Support for the involvement of TPH2 gene in affective disorders

The zebrafish for preclinical psilocybin research

In this review, we discuss the possible utility of zebrafish in research on psilocybin, a psychedelic drug whose recreational use as well as possible clinical application are gaining increasing interest. First, we review behavioral tests with zebrafish, focussing on anxiety and social behavior, which have particular relevance in the context of psilocybin research. Next, we briefly consider methods of genetic manipulations with which psilocybin's phenotypical effects and underlying mechanisms may be investigated in zebrafish. We briefly review the known mechanisms of psilocybin, and also discuss what we know about its safety and toxicity profile. Last, we discuss examples of how psilocybin may be employed for testing treatment efficacy in preclinical research for affective disorders in zebrafish. We conclude that zebrafish has a promising future in preclinical research on psychedelic drugs.

Rifaximin ameliorates depression-like behaviour in chronic unpredictable mild stress rats by regulating intestinal microbiota and hippocampal tryptophan metabolism

BACKGROUND

Chronic unpredictable mild stress (CUMS) can induce depressive behaviours and alter the composition of the gut microbiome. Although modulating gut microbiota can improve depression-like behaviour in rats, the mechanism of action is unclear. Additionally, gut microbiota can affect brain function through the neuroendocrine pathway. This pathway may function by regulating the secretion of neurotransmitters such as tryptophan (TRP). Metabolites of TRP, such as 5-hydroxytryptamine (5-HT) and kynurenine (KYN), are related to the pathophysiological process of depression. Indoleamine-2, 3-dioxygenase-1 (IDO1) and Tryptophan hydroxylase 2 (TPH2) are the key rate-limiting enzymes in TRP metabolism and play an important role in KYN and 5-HT metabolism.

METHODS

Rats were subjected to four weeks of CUMS and given rifaximin150 mg/kg by oral gavage daily. After modelling, we investigated the rat's behaviours, composition of the faecal microbiome, neurotransmitter metabolism and key metabolic enzymes of the TRP pathway in the hippocampus (HIP).

RESULTS

Rifaximin administration improved depressive behaviour in rats, corrected intestinal microbiota disorders and HIP TRP metabolism and regulated the expression of IDO1 and TPH2 in the HIP.

CONCLUSIONS

Rifaximin improves depression-like behaviour in CUMS rats by influencing the gut microbiota and tryptophan metabolism.

Meta-analysis of association between TPH2 single nucleotide poiymorphism and depression

Tryptophan hydroxylase 2 (TPH2) plays a crucial role in the human brain. Although the association between the TPH2 gene and depression has been suggested in previous meta-analyses, studies based on Chinese subjects are often neglected. Therefore, we included some previous studies based on Chinese subjects to explore the relationship between TPH2 polymorphisms and depression via conducting an extensive meta-analysis. We reviewed 40 research papers that included data on TPH2 gene single nucleotide polymorphisms (SNPs) from 5766 patients with depression and 5988 healthy subjects. The analysis showed an association between polymorphisms in the TPH2 gene and depression, and some results were significant in 24 studies that included Chinese Han study participants. The results of our meta-analysis showed that rs4570625, rs17110747, rs120074175, rs4290270, rs120074175, and rs4290270 may be significantly associated with depression, and that rs11178997 (A/A genotype) may be a significant risk factor for depression in the Chinese subjects. Based on the results of this study, biological experiments should be performed in the future to explore how different SNPs affect depression.

Identifying patients with major depressive disorder based on tryptophan hydroxylase-2 methylation using machine learning algorithms

OBJECTIVES

This study aimed to identify patients with major depressive disorder (MDD) by developing different machine learning (ML) models based on tryptophan hydroxylase-2 (TPH2) methylation and environmental stress.

METHODS

The data were collected from 291 patients with MDD and 100 healthy control participants: individual basic information, the Negative Life Events Scale (NLES) scores, the Childhood Trauma Questionnaire (CTQ) scores and the methylation level at 38 CpG sites in TPH2. Information gain was used to select critical input variables. Support vector machine (SVM), back propagation neural network (BPNN) and random forest (RF) algorithms were used to build recognition models, which were evaluated by the 10-fold cross-validation. SHapley Additive exPlanations (SHAP) method was used to evaluate features importance.

RESULTS

Gender, NLES scores, CTQ scores and 13 CpG sites in TPH2 gene were considered as predictors in the models. Three ML algorithms showed satisfactory performance in predicting MDD and the BPNN model indicated best prediction effects.

CONCLUSION

ML models with TPH2 methylation and environmental stress were identified to possess great performance in identifying patients with MDD, which provided precious experience for artificial intelligence to assist traditional diagnostic methods in the future.

TPH2 in the Dorsal Raphe Nuclei Regulates Energy Balance in a Sex-Dependent Manner

AbstractCentral 5-hydroxytryptamine (5-HT), which is primarily synthesized by tryptophan hydroxylase 2 (TPH2) in the dorsal Raphe nuclei (DRN), plays a pivotal role in the regulation of food intake and body weight. However, the physiological functions of TPH2 on energy balance have not been consistently demonstrated. Here we systematically investigated the effects of TPH2 on energy homeostasis in adult male and female mice. We found that the DRN harbors a similar amount of TPH2+ cells in control male and female mice. Adult-onset TPH2 deletion in the DRN promotes hyperphagia and body weight gain only in male mice, but not in female mice. Ablation of TPH2 reduces hypothalamic pro-opiomelanocortin (POMC) neuronal activity robustly in males, but only to a modest degree in females. Deprivation of estrogen by ovariectomy (OVX) causes comparable food intake and weight gain in female control and DRN-specific TPH2 knockout mice. Nevertheless, disruption of TPH2 blunts the anorexigenic effects of exogenous estradiol (E2) and abolishes E2-induced activation of POMC neurons in OVX female mice, indicating that TPH2 is indispensable for E2 to activate POMC neurons and to suppress appetite. Together, our study revealed that TPH2 in the DRN contributes to energy balance regulation in a sexually dimorphic manner.

Association between temperament related traits and single nucleotide polymorphisms in the serotonin and oxytocin systems in Merino sheep

Animal temperament is defined as the consistent behavioral and physiological differences that are seen between individuals in response to the same stressor. Neurotransmitter systems, like serotonin and oxytocin in the central nervous system, underlie variation in behavioral traits in humans and other animals. Variations like single nucleotide polymorphisms (SNPs) in the genes for tryptophan 5-hydroxylase (TPH2), the serotonin transporter (SLC6A4), the serotonin receptor (HTR2A), and the oxytocin receptor (OXTR) are associated with behavioral phenotype in humans. Thus, the objective of this study was to identify SNPs in those genes and to test if those variations are associated with the temperament in Merino sheep. Using ewes from the University of Western Australia temperament flock, which has been selected on emotional reactivity for more than 20 generations, eight SNPs (rs107856757, rs107856818, rs107856856 and rs107857156 in TPH2, rs20917091 in SLC6A4, rs17196799 and rs17193181 in HTR2A, and rs17664565 in OXTR) were found to be distributed differently between calm and nervous sheep. These eight SNPs were then genotyped in 260 sheep from a flock that has never been selected on emotional reactivity, followed by the estimation of the behavioral traits of those 260 sheep using an arena test and an isolation box test. We found that several SNPs in TPH2 (rs107856757, rs107856818, rs107856856 and rs107857156) were in strong linkage disequilibrium, and all were associated with behavioral phenotype in the nonselected sheep. Similarly, rs17196799 in HTR2A was also associated with the behavioral phenotype.

