Genetics of Attention-Deficit Hyperactivity Disorder

The controversial use of Ritalin in the treatment of attention-deficit hyperactivity disorder (ADHD) has brought the disorder into the public’s consciousness. Anyone who has ever dealt with children for longer than a few minutes knows that they all exhibit classic behaviors of ADHD – inability to sit still, concentrate, or control their behavior. The difference is that these behaviors are developmentally appropriate for toddlers and pre-school kids, but not for older children and adults.

It’s estimated that 3-4% of children and 1% of adults are affected by ADHD. In addition to the symptoms I mentioned above, children and adults with ADHD are also at increased risk of psychiatric disorders, including drug and alcohol abuse, antisocial behaviour, anxiety, depression, and general and specific learning difficulties.

ADHD tends to run in families – having a mother, father, or sibling with ADHD increases a person’s risk of ADHD four- to eight-fold. Clustering of ADHD in families suggests that genes are most likely involved. Here are some of the candidate genes that have been idenfied so far:

• dopamine D4 receptor gene


• dopamine transporter gene (DAT1)


• dopamine D5 receptor gene


• synaptosomal-associated protein gene (SNAP-25)


• dopamine {alpha}-hydroxylase gene


• serotonin 1B receptor gene


• serotonin transporter gene


• ß4-nicotinic receptor subunit gene


• noradrenergic transporter gene


• brain-derived neurotrophic factor gene


• chromosome regions 5p13, 6q12, 16p13 and 17p11

It’s not just genes, of course, that contribute to someone’s risk of developing ADHD. Environmental exposures are also important:

• Prematurity
• Low birth weight
• Pregnancy and/or birth complications
• Mother’s use of alcohol or tobacco during pregnancy
• Early and severe parental neglect
• Receving adequate emotional care and warmth
• Inconsistent parenting
• Parental divorce
• Family conflict
• Early institutional rearing
• Closed head trauma
• Exposure to lead

Neither genes nor environment act alone. For example, children exposed to maternal smoking during pregnancy AND had the genetic variation of DAT1 associated with ADHD were more likely to exhibit hyperactivity-impulsitivity and oppositional behaviour. And, sometimes genes only seem to have an effect in the presence of environmental exposures, i.e., DAT1 is associated with ADHD only in children whose mothers had drunk alcohol during pregnancy.

Like other neuropsychological disorders, such as autism, ADHD is a complex disease that has a wide range of symptoms and severity. Understanding the genes that come into play will help focus research, diagnosis, and treatment.

~The British Journal of Psychiatry (2005), 187:103-105