COMT genotype and its role on hippocampal–prefrontal regions in declarative memory

NeuroImage 53 (2010) 978–984

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COMT genotype and its role on hippocampal–prefrontal regions in declarative memory Sören Krach b,⁎,1, Andreas Jansen b,1, Axel Krug a, Valentin Markov c, Markus Thimm c, Abigail J Sheldrick c, Thomas Eggermann d, Klaus Zerres d, Tony Stöcker e, N Jon Shah e, Tilo Kircher a a

Department of Psychiatry and Psychotherapy, Philipps-University Marburg, Rudolf-Bultmann-Str. 8, 35039 Marburg, Germany Department of Psychiatry and Psychotherapy, Section of Brain Imaging, Philipps-University Marburg, Rudolf-Bultmann-Str. 8, 35039 Marburg, Germany Department of Psychiatry and Psychotherapy, RWTH Aachen University, Pauwelsstr. 30, 52074 Aachen, Germany d Institute of Human Genetics, RWTH Aachen University, Pauwelsstr. 30, 52074 Aachen, Germany e Institute of Neuroscience and Biophysics 3-Medicine, Research Center Jülich, Germany b c

a r t i c l e

i n f o

Article history: Received 4 August 2009 Revised 3 December 2009 Accepted 22 December 2009 Available online 11 January 2010 Keywords: COMT fMRI Schizophrenia Memory encoding Memory retrieval Hippocampus Prefrontal cortex

a b s t r a c t Introduction: Memory dysfunction is a prominent feature in schizophrenia. Impairments of declarative memory have been consistently linked to alterations especially within hippocampal–prefrontal regions. Due to the high heritability of schizophrenia, susceptibility genes and their modulatory impact on the neural correlates on memory are of major relevance. In the present study the influence of the COMT val158met status on the neural correlates of declarative memory was investigated in healthy subjects. Methods: From an initial behavioural sample of 522 healthy individuals (Sheldrick et al., 2008), 84 subjects underwent fMRI scanning while performing a memory encoding and a retrieval task. The COMT val158met status was determined for the whole sample and correlated with cortical activation within the group of n = 84 individuals. Results: There were no effects of COMT status on behavioural performance. For declarative memory processing the number of met alleles predicted circumscribed bilateral insula and anterior hippocampus activations during memory encoding as well as less deactivations within the bilateral posterior parahippocampal gyri during memory retrieval. Discussion: Although declarative memory performance was unaffected, the neural correlates within hippocampal–prefrontal regions demonstrate a link between COMT val158met carrier status and brain areas associated with declarative memory processing. The study contributes to a better understanding of the role that susceptibility genes might play in the aetiology of schizophrenia. © 2010 Elsevier Inc. All rights reserved.

Introduction Memory impairments are one of the key deficits in schizophrenia (Dickinson et al., 2007). Meta-analyses show that about two thirds of the patients perform below the median of aggregated patient/control samples (Heinrichs and Zakzanis 1998). These impairments are already present in the prodromal state of schizophrenia (e.g. Simon et al., 2007) as well as in relatives of patients (Barrantes-Vidal et al., 2007; Dickinson et al., 2007; Ma et al., 2007; Myles-Worsley et al., 2007), c.f. review in (Brewer et al., 2006). Especially, in tasks involving the long-term maintenance and retrieval of verbal or figural material patients with schizophrenia have significant impairments (Tracy et al., 2001). Lesion and more recently functional neuroimaging studies have consistently demonstrated the hippocampal formation (i.e. parahip⁎ Corresponding author. Fax: +49 6421 5868939. E-mail address: [email protected] (S. Krach). 1 Both authors contributed equally. 1053-8119/$ – see front matter © 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.neuroimage.2009.12.090

pocampal gyrus, subiculum, hippocampus proper) as well as the prefrontal cortex (PFC) as the main cortical sites that orchestrate memory processes (Brewer et al., 1998; Cabeza and Nyberg 2000; Callicott et al., 1999; Fernandez and Tendolkar 2001; Kircher et al., 2008; Nystrom et al., 2000; Wagner et al., 1998). The hippocampus proper is the central area implicated in the formation and retrieval of item–item associations (Henke et al., 1997) and contextual bindings (Boyer et al., 2007), while the PFC supports effortful associative processing strategies by suppressing irrelevant information (Cabeza and Nyberg 2000; Fernandez and Tendolkar 2001). Hence, hippocampal as well as prefrontal regions implement processes that support declarative (episodic/semantic encoding and retrieval) memory performance (Cabeza and Nyberg 2000; Callicott et al., 1999; Nystrom et al., 2000). Meta-analytical findings suggest that patients with schizophrenia exhibit profound hypoactivations of the bilateral prefrontal cortices and the hippocampus proper during memory encoding and retrieval tasks (Achim and Lepage 2005; Keshavan et al., 2002; Kircher et al., 2005; Leube et al., 2001; Leube et al., 2003a;b; Perlstein et al., 2003) and thus

