Chromosome missegregation causes colon cancer by APC loss of heterozygosity

HPV16 E6 induces chromosomal instability due to polar chromosomes caused by E6AP-dependent degradation of the mitotic kinesin CENP-E

Chromosome segregation during mitosis is highly regulated to ensure production of genetically identical progeny. Recurrent mitotic errors cause chromosomal instability (CIN), a hallmark of tumors. The E6 and E7 oncoproteins of high-risk human papillomavirus (HPV), which causes cervical, anal, and head and neck cancers (HNC), cause mitotic defects consistent with CIN in models of anogenital cancers, but this has not been studied in the context of HNC. Here, we show that HPV16 induces a specific type of CIN in patient HNC tumors, patient-derived xenografts, and cell lines, which is due to defects in chromosome congression. These defects are specifically induced by the HPV16 oncogene E6 rather than E7. We show that HPV16 E6 expression causes degradation of the mitotic kinesin CENP-E, whose depletion produces chromosomes that are chronically misaligned near spindle poles (polar chromosomes) and fail to congress. Though the canonical oncogenic role of E6 is the degradation of the tumor suppressor p53, CENP-E degradation and polar chromosomes occur independently of p53. Instead, E6 directs CENP-E degradation in a proteasome-dependent manner via the E6-associated ubiquitin protein ligase E6AP/UBE3A. This study reveals a mechanism by which HPV induces CIN, which may impact HPV-mediated tumor initiation, progression, and therapeutic response.

Reducing the aneuploid cell burden - cell competition and the ribosome connection

Aneuploidy, the gain or loss of chromosomes, is the cause of birth defects and miscarriage and is almost ubiquitous in cancer cells. Mosaic aneuploidy causes cancer predisposition, as well as age-related disorders. Despite the cell-intrinsic mechanisms that prevent aneuploidy, sporadic aneuploid cells do arise in otherwise normal tissues. These aneuploid cells can differ from normal cells in the copy number of specific dose-sensitive genes, and may also experience proteotoxic stress associated with mismatched expression levels of many proteins. These differences may mark aneuploid cells for recognition and elimination. The ribosomal protein gene dose in aneuploid cells could be important because, in Drosophila, haploinsufficiency for these genes leads to elimination by the process of cell competition. Constitutive haploinsufficiency for human ribosomal protein genes causes Diamond Blackfan anemia, but it is not yet known whether ribosomal protein gene dose contributes to aneuploid cell elimination in mammals. In this Review, we discuss whether cell competition on the basis of ribosomal protein gene dose is a tumor suppressor mechanism, reducing the accumulation of aneuploid cells. We also discuss how this might relate to the tumor suppressor function of p53 and the p53-mediated elimination of aneuploid cells from murine embryos, and how cell competition defects could contribute to the cancer predisposition of Diamond Blackfan anemia.

Identification and validation of a hub gene prognostic index for hepatocellular carcinoma

Aims: We aim to provide new insights into the mechanisms of hepatocellular carcinoma (HCC) and identify key genes as biomarkers for the prognosis of HCC. Materials & methods: Differentially expressed genes between HCC tissues and normal tissues were identified via the Gene Expression Omnibus tool. The top ten hub genes screened by the degree of the protein nodes in the protein-protein interaction network also showed significant associations with overall survival in HCC patients. Results: A prognostic model containing a five-gene signature was constructed to predict the prognosis of HCC via multivariate Cox regression analysis. Conclusion: This study identified a novel five-gene signature (CDK1, CCNB1, CCNB2, BUB1 and KIF11) as a significant independent prognostic factor.

