Genome-wide analysis of rare copy number variations reveals PARK2 as a candidate gene for attention-deficit/hyperactivity disorder

Abstract

Attention-deficit/hyperactivity disorder (ADHD) is a common, highly heritable neurodevelopmental disorder. Genetic loci have not yet been identified by genome-wide association studies. Rare copy number variations (CNVs), such as chromosomal deletions or duplications, have been implicated in ADHD and other neurodevelopmental disorders. To identify rare (frequency ≤1%) CNVs that increase the risk of ADHD, we performed a whole-genome CNV analysis based on 489 young ADHD patients and 1285 adult population-based controls and identified one significantly associated CNV region. In tests for a global burden of large (>500 kb) rare CNVs, we observed a nonsignificant (P=0.271) 1.126-fold enriched rate of subjects carrying at least one such CNV in the group of ADHD cases. Locus-specific tests of association were used to assess if there were more rare CNVs in cases compared with controls. Detected CNVs, which were significantly enriched in the ADHD group, were validated by quantitative (q)PCR. Findings were replicated in an independent sample of 386 young patients with ADHD and 781 young population-based healthy controls. We identified rare CNVs within the parkinson protein 2 gene (PARK2) with a significantly higher prevalence in ADHD patients than in controls (P=2.8 × 10(-4) after empirical correction for genome-wide testing). In total, the PARK2 locus (chr 6: 162 659 756-162 767 019) harboured three deletions and nine duplications in the ADHD patients and two deletions and two duplications in the controls. By qPCR analysis, we validated 11 of the 12 CNVs in ADHD patients (P=1.2 × 10(-3) after empirical correction for genome-wide testing). In the replication sample, CNVs at the PARK2 locus were found in four additional ADHD patients and one additional control (P=4.3 × 10(-2)). Our results suggest that copy number variants at the PARK2 locus contribute to the genetic susceptibility of ADHD. Mutations and CNVs in PARK2 are known to be associated with Parkinson disease.

Figure 1

Results for the PARK2 locus in the GWAS and in the replication sample. Each panel consists of four parts (called CNVs, PARK2 gene, probes analysed and association tests): CNVs: red (pink) bars represent duplications in an ADHD case (control), blue (lightblue) bars indicate a deletion of an ADHD case (control). PARK2 gene: the marks indicate the coding regions (NCBI36/hg18). Association tests: permutation-based one-sided −log₁₀-transformed P-values for association tests; the black (pink; lightblue) line represents association tests for an increased frequency of segmental CNV data independent of type (deletions; duplications) in cases compared with controls. The significance level P=0.05 is highlighted as a dashed red line. The chromosomal region offering genome-wide significantly more CNVs in ADHD patients than in controls is highlighted by grey vertical shading. (a) Results for the GWAS sample. The presented P-values are genome-wide empirically corrected. The chromosomal region covered by the qPCR assay used for validation of PennCNV's CNV calls is shown as a darkgrey vertical dashed line within this region of genome-wide significance. The duplication that could not be validated by qPCR analysis is marked by ‘x'. Results of association tests after exclusion of the non-validated case duplication are shown in Supplementary Figure S7. (b) Results for the replication sample. The presented P-values are pointwise corrected.