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Table of Contents
ORIGINAL ARTICLE
Year : 2015  |  Volume : 2  |  Issue : 2  |  Page : 34-39

TPH2 variant rs7305115 and its interaction with acute stressful life events in etiology of suicide attempt in Serbian psychiatric patients


1 Center for Human Molecular Genetics, Faculty of Biology, University of Belgrade, Studentski trg 16, PO box 52, Belgrade 11 000, Serbia
2 Clinic for Psychiatry, Clinical Centre of , Pasterova 2, Belgrade 11000; Medical School, University of Belgrade, Doktora Subotića 8, Belgrade 11000, Serbia
3 Clinic for Psychiatry, Clinical Centre of , Pasterova 2, Belgrade 11000, Serbia

Date of Web Publication5-Jul-2017

Correspondence Address:
Jelena Karanovic
Center for Human Molecular Genetics, Faculty of Biology, University of Belgrade, Studentski trg 16, PO box 52, Belgrade 11 000
Serbia
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Source of Support: None, Conflict of Interest: None


DOI: 10.5530/ami.2015.2.8

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  Abstract 


Introduction: Suicide attempt (SA) is in the middle of continuum of complex suicidal behavior phenotype. Psychiatric disorders and acute stressful life events (SLEs) are triggers for suicidal behavior. Serotonin system genes are often implicated in suicidal behavior. Tryptophan hydroxylase 2 (TPH2), exclusively expressed in the brain, is the rate-limiting enzyme for serotonin biosynthesis. TPH2 may be enrolled in stress-response mechanisms via hypothalamic–pituitary–adrenal axis, while TPH2 variant rs7305115 has been reported to affect gene expression in postmortem human pons. To date, only poor examination of this variant in etiology of suicidal behavior has reported conflicting results. The aim of the present study was to assess rs7305115 main effect and its interaction with acute SLEs in SA pathology among Serbian psychiatric patients. Methods: 165 suicide attempters and 188 suicide non-attempters, suffering from major psychiatric disorders, participated in the study. Acute SLEs score was assessed using the List of Threatening Experiences Questionnaire. Variant rs7305115 was genotyped using TaqMan-based allelic discrimination assay. Statistical analyses were done in SNPstats by applying logistic regression adjusted by psychiatric diagnoses. Results: Variant rs7305115 was not associated with SA in Serbian psychiatric patients, neither alone, nor in combination with acute SLEs, for all five models of inheritance tested (P>0.05). Discussion: Our finding does not support the main and moderating implication of TPH2 variant rs7305115 in SA liability among Serbian psychiatric patients. Further examination in larger samples of this variant in SA patology is necessary due to its functional relevance.

Keywords: Suicide attempted, Tryptophan hydroxylase 2, Stressful events, Gene-environment interaction, Risk factors


How to cite this article:
Karanovic J, Ivković M, Pantović M, Brajušković G, Romac S, Pavićević DS. TPH2 variant rs7305115 and its interaction with acute stressful life events in etiology of suicide attempt in Serbian psychiatric patients. Acta Med Int 2015;2:34-9

How to cite this URL:
Karanovic J, Ivković M, Pantović M, Brajušković G, Romac S, Pavićević DS. TPH2 variant rs7305115 and its interaction with acute stressful life events in etiology of suicide attempt in Serbian psychiatric patients. Acta Med Int [serial online] 2015 [cited 2019 May 19];2:34-9. Available from: http://www.actamedicainternational.com/text.asp?2015/2/2/34/209648




  Introduction Top


Suicidal behavior is a worldwide health problem. The continuum of suicidal behavior includes suicidal ideation, suicide attempt (SA) and completed suicide. SA is a self-harm act with at least some intent to take one's own life.[1] It is estimated that almost one million people commit suicide each year, while 20 times more people attempt suicide. Eastern European countries have the highest suicidal rates, over 30 per 100000 people.[2] In Serbia, suicidal rate is 18.15 per 100000 people.[3]

