DNA replication origin activation in space and time.

DNA replication begins with the assembly of pre-replication complexes (pre-RCs) at thousands of DNA replication origins during the G1 phase of the cell cycle. At the G1-S-phase transition, pre-RCs are converted into pre-initiation complexes, in which the replicative helicase is activated, leading to DNA unwinding and initiation of DNA synthesis. However, only a subset of origins are activated during any S phase. Recent insights into the mechanisms underlying this choice reveal how flexibility in origin usage and temporal activation are linked to chromosome structure and organization, cell growth and differentiation, and replication stress.

Functional characterization of 5p15.33 risk locus in uveal melanoma reveals rs452384 as a functional variant and NKX2.4 as an allele-specific interactor.

The TERT/CLPTM1L risk locus on chromosome 5p15.33 is a pleiotropic cancer risk locus in which multiple independent risk alleles have been identified, across well over ten cancer types. We previously conducted a genome-wide association study in uveal melanoma (UM), which uncovered a role for the TERT/CLPTM1L risk locus in this intraocular tumor and identified multiple highly correlated risk alleles. Aiming to unravel the biological mechanisms in UM of this locus, which contains a domain enriched in active chromatin marks and enhancer elements, we demonstrated the allele-specific enhancer activity of this risk region using reporter assays. In UM, we identified the functional variant rs452384, of which the C risk allele is associated with higher gene expression, increased CLPTM1L expression in UM tumors, and a longer telomere length in peripheral blood mononuclear cells. Electrophoretic mobility shift assays and quantitative mass spectrometry identified NKX2.4 as an rs452384-T-specific binding protein, whereas GATA4 preferentially interacted with rs452384-C. Knockdown of NKX2.4 but not GATA4 resulted in increased TERT and CLPTM1L expression. In summary, the UM risk conferred by the 5p locus is at least partly due to rs452384, for which NKX2.4 presents strong differential binding activity and regulates CLPTM1L and TERT expression. Altogether, our work unraveled some of the complex regulatory mechanisms at the 5p15.33 susceptibility region in UM, and this might also shed light on shared mechanisms with other tumor types affected by this susceptibility region.

Structural centrosome aberrations promote non-cell-autonomous invasiveness.

Centrosomes are the main microtubule-organizing centers of animal cells. Although centrosome aberrations are common in tumors, their consequences remain subject to debate. Here, we studied the impact of structural centrosome aberrations, induced by deregulated expression of ninein-like protein (NLP), on epithelial spheres grown in Matrigel matrices. We demonstrate that NLP-induced structural centrosome aberrations trigger the escape ("budding") of living cells from epithelia. Remarkably, all cells disseminating into the matrix were undergoing mitosis. This invasive behavior reflects a novel mechanism that depends on the acquisition of two distinct properties. First, NLP-induced centrosome aberrations trigger a re-organization of the cytoskeleton, which stabilizes microtubules and weakens E-cadherin junctions during mitosis. Second, atomic force microscopy reveals that cells harboring these centrosome aberrations display increased stiffness. As a consequence, mitotic cells are pushed out of mosaic epithelia, particularly if they lack centrosome aberrations. We conclude that centrosome aberrations can trigger cell dissemination through a novel, non-cell-autonomous mechanism, raising the prospect that centrosome aberrations contribute to the dissemination of metastatic cells harboring normal centrosomes.

MCM8- and MCM9-deficient mice reveal gametogenesis defects and genome instability due to impaired homologous recombination.

We generated knockout mice for MCM8 and MCM9 and show that deficiency for these genes impairs homologous recombination (HR)-mediated DNA repair during gametogenesis and somatic cells cycles. MCM8(-/-) mice are sterile because spermatocytes are blocked in meiotic prophase I, and females have only arrested primary follicles and frequently develop ovarian tumors. MCM9(-/-) females also are sterile as ovaries are completely devoid of oocytes. In contrast, MCM9(-/-) testes produce spermatozoa, albeit in much reduced quantity. Mcm8(-/-) and Mcm9(-/-) embryonic fibroblasts show growth defects and chromosomal damage and cannot overcome a transient inhibition of replication fork progression. In these cells, chromatin recruitment of HR factors like Rad51 and RPA is impaired and HR strongly reduced. We further demonstrate that MCM8 and MCM9 form a complex and that they coregulate their stability. Our work uncovers essential functions of MCM8 and MCM9 in HR-mediated DSB repair during gametogenesis, replication fork maintenance, and DNA repair.

