Lagging chromosomes entrapped in micronuclei are not 'lost' by cells

Fate of micronuclei and micronucleated cells after treatment of HeLa cells with different genotoxic agents

Although micronuclei are well-known biomarkers of genotoxic damage, the biological consequences of micronucleus induction are only poorly understood. To further elucidate these consequences, HeLa cells stably expressing histone 2B coupled with green fluorescent protein were used for long-term live cell imaging to investigate the fate of micronuclei and micronucleated cells after treatment of cells with various genotoxic agents (doxorubicin (20, 30 and nM), tert-butyl hydroperoxide (tBHP, 50, 100 and 150 µM), radiation (0.5, 1 and 2 Gy), methyl methanesulfonate (MMS, 20, 25 and 30 µg/ml) and vinblastine (1, 2 and 3 nM)). Most micronuclei persist for multiple cell cycles or reincorporate while micronucleated cells were more prone to cell death, senescence and fatal mitotic errors compared to non-micronucleated cells, which is consistent with previous studies using etoposide. No clear substance-related effects on the fate of micronuclei and micronucleated cells were observed. To further investigate the fate of micronuclei, extrusion of micronuclei was studied with treatments reported as inducing the extrusion of micronuclei. Since extrusion was not observed in HeLa cells, the relevance of extrusion of micronuclei remains unclear. In addition, degradation of micronuclei was analysed via immunostaining of γH2AX, which demonstrated a high level of DNA damage in micronuclei compared to the main nuclei. Furthermore, transduction with two reporter genes (LC3B-dsRed and LaminB1-dsRed) was conducted followed by long-term live cell imaging. While autophagy marker LC3B was not associated with micronuclei, Lamin B1 was found in approximately 50% of all micronuclei. While degradation of micronuclei was not observed to be a frequent fate of micronuclei, the results show impaired stability of DNA and micronuclear envelope indicating rupture of micronuclei as a pre-step to chromothripsis.

Centromere defects, chromosome instability, and cGAS-STING activation in systemic sclerosis

Centromere defects in Systemic Sclerosis (SSc) have remained unexplored despite the fact that many centromere proteins were discovered in patients with SSc. Here we report that lesion skin fibroblasts from SSc patients show marked alterations in centromeric DNA. SSc fibroblasts also show DNA damage, abnormal chromosome segregation, aneuploidy (only in diffuse cutaneous (dcSSc)) and micronuclei (in all types of SSc), some of which lose centromere identity while retaining centromere DNA sequences. Strikingly, we find cytoplasmic "leaking" of centromere proteins in limited cutaneous SSc (lcSSc) fibroblasts. Cytoplasmic centromere proteins co-localize with antigen presenting MHC Class II molecules, which correlate precisely with the presence of anti-centromere antibodies. CENPA expression and micronuclei formation correlate highly with activation of the cGAS-STING/IFN-β pathway as well as markers of reactive oxygen species (ROS) and fibrosis, ultimately suggesting a link between centromere alterations, chromosome instability, SSc autoimmunity, and fibrosis.

Absence of a robust mitotic timer mechanism in early preimplantation mouse embryos leads to chromosome instability

Preimplantation embryos often consist of a combination of euploid and aneuploid cells, suggesting that safeguards preventing the generation and propagation of aneuploid cells in somatic cells might be deficient in embryos. In somatic cells, a mitotic timer mechanism has been described, in which even a small increase in the duration of M phase can cause a cell cycle arrest in the subsequent interphase, preventing further propagation of cells that have undergone a potentially hazardously long M phase. Here, we report that cell divisions in the mouse embryo and embryonic development continue even after a mitotic prolongation of several hours. However, similar M-phase extensions caused cohesion fatigue, resulting in prematurely separated sister chromatids and the production of micronuclei. Only extreme prolongation of M phase caused a subsequent interphase arrest, through a mechanism involving DNA damage. Our data suggest that the simultaneous absence of a robust mitotic timer and susceptibility of the embryo to cohesion fatigue could contribute to chromosome instability in mammalian embryos.

This article has an associated ‘The people behind the papers’ interview.

