Failure to find linkage between a functional polymorphism in the dopamine D4 receptor gene and schizophrenia

Nonparametric linkage analysis between schizophrenia and candidate genes of dopaminergic and serotonergic systems

BACKGROUND

Alterations in dopaminergic and serotonergic systems have been implicated in the pathophysiology of schizophrenia for many years. This study was performed to assess the possible involvement of the dopamine receptor genes D2 (DRD2), D3, D4, serotonin receptor genes 1Da, 1Db, and 2A in the etiology of schizophrenia.

METHODS

We examined 33 multiplex schizophrenic families from Portugal.

RESULTS

Linkage analysis performed by GENEHUNTER showed nonsignificant linkage for these genes. A maximum nonparametric linkage score of 1.635 (P=.032) at DRD2 gene was observed, and this finding suggests DRD2 gene for further studies.

CONCLUSION

the polymorphisms studied at dopamine receptor genes D3, D4, serotonin receptor genes 1Da, 1Db, and 2A do not have a major effect in susceptibility to schizophrenia in a Portuguese population.

No evidence of association or linkage disequilibrium between polymorphisms in the 5' upstream and coding regions of the dopamine D4 receptor gene and schizophrenia in a Portuguese population

Alterations in dopaminergic system have been implicated in the pathophysiology of this disease for many years, and this study was performed to assess the possible involvement of the dopamine D4 receptor (DRD4) gene polymorphisms either in the 5' upstream or in the coding regions, in the etiology of schizophrenia. The approach included an association study with 90 Portuguese trios by doing the analysis of the individual alleles and the haplotypes. For the polymorphisms in the 5' upstream region (-C616G and -C521T) and in the coding region (48 bp repeat) of the DRD4 gene, negative results were obtained with both haplotype relative risk (HRR) and transmission disequilibrium test (TDT), as well as transmit. These data suggest that polymorphisms (-C616G, -C521T, and 48 bp repeat) at the DRD4 gene do not have a minor effect in the susceptibility to schizophrenia in our sample.

The new genetics of schizophrenia

Despite the genetic and phenotypic complexity of schizophrenia, much progress has been made. Research has largely excluded the possibility that genes of major effect exist; linkage analysis has provided independently replicated evidence for genes of moderate effect on several chromosomal regions. Association studies suggest that alleles of at least two genes, those encoding D3 and 5HT2A, confer a small rise in susceptibility to schizophrenia, and there are convergent findings from several different lines of research implicating regions such as 22q11, although no specific causative genes for schizophrenia have been definitively identified yet. There are strong grounds for optimism as larger samples are collected to increase the power of studies, and novel methods of statistical analysis and large-scale genotyping of SNPs are developed and refined. Although the difficulties and challenges of genetics research into schizophrenia are formidable, the devastating personal and social consequences of the illness make it imperative that these challenges are faced, because the identification of susceptibility genes for schizophrenia would result in further productive neurobiologic research and ultimately improvements in the prevention and treatment of schizophrenia.

Ethnic heterogeneity in allele variation in the DRD4 gene in schizophrenia

The goal of the present study was to use a meta-analysis on previous studies plus our own unpublished data to confirm and extend findings which indicate that the variation in the dopamine D4 receptor (DRD4) gene is best represented by a mixture of two different ethnic groups. The genotype distribution was divided into either a long or short form using a mixture analysis of normal controls of different ethnic origins under the assumption that there is a single major gene. The meta-analysis was based on the data from 19 independent samples, 18 association studies, and from our own unpublished data, including a total of 1431 schizophrenic patients (sporadic cases 1309, familial cases 122) and 1439 controls. No significant genotype differences were noted between patients and controls for the whole sample. However, reorganization of the studies into different groups by the geographical origin of samples revealed significant ethnic heterogeneity. In addition, there was a significant association between the long form of DRD4 gene and schizophrenia in Caucasians, especially those with familial schizophrenia.

