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

tubg1 Somatic Mutants Show Tubulinopathy-Associated Neurodevelopmental Phenotypes in a Zebrafish Model

Development of the multilayered cerebral cortex relies on precise orchestration of neurogenesis, neuronal migration, and differentiation, processes tightly regulated by microtubule dynamics. Mutations in tubulin superfamily genes have been associated with tubulinopathies, encompassing a spectrum of cortical malformations including microcephaly and lissencephaly. Here, we focus on γ-tubulin, a pivotal regulator of microtubule nucleation encoded by TUBG1. We investigate its role in brain development using a zebrafish model with somatic tubg1 mutation, recapitulating features of TUBG1-associated tubulinopathies in patients and mouse disease models. We demonstrate that γ-tubulin deficiency disrupts neurogenesis and brain development, mirroring microcephaly phenotypes. Furthermore, we uncover a novel potential regulatory link between γ-tubulin and canonical Wnt/β-catenin signaling, with γ-tubulin deficiency impairing Wnt activity. Our findings provide insights into the pathogenesis of cortical defects and suggest that γ-tubulin could be a potential target for further research in neurodevelopmental disorders, although challenges such as mode of action, specificity, and potential side effects must be addressed.

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.

LncRNA TubAR complexes with TUBB4A and TUBA1A to promote microtubule assembly and maintain myelination

A long-standing hypothesis proposes that certain RNA(s) must exhibit structural roles in microtubule assembly. Here, we identify a long noncoding RNA (TubAR) that is highly expressed in cerebellum and forms RNA-protein complex with TUBB4A and TUBA1A, two tubulins clinically linked to cerebellar and myelination defects. TubAR knockdown in mouse cerebellum causes loss of oligodendrocytes and Purkinje cells, demyelination, and decreased locomotor activity. Biochemically, we establish the roles of TubAR in promoting TUBB4A-TUBA1A heterodimer formation and microtubule assembly. Intriguingly, different from the hypomyelination-causing mutations, the non-hypomyelination-causing mutation TUBB4A-R2G confers gain-of-function for an RNA-independent interaction with TUBA1A. Experimental use of R2G/A mutations restores TUBB4A-TUBA1A heterodimer formation, and rescues the neuronal cell death phenotype caused by TubAR knockdown. Together, we uncover TubAR as the long-elusive structural RNA for microtubule assembly and demonstrate how TubAR mediates microtubule assembly specifically from αβ-tubulin heterodimers, which is crucial for maintenance of cerebellar myelination and activity.

Genetics and Pathogenesis of Dystonia

Dystonia is a clinically and genetically highly heterogeneous neurological disorder characterized by abnormal movements and postures caused by involuntary sustained or intermittent muscle contractions. A number of groundbreaking genetic and molecular insights have recently been gained. While they enable genetic testing and counseling, their translation into new therapies is still limited. However, we are beginning to understand shared pathophysiological pathways and molecular mechanisms. It has become clear that dystonia results from a dysfunctional network involving the basal ganglia, cerebellum, thalamus, and cortex. On the molecular level, more than a handful of, often intertwined, pathways have been linked to pathogenic variants in dystonia genes, including gene transcription during neurodevelopment (e.g., KMT2B, THAP1), calcium homeostasis (e.g., ANO3, HPCA), striatal dopamine signaling (e.g., GNAL), endoplasmic reticulum stress response (e.g., EIF2AK2, PRKRA, TOR1A), autophagy (e.g., VPS16), and others. Thus, different forms of dystonia can be molecularly grouped, which may facilitate treatment development in the future.

Molecular Pathogenic Mechanisms of Hypomyelinating Leukodystrophies (HLDs)

Hypomyelinating leukodystrophies (HLDs) represent a group of congenital rare diseases for which the responsible genes have been identified in recent studies. In this review, we briefly describe the genetic/molecular mechanisms underlying the pathogenesis of HLD and the normal cellular functions of the related genes and proteins. An increasing number of studies have reported genetic mutations that cause protein misfolding, protein dysfunction, and/or mislocalization associated with HLD. Insight into the mechanisms of these pathways can provide new findings for the clinical treatments of HLD.

Gross Motor Function in Pediatric Onset TUBB4A-Related Leukodystrophy: GMFM-88 Performance and Validation of GMFC-MLD in TUBB4A

TUBB4A pathogenic variants are associated with a spectrum of neurologic impairments including movement disorders and leukodystrophy. With the development of targeted therapies, there is an urgent unmet need for validated tools to measure mobility impairment. Our aim is to explore gross motor function in a pediatric-onset TUBB4A-related leukodystrophy cohort with existing gross motor outcome tools. Gross Motor Function Measure-88 (GMFM-88), Gross Motor Function Classification System (GMFCS-ER), and Gross Motor Function Classification-Metachromatic Leukodystrophy (GMFC-MLD) were selected through face validity. Subjects with a confirmed clinical and molecular diagnosis of TUBB4A-related leukodystrophy were enrolled. Participants' sex, age, genotype, and age at disease onset were collected, together with GMFM-88 and concurrent GMFCS-ER and GMFC-MLD. Performances on each measure were compared. GMFM-88 floor effect was defined as total score below 20%. A total of 35 subjects participated. Median performance by GMFM-88 was 16.24% (range 0-97.31), with 42.9% (n = 15) of individuals performing above the floor. GMFM-88 Dimension A (Lying and Rolling) was the best-performing dimension in the GMFM-88 (n = 29 above the floor). All levels of the Classification Scales were represented, with the exception of the GMFC-MLD level 0. Evaluation by GMFM-88 was strongly correlated with the Classification Scales (Spearman correlations: GMFCS-ER:GMFM-88 r = 0.90; GMFC-MLD:GMFM-88 r = 0.88; GMFCS-ER:GMFC-MLD: r = 0.92). Despite overall observation of a floor effect, the GMFM-88 is able to accurately capture the performance of individuals with attenuated phenotypes. GMFM-88 Dimension A shows no floor effect. GMFC-MLD shows a strong correlation with GMFCS-ER and GMFM-88, supporting its use as an age-independent functional score in TUBB4A-related leukodystrophy.