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provide valid demonstrations of ineffective processes in patients with schizophrenia when performing such memory tasks (Schneider et al., 2007). Relatives of patients display the same deficits, albeit to a lower degree (e.g. Snitz et al., 2006). Findings on a genetic influence of cognitive performance measures (Dickinson et al., 2007) and personality profiles (Jang et al., 1996) support this reasoning. A number of susceptibility genes for schizophrenia have recently been identified, among others Dysbindin (Schwab et al., 2003; Straub et al., 2002; Van Den Bogaert et al., 2003), RGS4 (Chen et al., 2004b; Morris et al., 2004; Williams et al., 2004), DISC1 and DISC2 (Hennah et al., 2003, 2005; Hodgkinson et al., 2004; Porteous et al., 2006), NRG1 (Li et al., 2006; Stefansson et al., 2002; Zhao et al., 2004) and the functional polymorphism of the human catechol-O-methyltransferase gene (COMT, located at 22q11.1-q11.2) (see also Allen et al., 2008). The polymorphism of the COMT gene (val158met) results in a substitution of methionine to valine. Due to the higher enzymatic activity in the synaptic cleft caused by the val allele, lower levels of extracellular dopamine are found, while the opposite holds true for the met allele (Chen et al., 2004a; Weinberger et al., 2001). COMT val158met genotype and, accordingly dopamine, are crucial factors in the regulation of hippocampal (Matsumoto et al., 2003) and prefrontal cortex (Gallinat et al., 2003; Palmatier et al., 1999) mediated cognition. Recent studies emphasize their role on memory encoding and retrieval (Bertolino et al., 2006a,b; de Frias et al., 2005; Schott et al., 2006), the processing of faces (Drabant et al., 2006), sentence completion (McIntosh et al., 2007), auditory odd-ball paradigms (Demiralp et al., 2007; Gallinat et al., 2003) and executive functions (Malhotra et al., 2002; Rosa et al., 2004). Furthermore, COMT val158met genotype status is associated with hippocampal volume reduction in homozygous val carriers, while the prefrontal gray matter volume is regarded as being greater among homozygous COMT schizophrenia risk gene carriers as opposed to homozygous met allele carriers (Cerasa et al., 2008). Taken together, the number of val alleles (heterozygous carriers have intermediate levels of enzymatic activity) is associated with poorer cognitive functioning (among others executive functions, memory, attention) in patients with schizophrenia (Ehlis et al., 2007; Minzenberg et al., 2006) and moreover predicts variation in gray matter volume in healthy individuals carrying the functional polymorphism of the human COMT gene (Aguilera et al., 2008). While recent studies mainly focused on the effect of COMT val158met genotype on neural correlates of working memory (Bertolino et al., 2006a,b; McIntosh et al., 2007; Meyer-Lindenberg et al., 2005, 2006), fewer studies addressed declarative memory processing and its complex modulation and heterogeneity within hippocampal–prefrontal regions (Bertolino et al., 2006a,b). In the present study the effect of the COMT val158met genotype on performance measures and their neural correlates was investigated in a memory encoding as well as memory retrieval paradigm. Healthy individuals were studied to establish results not biased due to psychopathology usually present in patients with schizophrenia. Based on previous findings that the COMT val158met genotype impacts on hippocampal–prefrontal regions via decreased/increased dopamine expression, it was hypothesized that the number of met alleles would predict hippocampal–prefrontal dysfunction. Methods Subjects Study sample The subjects (N = 84, 57 men, 27 women) of the present study are a sub-group of participants taking part in a prior behavioural study (N = 522) (Sheldrick et al., 2008). Participants had a mean age of 23.3 years (SD = 3.0), were all right handed (as tested with the Edinburgh Laterality Scale (Oldfield 1971)) and had 15.7 (2.6) years of

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Table 1 Sociodemographic variables and behavioural data (standard deviations in parentheses). Met/Met

Val/Met

Val/Val

F/t

p

fMRI sample (n = 84) Sex ratio (men/women) Age Education (years) Estimated IQ

n = 32 24/8 23.8 (3.6) 16.0 (3.0) 110.8 (13.0)

n = 28 16/12 22.4 (2.3) 15.1 (1.7) 114.4 (10.7)

n = 24 17/7 23.7 (2.6) 16.0 (2.6) 109.7 (12.9)

1.802 1.965 1.192 1.358

n.s. n.s. n.s. n.s.

