Smoking may leave traces on the genome for decades, scientists have found out. This"footprint" is known as DNA methylation, an epigenetic process by which methyl groups are added to DNA, modifying its function – for better or for worse.

Although the number of smokers is declining worldwide, smoking still causes the highest number of preventable deaths every year. People who have stopped can still be at risk of developing smoking-related diseases, even years after. These include cancers and cardiovascular diseases but also chronic obstructive pulmonary disease.

Health experts are still uncertain about what molecular mechanisms are to blame for this long-term development of diseases and how they work.

In this latest study, published in Circulation: Cardiovascular Genetics, an American Heart Association journal, scientists have investigated the role that DNA methylation could play in the development of smoking-related conditions – examining DNA methylation sites related to smoking and how they are linked to genes involved in these diseases.

Possible biomarkers

The study is a genome-wide analysis of DNA methylation sites across the human genome, using blood samples taken from nearly 16,000 participants. Some were current smokers, while others used to smoke but had stopped at the time of the research.

The scientists identified a number of DNA methylation sites which they say emerged as a result of smoking and which were associated with more than 7,000 genes of the human genome. While most DNA methylation sites disappeared after five years of quitting cigarettes, some remained even three decades on in former smokers. A number of these sites appeared to mark genes that had previously been incriminated in the development of cancers and cardiovascular diseases.

"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 first author Roby Joehanes, an instructor at Harvard Medical School. "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."

The findings could have other significant research implications: the DNA methylation sites identified could indeed help scientists develop biomarkers to assess a patient's smoking history, and serve as targets to develop new kinds of treatment.