Novel loss of function mutation in TUBA1A gene compromises tubulin stability and proteostasis causing spastic paraplegia and ataxia

Microtubules are dynamic cytoskeletal structures involved in several cellular functions, such as intracellular trafficking, cell division and motility. More than other cell types, neurons rely on the proper functioning of microtubules to conduct their activities and achieve complex morphologies. Pathogenic variants in genes encoding for α and β-tubulins, the structural subunits of microtubules, give rise to a wide class of neurological disorders collectively known as "tubulinopathies" and mainly involving a wide and overlapping range of brain malformations resulting from defective neuronal proliferation, migration, differentiation and axon guidance. Although tubulin mutations have been classically linked to neurodevelopmental defects, growing evidence demonstrates that perturbations of tubulin functions and activities may also drive neurodegeneration. In this study, we causally link the previously unreported missense mutation p.I384N in TUBA1A, one of the neuron-specific α-tubulin isotype I, to a neurodegenerative disorder characterized by progressive spastic paraplegia and ataxia. We demonstrate that, in contrast to the p.R402H substitution, which is one of the most recurrent TUBA1A pathogenic variants associated to lissencephaly, the present mutation impairs TUBA1A stability, reducing the abundance of TUBA1A available in the cell and preventing its incorporation into microtubules. We also show that the isoleucine at position 384 is an amino acid residue, which is critical for α-tubulin stability, since the introduction of the p.I384N substitution in three different tubulin paralogs reduces their protein level and assembly into microtubules, increasing their propensity to aggregation. Moreover, we demonstrate that the inhibition of the proteasome degradative systems increases the protein levels of TUBA1A mutant, promoting the formation of tubulin aggregates that, as their size increases, coalesce into inclusions that precipitate within the insoluble cellular fraction. Overall, our data describe a novel pathogenic effect of p.I384N mutation that differs from the previously described substitutions in TUBA1A, and expand both phenotypic and mutational spectrum related to this gene.

Insights on the Role of α- and β-Tubulin Isotypes in Early Brain Development

Tubulins are the highly conserved subunit of microtubules which involve in various fundamental functions including brain development. Microtubules help in neuronal proliferation, migration, differentiation, cargo transport along the axons, synapse formation, and many more. Tubulin gene family consisting of multiple isotypes, their differential expression and varied post translational modifications create a whole new level of complexity and diversity in accomplishing manifold neuronal functions. The studies on the relation between tubulin genes and brain development opened a new avenue to understand the role of each tubulin isotype in neurodevelopment. Mutations in tubulin genes are reported to cause brain development defects especially cortical malformations, referred as tubulinopathies. There is an increased need to understand the molecular correlation between various tubulin mutations and the associated brain pathology. Recently, mutations in tubulin isotypes (TUBA1A, TUBB, TUBB1, TUBB2A, TUBB2B, TUBB3, and TUBG1) have been linked to cause various neurodevelopmental defects like lissencephaly, microcephaly, cortical dysplasia, polymicrogyria, schizencephaly, subcortical band heterotopia, periventricular heterotopia, corpus callosum agenesis, and cerebellar hypoplasia. This review summarizes on the microtubule dynamics, their role in neurodevelopment, tubulin isotypes, post translational modifications, and the role of tubulin mutations in causing specific neurodevelopmental defects. A comprehensive list containing all the reported tubulin pathogenic variants associated with brain developmental defects has been prepared to give a bird's eye view on the broad range of tubulin functions.

Roles of the cytoskeleton in human diseases

Recently, researches have revealed the key roles of the cytoskeleton in the occurrence and development of multiple diseases, suggesting that targeting the cytoskeleton is a viable approach for treating numerous refractory diseases. The cytoskeleton is a highly structured and complex network composed of actin filaments, microtubules, and intermediate filaments. In normal cells, these three cytoskeleton components are highly integrated and coordinated. However, the cytoskeleton undergoes drastic remodeling in cytoskeleton-related diseases, causing changes in cell polarity, affecting the cell cycle, leading to senescent diseases, and influencing cell migration to accelerate cancer metastasis. Additionally, mutations or abnormalities in cytoskeletal proteins and their related proteins are closely associated with several congenital diseases. Therefore, this review summarizes the roles of the cytoskeleton in cytoskeleton-related diseases as well as its potential roles in disease treatment to provide insights regarding the physiological functions and pathological roles of the cytoskeleton.

Understanding molecular mechanisms and predicting phenotypic effects of pathogenic tubulin mutations

Cells rely heavily on microtubules for several processes, including cell division and molecular trafficking. Mutations in the different tubulin-α and -β proteins that comprise microtubules have been associated with various diseases and are often dominant, sporadic and congenital. While the earliest reported tubulin mutations affect neurodevelopment, mutations are also associated with other disorders such as bleeding disorders and infertility. We performed a systematic survey of tubulin mutations across all isotypes in order to improve our understanding of how they cause disease, and increase our ability to predict their phenotypic effects. Both protein structural analyses and computational variant effect predictors were very limited in their utility for differentiating between pathogenic and benign mutations. This was even worse for those genes associated with non-neurodevelopmental disorders. We selected tubulin-α and -β disease mutations that were most poorly predicted for experimental characterisation. These mutants co-localise to the mitotic spindle in HeLa cells, suggesting they may exert dominant-negative effects by altering microtubule properties. Our results show that tubulin mutations represent a blind spot for current computational approaches, being much more poorly predicted than mutations in most human disease genes. We suggest that this is likely due to their strong association with dominant-negative and gain-of-function mechanisms.

