Examination of the SLITRK1 gene in Caucasian patients with Tourette syndrome

Developmental control of noradrenergic system by SLITRK1 and its implications in the pathophysiology of neuropsychiatric disorders

SLITRK1 is a neuronal transmembrane protein with neurite development-and synaptic formation-controlling abilities. Several rare variants of SLITRK1 have been identified and implicated in the pathogenesis of Tourette's syndrome, trichotillomania, and obsessive-compulsive disorder, which can be collectively referred to as obsessive-compulsive-spectrum disorders. Recent studies have reported a possible association between bipolar disorder and schizophrenia, including a revertant of modern human-specific amino acid residues. Although the mechanisms underlying SLITRK1-associated neuropsychiatric disorders are yet to be fully clarified, rodent studies may provide some noteworthy clues. Slitrk1-deficient mice show neonatal dysregulation of the noradrenergic system, and later, anxiety-like behaviors that can be attenuated by an alpha 2 noradrenergic receptor agonist. The noradrenergic abnormality is characterized by the excessive growth of noradrenergic fibers and increased noradrenaline content in the medial prefrontal cortex, concomitant with enlarged serotonergic varicosities. Slitrk1 has both cell-autonomous and cell-non-autonomous functions in controlling noradrenergic fiber development, and partly alters Sema3a-mediated neurite control. These findings suggest that transiently enhanced noradrenergic signaling during the neonatal stage could cause neuroplasticity associated with neuropsychiatric disorders. Studies adopting noradrenergic signal perturbation via pharmacological or genetic means support this hypothesis. Thus, Slitrk1 is a potential candidate genetic linkage between the neonatal noradrenergic signaling and the pathophysiology of neuropsychiatric disorders involving anxiety-like or depression-like behaviors.

Lack of Association of FLT3 rs2504235 and Absence of SLITRK1 var321 in Patients with Tic Disorders from Guangdong Province, China

OBJECTIVE

Tic disorders (TDs) are highly polygenic and heritable neurodevelopmental disorders characterized by the presence of movements (motor tics) and/or vocalizations (phonic tics). SLITRK1 is a pathogenic variation of TD, and in a recent genome-wide association study in those of European ancestry, a single-nucleotide polymorphism (rs2504235) in the FLT3 gene was significantly associated with TDs/Tourette's syndrome. However, these results need to be proved in different populations. This study aimed to determine whether these two genetic variants were also associated with TD patients in south China.

METHODS

A total of 116 child TD patients and 114 healthy controls were included. All children underwent peripheral blood sampling for genomic DNA extraction. Gene fragments with two single-nucleotide polymorphisms were amplified by PCR and sequenced by Sanger chain termination before genotype analysis.

RESULTS

SLITRK1 var321 was not observed in any of the TD patients or controls. No significant difference was observed in allelic frequencies or genotypic distributions of rs2504235 between TD patients and controls.

CONCLUSION

Our results provide no evidence to support the previous conclusion that SLITRK1 var321 plays a major role in TDs, and FLT3 rs2504235 was not significantly associated with TDs in our cohort.

Candidate Genes and Pathways Associated with Gilles de la Tourette Syndrome-Where Are We?

Gilles de la Tourette syndrome (GTS) is a childhood-onset neurodevelopmental and -psychiatric tic-disorder of complex etiology which is often comorbid with obsessive-compulsive disorder (OCD) and/or attention deficit hyperactivity disorder (ADHD). Twin and family studies of GTS individuals have shown a high level of heritability suggesting, that genetic risk factors play an important role in disease etiology. However, the identification of major GTS susceptibility genes has been challenging, presumably due to the complex interplay between several genetic factors and environmental influences, low penetrance of each individual factor, genetic diversity in populations, and the presence of comorbid disorders. To understand the genetic components of GTS etiopathology, we conducted an extensive review of the literature, compiling the candidate susceptibility genes identified through various genetic approaches. Even though several strong candidate genes have hitherto been identified, none of these have turned out to be major susceptibility genes yet.

The Potential Role of miRNAs as Predictive Biomarkers in Neurodevelopmental Disorders

Neurodevelopmental disorders are defined as a set of abnormal brain developmental conditions marked by the early childhood onset of cognitive, behavioral, and functional deficits leading to memory and learning problems, emotional instability, and impulsivity. Autism spectrum disorder, attention-deficit/hyperactivity disorder, Tourette syndrome, fragile X syndrome, and Down's syndrome are a few known examples of neurodevelopmental disorders. Although they are relatively common in both developed and developing countries, very little is currently known about their underlying molecular mechanisms. Both genetic and environmental factors are known to increase the risk of neurodevelopmental disorders. Current diagnostic and screening tests for neurodevelopmental disorders are not reliable; hence, individuals with neurodevelopmental disorders are often diagnosed in the later stages. This negatively affects their prognosis and quality of life, prompting the need for a better diagnostic biomarker. Recent studies on microRNAs and their altered regulation in diseases have shed some light on the possible role they could play in the development of the central nervous system. This review attempts to elucidate our current understanding of the role that microRNAs play in neurodevelopmental disorders with the hope of utilizing them as potential biomarkers in the future.

