ScienceDaily (June 10, 2012) Decoding the DNA of patients with advanced breast cancer has allowed scientists to identify distinct cancer "signatures" that could help predict which women are most likely to benefit from estrogen-lowering therapy, while sparing others from unnecessary treatment.

Researchers at Washington University School of Medicine in St. Louis uncovered mutations linked to whether or not women respond to aromatase inhibitors, drugs often prescribed to shrink large tumors before surgery. These mutations also correlate with clinical features of breast tumors, including how likely they are to grow quickly and spread.

The research, which also involved physicians and scientists at the Alvin J. Siteman Cancer Center at Barnes-Jewish Hospital and Washington University School of Medicine and The Genome Institute, is published June 10 in the advance online edition of Nature.

"This is one of the first cancer genomics studies to move beyond cataloging mutations involved in cancer to finding those linked to treatment response and other clinical features," says senior author Elaine Mardis, PhD, co-director of The Genome Institute. "If our results are validated in larger studies, we think genomic information will be one more data point for physicians to consider when they select among several treatment options for their patients."

The study involved DNA from 77 post-menopausal women with stage 2 or 3 estrogen-receptor-positive breast cancer, the most common form of the disease. Estrogen stimulates the growth of these tumors, and all the women received aromatase inhibitors to lower estrogen in the body. The drugs can reduce the size of breast tumors, enabling many women to receive breast-conserving surgery rather than a mastectomy. But aromatase inhibitors only work in some women, and doctors don't know why.

To answer that question, the researchers compared the DNA in the tumor samples to matched DNA from the same patients' healthy cells, which allowed them to identify mutations that only occurred in the cancer cells. This "unbiased" approach finds all the mutations underlying a patient's cancer not just those that would be expected to occur.

The tumor samples came from women enrolled in one of two aromatase inhibitor clinical trials sponsored by the American College of Surgeons Oncology Group. As part of those trials, researchers had collected detailed information about the women's tumors and whether they responded to a four-month course of aromatase inhibitor therapy before surgery. Twenty-nine of the tumor samples came from women whose tumors were resistant to aromatase inhibitors, and 48 came from patients whose tumors responded.

Over all, the scientists noted that tumors in women who responded to the estrogen-lowering drugs had relatively few mutations, while those whose cancers were resistant to the treatment had higher mutation rates and were genomically more complex.

"This makes sense in hindsight but it's not something that we would have predicted," Mardis says.

The researchers identified 18 significantly mutated genes in the tumor samples, meaning the genes were altered more often than would have been expected. Some of these genes were already known to be important in breast cancer but others were completely unexpected, including a handful that are well-recognized for their role in leukemia.

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Decoding DNA finds breast tumor signatures that predict treatment response