Alandmark global study has, for the first time, homed in on 12specific genomic regions associated with attention deficithyperactivity disorder (ADHD). The research reveals the geneticunderpinnings of this highly heritable condition and suggests thisdiscovery is only a small part of the wider genetic puzzle that makes upADHD.
A large number of family, twin and adoption studies have revealedthat ADHD is incredibly heritable. Between 70 and 80 percent of casesdisplay some kind of family history yet no clear genes have so farbeen found to be significantly associated with the condition. It hasbeen suggested that ADHD is a polygenic condition, meaning no singlegenetic variant is the cause but rather it arises through a largecoalescence of many different variants.
This new research is the biggest genetic study into ADHD everconducted, comprising 20,000 people with ADHD compared to a controlof 35,000 healthy subjects.
"This is a landmark study because it involves patients from allover the world," explains Anita Thapar, one of the researchers onthe project from Cardiff University. "This large number of patientsamples has been lacking for ADHD, meaning our understanding of ADHDgenetics has lagged behind physical disorders and other psychiatricdisorders like schizophrenia and depression."
The research uncovered 12 common genomic regions that for the first time could beclearly associated with ADHD. While many of these regionsshare associations with other diseases, several of them play a key role in healthy brain development and learning.
One of the more interesting outcomes from the research seems tolend weight to the hypothesis that clinically diagnosed ADHD is theextreme end of a spectrum of genetically influenced behaviour. Astrong genetic similarity was found between people who are clinicallydiagnosed with ADHD and those who just display some of the samebehavioural traits, such as fidgetiness and inattention.
"The correlation between these rather different definitions ofADHD suggests that clinically diagnosed ADHD may be the severe end ofa continuous distribution of symptoms in the general population,"says Joanna Martin, another Cardiff researcher working on theproject.
The study also suggests that these 12 genomic regions are just the tipof a potentially massive genetic iceberg that is associated withADHD. It's hypothesised that common genetic variants may onlyaccount for around 22 percent of a person's ADHD risk, with yet-to-be-discovered rare genetic variants and environmental factorsaccounting for the rest.
Unfortunately, those looking for a single, easy genetic causebehind ADHD will be disappointed, however this study is a valuableaddition to a body of growing science tracking the genes that help usunderstand the heritability of ADHD.
"The genetic risk variants related to this condition play asignificant role in brain-related and other core biologicalprocesses," says Thapar. "The next step is to determine the exactrole of these genes in ADHD to help us inform better treatments tosupport those affected by the condition."
The new study was published in the journal Nature Genetics.
Source: Cardiff University