Developments in the pharmacological treatment of schizophrenia

Schizophrenia is, at once, a biological disease, a neuropsychological disorder and a dysfunction of social interactions. This presents clinicians with a series of problems with regards to therapy. In the first section of this article, some of the clinical challenges that face those attempting to develop new drugs, are summarised. Several potential pharmacological therapeutic targets that have been, and are continuing to be used, in the development of new antipsychotic drugs, are then considered. This is followed by an outline of the pharmacological and clinical profiles of some of the newer generation antipsychotics, as well as investigational drugs in the pipeline for schizophrenia. Finally, the implications of the introduction of these new drugs for the management of schizophrenia, are discussed.

Family-based association study of 5-HTTLPR, TPH, MAO-A, and DRD4 polymorphisms in mood disorders

Variants of the functional polymorphism in the serotonin transporter (upstream regulatory region: 5-HTTLPR), the tryptophan hydroxylase (TPH), the monoamine oxidase A (MAO-A), and the dopamine receptor D4 (DRD4) genes have all been associated with mood disorders. The aim of this study was to test those hypotheses by using a family-based association approach. Both diagnoses and psychopathology were used for phenotype definitions. A total of 134 nuclear families with mood disorders, with probands affected by bipolar (n = 103) or major depressive (n = 58) disorders, were included in the study. All subjects were typed for the above-mentioned gene variants using polymerase chain reaction (PCR) technique. No significant transmission disequilibrium was found in the overall sample for any polymorphism. A separate analysis of bipolar subjects only, or the use of continuous psychopathologic traits as affectation status did not influence the observed results. Our study did not support the involvement of 5-HTTLPR, TPH, MAO-A, or DRD4 polymorphisms in mood disorders.

DRD4 exon 3 variants associated with delusional symptomatology in major psychoses: a study on 2,011 affected subjects

We previously reported an association of DRD4 exon3 long allele variants with delusional symptomatology independently from diagnoses. The aim of this investigation was to study DRD4 in major psychoses and to test the association in a larger sample. We studied 2,011 inpatients affected by bipolar disorder (n = 811), major depressive disorder (n = 635), schizophrenia (n = 419), delusional disorder (n = 104), psychotic disorder not otherwise specified (n = 42), and 601 healthy controls. A subsample of 1,264 patients were evaluated using the OPCRIT checklist and differences of symptomatology factor scores among genetic variants were assessed using one-way analysis of variance (ANOVA). DRD4 allele and genotype frequencies in bipolars, schizophrenics, delusionals, and psychotic NOS were not significantly different from controls; major depressives showed a trend toward an excess of DRD4*Short and DRD4*Short/Short variants versus controls. The ANOVA on factor scores in the whole subsample of 1,264 subjects showed a significant difference on delusion factor in allele analysis (P = 0.007), and in genotype one (P = 0.018), with DRD4*Long containing variants associated with severe symptomatology. The analysis in the replication subjects only (n = 803) showed a trend in the same direction, though not reaching the significance level. This analysis in an enlarged sample suggests that DRD4*Long alleles exert a small but significant influence on the delusional symptomatology in subjects affected by major psychoses.

No association between a promoter dopamine D(4) receptor gene variant and schizophrenia

The dopamine D(4) receptor has been implicated in the pathogenesis of schizophrenia. An association between a putative functional promoter polymorphism (-521C/T) in the dopamine D(4) receptor gene (DRD4) and schizophrenia was recently reported. In the present study, patients with schizophrenia (n = 132) and control subjects (n = 388) were analyzed with respect to the DRD4 - 521C/T polymorphism. No significant case control differences emerged. The present results do not support a major role for DRD4 in the etiology of schizophrenia among Caucasians from Sweden.

Association between tryptophan hydroxylase gene polymorphism (A218C) and schizophrenic disorders

The human tryptophan hydroxylase (TPH) gene, the rate-limiting enzyme in serotonin biosynthesis, was localized on human chromosome 11p14-p15.3. Variation within intron 7 of the TPH gene was found to influence serotonin metabolism in the brain. To explore the possible role of TPH in the pathogenesis of schizophrenic disorders, we genotyped the TPH A218C polymorphism in 196 schizophrenic patients and 251 controls. The results demonstrated that genotype distribution was significantly different between schizophrenic patients and control subjects (P=0.002). No association was found between TPH genotypes and suicidal history in schizophrenic patients (P=0.239). The positive finding in this study suggests that the TPH 218A allele is a risk factor for schizophrenic disorders or is in linkage disequilibrium with the putative schizophrenia susceptibility locus in Han Chinese population.

