The genetics of Tourette disorder

Identity-by-descent analysis of a large Tourette's syndrome pedigree from Costa Rica implicates genes involved in neuronal development and signal transduction

Tourette Syndrome (TS) is a heritable, early-onset neuropsychiatric disorder that typically begins in early childhood. Identifying rare genetic variants that make a significant contribution to risk in affected families may provide important insights into the molecular aetiology of this complex and heterogeneous syndrome. Here we present a whole-genome sequencing (WGS) analysis from the 11-generation pedigree (>500 individuals) of a densely affected Costa Rican family which shares ancestry from six founder pairs. By conducting an identity-by-descent (IBD) analysis using WGS data from 19 individuals from the extended pedigree we have identified putative risk haplotypes that were not seen in controls, and can be linked with four of the six founder pairs. Rare coding and non-coding variants present on the haplotypes and only seen in haplotype carriers show an enrichment in pathways such as regulation of locomotion and signal transduction, suggesting common mechanisms by which the haplotype-specific variants may be contributing to TS-risk in this pedigree. In particular we have identified a rare deleterious missense variation in RAPGEF1 on a chromosome 9 haplotype and two ultra-rare deleterious intronic variants in ERBB4 and IKZF2 on the same chromosome 2 haplotype. All three genes play a role in neurodevelopment. This study, using WGS data in a pedigree-based approach, shows the importance of investigating both coding and non-coding variants to identify genes that may contribute to disease risk. Together, the genes and variants identified on the IBD haplotypes represent biologically relevant targets for investigation in other pedigree and population-based TS data.

Cerebellin-2 regulates a serotonergic dorsal raphe circuit that controls compulsive behaviors

Cerebellin-1 (Cbln1) and cerebellin-2 (Cbln2) are secreted glycoproteins that are expressed in distinct subsets of neurons throughout the brain. Cbln1 and Cbln2 simultaneously bind to presynaptic neurexins and postsynaptic GluD1 and GluD2, thereby forming trans-synaptic adhesion complexes. Genetic associations link cerebellins, neurexins and GluD's to neuropsychiatric disorders involving compulsive behaviors, such as Tourette syndrome, attention-deficit hyperactivity disorder (ADHD), and obsessive-compulsive disorder (OCD). Extensive evidence implicates dysfunction of serotonergic signaling in these neuropsychiatric disorders. Here, we report that constitutive Cbln2 KO mice, but not Cbln1 KO mice, display robust compulsive behaviors, including stereotypic pattern running, marble burying, explosive jumping, and excessive nest building, and exhibit decreased brain serotonin levels. Strikingly, treatment of Cbln2 KO mice with the serotonin precursor 5-hydroxytryptophan or the serotonin reuptake-inhibitor fluoxetine alleviated compulsive behaviors. Conditional deletion of Cbln2 both from dorsal raphe neurons and from presynaptic neurons synapsing onto dorsal raphe neurons reproduced the compulsive behaviors of Cbln2 KO mice. Finally, injection of recombinant Cbln2 protein into the dorsal raphe of Cbln2 KO mice largely reversed their compulsive behaviors. Taken together, our results show that Cbln2 controls compulsive behaviors by regulating serotonergic circuits in the dorsal raphe.

Networks in the Field of Tourette Syndrome

Gilles de la Tourette syndrome (TS) is a neuropsychiatric neurodevelopmental disorder with the cardinal clinical features of motor and phonic tics. Clinical phenomenology can be complex since, besides tics, there are other features including premonitory urges preceding tics, pali-, echo-, and coprophenomena, hypersensitivity to external stimuli, and symptom dependency on stress, attention, and other less well-defined factors. Also, the rate of comorbidities, particularly attention deficit hyperactivity disorder and obsessive-compulsive disorder, is high. Mirroring the complexities of the clinical course and phenomenology, pathophysiological findings are very diverse, and etiology is disputed. It has become clear, though, that abnormalities in the basal ganglia and their connections with cortical areas are key for the understanding of the pathophysiology and as regards etiology, genetic factors are crucial. Against this background, both adequate clinical management of TS and TS-related research require multidisciplinary preferably international cooperation in larger groups or networks to address the multiple facets of this disorder and yield valid and useful data. In particular, large numbers of patients are needed for brain imaging and genetic studies. To meet these requirements, a number of networks and groups in the field of TS have developed over the years creating an efficient, lively, and supportive international research community. In this review, we will provide an overview of these groups and networks.

Biochemical markers of striatal desensitization in cortical-limbic hyperglutamatergic TS- & OCD-like transgenic mice

