DNA lesions induced by replication stress trigger mitotic aberration and tetraploidy development

The hereditary N363K POLE exonuclease mutant extends PPAP tumor spectrum to glioblastomas by causing DNA damage and aneuploidy in addition to increased mismatch mutagenicity

The exonuclease domain of DNA polymerases epsilon's catalytic subunit (POLE) removes misincorporated nucleotides, called proofreading. POLE-exonuclease mutations cause colorectal- and endometrial cancers with an extreme burden of single nucleotide substitutions. We recently reported that particularly the hereditary POLE exonuclease mutation N363K predisposes in addition to aggressive giant cell glioblastomas. We knocked-in this mutation homozygously into human cell lines and compared its properties to knock-ins of the likewise hereditary POLE L424V mutation and to a complete proofreading-inactivating mutation (exo-null). We found that N363K cells have higher mutation rates as both L424V- or exo-null mutant cells. In contrast to L424V cells, N363K cells expose a growth defect, replication stress and DNA damage. In non-transformed cells, these burdens lead to aneuploidy but macroscopically normal nuclei. In contrast, transformed N363K cells phenocopy the enlarged and disorganized nuclei of giant cell glioblastomas. Taken together, our data characterize a POLE exonuclease domain mutant that not only causes single nucleotide hypermutation, but in addition DNA damage and chromosome instability, leading to an extended tumor spectrum. Our results expand the understanding of the polymerase exonuclease domain and suggest that an assessment of both the mutational potential and the genetic instability might refine classification and treatment of POLE-mutated tumors.

Genome destabilization-associated phenotypes arising as a consequence of therapeutic treatment are suppressed by Olaparib

Malignancy is often associated with therapeutic resistance and metastasis, usually arising after therapeutic treatment. These include radio- and chemo-therapies, which cause cancer cell death by inducing DNA double strand breaks (DSBs). However, it is still unclear how resistance to these DSBs is induced and whether it can be suppressed. Here, we show that DSBs induced by camptothecin (CPT) and radiation jeopardize genome stability in surviving cancer cells, ultimately leading to the development of resistance. Further, we show that cytosolic DNA, accumulating as a consequence of genomic destabilization, leads to increased cGAS/STING-pathway activation and, ultimately, increased cell migration, a precursor of metastasis. Interestingly, these genomic destabilization-associated phenotypes were suppressed by the PARP inhibitor Olaparib. Recognition of DSBs by Rad51 and genomic destabilization were largely reduced by Olaparib, while the DNA damage response and cancer cell death were effectively increased. Thus, Olaparib decreases the risk of therapeutic resistance and cell migration of cells that survive radio- and CPT-treatments.

Genomic Instability and Cancer Risk Associated with Erroneous DNA Repair

Many cancers develop as a consequence of genomic instability, which induces genomic rearrangements and nucleotide mutations. Failure to correct DNA damage in DNA repair defective cells, such as in BRCA1 and BRCA2 mutated backgrounds, is directly associated with increased cancer risk. Genomic rearrangement is generally a consequence of erroneous repair of DNA double-strand breaks (DSBs), though paradoxically, many cancers develop in the absence of DNA repair defects. DNA repair systems are essential for cell survival, and in cancers deficient in one repair pathway, other pathways can become upregulated. In this review, we examine the current literature on genomic alterations in cancer cells and the association between these alterations and DNA repair pathway inactivation and upregulation.

Biosecurity test of conjugated nanoparticles of chitosanprotoporphyrin IX-vitamin B9 for their use in photodynamic therapy

Nanotechnology proposes new applications for the development of nanotransporters and active targeting molecules with the use of biodegradable polymeric nanoparticles to improve the specificity towards target cells. However, these products must comply with safety tests to be endorsed as therapeutic alternatives by regulatory organizations. The goal of this work was to evaluate the biosafety (cytotoxicity and genotoxicity) of chitosan polymeric nanoparticles conjugate with protoporphyrin IX and vitamin B9 (CNPs-PpIX-B9) that were previously optimized from the established protocol by our laboratory and tested in CHO-K1 cells by bioassay following the recommendations of the chromosomal aberrations test by OECD 473 (2016) guideline. The conjugate did not show evidence of genotoxicity (clastogenicity). Surprisingly, the significant differences between the treatments performed and the negative control do not represent increases in chromosomal aberrations, whereby the safe concentrations to use the conjugate without inducing cytotoxic or genotoxic effects are less than 0.25 mg / mL. Since it induced a significant decrease of structural chromosomal aberrations, generating a positive effect on the genomic stability of CHO-K1 cells cultured in this test system.

Replication-stress-associated DSBs induced by ionizing radiation risk genomic destabilization and associated clonal evolution

Exposure to ionizing radiation is associated with cancer risk. Although multiple types of DNA damage are caused by radiation, it remains unknown how this damage is associated with cancer risk. Here, we show that after repair of double-strand breaks (DSBs) directly caused by radiation (dir-DSBs), irradiated cells enter a state at higher risk of genomic destabilization due to accumulation of replication-stress-associated DSBs (rs-DSBs), ultimately resulting in clonal evolution of cells with abrogated defense systems. These effects were observed over broad ranges of radiation doses (0.25–2 Gy) and dose rates (1.39–909 mGy/min), but not upon high-dose irradiation, which caused permanent cell-cycle arrest. The resultant genomic destabilization also increased the risk of induction of single-nucleotide variants (SNVs), including radiation-associated SNVs, as well as structural alterations in chromosomes. Thus, the radiation-associated risk can be attributed to rs-DSB accumulation and resultant genomic destabilization.

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Replication-stress-associated DSBs accumulate after exposure to ionizing radiation

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Such DSBs risk genomic destabilization and associated mutagenesis

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The resulting genomic rearrangements and mutations lead to clonal evolution

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The radiation-associated risks arise at wide ranges of radiation doses and dose rates

Genomic destabilization and its associated mutagenesis increase with senescence-associated phenotype expression

Cancer develops through multiple rounds of clonal evolution of cells with abrogated defense systems. Such clonal evolution is triggered by genomic destabilization with associated mutagenesis. However, what increases the risk of genomic destabilization remains unclear. Genomic instability is usually the result of erroneous repair of DNA double‐strand breaks (DSB); paradoxically, however, most cancers develop with genomic instability but lack mutations in DNA repair systems. In this manuscript, we review current knowledge regarding a cellular state that increases the risk of genomic destabilization, in which cells exhibit phenotypes often observed during senescence. In addition, we explore the pathways that lead to genomic destabilization and its associated mutagenesis, which ultimately result in cancer.