Genome-scale analysis of metazoan replication origins reveals their organization in specific but flexible sites defined by conserved features.

In metazoans, thousands of DNA replication origins (Oris) are activated at each cell cycle. Their genomic organization and their genetic nature remain elusive. Here, we characterized Oris by nascent strand (NS) purification and a genome-wide analysis in Drosophila and mouse cells. We show that in both species most CpG islands (CGI) contain Oris, although methylation is nearly absent in Drosophila, indicating that this epigenetic mark is not crucial for defining the activated origin. Initiation of DNA synthesis starts at the borders of CGI, resulting in a striking bimodal distribution of NS, suggestive of a dual initiation event. Oris contain a unique nucleotide skew around NS peaks, characterized by G/T and C/A overrepresentation at the 5' and 3' of Ori sites, respectively. Repeated GC-rich elements were detected, which are good predictors of Oris, suggesting that common sequence features are part of metazoan Oris. In the heterochromatic chromosome 4 of Drosophila, Oris correlated with HP1 binding sites. At the chromosome level, regions rich in Oris are early replicating, whereas Ori-poor regions are late replicating. The genome-wide analysis was coupled with a DNA combing analysis to unravel the organization of Oris. The results indicate that Oris are in a large excess, but their activation does not occur at random. They are organized in groups of site-specific but flexible origins that define replicons, where a single origin is activated in each replicon. This organization provides both site specificity and Ori firing flexibility in each replicon, allowing possible adaptation to environmental cues and cell fates.

Synergic reprogramming of mammalian cells by combined exposure to mitotic Xenopus egg extracts and transcription factors.

Transfer of somatic cell nuclei to enucleated eggs and ectopic expression of specific transcription factors are two different reprogramming strategies used to generate pluripotent cells from differentiated cells. However, these methods are poorly efficient, and other unknown factors might be required to increase their success rate. Here we show that Xenopus egg extracts at the metaphase stage (M phase) have a strong reprogramming activity on mouse embryonic fibroblasts (MEFs). First, they reset replication properties of MEF nuclei toward a replication profile characteristic of early development, and they erase several epigenetic marks, such as trimethylation of H3K9, H3K4, and H4K20. Second, when MEFs are reversibly permeabilized in the presence of M-phase Xenopus egg extracts, they show a transient increase in cell proliferation, form colonies, and start to express specific pluripotency markers. Finally, transient exposure of MEF nuclei to M-phase Xenopus egg extracts increases the success of nuclear transfer to enucleated mouse oocytes and strongly synergizes with the production of pluripotent stem cells by ectopic expression of transcription factors. The mitotic stage of the egg extract is crucial, because none of these effects is detected when using interphasic Xenopus egg extracts. Our data demonstrate that mitosis is essential to make mammalian somatic nuclei prone to reprogramming and that, surprisingly, the heterologous Xenopus system has features that are conserved enough to remodel mammalian nuclei.

Cytidine analogs are synthetic lethal with base excision repair default due to MBD4 deficiency.

Inactivating mutations of MBD4 have been reported in subsets of various tumors. A deficiency of this DNA glycosylase, recognizing specifically T:G mismatch resulting from the deamination of methyl-cytosine, results in a hypermutated phenotype due to the accumulation of CpG>TpG transitions. Here, we hypothesize that the difference in DNA metabolism consecutive to MBD4 deficiency may result in specific cytotoxicities in MBD4-deficient tumor cells in a synthetic lethality fashion. After a large-scale drug repurposing screen, we show in two isogenic MBD4 knock-out cell models that the inactivation of MBD4 sensitizes cancer cells to cytidine analogs. We further confirm the exquisite activity of gemcitabine in an MBD4-deficient co-clinical model as (i) it completely prevented the development of an MBD4-deficient uveal melanoma patient-derived xenograft and (ii) treatment in the corresponding patient resulted in an exceptional tumor response. These data suggest that patients harboring MBD4-deficient tumors may be treated efficiently by cytidine analogs.

