Children diagnosed with attention hyperactivity disorder (ADHD) are twice as likely to pick up smoking than youngsters without the disorder.
A group of Canadian researchers lead by Dr. Ridha Joober of the Douglas Mental Health University Institute in Montreal looked at genetic markers among children with ADHD and found a variant previously implicated in increasing the risk for smoking behavior may also increase the risk for ADHD. According to the study, published on line in Archives of Disease in Childhood, this may explain why people who suffer from ADHD are also much more prone to become addicted to cigarettes.
In addition, says Joober, “This may also explain why children with ADHD are more likely to be born to mothers who smoked during pregnancy. This genetic variant–and there are 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 to these shared genetic determinants.”
To identify the genetic factors at work, the researchers looked at five variations in DNA sequences–also called single nucleotide polymorphisms or SNPs–in different genes that influence different smoking behaviors, like the number of cigarettes a person smokes a day or the average age when smoking starts. The team collected DNA from 454 children aged 6 to 12 with confirmed ADHD. The researchers also assessed the children’s behavioral and emotional problems at home and at school through validated behavior tests.
To compare how the genetic variants were being passed on, the team also gathered DNA from the children’s parents and siblings. This also allowed the researchers to pinpoint which variants were most strongly associated with common mental and physical behaviors among the kids with ADHD.
Mothers of the participants also reported whether or not they smoked during pregnancy so the researchers could see if the environmental influence of tobacco played a role. Of the 394 mothers who responded, 171 smoked during pregnancy and 223 did not.
The researchers found that one of the SNPs was strongly associated with the number of cigarettes a person smokes as well as ADHD behaviors. Children with the variation were more likely to have behavioral problems and struggle with tasks involving more brain power and concentration, things that children with ADHD find more challenging. Kids were equally likely to inherit the allele if their mothers did or did not smoke during pregnancy, which ruled out environmental smoking exposure as a factor and confirmed the stronger genetic contribution.
The authors concluded that the allele potentially increases the risk of ADHD and smoking by encouraging behaviors and “cognitive deficits” in the brain that can manifest as ADHD during childhood and thus be a gateway to smoking later.
“For a long time, smoking during pregnancy has been associated with a higher risk of ADHD in the offspring. However, the nature of this association was very much debated and one the hypotheses was that these two [outcomes] are due to common factors that increase smoking in mothers and ADHD in their children. This study shows for the first time that this common link could be genes,” says Joober.
Joober says that while we are still far from using gene therapy to even consider modifying the genetic variant to avoid the tendency toward ADHD and smoking, the results could still help to identify people at a biological risk for developing ADHD and cigarette addiction. “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 curve down than in the general public,” he says.
In a corresponding editorial, Dr. Miriam Cooper and Professor Anita Thapar from Cardiff University School of Medicine, Wales, UK called the study “an intriguing starting point,” but caution that the findings are only preliminary. “Although the ﬁndings are potentially exciting, the authors are right to be tentative about their conclusions at this stage as there are certain cautions to be exercised when considering the implications,” they wrote.
The authors acknowledge that although their study population size was large, the study needs to be replicated before any conclusive links can be made. “The implications for the treatment of ADHD are not clear now. Once we will know the exact role of this variant, it might be possible to target this gene for new drug development,” says Joober.