Three-Dimensional Simulation of Whole-Genome Structuring Through the Transition from Anaphase to Interphase.

In order to analyze the three-dimensional genome architecture, it is important to simulate how the genome is structured through the cell cycle progression. In this chapter, we present the usage of our computation codes for simulating how the human genome is formed as the cell transforms from anaphase to interphase. We do not use the global Hi-C data as an input into the genome simulation but represent all chromosomes as linear polymers annotated by the neighboring region contact index (NCI), which classifies the A/B type of each local chromatin region. The simulated mitotic chromosomes heterogeneously expand upon entry to the G1 phase, which induces phase separation of A and B chromatin regions, establishing chromosome territories, compartments, and lamina and nucleolus associations in the interphase nucleus. When the appropriate one-dimensional chromosomal annotation is possible, using the protocol of this chapter, one can quantitatively simulate the three-dimensional genome structure and dynamics of human cells of interest.

Development and validation of a mitotic catastrophe-related genes prognostic model for breast cancer.

Background: Breast cancer has become the most common malignant tumor in women worldwide. Mitotic catastrophe (MC) is a way of cell death that plays an important role in the development of tumors. However, the exact relationship between MC-related genes (MCRGs) and the development of breast cancer is still unclear, and further research is needed to elucidate this complexity. Methods: Transcriptome data and clinical data of breast cancer were downloaded from the Cancer Genome Atlas (TCGA) database and the Gene Expression Omnibus (GEO) database. We identified differential expression of MCRGs by comparing tumor tissue with normal tissue. Subsequently, we used COX regression analysis and LASSO regression analysis to construct the prognosis risk model of MCRGs. Kaplan-Meier survival curve and receiver operating characteristic (ROC) curve were used to evaluate the predictive ability of prognostic model. Moreover, the clinical relevance, gene set enrichment analysis (GSEA), immune landscape, tumor mutation burden (TMB), and immunotherapy and drug sensitivity analysis between high-risk and low-risk groups were systematically investigated. Finally, we validated the expression levels of genes involved in constructing the prognostic model through real-time quantitative polymerase chain reaction (RT-qPCR) at the cellular and tissue levels. Results: We identified 12 prognostic associated MCRGs, four of which were selected to construct prognostic model. The Kaplan-Meier analysis suggested that patients in the high-risk group had a shorter overall survival (OS). The Cox regression analysis and ROC analysis indicated that risk model had independent and excellent ability in predicting prognosis of breast cancer patients. Mechanistically, a remarkable difference was observed in clinical relevance, GSEA, immune landscape, TMB, immunotherapy response, and drug sensitivity analysis. RT-qPCR results showed that genes involved in constructing the prognostic model showed significant abnormal expressions and the expression change trends were consistent with the bioinformatics results. Conclusions: We established a prognosis risk model based on four MCRGs that had the ability to predict clinical prognosis and immune landscape, proposing potential therapeutic targets for breast cancer.

BRCA2 stabilises RAD51 and DMC1 nucleoprotein filaments through a conserved interaction mode.

BRCA2 is essential for DNA repair by homologous recombination in mitosis and meiosis. It interacts with recombinases RAD51 and DMC1 to facilitate the formation of nucleoprotein filaments on resected DNA ends that catalyse recombination-mediated repair. BRCA2's BRC repeats bind and disrupt RAD51 and DMC1 filaments, whereas its PhePP motifs bind recombinases and stabilise their nucleoprotein filaments. However, the mechanism of filament stabilisation has hitherto remained unknown. Here, we report the crystal structure of a BRCA2-DMC1 complex, revealing how core interaction sites of PhePP motifs bind to recombinases. The interaction mode is conserved for RAD51 and DMC1, which selectively bind to BRCA2's two distinct PhePP motifs via subtly divergent binding pockets. PhePP motif sequences surrounding their core interaction sites protect nucleoprotein filaments from BRC-mediated disruption. Hence, we report the structural basis of how BRCA2's PhePP motifs stabilise RAD51 and DMC1 nucleoprotein filaments for their essential roles in mitotic and meiotic recombination.