Tics and compulsions in comorbid Tourette's syndrome (TS) and obsessive-compulsive disorder (OCD) are associated with chronic hyperactivity of parallel cortico/amygdalo-striato-thalamo-cortical (CSTC) loop circuits. Comorbid TS- & OCD-like behaviors have likewise been observed in D1CT-7 mice, in which an artificial neuropotentiating transgene encoding the cAMP-elevating intracellular subunit of cholera toxin (CT) is chronically expressed selectively in somatosensory cortical & amygdalar dopamine (DA) D1 receptor-expressing neurons that activate cortico/amygdalo-striatal glutamate (GLU) output. We've now examined in D1CT-7 mice whether the chronic GLU output from their potentiated cortical/limbic CSTC subcircuit afferents associated with TS- & OCD-like behaviors elicits desensitizing neurochemical changes in the striatum (STR). Microdialysis-capillary electrophoresis and in situ hybridization reveal that the mice's chronic GLU-excited STR exhibits pharmacodynamic changes in three independently GLU-regulated measures of output neuron activation, co-excitation, and desensitization, signifying hyperactive striatal CSTC output and compensatory striatal glial and neuronal desensitization: 1) Striatal GABA, an output neurotransmitter induced by afferent GLU, is increased. 2) Striatal d-serine, a glial excitatory co-transmitter inhibited by afferent GLU, is decreased. 3) Striatal Period1 (Per1), which plays a non-circadian role in the STR as a GLU + DA D1- (cAMP-) dependent repressor thought to feedback-inhibit GLU + DA- triggered ultradian urges and motions, is transcriptionally abolished. These data imply that chronic cortical/limbic GLU excitation of the STR desensitizes its co-excitatory d-serine & DA inputs while freezing its GABA output in an active state to mediate chronic tics and compulsions - possibly in part by abolishing striatal Per1-dependent ultradian extinction of urges and motions.

Genetic Approaches to Understanding Psychiatric Disease

Human genetic studies have been the driving force in bringing to light the underlying biology of psychiatric conditions. As these studies fill in the gaps in our knowledge of the mechanisms at play, we will be better equipped to design therapies in rational and targeted ways, or repurpose existing therapies in previously unanticipated ways. This review is intended for those unfamiliar with psychiatric genetics as a field and provides a primer on different modes of genetic variation, the technologies currently used to probe them, and concepts that provide context for interpreting the gene-phenotype relationship. Like other subfields in human genetics, psychiatric genetics is moving from microarray technology to sequencing-based approaches as barriers of cost and expertise are removed, and the ramifications of this transition are discussed here. A summary is then given of recent genetic discoveries in a number of neuropsychiatric conditions, with particular emphasis on neurodevelopmental conditions. The general impact of genetics on drug development has been to underscore the extensive etiological heterogeneity in seemingly cohesive diagnostic categories. Consequently, the path forward is not in therapies hoping to reach large swaths of patients sharing a clinically defined diagnosis, but rather in targeting patients belonging to specific "biotypes" defined through a combination of objective, quantifiable data, including genotype.

Slitrk Missense Mutations Associated with Neuropsychiatric Disorders Distinctively Impair Slitrk Trafficking and Synapse Formation

Slit- and Trk-like (Slitrks) are a six-member family of synapse organizers that control excitatory and inhibitory synapse formation by forming trans-synaptic adhesions with LAR receptor protein tyrosine phosphatases (PTPs). Intriguingly, genetic mutations of Slitrks have been associated with a multitude of neuropsychiatric disorders. However, nothing is known about the neuronal and synaptic consequences of these mutations. Here, we report the structural and functional effects on synapses of various rare de novo mutations identified in patients with schizophrenia or Tourette syndrome. A number of single amino acid substitutions in Slitrk1 (N400I or T418S) or Slitrk4 (V206I or I578V) reduced their surface expression levels. These substitutions impaired glycosylation of Slitrks expressed in HEK293T cells, caused retention of Slitrks in the endoplasmic reticulum and cis-Golgi compartment in COS-7 cells and neurons, and abolished Slitrk binding to PTPδ. Furthermore, these substitutions eliminated the synapse-inducing activity of Slitrks, abolishing their functional effects on synapse density in cultured neurons. Strikingly, a valine-to-methionine mutation in Slitrk2 (V89M) compromised synapse formation activity in cultured neuron, without affecting surface transport, expression, or synapse-inducing activity in coculture assays. Similar deleterious effects were observed upon introduction of the corresponding valine-to-methionine mutation into Slitrk1 (V85M), suggesting that this conserved valine residue plays a key role in maintaining the synaptic functions of Slitrks. Collectively, these data indicate that inactivation of distinct cellular mechanisms caused by specific Slitrk dysfunctions may underlie Slitrk-associated neuropsychiatric disorders in humans, and provide a robust cellular readout for the development of knowledge-based therapies.

The Genetic Etiology of Tourette Syndrome: Large-Scale Collaborative Efforts on the Precipice of Discovery

Gilles de la Tourette Syndrome (TS) is a childhood-onset neurodevelopmental disorder that is characterized by multiple motor and phonic tics. It has a complex etiology with multiple genes likely interacting with environmental factors to lead to the onset of symptoms. The genetic basis of the disorder remains elusive. However, multiple resources and large-scale projects are coming together, launching a new era in the field and bringing us on the verge of discovery. The large-scale efforts outlined in this report are complementary and represent a range of different approaches to the study of disorders with complex inheritance. The Tourette Syndrome Association International Consortium for Genetics (TSAICG) has focused on large families, parent-proband trios and cases for large case-control designs such as genomewide association studies (GWAS), copy number variation (CNV) scans, and exome/genome sequencing. TIC Genetics targets rare, large effect size mutations in simplex trios, and multigenerational families. The European Multicentre Tics in Children Study (EMTICS) seeks to elucidate gene-environment interactions including the involvement of infection and immune mechanisms in TS etiology. Finally, TS-EUROTRAIN, a Marie Curie Initial Training Network, aims to act as a platform to unify large-scale projects in the field and to educate the next generation of experts. Importantly, these complementary large-scale efforts are joining forces to uncover the full range of genetic variation and environmental risk factors for TS, holding great promise for identifying definitive TS susceptibility genes and shedding light into the complex pathophysiology of this disorder.

