XWL-5-11

The synergistic effects of drug combinations have emerged as a promising approach for achieving efficient cancer treatment. Through our exploration of drug combinations, we found that medroxyprogesterone acetate (MPA), a steroid, induced a synergistic antitumor effect in combination with the topoisomerase II inhibitor etoposide (ETP). In this study, we investigated the mechanisms underlying this synergistic effect for potential clinical applications. To elucidate the relevant mechanisms, we performed a cell viability assay, cell cycle analysis, DNA repair assays, detection of DNA double‐strand breaks (DSBs) and the nuclear localization of topoisomerase II (Top2), and genome‐wide detection of DSBs. MPA synergistically increased ETP‐induced DSBs, resulting in cell cycle arrest in the G2/M phase. Interestingly, this effect was not due to the inhibition of DSB repair but to a specific increase in the Top2‐DNA covalent complex formed by ETP. A genome‐wide search for DSB locations revealed that DSB formation was promoted near promoter regions, suggesting the involvement of MPA transcriptional modulation in this mechanism. We also found that various steroids promoted DSB formation when combined with ETP, strongly supporting our synergistic model. Therefore, this synergistic effect is based on an innovative mechanism that differs from conventional strategies targeting the DNA damage response and is expected to contribute toward novel therapeutic options.

Topoisomerase II (TOP2) inhibitors (TOP2i) are mainstay chemotherapeutic agents that undermine genome integrity by stabilizing TOP2-DNA complexes accompanied by DNA damage formation. Here, we reveal the uncharacterized protein CRAMP1 and H1 linker histones as key effectors of TOP2i tolerance in human cells. We demonstrate that CRAMP1 defines a dedicated histone H1 biogenesis factor stimulating transcription of both replicative and non-replicative H1 genes, driven by its concurrent targeting to histone gene loci and H1-specific promoter motifs. CRAMP1 promotes TOP2i tolerance by maintaining H1 supply, involving a novel mechanism uncoupled from TOP2i-induced DNA damage whereby reducing the H1 pool triggers unscheduled TOP2 substrate formation in low-accessibility chromatin states. This amplifies total demand for TOP2 activity, lowering the threshold for TOP2i-mediated exhaustion of TOP2. Our discoveries elucidate the mechanistic basis of histone H1 biogenesis in human cells, opening opportunities for selectively manipulating linker but not core histone supply and targeting cancer-associated H1 deficiency.