Distinct Orchestration and Dynamic Processes on γ-H2AX and p-H3 for Two Major Types of Genotoxic Chemicals Revealed by Mass Spectrometry Analysis

Genotoxic chemicals act by causing DNA damage, which, if left unrepaired, can have deleterious consequences for cell survival. DNA damage response (DDR) gets activated to repair or mitigate the effects of DNA damage. Histone H2AX and H3 phosphorylation biomarkers (γ-H2AX and p-H3) have attracted great attention as they play pivotal roles in the DDR. Simultaneous quantitation of γ-H2AX and p-H3 in exposed cells may monitor the toxicity of genotoxic chemicals and to some extent reflect the subsequent DDR process. Reported here is the first comprehensive characterization of distinct orchestration and dynamic processes on cellular γ-H2AX and p-H3 for two major types of genotoxic chemicals, clastogens and aneugens, by stable isotope dilution-liquid chromatography-tandem mass spectrometry (ID-LC-MS/MS). We find that clastogens significantly induce an increase in γ-H2AX and a decrease in p-H3; aneugens have no obvious effect on γ-H2AX, whereas induce either an increase or a decrease in p-H3. In addition, the specific profiles of clastogens and aneugens affecting DNA damage may be dynamically observed, which in turn provides insights into the processes involving DNA damage repair as well as transcription. Taken together, these results suggest that robust LC-MS/MS analysis of γ-H2AX and p-H3 can not only quantitatively differentiate mechanistic information on clastogens and aneugens but also dynamically present the detail profiles of DNA damage and repair processes.

DNA Replication Stress and Chromosomal Instability: Dangerous Liaisons

Chromosomal instability (CIN) is associated with many human diseases, including neurodevelopmental or neurodegenerative conditions, age-related disorders and cancer, and is a key driver for disease initiation and progression. A major source of structural chromosome instability (s-CIN) leading to structural chromosome aberrations is "replication stress", a condition in which stalled or slowly progressing replication forks interfere with timely and error-free completion of the S phase. On the other hand, mitotic errors that result in chromosome mis-segregation are the cause of numerical chromosome instability (n-CIN) and aneuploidy. In this review, we will discuss recent evidence showing that these two forms of chromosomal instability can be mechanistically interlinked. We first summarize how replication stress causes structural and numerical CIN, focusing on mechanisms such as mitotic rescue of replication stress (MRRS) and centriole disengagement, which prevent or contribute to specific types of structural chromosome aberrations and segregation errors. We describe the main outcomes of segregation errors and how micronucleation and aneuploidy can be the key stimuli promoting inflammation, senescence, or chromothripsis. At the end, we discuss how CIN can reduce cellular fitness and may behave as an anticancer barrier in noncancerous cells or precancerous lesions, whereas it fuels genomic instability in the context of cancer, and how our current knowledge may be exploited for developing cancer therapies.

Nucleosides Rescue Replication-Mediated Genome Instability of Human Pluripotent Stem Cells

Human pluripotent stem cells (PSCs) are subject to the appearance of recurrent genetic variants on prolonged culture. We have now found that, compared with isogenic differentiated cells, PSCs exhibit evidence of considerably more DNA damage during the S phase of the cell cycle, apparently as a consequence of DNA replication stress marked by slower progression of DNA replication, activation of latent origins of replication, and collapse of replication forks. As in many cancers, which, like PSCs, exhibit a shortened G1 phase and DNA replication stress, the resulting DNA damage may underlie the higher incidence of abnormal and abortive mitoses in PSCs, resulting in chromosomal non-dysjunction or cell death. However, we have found that the extent of DNA replication stress, DNA damage, and consequent aberrant mitoses can be substantially reduced by culturing PSCs in the presence of exogenous nucleosides, resulting in improved survival, clonogenicity, and population growth.

Resveratrol and its Related Polyphenols Contribute to the Maintenance of Genome Stability

Genomic destabilisation is associated with the induction of mutations, including those in cancer-driver genes, and subsequent clonal evolution of cells with abrogated defence systems. Such mutations are not induced when genome stability is maintained; however, the mechanisms involved in genome stability maintenance remain elusive. Here, resveratrol (and related polyphenols) is shown to enhance genome stability in mouse embryonic fibroblasts, ultimately protecting the cells against the induction of mutations in the ARF/p53 pathway. Replication stress-associated DNA double-strand breaks (DSBs) that accumulated with genomic destabilisation were effectively reduced by resveratrol treatment. In addition, resveratrol transiently stabilised the expression of histone H2AX, which is involved in DSB repair. Similar effects on the maintenance of genome stability were observed for related polyphenols. Accordingly, we propose that polyphenol consumption can contribute to the suppression of cancers that develop with genomic instability, as well as lifespan extension.

Mismatch repair dependence of replication stress-associated DSB recognition and repair

Most cancers develop with one of two types of genomic instability, namely, chromosomal instability (CIN) or microsatellite instability (MSI). Both are induced by replication stress-associated DNA double-strand breaks (DSBs). The type of genomic instability that arises is dependent on the choice of DNA repair pathway. Specifically, MSI is induced via a PolQ-dependent repair pathway called microhomology-mediated end joining (MMEJ) in a mismatch repair (MMR)-deficient background. However, it is unclear how the MMR status determines the choice of DSB repair pathway. Here, we show that replication stress-associated DSBs initially targeted by the homologous recombination (HR) system were subsequently hijacked by PolQ-dependent MMEJ in MMR-deficient cells, but persisted as HR intermediates in MMR-proficient cells. PolQ interacting with MMR factors was effectively loaded onto damaged chromatin in an MMR-deficient background, in which merged MRE11/γH2AX foci also effectively formed. Thus, the choice of DNA repair pathway according to the MMR status determines whether CIN or MSI is induced.

Genomic-Destabilization-Associated Mutagenesis and Clonal Evolution of Cells with Mutations in Tumor-Suppressor Genes

The development of cancer is driven by genomic instability and mutations. In general, cancer develops via multiple steps. Each step involves the clonal evolution of cells with abrogated defense systems, such as cells with mutations in cancer-suppressor genes. However, it remains unclear how cellular defense systems are abrogated and the associated clonal evolution is triggered and propagated. In this manuscript, we review current knowledge regarding mutagenesis associated with genomic destabilization and its relationship with the clonal evolution of cells over the course of cancer development, focusing especially on mechanistic aspects.