Influence of Xiaoyaosan on depressive-like behaviors in chronic stress-depressed rats through regulating tryptophan metabolism in hippocampus

BACKGROUND

Tryptophan metabolism has always been considered to play a vital role in mental disorder diseases, and how traditional Chinese formula Xiaoyaosan regulates the tryptophan metabolism is a complement to the pathogenesis of depression. This study established a depression rat model by the chronic immobilization stress (CIS) method and observed the change in tryptophan metabolism in hippocampus and the effects of Xiaoyaosan.

METHODS

Forty-eight male Sprague Dawley (SD) rats were randomly divided into the following four groups: control group, CIS group, Xiaoyaosan group, and fluoxetine group. The depression model was established by the 21-day CIS. The food intake and body weight were recorded, and the sucrose preference test (SPT), novelty suppressed feeding (NSF) test and open field test (OFT) were also used to evaluate the model. Then, the contents of tryptophan and 5-hydroxytryptamine (5-HT) in hippocampus were detected by the ELISA method, and the expression levels of tryptophan hydrogenase 2 (TPH2) and indoleamine 2,3-dioxygenase 1 (IDO1) in hippocampus were determined by quantitative reverse transcriptase polymerase chain reaction reaction (qRT-PCR) and Western blot methods.

RESULTS

The behavioral data showed a significant difference between the model group and the normal group. The 5-HT content in the hippocampi of CIS rats was significantly reduced, whereas the tryptophan content in the hippocampi of model rats was significantly increased. The TPH2 level in hippocampus of the model group was significantly decreased, and the IDO1 level was significantly increased. Xiaoyaosan and fluoxetine could significantly reverse these changes and had obvious curative effects.

CONCLUSION

The abnormal tryptophan metabolism existed in the hippocampi of chronic stress-depressed rats, which was closely related to the pathogenesis of depression. Xiaoyaosan could improve the tryptophan metabolism by regulating the expression levels of TPH2 and IDO1, thus exerting an antidepressant-like effect.

Serotonin depletion causes valproate-responsive manic-like condition and increased hippocampal neuroplasticity that are reversed by stress

Abnormal hippocampal neural plasticity has been implicated in behavioural abnormalities and complex neuropsychiatric conditions, including bipolar disorder (BD). However, the determinants of this neural alteration remain unknown. This work tests the hypothesis that the neurotransmitter serotonin (5-HT) is a key determinant of hippocampal neuroplasticity, and its absence leads to maladaptive behaviour relevant for BD. Depletion of brain 5-HT in Tph2 mutant mice resulted in reduced behavioural despair, reduced anxiety, marked aggression and lower habituation in novel environments, reminiscent of bipolar-associated manic behaviour. Treatment with valproate produced a substantial improvement of the mania-like behavioural phenotypes displayed by Tph2 mutants. Brain-wide fMRI mapping in mutants revealed functional hippocampal hyperactivity in which we also observed dramatically increased neuroplasticity. Importantly, remarkable correspondence between the transcriptomic profile of the Tph2 mutant hippocampus and neurons from bipolar disorder patients was observed. Chronic stress reversed the emotional phenotype and the hippocampal transcriptional landscape of Tph2 mutants. These changes were associated with inappropriate activation of transcriptional adaptive response to stress as assessed by gene set enrichment analyses in the hippocampus of Tph2 mutant mice. These findings delineate 5-HT as a critical determinant in BD associated maladaptive emotional responses and aberrant hippocampal neuroplasticity, and support the use of Tph2−/− mice as a new research tool for mechanistic and therapeutic research in bipolar disorder.

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 Novel Interaction between Tryptophan Hydroxylase 2 (TPH2) Gene Polymorphism (rs4570625) and BDNF Val66Met Predicts a High-Risk Emotional Phenotype in Healthy Subjects

Poor inhibitory processing of negative emotional content is central to many psychiatric disorders, including depression and anxiety. Moreover, increasing evidence suggests that core aspects of emotion-inhibitory processing are largely inherited and as such may represent a key intermediate or risk-related phenotype for common affective diseases (e.g., unipolar depressive, anxiety disorders). The current study employed a candidate-gene approach in order to most effectively examine this complex behavioral phenotype. We examined the novel interaction between BDNF (Val⁶⁶Met) and TPH2 (rs4570625) polymorphisms and their influence on behavioral inhibition of negative emotion in two independent investigations of healthy adults. BDNF Met carriers consistently report greater symptoms of affective disease and display corresponding behavioral rigidity, while TPH2 T carriers display poor inhibitory processing. These genotypes are traditionally perceived as ‘risk’ genotypes when compared to their respective major Val and G homozygous genotypes, but evidence is mixed. Recent studies in humans and mutant mouse models suggest biological epistasis between BDNF and genes involved in serotonin regulation. Moreover, polymorphisms in the TPH2 gene may have greater influence on serotonergic function than other more commonly studied polymorphisms (e.g., 5-HTTLPR). We observed consistent evidence across two different emotion-inhibition paradigms, one with high internal validity (Study 1, n = 119) and one with high ecological validity (Study 2, n = 115) that the combination of Val/Val and G/G genotypes was clearly associated with impaired inhibition of negative emotional content. This was followed by individuals carrying the BDNF—Met allele (including Met/Val and Met/Met) when combined with the TPH2—T allele (including T/G and T/T combinations). The consistency of these results across tasks and studies suggests that these two groups may be particularly vulnerable to the most common psychiatric disorders and should be targets for future clinical investigation.

TPH2 gene polymorphisms and bipolar disorder: A meta-analysis

BACKGROUND

Disturbance of the serotonergic system contributes to the etiology of bipolar disorder (BD). Tryptophan hydroxylase-2 (TPH2) is an important rate-limiting enzyme in the synthetic pathway for brain serotonin and has been suggested to play a role in BD.

MATERIALS AND METHODS

We performed a systematic review and meta-analysis of all studies to date investigating the association studies between TPH2 and BD published before Aug 2014. All studies were abstracted from PubMed, Embase, HuGNet, and China National Knowledge Infrastructure (CNKI). Manuscripts and the supplementary documents of published genome-wide association studies in the field were also included. Effect sizes of independent loci that have been studied in more than three articles were synthesized using fixed and random effects models.

RESULTS

Eight eligible studies addressed association between 63 TPH2 gene single nucleotide polymorphisms (SNPs) with BD, after linkage disequilibrium analysis, 12 independent SNPs were identified. Finally, three SNPs (rs4760820, rs11178998, and rs7954758) were found associated with BD using fixed effects models, and rs4760820 and rs11178998 were still associated with BD even with the more conservative random effects models.

CONCLUSIONS

rs4760820 and rs11178998 were identified to have strong genetic association with BD in present study though confirmation will require larger sample sizes and in additional populations.