Tubulin mutations in human neurodevelopmental disorders

Mutations causing dysfunction of tubulins and microtubule-associated proteins, also known as tubulinopathies, are a group of recently described entities that lead to complex brain malformations. Anatomical and functional consequences of the disruption of tubulins include microcephaly, combined with abnormal corticogenesis due to impaired migration or lamination and abnormal growth cone dynamics of projecting and callosal axons. Key imaging features of tubulinopathies are characterized by three major patterns of malformations of cortical development (MCD): lissencephaly, microlissencephaly, and dysgyria. Additional distinctive MRI features include dysmorphism of the basal ganglia, midline commissural structure hypoplasia or agenesis, and cerebellar and brainstem hypoplasia. Tubulinopathies can be diagnosed as early as 21-24 gestational weeks using imaging and neuropathology, with possible extreme microlissencephaly with an extremely thin cortex, lissencephaly with either thick or thin/intermediate cortex, and dysgyria combined with cerebellar hypoplasia, pons hypoplasia and corpus callosum dysgenesis. More than 100 MCD-associated mutations have been reported in TUBA1A, TUBB2B, or TUBB3 genes, whereas fewer than ten are known in other genes such TUBB2A, TUBB or TUBG1. Although these mutations are scattered along the α- and β-tubulin sequences, recurrent mutations are consistently associated with almost identical cortical dysgenesis. Much of the evidence supports that these mutations alter the dynamic properties and functions of microtubules in several fashions. These include diminishing the abundance of functional tubulin heterodimers, altering GTP binding, altering longitudinal and lateral protofilament interactions, and impairing microtubule interactions with kinesin and/or dynein motors or with MAPs. In this review we discuss the recent advances in our understanding of the effects of mutations of tubulins and microtubule-associated proteins on human brain development and the pathogenesis of malformations of cortical development.

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.

Functional Investigation of TUBB4A Variants Associated with Different Clinical Phenotypes

Dominant TUBB4A variants result in different phenotypes, including hypomyelination with atrophy of the basal ganglia and cerebellum (H-ABC), dystonia type 4 (DYT4), and isolated hypomyelination. Here, we report four new patients with a novel TUBB4A variant (p.K324T) and three new patients with previously reported variants (p.Q292K, p.V255I, p.E410K). The individual carrying the novel p.K324T variant exhibits epilepsy of infancy with migrating focal seizures (EIMFS), while the other three have isolated hypomyelination phenotype. We also present a study of the cellular effects of TUBB4A variants responsible for H-ABC (p.D249N), DYT4 (p.R2G), a severe combined phenotype with combination of hypomyelination and EIMFS (p.K324T), and isolated hypomyelination (p.Q292K and p.E410K) on microtubule stability and dynamics, neurite outgrowth, dendritic spine development, and kinesin binding. Cellular-based assays reveal that all variants except p.R2G increase microtubule stability, decrease microtubule polymerization rates, reduce axonal outgrowth, and alter the density and shape of dendritic spines. We also find that the p.K324T and p.E410K variants perturb the binding of TUBB4A to KIF1A, a neuron-specific kinesin required for transport of synaptic vesicle precursors. Taken together, our data suggest that impaired microtubule stability and dynamics, defected axonal growth, and dendritic spine development form the common molecular basis of TUBB4A-related leukodystrophy. Impairment of TUBB4A binding to KIF1A is more likely to be involved in the isolated hypomyelination phenotype, which suggests that alterations in kinesin binding may cause different phenotypes. In conclusion, our study extends the spectrum of TUBB4A mutations and related phenotypes and provides insight into why different TUBB4A variants cause distinct clinical phenotypes.

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.

A dominant tubulin mutation causes cerebellar neurodegeneration in a genetic model of tubulinopathy

Mutations in tubulins cause distinct neurodevelopmental and degenerative diseases termed "tubulinopathies"; however, little is known about the functional requirements of tubulins or how mutations cause cell-specific pathologies. Here, we identify a mutation in the gene Tubb4a that causes degeneration of cerebellar granule neurons and myelination defects. We show that the neural phenotypes result from a cell type-specific enrichment of a dominant mutant form of Tubb4a relative to the expression other β-tubulin isotypes. Loss of Tubb4a function does not underlie cellular pathology but is compensated by the transcriptional up-regulation of related tubulin genes in a cell type-specific manner. This work establishes that the expression of a primary tubulin mutation in mature neurons is sufficient to promote cell-autonomous cell death, consistent with a causative association of microtubule dysfunction with neurodegenerative diseases. These studies provide evidence that mutations in tubulins cause specific phenotypes based on expression ratios of tubulin isotype genes.

In-silico phenotype prediction by normal mode variant analysis in TUBB4A-related disease

TUBB4A-associated disorder is a rare condition affecting the central nervous system. It displays a wide phenotypic spectrum, ranging from isolated late-onset torsion dystonia to a severe early-onset disease with developmental delay, neurological deficits, and atrophy of the basal ganglia and cerebellum, therefore complicating variant interpretation and phenotype prediction in patients carrying TUBB4A variants. We applied entropy-based normal mode analysis (NMA) to investigate genotype–phenotype correlations in TUBB4A-releated disease and to develop an in-silico approach to assist in variant interpretation and phenotype prediction in this disorder. Variants included in our analysis were those reported prior to the conclusion of data collection for this study in October 2019. All TUBB4A pathogenic missense variants reported in ClinVar and Pubmed, for which associated clinical information was available, and all benign/likely benign TUBB4A missense variants reported in ClinVar, were included in the analysis. Pathogenic variants were divided into five phenotypic subgroups. In-silico point mutagenesis in the wild-type modeled protein structure was performed for each variant. Wild-type and mutated structures were analyzed by coarse-grained NMA to quantify protein stability as entropy difference value (ΔG) for each variant. Pairwise ΔG differences between all variant pairs in each structural cluster were calculated and clustered into dendrograms. Our search yielded 41 TUBB4A pathogenic variants in 126 patients, divided into 11 partially overlapping structural clusters across the TUBB4A protein. ΔG-based cluster analysis of the NMA results revealed a continuum of genotype–phenotype correlation across each structural cluster, as well as in transition areas of partially overlapping structural clusters. Benign/likely benign variants were integrated into the genotype–phenotype continuum as expected and were clearly separated from pathogenic variants. We conclude that our results support the incorporation of the NMA-based approach used in this study in the interpretation of variant pathogenicity and phenotype prediction in TUBB4A-related disease. Moreover, our results suggest that NMA may be of value in variant interpretation in additional monogenic conditions.

