Psychiatric disorders, myoclonus dystonia, and the epsilon-sarcoglycan gene: A systematic review

Rare genetic brain disorders with overlapping neurological and psychiatric phenotypes

Understanding rare genetic brain disorders with overlapping neurological and psychiatric phenotypes is of increasing importance given the potential for developing disease models that could help to understand more common, polygenic disorders. However, the traditional clinical boundaries between neurology and psychiatry result in frequent segregation of these disorders into distinct silos, limiting cross-specialty understanding that could facilitate clinical and biological advances. In this Review, we highlight multiple genetic brain disorders in which neurological and psychiatric phenotypes are observed, but for which in-depth, cross-spectrum clinical phenotyping is rarely undertaken. We describe the combined phenotypes observed in association with genetic variants linked to epilepsy, dystonia, autism spectrum disorder and schizophrenia. We also consider common underlying mechanisms that centre on synaptic plasticity, including changes to synaptic and neuronal structure, calcium handling and the balance of excitatory and inhibitory neuronal activity. Further investigation is needed to better define and replicate these phenotypes in larger cohorts, which would help to gain greater understanding of the pathophysiological mechanisms and identify common therapeutic targets.

Case report: atypical Silver-Russell syndrome patient with hand dystonia: the valuable support of the consensus statement to the wide syndromic spectrum

The amount of Insulin Growth Factor 2 (IGF2) controls the rate of embryonal and postnatal growth. The IGF2 and adjacent H19 are the imprinted genes of the telomeric cluster in the 11p15 chromosomal region regulated by differentially methylated regions (DMRs) or imprinting centers (ICs): H19/IGF2:IG-DMR (IC1). Dysregulation due to IC1 Loss-of-Methylation (LoM) or Gain-of-Methyaltion (GoM) causes Silver-Russell syndrome (SRS) or Beckwith-Wiedemann syndrome (BWS) disorders associated with growth retardation or overgrowth, respectively. Specific features define each of the two syndromes, but isolated asymmetry is a common cardinal feature, which is considered sufficient for a diagnosis in the BWS spectrum. Here, we report the case of a girl with right body asymmetry, which suggested BWS spectrum. Later, BWS/SRS molecular analysis identified IC1_LoM revealing the discrepant diagnosis of SRS. A clinical re-evaluation identified a relative macrocephaly and previously unidentified growth rate at lower limits of normal at birth, feeding difficulties, and asymmetry. Interestingly, and never previously described in IC1_LoM SRS patients, since the age of 16, she has developed hand-writer's cramps, depression, and bipolar disorder. Trio-WES identified a VPS16 heterozygous variant [NM_022575.4:c.2185C>G:p.Leu729Val] inherited from her healthy mother. VPS16 is involved in the endolysosomal system, and its dysregulation is linked to autosomal dominant dystonia with incomplete penetrance and variable expressivity. IGF2 involvement in the lysosomal pathway led us to speculate that the neurological phenotype of the proband might be triggered by the concurrent IGF2 deficit and VPS16 alteration.

Myoclonus- A Review

Myoclonus is a hyperkinetic movement disorder characterized by a sudden, brief, involuntary jerk. Positive myoclonus is caused by abrupt muscle contractions, while negative myoclonus by sudden cessation of ongoing muscular contractions. Myoclonus can be classified in various ways according to body distribution, relation to activity, neurophysiology, and etiology. The neurophysiological classification of myoclonus by means of electrophysiological tests is helpful in guiding the best therapeutic strategy. Given the diverse etiologies of myoclonus, a thorough history and detailed physical examination are key to the evaluation of myoclonus. These along with basic laboratory testing and neurophysiological studies help in narrowing down the clinical possibilities. Though symptomatic treatment is required in the majority of cases, treatment of the underlying etiology should be the primary aim whenever possible. Symptomatic treatment is often not satisfactory, and a combination of different drugs is often required to control the myoclonus. This review addresses the etiology, classification, clinical approach, and management of myoclonus.

