UvA-DARE (Digital Academic Repository) Genetic architecture of dystonia Groen, Justus. Link to publication

Transcription

1 UvA-DARE (Digital Academic Repository) Genetic architecture of dystonia Groen, Justus Link to publication Citation for published version (APA): Groen, J. L. (2014). Genetic architecture of dystonia General rights It is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), other than for strictly personal, individual use, unless the work is under an open content license (like Creative Commons). Disclaimer/Complaints regulations If you believe that digital publication of certain material infringes any of your rights or (privacy) interests, please let the Library know, stating your reasons. In case of a legitimate complaint, the Library will make the material inaccessible and/or remove it from the website. Please Ask the Library: or a letter to: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam, The Netherlands. You will be contacted as soon as possible. UvA-DARE is a service provided by the library of the University of Amsterdam ( Download date: 13 Dec 2018

2 1.1 General Introduction

3 12 Chapter one The genetic architecture of dystonia. For a reader who gets a hold of this thesis and intends to read it (if only in part), the title can make them change their mind quickly. Because what one does not know one cannot love. What does genetic architecture mean and what is dystonia? The two concepts used in the title line are not commonly used, not even in medical practice. Hence the objective of this general introduction, which is to make you, the reader, familiar with these phrases, introduce you to the main questions of this thesis and, foremost, to persuade you to read on. First off, what is genetic architecture? The word architecture here refers to the genetic structure underlying a disease. A genotype is the information we have inherited, the makeup of our genes, as passed on from our ancestors. This genetic information embodies over 3 billion base pairs. The variability in this information determines susceptibility to disease, but also determines our personality, the way we look, and much more. The amount of genetic variation is astonishing: each individual human genome contains around three and a half million variants, on average. In other words: in every 1,000 base pairs there is a variant present. 1 Some of these variants can cause protein dysfunction, create a new function or result in total loss of this protein. This can cause problems on a cellular level, ultimately leading to disease. The challenge is to identify the causal variants, responsible for the risk on a disease, in this thesis on dystonia. To create some order in the wilderness of variants as well as for the benefit of genetic research, three comprehensive frequency classes of genetic variants can be distinguished: rare variants, common variants and an intermediate group. 2 This spectrum of genetic variants is shown in Figure 1. Rare alleles can have a major genetic impact (high-risk rare alleles). It has been known for decades that these variants ( mutations ) can cause familial diseases (as the saying goes: it runs in the family ). Other genetic variants occur more frequently in the population. These common variants typically hold a small risk for developing disease (low-risk common variants). In between these classes there is another group of variants with a strong genetic risk (not strong enough, however, to cause a clear familial pattern.) This group (moderate-risk low-frequency alleles) are not as frequent as common variants, but are more frequent than the rare alleles. The described grouping of variants is a simplification of the real genetic spectrum, but grouping is helpful in our search for disease risk variants. What can be said here about the disease risk involved? Does a rare variant have more impact than common variants? In other words: is carrying a rare variant worse than having a common variant? Things are more complicated than that: small changes occurring in an essential hub protein can exert a large impact on cell function. Furthermore, a person can carry many harmless common variants, which together (cumulative or additive) can compound the risk for a disease even more. For the purposes of this thesis, we have studied the contribution of genetic variants from these different frequency classes to the risk of dystonia. The other word to be discussed is dystonia. Dystonia is defined as a neurological movement disorder characterized by sustained or intermittent muscle contractions causing

4 1.1 General Introduction 13 Figure 1. Spectrum of genetic variants by risk allele frequency and strength of genetic effect. Focus of this thesis lies in identifying associations with variants shown within diagonal dotted lines. GWA=Genome Wide Association study. Adapted from Manolio TA et al. Nature Courtesy of Dr.Teri Manolio, NIH. abnormal, often repetitive, movements, postures, or both. Dystonic movements are typically patterned, twisting, and may be tremulous. Dystonia is often initiated or worsened by voluntary action and associated with overflow muscle activation. 3 Several clinical descriptors can be utilized to specify the type of dystonia in a patient: age at onset, body distribution, pattern in time (temporal pattern), coexistence of other movement disorders or other neurological manifestations. 3 A younger age at onset is associated with a strong genetic factor and a more severe presentation. When dystonia occurs at a higher age, especially above fifty years of age, dystonia is most often mild and does not spread to other body parts. The distribution and spread of dystonia are both important prognostic factors. If one body region is affected, it is called a focal dystonia and includes a wide variability: cervical dystonia (dystonia of the neck muscles, the most frequent form), blepharospasm (dystonia of the eyes, involuntarily closing of the eyes), spasmodic dysphonia (laryngeal dystonia), oromandibular dystonia (including dystonia of the tongue and mandible musculature) and hand dystonia (most often writer s cramp). In segmental dystonia, two or more contiguous body regions are affected, in generalized dystonia the trunk and at least two other

