SCIENTISTS have solved the mystery of why some smokers get lung cancer while others escape — and developed a blood test to identify those most at risk.

They have found a genetic variation that makes some smokers ten times as likely as others to get the disease. It also explains why non-smokers are vulnerable. The explanation, says Zvi Livneh of the Weizmann Institute of Science in Israel, is that some people are better at repairing smoking-related damage to their DNA than others.

The blood test, which could be available in three to four years, measures how good the repair mechanism is and determines the risks of contracting cancer. The figures are remarkable: a smoker with a poor DNA repair mechanism is more than 200 times more likely to get lung cancer than a non-smoker with good DNA repair. Among non-smokers, the weaker gene raises the risk by more than 13 times.

Lung cancer kills more than 33,000 people a year in Britain, and Professor Livneh says that a large proportion of these cases result from a combination of smoking and poor DNA repair. “My vision is that people could be screened using our test and warned if they had a high risk,” he said.

“What is really striking is the extra risk you get from poor DNA repair. When I talk to doctors, they say that general warnings about smoking aren’t taken seriously. People don’t think it applies to them. But if you added something really personal, like saying that a person’s risk was 100 times higher, then you have a much more effective way of persuading them to give up.”

Cancer starts when DNA is damaged — whether by smoking, radiation or spontaneous mutations. The damage is almost always repaired, but the efficiency of the repair mechanism varies from person to person.

Professor Livneh and his team, who report in the Journal of the National Cancer Institute, looked at one particular repair mechanism, carried out by an enzyme called OGG, and developed a test for measuring OGG activity from a blood sample. When they compared 68 patients with lung cancer with 68 healthy people, they found that OGG activity was much lower in the patients.

They also found that OGG activity affected the risk in non-smokers, so that a non-smoker with low OGG activity is at almost as much risk of lung cancer as a smoker with normal OGG activity.

But those who make up the majority of lung cancer patients have a deadly combination: low OGG activity and heavy smoking. Persuading even some of those to give up cigarettes could have a big effect on lung cancer incidence.

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