“Stress-diathesis” model of suicidal behavior assumes that predisposition to suicidal outcome is determined by distal risk factors, including genetic and epigenetic factors, childhood trauma and personality traits, while proximal risk factors, including psychiatric disorders and acute stressful life events (SLEs), are suicidal triggers.[4] Having a psychiatric disorder, especially a mood disorder, is a very common, but not a necessary proximal risk factor in SA patients.[5],[6] Acute SLEs are strong proximal risk factors, even in the absence of any other psychiatric disorder.[7] Genetic (distal) risk factors are estimated to contribute to SA liability between 17 and 55%, but mostly they have not been identified yet.[8] Serotonin system genes are the most frequently studied and implicated in etiology of SA due to hypofunction of serotonin system in suicide.[9]

Serotonin regulates a wide spectrum of complex brain functions including aggression, anxiety, appetite, circadian rhythm, cognition, mood, pain sensitivity and sexual behavior.[10] The biosinthesis of serotonin is catalyzed by the rate-limiting enzyme tryptophan hydroxylase (TPH).[11] TPH1 and TPH2 isoforms are coded by two different genes which have highly homologous sequences.[12] Despite TPH1 and TPH2 being traditionally reffered to as peripheral and central isoform,[11],[13] respectively, a recent study has demonstrated the expression of both isoforms in various regions of the human brain.[14] TPH2 has been found to have 4-fold higher expression than TPH1 in raphe nuclei, and 7-fold higher expression in raphe nuclei than in other human brain regions.[14] Interestingly, 35% greater density and number of serotonergic neurons in the dorsal raphe nuclei compared to non-suicide controls have been observed in suicide victims.[15] Moreover, an increased TPH2 mRNA and protein levels have been observed in postmortem brain of depressed suicide victims,[16] while Tph2-null mice have displayed behavioral alternations, including aggression, anxiety-like and depression-like behavior,[17] suggesting its plausible role in suicidal behavior.

To date, common variants in TPH2 gene have been inconsistently associated with suicidal behavior, and functional variant rs7305115 has been poorly studied.[9] A-allele and AA genotype of the rs7305115 have been reported as less frequent among depressed Chinese patients with SA history.[18],[19] Moreover, TPH2 “risk” haplotype, including rs7305115, has been associated with SA[20] and with suicidal and parasuicidal behaviors.[21] However, other studies have reported neither direct nor indirect association between variant rs7305115 and suicidal behavior.[22],[23],[24],[25] Rs7305115 is located in exon 7 of TPH2 gene and has been reported to control gene expression in postmortem human pons.[26]

Since various risk factors alone are unlikely to explain a complex trait of suicidal behavior, the interaction between those factors should be considered.[27] It has been hypothesized that TPH2 could be involved in glucocorticoid-dependent stress-response mechanisms via hypothalamic–pituitary–adrenal (HPA) axis,[28],[29] suggesting that acute SLEs should be investigated in interaction with TPH2 in etiology of SA pathology. To date, only one study has examined the association of TPH2 rs7305115 interaction with acute SLEs in etiology of suicidal behavior. A-allele of rs7305115 in combination with acute SLEs has been identified as protective factor for suicide in depressed Chinese patients.[19]

The aim of the present study was to investigate the main and moderating effects of TPH2 variant rs7305115 on SA risk among Serbian patients with major psychiatric disorders.


  Methods Top


Subjects

A total of 353 unrelated psychiatric patients of Serbian origin with major depressive disorder, bipolar disorder or schizophrenia were included in the study. Patients were hospitalized for five weeks at the Department of Psychiatry at Clinical Centre of Serbia between 2006 and 2012. Psychiatric disorders were determined by two trained psychiatrists using Structured Clinical Interview for DSM- IV Axis I Disorders.[30] Psychiatric patients were divided into 165 (46.74%) suicide attempters (SAs) and 188 (53.26%) suicide non-attempters (non-SAs). SAs were hospitalized after SA, while non-SAs were hospitalized after recurrence of the primary psychiatric disorder and without history of SA. Demographic characteristics, including age and gender, and psychiatric diagnoses of the studied subjects were described in more detail in our previous study.[31]

All patients assigned a written informed consent for their participation in the study. The protocol was approved by Ethics Committee of the Clinical Centre of Serbia for human subjects. The study was conducted in accordance with the Helsinki Declaration of 1975, which was revised in 2000.