Mood and emotional disorders associated with parkinsonism, Huntington disease, and other movement disorders

This chapter provides a review of mood, emotional disorders, and emotion processing deficits associated with diseases that cause movement disorders, including Parkinson's disease, Lewy body dementia, multiple system atrophy, progressive supranuclear palsy, corticobasal degeneration, frontotemporal dementia with parkinsonism, Huntington's disease, essential tremor, dystonia, and tardive dyskinesia. For each disorder, a clinical description of the common signs and symptoms, disease progression, and epidemiology is provided. Then the mood and emotional disorders associated with each of these diseases are described and discussed in terms of clinical presentation, incidence, prevalence, and alterations in quality of life. Alterations of emotion communication, such as affective speech prosody and facial emotional expression, associated with these disorders are also discussed. In addition, if applicable, deficits in gestural and lexical/verbal emotion are reviewed. Throughout the chapter, the relationships among mood and emotional disorders, alterations of emotional experiences, social communication, and quality of life, as well as treatment, are emphasized.

Mutation in ε-Sarcoglycan Induces a Myoclonus-Dystonia Syndrome-Like Movement Disorder in Mice

Myoclonus dystonia syndrome (MDS) is an inherited movement disorder, and most MDS-related mutations have so far been found in the ε-sarcoglycan (SGCE) coding gene. By generating SGCE-knockout (KO) and human 237 C > T mutation knock-in (KI) mice, we showed here that both KO and KI mice exerted typical movement defects similar to those of MDS patients. SGCE promoted filopodia development in vitro and inhibited excitatory synapse formation both in vivo and in vitro. Loss of function of SGCE leading to excessive excitatory synapses that may ultimately contribute to MDS pathology. Indeed, using a zebrafish MDS model, we found that among 1700 screened chemical compounds, Vigabatrin was the most potent in readily reversing MDS symptoms of mouse disease models. Our study strengthens the notion that mutations of SGCE lead to MDS and most likely, SGCE functions to brake synaptogenesis in the CNS.

Combined dystonias: clinical and genetic updates

The genetic combined dystonias are a clinically and genetically heterogeneous group of neurologic disorders defined by the overlap of dystonia and other movement disorders such as parkinsonism or myoclonus. The number of genes associated with combined dystonia syndromes has been increasing due to the wider recognition of clinical features and broader use of genetic testing. Nevertheless, these diseases are still rare and represent only a small subgroup among all dystonias. Dopa-responsive dystonia (DYT/PARK-GCH1), rapid-onset dystonia-parkinsonism (DYT/PARK-ATP1A3), X-linked dystonia-parkinsonism (XDP, DYT/PARK-TAF1), and young-onset dystonia-parkinsonism (DYT/PARK-PRKRA) are monogenic combined dystonias accompanied by parkinsonian features. Meanwhile, MYC/DYT-SGCE and MYC/DYT-KCTD17 are characterized by dystonia in combination with myoclonus. In the past, common molecular pathways between these syndromes were the center of interest. Although the encoded proteins rather affect diverse cellular functions, recent neurophysiological evidence suggests similarities in the underlying mechanism in a subset. This review summarizes recent developments in the combined dystonias, focusing on clinico-genetic features and neurophysiologic findings. Disease-modifying therapies remain unavailable to date; an overview of symptomatic therapies for these disorders is also presented.

Dissociation in reactive and proactive inhibitory control in Myoclonus dystonia

Myoclonus-dystonia (MD) is a syndrome characterized by myoclonus of subcortical origin and dystonia, frequently associated with psychiatric comorbidities. The motor and psychiatric phenotypes of this syndrome likely result from cortico-striato-thamalo-cerebellar-cortical pathway dysfunction. We hypothesized that reactive and proactive inhibitory control may be altered in these patients. Using the Stop Signal Task, we assessed reactive and proactive inhibitory control in MD patients with (n = 12) and without (n = 21) deep brain stimulation of the globus pallidus interna and compared their performance to matched healthy controls (n = 24). Reactive inhibition was considered as the ability to stop an already initiated action and measured using the stop signal reaction time. Proactive inhibition was assessed through the influence of several consecutive GO or STOP trials on decreased response time or inhibitory process facilitation. The proactive inhibition was solely impaired in unoperated MD patients. Patients with deep brain stimulation showed impairment in reactive inhibition, independent of presence of obsessive–compulsive disorders. This impairment in reactive inhibitory control correlated with intrinsic severity of myoclonus (i.e. pre-operative score). The results point to a dissociation in reactive and proactive inhibitory control in MD patients with and without deep brain stimulation of the globus pallidus interna.

