A gene that is more active in women may explain why female smokers are more than twice as likely to develop lung cancer than their male counterparts, scientists believe.

Female smokers are more likely than males to develop lung cancer at an early age, and after smoking fewer cigarettes.

US researchers studying the genetic structure in lung tissue cells removed from both men and women found that a gene linked to abnormal growth of lung cells is much more active in women.

Biologist Dr Sharon Shriver, of Pennsylvania State University, said when the gene was active in men it led to a two to three-fold increased risk of lung cancer - but in women with the active gene the risk was increased 12-fold.

The research echoes previous studies that have shown that both smoking and non-smoking women are at much higher risk of developing lung cancer than their male counterparts.

Dr Shriver said: "Women are more likely to develop lung cancer after less smoking exposure than are men.

"Also a non-smoker who develops lung cancer is three times more likely to be female than male. Our study may provide an explanation for this."

Lung specimens

The researchers studied lung specimens taken from 38 women and 40 men. The group included both smokers and non-smokers.

Fifty-eight in the group were lung cancer patients. The rest were undergoing lung surgery for other reasons.

The researchers tested the lung tissue for activity by a gene called gastrin-releasing peptide receptor, or GRPR.

This gene prompts cells to link with the GRP hormone. When this happens, it causes cell growth that is linked to lung cancer.

The GRPR gene was found to be active in 55% of the non-smoking women and about 75% of the smoking women.

Among male non-smokers, the gene was not active at all, the researchers found. The gene was active in only 20% percent of the male smokers.

Lead researcher Dr Jill Siegfried, of the University of Pittsburgh, said: "Men have to smoke for this gene to kick in.

"But women don't have to smoke at all for the gene to be active. This may be why non-smoking women are more susceptible to lung cancer."

Key role in development

Dr Siegfried said the GRPR gene plays a key role in the development of the lungs and the bronchial tree, but the gene is usually inactive in adults.

It becomes active only in the presence of tobacco smoke or some other respiratory insult.

The gene is on the X chromosome. Women have two of these chromosomes, while men have one.

Dr Robin Rudd, a lung cancer expert and spokesman for the British Lung Foundation, said the research was an important breakthrough.

He said: "This opens up the possibility that one could seek to detect activity of this gene to give specific information to individual patients about the effect smoking was having on them.

"People with a high level of activity could be warned that it was all the more important that they give up smoking."

The research will be published in the Journal of the National Cancer Institute.