Support of the histaminergic hypothesis in Tourette syndrome: association of the histamine decarboxylase gene in a large sample of families

From Genetics to Epigenetics: New Perspectives in Tourette Syndrome Research

Gilles de la Tourette Syndrome (TS) is a neurodevelopmental disorder marked by the appearance of multiple involuntary motor and vocal tics. TS presents high comorbidity rates with other disorders such as attention deficit hyperactivity disorder (ADHD) and obsessive compulsive disorder (OCD). TS is highly heritable and has a complex polygenic background. However, environmental factors also play a role in the manifestation of symptoms. Different epigenetic mechanisms may represent the link between these two causalities. Epigenetic regulation has been shown to have an impact in the development of many neuropsychiatric disorders, however very little is known about its effects on Tourette Syndrome. This review provides a summary of the recent findings in genetic background of TS, followed by an overview on different epigenetic mechanisms, such as DNA methylation, histone modifications, and non-coding RNAs in the regulation of gene expression. Epigenetic studies in other neurological and psychiatric disorders are discussed along with the TS-related epigenetic findings available in the literature to date. Moreover, we are proposing that some general epigenetic mechanisms seen in other neuropsychiatric disorders may also play a role in the pathogenesis of TS.

Discovery and Preclinical Validation of [(11)C]AZ13153556, a Novel Probe for the Histamine Type 3 Receptor

The histamine type 3 receptor (H3) is a G protein-coupled receptor implicated in several disorders of the central nervous system. Herein, we describe the radiolabeling and preclinical evaluation of a candidate radioligand for the H3 receptor, 4-(1S,2S)-2-(4-cyclobutylpiperazine-1-carbonyl)cyclopropyl]-N-methyl-benzamide (5), and its comparison with one of the frontrunner radioligands for H3 imaging, namely, GSK189254 (1). Compounds 1 and 5 were radiolabeled with tritium and carbon-11 for in vitro and in vivo imaging experiments. The in vitro binding of [(3)H]1 and [(3)H]5 was examined by (i) saturation binding to rat and nonhuman primate brain tissue homogenate and (ii) in vitro autoradiography on tissue sections from rat, guinea pig, and human brain. The in vivo binding of [(11)C]1 and [(11)C]5 was examined by PET imaging in mice and nonhuman primates. Bmax values obtained from Scatchard analysis of [(3)H]1 and [(3)H]5 binding were in good agreement. Autoradiography with [(3)H]5 on rat, guinea pig, and human brain slices showed specific binding in regions known to be enhanced in H3 receptors, a high degree of colocalization with [(3)H]1, and virtually negligible nonspecific binding in tissue. PET measurements in mice and nonhuman primates demonstrated that [(11)C]5 binds specifically and reversibly to H3 receptors in vivo with low nonspecific binding in brain tissue. Whereas [(11)C]1 showed similar binding characteristics in vivo, the binding kinetics appeared faster for [(11)C]5 than for [(11)C]1.

CONCLUSIONS

[(11)C]5 has suitable properties for quantification of H3 receptors in nonhuman primate brain and has the potential to offer improved binding kinetics in man compared to [(11)C]1.

Recent Advances in Understanding and Managing Tourette Syndrome

Tourette syndrome (TS) is a neurologic and behavioral disorder consisting of motor and phonic tics with onset in childhood or adolescence. The severity of tics can range from barely perceptible to severely impairing due to social embarrassment, discomfort, self-injury, and interference with daily functioning and school or work performance. In addition to tics, most patients with TS have a variety of behavioral comorbidities, including attention deficit hyperactivity disorder and obsessive-compulsive disorder. Studies evaluating the pathophysiology of tics have pointed towards dysfunction of the cortico-striato-thalamo-cortical circuit, but the mechanism of this hyperkinetic movement disorder is not well understood. Treatment of TS is multidisciplinary, typically involving behavioral therapy, oral medications, and botulinum toxin injections. Deep brain stimulation may be considered for “malignant” TS that is refractory to conventional therapy. In this review, we will highlight recent developments in the understanding and management strategies of TS.

