECTIVE

To report 4 novel TUBB4A mutations leading to laryngeal and cervical dystonia with frequent generalization.

METHODS

We screened 4 families including a total of 11 definitely affected members with a clinical picture resembling the original description.

RESULTS

Four novel variants in the TUBB4A gene have been identified: D295N, R46M, Q424H, and R121W. In silico modeling showed that all variants have characteristics similar to R2G. The variants segregate with the disease in 3 of the families with evidence of incomplete penetrance in 2 of them. All 4 variants would be classified as likely pathogenic. The clinical picture particularly included laryngeal dystonia (often the site of onset), associated with cervical and upper limb dystonia and frequent generalization. Laryngeal dystonia was extremely prevalent (>90%) both in the original cases and in this case series. The hobby horse gait was evident in only 1 patient in this case series.

CONCLUSIONS

Our interpretation is that laryngeal involvement is a hallmark feature of DYT-TUBB4A. Nevertheless, TUBB4A mutations remain an exceedingly rare cause of laryngeal or other isolated dystonia.

Functional gait disorders: A sign-based approach

Functional gait disorders are common in clinical practice. They are also usually disabling for affected individuals. The diagnosis is challenging because no single walking pattern is pathognomonic for a functional gait disorder. Establishing a diagnosis is based not primarily on excluding organic gait disorders but instead predominantly on recognizing positive clinical features of functional gait disorders, such as an antalgic, a buckling, or a waddling gait. However, these features can resemble and overlap with organic gait disorders. It is therefore necessary to also look for inconsistency (variations in clinical presentation that cannot be reconciled with an organic lesion) and incongruity (combination of symptoms and signs that is not seen with organic lesions). Yet, these features also have potential pitfalls as inconsistency can occur in patients with dystonic gait or those with freezing of gait. Similarly, patients with dystonia or chorea can present with bizarre gait patterns that may falsely be interpreted as incongruity. A further complicating factor is that functional and organic gait disorders may coexist within the same patient. To improve the diagnostic process, we present a sign-based approach-supported by videos-that incorporates the diverse clinical spectrum of functional gait disorders. We identify 7 groups of supportive gait signs that can signal the presence of functional gait disorders. For each group of signs, we highlight how specific clinical tests can bring out the inconsistencies and incongruencies that further point to a functional gait disorder.

Phasic Knee Bending Dystonic and Parkinsonian Gait: A Characteristic Finding in X-Linked Dystonia Parkinsonism

Background

X-linked dystonia parkinsonism (XDP) is a rare disorder characterized by adult-onset, progressive dystonia that, over time, is combined with or replaced by features of parkinsonism. Gait impairment is common.

Methods

Case series of 4 XDP patients with a unique gait disorder.

Results

The patients displayed a characteristic gait disorder with combined dystonic and parkinsonian gait features, with phasic knee bending. Of these patients, all had parkinsonism and three-quarters had prominent dystonic features, but 1 had predominant parkinsonism and subtle dystonic features.

Conclusion

Although XDP is a classic form of dystonia parkinsonism, some cases can mimic idiopathic Parkinson's disease. We describe a gait disorder which appears unique to XDP, involving phasic dystonic knee bending superimposed on parkinsonian shuffling, and may help clinically differentiate one of our parkinsonian-predominant patients from more-common forms of parkinsonism. The gait is distinct from other complex dystonic disorders with gait involvement.

Predictors of alcohol responsiveness in dystonia

OBJECTIVE

To determine predictors of alcohol responsiveness in a large cohort of patients with dystonia.

METHODS

A total of 2,159 participants with dystonia were prospectively enrolled in the cross-sectional Dystonia Coalition multicenter study. Patients with secondary, combined, or confirmed genetic dystonia (total n = 164) or unknown alcohol responsiveness (n = 737) were excluded. Patients answered a standardized questionnaire and were clinically examined using a standardized video protocol and the Burke-Fahn-Marsden Dystonia Rating Scale. Alcohol responsiveness was determined by patients' self-report.

