International Journal of Neuropsychopharmacology (2013), 16, 83–90. f CINP 2012 doi:10.1017/S1461145712000223
ARTICLE
A variant of the neuronal amino acid transporter SLC6A15 is associated with ACTH and cortisol responses and cognitive performance in unipolar depression Anna Schuhmacher*, Leonhard Lennertz*, Michael Wagner, Susanne Ho¨fels, Ute Pfeiffer, Vera Guttenthaler, Wolfgang Maier, Astrid Zobel* and Rainald Mo¨ssner* Department of Psychiatry and Psychotherapy, University of Bonn, Germany
Abstract Major depressive disorder (MDD) is accompanied by both cognitive impairments and a hyperactivity of the hypothalamic–pituitary–adrenocortical (HPA) system, resulting in an enhanced glucocorticoid secretion. Cortisol acts via mineralocorticoid and glucocorticoid receptors densely located in the hippocampus, a brain area that is important regarding cognitive functions and especially memory functions. Recently, a variant (rs1545843) affecting transcription of the human SLC6A15 gene has been associated with depression in a genome-wide association study. In an animal model, the neuronal amino acid transporter SLC6A15 was found to be decreased in stress-susceptible mice. Against the background of stress impacting on the activity of the HPA axis, we have investigated alterations of adrenocorticotropic hormone (ACTH) and cortisol secretion in the combined dexamethasone/corticotrophin-releasing hormone (Dex/CRH) test as well as memory and attention performance in a sample of 248 patients with unipolar depression and 172 healthy control subjects genotyped for rs1545843. MDD patients carrying the depression-associated AA genotype showed enhanced maximum and area under the curve ACTH and cortisol answers (p=0.03) as well as an impaired memory and impaired sustained attention performance (p=0.04) compared to carriers of at least one G allele. No effects of the SLC6A15 variant were found in the healthy control group. Our findings argue for a role of the SLC6A15 gene in ACTH and cortisol secretion during the Dex/CRH test and furthermore in the occurrence of cognitive impairments in unipolar depression. Received 27 September 2011 ; Reviewed 26 November 2011 ; Revised 2 February 2012 ; Accepted 17 February 2012 ; First published online 4 April 2012 Key words : Attention, cognition, cortisol, depression, Dex/CRH test, HPA, memory, rs1545843, SLC6A15.
Introduction The hippocampus plays a major role in modulating the hypothalamic–pituitary–adrenocortical (HPA) axis, which is dysregulated in depressive disorders (Holsboer, 2000, 2001). Cortisol acts via mineralocorticoid and glucocorticoid receptors densely located in the hippocampus. This HPA dysregulation, which can be most specifically measured by the Address for correspondence : Dr A. Schuhmacher, Department of Psychiatry and Psychotherapy, University of Bonn, Sigmund-FreudStraße 25, D-53105 Bonn, Germany. Tel. : +49 228 287 15717 Fax : +49 228 287 16097 Email :
[email protected] * These authors contributed equally to this work.
combined dexamethasone/corticotrophin-releasing hormone (Dex/CRH) test, is supposed to reflect a pathophysiology causally related to major depressive disorder (MDD ; Holsboer, 2000, 2001). Due to the important role of the hippocampus in regulating the glucocorticoid feedback loop, hippocampal volume loss is thought to be associated with impaired HPA axis regulation. Furthermore, it is assumed that the neurotoxic effect of cortisol on hippocampal neurons is responsible for the cognitive dysfunction during depression (Egeland et al. 2005). Chronic stress is associated with the development of depressive disorders (Wang, 2005). In an animal model of chronically stressed mice, it could be shown that SLC6A15 mRNA levels in the hippocampal CA1
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region and the dentate gyrus were reduced in stresssusceptible mice compared to stress-resistant mice (Kohli et al. 2011). Recently Kohli et al. (2011) found the variant rs1545843 to be associated with depression in a genome-wide association study. The risk of homozygote AA genotype carriers to suffer from depression and depressive symptoms was 1.42-fold higher compared to carriers of at least one G allele. The closest annotated gene (287 kb) to the region of association is the neuron-specific neutral amino acid transporter gene SLC6A15. The SLC6A15 transporter belongs to the solute carrier 6 (SLC6) family (Broer et al. 2006). SLC6A15 is highly expressed in neurons in the hippocampus (Lein et al. 2007 ; Masson et al. 1996). The risk allele of rs1545843 was associated with a reduction of hippocampal expression of SLC6A15 transcript in temporal lobe epilepsy patients, thus suggesting a regulatory role of this variant on the SLC6A15 gene. Furthermore, reduced hippocampal volumes in depressed patients carrying the AA risk genotype in comparison to healthy controls directly link rs1545843 and the SLC6A15 gene to the pathophysiology of MDD (Kohli et al. 2011). The authors concluded that ‘patients with recurrent major depression and the risk genotype experience an exacerbated reduction in hippocampal volume ’. We therefore investigated the depression-associated variant of the SLC6A15 gene (rs1545843) with the adrenocorticotropic hormone (ACTH) and cortisol responses in the combined Dex/CRH test in depressive in-patients and healthy control subjects. Furthermore, we investigated neuropsychological performance because the hippocampus plays a major role especially in memory function.