Lack of association between TPH2 gene polymorphisms with major depressive disorder in multiethnic Malaysian population

INTRODUCTION

Numerous association studies of candidate genes studies with major depressive disorder (MDD) have been conducted for many years; however, the evidence of association between genes and the risk of developing MDD still remains inconclusive. In this study, we aimed to investigate the association between the tryptophan hydroxylase 2 (TPH2) gene and MDD in three ethnic groups (Malay, Chinese and Indian) within the Malaysian population.

METHODS

Two hundred and sixty five MDD patients who fulfilled the Diagnostic and Statistical Manual of Mental Disorders-IV criteria for MDD and 332 healthy controls were recruited for the study. All cases and controls were then genotyped for TPH2 polymorphisms rs1386494, rs1386495 and rs7305115.

RESULTS

Single locus analysis in pooled and ethnically stratified subjects revealed no association between each of the three variants of the TPH2 gene with susceptibility to MDD. Strong linkage disequilibrium was detected between rs1386495 and rs1386494 in pooled subjects; however, no significant association was found in the haplotype analysis.

DISCUSSIONS

In this study, we suggest that in both the Chinese and Indian populations, gender distribution differ significantly between cases and controls, showing that women are more at risk of developing MDD compared with men. Therefore, we suggest that the occurrence of MDD in both Chinese and Indians in the Malaysian population may be influenced by gender.

Pilot study of Biomarkers for predicting effectiveness of ramosetron in diarrhea-predominant irritable bowel syndrome: expression of S100A10 and polymorphisms of TPH1

BACKGROUND

Serotonin type 3 receptor (5-HT3 R) antagonists are potentially useful therapeutic agents for diarrhea-predominant irritable bowel syndrome (IBS-D). To identify biomarkers predicting effectiveness of the 5-HT3 R antagonist (ramosetron) in IBS-D.

METHODS

Irritable bowel syndrome-D Japanese subjects received 2.5 or 5 μg of ramosetron once daily for 4 weeks. Colonic mucosal S100A and tryptophan hydroxylase (TPH) mRNA expression levels were measured before treatment. Genomic DNA was extracted from blood and polymorphisms of TPH1 and TPH2 were analyzed.

KEY RESULTS

Forty-two patients (27 men and 15 women, mean age 42 years) with IBS-D were included for analysis. Improvement of IBS symptoms was seen in 26 (61.9%). Baseline S100A10 (p = 0.02) and TPH1 (p = 0.02) expression were significantly higher in the ramosetron responders than in the non-responders. The frequencies of the TPH1 rs4537731G allele in linkage disequilibrium with the TPH1 rs7130929 T allele (11.5% vs 50%, p = 0.003; OR: 12; 95% CI: 2.1-69) along with TPH1 rs211105 C allele (3.8% vs 43.8%, p = 0.0003; OR: 19; 95% CI: 2.1-181) were significantly lower in the responders than in the non-responders. The mean scores of diarrhea at baseline were significantly higher (5.2 vs 3.7, p = 0.005) in patients with TPH1 rs211105 T/T than those with the G allele.

CONCLUSIONS & INFERENCES

TPH1 gene polymorphisms and S100A10 expression, which correlate with 5-HT signaling were associated with ramosetron effectiveness in IBS-D, and may possibly lead to prospective identification of the resistance to treatment.

Association analysis of genes in serotonin pathway with attention and executive function in patients with bipolar affective disorder

BACKGROUND

The reason why it is difficult to identify susceptibility genes attributed to bipolar disorder (BPD) is the phenotypic heterogeneity. The use of endophenotypes has been advocated as one possible strategy to discovery cause variants of BPD.

METHODS

A total of 164 patients with BPD and 164 matched controls were employed in the present research. Fifty-two single nucleotide polymorphisms (SNPs) within the genes in serotonin pathway were selected for genotyping using the GoldenGate genotyping assay. All participants completed three neurocognitive tests including the tower of Hanoi (TOH), the Wisconsin card sorting test (WCST) and Trail making tests (TMTA and TMTB-M).

RESULTS

Patients with BPD demonstrated a wide range of deficits in mental activities of attention and speed of information processing, and executive function. Significant interactions between rs2760347 in 5HTR2A gene and diagnosis were found for the executive time of TOH, with β=11.82 and P=0.002 (adjusted P=0.03 after Bonferroni correction).

CONCLUSIONS

Cognitive impairments existing in BPD may be particularly notable in certain domains of attention and executive function, and 5HTR2A gene may be involved in modulating executive function of BP-I patients.

Genes in the serotonin pathway are associated with bipolar affective disorder in a Han Chinese population

Serotonin plays an important role in mood regulation, but the involvement of serotonin pathway genes in the development of bipolar I disorder (BP-I), a mood disorder, is not clear. We selected 21 single-nucleotide polymorphisms (SNPs) within the HTR2A gene, 8 within the SLC6A4 gene and 23 within the TPH2 gene for genotyping using the GoldenGate genotyping assay. A total of 375 patients with BP-I and 475 normal controls were recruited. Two out of 21 SNPs (rs1475196 and rs9567747) in the HTR2A gene and 1/23 SNPs (rs17110566) in the TPH2 gene were significantly associated with BP-I, both genotype-wise and allele-wise. Furthermore, a specific haplotype in the HTR2A gene showed a significant association with BP-I. Our results indicate that the HTR2A and TPH2 genes in the serotonin pathway play important roles in susceptibility to BP-I.

Use of the cross-translational model to study self-injurious behavior in human and nonhuman primates

Nonsuicidal self-injurious behavior occurs in the general human population, particularly among teenagers and young adults. Some rhesus macaques also develop self-injurious behavior (SIB) as adolescents or young adults. In both of these cases, the development of harmful behaviors is idiopathic, only coming to the attention of physicians or veterinarians after the disorder is established. Thus, a combination of retrospective, statistical, and empirical procedures are used to understand this disorder. Here, we identify concordances between macaques and humans across five different levels of analysis-(1) form and prevalence, (2) etiology, (3) triggering events, (4) function/maintenance, and (5) therapeutic intervention-and show the value of the cross-translational model (macaques to humans and humans to macaques) in understanding this phenomenon. Substantial concordance is present with respect to the range of severity, the presence of early life stress exposure, and emotional dysregulation. In the macaque model, additional information is available on the hypothalamic-pituitary-adrenal axis stress response system, possible genetic involvement, and the immediate contextual situations that appear to trigger or exacerbate SIB episodes. In contrast, considerably more information is available from human studies on the effectiveness of various treatment regimens. Veterinarians have drawn on this information to explore these therapeutic interventions in monkeys. We expect that models of SIB will continue to have cross-translational impact as scientists and practitioners move from preclinical to clinical research and treatment.

