Rhesus monkey tryptophan hydroxylase-2 coding region haplotypes affect mRNA stability

Methylation of the tryptophan hydroxylase‑2 gene is associated with mRNA expression in patients with major depression with suicide attempts

Tryptophan hydroxylase-2 (TPH2) contributes to alterations in the function of neuronal serotonin (5-HT), which are associated with various psychopathologies, including major depressive disorder (MDD) or suicidal behavior. The methylation of a single CpG site in the promoter region of TPH2 affects gene expression. Suicide and MDD are strongly associated and genetic factors are at least partially responsible for the variability in suicide risk. The aim of the present study was to investigate whether variations in TPH2 methylation in peripheral blood samples may predispose patients with MDD to suicide attempts. TPH2 mRNA expression levels differed significantly between 50 patients with MDD who had attempted suicide (MDD + suicide group) and 75 control patients with MDD (MDD group); TPH2 expression levels were significantly decreased (P=0.0005) in the patients who had attempted suicide. Furthermore, the frequency of TPH2 methylation was 36.0% in the MDD + suicide group, while it was 13.0% in the MDD group. The results of the present study demonstrated that methylation in the promoter region of TPH2 significantly affected the mRNA expression levels of TPH2, thus suggesting that methylation of the TPH2 promoter may silence TPH2 mRNA expression in MDD patients with or without suicidal behavior. In addition, there was a significant correlation between the methylation status of the TPH2 promoter and depression, hopelessness and cognitive impairment in the MDD + suicide group. In conclusion, the present study demonstrated that TPH2 expression was regulated by DNA methylation of the TPH2 promoter region in patients with MDD.

Functional studies of tyrosine hydroxylase missense variants reveal distinct patterns of molecular defects in Dopa-responsive dystonia

Congenital tyrosine hydroxylase deficiency (THD) is found in autosomal-recessive Dopa-responsive dystonia and related neurological syndromes. The clinical manifestations of THD are variable, ranging from early-onset lethal disease to mild Parkinson disease-like symptoms appearing in adolescence. Until 2014, approximately 70 THD patients with a total of 40 different disease-related missense mutations, five nonsense mutations, and three mutations in the promoter region of the tyrosine hydroxylase (TH) gene have been reported. We collected clinical and biochemical data in the literature for all variants, and also generated mutant forms of TH variants previously not studied (N = 23). We compared the in vitro solubility, thermal stability, and kinetic properties of the TH variants to determine the cause(s) of their impaired enzyme activity, and found great heterogeneity in all these properties among the mutated forms. Some TH variants had specific kinetic anomalies and phenylalanine hydroxylase, and Dopa oxidase activities were measured for variants that showed signs of altered substrate binding. p.Arg233His, p.Gly247Ser, and p.Phe375Leu had shifted substrate specificity from tyrosine to phenylalanine and Dopa, whereas p.Cys359Phe had an impaired activity toward these substrates. The new data about pathogenic mechanisms presented are expected to contribute to develop individualized therapy for THD patients.

Large-scale polymorphism discovery in macaque G-protein coupled receptors

BACKGROUND

G-protein coupled receptors (GPCRs) play an inordinately large role in human health. Variation in the genes that encode these receptors is associated with numerous disorders across the entire spectrum of disease. GPCRs also represent the single largest class of drug targets and associated pharmacogenetic effects are modulated, in part, by polymorphisms. Recently, non-human primate models have been developed focusing on naturally-occurring, functionally-parallel polymorphisms in candidate genes. This work aims to extend those studies broadly across the roughly 377 non-olfactory GPCRs. Initial efforts include resequencing 44 Indian-origin rhesus macaques (Macaca mulatta), 20 Chinese-origin rhesus macaques, and 32 cynomolgus macaques (M. fascicularis).