Differences in telomerase activity and the effects of AZT in aneuploid and euploid cells in colon cancer

Telomerase-targeted treatments for cancer have received a great deal of attention because telomerase is detected in nearly all cancer cells but is not expressed in most normal tissues. Aneuploidy refers to a chromosome number that is not a multiple of the base chromosome number and can indicate either hypo- or hyperploid chromosome numbers. Most solid tumors are aneuploid. In the present study, we sought to determine whether there are differences in telomerase activity and hTERT gene expression between aneuploid and euploid cells. Furthermore, we investigated telomerase inhibitor 3'-azido-3'-deoxythymidine (AZT)-induced cell apoptosis using the p53-Puma/Noxa/Bax pathway and cell cycle arrest using the p53-p21 pathway in both aneuploid and euploid cells. Our results demonstrate that telomerase activity and hTERT gene expression were higher in aneuploid than in euploid cells. In addition, AZT exerted time- and dose-dependent cytotoxic effects on both aneuploid and euploid cells, and aneuploid cells were more sensitive to AZT-induced cytotoxicity. In addition, both the p53-Puma/Noxa/Bax pathway and the cell cycle arrest-associated p53-p21 pathway were involved in the AZT-induced suppression of tumor cells. Importantly, aneuploid cells were more sensitive to AZT-induced cell cycle arrest (p53-p21) and DNA double-strand breaks (γ-H2AX), while euploid cells were more sensitive to AZT-induced apoptosis (p53-Puma/Bax/Noxa).

Living in CIN: Mitotic Infidelity and Its Consequences for Tumor Promotion and Suppression

Errors in chromosome segregation during mitosis have been recognized as a hallmark of tumor cells since the late 1800s, resulting in the long-standing hypothesis that mitotic abnormalities drive tumorigenesis. Recent work has shown that mitotic defects can promote tumors, suppress them, or do neither, depending on the rate of chromosome missegregation. Here we discuss the causes of chromosome missegregation, their effects on tumor initiation and progression, and the evidence that increasing the rate of chromosome missegregation may be an effective chemotherapeutic strategy.

Creation of Mice Bearing a Partial Duplication of HPRT Gene Marked with a GFP Gene and Detection of Revertant Cells In Situ as GFP-Positive Somatic Cells

It is becoming clear that apparently normal somatic cells accumulate mutations. Such accumulations or propagations of mutant cells are thought to be related to certain diseases such as cancer. To better understand the nature of somatic mutations, we developed a mouse model that enables in vivo detection of rare genetically altered cells via GFP positive cells. The mouse model carries a partial duplication of 3’ portion of X-chromosomal HPRT gene and a GFP gene at the end of the last exon. In addition, although HPRT gene expression was thought ubiquitous, the expression level was found insufficient in vivo to make the revertant cells detectable by GFP positivity. To overcome the problem, we replaced the natural HPRT-gene promoter with a CAG promoter. In such animals, termed HPRT-dup-GFP mouse, losing one duplicated segment by crossover between the two sister chromatids or within a single molecule of DNA reactivates gene function, producing hybrid HPRT-GFP proteins which, in turn, cause the revertant cells to be detected as GFP-positive cells in various tissues. Frequencies of green mutant cells were measured using fixed and frozen sections (liver and pancreas), fixed whole mount (small intestine), or by means of flow cytometry (unfixed splenocytes). The results showed that the frequencies varied extensively among individuals as well as among tissues. X-ray exposure (3 Gy) increased the frequency moderately (~2 times) in the liver and small intestine. Further, in two animals out of 278 examined, some solid tissues showed too many GFP-positive cells to score (termed extreme jackpot mutation). Present results illustrated a complex nature of somatic mutations occurring in vivo. While the HPRT-dup-GFP mouse may have a potential for detecting tissue-specific environmental mutagens, large inter-individual variations of mutant cell frequency cause the results unstable and hence have to be reduced. This future challenge will likely involve lowering the background mutation frequency, thus reducing inter-individual variation.