Tools used to assay genomic instability in cancers and cancer meiomitosis

Genomic instability is a defining characteristic of cancer and the analysis of DNA damage at the chromosome level is a crucial part of the study of carcinogenesis and genotoxicity. Chromosomal instability (CIN), the most common level of genomic instability in cancers, is defined as the rate of loss or gain of chromosomes through successive divisions. As such, DNA in cancer cells is highly unstable. However, the underlying mechanisms remain elusive. There is a debate as to whether instability succeeds transformation, or if it is a by-product of cancer, and therefore, studying potential molecular and cellular contributors of genomic instability is of high importance. Recent work has suggested an important role for ectopic expression of meiosis genes in driving genomic instability via a process called meiomitosis. Improving understanding of these mechanisms can contribute to the development of targeted therapies that exploit DNA damage and repair mechanisms. Here, we discuss a workflow of novel and established techniques used to assess chromosomal instability as well as the nature of genomic instability such as double strand breaks, micronuclei, and chromatin bridges. For each technique, we discuss their advantages and limitations in a lab setting. Lastly, we provide detailed protocols for the discussed techniques.

An anaphase surveillance mechanism prevents micronuclei formation from frequent chromosome segregation errors

Micronuclei are a hallmark of cancer and several other human disorders. Recently, micronuclei were implicated in chromothripsis, a series of massive genomic rearrangements that may drive tumor evolution and progression. Here, we show that Aurora B kinase mediates a surveillance mechanism that integrates error correction during anaphase with spatial control of nuclear envelope reassembly to prevent micronuclei formation. Using high-resolution live-cell imaging of human cancer and non-cancer cells, we uncover that anaphase lagging chromosomes are more frequent than previously anticipated, yet they rarely form micronuclei. Micronuclei formation from anaphase lagging chromosomes is prevented by a midzone-based Aurora B phosphorylation gradient that stabilizes kinetochore-microtubule attachments and assists spindle forces required for anaphase error correction while delaying nuclear envelope reassembly on lagging chromosomes, independently of microtubule density. We propose that a midzone-based Aurora B phosphorylation gradient actively monitors and corrects frequent chromosome segregation errors to prevent micronuclei formation during human cell division.

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Anaphase lagging chromosomes are frequent but rarely form micronuclei

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A midzone Aurora B activity gradient prevents micronuclei from segregation errors

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Midzone Aurora B assists spindle forces at the kinetochores to correct errors

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Aurora B spatially regulates nuclear envelope reformation on lagging chromosomes

Structural Chromosome Instability: Types, Origins, Consequences, and Therapeutic Opportunities

Chromosomal instability (CIN) refers to an increased rate of acquisition of numerical and structural changes in chromosomes and is considered an enabling characteristic of tumors. Given its role as a facilitator of genomic changes, CIN is increasingly being considered as a possible therapeutic target, raising the question of which variables may convert CIN into an ally instead of an enemy during cancer treatment. This review discusses the origins of structural chromosome abnormalities and the cellular mechanisms that prevent and resolve them, as well as how different CIN phenotypes relate to each other. We discuss the possible fates of cells containing structural CIN, focusing on how a few cell duplication cycles suffice to induce profound CIN-mediated genome alterations. Because such alterations can promote tumor adaptation to treatment, we discuss currently proposed strategies to either avoid CIN or enhance CIN to a level that is no longer compatible with cell survival.

Chromoanagenesis, the mechanisms of a genomic chaos

Designated under the name of chromoanagenesis, the phenomena of chromothripsis, chromanasynthesis and chromoplexy constitute new types of complex rearrangements, including many genomic alterations localized on a few chromosomal regions, and whose discovery over the last decade has changed our perception about the formation of chromosomal abnormalities and their etiology. Although exhibiting specific features, these new catastrophic mechanisms generally occur within a single cell cycle and their emergence is closely linked to genomic instability. Various non-exclusive exogenous or cellular mechanisms capable of generating chromoanagenesis have been evoked. However, recent experimental data shed light on 2 major processes, which following a defect in the mitotic segregation of chromosomes, can generate a cascade of cellular events leading to chromoanagenesis. These mechanisms are the formation of micronuclei integrating isolated chromosomal material, and the occurrence of chromatin bridges around chromosomal material resulting from telomeric fusions. In both cases, the cellular and molecular mechanisms of fragmentation, repair and transmission of damaged chromosomal material are consistent with the features of chromoanagenesis-related complex chromosomal rearrangements. In this review, we introduce each type of chromoanagenesis, and describe the experimental models that have allowed to validate the existence of chromoanagenesis events and to better understand their cellular mechanisms of formation and transmission, as well as their impact on the stability and the plasticity of the genome.

