Genes that make women more susceptible to breast cancer don't have any link to lifestyle factors that also raise their risk, a new study says.
Some experts previously thought there might be dangerous interactions between breast cancer mutations and other risk factors for the disease, like taking hormone replacement therapy - and that these women had a particularly high risk of breast cancer.
According to a study published on Wednesday in the medical journal, Lancet, that isn't the case.
British researchers studied 7610 women with breast cancer and 10,196 women without it. All of the women provided a blood sample for genetic testing and information about other risk factors like obesity, alcohol consumption and hormone replacement therapy.
The scientists used a statistical analysis to examine the relationship between genetic and lifestyle factors. They found that although genetic mutations and lifestyle choices both contribute to cancer, they do so separately and do not mix for a more deadly effect.
The genetic mutations studied are carried in up to 60% of women and increase a woman's breast cancer risk from 10 to 20%. The study did not include the rare BRCA genes which dramatically increase the risk of breast cancer. The study was paid for by Britain's Medical Research Council and Cancer Research UK.
Ruth Travis of the Cancer Epidemiology Unit at the University of Oxford and the study's lead author, said it was reassuring she and colleagues didn't find any proof of synergy between breast cancer mutations and lifestyle factors.
"There's a danger of feeling you're at the fate of your genes," Travis said. "But whatever you're born with, there are things you can do to modify your risk."
Experts said lifestyle factors are often more important in avoiding breast cancer than genetic ones. For example, being fat elevates your risk by 40 percent and taking hormone replacement therapy doubles it.
Susan Gapstur, vice president of epidemiology at the American Cancer Society, said the findings would not change the group's prevention messages to women, like avoiding weight gain, staying physically active, and minimising hormone replacement therapy. Gapstur was not connected to the study.
She said the research underlined the complexity of breast cancer and that scientists still don't completely understand what triggers it. "It likely won't be a single genetic factor (that causes breast cancer) but maybe several genetic variants in combination and some environmental factors," she said.