Even years after quitting, former smokers still have an increased risk of lung cancer -- and now Canadian scientists believe they know why.
It appears that though most smoking damage is repaired over time, the habit appears to permanently alter the activity of key genes.
Dr. Stephen Lam, chair of the B.C. Cancer Agency's lung tumour group, says the findings may explain why 50 per cent of Canadian patients with newly diagnosed lung cancer are former smokers.
Researchers from the B.C. Cancer Agency, writing in the journal BMC Genomics, looked at the lung tissue of 24 current, former, and non-smokers. Using a technique called serial analysis of gene expression (SAGE), they identified close to 600 genes that were differentially expressed between current and non-smokers.
Only about a fifth of the genes in a cell are switched on at any given time, but environmental factors such as smoking lead to changes in gene activity.
Of the 600 genes identified, changes in almost one third of them are irreversible in former smokers, the researchers found. Specifically, some DNA repair genes are irreversibly damaged by smoking. Smoking also switched off genes that help combat lung cancer development.
Nearly another third of the genes displayed changes that are reversible by stopping smoking.
"Those genes and functions which do not revert to normal levels upon smoking cessation may provide insight into why former smokers still maintain a risk of developing lung cancer," said Raj Chari, who led the research.
Chari says further investigation into the genes that do not revert to normal levels upon smoking cessation may offer new insight into lung cancer.
Lung cancer, the most preventable of all cancers, remains the leading cause of cancer death for both men and women in Canada. In 2007, an estimated 23,300 Canadians will be diagnosed with lung cancer and 19,900 will die of it.
