Inherited myoclonus-dystonia: evidence supporting genetic heterogeneity

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

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

SGCE and myoclonus dystonia: motor characteristics, diagnostic criteria and clinical predictors of genotype

Myoclonus dystonia syndrome (MDS) is a young-onset movement disorder. A proportion of cases are due to mutations in the maternally imprinted SGCE gene. We assembled the largest cohort of MDS patients to date, and determined the frequency and type of SGCE mutations. The aim was to establish the motor phenotype in mutation carriers and utility of current diagnostic criteria. Eighty-nine probands with clinical features compatible with MDS were recruited from the UK and Ireland. Patients were phenotypically classified as "definite", "probable" or "possible" MDS according to previous guidelines. SGCE was analyzed using direct sequencing and copy number variant analysis. In those where no mutation was found, DYT1 (GAG deletion), GCH1, THAP1 and NKX2.1 genes were also sequenced. Nineteen (21.3%) probands had an SGCE mutation. Three patterns of motor symptoms emerged: (1) early childhood onset upper body myoclonus and dystonia, (2) early childhood onset lower limb dystonia, progressing later to more pronounced myoclonus and upper body involvement, and (3) later childhood onset upper body myoclonus and dystonia with evident cervical involvement. Five probands had large contiguous gene deletions ranging from 0.7 to 2.3 Mb in size with distinctive clinical features, including short stature, joint laxity and microcephaly. Our data confirms that SGCE mutations are most commonly identified in MDS patients with (1) age at onset ≤10 years and (2) predominant upper body involvement of a pure myoclonus-dystonia. Cases with whole SGCE gene deletions had additional clinical characteristics, which are not always predicted by deletion size or gene involvement.

SGCZ mutations are unlikely to be associated with myoclonus dystonia

BACKGROUND

Myoclonus dystonia syndrome (MDS) is a hyperkinetic movement disorder caused, in a proportion of cases, by mutations of the maternally imprinted epsilon-sarcoglycan gene (SGCE). SGCE mutation rates vary between cohorts, suggesting genetic heterogeneity. E- and ζ-sarcoglycan are both expressed in brain tissue. In this study we tested whether zeta-sarcoglycan gene (SGCZ) mutations also contribute to this disorder.

METHODS

Patients with clinically suspected MDS and no SGCE mutation were recruited and classified, according to previously published criteria, as to their likelihood of the movement disorder. All SGCZ exons and intron/exon boundaries were screened by direct sequencing.

RESULTS

Fifty-four SGCE mutation-negative patients were recruited from the UK and the Netherlands. Subdivided according to the likelihood of the movement disorder resulted in 17 'definite', 16 'probable' and 21 'possible' cases. No pathogenic SGCZ mutations were identified.

CONCLUSIONS

SGCZ mutations are unlikely to contribute to the genetic heterogeneity in MDS.

Primary dystonias and genetic disorders with dystonia as clinical feature of the disease

Dystonia is probably the most common form of movement disorder encountered in the clinical practice. It is characterized by sustained muscle contractions, usually producing twisting and repetitive movements or abnormal postures or positions. Dystonias can be classified in several ways, including primarily by the clinical phenomenology or by the underlining etiology, in particular to understand if the presentation is genetically determined. By advances of genetics, including contemporary genomic technologies, there is a growing understanding of the molecular underpinnings of genetically determined dystonias. The intricacy of information requires a user friendly, novel database that may efficiently serve clinicians to inform of advances of the field and to diagnose and manage these often complex cases. Here we present an up to date, comprehensive review - in tabulated formats - of genetically determined primary dystonias and complex Mendelian disorders with dystonia as central feature. The detailed search up to December 24, 2012, identified 24 hereditary primary dystonias (DYT1 to DYT 25) that are mostly monogenic disorders, and a larger group (>70) of genetic syndromes in which dystonia is one of the characteristic clinical features. We organized the findings not only by individual information (name of the conditions, pattern of inheritance, chromosome and gene abnormality, clinical features, relevant ancillary tests and key references), but also provide symptom-oriented organization of the clinical entities for efficient inquiries.

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.