Acute cerebellar knockdown of Sgce reproduces salient features of myoclonus-dystonia (DYT11) in mice

Myoclonus dystonia (DYT11) is a movement disorder caused by loss-of-function mutations in SGCE and characterized by involuntary jerking and dystonia that frequently improve after drinking alcohol. Existing transgenic mouse models of DYT11 exhibit only mild motor symptoms, possibly due to rodent-specific developmental compensation mechanisms, which have limited the study of neural mechanisms underlying DYT11. To circumvent potential compensation, we used short hairpin RNA (shRNA) to acutely knock down Sgce in the adult mouse and found that this approach produced dystonia and repetitive, myoclonic-like, jerking movements in mice that improved after administration of ethanol. Acute knockdown of Sgce in the cerebellum, but not the basal ganglia, produced motor symptoms, likely due to aberrant cerebellar activity. The acute knockdown model described here reproduces the salient features of DYT11 and provides a platform to study the mechanisms underlying symptoms of the disorder, and to explore potential therapeutic options.

Twenty years on: Myoclonus-dystonia and ε-sarcoglycan - neurodevelopment, channel, and signaling dysfunction

Myoclonus-dystonia is a clinical syndrome characterized by a typical childhood onset of myoclonic jerks and dystonia involving the neck, trunk, and upper limbs. Psychiatric symptomatology, namely, alcohol dependence and phobic and obsessive-compulsive disorder, is also part of the clinical picture. Zonisamide has demonstrated effectiveness at reducing both myoclonus and dystonia, and deep brain stimulation seems to be an effective and long-lasting therapeutic option for medication-refractory cases. In a subset of patients, myoclonus-dystonia is associated with pathogenic variants in the epsilon-sarcoglycan gene, located on chromosome 7q21, and up to now, more than 100 different pathogenic variants of the epsilon-sarcoglycan gene have been described. In a few families with a clinical phenotype resembling myoclonus-dystonia associated with distinct clinical features, variants have been identified in genes involved in novel pathways such as calcium channel regulation and neurodevelopment. Because of phenotypic similarities with epsilon-sarcoglycan gene-related myoclonus-dystonia, these conditions can be collectively classified as "myoclonus-dystonia syndromes." In the present article, we present myoclonus-dystonia caused by epsilon-sarcoglycan gene mutations, with a focus on genetics and underlying disease mechanisms. Second, we review those conditions falling within the spectrum of myoclonus-dystonia syndromes, highlighting their genetic background and involved pathways. Finally, we critically discuss the normal and pathological function of the epsilon-sarcoglycan gene and its product, suggesting a role in the stabilization of the dopaminergic membrane via regulation of calcium homeostasis and in the neurodevelopmental process involving the cerebello-thalamo-pallido-cortical network. © 2019 International Parkinson and Movement Disorder Society.

