Linkage study of the dopamine D5 receptor gene and Gilles de la Tourette syndrome

Current Understanding of the Genetics of Tourette Syndrome

Gilles de la Tourette syndrome (TS) is a common, childhood-onset psychiatric disorder characterized by persistent motor and vocal tics. It is a heterogeneous disorder in which the phenotypic expression may be affected by environmental factors, such as immune responses. Furthermore, several studies have shown that genetic factors play a vital role in the etiology of TS, as well as its comorbidity with other disorders, including attention deficit hyperactivity disorder, obsessive-compulsive disorder, and autism spectrum disorder. TS has a complex inheritance pattern and, according to various genetic studies, several genes and loci have been correlated with TS. Genome-wide linkage studies have identified Slit and Trk-like 1 (SLITRK1) and histidine decarboxylase (HDC) genes, and candidate gene association studies have extensively investigated the dopamine and serotonin system genes, but there have been no consistent results. Moreover, genome-wide association studies have implicated several genetic loci; however, larger study cohorts are needed to confirm this. Copy number variations, which are polymorphisms in the number of gene copies due to chromosomal deletions or duplications, are considered another significant source of mutations in TS. In the last decade, whole genome/exome sequencing has identified several novel genetic mutations in patients with TS. In conclusion, more studies are needed to reveal the exact mechanisms of underlying TS, which may help to provide more information on the prognosis and therapeutic plans for TS.

Progress in Genetic Studies of Tourette's Syndrome

Tourette's Syndrome (TS) is a complex disorder characterized by repetitive, sudden, and involuntary movements or vocalizations, called tics. Tics usually appear in childhood, and their severity varies over time. In addition to frequent tics, people with TS are at risk for associated problems including attention deficit hyperactivity disorder (ADHD), obsessive-compulsive disorder (OCD), anxiety, depression, and problems with sleep. TS occurs in most populations and ethnic groups worldwide, and it is more common in males than in females. Previous family and twin studies have shown that the majority of cases of TS are inherited. TS was previously thought to have an autosomal dominant pattern of inheritance. However, several decades of research have shown that this is unlikely the case. Instead TS most likely results from a variety of genetic and environmental factors, not changes in a single gene. In the past decade, there has been a rapid development of innovative genetic technologies and methodologies, as well as significant progresses in genetic studies of psychiatric disorders. In this review, we will briefly summarize previous genetic epidemiological studies of TS and related disorders. We will also review previous genetic studies based on genome-wide linkage analyses and candidate gene association studies to comment on problems of previous methodological and strategic issues. Our main purpose for this review will be to summarize the new genetic discoveries of TS based on novel genetic methods and strategies, such as genome-wide association studies (GWASs), whole exome sequencing (WES) and whole genome sequencing (WGS). We will also compare the new genetic discoveries of TS with other major psychiatric disorders in order to understand the current status of TS genetics and its relationship with other psychiatric disorders.

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.

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.

Dopaminergic receptor D5 mRNA expression is increased in circulating lymphocytes of Tourette syndrome patients

Tourette syndrome (TS) is a neuropsychiatric disorder in which dopaminergic dysfunction and immune system abnormalities seem to coexist. Using real-time PCR, we determined mRNA expression of dopamine receptors (DRs) D1-5 in peripheral blood lymphocytes (PBLs) from 15 TS patients and 15 sex- and age-matched healthy controls (HCs). DRD5 mRNA levels in cells from TS were higher than in cells from HCs. In TS patients with obsessive-compulsive disorder, DRD5 mRNA levels in PBLs showed a highly positive correlation with the severity of compulsive symptoms. DRD5 mRNA upregulation in PBLs from TS patients may represent a peripheral marker of dopaminergic dysfunction and supports the involvement of the immune system in TS.

