ADHD Genes: DRD2

ADHD Genes

Dopamine D2 Receptor Gene (DRD2): TaqI A1 allele, chromosome 11 q22-q23

Based on the same overview study (Faraone and Khan, J Clin Psychiatry 2006; 67 (sup 8), 13-20) as the other seven ADHD genes, there has been some association between a gene on the 11th human chromosome and likelihood of developing ADHD. The form (also called "allele") of this gene associated with ADHD is called the Dopamine D2 Receptor Gene (DRD2) TaqI A1 allele. The findings from the main study on this gene were not replicated, but one study found that individuals possessing the above form of the gene showed an increased likelihood of having ADHD.

Interestingly, this form of the gene is also associated with at least two other disorders that are known to frequently occur alongside of ADD and ADHD. Individuals carrying the TaqI A1 form of the gene also showed a significant increased likelihood of having Tourette’s disorder. Tourette’s is a relatively common comorbid (meaning “occurring along with” or “occurring along side of”) disorder of ADD or ADHD. For those not familiar with the disorder, Tourette’s is a disorder that can result in involuntary behaviors such as “tics”, involuntary twitching, and, in some cases, outbursts of inappropriate speech and profanity. Along with ADD and ADHD, Tourette’s is also seen alongside of other disorders such as Obsessive Compulsive Disorder (OCD) at relatively high frequencies.

In addition to Tourette’s, there is evidence has linked the TaqI A1 form of DRD2 to Parkinson’s Disease (Grevle, et. al, Allelic association between the DRD2 TaqI A Polymorphism and Parkinson’s disease, Movement Disorders 2001, Volume 15, Issue 6, 1070-74). Several findings have pegged ADD and ADHD individuals to having a higher likelihood of developing Parkinson’s later in life. There is a distinct possibility that this form of the gene may be a significant underlying factor between the two disorders.

ADHD genes

ADHD Gene #2 DRD5

In another post, I described that there were at least 7 well-known genes that are associated with ADHD. In fact, since the publication of this paper, additional ones have been identified. Within the past couple of months, another key study on ADHD genes has been discussed.

ADHD Gene #2: Dopamine D5 receptor gene (DRD5): CA repeat, 148 bp

In a previous posting, we discussed an ADHD gene found on the 11th chromosome in humans, called the DRD4. One of the forms (also called alleles) of this gene was associated with the disorder of ADHD, in particular the inattentive component of ADHD.

A second ADHD gene called the "Dopamine D5 receptor gene", or DRD5, is also thought to have strong familial ties to the disorder. Like the DRD4 gene listed above, DRD5 has multiple forms (or alleles). This is located on chromosome number 4 for humans. The "ADHD allele" which is referred to as "CA repeat, 148 bp" (this notation is commonly used by geneticists and refers to the length and DNA makeup of the "ADHD form" of the gene, the exact details aren't entirely important) is slightly greater in length than the non-ADHD form(s). While different studies on this allele have produced different results, it appears that this form of the gene, like the form of the DRD4 ADHD gene listed above, is tied more towards the inattentive than hyperactive component of ADHD. Statistically, however, there appears to be a weaker association between the DRD5 gene and ADHD than the DRD4 gene.