Telomerase abrogates aneuploidy-induced telomere replication stress, senescence and cell depletion

The causal role of aneuploidy in cancer initiation remains under debate since mutations of euploidy-controlling genes reduce cell fitness but aneuploidy strongly associates with human cancers. Telomerase activation allows immortal growth by stabilizing telomere length, but its role in aneuploidy survival has not been characterized. Here, we analyze the response of primary human cells and murine hematopoietic stem cells (HSCs) to aneuploidy induction and the role of telomeres and the telomerase in this process. The study shows that aneuploidy induces replication stress at telomeres leading to telomeric DNA damage and p53 activation. This results in p53/Rb-dependent, premature senescence of human fibroblast, and in the depletion of hematopoietic cells in telomerase-deficient mice. Endogenous telomerase expression in HSCs and enforced expression of telomerase in human fibroblasts are sufficient to abrogate aneuploidy-induced replication stress at telomeres and the consequent induction of premature senescence and hematopoietic cell depletion. Together, these results identify telomerase as an aneuploidy survival factor in mammalian cells based on its capacity to alleviate telomere replication stress in response to aneuploidy induction.

Aneuploidy in health, disease, and aging

Aneuploidy, an aberrant number of chromosomes, has been recognized as a feature of human malignancies for over a century, but compelling evidence for causality was largely lacking until mouse models for chromosome number instability were used. These in vivo studies have not only uncovered important new insights into the extremely complex aneuploidy-cancer relationship but also into the molecular mechanisms underlying proper and aberrant chromosome segregation. A series of diverse mouse models for the mitotic checkpoint protein BubR1 has provided evidence for a provocative novel link between aneuploidization and the development of age-related pathologies.

Genomic instability and colon carcinogenesis: from the perspective of genes

Colon cancer is the second most lethal cancer; approximately 600,000 people die of it annually in the world. Colon carcinogenesis generally follows a slow and stepwise process of accumulation of mutations under the influence of environmental and epigenetic factors. To adopt a personalized (tailored) cancer therapy approach and to improve current strategies for prevention, diagnosis, prognosis, and therapy overall, advanced understanding of molecular events associated with colon carcinogenesis is necessary. A contemporary approach that combines genetics, epigenomics, and signaling pathways has revealed many genetic/genomic alterations associated with colon cancer progression and their relationships to a genomic instability phenotype prevalent in colon cancer. In this review, we describe the relationship between gene mutations associated with colon carcinogenesis and a genomic instability phenotype, and we discuss possible clinical applications of genomic instability studies. Colon carcinogenesis is associated with frequent mutations in several pathways that include phosphatidylinositol 3-kinase, adenomatous polyposis coli, p53 (TP53), F-box and WD repeat domain containing 7, transforming growth factor-β, chromosome cohesion, and K-RAS. These genes frequently mutated in pathways affecting colon cancer were designated colon cancer (CAN) genes. Aberrations in major colon CAN genes have a causal relationship to genomic instability. Conversely, genomic instability itself plays a role in colon carcinogenesis in experimental settings, as demonstrated in transgenic mouse models with high genomic instability. Thus, there is a feedback-type relationship between CAN gene mutations and genomic instability. These genetic/genomic studies have led to emerging efforts to apply the knowledge to colon cancer prognosis and to targeted therapy.

Mitotic Stress and Chromosomal Instability in Cancer: The Case for TPX2

Cell cycle deregulation is a common motif in human cancer, and multiple therapeutic strategies are aimed to prevent tumor cell proliferation. Whereas most current therapies are designed to arrest cell cycle progression either in G1/S or in mitosis, new proposals include targeting the intrinsic chromosomal instability (CIN, an increased rate of gain or losses of chromosomes during cell division) or aneuploidy (a genomic composition that differs from diploid) that many tumor cells display. Why tumors cells are chromosomally unstable or aneuploid and what are the consequences of these alterations are not completely clear at present. Several mitotic regulators are overexpressed as a consequence of oncogenic alterations, and they are likely to alter the proper regulation of chromosome segregation in cancer cells. In this review, we propose the relevance of TPX2, a mitotic regulator involved in the formation of the mitotic spindle, in oncogene-induced mitotic stress. This protein, as well as its partner Aurora-A, is frequently overexpressed in human cancer, and its deregulation may participate not only in chromosome numeric aberrations but also in other forms of genomic instability in cancer cells.