Surgical treatment of myoclonus dystonia syndrome

Myoclonus dystonia (M-D) syndrome is a heritable movement disorder characterized by myoclonic jerks and dystonia primarily of the upper extremities. M-D remains poorly responsive to pharmacological treatment. Emerging reports suggest good response to DBS of the internal globus pallidus (GPi) and ventral intermediate nucleus (VIM) of the thalamus. This study aimed to appraise the value of these two DBS targets by evaluating reports available in the literature. A systematic search of published case reports and case series was performed on Medline and Embase. Responses to DBS were evaluated. Myoclonus was assessed with the Unified Myoclonus Rating Scale (UMRS) and dystonia by the Burke-Fahn-Marsden dystonia rating scale (BFMDRS). The primary outcome of interest was the relative improvements noted with GPi, compared to VIM stimulation. A total of 17 publications yielded 40 unique cases, with mean follow-up of 27.2 months. All patients demonstrated improvements in myoclonus scores, with 93.5% showing at least a 50% improvement in UMRS. The mean improvement in myoclonus scores was 72.6%. In contrast, dystonia scores were improved in 87.9% of patients, with 72.7% reporting at least a 50% improvement in BFMDRS. The mean improvement in dystonia scores was 52.6%. Improvements in myoclonus scores were similar for both GPi (75.7%) and VIM (70.4%; P = 0.27). However, the improvements in dystonia scores were greater with GPi (60.2%), compared to VIM (33.3%; P = 0.03). Although both targets achieve similar improvements in myoclonus, GPi stimulation may be a preferred target because it may achieve greater improvements in dystonia, compared to VIM stimulation.

Dopamine D2 receptors as treatment targets in schizophrenia

The antipsychotic effectiveness of chlorpromazine and haloperidol started a search for their therapeutic targets. The antipsychotic receptor target turned out to be a dopamine receptor, now cloned as the dopamine D2 receptor. The D2 receptor is the common target for antipsychotics. Antipsychotic clinical doses correlate with their affinities for this receptor. Therapeutic doses of antipsychotics occupy 60 to 80% of brain D2 receptors in patients, but aripiprazole occupies up to 90%. While antipsychotics may take up to six hours to occupy D2 receptors, much clinical improvement occurs within a few days. The receptor has high- and low-affinity states. The D2High state is functional for dopamine-like agonists such as aripiprazole. Most individuals with schizophrenia are supersensitive to dopamine. Animal models of psychosis show that a variety of risk factors, genetic and nongenetic, are associated with behavioral supersensitivity to dopamine, reflected in elevated levels of dopamine D2High receptors. Although antipsychotics such as haloperidol alleviate psychosis and reverse the elevation of D2High receptors, long-term use of traditional antipsychotics can further enhance dopamine supersensitivity in patients. Therefore, switching from a traditional antipsychotic to an agonist antipsychotic such as aripiprazole can result in the emergence of psychotic signs and symptoms. Clozapine and quetiapine do not elicit parkinsonism and rarely result in tardive dyskinesia because they are released from D2 within 12 to 24 hours. Traditional antipsychotics remain attached to D2 receptors for days, preventing relapse, but allowing accumulation that can lead to tardive dyskinesia. Future goals include imaging D2High receptors and desensitizing them in early-stage psychosis.

Electrophysiological features of myoclonus-dystonia

Inherited myoclonus-dystonia (M-D) is an autosomal dominant disorder characterized by myoclonus and dystonia that often improves with alcohol. To examine the electrophysiologic characteristics of M-D, we studied 6 patients from 4 different families and 9 age-matched healthy subjects. Neurophysiological studies performed include electromyography (EMG)-electroencephalography (EEG) polygraphy, jerk-locked back-averaged EEG, somatosensory evoked potentials (SEP), long-latency reflex (LLR) to median and digital nerve stimulation, and transcranial magnetic stimulation studies with short-interval intracortical inhibition (SICI), intracortical facilitation (ICF), and long-interval intracortical inhibition (LICI). All 6 patients showed myoclonus and dystonia on clinical examination and EMG testing. The EMG burst durations ranged from 30.4 to 750.6 milliseconds (mean, 101.5 milliseconds). Jerk-locked back-averaged EEG failed to reveal any preceding cortical correlates. Median nerve SEP revealed no giant potential. No patients had exaggerated LLR to median or digital nerve stimulation. There was no significant difference in SICI, ICF, and LICI between M-D patients and normal subjects. Myoclonus in inherited M-D is likely of subcortical origin. Normal intracortical inhibition and facilitation suggest that the GABAergic circuits in the motor cortex are largely intact and that the mechanisms of myoclonus and dystonia are different from those for cortical myoclonus and other dystonic disorders.

