Activation of the maternally preset program of apoptosis by microinjection of 5-aza-2′-deoxycytidine and 5-methyl-2′-deoxycytidine-5′-triphosphate in Xenopus laevis embryos

The present study examines the effects on embryogenesis of microinjecting Xenopus laevis fertilized eggs with 5-aza-2′-deoxycytidine (5-Aza-CdR), which induces hypomethylation of DNA, and 5-methyl-2′-deoxycytidine-5′-triphosphate (5-methyl-dCTP), which induces hypermethylation of DNA. Embryos injected with either one of these analogs cleaved normally until the mid-blastula stage, but underwent massive cell dissociation and stopped development at the early gastrula stage. Dissociated cells that appeared here were positive by terminal deoxyribonucleotidyl transferase-mediated deoxyuridine triphosphate-digoxigenin nick end-labeling and contained fragmented nuclei with condensed chromatin. The DNA from these cells formed a 'ladder' on electrophoresis. Furthermore, the induction of cell dissociation by 5-Aza-CdR and 5-methyl-dCTP was postponed by 2-3 h by co-injection of Bcl-2 mRNA and the normal metabolite (CdR and dCTP, respectively). Using a specific antibody against 5-methyl-cytosine, we confirmed that 5-Aza-CdR induces hypomethylation, whereas 5-methyl-dCTP induces hypermethylation in X. laevis embryos before the onset of cell dissociation. Incorporation of radioactive precursors revealed that synthesis of DNA, and also RNA, is inhibited significantly in both 5-Aza-CdR-injected and 5-methyl-dCTP-injected embryos. These results show that 5-Aza-CdR and 5-methyl-dCTP are incorporated into DNA and induce apoptosis, probably through alteration of DNA methylation coupled with inhibition of DNA replication and/or transcription.