The interaction between TPH2 promoter haplotypes and clinical-demographic risk factors in suicide victims with major psychoses

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.

Epidemiology, neurobiology and pharmacological interventions related to suicide deaths and suicide attempts in bipolar disorder: Part I of a report of the International Society for Bipolar Disorders Task Force on Suicide in Bipolar Disorder

OBJECTIVES

Bipolar disorder is associated with elevated risk of suicide attempts and deaths. Key aims of the International Society for Bipolar Disorders Task Force on Suicide included examining the extant literature on epidemiology, neurobiology and pharmacotherapy related to suicide attempts and deaths in bipolar disorder.

METHODS

Systematic review of studies from 1 January 1980 to 30 May 2014 examining suicide attempts or deaths in bipolar disorder, with a specific focus on the incidence and characterization of suicide attempts and deaths, genetic and non-genetic biological studies and pharmacotherapy studies specific to bipolar disorder. We conducted pooled, weighted analyses of suicide rates.

RESULTS

The pooled suicide rate in bipolar disorder is 164 per 100,000 person-years (95% confidence interval = [5, 324]). Sex-specific data on suicide rates identified a 1.7:1 ratio in men compared to women. People with bipolar disorder account for 3.4-14% of all suicide deaths, with self-poisoning and hanging being the most common methods. Epidemiological studies report that 23-26% of people with bipolar disorder attempt suicide, with higher rates in clinical samples. There are numerous genetic associations with suicide attempts and deaths in bipolar disorder, but few replication studies. Data on treatment with lithium or anticonvulsants are strongly suggestive for prevention of suicide attempts and deaths, but additional data are required before relative anti-suicide effects can be confirmed. There were limited data on potential anti-suicide effects of treatment with antipsychotics or antidepressants.

CONCLUSION

This analysis identified a lower estimated suicide rate in bipolar disorder than what was previously published. Understanding the overall risk of suicide deaths and attempts, and the most common methods, are important building blocks to greater awareness and improved interventions for suicide prevention in bipolar disorder. Replication of genetic findings and stronger prospective data on treatment options are required before more decisive conclusions can be made regarding the neurobiology and specific treatment of suicide risk in bipolar disorder.

A common TPH2 haplotype regulates the neural processing of a cognitive control demand

The monoamine neurotransmitter, serotonin, critically regulates the function of the cerebral cortex and is involved in psychiatric disorders. Tryptophan hydroxylase (TPH) is the rate-limiting enzyme in the synthesis of serotonin with the neuron-specific TPH2 isoform present exclusively in the brain and encoded by the TPH2 gene on chromosome 12q21. The haplotype structure of TPH2 was defined for 16 single-nucleotide polymorphisms (SNPs) in a healthy subject population and a haplotype block analysis confirmed the presence of a six SNP haplotype in a yin configuration that has previously been associated with risk for suicidality, depression, and anxiety disorders. Functional magnetic resonance imaging (fMRI) was used to assess the influence of TPH2 variation on brain function related to cognitive control using the Multi-Source Interference Task (MSIT). The MSIT-related blood oxygen level-dependent (BOLD) response was increased with increasing copies of the TPH2 yin haplotype for the dorsal anterior cingulate cortex (dACC), right inferior frontal cortex (IFC), and anterior striatum. A functional connectivity analysis further revealed that increasing numbers of the TPH2 yin haplotype was associated with diminished functional coupling between the dACC and the right IFC, precentral gyrus, parietal cortex and dlPFC. A moderation analysis indicated that the relationship between neural processing networks and cognitive control was significantly modulated by allelic variation for the TPH2 yin haplotype. These findings suggest that the association of risk for psychiatric disorders with a common TPH2 yin haplotype is related to the inefficient functional engagement of cortical areas involved in cognitive control and alterations in the mode of functional connectivity of dACC pathways.

