Dopamine D2-like sites in schizophrenia, but not in Alzheimer's, Huntington's, or control brains, for [3H]benzquinoline

Identification of blood biomarkers for psychosis using convergent functional genomics

There are to date no objective clinical laboratory blood tests for psychotic disease states. We provide proof of principle for a convergent functional genomics (CFG) approach to help identify and prioritize blood biomarkers for two key psychotic symptoms, one sensory (hallucinations) and one cognitive (delusions). We used gene expression profiling in whole blood samples from patients with schizophrenia and related disorders, with phenotypic information collected at the time of blood draw, then cross-matched the data with other human and animal model lines of evidence. Topping our list of candidate blood biomarkers for hallucinations, we have four genes decreased in expression in high hallucinations states (Fn1, Rhobtb3, Aldh1l1, Mpp3), and three genes increased in high hallucinations states (Arhgef9, Phlda1, S100a6). All of these genes have prior evidence of differential expression in schizophrenia patients. At the top of our list of candidate blood biomarkers for delusions, we have 15 genes decreased in expression in high delusions states (such as Drd2, Apoe, Scamp1, Fn1, Idh1, Aldh1l1), and 16 genes increased in high delusions states (such as Nrg1, Egr1, Pvalb, Dctn1, Nmt1, Tob2). Twenty-five of these genes have prior evidence of differential expression in schizophrenia patients. Predictive scores, based on panels of top candidate biomarkers, show good sensitivity and negative predictive value for detecting high psychosis states in the original cohort as well as in three additional cohorts. These results have implications for the development of objective laboratory tests to measure illness severity and response to treatment in devastating disorders such as schizophrenia.

Case-control association study of 59 candidate genes reveals the DRD2 SNP rs6277 (C957T) as the only susceptibility factor for schizophrenia in the Bulgarian population

The development of molecular psychiatry in the last few decades identified a number of candidate genes that could be associated with schizophrenia. A great number of studies often result with controversial and non-conclusive outputs. However, it was determined that each of the implicated candidates would independently have a minor effect on the susceptibility to that disease. Herein we report results from our replication study for association using 255 Bulgarian patients with schizophrenia and schizoaffective disorder and 556 Bulgarian healthy controls. We have selected from the literatures 202 single nucleotide polymorphisms (SNPs) in 59 candidate genes, which previously were implicated in disease susceptibility, and we have genotyped them. Of the 183 SNPs successfully genotyped, only 1 SNP, rs6277 (C957T) in the DRD2 gene (P=0.0010, odds ratio=1.76), was considered to be significantly associated with schizophrenia after the replication study using independent sample sets. Our findings support one of the most widely considered hypotheses for schizophrenia etiology, the dopaminergic hypothesis.

Phenomic, convergent functional genomic, and biomarker studies in a stress-reactive genetic animal model of bipolar disorder and co-morbid alcoholism

We had previously identified the clock gene D-box binding protein (Dbp) as a potential candidate gene for bipolar disorder and for alcoholism, using a Convergent Functional Genomics (CFG) approach. Here we report that mice with a homozygous deletion of DBP have lower locomotor activity, blunted responses to stimulants, and gain less weight over time. In response to a chronic stress paradigm, these mice exhibit a diametric switch in these phenotypes. DBP knockout mice are also activated by sleep deprivation, similar to bipolar patients, and that activation is prevented by treatment with the mood stabilizer drug valproate. Moreover, these mice show increased alcohol intake following exposure to stress. Microarray studies of brain and blood reveal a pattern of gene expression changes that may explain the observed phenotypes. CFG analysis of the gene expression changes identified a series of novel candidate genes and blood biomarkers for bipolar disorder, alcoholism, and stress reactivity.