RESULTS

A total of 1,258 patients with isolated dystonia (mean age: 59.5 ± 12.2 years; 898 women) met the inclusion criteria; 369 patients (29.3%) reported improvement of dystonia after alcohol consumption. Alcohol responsiveness was not related to sex (p = 0.742), age (p = 0.715), or severity of dystonia (p = 0.623). Age at onset was lower in patients who responded to alcohol (p < 0.001). Alcohol responsiveness differed across dystonia subgroups (multifocal/generalized > segmental [p = 0.014]; cervical and laryngeal > cranial and limb [p < 0.001]) and was related to a positive family history of movement disorders (p = 0.001), and presence of tremor (p < 0.001).

CONCLUSION

The association of alcohol responsiveness with a positive family history for movement disorders, generalized dystonia, and an earlier age at onset suggests that patients with dystonia who have an underlying genetic contribution may be more likely to respond beneficially to alcohol. The fact that dystonic tremor may respond to alcohol is in keeping with the observation that the intake of GABAergic drugs may have a beneficial effect in a proportion of patients.

Diagnosis and treatment of pediatric onset isolated dystonia

Isolated dystonia refers to a genetic heterogeneous group of progressive conditions with onset of symptoms during childhood or adolescence, progressive course with frequent generalization and marked functional impairment. There are well-known monogenic forms of isolated dystonia with pediatric onset such as DYT1 and DYT6 transmitted with autosomal dominant inheritance and low penetrance. Genetic findings of the past years have widened the etiological spectrum and the phenotype. The recently discovered genes (GNAL, ANO-3, KTM2B) or variant of already known diseases, such as Ataxia-Teleangectasia, are emerging as another causes of pediatric onset dystonia, sometimes with a more complex phenotype, but their incidence is unknown and still a considerable number of cases remains genetically undetermined. Due to the severe disability of pediatric onset dystonia treatment remains unsatisfactory and still mainly based upon oral pharmacological agents. However, deep brain stimulation is now extensively applied with good to excellent results especially when patients are treated early during the course of the disease.

Identification of a novel genetic locus underlying tremor and dystonia

BACKGROUND

Five affected individuals with syndromic tremulous dystonia, spasticity, and white matter disease from a consanguineous extended family covering a period of over 24 years are presented. A positional cloning approach utilizing genome-wide linkage, homozygozity mapping and whole exome sequencing was used for genetic characterization. The impact of a calmodulin-binding transcription activator 2, (CAMTA2) isoform 2, hypomorphic mutation on mRNA and protein abundance was studied using fluorescent reporter expression cassettes. Human brain sub-region cDNA libraries were used to study the expression pattern of CAMTA2 transcript variants.

RESULTS

Linkage analysis and homozygozity mapping localized the disease allele to a 2.1 Mb interval on chromosome 17 with a LOD score of 4.58. Whole exome sequencing identified a G>A change in the transcript variant 2 5'UTR of CAMTA2 that was only 6 bases upstream of the translation start site (c.-6G > A) (NM_001171166.1) and segregated with disease in an autosomal recessive manner. Transfection of wild type and mutant 5'UTR-linked fluorescent reporters showed no impact upon mRNA levels but a significant reduction in the protein fluorescent activity implying translation inhibition.

CONCLUSIONS

Mutation of CAMTA2 resulting in post-transcriptional inhibition of its own gene activity likely underlies a novel syndromic tremulous dystonia.

Leukodystrophies: a proposed classification system based on pathological changes and pathogenetic mechanisms

Leukodystrophies are genetically determined disorders characterized by the selective involvement of the central nervous system white matter. Onset may be at any age, from prenatal life to senescence. Many leukodystrophies are degenerative in nature, but some only impair white matter function. The clinical course is mostly progressive, but may also be static or even improving with time. Progressive leukodystrophies are often fatal, and no curative treatment is known. The last decade has witnessed a tremendous increase in the number of defined leukodystrophies also owing to a diagnostic approach combining magnetic resonance imaging pattern recognition and next generation sequencing. Knowledge on white matter physiology and pathology has also dramatically built up. This led to the recognition that only few leukodystrophies are due to mutations in myelin- or oligodendrocyte-specific genes, and many are rather caused by defects in other white matter structural components, including astrocytes, microglia, axons and blood vessels. We here propose a novel classification of leukodystrophies that takes into account the primary involvement of any white matter component. Categories in this classification are the myelin disorders due to a primary defect in oligodendrocytes or myelin (hypomyelinating and demyelinating leukodystrophies, leukodystrophies with myelin vacuolization); astrocytopathies; leuko-axonopathies; microgliopathies; and leuko-vasculopathies. Following this classification, we illustrate the neuropathology and disease mechanisms of some leukodystrophies taken as example for each category. Some leukodystrophies fall into more than one category. Given the complex molecular and cellular interplay underlying white matter pathology, recognition of the cellular pathology behind a disease becomes crucial in addressing possible treatment strategies.

