Serotonin subtype 2 receptor genes and clinical response to clozapine in schizophrenia patients

No association between serotonin 2A receptor gene variants and personality traits

Among theories of biological underpinnings to personality traits, different mechanisms of the serotonergic system are perhaps the most common factors suggested to influence individual differences in personality traits. We have investigated two frequent variants in the serotonin 2A receptor gene (5-HT2A) and personality traits. Healthy Swedish subjects (n = 304) were assessed with the Karolinska Scales of Personality (KSP) inventory. After correction for multiple testing, no significant differences were found. We conclude that the investigated 5-HT2A gene variants do not significantly influence personality as assessed by the KSP in the present population.

Association between the serotonin 2A receptor gene and tardive dyskinesia in chronic schizophrenia

Tardive dyskinesia (TD) is a long-term adverse effect of antipsychotic drugs that are dopamine D2 receptor blockers.(1) Serotonin receptor antagonism has been proposed as a common mechanism contributing to the low extrapyramidal effects profile of atypical antipsychotic drugs.(2) We examined the association of three polymorphisms in the 5-HT2A receptor gene (HTR2A) with TD susceptibility--T102C(3) and his452tyr(4) in the coding region and A-1438G(5) in the promoter--in matched schizophrenia patients with (n = 59, SCZ-TD-Y) and without TD (n = 62, SCZ-TD-N) and normal control subjects (n = 96). The T102C and the A-1438G polymorphisms are in complete linkage disequilibrium but not his452tyr. There was a significant excess of 102C and -1438G alleles (62.7%) in the SCZ-TD-Y patients compared to SCZ-TD-N patients (41.1%) and controls (45.9%; chi(2) = 12.8, df = 2, P = 0.002; SCZ-TD-Y vs SCZ-TD-N, chi(2) = 11.4, df = 1, P = 0.0008, OR 2.41, 95% CI 1.43-3.99) and of 102CC and -1438GG genotypes (SCZ-TD-Y 42.4%, SCZ-TD-N, 16.1%, controls 20.8%, chi(2) = 13.3, df = 4, P = 0.01). The 102CC and the -1438GG genotypes were associated with significantly higher AIMS trunk dyskinesia scores (F = 3.9; df = 2, 116; P = 0.02) and more incapacitation (F = 5.0; df = 2, 115; P = 0.006). The his452tyr polymorphism showed no association with TD. These findings suggest that the 5-HT2A receptor gene is significantly associated with susceptibility to TD in patients with chronic schizophrenia. Previously reported association of the T102C and A-1438G polymorphisms with schizophrenia(6) may reflect association of a sub-group of patients with a susceptibility to abnormal involuntary movements related to antipsychotic drug exposure.

Lack of association between serotonin-2A receptor gene (HTR2A) polymorphisms and tardive dyskinesia in schizophrenia

Tardive dyskinesia (TD) is a disabling neurological side effect associated with long-term treatment with typical antipsychotics. Family studies and animal models lend evidence for hereditary predisposition to TD. The newer atypical antipsychotics pose a minimal risk for TD which is in part attributed to their ability to block the serotonin-2A (5-HT(2A)) receptor. 5-HT(2A) receptors were also identified in the basal ganglia; a brain region that plays a critical role in antipsychotic-induced movement disorders. We tested the significance of variation in the 5-HT(2A) receptor gene (HTR2A) in relation to the TD phenotype. Three polymorphisms in HTR2A, one silent (C102T), one that alters the amino acid sequence (his452tyr) and one in the promoter region (A-1437G) were investigated in 136 patients refractory or intolerant to treatment with typical antipsychotics and with a DSM-IIIR diagnosis of schizophrenia. We did not find any significant difference in allele, genotype or haplotype frequencies of polymorphisms in HTR2A among patients with or without TD (P > 0.05). Further analysis using the ANCOVA statistic with a continuous measure of the TD phenotype (Abnormal Involuntary Movement Scale (AIMS) score) found that the AIMS scores were not significantly influenced by HTR2A polymorphisms, despite controlling for potential confounders such as age, gender and ethnicity (P > 0.05). Theoretically, central serotonergic function can be subject to genetic control at various other mechanistic levels including the rate of serotonin synthesis (tryptophane hydroxylase gene), release, reuptake (serotonin transporter gene) and degradation (monoamine oxidase gene). Analyses of these other serotonergic genes are indicated. In summary, polymorphisms in HTR2A do not appear to influence the risk for TD. Further studies evaluating in tandem multiple candidate genes relevant for the serotonergic system are warranted to dissect the genetic basis of the complex TD phenotype.

