Response of radiation-sensitive human cells to defined DNA breaks

We have investigated the response of four human cell lines, representing a range of sensitivities to ionizing radiation, to enzymes which induce defined DNA double-strand breaks (dsbs). Cell lines were derived from a normal individual, from the cancer-prone disorders ataxia-telangiectasia (AT) and Bloom's syndrome (BS), and from an immunodeficient individual (46BR). The molecular defects in AT and BS are unknown, while 46BR is known to be DNA ligase I deficient. We assayed the clonogenic survival of the cell lines following in vivo scission of the DNA by the restriction endonucleases PvuII and BanI. These two enzymes differ in their action; PvuII gives rise to dsbs with blunt termini, while BanI generates staggered ends with a 4 bp overhang. We found a correlation between the sensitivity of the cell lines to X-rays and to the blunt-end cutter PvuII, but not to the cohesive-end cutter BanI.