Plasmodium NEK1 coordinates MTOC organisation and kinetochore attachment during rapid mitosis in male gamete formation.

Mitosis is an important process in the cell cycle required for cells to divide. Never in mitosis (NIMA)-like kinases (NEKs) are regulators of mitotic functions in diverse organisms. Plasmodium spp., the causative agent of malaria is a divergent unicellular haploid eukaryote with some unusual features in terms of its mitotic and nuclear division cycle that presumably facilitate proliferation in varied environments. For example, during the sexual stage of male gametogenesis that occurs within the mosquito host, an atypical rapid closed endomitosis is observed. Three rounds of genome replication from 1N to 8N and successive cycles of multiple spindle formation and chromosome segregation occur within 8 min followed by karyokinesis to generate haploid gametes. Our previous Plasmodium berghei kinome screen identified 4 Nek genes, of which 2, NEK2 and NEK4, are required for meiosis. NEK1 is likely to be essential for mitosis in asexual blood stage schizogony in the vertebrate host, but its function during male gametogenesis is unknown. Here, we study NEK1 location and function, using live cell imaging, ultrastructure expansion microscopy (U-ExM), and electron microscopy, together with conditional gene knockdown and proteomic approaches. We report spatiotemporal NEK1 location in real-time, coordinated with microtubule organising centre (MTOC) dynamics during the unusual mitoses at various stages of the Plasmodium spp. life cycle. Knockdown studies reveal NEK1 to be an essential component of the MTOC in male cell differentiation, associated with rapid mitosis, spindle formation, and kinetochore attachment. These data suggest that P. berghei NEK1 kinase is an important component of MTOC organisation and essential regulator of chromosome segregation during male gamete formation.

Dynamic Mitotic Localization of the Centrosomal Kinases CDK1, Plk, AurK, and Nek2 in Dictyostelium amoebae.

The centrosome of the amoebozoan model Dictyostelium discoideum provides the best-established model for an acentriolar centrosome outside the Opisthokonta. Dictyostelium exhibits an unusual centrosome cycle, in which duplication is initiated only at the G2/M transition and occurs entirely during the M phase. Little is known about the role of conserved centrosomal kinases in this process. Therefore, we have generated knock-in strains for Aurora (AurK), CDK1, cyclin B, Nek2, and Plk, replacing the endogenous genes with constructs expressing the respective green fluorescent Neon fusion proteins, driven by the endogenous promoters, and studied their behavior in living cells. Our results show that CDK1 and cyclin B arrive at the centrosome first, already during G2, followed by Plk, Nek2, and AurK. Furthermore, CDK1/cyclin B and AurK were dynamically localized at kinetochores, and AurK in addition at nucleoli. The putative roles of all four kinases in centrosome duplication, mitosis, cytokinesis, and nucleolar dynamics are discussed.

CDK5-cyclin B1 regulates mitotic fidelity.

CDK1 has been known to be the sole cyclin-dependent kinase (CDK) partner of cyclin B1 to drive mitotic progression1. Here we demonstrate that CDK5 is active during mitosis and is necessary for maintaining mitotic fidelity. CDK5 is an atypical CDK owing to its high expression in post-mitotic neurons and activation by non-cyclin proteins p35 and p392. Here, using independent chemical genetic approaches, we specifically abrogated CDK5 activity during mitosis, and observed mitotic defects, nuclear atypia and substantial alterations in the mitotic phosphoproteome. Notably, cyclin B1 is a mitotic co-factor of CDK5. Computational modelling, comparison with experimentally derived structures of CDK-cyclin complexes and validation with mutational analysis indicate that CDK5-cyclin B1 can form a functional complex. Disruption of the CDK5-cyclin B1 complex phenocopies CDK5 abrogation in mitosis. Together, our results demonstrate that cyclin B1 partners with both CDK5 and CDK1, and CDK5-cyclin B1 functions as a canonical CDK-cyclin complex to ensure mitotic fidelity.

The three Plasmodium falciparum Aurora-related kinases display distinct temporal and spatial associations with mitotic structures in asexual blood stage parasites and gametocytes.