Linking abnormal mitosis to the acquisition of DNA damage

Cellular defects that impair the fidelity of mitosis promote chromosome missegregation and aneuploidy. Increasing evidence reveals that errors in mitosis can also promote the direct and indirect acquisition of DNA damage and chromosome breaks. Consequently, deregulated cell division can devastate the integrity of the normal genome and unleash a variety of oncogenic stimuli that may promote transformation. Recent work has shed light on the mechanisms that link abnormal mitosis with the development of DNA damage, how cells respond to such affronts, and the potential impact on tumorigenesis.

Immunosurveillance against tetraploidization-induced colon tumorigenesis

Circumstantial evidence suggests that colon carcinogenesis can ensue the transient tetraploidization of (pre-)malignant cells. In line with this notion, the tumor suppressors APC and TP53, both of which are frequently inactivated in colon cancer, inhibit tetraploidization in vitro and in vivo. Here, we show that-contrarily to their wild-type counterparts- Tp53 (-/-) colonocytes are susceptible to drug-induced or spontaneous tetraploidization in vitro. Colon organoids generated from tetraploid Tp53 (-/-) cells exhibit a close-to-normal morphology as compared to their diploid Tp53 (-/-) counterparts, yet the colonocytes constituting these organoids are characterized by an increased cell size and an elevated expression of the immunostimulatory protein calreticulin on the cell surface. The subcutaneous injection of tetraploid Tp53 (-/-) colon organoids led to the generation of proliferating tumors in immunodeficient, but not immunocompetent, mice. Thus, tetraploid Tp53 (-/-) colonocytes fail to survive in immunocompetent mice and develop neoplastic lesions in immunocompromised settings only. These results suggest that tetraploidy is particularly oncogenic in the context of deficient immunosurveillance.

Developmental pathways in colon cancer: crosstalk between WNT, BMP, Hedgehog and Notch

A hallmark of cancer is reactivation/alteration of pathways that control cellular differentiation during developmental processes. Evidence indicates that WNT, Notch, BMP and Hedgehog pathways have a role in normal epithelial cell differentiation, and that alterations in these pathways accompany establishment of the tumorigenic state. Interestingly, there is recent evidence that these pathways are intertwined at the molecular level, and these nodes of intersection may provide opportunities for effective targeted therapies. This review will highlight the role of the WNT, Notch, BMP and Hedgehog pathways in colon cancer.

Losing balance: the origin and impact of aneuploidy in cancer

Most solid human tumours are aneuploid, that is, they contain an abnormal number of chromosomes. Paradoxically, however, aneuploidy has been reported to induce a stress response that suppresses cellular proliferation in vitro. Here, we review the progress in our understanding of the causes and effects of aneuploidy in cancer and discuss how, in specific contexts, aneuploidy can provide a growth advantage and facilitate cellular transformation. We also explore the emerging possibilities for targeting the cause or consequences of aneuploidy therapeutically.

Haploinsufficiency of SGO1 results in deregulated centrosome dynamics, enhanced chromosomal instability and colon tumorigenesis

Chromosome instability (CIN) is found in 85% of colorectal cancers. Defects in mitotic processes are implicated in high CIN and may be critical events in colorectal tumorigenesis. Shugoshin-1 (SGO1) aids in the maintenance of chromosome cohesion and prevents premature chromosome separation and CIN. In addition, integrity of the centrosome may be compromised due to the deficiency of Cohesin and Sgo1 through the disengagement of centrioles. We report here the generation and characterization of SGO1-mutant mice and show that haploinsufficiency of SGO1 leads to enhanced colonic tumorigenesis. Complete disruption of SGO1 results in embryonic lethality, whereas SGO1+/- mice are viable and fertile. Haploinsufficiency of SGO1 results in genomic instability manifested as missegregation of chromosomes and formation of extra centrosomal foci in both murine embryonic fibroblasts and adult bone marrow cells. Enhanced CIN observed in SGO1-deficient mice resulted in an increase in formation of aberrant crypt foci (ACF) and accelerated development of tumors after exposure to azoxymethane (AOM), a colon carcinogen. Together, these results suggest that haploinsufficiency of SGO1 causes enhanced CIN, colonic preneoplastic lesions and tumorigenesis in mice. SGO1 is essential for the suppression of CIN and tumor formation.

