Genome-wide association study identifies genetic variation in neurocan as a susceptibility factor for bipolar disorder

Abstract

We conducted a genome-wide association study (GWAS) and a follow-up study of bipolar disorder (BD), a common neuropsychiatric disorder. In the GWAS, we investigated 499,494 autosomal and 12,484 X-chromosomal SNPs in 682 patients with BD and in 1300 controls. In the first follow-up step, we tested the most significant 48 SNPs in 1729 patients with BD and in 2313 controls. Eight SNPs showed nominally significant association with BD and were introduced to a meta-analysis of the GWAS and the first follow-up samples. Genetic variation in the neurocan gene (NCAN) showed genome-wide significant association with BD in 2411 patients and 3613 controls (rs1064395, p = 3.02 × 10(-8); odds ratio = 1.31). In a second follow-up step, we replicated this finding in independent samples of BD, totaling 6030 patients and 31,749 controls (p = 2.74 × 10(-4); odds ratio = 1.12). The combined analysis of all study samples yielded a p value of 2.14 × 10(-9) (odds ratio = 1.17). Our results provide evidence that rs1064395 is a common risk factor for BD. NCAN encodes neurocan, an extracellular matrix glycoprotein, which is thought to be involved in cell adhesion and migration. We found that expression in mice is localized within cortical and hippocampal areas. These areas are involved in cognition and emotion regulation and have previously been implicated in BD by neuropsychological, neuroimaging, and postmortem studies.

Figure 1

Association Results for the GWAS and the Two Best-Supported Genes from the Follow-Up Study (A) Manhattan plot. (B) Regional association plots (RAPs) displaying NCAN and MAD1L1. The most associated marker from the GWAS (enlarged red diamond) is centered in a genomic window of 1 Mb (hg18, RefSeq genes); its p value from the combined analysis (meta) is shown (enlarged blue diamond). The LD strength (r²) between the sentinel SNP from the GWAS and its flanking markers is demonstrated by the red (high) to white (low) color bar. The recombination rate (cM/Mb; second y axis) is plotted in blue, according to HapMap-CEU. RAPs were generated with SNAP.

Figure 2

Genetic Effect Sizes and Significance Levels for NCAN rs1064395 at All Steps of Analysis Forest plot shows odds ratios (orange diamonds) and their 95% confidence intervals (horizontal lines) of individual study samples. The odds ratio referring to the meta-analysis of all study samples is represented by the enlarged blue diamond. BiH / SRB, Bosnia and Herzegovina / Serbia.

Figure 3

Expression Patterns of Neurocan in Mouse Brain Areas Previously Implicated in BD RNA in situ hybridization of Ncan in sagittal (A, C–E) and coronal (B) sections at E14.5 (A), E15.5 (B), E16.5 (C), and P46 (D and E). In the embryo, Ncan is observed exclusively in the CNS, with high expression in the cortical plate (Cp) and subventricular zone (Sz, arrow in C) of the neocortex and in the ventricular zone of the basal ganglia (Bg, A–C). Ncan was also detected in the hypothalamus (Ht) and amygdala (Am, B). In postnatal mice, Ncan expression in the cortex (Cx) is restricted to layer II (arrow in D). In the hippocampus (E), it was detected at lower intensity in the dentate gyrus (Dg) and CA1. Lv indicates the lateral ventricle.