A History of Dystonia: Ancient to Modern

Before 1911, when Hermann Oppenheim introduced the term dystonia, this movement disorder lacked a unifying descriptor. While words like epilepsy, apoplexy, and palsy have had their meanings since antiquity, references to dystonia are much harder to identify in historical documents. Torticollis is an exception, although there is difficulty distinguishing dystonic torticollis from congenital muscular torticollis. There are, nevertheless, possible representations of dystonia in literature and visual art from the pre-modern world. Eighteenth century systematic nosologists such as Linnaeus, de Sauvages, and Cullen had attempted to classify some spasmodic conditions, including torticollis. But only after Charcot's contributions to clinical neuroscience were the various forms of generalized and focal dystonia clearly delineated. They were categorized as névroses: Charcot's term for conditions without an identifiable neuroanatomical cause. For a time thereafter, psychoanalytic models of dystonia based on Freud's ideas about unconscious conflicts transduced into physical symptoms were ascendant, although there was always a dissenting "organic" school. With the rise of subspecialization in movement disorders during the 1970s, the pendulum swung strongly back toward organic causation. David Marsden's clinical and electrophysiological research on the adult-onset focal dystonias was particularly important in establishing a physical basis for these disorders. We are still in a period of "living history" of dystonia, with much yet to be understood about pathophysiology. Rigidly dualistic models have crumbled in the face of evidence of electrophysiological and psychopathological overlap between organic and functional dystonia. More flexible biopsychosocial frameworks may address the demand for new diagnostic and therapeutic rationales.

Genetic studies of alcohol dependence in the context of the addiction cycle

Family, twin and adoption studies demonstrate clearly that alcohol dependence and alcohol use disorders are phenotypically complex and heritable. The heritability of alcohol use disorders is estimated at approximately 50-60% of the total phenotypic variability. Vulnerability to alcohol use disorders can be due to multiple genetic or environmental factors or their interaction which gives rise to extensive and daunting heterogeneity. This heterogeneity makes it a significant challenge in mapping and identifying the specific genes that influence alcohol use disorders. Genetic linkage and (candidate gene) association studies have been used now for decades to map and characterize genomic loci and genes that underlie the genetic vulnerability to alcohol use disorders. These approaches have been moderately successful in identifying several genes that contribute to the complexity of alcohol use disorders. Recently, genome-wide association studies have become one of the major tools for identifying genes for alcohol use disorders by examining correlations between millions of common single-nucleotide polymorphisms with diagnosis status. Genome-wide association studies are just beginning to uncover novel biology; however, the functional significance of results remains a matter of extensive debate and uncertainty. In this review, we present a select group of genome-wide association studies of alcohol dependence, as one example of a way to generate functional hypotheses, within the addiction cycle framework. This analysis may provide novel directions for validating the functional significance of alcohol dependence candidate genes. This article is part of the Special Issue entitled "Alcoholism".

Functional dystonia and the borderland between neurology and psychiatry: New concepts

Mind-brain dualism has dominated historical commentary on dystonia, a dichotomous approach that has left our conceptual grasp of it stubbornly incomplete. This is particularly true of functional dystonia, most diagnostically challenging of all functional movement disorders, in which the question of inherent psychogenicity remains a focus of debate. Phenomenological signs considered in isolation lack the specificity to distinguish organic and nonorganic forms, and dystonia's variability has frustrated attempts to develop objective laboratory-supported standards. Diagnostic criteria for functional dystonia that place emphasis on psychiatric symptoms perform poorly in studies of reliability, partly explained by the high frequency of psychopathology in organic dystonia. Novel approaches from the cognitive neurosciences may offer a way forward. Theory on Bayesian statistical prediction in cognitive processing is supported by sufficient experimental evidence for this model to be taken seriously as a way of reconciling contradictory notions about voluntary and unconscious motor control in functional movement disorders. In a Bayesian formulation of functional dystonia, misallocation of attention and abnormal predictive beliefs generate movements that are executed without a sense of agency. Building on this framework, there is a consensus that a biopsychosocial approach is required and that a unified philosophy of brain and mind is the best way to locate dystonia in the neurology-psychiatry borderland. At a practical level, movement disorder neurologists are best placed to differentiate organic from functional dystonia. The main role of psychiatrists is in the diagnosis and management of the primarily psychiatric disorders that often accompany dystonia. © 2016 International Parkinson and Movement Disorder Society.

Progression of neuropsychiatric and cognitive features due to exons 2 to 5 deletion in the epsilon-sarcoglycan gene: a case report

Physical symptoms of myoclonus dystonia due to epsilon-sarcoglycan mutations are well documented; however, the progression of neuropsychiatric and cognitive symptoms remains unclear. We present a case of a 34-year-old woman with early childhood onset of myoclonic jerks, dystonic posture and developmental delay due to exons 2 to 5 deletion in the epsilon-sarcoglycan gene. Over time, she developed neuropsychiatric symptoms. She underwent bilateral deep brain stimulation of the ventral intermediate nucleus of the thalamus for her motor symptoms, which greatly improved but she exhibited slow deterioration of her neuropsychiatric and cognitive symptoms, particularly apathy, aggression and severe executive dysfunction.

