Advances in the Genetics of ADHD
Fig 1 (Russo, 2012)
ADHD or attention-deficit/hyperactivity disorder is common among children (Smith, Mick & Faraone, 2009). Symptoms include inappropriate attention, impulsivity, and hyperactivity and 50% of affected children would continue to have the symptoms during adulthood and are more likely to be unemployed, involved in crime and have difficulties in relationships (Harpin, 2005, cited in Smith et al., 2009; Smith, Mick & Faraone, 2009). The worldwide prevalence estimate of ADHD is 5.3% (Polanczyk et al., 2007, cited in Smith, Mick & Faraone, 2009).
The following studies demonstrate that ADHD is heritable (Smith, Mick & Faraone, 2009). The risk of ADHD in parents and siblings of an individual with ADHD is multiplied twofold to eightfold (Faraone & Doyle, 2000, cited in Smith, Mick & Faraone, 2009). Biological relatives are more likely to be hyperactive than adoptive relatives (Satterfield et al., 1974 cited in Smith, Mick & Faraone, 2009). From twenty different studies the mean heritability estimate for ADHD is 76% (Faraone & Doyle, 2000, cited in Smith, Mick & Faraone, 2009).
Genetic markers (variations) associated with ADHD are being investigated worldwide (Smith, Mick & Faraone, 2009). Studies that examine whether linkages involving genetic markers are more prevalent in children with ADHD and their unaffected siblings were performed (Smith, Mick & Faraone, 2009). Genetic linkage is when genes are located nearby on the same chromosomal region because they tend to be inherited together (Reece et al., 2012, p. 298). Many different studies “show some degree of overlap” in linkages at chromosomal regions: 5p, 9q, 16q and 17p (p and q determine which half of the centrosome) (Smith, Mick & Faraone, 2009).
Since dopamine and norepinephrine are used to treat ADHD, genes that involve these neurotransmitters (chemical messengers between synaptic cells or neurons) have been candidate genes in studies (Smith, Mick & Faraone, 2009)(Reece, 2012, p. 1070). The hormones, dopamine and serotonin affect the sleep, mood , attention and learning of an individual (Reece et al., 2012), while norepinephrine can also act as a stress hormone that affects the fight or flight response (Gunay, 2004).
Figure 2 Nuerotransmitters in between neurons (Reece et al., 2012, p. 1071)
DRD4 and DRD5 are genes which affect the dopamine receptors of postsynaptic cells (cells receiving the message) (Smith, Mick & Faraone, 2009)(Reece, 2012, p. 1070). A 7-repeat (repetition of nucleotide sequence) variation in DRD4 has been found to be related to ADHD symptoms and its combination with a variation in the DRD5 is associated with “persistent ADHD” (Langley, 2004 cited in Smith, Mick & Faraone, 2009). A certain variation in a serotonin transport gene, DAT1 has also been associated with ADHD symptoms. A variation in the promoter of serotonin transport gene, 5-HTT, reduces the transcription of the gene, thus affecting serotonin levels (Smith, Mick & Faraone, 2009). Polymorphisms (variations) in the norepinephrine transporter gene, NET1 affects an individual’s attention (Brookes, 2006 cited in Smith, Mick & Faraone, 2009).
ADHD is a common, heritable disorder affecting children. Genetic linkage studies are lacking in definitive findings while candidate gene studies demonstrate how variations in genes can affect neurotransmitters and possibly an individual’s mental function.
Günay, G, Akköse, C, Çolak, G 2004, Norepinephrine (NE) Epinephrine Epinephrine(E) Receptors, viewed March 18, 2012, < http://www.library.uq.edu.au/training/citation/harvard_6.pdf>
Reece, JB, Meyers N, Urry, LA, Cain, ML et al. 2012, Campbell Biology, 2nd edn, Pearson Australia Group, n.p.
Russo, J 2012, Contributions of physical activity to ADHD care, viewed March 20, 2012, < http://www.thecamreport.com/2012/01/contributions-of-physical-activity-to-adhd-care/>
Smith, AK, Mick, E, Faraone, SV 2009, ‘Advances in Genetic Studies of Attention-Defi cit/Hyperactivity Disorder’, Current Psychiatry Reports, no. 11, pp. 143-148, viewed March 16 2012, < http://www.springerlink.com.ezproxy.library.uq.edu.au/content/KK4J034764781835/fulltext.pdf>
U.S. Department of Energy 2003, Finding a gene on a chromosome map, viewed March 18 2012, http://www.ornl.gov/sci/techresources/Human_Genome/posters/chromosome/map.shtml