Clinical Instruments

All participants answered 12 questions relating stressful events with long-term threat using the List of Threatening Experiences Questionnaire (LTE-Q).[32] Only acute SLEs, occured 12 months prior to the SA or prior to the recurrence of the disease, were examined in this study, which was confirmed by two psychiatrists. LTE-Q score was calculated as a sum of reported items. The main effect of acute SLEs on SA risk among Serbian psychiatric patients was demonstrated in our previous study.[31]

Genotyping

DNA was isolated from subjects' blood samples using the QIAamp DNA Blood Mini Kit (Qiagen, Hilden, Germany). Genotyping of rs7305115 was performed by allelic discrimination assay using TaqMan® Pre-Designed SNP Genotyping Assay (Life Technologies, Grand Island, USA). Allelic discrimination assay was done in 10 μl volume containing the next components: 1xSNP Genotyping Assay Mix C___8376164_10 (Life Technologies, Grand Island, USA), 1xTaqMan® Universal PCR Master Mix (No AmpErase® UNG, Life Technologies, Grand Island, USA), ddH2O and 1-20 ng genomic DNA. Polymerase chain reaction (PCR) was performed using 7500 Real-Time PCR System (Applied Biosystems, Warrington, UK). PCR programe was the following: initial denaturation at 95°C for 10 min and 40 cycles of denaturation at 95°C for 15 s and primer annealing at 60°C for 1 min. Sequence Detection Systems (SDS) software 1.2 (Applied Biosystems, Warrington, UK) with a multi-component algorithm was used to calculate distinct allele signal contributions from fluorescent measurements for each sample. 10% of randomly selected samples were re-genotyped with a 100% concordance.

Statistical Analyses

Statistical analyses were done in SNPstats,[33] available at http://bioinfo.iconcologia.net/SNPstats. Logistic regression adjusted by psychiatric diagnoses was used for testing the genotypic association and GxE interaction analyses. Adjustment by psychiatric diagnoses was applied due to existing of statistic trend in its differences between SAs and non-SAs, as demonstrated in our previous study.[31] Other covariates were not included as covariates in statistical analyses since there was no significant difference in mean age, gender and female to male ratio between SAs and non- Sas.[31] A limit for significant association was set to P<0.05, while an association strength was measured by the odds ratio (OR) with a 95%-confidence interval (CI). Akaike information criterion (AIK) with the lowest value was a measure of the best genetic model of inheritance. Hardy- Weinberg equilibrium was not assessed since this was the case-only study.


  Results Top


In present study, TPH2 variant rs7305115 was not significantly associated with SA in Serbian psychiatric patients for any tested model of inheritance ([Table 1], P>0.05).
Table 1: Genotypic association of TPH2 rs7305115 with suicide attempt risk in Serbian patients with major psychiatric disorders

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The interaction between variant rs7305115 and LTE-Q score was not significantly associated with SA among Serbian psychiatric patients for all tested genetic models of inheritance ([Table 2], P>0.05).
Table 2: Interaction between TPH2 rs7305115 and acute stressful life events in etiology of suicide attempt in Serbian patients with major psychiatric disorders

Click here to view



  Discussion Top


Functional TPH2 variant rs7305115 had been rarely investigated in etiology of SA and suicidal behavior in general. Based on neurobiological evidence about implication of TPH2 in suicidal behavior, and few positive findings in association studies of rs7305115 with SA,[9] we examined weather this variant had main or moderating effect on SA liability among Serbian psychiatric patients. We tested if rs7305115 mediated the acute SLEs effect on SA outcome due to possible role of TPH2 in HPA axis-mediated response to stress.[28],[29]

We found lack of association between rs7305115 and SA in Serbian psychiatric patients. The same finding was reported in a few studies that had examined suicidal behavior. Single-locus and haplotype analyses revealed no association of rs7305115, as a tag variant, and completed suicide in Japanese population.[23] Studies that had examined the whole TPH2 gene covered with tag variants, not including rs7305115 directly, reported also negative results in German alcohol-dependent patients with SA,[22] in Estonian male suicide completers,[24] and in Brazilian whites patients with bipolar disorder and SA.[25] On the other hand, Zhou et al.[20] identified “risk” haplotype for SA in Finnish whites and African Americans, containing the G-allele of rs7305115, while single-locus analysis for rs7305115 revealed no association with SA in all four tested populations. Additionaly, this “risk” haplotype was associated with suicidal and parasuicidal behaviors in European whites subsample.[21] Moreover, A-allele and AA genotype of the rs7305115 had been shown as protective factor for SA in depressed patients of Chinese origin.[18],[19] Interestingly, A-allele of rs7305115, located in exon 7 of TPH2 gene, had been associated with increased TPH2 mRNA expression in postmortem human pons, probably by enhansing the efficiency of mRNA splicing.[26] This suggests that A-allele may really have a protective role for SA, but this assocation has been inconsistently demonstrated. Based on only few studies, final conclusion may not be drawn, especially because different populations and patients with various comorbid psychiatric disorders and different types od suicidal behavior have been investigated. Assuming functional relevance of rs7305115, further examination of this variant is needed to clarify its reliable implication in SA pathology.