Functional Evaluations of Genes Disrupted in Patients with Tourette's Disorder

Tourette's disorder (TD) is a highly heritable neurodevelopmental disorder with complex genetic architecture and unclear neuropathology. Disruptions of particular genes have been identified in subsets of TD patients. However, none of the findings have been replicated, probably due to the complex and heterogeneous genetic architecture of TD that involves both common and rare variants. To understand the etiology of TD, functional analyses are required to characterize the molecular and cellular consequences caused by mutations in candidate genes. Such molecular and cellular alterations may converge into common biological pathways underlying the heterogeneous genetic etiology of TD patients. Herein, we review specific genes implicated in TD etiology, discuss the functions of these genes in the mammalian central nervous system and the corresponding behavioral anomalies exhibited in animal models, and importantly, review functional analyses that can be performed to evaluate the role(s) that the genetic disruptions might play in TD. Specifically, the functional assays include novel cell culture systems, genome editing techniques, bioinformatics approaches, transcriptomic analyses, and genetically modified animal models applied or developed to study genes associated with TD or with other neurodevelopmental and neuropsychiatric disorders. By describing methods used to study diseases with genetic architecture similar to TD, we hope to develop a systematic framework for investigating the etiology of TD and related disorders.

"Hyperglutamatergic cortico-striato-thalamo-cortical circuit" breaker drugs alleviate tics in a transgenic circuit model of Tourette׳s syndrome

The brain circuits underlying tics in Tourette׳s syndrome (TS) are unknown but thought to involve cortico/amygdalo-striato-thalamo-cortical (CSTC) loop hyperactivity. We previously engineered a transgenic mouse "circuit model" of TS by expressing an artificial neuropotentiating transgene (encoding the cAMP-elevating, intracellular A1 subunit of cholera toxin) within a small population of dopamine D1 receptor-expressing somatosensory cortical and limbic neurons that hyperactivate cortico/amygdalostriatal glutamatergic output circuits thought to be hyperactive in TS and comorbid obsessive-compulsive (OC) disorders. As in TS, these D1CT-7 ("Ticcy") transgenic mice׳s tics were alleviated by the TS drugs clonidine and dopamine D2 receptor antagonists; and their chronic glutamate-excited striatal motor output was unbalanced toward hyperactivity of the motoric direct pathway and inactivity of the cataleptic indirect pathway. Here we have examined whether these mice׳s tics are countered by drugs that "break" sequential elements of their hyperactive cortical/amygdalar glutamatergic and efferent striatal circuit: anti-serotonoceptive and anti-noradrenoceptive corticostriatal glutamate output blockers (the serotonin 5-HT2a,c receptor antagonist ritanserin and the NE alpha-1 receptor antagonist prazosin); agmatinergic striatothalamic GABA output blockers (the presynaptic agmatine/imidazoline I1 receptor agonist moxonidine); and nigrostriatal dopamine output blockers (the presynaptic D2 receptor agonist bromocriptine). Each drug class alleviates tics in the Ticcy mice, suggesting a hyperglutamatergic CSTC "tic circuit" could exist in TS wherein cortical/amygdalar pyramidal projection neurons׳ glutamatergic overexcitation of both striatal output neurons and nigrostriatal dopaminergic modulatory neurons unbalances their circuit integration to excite striatothalamic output and create tics, and illuminating new TS drug strategies.

Histamine and histamine receptors in Tourette syndrome and other neuropsychiatric conditions

The potential contributions of dysregulation of the brain's histaminergic modulatory system to neuropsychiatric disease, and the potential of histamine-targeting medications as therapeutic agents, are gradually coming into focus. The H3R receptor, which is expressed primarily in the central nervous system, is a promising pharmacotherapeutic target. Recent evidence for a contribution of histamine dysregulation to Tourette syndrome and tic disorders is particularly strong; although specific mutations in histamine-associated genes are rare, they have led to informative studies in animal models that may pave the way for therapeutic advances. A controlled study of an H3R antagonist in Tourette syndrome is ongoing. Preclinical studies of H3R antagonists in schizophrenia, attention deficit disorder, and narcolepsy have all shown promise. Recently reported controlled studies have been disappointing in schizophrenia and attention deficit disorder, but the H3R antagonist pitolisant shows promise in the treatment of narcolepsy and excessive daytime sleepiness and is currently under regulatory review for these conditions. This article is part of the Special Issue entitled 'Histamine Receptors'.