Kinetically Stabilizing Mutations in Beta Tubulins Create Isotype-Specific Brain Malformations

Mutations in the family of genes encoding the tubulin subunits of microtubules are associated with a spectrum of human brain malformations known as tubulinopathies. How these mutations impact tubulin activity to give rise to distinct developmental consequences is poorly understood. Here we report two patients exhibiting brain malformations characteristic of tubulinopathies and heterozygous T178M missense mutations in different β-tubulin genes, TUBB2A or TUBB3. RNAseq analysis indicates that both TUBB2A and TUBB3 are expressed in the brain during development, but only TUBB2A maintains high expression in neurons into adulthood. The T178 residue is highly conserved in β-tubulins and located in the exchangeable GTP-binding pocket of β-tubulin. To determine the impact of T178M on β-tubulin function we created an analogous mutation in the β-tubulin of budding yeast and show that the substitution acts dominantly to produce kinetically stabilized microtubules that assemble and disassemble slowly, with fewer transitions between these states. In vitro experiments with purified mutant tubulin demonstrate that T178M decreases the intrinsic assembly activity of β-tubulin and forms microtubules that rarely transition to disassembly. We provide evidence that the T178M substitution disrupts GTPase-dependent conformational changes in tubulin, providing a mechanistic explanation for kinetic stabilization. Our findings demonstrate the importance of tubulin’s GTPase activity during brain development, and indicate that tubulin isotypes play different, important roles during brain development.

Acquisition of Developmental Milestones in Hypomyelination With Atrophy of the Basal Ganglia and Cerebellum and Other TUBB4A-Related Leukoencephalopathy

Mutations in TUBB4A are associated with a spectrum of neurologic disorders categorized as TUBB4A-related leukoencephalopathy. Affected children can present with global developmental delay or normal early development, followed by a variable loss of skills over time. Further research is needed to characterize the factors associated with the divergent developmental trajectories in this rare monogenic disorder because this phenotypic spectrum is not fully explained by genotype alone.To characterize early psychomotor features, developmental milestones and age of disease onset were collected from medical records (n=54 individuals). Three subcohorts were identified: individuals with the common p.Asp249Asn variant vs all other genotypes with either early (<12 months of age) or late onset of presentation. Individuals with the p.Asp249Asn variant or those with non-p.Asp249Asn genotypes with later disease onset attained key milestones, including head control, sitting, and independent walking. Subjects with early-onset, non-p.Asp249Asn-associated disease were less likely to achieve developmental milestones. Next, we defined the developmental severity as the percentage of milestones attained by age 2 years. The mild form was defined as attaining at least 75% of key developmental milestones. Among cohort categorized as mild, individuals with p.Asp249Asn variant were more likely to lose acquired abilities when compared with non-p.Asp249Asn individuals.Our results suggest multiple influences on developmental trajectory, including a strong contribution from genotype and age of onset. Further studies are needed to identify additional factors that influence overall outcomes to better counsel families and to design clinical trials with appropriate clinical endpoints.

Whispering dysphonia in TUBB4A-related disorders responsive to bipallidal deep brain stimulation

BACKGROUND

Mutations in TUBB4A are associated with a wide phenotypic spectrum including generalized dystonia with whispering dysphonia (DYT-TUBB4A).

METHODS

We report the case of a 44-year-old patient with DYT-TUBB4A with a clinical presentation of disabling progressive dystonia, with a prominent laryngeal, cervical and facial involvement.

RESULTS

Bipallidal deep brain stimulation (DBS) resulted in a 55% reduction of dystonia severity assessed by the Burke-Fahn-Marsden scale score 6 months after surgery. The effect was obvious on the cervical and facial components of dystonia.

CONCLUSION

We suggest that bipallidal DBS should be considered in patients with disabling dystonia related to TUBB4A variants.

Acquisition of Developmental Milestones in Hypomyelination With Atrophy of the Basal Ganglia and Cerebellum and Other TUBB4A-Related Leukoencephalopathy

Mutations in TUBB4A are associated with a spectrum of neurologic disorders categorized as TUBB4A-related leukoencephalopathy. Affected children can present with global developmental delay or normal early development, followed by a variable loss of skills over time. Further research is needed to characterize the factors associated with the divergent developmental trajectories in this rare monogenic disorder because this phenotypic spectrum is not fully explained by genotype alone.To characterize early psychomotor features, developmental milestones and age of disease onset were collected from medical records (n=54 individuals). Three subcohorts were identified: individuals with the common p.Asp249Asn variant vs all other genotypes with either early (<12 months of age) or late onset of presentation. Individuals with the p.Asp249Asn variant or those with non-p.Asp249Asn genotypes with later disease onset attained key milestones, including head control, sitting, and independent walking. Subjects with early-onset, non-p.Asp249Asn-associated disease were less likely to achieve developmental milestones. Next, we defined the developmental severity as the percentage of milestones attained by age 2 years. The mild form was defined as attaining at least 75% of key developmental milestones. Among cohort categorized as mild, individuals with p.Asp249Asn variant were more likely to lose acquired abilities when compared with non-p.Asp249Asn individuals.Our results suggest multiple influences on developmental trajectory, including a strong contribution from genotype and age of onset. Further studies are needed to identify additional factors that influence overall outcomes to better counsel families and to design clinical trials with appropriate clinical endpoints.

