Analysis of the novel TPH2 gene in bipolar disorder and suicidality

Molecular Biomarkers Predictive of Sertraline Treatment Response in Young Children With Autism Spectrum Disorder

Sertraline is one among several selective serotonin reuptake inhibitors (SSRIs) that exhibited improvement of language development in Autism Spectrum Disorder (ASD); however, the molecular mechanism has not been elucidated. A double blind, randomized, 6-month, placebo-controlled, clinical trial of low-dose sertraline in children ages (3–6 years) with ASD was conducted at the UC Davis MIND Institute. It aimed at evaluating the efficacy and benefit with respect to early expressive language development and global clinical improvement. This study aimed to identify molecular biomarkers that might be key players in the serotonin pathway and might be predictive of a clinical response to sertraline. Fifty eight subjects with the diagnosis of ASD were randomized to sertraline or placebo. Eight subjects from the sertraline arm and five from the placebo arm discontinued from the study. Furthermore, four subjects did not have a successful blood draw. Hence, genotypes for 41 subjects (20 on placebo and 21 on sertraline) were determined for several genes involved in the serotonin pathway including the serotonin transporter-linked polymorphic region (5-HTTLPR), the tryptophan hydroxylase 2 (TPH2), and the Brain-Derived Neurotrophic Factor (BDNF). In addition, plasma levels of BDNF, Matrix metallopeptidase 9 (MMP-9) and a selected panel of cytokines were determined at baseline and post-treatment. Intent-to-treat analysis revealed several primary significant correlations between molecular changes and the Mullen Scales of Early Learning (MSEL) and Clinical Global Impression Scale – Improvement (CGI-I) of treatment and control groups but they were not significant after adjustment for multiple testing. Thus, sertraline showed no benefit for treatment of young children with ASD in language development or changes in molecular markers in this study. These results indicate that sertraline may not be beneficial for the treatment of children with ASD; however, further investigation of larger groups as well as longer term follow-up studies are warranted.

TPH2 polymorphisms across the spectrum of psychiatric morbidity: A systematic review and meta-analysis

Tryptophan hydroxylase 2 (TPH2) is the rate-limiting enzyme in brain serotonin synthesis. The TPH2 gene has frequently been investigated in relation to psychiatric morbidity. The aim of the present review is to integrate results from association studies between TPH2 single nucleotide polymorphisms (SNPs) and various psychiatric disorders, which we furthermore quantified with meta-analysis. We reviewed 166 studies investigating 69 TPH2 SNPs in a broad range of psychiatric disorders, including over 30,000 patients. According to our meta-analysis, TPH2 polymorphisms show strongest associations with mood disorders, suicide (attempt) and schizophrenia. Despite small effect sizes, we conclude that TPH2 SNPs in the coding and non-coding areas (rs4570625, rs11178997, rs11178998, rs10748185, rs1843809, rs4290270, rs17110747) are each associated with one or more psychopathological conditions. Our findings highlight the possible common serotonergic mechanisms of the investigated psychiatric disorders. Yet, the functional relevance of most TPH2 polymorphisms is unclear. Characterizing how exactly the different TPH2 variants influence the serotonergic neurotransmission is a next necessary step in understanding the psychiatric disorders where serotonin is implicated.

A case-control association study of 12 candidate genes and attempted suicide in French adolescents

Background Suicide is the second leading cause of death for 10-19-year-olds. Evidence has shown that attempted suicide is a complex interplay of genes and environmental factors. In the adult population, possible associations between genetic polymorphisms and suicidal behaviors have been investigated for several genes, most often with inconsistent findings and poor replicability of significant associations. This study aimed to identify gene variants conferring risk for adolescent suicide attempt. Methods We selected the genes and variants after an analysis of the literature and a selection of the most significant associations identified. We performed analysis on 22 single nucleotide polymorphisms (SNPs) in 12 genes (COMT, CRHR1, FKBP5, SLC6A4, HTR1B, HTR2A, TPH1, TPH2, BDNF, NTRK2, NOS1 and IL28RA) for association with suicide attempt, hopelessness and impulsivity in an independent sample, composed of 98 adolescent suicide attempters who required hospitalization based on emergency assessments, and 150 healthy volunteers. Quality controls, deviations from the Hardy-Weinberg disequilibrium and statistical tests of association (case/control) were calculated using PLINK. Asymptotic p-values were corrected with the Benjamini-Hochberg method. The level of significance was set to 0.05. Results We identified four polymorphisms of interest, rs10868235 (NTRK2), rs1659400 (NTRK2), rs2682826 (NOS1) and rs7305115 (TPH2), with significant associations for suicide attempts or for the quantitative hopelessness or impulsivity phenotypes. However, none of the associations withstand statistical correction tests. Conclusion Our results do not support the role of the 22 SNPs selected in suicide attempt or hopelessness and impulsivity in adolescent population. However, the relatively small sample size and the probable effect of gene-gene interaction or gene-environment interaction on suicidal behavior could not be ruled out.

An overview of the neurobiology of suicidal behaviors as one meta-system

Suicidal behaviors (SB) may be regarded as the outmost consequence of mental illnesses, or as a distinct entity per se. Regardless, the consequences of SB are very large to both society and affected individuals. The path leading to SB is clearly a complex one involving interactions between the subject's biology and environmental influences throughout life. With the aim to generate a representative and diversified overview of the different neurobiological components hypothesized or shown implicated across the entire SB field up to date by any approach, we selected and compiled a list of 212 gene symbols from the literature. An increasing number of novel gene (products) have been introduced as candidates, with half being implicated in SB in only the last 4 years. These candidates represent different neuro systems and functions and might therefore be regarded as competing or redundant explanations. We then adopted a unifying approach by treating them all as parts of the same meta-system, using bioinformatic tools. We present a network of all components connected by physical protein-protein interactions (the SB interactome). We proceeded by exploring the differences between the highly connected core (~30% of the candidate genes) and its peripheral parts, observing more functional homogeneity at the core, with multiple signal transduction pathways and actin-interacting proteins connecting a subset of receptors in nerve cell compartments as well as development/morphology phenotypes and the stress-sensitive synaptic plasticity processes of long term potentiation/depression. We suggest that SB neurobiology might also be viewed as one meta-system and perhaps be explained as intrinsic unbalances acting within the core or as imbalances arising between core and specific peripheral components.