Pediatric Autoimmune Disorders Associated with Streptococcal Infections and Tourette's Syndrome in Preclinical Studies

Accumulating evidence suggests that Tourette's Syndrome (TS) - a multifactorial pediatric disorder characterized by the recurrent exhibition of motor tics and/or vocal utterances - can partly depend on immune dysregulation provoked by early repeated streptococcal infections. The natural and adaptive antibody-mediated reaction to streptococcus has been proposed to potentially turn into a pathological autoimmune response in vulnerable individuals. Specifically, in conditions of increased permeability of the blood brain barrier (BBB), streptococcus-induced antibodies have been proposed to: (i) reach neuronal targets located in brain areas responsible for motion control; and (ii) contribute to the exhibition of symptoms. This theoretical framework is supported by indirect evidence indicating that a subset of TS patients exhibit elevated streptococcal antibody titers upon tic relapses. A systematic evaluation of this hypothesis entails preclinical studies providing a proof of concept of the aforementioned pathological sequelae. These studies shall rest upon individuals characterized by a vulnerable immune system, repeatedly exposed to streptococcus, and carefully screened for phenotypes isomorphic to the pathological signs of TS observed in patients. Preclinical animal models may thus constitute an informative, useful tool upon which conducting targeted, hypothesis-driven experiments. In the present review we discuss the available evidence in preclinical models in support of the link between TS and pediatric autoimmune neuropsychiatric disorders associated with streptococcus infections (PANDAS), and the existing gaps that future research shall bridge. Specifically, we report recent preclinical evidence indicating that the immune responses to repeated streptococcal immunizations relate to the occurrence of behavioral and neurological phenotypes reminiscent of TS. By the same token, we discuss the limitations of these studies: limited evidence of behavioral phenotypes isomorphic to tics and scarce knowledge about the immunological phenomena favoring the transition from natural adaptive immunity to pathological outcomes.

Association of ADORA1 rs2228079 and ADORA2A rs5751876 Polymorphisms with Gilles de la Tourette Syndrome in the Polish Population

BACKGROUND

Gilles de la Tourette syndrome (GTS) is a neurodevelopmental disorder characterized by motor and vocal tics. Hyperactivity of dopaminergic transmission is considered a prime abnormality in the pathophysiology of tics. There are reciprocal antagonistic interactions between adenosine and dopamine transmission. The aim of the study was to analyze the association of two polymorphisms, rs2228079 in ADORA1 and rs5751876 in ADORA2A, with the risk of GTS and co-morbid disorders.

MATERIAL AND METHODS

A total of 162 Polish GTS patients and 270 healthy persons were enrolled in the study. Two polymorphisms were selected on the basis of knowledge of SNPs frequencies in ADORA1 and ADORA2A. Chi-square test was used for allelic and genotypic association studies. Association of genotypes with age of tic onset was analyzed with Mann-Whitney test. Multivariate logistic regression was used to find independent predictors of GTS risk.

RESULTS

We found that the risk of GTS was associated with rs2228079 and rs5751876 polymorphisms. The GG+GT genotypes of rs2228079 in ADORA1 were underrepresented in GTS patients (p = 0.011), whereas T allele of rs5751876 in ADORA2A was overrepresented (p = 0.017). The GG genotype of rs2228079 was associated with earlier age of tic onset (p = 0.046). We found also that the minor allele G of rs2228079 was more frequent in GTS patients with depression as compared to the patients without depression (p = 0.015). Also the genotype GG was significantly more frequent in patients with obsessive compulsive disorder/behavior (OCD/OCB, p = 0.021) and depression (p = 0.032), as compared to the patients without these co-morbidities. The minor allele T frequency of rs5751876 was lower in GTS patients with co-morbid attention deficit hyperactivity disorder (p = 0.022), and TT+TC genotypes were less frequent in the non-OCD anxiety disorder group (p = 0.045).

CONCLUSION

ADORA1 and ADORA2A variants are associated with the risk of GTS, co-morbid disorders, and may affect the age of tic onset.

The role of atypical antipsychotics for treatment of Tourette's syndrome: an overview

Tourette's syndrome (TS) is a neuropsychiatric disorder of childhood onset characterized by multiple motor and phonic tics that fluctuate over time. Tic symptoms often improve by late adolescence, but some children and adults with TS may experience significant tic-related morbidity, including social and family problems, academic difficulties, and pain. When more conservative interventions are not successful, and when certain psychiatric co-morbidities further complicate the clinical profile, treating TS with an atypical antipsychotic medication may be a reasonable second-tier approach. However, the evidence supporting efficacy and safety of the atypical antipsychotics for treatment of tics is still very limited. The objective of this paper is to provide an updated overview of the role of atypical antipsychotics for treatment of TS, with evidence-based guidance on their use. Evidence for efficacy of different typical and atypical antipsychotics for treatment of tics was examined by conducting a systematic, keyword-related search of 'atypical antipsychotics' and 'Tourette's syndrome' in PubMed (National Library of Medicine, Washington, DC, USA). Four recent treatment consensus publications were also reviewed. This review focused on literature published from 2000 to 2013 and on available randomized controlled trials in TS. Evidence supporting the use of atypical antipsychotics for treatment of TS is limited. There are few randomized medication treatment trials in TS (i.e. risperidone, aripiprazole, ziprasidone), which employed varying methodologies, thereby restricting meaningful comparisons among studies. Future collaborations among clinical sites with TS expertise employing high-quality study design may better elucidate the role of atypical antipsychotics for treatment of TS.