5 14 Chapter one sites are affected. The temporal pattern of the dystonia can differentiate between paroxysmal dystonias, task-specific dystonias, mobile or static dystonia. Is dystonia a paroxysm-like occurrence or persistent? Is there a diurnal pattern of complaints, or are they confined to one specific task (e.g. writing or playing the violin)? It is equally important to consider the question of whether there is a progression of the disease or if it remains stable across time. To complete the description one should identify any associated features like tremor, myoclonus, parkinsonism, psychiatric comorbidity and cognitive decline. Combining these characteristics of the dystonia is helpful in recognizing of clinical patterns. There is no pathognomonic presentation, however, that allows for reliable clinical-etiological correlations. Dystonia syndromes have a remarkable degree of phenotypic variability, with frequent overlapping among different syndromes. The majority of dystonia patients have a primary dystonia syndrome. In patients with primary dystonia no other motor symptoms are present, except in some cases a postural arm tremor. The most frequent forms of dystonia are the focal primary dystonias: in the general population approximately one in suffers from focal dystonia. 4 What is known about the genetic factors involved in dystonia? Most research into the genetics behind dystonia is carried out on the more rare familial dystonia syndromes. The different identified familial dystonia syndromes are classified as DYTs. These are ordered in chronological order, with DYT1 being the first described familial dystonia syndrome. If, aside from dystonia, additional movement disorders like myoclonus (jerks) or parkinsonism are present, it is then considered a dystonia plus or combined dystonia syndrome. 3;5 Myoclonus- Dystonia (M-D) syndrome is also subject of genetic studies as part of this thesis. M-D typically occurs at young age with myoclonus (jerky movements) of the head or arms, combined with dystonia of the upper body. In as much as 21-85% of M-D cases, mutations in the SGCE gene (epsilon-sarcoglycan, DYT11) can be identified. 6 An overview of the known primary dystonia and combined dystonia syndromes is presented in Table 1. In another group of dystonia patients, the secondary dystonias, dystonia is a symptom of an identified neurological condition and additional neurological deficits are often present. It was during the course of the diagnostic work-up (neurological examination, brain magnetic resonance imaging or blood tests) that an underlying cause was found. These secondary dystonias do not fall in the scope of this thesis. Primary dystonias are caused by interplay between genetic factors and environmental influences. As for the concept of interplay, we may best describe it thus: the genetic makeup (genotype) of the patient forms the risk, and (still unknown) environmental factors or developmental conditions trigger disease. This is not only the case for the sporadic forms, but it occurs also in most familial forms when a second hit is required to trigger the disease.

