Researchers found that women whose ATM gene had higher levels of a chemical modification called methylation were twice as likely to go on to develop breast cancer, compared to those with lower levels.
'We know that genetic variation contributes to a person's risk of disease', explained study leader Dr James Flanagan. 'With this new study we can now also say that epigenetic variation, or differences in how genes are modified, also has a role'.
Of the 1,380 women analysed in the study, published in Cancer Research, 640 developed breast cancer. These women were of various ages, but the results were most clear in those under 60.
In one case, high levels of ATM methylation pre-dated breast cancer diagnosis by eleven years, suggesting that it might be possible to identify the risk many years in advance. However, the average amount of time the blood test pre-dated cancer diagnosis was only three years.
This modification may therefore act as an early warning sign for the disease and the team hopes it will allow them to develop a simple blood test to monitor high-risk patients more closely.
Baroness Delyth Morgan of Breast Cancer Campaign said: 'By piecing together how this happens, we can look at ways of preventing the disease and detecting it earlier to give people the best possible chance of survival'.
However, more research will be needed to determine whether a blood test can predict breast cancer before it develops, meaning that it would be a still be a number of years before such a test becomes available.
Dr Flanagan is keen to look at epigenetic modifications elsewhere, saying that the 'next step […] is a genome-wide approach to try and find all the associated genes'.
'We hope that this research is just the beginning of our understanding about the epigenetic component of breast cancer risk', he added. 'The challenge will be how to incorporate all of this new information into the computer models that are currently used for individual risk prediction'.