ESCRT-III-mediated membrane fusion drives chromosome fragments through nuclear envelope channels

Mitotic cells must form a single nucleus during telophase or exclude part of their genome as damage-prone micronuclei. While research has detailed how micronuclei arise from cells entering anaphase with lagging chromosomes, cellular mechanisms allowing late-segregating chromosomes to rejoin daughter nuclei remain underexplored. Here, we find that late-segregating acentric chromosome fragments that rejoin daughter nuclei are associated with nuclear membrane but devoid of lamin and nuclear pore complexes in Drosophila melanogaster. We show that acentrics pass through membrane-, lamin-, and nuclear pore-based channels in the nuclear envelope that extend and retract as acentrics enter nuclei. Membrane encompassing the acentrics fuses with the nuclear membrane, facilitating integration of the acentrics into newly formed nuclei. Fusion, mediated by the membrane fusion protein Comt/NSF and ESCRT-III components Shrub/CHMP4B and CHMP2B, facilitates reintegration of acentrics into nuclei. These results suggest a previously unsuspected role for membrane fusion, similar to nuclear repair, in the formation of a single nucleus during mitotic exit and the maintenance of genomic integrity.

Long-term fate of etoposide-induced micronuclei and micronucleated cells in Hela-H2B-GFP cells

Micronuclei are small nuclear cellular structures containing whole chromosomes or chromosomal fragments. While there is a lot of information available about the origin and formation of micronuclei, less is known about the fate of micronuclei and micronucleated cells. Possible fates include extrusion, degradation, reincorporation and persistence. Live cell imaging was performed to quantitatively analyse the fates of micronuclei and micronucleated cells occurring in vitro. Imaging was conducted for up to 96 h in HeLa-H2B-GFP cells treated with 0.5, 1 and 2 µg/ml etoposide. While a minority of micronuclei was reincorporated into the main nucleus during mitosis, the majority of micronuclei persisted without any alterations. Degradation and extrusion were observed rarely or never. The presence of micronuclei affected the proliferation of the daughter cells and also had an influence on cell death rates. Mitotic errors were found to be clearly increased in micronucleus-containing cells. The results show that micronuclei and micronucleated cells can, although delayed in cell cycle, sustain for multiple divisions.

Understanding the birth of rupture-prone and irreparable micronuclei

Micronuclei are extra-nuclear bodies mainly derived from ana-telophase lagging chromosomes/chromatins (LCs) that are not incorporated into primary nuclei at mitotic exit. Unlike primary nuclei, most micronuclei are enclosed by nuclear envelope (NE) that is highly susceptible to spontaneous and irreparable rupture. Ruptured micronuclei act as triggers of chromothripsis-like chaotic chromosomal rearrangements and cGAS-mediated innate immunity and inflammation, raising the view that micronuclei play active roles in human aging and tumorigenesis. Thus, understanding the ways in which micronuclear envelope (mNE) goes awry acquires increased importance. Here, we review the data to present a general framework for this question. We firstly describe NE reassembly after mitosis and NE repair during interphase. Simultaneously, we briefly discuss how mNE is organized and how mNE rupture controls the fate of micronuclei and micronucleated cells. As a focus of this review, we highlight current knowledge about why mNE is rupture-prone and irreparable. For this, we survey observations from a series of elegant studies to provide a systematic overview. We conclude that the birth of rupture-prone and irreparable micronuclei may be the cumulative effects of their intracellular geographic origins, biophysical properties, and specific mNE features. We propose that DNA damage and immunogenicity in micronuclei increase stepwise from altered mNE components, mNE rupture, and refractory to repair. Throughout our discussion, we note interesting issues in mNE fragility that have yet to be resolved.

