53BP1-mediated activation of the tumor suppressor p53.

In recent years, the role of 53BP1 as a cell cycle regulator has come into the spotlight. 53BP1 is best understood for its role in controlling DNA double-strand break repair. However, 53BP1 was initially discovered as an interaction partner of the tumor suppressor p53, which proved to be independent of DNA repair. The importance of this interaction is becoming increasingly clear. 53BP1 responds to mitotic stress, which prolongs mitosis, or to DNA damage and triggers the stabilization of p53 by the deubiquitinase USP28 to stop the proliferation of potentially damaged cells. The ability of 53BP1 to respond to mitotic stress or DNA damage is controlled by cell cycle-specific post-translational modifications and is therefore restricted to specific cell cycle phases. 53BP1-mediated p53 activation is likely involved in tumor suppression and is associated with genetic diseases such as primary microcephaly. This review emphasizes the importance of these mechanisms for the development and maintenance of healthy tissues.

Control of cell proliferation by memories of mitosis.

Mitotic duration is tightly constrained, and extended mitosis is characteristic of problematic cells prone to chromosome missegregation and genomic instability. We show here that mitotic extension leads to the formation of p53-binding protein 1 (53BP1)-ubiquitin-specific protease 28 (USP28)-p53 protein complexes that are transmitted to, and stably retained by, daughter cells. Complexes assembled through a Polo-like kinase 1-dependent mechanism during extended mitosis and elicited a p53 response in G1 that prevented the proliferation of the progeny of cells that experienced an approximately threefold extended mitosis or successive less extended mitoses. The ability to monitor mitotic extension was lost in p53-mutant cancers and some p53-wild-type (p53-WT) cancers, consistent with classification of TP53BP1 and USP28 as tumor suppressors. Cancers retaining the ability to monitor mitotic extension exhibited sensitivity to antimitotic agents.

Centriole-independent mitotic spindle assembly relies on the PCNT-CDK5RAP2 pericentriolar matrix.

Centrosomes, composed of centrioles that recruit a pericentriolar material (PCM) matrix assembled from PCNT and CDK5RAP2, catalyze mitotic spindle assembly. Here, we inhibit centriole formation and/or remove PCNT-CDK5RAP2 in RPE1 cells to address their relative contributions to spindle formation. While CDK5RAP2 and PCNT are normally dispensable for spindle formation, they become essential when centrioles are absent. Acentriolar spindle assembly is accompanied by the formation of foci containing PCNT and CDK5RAP2 via a microtubule and Polo-like kinase 1-dependent process. Foci formation and spindle assembly require PCNT-CDK5RAP2-dependent matrix assembly and the ability of CDK5RAP2 to recruit γ-tubulin complexes. Thus, the PCM matrix can self-organize independently of centrioles to generate microtubules for spindle assembly; conversely, an alternative centriole-anchored mechanism supports spindle assembly when the PCM matrix is absent. Extension to three cancer cell lines revealed similar results in HeLa cells, whereas DLD1 and U2OS cells could assemble spindles in the absence of centrioles and PCNT-CDK5RAP2, suggesting cell type variation in spindle assembly mechanisms.

TRIM37 controls cancer-specific vulnerability to PLK4 inhibition.

Centrosomes catalyse the formation of microtubules needed to assemble the mitotic spindle apparatus1. Centrosomes themselves duplicate once per cell cycle, in a process that is controlled by the serine/threonine protein kinase PLK4 (refs. 2,3). When PLK4 is chemically inhibited, cell division proceeds without centrosome duplication, generating centrosome-less cells that exhibit delayed, acentrosomal spindle assembly4. Whether PLK4 inhibitors can be leveraged as a treatment for cancer is not yet clear. Here we show that acentrosomal spindle assembly following PLK4 inhibition depends on levels of the centrosomal ubiquitin ligase TRIM37. Low TRIM37 levels accelerate acentrosomal spindle assembly and improve proliferation following PLK4 inhibition, whereas high TRIM37 levels inhibit acentrosomal spindle assembly, leading to mitotic failure and cessation of proliferation. The Chr17q region containing the TRIM37 gene is frequently amplified in neuroblastoma and in breast cancer5-8, rendering these cancer types highly sensitive to PLK4 inhibition. We find that inactivating TRIM37 improves acentrosomal mitosis because TRIM37 prevents PLK4 from self-assembling into centrosome-independent condensates that serve as ectopic microtubule-organizing centres. By contrast, elevated TRIM37 expression inhibits acentrosomal spindle assembly through a distinct mechanism that involves degradation of the centrosomal component CEP192. Thus, TRIM37 is an essential determinant of mitotic vulnerability to PLK4 inhibition. Linkage of TRIM37 to prevalent cancer-associated genomic changes-including 17q gain in neuroblastoma and 17q23 amplification in breast cancer-may offer an opportunity to use PLK4 inhibition to trigger selective mitotic failure and provide new avenues to treatments for these cancers.

NDF, a nucleosome-destabilizing factor that facilitates transcription through nucleosomes.

Our understanding of transcription by RNA polymerase II (Pol II) is limited by our knowledge of the factors that mediate this critically important process. Here we describe the identification of NDF, a nucleosome-destabilizing factor that facilitates Pol II transcription in chromatin. NDF has a PWWP motif, interacts with nucleosomes near the dyad, destabilizes nucleosomes in an ATP-independent manner, and facilitates transcription by Pol II through nucleosomes in a purified and defined transcription system as well as in cell nuclei. Upon transcriptional induction, NDF is recruited to the transcribed regions of thousands of genes and colocalizes with a subset of H3K36me3-enriched regions. Notably, the recruitment of NDF to gene bodies is accompanied by an increase in the transcript levels of many of the NDF-enriched genes. In addition, the global loss of NDF results in a decrease in the RNA levels of many genes. In humans, NDF is present at high levels in all tested tissue types, is essential in stem cells, and is frequently overexpressed in breast cancer. These findings indicate that NDF is a nucleosome-destabilizing factor that is recruited to gene bodies during transcriptional activation and facilitates Pol II transcription through nucleosomes.

53BP1 and USP28 mediate p53 activation and G1 arrest after centrosome loss or extended mitotic duration.

In normal human cells, centrosome loss induced by centrinone-a specific centrosome duplication inhibitor-leads to irreversible, p53-dependent G1 arrest by an unknown mechanism. A genome-wide CRISPR/Cas9 screen for centrinone resistance identified genes encoding the p53-binding protein 53BP1, the deubiquitinase USP28, and the ubiquitin ligase TRIM37. Deletion of TP53BP1, USP28, or TRIM37 prevented p53 elevation in response to centrosome loss but did not affect cytokinesis failure-induced arrest or p53 elevation after doxorubicin-induced DNA damage. Deletion of TP53BP1 and USP28, but not TRIM37, prevented growth arrest in response to prolonged mitotic duration. TRIM37 knockout cells formed ectopic centrosomal-component foci that suppressed mitotic defects associated with centrosome loss. TP53BP1 and USP28 knockouts exhibited compromised proliferation after centrosome removal, suggesting that centrosome-independent proliferation is not conferred solely by the inability to sense centrosome loss. Thus, analysis of centrinone resistance identified a 53BP1-USP28 module as critical for communicating mitotic challenges to the p53 circuit and TRIM37 as an enforcer of the singularity of centrosome assembly.