Update on the Genetics of Dystonia

Mainly due to the advent of next-generation sequencing (NGS), the field of genetics of dystonia has rapidly grown in recent years, which led to the discovery of a number of novel dystonia genes and the development of a new classification and nomenclature for inherited dystonias. In addition, new findings from both in vivo and in vitro studies have been published on the role of previously known dystonia genes, extending our understanding of the pathophysiology of dystonia. We here review the current knowledge and recent findings in the known genes for isolated dystonia TOR1A, THAP1, and GNAL as well as for the combined dystonias due to mutations in GCH1, ATP1A3, and SGCE. We present confirmatory evidence for a role of dystonia genes that had not yet been unequivocally established including PRKRA, TUBB4A, ANO3, and TAF1. We finally discuss selected novel genes for dystonia such as KMT2B and VAC14 along with the challenges for gene identification in the NGS era and the translational importance of dystonia genetics in clinical practice.

Dystonia-Causing Mutations as a Contribution to the Etiology of Spasmodic Dysphonia

OBJECTIVE

Spasmodic dysphonia is a focal dystonia of the larynx with heterogeneous manifestations and association with familial risk factors. There are scarce data to allow precise understanding of etiology and pathophysiology. Screening for dystonia-causing genetic mutations has the potential to allow accurate diagnosis, inform about genotype-phenotype correlations, and allow a better understanding of mechanisms of disease.

STUDY DESIGN

Cross-sectional study.

SETTING

Tertiary academic medical center.

SUBJECTS AND METHODS

We enrolled patients presenting with spasmodic dysphonia to the voice clinic of our academic medical center. Data included demographics, clinical features, family history, and treatments administered. The following genes with disease-causing mutations previously associated with spasmodic dysphonia were screened: TOR1A (DYT1), TUBB4 (DYT4), and THAP1 (DYT6).

RESULTS

Eighty-six patients were recruited, comprising 77% females and 23% males. A definite family history of neurologic disorder was present in 15% (13 of 86). Average age (± standard deviation) of symptom onset was 42.1 ± 15.7 years. Most (99%; 85 of 86) were treated with botulinum toxin, and 12% (11 of 86) received oral medications. Genetic screening was negative in all patients for the GAG deletion in TOR1A (DYT1) and in the 5 exons currently associated with disease-causing mutations in TUBB4 (DYT4). Two patients tested positive for novel/rare variants in THAP1 (DYT6).

CONCLUSION

Genetic screening targeted at currently known disease-causing mutations in TOR1A, THAP1, and TUBB4 appears to have low diagnostic yield in sporadic spasmodic dysphonia. In our cohort, only 2 patients tested positive for novel/rare variants in THAP1. Clinicians should make use of genetic testing judiciously and in cost-effective ways.

The genetics of the dystonias--a review based on the new classification of the dystonias

The definition and classification of the dystonias was recently revisited. In the new 2013 classification, the dystonias are subdivided in terms of their etiology according to whether they are the result of pathological changes or structural damage, have acquired causes or are inherited. As hereditary dystonias are clinically and genetically heterogeneous, we sought to classify them according to the new recently defined criteria. We observed that although the new classification is still the subject of much debate and controversy, it is easy to use in a logical and objective manner with the inherited dystonias. With the discovery of new genes, however, it remains to be seen whether the new classification will continue to be effective.