CHK1-CENP B/MAD2 is associated with mild oxidative damage-induced sex chromosome aneuploidy of male mouse embryos during in vitro fertilization

A high incidence of aneuploidy is observed in vitro fertilization (IVF)-derived embryos, but the formation and repair mechanisms are unknown. Here, we investigated the effects of slightly increased reactive oxygen species (ROS) produced by in vitro culture conditions on embryo aneuploidy and the roles of the spindle assembly checkpoint (SAC) protein, mitotic arrest-deficient 2 (MAD2), and the DNA damage response (DDR) protein, checkpoint kinase 1 (CHK1), in aneuploidy repair. By assessing chromosome abnormalities via DAPI staining, karyotype analysis and next-generation sequencing technology, we demonstrated that mild oxidative damage mainly increased the risk of sex chromosome aneuploidy in male mouse embryos (41,XXY,+X and 41,XYY,+Y) through chromosome mis-segregation during the first mitosis. Isobaric tags for relative and absolute quantitation technology revealed that mild oxidative damage inhibited the expression of male reproduction-related proteins, including a kinase anchor protein 4 (AKAP4), whose gene is located on mouse/human Chromosome X. Under mild oxidative damage, abrogation of MAD2 by MAD2 inhibitor-1 (M2I-1) or CHK1 by siRNA microinjection increased sex chromosome mosaicism rate and reduced mitosis-promoting factor (MPF) activity. CHK1 inhibition also reduced kinetochore localization of centromere protein B (CENP B) and MAD2. These findings show that DDR and SAC are responsible for repair of sex chromosome mosaicism via the pCHK1 (S345)-CENP B/MAD2-MPF pathway; further, IVF may have negative effects on male offspring's reproduction ability, which ultimately depends on their continued repair capability. Therefore, we suggest that antioxidants, especially those targeting improved CHK1-MAD2 function, may be a promising therapeutic strategy to reduce aneuploidy formation of IVF-derived embryos and to maintain genome integrity of embryo and offspring.

Undifferentiated Sarcomas Develop through Distinct Evolutionary Pathways

Undifferentiated sarcomas (USARCs) of adults are diverse, rare, and aggressive soft tissue cancers. Recent sequencing efforts have confirmed that USARCs exhibit one of the highest burdens of structural aberrations across human cancer. Here, we sought to unravel the molecular basis of the structural complexity in USARCs by integrating DNA sequencing, ploidy analysis, gene expression, and methylation profiling. We identified whole genome duplication as a prevalent and pernicious force in USARC tumorigenesis. Using mathematical deconvolution strategies to unravel the complex copy-number profiles and mutational timing models we infer distinct evolutionary pathways of these rare cancers. In addition, 15% of tumors exhibited raised mutational burdens that correlated with gene expression signatures of immune infiltration, and good prognosis.

Chromoanagenesis: cataclysms behind complex chromosomal rearrangements

Background

During the last decade, genome sequencing projects in cancer genomes as well as in patients with congenital diseases and healthy individuals have led to the identification of new types of massive chromosomal rearrangements arising during single chaotic cellular events. These unanticipated catastrophic phenomenon are termed chromothripsis, chromoanasynthesis and chromoplexis., and are grouped under the name of “chromoanagenesis”.

Results

For each process, several specific features have been described, allowing each phenomenon to be distinguished from each other and to understand its mechanism of formation and to better understand its aetiology. Thus, chromothripsis derives from chromosome shattering followed by the random restitching of chromosomal fragments with low copy-number change whereas chromoanasynthesis results from erroneous DNA replication of a chromosome through serial fork stalling and template switching with variable copy-number gains, and chromoplexy refers to the occurrence of multiple inter-and intra-chromosomal translocations and deletions with little or no copy-number alterations in prostate cancer. Cumulating data and experimental models have shown that chromothripsis and chromoanasynthesis may essentially result from lagging chromosome encapsulated in micronuclei or telomere attrition and end-to-end telomere fusion.

Conclusion

The concept of chromanagenesis has provided new insight into the aetiology of complex structural rearrangements, the connection between defective cell cycle progression and genomic instability, and the complexity of cancer evolution. Increasing reported chromoanagenesis events suggest that these chaotic mechanisms are probably much more frequent than anticipated.