Psychiatric disorders, myoclonus dystonia and SGCE: an international study

Objective

Myoclonus‐dystonia (M‐D) is a hyperkinetic movement disorder, typically alcohol‐responsive upper body myoclonus and dystonia. The majority of autosomal dominant familial cases are caused by epsilon‐sarcoglycan gene (SGCE) mutations. Previous publications have observed increased rates of psychiatric disorders amongst SGCE mutation‐positive populations. We analyzed the psychiatric data from four international centers, forming the largest cohort to date, to further determine the extent and type of psychiatric disorders in M‐D.

Methods

Psychiatric data from SGCE mutation‐positive M‐D cohorts, collected by movement disorder specialists in the Netherlands, United Kingdom, United States, and Germany, were analyzed. These data were collected using standardized, systematic questionnaires allowing classification of symptoms according to Diagnostic and Statistical Manual of Mental Disorders, fourth edition (DSM‐IV) criteria. Based on motor findings and SGCE mutation analysis, participants were classified into one of three groups: manifesting carriers, nonmanifesting carriers and noncarriers.

Results

Data from 307 participants were evaluated (140 males, 167 females, mean age at examination: 42.5 years). Two‐thirds of motor affected mutation carriers (n = 132) had ≥1 psychiatric diagnosis, specific, and social phobias being most common followed by alcohol dependence and obsessive‐compulsive disorder (OCD). Compared to familial controls, affected mutation carriers had significantly elevated overall rates of psychiatric disorders (P < 0.001). The most significant differences were observed with alcohol dependence (P < 0.001), OCD (P < 0.001), social and specific phobias (P < 0.001).

Interpretation

M‐D due to SGCE mutations is associated with specific psychiatric disorders, most commonly OCD, anxiety‐related disorders, and alcohol dependence. These suggest either a potential pleiotropic function for SGCE within the central nervous system or a secondary effect of the motor disorder.

Non-motor symptoms in genetically defined dystonia: Homogenous groups require systematic assessment

INTRODUCTION

Dystonia is a movement disorder involving sustained or intermittent muscle contractions resulting in abnormal movements and postures. Identification of disease causing genes has allowed examination of genetically homogenous groups. Unlike the motor symptoms, non-motor characteristics are less clearly defined, despite their impact on a patient's quality of life. This review aims to examine the evidence for non-motor symptoms, addressing cohort size and methods of assessment in each study.

METHODS

A systematic and standardised search strategy was used to identify the published literature relating to psychiatric symptoms, cognition, sleep disorders, sensory abnormalities and pain in each of the genetically determined dystonias. Studies were divided according to cohort size, method of assessment and whether comparison was made to an appropriate control group.

RESULTS

Ninety-five articles were identified including reported clinical histories (n = 42), case reports and smaller case series (n = 12), larger case series (n = 23) and case-control cohorts (n = 18). Psychiatric symptoms were the most frequently investigated with anxiety, depression and Obsessive-Compulsive disorder being most common. Cognitive impairment involved either global deficits or isolated difficulties in specific domains. Disturbances to sleep were most common in the dopa-responsive dystonias. Sensory testing in DYT1 cases identified an intermediate subclinical phenotype.

CONCLUSION

Non-motor symptoms form an integral component of the dystonia phenotype. However, future studies should involve a complete assessment of all symptom subtypes in order to understand the frequency and gene-specificity of these symptoms. This will enable early symptom identification, appropriate clinical management, and provide additional outcome measures in future clinical trials.