Aurora kinases are crucial regulators of mitotic cell cycle progression in eukaryotes. The protozoan malaria parasite Plasmodium falciparum replicates via schizogony, a specialized mode of cell division characterized by consecutive asynchronous rounds of nuclear division by closed mitosis followed by a single cytokinesis event producing dozens of daughter cells. P. falciparum encodes three Aurora-related kinases (PfARKs) that have been reported essential for parasite proliferation, but their roles in regulating schizogony have not yet been explored in great detail. Here, we engineered transgenic parasite lines expressing GFP-tagged PfARK1-3 to provide a systematic analysis of their expression timing and subcellular localization throughout schizogony as well as in the non-dividing gametocyte stages, which are essential for malaria transmission. We demonstrate that all three PfARKs display distinct and highly specific and exclusive spatiotemporal associations with the mitotic machinery. In gametocytes, PfARK3 is undetectable, and PfARK1 and PfARK2 show male-specific expression in late-stage gametocytes, consistent with their requirement for endomitosis during male gametogenesis in the mosquito vector. Our combined data suggest that PfARK1 and PfARK2 have non-overlapping roles in centriolar plaque maturation, assembly of the mitotic spindle, kinetochore-spindle attachment and chromosome segregation, while PfARK3 seems to be exquisitely involved in daughter cell cytoskeleton assembly and cytokinesis. These important new insights provide a reliable foundation for future research aiming at the functional investigation of these divergent and possibly drug-targetable Aurora-related kinases in mitotic cell division of P. falciparum and related apicomplexan parasites.IMPORTANCEMalaria parasites replicate via non-conventional modes of mitotic cell division, such as schizogony, employed by the disease-causing stages in the human blood or endomitosis during male gametogenesis in the mosquito vector. Understanding the molecular mechanisms regulating cell division in these divergent unicellular eukaryotes is not only of scientific interest but also relevant to identify potential new antimalarial drug targets. Here, we carefully examined the subcellular localization of all three Plasmodium falciparum Aurora-related kinases (ARKs), distantly related homologs of Aurora kinases that coordinate mitosis in model eukaryotes. Detailed fluorescence microscopy-based analyses revealed distinct, specific, and exclusive spatial associations for each parasite ARK with different components of the mitotic machinery and at different phases of the cell cycle during schizogony and gametocytogenesis. This comprehensive set of results closes important gaps in our fragmentary knowledge on this important group of kinases and offers a valuable source of information for future functional studies.

miR-31-mediated local translation at the mitotic spindle is important for early development.

miR-31 is a highly conserved microRNA that plays crucial roles in cell proliferation, migration and differentiation. We discovered that miR-31 and some of its validated targets are enriched on the mitotic spindle of the dividing sea urchin embryo and mammalian cells. Using the sea urchin embryo, we found that miR-31 inhibition led to developmental delay correlated with increased cytoskeletal and chromosomal defects. We identified miR-31 to directly suppress several actin remodeling transcripts, including ß-actin, Gelsolin, Rab35 and Fascin. De novo translation of Fascin occurs at the mitotic spindle of sea urchin embryos and mammalian cells. Importantly, miR-31 inhibition leads to a significant a increase of newly translated Fascin at the spindle of dividing sea urchin embryos. Forced ectopic localization of Fascin transcripts to the cell membrane and translation led to significant developmental and chromosomal segregation defects, highlighting the importance of the regulation of local translation by miR-31 at the mitotic spindle to ensure proper cell division. Furthermore, miR-31-mediated post-transcriptional regulation at the mitotic spindle may be an evolutionarily conserved regulatory paradigm of mitosis.

Clinical, radiological, and pathological features of mitotically active cellular fibroma of ovary: A review of cases with literature review.

