A Model for the Diversity Explosion Fundamental to Metastasis

Abstract

Most deaths from cancer are caused by their metastases. Along with the ability to evade the immune system and to invade and disrupt very different tissue environments, metastases usually develop resistance to whatever therapeutic approaches are tried. Their extreme adaptability requires a diversity of traits from which natural selection can choose, but current models, such as the epithelial-to-mesenchymal transition (EMT), do not directly address how this diversity is generated. We observed that single cell clones of Panc1 human pancreas cancer cells that had had crispR knock outs (KO) of the gene for activin-like kinase 4 (ALK4) had developed a markedly diverse morphology. Time-lapse and fluorescence microscopy, FACS analysis and mitotic chromosome squashes provided evidence that the cells diversify profoundly in size, behavior and chromosome number. This diversity appears to develop through cytokinesis failure, the generation of very large multinuclear cells that exhibit a high range of nuclear configurations, coupled with continued cell divisions that sometimes generate three, four, and more daughter cells in one cell division. This astounding activity often resulted in cells that were much smaller than the normal Panc1 cells, and indirect evidence suggests they were significantly sub-ploidy, yet underwent regular cell divisions. A subset of the smaller cells were highly motile, and were observed to sometimes merge together or even into larger cells. This phenomenon could provide an unappreciated venue for transferring mutated genes, chromosome fragments, whole chromosomes or sets of chromosomes into an invaded cell. We suggest that these observations may be a serendipitous illustration of the missing link between the genetic mutations implicated in the development of primary carcinoma, and the extreme genomic diversity that characterizes malignant metastases. Graphical SummarySingle cell clones of Panc 1 ALK4 KO cultures changed from largely uniformly epithelioid in character (A) to cultures composed of an unexpectedly wide variety of cells, and that this variety is not explained by the traditional "EMT" (epithelial-to-mesenchymal-transition). These cells range from very small to very large, have varying degrees of motility, chromosome number, and cell division behavior. We describe the new morphologies and behaviors, and present time lapse movies and microscopic evidence that help explain the evolution of this variety. These new cell types integrate into colonies where the smallest cells gather into spherical masses (D). We believe that the heterogeneity in genomic content and behavior provides the variety needed for natural selection to select cells that are resistant to most treatments for metastatic cancers.