Performance fMRI task Correctly retrieved faces

24.0 (3.2)

24.3 (3.4)

23.5 (2.6)

0.414

n.s.

F values are given for age, education and IQ, t values are given for corresponding βweights of linear regression analyses of spatial span and letter–number-span. IQ was estimated using the MWT-B (Lehrl et al. 1995). n.s. = non significant (all p N 0.05).

education. Selection for the participation of the fMRI scanning was random and based on availability as well as scanner eligibility. Written informed consent was obtained from all subjects after thorough description of the study. The study protocol was approved by the local ethics committee. The characteristics of this sample are given in Table 1. Genetic analysis Genomic DNA was isolated from peripheral lymphocytes by a simple salting-out procedure. COMT val158met genotypes were determined by restriction fragment length polymorphism as previously described (Malhotra et al., 2002). A 109-base-pair polymerase chain reaction (PCR) product was generated in 40 cycles with an annealing temperature of 50 °C by using primers Comt1 nt 1881 5′-CTCATCACCATCGAGAGATCAA-3′ and Comt2 nt 1989 5′-CCAGGTCTGACAACGGGTCA-3′22. The val158 and met158 alleles were discriminated by digesting the PCR product with NlaIII at 37 °C for 4 hours, followed by a native 10% polyacrylamide gel electrophoresis. The val158 homozygotes (86 and 23 base pairs), met158 homozygotes (68 and 18 base pairs) and val158met heterozygotes (86, 68, 23 and 18 base pairs) were visualized by silver staining. FMRI tasks Memory encoding During the memory encoding task, either single pictures of neutral faces or the symbol “#” were presented on a black background for 4000 ms in a pseudo-randomized order using Presentation software package (Neurobehavioral Systems Inc., San Francisco, CA). Following this, stimuli were replaced by a blank screen for another 1000 ms completing one trial. Participants were instructed to actively memorize each face for later retrieval. In order to ensure continuous attention to the task, participants were asked to indicate the sex of the displayed person via button pressing (LUMItouch™ Lightwave Technologies, Richmond, B.C., Canada). During low-level baseline participants were enforced to press a button with the left index finger every time the symbol “#” appeared. There were five blocks of each condition with six responses in each block resulting in 30 faces to be encoded. Similarly, during baseline 30 button presses had to be accomplished. Each block lasted for 30 s. This task has been successfully applied in different previous studies (Kircher et al., 2005, 2008; Leube et al., 2001; Leube et al., 2003a,b). As there were only single slips during the sex discrimination task, no trials were excluded from the later analyses. Memory retrieval After a pause of 3–4 min a retrieval phase of equal length and structure to the encoding phase was administered. Here, pictures of previously encoded faces were randomly paired with new faces

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presented next to each other on the screen. At task, subjects had to decide which of the two faces resembled the previously memorized face and indicate their decision via button pressing (LUMItouch™ Lightwave Technologies, Richmond, B.C., Canada). The low-level baseline task was equal to the one applied during the encoding phase. Both task designs, face encoding and retrieval, resembled to the one used in a previous study (Leube et al., 2001). Data acquisition Imaging was performed on a 3-Tesla Tim Trio MR scanner (Siemens Medical Systems) in the Institute of Neuroscience and Biophysics-Medicine, Research Center Jülich. Functional images were collected with echo planar imaging (EPI) sensitive to BOLD contrast (T2⁎, 64x64 matrix, FoV 200 mm × 200 mm, 36 slices, 3 mm thickness, TR = 2.25 s, TE = 30 ms, flipangle = 90). Slices covered the whole brain and were positioned transaxially parallel to the anterior– posterior commissural line (AC–PC). The face encoding/retrieval task comprised 137 functional images. The initial three images were excluded from further analysis in order to remove the influence of T1 stabilization effects. Data analyses General data analyses Since there were sufficient data in all three groups, statistical analyses of genotype effects were performed in accordance with a codominant model (regression analysis), thus checking if the number of val/met alleles influences brain activation (either an increasing or decreasing effect). fMRI data analysis fMRI data analyses were calculated using SPM5 (www.fil.ion.uc.ac. uk/spm). After realignment and stereotaxic normalization (2 mm × 2 mm × 2 mm), a 6 mm full-width-at-half-maximum (FWHM) Gaussian smoothing kernel was applied to increase the signal-to-noise ratio and to compensate for inter-subject anatomical variation. Head movement was checked for each subject. All subjects had tolerable head movement smaller than one voxel size. Statistical analyses were performed based on a two-level approach. At the single-subject level, BOLD responses for memory encoding (encoding vs. low-level baseline) and memory retrieval (retrieval vs. low-level baseline) were modeled by a boxcar function convolved with the canonical hemodynamic response function employed by SPM5. Parameter estimate (β-) and tstatistic images were calculated for each subject. At the group level, the individual β-contrasts relating to activation differences between the experimental task and the low-level baseline

