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Editorial the Molecular Genetics of the Dystonias

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Editorial the Molecular Genetics of the Dystonias J Neurol Neurosurg Psychiatry 1998;64:427–429 427 J Neurol Neurosurg Psychiatry: first published as 10.1136/jnnp.64.4.427 on 1 April 1998. Downloaded from Editorial The molecular genetics of the dystonias Since the introduction of the term dystonia by Oppenheim in a large non-Jewish French Canadian family with gener- in 1911 to describe altering muscle tone and postural alised dystonia.15 Linkage to this region was also found in deformities seen in patients,1 there has been considerable Ashkenazi Jewish and other European non-Jewish controversy surrounding its aetiology, classification, and kindreds.16 17 In non-Jewish kindreds, genetic heterogeneity genetics. Dystonia can describe a symptom, a syndrome, or was detected, indicating that defects in more than one gene a clinical sign that is part of a more severe neurological dis- can cause familial generalised dystonia.17–21 An interesting order. Dystonia is now defined as a syndrome of sustained feature was that dystonia in chromosome 9q34 linked involuntary muscle contractions, often causing twisting families invariably began in a limb before subsequent gen- and repetitive movements or abnormal posture.2 eralisation, whereas, in most non-linked kindreds, the dys- The dystonias are a relatively common group of tonia often had onset in the craniocervical region. neurological disorders with conservative estimates of 30 The finding of strong allelic association in Ashkenazi 000 aVected people in the United Kingdom.3 Dystonia has Jewish patients suggested a founder mutation causing the been divided into primary, in which there is no identifiable dystonia in this particular population and localised DYT1 underlying cause, or secondary to other neurological con- to a 1cM region.22 The haplotype identified was specifically ditions, which include structural lesions of the basal associated with limb onset generalised dystonia.23 It was ganglia, exposure to drugs and toxins, and cerebral palsy. It estimated that this founder mutation arose about 350 years was from the study of secondary dystonia that it became ago in the historic Jewish Pale of settlement in Lithuania clear that disruption of basal ganglia motor circuits and Byelorussia.24 produces dystonia.245Primary dystonia is distinguished by Recently the DYT1 gene has been cloned and analysed the lack of other neurological involvement and absence of for mutations.25 The fascinating finding was ofa3bpdele- distinct neuropathology. Central dopaminergic pathways tion in the coding sequence found in all aVected and obli- have been implicated in the production of dystonic move- gate carriers with chromosome 9 linked primary dystonia, ments from the study of tardive dystonia6 (secondary to D2 regardless of ethnic background and surrounding haplo- receptor blockade) and dopa responsive dystonia (see type. The mutation specifically causes early limb onset later). generalised dystonia. Assuming this mutation arose In recent years, advances in molecular genetic tech- independently in diVerent ethnic groups, the finding niques have led to exciting discoveries which have suggests that only one variation in the encoded protein can expanded our knowledge of the pathogenesis of dystonia give rise to early onset dystonia. The deletion results in the and led to a revision of its classification (table).7 This loss of one of a pair of glutamic acid residues near the car- classification incorporates the advances in our understand- boxy terminus of a novel protein named torsinA. TorsinA ing of the genetics and aetiology of dystonia, and also the contains an ATP binding domain and a putative increasing number of distinguishable clinical phenotypes. N-terminal leader sequence. It has homology to three To date, eight genes causing dystonia have been mapped mammalian genes and a nematode gene on database http://jnnp.bmj.com/ and three of these have been cloned. The following sections searches, and distant similarity to the family of heat shock review these recent discoveries. proteins and Clp proteases. However, its function and putative role in the nervous system remains uncertain. Primary torsion dystonia The genetic studies of primary dystonia have enabled Primary torsion dystonia (also known as idiopathic torsion more accurate genetic counselling for families. For a famil- dystonia) is the commonest form of dystonia with an esti- ial case of generalised, segmental, or multifocal primary mated prevalence of 329 per million of which focal dysto- dystonia in the United Kingdom, the risk to first degree nia accounted for 294 per million.8 It consists of dystonic relatives is estimated at 21%. If the index case is isolated, on September 25, 2021 by guest. Protected