Spontaneous apoptosis in mouse F4N-S erythroleukemia cells induces a nonrandom fragmentation of DNA

This study characterizes the fragmented DNA of mouse erythroleukemia (MEL) cells spontaneously entering apoptosis. Fragmented DNA was isolated by introducing a novel procedure that ensured a complete extraction of the characteristic oligonucleosomal ladder. As the results show, less than 10% of DNA of apoptotic cells is fragmented in this form. The main conclusion from experiments to characterize the nature of fragmented DNA is that spontaneous apoptosis induces a nonrandom cleavage of genomic DNA. The Southern analysis performed with total apoptotic DNA revealed that it is strongly enriched in interspersed repetitive sequences. In situ hybridizations with such DNA showed further than in interphase nuclei these sequences flock together and form clusters spread throughout the whole nuclear area whereas in mitotic chromosomes they are located predominantly at their pericentromeric/peritelomeric ends. Partial cloning and sequencing reinforces the notion that the apoptotic DNA is representative for a heterochromatinic portion of the mouse genome. Support for such an unexpected conclusion is coming also from experiments indicating that this DNA is heavily methylated and poorly transcribed.