RESULTS

Using the Agilent target enrichment system, capture baits were designed for GPCRs off the human and rhesus exonic sequence. Using next generation sequencing technologies, nearly 25,000 SNPs were identified in coding sequences including over 14,000 non-synonymous and more than 9,500 synonymous protein-coding SNPs. As expected, regions showing the least evolutionary constraint show greater rates of polymorphism and greater numbers of higher frequency polymorphisms. While the vast majority of these SNPs are singletons, roughly 1,750 non-synonymous and 2,900 synonymous SNPs were found in multiple individuals.

CONCLUSIONS

In all three populations, polymorphism and divergence is highly concentrated in N-terminal and C-terminal domains and the third intracellular loop region of GPCRs, regions critical to ligand-binding and signaling. SNP frequencies in macaques follow a similar pattern of divergence from humans and new polymorphisms in primates have been identified that may parallel those seen in humans, helping to establish better non-human primate models of disease.

Tryptophan hydroxylase-2: an emerging therapeutic target for stress disorders

Serotonin (5-HT) has been long recognized to modulate the stress response, and dysfunction of 5-HT has been implicated in numerous stress disorders. Accordingly, the 5-HT system has been targeted for the treatment of stress disorders. Tryptophan hydroxylase (TPH) is the rate-limiting enzyme in 5-HT synthesis, and the recent identification of a second, neuron-specific TPH isoform (TPH2) opened up a new area of research. With a decade of extensive investigation, it is now recognized that: (1) TPH2 exhibits a highly flexible gene expression that is modulated by an increasing number of internal and external environmental factors including the biological clock, stressors, endogenous hormones, and antidepressant therapies; and (2) genetically determined TPH2 activity is linked to a growing body of stress-related neuronal correlates and behavioral traits. These findings reveal an active role of TPH2 in the stress response and provide new insights into the long recognized but not yet fully understood 5-HT-stress interaction. As a major modulator of 5-HT neurotransmission and the stress response, TPH2 is of both pathophysiological and pharmacological significance, and is emerging as a new therapeutic target for the treatment of stress disorders. Given that numerous antidepressant therapies influence TPH2 gene expression, TPH2 is already inadvertently targeted for the treatment of stress disorders. With increased understanding of the regulation of TPH2 activity we can now purposely utilize TPH2 as a target to develop new or optimize current therapies, which are expected to greatly improve the prevention and treatment of a wide variety of stress disorders.

Genetic load is associated with hypothalamic-pituitary-adrenal axis dysregulation in macaques

Dysregulation of the hypothalamic-pituitary-adrenal (HPA) axis pathway is associated with several neuropsychiatric disorders, including post-traumatic stress disorder (PTSD), major depressive disorder (MDD), schizophrenia and alcohol abuse. Studies have demonstrated an association between HPA axis dysfunction and gene variants within the cortisol, serotonin and opioid signaling pathways. We characterized polymorphisms in genes linked to these three neurotransmitter pathways and tested their potential interactions with HPA axis activity, as measured by dexamethasone (DEX) suppression response. We determined the percent DEX suppression of adrenocorticotropic hormone (ACTH) and cortisol in 62 unrelated, male rhesus macaques. While DEX suppression of cortisol was robust amongst 87% of the subjects, ACTH suppression levels were broadly distributed from -21% to 66%. Thirty-seven monkeys from the high and low ends of the ACTH suppression distribution (18 'high' and 19 'low' animals) were genotyped at selected polymorphisms in five unlinked genes (rhCRH, rhTPH2, rhMAOA, rhSLC6A4 and rhOPRM). Associations were identified between three variants (rhCRH-2610C>T, rhTPH2 2051A>C and rh5-HTTLPR) and level of DEX suppression of ACTH. In addition, a significant additive effect of the 'risk' genotypes from these three loci was detected, with an increasing number of 'risk' genotypes associated with a blunted ACTH response (P = 0.0009). These findings suggest that assessment of multiple risk alleles in serotonin and cortisol signaling pathway genes may better predict risk for HPA axis dysregulation and associated psychiatric disorders than the evaluation of single gene variants alone.