The molecular origins and pathophysiological consequences of micronuclei: New insights into an age-old problem

Micronuclei (MN), the small nucleus-like bodies separated from the primary nucleus, can exist in cells with numerical and/or structural chromosomal aberrations in apparently normal tissues and more so in tumors in humans. While MN have been observed for over 100 years, they were merely and constantly considered as passive indicators of chromosome instability (CIN) for a long time. Relatively little is known about the molecular origins and biological consequences of MN. Rapid technological advances are helping to close these gaps. Very recent studies provide exciting evidence that MN act as key platform for chromothripsis and a trigger of innate immune response, suggesting that MN could affect cellular functions by both genetic and nongenetic means. These previously unappreciated findings have reawakened widespread interests in MN. In this review, the diverse mechanisms leading to MN generation and the complex fate profiles of MN are discussed, together with the evidence for their contribution to CIN, inflammation, senescence and cell death. Moreover, we put this knowledge together into a speculative perspective on how MN may be responsible for cancer development and how their presence may influence the choice of treatment. We suggest that the heterogeneous responses to MN may function physiological to ensure the arrestment, elimination and immune clearance of damaged cells, but pathologically, may enable the survival and oncogenic transformation of cells bearing CIN. These insights not only underscore the complexity of MN biology, but also raise a host of new questions and provide fertile ground for future research.

Micronuclei Formation Is Prevented by Aurora B-Mediated Exclusion of HP1a from Late-Segregating Chromatin in Drosophila

While it is known that micronuclei pose a serious risk to genomic integrity by undergoing chromothripsis, mechanisms preventing micronucleus formation remain poorly understood. Here, we investigate how late-segregating acentric chromosomes that would otherwise form micronuclei instead reintegrate into daughter nuclei by passing through Aurora B kinase-dependent channels in the nuclear envelope of Drosophila melanogaster neuroblasts. We find that localized concentrations of Aurora B preferentially phosphorylate H3(S10) on acentrics and their associated DNA tethers. This phosphorylation event prevents HP1a from associating with heterochromatin and results in localized inhibition of nuclear envelope reassembly on endonuclease- and X-irradiation-induced acentrics, promoting channel formation. Finally, we find that HP1a also specifies initiation sites of nuclear envelope reassembly on undamaged chromatin. Taken together, these results demonstrate that Aurora B-mediated regulation of HP1a-chromatin interaction plays a key role in maintaining genome integrity by locally preventing nuclear envelope assembly and facilitating the incorporation of late-segregating acentrics into daughter nuclei.

Causes and consequences of chromosome segregation error in preimplantation embryos

Errors in chromosome segregation are common during the mitotic divisions of preimplantation development in mammalian embryos, giving rise to so-called ‘mosaic’ embryos possessing a mixture of euploid and aneuploid cells. Mosaicism is widely considered to be detrimental to embryo quality and is frequently used as criteria to select embryos for transfer in human fertility clinics. However, despite the clear clinical importance, the underlying defects in cell division that result in mosaic aneuploidy remain elusive. In this review, we summarise recent findings from clinical and animal model studies that provide new insights into the fundamental mechanisms of chromosome segregation in the highly unusual cellular environment of early preimplantation development and consider recent clues as to why errors should commonly occur in this setting. We furthermore discuss recent evidence suggesting that mosaicism is not an irrevocable barrier to a healthy pregnancy. Understanding the causes and biological impacts of mosaic aneuploidy will be pivotal in the development and fine-tuning of clinical embryo selection methods.

A novel TPR-BEN domain interaction mediates PICH-BEND3 association

PICH is a DNA translocase required for the maintenance of chromosome stability in human cells. Recent data indicate that PICH co-operates with topoisomerase IIα to suppress pathological chromosome missegregation through promoting the resolution of ultra-fine anaphase bridges (UFBs). Here, we identify the BEN domain-containing protein 3 (BEND3) as an interaction partner of PICH in human cells in mitosis. We have purified full length PICH and BEND3 and shown that they exhibit a functional biochemical interaction in vitro. We demonstrate that the PICH–BEND3 interaction occurs via a novel interface between a TPR domain in PICH and a BEN domain in BEND3, and have determined the crystal structure of this TPR–BEN complex at 2.2 Å resolution. Based on the structure, we identified amino acids important for the TPR–BEN domain interaction, and for the functional interaction of the full-length proteins. Our data reveal a proposed new function for BEND3 in association with PICH, and the first example of a specific protein–protein interaction mediated by a BEN domain.

