"Our results are significant because they could change the way we think about the course of cancer progression. The molecular results provide new insight into the pathological changes that we have been observing for decades," says Dennis Sgroi, MD, of the Department of Pathology at MGH, co-senior author of the current report.
"Correlating new methods for diagnosing cancer based on gene expression signatures with currently accepted and proven pathological classifications is an important step in developing more effective strategies for diagnosing and treating breast cancer," he adds. "A major goal of our research is to demonstrate that new molecular analysis technologies can generate accurate gene expression signatures that precisely characterize tumor cells and that these signatures can be used to determine the molecular origin of the criteria pathologists use to classify tumors." Sgroi is director of breast pathology at MGH and is an assistant professor of Pathology at Harvard Medical School.
The authors note that it is widely held that breast cancer begins at a premalignant stage called atypical ductal hyperplasia, progresses into the preinvasive stage of ductal carcinoma in situ and culminates in the potentially lethal stage of invasive ductal carcinoma. Contrary to the researchers' initial expectations, the different stages of breast cancer showed remarkably similar molecular signatures or patterns of gene expression. Specifically, the preinvasive stages of breast cancer within one patient demonstrated a gene expression pattern that was highly similar to the pattern seen in the invasive, potentially lethal stage. This suggests that the genes conferring invasive behavior may already be expressed in the pre-invasive stage.
Intriguingly, distinct signatures were found in malignant cells extracted from tumors of different cancer grades – grade being another measurement by which tumors are classified. In other words, while tumors at all three of the traditional pathological stages do not significantly differ in their molecular signatures, there is a strong correlation between different gene expression profiles and differences in tumor grades. Additionally, these researchers have uncovered a subset of genes associated with high tumor grade and with the transition from the non-lethal stage to the potentially lethal stage.
A major goal of breast cancer research has been to characterize accurately the molecular events associated with cancer progression. The microscopic size of early-stage breast tumors and the varied cell populations present in tumor biopsies have hindered progress towards this goal. The researchers combined a number of different techniques to overcome these problems.
The team used Laser Capture Microdissection technology to select pure populations of tumor cells from biopsy samples representing the distinct breast cancer stages. The RNA from these cells was extracted, amplified and measured, allowing precise determination of which genes were turned on or turned off in the microdissected cells. The researchers measured the expression levels of over 12,000 genes simultaneously using DNA microarray technology.
"By combining the molecular pathology expertise at MGH with the advanced molecular analysis capabilities of Arcturus, we were able to obtain accurate molecular signatures from microscopic samples of pure cell populations representing all of the distinct stages and grades of cancer," says Mark Erlander, PhD, chief scientific officer of Arcturus and co-senior author of the PNAS report. "We feel that a new generation of diagnostic tests based on molecular analysis of tumor biopsies could provide superior criteria for determining which drugs are best suited for a particular patient and whether more aggressive treatment is warranted."
The research teams are continuing their collaboration and are identifying unique molecular signatures correlating with other important properties of tumors, such as the probability of disease recurrence after treatment and tumor response to specific drug therapies.
Massachusetts General Hospital, established in 1811, is the original and largest teaching hospital of Harvard Medical School. The MGH conducts the largest hospital-based research program in the United States, with an annual research budget of more than $350 million and major research centers in AIDS, cardiovascular research, cancer, cutaneous biology, neurodegenerative disorders, transplantation biology and photomedicine. In 1994, MGH and BWH joined to form Partners HealthCare System, an integrated health care delivery system comprising the two academic medical centers, specialty and community hospitals, a network of physician groups and nonacute and home health services. Visit www.massgeneral.org for more information.
Arcturus, founded in 1996, is a private company located in Mountain View, Calif. Arcturus manufactures and markets a complete product line of integrated instruments and reagents for microgenomics – the extraction and analysis of nucleic acids and proteins from laser-captured cells. The company has developed a unique technology platform enabling molecular analysis of tissue biopsy samples and has applied its platform to discover cell-specific molecular signatures associated with human diseases such as cancer. The company's Laser Capture Microdissection systems are installed in over 500 laboratories worldwide. Additional information on Arcturus can be found at www.arctur.com.
Journal
Proceedings of the National Academy of Sciences