Relationship of DNA damage signaling to DNA replication following treatment with DNA topoisomerase inhibitors @entity9800 /topotecan, XXXX , or etoposide.

@entity10081 @entity5 @entity2664 @entity10080 @entity1191 @entity9800
DNA topoisomerase I (Top1) and topoisomerase II (Top2) inhibitors are widely used to treat a variety of @entity5 . Their mechanism of action involves stabilization of otherwise transient ("cleavable") complexes between Top1 or Top2 and DNA; collisions of DNA replication forks with such stabilized complexes lead to formation of DNA double-strand breaks (DSBs). In this study, using @entity10080 ( @entity10081 ) as a DNA precursor, we directly assessed the relationship between DNA replication and induction of DSBs revealed as yH2AX foci in A549 cells treated with Top1 inhibitors topotecan (Tpt) or @entity9800 ( @entity1191 ) and Top2 inhibitors @entity2664 (Mxt) and etoposide (Etp). Analysis of cells by multiparameter laser scanning cytometry following treatment with Tpt or Cpt revealed that only DNA replicating cells showed induction of yH2AX and a strong correlation between DNA replication and formation of DSBs (r = 0.86). In cells treated with Mxt or Etp, the correlation was weaker (r = 0.52 and 0.64). In addition, both Mtx and Etp caused induction of yH2AX in cells not replicating DNA. Confocal imaging of nuclei of cells treated with Tpt revealed the presence of yH2AX foci predominantly in DNA replicating cells and close association and co-localization of yH2AX foci with DNA replication sites. In cells treated with Mxt or Etp, the yH2AX foci were induced in DNA replicating as well as non-replicating cells but the close association between a large proportion of yH2AX foci and DNA replication sites was also apparent. The data are consistent with the view that collision of DNA replication forks with cleavable Top1-DNA complexes stabilized by Tpt/Cpt is the sole cause of induction of DSBs. Additional mechanisms such as involvement of transcription and/or generation of oxidative stress may contribute to DSBs induction by Mxt and Etp. The confocal analysis of the association between DNA replication sites and the sites of DSBs (yH2AX foci) opens a new approach for mechanistic studies of the involvement of DNA replication in induction of DNA damage.
