Genome-wide copy number variation study associates metabotropic glutamate receptor gene networks with attention deficit hyperactivity disorder

Abstract

Attention deficit hyperactivity disorder (ADHD) is a common, heritable neuropsychiatric disorder of unknown etiology. We performed a whole-genome copy number variation (CNV) study on 1,013 cases with ADHD and 4,105 healthy children of European ancestry using 550,000 SNPs. We evaluated statistically significant findings in multiple independent cohorts, with a total of 2,493 cases with ADHD and 9,222 controls of European ancestry, using matched platforms. CNVs affecting metabotropic glutamate receptor genes were enriched across all cohorts (P = 2.1 × 10(-9)). We saw GRM5 (encoding glutamate receptor, metabotropic 5) deletions in ten cases and one control (P = 1.36 × 10(-6)). We saw GRM7 deletions in six cases, and we saw GRM8 deletions in eight cases and no controls. GRM1 was duplicated in eight cases. We experimentally validated the observed variants using quantitative RT-PCR. A gene network analysis showed that genes interacting with the genes in the GRM family are enriched for CNVs in ∼10% of the cases (P = 4.38 × 10(-10)) after correction for occurrence in the controls. We identified rare recurrent CNVs affecting glutamatergic neurotransmission genes that were overrepresented in multiple ADHD cohorts.

Figure 1

A deletion directly affecting GRM5 that is exclusive to cases with ADHD and that was replicated in the IMAGE and PUWMa studies. Four hemizygous deletions in GRM5 in cases with ADHD from the CHOP study that were replicated by two deletions and three larger deletions found in the IMAGE study and one deletion found in the PUWMa study. The SNP coverage of the Illumina 550K, Perlegen 600K, Illumina 1M and Affymetrix 5.0 arrays is shown by vertical blue lines. M.Of.M.Cs., Massachusetts General Hospital offspring male case; W.Fa.M.Cn., Washington University father male control.

Figure 2

GRM receptor gene interaction networks affected in ADHD. (a) GRM receptor genes are shown as large diamond-shaped nodes, and other genes within two degrees of interaction with GRM genes are shown as smaller circular nodes. Nodes are colored to represent the enrichment of the CNVs: dark red represents deletions enriched in cases, light red represents deletions enriched in controls, dark turquoise represents duplications enriched in cases, light turquoise represents duplications enriched in controls, and gray represents diploids that are devoid of CNVs. Thick blue dashed lines highlight edges that are connected to at least one GRM gene, and thin gray lines represent all other gene interactions. Highly connected modules enriched for significant functional annotations are highlighted by blue shaded ellipses. Details on the gene-based CNV observations are included in Supplementary Table 16, and the respective gene functional clusters are listed in Supplementary Table 17. (b) A schematic overview showing the interaction of GRM receptors affected in ADHD with modules of genes enriched for functional significance. GRM receptor genes are shown as diamonds colored either turquoise or red to represent duplications and deletions, respectively, that were enriched in cases. Boxes highlight the functional modules defined by the network of interacting genes (a) that are significantly enriched for Gene Ontology annotations. The functional modules describe significant functional annotations and are labeled with the cluster name and the number of component genes in parenthesis. Functional annotations that may be particularly pertinent to the underlying pathophysiology of ADHD are shown in bold. The edges of the network connect GRM receptor genes to functional modules: solid lines indicate membership of the GRM-interacting gene in the functional module, and dotted lines indicate a first-degree relationship between GRM receptor genes and at least one component gene of a functional module