Recent advances in the genetics of dystonia

Dystonia, a common and genetically heterogeneous neurological disorder, was recently defined as "a movement disorder characterized by sustained or intermittent muscle contractions causing abnormal, often repetitive, movements, postures, or both." Via the application of whole-exome sequencing, the genetic landscape of dystonia and closely related movement disorders is becoming exposed. In particular, several "novel" genetic causes have been causally associated with dystonia or dystonia-related disorders over the past 2 years. These genes include PRRT2 (DYT10), CIZ1 (DYT23), ANO3 (DYT24), GNAL (DYT25), and TUBB4A (DYT4). Despite these advances, major gaps remain in identifying the genetic origins for most cases of adult-onset isolated dystonia. Furthermore, model systems are needed to study the biology of PRRT2, CIZ1, ANO3, Gαolf, and TUBB4A in the context of dystonia. This review focuses on these recent additions to the family of dystonia genes, genotype-phenotype correlations, and possible cellular contributions of the encoded proteins to the development of dystonia.

Genetics in dystonia: an update

The past year has been extremely successful with regard to the genetics of dystonia, with the identification of four new dystonia genes (CIZ1, ANO3, GNAL, and TUBB4A). This progress was primarily achieved because of the application of a new technology, next-generation DNA sequencing, which allows rapid and comprehensive assessment of a patient's genome. In addition, a combination of next-generation and traditional Sanger sequencing has expanded the phenotypic spectrum associated with some of the dystonia plus (ATP1A3) and paroxysmal (PRRT2) loci. This article reviews the newly identified genes and phenotypes and discusses the future applications of next-generation sequencing to dystonia research.

Hypomyelination with atrophy of the basal ganglia and cerebellum: further delineation of the phenotype and genotype-phenotype correlation

Hypomyelination with atrophy of the basal ganglia and cerebellum is a rare leukoencephalopathy that was identified using magnetic resonance imaging in 2002. In 2013, whole exome sequencing of 11 patients with the disease revealed that they all had the same de novo mutation in TUBB4A, which encodes tubulin β-4A. We investigated the mutation spectrum in a cohort of 42 patients and the relationship between genotype and phenotype. Patients were selected on the basis of clinical and magnetic resonance imaging abnormalities that are indicative of hypomyelination with atrophy of the basal ganglia and cerebellum. Genetic testing and a clinical inventory were performed, and sequential magnetic resonance images were evaluated using a standard protocol. The heterozygous TUBB4A mutation observed in the first 11 patients was the most common (25 patients). Additionally, 13 other heterozygous mutations were identified, located in different structural domains of tubulin β-4A. We confirmed that the mutations were de novo in all but three patients. In two of these three cases we lacked parental DNA and in one the mutation was also found in the mother, most likely due to mosaicism. Patients showed a phenotypic continuum ranging from neonatal to childhood disease onset, normal to delayed early development and slow to more rapid neurological deterioration. Neurological symptomatology consisted of extrapyramidal movement abnormalities, spasticity, ataxia, cognitive deficit and sometimes epilepsy. Three patients died and the oldest living patient was 29 years of age. The patients' magnetic resonance images showed an absent or disappearing putamen, variable cerebellar atrophy and highly variable cerebral atrophy. Apart from hypomyelination, myelin loss was evident in several cases. Three severely affected patients had similar, somewhat atypical magnetic resonance image abnormalities. The study results were strongly suggestive of a genotype-phenotype correlation. The 25 patients with the common c.745G>A mutation generally had a less rapidly progressive disease course than the 17 cases with other TUBB4A mutations. Overall, this work demonstrates that the distinctive magnetic resonance imaging pattern for hypomyelination with atrophy of the basal ganglia and cerebellum defines a homogeneous clinical phenotype of variable severity. Patients almost invariably have prominent extrapyramidal movement abnormalities, which are rarely seen in patients with hypomyelination of different origin. A dominant TUBB4A mutation is also associated with dystonia type 4, in which magnetic resonance images of the brain seem normal. It is highly likely that there is a disease continuum associated with TUBB4A mutations, of which hypomyelination with atrophy of the basal ganglia and cerebellum and dystonia type 4 are the extremes. This would indicate that extrapyramidal movement abnormalities constitute the core feature of the disease spectrum related to dominant TUBB4A mutations and that all other features are variable.