condition were separately analyzed based on the assumption of a gene–dose effect and thus were entered into a regression analysis using COMT val158met genotype as a covariate. Multiple regressions were calculated both ways in order to detect hypo-/ hyperactivations relative to baseline depending on the number of val or met alleles, respectively. In order to correct for multiple comparisons within a search volume we applied a cluster extent threshold determined by Monte Carlo simulations (Slotnick et al., 2003). For a threshold at the voxel level at p = 0.001 and spatial properties as present in this study, 10,000 simulations resulted in an extent threshold of 26 resampled voxels. This procedure prevented a false-positive rate above 5% due to multiple testing. Brain activations were plotted on the anatomical SPM template (Table 2). Results Behavioural data Within the fMRI sub-sample which was derived from a former study (Sheldrick et al., 2008) there was a levelled distribution of COMT val158met genotypes. We selected the subjects in order to have an equal amount of subjects within each single cell (this is the reason why the distribution is not in HWE). 32 subjects carried homozygous met alleles (24 male), 28 subjects heterozygous val/ met alleles (16 male) and 24 subjects the homozygous val alleles (17 male). The resulting groups and variables are displayed in Table 1. Analyses of behavioural data of the fMRI task in the 84 subjects revealed no significant differences in task performance between groups (see Table 1). Neuroimaging data Memory encoding Whole-group analyses for memory encoding depict the usually observed pattern of occipital, hippocampal, motor (which is evoked by the contrast of left/right pressing of the response buttons during gender differentiation tasks in comparison to only left-manual button pressing during the low-level baseline) and prefrontal activation (see Fig. 1). Regarding the gene–dose effect, significant correlations of the cerebellum bilaterally, the right anterior hippocampus (BA 35; x = 24, y = −22, z = −17) as well as the insula bilaterally (BA 13; × = 30/ –38, y = 7/17, z = −9/− 9) were found with the number of met alleles (see Fig. 2A). The parameter estimates derived at the local maximal activations demonstrate a clear and linear increase in each of the three cortical regions found to correlate with the COMT status (see Fig. 2B).

Table 2 Correlations of COMT val158met status with neural activations during memory encoding and retrieval processes. Only clusters of at least 26 voxels (see Monte Carlo simulation) are depicted. BA

Coordinates x

y

Memory encoding task [Correlations with an increase in the number of met alleles (val/valbval/metbmet/met)] R Cerebellum 18 − 36 R Insula Inferior frontal gyrus 30 7 R Anterior Hippocampus 35 24 − 22 L Cerebellum − 12 − 48 L Insula Inferior frontal gyrus − 38 17 Memory retrieval task [Correlations with an increase in the number of met alleles (val/valbval/metbmet/met)] L Posterior parahippocampal gyrus 36 − 30 − 36 R Posterior parahippocampal gyrus 36 32 − 33

t-value

Cluster size (in voxels)

− 30

4.65

167

−9 − 17 − 19

4.10 3.88 3.84

55 27 61

−9

3.77

46

− 12 −8

4.47 4.05

49 31

z

Coordinates are listed in Talairach and Tournoux (1988) atlas space. BA is the Brodmann area nearest to the coordinate and should be considered approximate.

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Fig. 1. Whole-group (N = 84) brain activations during memory encoding/retrieval (relative to baseline).