A functional Tph2 C1473G polymorphism causes an anxiety phenotype via compensatory changes in the serotonergic system

The association of single-nucleotide polymorphisms (SNPs) in the human tryptophan hydroxylase 2 (TPH2) gene with anxiety traits and depression has been inconclusive. Observed inconsistencies might result from the fact that TPH2 polymorphisms have been studied in a genetically heterogeneous human population. A defined genetic background, control over environmental factors, and the ability to analyze the molecular and neurochemical consequences of introduced genetic alterations constitute major advantages of investigating SNPs in inbred laboratory mouse strains. To investigate the behavioral and neurochemical consequences of a functional C1473G SNP in the mouse Tph2 gene, we generated congenic C57BL/6N mice homozygous for the Tph2 1473G allele. The Arg(447) substitution in the TPH2 enzyme resulted in a significant reduction of the brain serotonin (5-HT) in vivo synthesis rate. Despite decreased 5-HT synthesis, we could detect neither a reduction of brain region-specific 5-HT concentrations nor changes in baseline and stress-induced 5-HT release using a microdialysis approach. However, using a [(35)S]GTP-γ-S binding assay and 5-HT(1A) receptor autoradiography, a functional desensitization of 5-HT(1A) autoreceptors could be identified. Furthermore, behavioral analysis revealed a distinct anxiety phenotype in homozygous Tph2 1473G mice, which could be reversed with chronic escitalopram treatment. Alterations in depressive-like behavior could not be detected under baseline conditions or after chronic mild stress. These findings provide evidence for an involvement of functional Tph2 polymorphisms in anxiety-related behaviors, which are likely not caused directly by alterations in 5-HT content or release but are rather due to compensatory changes during development involving functional desensitization of 5-HT(1A) autoreceptors.

Association between tryptophan hydroxylase 2 gene polymorphism and completed suicide

The association between suicide and a single nucleotide polymorphism (rs1386483) was examined in the recently identified tryptophan hydroxylase 2 (TPH2) gene. Blood samples of 143 suicide victims and 162 age- and sex-matched controls were examined. The frequency of the TT genotype in the TPH2 polymorphism was higher in suicide victims than in controls (17.5% vs. 8.6%; p = 0.02), particularly in those with a history of repeated suicide attempts (53.3% vs. 8.6%; p < 0.0001). The examined TPH2 polymorphism was found to be associated with suicide. This genetic marker may be particularly important in understanding risk of multiple suicide attempts. Further analyses are needed to confirm these results.

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.

Risk-taking behavior in a gambling task associated with variations in the tryptophan hydroxylase 2 gene: relevance to psychiatric disorders

Decision making, choosing the best option from the possible outcomes, is impaired in many psychiatric conditions including affective disorders. We tested the hypothesis that variations in serotonergic genes (TPH2, TPH1, SLC6A4, HTR1A), which influence serotonin availability, affect choice behavior in a probabilistic gambling task. A population cohort (N=1035) completed a paper-and-pencil gambling task, filled out personality and symptom questionnaires and gave consent for the use of their DNA in a genetic association study. A subgroup of subjects (N=69) also completed a computer version of the task. The gambling task was designed to estimate an individual's tendency to take a risk when choosing between a smaller but more certain 'win' and a larger, less probable one. We genotyped seven haplotype tagging SNPs in the TPH2 gene, and previously reported functional polymorphisms from the other genes (rs1800532, 5HTTLPR, and rs6295). Carriers of the more prevalent TPH2 haplotype, which was previously associated with less active enzyme variant, showed reduced risk taking on both tasks compared with subjects not carrying the common haplotype. The effect of TPH2 haplotypes on risk-taking was independent of current depression and anxiety symptoms, neuroticism and impulsiveness scores. We did not find an association between functional polymorphisms in the TPH1, SLC6A4, HTR1A genes and risk-taking behavior. In conclusion, our study demonstrates the role of the TPH2 gene and the serotonin system in risk taking and suggests that TPH2 gene may contribute to the expression of psychiatric phenotypes through altered decision making.

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.

Management of schizophrenia with suicide risk

Suicidal behavior remains a major source of morbidity and mortality among schizophrenics. The National Institute of Mental Health Longitudinal Study of Chronic Schizophrenia found that over a mean of 6 years, 38% of the patients had at least one suicide attempt and 57% admitted to substantial suicidal ideation. Suicide is also a major issue among inpatients, with serious implications for clinical practice and patient-doctor relationships. The management of schizophrenic patients with suicide risk remains a difficult area for clinicians despite attempts to better understand it by gathering experts in the field. This article discusses the frequency of suicidal behavior in schizophrenia, offers a model for understanding it, and discusses various aspects of the management of the at-risk schizophrenic patient.