Dopamine D4 receptor (DRD4) gene polymorphism in the first psychotic episode

BACKGROUND

Dopamine D4 receptor (DRD4) has shown some interesting properties at genetic and possibly functional levels. It has been suggested that some molecular variants of the DRD4 gene (e.g., four and seven alleles) could be implicated in the pathogenesis of psychotic disorders. Additionally, the VNTR polymorphism could be implicated in part of the response to treatment with neuroleptics. This study tested the possible association between the 48-bp tandem repeats in exon 3 of the DRD4 gene and patients experiencing their first psychotic episode.

METHODS

Patients with a first psychotic episode (FPE, n = 37) were diagnosed and compared with a matched control group (n = 37). The FPE group was subdivided into two categories: those with nonaffective and those with affective psychoses. The variable number of tandem repeats (VNTR) region of the DRD4 gene was amplified by PCR procedures. Chi-square statistics and appropriate corrections and adjustments were used for data analysis.

CONCLUSIONS

A significantly lower frequency of the four repeat (4-R) carriers in the FPE group was observed. This association was sustained mainly by the affective psychotic group (chi2 = 9.99 df = 2, p = 0.0073). Although these results require testing with stringent methods, it is suggested that the DRD4-4R allele may confer some protection against psychosis, mainly of the affective subtype.

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.

The dopamine D(4) receptor: one decade of research

Dopamine is an important neurotransmitter involved in motor control, endocrine function, reward, cognition and emotion. Dopamine receptors belong to the superfamily of G protein-coupled receptors and play a crucial role in mediating the diverse effects of dopamine in the central nervous system (CNS). The dopaminergic system is implicated in disorders such as Parkinson's disease and addiction, and is the major target for antipsychotic medication in the treatment of schizophrenia. Molecular cloning studies a decade ago revealed the existence of five different dopamine receptor subtypes in mammalian species. While the presence of the abundantly expressed dopamine D(1) and D(2) receptors was predicted from biochemical and pharmacological work, the cloning of the less abundant dopamine D(3), D(4) and D(5) receptors was not anticipated. The identification of these novel dopamine receptor family members posed a challenge with respect to determining their precise physiological roles and identifying their potential as therapeutic targets for dopamine-related disorders. This review is focused on the accomplishments of one decade of research on the dopamine D(4) receptor. New insights into the biochemistry of the dopamine D(4) receptor include the discovery that this G protein-coupled receptor can directly interact with SH3 domains. At the physiological level, converging evidence from transgenic mouse work and human genetic studies suggests that this receptor has a role in exploratory behavior and as a genetic susceptibility factor for attention deficit hyperactivity disorder.

Dopamine D4 receptor 48-bp repeat polymorphism: no association with response to antipsychotic treatment, but association with catatonic schizophrenia

The dopamine D4 receptor (DRD4) may play a role in the pathogenesis of neuropsychiatric disease and in the action of dopaminergic drugs. The 48-bp repeat polymorphism (48-bp VNTR) coding for a 16-amino acid segment in the third cytoplasmatic loop of the DRD4 was studied as a predictor of the therapeutic response to antipsychotics and as susceptibility factor for schizophrenia. We included 638 in-patients with acute schizophrenic, schizoaffective (mainly schizophrenic), and other nonaffective psychoses, as well as two reference groups: one with 278 in-patients with non-psychiatric diseases, and one with 474 healthy volunteers. Catatonic patients (DSM-IV 295.2) more frequently carried the DRD4 D4.2 and D4.3 allele than did all other schizophrenic cases (P < 0.001; OR: 2.7; CI: 1.5-4.9) and controls (P < 0.004; OR: 2.3; CI: 1.3-4.2). We found no significant difference in the DRD4 allele or in genotype frequencies in our comparison of all schizophrenic patients and controls. The subgroups with affected family members, and the subgroups with early or late onset of disease, also did not differ from the controls in DRD4 allele frequencies. The 48-bp VNTR was not a predictor for therapeutic outcome measured by the positive and negative symptoms scale. A total of 1390 subjects showed between 1 and 10 repeats (D4. 1 and D4.10), with 25 different genotypes. These data exclude a major role of DRD4 48-bp VNTR in response to antipsychotic therapy and as susceptibility factor for schizophrenia, but catatonic schizophrenia may be associated with the D4.2 and D4.3 alleles.

