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

Homologous recombination suppresses transgenerational DNA end resection and chromosomal instability in fission yeast

Chromosomal instability (CIN) drives cell-to-cell heterogeneity, and the development of genetic diseases, including cancer. Impaired homologous recombination (HR) has been implicated as a major driver of CIN, however, the underlying mechanism remains unclear. Using a fission yeast model system, we establish a common role for HR genes in suppressing DNA double-strand break (DSB)-induced CIN. Further, we show that an unrepaired single-ended DSB arising from failed HR repair or telomere loss is a potent driver of widespread CIN. Inherited chromosomes carrying a single-ended DSB are subject to cycles of DNA replication and extensive end-processing across successive cell divisions. These cycles are enabled by Cullin 3-mediated Chk1 loss and checkpoint adaptation. Subsequent propagation of unstable chromosomes carrying a single-ended DSB continues until transgenerational end-resection leads to fold-back inversion of single-stranded centromeric repeats and to stable chromosomal rearrangements, typically isochromosomes, or to chromosomal loss. These findings reveal a mechanism by which HR genes suppress CIN and how DNA breaks that persist through mitotic divisions propagate cell-to-cell heterogeneity in the resultant progeny.

miRNA dysregulation is an emerging modulator of genomic instability

Genomic instability consists of a range of genetic alterations within the genome that contributes to tumor heterogeneity and drug resistance. It is a well-established characteristic of most cancer cells. Genome instability induction results from defects in DNA damage surveillance mechanisms, mitotic checkpoints and DNA repair machinery. Accumulation of genetic alterations ultimately sets cells towards malignant transformation. Recent studies suggest that miRNAs are key players in mediating genome instability. miRNAs are a class of small RNAs expressed in most somatic tissues and are part of the epigenome. Importantly, in many cancers, miRNA expression is dysregulated. Consequently, this review examines the role of miRNA dysregulation as a causal step for induction of genome instability and subsequent carcinogenesis. We focus specifically on mechanistic studies assessing miRNA(s) and specific subtypes of genome instability or known modes of genome instability. In addition, we provide insight on the existing knowledge gaps within the field and possible ways to address them.

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.

Karyotype Aberrations in Action: The Evolution of Cancer Genomes and the Tumor Microenvironment

Cancer is a disease of cellular evolution. For this cellular evolution to take place, a population of cells must contain functional heterogeneity and an assessment of this heterogeneity in the form of natural selection. Cancer cells from advanced malignancies are genomically and functionally very different compared to the healthy cells from which they evolved. Genomic alterations include aneuploidy (numerical and structural changes in chromosome content) and polyploidy (e.g., whole genome doubling), which can have considerable effects on cell physiology and phenotype. Likewise, conditions in the tumor microenvironment are spatially heterogeneous and vastly different than in healthy tissues, resulting in a number of environmental niches that play important roles in driving the evolution of tumor cells. While a number of studies have documented abnormal conditions of the tumor microenvironment and the cellular consequences of aneuploidy and polyploidy, a thorough overview of the interplay between karyotypically abnormal cells and the tissue and tumor microenvironments is not available. Here, we examine the evidence for how this interaction may unfold during tumor evolution. We describe a bidirectional interplay in which aneuploid and polyploid cells alter and shape the microenvironment in which they and their progeny reside; in turn, this microenvironment modulates the rate of genesis for new karyotype aberrations and selects for cells that are most fit under a given condition. We conclude by discussing the importance of this interaction for tumor evolution and the possibility of leveraging our understanding of this interplay for cancer therapy.