Association of functional DBH genetic variants with alcohol dependence risk and related depression and suicide attempt phenotypes: results from a large multicenter association study

OBJECTIVE

Dopamine-beta-hydroxylase (DBH) metabolizes the conversion of dopamine to noradrenaline. DBH, located on chromosome 9q34.2 has variants with potential functional consequences which may be related to alterations of neurotransmitter function and several psychiatric phenotypes, including alcohol dependence (AD), depression (MD) and suicidal behavior (SA). The aim of this association study in a large multicenter sample of alcohol-dependent individuals and controls is to investigate the role of DBH SNPs and haplotypes in AD risk and associated phenotypes (AD with MD or SA).

METHOD

1606 inpatient subjects with DSM-IV AD from four addiction treatment centers and 1866 control subjects were included. Characteristics of AD, MD and SA were obtained using standardized structured interviews. After subjects were genotyped for 4 DBH polymorphisms, single SNP case-control and haplotype analyses were conducted.

RESULTS

rs1611115 (near 5') C-allele and related haplotypes were significantly associated with alcohol dependence in females. This association with female alcohol dependence also accounts for the significant relationship between this variant and comorbid conditions and traits.

CONCLUSIONS

This study presents evidence for a potentially functional DBH variant influencing the risk for alcohol dependence while other comorbid conditions are not independently influenced by this SNP. However, the study also supports the possible role of the dopamine system in the etiology of female alcohol dependence.

Serotonergic genes and suicide: a systematic review

Suicide is one of the leading causes of death in the world. Its aetiology is complex and diverse, however, epidemiological studies show that suicidal behavior is partly heritable. Neurobiological evidence implicates serotonergic dysfunction in suicidality, stimulating genetic research to focus on genes related to the serotonergic system. In this paper, we review evidence from studies examining the association between various serotonergic genes (Tryptophan Hydroxylase genes: TPH1; TPH2, Serotonin Transporter gene: 5-HTTLPR in SLC6A4, Serotonin Receptor genes: HTR1A, HTR2A, HTR1B, HTR2C and Monoamine Oxidase A gene: MAOA) and suicidal behavior. The data show associations between variation on the TPH1 gene and 5-HTTLPR gene and violent suicidal behavior in Caucasian populations, with the least inconsistencies. Results are mixed for the TPH2 gene and serotonin receptor genes, but for some genes, studies that include haplotypic analyses or that examine a larger coding region of the genes tend to provide more reliable results. Findings on endophenotypes of suicidality, such as aggression and impulsivity traits, show positive associations for the TPH1, HTR2A, and MAOA genes, but need further replication, since negative associations are also occasionally reported. Since genes can only partially explain suicidal risk, several studies during the past decade have tried to incorporate environmental factors in the susceptibility model. Studies to date show that variation on the 5-HTTLPR, MAOA and HTR2A gene can interact with stressful life events to increase risk for suicidal behavior. Limitations of case-control studies are discussed and future considerations are put forward with regard to endophenotypic measurements and gene-environment interactions.

Associations between prefrontal γ-aminobutyric acid concentration and the tryptophan hydroxylase isoform 2 gene, a panic disorder risk allele in women

Associations between the central serotonergic and γ-aminobutyric acid (GABA) systems play key roles in the prefrontal cortical regulation of emotion and cognition and in the pathophysiology and pharmacotherapy of highly prevalent psychiatric disorders. The goal of this study was to test the effects of common variants of the tryptophan hydroxylase isoform 2 (TPH2) gene on GABA concentration in the prefrontal cortex (PFC) using magnetic resonance spectroscopy. In this study involving 64 individuals, we examined the associations between prefrontal cortical GABA concentration and 12 single nucleotide polymorphisms (SNPs) spanning the TPH2 gene, including rs4570625 (-703 G/T SNP), a potentially functional TPH2 polymorphism that has been associated with decreased TPH2 mRNA expression and panic disorder. Our results revealed a significant association between increased GABA concentration in the PFC and the T-allele frequencies of two TPH2 SNPs, namely rs4570625 (-703 G/T) and rs2129575 (p⩽0.0004) and the C-allele frequency of one TPH2 SNP, namely rs1386491 (p = 0.0003) in female subjects. We concluded that rs4570625 (-703 G/T), rs2129575 and rs1386491 play a significant role in GABAergic neurotransmission and may contribute to the sex-specific dysfunction of the GABAergic system in the PFC.

The 5-HT deficiency theory of depression: perspectives from a naturalistic 5-HT deficiency model, the tryptophan hydroxylase 2Arg439His knockin mouse

A decreased level of brain 5-hydroxytryptamine (5-HT) has been theorized to be a core pathogenic factor in depression for half a century. The theory arose from clinical observations that drugs enhancing extracellular levels of 5-HT (5-HT(Ext)) have antidepressant effects in many patients. However, whether such drugs indeed correct a primary deficit remains unresolved. Still, a number of anomalies in putative biomarkers of central 5-HT function have been repeatedly reported in depression patients over the past 40 years, collectively indicating that 5-HT deficiency could be present in depression, particularly in severely ill and/or suicidal patients. This body of literature on putative 5-HT biomarker anomalies and depression has recently been corroborated by data demonstrating that such anomalies indeed occur consequent to severely reduced 5-HT(Ext) levels in a mouse model of naturalistic 5-HT deficiency, the tryptophan hydroxylase 2 His(439) knockin (Tph2KI) mouse. In this review, we will critically assess the evidence for 5-HT deficiency in depression and the possible role of polymorphisms in the Tph2 gene as a causal factor in 5-HT deficiency, the latter investigated from a clinical as well as preclinical angle.

Tryptophan hydroxylase(2) gene polymorphisms predict brain serotonin synthesis in the orbitofrontal cortex in humans

Brain regional serotonin synthesis can be estimated in vivo using positron emission tomography (PET) and α-[((11))C]methyl-L-tryptophan ((11)C-AMT) trapping (K*) as a proxy. Recently, we reported evidence of lower normalized (11)C-AMT trapping in the orbitofrontal cortex (OBFC) of subjects meeting the criteria for an impulsive and/or aggressive behavioral phenotype. In this study, we examined whether part of the variance in OBFC serotonin synthesis is related to polymorphisms of the gene that encodes for the indoleamine's rate-limiting enzyme in the brain, tryptophan hydroxylase-2 (TPH(2)). In all, 46 healthy controls had PET (11)C-AMT scans and were genotyped for 11 single-nucleotide polymorphisms (SNPs) distributed across the TPH(2) gene and its 5' upstream region. Several TPH(2) SNPs were associated with lower normalized blood-to-brain clearance of (11)C-AMT in the OBFC. Dose-effect relationships were found for two variants (rs6582071 and rs4641527, respectively, located in the 5' upstream region and intron 1) that have previously been associated with suicide. Associations in the OBFC remained statistically significant in a mixed larger sample of patients and controls. These results suggest that in humans, genetic factors might partly account for variations in serotonin synthesis in the OBFC.

