This was a surprise because scientists have identified myriad different types of breast cancers based on their genetics and other characteristics.
In this case, one group showed high methylation and one showed low.
The high-methylation group "tended to be pretty stable at the genomic level and were composed primarily of hormone-positive cancers," Chan said. "The [low-methylation] group includes all the hormone-negative group and about half of hormone-positive cancers."
And women with high-methylation tumors did much better than the other group, independent of their hormone-receptor status.
The team is now working on a test to distinguish the two epigenetic types and are investigating what drives these changes, which have also been observed in colon cancer and the deadly brain cancer glioblastoma.
"What we're probably looking at is some fundamental process that becomes dysregulated and helps drive the cancers," Chan said.
"This [study] allows us to further subset breast cancers into those that are likely to metastasize and those that aren't likely to metastasize, and that's helpful," noted one expert, Dr. Patrick Borgen, chairman of the department of surgery at Maimonides Medical Center in New York City.
But, Borgen cautioned, "the technology that [does this categorization] is far from the grasp of day-to-day clinicians. [The study is] an important foundation for further research."
Learn more about epigenetics at the University of Utah.
SOURCES: Timothy A. Chan, M.D., Ph.D., lab head and attending physician, Memorial Sloan-Kettering Cancer Center, New York City; Patrick Borgen, M.D., chairman, department of surgery, Maimonides Medical Center, New York City; March 23, 2011, Science Translational Medicine
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