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

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

Advances in tryptophan hydroxylase-2 gene expression regulation: new insights into serotonin-stress interaction and clinical implications

Serotonin (5-HT) modulates the stress response by interacting with the hormonal hypothalamic-pituitary-adrenal (HPA) axis and neuronal sympathetic nervous system (SNS). Tryptophan hydroxylase (TPH) is the rate-limiting enzyme in 5-HT biosynthesis, and the recent identification of a second, neuron-specific TPH isoform (TPH2) opened up a new area of research. While TPH2 genetic variance has been linked to numerous behavioral traits and disorders, findings on TPH2 gene expression have not only reinforced, but also provided new insights into, the long-recognized but not yet fully understood 5-HT-stress interaction. In this review, we summarize advances in TPH2 expression regulation and its relevance to the stress response and clinical implications. Particularly, based on findings on rhesus monkey TPH2 genetics and other relevant literature, we propose that: (i) upon activation of adrenal cortisol secretion, the cortisol surge induces TPH2 expression and de novo 5-HT synthesis; (ii) the induced 5-HT in turn inhibits cortisol secretion by modulating the adrenal sensitivity to ACTH via the suprachiasmatic nuclei (SCN)-SNS-adrenal system, such that it contributes to the feedback inhibition of cortisol production; (iii) basal TPH2 expression or 5-HT synthesis, as well as early-life experience, influence basal cortisol primarily via the hormonal HPA axis; and (iv) 5'- and 3'-regulatory polymorphisms of TPH2 may differentially influence the stress response, presumably due to their differential roles in gene expression regulation. Our increasing knowledge of TPH2 expression regulation not only helps us better understand the 5-HT-stress interaction and the pathophysiology of neuropsychiatric disorders, but also provides new strategies for the treatment of stress-associated diseases.

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.

Genome-wide approaches to schizophrenia

Schizophrenia (SZ) is a common and severe psychiatric disorder with both environmental and genetic risk factors, and a high heritability. After over 20 years of molecular genetics research, new molecular strategies, primarily genome-wide association studies (GWAS), have generated major tangible progress. This new data provides evidence for: (1) a number of chromosomal regions with common polymorphisms showing genome-wide association with SZ (the major histocompatibility complex, MHC, region at 6p22-p21; 18q21.2; and 2q32.1). The associated alleles present small odds ratios (the odds of a risk variant being present in cases vs. controls) and suggest causative involvement of gene regulatory mechanisms in SZ. (2) Polygenic inheritance. (3) Involvement of rare (<1%) and large (>100kb) copy number variants (CNVs). (4) A genetic overlap of SZ with autism and with bipolar disorder (BP) challenging the classical clinical classifications. Most new SZ findings (chromosomal regions and genes) have generated new biological leads. These new findings, however, still need to be translated into a better understanding of the underlying biology and into causal mechanisms. Furthermore, a considerable amount of heritability still remains unexplained (missing heritability). Deep resequencing for rare variants and system biology approaches (e.g., integrating DNA sequence and functional data) are expected to further improve our understanding of the genetic architecture of SZ and its underlying biology.

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.

The effect of rearing experience and TPH2 genotype on HPA axis function and aggression in rhesus monkeys: a retrospective analysis

