Lifestyle and genes pose separate risks for breast cancer

Recent studies have identified several genetic variations found commonly among the population that carry a small but increased risk of breast cancer. Little is known about how the effects of these genes might be connected to known lifestyle and behavioural ('environmental') risk factors for breast cancer.

The Oxford team have examined this question in detail for the first time in a large study of women's health in the UK. The findings are reported in the medical journal The Lancet.

The study's main author, Dr Ruth Travis of Oxford University's Cancer Epidemiology Unit, says: 'Known risk factors for breast cancer include both lifestyle factors and inherited genetic factors. We looked at whether lifestyle factors for breast cancer, such as use of HRT, alcohol consumption and reproductive history, influence the genetic risks: and the answer is that they do not.'

The study included 7160 women who developed breast cancer and 10,196 women without breast cancer. All the women provided a blood sample for genetic testing as well as information on lifestyle factors.

The Oxford team looked at how the risk of breast cancer associated with 12 common genetic variants in the women's DNA might vary with 10 well-known environmental risk factors for breast cancer. These included number of children, age at birth of first child, breastfeeding, use of hormone replacement therapy, obesity, and alcohol consumption.

None of the 120 comparisons (12 genetic x 10 environmental risk factors) offered evidence of any interactions between genes and the environment. In particular, and contrary to previous suggestions, use of hormone replacement therapy did not affect the risk associated with these common genes.

The lack of interaction means that although both genetic and environmental factors separately increase breast cancer risk, they appear to do so independently.

The study did not include the well-known, but much rarer breast cancer susceptibility genes BRCA1 and BRCA2. These carry a high risk of breast cancer but affect relatively few women.

Dr Jane Green, also of the Cancer Epidemiology Unit, adds: 'This study provides another piece in the jigsaw, helping us to understand how genes and lifestyle affect breast cancer risk.

'Genes account for only a small proportion of breast cancers and for most women 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 behaviour women can alter their risk of breast cancer.'