Primary tumors in certain metastatic cases have potential dissemination mechanisms. However, they often lack the potential to colonize distant microenvironments, and consequently the disseminated cancer cells enter into a state of latency which can last for years. In order to investigate the metastatic colonization potential at the gene expression level, we compared such primary tumors with their matching, actively proliferating metastatic tumors. Six pairs of colon-to-lung metachronous tumor samples were examined for the expression levels of 84 well-known metastatic genes using the quantitative RT-PCR-based PCR Array technology. The unsupervised hierarchical clustering of all 12 samples together, resulted in the formation of one closely related cluster by the primary tumors, but highly diversified ones by the metastatic tumors. A pair-wise comparison of the matching primary-metastatic tumors showed that different groups of genes were activated in the lung metastases. Therefore we charted specific genes involved in the genetic diversification processes. A number of these genes showed similar differential expression (ΔCt) patterns in all the patients. These were the cancer cell-, the microenvironment- and the stem cell-specific gene groups. In conclusion, the results suggest that the primary colorectal cancer cells are diversified as regards colonization of the lung, which could explain why the effective therapies for primary colorectal cancers are often not appropriate for controling the growth of pulmonary metastases.

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