Challenges and Controversies in the Genetic Diagnosis of Hereditary Spastic Paraplegia

Purpose of Review

The hereditary spastic paraplegias (HSPs) are a group of disorders characterised by progressive lower limb weakness and spasticity. We address the challenges and controversies involved in the genetic diagnosis of HSP.

Recent Findings

There is a large and rapidly expanding list of genes implicated in HSP, making it difficult to keep gene testing panels updated. There is also a high degree of phenotypic overlap between HSP and other disorders, leading to problems in choosing the right panel to analyse. We discuss genetic testing strategies for overcoming these diagnostic hurdles, including the use of targeted sequencing gene panels, whole-exome sequencing and whole-genome sequencing. Personalised treatments for HSP are on the horizon, and a genetic diagnosis may hold the key to access these treatments.

Summary

Developing strategies to overcome the challenges and controversies in HSP may hold the key to a rapid and accurate genetic diagnosis.

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.

Mutational spectrum of GNAL, THAP1 and TOR1A genes in isolated dystonia: study in a population from Spain and systematic literature review

OBJECTIVE

We aimed to investigate the prevalence of TOR1A, GNAL and THAP1 variants as the cause of dystonia in a cohort of Spanish patients with isolated dystonia and in the literature.

METHODS

A population of 2028 subjects (including 1053 patients with different subtypes of isolated dystonia and 975 healthy controls) from southern and central Spain was included. The genes TOR1A, THAP1 and GNAL were screened using a combination of high-resolution melting analysis and direct DNA resequencing. In addition, an extensive literature search to identify original articles (published before 10 August 2020) reporting mutations in TOR1A, THAP1 or GNAL associated to dystonia was performed.

RESULTS

Pathogenic or likely pathogenic variants in TOR1A, THAP1 and GNAL were identified in 0.48%, 0.57% and 0.29% of our patients, respectively. Five patients carried the variation p.Glu303del in TOR1A. A very rare variant in GNAL (p.Ser238Asn) was found as a putative risk factor for dystonia. In the literature, variations in TOR1A, THAP1 and GNAL accounted for about 6%, 1.8% and 1.1% of published dystonia patients, respectively.

CONCLUSIONS

There is a different genetic contribution to dystonia of these three genes in our patients (about 1.3% of patients) and in the literature (about 3.6% of patients), probably due the high proportion of adult-onset cases in our cohort. As regards age at onset, site of dystonia onset, and final distribution, in our population there is a clear differentiation between DYT-TOR1A and DYT-GNAL, with DYT-THAP1 likely to be an intermediate phenotype.

Cross-sectional quantitative analysis of the natural history of TUBA1A and TUBB2B tubulinopathies

Purpose

TUBA1A and TUBB2B tubulinopathies are rare neurodevelopmental disorders characterized by cortical and extracortical malformations and heterogenic phenotypes. There is a need for quantitative clinical endpoints that will be beneficial for future diagnostic and therapeutic trials.

Methods

Quantitative natural history modeling of individuals with TUBA1A and TUBB2B tubulinopathies from clinical reports and database entries of DECIPHER and ClinVar. Main outcome measures were age at disease onset, survival, and diagnostic delay. Phenotypical, neuroradiological, and histopathological features were descriptively illustrated.

Results

Mean age at disease onset was 4 (TUBA1A) and 6 months (TUBB2B), respectively. Mortality was equally estimated with 7% at 3.2 (TUBA1A) and 8.0 years (TUBB2B). Diagnostic delay was significantly higher in TUBB2B (12.3 years) compared with TUBA1A tubulinopathy (4.2 years). We delineated the isotype-dependent clinical, neuroradiological, and histopathological phenotype of affected individuals and present brain malformations associated with epilepsy and an unfavorable course of disease.

Conclusion

The natural history of tubulinopathies is defined by the genotype and associated brain malformations. Defined data on estimated survival, diagnostic delay, and disease characteristics of TUBA1A and TUBB2B tubulinopathy will help to raise disease awareness and encourage future clinical trials to optimize genetic testing, family counseling, and supportive care.

Microtubule Dysfunction: A Common Feature of Neurodegenerative Diseases

Neurons are particularly susceptible to microtubule (MT) defects and deregulation of the MT cytoskeleton is considered to be a common insult during the pathogenesis of neurodegenerative disorders. Evidence that dysfunctions in the MT system have a direct role in neurodegeneration comes from findings that several forms of neurodegenerative diseases are associated with changes in genes encoding tubulins, the structural units of MTs, MT-associated proteins (MAPs), or additional factors such as MT modifying enzymes which modulating tubulin post-translational modifications (PTMs) regulate MT functions and dynamics. Efforts to use MT-targeting therapeutic agents for the treatment of neurodegenerative diseases are underway. Many of these agents have provided several benefits when tested on both in vitro and in vivo neurodegenerative model systems. Currently, the most frequently addressed therapeutic interventions include drugs that modulate MT stability or that target tubulin PTMs, such as tubulin acetylation. The purpose of this review is to provide an update on the relevance of MT dysfunctions to the process of neurodegeneration and briefly discuss advances in the use of MT-targeting drugs for the treatment of neurodegenerative disorders.

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.

How have advances in genetic technology modified movement disorder nosology?

The role of genetics and its technological development have been fundamental in advancing the field of movement disorders, opening the door to precision medicine. Starting from the revolutionary discovery of the locus of the Huntington's disease gene, we review the milestones of genetic discoveries in movement disorders and their impact on clinical practice and research efforts. Before the 1980s, early techniques did not allow the identification of genetic alteration in complex diseases. Further advances increasingly defined a large number of pathogenic genetic alterations. Moreover, these techniques allowed epigenomic, transcriptomic and microbiome analyses. In the 2020s, these new technologies are poised to displace phenotype-based classifications towards a nosology based on genetic/biological data. Advances in genetic technologies are engineering a reversal of the phenotype-to-genotype order of nosology development, replacing convergent clinicopathological disease models with the genotypic divergence required for future precision medicine applications.