Interactions of the histamine and hypocretin systems in CNS disorders

Histamine and hypocretin neurons are localized to the hypothalamus, a brain area critical to autonomic function and sleep. Narcolepsy type 1, also known as narcolepsy with cataplexy, is a neurological disorder characterized by excessive daytime sleepiness, impaired night-time sleep, cataplexy, sleep paralysis and short latency to rapid eye movement (REM) sleep after sleep onset. In narcolepsy, 90% of hypocretin neurons are lost; in addition, two groups reported in 2014 that the number of histamine neurons is increased by 64% or more in human patients with narcolepsy, suggesting involvement of histamine in the aetiology of this disorder. Here, we review the role of the histamine and hypocretin systems in sleep-wake modulation. Furthermore, we summarize the neuropathological changes to these two systems in narcolepsy and discuss the possibility that narcolepsy-associated histamine abnormalities could mediate or result from the same processes that cause the hypocretin cell loss. We also review the changes in the hypocretin and histamine systems, and the associated sleep disruptions, in Parkinson disease, Alzheimer disease, Huntington disease and Tourette syndrome. Finally, we discuss novel therapeutic approaches for manipulation of the histamine system.

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.

Tourette Syndrome research highlights 2014

About 200 journal articles reported research on Tourette syndrome and other tic disorders in 2014. Here we briefly summarize a few of the reports that seemed most important or interesting, ranging from animal models to human studies. Readers can comment on our choices or provide their own favorites using the tools on the online article.

Tourette syndrome and comorbid ADHD: causes and consequences

Attention deficit hyperactivity disorder (ADHD) is the most common comorbid condition in patients with Tourette syndrome (TS). The co-occurrence of ADHD and TS is in most cases associated with a higher social and psychopathological impairment. Comorbidity between Tourette and ADHD appears to have a complex and partially known pathogenesis in which genetic, environmental, and neurobiological factors can be implicated. Genetic studies have revealed an involvement of dopaminergic, catecholaminergic, and GABAergic genes that modulated the activity of neurotransmitters. Furthermore, there are a lot of networks implicated in the development of ADHD and TS, involving cortical and striatal areas and basal ganglia. Although a large number of studies tried to find a common pathogenesis, the complex pathways responsible are not clear. The genes implicated in both disorders are currently unidentified, but it is probable that epigenetic factors associated with neural modifications can represent a substrate for the development of the diseases.

CONCLUSION

In this paper, recent advances in neurobiology of ADHD and TS are reviewed, providing a basis for understanding the complex common pathogenesis underlying the frequent co-occurrence of the two conditions and the therapeutic choices.

Neurobehavioral aspects, pathophysiology, and management of Tourette syndrome

PURPOSE OF REVIEW

This update summarizes progress in understanding Tourette syndrome clinical characteristics, etiology, and treatment over the past year.

RECENT FINDINGS

Premonitory sensory phenomena were found to have important impacts on Tourette syndrome quality of life. A rare genetic form of Tourette syndrome due to L-histidine-decarboxylase mutation, with similar features in human and rodent, has inspired new research on functional anatomy of Tourette syndrome. In response to new data, treatment guidelines have been revised to include behavioral therapy as first-line treatment. Novel dopamine receptor antagonists aripiprazole and ecopipam have shown potential efficacy - as well as tolerability concerns. Recent work has suggested efficacy and tolerability of topiramate and fluphenazine, but more rigorous studies are needed to further understand their role in Tourette syndrome management. Recent consensus guidelines explain when deep brain stimulation can be considered for severe refractory cases under a multidisciplinary team.

SUMMARY

More research is needed to identify better tolerated treatments for, to understand pathophysiology or functional anatomy of, and to predict or influence longitudinal outcome of Tourette syndrome.

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.

Updates in medical and surgical therapies for Tourette syndrome

Tourette syndrome is a complex neurobehavioral disorder defined by multiple motor and at least 1 vocal tic, persisting over 1 year, waxing and waning in severity, and not explained by another condition. The condition may range from mild nuisance to debilitating and disabling in severity. Management includes counseling and reassurance, behavioral interventions, pharmacologic, and rarely, surgical interventions. Traditionally, alpha-2 agonists and dopamine receptor antagonists have been utilized. In addition, a number of different pharmacotherapies have been implemented in the search for improved management of tics with better tolerability. In rare, severely disabling cases, neuromodulation with deep brain stimulation may be indicated. Optimal brain targets and candidate selection are still in evolution. This article will review the evidence for current medical and surgical therapies with a focus on recent updates.

