The Genetics of ADHD

The 3-year-old is "into everything" and is forever playing with his hair. His mother finds the car radio "distracting" when she's driving. The boy's grandfather was a "daydreamer."

Such "generational" similarities have caused researchers to observe that ADHD runs in families. Up to 35 percent of children with Attention Deficit Hyperactivity Disorder have a direct relative with ADHD, say researchers, who add that an underlying genetic origin seems likely.

A clinical trial (http://www.clinicaltrials.gov/) is being conducted now to further research the family component in ADHD. Sponsored by the National Institutes of Health's National Human Genome Research Institute (NHGRI), participants are currently being recruited.

Previous studies of twins have found that ADHD is more likely to occur in identical twins than in fraternal twins, whose genetic makeup is similar but not identical. A deficiency in the neurotransmitter, dopamine, has been implicated. Two genes in particular are affected: a dopamine receptor gene on chromosome 11 and the dopamine transporter gene on chromosome 5. Dopamine is one of the brain's "feel good" chemicals. It also helps govern impulsivity - an ADHD symptom.

ADHD, the most common behavioral disorder in childhood, affects up to 5 percent of children ages 7 to 17, researchers say. Scientists describe it as a complex disorder. In the current clinical trial, the government hopes to study entire families - both parents and all siblings. Those accepted will have blood drawn locally by their own physician, using kits provided by and returned to the National Institutes of Health. There is no cost to participating families.

The hypothesis of the study is that ADHD is a genetically influenced brain disorder. Studied will be an isolated population in Columbia, South America and, in U.S. families, at least one affected child and at least one sibling (either affected or unaffected), and their parents.

DNA from these two groups will be analyzed to search for genes associated with ADHD. Scientists say the brain's prefrontal cortex is known to play a very important role in the processing of emotions and impulsivity. Researchers say measuring brain metabolites in this region could help establish the genetic marker for ADHD.

For more information about ADHD genetic marker research, contact Jules Asher of the National Institute of Mental Health, (301) 443-4536. To learn more about the clinical trial, reference study Identifier: NCT00046059 and call (800) 411-1222 or e-mail prpl@mail.cc.nih.gov?subject=NCT00046059,%20000058:%2000-HG-0058-%20%20Genetic%20Analysis%20of%20Attention%20Deficit%20Hyperactivity%20Disorder%20(ADHD).

Currently recruiting in California is the University of California, Irvine Medical Center, Orange, CA, 92668. In Maryland, participants are being recruited for the National Institutes of Health Clinical Center, 9000 Rockville Pike, Bethesda, MD, 20892.