Recent molecular genetic studies and methodological issues in suicide research

Suicide behavior (SB) spans a spectrum ranging from suicidal ideation to suicide attempts and completed suicide. Strong evidence suggests a genetic susceptibility to SB, including familial heritability and common occurrence in twins. This review addresses recent molecular genetic studies in SB that include case-control association, genome gene-expression microarray, and genome-wide association (GWA). This work also reviews epigenetics in SB and pharmacogenetic studies of antidepressant-induced suicide. SB fulfills criteria for a complex genetic phenotype in which environmental factors interact with multiple genes to influence susceptibility. So far, case-control association approaches are still the mainstream in SB genetic studies, although whole genome gene-expression microarray and GWA studies have begun to emerge in recent years. Genetic association studies have suggested several genes (e.g., serotonin transporter, tryptophan hydroxylase 2, and brain-derived neurotrophic factor) related to SB, but not all reports support these findings. The case-control approach while useful is limited by present knowledge of disease pathophysiology. Genome-wide studies of gene expression and genetic variation are not constrained by our limited knowledge. However, the explanatory power and path to clinical translation of risk estimates for common variants reported in genome-wide association studies remain unclear because of the presence of rare and structural genetic variation. As whole genome sequencing becomes increasingly widespread, available genomic information will no longer be the limiting factor in applying genetics to clinical medicine. These approaches provide exciting new avenues to identify new candidate genes for SB genetic studies. The other limitation of genetic association is the lack of a consistent definition of the SB phenotype among studies, an inconsistency that hampers the comparability of the studies and data pooling. In summary, SB involves multiple genes interacting with non-genetic factors. A better understanding of the SB genes by combining whole genome approaches with case-control association studies, may potentially lead to developing effective screening, prevention, and management of SB.

Overview of the genetics of major depressive disorder

Major depressive disorder (MDD) is a common psychiatric illness with high levels of morbidity and mortality. Despite intensive research during the past several decades, the neurobiological basis and pathophysiology of depressive disorders remain unknown. Genetic factors play important roles in the development of MDD, as indicated by family, twin, and adoption studies, and may reveal important information about disease mechanisms. This article describes recent developments in the field of psychiatric genetics, with a focus on MDD. Early twin studies, linkage studies, and association studies are discussed. Recent findings from genome-wide association studies are reviewed and future directions discussed. Despite all efforts, thus far, no single genetic variation has been identified to increase the risk of depression substantially. Genetic variants are expected to have only small effects on overall disease risk, and multiple genetic factors in conjunction with environmental factors are likely necessary for the development of MDD. Future large-scale studies are needed to dissect this complex phenotype and to identify pathways involved in the etiology of MDD.

Tryptophan-hydroxylase 2 haplotype association with borderline personality disorder and aggression in a sample of patients with personality disorders and healthy controls

BACKGROUND

There is a decreased serotonergic function in impulsive aggression and borderline personality disorder (BPD), and genetic association studies suggest a role of serotonergic genes in impulsive aggression and BPD. Only one study has analyzed the association between the tryptophan-hydroxylase 2 (TPH2) gene and BPD. A TPH2 "risk" haplotype has been described that is associated with anxiety, depression and suicidal behavior.

METHODS

We assessed the relationship between the previously identified "risk" haplotype at the TPH2 locus and BPD diagnosis, impulsive aggression, affective lability, and suicidal/parasuicidal behaviors, in a well-characterized clinical sample of 103 healthy controls (HCs) and 251 patients with personality disorders (109 with BPD). A logistic regression including measures of depression, affective lability and aggression scores in predicting "risk" haplotype was conducted.

RESULTS

The prevalence of the "risk" haplotype was significantly higher in patients with BPD compared to HCs. Those with the "risk" haplotype have higher aggression and affect lability scores and more suicidal/parasuicidal behaviors than those without it. In the logistic regression model, affect lability was the only significant predictor and it correctly classified 83.1% of the subjects as "risk" or "non-risk" haplotype carriers.

CONCLUSIONS

We found an association between the previously described TPH2 "risk" haplotype and BPD diagnosis, affective lability, suicidal/parasuicidal behavior, and aggression scores.

Effect of tryptophan hydroxylase-2 rs7305115 SNP on suicide attempts risk in major depression

Background

Suicide and major depressive disorders (MDD) are strongly associated, and genetic factors are responsible for at least part of the variability in suicide risk. We investigated whether variation at the tryptophan hydroxylase-2 (TPH2) gene rs7305115 SNP may predispose to suicide attempts in MDD.

Methods

We genotyped TPH2 gene rs7305115 SNP in 215 MDD patients with suicide and matched MDD patients without suicide. Differences in behavioral and personality traits according to genotypic variation were investigated by logistic regression analysis.

Results

There were no significant differences between MDD patients with suicide and controls in genotypic (AG and GG) frequencies for rs7305115 SNP, but the distribution of AA genotype differed significantly (14.4% vs. 29.3%, p < 0.001). The G-allele frequency was significantly higher in cases than control group (58.1% vs.45.6%, p < 0.001), but the A-allele carrier indicated a decreased trend in MDD with suicide behaviors than control group (41.9% vs.54.4%, p < 0.001). The multivariate logistic regression analysis indicated that TPH2 rs7305115 AA (OR 0.33, 95% CI 0.22-0.99), family history of suicide (OR 2.98, 95% CI 1.17-5.04), negative life events half year ago (OR 6.64, 95% CI 2.48-11.04) and hopelessness (OR 7.68, 95% CI 5.79-13.74) were significantly associated with the suicide behaviors in MDD patients.

Conclusions

The study suggested that hopelessness, negative life events and family history of suicide were risk factors of attempted suicide in MDD while the TPH2 rs7305115A remained a significant protective predictor of suicide attempts.

Association of polymorphisms of the tryptophan hydroxylase 2 gene with risk for bipolar disorder or suicidal behavior

Bipolar disorder (BD) is a severe psychiatric illness characterized by the occurrence of elevated mood alternating with depressive episodes, having a estimated lifetime prevalence of 0.4-1.6% using DSM-IV criteria. Disturbances of the central serotonergic system has been associated with the pathophysiology of affective disorders and suicidal behavior. Tryptophan hydroxylase 2 (TPH2) which is a rate limiting enzyme in the serotonin synthesis is considered an important candidate gene associated with psychiatric disorders. Our sample consisted of 527 subjects (303 diagnosed with bipolar disorder and 224 healthy controls) which were genotyped for eight tagSNPs (rs4448731, rs4565946, rs11179000, rs7955501, rs10506645, rs4760820, rs1487275 and rs10879357) covering the whole gene of the human TPH2. Statistical analyses were performed using UNPHASED version 3.0.12 and Haploview((R)). Single markers, genotype and haplotype association analysis did not show significant genetic association with bipolar disorder or suicidal behavior. Our findings do not support the association between diagnosis of BD or suicidal behavior and TPH2 polymorphisms.