Splicing Patterns in SF3B1-Mutated Uveal Melanoma Generate Shared Immunogenic Tumor-Specific Neoepitopes.

Disruption of splicing patterns due to mutations of genes coding splicing factors in tumors represents a potential source of tumor neoantigens, which would be both public (shared between patients) and tumor-specific (not expressed in normal tissues). In this study, we show that mutations of the splicing factor SF3B1 in uveal melanoma generate such immunogenic neoantigens. Memory CD8+ T cells specific for these neoantigens are preferentially found in 20% of patients with uveal melanoma bearing SF3B1-mutated tumors. Single-cell analyses of neoepitope-specific T cells from the blood identified large clonal T-cell expansions, with distinct effector transcription patterns. Some of these expanded T-cell receptors are also present in the corresponding tumors. CD8+ T-cell clones specific for the neoepitopes specifically recognize and kill SF3B1-mutated tumor cells, supporting the use of this new family of neoantigens as therapeutic targets. SIGNIFICANCE: Mutations of the splicing factor SF3B1 in uveal melanoma generate shared neoantigens that are uniquely expressed by tumor cells, leading to recognition and killing by specific CD8 T cells. Mutations in splicing factors can be sources of new therapeutic strategies applicable to diverse tumors.This article is highlighted in the In This Issue feature, p. 1861.

Metazoan DNA replication origins.

DNA replication starts with the opening of DNA at sites called DNA replication origins. From the single sequence-specific DNA replication origin of the small Escherichia coli genome, up to thousands of origins that are necessary to replicate the large human genome, strict sequence specificity has been lost. Nevertheless, genome-wide analyses performed in the recent years, using different mapping methods, demonstrated that there are precise locations along the metazoan genome from which replication initiates. These sites contain relaxed sequence consensus and epigenetic features. There is flexibility in the choice of origins to be used during a given cell cycle, probably imposed by evolution and developmental constraints. Here, we will briefly describe their main features.

Structural centrosome aberrations sensitize polarized epithelia to basal cell extrusion.

Centrosome aberrations disrupt tissue architecture and may confer invasive properties to cancer cells. Here we show that structural centrosome aberrations, induced by overexpression of either Ninein-like protein (NLP) or CEP131/AZI1, sensitize polarized mammalian epithelia to basal cell extrusion. While unperturbed epithelia typically dispose of damaged cells through apical dissemination into luminal cavities, certain oncogenic mutations cause a switch in directionality towards basal cell extrusion, raising the potential for metastatic cell dissemination. Here we report that NLP-induced centrosome aberrations trigger the preferential extrusion of damaged cells towards the basal surface of epithelial monolayers. This switch in directionality from apical to basal dissemination coincides with a profound reorganization of the microtubule cytoskeleton, which in turn prevents the contractile ring repositioning that is required to support extrusion towards the apical surface. While the basal extrusion of cells harbouring NLP-induced centrosome aberrations requires exogenously induced cell damage, structural centrosome aberrations induced by excess CEP131 trigger the spontaneous dissemination of dying cells towards the basal surface from MDCK cysts. Thus, similar to oncogenic mutations, structural centrosome aberrations can favour basal extrusion of damaged cells from polarized epithelia. Assuming that additional mutations may promote cell survival, this process could sensitize epithelia to disseminate potentially metastatic cells.

Impact of Centrosome Aberrations on Chromosome Segregation and Tissue Architecture in Cancer.

Centrosomes determine the disposition of microtubule networks and thereby contribute to regulate cell shape, polarity, and motility, as well as chromosome segregation during cell division. Additionally, centrioles, the core components of centrosomes, are required for the formation of cilia and flagella. Mutations in genes coding for centrosomal and centriolar proteins are responsible for several human diseases, foremost ciliopathies and developmental disorders resulting in small brains (primary microcephaly) or small body size (dwarfism). Moreover, a long-standing postulate implicates numerical and/or structural centrosome aberrations in the etiology of cancer. In this review, we will discuss recent work on the role of centrosome aberrations in the promotion of genome instability and the disruption of tissue architecture, two hallmarks of human cancers. We will emphasize recent studies on the impact of centrosome aberrations on the polarity of epithelial cells cultured in three-dimensional spheroid models. Collectively, the results from these in vitro systems suggest that different types of centrosome aberrations can promote invasive behavior through different pathways. Particularly exciting is recent evidence indicating that centrosome aberrations may trigger the dissemination of potentially metastatic cells through a non-cell-autonomous mechanism.