Association of the Slit and Trk-like 1 gene in Taiwanese patients with Tourette syndrome

Tourette syndrome is a neurologic disorder characterized by both motor and vocal tics. Recently, two variants, including a single-base deletion resulting in a truncated protein and a 3'-untranslated-region variant altering a binding site for micro-RNA in the Slit and Trk-like 1 gene, were found to be a genetic cause of Tourette syndrome. The Slit and Trk-like 1 family was identified as neuronal transmembrane proteins that control neurite outgrowth. This study aimed to determine whether mutations in the gene can be found in Taiwanese patients with Tourette syndrome. In total, 160 patients were included. All children underwent peripheral blood sampling for genotype analyses. We sequenced the whole Slit and Trk-like 1 gene, including the promoter, the 3'-untranslated region, the 5'-untranslated region, and the whole coding region. We found that none of the 160 samples revealed any mutation in the whole gene sequence. In addition, there was only one polymorphism, c.3225 T>C, detected in 10 individuals. We conclude that in rare variants, it may be difficult to establish an association with disorder. Therefore, genetic screening in the Slit and Trk-like 1 gene for the recently identified mutations does not appear to be of utility in the diagnosis of Tourette syndrome.

Tic disorders and obsessive compulsive disorder: where is the link?

Over the last years evidence on the overlap between tic-disorders (TD) and obsessive compulsive behavior/disorder (OCB/OCD) has increased. The main focus of research have been the phenomenological and epidemiological similarities and differences in samples of different age, primary diagnosis (TD vs. OCD) including the co-occurrence of both. Unfortunately, only a minority of studies included all three groups (TD, TD + OCD, OCD). Nevertheless, new insight concerning possible subtypes for both TD and OCD has been gained. While some authors concentrated on OCD with/without tics we will summarize the field of TD and OCB/OCD from the viewpoint of tics, since OCB plays an important role in patients with TD. Thereby we will not only sharpen the clinicans' awareness of known differences in phenomenology, epidemiology, genetics and neurobiology, aimed to improve their diagnoses and treatment but also highlight the gaps of knowledge and discuss possibilities for further research in this field.

Dopamine D5 receptor localization on cholinergic neurons of the rat forebrain and diencephalon: a potential neuroanatomical substrate involved in mediating dopaminergic influences on acetylcholine release

The study of dopaminergic influences on acetylcholine release is especially useful for the understanding of a wide range of brain functions and neurological disorders, including schizophrenia, Parkinson's disease, Alzheimer's disease, and drug addiction. These disorders are characterized by a neurochemical imbalance of a variety of neurotransmitter systems, including the dopamine and acetylcholine systems. Dopamine modulates acetylcholine levels in the brain by binding to dopamine receptors located directly on cholinergic cells. The dopamine D5 receptor, a D1-class receptor subtype, potentiates acetylcholine release and has been investigated as a possible substrate underlying a variety of brain functions and clinical disorders. This receptor subtype, therefore, may prove to be a putative target for pharmacotherapeutic strategies and cognitive-behavioral treatments aimed at treating a variety of neurological disorders. The present study investigated whether cholinergic cells in the dopamine targeted areas of the cerebral cortex, striatum, basal forebrain, and diencephalon express the dopamine D5 receptor. These receptors were localized on cholinergic neurons with dual labeling immunoperoxidase or immunofluorescence procedures using antibodies directed against choline acetyltransferase (ChAT) and the dopamine D5 receptor. Results from this study support previous findings indicating that striatal cholinergic interneurons express the dopamine D5 receptor. In addition, cholinergic neurons in other critical brain areas also show dopamine D5 receptor expression. Dopamine D5 receptors were localized on the somata, dendrites, and axons of cholinergic cells in each of the brain areas examined. These findings support the functional importance of the dopamine D5 receptor in the modulation of acetylcholine release throughout the brain.