Neuronal tryptophan hydroxylase (TPH2) in suicidal behaviour

Serotonin (5-hydroxytryptamine, 5-HT) is a signalling molecule involved in many brain functions as well as in a variety of psychiatric conditions, including suicidal behaviour. Tryptophan hydroxylase (TPH), the rate-limiting enzyme of 5-HT biosynthesis, plays a critical role in 5-HT metabolism and thus in the regulation of 5-HT neurotransmission. There are two isoforms of TPH, so-called peripheral (TPH1) and neuronal (TPH2), encoded by the two distinct genes. TPH1 is abundant in the peripheral tissues, while TPH2 was detected exclusively within the nervous system. Since its discovery in 2003, TPH2 gene has attracted great interest as a potential factor contributing to diathesis of suicidal behaviour. However, initial positive findings indicating an association of TPH2 genetic variants with suicide have not been replicated consistently. Here we provide an overview of published studies to date, that have evaluated the contribution of TPH2 gene to the etiopathogenesis of suicidal behaviour.

TPH2 is not a susceptibility gene for suicide in Japanese population

BACKGROUND

Serotonergic systems mediate a control of aggression and/or impulsivity in human and are suggested to be involved in suicidal behavior. The newly identified neuronal tryptophan hydroxylase isoform 2 (TPH2), the rate-limiting enzyme in serotonin synthesis, represents a prime candidate in numerous genetic association analyses of suicidal behavior; however, the results are still inconclusive. The discrepancy may result from the heterogeneity of pathogenesis of suicidal behavior and/or methodological mismatches. We, therefore, attempted to replicate the association of TPH2 gene with suicide using a case-control study of 234 completed suicides and 260 control subjects in Japanese population.

METHODS

We genotyped 15 tagging-single nucleotide polymorphisms (SNPs) including 4 SNPs, which were previously reported to be associated with suicidal behavior, using the TaqMan probe assays and the polymerase chain reaction and restriction fragment length polymorphism (PCR-RFLP) method.

RESULTS

We found no significant differences in genotypic distributions (uncorrected p=0.06-0.98) or allelic frequencies (uncorrected p=0.09-0.95) of the fifteen SNPs between the completed suicides and control groups. Haplotypes constructed with these SNPs were also not associated with suicide (uncorrected p=0.03-0.96 and corrected p=0.20-1.00). Even when we took sex and suicidal methods (violent or non-violent) into account for the analyses, no significant differences in genotypic distributions, allelic/haplotypic frequencies were found in the two groups.

CONCLUSION

Our results suggest that the common SNPs and haplotypes of the TPH2 gene are unlikely to contribute to the genetic susceptibility to suicidal behavior in Japanese population.

Case-control association study of 65 candidate genes revealed a possible association of a SNP of HTR5A to be a factor susceptible to bipolar disease in Bulgarian population

BACKGROUND

Bipolar affective disorder (BAD) is a psychiatric illness characterized by episodes of mania and depression. Although the etiology is not clear, epidemiological studies suggest it is a result of an interaction of genetic and environmental factors. Despite of enormous efforts and abundant studies conducted, none has yet been identified definitively a gene susceptible to bipolar disorder.

METHODS

Ninety-four Bulgarian patients diagnosed with bipolar disorder and 184 Bulgarian healthy individuals, were used for genotyping of 191 single nucleotide polymorphisms (SNPs) by TaqMan and/or Invader assays. Seventeen SNPs that revealed P value less than 0.05 in the first screening were genotyped using an additional independent set of samples, consisting of 78 BAD cases and 372 controls.

RESULTS

After applying the Bonferonni correction on genotyping results of 172 cases and 556 controls, only one SNP, rs1800883, in the HTR5A gene revealed a significant level of P value (P=0.000097; odds ratio=1.80 (95%CI, 1.27-2.54); corrected P=0.017).

CONCLUSIONS

Our findings suggest that HTR5A gene could play an important role in the pathogenesis of bipolar disorder in our population. However these findings should be viewed with caution and replication studies in other populations are necessary in support of these findings.

Variation in tryptophan hydroxylase-2 gene is not associated to male completed suicide in Estonian population

Dysfunction of the central serotonergic system has been related to a spectrum of psychiatric disorders, including suicidal behavior. Tryptophan hydroxylase isoform 2 (TPH2) is the rate-limiting enzyme in the biosynthetic pathway of serotonin, being expressed in serotonergic neurons of raphe nuclei. We investigated genetic variation in TPH2 gene in two samples of male subjects: 288 suicide completers and 327 volunteers, in order to reveal any associations between 14 single nucleotide polymorphisms and completed suicide. No associations were revealed neither on allelic nor haplotype level. Our finding does not support the hypothesis of TPH2 being a susceptibility factor for completed suicide in males of Estonian origin.

Tryptophan research in panic disorder

A considerable body of evidence suggests the involvement of serotonin neurotransmission in the pathogenesis of panic disorder. Research on pathways and functions of tryptophan, an essential amino acid converted into serotonin, may advance our understanding of serotonergic actions in panic disorder and related phenomena. The investigative approaches in this field include manipulations of tryptophan availability as well as genetic association and functional brain imaging studies. In this review we examine the principle findings of these studies and propose further research directions.

Common genetic variations in human brain-specific tryptophan hydroxylase-2 and response to antidepressant treatment

OBJECTIVE

Genetic variability within the serotoninergic system may predict the response to antidepressant drugs. Several polymorphisms in the gene coding for the brain-specific tryptophan hydroxylase (TPH2) have been associated with susceptibility to psychiatric diseases. In this study, we analyzed the correlation between TPH2 polymorphisms and response to antidepressant drugs.

METHODS

The study included 182 patients who received drug treatment for major depression. To assess the variability in the TPH2 gene, four single nucleotide polymorphisms (SNPs) tagging the common TPH2 haplotypes and six SNPs medically relevant according to data from other studies were analyzed in a multiplex single base primer extension reaction.

RESULTS

Two SNPs, rs10897346 and rs1487278, were significantly associated with response to therapy (P=0.003 and 0.007). The rs10897346 variant showed the highest predictive values with carriers of null C alleles showing a 2.6-fold increased risk (95% confidence interval 1.4-4.8) for nonresponse compared with the others. The effect was found in all major types of antidepressant medications administered in this study and was statistically significant in the subgroup on selective serotonin reuptake inhibitors. Multiple logistic regression analyses confirmed the rs10879346 polymorphism as an independent predictor of the antidepressant response (odds ratio: 3.86; 1.75-8.55, P=0.0008). The therapeutically relevant variant rs10897346 is completely linked with the functional Pro312Pro polymorphism, which is known to affect TPH2 expression and may influence serotonin synthesis in the brain.