Cytogenetic analysis of nuclear abnormalities in the erythrocytes of gecko lizards (Phyllopezus periosus) collected in a semi-arid region of northeast Brazil: Possible effects of natural background radioactivity

High natural-background radioactivity levels occur in the semi-arid region of the State of Rio Grande do Norte, northeastern Brazil. We have studied the lizard Phyllopezus periosus, an endemic species of the Brazilian caatinga with saxicolous habitat, as a bioindicator of environmental quality. Specimens were collected in three areas, an environmental protection area and two areas recognized as having high natural background radiation, one of these being a mining area. Level of metals and gamma radiation emitters present in the water sources potentially used by the lizards were measured. The biological endpoints assessed were micronuclei and nuclear abnormalities in blood samples. Significant differences in background radioactivity levels were found among the assessed areas. Statistically significant differences in micronuclei and nuclear abnormality frequencies were seen, among the study areas and a relationship between radioactivity level and genetic damage was observed.

Micronuclei as biomarkers of DNA damage, aneuploidy, inducers of chromosomal hypermutation and as sources of pro-inflammatory DNA in humans

Micronuclei (MNi) are among the most widely studied biomarkers of DNA damage and chromosomal instability in humans. They originate from chromosome fragments or intact chromosomes that are not included in daughter nuclei during mitosis. The main reasons for their formation are a lack of functional centromere in the chromosome fragments or whole chromosomes or defects in one or more of the proteins of the mitotic system that, consequently, fails to segregate chromosomes properly. Assays have been developed to measure MNi in peripheral blood lymphocytes, red blood cells as well as various types of epithelial cells such as buccal, nasal, urothelial and cervical cells. Some of the assays have been further developed into micronucleus (MN) cytome assays to include additional nuclear anomalies, cell death and nuclear division biomarkers. In addition, the use of molecular probes has been adopted widely for the purpose of understanding the mechanistic origin of MNi. MN assays in humans are used for the purpose of investigating the genotoxic effects of adverse environmental, life-style and occupational factors, genetic susceptibility to DNA damage, and for determining risk of accelerated aging and diseases affected by genomic instability such as developmental defects and cancer. The emerging new knowledge showing that chromosomes trapped in MNi can undergo a high rate of fragmentation and become massively re-arranged have highlighted the possibility that MN formation is not only a biomarker of induced DNA damage but also a mechanism that drives hypermutation. Furthermore, another line of recent research showed that DNA and chromatin leaking from disrupted MNi triggers the innate immune cGAS-STING mechanism that promotes inflammation which can cause a wide-range of age-related diseases if left unresolved. For these reasons, MN assays in humans have become an increasingly important biomarker of disease initiation and progression across all life-stages.

Single-Cell Analysis Reveals that Chronic Silver Nanoparticle Exposure Induces Cell Division Defects in Human Epithelial Cells

Multiple organizations have urged a paradigm shift from traditional, whole animal, chemical safety testing to alternative methods. Although these forward-looking methods exist for risk assessment and predication, animal testing is still the preferred method and will remain so until more robust cellular and computational methods are established. To meet this need, we aimed to develop a new, cell division-focused approach based on the idea that defective cell division may be a better predictor of risk than traditional measurements. To develop such an approach, we investigated the toxicity of silver nanoparticles (AgNPs) on human epithelial cells. AgNPs are the type of nanoparticle most widely employed in consumer and medical products, yet toxicity reports are still confounding. Cells were exposed to a range of AgNP doses for both short- and-long term exposure times. The analysis of treated cell populations identified an effect on cell division and the emergence of abnormal nuclear morphologies, including micronuclei and binucleated cells. Overall, our results indicate that AgNPs impair cell division, not only further confirming toxicity to human cells, but also highlighting the propagation of adverse phenotypes within the cell population. Furthermore, this work illustrates that cell division-based analysis will be an important addition to future toxicology studies.