Refractory Gilles de la Tourette Syndrome-Many Pieces That Define the Puzzle

Gilles de la Tourette syndrome (GTS) is a childhood onset neuropsychiatric disorder characterized by the presence of motor and vocal tics. The clinical spectrum of GTS is heterogeneous and varies from mild cases that do not require any medical attention to cases that are refractory to standard treatments. One of the unresolved issues is the definition of what constitutes treatment-refractory GTS. While for some other neuropsychiatric disorders, such as obsessive-compulsive disorder (OCD), a clear definition has been established, there is still no consensus with regard to GTS. One important issue is that many individuals with GTS also meet criteria for one or more other neurodevelopmental and neuropsychiatric disorders. In many individuals, the severity of these comorbid conditions contributes to the degree to which GTS is treatment refractory. The scope of this paper is to present the current state-of-the-art regarding refractory GTS and indicate possible approaches to define it. In closing, we discuss promising approaches to the treatment of individuals with refractory GTS.

Association of Genetic Variation in the 3'UTR of LHX6, IMMP2L, and AADAC With Tourette Syndrome

Background: Tourette Syndrome (TS) is a neurodevelopmental disorder that presents with motor and vocal tics early in childhood. The aim of this study was to investigate genetic variants in the 3' untranslated region (3'UTR) of TS candidate genes with a putative link to microRNA (miRNA) mediated regulation or gene expression. Methods: We used an in silico approach to identify 32 variants in the 3'UTR of 18 candidate genes putatively changing the binding site for miRNAs. In a sample composed of TS cases and controls (n = 290), as well as TS family trios (n = 148), we performed transmission disequilibrium test (TDT) and meta-analysis. Results: We found positive association of rs3750486 in the LIM homeobox 6 (LHX6) gene (p = 0.021) and rs7795011 in the inner mitochondrial membrane peptidase subunit 2 (IMMP2L) gene (p = 0.029) with TS in our meta-analysis. The TDT showed an over-transmission of the A allele of rs1042201 in the arylacetamide deacetylase (AADAC) gene in TS patients (p = 0.029). Conclusion: This preliminary study provides further support for the involvement of LHX6, IMMP2L, and AADAC genes, as well as epigenetic mechanisms, such as altered miRNA mediated gene expression regulation in the etiology of TS.

Tourette disorder and other tic disorders

Tourette disorder is a developmental neuropsychiatric condition characterized by vocal and motor tics that can range in severity from mild to disabling. It represents one end of a spectrum of tic disorders and is estimated to affect 0.5-0.7% of the population. Accumulated evidence supports a substantial genetic contribution to disease risk, but the identification of genetic variants that confer risk has been challenging. Positive findings in candidate gene association studies have not replicated, and genomewide association studies have not generated signals of genomewide significance, in large part because of inadequate sample sizes. Rare mutations in several genes have been identified, but their causality is difficult to establish. As in other complex neuropsychiatric disorders, it is likely that Tourette disorder risk involves a combination of common, low-effect and rare, larger-effect variants in multiple genes acting together with environmental factors. With the ongoing collection of larger patient cohorts and the emergence of affordable high-throughput genomewide sequencing, progress is expected to accelerate in coming years.

Investigation of previously implicated genetic variants in chronic tic disorders: a transmission disequilibrium test approach

Genetic studies in Tourette syndrome (TS) are characterized by scattered and poorly replicated findings. We aimed to replicate findings from candidate gene and genome-wide association studies (GWAS). Our cohort included 465 probands with chronic tic disorder (93% TS) and both parents from 412 families (some probands were siblings). We assessed 75 single nucleotide polymorphisms (SNPs) in 465 parent-child trios; 117 additional SNPs in 211 trios; and 4 additional SNPs in 254 trios. We performed SNP and gene-based transmission disequilibrium tests and compared nominally significant SNP results with those from a large independent case-control cohort. After quality control 71 SNPs were available in 371 trios; 112 SNPs in 179 trios; and 3 SNPs in 192 trios. 17 were candidate SNPs implicated in TS and 2 were implicated in obsessive-compulsive disorder (OCD) or autism spectrum disorder (ASD); 142 were tagging SNPs from eight monoamine neurotransmitter-related genes (including dopamine and serotonin); 10 were top SNPs from TS GWAS; and 13 top SNPs from attention-deficit/hyperactivity disorder, OCD, or ASD GWAS. None of the SNPs or genes reached significance after adjustment for multiple testing. We observed nominal significance for the candidate SNPs rs3744161 (TBCD) and rs4565946 (TPH2) and for five tagging SNPs; none of these showed significance in the independent cohort. Also, SLC1A1 in our gene-based analysis and two TS GWAS SNPs showed nominal significance, rs11603305 (intergenic) and rs621942 (PICALM). We found no convincing support for previously implicated genetic polymorphisms. Targeted re-sequencing should fully appreciate the relevance of candidate genes.

