Modulation of anger control in suicide attempters by TPH-1
*P. Baud, MD, Department of Psychiatry, Clinique de Belle-Idée, 2 chemin du Petit-Bel-Air, 1225 Chêne-Bourg, Switzerland. E-mail: firstname.lastname@example.org
A genetic association between the tryptophan hydroxylase gene (TPH)-1 A218C polymorphism and suicidal behaviour is supported by numerous case–control studies as well as recent meta-analyses. Some data suggest that this polymorphism could also influence individual differences in anger-related personality traits, a phenotype partially under genetic control and known to increase the risk of suicide ideation and attempt. The aim of the present study was to investigate whether the TPH-1 A218C polymorphism affected anger-related personality traits in suicide attempters (n = 544). We hypothesized that suicide attempters carrying the AA genotype would display different scores on a scale measuring anger-related traits compared with suicide attempters carrying the CC genotype. Indeed, the dimension of Anger Control was significantly affected by the TPH-1 A218C polymorphism: suicide attempters carrying the AA genotype scored significantly lower on the Anger Control subscale than suicide attempters carrying the AC and CC genotypes. This polymorphism did not display any influence on the other State-Trait Anger Expression Inventory subscales. This result confirms our working hypothesis and suggests that the TPH-1 genotype could confer a vulnerability to suicidal behaviour through a reduced capacity to control anger, which in turn may represent a common psychopathological and behavioural pathway to suicidal behaviour in an important subgroup of clinical subjects.
A genetic influence on suicidal behaviour is now widely acknowledged and has received convincing support from familial, twin and adoption studies (for a recent review, see Brent & Mann 2005), as well as from a growing body of data from molecular genetic studies, mostly conducted on serotonergic candidate genes (Bondy et al. 2006; Courtet et al. 2005). Yet, the functional steps through which multiple genes of small effect size could shape such a complex behaviour still remain elusive. Personality traits have been proposed as intermediary phenotypes between gene effects and suicidal behaviour (Baud 2005). The most relevant among them, beside impulsive–aggressive dispositions, are anger-related traits, which have been shown to play a role in suicide ideation (Velting 1999) and attempt (Brezo et al. 2006; Horesh et al. 1997) and to be partially under genetic control (Cates et al. 1993; Gustavsson et al. 1996; Rebollo & Boomsma 2006; Sluyter et al. 2000).
A non-functional polymorphism within the tryptophan hydroxylase gene (TPH-1 intron 7 A218C), which codes for the rate-limiting enzyme of serotonin (5-HT) biosynthesis, has been repeatedly associated with suicidal behaviour. Although association studies provided conflicting results, meta-analyses suggested that the A218 allele ‘has a dose-dependent effect on the risk of suicidal behavior’ (Bellivier et al. 2004; Li & He 2006).
The TPH-1 A218C polymorphism has also been associated with individual differences in anger-related personality traits. In a non-patient sample of community-derived volunteers, carriers of the A218 allele scored significantly higher on measures of aggression and tendency to experience unprovoked anger and were more likely to report expressing their anger outwardly (Manuck et al. 1999). Another investigation of anger-related personality traits in a population of community-based healthy volunteers and in a sample of suicide attempters found that carriers of the A218 allele in both groups scored higher for State Anger, Trait Anger and Angry Temperament, when investigated with the State-Trait Anger Expression Inventory (STAXI) (Rujescu et al. 2002).
The present study aims to investigate whether TPH-1 polymorphisms affected anger-related personality traits in suicide attempters. More specifically, assuming that the TPH-1 A218 allele increases the risk of suicidal behaviour, mainly in double dose (homozygote), and influences anger-related personality traits, we hypothesized that suicide attempters carrying the AA genotype would display different scores on a scale measuring anger-related traits from suicide attempters carrying the CC genotype.
The discovery of a second isoform of tryptophan hydroxylase, namely TPH-2, led to doubts about the expression of TPH-1 in the brain. In mice indeed, TPH-1 mRNA was mainly expressed in the periphery and in the pineal gland, whereas TPH-2 mRNA was exclusively detected in the brain (Walther et al. 2003). However, TPH-1 mRNA was recently shown to be expressed in several regions of the human brain (cortex, thalamus, hypothalamus, hippocampus, amygdala, cerebellum and raphe nuclei) (Zill et al. 2007). Therefore, TPH-1 polymorphisms may modulate TPH activity in adult brain. Secondly, peripheral 5-HT, synthesized by TPH-1, plays a major role in fetal brain development (Gaspar et al. 2003). In mice, TPH-1 suppression or genetic variations in TPH-1 have a strong influence on both brain development (Côtéet al. 2007) and behaviour later in life (Nakamura et al. 2006). These results validate TPH-1 as a candidate gene in association studies on psychiatric disorders.
Thus, in a large clinical population of suicide attempters, scores on the subscales of the STAXI were studied according to the subjects’TPH-1 genotype.
