TAIPEI: Children who are exposed to tobacco smoke from their fathers while they're in the womb may be more likely than those who are not to develop asthma by age 6, according to a study of chemical changes to DNA.
While prenatal smoke exposure has long been linked to an increased risk of childhood asthma, the current study offers fresh evidence that it's not just a pregnant mother's smoking that can cause harm.
Researchers followed 756 babies for six years. Almost one in four were exposed to tobacco by fathers who smoked while the child was developing in the womb; only three mothers smoked.
Overall, 31 per cent of kids with fathers who smoked during pregnancy developed asthma by age 6, compared with 23 per cent of kids without fathers who smoked, the study found.
Asthma was also more common among kids whose fathers were heavier smokers, senior study author Dr. Kuender Yang of the National Defense Medical Center in Taipei said by email.
"Children with prenatal paternal tobacco smoke exposure corresponding to more than 20 cigarettes per day had a significantly higher risk of developing asthma than those with less than 20 cigarettes per day and those without prenatal paternal tobacco smoke exposure," Yang said.
About 35 per cent of the kids with fathers who were heavier smokers developed asthma, compared with 25 per cent of children with fathers who were lighter smokers and 23 per cent of kids with fathers who didn't smoke at all during pregnancy.
Smoking by fathers during pregnancy was also associated with changes in methylation - a chemical code along the DNA strand that influences gene activity - on portions of genes involved in immune system function and the development of asthma.
Researchers extracted infants' DNA from cord blood immediately after birth and examined methylation along the DNA strand. The more fathers smoked during pregnancy, the more methylation increased on stretches of three specific genes that play a role in immune function.
Children who had the greatest methylation increases at birth, affecting all three of these genes, had up to almost twice the risk of having asthma by age 6 as other kids in the study.
While smoking by fathers during pregnancy was linked to childhood asthma, it didn't appear to impact children's sensitivity to allergens or total levels of IgE, an antibody associated with asthma.
This suggests that the risk of asthma from tobacco exposure is unlike allergic asthma, which is driven by allergies or allergic sensitization via IgE antibody, said Dr. Avni Joshi, a researcher at the Mayo Clinic Children's Center in Rochester, Minnesota, who wasn't involved in the study.
The study wasn't designed to prove whether or how prenatal smoking exposure might directly cause so-called epigenetic changes, or how those changes cause asthma in children.
It's not yet clear how the alterations seen along the DNA strand where methylation increased might cause asthma, the study team notes in Frontiers in Genetics.
Still, the message to parents should be clear, Joshi said by email.
"Smoking is bad at ANY point in time: before the baby is born and after the baby is born," Joshi said.
"Many parents defer quitting until the baby is born, but this study stresses that the prenatal exposure to tobacco creates changes to the unborn child's immune system, hence it is best to quit as a family decides to have children, even before the conception happens."