Transformation of non-tumor host cells during tumor progression: theories and evidence.

INTRODUCTION: Most cancer deaths are due to the development of metastases and this phenomenon is still a hard challenge for researchers. A number of theories have tried to unravel the metastatic machinery, but definitive results that link the evidence with conventional concepts of metastatic disease remain to be reported. AREAS COVERED: Considerable evidence suggests interactions between tumor cells and host cells that might be essential for tumor progression and metastasis. Most such evidence is suggestive of fusion phenomena, but some suggest the transfer of cell-free DNA (cfDNA). Such evidence is often ignored or overlooked in the assessment and management of malignancy. In this article, we review the available evidence for the importance of cell fusion and cfDNA in metastasis, and we present some preliminary data that support the hypothesis that tumor progression might be based not only on the division of tumor cells but also on the transformation of normal cells. EXPERT OPINION: Future success in the search for cancer therapies will surely require advances in our knowledge of the pathways of tumor invasion by unexpected mechanisms. Thus, no well supported evidence for roles of cell-free nucleic acids and fusion of cells or of cells with vesicles should be ignored.