Pharmacogenomics of psychiatric disorders

Pharmacogenomics, the utilization of genetic information to predict outcome of drug treatment (therapeutic and side-effects), holds great promise for clinical medicine. The pharmacotherapy of psychiatric disorders exhibits wide variability in therapeutic response with little scientific guidance for treatment on a patient-by-patient basis. The emerging field of pharmacogenomics holds great potential for refining and optimizing psychopharmacology. Key components for future development of the pharmacogenomics of psychiatric disorders include understanding the mechanism of drug action, identification of candidate genes and their variants, and well-conducted clinical trials. In this article, data from recent studies are examined with particular emphasis on methodological requirements and direction for future research.

Lack of association between the T-->C 267 serotonin 5-HT6 receptor gene (HTR6) polymorphism and prediction of response to clozapine in schizophrenia

The affinity of clozapine for 5-HT2A, 5-HT2C, 5-HT6, 5-HT7, and 5-HT1A receptors has been suggested to contribute to various aspects of its complex clinical actions. This study examined the hypothesis that genetic variation in 5-HT1A, 5-HT6, and 5-HT7 receptor genes is involved in the variability observed in response to clozapine. We employed a pharmacogenetic approach in a group (n=185) of schizophrenia patients that have been clinically well characterized for clozapine response. Polymorphisms in the 5-HT6 (HTR6), 5-HT1A (HTR1A) and 5-HT7 (HTR7) receptor genes were genotyped. No evidence for either an allelic or genotypic association of the T-->C 267 HTR6 polymorphism with response to clozapine was found in our sample (allele: chi(2)=0.06, 1 df, P=0.80; genotype: chi(2)=1.21, 2 df, P=0.55). The pro16leu HTR1A polymorphism was not observed in our sample; all individuals genotyped were pro/pro 16 homozygotes. With respect to the pro279leu HTR7 polymorphism, one Caucasian male responder to clozapine was observed to be heterozygous (pro/leu 279 genotype). This individual was clinically similar to the other clozapine responders. Overall, our findings do not support a role for the T-->C 267 polymorphism of the 5-HT6 receptor gene in response to clozapine, although replication is required to confirm this finding.

Perspectives in pharmacogenetics

The historical perspective of pharmacogenetics is presented to a large extent from a personal view, since I happened to get into this field of science at the time of its beginning, and since pharmacogenetics has remained the backbone of my scientific career. Pharmacogenetics initially emphasized observations of interpersonal variability, but the attention on interethnic differences soon followed. Technical advances led to the identification of many responsible gene alterations in both individuals and in populations. Included is a relatively extensive discussion in which the effects and the different consequences of monogenic (mendelian) and multigenic (multifactorial) variation are compared.

Pharmacogenetics and the serotonin system: initial studies and future directions

Serotonin (5-hydroxytryptamine, 5-HT) appears to play a role in the pathophysiology of a range of neuropsychiatric disorders, and serotonergic agents are of central importance in neuropharmacology. Genes encoding various components of the 5-HT system are being studied as risk factors in depression, schizophrenia, obsessive-compulsive disorder, aggression, alcoholism, and autism. Recently, pharmacogenetic research has begun to examine possible genetic influences on therapeutic response to drugs affecting the serotonin system. Genes regulating the synthesis (TPH), storage (VMAT2), membrane uptake (HTT), and metabolism (MAOA) of 5-HT, as well as a number of 5-HT receptors (HTR1A, HTR1B, HTR2A, HTR2C, and HTR5A), have been studied and this initial research is reviewed here. After a brief introduction to serotonin neurobiology and a general discussion of appropriate genetic methodology, each of the major 5-HT-related genes and their encoded proteins are reviewed in turn. For each gene, relevant polymorphisms and research on functional variants are discussed; following brief reviews of the disorder or trait association and linkage studies, pharmacogenetic studies performed to date are covered. The critical and manifold roles of the serotonin system, the great abundance of targets within the system, the wide range of serotonergic agents-available and in development-and the promising preliminary results suggest that the serotonin system offers a particularly rich area for pharmacogenetic research.