Pathogenic variants in TUBB4A are not found in primary dystonia

OBJECTIVE

To determine the contribution of TUBB4A, recently associated with DYT4 dystonia in a pedigree with "whispering dysphonia" from Norfolk, United Kingdom, to the etiopathogenesis of primary dystonia.

METHODS

High-resolution melting and Sanger sequencing were used to inspect the entire coding region of TUBB4A in 575 subjects with primary laryngeal, segmental, or generalized dystonia.

RESULTS

No pathogenic variants, including the exon 1 variant (c.4C>G) identified in the DYT4 whispering dysphonia kindred, were found in this study.

CONCLUSION

The c.4C>G DYT4 mutation appears to be private, and clinical testing for TUBB4A mutations is not justified in spasmodic dysphonia or other forms of primary dystonia. Moreover, given its allelic association with leukoencephalopathy hypomyelination with atrophy of basal ganglia and cerebellum and protean clinical manifestations (chorea, ataxia, dysarthria, intellectual disability, dysmorphic facial features, and psychiatric disorders), DYT4 should not be categorized as a primary dystonia.

Genetics of dystonia: what's known? What's new? What's next?

Although all forms of dystonia share the core clinical features of involuntary dystonic dyskinesia, there is not only marked phenotypic but also etiologic heterogeneity. Isolated dystonia can be caused by mutations in TOR1A (DYT1), TUBB4 (DYT4), THAP1 (DYT6), CIZ1 (DYT23), ANO3 (DYT24), and GNAL (DYT25). Combined dystonias (with parkinsonism or myoclonus) are further subdivided into persistent (TAF1 [DYT3], GCHI [DYT5], SGCE [DYT11], ATP1A3 [DYT12]), PRKRA (DYT16), and paroxysmal (MR-1 [DYT8], PRRT2 [DYT10], SLC2A1 [DYT18]. With the advent of next-generation sequencing, an unprecedented number of new dystonia genes have recently been described, including 4 in the past 12 months. Despite the need for independent confirmation, these recent findings raise 2 important questions regarding (1) the role of genetics in dystonia overall and (2) the role of different molecular mechanisms in dystonia pathogenesis. The genetic contribution to dystonia represents a continuum ranging from genetic susceptibility factors of small effect to causative genes with markedly reduced penetrance to those with full penetrance. Equally diverse and complex are the pathways and neuronal function(s) putatively involved in dystonia pathogenesis including dopamine signaling, intracellular transport, cytoskeletal dynamics, transcriptional regulation, cell-cycle control, ion channel function, energy metabolism, signal transduction, and detoxification mechanisms. In the next decade of dystonia research, we expect to see the discovery of additional dystonia genes and susceptibility factors. In this context, it will be of great interest to explore whether the diverse cellular functions of the known dystonia proteins may be linked to shared pathways and thus complete the complex puzzle of dystonia pathogenesis. © 2013 Movement Disorder Society.

Expansion of the spectrum of TUBB4A-related disorders: a new phenotype associated with a novel mutation in the TUBB4A gene

Mutations in the TUBB4A gene have been identified so far in two neurodegenerative disorders with extremely different clinical features and course: whispering dysphonia, also known as dystonia type 4 (DYT4), and hypomyelination with atrophy of the basal ganglia and cerebellum (H-ABC). We describe a patient with slowly progressive spastic paraparesis, segmental dystonia, intellectual disability, behavioral problems, and evidence of permanent, incomplete myelination associated with progressive cerebellar atrophy. Whole exome sequencing revealed a novel E410K de novo heterozygous mutation in the TUBB4A gene. The clinical and radiological picture of our patient is different from the classic phenotype; thus, it expands the phenotypic variation of TUBB4A-gene-related disorders.

Primary and secondary dystonic syndromes: an update

PURPOSE OF REVIEW

The dystonias are a common but complex group of disorders that show considerable variation in cause and clinical presentation. The purpose of this review is to highlight the most important discoveries and insights from across the field over the period of the past 18 months.

