The degradation profile of extrachromosomal circular DNA during cisplatin-induced apoptosis is consistent with preferential cleavage at matrix attachment regions

Extrachromosomal circular DNA molecules are prevalent in cancer cells and h arbor amplified genes, such as oncogenes and drug resistance genes, that ca n provide a selective growth advantage to cancer cells. These circular DNA structures include double minute chromosomes (dmin), which can be detected with light microscopy following Giemsa staining, and submicroscopic circula r DNA structures referred to as episomes. In this study, we investigated th e fate of dmin and episomes in multidrug-resistant human epidermoid KB-V1 c ells undergoing cisplatin-induced apoptosis - a mode of cell death initiall y characterized by the fragmentation of chromosomal DNA, while the nuclear membrane remains intact. The circular DNA structures carry amplified copies of the multidrug resistance gene (MDR1). During cisplatin-induced apoptoti c cell death, episomes and dmin, as well as native chromosomes, were degrad ed into high molecular weight DNA fragments of approximately 50 kb in lengt h. DNA fragments in this size range appear to result from the preferential cleavage of matrix-associated regions in chromatin with the subsequent rele ase of 20-30 nm loop domains of chromatin from the nuclear scaffold. Scanni ng electron microscopy studies were performed and confirmed the presence of 30 nm filaments in a higher-order DNA packing of MDR1-containing dmin and episomes. These combined data provide strong evidence that the higher-order DNA packing of episomes, as well as dmin, is similar to that of native chr omosomes and underscore the potential for extrachromosomal DNA amplicons to study the structural and functional organization of chromatin. We discuss the implications of extrachromosomal DNA matrix associated regions competin g with native chromosomal DNA for binding to the nuclear matrix in tumor ce lls.