Variable number of tandem repeat polymorphisms of DRD4: re-evaluation of selection hypothesis and analysis of association with schizophrenia

DRD4 VNTR 4/4 homozygosity as a genetic biomarker for treatment selection in patients with schizophrenia

OBJECTIVE

There seems to be an association between the DRD4 48-bp VNTR polymorphisms and antipsychotic treatment response, but there is a rare reference to confirm this finding. Hence, the present study tried to investigate the association between DRD4 48-bp VNTR polymorphisms and the treatment response of antipsychotics in patients with schizophrenia in Taiwan, using a propensity score matching (PSM) method.

METHODS

A total of 882 participants were enrolled in this study and completed informed consent, research questionnaires, including demographic information and the revised Chinese version Beliefs about Voices Questionnaire, and blood sampling. For descreasing of the selection bias and confounding variables, the PSM nearest neighbor matching method was used to select 765 paitents with schizophrenia (ratio of 1:8 between 85 persistent auditory hallucination and 680 controls) with matched and controlled the age and gender.

RESULTS

Schizophrenia patients with DRD4 4 R homozygosity had a lower rate of good antipsychotic treatment response than the other DRD4 genotype carriers (DRD4 non-4/4). Among those 4 R homozygosity carriers, 60 cases of 503 (11.9%) retain persistent auditory hallucinations. Furthermore, this subgroup of patients is accounted for up to 70.6% of cases with poor neuroleptic treatment response.

CONCLUSIONS

A poor treatment outcome for patients with the 4 R homozygosity had presented,that comparing with those DRD non-4/4 genotype carriers. DRD4 VNTR 4 R homozygosity could be a genetic biomarker to predict poor antipsychotic treatment response in schizophrenia. Patients with DRD 4/4 probably receive novel antipsychotic medications preferentially or in combination with alternative therapy, such as psychotherapy or milieu therapy.

Revisiting tandem repeats in psychiatric disorders from perspectives of genetics, physiology, and brain evolution

Genome-wide association studies (GWASs) have revealed substantial genetic components comprised of single nucleotide polymorphisms (SNPs) in the heritable risk of psychiatric disorders. However, genetic risk factors not covered by GWAS also play pivotal roles in these illnesses. Tandem repeats, which are likely functional but frequently overlooked by GWAS, may account for an important proportion in the "missing heritability" of psychiatric disorders. Despite difficulties in characterizing and quantifying tandem repeats in the genome, studies have been carried out in an attempt to describe impact of tandem repeats on gene regulation and human phenotypes. In this review, we have introduced recent research progress regarding the genomic distribution and regulatory mechanisms of tandem repeats. We have also summarized the current knowledge of the genetic architecture and biological underpinnings of psychiatric disorders brought by studies of tandem repeats. These findings suggest that tandem repeats, in candidate psychiatric risk genes or in different levels of linkage disequilibrium (LD) with psychiatric GWAS SNPs and haplotypes, may modulate biological phenotypes related to psychiatric disorders (e.g., cognitive function and brain physiology) through regulating alternative splicing, promoter activity, enhancer activity and so on. In addition, many tandem repeats undergo tight natural selection in the human lineage, and likely exert crucial roles in human brain evolution. Taken together, the putative roles of tandem repeats in the pathogenesis of psychiatric disorders is strongly implicated, and using examples from previous literatures, we wish to call for further attention to tandem repeats in the post-GWAS era of psychiatric disorders.

A meta-analysis of data associating DRD4 gene polymorphisms with schizophrenia

To explore the association between DRD4 polymorphisms and schizophrenia risk, a meta-analysis was carried out with 41 case-control articles. Specifically, we included 28 articles (5,735 cases and 5,278 controls) that pertained to the 48 bp variable number tandem repeat (VNTR) polymorphism, nine articles (1,517 cases and 1,746 controls) that corresponded to the 12 bp tandem repeat (TR), six articles (1,912 cases and 1,836 controls) that addressed the 120 bp TR, 10 articles (2,927 cases and 2,938 controls) that entailed the -521 C>T polymorphism, six articles (1,735 cases and 1,724 controls) that pertained to the -616 C>G polymorphism, and four articles (1,191 cases and 1,215 controls) that involved the -376 C>T polymorphism. Pooled analysis, subgroup analysis, and sensitivity analysis were performed, and the data were visualized by means of forest and funnel plots. Results of pooled analysis indicated that the -521 CC variant (Pz=0.009, odds ratio [OR] =1.218, 95% confidence interval [CI] =1.050-1.413) and genotype L/L (ie, long allele) of the 120 bp TR were risk factors of schizophrenia (Pz=0.004, OR =1.275, 95% CI =1.081-1.504). The 48 bp VNTR, the 12 bp TR, the -616 C>G polymorphism, and the -376 C>T polymorphism were not associated with schizophrenia. Additional research is warranted to explore the association between polymorphisms of DRD4 and schizophrenia risk.