Histidine decarboxylase knockout mice, a genetic model of Tourette syndrome, show repetitive grooming after induced fear

Tics, such as are seen in Tourette syndrome (TS), are common and can cause profound morbidity, but they are poorly understood. Tics are potentiated by psychostimulants, stress, and sleep deprivation. Mutations in the gene histidine decarboxylase (Hdc) have been implicated as a rare genetic cause of TS, and Hdc knockout mice have been validated as a genetic model that recapitulates phenomenological and pathophysiological aspects of the disorder. Tic-like stereotypies in this model have not been observed at baseline but emerge after acute challenge with the psychostimulant d-amphetamine. We tested the ability of an acute stressor to stimulate stereotypies in this model, using tone fear conditioning. Hdc knockout mice acquired conditioned fear normally, as manifested by freezing during the presentation of a tone 48h after it had been paired with a shock. During the 30min following tone presentation, knockout mice showed increased grooming. Heterozygotes exhibited normal freezing and intermediate grooming. These data validate a new paradigm for the examination of tic-like stereotypies in animals without pharmacological challenge and enhance the face validity of the Hdc knockout mouse as a pathophysiologically grounded model of tic disorders.

Corticostriatal interactions in the generation of tic-like behaviors after local striatal disinhibition

The pathophysiology of the tics that define Gilles de la Tourette syndrome (TS) is not well understood. Local disinhibition within the striatum has been hypothesized to play a pathogenic role. In support of this, experimental disinhibition by local antagonism of GABA-A receptors within the striatum produces tic-like phenomenology in monkey and rat. We replicated this effect in mice via local picrotoxin infusion into the dorsal striatum. Infusion of picrotoxin into sensorimotor cortex produced similar movements, accompanied by signs of behavioral activation; higher-dose picrotoxin in the cortex produced seizures. Striatal inhibition with local muscimol completely abolished tic-like movements after either striatal or cortical picrotoxin, confirming their dependence on the striatal circuitry; in contrast, cortical muscimol attenuated but did not abolish movements produced by striatal picrotoxin. Striatal glutamate blockade eliminated tic-like movements after striatal picrotoxin, indicating that glutamatergic afferents are critical for their generation. These studies replicate and extend previous work in monkey and rat, providing additional validation for the local disinhibition model of tic generation. Our results reveal a key role for corticostriatal glutamatergic afferents in the generation of tic-like movements in this model.

The Tourette International Collaborative Genetics (TIC Genetics) study, finding the genes causing Tourette syndrome: objectives and methods

Tourette syndrome (TS) is a neuropsychiatric disorder characterized by recurrent motor and vocal tics, often accompanied by obsessive-compulsive disorder and/or attention-deficit/hyperactivity disorder. While the evidence for a genetic contribution is strong, its exact nature has yet to be clarified fully. There is now mounting evidence that the genetic risks for TS include both common and rare variants and may involve complex multigenic inheritance or, in rare cases, a single major gene. Based on recent progress in many other common disorders with apparently similar genetic architectures, it is clear that large patient cohorts and open-access repositories will be essential to further advance the field. To that end, the large multicenter Tourette International Collaborative Genetics (TIC Genetics) study was established. The goal of the TIC Genetics study is to undertake a comprehensive gene discovery effort, focusing both on familial genetic variants with large effects within multiply affected pedigrees and on de novo mutations ascertained through the analysis of apparently simplex parent-child trios with non-familial tics. The clinical data and biomaterials (DNA, transformed cell lines, RNA) are part of a sharing repository located within the National Institute for Mental Health Center for Collaborative Genomics Research on Mental Disorders, USA, and will be made available to the broad scientific community. This resource will ultimately facilitate better understanding of the pathophysiology of TS and related disorders and the development of novel therapies. Here, we describe the objectives and methods of the TIC Genetics study as a reference for future studies from our group and to facilitate collaboration between genetics consortia in the field of TS.

Dysregulated intracellular signaling in the striatum in a pathophysiologically grounded model of Tourette syndrome

Tic disorders produce substantial morbidity, but their pathophysiology remains poorly understood. Convergent evidence suggests that dysregulation of the cortico-basal ganglia circuitry is central to the pathogenesis of tics. Tourette syndrome (TS), the most severe end of the continuum of tic disorders, is substantially genetic, but causative mutations have been elusive. We recently described a mouse model, the histidine decarboxylase (Hdc) knockout mouse, that recapitulates a rare, highly penetrant mutation found in a single family; these mice exhibit TS-like phenomenology. These animals have a global deficit in brain histamine and a consequent dysregulation of DA in the basal ganglia. Histamine modulation of DA effects is increasingly appreciated, but the mechanisms underlying this modulation remain unclear; the consequences of modest DA elevation in the context of profound HA deficiency are difficult to predict, but understanding them in the Hdc knockout mouse may provide generalizable insights into the pathophysiology of TS. Here we characterized signaling pathways in striatal cells in this model system, at baseline and after amphetamine challenge. In vivo microdialysis confirms elevated DA in Hdc-KO mice. We find dephosphorylation of Akt and its target GSK3β and activation of the MAPK signaling cascade and its target rpS6; these are characteristic of the effects of DA on D2- and D1-expressing striatal neurons, respectively. Strikingly, there is no alteration in mTOR signaling, which can be regulated by DA in both cell types. These cellular effects help elucidate striatal signaling abnormalities in a uniquely validated mouse model of TS and move towards the identification of new potential therapeutic targets for tic disorders.