Material and method Subjects Consecutively admitted patients with first or recurrent unipolar depression (DSM-IV) aged 18–60 yr were recruited (Department of Psychiatry at the University of Bonn). Diagnosis was based on the clinical diagnosis and SCID interview (Wittchen et al. 1997) administered by an independent, experienced senior consultant. Reliability for the diagnosis of major depression was 0.95. To create a diagnostically and ethnically homogeneous sample, only Caucasian – mostly German – patients showing a minimum score of 18 points on the Hamilton Rating Scale for Depression (HAMD ; Hamilton, 1960) were included in this study. Exclusion criteria were psychotic disorders, bipolar I and II disorder, antisocial or borderline personality
disorder or substance dependence and abuse as well as other serious medical disorders. Co-morbid disorders diagnosed in MDD patients were mainly anxiety disorders [including panic disorder with or without agoraphobia, obsessive–compulsive disorder, generalized anxiety disorder, phobias and post-traumatic stress disorder (PTSD)] but also eating disorders (anorexia and bulimia) and somatoform disorders (pain disorder, somatization disorder). Caucasian controls were systematically recruited from the general population living in the same region (Bonn) using the register of inhabitants. Only healthy Caucasian controls without severe acute physical illness and without a lifetime history of psychiatric disorder were included. Treatment All patients received a standardized antidepressant routine treatment after tapering off the previous antidepressant medication within a period of up to 4 d immediately after admission. A wash-out interval was not planned in order to guarantee the naturalistic nature of the treatment. Treatment followed an internal quality assurance programme, conducted according to evidence-based clinical guidelines. The following procedure was applied : (a) Patients received citalopram, mirtazapine or nortriptyline in a standardized monotherapy. (b) Stepwise increase of dosage from day 1 to day 4 (dosage by clinical decision). (c) Fixed dosage day 4 to day 8. (d) Exclusively lorazepam up to 3 mg (fixed dosage from day 4 to day 8) was allowed as additional psychotropic medication. Severity of depression (HAMD) was measured on day 8. According to the study by Zobel et al. (2010), day 8 was chosen for all parameters (HAMD, ACTH and cortisol measurements) in order to avoid confounding effects by admission, previous medication or change of medication. The mean baseline HAMD (t1) score of 23.02+x5.71 points indicates a severe to moderate level of disease in the study group. The study was approved by the ethical committee of the university and was conducted according to the declaration of Helsinki. All participants had given written informed consent. Genotyping DNA used for the single nucleotide polymorphism (SNP) genotyping was isolated either from EDTA acid anticoagulated blood or permanent cell cultures that
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were received after transforming the lymphocytes with Epstein–Barr virus. Isolation of DNA followed the Qiagen protocol for the Blood & Cell Culture DNA Isolation kit (Qiagen, Germany). For the polymerase chain reaction (PCR), we used 12.5 ng DNA pipetted in a black 384-well plate (Biozym Scientific, Germany). In each well, 5 ml solution containing the Universal PCR MasterMix, No AmpErase UNG and the SNP Genotyping Assay (both provided by Applied Biosystems, USA) were added according to SNP genotyping protocol. PCR was performed following the protocol for this assay. Both alleles were scored in a single well by measuring the fluorescence at the end of the PCR using a Tecan Ultra 384 reader (Tecan, Germany). Excitation and emission wavelengths for the fam labelled probes were 485 and 535 nm and for the vic labelled probes 535 and 590 nm, respectively.