Dopamine receptor D4 is not associated with antidepressant activity of sleep deprivation

Total sleep deprivation (TSD) is an effective treatment for mood disorders which is thought to act through an enhancement in several neurotransmitter pathways including dopaminergic transmission. However, not all patients respond to TSD and genetic factors are likely to play a major role in determining TSD response. The aim of this study is to investigate the influence of dopamine receptor D4 exon 3 (DRD4) variants on TSD antidepressant efficacy in bipolar disorder. One hundred and twenty-four depressed inpatients affected by bipolar disorder (DSM-IV) were treated with repeated cycles of TSD and were typed for DRD4 variants at the third exon using polymerase chain reaction (PCR) techniques. DRD4 variants were not associated with TSD outcome. Consideration of possible stratification effects such as gender, age at onset and duration of illness did not reveal any association either. DRD4 exon 3 variants are not a main factor influencing TSD outcome in bipolar disorder.

A genetic polymorphism in the promoter region of DRD4 associated with expression and schizophrenia

The human dopamine D4 receptor gene (DRD4) is an important candidate gene for schizophrenia. We identified a novel -521C>T polymorphism in the 5'-promoter region of DRD4. A transient expression method revealed that the T allele of this polymorphism reduces the transcriptional efficiency by 40% compared with the C allele. This polymorphism is of interest because of reported elevation of D4-like sites and DRD4 mRNA in the postmortem schizophrenic brain. The C allele frequency was significantly higher in 252 Japanese schizophrenics (0.48) than in 269 Japanese controls (0.41) (p = 0. 02) [odds ratio = 1.35 (95% confidence interval 1.05 - 1.72)]. Although the association is weak and should be considered tentative until other studies replicate it, this polymorphism provides a tool with the potential to examine whether DRD4 is related to susceptibility to and neuroleptic response in schizophrenia.

Lack of allelic association of dopamine D4 receptor gene polymorphisms with Parkinson's disease in a Chinese population

Parkinson's disease (PD) is a neurodegenerative disease caused by a multitude of environmental, neurochemical, and genetic factors. The gene for human dopamine D4 receptor (DRD4) has been considered as a plausible candidate for the pathogenesis of PD. Different dopamine D4 receptor allelic forms have variable affinity toward certain neuroleptics such as clozapine, suggesting a role for dopamine D4 receptors in neurologic disorders. To test the hypothesis that the DRD4 polymorphism is associated with the susceptibility to Parkinson's disease, we have examined differences in allele frequencies of different DRD4 polymorphisms in 101 Chinese patients with PD and in 105 age-matched control subjects in Hong Kong. The DRD4 gene was analyzed by a non-radioactive polymerase chain reaction-based Southern hybridization with chemiluminescence detection. The number of polymorphic 48 base pair tandem repeats in exon 3 was identified in each study subject. The DRD4 alleles with high frequencies in the control subjects are 4-repeat allele (72.4%), 2-repeat allele (21.4%), and 7-repeat allele (3.8%) which accounted for over 97% of the total alleles in the elderly Chinese population. The most prevalent genotype in the control subjects is the 4/4 (47.6%), followed by 4/2 (38.6), 4/7 (7.6%), and 2/2 (3.0%). None of the variable number tandem repeat polymorphism showed evidence for genetic association with Parkinson's disease.