Association of the KCNJ5 gene with Tourette Syndrome and Attention-Deficit/Hyperactivity Disorder

Linkage and association of Tourette Syndrome (TS) and Attention-Deficit/Hyperactivity Disorder (ADHD) have previously been reported in the 11q24 chromosomal region. To identify the risk gene within the region we studied the potassium inwardly-rectifying channel J5 (KCNJ5) gene in a sample of 170 nuclear families with TS. We genotyped eight markers across the gene and observed biased transmission of haplotypes from parents to probands in this sample. We then tested these markers in an independent sample of 242 nuclear families with ADHD and found the same haplotype was significantly over transmitted to ADHD probands. Screening of the coding region of KCNJ5 in 48 probands with TS did not identify any variation that could explain the association of the haplotype. We also genotyped two microsatellite markers, one in the promoter and the other in the 3' region and found no evidence for association for either marker for TS, however, we found significant evidence for association with the 3' repeat and ADHD. A small gene (c11orf45) of unknown function lies within the first intron of KCNJ5 that is transcribed in the opposite orientation and this gene may regulate the expression of KCNJ5. We studied the correlation of the expression of KCNJ5 and the antisense transcript in brain tissues from control individuals and found that the antisense transcript and the short KCNJ5 isoform are co-expressed in three brain regions. The results of this study indicate that KCNJ5 is associated with TS and ADHD in our samples, however, the functional variant(s) remain to be identified.

A behavioural test battery to investigate tic-like symptoms, stereotypies, attentional capabilities, and spontaneous locomotion in different mouse strains

The preclinical study of human disorders associated with comorbidities and for which the aetiology is still unclear may substantially benefit from multi-strain studies conducted in mice. The latter can help isolating experimental populations (strains) exhibiting distinct facets in the parameters isomorphic to the symptoms of a given disorder. Through a reverse-translation approach, multi-strain studies can inform both natural predisposing factors and environmental modulators. Thus, mouse strains selected for a particular trait may be leveraged to generate hypothesis-driven studies aimed at clarifying the potential role played by the environment in modulating the exhibition of the symptoms of interest. Tourette's syndrome (TS) constitutes a paradigmatic example whereby: it is characterized by a core symptom (tics) often associated with comorbidities (attention-deficit-hyperactivity and obsessive-compulsive symptoms); it has a clear genetic origin though specific genes are, as yet, unidentified; its course (exacerbations and remissions) is under the influence of environmental factors. Based on these considerations, we tested four mouse strains (ABH, C57, CD1, and SJL) - varying along a plethora of behavioural, neurochemical, and immunological parameters - on a test battery tailored to address the following domains: tics (through the i.p. administration of the selective 5-HT2 receptor agonist DOI, 5mg/kg); locomotion (spontaneous locomotion in the home-cage); perseverative responding in an attentional set shifting task; and behavioural stereotypies in response to a single amphetamine (10mg/kg, i.p.) injection. Present data demonstrate that while ABH and SJL mice respectively exhibit selective increments in amphetamine-induced sniffing behaviour and DOI-induced tic-like behaviours, C57 and CD1 mice show a distinct phenotype, compared to other strains, in several parameters.

Histidine decarboxylase deficiency causes tourette syndrome: parallel findings in humans and mice

Tourette syndrome (TS) is characterized by tics, sensorimotor gating deficiencies, and abnormalities of cortico-basal ganglia circuits. A mutation in histidine decarboxylase (Hdc), the key enzyme for the biosynthesis of histamine (HA), has been implicated as a rare genetic cause. Hdc knockout mice exhibited potentiated tic-like stereotypies, recapitulating core phenomenology of TS; these were mitigated by the dopamine (DA) D2 antagonist haloperidol, a proven pharmacotherapy, and by HA infusion into the brain. Prepulse inhibition was impaired in both mice and humans carrying Hdc mutations. HA infusion reduced striatal DA levels; in Hdc knockout mice, striatal DA was increased and the DA-regulated immediate early gene Fos was upregulated. DA D2/D3 receptor binding was altered both in mice and in humans carrying the Hdc mutation. These data confirm histidine decarboxylase deficiency as a rare cause of TS and identify HA-DA interactions in the basal ganglia as an important locus of pathology.

Practice parameter for the assessment and treatment of children and adolescents with tic disorders

Tic disorders, including Tourette's disorder, present with a wide range of symptom severity and associated comorbidity. This Practice Parameter reviews the evidence from research and clinical experience in the evaluation and treatment of pediatric tic disorders. Recommendations are provided for a comprehensive evaluation to include common comorbid disorders and for a hierarchical approach to multimodal interventions.