(Austin et al. 2001). Working and episodic memory was assessed in standard tasks using digits or verbal material. The first was measured by the Digit Span forward and backward (part of the Wechsler Intelligence Test) ; the Digit Span requires the subject to recall a series of verbally presented digits in forward or reverse order, with the number of digits increasing until the maximum span is reached. Episodic memory was measured by a German version of the Rey Auditory Verbal Learning Test (VLMT ; Helmstaedter et al. 2001). Delayed recall after 30 min and the global learning performance were tested. For measurement of sustained attention the d2 test (Brickenkamp, 1994) was used. Neuropsychological assessments were performed 1 d before the Dex/CRH-test at day 7 in order to exclude an influence of Dex to CRH on cognitive functions.
Dex/CRH test
Statistical analysis
The Dex/CRH-test was conducted according to a wellestablished protocol listed elsewhere (Heuser et al. 1996) 1 wk after admission. After pretreatment with an oral dose of 1.5 mg Dex at 23 : 00 hours the evening before, the test blood samples were collected at 15 : 00, 15 : 30, 15 : 45, 16 : 00 and 16 : 15 hours through an i.v. catheter. The HPA system was challenged with an infusion of 100 mg human CRH (Schering, Germany) at 15.02. The data analysis relies on the maximum value and the area under the curve (AUC) of ACTH and cortisol, determined by the trapezoidal rule, reflecting one of the most sensitive values in interpreting cortisol answers in the Dex/CRH test (Heuser et al. 1994, 1996).
For the statistical analysis, subjects carrying the AA genotype were compared to carriers of at least one G allele because Kohli et al. (2011) found an increased risk of suffering from depression for AA genotype carriers compared to the other two genotypes. To test for the distribution of sex, co-morbidity, medication, use of lorazepam, age, age at onset, the number of previous episodes and HAMD value at t1 between the genotypes, we made use of Pearson’s x2 tests and analyses of variance (ANOVAs). The dependence of the ACTH and cortisol secretion from SLC6A15 genotype was assessed by multivariate ANOVA (MANOVA), whereby maximum and AUC ACTH and cortisol levels served as dependent variables and the allelic variant (AA vs. AG and GG combined) of the rs1545843 served as factor. The influence of the SLC6A15 variant on the neuropsychological performance was tested by MANOVA, whereby the test scores digits forward, digits backward, VLMT number of words and VLMT delayed recall and the d2 performance score served as dependent variables and the allelic variant of the rs1545843 served as factor. Results were corrected for multiple testing according to Bonferroni. The level of significance was set at 0.05 (two-tailed). All statistical analyses were performed using SPSS statistical package 15.0 (SPSS Corp., USA) and power analyses were performed using G Power (Erdfelder et al. 1996).
Hormone assays Commercially available radioimmunoassay kits (Immulite 2000 ; Biermann, Germany) were used for cortisol measurements with a detection limit of 0.2 mg/dl plasma and intra- and interassay coefficients of variation of 8.1 % and 3.65 %, respectively. For analysis of ACTH concentrations, an immunoradiometric assay without extraction was used. The detection limit was 5.0 pg/ml and the intra- and interassay coefficients of variation were 7.4 % and 5.55 % respectively. ACTH and cortisol analyses accuracy of the laboratory of biochemistry was controlled by crosslaboratory validation studies. Neuropsychological testing
Results
The two major areas of cognitive impairment in depression, memory and attention were investigated
The genotypes of the MDD patients and the healthy control subjects were in Hardy Weinberg Equilibrium
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Table 1. Clinical and demographic characteristics of the major depressive disorder (MDD) sample (n=248) and the healthy control group (n=172)
Genotype SLC6A15 rs1545843 MDD patients Genotype distributiona Age (yr¡S.D.) Gender (male/female) Age at onset (yr)b Number of previous depressive episodesb Medication (mirtazapine/ citalopram/nortriptyline) Lorazepam (yes/no) Comorbidity (yes/no)b Hamilton t1b Healthy control subjects Genotype distributiona Age (yr¡S.D.) Gender (male/female)