Pharmacogenomics and schizophrenia

Although antipsychotic drugs are effective in alleviating schizophrenic symptoms, individual differences in patient response suggest that genetic components play a major role, and pharmacogenetic studies have indicated the possibility for a more individually based pharmacotherapy. The new field of pharmacogenomics, which focuses on genetic determinants of drug response at the level of the entire human genome, is important for development and prescription of safer and more effective individually tailored drugs. DNA microarray (DNA chip) analysis enables genome-wide scanning, using the high-density single nucleotide polymorphisms map. Pharmacogenomics will aid in understanding how genetics influence disease development and drug response, and contribute to discovery of new treatments. The rate of discovery of those polymorphisms will depend on the quality of the drug response phenotype. Prospective genotyping of schizophrenic patients for the many genes at the level of the drug target, drug metabolism, and disease pathways will contribute to individualized therapy matching the patient's unique genetic make-up with an optimally effective drug.

Neurotransmitter-related genes and antipsychotic response: pharmacogenetics meets psychiatric treatment

Pharmacogenetic research into neurotransmitter-related genes is helping to unravel genetic factors that determine antipsychotic response. Several genetic mutations in neurotransmitter receptors targeted by antipsychotic drugs have been found to be related to clinical response. Modern molecular genetic techniques will facilitate the identification of those mutations that determine treatment response. Future psychiatric prescription will include the genetic characterization of neurotransmitter receptors for the selection of the most beneficial drug according to the individual's pharmacogenetic profile.

Apolipoprotein E4: an allele associated with many diseases

Apolipoprotein E (apoE) was discovered as a plasma protein involved in lipoprotein metabolism. ApoE is synthesized by the liver and is also made locally in the brain. There are three common variants of apoE, resulting from common genetic variation, called E2, E3 and E4. The E3 allele is the most prevalent form, and the proportion of the three alleles differs between populations. Epidemiological studies have found that the E4 allele is associated with decreased longevity, increased plasma cholesterol levels and increased prevalence for cardiovascular disease and particularly for Alzheimer's disease. The apoE polymorphism also affects response to head trauma, cognitive decline upon ageing and several other disorders. Thus, common genetic variation in the apoE gene may be associated with successful ageing.

Association of polymorphism of serotonin 2A receptor gene with suicidal ideation in major depressive disorder

There is evidence indicating that density of 5-HT2A receptors is altered in brain regions of depressed suicide victims and in platelets of suicidal subjects with major depression or schizophrenia. Recent studies have also shown an association between the allele C of 102T/C polymorphism in the 5-HT2A receptor gene and schizophrenia. The present investigation tested the hypothesis that the observed changes in 5-HT2A receptor density in platelets of patients with major depression are a trait rather than state phenomenon and are associated with the 102 C allele in 5-HT2A receptor gene in a sample of 120 patients with major depression and a group of 131 control subjects comparable with respect to age, sex, and ethnic background. The allele and genotype frequencies of 102T/C polymorphism in 5-HT2A receptor gene were compared between patients and control subjects and between suicidal and non-suicidal patient groups. The major finding of this study was a significant association between the 102 C allele in 5-HT2A receptor gene and major depression, chi(2) = 4.5, df = 1, P = 0.03, particularly in patients with suicidal ideation, chi(2) = 8.5, df = 1, P < 0.005. Furthermore, we found that patients with a 102 C/C genotype had a significantly higher mean HAMD item 3 score (indication of suicidal ideation) than T/C or T/T genotype patients. Our results suggest that the 102T/C polymorphism in 5-HT2A receptor gene is primarily associated with suicidal ideation in patients with major depression. Am. J. Med. Genet. (Neuropsychiatr. Genet.) 96:56-60, 2000.

Pharmacogenetics of antipsychotic treatment: lessons learned from clozapine

The reintroduction of clozapine, the prototype of atypical antipsychotics, in the late 1980s has led to significant advances in the pharmacological management of schizophrenia. Since then, there has been a rapid development of novel "atypical" antipsychotic agents that have been pharmacologically modeled, to a certain extent, after their predecessor clozapine. As with all antipsychotics, there is variability among individuals in their response to these "atypical" drugs. Pharmacogenetics can provide a foundation for understanding this interindividual variability in antipsychotic response. This review first provides a rationale for the pharmacogenetic investigation of this variable trait. Studies of pharmacokinetic and pharmacodynamic factors of antipsychotic therapy are considered in the development of this rationale. Next, the molecular genetic techniques used to study this interindividual variation in response are described. This is followed by a review and discussion of the published studies examining genetic factors involved in clozapine response. From this, several recommendations for future pharmacogenetic investigations of antipsychotic response are proposed. Although still in its early stages, psychiatric pharmacogenetics should provide a basis for individualized pharmacotherapy of schizophrenia, and may also lead to the development of newer, more efficacious antipsychotic agents.