Targets and mechanisms of chemically induced aneuploidy. Part 1 of the report of the 2017 IWGT workgroup on assessing the risk of aneugens for carcinogenesis and hereditary diseases

An aneuploidy workgroup was established as part of the 7th International Workshops on Genotoxicity Testing. The workgroup conducted a review of the scientific literature on the biological mechanisms of aneuploidy in mammalian cells and methods used to detect chemical aneugens. In addition, the current regulatory framework was discussed, with the objective to arrive at consensus statements on the ramifications of exposure to chemical aneugens for human health risk assessment. As part of these efforts, the workgroup explored the use of adverse outcome pathways (AOPs) to document mechanisms of chemically induced aneuploidy in mammalian somatic cells. The group worked on two molecular initiating events (MIEs), tubulin binding and binding to the catalytic domain of aurora kinase B, which result in several adverse outcomes, including aneuploidy. The workgroup agreed that the AOP framework provides a useful approach to link evidence for MIEs with aneuploidy on a cellular level. The evidence linking chemically induced aneuploidy with carcinogenicity and hereditary disease was also reviewed and is presented in two companion papers. In addition, the group came to the consensus that the current regulatory test batteries, while not ideal, are sufficient for the identification of aneugens and human risk assessment. While it is obvious that there are many different MIEs that could lead to the induction of aneuploidy, the most commonly observed mechanisms involving chemical aneugens are related to tubulin binding and, to a lesser extent, inhibition of mitotic kinases. The comprehensive review presented here should help with the identification and risk management of aneugenic agents.

Genomic instability in a chronic lymphocytic leukemia patient with mono-allelic deletion of the DLEU and RB1 genes

Background

The most frequent cytogenetic abnormality detected in chronic lymphocytic leukemia (CLL) patients is the presence of a deletion within the chromosome band 13q14. Deletions can be heterogeneous in size, generally encompassing the DLEU1 and DLEU2 genes (minimal deleted region), but at times also including the RB1 gene. The latter, larger type of deletions are associated with worse prognosis.

Genomic instability is a characteristic of most cancers and it has been observed in CLL patients mainly associated with telomere shortening.

Case presentation

Cytogenetic and fluorescence in situ hybridization studies of a CLL patient showed a chromosomal translocation t(12;13)(q15;q14), a mono-allelic 13q14 deletion encompassing both the DLEU and RB1 genes, and genomic instability manifested as chromosomal breaks, telomeric associations, binucleated cells, nucleoplasmic bridges, and micronucleated cells.

In conclusion, our CLL patient showed genomic instability in conjunction with a 13q14 deletion of approximately 2.6 megabase pair involving the DLEU and RB1 genes, as well as other genes with potential for producing genomic instability due to haploinsufficiency.

Chromosome Missegregation and Aneuploidy Induction in Human Peripheral Blood Lymphocytes In vitro by Low Concentrations of Chlorpyrifos, Imidacloprid and α-Cypermethrin

Chlorpyrifos, imidacloprid, and α-cypermethrin are some of the most widely used insecticides in contemporary agriculture. However, their low-dose, nontarget genotoxic effects have not been extensively assayed. As one of the most relevant cancer biomarkers, we aimed to assess the aneuploidy due to chromosome missegregation during mitosis. To aim it we treated human lymphocytes in vitro with three concentrations of insecticides equivalents relevant for real scenario exposure assessed by regulatory agencies. We focused on chlorpyrifos as conventional and imidacloprid and α-cypermethrin as sustainable use insecticides. Cytokinesis-blocked micronucleus assay was performed coupled with fluorescence in situ hybridization (FISH) with directly labeled pancentromeric probes for chromosomes 9, 18, X and Y. None of the insecticides induced significant secondary DNA damage in terms of micronuclei (MN), nuclear buds (NB), or nucleoplasmic bridges (NPB). However, significant disbalances in chromosomes 9, 18, X and Y, and in insecticide-treated cells has been observed. According to recent studies, these disbalances in chromosome numbers may be atributted to defect sister chromatid cohesion which contribute to the increase of chromosome missegregation but not to micronuclei incidence. We conclude that tested insecticidal active substances exert chromosome missegregation effects at low concentrations, possibly by mechanism of sister chromatid cohesion. These findings may contribute to future risk assesments and understanding of insecticide mode of action on human genome. Environ. Mol. Mutagen. 60:72-84, 2019. © 2018 Wiley Periodicals, Inc.