p value genotype (AA, AG, GG) (p value AA vs. AG and GG)
AA
AG
GG
AG+GG combined (n=193)
n=55 48.9¡14.3 17/38 34.69¡14.16 3.82¡4.89
n=128 45.6¡13.4 45/83 35.28¡12.02 3.71¡4.29
n=65 45.9¡13.4 33/32 38.53¡13.44 3.96¡4.87
n=193 45.7¡13.3 78/115 36.38¡12.58 3.79¡4.48
0.29 (0.12)1 0.05 (0.20)2 0.19 (0.40)1 0.94 (0.97)1
8/40/7
27/78/23
19/37/9
46/115/32
0.27 (0.19)2
29/24 29/26 23.92¡5.72
74/52 74/54 23.08¡5.52
36/26 33/31 22.17¡6.06
110/78 107/85 22.77¡5.71
0.88 (0.62)2 0.66 (0.69)2 0.28 (0.21)1
n=33 39.8¡11.8 17/16
n=89 39.4¡12.7 44/45
n=50 43.4¡11.3 24/26
n=139 40.8¡12.3 68/71
0.16 (0.66)1 0.95 (0.79)2
1
One-way analysis of variance. Pearson’s x2 test. a Allele frequencies (A/G) : MDD patients 0.48/0.52, healthy control subjects 0.45/0.55. b Data not available for all 248 patients (age at onset : 237, number of previous episodes : 210, co-morbidity : 247, Hamilton t1 : 229, lorazepam : 241). Values are given as means and S.D. 2
(p=0.62 and p=0.55, respectively). There were no differences between the allelic variants of the SLC6A15 genotype concerning age, sex, age at onset, number of previous depressive episodes, medication, taking lorazepam, co-morbidity and depression severity at baseline (HAMD t1) for the MDD patients. In particular, the presence of co-morbid PTSD potentially confounding the analyses of the cortisol response was not associated with genotype in the MDD patients [x2(2)=0.66 ; p=0.78]. In the healthy control subjects, SLC6A15 genotype was not related to age and sex (p>0.1). The SLC6A15 variant was not associated with MDD comparing genotypes of patients and healthy controls [x2(2)=0.74 ; p=0.69]. Due to the fact that the risk of homozygote AA genotype carriers of suffering from depression and depressive symptoms was found to be 1.42-fold higher compared to carriers of at least one G allele (Kohli et al. 2011), the subjects were classified in carriers of one or two G alleles vs. AA genotype carriers for further analysis. A description of the clinical and demographic characteristics of the 248 patients and the 172 healthy control subjects stratified
according to SLC6A15 genotype rs1545843 (AA, AG and GG) and the genotype groups of the SLC6A15 variant (AA and AG/GG) is given in Table 1. In a MANOVA ACTH and cortisol max and ACTH and cortisol AUC values of the MDD patients differed significantly between the genotypes at t1 (F4,243=2.75 ; p=0.03). Pairwise Bonferroni’s corrected comparisons revealed that AA genotype carriers expressed higher ACTH and cortisol answers than carriers of at least one G allele (cortisol AUC : p=0.04 ; cortisol max : p=0.1 ; ACTH max : p=0.03 ; ACTH AUC : p=0.02). Figure 1 gives an overview of ACTH and cortisol values stratified by SLC6A15 variant in the MDD patients. In the smaller sample of healthy control subjects, the effect of the SLC6A15 variant was not replicated (all p>0.10). Power analyses indicated that power for the multivariate analysis comparing AA genotype carriers to G allele carriers was 0.76 in the MDD patients and 0.53 in the healthy control subjects. To achieve a statistical power of 0.8, effect sizes of f=0.22 and f=0.27 would be needed in the MDD patients and the healthy control subjects, respectively.
SLC6A15 gene, HPA system, memory and attention in depression (a)
Mean AUC value (S.E.M)
AA
AG
GG
p = 0.04
4000 3000 p = 0.02
2000 1000 0
AUC ACTH
AUC Cortisol
Mean number sequences (S.E.M)
(a) 5000
10
AA
AG
87
GG
9 8
p = 0.01
7 6 5 4 3 2 1 0 Digits forward
Digits backward
(b)
(b)
90 AA
AG
60
GG
70
Mean words (S.E.M)
Mean maximum (S.E.M)
80
60 50 40
p = 0.03
30 20 10
p = 0.003
AA
AG
GG
50 40 30 p = 0.02
20 10
0 Maximum cortisol
Fig. 1. (a) Mean area under the curve (AUC) and (b) maximum values of adrenocorticotropic hormone (ACTH) and cortisol in 248 unipolar depressed in-patients 1 wk after admission stratified by SLC6A15 genotype rs1545843. Pairwise Bonferroni’s corrected comparisons within the multivariate analysis of variance showed that AA genotype carriers expressed higher ACTH and cortisol answers than carriers of at least one G allele. Data are expressed as means (S.E.M.).