Molecular Signatures of Natural Selection for Polymorphic Genes of the Human Dopaminergic and Serotonergic Systems: A Review

A large body of research has examined the behavioral and mental health consequences of polymorphisms in genes of the dopaminergic and serotonergic systems. Along with this, there has been considerable interest in the possibility that these polymorphisms have developed and/or been maintained due to the action of natural selection. Episodes of natural selection on a gene are expected to leave molecular “footprints” in the DNA sequences of the gene and adjacent genomic regions. Here we review the research literature investigating molecular signals of selection for genes of the dopaminergic and serotonergic systems. The gene SLC6A4, which codes for a serotonin transport protein, was the one gene for which there was consistent support from multiple studies for a selective episode. Positive selection on SLC6A4 appears to have been initiated ∼ 20–25,000 years ago in east Asia and possibly in Europe. There are scattered reports of molecular signals of selection for other neurotransmitter genes, but these have generally failed at replication across studies. In spite of speculation in the literature about selection on these genes, current evidence from population genomic analyses supports selectively neutral processes, such as genetic drift and population dynamics, as the principal drivers of recent evolution in dopaminergic and serotonergic genes other than SLC6A4.

Liat1, an arginyltransferase-binding protein whose evolution among primates involved changes in the numbers of its 10-residue repeats

The arginyltransferase Ate1 is a component of the N-end rule pathway, which recognizes proteins containing N-terminal degradation signals called N-degrons, polyubiquitylates these proteins, and thereby causes their degradation by the proteasome. At least six isoforms of mouse Ate1 are produced through alternative splicing of Ate1 pre-mRNA. We identified a previously uncharacterized mouse protein, termed Liat1 (ligand of Ate1), that interacts with Ate1 but does not appear to be its arginylation substrate. Liat1 has a higher affinity for the isoforms Ate1(1A7A) and Ate1(1B7A). Liat1 stimulated the in vitro N-terminal arginylation of a model substrate by Ate1. All examined vertebrate and some invertebrate genomes encode proteins sequelogous (similar in sequence) to mouse Liat1. Sequelogs of Liat1 share a highly conserved ∼30-residue region that is shown here to be required for the binding of Liat1 to Ate1. We also identified non-Ate1 proteins that interact with Liat1. In contrast to Liat1 genes of nonprimate mammals, Liat1 genes of primates are subtelomeric, a location that tends to confer evolutionary instability on a gene. Remarkably, Liat1 proteins of some primates, from macaques to humans, contain tandem repeats of a 10-residue sequence, whereas Liat1 proteins of other mammals contain a single copy of this motif. Quantities of these repeats are, in general, different in Liat1 of different primates. For example, there are 1, 4, 13, 13, 17, and 17 repeats in the gibbon, gorilla, orangutan, bonobo, neanderthal, and human Liat1, respectively, suggesting that repeat number changes in this previously uncharacterized protein may contribute to evolution of primates.

Haplotype and nucleotide variation in the exon 3-VNTR of the DRD4 gene from indigenous and urban populations of Mexico

OBJECTIVE

To describe the population structure of the 48-bp variable number of tandem repeats (VNTR), located in exon 3 of the dopamine receptor D4 gene (DRD4), in 41 Tarahumara from northern Mexico, 20 Mixe from southern Mexico, and 169 people from Mexico City.

METHODS

Genotypes for the DRD4-VNTR were determined, from which 15 Tarahumara, eight Mixe, and 37 urban homozygous individuals were sequenced. Repeat-allele frequencies were compared with other world populations.

RESULTS

The DRD4-VNTR variation in Mexico City appeared similar to the world mean. For the Mixe and Maya, DRD4-VNTR diversity appeared closer to South American groups whereas the Tarahumara were similar to North American groups. People from Mexico City and the Mixe exhibited attributes of a large and admixed population and an isolated population, respectively. The Tarahumara showed endogamy associated with a substructure as suggested by a preliminary regional differentiation. For the DRD4-VNTR and/or the adjacent 5'-173 bp sequence, the three populations exhibited negative Tajima's D. Two new VNTR haplotypes were discovered: one in Mexico City and another among the Tarahumara.

CONCLUSIONS

A differentiation in the DRD4-VNTR of global relevance occurs between northern and southern populations of Mexico suggesting that the Mexican Trans-volcanic Belt has been a major frontier for human dispersion in the Americas. Ancient trespass of this barrier appears thus related to a major change in the population structure of the DRD4-VNTR. Distinctive and independent patterns of DRD4-VNTR diversity occur among the two Mexican indigenous populations by a still undefined combination of drift and selection.

