Genomic DNA transfer from apoptotic bodies
Department of Oncology, Karolinska Institutet, SE171 76 Stockholm Sweden
AIM: We have previously shown that genomic DNA may be transferred to one cell to another by the uptake of apoptotic bodies. Replication of transferred DNA could only be detected in cells deficient in p53 or its target gene p21. Here we have analyzed the role of the DNA damage response pathways in the protection against replication of DNA transferred by uptake of apoptotic bodies. METHODS: We have studied the transfer of the ras and myc oncogenes as well as the gene encoding hygromycin resistance to mouse embryonic fibroblasts deficient in Chk2 or DNase II genes. The activation of the p53 tumor suppressor protein was analyzed by immunofluorescence and western blot. RESULTS: We show that uptake of apoptotic bodies results in p53 and p21 accumulation in the phagocyte. This activation of p53 is dependent on Chk2 as no induction of p53 could be detected in Chk-2 null cells. Consistently, Chk-2 deficient cells were able to take up the hygromycine resistance gene from apoptotic bodies and thereby generate drug resistant colonies. We hypothesized that DNA fragmentation during apoptosis could generate double strand breaks that would activate the DNA damage response in the engulfing cell. We could show that joint inhibition of DNA fragmentation in the apoptotic body by ICADldm as inhibition of lysosomal DNA degradation by genetic ablation of DNase II totally blocked p53 activation. Interestingly, Inhibition of the DNase II allowed gene transfer in cells with a otherwise intact DNA repair response. CONCLUSIONS: Here we provide evidence that DNA fragmentation in the apoptotic cells activate the DNA damage response in the phagocyte. DNA double strand breaks results in Chk-2 dependent accumulation of p53 and p21 resulting in cell cycle arrest. We propose that the DNases together with the DNA damage response pathway constitute a genetic barrier preventing replication of foreign genomic DNA acquired by the uptake of apoptotic bodies. Since inactivation of the Chk-2, p53 is common in human cancer we argue that tumor cells may have the ability to exchange DNA via horizontal gene transfer which may be an efficient mechanism of scrambling DNA and thus adjusting to new selection pressures.
Paper presented at the International Symposium on Predictive Oncology and Intervention Strategies; Nice, France; February 7 - 10, 2004; in plenary session 703 (Oncogenic pathways).