Background: Previous research found an association between SNPs in the promoter region of DRD4 and statistically-derived phenotypes generated from ADHD symptoms. We sought to replicate this finding by using the same methodology in an independent sample of ADHD individuals.

Methods: Four SNPs were genotyped in and around DRD4 in 2,631 individuals in 642 families. We developed a quantitative phenotype at each SNP by weighting nine inattentive and nine hyperactive-impulsive symptoms. The weights were selected to maximize the heritability at each SNP. Once a quantitative phenotype was generated at each SNP, the screening procedure implemented in PBAT was used to select and test the five SNPs / genetic model combinations with the greatest power to detect an association within each candidate gene.

Results: One of the four SNPs was associated with the quantitative phenotypes generated from the ADHD symptoms (corrected p-values = 0.02). A rank ordering of the correlation between each of the ADHD symptoms and the quantitative phenotype suggested that hyperactive-impulsive symptoms were more strongly correlated with the phenotype; however, including inattentive symptoms was necessary to achieve a significant result.

Conclusions: This study partially replicated a previous finding by identifying an association between rs7124601 and a quantitative trait generated from ADHD symptoms. Rs7124601 is LD with the SNPs identified previously. In contrast to the previous study, this finding suggests that both hyperactive-impulsive and inattentive symptoms are important in the association.