Molecular genetics of metastasis.

We have adopted various approaches to identifying the genes(s) involved in metastasis. The first has been to observe whether introducing a defined activated oncogene (c-Ha-ras 1) into non-neoplastic cells confers not only tumorigenicity but other characteristics of malignancy. A second approach involves transfection of total genomic DNA from highly metastatic into nonmetastatic tumour cells. Thirdly, we are studying whether treatment of weakly metastatic tumour cells with agents known to influence tumour progression and gene expression (e.g. 12-O-tetradecanoylphorbol-13-acetate or 2'-deoxy-5-azacytidine) can affect metastatic capability. It was found that 3T3 fibroblasts which incorporated and expressed the activated rasH oncogene became tumorigenic and capable of lung colonization but not spontaneously metastatic. Additionally, transfection of inert tumour cells with DNA from highly metastatic human and animal cell lines sometimes markedly augmented their spontaneous metastatic capability and their lung colony-forming potential and induced them to form deposits in many extrapulmonary sites. Treatment of some tumour cell lines with azacytidine and 12-O-tetradecanoylphorbol-13-acetate markedly increased their metastatic behaviour after subcutaneous inoculation. Because several cell divisions occurred to produce the subcutaneous tumour before the cells disseminated, we consider the changed phenotype to be heritable and probably caused by alterations in gene expression. These results suggest that components of the metastatic phenotype are heritable and highly conserved in evolution and can be conferred on previously non-metastatic tumour cells by transfer of genomic DNA. In other studies we found that metastasizing tumour cells reach all organs in the body within minutes of entry into the blood but that the distribution of subsequent secondary tumours is neither uniform nor proportional to the numbers of cells retained in each site. The patterns of distribution of metastases tend to be related to the tissue of origin of the primary tumour. This was confirmed in observations on patients with intractable malignant ascites treated with peritoneo-venous shunts. Co-culture of tumour cells with fragments of various organs in vitro supported the conclusion that the normal cells of organs can support or inhibit secondary tumour formation. These observations collectively indicate that metastasis results from acquired abnormalities in gene regulation in tumour cells, but that the resulting abnormal cell behaviour can sometimes be modified or inhibited by local or systemic conditions in the host.