Additive effect of three noradrenergic genes (ADRA2A, ADRA2C, DBH) on attention-deficit hyperactivity disorder and learning disabilities in Tourette syndrome subjects

Halperin et al. (Halperin JM, Newcorn JH, Koda VH. Pick L, McKay KE, Knott P. Noradrenergic mechanisms in ADHD children with and without reading disabilities: a replication and extension. J Am Acad Child Adolesc Psychiatry 1997. 36. 1688-1696) reported a significant increase in plasma norepinephrine (NE) in attention-deficit hyperactivity disorder (ADHD) children with reading and other cognitive disabilities compared to ADHD children without learning disabilities (LD). We examined the hypothesis that ADHD +/- LD was associated with NE dysfunction at a molecular genetic level by testing for associations and additive effects between polymorphisms at three noradrenergic genes - the adrenergic alpha 2A receptor (ADXA2A), adrenergic alpha 2C receptor (ADRA2C), and dopamine beta-hydroxylase (DBH) genes. A total of 336 subjects consisting of 274 individuals with Tourette syndrome (TS) and 62 normal controls were genotyped. Regression analysis showed a significant correlation between scores for ADHD, a history of LD, and poor grade-school academic performance that was greatest for the additive effect of all three genes. Combined, these three genes accounted for 3.5% of the variance of the ADHD score (p = 0.0005). There was a significant increase in the number of variant NE genes progressing from subjects without ADI-ID (A -) or learning disorders (LD -) to A + LD - to A - LD + to A + LD + (p = 0.0017), but no comparable effect for dopamine genes. These data support an association between NE genes and ADHD, especially in ADHD + LD subjects.