MYC assembles and stimulates topoisomerases 1 and 2 in a "topoisome".

High-intensity transcription and replication supercoil DNA to levels that can impede or halt these processes. As a potent transcription amplifier and replication accelerator, the proto-oncogene MYC must manage this interfering torsional stress. By comparing gene expression with the recruitment of topoisomerases and MYC to promoters, we surmised a direct association of MYC with topoisomerase 1 (TOP1) and TOP2 that was confirmed in vitro and in cells. Beyond recruiting topoisomerases, MYC directly stimulates their activities. We identify a MYC-nucleated "topoisome" complex that unites TOP1 and TOP2 and increases their levels and activities at promoters, gene bodies, and enhancers. Whether TOP2A or TOP2B is included in the topoisome is dictated by the presence of MYC versus MYCN, respectively. Thus, in vitro and in cells, MYC assembles tools that simplify DNA topology and promote genome function under high output conditions.

Topoisomerase 1 activity during mitotic transcription favors the transition from mitosis to G1.

As cells enter mitosis, chromatin compacts to facilitate chromosome segregation yet remains transcribed. Transcription supercoils DNA to levels that can impede further progression of RNA polymerase II (RNAPII) unless it is removed by DNA topoisomerase 1 (TOP1). Using ChIP-seq on mitotic cells, we found that TOP1 is required for RNAPII translocation along genes. The stimulation of TOP1 activity by RNAPII during elongation allowed RNAPII clearance from genes in prometaphase and enabled chromosomal segregation. Disruption of the TOP1-RNAPII interaction impaired RNAPII spiking at promoters and triggered defects in the post-mitotic transcription program. This program includes factors necessary for cell growth, and cells with impaired TOP1-RNAPII interaction are more sensitive to inhibitors of mTOR signaling. We conclude that TOP1 is necessary for assisting transcription during mitosis with consequences for growth and gene expression long after mitosis is completed. In this sense, TOP1 ensures that cellular memory is preserved in subsequent generations.

Coinhibition of topoisomerase 1 and BRD4-mediated pause release selectively kills pancreatic cancer via readthrough transcription.

Pancreatic carcinoma lacks effective therapeutic strategies resulting in poor prognosis. Transcriptional dysregulation due to alterations in KRAS and MYC affects initiation, development, and survival of this tumor type. Using patient-derived xenografts of KRAS- and MYC-driven pancreatic carcinoma, we show that coinhibition of topoisomerase 1 (TOP1) and bromodomain-containing protein 4 (BRD4) synergistically induces tumor regression by targeting promoter pause release. Comparing the nascent transcriptome with the recruitment of elongation and termination factors, we found that coinhibition of TOP1 and BRD4 disrupts recruitment of transcription termination factors. Thus, RNA polymerases transcribe downstream of genes for hundreds of kilobases leading to readthrough transcription. This occurs during replication, perturbing replisome progression and inducing DNA damage. The synergistic effect of TOP1 + BRD4 inhibition is specific to cancer cells leaving normal cells unaffected, highlighting the tumor's vulnerability to transcriptional defects. This preclinical study provides a mechanistic understanding of the benefit of combining TOP1 and BRD4 inhibitors to treat pancreatic carcinomas addicted to oncogenic drivers of transcription and replication.

TOP1 CAD-seq: A protocol to map catalytically engaged topoisomerase 1 in human cells.

TOP1 CAD-seq enables mapping of TOP1 sites of covalent engagement with DNA. The procedure depends upon enrichment of DNA-covalent adducts using chaotropic salts and immunoprecipitation with an antibody specific for TOP1. Here, we describe a step-by-step protocol compatible with Illumina sequencing and bioinformatic pipeline for preliminary data analysis. Compared to other approaches for the genomic study of topoisomerases, TOP1 CAD-seq provides information about active TOP1 engaged on the DNA, taking advantage of low background due to absence of crosslinking. For complete details on the use and execution of this protocol, please refer to Das et al. (2022).

Analysis of Myc Chromatin Binding by Calibrated ChIP-Seq Approach.

Here, we present a strategy to map and quantify the interactions between Myc and chromatin using a calibrated Myc ChIP-seq approach. We recommend the use of an internal spike-in control for post-sequencing normalization to enable detection of broad changes in Myc binding as can occur under conditions with varied Myc abundance. We also highlight a range of bioinformatic analyses that can dissect the downstream effects of Myc binding. These methods include peak calling, mapping Myc onto an integrated metagenome, juxtaposing ChIP-seq data with matching RNA-seq data, and identifying gene ontologies enriched for genes with high Myc binding. Our aim is to provide a guided strategy, from cell harvest through to bioinformatic analysis, to elucidate the global effects of Myc on transcription.

Cyclin A2 localises in the cytoplasm at the S/G2 transition to activate PLK1.

Cyclin A2 is a key regulator of the cell cycle, implicated both in DNA replication and mitotic entry. Cyclin A2 participates in feedback loops that activate mitotic kinases in G2 phase, but why active Cyclin A2-CDK2 during the S phase does not trigger mitotic kinase activation remains unclear. Here, we describe a change in localisation of Cyclin A2 from being only nuclear to both nuclear and cytoplasmic at the S/G2 border. We find that Cyclin A2-CDK2 can activate the mitotic kinase PLK1 through phosphorylation of Bora, and that only cytoplasmic Cyclin A2 interacts with Bora and PLK1. Expression of predominately cytoplasmic Cyclin A2 or phospho-mimicking PLK1 T210D can partially rescue a G2 arrest caused by Cyclin A2 depletion. Cytoplasmic presence of Cyclin A2 is restricted by p21, in particular after DNA damage. Cyclin A2 chromatin association during DNA replication and additional mechanisms contribute to Cyclin A2 localisation change in the G2 phase. We find no evidence that such mechanisms involve G2 feedback loops and suggest that cytoplasmic appearance of Cyclin A2 at the S/G2 transition functions as a trigger for mitotic kinase activation.