Although both homologous recombination (HR) and nonhomologous end joining can repair DNA double-strand breaks (DSBs), the mechanisms by which one of these pathways is chosen over the other remain unclear. Here we show that transcriptionally active chromatin is preferentially repaired by HR. Using chromatin immunoprecipitation-sequencing (ChIP-seq) to analyze repair of multiple DSBs induced throughout the human genome, we identify an HR-prone subset of DSBs that recruit the HR protein RAD51, undergo resection and rely on RAD51 for efficient repair. These DSBs are located in actively transcribed genes and are targeted to HR repair via the transcription elongation-associated mark trimethylated histone H3 K36. Concordantly, depletion of SETD2, the main H3 K36 trimethyltransferase, severely impedes HR at such DSBs. Our study thereby demonstrates a primary role in DSB repair of the chromatin context in which a break occurs.
CDC25B phosphatases must activate cyclin B-CDK1 complexes to restart the cell cycle after an arrest in G2 phase caused by DNA damage. However, little is known about the precise mechanisms involved in this process, which may exert considerable impact on cancer susceptibility and therapeutic responses. Here we report the discovery of novel N-terminally truncated CDC25B isoforms, referred to as ?N-CDC25B, with an exclusively nuclear and nonredundant function in cell cycle re-initiation after DNA damage. ?N-CDC25B isoforms are expressed from a distinct promoter not involved in expression of canonical full-length isoforms. Remarkably, in contrast to the high lability and spatial dynamism of the full-length isoforms, ?N-CDC25B isoforms are highly stable and exclusively nuclear, strongly suggesting the existence of two pools of CDC25B phosphatases in the cell that have functionally distinct properties. Using isoform-specific siRNA, we found that depleting full-length isoforms, but not ?N-CDC25B isoforms, delays entry into mitosis. Thus, in an unperturbed cell cycle, the full-length isoforms are exclusively responsible for activating cyclin B-CDK1. Strikingly, in the late response to DNA damage, we found a CHK1-dependent shift in accumulation of CDC25B isoforms toward the ?N-CDC25B species. Under this physiological stress condition, the ?N-CDC25B isoform was found to play a crucial, nonredundant function in restarting the cell cycle after DNA damage-induced G2 phase arrest. Our findings reveal the existence of a previously unrecognized CDC25B isoform that operates specifically in the nucleus to reinitiate G2/M transition after DNA damage.
Tight regulation of cell cycle progression is essential for the maintenance of genomic integrity in response to DNA injury. The aim of this study was to identify new deubiquitinating enzymes (DUBs) involved in the regulation of the G?/M checkpoint. By using an siRNA-based screen to identify DUBs with an inherent ability to enhance a CDC25B-dependent G?/M checkpoint bypass, we have identified 11 candidates whose invalidation compromises checkpoint stringency. We subsequently focused our attention on one of these, the previously uncharacterized USP50. Using a TAP-tag approach associated to mass spectrometry, in addition to a yeast-two-hybrid screen, we identified HSP90 as a major interacting partner for USP50. We also demonstrate USP50 depletion causes a loss in accumulation of the HSP90 client Wee1, which is an essential component of the G?/M cell cycle arrest. Finally, we show that in response to DNA damaging agents, USP50 accumulates in the nucleus. We propose that USP50 may act through a HSP90-dependent mechanism to counteract CDC25B mitotic inducing activity and prevent Wee1 degradation, thereby repressing entry into mitosis following activation of the DNA damage checkpoint.
CDC25B phosphatase is a cell cycle regulator that plays a critical role in checkpoint control. Up-regulation of CDC25B expression has been documented in a variety of human cancers, however, the relationships with the alteration of the molecular mechanisms that lead to oncogenesis still remain unclear. To address this issue we have investigated, in model cell lines, the consequences of unscheduled and elevated CDC25B levels.
Chromatin undergoes major remodeling around DNA double-strand breaks (DSB) to promote repair and DNA damage response (DDR) activation. We recently reported a high-resolution map of ?H2AX around multiple breaks on the human genome, using a new cell-based DSB inducible system. In an attempt to further characterize the chromatin landscape induced around DSBs, we now report the profile of SMC3, a subunit of the cohesin complex, previously characterized as required for repair by homologous recombination. We found that recruitment of cohesin is moderate and restricted to the immediate vicinity of DSBs in human cells. In addition, we show that cohesin controls ?H2AX distribution within domains. Indeed, as we reported previously for transcription, cohesin binding antagonizes ?H2AX spreading. Remarkably, depletion of cohesin leads to an increase of ?H2AX at cohesin-bound genes, associated with a decrease in their expression level after DSB induction. We propose that, in agreement with their function in chromosome architecture, cohesin could also help to isolate active genes from some chromatin remodelling and modifications such as the ones that occur when a DSB is detected on the genome.
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