Dopamine receptor D2 gene polymorphisms are associated in Taiwanese children with Tourette syndrome

The pathophysiology of Tourette syndrome may involve the dopamine system. Dysfunction of the dopamine receptor D2 gene leads to many neuropsychiatric disorders. The objective of this study is to test the hypothesis that the dopamine receptor D2 gene may play a role in Tourette syndrome. A total of 151 children with Tourette syndrome and 183 normal control subjects were included in the study. Polymerase chain reaction was used to identify the Taq I DRD2 and DRD2 (H313H) polymorphisms of the dopamine receptor D2 gene. The genotype proportions of Taq I DRD2 and DRD2 (H313H) polymorphisms in the two groups were significantly different (P < 0.01 for both). The odds ratio for developing Tourette syndrome in individuals with the Taq I DRD2 A1 homozygote was 2.253 (95% confidence interval, 1.124-4.517) compared with individuals with the Taq I DRD2 A2 homozygote. The odds ratio for developing Tourette syndrome in individuals with the DRD2 (H313H) C homozygote was 2.96 (95% confidence interval, 1.398-6.269) compared with individuals with DRD2 (H313H) T homozygote. This study has demonstrated an association between the dopamine receptor D2 gene and Tourette syndrome. These data suggest that the dopamine receptor D2 gene or a closely linked gene might be one of the susceptibility factors for Tourette syndrome.

An update on the genetics of Gilles de la Tourette syndrome

OBJECTIVES

To summarize the current data suggesting that Gilles de la Tourette syndrome (GTS) is inherited and genetic.

METHODS

The extant literature on family studies, segregation analyses, candidate genes studies and linkage studies of GTS was reviewed and summarized.

RESULTS AND CONCLUSIONS

There is considerable data that suggests that: (1). genetic factors play an important role in the manifestation of GTS; (2). several genes are important with some possibly having major effect; and (3). several regions of the genome have been identified as potential locations of these susceptibility genes.

The genetics of Tourette syndrome

Twin and family studies demonstrate that Tourette syndrome (TS) is a genetic disorder. Early segregation analyses of family data were consistent with the hypothesis of autosomal dominant transmission; however, more recent studies suggest that the mode of inheritance is more complex. Current findings suggest that there are genes of major effect with other genes acting as modifiers. Several genome scans have been completed and several regions of interest have been identified that may harbor susceptibility genes for TS. Work is currently underway to replicate and extend these initial results.

An association study between 5-HTTLPR polymorphism, COMT polymorphism, and Tourette's syndrome

Several lines of evidence suggest that a genetic component underlies Tourette's syndrome (TS). We investigated both the role of the insertion/deletion polymorphism in the promoter region of the serotonin transporter gene (5-HTTLPR) and that of the Val-158-Met substitution in the catechol-O-methyl-transferase (COMT) gene in conferring susceptibility to TS. Fifty-two TS patients were recruited and compared with a control group of 63 healthy subjects. Neither a genotypic nor an allelic association was found; subdividing TS patients according to clinical variables, such as a co-diagnosis of obsessive-compulsive disorder (OCD) and a positive family history for obsessive compulsive disorder or tics, also failed to reveal a significant association. The lack of significance for 5-HTTLPR and COMT polymorphisms in conferring liability to TS does not exclude a role of different functional polymorphisms in genes coding for serotonergic or dopaminergic structures in the etiology of TS. In fact, TS is a complex disorder and these genes most likely have only a minor genetic effect in its etiology.

Antisociality, substance dependence, and the DRD5 gene: a preliminary study

A pilot population-based study of a microsatellite polymorphism at the DRD5 locus in adult European-Americans showed its association with childhood symptom counts for oppositional defiant disorder (ODD) in males and females and adult antisocial personality disorder (ASPD) in females. No association with childhood conduct disorder symptom count was observed. ODD mediated the genotype-ASPD relationship in females. Neither ODD nor ASPD significantly mediated the relationship between the genotype and the liability to substance dependence (SD). The data suggest involvement of the DRD5 locus in the variation and sexual dimorphism of SD liability and antisociality and in the developmental continuity of antisociality.

