Metastasis-Associated genes and metastatic tumor progression.

The morbidity in most cancer patients is not due to their primary cancers; it is due to metastatic disease. Thus understanding the progression of tumors to the metastatic state and the changes that take place in highly malignant cells are important in the development of new therapeutic approaches to diagnose, prognostically assess and treat highly progressive malignancies. Changes in the expression of certain genes or alterations of gene structures and encoded products can result in benign tumor cells progressing to the invasive and metastatic states. This has been shown in the laboratory by transferring dominantly acting oncogenes into susceptible cells and then testing the malignant properties of these cells in vivo. Usually such rapid qualitative changes in malignant state occur only rarely; the natural progression of tumor cells to the invasive or metastatic state occurs through a slow stepwise process of change. Tumor progression, in some instances, can be reversible, involving changes in dominantly acting oncogenes or tumor suppressor genes. The natural progression of tumors to highly malignant states also involves their ability to circumvent host microenvironmental controls that regulate cellular growth and diversity. Quantitative changes in gene expression rather than qualitative changes in gene structure are important in microenvironmental effects on progression. One of the important mechanisms of cellular regulation in epithelial tissues, such as breast epithelium, appears to be mediated by intercellular junctional communication. Changes in gene expression can result in loss of junctional communication, followed by cellular diversification and progression. Highly malignant tumor cells that have slowly evolved in vivo with only a few qualitative changes in gene structure have probably undergone extensive cycles of diversification and have multiple quantitative differences in gene expression. Some of these genes are related to metastasis. For example, we have identified a novel gene called mta1 (rat) or MTA1 (human) that appears to be involved in mammary cell motility and growth regulation. This may be an example of a gene that regulates highly malignant cellular phenotypes. When coupled with other cellular changes, such as loss of intercellular communication, specific changes in gene expression may result in cellular diversification and tumor progression.