New approaches to discovering drugs that treat dystonia

Introduction: Dystonia consists of involuntary movements, abnormal posturing, and pain. In adults, dystonia presents in a particular region of the body and causes significant disability due to pain as well as impairment in activities of daily living and employment. The current gold standard treatment, botulinum toxin (BoNT), has limitations - painful, frequent injections due to 'wearing off' of treatment effect; expense; and expected side effects like swallowing difficulty and weakness. There is a clear therapeutic gap in our current treatment options for dystonia and also a clear need for an effective novel treatment. Testing any novel treatment is complicated because most adults with focal dystonia are treated with BoNT. Areas covered: This review focuses on establishing the need for novel therapeutics. It also suggests potential leads from preclinical studies; and, discusses the issue of clinical trial readiness in the dystonia field. Expert opinion: Identifying a novel therapeutic intervention for dystonia patients faces two major challenges. The first is acknowledging the therapeutic gap that currently exists. Second, shifting some of our research aims in dystonia to clinical trial readiness is imperative if we are to be ready to test novel therapeutic agents.

Identification of Disease Susceptibility Alleles in the Next Generation Sequencing Era

The development of next generation sequencing (NGS) technologies has transformed the study of human genetic variation. In less than a decade, NGS has facilitated the discovery of causal mutations in both rare, monogenic diseases and common, heterogeneous disorders, leading to unprecedented improvements in disease diagnosis and treatment strategies. Given the rapid evolution of NGS platforms, it is now possible to analyze whole genomes and exomes quickly and affordably. Further, emerging NGS applications, such as single-cell sequencing, have the power to address specific issues like somatic variation, which is yielding new insights into the role of somatic mutations in cancer and late-onset diseases. Despite limitations associated with current iterations of NGS technologies, the impact of this approach on identifying disease-causing variants has been significant. This chapter provides an overview of several NGS platforms and applications and discusses how these technologies can be used in concert with experimental and computational strategies to identify variants with a causative effect on disease development and progression.

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.

Motor protein binding and mitochondrial transport are altered by pathogenic TUBB4A variants

Mutations in TUBB4A have been identified to cause a wide phenotypic spectrum of diseases ranging from hereditary generalized dystonia with whispering dysphonia (DYT-TUBB4A) and hereditary spastic paraplegia (HSP) to leukodystrophy hypomyelination with atrophy of the basal ganglia and cerebellum (H-ABC). TUBB4A encodes the brain-specific β-tubulin isotype, β-tubulin 4A. To elucidate the pathogenic mechanisms conferred by TUBB4A mutations leading to the different phenotypes, we functionally characterized three pathogenic TUBB4A variants (c.4C>G,p.R2G; c.745G>A,p.D249N; c.811G>A, p.A271T) as representatives of the mutational and disease spectrum) in human neuroblastoma cells and human induced pluripotent stem cell (iPSC)-derived neurons. We showed that mRNA stability was not affected by any of the TUBB4A variants. Although two mutations (p.R2G and p.D249N) are located at the α/β-tubulin interdimer interface, we confirmed incorporation of all TUBB4A mutants into the microtubule network. However, we showed that the mutations p.D249N and p.A271T interfered with motor protein binding to microtubules and impaired neurite outgrowth and microtubule dynamics. Finally, TUBB4A mutations, as well as heterozygous knockout of TUBB4A, disrupted mitochondrial transport in iPSC-derived neurons. Taken together, our findings suggest that functional impairment of microtubule-associated transport is a shared pathogenic mechanism by which the TUBB4A mutations studied here cause a spectrum of diseases.

Clinical and Functional Characterization of the Recurrent TUBA1A p.(Arg2His) Mutation

The TUBA1A gene encodes tubulin alpha-1A, a protein that is highly expressed in the fetal brain. Alpha- and beta-tubulin subunits form dimers, which then co-assemble into microtubule polymers: dynamic, scaffold-like structures that perform key functions during neurogenesis, neuronal migration, and cortical organisation. Mutations in TUBA1A have been reported to cause a range of brain malformations. We describe four unrelated patients with the same de novo missense mutation in TUBA1A, c.5G>A, p.(Arg2His), as found by next generation sequencing. Detailed comparison revealed similar brain phenotypes with mild variability. Shared features included developmental delay, microcephaly, hypoplasia of the cerebellar vermis, dysplasia or thinning of the corpus callosum, small pons, and dysmorphic basal ganglia. Two of the patients had bilateral perisylvian polymicrogyria. We examined the effects of the p.(Arg2His) mutation by computer-based protein structure modelling and heterologous expression in HEK-293 cells. The results suggest the mutation subtly impairs microtubule function, potentially by affecting inter-dimer interaction. Based on its sequence context, c.5G>A is likely to be a common recurrent mutation. We propose that the subtle functional effects of p.(Arg2His) may allow for other factors (such as genetic background or environmental conditions) to influence phenotypic outcome, thus explaining the mild variability in clinical manifestations.

Recent advances in understanding and managing dystonia

Within the field of movement disorders, the conceptual understanding of dystonia has continued to evolve. Clinical advances have included improvements in recognition of certain features of dystonia, such as tremor, and understanding of phenotypic spectrums in the genetic dystonias and dystonia terminology and classification. Progress has also been made in the understanding of underlying biological processes which characterize dystonia from discoveries using approaches such as neurophysiology, functional imaging, genetics, and animal models. Important advances include the role of the cerebellum in dystonia, the concept of dystonia as an aberrant brain network disorder, additional evidence supporting the concept of dystonia endophenotypes, and new insights into psychogenic dystonia. These discoveries have begun to shape treatment approaches as, in parallel, important new treatment modalities, including magnetic resonance imaging-guided focused ultrasound, have emerged and existing interventions such as deep brain stimulation have been further refined. In this review, these topics are explored and discussed.

