Elsevier

NeuroImage

Volume 94, 1 July 2014, Pages 147-154
NeuroImage

Replication of brain function effects of a genome-wide supported psychiatric risk variant in the CACNA1C gene and new multi-locus effects

https://doi.org/10.1016/j.neuroimage.2014.03.007Get rights and content

Abstract

Variation in the CACNA1C gene has consistently been associated with psychosis in genome wide association studies. We have previously shown in a sample of n = 110 healthy subjects that carriers of the CACNA1C rs1006737 risk variant exhibit hippocampal and perigenual anterior cingulate dysfunction (pgACC) during episodic memory recall. Here, we aimed to replicate our results, by testing for the effects of the rs1006737 risk variant in a new large cohort of healthy controls. We furthermore sought to refine these results by identifying the impact of a CACNA1C specific, gene-wide risk score in the absence of clinical pathology.

An independent sample of 179 healthy subjects genotyped for rs1006737 underwent functional magnetic resonance imaging (fMRI) while performing an associative episodic memory task and underwent psychological testing similar to the discovery sample. The effect of gene-wide risk scores was analyzed in the combined sample of 289 subjects.

We replicated our discovery findings of hippocampal and pgACC dysfunction in carriers of the rs1006737 risk variant. Additionally, we observed diminished activation of the dorsolateral prefrontal cortex, in the replication sample. Our replicated results as well as this new effect were also observable in the combined sample. Moreover, the same system-level phenotypes were significantly associated with the individual gene-based genetic risk score.

Our findings suggest that altered hippocampal and frontolimbic function is associated with variants in the CACNA1C gene. Since CACNA1C variants have been associated repeatedly with psychosis at a genome-wide level, and preclinical data provide convergent evidence for the relevance of the CACNA1C gene for hippocampal and frontolimbic plasticity and adaptive regulation of stress, our data suggest a potential pathophysiological mechanism conferred by CACNA1C variants that may mediate risk for symptom dimensions shared among bipolar disorder, major depression, and schizophrenia.

Introduction

Severe psychiatric disorders are both common and disabling, and new treatments are urgently needed to enhance the currently unacceptable rates of morbidity, non-response, and relapse. An essential limiting factor is our lack of understanding of the molecular and system-level pathophysiology. Since the heritability of common psychiatric disorders is high, characterizing mechanisms of genetic risk is a promising route.

Genome-wide association studies (GWAS) have reported strong evidence for an association of bipolar disorder (Craddock et al., 2009, Ferreira et al., 2008, Green et al., 2012, Keers et al., 2009, Lett et al., 2011, Sklar et al., 2008), unipolar depression (Green et al., 2010, Liu et al., 2011, Wray et al., 2012) and schizophrenia (Green et al., 2010, Hamshere et al., 2012, Moskvina et al., 2009, Nyegaard et al., 2010) with polymorphism rs1006737 located within the CACNA1C gene on chromosome 12p13. Further, CACNA1C is one of the most robust findings when combining multiple disease categories (Moskvina et al., 2009, Ripke et al., 2011, Sklar et al., 2011, Smoller et al., 2013, Williams et al., 2011) and combining schizophrenia and bipolar disorder as a single phenotype (Ruderfer et al., in press), implicating that CACNA1C variants belong to a class of shared susceptibility factors across the mood–psychosis spectrum (Bhat et al., 2012, Craddock et al., 2009, Smoller et al., 2013).

CACNA1C encodes the alpha subunit of the l-type voltage-dependent calcium channel Cav1.2 and calcium influx through Cav1.2 channels triggers processes that underlie hippocampus dependent memory (Moosmang et al., 2005, White et al., 2008, Woodside et al., 2004). In particular, mice with a selective inactivation of the CACNA1C gene, lacking Cav1.2 in the hippocampus, show a defect in NMDA-receptor independent long term potentiation (LTP) in the CA1 region of the hippocampus that is paralleled by a severe memory deficit (Moosmang et al., 2005). Notably, clinical studies identified structural and functional abnormalities within the hippocampal formation as an important intermediate phenotype (Gottesman and Gould, 2003, Meyer-Lindenberg and Weinberger, 2006) for schizophrenia (Harrison and Eastwood, 2001, Heckers and Konradi, 2010, Tamminga et al., 2010), as well as major mood disorders (Frey et al., 2007, Hasler et al., 2006, MacQueen and Frodl, 2011, Savitz and Drevets, 2009). Importantly, Cav1.2 is an established drug target because it is the binding site for calcium channel blockers such as verapamil, which has shown some evidence of efficacy in mood stabilization in bipolar and schizoaffective psychosis (Keers et al., 2009, Levy and Janicak, 2000).

