The Bal I and Msp I polymorphisms in the dopamine D3 receptor gene display, linkage disequilibrium with each other but no association with 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.

Association between Tourette syndrome and the dopamine D3 receptor gene rs6280

BACKGROUND

Tourette syndrome (TS) is a complex, heterozygous genetic disorder. The number of molecular genetic studies have investigated several candidate genes, particularly those implicated in the dopamine system. The dopamine D3 receptor (DRD3) gene has been considered as a candidate gene in TS. There was not any report about the association study of TS and DRD3 gene in Han Chinese population. We combined a case-control genetic association analysis and nuclear pedigrees transmission disequilibrium test (TDT) analysis to investigate the association between DRD3 gene rs6280 single nucleotide polymorphisms (SNPs) and TS in a Han Chinese population.

METHODS

A total of 160 TS patients was diagnosed by the diagnostic criteria of the Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition. The DRD3 gene rs6280 SNPs were genotyped by TaqMan SNP genotyping assay technique in all subjects. We used a case-control genetic association analysis to compare the difference in genotype and allele frequencies between 160 TS patients and 90 healthy controls. At the same time, we used TDT analysis to identify the DRD3 gene rs6280 transmission disequilibrium among 101 nuclear pedigrees.

RESULTS

The genotype and allele frequency of DRD3 gene rs6280 SNPs had no statistical difference between control group (90) and TS group (160) (χ2 = 3.647, P = 0.161; χ2 = 0.643, P = 0.423) using Chi-squared test. At the basis of the 101 nuclear pedigrees, TDT analysis showed no transmission disequilibrium of DRD3 gene rs6280 SNPs (χ2 = 0; P = 1).

CONCLUSIONS

Our findings provide no evidence for an association between DRD3 gene rs6280 and TS in the Han Chinese population.

Autism, DRD3 and repetitive and stereotyped behavior, an overview of the current knowledge

The SNP rs167771 of the dopamine-3-receptor gene (DRD3) has been associated with autism spectrum disorder (ASD) in samples from the United Kingdom, The Netherlands and Spain. The DRD3 polymorphisms of rs167771 are significantly associated with a specific type of repetitive and stereotyped behavior, called sameness. Repetitive and stereotyped behavior occurs in several neuropsychiatric disorders and the combined picture across these disorders strongly suggests the involvement of the basal ganglia - frontal lobe circuitry. In autism, abnormalities of the basal ganglia, in particular the caudate nucleus, are the best replicated findings in neuroimaging studies. Interestingly, the DRD3 gene is highly expressed in the basal ganglia, most notably the caudate nucleus. The rs167771 SNP was recently also found to be related to risperidone-induced extra-pyramidal side effects (EPS) in patients with autism, which is important since risperidone is approved for the treatment of aggression, irritability and rigid behavior in ASD. To conclude, striatum abnormalities in autism are associated with repetitive and stereotyped behavior in autism and may be related to DRD3 polymorphisms.

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.

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.

Alterations in dopamine D3 receptors in the circling (ci3) rat mutant

We have previously described a black-hooded mutant rat (BH.7A/Ztm-ci3/ci3) that displays abnormal lateralized circling behavior, but normal auditory and vestibular functions. Neurochemical determination of dopamine and dopamine metabolite levels in striatum, nucleus accumbens and substantia nigra showed that ci3 rats have a significant asymmetry in striatal dopamine in that dopamine levels were significantly lower in the hemisphere contralateral to the preferred direction of turning. Consistent with this finding, immunohistological examination of dopaminergic neurons in substantia nigra and ventral tegmental area yielded a significant laterality in the medial part of substantia nigra pars compacta with a lower density of tyrosine hydroxylase-positive neurons in the contralateral hemisphere of mutant circling rats, while no laterality was seen in unaffected rats of the background strain. In the present study, quantitative autoradiography was used to examine the binding of [(3)H]SCH 23390, [(3)H]raclopride and [(3)H]7-OH-DPAT (7-hydroxy-N,N-di-n-propyl-2-aminotetralin) to dopamine D1, D2, and D3 receptors, respectively, in various brain regions of ci3 rats and unaffected rats of the background strain (BH.7A(LEW)/Won). No significant differences between circling rats and controls were obtained for D1 and D2 receptor binding in any region, but mutant rats differed from controls in dopamine D3 binding in several regions. A significant decrease in D3 binding was seen in the shell of the nucleus accumbens, the islands of Calleja, and the subependymal zone of ci3 mutant rats. Furthermore, a significant laterality in D3 binding was determined in ci3 rats in that binding was lower in the contralateral hemisphere in the shell of the nucleus accumbens and the islands of Calleja. Our data indicate that alterations of dopamine D3 receptors may be involved in the behavioral phenotype of the ci3 rat, thus substantiating the findings from a recent genetic linkage analysis that indicated the D3 receptor gene as a candidate gene in this rat mutant.

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.