Materials and methods
Suicidal patients (n = 544) were recruited in the Psychiatric University Hospitals of Geneva (Switzerland) and Montpellier (France) on the basis of a history of suicide attempt, without considering whether suicidal behaviour was, or was not, the immediate reason for hospitalization. Indeed, patients from Montpellier were included in the study shortly after having attempted suicide, whereas Geneva patients were recruited on the basis of a past suicide attempt.
Psychiatric and behavioural assessments
Patients were assessed for psychiatric diagnoses using the French version of the Diagnostic Interview for Genetic Studies, which has a good interrater reliability (Nurnberger et al. 1994).
Anger-related traits were assessed with the STAXI (Spielberger 1988). The STAXI is a 44-item self-report questionnaire measuring the experience and expression of anger, in accordance with the state-trait personality theory (Eckhardt et al. 2004). The experience of anger comprises State Anger (the current feelings experienced by an individual) and Trait Anger (the individual disposition to experience anger or the ease, frequency and intensity of becoming angered), whereas the expression of anger comprises the three components of Anger-In (which measures the tendency for an individual to suppress angry feelings), Anger-Out (the tendency to outwardly express anger towards people or objects) and Anger Control (the capacity of an individual to regulate or control his anger).
Genotyping and haplotyping were carried out following previous protocols for TPH-1 intron 7 A218C (rs1800532), intron 8 (rs10741734 T/C polymorphism) and 3′ UTR (untranslated region) (CTCA repeat, dichotomised in short, S [194 and less] and long, L [more than 194] repeats) polymorphisms (Paoloni-Giacobino et al. 2000). Because of the complete linkage disequilibrium between these three markers, the genotyping of rs10741734 and 3′ UTR CTCA repeat allows us to control for genotyping of rs1800532.
As each STAXI subscale displayed non-parametric curves, subjects were distributed into three groups (tertiles) according to their scores (low, medium and high scorers). On the State Anger subscale, low scores were between 10 and 15, medium scores between 16 and 24 and high scores between 25 and 40. On the Trait Anger subscale, low scores were between 10 and 20, medium scores between 21 and 27 and high scores between 28 and 40. On the Anger-In subscale, low scores were between 8 and 18, medium between 19 and 24 and high between 25 and 32. On the Anger-Out subscale, low scores were between 8 and 13, medium between 14 and 18 and high between 19 and 32. On the Anger Control subscale, low scores were between 10 and 18, medium between 19 and 23 and high between 24 and 32.
A multinomial regression was used to estimate the odds ratio (OR) at which high scorers compared with medium and with low scorers, as well as medium scorers compared with low scorers, were associated with a particular genotype.
Multinomial regression was used because it allowed us to compare, for each STAXI subscale, the high, medium and low scorers within the same model, while adjusting for gender and age. In a second analysis, we also adjusted for diagnoses and recruitment centre.
An interaction term was also introduced in the model. We looked at a possible interaction of genotype with gender.
The calculated coefficients represented the OR for having a particular genotype for suicide attempters belonging to the high scorers group compared with the two other groups, as well as for those belonging to the medium scorers group compared with those from the group of low scorers. This calculation was made for each STAXI subscale. We used the statistical package stata v.8.
For the haplotype frequencies and between-group comparisons, we used the Haploview v.3.3 (Stephens et al. 2001) and phase v.2.1 (Barrett et al. 2005) programs. Linkage disequilibrium between markers was calculated using Haploview.
As we compared the genotypic distribution of one polymorphic marker and one haplotype on the five STAXI subscales, a correction for multiple testing was required. Correcting for multiple testing in a multistage analysis is difficult. The primary purpose of our research was to detect an effect of TPH-1 A218C on the STAXI subscales and secondarily to confirm the results in a haplotype analysis. As the STAXI subscales were all highly correlated, we considered this equivalent to two independent measures. For a multiple testing correction on the P values, we therefore used P = 0.05/2 = 0.025 as a threshold for significance. As the haplotype analysis was a confirmatory result encompassing polymorphisms in high linkage disequilibrium with the TPH-1 A218C polymorphism, it was not taken into account in the correction for multiple testing.
Clinical and demographic data for the population of suicide attempters are presented in Table 1.
Table 1. Clinical and demographic characteristics of suicide attempters (n = 544)
| Female||378 (69.5)|
| Male||166 (30.5)|
| MDD||360 (66.2)|
| BP||113 (20.8)|
| Substance abuse||21 (3.9)|
| SZ/SAD/psychotic disorder||18 (3.3)|
| Anxiety disorder||9 (1.7)|
| Anorexia||1 (0.2)|
| Unknown||22 (4.0)|
| ||Mean (SD)/min–max|
Male and female suicide attempters did not differ according to any STAXI subscale.
TPH-1 A218C polymorphism was in Hardy–Weinberg equilibrium. High linkage disequilibrium was found between the three polymorphisms ranging from 0.93 to 1, with two observed haplotypes: ATS and CCL.
