Differential effects of DAAO on regional activation and functional connectivity in schizophrenia, bipolar disorder and controls

Abstract

Recent studies have identified DAAO as a probable susceptibility gene for schizophrenia and bipolar disorder. However, little is known about how this gene affects brain function to increase vulnerability to these disorders. We examined the impact of DAAO genotype (rs3918346) on brain function in patients with schizophrenia, patients with bipolar I disorder and healthy controls. We tested the hypothesis that a variation in DAAO genotype would be associated with altered prefrontal function and altered functional connectivity in schizophrenia and bipolar disorder. We used functional magnetic resonance imaging to measure brain responses during a verbal fluency task in a total of 121 subjects comprising 40 patients with schizophrenia, 33 patients with bipolar disorder and 48 healthy volunteers. We then used statistical parametric mapping (SPM) and psycho-physiological interaction (PPI) analyses to estimate the main effects of diagnostic group, the main effect of genotype, and their interaction on brain activation and on functional connectivity. Inferences were made at p < 0.05, after correction for multiple comparisons across the whole brain. In the schizophrenia group relative to the control group, patients with one or two copies of the T allele showed lower deactivation in the left precuneus and greater activation in the right posterior cingulate gyrus than patients with two copies of the C allele. This diagnosis × genotype interaction was associated with differences in the functional connectivity of these two regions with other cortical and subcortical areas. In contrast, there were no significant effects of diagnosis or of genotype in comparisons involving bipolar patients. Our results suggest that genetic variation in DAAO has a significant impact on both regional activation and functional connectivity, and provide evidence for a diagnosis-dependent pattern of gene action.

Introduction

Schizophrenia and bipolar disorder are severe psychiatric disorders with a significant genetic component (Owen et al., 2007, Craddock et al., 2006, Berrettini, 2003, Cardno et al., 2002). The identification of susceptibility genes has been problematic because of phenotypic complexity and a mode of transmission compatible with multiple loci. However, association studies have provided converging evidence in favour of a number of positional genes mapped to both schizophrenia and bipolar disorder linkage regions (Park et al., 2004, Lewis et al., 2003, Segurado et al., 2003); one such candidate is the d-Amino Acid Oxidase (DAAO) gene (Prata et al., 2008a, Prata et al., 2008b).

The DAAO gene is located at 12q23, a region linked to schizophrenia and bipolar disorder (Sklar, 2002). It was originally associated with schizophrenia by Chumakov et al. (2002) using Canadian samples. Further evidence of association comes from several replication studies (Schumacher et al., 2004, Liu et al., 2004, Kapoor et al., 2006, Corvin et al., 2007, Madeira et al., 2008, Bass et al., 2009). Recently, two genetic studies (Prata et al., 2008a, Prata et al., 2008b, Fallin et al., 2005) have also implicated the DAAO gene in bipolar disorder. First, Fallin et al. reported a significant association of this gene with bipolar I disorder but not with schizophrenia (Fallin et al., 2005). Prata et al. subsequently found significant association of a two-SNP risk haplotype for DAAO with bipolar disorder (Prata et al., 2008a, Prata et al., 2008b). Taken collectively, these studies suggest that DAAO may confer biological susceptibility to psychosis across the traditional Kraepelian dichotomy of schizophrenia and bipolar disorder (Craddock et al., 2007).

The DAAO gene encodes the DAAO enzyme that oxidises d-serine, a co-agonist for the NMDA glutamate receptor (Boks et al., 2007). The NMDA receptor is known to play an important role in synaptic plasticity, neurodevelopment and excitotoxicity (Tuominen et al., 2005). It is characterised by two distinct sub-units known as NR1 and NR2; NR1 is a binding site for co-agonists glycine and d-serine while NR2 is the agonist binding site for glutamate. NR1 must be occupied for glutamate to be able to open the channel; this requires the availability of glycine and, to a greater extent, d-serine. Thus, selective degradation of d-serine by the DAAO enzyme results in reduced NMDA neurotransmission. There is growing evidence that NMDA neurotransmission is altered in psychosis (Verrall et al., 2010, Stone et al., 2009, Boks et al., 2007, Hashimoto et al., 2003, Coyle et al., 2003, Tanii et al., 1994, Javitt and Zukin, 1991). For example, d-serine levels are decreased in the cerebrospinal fluid and serum of patients with schizophrenia (Hashimoto et al., 2005) and administration of d-serine may reduce negative, positive and cognitive symptoms in schizophrenia (Heresco-Levy, 2005). It has therefore been proposed that increased activity of the DAAO enzyme may lead to increased degradation of d-serine in schizophrenia, resulting in relative NMDA hypofunction (Boks et al., 2007). Dysfunction of NMDA neurotransmission has also been suggested in bipolar disorder, given the glutamatergic action of mood stabilisers (Li et al., 2002) and increased glutamine/glutamate concentration in the dorsolateral prefrontal cortex and cingulate gyrus in bipolar disorder (Michael et al., 2003, Dager et al., 2004).

While the effects of DAAO at molecular and cellular levels are emerging, little is known about this gene's effects on the brain at macroscopic or systems levels. There have been only two previous studies investigating the association between DAAO genotype and cognitive function (Stefanis et al., 2007, Goldberg et al., 2006). Stefanis et al. (2007) found an effect of DAAO haplotype on spatial working memory in a population of 2243 male military conscripts. In contrast, in a sample of over 600 healthy controls, patients with schizophrenia and their nonpsychotic siblings, Goldberg et al. (2006) found no genotype or genotype × diagnosis interactions for any of the three DAAO SNPs used (MDAAO 5–7, using the nomenclature of Chumakov). To date, there have been no neuroimaging studies that investigated the impact of DAAO genotype on brain function in healthy controls or psychotic patients. Thus, little is known about how this gene affects brain function at a macroscopic or system level. Furthermore, it is unclear whether or not the impact of DAAO on brain function is expressed consistently across different diagnostic categories, given that recent studies on other candidate genes for schizophrenia and bipolar disorder have provided evidence for a disease-dependent pattern of gene action (Mechelli et al., 2008, Prata et al., 2008a, Prata et al., 2008b).

The aim of the present study was therefore to investigate the impact of genetic variation in DAAO (the rs3918346 polymorphism) on brain function in 3 diagnostic groups: healthy volunteers, patients with schizophrenia and patients with bipolar I disorder. We used functional magnetic resonance imaging (fMRI) to measure brain responses during a verbal fluency task that required subjects to generate and articulate words in response to letter cues. This task was chosen for two main reasons: first, it engages a distributed network of fronto-temporal cortical and sub-cortical brain regions that have been implicated in schizophrenia and bipolar disorder (Fu et al., 2005, Fu et al., 2002, Curtis et al., 2001); second, it taps into executive cognitive processes that are compromised in both schizophrenia and bipolar disorder (Daban et al., 2006, Kravariti et al., 2005, Krabbendam et al., 2005, Curtis et al., 2001). A “clustered” image acquisition sequence was used in order to record overt vocal responses in the absence of scanner noise (Fu et al., 2005, Fu et al., 2002) and then model correct and incorrect trials separately in the statistical analysis.

Our first hypothesis was that DAAO genotype would have a measurable impact on activation during the verbal fluency task. While there is evidence for DAAO's action in the prefrontal cortex (Verrall et al., 2010, Hashimoto et al., 2009, Verrall et al., 2007), this gene is thought to be expressed throughout the cortex; we therefore tested for its effects in the whole brain using an appropriate correction for multiple comparisons. Our second hypothesis was that the impact of DAAO on the prefrontal function would vary across the three diagnostic groups as revealed by significant genotype × diagnosis interactions.