Risks of aneuploidy induction from chemical exposure: Twenty years of collaborative research in Europe from basic science to regulatory implications

Although Theodor Boveri linked abnormal chromosome numbers and disease more than a century ago, an in-depth understanding of the impact of mitotic and meiotic chromosome segregation errors on cell proliferation and diseases is still lacking. This review reflects on the efforts and results of a large European research network that, from the 1980's until 2004, focused on protection against aneuploidy-inducing factors and tackled the following problems: 1) the origin and consequences of chromosome imbalance in somatic and germ cells; 2) aneuploidy as a result of environmental factors; 3) dose-effect relationships; 4) the need for validated assays to identify aneugenic factors and classify them according to their modes of action; 5) the need for reliable, quantitative data suitable for regulating exposure and preventing aneuploidy induction; 6) the need for mechanistic insight into the consequences of aneuploidy for human health. This activity brought together a consortium of experts from basic science and applied genetic toxicology to prepare the basis for defining guidelines and to encourage regulatory activities for the prevention of induced aneuploidy. Major strengths of the EU research programmes on aneuploidy were having a valuable scientific approach based on well-selected compounds and accurate methods that allow the determination of precise dose-effect relationships, reproducibility and inter-laboratory comparisons. The work was conducted by experienced scientists stimulated by a fascination with the complex scientific issues surrounding aneuploidy; a key strength was asking the right questions at the right time. The strength of the data permitted evaluation at the regulatory level. Finally, the entire enterprise benefited from a solid partnership under the lead of an inspired and stimulating coordinator. The research programme elucidated the major modes of action of aneugens, developed scientifically sound assays to assess aneugens in different tissues, and achieved the international validation of relevant assays with the goal of protecting human populations from aneugenic chemicals. The role of aneuploidy in tumorigenesis will require additional research, and the study of effects of exposure to multiple agents should become a priority. It is hoped that these reflections will stimulate the implementation of aneuploidy testing in national and OECD guidelines.

Complexity of the 5' Untranslated Region of EIF4A3, a Critical Factor for Craniofacial and Neural Development

Repeats in coding and non-coding regions have increasingly been associated with many human genetic disorders, such as Richieri-Costa-Pereira syndrome (RCPS). RCPS, mostly characterized by midline cleft mandible, Robin sequence and limb defects, is an autosomal-recessive acrofacial dysostosis mainly reported in Brazilian patients. This disorder is caused by decreased levels of EIF4A3, mostly due to an increased number of repeats at the EIF4A3 5′UTR. EIF4A3 5′UTR alleles are CG-rich and vary in size and organization of three types of motifs. An exclusive allelic pattern was identified among affected individuals, in which the CGCA-motif is the most prevalent, herein referred as “disease-associated CGCA-20nt motif.” The origin of the pathogenic alleles containing the disease-associated motif, as well as the functional effects of the 5′UTR motifs on EIF4A3 expression, to date, are entirely unknown. Here, we characterized 43 different EIF4A3 5′UTR alleles in a cohort of 380 unaffected individuals. We identified eight heterozygous unaffected individuals harboring the disease-associated CGCA-20nt motif and our haplotype analyses indicate that there are more than one haplotype associated with RCPS. The combined analysis of number, motif organization and haplotypic diversity, as well as the observation of two apparently distinct haplotypes associated with the disease-associated CGCA-20nt motif, suggest that the RCPS alleles might have arisen from independent unequal crossing-over events between ancient alleles at least twice. Moreover, we have shown that the number and sequence of motifs in the 5′UTR region is associated with EIF4A3 repression, which is not mediated by CpG methylation. In conclusion, this study has shown that the large number of repeats in EIF4A3 does not represent a dynamic mutation and RCPS can arise in any population harboring alleles with the CGCA-20nt motif. We also provided further evidence that EIF4A3 5′UTR is a regulatory region and the size and sequence type of the repeats at 5′UTR may contribute to clinical variability in RCPS.

