Researchers at the University of California, San Diego have used a new strategy to identify differences between non-metastatic and highly metastatic breast cancer cells. The article by Valerie Montel et al., "Expression profiling of primary tumors and matched lymphatic and lung metastases in a xenogeneic breast cancer model," appears in the May 2005 issue of The American Journal of Pathology and is accompanied by a commentary.
The significance of the study's findings lies in how the microarray method was employed. Previous studies have examined the patterns of genes that are active in primary human tumors, but the genetic differences that exist between individual patients can make interpretation of such results difficult. The beauty of the Montel et al. study is the use of microarrays to analyze variations in gene activity between cancer cell lines with differing capability to spread to distant organs (metastasize) but derived from the same human breast cancer. This eliminates the problem of irrelevant genetic variability among tumors derived from different patients.
The study, performed in the lab of Dr. David Tarin, used three cell lines that were weakly, moderately, or highly metastatic when injected into mice with compromised immune systems. Because the injected cells were labeled with green fluorescence protein (GFP), dissemination of the cancer cells could be tracked accurately due to their green glow.
Each of the three cell lines was injected into the mammary pads of mice, and metastasis was monitored by examining migration of cells to the lymph nodes and lungs. As expected, the weakly metastatic cells rarely moved to other sites while the moderately and highly metastatic cells migrated at increasing frequencies. The resulting primary and secondary (metastatic) tumors were then harvested for gene expression analysis by microarray technology.
Using a gene chip of 22,000 genes, the researchers determined which genes were turned "on" and "off" in primary versus metastatic tumors. Interestingly, few differences existed between the genes expressed in primary and secondary tumors originating from the same injected cell line. However, comparisons between non-metastatic and highly metastatic tumors identified several genes with altered expression patterns. This was further confirmed by analyzing RNA and protein levels from the tumors in vivo and the original cell lines in culture.