Lasky-Su and colleagues also reported P values for a selection of predefined (classic) ADHD candidate genes (Brookes et al. 2006). Most significant findings with P values in the range of 10−5 were observed for SLC9A9. This gene (which was with a total of 181 SNPs the largest gene analyzed) also contained the largest number of associations in terms of SNPs and phenotypes. Other findings at P values of 10−4 were for DDC and SNAP25. The SLC6A1, ADRB2, HTR1E, ADRA1A, DBH, BDNF, DRD2, TPH2, HTR2A, SLC6A2, PER1, CHRNA4, COMT and SYT1 genes showed association at 10−3. SLC9A9 encodes a sodium/hydrogen exchanger and may be a particularly viable candidate gene for ADHD. It was originally reported disrupted in a patient with ADHD (de Silva et al. 2003) and also shows mutations in patients with autism (Morrow et al. 2008) as well as having been found in multiple GWAS for addiction-related disorders (Uhl et al. 2008a, b).