OBJECTIVE: Mitotically active cellular fibroma (MACF) of the ovary, characterized by relatively high mitotic activity without severe atypia, was first described in the WHO classification in 2014. However, due to its rarity, the clinicopathological characteristics of ovarian MACF have not been established. This study was performed to describe the clinical, radiological, and pathological features of MACF by analyzing 11 cases of ovarian MACF. MATERIALS AND METHODS: Between 2015 and 2022, 11 patients with ovarian MACFs underwent surgical treatment at our institution. Clinicopathologic data of the patients were retrospectively reviewed from their medical records. RESULTS: Median patient age was 53.7 years (range 21-77 years), and median tumor diameter was 7.8 cm (range 4.3-14.0 cm). Preoperative CA125 was elevated in 4 cases. Four of the eleven patients had abdominal pain, and two presented with vulvar pain or a palpable abdominal mass, respectively. Preoperative radiological impressions included fibroma, fibrothecoma, stromal tumor, and cystadenocarcinoma. A laparoscopic approach was adopted in 7 cases (64%). Intraoperative frozen section was performed in 5 patients, and all demonstrated the presence of a benign, fibromatous stromal tumor. Three patients underwent fertility-sparing surgery, including laparoscopic ovarian cystectomy and unilateral salpingo-oophorectomy. Median follow-up was 37.7 months (range 2-84 months), and no patient experienced disease relapse or died of their disease. CONCLUSION: This study shows that ovarian MACF has a benign clinical course. Fertility-sparing surgery provides a safe therapeutic option for MACF, which can be managed safely by laparoscopy. Imaging findings and final pathological diagnosis were not well matched. Intraoperative frozen section is important for determining surgical extent in mitotically active cellular fibroma of the ovary.

Dynamic phosphorylation of FOXA1 by Aurora B guides post-mitotic gene reactivation.

FOXA1 serves as a crucial pioneer transcription factor during developmental processes and plays a pivotal role as a mitotic bookmarking factor to perpetuate gene expression profiles and maintain cellular identity. During mitosis, the majority of FOXA1 dissociates from specific DNA binding sites and redistributes to non-specific binding sites; however, the regulatory mechanisms governing molecular dynamics and activity of FOXA1 remain elusive. Here, we show that mitotic kinase Aurora B specifies the different DNA binding modes of FOXA1 and guides FOXA1 biomolecular condensation in mitosis. Mechanistically, Aurora B kinase phosphorylates FOXA1 at Serine 221 (S221) to liberate the specific, but not the non-specific, DNA binding. Interestingly, the phosphorylation of S221 attenuates the FOXA1 condensation that requires specific DNA binding. Importantly, perturbation of the dynamic phosphorylation impairs accurate gene reactivation and cell proliferation, suggesting that reversible mitotic protein phosphorylation emerges as a fundamental mechanism for the spatiotemporal control of mitotic bookmarking.

The Mitotic Checkpoint Complex controls the association of Cdc20 regulatory protein with the ubiquitin ligase APC/C in mitosis.

The ubiquitin ligase Anaphase-Promoting Complex/Cyclosome (APC/C) and its regulatory protein Cdc20 play important roles in the control of different stages of mitosis. APC/C associated with Cdc20 is active and promotes metaphase-anaphase transition by targeting for degradation inhibitors of anaphase initiation. Earlier in mitosis, premature action of APC/C is prevented by the mitotic checkpoint (or spindle assembly checkpoint) system, which ensures that anaphase is not initiated until all chromosomes are properly attached to the mitotic spindle. The active mitotic checkpoint system promotes the assembly of a Mitotic Checkpoint Complex (MCC), which binds to APC/C and inhibits its activity. The interaction of MCC with APC/C is strongly enhanced by Cdc20 bound to APC/C. While the association of Cdc20 with APC/C was known to be essential for both these stages of mitosis, it was not known how Cdc20 remains bound in spite of ongoing processes, phosphorylation and ubiquitylation, that stimulate its release from APC/C. We find that MCC strongly inhibits the release of Cdc20 from APC/C by the action of mitotic protein kinase Cdk1-cyclin B. This is not due to protection from phosphorylation of specific sites in Cdc20 that affect its interaction with APC/C. Rather, MCC stabilizes the binding to APC/C of partially phosphorylated forms of Cdc20. MCC also inhibits the autoubiquitylation of APC/C-bound Cdc20 and its ubiquitylation-promoted release from APC/C. We propose that these actions of MCC to maintain Cdc20 bound to APC/C in mitosis are essential for the control of mitosis during active mitotic checkpoint and in subsequent anaphase initiation.