Replication stress triggers microsatellite destabilization and hypermutation leading to clonal expansion in vitro

Mismatch repair (MMR)-deficient cancers are characterized by microsatellite instability (MSI) and hypermutation. However, it remains unclear how MSI and hypermutation arise and contribute to cancer development. Here, we show that MSI and hypermutation are triggered by replication stress in an MMR-deficient background, enabling clonal expansion of cells harboring ARF/p53-module mutations and cells that are resistant to the anti-cancer drug camptothecin. While replication stress-associated DNA double-strand breaks (DSBs) caused chromosomal instability (CIN) in an MMR-proficient background, they induced MSI with concomitant suppression of CIN via a PARP-mediated repair pathway in an MMR-deficient background. This was associated with the induction of mutations, including cancer-driver mutations in the ARF/p53 module, via chromosomal deletions and base substitutions. Immortalization of MMR-deficient mouse embryonic fibroblasts (MEFs) in association with ARF/p53-module mutations was ~60-fold more efficient than that of wild-type MEFs. Thus, replication stress-triggered MSI and hypermutation efficiently lead to clonal expansion of cells with abrogated defense systems.

Mismatch repair (MMR)-deficient cancers are characterized by microsatellite instability (MSI) and hypermutation. Here authors reveal a mechanism by which replication stress induces MSI and associated induction of mutations in vitro.

The emerging links between chromosomal instability (CIN), metastasis, inflammation and tumour immunity

Many cancers possess an incorrect number of chromosomes, a state described as aneuploidy. Aneuploidy is often caused by Chromosomal Instability (CIN), a process of continuous chromosome mis-segregation. CIN is believed to endow tumours with enhanced evolutionary capabilities due to increased intratumour heterogeneity, and facilitating adaptive resistance to therapies. Recently, however, additional consequences and associations with CIN have been revealed, prompting the need to understand this universal hallmark of cancer in a multifaceted context. This review is focused on the investigation of possible links between CIN, metastasis and the host immune system in cancer development and treatment. We specifically focus on these links since most cancer deaths are due to the consequences of metastasis, and immunotherapy is a rapidly expanding novel avenue of cancer therapy.

Cytokinesis defects and cancer

Whole-genome and centrosome duplication as a consequence of cytokinesis failure can drive tumorigenesis in experimental model systems. However, whether cytokinesis failure is in fact an important cause of human cancers has remained unclear. In this Review, we summarize evidence that whole-genome-doubling events are frequently observed in human cancers and discuss the contribution that cytokinesis defects can make to tumorigenesis. We provide an overview of the potential causes of cytokinesis failure and discuss how tetraploid cells that are generated through cytokinesis defects are used in cancer as a transitory state on the route to aneuploidy. Finally, we discuss how cytokinesis defects can facilitate genetic diversification within the tumour to promote cancer development and could constitute the path of least resistance in tumour evolution.

Onset of deaminase APOBEC3B induction in response to DNA double-strand breaks

Deamination of 5-methyl cytosine is a major cause of cancer-driver mutations in inflammation-associated cancers. The deaminase APOBEC3B is expressed in these cancers and causes mutations under replication stress; however, the mechanisms by which APOBEC3B mediates deamination and its association with genomic disorders are still unclear. Here, we show that APOBEC3B is stabilized to induce deamination reaction in response to DNA double-strand breaks (DSBs), resulting in the formation of long-lasting DSBs. Uracil, the major deamination product, is subsequently targeted by base excision repair (BER) through uracil-DNA glycosylase 2 (UNG2); hence late-onset DSBs arise as by-products of BER. The frequency of these delayed DSBs was increased by treatment of cells with a PARP inhibitor, and was suppressed following knock-down of UNG2. The late-onset DSBs were induced in an ATR-dependent manner. Those secondary DSBs were persistent, unlike DSBs directly caused by γ-ray irradiation. Overall, these results suggest that the deaminase APOBEC3B is induced in response to DSBs, leading to long-lasting DSB formation in addition to mutagenic 5me-C>T transition induction.

Impaired Notch Signaling Leads to a Decrease in p53 Activity and Mitotic Catastrophe in Aged Muscle Stem Cells

The decline of tissue regenerative potential with age correlates with impaired stem cell function. However, limited strategies are available for therapeutic modulation of stem cell function during aging. Using skeletal muscle stem cells (MuSCs) as a model system, we identify cell death by mitotic catastrophe as a cause of impaired stem cell proliferative expansion in aged animals. The mitotic cell death is caused by a deficiency in Notch activators in the microenvironment. We discover that ligand-dependent stimulation of Notch activates p53 in MuSCs via inhibition of Mdm2 expression through Hey transcription factors during normal muscle regeneration and that this pathway is impaired in aged animals. Pharmacologic activation of p53 promotes the expansion of aged MuSCs in vivo. Altogether, these findings illuminate a Notch-p53 signaling axis that plays an important role in MuSC survival during activation and is dysregulated during aging, contributing to the age-related decline in muscle regenerative potential.

Mechanisms of Oncogene-Induced Replication Stress: Jigsaw Falling into Place

Oncogene activation disturbs cellular processes and accommodates a complex landscape of changes in the genome that contribute to genomic instability, which accelerates mutation rates and promotes tumorigenesis. Part of this cellular turmoil involves deregulation of physiologic DNA replication, widely described as replication stress. Oncogene-induced replication stress is an early driver of genomic instability and is attributed to a plethora of factors, most notably aberrant origin firing, replication-transcription collisions, reactive oxygen species, and defective nucleotide metabolism.Significance: Replication stress is a fundamental step and an early driver of tumorigenesis and has been associated with many activated oncogenes. Deciphering the mechanisms that contribute to the replication stress response may provide new avenues for targeted cancer treatment. In this review, we discuss the latest findings on the DNA replication stress response and examine the various mechanisms through which activated oncogenes induce replication stress. Cancer Discov; 8(5); 537-55. ©2018 AACR.

