Evidence that metastases arise de novo as new cancers from cells of target organs

Introduction Based on our earlier finding that cell-free chromatin particles (cfChPs) released from dying cancer cells are potentially oncogenic, we hypothesised that metastases arise as new cancers from the cells of target organs transformed by cfChPs released from dying cancer cells. Methods We fluorescently dually-labelled MDA-MB-231 human breast cancer cells and A-375 human melanoma cells in their DNA with BrdU and in their histones with CellLight Histone 2BGFP. One hundred thousand dually-labelled cells were intravenously injected into SCID mice. In other experiments unlabelled cells were injected for detection of lung metastasis. Also intravenously injected were 700 ng of purified cfChPs isolated from radiation-killed MDA-MB-231 cells. Results We observed that the fluorescently dually-labelled MDA-MB-231 and A-375 cells died upon reaching the lungs and released dually-labelled fluorescent chromatin particles that accumulated in the nuclei of lung cells at 48 h. Injection of unlabelled cfChPs led to the activation of 10 hallmarks of cancer and immune checkpoints in lung cells at 72 h, suggesting that the lung cells had rapidly transformed into incipient cancer cells. Fluorescent in situ hybridisation analysis of lung metastases that subsequently developed using mouse and human specific DNA probes revealed that the tumour cells contained both mouse and human DNA in almost equal proportions. Similarly, immune-fluorescence analysis using species specific mAbs revealed that the tumour cells co-expressed mouse and human specific proteins. Metaphase preparations and single-cell clones developed from cell cultures of lung metastases were found to contain chimeric chromosomes containing both mouse and human DNA, and the cells to co-express both human- and mouse-specific proteins. The Intravenously injected purified cfChPs isolated from radiation-killed MDA-MB-231 cells also induced lung metastasis which predominantly contained mouse DNA strongly suggesting that the metastatic tumours had arisen from the mouse lung cells. Conclusion These results provide strong evidence that cfChPs released from dying cancer cells integrate into genomes of target lung cells to transform them into new cancers that masquerade as metastasis. They support our hypothesis that metastases arise from the cells of target organs and not from those of the primary tumour. These findings have implications for the principles of cancer therapy.