Fluid shear stress impacts ovarian cancer cell viability, subcellular organization, and promotes genomic instability

Ovarian cancer cells are exposed to physical stress in the peritoneal cavity during both tumor growth and dissemination. Ascites build-up in metastatic ovarian cancer further increases the exposure to fluid shear stress. Here, we used a murine, in vitro ovarian cancer progression model in parallel with immortalized human cells to investigate how ovarian cancer cells of increasing aggressiveness respond to <1dynecm2 of fluid-induced shear stress. This biophysical stimulus significantly reduced cell viability in all cells exposed, independent of disease stage. Fluid shear stress induced spheroid formation and altered cytoskeleton organization in more tumorigenic cell lines. While benign ovarian cells appeared to survive in higher numbers under the influence of fluid shear stress, they exhibited severe morphological changes and chromosomal instability. These results suggest that exposure of benign cells to low magnitude fluid shear stress can induce phenotypic changes that are associated with transformation and ovarian cancer progression. Moreover, exposure of tumorigenic cells to fluid shear stress enhanced anchorage-independent survival, suggesting a role in promoting invasion and metastasis.

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.

M2 macrophages are more resistant than M1 macrophages following radiation therapy in the context of glioblastoma

In some highly inflammatory tumors, such as glioblastoma (GB), macrophages (MΦ) represent the most abundant population of reactive cells. MΦ, initially denoted as M0 MΦ, can be polarized into two further phenotypes: the antitumor M1 MΦ, and the protumor M2 MΦ. The three phenotypes can reside simultaneously in the tumor mass and various external factors may influence MΦ polarization. Radiotherapy is a common modality of cancer treatment aiming to target tumor cells. However, the specific effects of X-ray radiation on the inflammatory cells are, so far, controversial and not fully understood. In the present investigation, we have first analyzed, in vivo, the effect of X-ray radiation on MΦ present in GB tumors. We have observed a decrease in MΦ number paralleled by an increase in the proportion of M2 MΦ. To understand this phenomenon, we then evaluated, in vitro, the effects of X-rays on the MΦ phenotypes and survival. We have found that X-ray radiation failed to modify the phenotype of the different MΦ. However, M1 MΦ were more sensitive to ionizing radiation than M2 MΦ, both in normoxia and in hypoxia, which could explain the in vivo observations. To conclude, M2 MΦ are more radioresistant than M0 and M1 MΦ and the present study allows us to propose that X-ray radiotherapy could contribute, along with other phenomena, to the increased density in the protumor M2 MΦ in GB.

Atrazine exposure elicits copy number alterations in the zebrafish genome

Atrazine is an agricultural herbicide used throughout the Midwestern United States that frequently contaminates potable water supplies resulting in human exposure. Using the zebrafish model system, an embryonic atrazine exposure was previously reported to decrease spawning rates with an increase in progesterone and ovarian follicular atresia in adult females. In addition, alterations in genes associated with distinct molecular pathways of the endocrine system were observed in brain and gonad tissue of the adult females and males. Current hypotheses for mechanistic changes in the developmental origins of health and disease include genetic (e.g., copy number alterations) or epigenetic (e.g., DNA methylation) mechanisms. As such, in the current study we investigated whether an atrazine exposure would generate copy number alterations (CNAs) in the zebrafish genome. A zebrafish fibroblast cell line was used to limit detection to CNAs caused by the chemical exposure. First, cells were exposed to a range of atrazine concentrations and a crystal violet assay was completed, showing confluency decreased by ~60% at 46.3μM. Cells were then exposed to 0, 0.463, 4.63, or 46.3μM atrazine and array comparative genomic hybridization completed. Results showed 34, 21, and 44 CNAs in the 0.463, 4.63, and 46.3μM treatments, respectively. Furthermore, CNAs were associated with previously reported gene expression alterations in adult male and female zebrafish. This study demonstrates that atrazine exposure can generate CNAs that are linked to gene expression alterations observed in adult zebrafish exposed to atrazine during embryogenesis providing a mechanism of the developmental origins of atrazine endocrine disruption.