copyright. movements and postures (with or without tremor) with no the risks are 14% to children and 8% to siblings.13 The other neurological features or identifiable cause.9 The identification of a single mutation will facilitate both range of severity is wide, including severe generalised dys- predictive and prenatal genetic testing with appropriate tonia (formerly known as dystonia musculorum deform- counselling. However, the low penetrance and variable ans), segmental and multifocal dystonia, and focal dystonia expression of the DYT1 gene means that such tests should (including torticollis, blepharospasm, and writer’s cramp). be performed with caution, as the phenotype cannot be Dystonia can develop at almost any age and this often accurately predicted. determines its severity. Onset in childhood, particularly in For focal and segmental primary dystonia, the role of a limb, is strongly predictive of subsequent DYT1 and other genes is less clear. Focal primary is the generalisation,29and this form of dystonia is reported to be most common form of dystonia, has onset in adult life, and more prevalent in the Ashkenazi Jewish population.10 By often appears to be sporadic.11 There is, however, evidence contrast, dystonia arising in cranial or axial muscles tends that genetic components play a part in focal dystonia with to develop in adult life and remain focal or segmental in families showing autosomal dominant inheritance.26–28 In distribution.11 addition, a genetic study of index cases with focal dystonia Genetic studies in both Jewish and non-Jewish popula- found aVected relatives in 25% of patients.29 Linkage stud- tions have shown that early onset severe primary dystonia ies have excluded the DYT1 gene as a cause for focal dys- is caused by an autosomal dominant gene(s) with reduced tonia in two families with cervical dystonia30 and, in penetrance (30%-40%) and variable expression.12–14 In another study, no persons with craniocervical dystonia or 1989, Ozelius et al reported linkage between a dystonia writer’s cramp were found to have the DYT1 GAG locus (DYT1) and a polymorphism on chromosome 9q34 deletion.25 428 Warner, Jarman J Neurol Neurosurg Psychiatry: first published as 10.1136/jnnp.64.4.427 on 1 April 1998. Downloaded from Classification of dystonia homovanillic acid concentrations. To date, no linkage for the gene(s) involved has been found. Primary dystonia Phenotype of dystonia alone (+/− tremor) Dystonia-plus syndromes Phenotype of dystonia plus additional neurological features (for example, plus MYOCLONIC DYSTONIA parkinsonism (dopa responsive dystonia Myoclonic dystonia is a rare variant characterised by dys- and rapid onset dystonia-parkinsonism)) tonia and myoclonus with onset in adolescence or early Plus myoclonus (myoclonic dystonia) Secondary dystonias Result from environmental factors (for adult life, aVecting predominantly the arms and axial mus- example, stroke, drugs, toxins, and cles. It is extremely sensitive to alcohol. The families perinatal asphyxia) described exhibit autosomal dominant inheritance with Heredodegenerative diseases Present as dystonia-plus syndromes with 43 44 underlying neurodegeneration reduced penetrance and variable expression. Linkage Paroxysmal dystonia Paroxysmal kinisigenic dystonia analysis with chromosome 9q markers has excluded the Paroxysmal dystonic choreoathetosis DYT1 region in a Swedish family.45 Linkage to chromosome 18p has been found in a large Secondary dystonia German kindred with adult onset cervical dystonia and The term secondary dystonia implies an identifiable cause spasmodic dysphonia.31 A follow up study of apparently such as neuroleptic exposure, physical insult to the brain sporadic cases of focal dystonia in northern Germany (stroke, tumour), and cerebral palsy. Bressman et al exam- showed linkage disequilibrium for a common haplotype of ined the possible role of genetic susceptibility in the devel- 46 markers over a 6cM region around this locus, suggesting opment of secondary dystonia. They analysed chromo- that a dominant founder mutation of low penetrance may some 9q34 haplotypes in 40 Ashkenazi Jewish patients with be an important cause of focal dystonia in this population.32 secondary dystonia; the majority due to neuroleptic expo- Whether this finding is applicable to other European sure or perinatal asphyxia. Nine patients were considered populations is uncertain, but, if replicated, it would suggest phenocopies of dystonia caused by DYT1. No evidence that genetic factors play a larger part in the causation of that the DYT1 founder mutation contributed to secondary primary focal dystonia than previously thought. dystonia was found. It was emphasised that accurate diag- A further dystonia locus has been identified on chromo- nostic criteria which included questioning about dopamine some 8 in two German American Mennonite
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