The expanding universe of disorders of the basal ganglia

The basal ganglia were originally thought to be associated purely with motor control. However, dysfunction and pathology of different regions and circuits are now known to give rise to many clinical manifestations beyond the association of basal ganglia dysfunction with movement disorders. Moreover, disorders that were thought to be caused by dysfunction of the basal ganglia only, such as Parkinson's disease and Huntington's disease, have diverse abnormalities distributed not only in the brain but also in the peripheral and autonomic nervous systems; this knowledge poses new questions and challenges. We discuss advances and the unanswered questions, and ways in which progress might be made.

Myoclonus

PURPOSE OF REVIEW

Myoclonus remains a challenging movement phenotype to characterize, evaluate, and treat. A systematic assessment of the temporal sequence, phenomenology, and distribution of movements can assist in the rational approach to diagnosis and management.

RECENT FINDINGS

Cortical forms of myoclonus are increasingly recognized as primarily cerebellar disorders. A syndrome of orthostatic myoclonus has been recognized by electrophysiology in patients with neurodegenerative disorders, mainly in Alzheimer disease, accounting for impairments in gait and balance previously mischaracterized as normal pressure hydrocephalus or orthostatic tremor. Tyrosine hydroxylase deficiency and Silver-Russell syndrome (uniparental disomy of chromosome 7) have been established as two novel causes of the myoclonus-dystonia syndrome. Mutations in the glycine receptor (GlyR) α1-subunit gene (GLRA1) explain the major expression of hyperekplexia, an inherited excessive startle disorder, butnewly identified mutations in GlyR β-subunit (GLRB) and glycine transporter 2 (GlyT2) genes (SLC6A5) account for "minor" forms of this disorder manifested as excessive startle and hypnic jerks. The entity previously known as palatal myoclonus has been reclassified as palatal tremor in recognition of its clinical and electromyographic features and no longer enters the differential diagnosis of myoclonic disorders. Increasing documentation of psychogenic features in patients previously characterized as having propriospinal myoclonus has cast doubts on the existence of this distinctive disorder.

SUMMARY

Myoclonus can be a prominent manifestation of a wide range of disorders. Electrophysiologic testing aids in distinguishing myoclonus from other mimics and classifying them according to cortical, subcortical, or spinal origin, which assists the choice of treatment. Despite the lack of randomized clinical trials, levetiracetam appears most effective in patients with cortical myoclonus, whereas clonazepam remains the only first-line therapeutic option in subcortical and spinal myoclonus.

SGCE mutations cause psychiatric disorders: clinical and genetic characterization

Myoclonus dystonia syndrome is a childhood onset hyperkinetic movement disorder characterized by predominant alcohol responsive upper body myoclonus and dystonia. A proportion of cases are due to mutations in the maternally imprinted SGCE gene. Previous studies have suggested that patients with SGCE mutations may have an increased rate of psychiatric disorders. We established a cohort of patients with myoclonus dystonia syndrome and SGCE mutations to determine the extent to which psychiatric disorders form part of the disease phenotype. In all, 89 patients with clinically suspected myoclonus dystonia syndrome were recruited from the UK and Ireland. SGCE was analysed using direct sequencing and for copy number variants. In those patients where no mutation was found TOR1A (GAG deletion), GCH1, THAP1 and NKX2-1 were also sequenced. SGCE mutation positive cases were systematically assessed using standardized psychiatric interviews and questionnaires and compared with a disability-matched control group of patients with alcohol responsive tremor. Nineteen (21%) probands had a SGCE mutation, five of which were novel. Recruitment of family members increased the affected SGCE mutation positive group to 27 of whom 21 (77%) had psychiatric symptoms. Obsessive-compulsive disorder was eight times more likely (P < 0.001) in mutation positive cases, compulsivity being the predominant feature (P < 0.001). Generalized anxiety disorder (P = 0.003) and alcohol dependence (P = 0.02) were five times more likely in mutation positive cases than tremor controls. SGCE mutations are associated with a specific psychiatric phenotype consisting of compulsivity, anxiety and alcoholism in addition to the characteristic motor phenotype. SGCE mutations are likely to have a pleiotropic effect in causing both motor and specific psychiatric symptoms.

