Smoking a pack a day causes 150 mutations a year in every lung cell , research shows

Scientists have found that smokinga pack a day of cigarettes can cause 150 damaging changes to asmoker's lung cells each year.

The findings come from a study of the devastating geneticdamage, or mutations, caused by smoking in various organs in thebody.

Publishing in the journal Science on Thursday, theresearchers said the findings show a direct link between thenumber of cigarettes smoked in a lifetime and the number ofmutations in the DNA of cancerous tumours.

The highest mutation rates were seen in lung cancers, buttumours in other parts of the body including the bladder,liver and throat also had smoking-associated mutations, theysaid. This explains why smoking also causes many other types ofcancer beside lung cancer.

Smoking kills six million people a year worldwide and, ifcurrent trends continue, the World Health Organization predictsmore than 1 billion tobacco-related deaths this century.

Cancer is caused by mutations in the DNA of a cell. Smokinghas been linked with at least 17 types of cancer, but until nowscientists were not clear on the mechanisms behind many of them.

Ludmil Alexandrov of Los Alamos National Laboratory in theUnited States, one of those who carried out the research,explained that in particular, it had until now been difficult toexplain how smoking increases the risk of cancer in parts of thebody that don't come into direct contact with smoke.

"Before now, we had a large body of epidemiological evidencelinking smoking with cancer, but now we can actually observe andquantify the molecular changes in the DNA," he said.

This study analysed over 5,000 tumours, comparing cancersfrom smokers with those from people who had never smoked.

It found certain molecular fingerprints of DNA damage called mutational signatures in the smokers' DNA, and thescientists counted how many of these were in different tumours.

In lung cells, they found that on average, smoking a pack ofcigarettes a day led to 150 mutations in each cell every year.

Each mutation is a potential start point for a "cascade ofgenetic damage" that can eventually lead to cancer, they said.

The results also showed that a smoking a pack of cigarettesa day led to an average 97 mutations in each cell in the larynx,39 mutations for the pharynx, 23 for the mouth, 18 for thebladder, and six mutations in every cell of the liver each year.

Mike Stratton, who co-led the work at Britain's WellcomeTrust Sanger Institute, said it was a bit like digging in to thearchaeology of each tumour.

"The genome of every cancer provides a kind ofarchaeological record, written in the DNA code itself, of theexposures that caused the mutations," he said. "Looking in theDNA of cancers can provide provocative new clues to how [they]develop and thus, potentially, how they can be prevented."