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.

Growth, progression and chromosome instability of Neuroblastoma: a new scenario of tumorigenesis?

BACKGROUND

Neuroblastoma is a pediatric cancer with a low survival rate of patients with metastatic stage 4 disease. Tumor aggressiveness and progression have been associated with structural copy number variations (CNVs) that are observed in malignant cells. In contrast, localized Neuroblastomas, which are associated with a low number of structural CNVs but frequent numerical CNVs, are less aggressive, and patients have good outcomes. Finally, whole-genome and whole-exome sequencing of Neuroblastoma tissues have shown few damaging mutations in these tumors.

CONCLUSIONS

In the present report it is proposed that chromosome instability (CIN) plays a major role in Neuroblastoma tumorigenesis and that CIN is already present in the early phases of tumor development. High CIN can promote several types of chromosomal damage including chromothripsis, gene deletion, amplification and rearrangements, which deregulate gene expression. Indeed, gene rearrangements have been reported as a new scenario in the development of Neuroblastoma, which supports the hypothesis that CIN is an early step preliminary to the late catastrophic events leading to tumor development.

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.

Whole Chromosome Instability induces senescence and promotes SASP

Age-related accumulation of ploidy changes is associated with decreased expression of genes controlling chromosome segregation and cohesin functions. To determine the consequences of whole chromosome instability (W-CIN) we down-regulated the spindle assembly checkpoint component BUB1 and the mitotic cohesin SMC1A, and used four-color-interphase-FISH coupled with BrdU incorporation and analyses of senescence features to reveal the fate of W-CIN cells. We observed significant correlations between levels of not-diploid cells and senescence-associated features (SAFs). W-CIN induced DNA double strand breaks and elevated oxidative stress, but caused low apoptosis. SAFs of W-CIN cells were remarkably similar to those induced by replicative senescence but occurred in only 13 days versus 4 months. Cultures enriched with not-diploid cells acquired a senescence-associated secretory phenotype (SASP) characterized by IL1B, CXCL8, CCL2, TNF, CCL27 and other pro-inflammatory factors including a novel SASP component CLEC11A. These findings suggest that W-CIN triggers premature senescence, presumably to prevent the propagation of cells with an abnormal DNA content. Cells deviating from diploidy have the ability to communicate with their microenvironment by secretion of an array of signaling factors. Our results suggest that aneuploid cells that accumulate during aging in some mammalian tissues potentially contribute to age-related pathologies and inflammation through SASP secretion.

Contrasting mechanisms of de novo copy number mutagenesis suggest the existence of different classes of environmental copy number mutagens

While gene copy number variations (CNVs) are abundant in the human genome, and often are associated with disease consequences, the mutagenic pathways and environmental exposures that cause these large structural mutations are understudied relative to conventional nucleotide substitutions in DNA. The members of the environmental mutagenesis community are currently seeking to remedy this deficiency, and there is a renewed interest in the development of mutagenicity assays to identify and characterize compounds that may induce de novo CNVs in humans. To achieve this goal, it is critically important to acknowledge that CNVs exist in two very distinct classes: nonrecurrent and recurrent CNVs. The goal of this commentary is to emphasize the deep contrasts that exist between the proposed pathways that lead to these two mutation classes. Nonrecurrent de novo CNVs originate primarily in mitotic cells through replication-dependent DNA repair pathways that involve microhomologies (<10 bp), and are detected at higher frequency in children of older fathers. In contrast, recurrent de novo CNVs are most often formed in meiotic cells through homologous recombination between nonallelic large low-copy repeats (>10,000 bp), without an associated paternal age effect. Given the biological differences between the two CNV classes, it is our belief that nonrecurrent and recurrent CN mutagens will probably differ substantially in their modes of action. Therefore, each CNV class may require their own uniquely designed assays, so that we as a field may succeed in uncovering the broadest possible spectrum of environmental CN mutagens.