Myoclonus-dystonia and Silver-Russell syndrome resulting from maternal uniparental disomy of chromosome 7

Myoclonus-dystonia (M-D) is a movement disorder that is often associated with mutations in epsilon-sarcoglycan (SGCE), a maternally imprinted gene at 7q21.3. We report a 24-year-old male with short stature (<5th percentile) and a movement disorder clinically consistent with M-D. Single nucleotide polymorphism (SNP) array did not identify significant copy number changes, but revealed three long continuous stretches of homozygosity on chromosome 7 suggestive of uniparental disomy. Parental SNP arrays confirmed that the proband had maternal uniparental disomy of chromosome 7 (mUPD7) with regions of heterodisomy and isodisomy. mUPD7 is the cause of approximately 5-10% of Silver-Russell syndrome (SRS), a disorder characterized by prenatal and postnatal growth retardation. Although SRS was not suspected in our patient, these findings explain his short stature. SGCE methylation testing showed loss of the unmethylated paternal allele. Our findings provide a unifying diagnosis for his short stature and M-D and help to optimize his medication regimen. In conclusion, we show that M-D is a clinical feature that may be associated with SRS due to mUPD7. Individuals with mUPD7 should be monitored for the development of movement disorders. Conversely, individuals with M-D and short stature should be evaluated for SRS.

Prominent psychiatric comorbidity in the dominantly inherited movement disorder myoclonus-dystonia

BACKGROUND

Neurological and psychiatric disorders show clinical overlap suggesting a shared pathophysiological background. We evaluated myoclonus-dystonia, a monogenic movement disorder as a disease model for inherited psychopathology.

METHOD

We investigated 12 SGCE mutation carriers using standardized neurological and psychiatric examinations to assign DSM-IV diagnoses. Furthermore, we analyzed all studies in the Medline database which included psychiatric information on SGCE mutation-positive patients.

RESULTS

Of our twelve SGCE mutation carriers, 10 were older than 16 years. Two of them (20%) reported psychiatric diagnoses before our examination, which resulted in at least one psychiatric diagnosis in seven (70%) patients, most frequently anxiety (60%), depression (30%) or both. Substance abuse was observed in 20%, whereas obsessive-compulsive disorders were absent. One mutation carrier showed Axis 2 features. In the literature analysis, the ten studies using standardized tools covering DSM-IV criteria reported prevalences similar to those in our sample. This was three times the frequency of psychiatric disorders detected in 13 studies using clinical history or patient report only.

CONCLUSION

About two thirds of SGCE mutation carriers develop psychiatric comorbidity and >80% are previously undiagnosed.

A gain-of-glycosylation mutation associated with myoclonus-dystonia syndrome affects trafficking and processing of mouse ε-sarcoglycan in the late secretory pathway

Missense mutations in the SGCE gene encoding ε-sarcoglycan account for approximately 15% of SGCE-positive cases of myoclonus-dystonia syndrome (MDS) in humans. In this study, we show that while the majority of MDS-associated missense mutants modeled with a murine ε-sarcoglycan cDNA are substrates for endoplasmic reticulum-associated degradation, one mutant, M68T (analogous to human c.275T>C, p.M92T), located in the Ig-like domain of ε-sarcoglycan, results in a gain-of-glycosylation mutation producing a protein that is targeted to the plasma membrane, albeit at reduced levels compared to wild-type ε-sarcoglycan. Removal of the ectopic N-linked glycan failed to restore efficient plasma membrane targeting of M68T demonstrating that the substitution rather than the glycan was responsible for the trafficking defect of this mutant. M68T also colocalized with CD63-positive vesicles in the endosomal-lysosomal system and was found to be more susceptible to lysosomal proteolysis than wild-type ε-sarcoglycan. Finally, we demonstrate impaired ectodomain shedding of M68T, a process that occurs physiologically for ε-sarcoglycan resulting in the lysosomal trafficking of the intracellular C-terminal domain of the protein. Our findings show that functional analysis of rare missense mutations can provide a mechanistic insight into the pathogenesis of MDS and the physiological role of ε-sarcoglycan.