Breast cancer is not one disease, but 10, according to the largest ever genetic analysis of breast tumours writes Science Editor Tom Clarke.
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The finding could have profound implications for the diagnosis and treatment of breast cancer which kills more than 30 women in Britain every day.
The analysis looked at 2,000 breast tumour samples from women in the UK and Canada. The researchers then screened them for thousands of genetic markers. They found nearly all of the tumours fell into 10 distinct genetic groups.
"This makes breast cancer an umbrella term for an even greater number of diseases," said Professor Carlos Caldas from the University of Cambridge who led the study. "Our results will pave the way for doctors in the future to diagnose the type of breast cancer a woman has, the types of drugs that will work, and those that won't, in a much more precise way than is currently possible."
The study, published today in the science journal Nature, shows how this genetic classification of breast cancers could spare some women toxic and expensive treatment, or focus attention on those in particular need.
Two key tests tend to predict how well a breast cancer will respond to treatment. The first, the estrogen receptor (ER test), sees whether the tumour will respond to hormone therapy using drugs like Tamoxifen.
Another, the HER2 test, will show if drugs like herceptin will slow the tumour's growth.
In general, women who are positive for both these tests have the easiest to treat, and most survivable cancers. Those who are negative for both, can have cancers that are much harder to control.
But the new study explains why a few women who are both ER and HER2 positive can have very aggressive cancers. Their tumours fall into one of the 10 new genetic groups that correspond to a very aggressive form of tumour. New tests could ensure these cancers aren't mis-diagnosed as "easy to treat".
Conversely the study also finds another group of very non-aggressive tumours that are negative for ER and HER2. Currently women with this tumour type might undergo toxic chemotherapy that they don't actually need.
The researchers point out however it will take a few years to confirm what this lab-based study has found before hospital tests are available for the new sub-types identified. But they are confident breast cancer cases in future will be diagnosed based on their genetic profile.
"This landmark study will completely change the way we look at breast cancer," said Dr Harpal Kumar, Chief Executive of Cancer Research UK, which funded the study. The research is one of the first to take cancer treatment towards "personalised medicine." The idea that in future many diseases will be characterised and treated based on a patient's own unique genetic background.
In addition to re-classifying breast cancers, the study also indentified a number of new genes that seem to be involved in either speeding up, or holding back, the progression of breast cancer.
Although some years away, each could provide important targets for new drugs to treat breast cancer.