Genetic and neurocognitive foundations of emotion abnormalities in bipolar disorder

INTRODUCTION

Bipolar Disorder (BD) is a serious mood disorder, the aetiology of which is still unclear. The disorder is characterised by extreme mood variability in which patients fluctuate between markedly euphoric, irritable, and elevated states to periods of severe depression. The current research literature shows that BD patients demonstrate compromised neurocognitive ability in addition to these mood symptoms. Viable candidate genes implicated in neurocognitive and socioemotional processes may explain the development of these core emotion abnormalities. Additionally, links between faulty neurocognition and impaired socioemotional ability complement genetic explanations of BD pathogenesis. This review examines associations between cognition indexing prefrontal neural regions and socioemotional impairments including emotion processing and regulation. A review of the effect of COMT and TPH2 on these functions is also explored.

METHODS

Major computer databases including PsycINFO, Google Scholar, and Medline were consulted in order to conduct a comprehensive review of the genetic and cognitive literature in BD.

RESULTS

This review determines that COMT and TPH2 genetic variants contribute susceptibility to abnormal prefrontal neurocognitive function which oversees the processing and regulation of emotion. This provides for greater understanding of some of the emotional and cognitive symptoms in BD.

CONCLUSIONS

Current findings in this direction show promise, although the literature is still in its infancy and further empirical research is required to investigate these links explicitly.

TPH gene polymorphisms are associated with disease perception and quality of life in women with irritable bowel syndrome

The aims of this exploratory study were to examine whether tryptophan hydroxylase (TPH) gene polymorphisms are associated with psychosocial factors in women with irritable bowel syndrome (IBS). TPH is the rate-limiting enzyme in the biosynthesis of serotonin and has two isoforms, TPH1 and TPH2. Four single nucleotide polymorphisms (SNPs) in the TPH1 gene and one SNP in the TPH2 gene were selected based on previous studies investigating associations between these SNPs and psychiatric or behavioral disorders. One hundred ninety-nine Caucasian women with IBS were included. Results of univariate analysis showed no association between TPH1and TPH2 gene SNPs and current level of psychological distress or psychiatric illness. However, TPH1 gene SNPs were associated with IBS-related cognitions (rs4537731 and rs21105) and quality of life (rs684302 and rs1800532), in particular the mental health and energy subscales. These associations were independent of the subjects' levels of gastrointestinal symptoms. These results suggest that patients' perception of their illness, and of the impact it has on their lives, may be subject to genetic influences, in this case sequence variants in TPH1. However, caution should be used in interpreting these results given the large number of hypothesis tests performed in this exploratory hypothesis-generating study, and the results should be considered tentative until confirmed in an independent sample.

Association of TPH1, TPH2, and 5HTTLPR with PTSD and depressive symptoms

OBJECTIVE

To examine the potential contribution of the serotonin hydroxylase (TPH1 and TPH2) genes, and the serotonin transporter promoter polymorphism (5HTTLPR) to the unique and pleiotropic risk of PTSD symptoms and depressive symptoms.

METHODS

Participants included 200 adults exposed to the 1988 Spitak earthquake from 12 multigenerational families (3 to 5 generations). Severity of trauma exposure, PTSD, and depressive symptoms were assessed using standard psychometric instruments. Pedigree-based variance component analysis was used to assess the association between select genes and the phenotypes.

RESULTS

After adjusting for age, sex, exposure and environmental variables, there was a significant association of PTSD symptoms with the 't' allele of TPH1 SNP rs2108977 (p<0.004), explaining 3% of the phenotypic variance. This allele also showed a non-significant trend for an association with depressive symptoms (p=0.08). Also, there was a significant association of PTSD symptoms and the 't' allele of TPH2 SNP rs11178997 (p=0.03), explaining 4% of the variance. Depressive symptoms were significantly associated with the 's' allele of 5HTTLPR (p=0.03), explaining 4% of the variance.

LIMITATIONS

Retrospective rating of exposure may have been subject to memory failure leading to misestimation of symptom severities. Second, findings may not be generalizable to other ethnic/racial populations.

CONCLUSION

To our knowledge, this is the first published report showing that variants in TPH1 and TPH2 genes constitute risk factors for PTSD symptoms. Additionally, the TPH1 gene may be associated pleiotropically with PTSD and depressive symptoms. The association of the 's' allele of 5HTTLPR polymorphism with depression adds to similar findings from case/case-control studies.

Association analysis of the tryptophan hydroxylase 2 gene polymorphisms in patients with methamphetamine dependence/psychosis

There is a growing evidence that serotoninergic systems modulate dopaminergic neurotransmission. We analyzed the association between the variations in the brain tryptophan hydroxylase 2 (TPH2) gene, a rate limiting enzyme for serotonin biosynthesis, and methamphetamine (METH) dependence/psychosis in a Japanese population. We found ten single nucleotide polymorphisms (SNPs) and two polynucleotide polymorphisms in TPH2 gene exons and exon-intron boundaries. A total of 162 patients and 243 controls were used for the association analysis between these polymorphisms and METH dependence/psychosis. No significant differences were observed in either genotypic or allelic frequencies between METH dependent/psychotic patients and controls. A global test of differentiation among samples based on haplotype frequencies showed no significant association. With respect to latency of psychosis, prognosis of psychosis, and spontaneous relapse, we found no significant association with these SNPs. These results suggest that the TPH2 gene variants may not be a factor in vulnerability to METH dependence/psychosis.

Tryptophan hydroxylase 2 gene is associated with major depressive disorder in a female Chinese population

BACKGROUND

Previous candidate gene studies of major depressive disorder (MDD) have provided inconclusive evidence of association for genes with strong biological rationale for MDD. In this study, we aimed to investigate the association of tryptophan hydroxylase 2 gene with MDD and its treatment response in the Chinese Han population.

METHODS

Three hundred and sixty eight depressed patients who met DSM-IV criteria for major depressive disorder were recruited for the study. 371 normal controls were recruited from local community. Patients and normal controls were genotyped for TPH2 (rs4290270 and rs7305115) variants by polymerase chain reaction. Male and female subjects were analyzed separately.

RESULTS

No differences were found in the frequencies of the single alleles and genotypes of the tested polymorphisms between MDD patients and normal group. The frequency of the A-A haplotype was significantly higher in female MDD compared to healthy female controls (P<0.05). No significant association with treatment response was discovered in haplotype and single-marker analysis.

LIMITATIONS

This study lacks a placebo control and we cannot definitively exclude the possibility that some patients in the responder group responded to the placebo effect alone.

CONCLUSION

The result suggests that TPH2 gene may have a gender dependent effect on susceptibility to MDD but not with its treatment response in Chinese Han population. Further studies are needed to replicate the association that we observed.

Influence of TPH2 variants on diagnosis and response to treatment in patients with major depression, bipolar disorder and schizophrenia

The present study is aimed at exploring whether some single nucleotide polymorphisms (SNPs) within the tryptophan hydroxylase 2 gene (TPH2) could be associated with major depression (MD), bipolar disorder (BD) and schizophrenia and whether they could predict clinical outcomes in Korean in-patients treated with antidepressants, mood stabilizers and antipsychotics, respectively. One hundred forty-five patients with MD, 132 patients with BD, 221 patients with schizophrenia and 170 psychiatrically healthy controls were genotyped for six TPH2 SNPs (rs4570625, rs10748185, rs11179027, rs1386498, rs4469933, and rs17110747). Baseline and final clinical measures, including the Montgomery-Åsberg Depression Rating Scale (MADRS), Young Mania Rating Scale and Positive and Negative Syndrome Scale, for patients with MD, BD and schizophrenia, respectively were recorded. None of the SNPs under investigation were associated with MD, BD and schizophrenia. However, in patients with MD, the rs4570625-rs10748185 G-A haplotype was significantly associated with higher endpoint MADRS severity, though not with response. Our results suggest that TPH2 variants neither have a major role in MD, BD and schizophrenia nor in response to treatments.

