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Researchers announced on Wednesday a discovery that could open the door to personalized, targeted therapies for breast cancer.

A team of researchers in Canada collaborated over nearly five years with scientists in Britain to determine the genetic underpinnings of breast-cancer tumours.

In what is being described as the most ambitious study of its kind ever conducted, the teams analyzed the DNA and RNA of about 2,000 breast-cancer tumours taken from women in Canada and Britain in the past decade. After identifying patterns and genetic characteristics in each, the researchers have declared that breast cancer can be divided into 10 distinct subtypes.

The findings, published in the journal Nature, won't be of any immediate help to breast-cancer patients. But they represent an important step in the movement toward personalized medicine, a burgeoning field in which doctors use specific information about patients to diagnose disease and deliver treatment customized for the individual.

"It really is leading us into exploring new ways of developing drugs, not just single drugs, but combinations of drugs," said Sam Aparicio, co-lead author of the study and chair of breast-cancer research at the B.C. Cancer Agency.

Until now, breast cancer has been classified into three broad categories and treated accordingly: hormone receptor positive; human epidermal growth factor receptor 2 (HER2) positive; or triple negative (meaning not positive for estrogen) progesterone or HER2.

But the medical community has long been confounded by the fact that it is difficult to predict how patients will fare; breast cancer seems to behave differently in the majority of patients and no one fully understood why.

In recent years, it has become increasingly clear that breast cancer is an umbrella term for numerous different diseases, and that understanding the genetic underpinnings of each is the key to creating targeted treatments for each individual type.

"Basically I think the message is that breast cancer is very heterogeneous," said Philippe Bedard, a medical oncologist at Toronto's Princess Margaret Hospital who was not involved with the study. "It's not one disease, it's many different diseases."

Dr. Aparicio and colleagues at the Cancer Research UK Cambridge Research Institute, the University of British Columbia and the Manitoba Institute of Cell Biology, pored over the genetic data of about 2,000 breast-cancer tumours to detect patterns or common characteristics that could provide clues about the disease.

They reclassified the tumours according to common genetic traits and concluded that breast cancer could be divided into 10 subtypes.

But it's not just breast cancer. Researchers believe that virtually all types of cancer have multiple subcategories that develop based on a variety of factors. This has given rise to hope for new drugs that can target the specific genetic pathways that are involved in the development or growth of cancer.

Dr. Aparicio, who is also a professor in the department of pathology and laboratory medicine at UBC, said there is no doubt that more subtypes of breast cancer exist.

But the discovery gives researchers a starting point for studying how various genes are involved in the development of breast cancer and how they may be targeted for personalized therapy. For instance, scientists may be able to focus on the genetic changes involved in aggressive or difficult-to-treat forms of breast cancer to develop possible treatments.

But how useful that information will ever be in a real-world setting is anyone's guess.

Steven Narod, Canada Research Chair in breast cancer and senior scientist at the Women's College Research Institute in Toronto, said it's one thing to identify genes involved in a particular form of breast cancer, but another to develop a drug that actually represents a breakthrough.

In the past few decades, Dr. Narod said, there have been few important discoveries for the treatment of breast cancer. They include tamoxifen, a common hormone therapy, and trastuzumab, known better under the brand name Herceptin, which is given to some patients with HER2 breast cancer.

New drugs continue to come on the market, but with marginal benefits, such as increasing life expectancy by a matter of a few weeks.

Last year, Avastin was at the centre of controversy over whether health regulators should pull its approval to treat breast cancer. Research showed that it did not increase overall life expectancy and had a risk of serious side effects. In November, Health Canada revoked its approval of the drug for breast cancer.

Given the difficulty of drug development, it's important to remain cautious when thinking that a genetic analysis of tumours means that a treatment breakthrough is around the corner, Dr. Narod said. "It's easy to predict things, but so far … it's a lot of speculation."

Dr. Aparicio acknowledges the difficulty that lies ahead for the future of cancer and targeted therapy. But at least their research is a starting point, he said. "The personalization of cancer medicine is happening," he said. "It's just that it's happening step by step rather than in one giant leap."