Suicide neurobiology

In this review, we examine the history of the neurobiology of suicide, as well as the genetics, molecular and neurochemical findings in suicide research. Our analysis begins with a summary of family, twin, and adoption studies, which provide support for the investigation of genetic variation in suicide risk. This leads to an overview of neurochemical findings restricted to neurotransmitters and their receptors, including recent findings in whole genome gene expression studies. Next, we look at recent studies investigating lipid metabolism, cell signalling with a particular emphasis on growth factors, stress systems with a focus on the role of polyamines, and finally, glial cell pathology in suicide. We conclude with a description of new ideas to study the neurobiology of suicide, including subject-specific analysis, protein modification assessment, neuroarchitecture studies, and study design strategies to investigate the complex suicide phenotype.

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.

Suicide and suicidal behavior

Suicidal behavior is a leading cause of injury and death worldwide. Information about the epidemiology of such behavior is important for policy-making and prevention. The authors reviewed government data on suicide and suicidal behavior and conducted a systematic review of studies on the epidemiology of suicide published from 1997 to 2007. The authors' aims were to examine the prevalence of, trends in, and risk and protective factors for suicidal behavior in the United States and cross-nationally. The data revealed significant cross-national variability in the prevalence of suicidal behavior but consistency in age of onset, transition probabilities, and key risk factors. Suicide is more prevalent among men, whereas nonfatal suicidal behaviors are more prevalent among women and persons who are young, are unmarried, or have a psychiatric disorder. Despite an increase in the treatment of suicidal persons over the past decade, incidence rates of suicidal behavior have remained largely unchanged. Most epidemiologic research on suicidal behavior has focused on patterns and correlates of prevalence. The next generation of studies must examine synergistic effects among modifiable risk and protective factors. New studies must incorporate recent advances in survey methods and clinical assessment. Results should be used in ongoing efforts to decrease the significant loss of life caused by suicidal behavior.

Stress, genes and the biology of suicidal behavior

Suicidal behavior is partly heritable. Studies seeking the responsible candidate genes have examined genes involved in neurotransmitter systems shown to have altered function in suicide and attempted suicide. These neurotransmitter systems include the serotonergic, noradrenergic, and dopaminergic systems and the hypothalamic-pituitary-adrenal axis. With some exceptions, most notably the serotonin transporter gene promoter polymorphism (HTTLPR), replication of candidate gene association studies findings has been difficult. This article reviews current knowledge of specific gene effects and gene-environment interactions that influence risk for suicidal behavior. Effects of childhood stress on development and how it influences adult responses to current stress are shown to be relevant for mood disorders, aggressive/impulsive traits, and suicidal behavior.

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.

Characterization of a functional promoter polymorphism of the human tryptophan hydroxylase 2 gene in serotonergic raphe neurons

BACKGROUND

Tryptophan hydroxylase 2 (TPH2) is the rate-limiting enzyme in brain serotonin (5-HT) biosynthesis. Although dysfunction of 5-HT neurotransmission has been implicated in a variety of neuropsychiatric conditions, the human TPH2 promoter has not been characterized in vitro.

METHODS

The functional relevance of TPH2 promoter polymorphisms was determined with luciferase assays in primary serotonergic neurons from rat raphe nuclei and in human small cell lung carcinoma cells (SHP-77 cells). We also investigated transcription factor binding to the variant promoter sequence with electrophoretic mobility shift assay (EMSA).

RESULTS

The polymorphism rs11178997 of the human TPH2 promoter significantly reduced TPH2 transcriptional activity by 22% and 7% in primary serotonergic neurons and in SHP-77 cells, respectively. In contrast, no significant differences in promoter activity were observed for the G- and T-alleles of rs4570625. The EMSA revealed reduced binding of the transcription factor POU3F2 (also known as Brn-2, N-Oct-3) to the A-allele of the polymorphism rs11178997. Overexpression of POU3F2 resulted in a robust activation of the TPH2 promoter (2.7-fold).

CONCLUSIONS

Our data suggest that the human TPH2 promoter polymorphism rs11178997 impacts on gene expression, which might have implications for the development and function of the serotonergic system in the brain.

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.

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.