Development by environment interactions controlling tryptophan hydroxylase expression

Tryptophan hydroxylase is the rate-limiting enzyme in the biosynthesis of serotonin (5-hydroxytryptamine; 5-HT). Two isoforms of tryptophan hydroxylase, derived from different genes, tph1 and tph2, have been identified. The tph1 isoform is expressed in peripheral tissues, whereas tph2 is brain and neuron-specific. Recent studies suggest that tph2 expression and brain serotonin turnover are upregulated in depressed suicide patients, and drug-free depressed patients, respectively. Increased tph2 expression could result from genetic influences, early life developmental influences, adverse experience during adulthood, or interactions among these factors. Studies in rodents support the hypothesis that interactions between early life developmental influences and adverse experience during adulthood play an important role in determining tph2 expression. In this review, we highlight the evidence for the effects of adverse early life experience and stressful experience during adulthood on both tph1 and tph2 expression.

TPH2 G/T polymorphism is associated with hyperphagia, IQ, and internalizing problems in Prader-Willi syndrome

BACKGROUND

Prader-Willi syndrome (PWS) is a genetic, neurodevelopmental disorder characterized by intellectual disabilities, growth hormone dysregulation, hyperphagia, increased risks of morbid obesity, compulsive behaviors, and irritability. As aberrant serotonergic functioning is strongly implicated in PWS, we examined associations between the PWS phenotype and polymorphisms in tryptophan hydroxylase 2 (TPH2), the rate-limiting enzyme in the biosynthesis of serotonin in the brain.

METHODS

Ninety-two individuals with PWS aged 4 to 50 years (M = 21.97) were genotyped for the TPH2 G703-T polymorphism. IQ testing was conducted in offspring, and parents completed questionnaires that tapped their child's compulsivity, hyperphagia, and other behavior problems.

RESULTS

As expected, the frequency of G/T or T/T polymorphisms in participants with PWS (39%) was similar to rates found in the general population (38%). Compared to those with a homozygous (G/G) genotype, individuals with a T allele had significantly higher hyperphagic behavior, drive, and severity scores, and they also had a younger age of onset of hyperphagia. Those with a T allele also had higher IQ scores than their counterparts. Females with a T allele had significantly higher internalizing symptoms, primarily anxiety and depression, than all others.

CONCLUSIONS

TPH2 G/T polymorphisms, and presumed loss of enzyme function, were associated with specific aspects of the PWS phenotype. Aberrant serotonergic functioning is strongly implicated in hyperphagia in PWS, and females with TPH2 T alleles may be at higher risk for affective or mood disorders. Findings hold promise for examining other serotonin-altering genes in PWS, and for future serotonin-altering treatment trials.

Tryptophan hydroxylase-2 (TPH2) in disorders of cognitive control and emotion regulation: a perspective

Based on genetic variation, there is accumulating evidence that altered function of tryptophan hydroxylase-2 (TPH2), the enzyme critical for synthesis of serotonin (5-HT) in the brain, plays a role in anxiety-, aggression- and depression-related personality traits and in the pathogenesis of disorders featuring deficits in cognitive control and emotion regulation. Here, we appraise the genetic and neurobiological evidence to illustrate the critical role of TPH2 in central 5-HT system function and in the pathophysiology of a wide spectrum of disorders of cognitive control and emotion regulation, ranging from depression to attention-deficit/hyperactivity disorder (ADHD), a phenotype commonly associated with difficulties in the control of emotion and with a high co-morbidity of depression. Findings from psychophysiological and functional imaging studies are indicative of various TPH2 polymorphisms directly influencing serotonergic function and thus impacting on mood disorders and on the response to antidepressant treatment. Especially a combination with uncontrollable stress seems to potentiate these effects linking gene-environment interaction directly with behavioral dysfunction in human and animal models. TPH2-deficient mice display alterations in anxiety-like behavior which is accompanied by adaptational changes of 5-HT(1A) receptors and its associated signaling pathway. Mouse models in conjunction with cognitive neuroscience approaches in humans are providing unexpected results and it may well be that future research on TPH2 will provide an entirely new view of 5-HT in brain development and function related to neuropsychiatric disorders.

Associations between polymorphic variants of the tryptophan hydroxylase 2 gene and obsessive-compulsive disorder

OBJECTIVE: A substantial body of evidence suggests that obsessive-compulsive disorder has a genetic component, and substantial candidate genes for the disorder have been investigated through association analyses. A particular emphasis has been placed on genes related to the serotonergic system, which is likely to play an important role in the pathogenesis of obsessive-compulsive disorder. The gene for tryptophan hydroxylase 2, which is a rate limiting enzyme in serotonin synthesis is considered an important candidate gene associated with psychiatric disorders. METHOD: Our sample consisted of 321 subjects (107 diagnosed with obsessive-compulsive disorder and 214 healthy controls), which were genotyped for eight tagSNPs (rs4448731, rs4565946, rs11179000, rs7955501, rs10506645, rs4760820, rs1487275 and rs10879357) covering the entire human tryptophan hydroxylase 2 gene. Statistical analyses were performed using UNPHASED, version 3.0.12, and Haploview ((R)). RESULTS: Single markers, genotype analysis did not show a significant genetic association with obsessive-compulsive disorder. A significant association between the T-C-T (rs4448731, rs4565946, rs10506645) and C-A-T (rs4565946, rs7955501, rs10506645) haplotypes and obsessive-compulsive disorder was observed, as well as a strong linkage disequilibrium between SNPs rs4448731 and rs4565946, and SNPs rs10506645 and 4760820. DISCUSSION: Our research has not demonstrated the existence of associations between the eight SNPs of TPH2 and obsessive-compulsive disorder. However, two LD and two haplotypes areas were demonstrated, thus suggesting that more studies in TPH2 are needed to investigate the role of tryptophan hydroxylase 2 variants in obsessive-compulsive disorder.

Antidepressant-like responses to lithium in genetically diverse mouse strains

A mood stabilizing and antidepressant response to lithium is only found in a subgroup of patients with bipolar disorder and depression. Identifying strains of mice that manifest differential behavioral responses to lithium may assist in the identification of genomic and other biologic factors that play a role in lithium responsiveness. Mouse strains were tested in the forced swim test (FST), tail suspension test (TST) and open-field test after acute and chronic systemic and intracerebroventricular (ICV) lithium treatments. Serum and brain lithium levels were measured. Three (129S6/SvEvTac, C3H/HeNHsd and C57BL/6J) of the eight inbred strains tested, and one (CD-1) of the three outbred strains, showed an antidepressant-like response in the FST following acute systemic administration of lithium. The three responsive inbred strains, as well as the DBA/2J strain, displayed antidepressant-like responses to lithium in the FST after chronic administration of lithium. However, in the TST, acute lithium resulted in an antidepressant-like effect only in C3H/HeNHsd mice. Only C57BL/6J and DBA/2J showed an antidepressant-like response to lithium in the TST after chronic administration. ICV lithium administration resulted in a similar response profile in BALB/cJ (non-responsive) and C57BL/6J (responsive) strains. Serum and brain lithium concentrations showed that behavioral results were not because of differential pharmacokinetics of lithium in individual strains, suggesting that genetic factors likely regulate these behavioral responses to lithium. Our results indicate that antidepressant-like responses to lithium in tests of antidepressant efficacy varies among genetically diverse mouse strains. These results will assist in identifying genomic factors associated with lithium responsiveness and the mechanisms of lithium action.