Dopamine D4 receptor gene polymorphisms and neuroleptic response in schizophrenia

BACKGROUND

Dopamine D4 receptor (DRD4) gene polymorphisms are associated with various pharmacologic activities. This study investigated whether polymorphisms of 48-bp tandem repeats in the exon 3 of the DRD4 gene are related to neuroleptic response.

METHODS

The neuroleptic response at the acute stage of schizophrenia was assessed in 80 (48 men, 32 women) schizophrenic patients. The negative symptoms at remission were also rated. DRD4 genotype was established using the polymerase chain reaction. Patients with genotypes containing an allele with only two repeats (2-2, 2-3, 2-4, 2-6) were assigned to group I (n = 38). Those homozygous for four 48-bp repeats were assigned to group II (n = 42).

RESULTS

Thirteen (34.2%) of the 38 group I subjects and 26 (61.9%) of the 42 group II subjects had good neuroleptic response during acute stage treatment (chi 2 6.12, df = 1, p < .02). In remission, the rates of negative symptoms of blunt affect, avolition, and global negative rating were higher in group I than in group II. This was more prominent in men than in women.

CONCLUSIONS

The presence of homozygous four 48-bp repeats in both alleles in exon 3 of the DRD4 gene is associated with good neuroleptic response during acute treatment, and with a lower prevalence of negative symptoms at remission, especially in male schizophrenic patients.

Dopamine receptor D4 gene is associated with delusional symptomatology in mood disorders

Disturbances of the dopaminergic neurotransmitter system have been implicated in the pathogenesis of depressive symptoms. Many studies have, however, failed to detect any association between genetic markers for the dopamine system and mood disorders. A possible reason for this may lie in the definition of phenotype, which is traditionally based on psychiatric diagnosis. In this study, we investigated the possibility that functional variants of the dopamine D4 receptor (DRD4) gene might be associated with depressive symptomatology in a sample of mood disorder subjects. Seventy-nine inpatients affected by bipolar (n=37) and major depressive (n=42) disorder (DSM-IV) were assessed at admission by the Hamilton Depression Rating Scale and were typed for DRD4 variants at the third exon using polymerase chain reaction (PCR) techniques. DRD4 was associated with delusional symptoms (F=5.56; d.f.=2,145; P=0.005), with DRD4*7 exhibiting higher scores when compared to DRD4*4 (P=0.006) variants. Polarity of mood disorder did not influence results significantly. The findings are in accordance with our previous report of an association of the DRD4 gene with delusional symptomatology of major psychoses. DRD4*7 should, therefore, be considered a liability factor for delusional symptoms in mood disorders.

Twelve-nucleotide repeat polymorphism of D4 dopamine receptor gene in Chinese familial schizophrenic patients

BACKGROUND

Disturbances in dopaminergic transmission have been implicated in the etiology of schizophrenia. Catalano et al reported an association between delusional disorder and the number of a 12-nucleotide (bp) repeat sequence in the first exon of dopamine D4 receptor gene (DRD4), which indicated a possible role of this polymorphism in the pathogenesis of psychotic disorders.

METHODS

DNA of 42 Chinese controls, 50 sporadic schizophrenic patients, and 30 familial schizophrenic probands were collected. Genotype of the 12-bp repeat polymorphism of DRD4 was determined with polymerase chain reaction and agarose gel electrophoresis. Genotypic and allelic frequencies were compared among the three groups using the chi-square test.

RESULTS

Forty-three (86%) sporadic schizophrenic patients, 25 (83.3%) familial schizophrenic probands, and 35 (83.3%) controls were A1 (two 12-bp repeat) homozygotes. One (2.0%) sporadic schizophrenic and 1 familial schizophrenic patient were A2 (one 12-bp repeat) homozygotes. There was no significant difference in allelic and genotypic distributions among the three groups.

CONCLUSIONS

The present data do not support an association between schizophrenia and any specific allele of the 12-bp repeat polymorphism of DRD4. Significance of familial/sporadic division of schizophrenia cannot be supported regarding this repeat polymorphism.