Targeted Re-Sequencing Approach of Candidate Genes Implicates Rare Potentially Functional Variants in Tourette Syndrome Etiology

Although the genetic basis of Tourette Syndrome (TS) remains unclear, several candidate genes have been implicated. Using a set of 382 TS individuals of European ancestry we investigated four candidate genes for TS (HDC, SLITRK1, BTBD9, and SLC6A4) in an effort to identify possibly causal variants using a targeted re-sequencing approach by next generation sequencing technology. Identification of possible disease causing variants under different modes of inheritance was performed using the algorithms implemented in VAAST. We prioritized variants using Variant ranker and validated five rare variants via Sanger sequencing in HDC and SLITRK1, all of which are predicted to be deleterious. Intriguingly, one of the identified variants is in linkage disequilibrium with a variant that is included among the top hits of a genome-wide association study for response to citalopram treatment, an antidepressant drug with off-label use also in obsessive compulsive disorder. Our findings provide additional evidence for the implication of these two genes in TS susceptibility and the possible role of these proteins in the pathobiology of TS should be revisited.

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.

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.

Tourette Syndrome: Bridging the Gap between Genetics and Biology

Tourette syndrome is a childhood neuropsychiatric disorder, which presents with disruptive motor and vocal tics. The disease also has a high comorbidity with obsessive-compulsive disorder and attention deficit hyperactivity disorder, which may further increase the distress experienced by patients. Current treatments act with varying efficacies in alleviating symptoms, as the underlying biology of the disease is not fully understood to provide precise therapeutic targets. Moreover, the genetic complexity of the disorder presents a substantial challenge to the identification of genetic alterations that contribute to the Tourette's phenotype. Nevertheless, genetic studies have suggested involvement of dopaminergic, serotonergic, glutamatergic, and histaminergic pathways in the pathophysiology of at least some cases. In addition, genetic overlaps with other neuropsychiatric disorders may point toward a shared biology. The findings that are emerging from genetic studies will allow researchers to piece together the underlying components of the disease, in the hopes that a deeper understanding of Tourette's can lead to improved treatments for those affected by it.

Association study of the SLITRK5 gene and Tourette syndrome

Tourette syndrome (TS) and obsessive-compulsive disorder commonly occur together. Family studies indicate shared genetic risk factors. SLITRK5, one of a family of six SLITRK genes, has been suggested as a possible candidate gene contributing towards obsessive-compulsive disorder on the basis of the mouse knockout model that shows excessive grooming behaviours that are alleviated with fluoxetine. In this study, we tested the SLITRK5 gene as a candidate for TS in a family-based sample with 377 affected children. Using single nucleotide polymorphisms tagging the gene, we did not find any evidence supporting the association of TS and SLITRK5.

Porcine SLITRK1: Molecular cloning and characterization

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Molecular cloning of the porcine SLITRK1 gene is reported.

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A SLITRK1 transcript variant encoding a truncated protein was identified.

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The SLITRK1 transcript was exclusively expressed in brain tissues.

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There was low methylation of both the SLITRK1 gene body and its promoter.

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SLITRK1 was mapped to pig chromosome 11.

The membrane protein SLITRK1 functions as a developmentally regulated stimulator of neurite outgrowth and variants in this gene have been implicated in Tourette syndrome. In the current study we have cloned and characterized the porcine SLITRK1 gene. The genomic organization of SLITRK1 lacks introns, as does its human and mouse counterparts. RT-PCR cloning revealed two SLITRK1 transcripts: a full-length mRNA and a transcript variant that results in a truncated protein. The encoded SLITRK1 protein, consisting of 695 amino acids, displays a very high homology to human SLITRK1 (99%). The porcine SLITRK1 gene is expressed exclusively in brain tissues.

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.

