Investigating the increased risk of schizophrenia and bipolar disorders in relatives of ADHD probands using colocalization analysis of common genetic variants.

Background: Relatives of ADHD probands are known to be at increased risk of schizophrenia and bipolar disorder, suggesting shared genetic factors. In this study, we aim to identify shared common risk variants (i.e., Single-Nucleotide Polymorphisms, SNPs) between ADHD and schizophrenia, and between ADHD and bipolar disorder.
Methods: With the summary data from three GWAS, one on ADHD (20,183 cases with ADHD and 35,191 controls), another on schizophrenia (76,755 cases with schizophrenia and 243,649 controls) and another on bipolar disorder (41,917 cases with bipolar disorder and 371,549 controls), we used colocalization analysis to identify SNPs shared by ADHD and schizophrenia, and SNPs shared by ADHD and bipolar disorder. Functional genomic analyses were then conducted on these two sets of shared common genetic variants.
Results: We found that three of the 12 SNPs associated with ADHD colocalized with schizophrenia SNPs and one of the 12 SNPs associated with ADHD colocalized with bipolar disorder. Only 0.4% of the SNPs associated with schizophrenia (2 out of 431) and 2.3% of the SNPs associated with bipolar disorder (2 out of 86), colocalized with ADHD SNPs. Some genes mapped to these shared genetic variants (SCN2A and UNC5D) are involved in the development of the nervous system.
Conclusions: Using colocalization analysis, the present study uncovers shared genetic variants associated with ADHD and schizophrenia as well as ADHD and bipolar disorder, and may at least partially explain the increased risk of schizophrenia and bipolar disorder in relatives of ADHD probands.
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