Distribution of the C1473G polymorphism in tryptophan hydroxylase 2 gene in laboratory and wild mice

The neurotransmitter serotonin is implicated in the regulation of various forms of behavior, including aggression, sexual behavior and stress response. The rate of brain serotonin synthesis is determined by the activity of neuronal-specific enzyme tryptophan hydroxylase 2. The missense C1473G substitution in mouse tryptophan hydroxylase 2 gene has been shown to lower the enzyme activity and brain serotonin level. Here, the C1473G polymorphism was investigated in 84 common laboratory inbred strains, 39 inbred and semi-inbred strains derived from wild ancestors (mostly from Eurasia) and in 75 wild mice trapped in different locations in Russia and Armenia. Among all the classical inbred strains studied, only substrains of BALB/c, A and DBA, as well as the IITES/Nga and NZW/NSlc strains were homozygous for the 1473G allele. In contrast to laboratory strains, the 1473G allele was not present in any of the samples from wild and wild-derived mice, although the wild mice varied substantially in the C1477T neutral substitution closely linked to the C1473G polymorphism. According to these results, the frequency of the 1473G allele in natural populations does not exceed 0.5%, and the C1473G polymorphism is in fact a rare mutation that is possibly eliminated by the forces of natural selection.

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.

Effect of tryptophan hydroxylase-2 gene variants on amygdalar and hippocampal volumes

Tryptophan hydroxylase (TPH) is the rate-limiting enzyme in the synthesis of serotonin (5-HT). Genetic variations in human TPH2, a newly identified isoform of TPH, have been shown to impact on enzymatic activity of TPH and to be associated with emotion-related personality traits and mood/anxiety disorders. Identification of an intermediate phenotype that bridges the relationship between genes and behavior may be of great importance in the further clarification of how hTPH2 contributes to emotional regulation. Previous studies have shown that a polymorphism in the upstream regulatory region of hTPH2 (SNP G-703T, rs4570625) correlates functional MRI response of the amygdala. In this study, we examined the effect of this genotype on amygdalar and hippocampal volumes in 208 mentally healthy individuals. To measure volumes of amygdala and hippocampus, gray matter regions of interest were outlined manually on three-dimensional MRI data obtained using a 1.5-T scanner. Additionally, personality traits were evaluated using the Temperament and Character Inventory (TCI). Those subjects with T allele carriers were associated with significantly smaller volumes in bilateral amygdala and hippocampus and higher reward dependence than those with G allele homozygotes. These results suggest that amygdalar and hippocampal volumes assessed using MRI may be a useful intermediate phenotype that will uncover the biological pathway linking 5-HT synthesis and emotional behaviors and affective disorders.

Alternative splicing and extensive RNA editing of human TPH2 transcripts

Brain serotonin (5-HT) neurotransmission plays a key role in the regulation of mood and has been implicated in a variety of neuropsychiatric conditions. Tryptophan hydroxylase (TPH) is the rate-limiting enzyme in the biosynthesis of 5-HT. Recently, we discovered a second TPH isoform (TPH2) in vertebrates, including man, which is predominantly expressed in brain, while the previously known TPH isoform (TPH1) is primarly a non-neuronal enzyme. Overwhelming evidence now points to TPH2 as a candidate gene for 5-HT-related psychiatric disorders. To assess the role of TPH2 gene variability in the etiology of psychiatric diseases we performed cDNA sequence analysis of TPH2 transcripts from human post mortem amygdala samples obtained from individuals with psychiatric disorders (drug abuse, schizophrenia, suicide) and controls. Here we show that TPH2 exists in two alternatively spliced variants in the coding region, denoted TPH2a and TPH2b. Moreover, we found evidence that the pre-mRNAs of both splice variants are dynamically RNA-edited in a mutually exclusive manner. Kinetic studies with cell lines expressing recombinant TPH2 variants revealed a higher activity of the novel TPH2B protein compared with the previously known TPH2A, whereas RNA editing was shown to inhibit the enzymatic activity of both TPH2 splice variants. Therefore, our results strongly suggest a complex fine-tuning of central nervous system 5-HT biosynthesis by TPH2 alternative splicing and RNA editing. Finally, we present molecular and large-scale linkage data evidencing that deregulated alternative splicing and RNA editing is involved in the etiology of psychiatric diseases, such as suicidal behaviour.

Epistasis between IL1A, IL1B, TNF, HTR2A, 5-HTTLPR and TPH2 variations does not impact alcohol dependence disorder features

We assessed a set of biological (HDL, LDL, SGOT, SGPT, GGT, HTc, Hb and T levels) and psychometric variables (investigated through HAM-D, HAM-A, GAS, Liebowitz Social Anxiety Scale, Mark & Mathews Scale, Leyton scale, and Pilowski scale) in a sample of 64 alcohol dependent patients, at baseline and after a detoxification treatment. Moreover, we recruited 47 non-consanguineous relatives who did not suffer alcohol related disorders and underwent the same tests. In both groups we genotyped 11 genetic variations (rs1800587; rs3087258; rs1799724; 5-HTTLPR; rs1386493; rs1386494; rs1487275; rs1843809; rs4570625; rs2129575; rs6313) located in genes whose impact on alcohol related behaviors and disorders has been hypothesized (IL1A, IL1B, TNF, 5-HTTLPR, TPH2 and HTR2A). We analyzed the epistasis of these genetic variations upon the biological and psychological dimensions in the cases and their relatives. Further on, we analyzed the effects of the combined genetic variations on the short - term detoxification treatment efficacy. Finally, being the only not yet investigated variation within this sample, we analyzed the impact of the rs6313 alone on baseline assessment and treatment efficacy. We detected the following results: the couple rs6313 + rs2129575 affected the Leyton -Trait at admission (p = 0.01) (obsessive-compulsive trait), whilst rs1800587 + 5-HTTLPR impacted the Pilowski test at admission (p = 0.01) (hypochondriac symptoms). These results did not survive Bonferroni correction (p < or = 0.004). This lack of association may depend on the incomplete gene coverage or on the small sample size which limited the power of the study. On the other hand, it may reflect a substantial absence of relevance of the genotype variants toward the alcohol related investigated dimensions. Nonetheless, the marginal significance we detected could witness an informative correlation worth investigating in larger samples.