In subsamples consisting of 213 MDD in-patients and 126 healthy control subjects, memory and attention performance stratified by SLC6A15 genotype was tested. Again the descriptive variables did not differ between the allelic classifications of the genotype (all p>0.10). In a MANOVA, the different domains of cognitive performance served as dependent variables, whereas the SLCA15 variant served as factor. With regard to the MDD patients, the results of the multivariate analysis showed that the overall cognitive performance in the five tests differed between the genotypes (F5,207=2.44 ; p=0.036). Pairwise Bonferroni’s corrected comparisons revealed that AA genotype carriers remembered less digits than carriers of at least one G allele (digits forward : p=0.07 ; digits backward : p=0.01). Furthermore, AA genotype carriers showed a worse episodic memory performance than carriers of at least one G allele, remembering less words in the global condition
0 VLMT total learning
VLMT delayed recall
(c) Mean coded symbols (S.E.M)
Maximum ACTH
170
p = 0.005
AA
AG
GG
150 130 110 90 70 50 d2 performance
Fig. 2. Mean neuropsychological performance of 213 unipolar depressed in-patients 1 wk after admission stratified by SLC6A15 genotype rs1545843. (a) Digits Span ; (b) Rey Auditory Verbal Learning Test (VLMT) ; (c) d2 performance. Pairwise Bonferroni’s corrected comparisons within the multivariate analysis of variance revealed that AA genotype carriers showed worse memory and attention performance than carriers of at least one G allele. Data are expressed as means (S.E.M.).
as well as in the delayed recall condition (VLMT number of words : p=0.003 ; VLMT delayed recall : p=0.02). Sustained attention measured with the d2 test was also better in carriers of at least one G allele (p=0.002). No significant differences were found for SLC6A15 genotypes in the healthy control subjects (all p>0.05 ; Fig. 2).
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For this analysis, comparing AA genotype carriers to G allele carriers, power of the multivariate analyses were 0.69 in the MDD patients and 0.57 in the healthy control subjects. Effect sizes of f=0.31 (MDD patients) and f=0.41 (healthy control subjects) would be necessary to achieve a statistical power of 0.8. Discussion SLC6A15 has recently been found to be associated with MDD. Especially, SNP rs1545843 reached significance in a genome-wide association study (Kohli et al. 2011). We investigated the influence of the SLC6A15 variant rs1545843 on ACTH and cortisol secretion in the Dex/CRH test and on neuropsychological performance in unipolar depressed in-patients and a group of healthy control subjects. In the MDD patients, AA genotype carriers expressed higher ACTH and cortisol answers than carriers of at least one G allele. Furthermore, AA genotype carriers showed worse memory (digits backward, VLMT number of words and VLMT delayed recall) and attention (d2 performance score) performance than carriers of at least one G allele. Variant rs1545843 has been found to be associated with MDD in a genome-wide association analysis (Kohli et al. 2011). To test the robustness of their finding, the authors made use of five independent replication samples. Three of them were German case–control samples of patients with recurrent depression and healthy controls. The fourth Netherlands sample consisted of persons scoring high on the Centre for Epidemiologic Studies Depression Scale (CES-D) and persons rating in the lower quartile of the CES-D. The fifth sample included Afro-American subjects of an urban hospital who were classified as depressive or as non-depressive (controls) using Beck’s Depression Inventory (Beck et al. 1961). The recessive model revealed a nominally significant association of the rs1545843 with depression in four replication samples. A meta-analysis across all five samples showed that the risk of homozygote AA genotype carriers to suffer from depression and depressive symptoms was 1.42-fold higher compared to carriers of at least one G allele. In a sixth replication sample of UK cases and controls of the RADIANT study and controls of the Wellcome Trust Case-Control Consortium 2 study, the AA genotype carriers again showed a higher risk of suffering from MDD (p=0.008) than carriers of the other two genotypes. In the present study, we were not able to directly replicate the case–control association reported
