Women who carry a faulty BRCA1 or BRCA2 gene - which puts them at high risk of developing breast cancer – will further double their risk if they smoke, says a new international study.

University of Melbourne researchers were key collaborators on the study, published online in the journal Breast Cancer Research and Treatment.

The study included 780 women from large breast cancer studies, and more than 40 per cent were from the Australian Breast Cancer Family study, led by Professor John Hopper, from the University of Melbourne’s School of Population Health, and the Kathleen Cuningham Consortium for Research into Familial Breast Cancer (kConFab), run from Melbourne’s Peter McCallum Cancer Centre. International studies included the Breast Cancer Family Registry and the Ontario Cancer Genetic Network.

It is the first-ever study to demonstrate the increased breast cancer risk due to smoking for women with faulty BRCA1 and BRCA2 genes.

All of the women in the study were younger than 50, and all had a faulty breast cancer gene. Half of the group had cancer, the other half did not.

After analysing the lifestyles of the two groups, the study found that the women who developed breast cancer were more likely to have smoked than those who were cancer free.

Women with a faulty BRCA1 gene had 2.3 times the risk of developing cancer if they smoked for five years or more, while those with a faulty BRCA2 gene had a 2.6 increase in breast cancer risk if they smoked.

The longer the women smoked, the more their risk increased, rising by about 7 per cent for each year of smoking.

Researcher Dr Mark Jenkins, from the University of Melbourne, says although women who have a faulty BRCA1 or BRCA2 gene have a high risk of developing breast cancer, the new research suggests they can markedly reduce their risk by not smoking.

“This study suggests that women with faulty genes can effectively halve their risk of developing breast cancer by not smoking,’’ he says.

Dr Jenkins says BRCA1 and BRCA2 genes, when functioning normally, repair the type of DNA damage caused by carcinogens such as cigarette smoke.

Women who inherit a faulty copy of the genes are less able to repair DNA damage.

Dr Jenkins says the fact that not all women with faulty genes will develop breast cancer suggests that environmental factors influence their cancer risk.

“This study suggests that smoking plays a major role,” he says.

About 1 in 300 women have a faulty BRCA1 or BRCA2 gene. Of those, about 20-40 per cent will develop breast cancer by the age of 50. By the age of 70, about 60 per cent of these women will develop breast cancer.

Some women with faulty genes undergo mastectomies, have their ovaries removed, or undergo preventative chemotherapy to avoid developing breast cancer.

Dr Jenkins says this new study provides another practical way that women can reduce their risk of breast cancer.

“It adds to the growing body of evidence about the health dangers of smoking,’’ he says.

“If you have a faulty breast cancer gene – or a strong family history of breast cancer – not smoking may reduce your risk of developing breast cancer – and you will also get all the other known health benefits of not smoking.”