A breast cancer patient prepares for a radiological medical examination in an Athens hospital October 29, 2008. Credit: Reuters/Yannis Behrakis

In a study of more than 17,000 women, researchers found that although certain common gene variants raise the risk of developing breast cancer, they add to, but do not multiply, the risks posed by lifestyle factors like obesity or drinking.

The findings did not include the breast cancer genes known as BRCA-1 and BRCA-2 that occur much more rarely but which confer a high risk on women who have them.

This is reassuring because...it means that whatever you inherit in terms of common gene variants, the effect of things such as maintaining a healthy weight, limiting alcohol intake and being careful about HRT are still really important for reducing breast cancer risk, Ruth Travis of Oxford University's cancer epidemiology unit said in telephone interview.

Breast cancer is the most common type of cancer in women in wealthy nations. Worldwide, it kills around half a million people each year.

British scientists said last month they had found five common genetic factors linked to the risk of developing breast cancer, adding to 13 other common genetic risk variants. Combined, these common gene factors explain around eight percent of the risk of getting the disease.

Travis worked with Jane Green, also of Oxford University, and colleagues to study 7,160 women with breast cancer and 10,196 women without. The women gave blood samples for genetic testing and information about their lifestyles.

The researchers looked at the risk of breast cancer for 12 genetic variants -- known as single nucleotide polymorphisms, (SNPs) -- in the women's DNA.

They related these to 10 known environmental breast cancer risk factors: age at puberty onset, number of births, age at first birth, breastfeeding, menopausal status, age at menopause, use of hormone replacement therapy (HRT), weight, height, and alcohol consumption.

None of the 120 comparisons yielded significant evidence of gene-environment interactions, they wrote in their study, which was published in The Lancet medical journal.

Genes account for only a small proportion of breast cancers, Green said. The main risk factors remain the lifestyle factors such as childbearing, use of HRT, obesity and alcohol consumption.

The good news is that some of these are modifiable, so by changing their behavior women can alter their risk of breast cancer, she added.

A study published in the United States last year found that nearly 40 percent of all breast cancer cases there could be prevented if women kept a healthy weight, drank less alcohol, exercised more and breastfed their babies.