Genetic defect protects against smoking

PARIS, June 24 (AFP) - People with a common genetic defect are less likely to become smokers and, if they do, will smoke fewer cigarettes, according to a University of Toronto study published in Thursday's edition of Britain's "Nature" magazine.

Researchers believe the discovery of the effects of the defective gene, which reduces people's ability to metabolise nicotine, may lead to a treatment for tobacco dependence.

"People with a defective gene are twice as likely to avoid smoking altogether," said Professor Rachel Tyndale at the University's Centre for Addiction and Mental Health.

"In North America, where 30 per cent of the adult population smokes, about 7.5 million people are protected against smoking by carrying a single copy of this gene defect," she said.

"Even a single copy is sufficient to significantly reduce the risk of tobacco-dependence."

Researchers, whose work is based on last year's identification of a gene which plays a key role in nicotine metabolism, have discovered that 20 to 24 per cent of non-smokers carry a defective version of the gene, baptised "CYPA26," compared to around 10 per cent of smokers.

Dependent smokers adjust their smoking to maintain constant blood and brain nicotine levels. Those with the defective gene resulting in impaired nicotine metabolism smoked an average of 129 cigarettes a week compared to the 159 cigarettes a week smoked by people without the defect -- a decrease of 20 per cent. Less than one per cent of the population has two copies of the defective gene.

The study was carried out on 248 regular smokers addicted to nicotine, and 184 non-smokers.

Professor Edward Sellers, co-author of the report, wrote that "with these findings come the possibility of developing a method to chemically inhibit the function of the enzyme produced from the gene."

In other words, the next step is "a prevention and treatment for tobacco smoking."

Scientists say there may also be implications for nicotine replacement therapies such as gum, patches and spray as some smokers will metabolize the nicotine faster than others, so altering their effectiveness.

They also note that smokers carrying a CYP2A6 defective gene "may have a decreased risk of developing tobacco-related cancers".