Refinement of the DYT15 locus in myoclonus dystonia

Inherited myoclonus dystonia (MD) is an autosomal dominant disorder in which we previously mapped a novel locus to chromosome18p11 (OMIM number: 607488). Since no further informative STS markers were found within the flanking shared regions, we utilized single nucleotide polymorphisms (SNP) for fine-mapping. All known or predicted genes within this region were directly sequenced. We identified three recombinant SNPs in the distal region but none from the proximal region. Our previous linked region has now been reduced to 3.18 Mb but direct sequencing of all seven known and four predicted genes with EST support did not identify any mutations..

Movement disorders and alcohol misuse

Many movement disorders, including tics, chorea, tremor, myoclonus and parkinsonism, may result from substance abuse. However, alcohol in particular is associated in a more complex manner with two specific movement disorders, essential tremor (ET) and myoclonus-dystonia (M-D). In this review we discuss the comorbidity of alcohol abuse in both ET and M-D, the ameliorative effects of alcohol in both diseases, and review the data evaluating alcohol abuse secondary to self-medication. We also discuss shared pathophysiologic mechanisms in the understanding of both of these disorders, as the elucidation of the mechanisms by which alcohol exerts its effects may lead to novel therapeutic approaches.

Dystonia

BACKGROUND

Dystonia refers to a syndrome of sustained muscle contractions, frequently causing twisting and repetitive movements or abnormal postures. Although age at onset, anatomic distribution, and family history are essential elements in the evaluation of dystonia, new classification increasingly relies on etiologic and genetic data. In recent years, much progress has been made on the genetics of various forms of dystonia and its pathophysiology underlying the clinical signs. The treatment of dystonia has continued to evolve to include newer medications, different forms of botulinum toxin, and various surgical procedures.

REVIEW SUMMARY

In this article, the author reviewed and summarized the history of dystonia, its evolving classification, and recent genetic data, as well as its clinical investigation and treatment.

CONCLUSIONS

Recent advances in molecular biology have led to the discovery of novel dystonia genes and loci, updating classification schemes, and better understanding of underlying pathophysiology. Treatment strategies for dystonia have significantly been updated with the introduction of different forms of botulinum toxin therapy, new pharmacologic agents, and most recently pallidal deep brain stimulation. A systematic approach to the diagnosis and treatment evaluation of dystonic patients provides optimal care for long-term management.

Lack of mutations in the epsilon-sarcoglycan gene in patients with different subtypes of primary dystonias

Primary dystonias represent a clinically and genetically heterogeneous group of movement disorders. Mutations in the epsilon-sarcoglycan (SGCE) gene have been found recently to cause myoclonus-dystonia (MD). Considerable clinical variation of SGCE mutation carriers leads to the hypothesis that mutations in the SGCE gene might also be relevant for other subtypes of dystonias. To determine the contribution of mutations in the SGCE gene in patients with different subtypes of dystonias, we analyzed the coding sequence of the SGCE gene in a group of 296 patients with a clinical phenotype of primary dystonia and in 2 patients with a clinical phenotype of myoclonus-dystonia. Patients with mutations in the DYT1 gene were excluded. We could not detect a mutation in the SGCE gene in any of the 298 patients. Our results suggest that mutations in the SGCE gene cannot be held responsible for other subtypes of primary dystonia.

Analysis of the ?-sarcoglycan gene in familial and sporadic myoclonus-dystonia: Evidence for genetic heterogeneity

The epsilon-sarcoglycan gene (SGCE) on human chromosome 7q21 has been reported to be a major locus for inherited myoclonus-dystonia. Linkage to the SGCE locus has been detected in the majority of families tested, and mutations in the coding region have been found recently in families with autosomal dominant myoclonus-dystonia. To evaluate the relevance of SGCE in myoclonus-dystonia, we sequenced the entire coding region of the epsilon-sarcoglycan gene in 16 patients with either sporadic or familial myoclonus-dystonia. No mutations were found. This study suggests that epsilon-sarcoglycan does not play an important role in sporadic myoclonus-dystonia and supports genetic heterogeneity in familial cases.

