WEDNESDAY, Dec. 6 (HealthDay News) -- A new tumor-cell biomarker may predict how well women do after they're diagnosed with breast cancer, researchers report.

It might also prove to be a valuable target for therapy, they added.

When expression of the marker, a protein called p27, is low, especially among patients with hormone-receptor-positive tumors, prognosis is poor.

However, "it remains to be seen if this is useful in clinical practice," said study author Dr. Peggy Porter, head of the breast cancer research program at Fred Hutchinson Cancer Research Center in Seattle. "It's clearly an important marker. We want to follow up on it as a target for therapy."

"It's another marker to help predict survival in patients after treatment," added Dr. Jay Brooks, chairman of hematology/oncology at Ochsner Health System in Baton Rouge, La. "The question is whether or not we can use this before we treat someone. If this is reproduced in other studies, this may represent an opportunity to use this to pick out a group of people who may be able to benefit from additional therapies."

The findings are published in the Dec. 6 issue of the Journal of the National Cancer Institute.

P27, which prevent cells from dividing, was first identified by Porter and colleagues more than a decade ago. But its value as a prognostic marker to guide treatment is still unclear. That's because studies that had been conducted up until now had failed to give all participants the same treatment, so researchers couldn't arrive at any firm conclusions -- it was possible that differences in outcome could have been due to the treatments and not p27.

In this study, researchers gave the same course of chemotherapy to more than 3,100 women nationwide. All of the women had newly diagnosed, moderate-risk breast cancer.

More than 2,000 breast tumor samples were analyzed via tissue microarray for evidence of p27 expression.

In women whose tumors had high p27 expression, five-year survival was 91 percent, the researchers reported. In women whose tumors had low p27 expression, five-year survival was 85 percent.

They also found no association between p27 expression and survival among women with hormone-receptor-negative tumors -- only among those with hormone-receptor-positive tumors. Hormone-receptor-positive tumors need the hormones estrogen or progesterone to grow.

Now researchers need to better define which women will benefit from this information.

"This is the first study to test this in patients who were treated similarly, and it's a big step in our understanding," Porter said. "We now want to look at people with hormone-positive cancer."

And, one day, p27 may also prove to be a target for new therapies, she said.

In related news, a second study in the same issue of the journal found that the BRCA1 and BRCA2 mutations, which increase risk for breast, ovarian and possibly other cancers, may be more common than originally thought.

Canadian researchers estimated that about 1 percent of people in the general Ontario population have these mutations, higher than previously imagined.