Rebuilding Chromosomes After Catastrophe: Emerging Mechanisms of Chromothripsis

Cancer genome sequencing has identified chromothripsis, a complex class of structural genomic rearrangements involving the apparent shattering of an individual chromosome into tens to hundreds of fragments. An initial error during mitosis, producing either chromosome mis-segregation into a micronucleus or chromatin bridge interconnecting two daughter cells, can trigger the catastrophic pulverization of the spatially isolated chromosome. The resultant chromosomal fragments are religated in random order by DNA double-strand break repair during the subsequent interphase. Chromothripsis scars the cancer genome with localized DNA rearrangements that frequently generate extensive copy number alterations, oncogenic gene fusion products, and/or tumor suppressor gene inactivation. Here we review emerging mechanisms underlying chromothripsis with a focus on the contribution of cell division errors caused by centromere dysfunction.

Chromosome nondisjunction during bipolar mitoses of binucleated intermediates promote aneuploidy formation along with multipolar mitoses rather than chromosome loss in micronuclei induced by asbestos

Asbestos is a well-known occupational carcinogen that can cause aneuploidy during the early stages of neoplastic development. To explore the origins of asbestos-induced aneuploidy, we performed long-term live-cell imaging followed by fluorescence in situ hybridization of chromosomes 8 and 12 in human bronchial epithelial (HBEC) and mesothelial (MeT5A) cells. We demonstrate that asbestos induces aneuploidy via binucleated intermediates resulting from cytokinesis failure. On the one hand, asbestos increases chromosome nondisjunction during bipolar divisions of binucleated intermediates and produces near-tetraploidy. On the other hand, asbestos increases multipolar divisions of binucleated intermediates to produce aneuploidy. Surprisingly, chromosomes in asbestos-induced micronucleated cells are not truly lost by the cells, and do not contribute to aneuploid cell formation in either cell type. These results clarify the cellular source of asbestos-induced aneuploidy. In particular, they show the asbestos-induced disruption of bipolar chromosomal segregation in tetraploid cells, thereby demonstrating the causality between binucleated intermediates and aneuploidy evolution, rather than chromosome loss in micronuclei.

Fate of micronuclei and micronucleated cells

The present review describes available evidence about the fate of micronuclei and micronucleated cells. Micronuclei are small, extranuclear chromatin bodies surrounded by a nuclear envelope. The mechanisms underlying the formation of micronuclei are well understood but not much is known about the potential fate of micronuclei and micronucleated cells. Many studies with different experimental approaches addressed the various aspects of the post-mitotic fate of micronuclei and micronucleated cells. These studies are reviewed here considering four basic possibilities for potential fates of micronuclei: degradation of the micronucleus or the micronucleated cell, reincorporation into the main nucleus, extrusion from the cell, and persistence in the cytoplasm. Two additional fates need to be considered: premature chromosome condensation/chromothripsis and the elimination of micronucleated cells by apoptosis, yielding six potential fates for micronuclei and/or micronucleated cells. The available data is still limited, but it can be concluded that degradation and extrusion of micronuclei might occur in rare cases under specific conditions, reincorporation during the next mitosis occurs more frequently, and the majority of the micronuclei persist without alteration at least until the next mitosis, possibly much longer. Overall, the consequences of micronucleus formation on the cellular level are still far from clear, but they should be investigated further because micronucleus formation may contribute to the initial and later steps of malignant cell transformation, by causing gain or loss of genetic material in the daughter cells and by the possibility of massive chromosome rearrangement in chromosomes entrapped within a micronucleus by the mechanisms of chromothripsis and chromoanagenesis.

