Clustered DNA damage in irradiated human diploid fibroblasts: influence of chromatin organization

Clustered DNA damages are induced by ionizing radiation and are defined as two or more lesions within one or two helical turns. The aim of this study was to investigate the induction and repair of clustered DNA damage in cells with emphasis on the influence of structural differences in the chromatin organization. Human fibroblasts were irradiated with X rays and induced DSBs and clustered damages were quantified using pulsed-field gel electrophoresis combined with postirradiation incubation with the base excision repair endonuclease Fpg, which recognizes oxidized purines and cleaves the strand at sites inducing strand breaks. Hence clustered damages appear in enzyme-treated samples as additional DSBs. The chromatin was modified by different pretreatments that resulted in structures with varying compactness and levels of free radical scavenging capacity. We found that the induction of DSBs and clustered damages increased linearly with dose in all structures and that both types of lesions were allocated randomly within the nucleus. The induction yields increased with decreasing compactness of chromatin, and the chromatin effect was larger for clustered lesions than for DSBs. Clustered damages were processed efficiently with a fast and a slow repair component similar to that for induced DSBs.