Epsilon-sarcoglycan mutations found in combination with other dystonia gene mutations

Myoclonus-dystonia is a movement disorder associated with mutations in the epsilon-sarcoglycan gene (SGCE) in most families and in the DRD2 and DYT1 genes in two single families. In both of the latter families, we also found a mutation of SGCE. The molecular mechanisms through which the detected mutations may contribute to myoclonus-dystonia remain to be determined.

Classification and genetics of dystonia

Dystonia is a syndrome characterised by sustained muscle contractions, producing twisting, repetitive, and patterned movements, or abnormal postures. The dystonic syndromes include a large group of diseases that have been classified into various aetiological categories, such as primary, dystonia-plus, heredodegenerative, and secondary. The diverse clinical features of these disorders are reflected in the traditional clinical classification based on age at onset, distribution of symptoms, and site of onset. However, with an increased awareness of the molecular and environmental causes, the classification schemes have changed to reflect different genetic forms of dystonia. To date, at least 13 dystonic syndromes have been distinguished on a genetic basis and their loci are referred to as DYT1 to DYT13. This review focuses on the molecular and phenotypic features of the hereditary dystonias, with emphasis on recent advances.

Dystonia: clinical features, genetics, and treatment

PURPOSE OF REVIEW

The present review covers recent advances in dystonia research related to dystonia genetics and treatment. These have led to the discovery of novel dystonia genes and loci, to changing classification schemes, and to the introduction of improved and new treatment options.

RECENT FINDINGS

Currently 13 different forms of dystonia can be distinguished on a genetic basis (dystonia types 1-13). Recently, a novel gene locus (DYT13) was detected in a family with segmental dystonia, and the gene causing myoclonus-dystonia was identified (SGCE). Furthermore, a novel mutation in the DYT1 gene is associated with a myoclonus-dystonia phenotype. Regarding dystonia treatment, patients refractory to botulinum toxin type A can now be treated with botulinum toxin type B. Selective peripheral denervation remains an effective form of treatment for patients with secondary, but probably not with primary botulinum toxin treatment failure. Finally, a renaissance of functional surgical ablative procedures has taken place, with high frequency deep brain stimulation being introduced in dystonia treatment. Bilateral pallidotomy or pallidal stimulation may provide major benefit especially in patients with generalized, disabling dystonia with the most dramatic improvements in dystonia type 1 patients. Neurostimulation may also be effective in primary segmental axial dystonia, myoclonus-dystonia, and tardive dystonia.

SUMMARY

The recent mapping of additional dystonia gene loci, the identification of novel dystonia genes, and the characterization of proteins encoded by these genes have enhanced our understanding of various forms and aspects of the dystonias and have opened up new avenues for research. Treatment options include both medical and surgical therapies, with deep brain simulation being the most recent development.

Mutations in the gene encoding epsilon-sarcoglycan cause myoclonus-dystonia syndrome

The dystonias are a common clinically and genetically heterogeneous group of movement disorders. More than ten loci for inherited forms of dystonia have been mapped, but only three mutated genes have been identified so far. These are DYT1, encoding torsin A and mutant in the early-onset generalized form, GCH1 (formerly known as DYT5), encoding GTP-cyclohydrolase I and mutant in dominant dopa-responsive dystonia, and TH, encoding tyrosine hydroxylase and mutant in the recessive form of the disease. Myoclonus-dystonia syndrome (MDS; DYT11) is an autosomal dominant disorder characterized by bilateral, alcohol-sensitive myoclonic jerks involving mainly the arms and axial muscles. Dystonia, usually torticollis and/or writer's cramp, occurs in most but not all affected patients and may occasionally be the only symptom of the disease. In addition, patients often show prominent psychiatric abnormalities, including panic attacks and obsessive-compulsive behavior. In most MDS families, the disease is linked to a locus on chromosome 7q21 (refs. 11-13). Using a positional cloning approach, we have identified five different heterozygous loss-of-function mutations in the gene for epsilon-sarcoglycan (SGCE), which we mapped to a refined critical region of about 3.2 Mb. SGCE is expressed in all brain regions examined. Pedigree analysis shows a marked difference in penetrance depending on the parental origin of the disease allele. This is indicative of a maternal imprinting mechanism, which has been demonstrated in the mouse epsilon-sarcoglycan gene.