TORONTO - In a world first, Canadian scientists have decoded all three billion letters in the DNA sequence of a metastatic breast cancer tumour and identified the mutations that caused the malignancy to spread.

The landmark study by researchers at the B.C. Cancer Agency is a major step towards unravelling the mysteries of how cancer begins and what makes it move to other parts of the body.

"I never thought I would see it in my lifetime," co-principal investigator Dr. Samuel Aparicio, head of the agency's breast cancer research program, said of the ability to decode the breast tumour's DNA.

Aparicio said the DNA-mapping should help researchers develop new breast cancer treatments based on the genetic makeup of both primary and metastatic tumours.

The B.C. team sequenced the genome of a patient's original breast tumour and one that arose in her pleural cavity (between the lungs and chest wall) nine years later.

When the researchers looked at DNA in cells from the tumours, they were able to tease out 32 mutations - or spelling mistakes - among the three billion-letter alphabet of the genome. Only five of the mutations were present in the original tumour.

"And there were 19 of the mutations that we just didn't see (in the original tumour)," said Aparicio. "So what that told us was the tumour had evolved considerably from the primary to the eventual metastasis nine years later."

He said the discovery has big implications for the development of new cancer drugs, which may need to be designed to deal with the various genetic mutations.

"What the new approaches are leading us to now look at are can we associate mutations in the tumours with the response to drugs?"

Co-principal author Dr. Marco Marra said new-generation technology allowed the researchers to decode the breast cancer DNA in just weeks and at a fraction of what it would have cost only a few years ago.

The study is the cover story in this week's issue of the prestigious journal Nature.