Support for tryptophan hydroxylase-2 as a susceptibility gene for bipolar affective disorder

Bipolar affective disorder (BPAD) is a highly inherited genetic disorder and may be caused in part by deficits in serotonergic neurotransmission. We investigated whether variants within the tryptophan hydroxylase-2 (TPH2) gene, which is required for the synthesis of serotonin (5-HT), are associated with susceptibility to developing BPAD. Thirteen single nucleotide polymorphisms (SNPs) within TPH2 were genotyped in a collection of 151 Irish BPAD type I trios and were tested for association using the transmission disequilibrium test. SNPs rs1386482 and rs1386486, which are in very strong linkage disequilibrium, were associated with BPAD (P=0.006). The association retained significance after a correction for multiple testing. The associated SNPs are in perfect linkage disequilibrium with SNPs previously associated with BPAD (rs4290270) and impulsivity (rs1386483), a core trait of BPAD. These results strongly support a role for TPH2 in the aetiology of BPAD.

Growth retardation and altered autonomic control in mice lacking brain serotonin

Serotonin synthesis in mammals is initiated by 2 distinct tryptophan hydroxylases (TPH), TPH1 and TPH2. By genetically ablating TPH2, we created mice (Tph2(-/-)) that lack serotonin in the central nervous system. Surprisingly, these mice can be born and survive until adulthood. However, depletion of serotonin signaling in the brain leads to growth retardation and 50% lethality in the first 4 weeks of postnatal life. Telemetric monitoring revealed more extended daytime sleep, suppressed respiration, altered body temperature control, and decreased blood pressure (BP) and heart rate (HR) during nighttime in Tph2(-/-) mice. Moreover, Tph2(-/-) females, despite being fertile and producing milk, exhibit impaired maternal care leading to poor survival of their pups. These data confirm that the majority of central serotonin is generated by TPH2. TPH2-derived serotonin is involved in the regulation of behavior and autonomic pathways but is not essential for adult life.

Antagonism of serotonergic 5-HT2A/2C receptors: mutual improvement of sleep, cognition and mood?

Serotonin [5-hydroxytryptamine (5-HT)] and 5-HT receptors are involved in sleep and in waking functions such as cognition and mood. Animal and human studies support a particular role for the 5-HT(2A) receptor in sleep, which has led to renewed interest in this receptor subtype as a target for the development of novel pharmacological agents to treat insomnia. Focusing primarily on findings in healthy human volunteers, a review of the available data suggests that antagonistic interaction with 5-HT(2A) receptors (and possibly also 5-HT(2C) receptors) prolongs the duration of slow wave sleep and enhances low-frequency (< 7 Hz) activity in the sleep electroencephalogram (EEG), a widely accepted marker of sleep intensity. Despite certain differences, the changes in sleep and the sleep EEG appear to be remarkably similar to those of physiologically more intense sleep after sleep deprivation. It is currently unclear whether these changes in sleep are associated with improved vigilance, cognition and mood during wakefulness. While drug-induced interaction with sleep must be interpreted cautiously, too few studies are available to provide a clear answer to this question. Moreover, functional relationships between sleep and waking functions may differ between healthy controls and patients with sleep disorders. A multimodal approach investigating subjective and objective aspects of sleep and wakefulness provides a promising research avenue for shedding light on the complex relationships among 5-HT(2A/2C) receptor-mediated effects on sleep, the sleep EEG, cognition and mood in health and various diseases associated with disturbed sleep and waking functions.

Spatio-temporal expression of tryptophan hydroxylase isoforms in murine and human brain: convergent data from Tph2 knockout mice

Dysregulation of tryptophan hydroxylase (TPH)-dependent serotonin (5-HT) synthesis, has been implicated in various neuropsychiatric disorders, although the differential expression pattern of the two isoforms is controversial. Here, we report a comprehensive spatio-temporal isoform-specific analysis of TPH1 and TPH2 expression during pre- and postnatal development of mouse brain and in adult human brain. TPH2 expression was consistently detected in the raphe nuclei, as well as in fibers in the deep pineal gland and in small intestine. Although TPH1 expression was found in these peripheral tissues, no significant TPH1 expression was detected in the brain, neither during murine development, nor in mouse and human adult brain. In support of TPH2 specificity in brain 5-HT synthesis, raphe neurons of Tph2 knockout mice were completely devoid of 5-HT, with no compensatory activation of Tph1 expression. In conclusion, our findings indicate that brain 5-HT synthesis across the lifespan is exclusively maintained by TPH2.

5'-Untranslated region of the tryptophan hydroxylase-2 gene harbors an asymmetric bidirectional promoter but not internal ribosome entry site in vitro

Tryptophan hydroxylase-2 (TPH2) catalyzes the synthesis of neuronal serotonin, a major neurotransmitter involved in many brain functions and psychiatric disorders. We have previously revealed a critical role of the human TPH2 (hTPH2) 5'-UTR in gene expression regulation. This study aimed to further characterize mechanism(s) by which the hTPH2 5'-UTR regulates gene expression. An internal ribosome entry site (IRES) activity in hTPH2 5'-UTR was suggested by the conventional bicistronic reporter assay; however, further stringent experiments, including in vitro translation, quantitative real-time PCR, Northern blot, ribonuclease protection assay, and monocistronic reporter assay, demonstrated that the hTPH2 5'-UTR harbors a bidirectional promoter, but not IRES, within its downstream segment (61-141). The antisense promoter is much stronger than the sense promoter, but the strength of both promoters are cell-line dependent, with the highest and lowest activities being observed in HEK-293T and SK-N-MC cells, respectively. In accordance with our previous findings, the upstream segment (1-60) of hTPH2 5'-UTR suppresses the neighboring promoter of both direction, independent of the cell line and its location in the 5'- or 3'-flanking regions of the gene. In summary, this study demonstrates that no IRES but an asymmetric bidirectional promoter is present in the downstream segment of hTPH2 5'-UTR, and this promoter is susceptible to a gene silencing effect caused by the upstream segment (1-60) of hTPH2 5'-UTR. Our findings point to the potential involvement of antisense transcription and non-coding RNA in the regulation of TPH2 gene expression.

Mutations in human monoamine-related neurotransmitter pathway genes

Biosynthesis and metabolism of serotonin and catecholamines involve at least eight individual enzymes that are mainly expressed in tissues derived from the neuroectoderm, e.g., the central nervous system (CNS), pineal gland, adrenal medulla, enterochromaffin tissue, sympathetic nerves, and ganglia. Some of the enzymes appear to have additional biological functions and are also expressed in the heart and various other internal organs. The biosynthetic enzymes are tyrosine hydroxylase (TH), tryptophan hydroxylases type 1 and 2 (TPH1, TPH2), aromatic amino acid decarboxylase (AADC), dopamine beta-hydroxylase (DbetaH), and phenylethanolamine N-methyltransferase (PNMT), and the specific catabolic enzymes are monoamine oxidase A (MAO-A) and catechol O-methyltransferase (COMT). For the TH, DDC, DBH, and MAOA genes, many single nucleotide polymorphisms (SNPs) with unknown function, and small but increasing numbers of cases with autosomal recessive mutations have been recognized. For the remaining genes (TPH1, TPH2, PNMT, and COMT) several different genetic markers have been suggested to be associated with regulation of mood, pain perception, and aggression, as well as psychiatric disturbances such as schizophrenia, depression, suicidality, and attention deficit/hyperactivity disorder. The genetic markers may either have a functional role of their own, or be closely linked to other unknown functional variants. In the future, molecular testing may become important for the diagnosis of such conditions. Here we present an overview on mutations and polymorphisms in the group of genes encoding monoamine neurotransmitter metabolizing enzymes. At the same time we propose a unified nomenclature for the nucleic acid aberrations in these genes. New variations or details on mutations will be updated in the Pediatric Neurotransmitter Disorder Data Base (PNDDB) database (www.bioPKU.org).