Human diseases caused by germline and somatic abnormalities in microRNA and microRNA-related genes

The human genome harbors approximately 2000 genes that encode microRNAs (miRNAs), small non-coding RNAs of approximately 20-22 nt that mediate post-transcriptional gene silencing. MiRNAs are generated from long transcripts through stepwise processing by the Drosha/DGCR8, Exportin-5/RanGTP and Dicer/TRBP complexes. Given that the expression of each individual miRNA is tightly regulated, the altered expression of certain miRNAs plays a pivotal role in human diseases. For instance, germline and somatic mutations in the genes encoding the miRNA processing machinery have been reported in different cancers. Furthermore, certain miRNA genes are encoded within regions that are deleted or duplicated in individuals with chromosomal abnormalities, and the fact that the knockout of these miRNAs in animal models results in lethality or the abnormal development of certain tissues indicates that these miRNA genes contribute to the disease phenotypes. It has also been reported that mutations in miRNA genes or in miRNA-binding sites, which result in the impairment of tight regulation of target mRNA expression, cause human genetic diseases, although these cases are rare. This is in contrast to the aberrant expression of certain miRNAs that results from the impairment of transcriptional or post-transcriptional regulation, which has been reported frequently in various human diseases. The present review focuses on human diseases caused by mutations in genes encoding miRNAs and the miRNA processing machinery as well as in miRNA-binding sites. Furthermore, human diseases caused by chromosomal abnormalities that involve the deletion or duplication of regions harboring genes that encode miRNAs or the miRNA processing machinery are also introduced.

Prenatal risk factors for Tourette Syndrome: a systematic review

BACKGROUND

Tourette Syndrome (TS) appears to be an inherited disorder, although genetic abnormalities have been identified in less than 1% of patients, and the mode of inheritance is uncertain. Many studies have investigated environmental factors that might contribute to the onset and severity of tics and associated comorbidities such as obsessive compulsive disorder (OCD) and attention deficit hyperactive disorder (ADHD). A systematic review and qualitative analysis were performed to provide a broad view of the association between pre- and perinatal factors and TS.

METHODS

The Medline, Embase and PsycINFO databases were searched using terms specific to Tourette's syndrome and keywords such as "pregnancy", "prenatal", "perinatal", "birth" and "neonatal". Studies were limited to studies on human subjects published in English or French through October 2012.

RESULTS

22 studies were included. Studies were of limited methodological quality, with most samples derived from specialty clinics, and most exposures ascertained retrospectively. The majority of the results for demographic factors of parents, including age, education, socioeconomic status, and marital status, revealed no significant association with the onset of TS, or the presence of comorbidity. Many factors were reported to be significantly associated with the onset of TS, the presence of comorbidity and symptom severity, but the most consistently reported factors were maternal smoking and low birth weight.

CONCLUSIONS

There are few studies evaluating the relationship between pre and perinatal events and TS, and existing studies have major limitations, including the use of clinic rather than epidemiologically derived samples, retrospective data collection on pre and perinatal events and multiple hypothesis testing without appropriate statistical correction. The mechanism by which prenatal and perinatal adversities could lead to TS onset or symptom severity is unknown, but may be related to changes in the dopaminergic system as a result of early brain injury.

Neural cell adhesion molecules belonging to the family of leucine-rich repeat proteins

Leucine-rich repeats (LRRs) are motifs that form protein-ligand interaction domains. There are approximately 140 human genes encoding proteins with extracellular LRRs. These encode cell adhesion molecules (CAMs), proteoglycans, G-protein-coupled receptors, and other types of receptors. Here we give a brief description of 36 proteins with extracellular LRRs that all can be characterized as CAMs or putative CAMs expressed in the nervous system. The proteins are involved in multiple biological processes in the nervous system including the proliferation and survival of cells, neuritogenesis, axon guidance, fasciculation, myelination, and the formation and maintenance of synapses. Moreover, the proteins are functionally implicated in multiple diseases including cancer, hearing impairment, glaucoma, Alzheimer's disease, multiple sclerosis, Parkinson's disease, autism spectrum disorders, schizophrenia, and obsessive-compulsive disorders. Thus, LRR-containing CAMs constitute a large group of proteins of pivotal importance for the development, maintenance, and regeneration of the nervous system.

Sequence analysis of SLITRK1 for var321 in Danish patients with Tourette syndrome and review of the literature

Tourette syndrome (TS) is a complex neuropsychiatric disorder characterized by multiple motor and vocal tics and is often accompanied by comorbidities such as attention deficit hyperactivity disorder and obsessive-compulsive disorder. The complex etiology of TS and its co-occurrence with other disorders impedes linking genetic changes with disease segregation. One of the few genes that has been linked to TS is the SLITRK1 (Slit and Trk-like 1) gene, where four variations have been suggested as possible disease-associated changes. One of these variations, which has been reported in six unrelated TS patients, was a noncoding variant (var321) at the 3'-untranslated region of SLITRK1 within a conserved binding site for microRNA has-mir-189. To elucidate the potential role of var321 in disease pathogenesis, a cohort of 112 deeply phenotyped Danish TS patients was investigated for this variation. We could not detect var321 in the present cohort, suggesting that this is not a common variant among Danish TS patients.

