Estrogen receptor α-mediated transcription induces cell cycle-dependent DNA double-strand breaks

Prolonged exposure to estrogen increases breast cancer risk. Estrogen is known to induce chromosomal aberrations, yet the mechanisms by which estrogen promotes genomic instability are not fully understood. Here, we show that exposure of MCF-7 cells to 17 beta-estradiol (E2) induces DNA double-strand breaks (DSBs), as determined by the formation of gamma H2AX foci. Foci formation was dependent upon estrogen receptor-alpha (ER alpha) and the catalytic activity of the type II topoisomerase, topoisomerase II beta (topoII beta). Moreover, we show by chromatin immunoprecipitation that topoII beta-dependent E2-induced gamma H2AX localizes to the promoter of the estrogen-inducible gene, trefoil factor 1. E2-induced foci were associated with cyclin A expression and inhibited by pre-incubation with the DNA polymerase inhibitor aphidicolin suggesting that E2-induced DSBs are mediated by progression through S phase. Furthermore, E2-induced gamma H2AX foci colocalized with Rad51, suggesting that E2-induced DSBs are repaired by homologous recombination. We propose that DNA DSBs formed by the strand-cleaving activity of the topoII beta-DNA cleavage complex at estrogen-inducible genes can present a barrier to DNA replication, leading to persistent DNA DSBs in ER alpha-positive breast cancer cells.