Cooperative effects of Akt-1 and Raf-1 on the induction of cellular senescence in doxorubicin or tamoxifen treated breast cancer cells

Escape from cellular senescence induction is a potent mechanism for chemoresistance. Cellular senescence can be induced in breast cancer cell lines by the removal of estrogen signaling with tamoxifen or by the accumulation of DNA damage induced by the chemotherapeutic drug doxorubicin. Long term culturing of the hormone-sensitive breast cancer cell line MCF-7 in doxorubicin (MCF-7/DoxR) reduced the ability of doxorubicin, but not tamoxifen, to induce senescence. Two pathways that are often upregulated in chemo- and hormonal-resistance are the PI3K/PTEN/Akt/mTOR and Ras/Raf/MEK/ERK pathways. To determine if active Akt-1 and Raf-1 can influence drug-induced senescence, we stably introduced activated ΔAkt-1(CA) and ΔRaf-1(CA) into drug-sensitive and doxorubicin-resistant cells. Expression of a constitutively-active Raf-1 construct resulted in higher baseline senescence, indicating these cells possessed the ability to undergo oncogene-induced-senescence. Constitutive activation of the Akt pathway significantly decreased drug-induced senescence in response to doxorubicin but not tamoxifen in MCF-7 cells. However, constitutive Akt-1 activation in drug-resistant cells containing high levels of active ERK completely escaped cellular senescence induced by doxorubicin and tamoxifen. These results indicate that up regulation of the Ras/PI3K/PTEN/Akt/mTOR pathway in the presence of elevated Ras/Raf/MEK/ERK signaling together can contribute to drug-resistance by diminishing cell senescence in response to chemotherapy. Understanding how breast cancers containing certain oncogenic mutations escape cell senescence in response to chemotherapy and hormonal based therapies may provide insights into the design of more effective drug combinations for the treatment of breast cancer.

Molecular damage in cancer: an argument for mTOR-driven aging

Despite common belief, accumulation of molecular damage does not play a key role in aging. Still, cancer (an age-related disease) is initiated by molecular damage. Cancer and aging share a lot in common including the activation of the TOR pathway. But the role of molecular damage distinguishes cancer and aging. Furthermore, an analysis of the role of both damage and aging in cancer argues against "a decline, caused by accumulation of molecular damage" as a cause of aging. I also discuss how random molecular damage, via rounds of multiplication and selection, brings about non-random hallmarks of cancer.

Aurora B hyperactivation by Bub1 overexpression promotes chromosome missegregation

High expression of the mitotic kinase Bub1 is associated with a variety of human cancers and correlates with poor clinical prognosis, but whether Bub1 alone can drive tumorigenesis was unknown. We provided conclusive evidence that Bub1 has oncogenic properties by generating transgenic mice that overexpress Bub1 in a wide variety of tissues, resulting in aneuploidization. Consistently, Bub1 transgenic mice developed various kinds of spontaneous tumors as well as accelerated Myc-induced lymphomagenesis. While the mitotic checkpoint was robust in Bub1 overexpressing cells, misaligned and lagging chromosomes were observed. These defects originated from increased Aurora B activity and could be suppressed by inhibition of Aurora B. Taken together, this indicates that Bub1 has oncogenic properties and imply that aneuploidization and tumorigenesis result from Aurora B-dependent missegregation. Here, we focus on the complex relationship between Bub1 and Aurora B and discuss the broader implications of Bub1-dependent Aurora B activation in mediating error correction.

The aneuploidy paradox: costs and benefits of an incorrect karyotype

Aneuploidy has a paradoxical effect on cell proliferation. In all normal cells analyzed to date, aneuploidy has been found to decrease the rate of cell proliferation. Yet, aneuploidy is also a hallmark of cancer, a disease of enhanced proliferative capacity, and aneuploid cells are frequently recovered following the experimental evolution of microorganisms. Thus, in certain contexts, aneuploidy might also have growth-advantageous properties. New models of aneuploidy and chromosomal instability have shed light on the diverse effects that karyotypic imbalances have on cellular phenotypes, and suggest novel ways of understanding the role of aneuploidy in development and disease.

