Researchers have identified a specific allele that may raise the risk of both smoking and ADHD by influencing a patient’s behavior and cognitive function
--by Heather Kathryn Ross
Why is it that ADHD patients are so much more likely to smoke, and so much less likely to quit?
Smokers say that cigarettes have a calming effect on the body and brain, which may motivate some young people with attention deficit hyperactivity disorder (ADHD) to start smoking. However, new research suggests that a common genetic risk factor may also be to blame.
Scientists in Montreal, Canada focused on five small variations in DNA sequences called single nucleotide polymorphisms, or SNPs, that are known to influence smoking behaviors, such as how much a person smokes and his or her likelihood of becoming a smoker. They were curious to know whether these same genetic variations are also associated with the hyperactivity, impaired cognitive function, and lack of concentration that are the hallmarks of ADHD.
Previous research has shown that people with ADHD are almost twice as likely to smoke as their peers. They also tend to smoke a greater number of cigarettes than those without the behavioral disorder. This jibes with the findings of lead study author Dr. Ridha Joober of the Douglas Mental Health University Institute in Montreal.
Joober’s team discovered that one high-risk allele, or version, of the SNP rs 1329650 is greatly associated with both ADHD behaviors and the number of cigarettes a person smokes. This allele is significantly more likely to be passed down from ADHD parents to their children, and is associated with severe ADHD symptoms, including disinhibited behavior and lack of concentration. They also found that the allele in question was present in many of the mothers of study participants who had smoked during pregnancy.
“This [genetic connection] may explain why people who suffer from ADHD are also much more prone to becoming addicted to cigarettes,” Joober said. “It may also explain why children with ADHD are more likely to be born to mothers who smoked during pregnancy: this genetic variant (and probably many others) increases the risk for smoking in mothers and the risk for ADHD in their children, suggesting that smoking during pregnancy and ADHD are, at least in part, due these shared genetic determinants.”
Researchers don’t yet know precisely how this allele influences both ADHD behaviors and the propensity to smoke, but its effect is marked.
“Although the exact molecular mechanisms of this association need to be elucidated, we have observed that this genetic variant is associated with weaker executive control,” Joober said. “Carriers of this allele have more difficulties in tests exploring their mental flexibility, their impulsivity, and their working memory.”
More and larger studies will be necessary to bolster the theory that the same genetic risk factors influence both ADHD and smoking. However, Joober’s research is a novel and promising approach to explaining why certain behaviors and medical disorders tend to co-exist.
Joober’s team studied a population of 454 children ages six to 12 who had been diagnosed with ADHD. They used a variety of clinical tests and surveys to evaluate the degree of each child’s emotional and behavioral problems and cognitive impairments.
Researchers also collected blood samples for DNA testing from the children, their parents, and their siblings to see which, if any, of the five SNPs tied to smoking had been passed on and whether these SNPs were associated with more severe ADHD symptoms.
Though it will undoubtedly be years before this genetic variant can be targeted with new drug therapies, a better understanding of the genetic underpinnings of ADHD and smoking addiction may help medical providers target at-risk patients.
“The results may help in the earlier identification of subjects who are more prone to developing dependence on cigarette smoking,” Joober said. “This genetic information would be crucial, once confirmed and furthered, to develop preventive strategies, especially since smoking in ADHD patients tends to start earlier in life and, once initiated, is much more severe and harder to curb than in the general public.”
In 1995, researchers at the University of Michigan studying adult ADHD patients found that they were not only more likely to smoke than their non-ADHD peers, but also recalled experiencing far more ADHD symptoms as children. The authors concluded that “ADHD patients overinclude smokers, and these smokers find it extremely difficult to quit. For ADHD smokers, smoking may have begun as an attempt to manage deficits in attention and concentration, as suggested by greater childhood symptomatology in these patients.”
A 1997 study also concluded that adolescents with ADHD smoked much more than their peers, and that this association was likely the result of some underlying risk factor for both conditions.
Duke University scientists went a step further in 1998 when they theorized that the common risk factor was genetic, and that this unknown factor leads to dysfunction in the dopaminergic system in the brain, which controls the release of chemical “rewards.” They proposed that ADHD patients might receive more-than-typical amount of “rewards” in the form of higher dopamine levels when they ingest nicotine.
Finally, in 2011, scientists also from Duke University found that ADHD smokers experience more severe nicotine withdrawal symptoms than their non-ADHD peers, making it more difficult for them to quit.