The commonly investigated TPH-1 A218C polymorphism significantly influences the scores for Anger Control: suicide attempters carrying the AA genotype scored significantly lower on the Anger Control subscale than suicide attempters carrying the AC and CC genotypes. Indeed, the probability of having a high score compared with a low score was 2.17 for the AC carriers relative to the AA carriers [AC/AA: OR 2.17 (95% CI: 1.2–4.1), P = 0.017] and 2.13 for the CC carriers relative to the AA [CC/AA: OR 2.13 (95% CI: 1.1–4.1), p = 0.024]. Similarly, the probability of having a high score compared with a medium score was 1.59 for the AC carriers relative to the AA (AC/AA: ns) and 2.0 for the CC carriers relative to the AA (CC/AA: OR 2.0 (95% CI: 1.1–3.7), P = 0.03).
Table 2 shows the results obtained with multinomial regression and with the non-parametric Kruskal–Wallis test. The TPH-1 haplotype did not increase the significance of the results. The TPH-1 A218C haplotype did not display any influence on the other STAXI subscales (data not shown).
Table 2. Anger control scores as a function of TPH intron 7 polymorphism and haplotype
|Intron 7 A218C (rs1800532)|| |
| Intron 7 AA||36 (23.5)||42 (19.7)||21 (12.3)||1||1||1||1|
| Intron 7 AC||65 (42.5)||106 (49.8)||85 (49.7)||1.36||2.17 (P = 0.017)||1.59||5.26; 1; 0.022|
| Intron 7 CC||52 (34.0)||65 (30.5)||65 (38.0)||1.06||2.13 (P = 0.024)||2 (P = 0.03)||4.11; 1; 0.042|
| ATS/ATS||37 (23.7)||41 (19.2)||20 (11.8)||1||1||1||1|
| ATS/CCL||60 (38.5)||96 (44.9)||76 (44.7)||1.4||2.25 (P = 0.014)||1.6||5.51; 1; 0.018|
| CCL/CCL||59 (37.8)||77 (36.0)||74 (43.5)||1.17||2.29 (P = 0.012)||1.96 (P = 0.034)||4.81; 1; 0.028|
We here report a significant effect of the commonly investigated TPH-1 A218C polymorphism and a TPH-1 haplotype on an anger-related trait in a large population of suicide attempters.
Subjects carrying the AA genotype showed lower Anger Control scores than the AC and CC carriers. This result suggests a reduced capacity for the subjects carrying the AA genotype to control the expression of anger when they experience this emotion. Conversely, the alternative genotypes (AC and CC) can be considered as ‘protective’ against the tendency to lose control of the expression of anger. The fact that only AA homozygotes are at risk for lower anger control suggests an absence of dose–response relationship on this outcome.
Besides Anger Control, no other STAXI subscale was affected by the TPH-1 intron 7 genotype. As the allele A218 has been previously associated with suicide behaviour, the present result confirms our working hypothesis and suggests that the TPH-1 genotype could confer a vulnerability to suicidal behaviour through a reduced capacity to control anger. Thus, anger dyscontrol may represent a common psychopathological and behavioural pathway to suicidal behaviour in an important subsample of subjects attempting suicide. As angry feelings and related behaviours are particularly intense and frequent in borderline personality disorder (BPD) patients, it is highly plausible that an important subset of our population of suicide attempters – those patients with a reduced capacity to control the expression of anger – are in fact represented by BPD patients. A recently reported association between TPH-1 variants and suicidal borderline women adds credibility to this assumption and warrants closer investigation (Zaboli et al. 2006). However, anger proneness is also common in bipolar patients, who represent 20.8% of our population of suicide attempters, therefore preventing any assimilation of anger-prone suicide attempters with BPD patients.
The results of our study diverge from the study by Rujescu et al. (2002). These authors compared genotypes AA and AC with the CC genotype in a sample of suicide attempters and found the former to increase significantly the scores for State Anger, Trait Anger and Anger-In, whereas Anger Control scores were not affected by the TPH-1 A218C genotype. The discrepancies between both studies could be explained by the small size of the sample (n = 57) in the study performed by Rujescu et al. (2002) compared with the present population (n = 544) and by differences between the statistical methods used.
Interestingly, TPH-1 polymorphisms and haplotype similarly affect anger control in male and female suicide attempters, a result contrasting with our recent work reporting a gender effect of a catechol-O-methyltransferase gene (COMT) polymorphism on anger-related personality traits. We found that the COMT high activity genotype (Val158Val) markedly affected the scores on two STAXI subscales – Trait Anger and Anger Control – specifically in female suicide attempters (Baud et al. 2007). This would suggest that TPH-1 and COMT could differentially affect anger-related traits. While the COMT effect may be gender dependent and influence a stable personality trait, TPH-1 could modulate control over the expression of anger in both genders, without affecting the tendency to experience anger. Thus anger expression and control could be finely regulated by the serotonergic and possibly catecholaminergic systems.