Consistent with these data, we recently showed that healthy carriers of the CACNA1C rs1006737 risk variant exhibit a profound reduction of bilateral hippocampal activation during episodic memory recall accompanied by reduced activation of the subgenual part of the pgACC (Erk et al., 2010), one of the key regions for the mediation of stress-related responses (Ongur and Price, 2000, Phillips et al., 2008) that forms a convergence site for genetic and environmental risk factors (Meyer-Lindenberg and Tost, 2012). During memory processing the pgACC integrates and controls information from limbic areas by monitoring and selecting retrieval information for task appropriate responses and by inhibiting inappropriate information (Nieuwenhuis and Takashima, 2011).

We recently reported a similar pattern of hippocampal and pgACC dysfunction in first-degree relatives of patients with bipolar disorder, major depression and schizophrenia thus implicating abnormal perigenual and hippocampal activation as a promising intermediate phenotype for psychosis (Erk et al., in press).

Since replication is of crucial importance for imaging genetics studies, and far too few results have actually been replicated (Abbott, 2008), we here aimed to validate our previous findings in healthy controls in an independent cohort. The accumulation of many common and less common genetic variants is known to form a substantial proportion of genetic liability to psychiatric disease (Gottesman and Shields, 1967). Recent genetic studies have estimated this proportion at about a third (Dudbridge, 2013, Purcell et al., 2009). To capture this polygenic variation, Purcell et al. proposed the use of the polygenic score, a linear sum across multiple SNPs of risk alleles derived from an independent study, weighted by their effect size in that study. This was found to have a highly significant correlation with schizophrenia risk. Although the estimated proportion of genetic liability to schizophrenia explained by the polygenic score is smaller than the total polygenic contribution, due mainly to imprecision in estimating effect sizes (Dudbridge, 2013), it is still higher than that explained by any single variant. Concerning the particular influence of the CACNA1C gene on risk for psychiatric disease, which is based on evidence from clinical and preclinical data, a gene-based genetic risk score might capture a greater proportion of the shared genetic architecture of both the disease and imaging phenotypes than a single variant. Thus, we applied this approach by generating a gene-wide genetic risk score for the CACNA1C gene based on the available Psychiatric GWAS Consortium (PGC) data and individual genotype data extracted from a genome-wide dataset. We then sought to determine the impact of this score on hippocampal and pgACC activation during episodic memory recall.

Section snippets

Replication sample

A total of 179 German volunteers with grandparents of European ancestry were recruited at Bonn, Mannheim and Berlin and genotyped for rs1006737 (Table 1). No participant reported a lifetime or family history of schizophrenia or affective disorder or drug or alcohol dependence. After complete description of the study to the subjects, written informed consent was obtained. The study was approved by the local ethics committees of the Universities of Bonn, Heidelberg and Berlin. Of our sample 81

Effects of CACNA1C rs1006737 on brain function and behavioral and psychopathology measures in the replication sample

In general, task related brain activation, irrespective of genotype, was similar for the discovery and replication sample, and included the bilateral hippocamus, areas of the prefrontal cortex, anterior and posterior cingulate cortex, ventral striatum, brainstem, and temporal and occipital gyrus (see Supplementary Tables 1 and 2). Consistent with findings in our discovery sample, we observed a significant effect of rs1006737 genotype in the left hippocampus (x =  18, y =  12, z =  24, Z = 3.76, p

Discussion

The present study aimed to replicate previous findings implicating hippocampal and pgACC dysfunction in the neurogenetic action of the CACNA1C rs1006737 risk allele, and to determine the impact of a gene-based genetic risk score for CACNA1C on brain function in healthy subjects.

Since one of the criticisms of imaging genetics is that far too few results have been replicated (Abbott, 2008), and since we believe that CACNA1C variation adds to a highly treatment-relevant mechanism in psychiatric

Conclusion

Our results implicate the CACNA1C gene in the observed alteration of hippocampal, pgACC and DLPFC function, suggesting a dysfunctional regulatory circuit including limbic and prefrontal brain regions. Because variation in rs1006737 has been found to be associated with bipolar disorder (Craddock et al., 2009, Ferreira et al., 2008, Green et al., 2012, Keers et al., 2009, Lett et al., 2011, Sklar et al., 2008), major depression (Green et al., 2010, Liu et al., 2011, Wray et al., 2012) and

Acknowledgments

We thank Dagmar Gass, Alice Meisen, and Josephine Klambt for help with data acquisition, and Maria Garbusow for assistance in data analysis. All authors contributed substantially to the study and critically reviewed and approved the final manuscript.

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    Funding for this study was provided by the German Ministry for Education and Research (BMBF) grant NGFNplus MooDS and by the German Research Foundation (DFG) grant SFB 636-B7. This work was supported by NIH/NHLBI U01 HL089856, RO1 MH087590 and R01 MH081862. M.M.N. is a Member of the DFG funded Excellence Cluster ImmunoSensation and also received support from the Alfried Krupp von Bohlen und Halbach-Stiftung.

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