RNF8 and SCML2 cooperate to regulate ubiquitination and H3K27 acetylation for escape gene activation on the sex chromosomes

The sex chromosomes are enriched with germline genes that are activated during the late stages of spermatogenesis. Due to meiotic sex chromosome inactivation (MSCI), these sex chromosome-linked genes must escape silencing for activation in spermatids, thereby ensuring their functions for male reproduction. RNF8, a DNA damage response protein, and SCML2, a germline-specific Polycomb protein, are two major, known regulators of this process. Here, we show that RNF8 and SCML2 cooperate to regulate ubiquitination during meiosis, an early step to establish active histone modifications for subsequent gene activation. Double mutants of Rnf8 and Scml2 revealed that RNF8-dependent monoubiquitination of histone H2A at Lysine 119 (H2AK119ub) is deubiquitinated by SCML2, demonstrating interplay between RNF8 and SCML2 in ubiquitin regulation. Additionally, we identify distinct functions of RNF8 and SCML2 in the regulation of ubiquitination: SCML2 deubiquitinates RNF8-independent H2AK119ub but does not deubiquitinate RNF8-dependent polyubiquitination. RNF8-dependent polyubiquitination is required for the establishment of H3K27 acetylation, a marker of active enhancers, while persistent H2AK119ub inhibits establishment of H3K27 acetylation. Following the deposition of H3K27 acetylation, H3K4 dimethylation is established as an active mark on poised promoters. Together, we propose a model whereby regulation of ubiquitin leads to the organization of poised enhancers and promoters during meiosis, which induce subsequent gene activation from the otherwise silent sex chromosomes in postmeiotic spermatids.

Near-tetraploid cancer cells show chromosome instability triggered by replication stress and exhibit enhanced invasiveness

A considerable proportion of tumors exhibit aneuploid karyotypes, likely resulting from the progressive loss of chromosomes after whole-genome duplication. Here, by using isogenic diploid and near-tetraploid (4N) single-cell-derived clones from the same parental cell lines, we aimed at exploring how polyploidization affects cellular functions and how tetraploidy generates chromosome instability. Gene expression profiling in 4N clones revealed a significant enrichment of transcripts involved in cell cycle and DNA replication. Increased levels of replication stress in 4N cells resulted in DNA damage, impaired proliferation caused by a cell cycle delay during S phase, and higher sensitivity to S phase checkpoint inhibitors. In fact, increased levels of replication stress were also observed in nontransformed, proliferative posttetraploid RPE1 cells. Additionally, replication stress promoted higher levels of intercellular genomic heterogeneity and ongoing genomic instability, which could be explained by high rates of mitotic defects, and was alleviated by the supplementation of exogenous nucleosides. Finally, our data found that 4N cancer cells displayed increased migratory and invasive capacity, both in vitro and in primary colorectal tumors, indicating that tetraploidy can promote aggressive cancer cell behavior.-Wangsa, D., Quintanilla, I., Torabi, K., Vila-Casadesús, M., Ercilla, A., Klus, G., Yuce, Z., Galofré, C., Cuatrecasas, M., Lozano, J. J., Agell, N., Cimini, D., Castells, A., Ried, T., Camps, J. Near-tetraploid cancer cells show chromosome instability triggered by replication stress and exhibit enhanced invasiveness.

DNA damage signaling and polyploid macrophages in chronic inflammation

Whole genome duplications, an important step in cancer development, also occur in the macrophage lineage in disease: large multinucleated macrophages found within compact, ordered aggregates of immune cells, called granulomas, are a well-known histologic entity. Very recent work suggests that granuloma macrophages remarkably acquire epithelial cell features and the genotoxic stress response instructs granuloma macrophage genome duplications, suggesting that granuloma macrophages and pre-malignant epithelial cells may share common mechanisms of adaptation to chronic genotoxic stress. Exploring these mechanisms is key for a better understanding of the pathogenesis of chronic inflammatory diseases. Here we review the mechanisms of macrophage polyploidization, the role of DNA damage signaling in this process and the function of polyploid macrophages, with a focus on chronic inflammation.

Genomic Destabilization Triggered by Replication Stress during Senescence

Most cancers develop after middle age, and are often associated with multiple mutations and genomic instability, implying that genomic destabilization is critical for age-related tumor development. In this manuscript, we review current knowledge regarding (1) the senescent cellular background, which is associated with a higher risk of genomic destabilization; and (2) the contributions of genomic destabilization to cancer development.

Cyclin A2 regulates homologous recombination DNA repair and sensitivity to DNA damaging agents and poly(ADP-ribose) polymerase (PARP) inhibitors in human breast cancer cells

Defects in homologous recombination (HR) repair are found in breast cancers. Intriguingly, breast cancers with defective HR show increased sensitivity to DNA crosslinking agents and poly(ADP-ribose) polymerase (PARP) inhibitors. As such, genes that can affect HR functions have been of high interest in studies aiming to develop biomarkers for predicting response to treatment with these agents. Cyclin A2 is a key component of the core cell cycle machinery. However, whether cyclin A2 dysfunctions could cause HR defect and mediate sensitivity to DNA damaging agents remain unclear. Here we show that loss of cyclin A2 causes high rates of double-strand breaks (DSB) in MCF-7 and MDA-MB-231 cells. The increased DSB was due to defective HR-mediated repair of the breaks, resulting from reduced MRE11 and RAD51 proteins. Cyclin A2 mediates MRE11 abundance through its MRE11 mRNA binding property and RAD51 abundance through inhibition of proteasome degradation of RAD51. Moreover, cyclin A2 depletion hypersensitized the cells to DNA damaging agents, such as cisplatin and melphalan. Our results demonstrate novel roles for cyclin A2 in regulating HR repair and determining sensitivity to DNA cross linkers and PARP inhibitors in breast cancer cells.

Progression through mitosis promotes PARP inhibitor-induced cytotoxicity in homologous recombination-deficient cancer cells

Mutations in homologous recombination (HR) genes BRCA1 and BRCA2 predispose to tumorigenesis. HR-deficient cancers are hypersensitive to Poly (ADP ribose)-polymerase (PARP) inhibitors, but can acquire resistance and relapse. Mechanistic understanding how PARP inhibition induces cytotoxicity in HR-deficient cancer cells is incomplete. Here we find PARP inhibition to compromise replication fork stability in HR-deficient cancer cells, leading to mitotic DNA damage and consequent chromatin bridges and lagging chromosomes in anaphase, frequently leading to cytokinesis failure, multinucleation and cell death. PARP-inhibitor-induced multinucleated cells fail clonogenic outgrowth, and high percentages of multinucleated cells are found in vivo in remnants of PARP inhibitor-treated Brca2-/-;p53-/- and Brca1-/-;p53-/- mammary mouse tumours, suggesting that mitotic progression promotes PARP-inhibitor-induced cell death. Indeed, enforced mitotic bypass through EMI1 depletion abrogates PARP-inhibitor-induced cytotoxicity. These findings provide insight into the cytotoxic effects of PARP inhibition, and point at combination therapies to potentiate PARP inhibitor treatment of HR-deficient tumours.

