Mechanisms involved in rejoining DNA double-strand breaks induced by ionizing radiation and restriction enzymes.

DNA double-strand breaks are considered to be the most deleterious lesion induced by ionizing radiation. However, the mechanism of rejoining of these lesions has not been extensively studied at the molecular level. We have used a shuttle vector, pHAZE, to analyze the mechanism of rejoining of DNA double-strand breaks in human cells. The advantage of this vector system is that, unlike many previously described shuttle vectors, it has a large target gene for the detection of deletions and it is maintained as a freely replicating episome with chromatin conformation in the nucleus of human cells. In this study we compare data obtained on the spectrum of mutations induced in pHAZE by ionizing radiation (alpha-particles) and restriction enzymes (PvuII, ClaI, and PvuI). Unlike ionizing radiation, restriction enzymes induce double-strand breaks in DNA with known end structures at defined locations and therefore provide a model system for analyzing cellular responses to DNA double-strand breaks. Exposure of human cells containing the vector to alpha-particle irradiation produced both point mutations and large deletions in pHAZE. When the junction regions of the deletions were sequenced it was found that 65% were rejoined with up to 6 bp of homology at the junction region. Analysis of restriction-enzyme-induced mutations suggests that double-strand break ends are modified to facilitate rejoining and that the type of modification is characteristic for different end structures. Double-strand breaks with cohesive ends appear to have fewer modifications introduced at the break points before rejoining than breaks with blunt ends. When considered in relation to the data obtained with ionizing radiation this suggests that the presence of cohesive sequences either at, or in proximity to, the ends enhances rejoining of DNA double-strand breaks.