by Kohli et al. (2011) because of our small sample size with its limited statistical power. The closest annotated gene with a distance of approximately 287 kb to the region of association is SLC6A15 coding for a sodium-dependent branched chain amino acid transporter (Kohli et al. 2011). Of note, the MDD risk allele of rs1545843 was associated with a down-regulation of the expression of the SLC6A15 gene in the hippocampus of human premortem individuals with temporal lobe epilepsy, thus suggesting a regulatory role of this variant. Transporters of the SLC6 family, which is one of the largest transporter families, modulate neurotransmission and amino acid transport in epithelial cells (Broer et al. 2006). These amino acid transporters are of great importance regarding neurotransmission processes because the transported amino acids are precursors for neurotransmitters, such as serotonin, noradrenaline and dopamine (Broer et al. 2006). The transporters are grouped into monoamine transporters, the c-aminobutyric transporters, the amino acid transporters, the orphan transporters and the bacterial transporters (Nelson, 1998). The SLC6A15 was previously assumed to be an orphan transporter and was named v7-3 (Broer et al. 2006). The transporter did not show active uptake of neurotransmitters, such as dopamine, noradrenaline, 5-hydroxytryptamine and glutamate (Uhl et al. 1992). In 2004, Broer et al. assumed that orphan transporters could be amino acid transporters and based their assumption on their own identification of B0AT1 (SLC6A19) as Na+-dependent neutral amino acid transporter and on the identification of the Na+- and Clx-dependent IMINO transporter (SLC6A20), which was previously named orphan transporter XT3 or XTRP3 (Takanaga et al. 2005). Kohli et al. (2011) assumed that SLC6A15 could be involved in glutamate transmission because SLC6A15 contributes to leucine and total proline transport, both acting as precursors for glutamate synthesis (Kohli et al. 2011). Kohli et al. (2011) have intensively analysed biological correlates of the SLC6A15 variant rs1545843. Healthy controls carrying the risk allele showed lower N-acetylaspartate (NAA) levels in the hippocampus in magnet resonance spectroscopy (1H-NMR) than carriers homozygous for the protective allele. The authors remark that higher NAA levels constitute a functional correlate for neuronal integrity. Consistent with this finding, the authors could furthermore show that stress-susceptible mice have a lower SLC6A15 mRNA expression in the hippocampal CA1 region. Out of 120 mice, the six most stress-resistant and the six most stress-susceptible mice were selected.
SLC6A15 gene, HPA system, memory and attention in depression In stress-susceptible animals, the SLC6A15 mRNA levels in the CA1 region of the hippocampus were 1.9-fold reduced compared to stress-resilient mice. In depressed patients carrying the AA risk genotype, reduced hippocampal volumes were found in comparison to healthy controls in the subsamples of the German discovery and replication samples (Kohli et al. 2011). It is assumed that the neurotoxic effect of cortisol on hippocampal neurons is responsible for the cognitive dysfunction during depression (Egeland et al. 2005). Therefore, enhanced cortisol secretion has been thought to play an important role in inducing impairments in neuropsychological performance often seen in depression (Bearden et al. 2006 ; Sarosi et al. 2007 ; Zihl et al. 1998). Deficits in visuospatial performance, attention, memory and executive functioning are predominant in depression (Basso & Bornstein, 1999 ; Veiel, 1997). These impairments can persist even after remission from psychopathological symptoms (Marcos et al. 1994 ; Neu et al. 2001 ; Reppermund et al. 2007). The results of our accurately conducted study additionally link the AA genotype to higher ACTH and cortisol responses in the Dex/CRH test and to a worse cognitive performance in MDD patients. Therefore, SLC6A15 seems to be a promising candidate gene regarding HPA dysfunction and cognitive performance as it is mainly expressed in the hippocampus. Our findings strengthen the validity of the results of the study conducted by Kohli et al. (2011), who found the AA genotype associated with depressive symptomatology and decreased hippocampal volumes in humans as well as a reduced mRNA expression of this gene in the hippocampus in human risk allele carriers and stress-susceptible mice. A reduced hippocampal volume has previously been found to be associated with enhanced cortisol secretion (Knoops et al. 2010) as well as with impaired memory performance (Kirschbaum et al. 1996). It could be assumed that the alterations in hippocampal volume and neuronal integrity of the risk allele carriers form the basis of the dysregulation of the HPA axis and cognitive impairment in MDD. These highly interesting effects were not found in the healthy control group. However, the analyses in the healthy control group were clearly underpowered due to the smaller sample size as compared to the MDD patient sample. This notion was supported by post-hoc analyses combining the MDD patients and the healthy control group to increase power. All analyses (data not shown) indicated comparable effects as reported in the MDD patients alone, thus further corroborating our main findings.
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Conclusions In conclusion, our findings reveal a role of the SLC6A15 variant rs1545843 on the regulation of the HPA axis and on memory and attention performance in depressed in-patients. Further research is needed to replicate this finding.
Acknowledgements This study was funded by the German Federal Ministry for Education and Research BMBF (grants 01GV0907 and NGFN+ MooDs PNM-01GS08146-3) and by the 7th framework programme of the European Union (ADAMS project, HEALTH-F4-2009242257).
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