Early-onset schizophrenia and dopamine-related gene polymorphism

Schizophrenic patients with an onset before age 16 years (early-onset schizophrenia, EOS) would be a rare but attractive subpopulation for genetic studies. This study explored the relationship between the polymorphism of four dopamine-regulating-enzymes (tyrosine hydroxylase, dopamine-beta-hydroxylase, catechol-O-methyltransferase, monoamine oxidase-A) genes, four dopamine-receptors (dopamine D1, D2, D3, D4 receptors) genes and susceptibility to EOS in a Japanese sample. Subjects comprised 51 Japanese patients who met DSM-IV criteria for schizophrenia with an onset before age 16 (by age 15) and 148 Japanese healthy controls. DNA was extracted from whole blood and genotyping was carried out by PCR-RELP using each restriction endonuclease. No significant difference was found in the allele frequencies or genotype distributions of any of the eight genes examined between EOS and the control groups. We did not find the relationship between the polymorphism of eight dopamine-related genes and susceptibility to EOS in a Japanese sample.

Characterisation, mutation detection, and association analysis of alternative promoters and 5' UTRs of the human dopamine D3 receptor gene in schizophrenia

The dopamine D(3) receptor gene (DRD3) is a candidate for a number of psychiatric conditions including schizophrenia, bipolar disorder and alcohol and drug abuse. Previous studies have reported associations between polymorphisms in DRD3 and these disorders, but these findings may have reflected linkage disequilibrium with pathogenic variants that are further upstream. We have isolated and sequenced approximately 9 kb of genomic sequence upstream of the human DRD3 translational start site. Using 5' RACE, we have identified within this region three additional exons and two putative promoter regions which show promoter activity in three different cell lines. A 5' UTR identified only in lymphoblasts is spread over three exons and is 353 bp long. A second 5' UTR, found in adult and fetal brain, lymphocytes, kidney and placenta is spread over two exons and is 516 bp long. A 260-bp sequence within this 9 kb corresponds to a previously reported EST, but corresponding mRNA could not be found in the tissues above. The EST, 5' UTRs and putative promoter regions have been analysed for polymorphisms, revealing 10 single nucleotide polymorphisms, seven of which were tested for association in a large sample of unrelated patients with schizophrenia and matched controls. No associations were observed with schizophrenia. In addition we failed to replicate previous findings of association with homozygosity of the Ser9Gly variant. The results from this study imply that neither the coding nor the regulatory region of DRD3 plays a major role in predisposition to schizophrenia.

Polymorphisms in dopamine receptors: what do they tell us?

Many genetic studies have focussed on dopamine receptors and their relationship to neuropsychiatric disease. Schizophrenia, bipolar disorder, and substance abuse have been the most studied, but no conclusive linkage or association has been found. The possible influence of dopamine receptor variants on drug response has not received as much attention. While there is some evidence that polymorphisms and mutations in dopamine receptors can alter functional activity and pharmacological profiles, no conclusive data link these gene variants to drug response or disease. The lack of unequivocal findings may be related, in part, to the subtle changes in receptor pharmacology that these polymorphisms and mutations mediate. These subtle effects may be obscured by the influence of genes controlling drug metabolism and kinetics. Further insight into the pharmacogenetics of dopamine receptors may require not just more studies, but novel approaches to the study of complex genetic traits and diseases.

Reward deficiency syndrome: a biogenetic model for the diagnosis and treatment of impulsive, addictive, and compulsive behaviors

The dopaminergic system, and in particular the dopamine D2 receptor, has been implicated in reward mechanisms. The net effect of neurotransmitter interaction at the mesolimbic brain region induces "reward" when dopamine (DA) is released from the neuron at the nucleus accumbens and interacts with a dopamine D2 receptor. "The reward cascade" involves the release of serotonin, which in turn at the hypothalmus stimulates enkephalin, which in turn inhibits GABA at the substania nigra, which in turn fine tunes the amount of DA released at the nucleus accumbens or "reward site." It is well known that under normal conditions in the reward site DA works to maintain our normal drives. In fact, DA has become to be known as the "pleasure molecule" and/or the "antistress molecule." When DA is released into the synapse, it stimulates a number a DA receptors (D1-D5) which results in increased feelings of well-being and stress reduction. A consensus of the literature suggests that when there is a dysfunction in the brain reward cascade, which could be caused by certain genetic variants (polygenic), especially in the DA system causing a hypodopaminergic trait, the brain of that person requires a DA fix to feel good. This trait leads to multiple drug-seeking behavior. This is so because alcohol, cocaine, heroin, marijuana, nicotine, and glucose all cause activation and neuronal release of brain DA, which could heal the abnormal cravings. Certainly after ten years of study we could say with confidence that carriers of the DAD2 receptor A1 allele have compromised D2 receptors. Therefore lack of D2 receptors causes individuals to have a high risk for multiple addictive, impulsive and compulsive behavioral propensities, such as severe alcoholism, cocaine, heroin, marijuana and nicotine use, glucose bingeing, pathological gambling, sex addiction, ADHD, Tourette's Syndrome, autism, chronic violence, posttraumatic stress disorder, schizoid/avoidant cluster, conduct disorder and antisocial behavior. In order to explain the breakdown of the reward cascade due to both multiple genes and environmental stimuli (pleiotropism) and resultant aberrant behaviors, Blum united this hypodopaminergic trait under the rubric of a reward deficiency syndrome.