RECENT FINDINGS

Five new genes for primary dystonia (PRRT2, CIZ1, ANO3, TUBB4A and GNAL) have made their appearance in the literature. New subtypes of neuronal brain iron accumulation have been delineated and linked to mutations in C19orf12 and WDR45, while a new treatable form of dystonia with brain manganese deposition related to mutations in SLC30A10 has been described. At the same time, the phenotypes of other forms of dystonic syndromes have been expanded or linked together. Finally, there has been increasing recognition of both the extramotor phenotype in dystonia and the part played by the cerebellum in its pathophysiology.

SUMMARY

Recently, there has been unprecedented change in the scientific landscape with respect to the cause of various dystonic syndromes that is likely to make a direct impact on clinical practice in the near future. Understanding the genetic cause of these syndromes and the often wide phenotypic variation in their presentations will improve diagnosis and treatment. With time, these discoveries may also lead to much-needed progress in elucidating the underlying pathophysiology of dystonia.

The genetics of dystonia: new twists in an old tale

Dystonia is a common movement disorder seen by neurologists in clinic. Genetic forms of the disease are important to recognize clinically and also provide valuable information about possible pathogenic mechanisms within the wider disorder. In the past few years, with the advent of new sequencing technologies, there has been a step change in the pace of discovery in the field of dystonia genetics. In just over a year, four new genes have been shown to cause primary dystonia (CIZ1, ANO3, TUBB4A and GNAL), PRRT2 has been identified as the cause of paroxysmal kinesigenic dystonia and other genes, such as SLC30A10 and ATP1A3, have been linked to more complicated forms of dystonia or new phenotypes. In this review, we provide an overview of the current state of knowledge regarding genetic forms of dystonia—related to both new and well-known genes alike—and incorporating genetic, clinical and molecular information. We discuss the mechanistic insights provided by the study of the genetic causes of dystonia and provide a helpful clinical algorithm to aid clinicians in correctly predicting the genetic basis of various forms of dystonia.

The focal dystonias: current views and challenges for future research

The most common forms of dystonia are those that develop in adults and affect a relatively isolated region of the body. Although these adult-onset focal dystonias are most prevalent, knowledge of their etiologies and pathogenesis has lagged behind some of the rarer generalized dystonias, in which the identification of genetic defects has facilitated both basic and clinical research. This summary provides a brief review of the clinical manifestations of the adult-onset focal dystonias, focusing attention on less well understood clinical manifestations that need further study. It also provides a simple conceptual model for the similarities and differences among the different adult-onset focal dystonias as a rationale for lumping them together as a class of disorders while at the same time splitting them into subtypes. The concluding section outlines some of the most important research questions for the future. Answers to these questions are critical for advancing our understanding of this group of disorders and for developing novel therapeutics.

A de novo mutation in the β-tubulin gene TUBB4A results in the leukoencephalopathy hypomyelination with atrophy of the basal ganglia and cerebellum

Hypomyelination with atrophy of the basal ganglia and cerebellum (H-ABC) is a rare hereditary leukoencephalopathy that was originally identified by MRI pattern analysis, and it has thus far defied all attempts at identifying the causal mutation. Only 22 cases are published in the literature to date. We performed exome sequencing on five family trios, two family quartets, and three single probands, which revealed that all eleven H-ABC-diagnosed individuals carry the same de novo single-nucleotide TUBB4A mutation resulting in nonsynonymous change p.Asp249Asn. Detailed investigation of one of the family quartets with the singular finding of an H-ABC-affected sibling pair revealed maternal mosaicism for the mutation, suggesting that rare de novo mutations that are initially phenotypically neutral in a mosaic individual can be disease causing in the subsequent generation. Modeling of TUBB4A shows that the mutation creates a nonsynonymous change at a highly conserved asparagine that sits at the intradimer interface of α-tubulin and β-tubulin, and this change might affect tubulin dimerization, microtubule polymerization, or microtubule stability. Consistent with H-ABC's clinical presentation, TUBB4A is highly expressed in neurons, and a recent report has shown that an N-terminal alteration is associated with a heritable dystonia. Together, these data demonstrate that a single de novo mutation in TUBB4A results in H-ABC.

