Monoamine oxidase: A gene polymorphism, tryptophan hydroxylase gene polymorphism and antidepressant response to fluvoxamine in Japanese patients with major depressive disorder

Pharmacogenetics in the treatment of depression: pharmacodynamic studies

The pharmacological treatment of mood disorders has reduced their morbidity and improved mental health for millions of individuals worldwide, favouring a considerable reduction of the direct and indirect costs caused by these common pathologies. Unfortunately, not all individuals benefit, and 30-40% of patients do not show a complete response to treatment. Efficient clinical predictors are not available, although genetic factors are thought to play a substantial (but complex) role in the antidepressant response. Pharmacogenetics, which investigates the influence of genetic features on the pharmacological response, has gained increasing attention and holds great promise for clinical psychiatry. Here, a brief overview is provided on the various pharmacogenetic studies published to date that analyse the commonest treatments for depression: antidepressants, sleep deprivation and lithium salts.

Investigation of serotonin-related genes in antidepressant response

In this study, we sought out to test the hypothesis that genetic factors may influence antidepressant response to fluoxetine. The investigation focused on seven candidate genes in the serotonergic pathway involved in the synthesis, transport, recognition, and degradation of serotonin. Our clinical sample consisted of 96 subjects with unipolar major depression treated with fluoxetine with response variables assessed after a 12-week trial. Patient data were also collected to investigate the pattern of drug response. Using a high-throughput single-nucleotide polymorphism (SNP) genotyping platform and capillary electrophoresis, we genotyped patients at 110 SNPs and four repeat polymorphisms located in seven candidate genes (HTR1A, HTR2A, HTR2C, MAOA, SLC6A4, TPH1, and TPH2). Statistical tests performed included single-locus and haplotype association tests, and linkage disequilibrium (LD) estimation. Little evidence of population stratification was observed in the sample with 20 random SNPs using a genomic control procedure. Our most intriguing result involved three SNPs in the TPH1 gene and one SNP in the SLC6A4 gene, which show significant single-locus association when response to fluoxetine is compared to nonresponse (P=0.02-0.04). All odds ratios indicated an increased risk of not responding to fluoxetine. In the specific response vs nonspecific and nonresponse comparison, three SNPs in the TPH2 gene (P=0.02-0.04) were positively associated and one SNP in the HTR2A gene (P=0.02) was negatively associated. When comparing specific response to nonspecific response, we found significant negative associations in three SNPs in the HTR2A gene (P=0.001-0.03) and two SNPs in the MAOA gene (P=0.03-0.05). We observed variable, although strong LD, in each gene and unexpectedly low numbers of estimated haplotypes, formed from tagged SNPs. Significant haplotype associations were found in all but the HTR1A and HTR2C genes. Although these data should be interpreted cautiously due to the small sample size, these results implicate TPH1 and SLC6A4 in general response, and HTR2A, TPH2, and MAOA in the specificity of response to fluoxetine. Intriguingly, we observe that a number of the less frequent alleles of many of the SNP markers were associated with the nonresponse and nonspecific phenotypes.

Further evidence of a combined effect of SERTPR and TPH on SSRIs response in mood disorders

We reported an independent association of the short variant of the serotonin transporter gene-linked polymorphic region (SERTPR) and tryptophan hydroxylase (TPH) genes with antidepressant response to selective serotonin reuptake inhibitors (SSRIs). The aim of the present study was to confirm the effect of the SERTPR and TPH gene variants on the SSRIs antidepressant activity in a new sample of major and bipolar depressives. Two hundred and twenty one inpatients (major depressives = 128, bipolar disorder = 93) were treated with SSRIs (fluvoxamine or paroxetine) for 6 weeks; the severity of depressive symptoms was weekly assessed with the Hamilton Rating Scale for Depression (HAMD). SERTPR and TPH variants were determined using PCR-based techniques, 220 subjects genotyped for SERTPR and 221 for TPH that were never included in previous studies. SERTPR*s/s variant association with a poor response to SSRI treatment was confirmed, even if with less significant P values (P = 0.034), independently from clinical variables; pooling the present sample with previous ones we observed a highly significant effect (P < 0.000001). TPH*A/A variants showed higher HAMD scores throughout the trial but with only a trend in the same direction of our previous study in terms of a worse response of A/A genotypes. Thus, the previous positive association was not fully replicated for TPH. The present independent replication confirms SERTPR variants as a liability factor for antidepressant efficacy while the TPH effect is not unequivocal.