Connecting high-resolution 3D chromatin maps with cell division and cell differentiation at the root apical meristem.

KEY MESSAGE: We used marker-free technologies to study chromatin at cellular resolution. Our results show asymmetric chromatin distribution, explore chromatin dynamics during mitosis, and reveal structural differences between trichoblast and atrichoblast cell. The shapes, sizes, and structural organizations of plant nuclei vary considerably among cell types, tissues, and species. This diversity is dependent on various factors, including cellular function, developmental stage, and environmental or physiological conditions. The differences in nuclear structure reflect the state of chromatin, which, in turn, controls gene expression and regulates cell fate. To examine the interrelationship between nuclear structure, cell morphology, and tissue-specific cell proliferation and differentiation processes, we conducted multiple visualizations of H3K4me1, H3K9me2, 4',6-diamidino-2-phenylindole, 5-ethynyl 2'-deoxyuridine, and SCRI Renaissance 2200, followed by subsequent quantitative analysis of individual cells and nuclei. By assigning cylindrical coordinates to the nuclei in the iRoCS toolbox, we were able to construct in situ digital three-dimensional chromatin maps for all the tissue layers of individual roots. A detailed analysis of the nuclei features of H3K4me1 and H3K9me2 in the mitotic and the elongation zones in trichoblast and atrichoblast cells at the root apical meristem revealed cell type-specific chromatin dynamics with asymmetric distribution of euchromatin and heterochromatin marks that may be associated with cell cycle and cell differentiation characteristics of specific cells. Furthermore, the spatial distribution of nuclei stained with 5-ethynyl 2'-deoxyuridine in the epidermis and cortex tissues suggests short-range coordination of cell division and nuclear migration in a linear sequence through an unknown regulatory mechanism.

Aurora A Kinase Begins to Localize to the Centrosome in the S-phase of the Cell Cycle in the XL2 Cell Line.

BACKGROUND: The centrosome is one of the principal cell hubs, where numerous proteins important for intracellular regulatory processes are concentrated. One of them, serine-threonine kinase 6, alias Aurora A, is involved in centrosome duplication and mitotic spindle formation and maintenance. METHODS: Long-term vital observations of cells, immunofluorescence analysis of protein localization, synchronization of cells at different phases of the cell cycle, Western blot analysis of protein content were used in the work. RESULTS: In this study, we investigated the dynamics of Aurora A protein accumulation and degradation in the XL2 Xenopus cell line during its 28-hour cell cycle. Using Western blot and immunofluorescence analyses, we demonstrated that Aurora A disappeared from the centrosome within one hour following mitosis and was not redistributed to other cell compartments. Using double Aurora A/Bromodeoxyuridine immunofluorescence labeling of the cells with precisely determined cell cycle stages, we observed that Aurora A reappeared in the centrosome during the S-phase, which was earlier than reported for all other known proteins with mitosis-specific centrosomal localization. Moreover, Aurora A accumulation in the centrosomal region and centrosome separation were asynchronous in the sister cells. CONCLUSIONS: The reported data allowed us to hypothesize that Aurora A is one of the primary links in coordinating centrosome separation and constructing the mitotic spindle.

Thiostrepton induces spindle abnormalities and enhances Taxol cytotoxicity in MDA-MB-231 cells.