Whispering dysphonia (DYT4 dystonia) is caused by a mutation in the TUBB4 gene

OBJECTIVE

A study was undertaken to identify the gene underlying DYT4 dystonia, a dominantly inherited form of spasmodic dysphonia combined with other focal or generalized dystonia and a characteristic facies and body habitus, in an Australian family.

METHODS

Genome-wide linkage analysis was carried out in 14 family members followed by genome sequencing in 2 individuals. The index patient underwent a detailed neurological follow-up examination, including electrophysiological studies and magnetic resonance imaging scanning. Biopsies of the skin and olfactory mucosa were obtained, and expression levels of TUBB4 mRNA were determined by quantitative real-time polymerase chain reaction in 3 different cell types. All exons of TUBB4 were screened for mutations in 394 unrelated dystonia patients.

RESULTS

The disease-causing gene was mapped to a 23cM region on chromosome 19p13.3-p13.2 with a maximum multipoint LOD score of 5.338 at markers D9S427 and D9S1034. Genome sequencing revealed a missense variant in the TUBB4 (tubulin beta-4; Arg2Gly) gene as the likely cause of disease. Sequencing of TUBB4 in 394 unrelated dystonia patients revealed another missense variant (Ala271Thr) in a familial case of segmental dystonia with spasmodic dysphonia. mRNA expression studies demonstrated significantly reduced levels of mutant TUBB4 mRNA in different cell types from a heterozygous Arg2Gly mutation carrier compared to controls.

INTERPRETATION

A mutation in TUBB4 causes DYT4 dystonia in this Australian family with so-called whispering dysphonia, and other mutations in TUBB4 may contribute to spasmodic dysphonia. Given that TUBB4 is a neuronally expressed tubulin, our results imply abnormal microtubule function as a novel mechanism in the pathophysiology of dystonia.

Genetic issues in the diagnosis of dystonias

Dystonias are heterogeneous hyperkinetic movement disorders characterized by involuntary muscle contractions which result in twisting and repetitive movements and abnormal postures. Several causative genes have been identified, but their genetic bases still remain elusive. Primary Torsion Dystonias (PTDs), in which dystonia is the only clinical sign, can be inherited in a monogenic fashion, and many genes and loci have been identified for autosomal dominant (DYT1/TOR1A; DYT6/THAP1; DYT4/TUBB4a; DYT7; DYT13; DYT21; DYT23/CIZ1; DYT24/ANO3; DYT25/GNAL) and recessive (DYT2; DYT17) forms. However most sporadic cases, especially those with late-onset, are likely multifactorial, with genetic and environmental factors interplaying to reach a threshold of disease. At present, genetic counseling of dystonia patients remains a difficult task. Recently non-motor clinical findings in dystonias, new highlights in the pathophysiology of the disease, and the availability of high-throughput genome-wide techniques are proving useful tools to better understand the complexity of PTD genetics. We briefly review the genetic basis of the most common forms of hereditary PTDs, and discuss relevant issues related to molecular diagnosis and genetic counseling.

Mutations in the autoregulatory domain of β-tubulin 4a cause hereditary dystonia

Dystonia type 4 (DYT4) was first described in a large family from Heacham in Norfolk with an autosomal dominantly inherited whispering dysphonia, generalized dystonia, and a characteristic hobby horse ataxic gait. We carried out a genetic linkage analysis in the extended DYT4 family that spanned 7 generations from England and Australia, revealing a single LOD score peak of 6.33 on chromosome 19p13.12-13. Exome sequencing in 2 cousins identified a single cosegregating mutation (p.R2G) in the β-tubulin 4a (TUBB4a) gene that was absent in a large number of controls. The mutation is highly conserved in the β-tubulin autoregulatory MREI (methionine-arginine-glutamic acid-isoleucine) domain, highly expressed in the central nervous system, and extensive in vitro work has previously demonstrated that substitutions at residue 2, specifically R2G, disrupt the autoregulatory capability of the wild-type β-tubulin peptide, affirming the role of the cytoskeleton in dystonia pathogenesis.