Dopaminergic polymorphisms in Tourette syndrome: association with the DAT gene (SLC6A3)

Tourette syndrome (TS) is a chronic neuropsychiatric disorder characterized by involuntary motor and phonic tics. The pattern of inheritance and associated genetic abnormality has yet to be fully characterized. A dopaminergic abnormality in this disorder is supported by response to specific therapies, nuclear imaging, and postmortem studies. In this protocol, dopaminergic polymorphisms were examined for associations with TS and attention-deficit hyperactivity disorder (ADHD). Polymorphisms investigated included the dopamine transporter (DAT1 DdeI and DAT1 VNTR), dopamine receptor (D4 Upstream Repeat and D4 VNTR), dopamine converting enzyme (dopamine beta-hydroxylase), and the acid phosphatase locus 1 (ACP1) gene. DNA was obtained from 266 TS individuals +/- ADHD and 236 controls that were ethnicity-matched. A significant association, using a genotype-based association analysis, was identified for the TS-total and TS-only versus control groups for the DAT1 DdeI polymorphism (AG vs. AA, P = 0.004 and P = 0.01, respectively). Population structure, estimated by the genotyping of 27 informative SNP markers, identified 3 subgroups. A statistical re-evaluation of the DAT1 DdeI polymorphism following population stratification confirmed the association for the TS-total and TS-only groups, but the degree of significance was reduced (P = 0.017 and P = 0.016, respectively). This study has identified a significant association between the presence of TS and a DAT polymorphism. Since abnormalities of the dopamine transporter have been hypothesized in the pathophysiology of TS, it is possible that this could be a functional allele associated with clinical expression.

Towards understanding the schizophrenia code: an expanded convergent functional genomics approach

Identifying genes for schizophrenia through classical genetic approaches has proven arduous. Here, we present a comprehensive convergent analysis that translationally integrates brain gene expression data from a relevant pharmacogenomic mouse model (involving treatments with a psychomimetic agent - phencyclidine (PCP), and an anti-psychotic - clozapine), with human genetic linkage data and human postmortem brain data, as a Bayesian strategy of cross validating findings. Topping the list of candidate genes, we have three genes involved in GABA neurotransmission (GABRA1, GABBR1, and GAD2), one gene involved in glutamate neurotransmission (GRIA2), one gene involved in neuropeptide signaling (TAC1), two genes involved in synaptic function (SYN2 and KCNJ4), six genes involved in myelin/glial function (CNP, MAL, MBP, PLP1, MOBP and GFAP), and one gene involved in lipid metabolism (LPL). These data suggest that schizophrenia is primarily a disorder of brain functional and structural connectivity, with GABA neurotransmission playing a prominent role. These findings may explain the EEG gamma band abnormalities detected in schizophrenia. The analysis also revealed other high probability candidates genes (neurotransmitter signaling, other structural proteins, ion channels, signal transduction, regulatory enzymes, neuronal migration/neurite outgrowth, clock genes, transcription factors, RNA regulatory genes), pathways and mechanisms of likely importance in pathophysiology. Some of the pathways identified suggest possible avenues for augmentation pharmacotherapy of schizophrenia with other existing agents, such as benzodiazepines, anticonvulsants and lipid modulating agents. Other pathways are new potential targets for drug development. Lastly, a comparison with our earlier work on bipolar disorder illuminates the significant molecular overlap between schizophrenia and bipolar disorder.

The polymorphic nature of the human dopamine D4 receptor gene: a comparative analysis of known variants and a novel 27 bp deletion in the promoter region

BACKGROUND

The human dopamine D4 receptor (DRD4) is a candidate gene of great interest in molecular studies of human personality and psychiatric disorders. This gene is unique in having an exceptionally high amount of polymorphic sites both in the coding and in the promoter region.

RESULTS

We report the identification of a new 27 bp deletion starting 524 bp upstream of the initiation codon (27 bp del) of the dopamine D4 receptor (DRD4) gene, in the close vicinity of the -521C>T SNP. The presence of the 27 bp deletion leads to the misgenotyping of the -616C>G SNP by the Sau96 I RFLP method, thus the genotype determination of the mutation is of additional importance. The frequency of this novel sequence variation is considerably low (allele frequency is = 0.16%), as no homozygotes, and only 3 heterozygote carriers were found in a healthy, unrelated Caucasian sample (N = 955).