Characterization of SLITRK1 variation in obsessive-compulsive disorder

Obsessive compulsive disorder (OCD) is a syndrome characterized by recurrent and intrusive thoughts and ritualistic behaviors or mental acts that a person feels compelled to perform. Twin studies, family studies, and segregation analyses provide compelling evidence that OCD has a strong genetic component. The SLITRK1 gene encodes a developmentally regulated stimulator of neurite outgrowth and previous studies have implicated rare variants in this gene in disorders in the OC spectrum, specifically Tourette syndrome (TS) and trichotillomania (TTM). The objective of the current study was to evaluate rare genetic variation in SLITRK1 in risk for OCD and to functionally characterize associated coding variants. We sequenced SLITRK1 coding exons in 381 individuals with OCD as well as in 356 control samples and identified three novel variants in seven individuals. We found that the combined mutation load in OCD relative to controls was significant (p = 0.036). We identified a missense N400I change in an individual with OCD, which was not found in more than 1000 control samples (P<0.05). In addition, we showed the the N400I variant failed to enhance neurite outgrowth in primary neuronal cultures, in contrast to wildtype SLITRK1, which enhanced neurite outgrowth in this assay. These important functional differences in the N400I variant, as compared to the wildtype SLITRK1 sequence, may contribute to OCD and OC spectrum symptoms. A synonymous L63L change identified in an individual with OCD and an additional missense change, T418S, was found in four individuals with OCD and in one individual without an OCD spectrum disorder. Examination of additional samples will help assess the role of rare SLITRK1 variation in OCD and in related psychiatric illness.

The leucine-rich repeat superfamily of synaptic adhesion molecules: LRRTMs and Slitrks

Synapses are asymmetric intercellular junctions connected by multiple synaptic cell adhesion molecules (CAMs). Synaptic CAMs function in various stages of synaptogenesis - the process of synapse creation - encompassing synapse formation, maturation, refinement, plasticity, and elimination. The list of synaptic CAMs has rapidly grown, although their precise functions of most CAMs at synapses remain incomplete. Members of an emerging class of transmembrane proteins containing leucine-rich repeat (LRR) domains have received considerable recent research attention. In this minireview, I discuss recent findings on LRR-containing synaptic CAMs that impact synapse development and circuit formation, focusing on two families of LRR synaptic CAMs: leucine-rich transmembrane proteins (LRRTMs) and Slit and Trk-like family (Slitrks). Their basic biochemical properties, proposed functions at synapses, physiological significances, and open questions are summarized.

MicroRNAs in stress signaling and human disease

Disease is often the result of an aberrant or inadequate response to physiologic and pathophysiologic stress. Studies over the last 10 years have uncovered a recurring paradigm in which microRNAs (miRNAs) regulate cellular behavior under these conditions, suggesting an especially significant role for these small RNAs in pathologic settings. Here, we review emerging principles of miRNA regulation of stress signaling pathways and apply these concepts to our understanding of the roles of miRNAs in disease. These discussions further highlight the unique challenges and opportunities associated with the mechanistic dissection of miRNA functions and the development of miRNA-based therapeutics.

MicroRNA dysregulation in neuropsychiatric disorders and cognitive dysfunction

MicroRNAs (miRNA), a class of non-coding RNAs, are emerging as important modulators of neuronal development, structure and function. A connection has been established between abnormalities in miRNA expression and miRNA-mediated gene regulation and psychiatric and neurodevelopmental disorders as well as cognitive dysfunction. Establishment of this connection has been driven by progress in elucidating the genetic etiology of these phenotypes and has provided a context to interpret additional supporting evidence accumulating from parallel expression profiling studies in brains and peripheral blood of patients. Here we review relevant evidence that supports this connection and explore possible mechanisms that underlie the contribution of individual miRNAs and miRNA-related pathways to the pathogenesis and pathophysiology of these complex clinical phenotypes. The existing evidence provides useful hypotheses for further investigation as well as important clues for identifying novel therapeutic targets.