Analysis and Its Application for Prevention of Side-Effects of Drugs and for Evaluation of Drug Responsiveness

The response of patients to drugs can be affected by genetic polymorphisms/defects in drug metabolizing enzymes, transporters, and receptors. Genetic polymorphisms/defects are generated by mutation of coding regions and/or noncoding regions of target genes, such as single-point mutations, deletions/insertions, variation in the number of tandem repeats, etc. If a genetic defect in a patient which affects drug response were known, it would be possible to optimize medications individually. The author developed two improved methods for detecting CYP2C19(*)2 and CYP2C19(*)3. Using the methods, the type of CYP2C19 gene was examined in 80 inpatients, and the medication status of patients with the mutation was examined focusing on dosage and side effects. The author also examined polymorphisms of the serotonin transporter/biosynthetic or metabolizing enzymes in depressive patients treated with fluvoxamine, a selective serotonin reuptake inhibitor, and the relationship between clinical efficacy and polymorphisms was investigated. As a result, patients with the S/S genotype of 5-HTTLPR were found to experience better clinical efficacy.

Pharmacogenetics of antidepressants and antipsychotics: the contribution of allelic variations to the phenotype of drug response

Genetic factors contribute to the phenotype of drug response. We systematically analyzed all available pharmacogenetic data from Medline databases (1970-2003) on the impact that genetic polymorphisms have on positive and adverse reactions to antidepressants and antipsychotics. Additionally, dose adjustments that would compensate for genetically caused differences in blood concentrations were calculated. To study pharmacokinetic effects, data for 36 antidepressants were screened. We found that for 20 of those, data on polymorphic CYP2D6 or CYP2C19 were found and that in 14 drugs such genetic variation would require at least doubling of the dose in extensive metabolizers in comparison to poor metabolizers. Data for 38 antipsychotics were examined: for 13 of those CYP2D6 and CYP2C19 genotype was of relevance. To study the effects of genetic variability on pharmacodynamic pathways, we reviewed 80 clinical studies on polymorphisms in candidate genes, but those did not for the most part reveal significant associations between neurotransmitter receptor and transporter genotypes and therapy response or adverse drug reactions. In addition associations found in one study could not be replicated in other studies. For this reason, it is not yet possible to translate pharmacogenetic parameters fully into therapeutic recommendations. At present, antidepressant and antipsychotic drug responses can best be explained as the combinatorial outcome of complex systems that interact at multiple levels. In spite of these limitations, combinations of polymorphisms in pharmacokinetic and pharmacodynamic pathways of relevance might contribute to identify genotypes associated with best and worst responders and they may also identify susceptibility to adverse drug reactions.

The pharmacogenomics of selective serotonin reuptake inhibitors

The introduction of selective serotonin (5-HT) reuptake inhibitors (SSRIs) has significantly improved the pharmacological treatment of a range of psychiatric disorders. Nevertheless, despite the undoubted advantages of antidepressant treatment in terms of improved tolerability to therapy while maintaining a high level of efficacy, not all patients benefit from it; an appreciable proportion do not respond adequately, while others may show adverse reactions. The necessary change of the initial treatment choice often requires extended periods for the remission of symptomatology. Such difficulties could be avoided if it should be possible to determine more quickly the most suitable drug. Several factors have been thought to influence the outcome of antidepressant therapy. Among the factors influencing the interindividual variability in response to treatment with SSRI, differences in genetic features may play a significant role. Several genetic polymorphisms have been associated with therapeutic SSRI response, including genetic variants of the 5-HT transporter, 5-HT-2A-receptor, tryptophan hydroxylase, brain-derived neurotrophic factor, G-protein beta3 subunit, interleukin-1beta and angiotensin-converting enzyme, although with conflicting results; also cytochrome P450 drug-metabolising enzymes may bear a particular importance, although further corroboration of the findings is necessary, and further key participating genes remain to be identified. The hope is that the identification of these genetic components will eventually facilitate the development of a customised SSRI treatment.