CONCLUSION

The polymorphisms rs10897346 and Pro312Pro in the TPH2 gene might play an important role for TPH2 expression and antidepressant drug response.

The genetics of bipolar disorder: genome 'hot regions,' genes, new potential candidates and future directions

Bipolar disorder (BP) is a complex disorder caused by a number of liability genes interacting with the environment. In recent years, a large number of linkage and association studies have been conducted producing an extremely large number of findings often not replicated or partially replicated. Further, results from linkage and association studies are not always easily comparable. Unfortunately, at present a comprehensive coverage of available evidence is still lacking. In the present paper, we summarized results obtained from both linkage and association studies in BP. Further, we indicated new potential interesting genes, located in genome 'hot regions' for BP and being expressed in the brain. We reviewed published studies on the subject till December 2007. We precisely localized regions where positive linkage has been found, by the NCBI Map viewer (http://www.ncbi.nlm.nih.gov/mapview/); further, we identified genes located in interesting areas and expressed in the brain, by the Entrez gene, Unigene databases (http://www.ncbi.nlm.nih.gov/entrez/) and Human Protein Reference Database (http://www.hprd.org); these genes could be of interest in future investigations. The review of association studies gave interesting results, as a number of genes seem to be definitively involved in BP, such as SLC6A4, TPH2, DRD4, SLC6A3, DAOA, DTNBP1, NRG1, DISC1 and BDNF. A number of promising genes, which received independent confirmations, and genes that have to be further investigated in BP, have been also systematically listed. In conclusion, the combination of linkage and association approaches provided a number of liability genes. Nevertheless, other approaches are required to disentangle conflicting findings, such as gene interaction analyses, interaction with psychosocial and environmental factors and, finally, endophenotype investigations.

No association between a TPH2 promoter polymorphism and mood disorders or monoamine turnover

BACKGROUND

Tryptophan hydroxylase (TPH) is the rate-limiting enzyme in serotonin synthesis. TPH2 is a recently discovered isoform that is expressed predominantly in serotonin neurons. Associations are reported of TPH2 polymorphisms with MDD, bipolar disorder and suicidal behavior. This study examines a single nucleotide polymorphism in the putative promoter region of the TPH2 gene.

METHODS

One hundred nine bipolar, 324 major depressive disorder, and 130 healthy volunteers were genotyped for the rs4131347 (-C8347G) promoter SNP. Association was assessed with diagnosis, suicide attempt status, severity of psychopathology and cerebrospinal fluid monoamine metabolite levels of 5-HIAA, HVA, and MHPG. General linear models and logistic regression tested the effect of genotype*childhood abuse interactions on psychopathology severity and suicide attempt.

RESULTS

There was no association between genotype and either mood disorder, suicide attempt status, psychopathology severity or CSF monoamine metabolite levels.

CONCLUSIONS

No association was detected between the rs4131347 (-C8347G) SNP in the promoter region of the TPH2 gene and mood disorders, suicidal behavior or monoamine function.

Association between monoamine oxidase A polymorphisms and anger-related personality traits in Korean women

It has been suggested that polymorphisms in the monoamine oxidase A (MAO-A) gene are associated with aggressive and impulsive behaviors. In the present study, we investigated the association of the MAO-A variable number of tandem repeat polymorphism in the promoter region (MAO-A uVNTR) with anger-related personality traits. Specifically, MAO-A uVNTR polymorphisms were examined for associations with the State-Trait Anger Expression Inventory (STAXI) scores in 211 normal Korean women. All subjects were assessed using the STAXI and genotyped for MAO-A uVNTR status. The scores on the STAXI subscales differed significantly among the MAO-A uVNTR polymorphism genotypes in terms of anger expression-out (AX-Out) scores. Post hoc comparisons revealed significant differences between the 3/3 and 4/4, and between 3/4 and 4/4 polymorphisms. However, no significant difference was observed in other STAXI subscale scores among these genotypes. Subjects with the high-activity MAO-A uVNTR had significantly higher AX-Out scores than subjects with other genotypes. MAO-A uVNTR polymorphisms may contribute in part to the expression of anger. These findings support the hypothesis that this polymorphism in the MAO-A gene may be associated with anger-related personality traits in Korean women.

Power based association analysis (PBAT) of serotonergic and noradrenergic polymorphisms in bipolar patients with suicidal behaviour

BACKGROUND AND AIMS

Suicidality is a major health concern worldwide particularly in affective disorder patients. Attempted suicide is familial. There is strong neurobiological evidence showing that serotonergic and noradrenergic dysfunction is implicated in suicidal behaviours. We will apply now a new family based association strategy aimed to explain the genetic influence in suicidal behaviour by power based association test statistics (PBAT) in 336 bipolar patients assessed for suicidality within nuclear families.

METHODS

By use of conditional power calculations, the approach screens all possible null hypotheses without biasing the nominal significance level, and it identifies the subset of phenotypes that has optimal power when tested for association by either univariate or multivariate family based association test (FBAT). Using this statistical approach (PBAT) we investigated polymorphisms in serotonergic and noradrenergic genes, considering suicidal behaviour severity instead of the dichotomous phenotype (presence of suicide attempt).

RESULTS

COMT Val/Met polymorphism was not associated with suicide with high confidence (power=91%). On the other hand, the analysis of the other 12 markers in the adrenergic and serotonergic genes revealed that the TH allele tended towards association with higher severity of suicidal behaviour (p=0.060) but the power obtained was very low.

CONCLUSIONS

The marginal finding of association between TH and severe suicidal behaviour are convergent with previous reports. On the other hand, our sample has enough power to exclude the other polymorphisms investigated as major candidate for suicidality in bipolar disorder.

Neurobiology of suicidal behavior. An integration of biological and clinical findings

Suicide is among the top ten leading causes of death in individuals of all ages. An explanatory model for suicidal behavior that links clinical and psychological risk factors or endophenotypes, to the underlying neurobiological abnormalities associated with suicidal behavior may enhance prediction, help identify treatment options and have heuristic value. Our explanatory model proposes that developmental factors that are biological (genetics) and psychological or clinical (early childhood adversity) may have causal relevance to the disturbances found in subjects with suicidal behavior. In this way, our model integrates findings from several perspectives in suicidology and attempts to explain the relationship between various neurobiological, genetic, and clinical observations in suicide research, offering a comprehensive hypothesis to facilitate understanding of this complex outcome.