Combined defects of muscle phosphofructokinase and AMP deaminase in a child with myoglobinuria

A 14-year-old boy with exercise-related myalgia and cramps had several episodes of myoglobinuria since early childhood. An episode at 2 years of age caused acute renal failure. Histochemical and biochemical analysis of muscle showed a combined defect of phosphofructokinase (PFK) and adenosine monophosphate (AMP) deaminase. DNA analysis showed that the patient was homozygous for a G-to-C substitution at codon 39 of the PFK gene (previously described in an Italian patient) and for the common mutation found in AMP deaminase deficiency.

Review the role of dopamine D4 receptors in schizophrenia and antipsychotic action

For the past 20 years the most enduring explanation for schizophrenia has been the dopamine hypothesis, which proposes that the dopaminergic system is overactive in this widespread disease. Classically, the D2 receptor formed the core of the dopamine hypothesis since there was considerable evidence for elevations of D2 receptor levels in the brains of schizophrenic patients, and because these receptors served as the primary target in mediating antipsychotic effects of most neuroleptics. However, the dopamine D4 receptor has recently received particular attention in this context. This is because the atypical antipsychotic, clozapine, which is effective in treating refractory schizophrenics without the side-effect profile of typical neuroleptics, displays a 10-fold higher affinity for D4 compared to D2 or D3 receptors. Furthermore, the concentration in plasma water of clinical doses ofclozapine correlates well with its in vitro binding affinity for D4, but not D2 or D3 receptors, suggesting that D4 is a potential target in mediating clozapine's antipsychotic effects. As well, marked elevations in the level of a D4-like site (not identical to the D4 receptor) has been seen in the striatum of postmortem schizophrenic brains, but not in control brains. Finally, the most interesting feature of the D4 receptor is perhaps the array of polymorphisms associated with it, creating structural diversity in this receptor that supercedes all other known catecholamine receptors. The existence of these D4 polymorphisms raises the possibility that structural variations of this receptor may be associated with an increased susceptibility to schizophrenia, or observed variations in individual response to clozapine treatment. However, several studies aimed at investigating these hypotheses could not establish a direct role of D4 in schizophrenia. Furthermore, no association was evident between the polymorphic forms of D4 and susceptibility to schizophrenia, or variable clozapine response. Nevertheless, investigations surrounding this receptor has been far from futile. The observations which support the idea that D4 might serve as a target for clozapine have significantly modified and extended our understanding of mechanisms underlying atypical antipsychotic treatment of schizophrenia, as well as the dopamine hypothesis for schizophrenia. Further characterization of this receptor may prove to be crucial in designing highly effective antipsychotic drugs with minimal contraindications.

Rapid molecular haplotyping of the first exon of the human dopamine D4 receptor gene by heteroduplex analysis

The dopamine D4 receptor gene (DRD4) and its products are of great interest in many neurospsychiatric disorders. There are at least five expressed polymorphisms in exons 1 and 3, plus rare expressed variants, all of which may have functional relevance. Several studies have described methods for studying the exon 3 polymorphisms, especially the VNTR; fewer reports have documented the exon 1 polymorphisms and variants of DRD4. We report here a simple, rapid, nonisotopic, nondenaturing heteroduplex method for determining the molecular haplotype composed of the two more polymorphic systems of the first exon of DRD4: the 12 bp duplication and 13 bp deletion. This method will facilitate future research on expressed variation of this gene.

Dopamine D4 receptor gene polymorphisms: relation to ethnicity, no association with schizophrenia and response to clozapine in Israeli subjects

The dopamine D4 receptor (DRD4) is a candidate gene in the search for a genetic etiology of schizophrenia and for pharmacogenetic factors in the response to antipsychotic treatment. Previous work has not found linkage or association of a polymorphism in exon 3 of this gene with diagnosis of schizophrenia or response to clozapine. In this study we examined this association in Israeli schizophrenic subjects treated with clozapine, compared to ethnically matched controls. Another polymorphism of this gene, in exon 1, was also studied. Both polymorphisms showed no association with schizophrenia or treatment response. A significant difference in allelic distribution of DRD/ exon 3 polymorphism was found between Ashkenazi and non-Ashkenazi control subjects.