Common and rare alleles of the serotonin transporter gene, SLC6A4, associated with Tourette's disorder

To evaluate the hypothesis that functionally over-expressing alleles of the serotonin transporter (SERT) gene (solute carrier family 6, member 4, SLC6A4) are present in Tourette's disorder (TD), just as we previously observed in obsessive compulsive disorder (OCD), we evaluated TD probands (N = 151) and controls (N = 858). We genotyped the refined SERT-linked polymorphic region 5-HTTLPR/rs25531 and the associated rs25532 variant in the SLC6A4 promoter plus the rare coding variant SERT isoleucine-to-valine at position 425 (I425V). The higher expressing 5-HTTLPR/rs25531 LA allele was more prevalent in TD probands than in controls (χ(2)  = 5.75; P = 0.017; odds ratio [OR], 1.35); and, in a secondary analysis, surprisingly, it was significantly more frequent in probands who had TD alone than in those who had TD plus OCD (Fisher's exact test; P = 0.0006; OR, 2.29). Likewise, the higher expressing LAC haplotype (5-HTTLPR/rs25531/rs25532) was more frequent in TD probands than in controls (P = 0.024; OR, 1.33) and also in the TD alone group versus the TD plus OCD group (P = 0.0013; OR, 2.14). Furthermore, the rare gain-of-function SERT I425V variant was observed in 3 male siblings with TD and/or OCD and in their father. Thus, the cumulative count of SERT I425V becomes 1.57% in OCD/TD spectrum conditions versus 0.15% in controls, with a recalculated, family-adjusted significance of χ(2) = 15.03 (P < 0.0001; OR, 9.0; total worldwide genotyped, 2914). This report provides a unique combination of common and rare variants in one gene in TD, all of which are associated with SERT gain of function. Thus, altered SERT activity represents a potential contributor to serotonergic abnormalities in TD. The present results call for replication in a similarly intensively evaluated sample. © 2013 Movement Disorder Society.

Theoretical and practical considerations behind the use of laboratory animals for the study of Tourette syndrome

In the present manuscript we review a substantial body of literature describing several pre-clinical animal models designed and developed with the purpose of investigating the biological determinants of Tourette syndrome (TS). In order to map the animal models onto the theoretical background upon which they have been devised, we first define phenomenological and etiological aspects of TS and then match this information to the available pre-clinical models. Thus, we first describe the characteristic symptoms exhibited by TS patients and then a series of hypotheses attempting to identify the multifactorial causes of TS. With respect to the former, we detail the phenomenology of abnormal repetitive behaviors (tics and stereotypies), obsessive-compulsive behaviors and aberrant sensory-motor gating. With respect to the latter, we describe both potential candidate vulnerability genes and environmental factors (difficult pregnancies, psychosocial stressors and infections). We then discuss how this evidence has been translated in pre-clinical research with respect to both dependent (symptoms) and independent (etiological factors) variables. Thus, while, on the one hand, we detail the methodologies adopted to measure abnormal repetitive and obsessive-compulsive behaviors, and sensory-motor gating, on the other hand, we describe genetic engineering studies and environmental modulations aimed at reproducing the proposed biological determinants in laboratory rodents. A special emphasis is placed upon "programming" events, occurring during critical stages of early development and exerting organizational delayed consequences. In the final section, we outline a heuristic model with the purpose of integrating clinical and pre-clinical evidence in the study of TS.

CNV analysis in Tourette syndrome implicates large genomic rearrangements in COL8A1 and NRXN1

Tourette syndrome (TS) is a neuropsychiatric disorder with a strong genetic component. However, the genetic architecture of TS remains uncertain. Copy number variation (CNV) has been shown to contribute to the genetic make-up of several neurodevelopmental conditions, including schizophrenia and autism. Here we describe CNV calls using SNP chip genotype data from an initial sample of 210 TS cases and 285 controls ascertained in two Latin American populations. After extensive quality control, we found that cases (N = 179) have a significant excess (P = 0.006) of large CNV (>500 kb) calls compared to controls (N = 234). Amongst 24 large CNVs seen only in the cases, we observed four duplications of the COL8A1 gene region. We also found two cases with ∼400 kb deletions involving NRXN1, a gene previously implicated in neurodevelopmental disorders, including TS. Follow-up using multiplex ligation-dependent probe amplification (and including 53 more TS cases) validated the CNV calls and identified additional patients with rearrangements in COL8A1 and NRXN1, but none in controls. Examination of available parents indicates that two out of three NRXN1 deletions detected in the TS cases are de-novo mutations. Our results are consistent with the proposal that rare CNVs play a role in TS aetiology and suggest a possible role for rearrangements in the COL8A1 and NRXN1 gene regions.

Genetic susceptibility and neurotransmitters in Tourette syndrome

Family studies have consistently shown that Tourette syndrome (TS) is a familial disorder and twin studies have clearly indicated a genetic contribution in the etiology of TS. Whereas early segregation studies of TS suggested a single-gene autosomal dominant disorder, later studies have pointed to more complex models including additive and multifactorial inheritance and likely interaction with genetic factors. While the exact cellular and molecular base of TS is as yet elusive, neuroanatomical and neurophysiological studies have pointed to the involvement of cortico-striato-thalamocortical circuits and abnormalities in dopamine, glutamate, gamma-aminobutyric acid, and serotonin neurotransmitter systems, with the most consistent evidence being available for involvement of dopamine-related abnormalities, that is, a reduction in tonic extracellular dopamine levels along with hyperresponsive spike-dependent dopamine release, following stimulation. Genetic and gene expression findings are very much supportive of involvement of these neurotransmitter systems. Moreover, intriguingly, genetic work on a two-generation pedigree has opened new research pointing to a role for histamine, a so far rather neglected neurotransmitter, with the potential of the development of new treatment options. Future studies should be aimed at directly linking neurotransmitter-related genetic and gene expression findings to imaging studies (imaging genetics), which enables a better understanding of the pathways and mechanisms through which the dynamic interplay of genes, brain, and environment shapes the TS phenotype.