The role of D4 receptor gene exon III polymorphisms in shaping human altruism and prosocial behavior

Human beings are an extraordinarily altruistic species often willing to help strangers at a considerable cost (sometimes life itself) to themselves. But as Darwin noted "… he who was ready to sacrifice his life, as many a savage has been, rather than betray his comrades, would often leave no offspring to inherit his noble nature." Hence, this is the paradox of altruism. Twin studies have shown that altruism and other prosocial behavior show considerable heritability and more recently a number of candidate genes have been identified with this phenotype. Among these first provisional findings are genes encoding elements of dopaminergic transmission. In this article we will review the evidence for the involvement of one of these, the dopamine D4 receptor (DRD4) gene, in shaping human prosocial behavior and consider the methodologies employed in measuring this trait, specific molecular genetic findings and finally, evidence from several Gene × Environment (G × E) studies that imply differential susceptibility of this gene to environmental influences.

Is there a role for rare variants in DRD4 gene in the susceptibility for ADHD? Searching for an effect of allelic heterogeneity

Although several studies have demonstrated an association between the 7-repeat (7R) allele in the 48-bp variable number of tandem repeats (VNTRs) in the exon 3 at dopamine receptor D4 (DRD4) gene and attention-deficit/hyperactivity disorder (ADHD), others failed to replicate this finding. In this study, a total of 786 individuals with ADHD were genotyped for DRD4 exon 3 VNTR. All 7R homozygous subjects were selected for VNTR re-sequencing. Subjects homozygous for the 4R allele were selected paired by age, ancestry and disorder subtypes in order to have a sample as homogeneous as possible with 7R/7R individuals. Using these criteria, 103 individuals (66 with ADHD and 37 control individuals) were further investigated. An excess of rare variants were observed in the 7R alleles of ADHD patient when compared with controls (P=0.031). This difference was not observed in 4R allele. Furthermore, nucleotide changes that predict synonymous and non-synonymous substitutions were more common in the 7R sample (P=0.008 for total substitutions and P=0.043 for non-synonymous substitutions). In silico prediction of structural/functional alterations caused by these variants have also been observed. Our findings suggest that not only repeat length but also DNA sequence should be assessed to better understand the role of DRD4 exon 3 VNTR in ADHD genetic susceptibility.

No evidence for strong recent positive selection favoring the 7 repeat allele of VNTR in the DRD4 gene

The human dopamine receptor D4 (DRD4) gene contains a 48-bp variable number of tandem repeat (VNTR) in exon 3, encoding the third intracellular loop of this dopamine receptor. The DRD4 7R allele, which seems to have a single origin, is commonly observed in various human populations and the nucleotide diversity of the DRD4 7R haplotype at the DRD4 locus is reduced compared to the most common DRD4 4R haplotype. Based on these observations, previous studies have hypothesized that positive selection has acted on the DRD4 7R allele. However, the degrees of linkage disequilibrium (LD) of the DRD4 7R allele with single nucleotide polymorphisms (SNPs) outside the DRD4 locus have not been evaluated. In this study, to re-examine the possibility of recent positive selection favoring the DRD4 7R allele, we genotyped HapMap subjects for DRD4 VNTR, and conducted several neutrality tests including long range haplotype test and iHS test based on the extended haplotype homozygosity. Our results indicated that LD of the DRD4 7R allele was not extended compared to SNP alleles with the similar frequency. Thus, we conclude that the DRD4 7R allele has not been subjected to strong recent positive selection.

Role of functional dopaminergic gene polymorphisms in the etiology of idiopathic intellectual disability

Intellectual disability (ID) is of major concern throughout the world, though in ~40% of cases etiology remains unknown (idiopathic ID or IID). Cognitive impairment and behavioral problems are of common occurrence in these subjects and dopamine is known to play an important role in regulating these traits. In the present study the role of functional polymorphisms in three dopaminergic genes, dopamine receptor D4 (DRD4: exon3 VNTR and rs1800955), dopamine transporter (DAT1: 3'UTR VNTR and intron8 VNTR) and catechol-O-methyl transferase (COMT: rs4680 and rs165599), was explored in IID. Probands (n=225), parents (n=298) and ethnically matched controls (n=175) were recruited following DSM-IV. Genotype data obtained was used for population- and family-based statistical analyses. Population-based analysis showed significant association of DRD4 exon3 VNTR 6R allele (P=0.01), DAT1 3'UTR VNTR lower repeat (6R and 7R) alleles (P<0.02) and intron8 VNTR 5R allele (P=0.0012) with IID. Stratified analysis revealed significant association of these alleles (P<0.05) with IID individuals exhibiting severe behavioral problems. On the other hand, preferential transmission of COMT rs4680 A allele and A-A haplotype (P<0.05) was observed specifically in male IID probands without any behavioral problem. Markers failed to show any significant epistatic interaction by MDR analysis. Alleles showing positive association were all reported to confer suboptimal activity to the transcribed proteins. Therefore, an alteration in dopaminergic neurotransmission could be predicted that may lead to impairments in cognition and behavioral problems.