Dopamine D3 receptor gene polymorphism and response to clozapine in schizophrenic Pakastani patients

The dopamine D3 receptor (DRD3) appears to play an important role in the mediation of antipsychotic drug action. Genetic association of treatment response to the atypical antipsychotic drug clozapine with the DRD3 polymorphism Ser9Gly was investigated in a sample of 32 schizophrenic patients. We found association of treatment response with allele Gly-9 (P=0.0058) and with genotypes consisting of Gly-9 (P=0.033) by this pharmacogenetic approach. A combined analysis with two previous studies (Shaikh et al., Hum. Genet. 97 (1996) 714-719; Malhotra et al., Mol. Psychiatry 3 (1998) 72-75) further substantiates these results (P=0.0041).

Cellular and subcellular distribution of the serotonin 5-HT2A receptor in the central nervous system of adult rat

Light and electron microscope immunocytochemistry with a monoclonal antibody against the N-terminal domain of the human protein was used to determine the cellular and subcellular localization of serotonin 5-HT2A receptors in the central nervous system of adult rat. Following immunoperoxidase or silver-intensified immunogold labeling, neuronal, somatodendritic, and/or axonal immunoreactivity was detected in numerous brain regions, including all those in which ligand binding sites and 5-HT2A mRNA had previously been reported. The distribution of 5-HT2A-immunolabeled soma/dendrites was characterized in cerebral cortex, olfactory system, septum, hippocampal formation, basal ganglia, amygdala, diencephalon, cerebellum, brainstem, and spinal cord. Labeled axons were visible in every myelinated tract known to arise from immunoreactive cell body groups. In immunopositive soma/dendrites as well as axons, the 5-HT2A receptor appeared mainly cytoplasmic rather than membrane bound. Even though the dendritic labeling was generally stronger than the somatic, it did not extend to dendritic spines in such regions as the cerebral and piriform cortex, the neostriatum, or the molecular layer of the cerebellum. Similarly, there were no labeled axon terminals in numerous regions known to be strongly innervated by the immunoreactive somata and their axons (e.g., molecular layer of piriform cortex). It was concluded that the 5-HT2A receptor is mostly intracellular and transported in dendrites and axons, but does not reach into dendritic spines or axon terminals. Because it has previously been shown that this serotonin receptor is transported retrogradely as well as anterogradely, activates intracellular transduction pathways and intervenes in the regulation of the expression of many genes, it is suggested that one of its main functions is to participate in retrograde signaling systems activated by serotonin.

Prolactin hyperresponsiveness to D-fenfluramine in drug-free schizophrenic patients: a placebo-controlled study

BACKGROUND

Functional alterations in the central serotonergic system have been reported in schizophrenia but no conclusive data have been provided. In the present study, we investigated the prolactin (PRL) response to the selective serotonin (5-HT) releasing agent D-fenfluramine in both patients with schizophrenia and matched healthy subjects.

METHODS

Sixteen drug-free schizophrenics and 16 healthy subjects were randomized in a double-blind neuroendocrine test to D-fenfluramine (30 mg p.o.) or placebo. Blood PRL and cortisol concentrations were determined by radioimmunoassay, while plasma levels of D-fenfluramine were measured by mass spectrometry.

RESULTS

In schizophrenic patients, baseline plasma PRL levels were not different from controls, whereas plasma cortisol concentrations were significantly increased (p < .03). The PRL response to D-fenfluramine was significantly enhanced in patients as compared to matched control subjects (p < .005). Schizophrenics meeting Kane's criteria for previous nonresponse to typical neuroleptics exhibited a PRL response to D-fenfluramine significantly higher than non-drug-resistant patients (p < .04). No significant difference in plasma D-fenfluramine concentrations was observed between schizophrenic and healthy subjects.

CONCLUSIONS

These findings suggest a serotonergic hypersensitivity in chronic schizophrenia. This alteration seems to be peculiar to those patients refractory to typical neuroleptics.