BACKGROUND: Thiostrepton (TST) is a known inhibitor of the transcription factor Forkhead box M1 (FoxM1) and inducer of heat shock response (HSR) and autophagy. TST thus may be one potential candidate of anticancer drugs for combination chemotherapy. METHODS AND RESULTS: Immunofluorescence staining of mitotic spindles and flow cytometry analysis revealed that TST induces mitotic spindle abnormalities, mitotic arrest, and apoptotic cell death in the MDA-MB-231 triple-negative breast cancer cell line. Interestingly, overexpression or depletion of FoxM1 in MDA-MB-231 cells did not affect TST induction of spindle abnormalities; however, TST-induced spindle defects were enhanced by inhibition of HSP70 or autophagy. Moreover, TST exhibited low affinity for tubulin and only slightly inhibited in vitro tubulin polymerization, but it severely impeded tubulin polymerization and destabilized microtubules in arrested mitotic MDA-MB-231 cells. Additionally, TST significantly enhanced Taxol cytotoxicity. TST also caused cytotoxicity and spindle abnormalities in a Taxol-resistant cell line, MDA-MB-231-T4R. CONCLUSIONS: These results suggest that, in addition to inhibiting FoxM1, TST may induce proteotoxicity and autophagy to disrupt cellular tubulin polymerization, and this mechanism might account for its antimitotic effects, enhancement of Taxol anticancer effects, and ability to overcome Taxol resistance in MDA-MB-231 cells. These data further imply that TST may be useful to improve the therapeutic efficacy of Taxol.

ARID1 is required to regulate and reinforce H3K9me2 in sperm cells in Arabidopsis.

Heterochromatin de-condensation in companion gametic cells is conserved in both plants and animals. In plants, microspore undergoes asymmetric pollen mitosis (PMI) to produce a vegetative cell (VC) and a generative cell (GC). Subsequently, the GC undergoes pollen mitosis (PMII) to produce two sperm cells (SC). Consistent with heterochromatin de-condensation in the VC, H3K9me2, a heterochromatin mark, is barely detected in VC. However, how H3K9me2 is differentially regulated during pollen mitosis remains unclear. Here, we show that H3K9me2 is gradually evicted from the VC since PMI but remain unchanged in the GC and SC. ARID1, a pollen-specific transcription factor that facilitates PMII, promotes H3K9me2 maintenance in the GC/SC but slows down its eviction in the VC. The genomic targets of ARID1 mostly overlaps with H3K9me2 loci, and ARID1 recruits H3K9 methyltransferase SUVH6. Our results uncover that differential pattern of H3K9me2 between two cell types is regulated by ARID1 during pollen mitosis.

Carfilzomib shows therapeutic potential for reduction of liver fibrosis by targeting hepatic stellate cell activation.

Because hepatic stellate cells (HSCs) play a major role in fibrosis, we focused on HSCs as a potential target for the treatment of liver fibrosis. In this study, we attempted to identify drug candidates to inactivate HSCs and found that several proteasome inhibitors (PIs) reduced HSC viability. Our data showed that a second-generation PI, carfilzomib (CZM), suppressed the expression of fibrotic markers in primary murine HSCs at low concentrations of 5 or 10 nM. Since CZM was not toxic to HSCs up to a concentration of 12.5 nM, we examined its antifibrotic effects further. CZM achieved a clear reduction in liver fibrosis in the carbon tetrachloride (CCl4)-induced mouse model of liver fibrosis without worsening of liver injury. Mechanistically, RNA sequence analysis of primary HSCs revealed that CZM inhibits mitosis in HSCs. In the CCl4-injured liver, amphiregulin, which is known to activate mitogenic signaling pathways and fibrogenic activity and is upregulated in murine and human metabolic dysfunction-associated steatohepatitis (MASH), was downregulated by CZM administration, leading to inhibition of mitosis in HSCs. Thus, CZM and next-generation PIs in development could be potential therapeutic agents for the treatment of liver fibrosis via inactivation of HSCs without liver injury.

Heterotypic interaction promotes asymmetric division of human hematopoietic progenitors.

Hematopoietic stem and progenitor cells (HSPCs) give rise to all cell types of the hematopoietic system through various processes, including asymmetric divisions. However, the contribution of stromal cells of the hematopoietic niches in the control of HSPC asymmetric divisions remains unknown. Using polyacrylamide microwells as minimalist niches, we show that specific heterotypic interactions with osteoblast and endothelial cells promote asymmetric divisions of human HSPCs. Upon interaction, HSPCs polarize in interphase with the centrosome, the Golgi apparatus, and lysosomes positioned close to the site of contact. Subsequently, during mitosis, HSPCs orient their spindle perpendicular to the plane of contact. This division mode gives rise to siblings with unequal amounts of lysosomes and of the differentiation marker CD34. Such asymmetric inheritance generates heterogeneity in the progeny, which is likely to contribute to the plasticity of the early steps of hematopoiesis.

