Cell Fusion Connects Oncogenesis with Tumor Evolution

Abstract

Cell fusion likely drives tumor evolution by undermining chromosomal and DNA stability and/or by generating phenotypic diversity; however, whether a cell fusion event can initiate malignancy and direct tumor evolution is unknown. We report that a fusion event involving normal, nontransformed, cytogenetically stable epithelial cells can initiate chromosomal instability, DNA damage, cell transformation, and malignancy. Clonal analysis of fused cells reveals that the karyotypic and phenotypic potential of tumors formed by cell fusion is established immediately or within a few cell divisions after the fusion event, without further ongoing genetic and phenotypic plasticity, and that subsequent evolution of such tumors reflects selection from the initial diverse population rather than ongoing plasticity of the progeny. Thus, one cell fusion event can both initiate malignancy and fuel evolution of the tumor that ensues.

Genomic Catastrophe and Neoplastic Transformation

“…In addition to chromosomal instability, fusion-derived IEC-6 cell clones exhibited DNA damage in the form of doublestrand breaks; 35% to 42% of cells from fusion-derived clones exhibited evidence of double-strand breaks compared to only 4% to 9% of nonfused clones. 4 Despite DNA damage in fusion-derived IEC-6 cell clones, activated caspase 3 was rarely detected, indicating that DNA damage does not lead to apoptosis in these cells.…”

“…4 Fusion-derived IEC-6 cell clones were also heterogeneous with respect to ploidy; near-diploid clones with a modal chromosome number of 42 exhibited a wide range of chromosome numbers indicating the presence of significant subpopulations of aneuploid cells within the cell population. 4 These observations combine to suggest that early after a cell fusion event, the genomes of fusion-derived clones exhibit instability that manifests as changes in ploidy, with generation of aneuploid clones, neardiploid clones that likely have aberrant karyotypes despite a near-normal chromosome number, and near-diploid clones that contain significant subpopulations of aneuploid cells. In addition to chromosomal instability, fusion-derived IEC-6 cell clones exhibited DNA damage in the form of doublestrand breaks; 35% to 42% of cells from fusion-derived clones exhibited evidence of double-strand breaks compared to only 4% to 9% of nonfused clones.…”

“…Among the fusion-derived IEC-6 cell clones examined, 41% were aneuploid and 56% were near-diploid, whereas 86% of non-fused cell clones were diploid. 4 This suggests that cell fusion engenders genomic instability.…”

“…Fusion-derived IEC-6 cell clones were successfully established from 19% of FACS-sorted cells, 4 suggesting that a subset of cell fusion events result in clones that can proliferate and expand. Among the fusion-derived IEC-6 cell clones examined, 41% were aneuploid and 56% were near-diploid, whereas 86% of non-fused cell clones were diploid.…”

“…Fused cells were larger in size and exhibited dual fluorescent signal, whereas non-fused cells were of normal size and displayed only one fluorescent marker. 4 Fused cells were sorted and clonal populations were established and subsequently characterized for growth properties in vitro, markers of DNA damage, and tumorigenic potential in vivo. 4…”