Tubulin genes and malformations of cortical development

A large number of genes encoding for tubulin proteins are expressed in the developing brain. Each is subject to specific spatial and temporal expression patterns. However, most are highly expressed in post-mitotic neurons during stages of neuronal migration and differentiation. The major tubulin subclasses (alpha- and beta-tubulin) share high sequence and structural homology. These globular proteins form heterodimers and subsequently co-assemble into microtubules. Microtubules are dynamic, cytoskeletal polymers which play key roles in cellular processes crucial for cortical development, including neuronal proliferation, migration and cortical laminar organisation. Mutations in seven genes encoding alpha-tubulin (TUBA1A), beta-tubulin (TUBB2A, TUBB2B, TUBB3, TUBB4A, TUBB) and gamma-tubulin (TUBG1) isoforms have been associated with a wide and overlapping range of brain malformations or "Tubulinopathies". The majority of cortical phenotypes include lissencephaly, polymicrogyria, microlissencephaly and simplified gyration. Well-known hallmarks of the tubulinopathies include dysmorphism of the basal ganglia (fusion of the caudate nucleus and putamen with absence of the anterior limb of the internal capsule), midline commissural structures hypoplasia and/or agenesis (anterior commissure, corpus callosum and fornix), hypoplasia of the oculomotor and optic nerves, cerebellar hypoplasia or dysplasia and dysmorphism of the hind-brain structures. The cortical and extra-cortical brain phenotypes observed are largely dependent on the specific tubulin gene affected. In the present review, all the published data on tubulin family gene mutations and the associated cortical phenotypes are summarized. In addition, the most typical neuroimaging patterns of malformations of cortical development associated with tubulin gene mutations detected on the basis of our own experience are described.

The Twists of Pediatric Dystonia: Phenomenology, Classification, and Genetics

This article aims to provide a practical review of pediatric dystonia from a clinician's perspective. The focus is on the underlying genetic causes, recent findings, and treatable conditions. Dystonia can occur in an isolated fashion or accompanied by other neurological or systemic features. The clinical presentation is often a complex overlap of neurological findings with a large differential diagnosis. We recommend an approach guided by thorough clinical evaluation, brain magnetic resonance imaging (MRI), biochemical analysis, and genetic testing to hone in on the diagnosis. This article highlights the clinical and genetic complexity of pediatric dystonia and underlines the importance of a genetic diagnosis for therapeutic considerations.

TUBB4A mutations result in specific neuronal and oligodendrocytic defects that closely match clinically distinct phenotypes

Hypomyelinating leukodystrophies are heritable disorders defined by lack of development of brain myelin, but the cellular mechanisms of hypomyelination are often poorly understood. Mutations in TUBB4A, encoding the tubulin isoform tubulin beta class IVA (Tubb4a), result in the symptom complex of hypomyelination with atrophy of basal ganglia and cerebellum (H-ABC). Additionally, TUBB4A mutations are known to result in a broad phenotypic spectrum, ranging from primary dystonia (DYT4), isolated hypomyelination with spastic quadriplegia, and an infantile onset encephalopathy, suggesting multiple cell types may be involved. We present a study of the cellular effects of TUBB4A mutations responsible for H-ABC (p.Asp249Asn), DYT4 (p.Arg2Gly), a severe combined phenotype with hypomyelination and encephalopathy (p.Asn414Lys), as well as milder phenotypes causing isolated hypomyelination (p.Val255Ile and p.Arg282Pro). We used a combination of histopathological, biochemical and cellular approaches to determine how these different mutations may have variable cellular effects in neurons and/or oligodendrocytes. Our results demonstrate that specific mutations lead to either purely neuronal, combined neuronal and oligodendrocytic or purely oligodendrocytic defects that closely match their respective clinical phenotypes. Thus, the DYT4 mutation that leads to phenotypes attributable to neuronal dysfunction results in altered neuronal morphology, but with unchanged tubulin quantity and polymerization, with normal oligodendrocyte morphology and myelin gene expression. Conversely, mutations associated with isolated hypomyelination (p.Val255Ile and p.Arg282Pro) and the severe combined phenotype (p.Asn414Lys) resulted in normal neuronal morphology but were associated with altered oligodendrocyte morphology, myelin gene expression, and microtubule dysfunction. The H-ABC mutation (p.Asp249Asn) that exhibits a combined neuronal and myelin phenotype had overlapping cellular defects involving both neuronal and oligodendrocyte cell types in vitro. Only mutations causing hypomyelination phenotypes showed altered microtubule dynamics and acted through a dominant toxic gain of function mechanism. The DYT4 mutation had no impact on microtubule dynamics suggesting a distinct mechanism of action. In summary, the different clinical phenotypes associated with TUBB4A reflect the selective and specific cellular effects of the causative mutations. Cellular specificity of disease pathogenesis is relevant to developing targeted treatments for this disabling condition.

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.

NGS Technologies as a Turning Point in Rare Disease Research , Diagnosis and Treatment

Approximately 25-50 million Americans, 30 million Europeans, and 8% of the Australian population have a rare disease. Rare diseases are thus a common problem for clinicians and account for enormous healthcare costs worldwide due to the difficulty of establishing a specific diagnosis. In this article, we review the milestones achieved in our understanding of rare diseases since the emergence of next-generation sequencing (NGS) technologies and analyze how these advances have influenced research and diagnosis. The first half of this review describes how NGS has changed diagnostic workflows and provided an unprecedented, simple way of discovering novel disease-associated genes. We focus particularly on metabolic and neurodevelopmental disorders. NGS has enabled cheap and rapid genetic diagnosis, highlighted the relevance of mosaic and de novo mutations, brought to light the wide phenotypic spectrum of most genes, detected digenic inheritance or the presence of more than one rare disease in the same patient, and paved the way for promising new therapies. In the second part of the review, we look at the limitations and challenges of NGS, including determination of variant causality, the loss of variants in coding and non-coding regions, and the detection of somatic mosaicism variants and epigenetic mutations, and discuss how these can be overcome in the near future.