Effect of tryptophan hydroxylase-2 gene variants on suicide risk in major depression

BACKGROUND

Suicide and depressive disorders are strongly associated, yet not all depressed patients commit suicide. Genetic factors may partly explain this difference. We investigated whether variation at the tryptophan hydroxylase-2 (TPH2) gene and its 5' upstream region may predispose to suicide in major depressive disorder (MDD) and whether this predisposition is mediated by impulsive-aggressive behaviors (IABs).

METHODS

We genotyped 14 single nucleotide polymorphisms (SNPs) in 259 depressed subjects, 114 of which committed suicide while depressed. Phenotypic assessments were carried out by means of proxy-based interviews. Single-marker and haplotype association analyses were conducted. Differences in behavioral and personality traits according to genotypic variation were investigated, as well as genetic and clinical predictors of suicide.

RESULTS

We found two upstream and two intronic SNPs associated with suicide. No direct effect of these variants was observed on IABs. However, a slight association with reward dependence scores was found. Controlling for suicide risk factors, two SNPs (rs4448731 and rs4641527) significantly predicted suicide, along with cluster B personality disorders and family history of suicide.

CONCLUSIONS

The TPH2 gene and its 5' upstream region variants may be involved in the predisposition to suicide in MDD; however, our findings do not support the role of IABs as mediators.

Tryptophan hydroxylase 2 (TPH2) haplotypes predict levels of TPH2 mRNA expression in human pons

Tryptophan hydroxylase isoform 2 (TPH2) is expressed in serotonergic neurons in the raphe nuclei, where it catalyzes the rate-limiting step in the synthesis of the neurotransmitter serotonin. In search for functional polymorphisms within the TPH2 gene locus, we measured allele-specific expression of TPH2 mRNA in sections of human pons containing the dorsal and median raphe nuclei. Differences in allelic mRNA expression--referred to as allelic expression imbalance (AEI)--are a measure of cis-acting regulation of gene expression and mRNA processing. Two marker SNPs, located in exons 7 and 9 of TPH2 (rs7305115 and rs4290270, respectively), served for quantitative allelic mRNA measurements in pons RNA samples from 27 individuals heterozygous for one or both SNPs. Significant AEI (ranging from 1.2- to 2.5-fold) was detected in 19 out of the 27 samples, implying the presence of cis-acting polymorphisms that differentially affect TPH2 mRNA levels in pons. For individuals heterozygous for both marker SNPs, the results correlated well (r=0.93), validating the AEI analysis. AEI is tightly associated with the exon 7 marker SNP, in 17 of 18 subjects. Remarkably, expression from the minor allele exceeded that of the major allele in each case, possibly representing a gain-of-function. Genotyping of 20 additional TPH2 SNPs identified a haplotype block of five tightly linked SNPs for which heterozygosity is highly correlated with AEI and overall expression of TPH2 mRNA. These results reveal the presence of a functional cis-acting polymorphism, with high frequency in normal human subjects, resulting in increased TPH2 expression levels. The SNPs that correlate with AEI are closely linked to TPH2 SNPs previously shown to associate with major depression and suicide.

Molecular genetics of bipolar disorder and depression

In this review, all papers relevant to the molecular genetics of bipolar disorder published from 2004 to the present (mid 2006) are reviewed, and major results on depression are summarized. Several candidate genes for schizophrenia may also be associated with bipolar disorder: G72, DISC1, NRG1, RGS4, NCAM1, DAO, GRM3, GRM4, GRIN2B, MLC1, SYNGR1, and SLC12A6. Of these, association with G72 may be most robust. However, G72 haplotypes and polymorphisms associated with bipolar disorder are not consistent with each other. The positional candidate approach showed an association between bipolar disorder and TRPM2 (21q22.3), GPR50 (Xq28), Citron (12q24), CHMP1.5 (18p11.2), GCHI (14q22-24), MLC1 (22q13), GABRA5 (15q11-q13), BCR (22q11), CUX2, FLJ32356 (12q23-q24), and NAPG (18p11). Studies that focused on mood disorder comorbid with somatic symptoms, suggested roles for the mitochondrial DNA (mtDNA) 3644 mutation and the POLG mutation. From gene expression analysis, PDLIM5, somatostatin, and the mtDNA 3243 mutation were found to be related to bipolar disorder. Whereas most previous positive findings were not supported by subsequent studies, DRD1 and IMPA2 have been implicated in follow-up studies. Several candidate genes in the circadian rhythm pathway, BmaL1, TIMELESS, and PERIOD3, are reported to be associated with bipolar disorder. Linkage studies show many new linkage loci. In depression, the previously reported positive finding of a gene-environmental interaction between HTTLPR (insertion/deletion polymorphism in the promoter of a serotonin transporter) and stress was not replicated. Although the role of the TPH2 mutation in depression had drawn attention previously, this has not been replicated either. Pharmacogenetic studies show a relationship between antidepressant response and HTR2A or FKBP5. New technologies for comprehensive genomic analysis have already been applied. HTTLPR and BDNF promoter polymorphisms are now found to be more complex than previously thought, and previous papers on these polymorphisms should be treated with caution. Finally, this report addresses some possible causes for the lack of replication in this field.

Nested association between genetic variation in tryptophan hydroxylase II, bipolar affective disorder, and suicide attempts

BACKGROUND

Bipolar affective disorder (BPAD) is a common mental illness that is strongly associated with suicide. Suicidal behavior is thought to result from an interaction of genetic, neurobiological, and psychosocial factors and tends to cluster in families, suggesting specific familial factors distinct from those that underlie BPAD itself. Serotonin signaling has long been implicated in both BPAD and suicide, and the gene encoding the brain-expressed isoform of tryptophan hydroxlyase (TPH2) has been described. Markers in TPH2 have been implicated in suicide and major depressive disorder, but the results across studies are inconsistent. No studies have examined TPH2 in large samples of subjects with BPAD and suicide attempts (SA). We tested for a relationship between genetic variation in TPH2 and risk for BPAD and SA in a large family sample.

METHODS

The sample consisted of 2018 members of 670 families, ascertained through a sibling pair affected with bipolar I, bipolar II, or schizoaffective-bipolar disorder and diagnosed under DSM-III/IV criteria. Three single nucleotide polymorphisms representing the common haplotypes spanning TPH2 were analyzed.

RESULTS

Single-marker analysis failed to detect significant genetic association with BPAD or SA, but the number of informative families was small. Haplotype analysis showed significant association with both BPAD and SA, and the same haplotype was significantly associated with both BPAD and SA in a replication sample. Case-only analysis, stratified by SA, suggested that TPH2 was not an independent genetic risk factor for SA in this sample.