In our previous study, acute SLEs were found to be significant environmental risk factor (P=0.014) for SA in Serbian psychiatric patients.[31] However, in our current study, we revealed no significant interaction between rs7305115 and acute SLEs in etiology of SA in the same sample. Therefore, our results could not support the hypothesis involving TPH2 in stress-response mechanisms via HPA axis.[28],[29] Even though TPH2 is predominant isoform in the dorsal raphe nuclei, only TPH1 mRNA showed upregulation in this brain region of rats after chronic social stress,[34] which might explain our negative gene-environment association finding. As a support to our result, only variants in 3'-end of TPH2 showed significant association with HPA axis activity in rhesus monkeys, probably by regulating the level of TPH2 expression.[28] Rhesus monkeys were proposed to be non-human primate model for genotype-phenotype associations relevant to human diseases due to at least 98% homology in coding sequence between two species.[28] Nevertheless, one study revealed A-allele of rs7305115 in combination with negative acute SLEs, family history of suicide and hopelessness as protective from suicide in depressed patients.[19] Additionally, AA genotype was found protective from depressive disorders in three-way interaction with variant in serotonin transporter and early-life abuse,[35] and A-allele was also demonstrated as favorable factor in response to antidepressant therapy in combination with early-life abuse.[36] According to this, A-allele could be protective factor from both, adult and childhood stressful events in wide spectar of psychiatric disorders. Lim et al.[26] discussed about positive effect on higher frequency of A-allele on mood or mental activity as a result of positive selection. We used LTE-Q questionairre that included positive and negative acute SLEs, and this could be the reason we did not confirme Zhang et al. finding related to only negative SLEs.[19] Since mRNA and protein level of TPH2 were demonstrated to be glucocorticoid-dependent in raphe nuclei of mice[37],[38] and rats,[39] three-way interaction combining acute SLEs and genetic variants from TPH2 and glucocorticoid pathway should be investigated in the future.

In current study, we tested only SA among psychiatric patients, not suicidal behavior in general. This is the main advance of our study, since SA is recommended to be studied separately from completed suicide due to great heterogenity of suicidal behavior.[1] On the other hand, the main limitation of our study is a small sample size that can not powerfully detect small genetic effects of common variants.[40] However, all possible confounders were controlled appropriate. We applied adjustment by psychiatric diagnoses in logistic regression analyses to avoid comorbidities due to observed statistical trend in psychiatric status between SAs and non-SAs (P=0.066) in our previous study.[31] Since ethnicity, age, gender and female to male ratio were uniform (P>0.05) among studied groups,[31] we did not include them as covariates in statistical analyses. Thus, replication association studies with sufficient power and correct statistical approaches should be conducted to allow further comparison of obtained results. Another limitation of our study is performing of only single-locus analyses, while haplotype analyses have provided more consistent results for TPH2 gene implication in suicidal behavior.[9] Also, limitations in obtaining accurate information about traumatic events in psychiatric patients should be noted, even though the reliability about endorsed LTE-Q items have been estimated independently by two psychiatrists.

Our result suggests that TPH2 functional variant rs7305115 has neither main nor moderating effect on SA liability among Serbian patients with major psychiatric disorders. Nevertheless, this variant has been poorly examined in etiology of suicidal behavior, and our study is one small step forward in contribution to riddle genetic background of this complex phenotype. Further examination in larger samples of this and other TPH2 variants, in single and haplotype approach, is necessary in order to elucidate reliable role of this gene in etiology of suicidal behavior.


  Acknowledgements Top


This study was supported by the grant no. 173016 from the Ministry of Education, Science and Technological Development, Republic of Serbia.



 
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