New York, Sep 22 (IANS) Smoking, a leading preventable cause of deaths worldwide, impacts the human DNA for more than 30 years even after one quits, a study has found.

The findings showed that smoking leaves its "footprint" on the human genome in the form of DNA methylation -- a process by which cells control gene activity.

Methylation, 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.

"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 lead author Roby Joehanes, instructor at Harvard Medical School in Massachusetts, US.

For people who stopped smoking, the majority of DNA methylation sites returned to the levels that are seen in those who never smoked within five years of quitting it.

However, some DNA methylation sites persisted even after 30 years of quitting.

Even decades after stopping, former smokers are at long-term risk of developing diseases including cancers, chronic obstructive pulmonary disease, and stroke. 

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.

DNA methylation could be an important sign that reveals an individual's smoking history, and could provide researchers with potential targets for new therapies, the researchers said.

For the study, the team conducted a meta-analysis of DNA methylation sites across the human genome using blood samples taken from nearly 16,000 participants.

The researchers compared DNA methylation sites in current and former smokers to those who never smoked.

Smoking-associated DNA methylation sites were associated with more than 7,000 genes, or one-third of known human genes.

The researchers suggest that some of these long-lasting methylation sites may be marking genes potentially important for former smokers who are still at increased risk of developing certain diseases. 

The discovery of smoking-related DNA methylation sites raises the possibility of developing biomarkers to evaluate a patient's smoking history, as well as potentially developing new treatments targeted at these methylation sites.

The results were published in the journal Circulation: Cardiovascular Genetics.