Animal models recapitulating the multifactorial origin of Tourette syndrome

Tourette Syndrome (TS) is a neurological disorder characterized by motor and phonic tics affecting approximately 1% of the pediatric population. Behavioral comorbidities often include obsessive-compulsive behavior and impaired attention. The neurobiological substrates associated with TS generally entail abnormalities in neurotransmitter circuitry regulating basal ganglia activity. The neurotransmitters most often associated with TS are dopamine, serotonin, and GABA. TS origin roots in genetic predisposing factors, and environmental variables favoring tic onset and exacerbation. Among the latter, repeated infections with group A beta-hemolytic Streptococcus and psychosocial stressors encountered during development have been proposed to constitute likely susceptibility factors. In this chapter, we describe how this clinical/epidemiological knowledge has been translated into animal models of TS. Specifically, we review several studies attempting to reproduce TS-like symptoms (tics and behavioral stereotypies) and comorbidities (impaired attention, increased locomotion, and perseverative responding) in laboratory rodents. Additionally, we discuss studies in which the genetic and environmental predisposing factors have been modeled in experimental subjects. Ultimately, we propose a unifying perspective recapitulating dependent and independent variables in the preclinical study of TS and discuss its potential theoretical and heuristic implications.

Tourette syndrome and other tic disorders in childhood, adolescence and adulthood

BACKGROUND

Tourette syndrome is a combined motor and vocal tic disorder that begins in childhood and takes a chronic course. It arises in about 1% of all children, with highly varying severity. Transient and usually mild tics are seen in as many as 15% of all children in elementary school. The diagnosis is often delayed by several years.

METHODS

We selectively reviewed the pertinent literature, including the guidelines of the European Society for the Study of Tourette Syndrome for the diagnosis and treatment of tic disorders.

RESULTS

Tic disorders usually take a benign course, with spontaneous improvement in adolescence in about 90% of patients. Psychoeducation is the basis of treatment in each case and almost always brings marked emotional relief. Specific treatment is needed only for more severe tics and those that cause evident psychosocial impairment. 80-90% of patients with Tourette syndrome have comorbidities (attention deficit-hyperactivity disorder, obsessive-compulsive disorder, depression, anxiety, emotional dysregulation, autoaggression), which often impair their quality of life more than the tics do and therefore become the main target of treatment. There is little evidence for the efficacy of treatment for tics. Small-scale controlled studies with a brief follow-up period have been carried out for some neuroleptic drugs. Behavior therapy should be tried before drug treatment. A further option for very severely affected adults is deep brain stimulation.

CONCLUSION

Because of the low level of the available evidence, no definitive recommendations can be made for the treatment of tics.

Long term clinical course of Tourette syndrome

BACKGROUND

Recent studies using cluster analysis and factor analysis have suggested that Tourette Syndrome (TS) should no longer be considered a unitary condition.

MATERIAL AND METHODS

We retrospectively studied the long term clinical course of 100 TS patients. The patients were assessed at the onset and after 10 years follow-up to evaluate the severity of tic, the Obsessive Compulsive Disorder (OCD), the Attention Deficit Hyperactivity Disorder (ADHD) and the presence of anxiety and depression, rage attacks, self injuries behavior. Moreover at the follow-up they completed an evaluation scale on quality of life to assess the impairment in everyday life after 10 years of illness.

RESULTS

The "pure TS" clinical group (38 subjects) showed after 10 years follow-up that 58% carried on with the same clinical phenotype, whereas 42% changed in "TS+OCD" phenotype. Fifty-five percentage required pharmacological treatment. All the "TS+ADHD" clinical group (48 subjects) showed after 10 years follow-up a different clinical phenotype: 62% "TS pure" phenotype, 35% "TS+OCD" phenotype, 2% "TS+ADHD+OCD" phenotype. Sixty-five percentage of the subject required pharmacological treatment. The "TS+ADHD+OCD" clinical group (14 subjects) after 10 years follow-up showed that 14% carried on with the same clinical phenotype, whereas 8.3% presented "TS pure" phenotype and 92% presented "TS+OCD" phenotype. Seventy-one percentage were in need of therapy. With regards to quality of life, patients presented widespread impairment correlated to the presence of comorbid conditions.

CONCLUSION

Our findings suggest that pure TS has quite a good long-term clinical course. By contrast, those who presented comorbid condition at the onset showed a more severe prognosis.

