On the mechanisms and consequences of cell to cell DNA transfer

Sammanfattning: Development of cancer is a multistep process where an accumulation of genetic alterations drives normal cells to malignancy. In most cancer cells genetic changes can be observed both at the level of point mutations and as gains and losses of chromosomes. The eukaryotic cell has a complex protection system to recognize genetic alterations and, in the case of oncogene activation or severe DNA damage, direct the cell to undergo apoptosis. One of the morphological attributes initially used to define apoptosis is the fragmentation of DNA and the formation of apoptotic bodies. In healthy tissues apoptotic cells are rapidly phagocytosed by neighboring cells. Dying cells can thereby be cleared without leakage of harmful enzymes or eliciting an inflammatory response. The fragmentation of DNA that occurs in apoptotic cells is complemented by DNases in the phagocytosing cell, which ensure the destruction of potentially harmful DNA. However, it has been recently demonstrated that genes from apoptotic cells can avoid fragmentation and be salvaged and reutilized by the phagocytosing cell. This newly recognized mechanism of introducing foreign DNA is efficient since 15 percent of bovine aortic endothelial cells have been demonstrated to display uptake of apoptotic DNA and express this DNA. Furthermore, normal cells have been reported to be protected from propagation of DNA recovered from dying cells by the p53-p21 signaling pathway. This thesis demonstrates that DNA from apoptotic cells can enter the nucleus of phagocytosing cells by a previously undescribed mechanism of nuclear fusion named Pirinosis as an acronym of the Greek words Pirinas (nucleus) and Enosis (union). The degree of DNA fragmentation after phagocytosis is demonstrated to be crucial for signaling via the Chk2-p53-p21 pathway. Furthermore, DNA fragments within the nucleus of phagocytes are demonstrated to co-localize with early markers of the DNA damage pathway. Others have shown that endothelial cells in the tumor microenvironment display genetic alterations and aneuploidy and hence the dogma that diploid cells of the blood vessel wall are stable is challenged. This turned our focus to horizontal gene transfer between tumor cells and the surrounding stroma. Tumor associated endothelial cells isolated from rat tumors grown in mice displayed the same karyotype and inter-species chromosome fusions as detected after uptake of DNA from apoptotic cells in vitro. The hybrid tumor associated endothelial cells are shown to be functional in forming blood vessels that anastamose with the host circulatory system. We argue that phagocytosis of dying tumor cells by endothelial cells may create viable hybrid cells that are capable of creating functional vessels in vivo.