CONCLUSION

Remarkably, the deleted region contains consensus sequences of binding sites for several known transcription factors, suggesting that the different alleles may affect the transcriptional regulation of the gene. A comparison of methods and results for the allelic variations of the DRD4 gene in various ethnic groups is also discussed, which has a high impact in psychiatric genetic studies.

Certain 1,4-Disubstituted Aromatic Piperidines and Piperazines with Extreme Selectivity for the Dopamine D4 Receptor Interact with a Common Receptor Microdomain

We previously demonstrated that, in the D4 dopamine receptor, the aromatic microdomain that spans the interface of the second and third transmembrane segments influences the high-affinity interactions with the D4-selective ligand L750,667 [3-[[4-(4-iodophenyl) piperazin-1-yl]methyl]-1H-pyrrolo[2,3-b]pyridine] and the D2-selective ligands methylspiperone, aripiprazole, and its congener OPC4392 [7-[3-(4-(2,3-dimethylphenyl) piperazinyl) propoxy] 2-(1H)-quinolinone] (Schetz et al., 2000). Here we tested a variety of 1,4-disubstituted aromatic piperidines/piperazines (1,4-DAPs) with different subtype selectivities and functional properties against a panel of D4 receptor mutations in the aromatic microdomain to ascertain whether these ligands recognize this common site. Mutant D4 receptors were constructed by substituting the nonconserved amino acid(s) from the corresponding locations in the D2 receptor. The D4-L2.60W, D4-F2.61V, and D4-LM3.28-3.29FV substitutions result in alterations of the relative position of members of the aromatic microdomain. From these results and molecular models of the ligand-receptor complexes, we conclude that 9 of the 11 D4-selective 1,4-DAPs, including L750,667, have a common pattern of ligand-receptor recognition that depends upon favorable interactions with the phenylalanine at position 2.61 (D4-F2.61V, 20-96-fold decrease). Like methylspiperone, aripiprazole, and OPC4392, the two D4-selective 1,4-DAPs that are insensitive to the D4-F2.61V mutation are sensitive to aromatics at position 2.60 (D4-L2.60W, 7-20-fold increase), and they all have longer spacer arms that permit their tethered aromatics to adopt alternative orientations in the binding-site crevice. All 11 of the D4-selective 1,4-DAPs were sensitive to the D4-LM3.28-3.29FV mutation (13-494-fold decrease) but not the moderately D2-selective methylspiperone. The inferences suggest that subtype selectivity involves two different modes of interaction with the microdomain for the D4-selective 1,4-DAPs and a third mode for D2-selective 1,4-DAPs.

The dopamine D4 receptors and mechanisms of antipsychotic atypicality

The dopamine D4 receptor (D4) is a target for most common neuroleptic medications. After its initial discovery, it was found to possess the highest affinity of all dopamine receptor subtypes for the archetypical, atypical, antipsychotic clozapine. Nevertheless, initial clinical trials have not provided evidence that this receptor is a primary target for antipsychotic drugs. Considering the accumulated in vivo evidence that at least a subgroup of psychotic patients have altered dopamine signaling, all dopamine receptor subtypes likely contribute to the phenotypic expression of schizophrenia. New insights into the function of this receptor and its role in the modulation of excitatory signaling support the view that this dopamine receptor may affect attention and cognition. In this review, the authors outline some recent developments that provide insight into D4 receptor physiology, function and its possible relationship to schizophrenia treatment.

