We have known for a long time that smoking is bad for us! However, there is now evidence of exactly how much damage is caused as we can precisely count how many cancer-related DNA mutations accumulate in smokers’ organs over time.
Studies have found that on average, there is one DNA mutation per lung cell for every 50 cigarettes smoked. People who smoke a pack of 20 a day for a year generate 150 mutations per lung cell, 97 per larynx cell, 39 per pharynx cell, 18 per bladder cell and six per liver cell.
Tobacco smoking has previously been linked with at least 17 types of cancer, but this is the first time we have been able to quantify the exact molecular damage to our DNA.
A team at Los Alamos National Laboratory in New Mexico lead by Ludmil Alexandrov achieved this by comparing tumour DNA from 2500 smokers and 1000 non-smokers. This allowed them to identify which mutations were associated with smoking. Theoretically, every DNA mutation has the potential to trigger a cascade of genetic damage that causes cells to become cancerous. However, we they don’t know what the probability is of a single smoking-related DNA mutation turning into cancer, or which mutation types are likely to be more malignant. Research to answer this is currently underway.
Some smokers never develop cancer despite accruing thousands of mutations, but this is purely down to luck, Alexandrov says. “Smoking is like playing Russian roulette: the more you play, the higher the chance the mutations will hit the right genes and you will develop cancer,” he says. “However, there will always be people who smoke a lot but the mutations do not hit the right genes.”
The team hopes their findings will deter people from taking up smoking and debunk the myth that social smoking is harmless. Every cigarette has the potential to cause genetic mutations, Alexandrov says. Quitting smoking will not reverse these mutations – they leave permanent scars on DNA – but it will prevent the added risk of more mutations, he says.
There is good evidence that people who stop smoking have a significantly lower risk of premature death than those who continue, says Simon Chapman at the University of Sydney, Australia. For example, a UK study that followed 35,000 men for half a century found that smoking shaved 10 years off average life expectancy. But quitting at age 30 mostly erased the risk of premature death, and giving up at 50 halved it.
“Many smokers believe there’s no point in quitting because the damage is already done,” says Chapman. “But if smokers quit by middle age, they can avoid nearly all the excess risk of tobacco-caused deaths.”
Journal reference: Science, DOI: 10.1126/science.aag0299