A critical review of genetic studies of schizophrenia. II. Molecular genetic studies

Recent molecular genetic studies of schizophrenia have, until now, been unable to demonstrate any specific major gene for schizophrenia. On the contrary, linkage and association studies have yielded almost exclusively negative or contradictory results. Such studies have involved certain candidate genes, such as the genes for dopamine receptors and other brain neurotransmitters. Some of these candidate genes have now actually been excluded as specific aetiological factors in schizophrenia. Similarly, studies searching for a major gene for susceptibility to schizophrenia involving the whole human genome or large parts of chromosomes have not yielded unambiguously positive results. However, the most recent empirical evidence suggests that many polygenes, acting together, could constitute a risk factor for schizophrenia. It is thus most probable that genetic susceptibility to schizophrenic psychoses is polygenic, and that their effects are dependent on interaction with physical and psychosocial environmental factors.

No difference in the expression of the D4 gene in post-mortem frontal cortex from controls and schizophrenics

The primary biochemical theory of schizophrenia has centered on the role of dopaminergic dysfunction in the illness. The D2 receptor has been primarily indicated however, some atypical neuroleptics may not act at D2. The D4 receptor has a high affinity for the atypical antipsychotic clozapine and is therefore a potential target for drug design. The role of D4 in the aetiology of schizophrenia has, however, been the subject of controversy. Using radioligand binding assays some researchers have detected an elevation of D4-like receptors in schizophrenic striatum whilst conversely other workers claim that D4 receptors are undetectable in this region. Analysis of receptor levels is difficult due to the lack of a ligand selective for D4. We have therefore examined D4 at the level of gene expression. D4 mRNA levels have been examined in post-mortem frontal cortex from nine controls and eight schizophrenics using a reverse transcription-polymerase chain reaction (RT-PCR) method. No significant difference in D4 mRNA levels was found between the two groups. This result does not support a major role for variability of D4 gene expression in the aetiology of schizophrenia.

The molecular genetics of schizophrenia

There is overwhelming evidence for a significant genetic contribution to the etiology of schizophrenia. Molecular genetic techniques are now sufficiently advanced to be applied to complex genetic disorders with uncertain phenotypes, such as schizophrenia. In this article we first briefly discuss certain pertinent background issues: the evidence that schizophrenia has a heritable basis, the possible modes of inheritance involved, and how best to define schizophrenia in the light of this evidence; we then review the current status of research in the field. Large collaborative studies are currently underway that pave the way for the detection of genes of both major and minor effects. We may now be seeing the first consistently replicated results from chromosome 6 and 22 and from candidate genes, such as the dopamine D3 receptor gene.

A search for association between schizophrenia and dopamine-related alleles

Dopamine receptor dysfunction and altered tyrosine hydroxylase activity have both been implicated in the pathophysiology of schizophrenia. Schizophrenic patients and control subjects were examined for allele frequencies in the tyrosine hydroxylase and dopamine D2 and D4 receptor genes. No significant differences of allele or genotype frequencies were found between the two groups after adjustment for multiple comparisons. Neither were any significant relationships observed between allele frequencies and a number of clinical variables within the schizophrenic subsample. When no adjustment was made for multiple testing a few significant tendencies were obtained which warrant further research in extended patient and control materials. The results are compatible with the view that the tyrosine hydroxylase, dopamine receptor D2 and D4 gene polymorphisms examined are not of major importance in the aetiology or pathophysiology of schizophrenia.