Inefficient differentiation response to cell cycle stress leads to genomic instability and malignant progression of squamous carcinoma cells

Squamous cell carcinoma (SCC) or epidermoid cancer is a frequent and aggressive malignancy. However in apparent paradox it retains the squamous differentiation phenotype except for very dysplastic lesions. We have shown that cell cycle stress in normal epidermal keratinocytes triggers a squamous differentiation response involving irreversible mitosis block and polyploidisation. Here we show that cutaneous SCC cells conserve a partial squamous DNA damage-induced differentiation response that allows them to overcome the cell division block. The capacity to divide in spite of drug-induced mitotic stress and DNA damage made well-differentiated SCC cells more genomically instable and more malignant in vivo. Consistently, in a series of human biopsies, non-metastatic SCCs displayed a higher degree of chromosomal alterations and higher expression of the S phase regulator Cyclin E and the DNA damage signal γH2AX than the less aggressive, non-squamous, basal cell carcinomas. However, metastatic SCCs lost the γH2AX signal and Cyclin E, or accumulated cytoplasmic Cyclin E. Conversely, inhibition of endogenous Cyclin E in well-differentiated SCC cells interfered with the squamous phenotype. The results suggest a dual role of cell cycle stress-induced differentiation in squamous cancer: the resulting mitotic blocks would impose, when irreversible, a proliferative barrier, when reversible, a source of genomic instability, thus contributing to malignancy.

Characterisation of cell cycle arrest and terminal differentiation in a maximally proliferative human epithelial tissue: Lessons from the human hair follicle matrix

Human hair follicle (HF) growth and hair shaft formation require terminal differentiation-associated cell cycle arrest of highly proliferative matrix keratinocytes. However, the regulation of this complex event remains unknown. CIP/KIP family member proteins (p21^CIP1, p27^KIP1 and p57^KIP2) regulate cell cycle progression/arrest, endoreplication, differentiation and apoptosis. Since they have not yet been adequately characterized in the human HF, we asked whether and where CIP/KIP proteins localise in the human hair matrix and pre-cortex in relation to cell cycle activity and HF-specific epithelial cell differentiation that is marked by keratin 85 (K85) protein expression. K85 expression coincided with loss or reduction in cell cycle activity markers, including in situ DNA synthesis (EdU incorporation), Ki-67, phospho-histone H3 and cyclins A and B1, affirming a post-mitotic state of pre-cortical HF keratinocytes. Expression of CIP/KIP proteins was found abundantly within the proliferative hair matrix, concomitant with a role in cell cycle checkpoint control. p21^CIP1, p27^KIP1 and cyclin E persisted within post-mitotic keratinocytes of the pre-cortex, whereas p57^KIP2 protein decreased but became nuclear. These data imply a supportive role for CIP/KIP proteins in maintaining proliferative arrest, differentiation and anti-apoptotic pathways, promoting continuous hair bulb growth and hair shaft formation in anagen VI. Moreover, post-mitotic hair matrix regions contained cells with enlarged nuclei, and DNA in situ hybridisation showed cells that were >2N in the pre-cortex. This suggests that CIP/KIP proteins might counterbalance cyclin E to control further rounds of DNA replication in a cell population that has a propensity to become tetraploid. These data shed new light on the in situ-biography of human hair matrix keratinocytes on their path of active cell cycling, arrest and terminal differentiation, and showcase the human HF as an excellent, clinically relevant model system for cell cycle physiology research of human epithelial cells within their natural tissue habitat.

Mechanisms and consequences of aneuploidy and chromosome instability in the aging brain

Aneuploidy and polyploidy are a form of Genomic Instability (GIN) known as Chromosomal Instability (CIN) characterized by sporadic abnormalities in chromosome copy numbers. Aneuploidy is commonly linked to pathological states. It is a hallmark of spontaneous abortions and birth defects and it is observed virtually in every human tumor, therefore being generally regarded as detrimental for the development or the maturation of tissues under physiological conditions. Polyploidy however, occurs as part of normal physiological processes during maturation and differentiation of some mammalian cell types. Surprisingly, high levels of aneuploidy are present in the brain, and their frequency increases with age suggesting that the brain is able to maintain its functionality in the presence of high levels of mosaic aneuploidy. Because somatic aneuploidy with age can reach exceptionally high levels, it is likely to have long-term adverse effects in this organ. We describe the mechanisms accountable for an abnormal DNA content with a particular emphasis on the CNS where cell division is limited. Next, we briefly summarize the types of GIN known to date and discuss how they interconnect with CIN. Lastly we highlight how several forms of CIN may contribute to genetic variation, tissue degeneration and disease in the CNS.

Transcriptome analysis of tetraploid cells identifies cyclin D2 as a facilitator of adaptation to genome doubling in the presence of p53

Tetraploidization, or genome doubling, is a prominent event in tumorigenesis, primarily because cell division in polyploid cells is error-prone and produces aneuploid cells. This study investigates changes in gene expression evoked in acute and adapted tetraploid cells and their effect on cell-cycle progression. Acute polyploidy was generated by knockdown of the essential regulator of cytokinesis anillin, which resulted in cytokinesis failure and formation of binucleate cells, or by chemical inhibition of Aurora kinases, causing abnormal mitotic exit with formation of single cells with aberrant nuclear morphology. Transcriptome analysis of these acute tetraploid cells revealed common signatures of activation of the tumor-suppressor protein p53. Suppression of proliferation in these cells was dependent on p53 and its transcriptional target, CDK inhibitor p21. Rare proliferating tetraploid cells can emerge from acute polyploid populations. Gene expression analysis of single cell-derived, adapted tetraploid clones showed up-regulation of several p53 target genes and cyclin D2, the activator of CDK4/6/2. Overexpression of cyclin D2 in diploid cells strongly potentiated the ability to proliferate with increased DNA content despite the presence of functional p53. These results indicate that p53-mediated suppression of proliferation of polyploid cells can be averted by increased levels of oncogenes such as cyclin D2, elucidating a possible route for tetraploidy-mediated genomic instability in carcinogenesis.