6 1.1 General Introduction 15 Table 1. Overview of previously discovered familial primary dystonia syndromes. Primary, pure dystonia syndromes and dystonia-plus syndromes (combined dystonia) are shown. The first column depicts a short description of the clinical picture and the DYT - number, a chronological classification of dystonia syndromes. Syndromes are clustered on phenotype. Some syndromes are only clinically characterized and no locus or gene is known. #Last column shows possible dystonia-related pathway. *Genes are recently discovered with exome sequencing. From Tanabe et al. 10 and Lohmann et al. 11, adapted. Clinical description of syndrome (DYT) MOI Gene/Protein OMIM Putative Dystonia Pathway# Pure dystonia, segmental and generalized Early-onset, generalized (DYT1) AD TOR1A/TorsinA synaptic vesicle regulation, stress response, nuclear envelope Early-onset, generalized (DYT2) AR? Early onset, whispering dysphonia (DYT4) AD TUBB4/Beta-tubulin4a* / ; Adolescent-onset, mixed phenotype (DYT6) AD THAP1/Thanatos-associated domain- containing apoptosis associated protein 1 microtubule dynamics and intracellular trafficking transcription Adult-onset, generalized/multifocal (DYT21) AD? Pure dystonia, focal and segmental Adult-onset, focal (DYT7) AD? Adolescent-onset, multifocal/segmental (DYT13) AD? Adolescent-onset, cervical and laryngeal (DYT17) AR? Adult-onset, cervical dystonia (DYT23) AD CIZ1/Cip1-interacting zinc finger protein 1* cell cycle Adult-onset, craniocervical dystonia (DYT24) AD ANO3/ Anoctamin 3* ion channel Adult-onset, cervical dystonia (DYT25) AD GNAL/Alpha subunit of G protein* dopamine (processing) Dystonia, combined with myoclonus Myoclonus-dystonia (DYT11) AD SGCE/ Epsilon-sarcoglycan synaptic vesicle regulation Myoclonus-dystonia (DYT15) AD? Dystonia, combined with parkinsonism X-linked dystonia parkinsonism; (DYT3) X-Chr TAF1/TATA box-binding protein ass. factor cell cycle Dopa-responsive dystonia; diurnal variation (DYT5/DYT14) AD GCH1/GTP cyclohydrolase dopamine (synthesis) Early-onset, generalized with parkinsonism(dyt16) AR PRKRA/ Protein kinase, interferon-inducible doublestranded RNA dependent activator stress response Rapid-onset dystonia-parkinsonism (DYT12) AD ATP1A3/ Alpha 3 subunit ofna/k ATPase ion channel Dystonia, paroxysmal Paroxysmal nonkinesigenic dyskinesia (DYT8) AD MR-1/Myofibrillo-genesis regulator stress response Paroxysmal kinesigenic dyskinesia 1 (DYT10) AD PRRT2/Proline-rich transmembrane protein synaptic vesicle regulation Paroxysmal exertion-induced dyskinesia (DYT18/DYT9) AD SLC2A1 (GLUT1)/Glucose transporter type glucose uptake Paroxysmal kinesigenic dyskinesia 2 (DYT19) AD? Paroxysmal nonkinesigenic dyskinesia 2 (DYT20) AD?

7 16 Chapter one At the start of this research project, two genes were found for primary pure dystonia (TOR1A/DYT1 7 and THAP1/DYT6 8 ) and one gene for Myoclonus Dystonia (SGCE/DYT11 9 ). TOR1A mutations account for approximately 10% of familial dystonia patients. THAP1 was identified in 2008 for DYT6 and is the cause of dystonia in around 25% of the Dutch familial primary dystonias. For focal dystonia, around 1% is associated with mutations in these two genes, while over 25% shows a positive family history of dystonia. In this sporadic focal dystonia group, the role of common variations is assessed in (small) focal dystonia cohorts using candidate gene, case-control association studies. No (strong) genetic association was found so far and results were even contradictive, which is indicative of selection bias or lack of power. Key factors missing in these studies were clinically well described cohorts of sufficient size. This leaves us with a missing heritability in familial and sporadic dystonia. Aim and outline The aim of this thesis is to shed light on the genetic architecture of primary dystonias and identify causal and phenotype-modifying genetic variants. In Chapter 1, we describe the clinical diversity of focal primary torsion dystonia. We use the detailed clinical information to endorse previously suggested relations between family history, age at onset and the site of dystonia in this single, large cohort. Furthermore, we describe how phenotypic subgroups can guide future genetic research into focal dystonia. At the time of patient selection, we encountered several familial (myoclonus) dystonia cases, which did not carry a mutation in any of the known dystonia genes. In Chapter 2, we search for new rare genetic variants in dystonia families and screen dystonia cohorts to determine the phenotype of previously discovered dystonia genes. One family presented an unique myoclonus dystonia phenotype, showing previously unknown features, including an invalidating orthostatic myoclonus in the lower extremities. Clinical and electrophysiological evaluation of affected members of this family is presented in Paragraph 2.1A. In order to identify the causal gene defect for this syndrome, we performed family genetic linkage analysis combined with exome sequencing in this three-generation family ( 2.1B). Paragraph 2.2 describes the results of linkage combined with exome sequencing in a large SGCE-negative M-D family. Aside from the discovery of new dystonia genes, the phenotypic range, clinical presentation and treatment response of mutation carriers is also important for clinical practice. Therefore, we screened the DYT6 gene THAP1 in a large dystonia subgroups.( 2.3 and 2.4) Additionally, to assess the relevance of rare variants in known dystonia genes for the rare task-specific dystonia syndrome writer s cramp (WC) we screened SGCE (DYT11), TOR1A (DYT1) and PRKRA (DYT16) in 43 WC patients.( 2.5)