CONCLUSIONS

The TPH2 might contribute to the risk of both BPAD and SA in families with BPAD. Further studies are needed to uncover the functional genetic variation that accounts for the observed associations.

Molecular genetic findings in suicidal behavior: what is beyond the serotonergic system?

Various studies provide consistent evidence for a genetic component in suicidal behavior. First molecular genetic studies concentrated on genes of the serotonergic system based on the biochemical evidence that serotonergic neurotransmission is implicated in this behavior. Furthermore, genes of the dopaminergic and noradrenergic neurotransmitter systems have also been the subjects of investigations in this context. Some epidemical and clinical studies showed that low serum cholesterol levels are associated with suicidal behavior and genes involved in these pathways have been investigated. Microarray experiments provide the possibility of genome-wide gene expression analysis and help to investigate associated molecular mechanisms. The aim of this article is to review molecular genetic studies in suicidal behavior and to emphasize findings on new genes.

Effect of SNP at position 40237 in exon 7 of the TPH2 gene on susceptibility to suicide

OBJECTIVE

The novel tryptophan hydroxylase isoform (TPH2), being the rate-limiting enzyme in the biosynthesis of serotonin, has many biological functions and plays an important role as candidate gene in several psychiatric disorders. Here, an A-G single nucleotide polymorphism (SNP) at the position 40237 relative to 5'-end of TPH2 gene was selected for the association with suicide behavior.

METHODS

The study population consisted of 102 MD patients with suicide behavior as attempters and 123 MD patients without suicide behavior as non-attempters. The TPH2 SNP was determined by sequencing on an ABI 3700 automated DNA sequencer.

RESULTS

The A allele was significantly less frequent in attempters than in non-attempters (p=0.0067). Individuals with the A/A genotype showed a significantly lower risk of suicide behavior than those with the A/G or G/G genotype (OR=0.35).

CONCLUSIONS

Our findings suggest that the A-G polymorphism of TPH2 may confer susceptibility to suicide behavior in MD patients.

Association study of tryptophan hydroxylase 2 gene polymorphisms in panic disorder

Experimental studies on serotonin (5-HT) availability suggest a role for 5-HT synthesis rate in panicogenesis. Recently, it has been discovered that the tryptophan hydroxylase gene isoform 2 (TPH2), rather than TPH1, is preferentially expressed in the neuronal tissue and, therefore, is primarily responsible for the regulation of brain 5-HT synthesis. In the present case-control genetic association study we investigated whether panic disorder (PD) phenotypes are related to two single nucleotide polymorphisms (SNP) of TPH2, rs1386494 A/G and rs1386483 C/T. The study sample consisted of 213 (163 females and 50 males) PD patients with or without affective comorbidity and 303 (212 females and 91 males) matched healthy control subjects. The allelic and genotypic analyses in the total sample did not demonstrate significant association of PD with the studied SNPs, suggesting that these polymorphisms may not play a robust role in predisposition to PD. However, an association with rs1386494 SNP was observed in the subgroup of female patients with pure PD phenotype, indicating a possible gender-specific effect of TPH2 gene variants in PD.

A new gain-of-function allele in chimpanzee tryptophan hydroxylase 2 and the comparison of its enzyme activity with that in humans and rats

Tryptophan hydroxylase 2 (TPH2) is a rate-limiting enzyme of neuronal serotonin biosynthesis. Recently, two single nucleotide polymorphisms (SNPs) at the exon 11 coding region that resulted in amino acid substitutions in the C-terminal domain have been reported to affect enzyme activity in humans and mice. We determined 175 base-pair sequences of the exon 11 region in nine primate species from all recognized lineages. All nucleotide sequence substitutions were synonymous, with the exception of one adenine (A) to guanine (G) substitution at the 1404th position in the open reading frame (ORF). This substitution leads to a glutamine (Q) to arginine (R) amino acid substitution at the 468th position within chimpanzee sequences. The frequency of the G allele was 0.24 among 66 chimpanzees. Therefore, it is a novel SNP observed in chimpanzees, and we have named these two alleles as ch468Q and ch468R, respectively. When expressed in HeLa cells, ch468R caused an approximate 20% increase in enzyme function during L-5-hydroxytryptophan (5HTP) production (P<0.001). We also surveyed the interspecies difference in enzyme activity among human, chimpanzee, and rat. Although the rat showed an identical amino acid sequence at the C-terminal region as those of human and ch468Q, the rat enzyme was more active than those of human or chimpanzee (P<0.001), indicating the importance of substitutions in other regions. Our findings on the chimpanzee SNP will be a useful genetic marker in understanding the individual difference in the serotonin-related behavior.

Analysis of variations in the tryptophan hydroxylase-2 (TPH2) gene in cocaine dependence

While the exact physiological mechanisms underlying cocaine dependence remain unclear, a growing body of evidence indicates a role for the serotonergic neurotransmitter system in the pathology of this substance use disorder. The focus of the present study is to determine whether genetic variation in the tryptophan hydroxylase-2 (TPH2) gene, which encodes the enzyme responsible for synthesis of the majority of the serotonin contained in neurons of the central nervous system, contributes to the pathophysiology of cocaine dependence. To examine this hypothesis, we used a case-control study design in which the genotype and allele distributions for six single nucleotide polymorphisms (SNPs) in the TPH2 gene were compared between cocaine-dependent (n = 299) and control individuals (n = 208) of African descent. The results indicate that none of the SNPs in the TPH2 gene examined in this study associate with the cocaine-dependent phenotype. This work suggests that variations in the TPH2 gene are not a risk factor for the development of cocaine dependence, but these findings require confirmation in larger, independent samples of cocaine-dependent and control subjects.

Mouse tryptophan hydroxylase isoform 2 and the role of proline 447 in enzyme function

Tryptophan hydroxylase (TPH) is the rate-limiting enzyme in the synthesis of the neurotransmitter serotonin (5-HT). Once thought to be a single gene product, TPH is now known to exist in two isoforms. Isoform 1 (TPH1) is found in the pineal gland and gut, and isoform 2 (TPH2) is selectively expressed in brain. A single-nucleotide polymorphism in TPH2 results in a proline-to-arginine mutation at residue 447 and substantially lowers catalytic activity. In view of the importance of TPH in determining brain 5-HT function, we cloned TPH2 and produced the P447R mutant to assess the importance of this proline in enzyme function. Catalytically active TPH2 and the P447R mutant were expressed at the predicted subunit molecular mass of 56 kDa. The P447R mutant expressed less than 50% of the activity of TPH2. Mutation of this conserved proline in TPH1 (P403R) also resulted in an enzyme with significantly lower activity than the wild-type enzyme. The P447R mutant had a V(max) 50% lower than that of TPH2. The P447R mutation did not alter the oligomeric assembly of the protein, nor change its responsiveness to cysteine modification. The P447R mutation did not alter enzyme substrate specificity or stability, but conferred slightly enhanced sensitivity to inhibition by dopamine and diminished sensitivity to iron in catalysis. The conserved proline in TPH (residue 447 in TPH2 and 403 in TPH1) plays an important role in enzyme function by regulating V(max) of the catalytic reaction.