Pathogenetic model for Tourette syndrome delineates overlap with related neurodevelopmental disorders including Autism

Tourette syndrome (TS) is a highly heritable neuropsychiatric disorder characterised by motor and vocal tics. Despite decades of research, the aetiology of TS has remained elusive. Recent successes in gene discovery backed by rapidly advancing genomic technologies have given us new insights into the genetic basis of the disorder, but the growing collection of rare and disparate findings have added confusion and complexity to the attempts to translate these findings into neurobiological mechanisms resulting in symptom genesis. In this review, we explore a previously unrecognised genetic link between TS and a competing series of trans-synaptic complexes (neurexins (NRXNs), neuroligins (NLGNs), leucine-rich repeat transmembrane proteins (LRRTMs), leucine rich repeat neuronals (LRRNs) and cerebellin precursor 2 (CBLN2)) that links it with autism spectrum disorder through neurodevelopmental pathways. The emergent neuropathogenetic model integrates all five genes so far found to be uniquely disrupted in TS into a single pathogenetic chain of events described in context with clinical and research implications.

Evaluation of the LIM homeobox genes LHX6 and LHX8 as candidates for Tourette syndrome

The etiology and pathophysiology of Tourette Syndrome (TS) remain poorly understood. Multiple lines of evidence suggest that a complex genetic background and the cortico-striato-thalamo-cortical circuit are involved. The role of Lhx6 and Lhx8 in the development of the striatal interneurons, prompted us to investigate them as novel candidate genes for TS. We performed a comparative study of the expression of Lhx6 and Lhx8 and investigated genetic association with TS using two samples of trios (TSGeneSEE and German sample - 222 families). We show that Lhx6 and Lhx8 expression in the forebrain is evolutionarily conserved, underlining their possible importance in TS-related pathophysiological pathways. Our tagging-single nucleotide polymorphism (tSNP)-based association analysis was negative for association with LHX8. However, we found positive association with LHX6 in the TSGeneSEE sample (corrected P-value = 0.006 for three-site haplotype around SNP rs3808901) but no association in the sample of German families. Interestingly, the SNP allele that was identified to be significantly associated in the TSGeneSEE dataset, showed an opposite trend of transmission in the German dataset. Our analysis of the correlation of the LHX6 region with individual ancestry within Europe, revealed the fact that this particular SNP demonstrates a high degree of population differentiation and is correlated with the North to South axis of European genetic variation. Our results indicate that further study of the LHX6 gene in relation to the TS phenotype is warranted and suggest the intriguing hypothesis that different genetic factors may contribute to the etiology of TS in different populations, even within Europe.

Rare copy number variants in tourette syndrome disrupt genes in histaminergic pathways and overlap with autism

BACKGROUND

Studies of copy number variation (CNV) have characterized loci and molecular pathways in a range of neuropsychiatric conditions. We analyzed rare CNVs in Tourette syndrome (TS) to identify novel risk regions and relevant pathways, to evaluate burden of structural variation in cases versus controls, and to assess overlap of identified variations with those in other neuropsychiatric syndromes.

METHODS

We conducted a case-control study of 460 individuals with TS, including 148 parent-child trios and 1131 controls. CNV analysis was undertaken using 370 K to 1 M probe arrays, and genotyping data were used to match cases and controls for ancestry. CNVs present in < 1% of the population were evaluated.

RESULTS

While there was no significant increase in the number of de novo or transmitted rare CNVs in cases versus controls, pathway analysis using multiple algorithms showed enrichment of genes within histamine receptor (subtypes 1 and 2) signaling pathways (p = 5.8 × 10(-4) - 1.6 × 10(-2)), as well as axon guidance, cell adhesion, nervous system development, and synaptic structure and function processes. Genes mapping within rare CNVs in TS showed significant overlap with those previously identified in autism spectrum disorders but not intellectual disability or schizophrenia. Three large, likely pathogenic, de novo events were identified, including one disrupting multiple gamma-aminobutyric acid receptor genes.

CONCLUSIONS

We identify further evidence supporting recent findings regarding the involvement of histaminergic and gamma-aminobutyric acidergic mechanisms in the etiology of TS and show an overlap of rare CNVs in TS and autism spectrum disorders.

Genetic animal models of Tourette syndrome: The long and winding road from lab to clinic

Tourette syndrome (TS) is a disabling neuropsychiatric disorder characterised by persistent motor and vocal tics. TS is a highly comorbid state, hence, patients might experience anxiety, obsessions, compulsions, sleep abnormalities, depression, emotional liability, learning problems, and attention deficits in addition to tics. In spite of its complex heterogeneous genetic aetiology, recent studies highlighted a strong link between TS and genetic lesions in the HDC (L-histidine decarboxylase) gene, which encodes the enzyme that synthetises histamine, and the SLITRK1 (SLIT and TRK-like family member 1) gene, which encodes a transmembrane protein that was found to regulate neurite outgrowth. In addition to validating the contribution of a specific genetic aberration to the development of a particular pathology, animal models are crucial to dissect the function of disease-linked proteins, expose disease pathways through examination of genetic modifiers and discover as well as assess therapeutic strategies. Mice with a knockout of either Hdc or Slitrk1 exhibit anxiety and those lacking Hdc, display dopamine agonist-triggered stereotypic movements. However, the mouse knockouts do not spontaneously display tics, which are recognised as the hallmark of TS. In this review, we explore the features of the present genetic animal models of TS and identify reasons for their poor resemblance to the human condition. Importantly, we highlight ways forward aimed at developing a valuable genetic model of TS or a model that has good predictive validity in developing therapeutic drugs for the treatment of tics, hence potentially accelerating the arduous journey from lab to clinic.