Evolutionary genomics of animal personality

Research on animal personality can be approached from both a phenotypic and a genetic perspective. While using a phenotypic approach one can measure present selection on personality traits and their combinations. However, this approach cannot reconstruct the historical trajectory that was taken by evolution. Therefore, it is essential for our understanding of the causes and consequences of personality diversity to link phenotypic variation in personality traits with polymorphisms in genomic regions that code for this trait variation. Identifying genes or genome regions that underlie personality traits will open exciting possibilities to study natural selection at the molecular level, gene-gene and gene-environment interactions, pleiotropic effects and how gene expression shapes personality phenotypes. In this paper, we will discuss how genome information revealed by already established approaches and some more recent techniques such as high-throughput sequencing of genomic regions in a large number of individuals can be used to infer micro-evolutionary processes, historical selection and finally the maintenance of personality trait variation. We will do this by reviewing recent advances in molecular genetics of animal personality, but will also use advanced human personality studies as case studies of how molecular information may be used in animal personality research in the near future.

Dopamine receptor D4 allele distribution in Amerindians: a reflection of past behavior differences?

The DRD4 variable number of tandem repeats (VNTR) allele distribution of 172 Guarani (Kaiowá and Ñandeva subgroups) and Kaingang Brazilian Amerindians is reported. These results are integrated with those previously obtained for this ethnic group. Allele frequencies for the three populations are within the interval observed for 15 other Native American populations and show intermediate values between those observed in Amazonia and Patagonia. Significant differences in allele distribution between recent past hunter-gatherer and agriculturalist populations are observed, with an increase of the 7R allele among hunter-gatherers (P < 0.001). Analysis of molecular variance (AMOVA) and pairwise F(ST) data suggest three distinct sectors for the genetic landscape of Native South America: Andes, Center/Southeast region, and Amazonia. Common traits among hunter-gatherers such as novelty-seeking temperament, hyperactivity, and impulsivity could have been important and advantageous in new environments during America's prehistoric colonization.

Genomics and pharmacogenomics of schizophrenia

Schizophrenia (SCZ) is among the most disabling of mental disorders. Several neurobiological hypotheses have been postulated as responsible for SCZ pathogenesis: polygenic/multifactorial genomic defects, intrauterine and perinatal environment-genome interactions, neurodevelopmental defects, dopaminergic, cholinergic, serotonergic, gamma-aminobutiric acid (GABAergic), neuropeptidergic and glutamatergic/N-Methyl-D-Aspartate (NMDA) dysfunctions, seasonal infection, neuroimmune dysfunction, and epigenetic dysregulation. SCZ has a heritability estimated at 60-90%. Genetic studies in SCZ have revealed the presence of chromosome anomalies, copy number variants, multiple single-nucleotide polymorphisms of susceptibility distributed across the human genome, aberrant single nucleotide polymorphisms (SNPs) in microRNA genes, mitochondrial DNA mutations, and epigenetic phenomena. Pharmacogenetic studies of psychotropic drug response have focused on determining the relationship between variation in specific candidate genes and the positive and adverse effects of drug treatment. Approximately, 18% of neuroleptics are major substrates of CYP1A2 enzymes, 40% of CYP2D6, and 23% of CYP3A4; 24% of antidepressants are major substrates of CYP1A2 enzymes, 5% of CYP2B6, 38% of CYP2C19, 85% of CYP2D6, and 38% of CYP3A4; 7% of benzodiazepines are major substrates of CYP2C19 enzymes, 20% of CYP2D6, and 95% of CYP3A4. About 10-20% of Western populations are defective in genes of the CYP superfamily. Only 26% of Southern Europeans are pure extensive metabolizers for the trigenic cluster integrated by the CYP2D6+CYP2C19+CYP2C9 genes. The pharmacogenomic response of SCZ patients to conventional psychotropic drugs also depends on genetic variants associated with SCZ-related genes. Consequently, the incorporation of pharmacogenomic procedures both to drugs in development and drugs on the market would help to optimize therapeutics in SCZ and other central nervous system (CNS) disorders.