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WASHINGTON, Nov. 5 (Xinhua) -- People who smoke one pack of cigarettes a day develop an average of 150 extra mutations in their lungs every year, according to a new study that helps explain why smokers have such a higher risk of developing lung cancer.
Other organs were also affected, with the study showing that a pack a day led to an estimated average 97 mutations in each cell in the larynx, 39 mutations for the pharynx, 23 mutations for mouth, 18 mutations for bladder, and 6 mutations in every cell of the liver each year.
"Before now, we had a large body of epidemiological evidence linking smoking with cancer, but now we can actually observe and quantify the molecular changes in the DNA due to cigarette smoking," Ludmil Alexandrov, co-led author from the Los Alamos National Laboratory said in a statement.
The findings were published this week in the U.S. journal Science.
Cigarettes contain more than 7,000 chemicals, including over 70 known to cause cancer. Although previous studies have associated cigarette smoking with increased risk for 17 different types of cancer, including cancer in tissue not directly exposed to smoke, it has remained unclear how smoking causes cancers.
For this study, researchers looked at over 5,000 tumours, comparing cancers from smokers with cancers from people who had never smoked.
They found particular molecular fingerprints of DNA damage -- called mutational signatures -- in the smokers' DNA, and they counted the number of these particular mutations in different tumours.
The study revealed at least five distinct processes of DNA damage due to cigarette smoking.
The most widespread of these is a mutational signature already found in all cancers, which "seems to accelerate the speed of a cellular clock that mutates DNA prematurely," they said.
"The genome of every cancer provides a kind of 'archaeological record,' written in the DNA code itself, of the exposures that caused the mutations that lead to the cancer," Professor Sir Mike Stratton, joint lead author from the Wellcome Trust Sanger Institute, explained. "Our research indicates that the way tobacco smoking causes cancer is more complex than we thought."
Tobacco smoking claims the lives of at least six million people every year and, if current trends continue, the World Health Organization predicts more than one billion tobacco-related deaths in this century.