Analysis of the MRPL3, DNAJC13 and OFCC1 variants in Chinese Han patients with TS-CTD

Tourette syndrome/chronic tic phenotype (TS-CTD) is a neurological disorder manifested particularly by motor and vocal tics and associated with a variety of behavioral abnormalities. Recently, the mitochondrial ribosomal protein L3 gene (MRPL3) S75N, the DnaJ (Hsp40) homolog subfamily C member 13 gene (DNAJC13) A2057S, the orofacial cleft 1 candidate 1 gene (OFCC1) R129G and c.-5A>G variants are reported to be associated with Tourette syndrome/chronic tic phenotype (TS-CTD) in patients of European ancestry. To evaluate whether these variants are associated with TS-CTD in Chinese Han patients, we screened 132 Chinese Han patients from Mainland China. None of the 132 samples from patients with TS-CTD showed the MRPL3 S75N, DNAJC13 A2057S, OFCC1 R129G and c.-5A>G variants, and these variants probably are a rare cause of TS-CTD in a Chinese Han ethnic group. Genetic heterogeneity of TS should be considered and tests designed to detect these variants in Chinese Han ethnic group probably will not have a diagnostic utility in clinical practice.

Slitrks as emerging candidate genes involved in neuropsychiatric disorders

Slitrks are a family of structurally related transmembrane proteins belonging to the leucine-rich repeat (LRR) superfamily. Six family members exist (Slitrk1-6) and all are highly expressed in the central nervous system (CNS). Slitrks have been implicated in mediating basic neuronal processes, ranging from neurite outgrowth and dendritic elaboration to neuronal survival. Recent studies in humans and genetic mouse models have led to the identification of Slitrks as candidate genes that might be involved in the development of neuropsychiatric conditions, such as obsessive compulsive spectrum disorders and schizophrenia. Although these system-level approaches have suggested that Slitrks play prominent roles in CNS development, key questions remain regarding the molecular mechanisms through which they mediate neuronal signaling and connectivity.

Mutation screening of the HDC gene in Chinese Han patients with Tourette syndrome

Tourette Syndrome (TS) is a complex neuropsychiatric disorder characterized by vocal and motor tics. While environmental causes have been proposed to play a role, genetic factors are believed to be the main determinants of the disorder and its clinical manifestations. Recently, a heterozygous W317X mutation in the histidine decarboxylase gene (HDC) was reported to be responsible for TS in a two-generation pedigree. To investigate whether the HDC gene play a role in TS in Chinese Han population, we performed genetic analysis of the coding region of the HDC gene in 100 Chinese Han patients with TS. Three variants were found including a C > T transition (IVS1 + 52C > T), a novel C > A transition (c.426C > A) in exon 4, and a novel G > A transition (c.1743G > A) in exon 12, both predicted with no amino acid change. Extended analysis was conducted in a total of 120 TS patients and 240 sex, age, and ethnicity matched healthy controls. No significant differences in genotypic and allele distribution between patients and controls for these three variants (P = 0.274, P = 1.000 and P = 0.632 for genotypic distribution, respectively; P = 0.143, P = 1.000 and P = 0.582 for allele distribution, respectively) were observed, suggesting variants in the HDC gene may play little or no role in TS susceptibility in Chinese Han population.

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.

Tourette syndrome and other tic disorders

Tic disorders, including Tourette syndrome, are an intriguing group of paroxysmal movement abnormalities that begin in childhood, have a fluctuating course, are capable of causing psychosocial and physical problems, and often improve by early adulthood. These disorders are frequently associated with a variety of comorbid problems whose negative effects may exceed those of tics. Therapy is strictly symptomatic and usually includes educational, behavioral, and a variety of pharmacological therapies. Although there is strong evidence supporting an inherited basis, the precise genetic abnormality remains unknown. A proposed poststreptococcal autoimmune etiology remains controversial. Pathophysiologically, tics appear to arise from an alteration within cortico-striatal-thalamo-cortical circuits, but the definitive site is unknown. Evidence supports an abnormality of synaptic neurotransmission, likely involving the dopaminergic system.

The genetics of Tourette disorder

Tourette disorder (TD) is a childhood onset neuropsychiatric syndrome defined by persistent motor and vocal tics. Despite a long-standing consensus for a strong genetic contribution, the pace of discovery compared to other disorders of similar prevalence has been slow, due in part to a paucity of studies and both clinical heterogeneity and a complex genetic architecture. However, the potential for rapid progress is high. Recent rare variant findings have pointed to the importance of copy number variation, the overlap of risks among distinct diagnostic entities, the contribution of novel molecular mechanisms, and the value of family based studies. Finally, analysis of a cohort of sufficient size to identify common polymorphisms of plausible effect is underway, promising key information regarding the contribution of common alleles to TD.

