Maria Arranz

Association between clozapine response and allelic variation in 5-HT2A receptor gene

We report allelic association between a polymorphism (T102C) within the coding region of the 5-HT2A gene (HTR2A, 13q14-21) and response to clozapine in schizophrenic patients. Homozygosity for the C102 allele was more frequent (30/57, 53%) among patients who did not respond to clozapine than in those who responded (23/92, 25%). This finding is evidence that allelic variation of genes which encode neurotransmitter receptors can influence clinical response to antipsychotic drugs.

Pharmacogenetic prediction of clozapine response

We did association studies in multiple candidate genes to find the combination of polymorphisms that give the best predictive value of response to clozapine in schizophrenic patients. A combination of six polymorphisms in neurotransmitter-receptor-related genes resulted in 76.7% success in the prediction of clozapine response (p=0.0001) and a sensitivity of 95% (+/- 0.04) for satisfactory response. These results will form the basis for a simple test to enhance the usefulness of clozapine in psychiatric treatment.

Pharmacogenetics and pharmacogenomics of schizophrenia: a review of last decade of research

The last decade of research into the pharmacogenetics of antipsychotics has seen the development of genetic tests to determine the patients’ metabolic status and the first attempts at personalization of antipsychotic treatment. The most significant results are the association between drug metabolic polymorphisms, mainly in cytochrome P450 genes, with variations in drug metabolic rates and side effects. Patients with genetically determined CYP2D6 poor metabolizer (PMs) status may require lower doses of antipsychotic. Alternatively, CYP2D6 ultrarapid matabolizers (UMs) will need increased drug dosage to obtain therapeutic response. Additionally, polymorphisms in dopamine and serotonin receptor genes are repeatedly found associated with response phenotypes, probably reflecting the strong affinities that most antipsychotics display for these receptors. In particular, there is important evidence suggesting association between dopamine 2 receptor (D2) polymorphisms (Taq I and −141-C Ins/Del) and a dopamine 3 receptor (D3) polymorphism (Ser9Gly) with antipsychotic response and drug-induced tardive dyskinesia. Additionally, there is accumulating evidence indicating the influence of a 5-HT2C polymorphism (−759−T/C) in antipsychotic-induced weight gain. Application of this knowledge to clinical practice is slowly gathering pace, with pretreatment determination of individuals drug metabolic rates, via CYP genotyping, leading the field. Genetic determination of patients’ metabolic status is expected to bring clinical benefits by helping to adjust therapeutic doses and reduce adverse reactions. Genetic tests for the pretreatment prediction of antipsychotic response, although still in its infancy, have obvious implications for the selection and improvement of antipsychotic treatment. These developments can be considered as successes, but the objectives of bringing pharmacogenetic and pharmacogenomic research in psychiatric clinical practice are far from being realized. Further development of genetic tests is required before the concept of tailored treatment can be applied to psychopharmatherapy. This review aims to summarize the key findings from the last decade of research in the field. Current knowledge on genetic prediction of drug metabolic status, general response and drug-induced side effects will be reviewed and future pharmacogenomic and epigenetic research will be discussed.

Meta-analysis of studies on genetic variation in 5-HT2A receptors and clozapine response

Serotonin (5-HT) neurotransmitter receptors are targeted by atypical antipsychotic drugs. We hypothesized that genetic variation in these receptors may affect clinical response to the drugs targeting them. This hypothesis has been tested by several studies in which the correlation between polymorphic variants in the 5-HT2A receptor gene and clinical response to the atypical antipsychotic clozapine was investigated. The results of these studies either found association between 5-HT2A genetic variants and clozapine response or found differences in the same direction which did not reach statistical significance. Meta-analysis of these studies including 373 patients who responded to the treatment and 360 non-responders showed association between two 5-HT2A polymorphisms, 102-T/C and His452Tyr, and clozapine response. Statistical analysis of extreme responders showed a clearer association of the 102-T/C with clozapine response. These results reinforce the hypothesis and strengthen the candidacy of these receptors as important therapeutic targets.

Evidence for association between polymorphisms in the promoter and coding regions of the 5-HT2A receptor gene and response to clozapine

Clozapine is a potent atypical antipsychotic which binds to a variety of neurotransmitter receptors including serotonin (5-HT) receptors. However, the precise neurochemical site of clozapines therapeutic action is unknown. We hypothesize that genetic variation in the neurotransmitter receptors to which the drug binds may influence clozapine response. To test this hypothesis we genotyped a novel −1438-G/A polymorphism detected in the promoter region, and a His452Tyr polymorphism described in the coding region of the 5-HT2A receptor gene in two independent samples of clozapine-treated patients including responders and non-responders. Although the strong association between these polymorphisms and clozapine response observed in the first sample (sample I) was not statistically significant in the second sample (sample II), the results in both samples were in the same direction. Homozygosity for the allele G-1438 was higher among non-responders (56% in sample I, 43% in sample II) than in responders (28% in sample I and 32% in sample II) in both samples. Similarly, the frequency of allele Tyr452 was higher in non-responders (11% in sample I, 16% in sample II) than in responders (6% in sample I and 10% in sample II). A combined analysis of both samples showed association between both polymorphisms and clozapine response. These results provide further evidence suggesting that genetic variation at 5-HT2A receptors may influence clozapine response and strengthen the candidacy of these receptors as important therapeutic targets.