Pixantrone induces cell death through mitotic perturbations and subsequent aberrant cell divisions

Pixantrone is a novel aza-anthracenedione active against aggressive lymphoma and is being evaluated for use against various hematologic and solid tumors. The drug is an analog of mitoxantrone, but displays less cardiotoxicity than mitoxantrone or the more commonly used doxorubicin. Although pixantrone is purported to inhibit topoisomerase II activity and intercalate with DNA, exact mechanisms of how it induces cell death remain obscure. Here we evaluated the effect of pixantrone on a panel of solid tumor cell lines to understand its mechanism of cell killing. Initial experiments with pixantrone showed an apparent discrepancy between its anti-proliferative effects in MTS assays (short-term) compared with clonogenic assays (long-term). Using live cell videomicroscopy to track the fates of cells, we found that cells treated with pixantrone underwent multiple rounds of aberrant cell division before eventually dying after approximately 5 d post-treatment. Cells underwent abnormal mitosis in which chromosome segregation was impaired, generating chromatin bridges between cells or within cells containing micronuclei. While pixantrone-treated cells did not display γH2AX foci, a marker of DNA damage, in the main nuclei, such foci were often detected in the micronuclei. Using DNA content analysis, we found that pixantrone concentrations that induced cell death in a clonogenic assay did not impede cell cycle progression, further supporting the lack of canonical DNA damage signaling. These findings suggest pixantrone induces a latent type of DNA damage that impairs the fidelity of mitosis, without triggering DNA damage response or mitotic checkpoint activation, but is lethal after successive rounds of aberrant division.

Gamma-irradiated quiescent cells repair directly induced double-strand breaks but accumulate persistent double-strand breaks during subsequent DNA replication

H2AX is expressed at very low levels in quiescent normal cells in vivo and in vitro. Such cells repair DNA double-strand breaks (DSBs) induced by γ-irradiation through a transient stabilization of H2AX. However, the resultant cells accumulate small numbers of irreparable (or persistent) DSBs via an unknown mechanism. We found that quiescent cells that had repaired DSBs directly induced by γ-rays were prone to accumulate DSBs during the subsequent DNA replication. Unlike directly induced DSBs, secondary DSBs were not efficiently repaired, although Rad51 and 53BP1 were recruited to these sites. H2AX was dramatically stabilized in response to DSBs directly caused by γ-rays, enabling γH2AX foci formation and DSB repair, whereas H2AX was barely stabilized in response to secondary DSBs, in which γH2AX foci were small and DSBs were not efficiently repaired. Our results show a pathway that leads to the persistent DSB formation after γ-irradiation.

Genomic instability: Crossing pathways at the origin of structural and numerical chromosome changes

Genomic instability leads to a wide spectrum of genetic changes, including single nucleotide mutations, structural chromosome alterations, and numerical chromosome changes. The accepted view on how these events are generated predicts that separate cellular mechanisms and genetic events explain the occurrence of these types of genetic variation. Recently, new findings have shed light on the complexity of the mechanisms leading to structural and numerical chromosome aberrations, their intertwining pathways, and their dynamic evolution, in somatic as well as in germ cells. In this review, we present a critical analysis of these recent discoveries in this area, with the aim to contribute to a deeper knowledge of the molecular networks leading to adverse outcomes in humans following exposure to environmental factors. The review illustrates how several technological advances, including DNA sequencing methods, bioinformatics, and live-cell imaging approaches, have contributed to produce a renewed concept of the mechanisms causing genomic instability. Special attention is also given to the specific pathways causing genomic instability in mammalian germ cells. Remarkably, the same scenario emerged from some pioneering studies published in the 1980s to 1990s, when the evolution of polyploidy, the chromosomal effects of spindle poisons, the fate of micronuclei, were intuitively proposed to share mechanisms and pathways. Thus, an old working hypothesis has eventually found proper validation.

Development of cancer-initiating cells and immortalized cells with genomic instability

Cancers that develop after middle age usually exhibit genomic instability and multiple mutations. This is in direct contrast to pediatric tumors that usually develop as a result of specific chromosomal translocations and epigenetic aberrations. The development of genomic instability is associated with mutations that contribute to cellular immortalization and transformation. Cancer occurs when cancer-initiating cells (CICs), also called cancer stem cells, develop as a result of these mutations. In this paper, we explore how CICs develop as a result of genomic instability, including looking at which cancer suppression mechanisms are abrogated. A recent in vitro study revealed the existence of a CIC induction pathway in differentiating stem cells. Under aberrant differentiation conditions, cells become senescent and develop genomic instabilities that lead to the development of CICs. The resulting CICs contain a mutation in the alternative reading frame of CDKN2A (ARF)/p53 module, i.e., in either ARF or p53. We summarize recently established knowledge of CIC development and cellular immortality, explore the role of the ARF/p53 module in protecting cells from transformation, and describe a risk factor for genomic destabilization that increases during the process of normal cell growth and differentiation and is associated with the downregulation of histone H2AX to levels representative of growth arrest in normal cells.

PARP-2 sustains erythropoiesis in mice by limiting replicative stress in erythroid progenitors

Erythropoiesis is a tightly regulated process in which multipotential hematopoietic stem cells produce mature red blood cells. Here we show that deletion of poly(ADP-ribose) polymerase-2 (PARP-2) in mice leads to chronic anemia at steady state, despite increased erythropoietin plasma levels, a phenomenon not observed in mice lacking PARP-1. Loss of PARP-2 causes shortened lifespan of erythrocytes and impaired differentiation of erythroid progenitors. In erythroblasts, PARP-2 deficiency triggers replicative stress, as indicated by the presence of micronuclei, the accumulation of γ-H2AX (phospho-histone H2AX) in S-phase cells and constitutive CHK1 and replication protein A phosphorylation. Transcriptome analyses revealed the activation of the p53-dependent DNA-damage response pathways in PARP-2-deficient cells, culminating in the upregulation of cell-cycle and cell death regulators, concomitant with G2/M arrest and apoptosis. Strikingly, while loss of the proapoptotic p53 target gene Puma restored hematocrit levels in the PARP-2-deficient mice, loss of the cell-cycle regulator and CDK inhibitor p21 leads to perinatal death by exacerbating impaired fetal liver erythropoiesis in PARP-2-deficient embryos. Although the anemia displayed by PARP-2-deficient mice is compatible with life, mice die rapidly when exposed to stress-induced enhanced hemolysis. Our results pinpoint an essential role for PARP-2 in erythropoiesis by limiting replicative stress that becomes essential in the absence of p21 and in the context of enhanced hemolysis, highlighting the potential effect that might arise from the design and use of PARP inhibitors that specifically inactivate PARP proteins.