Deep brain stimulation for monogenic dystonia

PURPOSE OF REVIEW

Deep brain stimulation (DBS) has recently emerged as an important management option in children with medically refractory dystonia. DBS is most commonly used, best studied, and thought to be most efficacious for a select group of childhood or adolescent onset monogenic dystonias (designated with a standard 'DYT' prefix). We review how to clinically recognize these types of dystonia and the relative efficacy of DBS for key monogenic dystonias.

RECENT FINDINGS

Though used for dystonia in adults for several years, DBS has only lately been used in children. Recent evidence shows that patients with shorter duration of dystonia often experience greater benefit following DBS. This suggests that early recognition of the appropriate dystonic phenotypes and consideration of DBS in these patients may improve the management of dystonia.

SUMMARY

DBS should be considered early in patients who have medically refractory dystonia, especially for the monogenic dystonias that have a high response rate to DBS. It is important to differentiate between these monogenic dystonias and dystonias of other causes to properly prognosticate for these patients and to determine whether DBS is an appropriate management option.

Uner Tan syndrome caused by a homozygous TUBB2B mutation affecting microtubule stability

The integrity and dynamic properties of the microtubule cytoskeleton are indispensable for the development of the mammalian brain. Consequently, mutations in the genes that encode the structural component (the α/β-tubulin heterodimer) can give rise to severe, sporadic neurodevelopmental disorders. These are commonly referred to as the tubulinopathies. Here we report the addition of recessive quadrupedalism, also known as Uner Tan syndrome (UTS), to the growing list of diseases caused by tubulin variants. Analysis of a consanguineous UTS family identified a biallelic TUBB2B mutation, resulting in a p.R390Q amino acid substitution. In addition to the identifying quadrupedal locomotion, all three patients showed severe cerebellar hypoplasia. None, however, displayed the basal ganglia malformations typically associated with TUBB2B mutations. Functional analysis of the R390Q substitution revealed that it did not affect the ability of β-tubulin to fold or become assembled into the α/β-heterodimer, nor did it influence the incorporation of mutant-containing heterodimers into microtubule polymers. The 390Q mutation in S. cerevisiae TUB2 did not affect growth under basal conditions, but did result in increased sensitivity to microtubule-depolymerizing drugs, indicative of a mild impact of this mutation on microtubule function. The TUBB2B mutation described here represents an unusual recessive mode of inheritance for missense-mediated tubulinopathies and reinforces the sensitivity of the developing cerebellum to microtubule defects.

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.

Spasmodic Dysphonia: A Review. Part 1: Pathogenic Factors

Objective The purpose of this review is to describe the recent advances in identifying possible factors involved in the pathogenesis of spasmodic dysphonia. Spasmodic dysphonia is a task-specific focal laryngeal dystonia characterized by irregular and uncontrolled voice breaks. Pathogenesis of the disorder is poorly understood. Data Sources PubMed, Google Scholar, and Cochrane Library. Review Methods The data sources were searched using the following search terms: ( spasmodic dysphonia or laryngeal dystonia) and ( etiology, aetiology, diagnosis, pathogenesis, or pathophysiology). Conclusions Several potential etiological factors have been proposed by epidemiological, genetic, and neuropathological studies. Spasmodic dysphonia is a rare disorder primarily affecting females beginning in their 40s. Vocal tremor co-occurs in 30% to 60%. Large cohort studies identified risk factors such as a family history of neurological disorders including dystonia and tremor, recent viral illness, and heavy voice use. As none are rare events, a complex interactive process may contribute to pathogenesis in a small proportion of those at risk. Consequences to pathogenesis are neurological processes found in spasmodic dysphonia: loss of cortical inhibition, sensory processing disturbances, and neuroanatomical and physiological differences in the laryngeal motor control system. Implications for Practice Diagnosis of spasmodic dysphonia usually includes speech and laryngoscopic assessment. However, as diagnosis is sometimes problematic, measurement of neurophysiological abnormalities may contribute useful adjuncts for the diagnosis of spasmodic dysphonia in the future.

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.

A novel TUBB4A mutation G96R identified in a patient with hypomyelinating leukodystrophy onset beyond adolescence

The tubulin beta-4A gene (TUBB4A) is associated with two different clinical conditions, dystonia type 4 (DYT4) and hypomyelination with atrophy of the basal ganglia and cerebellum (H-ABC). We identified a novel TUBB4A mutation, c.286G>A (p.G96R), in an adult male patient who suffered neurological symptoms beyond adolescence. This patient shows intermediate clinical features between DYT4 and H-ABC, suggesting that the TUBB4A disorder would constitute a spectrum disorder.

Screening study of TUBB4A in isolated dystonia

Mutations in TUBB4A have been identified to cause a wide phenotypic spectrum ranging from hereditary generalized dystonia with whispering dysphonia (DYT4) to the leukodystrophy hypomyelination syndrome with atrophy of the basal ganglia and cerebellum (H-ABC). To test for the contribution of TUBB4A mutations in different ethnicities (Spanish, Italian, Korean, Japanese), we screened 492 isolated dystonia cases for mutations in this gene and for the first time determined TUBB4A copy number variations in 336 dystonia patients. A potentially pathogenic rare 3bp-in-frame deletion was found in a patient with cervical dystonia but no copy number variations were detected in this study, suggesting that TUBB4A mutations exceedingly rarely contribute to the etiology of isolated dystonia.