The Inheritance of Tourette Disorder: A review

Georges Gilles de la Tourette, in describing the syndrome that now bears his name, observed that the condition aggregated within families. Over the last three decades, numerous studies have confirmed this observation, and demonstrated that familial clustering is due in part to genetic factors. Recent studies are beginning to provide clues about the underlying genetic mechanisms important for the manifestation of some cases of Tourette Disorder (TD). Evidence has come from different study designs, such as nuclear families, twins, multigenerational families, and case-control samples, together examining the broad spectrum of genetic variation including cytogenetic abnormalities, copy number variants, genome-wide association of common variants, and sequencing studies targeting rare and/or de novo variation. Each of these classes of genetic variation holds promise for identifying the causative genes and biological pathways contributing to this paradigmatic neuropsychiatric disorder.

Animal models of tic disorders: a translational perspective

Tics are repetitive, sudden movements and/or vocalizations, typically enacted as maladaptive responses to intrusive premonitory urges. The most severe tic disorder, Tourette syndrome (TS), is a childhood-onset condition featuring multiple motor and at least one phonic tic for a duration longer than 1 year. The pharmacological treatment of TS is mainly based on antipsychotic agents; while these drugs are often effective in reducing tic severity and frequency, their therapeutic compliance is limited by serious motor and cognitive side effects. The identification of novel therapeutic targets and development of better treatments for tic disorders is conditional on the development of animal models with high translational validity. In addition, these experimental tools can prove extremely useful to test hypotheses on the etiology and neurobiological bases of TS and related conditions. In recent years, the translational value of these animal models has been enhanced, thanks to a significant re-organization of our conceptual framework of neuropsychiatric disorders, with a greater focus on endophenotypes and quantitative indices, rather than qualitative descriptors. Given the complex and multifactorial nature of TS and other tic disorders, the selection of animal models that can appropriately capture specific symptomatic aspects of these conditions can pose significant theoretical and methodological challenges. In this article, we will review the state of the art on the available animal models of tic disorders, based on genetic mutations, environmental interventions as well as pharmacological manipulations. Furthermore, we will outline emerging lines of translational research showing how some of these experimental preparations have led to significant progress in the identification of novel therapeutic targets for tic disorders.

Genetics of obsessive-compulsive disorder and related disorders

Twin and family studies support a significant genetic contribution to obsessive-compulsive disorder (OCD) and related disorders, such as chronic tic disorders, trichotillomania, skin-picking disorder, body dysmorphic disorder, and hoarding disorder. Recently, population-based studies and novel laboratory-based methods have confirmed substantial heritability in OCD. Genome-wide association studies and candidate gene association studies have provided information on specific gene variations that may be involved in the pathobiology of OCD, though a substantial portion of the genetic risk architecture remains unknown.

Genetic association signal near NTN4 in Tourette syndrome

Tourette syndrome (TS) is a neurodevelopmental disorder with a complex genetic etiology. Through an international collaboration, we genotyped 42 single nucleotide polymorphisms (p < 10(-3) ) from the recent TS genomewide association study (GWAS) in 609 independent cases and 610 ancestry-matched controls. Only rs2060546 on chromosome 12q22 (p = 3.3 × 10(-4) ) remained significant after Bonferroni correction. Meta-analysis with the original GWAS yielded the strongest association to date (p = 5.8 × 10(-7) ). Although its functional significance is unclear, rs2060546 lies closest to NTN4, an axon guidance molecule expressed in developing striatum. Risk score analysis significantly predicted case-control status (p = 0.042), suggesting that many of these variants are true TS risk alleles.

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.

Mouse models of neurodevelopmental disease of the basal ganglia and associated circuits

This chapter focuses on neurodevelopmental diseases that are tightly linked to abnormal function of the striatum and connected structures. We begin with an overview of three representative diseases in which striatal dysfunction plays a key role--Tourette syndrome and obsessive-compulsive disorder, Rett's syndrome, and primary dystonia. These diseases highlight distinct etiologies that disrupt striatal integrity and function during development, and showcase the varied clinical manifestations of striatal dysfunction. We then review striatal organization and function, including evidence for striatal roles in online motor control/action selection, reinforcement learning, habit formation, and action sequencing. A key barrier to progress has been the relative lack of animal models of these diseases, though recently there has been considerable progress. We review these efforts, including their relative merits providing insight into disease pathogenesis, disease symptomatology, and basal ganglia function.

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.