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- MATERIALS AND METHODS
Aims: Angiotensins were shown to have some role in the development of panic disorder (PD). In this study, we aimed to determine the frequency of polymorphisms in two angiotensin-related genes, angiotensin I-converting enzyme (ACE) and angiotensin II type I receptor (ATr1), in a sample of Turkish patients with PD and to evaluate their association with PD development.
Methods: Polymerase chain reaction and restriction fragment length polymorphism was used to analyze ATr1 A1166C polymorphism, and only polymerase chain reaction was used to analyze functional ACE insertion/deletion polymorphism in 123 patients with PD and in 169 similarly aged disease-free controls.
Results: There was no significant difference in the genotype distribution between PD patients and controls for each polymorphism (P > 0.05). Allele frequency of ACE insertion/deletion was borderline statistically significant between the groups (P = 0.055; odds ratio: 1.39; 95% confidence interval: 0.99–1.95), and allele frequency of ATr1 A1166C was not significantly different between the groups (P = 0.32; odds ratio: 0.81; 95% confidence interval: 0.53–1.22).
Conclusion: This study suggests that polymorphisms of ACE I/D and ATr1 A1166C are not associated with risk of PD in Turkish patients. However, in ACE insertion/deletion polymorphism, the insertion allele was found to be more frequent in the male subgroup of patients (χ2 = 4.61, P = 0.032) than in controls, suggesting a potential male-specific role of the less active ACE insertion allele in the pathogenesis of PD.
PANIC DISORDER (PD) is an anxiety disorder characterized by sudden attacks of intense fear and often accompanied with agoraphobia and loss of control. PD has a life-time prevalence of 1–3%.1 Family and twin studies suggest that, there is a strong genetic contribution to the pathogenesis of PD, with an estimated heritability of 50%.2–4 Women are affected 2:1 relative to men.5 There is no simple pattern of inheritance in segregation studies,6 so PD can be considered a polygenic, multifactorial genetically complex disorder.
Although sensitivity to anxiety is the major risk factor for PD,7 other concomitant factors play important roles in the development of PD, including family history, age, gender, personality type and stressful life events. Some factors affecting the respiratory system are the characteristic features of panic attacks including shortness of breath, a feeling of being smothered and hyperventilation, suggesting the involvement of central or peripheral regulation of respiration in the pathophysiology of PD.8 Other accompanying physical symptoms may also be observed during panic attacks such as chest pain, heart palpitations, dizziness and abdominal distress.9 The existence of a significant correlation between respiratory symptoms and increased responsiveness to CO2 in PD patients has been reported.10
Recently, some studies have hypothesized that polymorphisms in angiotensin-related genes affect anxiety disorders such as PD.7,8,11,12 Although the exact pathogenetic mechanism of PD has not been fully clarified, the possible involvement of polymorphisms on angiotensin-related genes in PD pathogenesis may indicate the role of enzymes of the renin-angiotensin system in psychiatric disorders. Consistent with this hypothesis, Olsson et al. and Bandelow et al. stated that polymorphisms involved in the angiotensin I-converting enzyme (ACE) gene may be one of the genetic factors for predisposition to an increased risk of PD.8,11
Angiotensins are known to act on cardiovascular, renal, endocrine and peripheral autocrine nervous systems. In addition, angiotensins are known to have a central effect within the brain. ACE catalyzes the conversion of angiotensin I to the active octapeptide angiotensin II. Angiotensin II controls blood pressure and the fluid-electrolyte balance.13,14 Some previous studies have indicated that angiotensin II is a potential risk factor in the pathogenesis of panic attacks in animal models.15,16
ACE has been shown to degrade a neurotransmitter called substance P (SP),17 which belongs to the tachykinin neuropeptide family. SP interacts selectively with the SP receptor and is degraded by ACE.18 It has been reported that an increase in SP concentration produces anxiogenic-like responses in rats.19,20
Polymorphisms of the ACE gene may play important roles in anxiety disorders. Data concerning the role of the functional insertion (I)/deletion (D) polymorphism in PD risk in different populations are conflicting.7,8,11 The functional I/D polymorphism is characterized by the presence (allele I) or absence (allele D) of a 287-bp Alu repeat within intron 16 of the ACE gene located on chromosome 17q23.21ACE I/D polymorphism is associated with serum and tissue ACE levels whereas the DD genotype shows the highest levels.21–24
The human angiotensin II type I receptor (ATr1) gene encodes ATr1, which is activated by angiotensin II in the brain. ATr1 mediates most of the effects of angiotensin II.25 It has also been reported that many of the classical and hypothetical functions of brain angiotensin II are mediated by stimulation of ATr1.26 An A/C substitution at position 1166 of the ATr1 gene has been identified in the 3′-untranslated region.27,28 The single nucleotide polymorphism (SNP) A1166C in the ATr1 gene was hypothesized to be involved in PD pathogenesis.8
Polymorphisms in the ACE and ATr1 genes may have some phenotypic significance in the development of PD. Screening for the possible relationship between polymorphisms of angiotensin-related genes and PD may contribute to an understanding of the pathogenesis of PD and may be useful in the prevention of this disease. There have been few studies exploring the relationship between angiotensin-related gene polymorphisms and PD susceptibility.7,8,11 However, there have been no studies examining the relationship between angiotensin-related gene polymorphisms and PD in a sample of Turkish patients. As the polymorphisms in ACE I/D and ATr1 A1166C are common in the population and have functional significance, we determined the frequency of the polymorphisms in a sample of Turkish patients with PD and evaluated their association with PD development.