ATM signalling and cancer

ATM, the protein kinase mutated in the rare human disease ataxia telangiectasia (A-T), has been the focus of intense scrutiny over the past two decades. Initially this was because of the unusual radiosensitive phenotype of cells from A-T patients, and latterly because investigating ATM signalling has yielded valuable insights into the DNA damage response, redox signalling and cancer. With the recent explosion in genomic data, ATM alterations have been revealed both in the germline as a predisposing factor for cancer and as somatic changes in tumours themselves. Here we review these findings, as well as advances in the understanding of ATM signalling mechanisms in cancer and ATM inhibition as a strategy for cancer treatment.

MYC and the control of DNA replication

The MYC oncogene is a multifunctional protein that is aberrantly expressed in a significant fraction of tumors from diverse tissue origins. Because of its multifunctional nature, it has been difficult to delineate the exact contributions of MYC's diverse roles to tumorigenesis. Here, we review the normal role of MYC in regulating DNA replication as well as its ability to generate DNA replication stress when overexpressed. Finally, we discuss the possible mechanisms by which replication stress induced by aberrant MYC expression could contribute to genomic instability and cancer.

Cre recombinase induces DNA damage and tetraploidy in the absence of loxP sites

The spatiotemporal manipulations of gene expression by the Cre recombinase (Cre) of bacteriophage P1 has become an essential asset to understanding mammalian genetics. Accumulating evidence suggests that Cre activity can, in addition to excising targeted loxP sites, induce cytotoxic effects, including abnormal cell cycle progression, genomic instability, and apoptosis, which can accelerate cancer progression. It is speculated that these defects are caused by Cre-induced DNA damage at off-target sites. Here we report the formation of tetraploid keratinocytes in the epidermis of keratin 5 and/or keratin 14 promoter-driven Cre (KRT5- and KRT14-Cre) expressing mouse skin. Biochemical analyses and flow cytometry demonstrated that Cre expression also induces DNA damage, genomic instability, and tetraploidy in HCT116 cells, and live-cell imaging revealed an extension of the G 2 cell cycle phase followed by defective or skipping of mitosis as cause for the tetraploidy. Since tetraploidy eventually leads to aneuploidy, a hallmark of cancer, our findings highlight the importance of distinguishing non-specific cytopathic effects from specific Cre/loxP-driven genetic manipulations when using Cre-mediated gene deletions.

Damage-induced DNA replication stalling relies on MAPK-activated protein kinase 2 activity

DNA damage can obstruct replication forks, resulting in replicative stress. By siRNA screening, we identified kinases involved in the accumulation of phosphohistone 2AX (γH2AX) upon UV irradiation-induced replication stress. Surprisingly, the strongest reduction of phosphohistone 2AX followed knockdown of the MAP kinase-activated protein kinase 2 (MK2), a kinase currently implicated in p38 stress signaling and G2 arrest. Depletion or inhibition of MK2 also protected cells from DNA damage-induced cell death, and mice deficient for MK2 displayed decreased apoptosis in the skin upon UV irradiation. Moreover, MK2 activity was required for damage response, accumulation of ssDNA, and decreased survival when cells were treated with the nucleoside analogue gemcitabine or when the checkpoint kinase Chk1 was antagonized. By using DNA fiber assays, we found that MK2 inhibition or knockdown rescued DNA replication impaired by gemcitabine or by Chk1 inhibition. This rescue strictly depended on translesion DNA polymerases. In conclusion, instead of being an unavoidable consequence of DNA damage, alterations of replication speed and origin firing depend on MK2-mediated signaling.

Replication stress and mitotic dysfunction in cells expressing simian virus 40 large T antigen

We previously demonstrated that simian virus 40 (SV40) large T antigen (LT) binds to the Bub1 kinase, a key regulator of the spindle checkpoint and chromosome segregation. Bub1 mutations or altered expression patterns are linked to chromosome missegregation and are considered to be a driving force in some human cancers. Here we report that LT, dependent on Bub1 binding, causes micronuclei, lagging chromatin, and anaphase bridges, which are hallmarks of chromosomal instability (CIN) and Bub1 insufficiency. Using time-lapse microscopy, we demonstrate that LT imposes a Bub1 binding-dependent delay in the metaphase-to-anaphase transition. Kinetochore fibers reveal that LT, via Bub1 binding, causes aberrant kinetochore (KT)-microtubule (MT) attachments and a shortened interkinetochore distance, consistent with a lack of tension. Previously, we showed that LT also induces the DNA damage response (DDR) via Bub1 binding. Using inducible LT cell lines, we show that an activated DDR was observed before the appearance of anaphase bridges and micronuclei. Furthermore, LT induction in serum-starved cells demonstrated γ-H2AX accumulation in cells that had not yet entered mitosis. Thus, DDR activation can occur independently of chromosome segregation defects. Replication stress pathways may be responsible, because signatures of replication stress were observed, which were attenuated by exogenous supplementation with nucleosides. Our observations allow us to propose a model that explains and integrates the diverse manifestations of genomic instability induced by LT.

E2F1-mediated DNA damage is implicated in 8-Cl-adenosine-induced chromosome missegregation and apoptosis in human lung cancer H1299 cells

Although E2F1-mediated DNA double-stranded breaks (DSBs) and tetraploid have been extensively studied, the role of E2F1 in mitotic catastrophe is still unknown. We have previously shown that 8-chloro-adenosine (8-Cl-Ado) induces DNA DSBs and aberrant mitosis in human lung cancer cells, followed by delayed apoptosis. Here, we demonstrate that E2F1-mediated DNA damage is implicated in 8-Cl-Ado-induced chromosome missegregation and apoptosis in lung cancer H1299 cells. We showed that E2F1 was accumulated upon 8-Cl-Ado-induced DNA DSBs. Induction of E2F1 by 8-Cl-Ado caused DNA damage in cycling cells including M cells. In contrast, silencing of E2F1 expression decreased 8-Cl-Ado-induced DNA DSBs, particularly eliminated E2F1-mediated mitotic DNA damage. Over-expression of E2F1 and/or 8-Cl-Ado exposure resulted in aberrant mitotic spindles and chromosome segregation errors. Furthermore, over-expression of E2F1 expression enhanced 8-Cl-Ado-induced apoptosis. Together, our data indicate that E2F1-mediated DNA damage, in particular mitotic DNA damage, is an important fraction of 8-Cl-Ado-induced DNA damage, which is implicated in 8-Cl-Ado-induced mitotic catastrophe and delayed apoptosis. Induction of E2F1 by 8-Cl-Ado may contribute at least partly to the drug-inhibited proliferation of cancer cells.