The genetics of child psychiatric disorders: focus on autism and Tourette syndrome

Investigations into the genetics of child psychiatric disorders have finally begun to shed light on molecular and cellular mechanisms of psychopathology. The first strains of success in this notoriously difficult area of inquiry are the result of an increasingly sophisticated appreciation of the allelic architecture of common neuropsychiatric and neurodevelopmental disorders, the consolidation of large patient cohorts now beginning to reach sufficient size to power reliable studies, the emergence of genomic tools enabling comprehensive investigations of rare as well as common genetic variation, and advances in developmental neuroscience that are fueling the rapid translation of genetic findings.

Tourette syndrome: gene expression as a tool to discover drug targets

Gene expression microarrays are a high-throughput, cost-effective method for measuring the expression of all genes in a sample. By comparing the expression patterns of healthy controls to diseased subjects, the genetic regulatory pathways underlying and affected by the disease can be elucidated. Furthermore, dysregulated genes are possible candidates for pharmaceutical therapy. Here, we consider the possibility of applying this approach to Tourette syndrome. We also review current theories of Tourette syndrome etiology and pharmacology.

MicroRNAs in psychiatric and neurodevelopmental disorders

Abnormalities in microRNA (miRNA)-mediated gene regulation have been observed in a variety of human diseases, especially in cancer. Here, we provide an account of newly emerging connections between miRNAs with various psychiatric and neurodevelopmental disorders, including recent findings of miRNA dysregulation in the 22q11.2 microdeletion syndrome, a well-established genetic risk factor for schizophrenia. miRNAs appear to be components of both the genetic architecture of these complex phenotypes as well as integral parts of the biological pathways that mediate the effects of primary genetic deficits. Therefore, they may contribute to both genetic heterogeneity and phenotypic variation of psychiatric and neurodevelopmental disorders and could serve as novel therapeutic targets.

Slitrk5 deficiency impairs corticostriatal circuitry and leads to obsessive-compulsive-like behaviors in mice

Obsessive-compulsive disorder (OCD) is a common psychiatric disorder defined by the presence of obsessive thoughts and repetitive compulsive actions, and it often encompasses anxiety and depressive symptoms. Recently, the corticostriatal circuitry has been implicated in the pathogenesis of OCD. However, the etiology, pathophysiology and molecular basis of OCD remain unknown. Several studies indicate that the pathogenesis of OCD has a genetic component. Here we demonstrate that loss of a neuron-specific transmembrane protein, SLIT and NTRK-like protein-5 (Slitrk5), leads to OCD-like behaviors in mice, which manifests as excessive self-grooming and increased anxiety-like behaviors, and is alleviated by the selective serotonin reuptake inhibitor fluoxetine. Slitrk5(-/-) mice show selective overactivation of the orbitofrontal cortex, abnormalities in striatal anatomy and cell morphology and alterations in glutamate receptor composition, which contribute to deficient corticostriatal neurotransmission. Thus, our studies identify Slitrk5 as an essential molecule at corticostriatal synapses and provide a new mouse model of OCD-like behaviors.

Differential expression of Slitrk family members in the mouse nervous system

The Slitrk family of transmembrane proteins is composed of six members that are highly expressed in the nervous system. To date, the function of Slitrks during development of the nervous system has yet to be defined. The high homology between the extracellular region of Slitrks and the repulsive axon guidance molecules Slits suggests that Slitrks may regulate axon outgrowth during development. To begin to evaluate their role during development, we have examined the expression of the Slitrk genes in the developing murine nervous system using in situ hybridization. Here, we show that despite some overlap in expression, the Slitrks display distinct patterns of expression in the olfactory system, the eye, forebrain structures, the cerebellum, the spinal cord, and dorsal root ganglia. These diverse patterns of expression suggest that Slitrk family members may have different functions during development of the nervous system.

The genetics of Tourette syndrome: a review

OBJECTIVES

This article summarizes and evaluates recent advances in the genetics of Gilles de la Tourette syndrome (GTS).

METHODS

This is a review of recent literature focusing on (1) the genetic etiology of GTS; (2) common genetic components of GTS, attention deficit hyperactivity disorder (ADHD), and obsessive compulsive disorder (OCD); (3) recent linkage studies of GTS; (4) chromosomal translocations in GTS; and (5) candidate gene studies.

RESULTS

Family, twin, and segregation studies provide strong evidence for the genetic nature of GTS. GTS is a heterogeneous disorder with complex inheritance patterns and phenotypic manifestations. Family studies of GTS and OCD indicate that an early-onset form of OCD is likely to share common genetic factors with GTS. While there apparently is an etiological relationship between GTS and ADHD, it appears that the common form of ADHD does not share genetic factors with GTS. The largest genome wide linkage study to date observed evidence for linkage on chromosome 2p23.2 (P=3.8x10(-5)). No causative candidate genes have been identified, and recent studies suggest that the newly identified candidate gene SLITRK1 is not a significant risk gene for the majority of individuals with GTS.