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.

Deficiency of brain 5-HT synthesis but serotonergic neuron formation in Tph2 knockout mice

The relative contribution of the two tryptophan hydroxylase (TPH) isoforms, TPH1 and TPH2, to brain serotonergic system function is controversial. To investigate the respective role of TPH2 in neuron serotonin (5-HT) synthesis and the role of 5-HT in brain development, mice with a targeted disruption of Tph2 were generated. The preliminary results indicate that in Tph2 knockout mice raphe neurons are completely devoid of 5-HT, whereas no obvious alteration in morphology and fiber distribution are observed. The findings confirm the exclusive specificity of Tph2 in brain 5-HT synthesis and suggest that Tph2-synthesized 5-HT is not required for serotonergic neuron formation.

Genetic variation of serotonin function and cognitive control

Although it is widely accepted that serotonin plays a pivotal role in the modulation of anxiety- and depression-related personality traits as well as in the pathogenesis of anxiety disorders and depression, the role of serotonin in cognition is less clear. In the present study, we investigated the involvement of serotonin in cognitive behaviors by examining the impact of genetic variation in key regulators of serotonergic neurotransmission on behavioral measures in a cognitive control task. Eighty-five healthy participants performed a cued continuous performance task (the AX Continuous Performance Task [AXCPT]) and were genotyped for polymorphisms in the transcriptional control regions of the tryptophan hydroxylase 2 gene (TPH2 G-703T; rs4570625) and the serotonin transporter gene (5-HTTLPR). The core result was that individuals lacking the rare TPH2 T allele were not faster than T allele carriers, but committed fewer errors and were less variable in responding. These findings parallel those of a recent study where an enhancement of executive control in individuals without the rare TPH2 T/T genotype was observed. Together with recent evidence that individuals without the T allele exhibit higher scores in anxiety- and depression-related personality traits, our results underscore the role of the TPH2 G-703T polymorphism in the modulation of behavior and raise the intriguing possibility that genetic variants associated with higher negative emotionality may have beneficial effects on some cognitive functions.

A regulatory domain in the N terminus of tryptophan hydroxylase 2 controls enzyme expression

Serotonin is involved in a variety of physiological processes in the central nervous system and the periphery. As the rate-limiting enzyme in serotonin synthesis, tryptophan hydroxylase plays an important role in modulating these processes. Of the two variants of tryptophan hydroxylase, tryptophan hydroxylase 2 (TPH2) is expressed predominantly in the central nervous system, whereas tryptophan hydroxylase 1 (TPH1) is expressed mostly in peripheral tissues. Although the two enzymes share considerable sequence homology, the regulatory domain of TPH2 contains an additional 41 amino acids at the N terminus that TPH1 lacks. Here we show that the extended TPH2 N-terminal domain contains a unique sequence involved in the regulation of enzyme expression. When expressed in cultured mammalian cells, TPH2 is synthesized less efficiently and is also less stable than TPH1. Removal of the unique portion of the N terminus of TPH2 results in expression of the enzyme at a level similar to that of TPH1, whereas protein chimeras containing this fragment are expressed at lower levels than their wild-type counterparts. We identify a region centered on amino acids 10-20 that mediates the bulk of this effect. We also demonstrate that phosphorylation of serine 19, a protein kinase A consensus site located in this N-terminal domain, results in increased TPH2 stability and consequent increases in enzyme output in cell culture systems. Because this domain is unique to TPH2, these data provide evidence for selective regulation of brain serotonin synthesis.

Glucocorticoid modulation of tryptophan hydroxylase-2 protein in raphe nuclei and 5-hydroxytryptophan concentrations in frontal cortex of C57/Bl6 mice

Considerable attention has focused on regulation of central tryptophan hydroxylase (TPH) activity and protein expression. At the time of these earlier studies, it was thought that there was a single central TPH isoform. However, with the recent identification of TPH2, it becomes important to distinguish between regulatory effects on the protein expression and activity of the two isoforms. We have generated a TPH2-specific polyclonal antiserum (TPH2-6361) to study regulation of TPH2 at the protein level and to examine the distribution of TPH2 expression in rodent and human brain. TPH2 immunoreactivity (IR) was detected throughout the raphe nuclei, in lateral hypothalamic nuclei and in the pineal body of rodent and human brain. In addition, a prominent TPH2-IR fiber network was found in the human median eminence. We recently reported that glucocorticoid treatment of C57/Bl6 mice for 4 days markedly decreased TPH2 messenger RNA levels in the raphe nuclei, whereas TPH1 mRNA was unaffected. The glucocorticoid-elicited inhibition of TPH2 gene expression was blocked by co-administration of the glucocorticoid receptor antagonist mifepristone (RU-486). Using TPH2-6361, we have extended these findings to show a dose-dependent decrease in raphe TPH2 protein levels in response to 4 days of treatment with dexamethasone; this effect was blocked by co-administration of mifepristone. Moreover, the glucocorticoid-elicited inhibition of TPH2 was functionally significant: serotonin synthesis was significantly reduced in the frontal cortex of glucocorticoid-treated mice, an effect that was blocked by mifepristone co-administration. This study provides further evidence for the glucocorticoid regulation of serotonin biosynthesis via inhibition of TPH2 expression, and suggest that elevated glucocorticoid levels may be relevant to the etiology of psychiatric diseases, such as depression, where hypothalamic-pituitary-adrenal axis dysregulation has been documented.

TPH2 and TPH1: association of variants and interactions with heroin addiction

The tryptophan hydroxylase 2 gene (TPH2) was resequenced at the 5' upstream, coding, and 3' downstream regions, including all 11 exons in 185 subjects. Twenty-three novel and 14 known variants were identified. In a cohort of 583 consecutively ascertained subjects, including normal volunteers and those with specific addictive diseases, six common TPH2 and one TPH1 variant were genotyped. Allele frequencies of three TPH2 variants and the TPH1 variant varied significantly among the four ethnic groups within the control subjects. Of these subjects, 385 who met heroin addiction or control criteria and were of Caucasian, African-American, or Hispanic ethnicity were examined for potential association with vulnerability to develop heroin addiction. At the two locus genotype level in Hispanics, the TPH1 rs1799913 variant was found to significantly interact with the TPH2 rs7963720 variant and heroin addiction (P=0.022), and with the TPH2 rs4290270 variant and heroin addiction (P=0.011). In the African-American group, a significant association of a specific TPH2 haplotype with heroin addiction also was found (SNPHAP, P=0.004; PHASE P=0.036).

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.