Smoking leaves "footprint" on DNA for over 30 years: study

Smoking leaves its "footprint" on the human genes for more than 30 years, scientists have found, suggesting that people may be at risk of cancers, stroke and other diseases, even decades after kicking the butt.
The new study suggests that DNA methylation, a process by which cells control gene activity, could be an important sign that unveils an individual's smoking history and could provide researchers with potential targets for new therapies.
"These results are important because methylation, as one of the mechanisms of the regulation of gene expression, affects what genes are turned on, which has implications for the development of smoking-related diseases," said Stephanie J London, from the US National Institute of Environmental Health Sciences of National Institutes of Health.

"Equally important is our finding that even after someone stops smoking, we still see the effects of smoking on their DNA," she said.
Even decades after stopping, former smokers are at long-term risk of developing diseases including some cancers, chronic obstructive pulmonary disease and stroke, researchers said.
While the molecular mechanisms responsible for these long-term effects remain poorly understood, previous studies linking DNA methylation sites to genes involved with coronary heart disease and pulmonary disease suggest it may play an important role.

Researchers conducted a meta-analysis of DNA methylation sites across the human genome using blood samples taken from nearly 16,000 participants from 16 groups of the Cohorts for Heart and Ageing Research in Genetic Epidemiology (CHARGE) Consortium, including a group of the Framingham Heart Study that has been followed by researchers since 1971.

They compared DNA methylation sites in current and former smokers to those who never smoked. The comparison suggests that smoking-associated DNA methylation sites were associated with more than 7,000 genes, or one-third of known human genes.
For people who stopped smoking, the majority of DNA methylation sites returned to levels seen in never smokers within five years of quitting smoking. However, some DNA methylation sites persisted even after 30 years of quitting.
The most statistically significant methylation sites were linked to genes enriched for association with numerous diseases caused by cigarette smoking, such as cardiovascular diseases and certain cancers.
"Our study has found compelling evidence that smoking has a long-lasting impact on our molecular machinery, an impact that can last more than 30 years," said Roby Joehanes, from the Harvard Medical School in the US.

"The encouraging news is that once you stop smoking, the majority of DNA methylation signals return to never smoker levels after five years, which means your body is trying to heal itself of the harmful impacts of tobacco smoking," said Joehanes.
The study appears in the journal Circulation: Cardiovascular Genetics.