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- MATERIALS AND METHODS
In addition to its endocrine functions, the renin-angiotensin system (RAS) plays an important role within individual tissues such as the brain. The brain RAS is thought to control blood pressure and may regulate metabolism through mechanisms that remain undefined.32 RAS has been studied in numerous psychiatric diseases including schizophrenia, bipolar disorders, major depression, migraine development,33–37 and PD.7,8,11,12
Many anxiety disorders, including PD, have genetic components, and an association between angiotensin-related genes and PD has been shown in previous findings based on the defects in respiratory and cardiovascular control in PD patients. Busjahn et al. reported lower heart rate variability in II and ID genotype carriers of the ACE gene compared to DD variants.38 As such, the existence of a significant correlation between regulation of respiration and pathophysiology of PD can be suggested. However, future studies of PD patients must divide patients into subgroups with and without any aberrations in respiration or heart rate variability.
Olsson et al. reported that the II or ID variants of the ACE gene were more common in PD patients compared to controls, which is consistent with our results.8 However, our results were not statistically significant. No statistically significant association was found between PD and ACE I/D, and ATr1 A1166C polymorphisms in this case-control study. In this study, D allele frequency of ACE gene in Turkish Caucasians (D allele 64 %) was confirmed as described previously.36 The C allele frequency of ATr1 gene in Turkish population (18%) was also confirmed as reported previously in a Turkish hypertension study.39
In the general population, PD occurs more frequently in women than in men.40 From this gender difference, candidate genes may emerge in PD pathogenesis. Thus, some studies expressing gonadal effects on ACE activity reported that women may be more influenced by the ACE gene than mens.41 Another study on ACE I/D polymorphism reported that the less-active I allele was found to be associated with PD in the male subgroup of patients, suggesting a potential gender-specific effect, which is in line with another study's report of a Caucasian sample.8,11 Furthermore, Erhardt et al. have identified an association of two SNP (rs4311 and rs4333), other than I/D polymorphism (rs4340), in the ACE gene with syndromal panic attacks.12 No statistically significant association was found between genotype distributions of genders and PD in this case-control study. This may be ascribed to ethnic, genetic and environmental differences in allele frequency for the investigated polymorphisms, which might affect the results in genetic studies. However, a statistically significant difference was found in the less-active I allele frequency in the male PD subgroup, which is consistent with the findings of Olsson et al. and Bandelow et al.8,11 A previous negative association was found in a Japanese sample of patients with PD.7 This negative finding can be attributed to allelic differences in the ACE I/D polymorphism between ethnic Japanese and Caucasian populations.42
Previous studies have shown that ACE catalyzes the degradation of SP by hydrolysation.43 The less-active I allele can lead to decreased degradation of SP, and thus, increased SP levels have been reported to induce anxiogenic-like responses.19,20 SP receptor (neurokinin 1 receptor) antagonists have been suggested as anxiolytic compounds that avail themselves to a specific therapy based on the ACE I/D genotype, where homozygous carriers of the less-active I allele might profit most.11
It has been suggested that angiotensin II affects central serotonin and dopamine turnover.44 RAS also has a complex influence on brain dopaminergic transmission. It has been shown that ACE inhibitors reduce the formation of angiotensin II and enhance dopamine release and turnover.45,46 Angiotensin II has been reported to facilitate the formation of dopaminergic nerve cells.47 Few studies have explored the interaction between angiotensin and serotonin. However, it has been shown that angiotensin II reduces central serotonin release in rats,48,49 and the ATr1 antagonist enhances serotonin formation.45 Thus, angiotensin-related gene polymorphisms may affect the serotonergic and dopaminergic system in PD patients. Thus, the role of the polymorphisms affecting the serotonergic and dopaminergic system may shed light on the pathogenesis of PD.
The results of this investigation suggest that the I/D polymorphism of the ACE gene and the A1166C polymorphism of the ATr1 gene may not be associated with PD pathogenesis. However, the allele frequency of the ACE gene has a borderline significance in PD patients. Furthermore, with respect to the ACE I/D polymorphism, the I allele was found to be more frequent in the male subgroup of PD patients than in controls, suggesting a potential gender-specific role for the less-active ACE I allele in the pathogenesis of PD. In contrast, no evidence for an association between ATr1 gene allele frequency and PD was found.
In conclusion, although the sample sizes of the groups of PD patients and healthy controls were not large enough to detect the significance between the groups, this is the first study to evaluate the possible association between angiotensin-related genes and PD development in a sample of Turkish patients. The results of our investigation suggest that the two well-known angiotensin-related genes, ACE and ATr1 polymorphisms, are not significantly associated with PD development in the study population. However, D allele frequency was borderline significant in patients with PD, and the less-active I allele frequency of the male subgroup suggests a potential gender-specific effect in the pathogenesis of PD.
The genes involved in the susceptibility and development of PD are still unknown. Given the findings, genome-wide association studies may enable the exploration of a large number of genes together. Furthermore, expression of non-coding RNA may lead to new genomic mechanisms of PD in future studies.