Enzymatic Restriction of Mammalian Cell DNA: Evidence for Double-strand Breaks as Potentially Lethal Lesions

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Abstract
Permeabilized Chinese hamster cells were treated with the restriction endonucleases Pvu II and Bam H1 which generate blunt-ended and cohesive-ended DNA double-strand breaks (dsb), respectively. Cells were then assayed for their clonogenic ability. These experiments were performed to test the hypothesis that mammalian cell death following X-ray exposure arises from the induction of dsb in DNA, and via the formation of chromosomal aberrations. It was shown previously that Pvu II induces chromosome aberrations whereas Bam H1 was ineffective in this respect. The results reported here show that Pvu II simulates X-ray exposure, in causing a dose-dependent loss of the reproductive integrity of mammalian cells. Dsb generated by Pvu II, i.e. with blunt ends, can therefore be regarded as potentially clastogenic as well as potentially lethal. Bam H1 was found not to reduce cell survival in the same enzyme dose range. These results support the notion that X-irradiated mammalian cells undergo a mode of death in which dsb in the DNA cause chromosomal aberrations which are lethal as a result of loss of genetic material in the form of chromosome fragments, or as a result of chromosome bridge formation.