There was no region specific BOLD activation increase correlated with the number of val alleles. Memory retrieval Again, whole-group analyses revealed the typically observed pattern of memory retrieval tasks encompassing visual, hippocampal as well as bilateral dorso-lateral prefrontal cortical (DLPFC) activations (see Fig. 1). A significant gene–dose effect (correlation with the number of met alleles) was found in the posterior parahippocampal gyri bilaterally (BA 36; x = 32/−30, y = −33/−36, z = −8/−12) (see Fig. 3A). Notably, no further cortical structures were associated with the increase in the number of met alleles during face retrieval. The parameter estimates derived at the local maximal activations document that the association between the neural activation pattern and COMT status was based on stronger bilateral posterior parahippocampal gyri decreases in the homozygous val allele carriers as opposed to the intermediate group and the homozygous met allele carriers (see Fig. 3B). As demonstrated in the whole-group activation/ deactivation map, the hippocampus proper clearly is activated during task (retrieval N baseline), while the parahippocampal gyri are at the same time deactivated (baseline N retrieval) (see Fig. 4). Similar to the memory encoding task there was no regional specific BOLD activation increase with the number of val alleles when compared to baseline. Discussion In the present study, the behavioural effects and neural correlates of the COMT (val158met) polymorphism on memory encoding and memory retrieval were investigated in a large sample of healthy subjects. While on a behavioral level declarative memory performance was unaffected by the COMT val158met polymorphism, genotype differences associated with the neural activation pattern during memory encoding and memory retrieval were (among cerebellar activity) exclusively linked to (para)-hippocampal– prefrontal regions. These regions play a central role in memory processing (Brewer et al., 1998; Cabeza and Nyberg 2000; Callicott et al., 1999; Fernandez and Tendolkar 2001; Kircher et al., 2008; Nystrom et al., 2000; Wagner et al., 1998) and are considered to be profoundly affected in schizophrenia (Achim and Lepage 2005; Keshavan et al., 2002; Kircher et al., 2005, 2008; Leube et al., 2001, 2003a,b; Perlstein et al., 2003).

Regarding memory encoding processes previous findings of profound hypoactivations in hippocampal–prefrontal regions in COMT val158met polymorphism carriers could be corroborated in the present study (Achim and Lepage 2005). Bilateral and almost mirrorimage like insula hypoactivations extending into the inferior frontal lobe were detected in healthy subjects carrying the homozygous val alleles. The inferior prefrontal cortex (incorporating the insula) is considered the main cortical region that distinguished schizophrenia patients from healthy controls in a recent meta-analysis (Achim and Lepage 2005) and is implicated in strategy use and depth of processing during episodic memory encoding (Kapur et al., 1994; Petersson et al., 2003; Simons and Spiers 2003). Thus, already in healthy participants at genetic risk for schizophrenia the very same structures are less activated the more val alleles the subjects carried. In healthy participants another structure, the hippocampus proper, is considered a key area in both, memory encoding and retrieval (Lepage et al., 1998; Schacter and Wagner 1999). In the present study, the hippocampus was clearly activated (during encoding as well as retrieval) in the whole group. However, during encoding the significant gene–dose effect (val/val b val/met b met/ met) within the right anterior hippocampus reveals an underlying processing deficit in the homozygous val allele carriers which, however, does not yet lead to behavioral consequences in our very simple task. Generally, the hippocampus accounts for the formation of meaningful associations (Davachi and Wagner 2002; Henke et al., 1997) and contextual binding (Boyer et al., 2007) during encoding. Activation in this region has been shown to predict subsequent retrieval success (Brewer et al., 1998; Davachi et al., 2003; Wagner et al., 1998). The meta-analysis of Achim and Lepage (2005) revealed a significant agglomeration of foci of reduced activation in patients with schizophrenia in the right hippocampus (though located slightly more posterior than in our study). Thus, the activation pattern of the present study on healthy subjects perfectly resembles these metaanalytical findings and suggests that even at the level of a genetic predisposition for schizophrenia (i.e. homozygous val allele carriers) less efficient and less associative encoding strategies are applied (Achim and Lepage 2005). Unfortunately, for the present encoding paradigm there were no possibilities to directly assess behavioural performance measures, as e.g. by means of the subsequent memory effect (Fernandez and Tendolkar 2001; Kircher et al., 2008). It can, however, be speculated that if task demands were more challenging these subtle processing deficits might even lead to consequences on

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Fig. 3. (A) Correlations of COMT val158met status (val/val b val/met b met/met) with the bilateral posterior parahippocampal gyri during memory retrieval (p b 0.001, corrected by Monte Carlo simulations; cluster extend = 26 voxels). (B) Parameter estimates extracted from the left/right posterior parahippocampal gyri document less decreased cortical activation with the number of met alleles during memory retrieval.