Sequential steps in DNA replication are inhibited to ensure reduction of ploidy in meiosis.

Meiosis involves two successive rounds of chromosome segregation without an intervening S phase. Exit from meiosis I is distinct from mitotic exit, in that replication origins are not licensed by Mcm2-7 chromatin binding, but spindle disassembly occurs during a transient interphase-like state before meiosis II. The absence of licensing is assumed to explain the block to DNA replication, but this has not been formally tested. Here we attempt to subvert this block by expressing the licensing control factors Cdc18 and Cdt1 during the interval between meiotic nuclear divisions. Surprisingly, this leads only to a partial round of DNA replication, even when these factors are overexpressed and effect clear Mcm2-7 chromatin binding. Combining Cdc18 and Cdt1 expression with modulation of cyclin-dependent kinase activity, activation of Dbf4-dependent kinase, or deletion of the Spd1 inhibitor of ribonucleotide reductase has little additional effect on the extent of DNA replication. Single-molecule analysis indicates this partial round of replication results from inefficient progression of replication forks, and thus both initiation and elongation replication steps may be inhibited in late meiosis. In addition, DNA replication or damage during the meiosis I-II interval fails to arrest meiotic progress, suggesting absence of checkpoint regulation of meiosis II entry.

DNA replication fading as proliferating cells advance in their commitment to terminal differentiation.

Terminal differentiation is the process by which cycling cells stop proliferating to start new specific functions. It involves dramatic changes in chromatin organization as well as gene expression. In the present report we used cell flow cytometry and genome wide DNA combing to investigate DNA replication during murine erythroleukemia-induced terminal cell differentiation. The results obtained indicated that the rate of replication fork movement slows down and the inter-origin distance becomes shorter during the precommitment and commitment periods before cells stop proliferating and accumulate in G1. We propose this is a general feature caused by the progressive heterochromatinization that characterizes terminal cell differentiation.

New cell or new cycle?

In mammals, trophoblast giant (TG) cell differentiation is characterized by a physiological endoreduplication, resulting in genome size augmentation. A recent study by Ullah and colleagues (pp. 3024-3036), published in this issue of Genes & Development, now elucidates the role of the cyclin-dependent kinase inhibitors (CKIs), p21 and p57, in mammalian endocycle regulation.

Flow cytometry to sort mammalian cells in cytokinesis.

BACKGROUND: Cell division or cytokinesis, which results from a series of events starting in metaphase, is the mechanism by which the mother cell cytoplasm is divided between the two daughter cells. Hence it is the final step of the cell division cycle. The aim of the present study was to demonstrate that mammalian cells undergoing cytokinesis can be sorted selectively by flow cytometry. MATERIALS AND METHODS: Cultures of HeLa cells were arrested in prometaphase by nocodazole, collected by mitotic shake-off and released for 90 min into fresh medium to enrich for cells undergoing cytokinesis. After ethanol fixation and DNA staining, cells were sorted based on DNA content and DNA fluorescence signal height. RESULTS: We define a cell population that transiently accumulates when synchronized cells exit mitosis before their entry into G1. We show that this population is highly enriched in cells undergoing cytokinesis. In addition, this population of cells can be sorted and analyzed by immunofluorescence and western blotting. CONCLUSIONS: This method of cell synchronization and sorting provides a simple means to isolate and biochemically analyze cells in cytokinesis, a period of the cell cycle that has been difficult to study by cell fractionation.

Before and after the spindle assembly checkpoint--an APC/C point of view.

On May 17-21, 2006 the Cold Spring Harbor Laboratory meeting on the cell cycle reunited over 350 researchers to discuss new findings in the cell cycle field. A common thread that connected numerous presentations was the regulation of the anaphase promoting complex/cyclosome (APC/C). This was also the main theme of the lecture given by the keynote speaker, Marc Kirschner (Harvard), who talked about "The unexpected importance of UbcH10 in both the G(1)/S transition and the initiation of anaphase", and it is also the main focus in this summary.