Schizophrenia: from phenomenology to neurobiology

Schizophrenia is a common and debilitating illness, characterized by chronic psychotic symptoms and psychosocial impairment that exact considerable human and economic costs. The literature in electronic databases as well as citations and major articles are reviewed with respect to the phenomenology, pathology, treatment, genetics and neurobiology of schizophrenia. Although studied extensively from a clinical, psychological, biological and genetic perspective, our expanding knowledge of schizophrenia provides only an incomplete understanding of this complex disorder. Recent advances in neuroscience have allowed the confirmation or refutation of earlier findings in schizophrenia, and permit useful comparisons between the different levels of organization from which the illness has been studied. Schizophrenia is defined as a clinical syndrome that may include a collection of diseases that share a common presentation. Genetic factors are the most important in the etiology of the disease, with unknown environmental factors potentially modulating the expression of symptoms. Schizophrenia is a complex genetic disorder in which many genes may be implicated, with the possibility of gene-gene interactions and a diversity of genetic causes in different families or populations. A neurodevelopmental rather than degenerative process has received more empirical support as a general explanation of the pathophysiology, although simple dichotomies are not particularly helpful in such a complicated disease. Structural brain changes are present in vivo and post-mortem, with both histopathological and imaging studies in overall agreement that the temporal and frontal lobes of the cerebral cortex are the most affected. Functional imaging, neuropsychological testing and clinical observation are also generally consistent in demonstrating deficits in cognitive ability that correlate with abnormalities in the areas of the brain with structural abnormalities. The dopamine and other neurotransmitter systems are certainly involved in the treatment or modulation of psychotic symptoms. These broad findings represent the distillation of a large body of disparate data, but firm and specific findings are sparse, and much about schizophrenia remains unknown.

Tandem duplication polymorphism upstream of the dopamine D4 receptor gene (DRD4)

The dopamine D4 receptor (DRD4) is a member of the D2-like dopamine receptor family. Polymorphisms at the DRD4 gene have been examined for association with a wide range of neuropsychiatric disorders and normal behavioral variation. The DRD4 gene is unusual in its high amount of expressed polymorphism in humans. Here we study the identification of a polymorphic tandem duplication of 120 bp located 1.2 kb upstream of the initiation codon. The duplicated region contains consensus sequences of binding sites for several known transcription factors, suggesting that different alleles may differ in their transcriptional activity. Because chimpanzees, gorillas, and orangutans lack the duplication, the duplicated allele is inferred to be derived. The frequency of this derived duplication allele ranges from 0.40-0.81 in the 11 populations from around the world typed for this polymorphism. Am. J. Med. Genet. (Neuropsychiatr. Genet.) 88:705-709, 1999.

Cortically driven Fos induction in the striatum is amplified by local dopamine D2-class receptor blockade

Dopamine D2-class receptors have been shown to control the excitability of striatal neurons in response to cortical activation. It has been unclear, however, whether such receptors could regulate the number of striatal neurons activated by cortical stimulation, and thus affect the population response of the striatum to its cortical inputs. We used Fos induction as a readout to measure the ensemble response of striatal neurons to localized stimulation of the frontal cortex and tested for the effects of D2-class dopamine receptor blockade on this response. In freely moving rats, we stimulated the frontal cortex by local epidural application of a dose of a GABAA receptor antagonist (picrotoxin) just threshold for inducing Fos in the striatum. We combined this treatment with D2-class dopamine receptor antagonist treatments at dose levels also just threshold for inducing Fos, using either (i) systemic haloperidol or (ii) intrastriatal (-)sulpiride. Both systemic and intrastriatal blockade of D2-class receptors sharply increased the numbers of striatal neurons exhibiting cortically evoked Fos induction. These findings suggest that local activation of intrastriatal D2-class dopamine receptors can regulate the number of striatal neurons responsive to cortical inputs, thus dynamically shaping the flow of information through the striatum.

Neurochemical brain imaging investigations of schizophrenia

Neurochemical brain imaging methods developed over the past 20 years offer significant promise for elucidating the biochemical underpinnings of schizophrenia. The two general methodologies used for these studies have been: 1) radiotracer imaging: PET (positron emission tomography) and SPECT (single photon emission computed tomography); and 2) NMR (nuclear magnetic resonance) imaging: fMRI (functional magnetic resonance imaging) and MRS (magnetic resonance spectroscopy). Despite conflicting findings, striatal D2 receptor density may be elevated in some, but not all patients. Elevated synthesis, and increased release of dopamine after amphetamine challenge have also been reported. Imaging of cortical 5-HT2A receptors suggests that this system is unaffected, in conflict with findings of postmortem studies. Although prior postmortem studies suggested an increase in cortical GABAA receptors, three SPECT studies have found no significant changes. MRS studies have shown decreased levels of NAA (N-acetyl-aspartate) moieties in hippocampus and frontal cortex of schizophrenic patients, which is consistent with the reported loss of neurons and neuropil in postmortem brains. In conclusion, developments in radiotracer and NMR imaging have provided promising leads to the biochemical abnormalities associated with schizophrenia. Future significant understanding is likely to occur with the development of new probes and enhanced instrument technology, when applied with an appreciation of the heterogeneity of the disorder and the need for careful clinical assessment of patients.