Fig. 2. (A) Correlations of COMT val158met status (val/val b val/met b met/met) with the right anterior hippocampus (A) and bilateral insula (B + C) during memory encoding (p b 0.001, corrected by Monte Carlo simulations; cluster extend = 26 voxels). (B) Parameter estimates extracted from the left/right insula and the right anterior hippocampus during memory encoding document increased activation with the number of met alleles.

behavioural measures in healthy subjects carrying the homozygous val alleles. Finally, during active retrieval the hippocampus proper was activated, while the parahippocampal gyri were deactivated. This finding is supported by earlier studies and implicates that the parahippocampal gyri are part of a “default mode network” of the

Fig. 4. Whole-group retrieval activations (red) and deactivations (blue) superimposed onto the standard canonical SPM template (in this example the left hemisphere is depicted). The white circle indicates the local maxima within the posterior parahippocampal gyrus used for correlation with the number of met alleles.

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brain (Fox et al., 2005; Fransson 2005; Greicius et al., 2003), thus showing increased firing rates during ongoing internally generated processes that occur during any baseline, but are disrupted when an exogenous task is imposed on the subject (Mechelli et al., 2008). Interestingly, activity measures within the bilateral posterior parahippocampal gyri were correlated with the number of met alleles in the healthy sample of the present study. Having a closer look at the parameter estimates derived at the left and right posterior parahippocampal gyri we could demonstrate that the gene–dose effect is based on significantly stronger deactivations the more val alleles the participants carried. It can be speculated that in homozygous val allele carriers the increased withdrawal of posterior parahippocampal gyri firing may help to reallocate cognitive resources at task in order to accommodate for task-relevant demands that might otherwise manifest themselves in decreased performance (Mechelli et al., 2008). In a recent restingstate fMRI study of Zhou et al. (2008) the parahippocampal gyri (the local maxima in their study resembled the coordinates within the parahippocampal gyri of the present study) showed reduced functional connectivities to the left and right hippocampus within a sample of schizophrenic patients. They speculate that the reduced connectivity between the anterior hippocampus and, among others, the parahippocampal gyri may reflect a breakdown of episodic memory processes in schizophrenic patients centred within the hippocampus proper (Zhou et al., 2008). The present findings now suggest that the stronger posterior parahippocampal gyri deactivations observed in homozygous val allele carriers may point to a vulnerability in episodic memory retrieval that resembles the neural pattern of schizophrenia patients, though on a behavioural performance level no breakdown is detectable. Further, the presents study stresses the role of the parahippocampal gyri, as opposed to the usually more prominent role of the hippocampus proper, in episodic memory processing. However, it is possible that the reallocations of cognitive resources from the parahippocampal gyri in the val allele carriers might have helped to equalize the reduced functioning of the hippocampus proper. This speculation might in part be supported by the finding of hippocampal (not parahippocampal) volume reductions in homozygous val allele carriers that, however, did not lead to apparent differences in the neural pattern within this region (Cerasa et al., 2008). In summary, COMT val158met status did not affect task performance, however, there was a strong and significant linear effect of COMT val158met status on hippocampal–prefrontal regions which are typically associated with a declarative memory task. Notably, the present findings are based on whole-brain analyses documenting that the effects are highly specific, while on the other hand being robust to false-positives. The data may point to COMT val158met status influencing hippocampal–prefrontal regions during declarative memory tasks in healthy subjects which may make them more liable to the outbreak of schizophrenia. Furthermore, the study indicates that next to general hypoactivations within prefrontal–hippocampal regions in val allele carriers, the exact interpretation of such results need to be reassessed with respect to the “default mode” of the brain. The study may contribute to a better understanding of the role susceptibility genes play in the pathophysiology of cognitive deficits in schizophrenia. Acknowledgments This work was supported by the Federal Ministry of Education and Research (Brain Imaging Centre West, 01GO0204). References Achim, A.M., Lepage, M., 2005. Episodic memory-related activation in schizophrenia: meta-analysis. Br. J. Psychiatry 187, 500–509.

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