Dopamine receptors and brain function

In the central nervous system (CNS), dopamine is involved in the control of locomotion, cognition, affect and neuroendocrine secretion. These actions of dopamine are mediated by five different receptor subtypes, which are members of the large G-protein coupled receptor superfamily. The dopamine receptor subtypes are divided into two major subclasses: the D1-like and D2-like receptors, which typically couple to Gs and Gj mediated transduction systems. In the CNS, the various receptor subtypes display specific anatomical distributions, with D1-like receptors being mainly post-synaptic and D2-like receptors being both pre- and post-synaptic. D1 and D2 dopamine receptors, the most abundant subtypes in the CNS, appear to be expressed largely in distinct neurons. Substance P and dynorphin, which are expressed in D1 receptor-containing neurons, as well as pre-proenkephalin in D2 receptor-containing neurons, have been used as monitors of dopaminergic activity in the CNS. Expression of immediate early genes, in particular fos, has also been found to correlate with dopaminergic transmission. Dopamine released from the hypothalamus controls the synthesis and secretion of prolactin from the anterior pituitary via D2 dopamine receptors. As yet none of the dopamine receptor subtypes have been associated with the etiology of psychotic disorders, such as schizophrenia. However, the recent characterization of D3 and D4 receptors which are, interestingly, expressed in areas of the CNS mediating cognition and affect or showing increased affinity for certain neuroleptics, have renewed the interest and hope of finding effective neuroleptics devoid of side effects. Finally, the recent production of genetically-derived animals lacking several of these receptor genes should help elucidate which specific physiological paradigms the receptors mediate.

Analysis of clozapine response and polymorphisms of the dopamine D4 receptor gene (DRD4) in schizophrenic patients

We have examined the hypothesis that a variable number of tandem repeats in the third cytoplasmic loop of the dopamine D4 receptor influences clinical response to clozapine using a sample of 189 schizophrenic patients. Alleles of the 48-bp repeat, which range from two to ten copies in the normal human population, were analysed by the polymerase chain reaction using genomic DNA as template. Association between these alleles and response to clozapine was tested using the difference in pre- and post-treatment GAS scores as a measure of response. We found no statistically significant variation between genotypic groups and response by analysis of variance. We conclude that the variation of the number of 48-bp repeats alone does not determine response to clozapine. Larger studies are underway to determine if there is a more subtle relationship with sequence variation within the repeats or at other polymorphic sites within the gene that may provide evidence for a component of clozapine's action being at D4 receptors.

Association study between the dopamine D4 receptor gene and schizophrenia

The dopamine D4 receptor is of major interest in schizophrenia research due to its high affinity for the atypical neuroleptic clozapine and a high degree of variability in the receptor gene (DRD4). Although several genetic linkage analyses performed on schizophrenia multiplex families from different regions of the world have either excluded or failed to prove that DRD4 is a major genetic factor for the development of schizophrenia, analyses for moderate predisposing effects are still of significant interest. We performed a study examining differences in allele frequencies of 4 different DRD4 polymorphisms in schizophrenia patients and age, sex, and ethnic origin matched controls. None of these 4 polymorphisms showed evidence for genetic association with schizophrenia, although a trend towards excess of the allele with 7 repeats in the (48)n bp exon III polymorphism was observed. Complexities in the DRD4 genetic investigation and further analytic approaches are discussed.

Turning point in the design of linkage studies of schizophrenia

Despite extensive genomic scans, linkage studies of multiplex pedigrees have been unable to produce replicable evidence of genes predisposing to schizophrenia. This indicates that it is unlikely that a single gene accounts for a majority of cases of schizophrenia, even in multiplex pedigrees. It is most likely that schizophrenia is caused by the nonlinear interaction of multiple genetic and environmental factors influencing brain development and function. This conclusion has strong implications for the design of linkage and association studies. Recently designed linkage studies involve several improvements to deal with extensive locus heterogeneity and multiplicative interaction. These improvements include much larger samples of pedigrees, systematic ascertainment and sequential extension rules, and standardized procedures at multiple sites to facilitate collaboration and replication. Future improvements are likely to require advances in the assessment of clinical and neurobiological variability in multiplex pedigrees, more systematic environmental assessment, and advances in analytic methods to deal with multiplicative interaction. Rather than focusing only on schizophrenia as one or more discrete disorders, future linkage efforts should also consider the etiology of individual clinical syndromes or dimensional components of risk that interact to cause the complex pattern of syndromal comorbidity observed within schizophrenics and their families.