The serotonin transporter is a potential susceptibility factor for bipolar affective disorder

The serotonin transporter is a strong candidate for aetiological involvement in affective disorders and psychosis. We analysed a VNTR in intron 2 of the human serotonin transporter gene (hSERT) for allelic association with bipolar affective disorder, unipolar depression and schizophrenia. An increased frequency of allele 12 of the VNTR was observed in subjects with bipolar affective disorder (n = 191; chi 2 p = 0.00048 by allele) but not unipolar depression (n = 86; chi 2 p = 0.18, ns) or schizophrenia (n = 129; chi 2 p = 0.08, ns), although a trend towards an excess of allele 12 was observed for the latter. There was also a significant difference in the frequency of allele 12 between bipolar affective disorder and unipolar depression (p = 0.0087). The relative risk for bipolar affective disorder with respect to allele 12 was 1.84 (95% CI 0.97-3.56) for heterozygotes, and 3.10 (95% CI 1.60-6.07) for homozygotes, with evidence for a gene-dosage effect. Because allele 12 is common in the population, the attributable risk is 50.8% (95% CI 14.5%-73.3%). We hypothesize that either the VNTR affects regulation of expression of hSERT at the transcriptional level or it is in linkage disequilibrium with another functional polymorphism in the gene, and this results in an increased risk for the development of bipolar affective disorder.

Association between clozapine response and allelic variation in the 5-HT2C receptor gene

A cysteine to serine substitution at amino acid 23 in the 5-HT2C receptor gene alters the pharmacological properties of the protein. We investigated this polymorphism in subjects with schizophrenia resistant to conventional neuroleptic drugs, and analysed our data for allelic association between the disease state or clinical response to the atypical antipsychotic drug, clozapine. Ninety percent of subjects who had one or more 5-HT2Cser alleles (19/21) were classified as clozapine responders compared with 59% (84/141) without this allele (χ2 = 7.7, p = 0.005), suggesting that this mutation is a predictor of good response to clozapine. There was no association between schizophrenia and the 5-HT2Cser allele, but our results indicate that the 5-HT2C receptor may contain the major site of action through which clozapine mediates its antipsychotic effects.

Low activity allele of catechol-O-methyltransferase gene associated with rapid cycling bipolar disorder

Catechol-O-methyltransferase (COMT) plays a major role in the breakdown of catecholamines.1 An amino acid polymorphism (val-108-met) determines high and low activity of the enzyme.2,3 A recent study in a small sample of patients with velo-cardio-facial syndrome who had bipolar affective disorder suggested that the Met (low activity) COMT allele might be associated with rapid-cycling in this population.4 We therefore tested the hypothesis that the Met allele might be associated with rapid cycling bipolar disorder in the wider population. We studied a sample of British Caucasian DSM-IV bipolar patients, of whom 55 met criteria for rapid cycling at some time during the illness and 110 met stringent criteria for a definite non-rapid cycling course. The COMT genotype was determined using a PCR assay. The low activity allele was more frequent in the group of rapid cyclers: 0.55 vs 0.42 (one-tailed χ2 = 5.12, d.f. = 1, P = 0.012), and bearers of low activity alleles showed a dose-dependent increased risk of lifetime occurrence of rapid cycling: χ2 test of linear association = 4.84, d.f. = 1, P = 0.014. Our data support the hypothesis that variation in the COMT gene modifies the course of bipolar disorder.

Analysis of a structural polymorphism in the 5-HT2A receptor and clinical response to clozapine.

Clozapine is an atypical antipsychotic with affinity for a broad range of receptors, including serotonin (5-HT) and dopamine receptors. It is successful in treating about 60% of patients refractory to other antipsychotic drugs. Since genetic variation in clozapines neurotransmitter receptor targets may affect clinical response through altering drug binding or receptor expression, we have studied a His452Tyr polymorphism in the 5-HT2A receptor (HTR2A) in a sample of 153 schizophrenic patients undergoing clozapine treatment and 178 normal controls. An association was found between the allele Tyr452 and poor clinical response.

High and low activity alleles of catechol-O-methyltransferase gene: ethnic difference and possible association with Parkinson's disease

Catechol-O-methyltransferase (COMT) is an enzyme that inactivates catecholamines such as adrenaline, noradrenaline, dopamine, and levodopa. Recently an amino acid change (Val-108-Met) of the COMT protein was found to determine high and low activity alleles of the COMT gene. We genotyped 109 Japanese patients with Parkinsons disease (PD) and 153 controls by using polymerase chain reaction (PCR) amplification and digestion by the restriction enzyme NlaIII. The frequency of low activity allele in the controls was 0.29, which was significantly different from that reported in Caucasians (0.50). When comparison was made between patients with PD and controls, homozygosity for the low activity allele was significantly more common among the patients than among the controls (P = 0.017; odds ratio, 2.8, 95% CI 1.2-6.5), suggesting that homozygosity for the low activity allele may increase susceptibility to PD.