Gene-environment (GxE) interactions contribute to the development of many neuropsychiatric disorders. Tryptophan hydroxylase-2 (TPH2) synthesizes neuronal serotonin and is closely related to the hypothalamic-pituitary-adrenal (HPA) axis, while early life experience is a critical environmental factor programming the HPA axis response to stress. This retrospective study investigated GxE interaction at the TPH2 locus in rhesus monkeys. Twenty-eight adult, male rhesus monkeys of Indian origin, either mother-reared or peer-reared as infants, were involved in this study. These monkeys have been previously genotyped for the functional A2051C polymorphism in rhTPH2, and had been physiologically and behaviorally characterized. rhTPH2 A2051C exerted a significant main effect (CC>AA&AC) on the cerebrospinal fluid (CSF) level of 5-hydroxyindole-3-acetic acid (5-HIAA; F((1,14))=6.42, p=0.024), plasma cortisol level in the morning (F((1,18))=14.63, p=0.002) and cortisol response to ACTH challenge (F((1,17))=6.87, p=0.018), while the rearing experience showed a significant main effect (PR>MR) on CSF CRH (F((1,20))=11.66, p=0.003) and cage shaking behavior (F((1,27))=4.45, p=0.045). The effects of rhTPH2 A2051C on the afternoon cortisol level, plasma ACTH level, dexamethasone suppression of urinary cortisol excretion, and aggression were dependent upon the rearing experience. These results were not confounded by the functional C77G polymorphism in the mu-opioid receptor (MOR). The present study supports the hypothesis that rearing experience and rhTPH2 A2051C interact to influence central 5-HT metabolism, HPA axis function, and aggressive behaviors. Our findings strengthen the involvement of G x E interactions at the loci of serotonergic genes and the utility of the nonhuman primate to model G x E interactions in the development of human neuropsychiatric diseases.

Functional properties of missense variants of human tryptophan hydroxylase 2

Tryptophan hydroxylase 2 (TPH2) catalyzes the rate-limiting step in serotonin biosynthesis in the nervous system. Several variants of human TPH2 have been reported to be associated with a spectrum of neuropsychiatric disorders such as unipolar major depression, bipolar disorder, suicidality, and attention-deficit/hyperactivity disorder (ADHD). We used three different expression systems: rabbit reticulocyte lysate, Escherichia coli, and human embryonic kidney cells, to identify functional effects of all human TPH2 missense variants reported to date. The properties of mutants affecting the regulatory domain, that is, p.Leu36Val, p.Leu36Pro, p.Ser41Tyr, and p.Arg55Cys, were indistinguishable from the wild-type (WT). Moderate loss-of-function effects were observed for mutants in the catalytic and oligomerization domains, that is, p.Pro206Ser, p.Ala328Val, p.Arg441His, and p.Asp479Glu, which were manifested via stability and solubility effects, whereas p.Arg303Trp had severely reduced solubility and was completely inactive. All variants were tested as substrates for protein kinase A and were found to have similar phosphorylation stoichiometries. A standardized assay protocol as described here for activity and solubility screening should also be useful for determining properties of other TPH2 variants that will be discovered in the future.

A predicted functional single-nucleotide polymorphism of bone morphogenetic protein-4 gene affects mRNA expression and shows a significant association with cutaneous melanoma in Southern Italian population

PURPOSE

An altered expression of bone morphogenetic protein 4 (BMP4) has been found in malignant melanoma cells. We performed an association study to investigate the effect of putative functional single nucleotide polymorphisms (SNPs) of BMP4 on development of cutaneous melanoma (CM).

METHODS

We selected the predicted functional SNPs 6007 C/T (rs17563) and -3445 T/G (rs4898820) by the combination of three computational tools (FASTSNP, F-SNP and SNP Function Portal) plus another tool (SNP@promoter) skilled in identifying SNPs in transcription regulatory regions. Both SNPs were genotyped in a case-control study of 215 individuals with CM and 342 controls. We also evaluated the BMP4 hypothetical mRNA secondary structure by GeneBee program, the BMP4 mRNA levels and protein concentrations according to the genotype of two selected SNPs in transformed B-cells of 80 controls and in plasma samples of 38 controls, respectively.

RESULTS

The BMP4 T-allele was associated with CM (OR: 1.39, 95% CI: 1.09-1.78, P = 0.007). The T-allele was predicted to change mRNA structure and the BMP4 mRNA levels were significantly higher in T-allele carriers compared with C-allele carriers (P = 0.01), even the BMP4 protein plasma levels were higher among T-allele carries, but without reaching the statistical significance. No significant association was found between the SNP -3445 T/G alleles and either the risk of CM, or the mRNA levels of BMP4.

CONCLUSIONS

This study evidences the relevance of using bioinformatics tools in searching for cancer-associated gene polymorphisms and suggests that the predicted functional SNP 6007 C/T affects BMP4 gene expression and the risk to development of CM.