The Genetics of ADHD
A great deal of research has been carried out on the genetic factors that may play a role in attention deficit hyperactivity disorder (ADHD). Over 1,800 studies have been published on the subject to date.
These studies, including family studies as well as those centered on specific genes or genome-wide screening, have produced strong evidence that genes play a role in susceptibility to ADHD. A 2009 review concluded that genetics account for 70 to 80 percent of the risk, with a mean estimate of 76 percent.
Specific gene studies have produced good evidence linking certain genes to the disorder, particularly the dopamine D4 (DRD4) and dopamine D5 (DRD5) genes. However, it is difficult to implicate any specific gene in ADHD “beyond reasonable doubt,” due to the diversity and complexity of the condition.
Dr. Tobias Banaschewski of the Central Institute of Mental Health in Mannheim, Germany, explains that, “Twin and adoption studies show ADHD to be highly heritable.” He writes, “In recent years, a large number of studies on different candidate genes for ADHD have been published. Most have focused on genes involved in the dopaminergic neurotransmission system.”
ADHD is linked to deficits in the functioning of several brain areas, including the prefrontal cortex, the basal ganglia, cerebellum, temporal and parietal cortex. These areas are important in brain activities that may be impaired in ADHD, such as response inhibition, memory, planning and organization, motivation, processing speed, inattention and impulsivity.
Gene studies, whether focusing on specific genes or scanning the whole genome, aim to link DNA variations with these observable symptoms. They also endeavor to locate the relevant chromosome regions.
A recent 2010 analysis of genome-wide studies found only one confirmed location on one chromosome (chromosome 16) that has been repeatedly linked to ADHD. The authors say, “This is not unexpected because the power of individual scans is likely to be low for a complex trait such as ADHD which may only have genes of small to moderate effects.”
While current results from ADHD genome-wide studies are far from conclusive, they do provide new directions and suggest research avenues to follow, the analysts say. Dr. Banaschewski comments, “To date, the findings from genetic studies in ADHD have been somewhat inconsistent and disappointing. Specific gene-based studies have similarly only explained a small percentage of the genetic component of ADHD. Despite the high heritability of the disorder, genome-wide studies have not shown extensive overlaps, with only one significant finding in the meta-analysis of studies [chromosome 16].” But he adds that “the latter approach is likely to redirect future ADHD research, given the apparent involvement of new gene systems and processes.”