Sumo-regulatory SENP2 controls the homeostatic squamous mitosis-differentiation checkpoint.

Squamous or epidermoid cancer arises in stratified epithelia but also is frequent in the non-epidermoid epithelium of the lung by unclear mechanisms. A poorly studied mitotic checkpoint drives epithelial cells bearing irreparable genetic damage into epidermoid differentiation. We performed an RNA-sequencing gene search to target unknown regulators of this response and selected the SUMO regulatory protein SENP2. Alterations of SENP2 expression have been associated with some types of cancer. We found the protein to be strongly localised to mitotic spindles of freshly isolated human epidermal cells. Primary cells rapidly differentiated after silencing SENP2 with specific shRNAs. Loss of SENP2 produced in synchronised epithelial cells delays in mitotic entry and exit and defects in chromosomal alignment. The results altogether strongly argue for an essential role of SENP2 in the mitotic spindle and hence in controlling differentiation. In addition, the expression of SENP2 displayed an inverse correlation with the immuno-checkpoint biomarker PD-L1 in a pilot collection of aggressive lung carcinomas. Consistently, metastatic head and neck cancer cells that do not respond to the mitosis-differentiation checkpoint were resistant to depletion of SENP2. Our results identify SENP2 as a novel regulator of the epithelial mitosis-differentiation checkpoint and a potential biomarker in epithelial cancer.

Irc20 modulates LOH frequency and distribution in S. cerevisiae.

Loss of Heterozygosity (LOH) due to mitotic recombination is frequently associated with the development of various cancers (e.g. retinoblastoma). LOH is also an important source of genetic diversity, especially in organisms where meiosis is infrequent. Irc20 is a putative helicase, and E3 ubiquitin ligase involved in DNA double-strand break repair pathway. We analyzed genome-wide LOH events, gross chromosomal changes, small insertion-deletions and single nucleotide mutations in eleven S. cerevisiae mutation accumulation lines of irc20∆, which underwent 50 mitotic bottlenecks. LOH enhancement in irc20∆ was small (1.6 fold), but statistically significant as compared to the wild type. Short (≤ 1 kb) and long (> 10 kb) LOH tracts were significantly enhanced in irc20∆. Both interstitial and terminal LOH events were also significantly enhanced in irc20∆ compared to the wild type. LOH events in irc20∆ were more telomere proximal and away from centromeres compared to the wild type. Gross chromosomal changes, single nucleotide mutations and in-dels were comparable between irc20∆ and wild type. Locus based and genome-wide analysis of meiotic recombination showed that meiotic crossover frequencies are not altered in irc20∆. These results suggest Irc20 primarily regulates mitotic recombination and does not affect meiotic crossovers. Our results suggest that the IRC20 gene is important for regulating LOH frequency and distribution.

ADARp150 counteracts whole genome duplication.

Impaired control of the G1/S checkpoint allows initiation of DNA replication under non-permissive conditions. Unscheduled S-phase entry is associated with DNA replication stress, demanding for other checkpoints or cellular pathways to maintain proliferation. Here, we uncovered a requirement for ADARp150 to sustain proliferation of G1/S-checkpoint-defective cells under growth-restricting conditions. Besides its well-established mRNA editing function in inversely oriented short interspersed nuclear elements (SINEs), we found ADARp150 to exert a critical function in mitosis. ADARp150 depletion resulted in tetraploidization, impeding cell proliferation in mitogen-deprived conditions. Mechanistically we show that ADAR1 depletion induced aberrant expression of Cyclin B3, which was causative for mitotic failure and whole-genome duplication. Finally, we find that also in vivo ADAR1-depletion-provoked tetraploidization hampers tumor outgrowth.