Horizontal gene transfer by uptake of apoptotic bodies

Abstract

In adults as well as during embryogenesis, apoptosis plays a key role in the elimination of cells in multicellular organisms. Apoptosis is important for the maintenance of tissue homeostasis. It is also of importance in tumor development, where tumor cells die by apoptosis due to low levels of oxygen and nutrients. Tumor progression is a multistep process where the accumulation of several genetic alterations can drive the transformation of a normal cell into a highly malignant tumor cell. In most malignancies, genetic changes can be observed at the chromosomal level as losses or gains of chromosomes leading to genetically unstable tumor cells. In human cancers chromosomal instabilities such as aneuplody, translocations and gene amplifications have been observed.

Today, it is well known that apoptotic cells will be rapidly cleared by phagocytosing cells. It is important that genetic material from apoptotic cells will not interact with DNA in the phagocytes since such genetic alterations could generate a malignant phenotype. It is therefore necessary for a cell to protect its own genome from incoming apopotic DNA. Until recently it was thought that the clearance of engulfed apoptotic bodies was only serving as a way to eliminate dead cells in order to prevent an inflammatory response, and that destruction of apoptotic cells always included destruction of the genes from the engulfed cell.

However, in this thesis we show that genes from apoptotic cells can be transferred and reutilized by phagocytosing cells. We found that only cells with impaired Chk2p53-p21 DNA damage pathway were able to propagate the horizontally transferred DNA, and a positive selection pressure was required as well. Normal phagocytosing cells with an intact DNA damage pathway were not able to Propagate the horizontally transferred DNA, but became senescent or died. This shows that the DNA damage pathway is protecting normal cells from replicating foreign apoptotic DNA. Furthermore, uptake of apoptotic bodies from transformed cells resulted in transformation of p53-/- and p21-/- recipient cells leading to a tumorigenic phenotype. Analyses of tumors developed from these transformed p53-/- and p2l-/- recipient cells showed that large amounts of genetic material had been transferred from the apoptotic donor cell. We found large fragments of chromosomes and even whole chromosomes as well as fusion-chromosomes within the tumor cells. These genetic alterations are similar to the ones frequently observed in human tumors and therefore we speculate that horizontal gene transfer of DNA between tumor cells may contribute to the genomic instability so commonly described in tumor cells.