Breast tumors that killed an American woman with so-called "triple negative" cancer had 50 separate mutations, including 20 that helped them spread, researchers reported on Wednesday.
New techniques for sequencing the entire genetic map of cells helped the team figure out the changes needed for cancer to spread and kill.
The findings may lead to new tests and new treatments for cancer, they reported in the journal Nature.
Rick Wilson of Washington University in St. Louis and colleagues have been at the forefront of efforts to sequence first the entire human genome and now various types of diseased cells.
They looked at the DNA in four samples from a 44-year-old woman who died when her "triple negative" cancer spread to her brain. This type of cancer disproportionately affects blacks and younger women.
"We've learned some significant lessons about cancer from sequencing the genomes of individual patients and their tumors, but it's clearly just the tip of the iceberg," Wilson said in a statement.
"Moving forward, we'll be comparing tumor genomes from many patients with the same type of cancer to find common genetic alterations. This comprehensive understanding of cancer can aid in the development of new approaches to cancer diagnosis and treatment."
The samples showed that cancer is just as complex as experts had predicted it would be and show that designing drugs to target one or two mutations is unlikely to be useful.
The patient, the first African-American woman to have her entire genome sequenced, had undergone chemotherapy and radiation, but the tumors spread anyway and she died within a year of being diagnosed.
They found 20 mutations that were not common in the early, primary tumors but very common in tumors that popped up outside the breast.
"This indicates that a small subset of cells with a lethal mutation repertoire break free from the primary tumor, circulate in the body, set up residence in other organs and grow aggressively," Dr. Matthew Ellis, another researcher from Washington University who worked on the study, said in a statement.