KDM4C Activity Modulates Cell Proliferation and Chromosome Segregation in Triple-Negative Breast Cancer

The Jumonji-containing domain protein, KDM4C, is a histone demethylase associated with the development of several forms of human cancer. However, its specific function in the viability of tumoral lineages is yet to be determined. This work investigates the importance of KDM4C activity in cell proliferation and chromosome segregation of three triple-negative breast cancer cell lines using a specific demethylase inhibitor. Immunofluorescence assays show that KDM4C is recruited to mitotic chromosomes and that the modulation of its activity increases the number of mitotic segregation errors. However, 3-(4,5-Dimethylthiazol-2-yl)-2,5-Diphenyltetrazolium Bromide (MTT) cell proliferation assays demonstrate that the demethylase activity is required for cell viability. These results suggest that the histone demethylase activity of KDM4C is essential for breast cancer progression given its role in the maintenance of chromosomal stability and cell growth, thus highlighting it as a potential therapeutic target.

Micronucleus formation causes perpetual unilateral chromosome inheritance in mouse embryos

Chromosome segregation defects in cancer cells lead to encapsulation of chromosomes in micronuclei (MN), small nucleus-like structures within which dangerous DNA rearrangements termed chromothripsis can occur. Here we uncover a strikingly different consequence of MN formation in preimplantation development. We find that chromosomes from within MN become damaged and fail to support a functional kinetochore. MN are therefore not segregated, but are instead inherited by one of the two daughter cells. We find that the same MN can be inherited several times without rejoining the principal nucleus and without altering the kinetics of cell divisions. MN motion is passive, resulting in an even distribution of MN across the first two cell lineages. We propose that perpetual unilateral MN inheritance constitutes an unexpected mode of chromosome missegregation, which could contribute to the high frequency of aneuploid cells in mammalian embryos, but simultaneously may serve to insulate the early embryonic genome from chromothripsis.

Chromosomal instability in mammalian pre-implantation embryos: potential causes, detection methods, and clinical consequences

Formation of a totipotent blastocyst capable of implantation is one of the first major milestones in early mammalian embryogenesis, but less than half of in vitro fertilized embryos from most mammals will progress to this stage of development. Whole chromosomal abnormalities, or aneuploidy, are key determinants of whether human embryos will arrest or reach the blastocyst stage. Depending on the type of chromosomal abnormality, however, certain embryos still form blastocysts and may be morphologically indistinguishable from chromosomally normal embryos. Despite the implementation of pre-implantation genetic screening and other advanced in vitro fertilization (IVF) techniques, the identification of aneuploid embryos remains complicated by high rates of mosaicism, atypical cell division, cellular fragmentation, sub-chromosomal instability, and micro-/multi-nucleation. Moreover, several of these processes occur in vivo following natural human conception, suggesting that they are not simply a consequence of culture conditions. Recent technological achievements in genetic, epigenetic, chromosomal, and non-invasive imaging have provided additional embryo assessment approaches, particularly at the single-cell level, and clinical trials investigating their efficacy are continuing to emerge. In this review, we summarize the potential mechanisms by which aneuploidy may arise, the various detection methods, and the technical advances (such as time-lapse imaging, "-omic" profiling, and next-generation sequencing) that have assisted in obtaining this data. We also discuss the possibility of aneuploidy resolution in embryos via various corrective mechanisms, including multi-polar divisions, fragment resorption, endoreduplication, and blastomere exclusion, and conclude by examining the potential implications of these findings for IVF success and human fecundity.

Mammalian pre-implantation chromosomal instability: species comparison, evolutionary considerations, and pathological correlations

Pre-implantation embryo development in mammals begins at fertilization with the migration and fusion of the maternal and paternal pro-nuclei, followed by the degradation of inherited factors involved in germ cell specification and the activation of embryonic genes required for subsequent cell divisions, compaction, and blastulation. The majority of studies on early embryogenesis have been conducted in the mouse or non-mammalian species, often requiring extrapolation of the findings to human development. Given both conserved similarities and species-specific differences, however, even comparison between closely related mammalian species may be challenging as certain aspects, including susceptibility to chromosomal aberrations, varies considerably across mammals. Moreover, most human embryo studies are limited to patient samples obtained from in vitro fertilization (IVF) clinics and donated for research, which are generally of poorer quality and produced with germ cells that may be sub-optimal. Recent technical advances in genetic, epigenetic, chromosomal, and time-lapse imaging analyses of high quality whole human embryos have greatly improved our understanding of early human embryogenesis, particularly at the single embryo and cell level. This review summarizes the major characteristics of mammalian pre-implantation development from a chromosomal perspective, in addition to discussing the technological achievements that have recently been developed to obtain this data. We also discuss potential translation to clinical applications in reproductive medicine and conclude by examining the broader implications of these findings for the evolution of mammalian species and cancer pathology in somatic cells.