Functional polymorphisms of the brain serotonin synthesizing enzyme tryptophan hydroxylase-2

Many neuropsychiatric disorders are considered to be related to the dysregulation of brain serotonergic neurotransmission. Tryptophan hydroxylase-2 (TPH2) is the neuronal-specific enzyme that controls brain serotonin synthesis. There is growing genetic evidence for the possible involvement of TPH2 in serotonin-related neuropsychiatric disorders; however, the degree of genetic variation in TPH2 and, in particular, its possible functional consequences remain unknown. In this short review, we will summarize some recent findings with respect to the functional analysis of TPH2.

Association between Tph2 gene polymorphism, brain tryptophan hydroxylase activity and aggressiveness in mouse strains

The brain neurotransmitter serotonin is involved in the regulation of aggressive behavior. The main factor determining the brain serotonin level is the activity of the rate-limiting enzyme in the biosynthesis of the neurotransmitter--tryptophan hydroxylase isoform (TPH) 2 encoded by the Tph2 gene. Recently the C1473G single-nucleotide polymorphism in the Tph2 gene was reported. Here we study the C1473G polymorphism in 10 inbred mouse strains (C57BL/6J, AKR/J, DD/He, C3H/HeJ, YT/Y, BALB/cJLac, CC57BR/Mv and A/He) and demonstrate the association of the polymorphism with brain TPH activity and intermale aggressiveness. TPH activity in the midbrain of mice homozygous for the 1473C allele was higher than that in mice carrying 1473G alleles. A close association of the 1473C allele with increased number of attacks towards another male was found. The results support a link between the C1473G polymorphism in Tph2 gene, tryptophan hydroxylase activity and intensity of intermale aggression.

Possible association between autism and variants in the brain-expressed tryptophan hydroxylase gene (TPH2)

We report a possible association between autism in our sample and a recently described brain-expressed tryptophan hydroxylase gene (TPH2). The well-replicated involvement of the serotonin neurotransmitter system in autism has stimulated interest in many genes in the serotonin pathway as possible candidates for mutations leading to autism susceptibility. Serotonin synthesis is controlled by the rate-limiting enzyme tryptophan hydroxylase. A mouse study of the original tryptophan hydroxylase gene (TPH1) and the new isoform (TPH2) showed that while TPH1 is primarily expressed peripherally, TPH2 is found exclusively in brain tissue. We searched for human sequence variants in 6,467 nucleotides covering all 11 exons of TPH2, and also 248 nucleotides upstream of the start codon, and 935 nucleotides downstream of the stop codon. Eighteen variants were characterized in 88 subjects with autism studied at our two centers, and 95 unrelated control subjects. Using a model-free association method and empirical P value estimation, two variants showed frequency differences between autism and control subjects (P = 0.01 for a T-G variant in intron 1, and P = 0.02 for a A-T variant in intron 4). A haplotype including these variants showed slightly increased significance (P = 0.005). Further investigation of clinical phenotypes showed a possible association between presence of the variants at these two SNPs and higher scores on the Autism Diagnostic Interview (ADI) domain describing repetitive and stereotyped behaviors (P = 0.007). We conclude that TPH2 may play a modest role in autism susceptibility, perhaps relating specifically to repetitive behaviors, pending replication of this result.

Differential hormonal regulation of tryptophan hydroxylase-2 mRNA in the murine dorsal raphe nucleus

BACKGROUND

Recently a novel tryptophan hydroxylase isoform (TPH2) was identified and shown to be highly expressed in the central nervous system (CNS). Hormonal effects on TPH2 mRNA expression in the rodent dorsal raphe nucleus (DRN) are unknown.

METHODS

In situ hybridization histochemistry and real-time reverse transcriptase-polymerase chain reaction (RT-PCR) were used to assess the effects of dexamethasone or estradiol on TPH2 mRNA levels in the DRN of C57/Bl6 mice.

RESULTS

Dexamethasone reduced TPH2 mRNA levels in the DRN of both ovx female and intact male mice. Reduction of TPH2 mRNA in the DRN was blocked by co-administration of mifepristone. Estradiol had no detectable effect on TPH2 mRNA levels in the DRN.

CONCLUSIONS

TPH2 mRNA is regulated by glucocorticoids but not estradiol in the mouse DRN. Glucocorticoid-mediated reduction of TPH2 message may have relevance to the etiology of major depression, psychotic major depression in particular, where elevated glucocorticoids are one hallmark of the disease.

Serotonin gene polymorphisms and bipolar I disorder: focus on the serotonin transporter

The pathogenesis of bipolar disorder may involve, at least in part, aberrations in serotonergic neurotransmission. Hence, serotonergic genes are attractive targets for association studies of bipolar disorder. We have reviewed the literature in this field. It is difficult to synthesize results as only one polymorphism per gene was typically investigated in relatively small samples. Nevertheless, suggestive associations are available for the 5HT2A receptor and the serotonin transporter genes. With the availability of extensive polymorphism data and high throughput genotyping techniques, comprehensive evaluation of these genes using adequately powered samples is warranted. We also report on our investigations of the serotonin transporter, SLC6A4 (17q11.1-q12). An insertion/deletion polymorphism (5HTTLPR) in the promoter region of this gene has been investigated intensively. However, the results have been inconsistent. We reasoned that other polymorphism/s may contribute to the associations and the inconsistencies may be due to variations in linkage disequilibrium (LD) patterns between samples. Therefore, we conducted LD analyses, as well as association and linkage using 12 polymorphisms, including 5HTTLPR. We evaluated two samples. The first sample consisted of 135 US Caucasian nuclear families having a proband with bipolar I disorder (BDI, DSM IV criteria) and available parents. For case-control analyses, the patients from these families were compared with cord blood samples from local Caucasian live births (n = 182). Our second, independent sample was recruited through the Systematic Treatment Enhancement Program for Bipolar Disorder (STEP-BD, 545 cases, 548 controls). No significant associations were detected at the individual polymorphism or haplotype level using the case-control or family-based analyses. Our analyses do not support association between SLC6A4 and BDI families. Further studies using sub-groups of BDI are worthwhile.