The scientists, from McGill University, Canada, reported in the Archives of Disease in Childhood that a variation of a gene may be associated with childhood ADHD (attention deficit hyperactivity disorder) and smoking addiction.
In this latest study, the authors write as background information that ADHD during childhood and subsequent smoking later in life go hand in hand. Children who have been diagnosed with ADHD have a considerably greater risk of starting to smoke early, and then becoming long-term heavy smokers.
The scientists concentrated on five variations in DNA sequences in various genes that are closely linked to different aspects of smoking behavior, such as quitting, starting smoking, and how many cigarettes are smoked each day.
They wanted to determine whether these five variations might be present in 454 children aged from 6 to 12 years who had been diagnosed with ADHD and referred to specialist treatment centers.
Their mothers were asked whether they smoked while they were pregnant. 171 of the 394 mothers who responded smoked when pregnant, while 223 did not.
They also assessed the children's emotions and behavioral problems at school and at home, as well as their intellectual capacity.
Blood samples were taken from the children, parents and brothers and sisters to determine who had high risk variants (alleles) of the five genetic markers that may have been passed on. If some did have them, they also wanted to see whether they were more strongly linked to "the externalising behaviours/disinhibition and impaired cognitive performance characteristic of ADHD."
One of the five variations in DNA sequences (single nucleotide polymorphisms or SNPs), rs 1329650, which was linked to how much people smoke, was more likely to be associated with ADHD.
The authors explained that "The high risk C allele of rs 1329650 was significantly more likely to be passed on from the parents and to be associated with the more severe form of ADHD. It was much more common among children who had higher scores on the validated behavioral tests."
The inherited risk allele was more common in the kids who performed poorly on tasks requiring brain power and concentration.
They also found that mothers who smoked during pregnancy were no more likely to give birth to babies with the inherited risk allele than the non-smoking mothers, suggesting that secondhand smoking does not alter the risk allele.
The researchers concluded that the C allele of rs1329650:
- May increase the risk of ADHD
- May also increase the risk of smoking through prompting behaviors and impaired higher brain functions that are common in childhood ADHD
Accompanying EditorialDr Miriam Cooper and Professor Anita Thapar sounded a word of caution. They emphasized that this study should be taken as preliminary evidence which suggests that a shared molecular genetic risk for smoking and ADHD is theoretically plausible. Large-scale tests are needed to confirm this.
"This is an intriguing starting point from which to conduct further related analyses. Acknowledging that the same genetic risk variants can have different observable effects in people could help inform discovery of risk variants for childhood developmental/psychiatric disorders for which it can be difficult to assemble very large sample sizes.
Such efforts could help uncover novel biological risk pathways and contribute to explaining why different behaviours and disorders commonly co-occur."
Written by Christian Nordqvist