Imaging dopamine D4 receptors in the living primate brain: a positron emission tomography study using the novel D1/D4 antagonist [11C]SDZ GLC 756

The dopamine D4 receptor has lately attracted interest since it has been hypothesized to be involved in the pathogenesis and pharmacotherapy of neuropsychiatric diseases. The present study provides first in vivo evidence of dopamine D4 receptors in primate brain using a [11C]benzo[g]quinoline, the novel radioligand [11C]SDZ GLC 756 ([11C]GLC: in vitro dissociation constants at human receptor clones [nM]: 1.10 at D1; 0.40 at D2; 25 at D3; 0.18 at D4.2; 6.03 at D5). Dynamic positron emission tomography scans were performed on healthy baboons (Papio hamadryas, n = 3). Specific receptor binding (SB) was calculated for striatum and neocortex (frontal, temporal, parietal, and occipital) based on the differences between the regional and the cerebellar concentration of [11C]. Blockade of D1 and D5 receptors by SCH23390 (1.7 pmol/kg) diminished SB in the striatum by 55 +/- 4% (mean +/- standard deviation, P < 0.05) and in the frontal cortex by 13 +/- 8% (P < 0.05) when compared to SB in the unblocked state (SB(D1-D5)). In the presence of the dopamine antagonists SCH23390 (1.7 micromol/kg) and raclopride (5.7 pmol/kg)--which mask the D1, D2, D3, and D5 subtypes--SB of [11C]GLC to D4 receptors (SB(D4)) was demonstrated in the striatum and all cortical regions of interest. In the striatum, the ratio of SB(D4)/SB(D1-D5) was 0.13 +/- 0.07. In the neocortex, SB(D4)/SB(D1-D5) was notably higher (0.77 +/- 0.29; mean of all cortical regions of interest). The widespread distribution of dopamine D4 receptors suggests a basic functional role of this receptor subtype in the modulation of cortical and subcortical neuronal activity.

Dopamine D2 and D4 receptor ligands: relation to antipsychotic action

Since the discovery that the antipsychotic action of phenothiazines was mediated by dopamine D2 receptors, the dopamine system has been scrutinized for schizophrenia related abnormalities. The focus has been to create neuroleptics with improved antipsychotic profiles and reduced side effects. With the identification of multiple dopamine receptor subtypes, the hypotheses regarding the role of dopamine in schizophrenia and antipsychotic action of neuroleptics have been refined. Even after the molecular identification of newer dopamine D2-like receptor subtypes (D3 and D4), the dopamine D2 receptor is still considered the predominant site for antipsychotic action. However, there has been much debate concerning the modulatory role of other dopamine receptor sites in the mechanism of action of antipsychotic drugs. Specifically, the dopamine D4 receptor has received much attention in this regard, since the atypical antipsychotic agent, clozapine, preferentially blocks this receptor subtype as compared with dopamine D2 and D3 receptors. In this review we will highlight some of the observations and arguments regarding the involvement of the dopamine D2 and D4 receptor sites in the therapeutic efficacy of antipsychotic medication.

A serotonin-4 receptor-like pseudogene in humans

During a search for new G-protein-linked receptors for dopamine and serotonin, we found a serotonin-4 receptor-like pseudogene. This receptor-like pseudogene is intronless, contains an in-frame stop codon following transmembrane-3, and has two one-nucleotide insertions between transmembrane-5 and -6 regions which alter the reading frame. The predicted amino acid sequence of the human pseudogene is about 35% identical with that of the rat serotonin-4 receptor.