Tumor formation involves the accumulation of a series of genetic alterations that are required for malignant growth. In most malignancies, genetic changes can be observed at the chromosomal level as losses or gains of whole or large portions of chromosomes. Here we provide evidence that tumor DNA may be horizontally transferred by the uptake of apoptotic bodies. Phagocytosis of apoptotic bodies derived from H-ras V12 -and human c-myc-transfected rat fibroblasts resulted in loss of contact inhibition in vitro and a tumorigenic phenotype in vivo. Fluorescence in situ hybridization analysis revealed the presence of rat chromosomes or of rat and mouse fusion chromosomes in the nuclei of the recipient murine cells. The transferred DNA was propagated, provided that the transferred DNA conferred a selective advantage to the cell and that the phagocytotic host cell was p53-negative. These results suggest that lateral transfer of DNA between eukaryotic cells may result in aneuploidy and the accumulation of genetic changes that are necessary for tumor formation.

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Tumor endothelial cells have long been regarded as genomically stable and therefore less likely to develop resistance to antiangiogenic therapies. However, recent findings have challenged this notion. We have shown that DNA can be transferred between cells through phagocytosis of apoptotic bodies by adjacent viable cells. Propagation of the ingested DNA is prevented by the activation of the p53-p21 pathway. In this study, we examined whether concomitant transfer of tumor DNA with genes that inactivate the p53 pathway could overcome the barrier to tumor DNA propagation. Our results demonstrate that fibroblasts and endothelial cells are capable of acquiring and replicating tumor DNA when the apoptotic tumor cells contain the SV40 large T antigen. Analysis of the tumor stroma of xenotransplanted tumors in severe combined immunodeficient mice revealed that a sub-population of the endothelial cells contained tumor DNA. These cells maintained the ability to form functional vessels in an in vivo assay and concurrently express tumor-encoded and endothelial-specific genes. Malignant and non-malignant cells are dependent on a functional blood circulation for the supply of oxygen and nutrients.1 Evidence from transgenic mouse models demonstrate that tumors initially lack the ability to induce blood vessel formation but subsequently acquire the capacity to recruit vessels from the adjoining stroma.2 This 'angiogenic switch' coincides with increase in tumor mass and invasiveness. Tumor cells secrete a plethora of angiogenic factors that induce sprouting angiogenesis (i.e. the formation of new capillaries from pre-existing capillaries) and the differentiation of hematopoietic stem cells into endothelial cells, which then contribute to tumor vessel formation. 3A potential advantage in targeting the endothelial cells of the tumor vasculature is that these cells are regarded as diploid and genetically stable. However, recent data have challenged this notion. Uveal melanomas have been reported to form their own circulatory network by the formation of vascular channels lined by tumor cells. 4 These vasculogenic channels apparently link directly to normal vessels without evidence of angiogenesis. Streubel et al.5 analyzed the cellular origin of the microvascular endothelial cells of B-cell lymphomas in patients. Chromosomal translocations specific for the tumor cells were detected in the tumor endothelium, suggesting a close relationship between the two cell types. Furthermore, analyses of human tumors transplanted in severe combined immunodeficient (SCID) mice demonstrate that the endothelial cells of solid human tumors grown in mice are genetically unstable and exhibit aneuploidy with multiple chromosomes and centrosomes. 6 Cumulatively, these studies potentially imply that tumor endothelial cells exhibit genetic instability albeit the mechanisms by which genetic alterations are induced are presently unclear.Our previous studies and those of others have demonstrated that viral and chromosomal DNA can be efficiently transferred be...

We have previously shown that DNA from dying tumor cells may be transferred to living cells via the uptake of apoptotic bodies and may contribute to tumor progression. DNA encoding H-ras V12 and c-myc oncogenes may be transferred to the nucleus of the phagocyte but will only integrate and propagate in p53-and p21-deficient mouse embryonic fibroblasts, whereas normal cells are resistant to transformation. Here, we show that this protective mechanism (activation of p53 and p21 after uptake of apoptotic bodies) is dependent on DNA fragmentation, where inhibition of the caspase-activated DNase in the apoptotic cells, in conjunction with genetic ablation of lysosomal DNase II in the phagocytes, completely blocks p53 activation and consequently allows DNA replication of transferred DNA. We, therefore, suggest that there is a causal relationship between DNA degradation during apoptosis and p53 activation. In addition, we could further show that Chk2 À/À cells were capable of replicating the hyg R gene taken up from engulfed apoptotic cells, suggesting involvement of the DNA damage response. These data show that the phagocytosing cell is sensing the degraded DNA within the apoptotic cell, hence preventing these genes from being replicated, probably through activation of the DNA damage response. We, therefore, hypothesize that DNase II together with the Chk2, p53, and p21 pathway form a genetic barrier blocking the replication of potentially harmful DNA introduced via apoptotic bodies, thereby preventing transformation and malignant development. (Mol Cancer Res 2006;4(3):187 -95)