8 1.1 General Introduction 17 In Chapter 3, we performed case-control association studies in adult onset, sporadic dystonia patient groups. Paragraph 3.1 looks at common variants in TOR1A, the gene responsible for DYT1 dystonia. Common variants in TOR1A were previously studied with contradictory results. To determine the relevance of TOR1A common variants in focal dystonia, we performed an association study, comparing cervical dystonia patients with controls, and subsequently carried out a meta-analysis. Changes in dopamine signalling have been involved in the pathogenesis of dystonia. We therefore performed a pathway based association study and assessed whether common variation within genes that regulate dopamine levels and in key genes of the dopamine metabolic pathway, modulate the risk for cervical dystonia.( 3.2) Cervical dystonia (CD) may be accompanied by a bilateral postural tremor of the arms. Paragraph 3.3 describes an association study with the brain-derived neurotrophic factor Val66Met variant (rs6265) in cervical dystonia patients, and studied the possible disease-modifying effect of this variant on the spread of dystonia and CD-related postural tremor. As for Chapter 4, which is the last part of this thesis, it deals with genetic variants in the twilight zone : here, we search for variants falling in between the two previously described groups. We were able to study these variants thanks to the development of new next-generation sequencing techniques. We present a pilot sequencing study testing the hypothesis that genetic variation in candidate pathways increases risk for dystonia and holds (part of) the unknown portion of genetic risk factors for dystonia. The results presented in this thesis are based on the research carried out on our Dutch dystonia patient cohort, specifically collected for the purpose of genetic research. The study was approved by the local ethics committees of the enrolled centres and all participants have given their informed consent. The author of this thesis examined and included all patients. DNA samples were stored and genotyping was performed at the Department of Genome analysis, Academic Medical Center Amsterdam.

9 18 Chapter one References (1) Conrad DF, Pinto D, Redon R et al. Origins and functional impact of copy number variation in the human genome. Nature 2010;464: (2) Manolio TA, Collins FS, Cox NJ et al. Finding the missing heritability of complex diseases. Nature 2009;461: (3) Albanese A, Bhatia K, Bressman SB et al. Phenomenology and classification of dystonia: a consensus update. Mov Disord 2013;28: (4) Butler AG, Duffey PO, Hawthorne MR, Barnes MP. An epidemiologic survey of dystonia within the entire population of northeast England over the past nine years. Adv Neurol 2004;94: (5) Fahn S, Bressman SB, Marsden CD. Classification of dystonia. Adv Neurol 1998;78:1-10. (6) Carecchio M, Magliozzi M, Copetti M et al. Defining the epsilon-sarcoglycan (SGCE) gene phenotypic signature in myoclonus-dystonia: a reappraisal of genetic testing criteria. Mov Disord 2013;28: (7) Ozelius LJ, Hewett JW, Page CE et al. The early-onset torsion dystonia gene (DYT1) encodes an ATP-binding protein. Nat Genet 1997;17: (8) Fuchs T, Gavarini S, Saunders-Pullman R et al. Mutations in the THAP1 gene are responsible for DYT6 primary torsion dystonia. Nat Genet 2009;41: (9) Zimprich A, Grabowski M, Asmus F et al. Mutations in the gene encoding epsilonsarcoglycan cause myoclonus-dystonia syndrome. Nat Genet 2001;29: (10) Tanabe LM, Kim CE, Alagem N, Dauer WT. Primary dystonia: molecules and mechanisms. Nat Rev Neurol 2009;5: (11) Lohmann K, Klein, C. Genetics of Dystonia:What s Known? What s New? What s Next? Mov Disord 2013;28: