Giving up cigarettes is notoriously hard to do. Now researchers report that some people carry a gene variant that leaves them especially hard-pressed to kick the habit. The findings appear in a paper published today in the journal Thorax.

The addictive nature of cigarettes stems from nicotine, which is metabolized in the body primarily through an enzyme known as CYP2A6. Previous work has revealed a number of varieties of the gene that encodes this enzyme, CYP2A6. Hidetoshi Nakamura of Keio University in Tokyo and colleagues studied 203 current or former smokers suffering from chronic obstructive pulmonary disease (COPD), a disease for which smoking is a risk factor, as well as 123 healthy, nonsmoking volunteers to determine what type of CYP2A6 they carried. The researchers found that the percentage of people with the D genotype of CYP2A6 was significantly higher among current smokers than it was among ex-smokers. What is more, genotype D was associated with an inability to quit smoking independent of age and lifelong cigarette consumption.

The scientists also note that genotype D appears to function as a protective factor against developing emphysema. "These findings suggest that determination of the genotype will be useful in efficiently withdrawing patients from nicotine dependence," they write, adding that DNA analysis could lead to more effective treatments for patients who are trying to butt out.