Chromothripsis: potential origin in gametogenesis and preimplantation cell divisions. A review

OBJECTIVE

To review the discovery of chromothripsis and analyze its impact on human reproduction.

DESIGN

Database and literature analysis.

SETTING

University hospital.

PATIENT(S)

Carriers of massive and complex chromosomal rearrangements.

INTERVENTION(S)

Cytogenetic analysis and molecular testing (fluorescence in situ hybridization, microarray, whole-genome sequencing).

MAIN OUTCOME MEASURE(S)

Chromothripsis occurrence in human gametes and preimplantation embryos, with regard to the potential causative mechanisms described in literature.

RESULT(S)

Databases were searched for the literature published up to March 2014. Chromothripsis is characterized by the shattering of one (or a few) chromosome segments followed by a haphazard reassembly of the fragments generated, arising through a single initial catastrophic event. Several mechanisms involving abortive apoptosis, telomere erosion, mitotic errors, micronuclei formation, and p53 inactivation might cause chromothripsis. The remarkable point is that all these plausible mechanisms have been identified in the field of human reproduction as causal factors for reproductive failures and the genesis of chromosomal abnormalities. Specific features of gametogenesis and early embryonic development such as the weakness of cell cycle and mitosis checkpoints and the rapid kinetics of division in germ cells and early cleavage embryos may contribute to the emergence of chromothripsis.

CONCLUSION(S)

The discovery of this new class of massive chromosomal rearrangement has deeply modified our understanding on the genesis of complex genomic rearrangements. Data presented in this review support the assumption that chromothripsis could operate in human germlines and during early embryonic development. Chromothripsis might arise more frequently than previously thought in both gametogenesis and early human embryogenesis.

Could contaminant induced mutations lead to a genetic diversity overestimation?

Contaminant driven genetic erosion reported through the inspection of selectable traits can be underestimated using neutral markers. This divergence was previously reported in the aquatic system of an abandoned pyrite mine. The most sensitive genotypes of the microcrustacean cladoceran Daphnia longispina were found to be lacking in the impacted reservoir near the entrance of the metal rich acid mine drainage (AMD). Since that divergence could be, at least partially, accounted for by mutagenicity and genotoxicity of the AMD, the present study aimed at providing such a characterization. The Allium cepa chromosomal aberration assay, using root meristematic cells, was carried out, by exposing seeds to 100, 10, 1, and 0.1 % of the local AMD. Chromosomal aberrations, cell division phases and cell death were quantified after the AMD exposure and after 24 and 48 h recovery periods. The AMD revealed to be mutagenic and genotoxic, even after diluting it to 1 and 0.1 %. Dilutions within this range were previously found to be below the lethality threshold and to elicit sublethal effects on reproduction of locally collected D. longispina clonal lineages Significant mutagenic effects (micronuclei and chromosomal breaks) were also found at 0.1 % AMD, supporting that exposure may induce permanent genetic alterations. Recovery tests showed that AMD genotoxic effects persisted after the exposure.

Chromothripsis and cancer: causes and consequences of chromosome shattering

Genomic alterations that lead to oncogene activation and tumour suppressor loss are important driving forces for cancer development. Although these changes can accumulate progressively during cancer evolution, recent studies have revealed that many cancer cells harbour chromosomes bearing tens to hundreds of clustered genome rearrangements. In this Review, we describe how this striking phenomenon, termed chromothripsis, is likely to arise through chromosome breakage and inaccurate reassembly. We also discuss the potential diagnostic, prognostic and therapeutic implications of chromothripsis in cancer.