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Functional promoter polymorphism of the neuronal isoform of tryptophan
hydroxylase (Tph2) in suicide
Jasminka Stefulja,⁎, Gordana Mokrovica, Dubravka Hranilovicb, Tatjana Bordukalo-Niksica, Mirko Bakulac,
Milovan Kubatc, Branimir Jerneja,d,1
aLaboratory of Neurochemistry and Molecular Neurobiology, Rudjer Boskovic Institute, Zagreb, Croatia
bDepartment of Animal Physiology, Faculty of Science, University of Zagreb, Zagreb, Croatia
cDepartment of Forensic Medicine and Criminology, Medical Faculty, University of Zagreb, Zagreb, Croatia
dCroatian Institute of Brain Research, Medical Faculty, University of Zagreb, Zagreb, Croatia
a b s t r a c t a r t i c l ei n f o
Received 27 October 2009
Received in revised form 26 August 2010
Accepted 28 August 2010
The association between suicide and G-703T polymorphism of the tryptophan hydroxylase 2 (TPH2), the rate-
limiting enzyme in the biosynthesis of the neurotransmitter serotonin, was studied in a sample of 291 suicide
victims and 280 healthy subjects of Croatian origin. No significant differences were found between the groups.
Obtained results do not support involvement of the investigated polymorphism in the susceptibility to suicide
© 2010 Elsevier Ireland Ltd. All rights reserved.
Tryptophan hydroxylase (TPH) is the rate-limiting enzyme in the
biosynthesis of 5HT. There are two isoforms of TPH, so-called
peripheral (TPH1) and neuronal (TPH2), the latter being expressed
exclusively within the nervous system (Walther et al., 2003). A single
nucleotide polymorphism (SNP) G-703T (rs4570625 in dbSNP, http://
www.ncbi.nlm.nih.gov/projects/SNP/), locatedin the promoter region
regulation of 5HT signaling. Respective SNP was shown to have
significant effect on the TPH2 gene expression in different cellular
systems (Lin et al., 2007; Chen et al., 2008), although data were not
unequivocal (Scheuch et al., 2007). Furthermore, it influenced in vivo
reactivity of limbic system, i.e. amygdalae (Brown et al., 2005; Canli
et al., 2005), as well as prefrontal and parietal cortices (Reuter et al.,
2008). Molecular genetic studies revealed association of G-703T
polymorphism with individual differences in emotion-related (Gut-
knecht et al., 2007; Reuter et al., 2007a) and executive (Reuter et al.,
2007b; Osinsky et al. 2009) functions, as well as with certain
neuropsychiatric conditions, including suicidal attempt (Yoon and
Kim, 2009). The present study, based on a relatively large sample of
suicide victims belonging to a Croatian population, re-evaluated
potential involvement of G-703T polymorphism in the etiopathogen-
esis of suicidal behavior.
2. Materials and methods
Data on our cohorts of suicide victims and control subjects, as well as the procedures
for genotyping and statistical analyses, are given in the expanded Materials and methods
section in the online data supplement, (doi:10.1016/j.psychres.2010.08.034).
The genotype frequencies of G-703T polymorphism accorded well
with Hardy–Weinberg equilibrium in both control (χ2=1.637, d.f.=2,
P=0.4411) and victim (χ2=0.060, d.f.=2, P=0.9702) group. There
were no significant differences between the groups in either genotype
(χ2=1.545, d.f.=2; P=0.4620) or allele (FET, P=0.7159) frequencies
The present study focuses research on the involvement of TPH2
gene in suicidal behavior to ethnically homogenous population of
Croatian origin. The study was based on the relatively large sample
(N=291) of subjects who have completed suicide, mainly (97%) by
violent means. The statistical power of our sample to detect
differences associated with odds ratios (OR) of 2.0 and 1.5, at the
level of significance of 0.05, amounted to 94.4% and 49.9%,
respectively. No differences were found between the control subjects
and suicide victims in the distribution of either genotype or allele
Psychiatry Research 186 (2011) 446–447
⁎ Corresponding author. Laboratory of Neurochemistry and Molecular Neurobiology,
Rudjer Boskovic Institute, Bijenicka 54, HR-10000 Zagreb, Croatia. Tel.: +385 1 4561
015; fax: +385 1 4561 177.
E-mail address: email@example.com (J. Stefulj).
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frequencies of G-703T polymorphism (Table 1), suggesting that the
polymorphism has no influence on suicide completion in our
population. Even when only violent suicide victims (N=282) were
taken into consideration, no association was found (data not shown).
The very recent study on suicide victims of Japanese origin (Mouri
et al., 2009), as well as the two previous studies on suicide attempters
(Zhou et al., 2005; Zill et al., 2007) also found no evidence for the
effect of G-703T polymorphism on suicidal behavior. Yoon and Kim
(2009), on the other hand, reported excess of GG homozygotes among
suicidal depressed patients as compared to healthy controls. Since no
differences between depressive patients with and without history of
suicide attempt or between the groups of non-suicidal depressive
patients and healthy controls were found, it seems plausible that the
observed association was not related to suicidal attempt itself (as
concluded by the authors), but possibly to depression or depression
associated with suicidal attempt.
Frequency of –703T allele of G-703T polymorphism in our control
sample, being 20.4%, fits well in the range of 18%–23%, reported for
other Caucasian populations (Zhou et al., 2005; Reuter et al., 2007a;
Zill et al., 2007). It should be noted that significantly higher
frequencies of −703T allele were found in non-Caucasian populations
such as African American (39%) (Zhou et al., 2005), American Indian
(48%) (Zhou et al., 2005), Korean (57%) (Yoon and Kim, 2009) and
Japanese (45%) (Mouri et al., 2009).
Complex disorders such as suicide are likely to involve multiple
genes along with epigenetic influences and the main limitation of our
study is that it considers effect of a single genetic factor. Another
limitation, the lack of records for suicide victims on the psychiatric co-
morbidity, should also be mentioned. Nevertheless, our results, as
well as findings on other populations (Zhou et al., 2005; Zill et al.,
2007; Mouri et al., 2009), even ethnically very distant ones, clearly
speak against the major role of the functional variant G-703T of the
TPH2 gene in the susceptibility to suicide. Serotonergic dysfunction
reportedly plays a major role in the etiopathogenesis of suicidal
behavior (for review see Mann, 2003). Our earlier research has also
given some support to this concept (Hranilovic et al., 2003; Jernej
et al., 2004; Stefulj et al., 2006). Therefore, association studies on
genes encoding synaptic proteins of this transmitter, including the
recently discovered TPH2, should be encouraged, with negative
reports being as important as positive ones in obtaining a realistic
picture of true genetic influences.
This work was supported by the Croatian Ministry of Science, Education and Sport,
grant 098-1081870-2395 (to B.J.).
Appendix A. Supplementary data
Supplementary data to this article can be found online at
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Allele and genotype counts and frequencies of the G-703T polymorphism of tryptophan
hydroxylase 2 (TPH2) promoter in Croatian control population and suicide victims.
aP values for differences in genotype and allele distributions between the control
and victim groups were calculated by Chi-square and Fisher's Exact Test, respectively.
J. Stefulj et al. / Psychiatry Research 186 (2011) 446–447