CONCLUSION

The genetics of GTS are complex and not well understood. The Genome Wide Association Study (GWAS) design can hopefully overcome the limitations of linkage and candidate gene studies. However, large-scale collaborations are needed to provide enough power to utilize the GWAS design for discovery of causative mutations. Knowledge of susceptibility mutations and biological pathways involved should eventually lead to new treatment paradigms for GTS.

Patrocles: a database of polymorphic miRNA-mediated gene regulation in vertebrates

The Patrocles database (http://www.patrocles.org/) compiles DNA sequence polymorphisms (DSPs) that are predicted to perturb miRNA-mediated gene regulation. Distinctive features include: (i) the coverage of seven vertebrate species in its present release, aiming for more when information becomes available, (ii) the coverage of the three compartments involved in the silencing process (i.e. targets, miRNA precursors and silencing machinery), (iii) contextual information that enables users to prioritize candidate ‘Patrocles DSPs’, including graphical information on miRNA-target coexpression and eQTL effect of genotype on target expression levels, (iv) the inclusion of Copy Number Variants and eQTL information that affect miRNA precursors as well as genes encoding components of the silencing machinery and (v) a tool (Patrocles finder) that allows the user to determine whether her favorite DSP may perturb miRNA-mediated gene regulation of custom target sequences. To support the biological relevance of Patrocles' content, we searched for signatures of selection acting on ‘Patrocles single nucleotide polymorphisms (pSNPs)’ in human and mice. As expected, we found a strong signature of purifying selection against not only SNPs that destroy conserved target sites but also against SNPs that create novel, illegitimate target sites, which is reminiscent of the Texel mutation in sheep.

Association of SLITRK1 to Gilles de la Tourette Syndrome

Previously the Slit and Trk-like family member 1 (SLITRK1) gene was identified as a candidate gene for Gilles de la Tourette Syndrome (GTS) based on a patient that carried a chromosomal inversion on 13q, as well as the identification of two rare DNA variants in the SLITRK1 gene. Since that report, studies have tested for the two rare variants in GTS and either did not find them, or when found, they did not segregate with the disorder in families, casting doubt on the relationship of this gene to GTS. We tested for these two rare variants and genotyped three polymorphisms that tag the currently identified major haplotypes of this gene in a sample of 154 nuclear families with GTS. In addition, the entire coding region was screened for novel DNA variants. We did not find the two reported rare variants in any of the probands or siblings in these families. We did however find significant evidence for association of a single polymorphism and of haplotypes of the three tagging polymorphisms. These findings provide the first support for the original finding indicating SLITRK1 as a susceptibility gene for GTS and indicate that further study of this gene in GTS is warranted.

Sequence analysis of the complete SLITRK1 gene in Austrian patients with Tourette's disorder

Mutations in the gene SLITRK1 (Slit and Trk-like 1) have been reported in patients with Tourette's disorder (TD). We sequenced the entire SLITRK1 gene including the coding region the 5' and 3' untranslated region in 92 Austrian patients with TD. No nucleotide changes within the protein-coding region were identified. One patient was found to carry a variant within the 3' untranslated region (3383g>a), which was absent in 192 control individuals and which segregated in two additional family members with tic symptoms. In conclusion, our results provide no evidence for SLITRK1 playing a major role in TD.

Chromosome 5 and Gilles de la Tourette syndrome: Linkage in a large pedigree and association study of six candidates in the region

Gilles de la Tourette Syndrome (TS) is a neuropsychiatric disorder characterized by both motor and vocal tics. In our previous genome scan for TS we identified evidence for linkage to the centromeric region of chromosome 5 in a single large family of 32 individuals with 10 family members with TS or chronic multiple tics (CMT). In this paper we report further analyses of the 5p-centromeric region in this pedigree. An additional 11 family members were identified and screened for TS. Using a set of 14 microsatellite markers we refined the linked region to a approximately 28 Mb interval between the markers D5S1506 and D5S76. A set of six candidate genes located in this region were selected to be tested for genetic association with TS. These genes were GDNF, ITGA1, ISL1, FGF10, HCN1 and SLC1A3. The TDT statistic was used for the association tests in a sample of 171 independent nuclear families with 241 affected children with TS. We found no evidence for an association between TS and markers in these genes in this sample of families. This study represents the first efforts to narrow the linkage region in the extended pedigree and the first tests of candidate genes in the chromosome 5 region linked to TS.