Mammary tumor cells disseminate during atypical hyperplasia in Her2-transgenic mice

2399 It has been known for long that many patients with very small tumors will eventually succumb to their disease despite curative surgery. Moreover, up to 7% of cancer patients are diagnosed with cancer of unknown primary, i.e. metastasis in the absence of primary cancer. Interestingly, also for sporadic breast cancer clinical and experimental data point to dissemination of human breast cancer cells at a very early stage of tumor development. All these findings suggest that the popular linear progression model, predicting metastatic spread to occur late in disease progression, may not be applicable to all cancer patients. To address this question we analyzed systemic breast cancer progression in a model of human breast cancer, the Her-2 transgenic mouse from epithelial transformation to metastasis. BALB/c mice transgenic for the activated rat HER-2/neu closely mimic the progression and gene expression profiles of human breast cancer. We screened lungs and bone marrow samples of mice with antibody directed against the transgene Her-2 and additionally antibody against cytokeratin (CK) from various time points to assess the onset of metastatic spread. The isolated DNA of single cells, areas of primary tumors and of manifest pulmonary metastases was analyzed by comparative genomic hybridization (CGH). To our great surprise we detected metastatic cells in lung and bone marrow at a time when mammary glands showed histologically non-invasive atypical hyperplasia only but no tumors. We carefully analyzed primary lesions at this stage of disease by transmission electron microscopy and indeed observed tumor cells sneaking through the basement membrane. To confirm the malignant potential of early-disseminated cancer cells we performed bone marrow transplantation. Bone marrow from mice displaying only atypical hyperplasia was transplanted into wild-type siblings. Strikingly, transplantation of only 80 disseminated cancer cells resulted in bone marrow carcinosis in the recipient animals.These findings suggest that particular genetic changes in a permissive genetic background initiate systemic cancer early during carcinogenesis. Further analysis of Her2-transgenic mice may reveal mechanisms of metastatic spread that are at variance with conventional models but may explain many clinical courses observed in human cancer.