Super-stereotypy II: enhancement of a complex movement sequence by intraventricular dopamine D1 agonists

This study compared the effect of intraventricular administration of dopamine D1 or D2 agonists or of ACTH on the sequential stereotypy of a serial pattern of grooming movements ("syntactic chain"). In a previous study, we showed that peripheral administration of D1 agonists increased the probability of occurrence and enhanced the stereotypy of the already-stereotyped movement pattern. Here we made microinjections of either SKF 38393 (a partial D1 agonist; 5, 10, 15, 20, 40 microg), SKF 82958 (a full D1 agonist; 5, 10, 20 microg), quinpirole (a D2 agonist; 5, 10, 20 microg), or ACTH-(1-24) (2, 5, 10 microg) into the lateral ventricles of rats. We measured the amount of grooming, the relative probability that the complex sequence pattern would occur, and the degree to which the syntactic pattern was completed faithfully. The total amount of grooming behavior was increased by intraventricular SKF 82958 and by ACTH, but was not changed by SKF 38393 and was decreased by quinpirole. Super-stereotypy of the sequential pattern was produced only by dopamine D1 agonists. The relative probability of initiating the syntactical sequence was increased by both SKF 38393 and SKF 82958, but was reduced by quinpirole and ACTH. The full D1 agonist, SKF 82958, also increased the likelihood that the pattern would be completed, thus causing sequential super-stereotypy in the strongest sense. Our results highlight a role for dopamine D1 receptors, probably within the basal ganglia, in the production of sequential super-stereotypy of complex behavioral patterns.

Attention-deficit hyperactivity disorder and the gene for the dopamine D5 receptor

A recent study has suggested a possible association of a polymorphism near the dopamine D5 receptor gene (DRD5) and attention-deficit hyperactivity disorder. The polymorphism studied was a (CA)n repeat located in the cosmid containing the D5 receptor gene2 and the allele that was reported to be associated with attention-deficit hyperactivity disorder (ADHD) was the 148-bp allele. In this study we sought to replicate this finding by testing for biased transmission of the alleles at this same polymorphism in a sample of 92 families with an ADHD proband. We did not observe significant evidence for biased transmission of the 148-bp allele, however we did observe biased transmission of two other alleles, the 136-bp allele and the 146-bp allele. For these two alleles the bias was for these two alleles not to be transmitted to the ADHD children. The number of informative transmissions for these two alleles was small, therefore it would be premature to make any conclusions from our study concerning the role of DRD5 in ADHD.

Complex segregation analysis of families ascertained through Gilles de la Tourette syndrome

Although family and twin studies suggest that genetic factors are involved in the etiology of Tourette syndrome and other related tic disorders, further evidence is needed to demonstrate that the familial transmission is consistent with known genetic factors. We performed a complex segregation analysis that allowed for a variable age of onset of Gilles de la Tourette, other tic disorders and obsessive compulsive phenotype information on 108 extended families, each ascertained through one Tourette proband by using regressive models that are able to incorporate additional explanatory variables and major gene effects. A special version of the S.A.G.E. program, REGTLhunt, was used to explore the likelihood surface of all examined models. Results indicated that the pattern of Tourette and other related tic disorders in our data sample is not consistent with Mendelian inheritance even after modelling explanatory variables such as obsessive compulsive symptomatology.

Tic disorders: new developments in Tourette syndrome and related disorders

The constellation of motor and vocal tics and certain of the other neuropsychiatric symptoms seen in Tourette syndrome are thought to have an organic basis, although the nature of the neurobiological lesion is uncertain. The syndrome is usually familial but the presumed genetic substrate has not been identified. A number of models currently under debate include a proposed autoimmune contribution.

Current status of genetic studies of Gilles de la Tourette syndrome

Gilles de la Tourette syndrome (TS) is a familial, neuropsychiatric disorder characterized by chronic, intermittent motor and vocal tics. Despite strong evidence for a genetic basis of this disorder from family, twin, and adoption studies, no convincing evidence for genetic linkage has been reported. Numerous groups world-wide have searched for genetic susceptibility factors for TS, testing specific candidate genes in neurotransmitter systems as well as DNA markers with known locations. Several factors may complicate the search for genes for this disorder, including diagnostic uncertainties, definition of the TS phenotypic spectrum as it relates to genetic susceptibility, assortative mating, genetic heterogeneity, and unclear mode of inheritance. In this article, we review the evidence for the genetic basis of TS and the current status of genetic studies.