The Arf/p53 protein module, which induces apoptosis, down-regulates histone H2AX to allow normal cells to survive in the presence of anti-cancer drugs

BACKGROUND

It is unclear how DNA-damaging agents target cancer cells over normal somatic cells.

RESULTS

Arf/p53-dependent down-regulation of H2AX enables normal cells to survive after DNA damage.

CONCLUSION

Transformed cells, which harbor mutations in either Arf or p53, are more sensitive to DNA-damaging agents.

SIGNIFICANCE

Cellular transformation renders cells more susceptible to some DNA-damaging agents. Anti-cancer drugs generally target cancer cells rather than normal somatic cells. However, the factors that determine this differential sensitivity are poorly understood. Here we show that Arf/p53-dependent down-regulation of H2AX induced the selective survival of normal cells after drug treatment, resulting in the preferential targeting of cancer cells. Treatment with camptothecin, a topoisomerase I inhibitor, caused normal cells to down-regulate H2AX and become quiescent, a process mediated by both Arf and p53. In contrast, transformed cells that harbor mutations in either Arf or p53 do not down-regulate H2AX and are more sensitive to drugs unless they have developed drug resistance. Such transformation-associated changes in H2AX expression rendered cancer cells more susceptible to drug-induced damage (by two orders of magnitude). Thus, the expression of H2AX and γH2AX (phosphorylated form of H2AX at Ser-139) is a critical factor that determines drug sensitivity and should be considered when administering chemotherapy.

Oncogenes induce genotoxic stress by mitotic processing of unusual replication intermediates

Processing of unusual replication intermediates such as reversed forks by MUS81 contributes to oncogene-induced double-strand breaks and depends on mitotic entry.

Oncogene-induced DNA replication stress activates the DNA damage response (DDR), a crucial anticancer barrier. DDR inactivation in these conditions promotes genome instability and tumor progression, but the underlying molecular mechanisms are elusive. We found that overexpression of both Cyclin E and Cdc25A rapidly slowed down replication forks and induced fork reversal, suggestive of increased topological stress. Surprisingly, these phenotypes, per se, are neither associated with chromosomal breakage nor with significant DDR activation. Oncogene-induced DNA breakage and DDR activation instead occurred upon persistent G2/M arrest or, in a checkpoint-defective context, upon premature CDK1 activation. Depletion of MUS81, a cell cycle–regulated nuclease, markedly limited chromosomal breakage and led to further accumulation of reversed forks. We propose that nucleolytic processing of unusual replication intermediates mediates oncogene-induced genotoxicity and that limiting such processing to mitosis is a central anti-tumorigenic function of the DNA damage checkpoints.

Involvement of multiple cell cycle aberrations in early preneoplastic liver cell lesions by tumor promotion with thioacetamide in a two-stage rat hepatocarcinogenesis model

Thioacetamide (TAA) induces oxidative stress and hepatocarcinogenicity in rats. We previously reported that TAA promotion caused various disruptions in cell cycle protein expression in rats, including downregulation of p16(Ink4a), which is associated with intraexonic hypermethylation in hepatocellular proliferative lesions. This study further investigated the contribution of cell cycle aberrations associated with early hepatocarcinogenic processes induced by TAA using antioxidants, enzymatically modified isoquercitrin (EMIQ) and α-lipoic acid (ALA), in a two-stage rat hepatocarcinogenesis model. TAA-promotion after initiation with N-diethylnitrosamine increased the number and area of hepatocellular foci immunoreactive for glutathione S-transferase placental form (GST-P) and the numbers of proliferating and apoptotic cells. Co-treatment with EMIQ and ALA suppressed these increases. TAA-induced formation of p16(Ink4a-) foci in concordance with GST-P(+) foci was not suppressed by co-treatment with EMIQ or ALA. TAA-promotion increased cellular distributions of cell proliferation marker Ki-67, G2/M and spindle checkpoint proteins (phosphorylated checkpoint kinase 1 and Mad2), the DNA damage-related protein phosphorylated histone H2AX, and G2-M phase-related proteins (topoisomerase IIα, phosphorylated histone H3 and Cdc2) within GST-P(+) foci, and co-treatment with EMIQ or ALA suppressed these increases. These results suggest that downregulation of p16(Ink4a) may allow selective proliferation of preneoplastic cells by TAA promotion. However, antioxidants did not counteract this gene control. Moreover, effective suppression of TAA-induced cellular population changes within preneoplastic lesions by antioxidants may reflect facilitation of cell cycling and accumulation of DNA damage causing the activation of cell cycle checkpoints, leading to G2 and M phase arrest at the early stages of hepatocarcinogenesis promoted by TAA.

Replication stress links structural and numerical cancer chromosomal instability

Cancer chromosomal instability (CIN) results in an increased rate of change of chromosome number and structure and generates intratumour heterogeneity. CIN is observed in most solid tumours and is associated with both poor prognosis and drug resistance. Understanding a mechanistic basis for CIN is therefore paramount. Here we find evidence for impaired replication fork progression and increased DNA replication stress in CIN(+) colorectal cancer (CRC) cells relative to CIN(-) CRC cells, with structural chromosome abnormalities precipitating chromosome missegregation in mitosis. We identify three new CIN-suppressor genes (PIGN (also known as MCD4), MEX3C (RKHD2) and ZNF516 (KIAA0222)) encoded on chromosome 18q that are subject to frequent copy number loss in CIN(+) CRC. Chromosome 18q loss was temporally associated with aneuploidy onset at the adenoma-carcinoma transition. CIN-suppressor gene silencing leads to DNA replication stress, structural chromosome abnormalities and chromosome missegregation. Supplementing cells with nucleosides, to alleviate replication-associated damage, reduces the frequency of chromosome segregation errors after CIN-suppressor gene silencing, and attenuates segregation errors and DNA damage in CIN(+) cells. These data implicate a central role for replication stress in the generation of structural and numerical CIN, which may inform new therapeutic approaches to limit intratumour heterogeneity.