Gene mutations in breast tissue may make cancer detection more difficult

Until now, researchers thought that breast cancer nearly always began when cells in the epithelium went haywire. But new research suggests that genetic mutations can ? and do ? occur initially in a deeper layer of breast tissue, called the stroma.

This presents a serious concern for physicians, who believed that breast tumors spread from epithelial tissue.
“Genetic alterations in carcinomas, including breast cancers, have always been attributed to epithelial cells,” said Charis Eng, Klotz professor and director of the Clinical Cancer Genetics Program at Ohio State University. She co-authored a new study that looks at genetic mutations in breast tissue.
“Breast cancer therapy and detection have always targeted the epithelium,” she said. “We need to rethink the conventional methods of treating and detecting the disease. Stromal tissue may be too deep for a mammogram to detect a tumor.”

Eng and her colleagues looked for breast tissue that had undergone loss of heterozygosity ? a genetic change that could lead to tumor development. They looked for such changes in epithelial and stromal breast tissue. The epithelium lines the outer surfaces of the body and organs and, in the breast, contains milk ducts and glands. Stroma is the connective tissue that supports this network.

“We think this kind of genetic change (loss of heterozygosity) might be the smoking gun in the development of certain types of cancer,” said Eng, who is also a professor of molecular virology, immunology and medical genetics and director of the division of human genetics in the department of internal medicine at Ohio State.

The study appears in a recent issue of the journal Nature Genetics.

The researchers analyzed 50 breast tissue samples taken from epithelial and stromal tumors. They looked for loss of heterozygosity in two known tumor-suppressor genes, TP53 and PTEN. Altered forms of either gene have been linked to the development of breast cancer.
The genes were isolated using a laser technique that allowed the researchers to dissect the tissue samples cell by cell.

Half of the tumor samples contained mutated TP53 genes: 11 samples had mutations in the epithelium alone, nine had mutations in the stroma alone and five had mutations in both tissue layers.

Nearly one-third of the breast cancer samples contained PTEN mutations. Yet these alterations were found in either the stroma or the epithelium only; just one tissue sample contained PTEN mutations in both types of tissue.

In addition to finding variations in the frequency and distribution of genetic mutations, the researchers also discovered that, with the exception of two samples, PTEN and TP53 mutations didn’t co-occur in the same tissue compartment.

“Yet when genetic mutations were found in both tissue compartments, at least three of the samples had different types of mutations between epithelium and the respective stroma,” Eng said. “This helps rule out cross-contamination between the tissues.”

Their exclusive nature and the presence of discordant mutations even within the same gene suggest that genetic alterations occur independently in the stroma and the epithelium. That is, a mutation could first occur in the stroma, rather than in the epithelium.

“These mechanisms aren’t limited to breast cancer,” Eng said. She and her colleagues are also looking at similar genetic changes in other solid tumors.

This research was supported by the Jimmy V Golf Classic Award for Translational Cancer Research from the V Foundation; the American Cancer Society; the Department of Defense U.S. Army Breast Cancer Research Program; and the National Cancer Institute.