Chromium induces chromosomal instability, which is partly due to deregulation of BubR1 and Emi1, two APC/C inhibitors

Disruption of cell cycle checkpoints and interference with the normal cell cycle progression frequently result in cell death or malignant transformation. Hexavalent chromium [Cr(VI)] is a well-known carcinogen that has been implicated in the occurrence of many types of human malignancies, including lung cancer. However, the exact mechanism by which Cr(VI) causes malignant transformation in the lung remains unknown. We have demonstrated that chronic exposure to a non-cytotoxic concentration of Cr(VI) induced a variety of chromosomal abnormalities, including premature sister chromatid separation, chromosomal breakage and the presence of lagging/misaligned chromosomes. After treatment with nocodazole, both HeLa and normal lung bronchial epithelial cells were arrested at mitosis. However, Cr(VI) significantly compromised M-phase arrest induced by nocodazole. Cr(VI) suppressed BubR1 activation and reduced expression of Emi1, leading to an unscheduled activation of APC/C. Consistent with this observation, Cr(VI) treatment caused enhanced polyubiquitination of geminin during mitotic release, while it deregulated the activity of Cdt1, a DNA replication licensing factor. Combined, these results suggest that Cr(VI)-induced chromosomal instability is partly due to a perturbation of APC/C activities, leading to chromosomal instability.

Bub1 overexpression induces aneuploidy and tumor formation through Aurora B kinase hyperactivation

High expression of the protein kinase Bub1 has been observed in a variety of human tumors and often correlates with poor clinical prognosis, but its molecular and cellular consequences and role in tumorigenesis are unknown. Here, we demonstrate that overexpression of Bub1 in mice leads to near-diploid aneuploidies and tumor formation. We found that chromosome misalignment and lagging are the primary mitotic errors responsible for the observed aneuploidization. High Bub1 levels resulted in aberrant Bub1 kinase activity and hyperactivation of Aurora B kinase. When Aurora B activity is suppressed, pharmacologically or via BubR1 overexpression, chromosome segregation errors caused by Bub1 overexpression are largely corrected. Importantly, Bub1 transgenic mice overexpressing Bub1 developed various kinds of spontaneous tumors and showed accelerated Myc-induced lymphomagenesis. Our results establish that Bub1 has oncogenic properties and suggest that Aurora B is a critical target through which overexpressed Bub1 drives aneuploidization and tumorigenesis.

CINister thoughts

Chromosome instability (CIN) is the process that leads to aneuploidy, a known hallmark of human tumours for over a century. Nowadays, it is believed that CIN promotes tumorigenesis by shuffling the genome into a malignant order through translocations, amplifications, deletions (structural CIN), and gains and losses of whole chromosomes (numerical CIN or nCIN). The present review focuses on the causes and consequences of nCIN. Several roads can lead to nCIN, including a compromised spindle assembly checkpoint, cohesion defects, p53 deficiency and flawed microtubule-kinetochore attachments. Whereas the link between nCIN and tumorigenesis is becoming more evident, indications have emerged recently that nCIN can suppress tumour formation as well. To understand these paradoxical findings, novel reagents and more sophisticated mouse models are needed. This will provide us with a better understanding of nCIN and eventually with therapies that exploit this characteristic of human tumours.

Sporadic aneuploidy in PHA-stimulated lymphocytes of trisomies 21, 18, and 13

Following the observation detected in a previous study that X chromosome monosomy in Turner's syndrome genotypes was associated with a sporadic loss and/or gain of other chromosomes, we studied here whether this instability is a consistent finding in constitutional autosomal trisomies. We used PHA-stimulated lymphocytes derived from 14 patients (10 patients with trisomy 21, 2 with trisomy 18, and 2 with trisomy 13). Fourteen healthy controls were compared. Fluorescence in situ hybridization, applied at interphase cells, was used to evaluate the level of aneuploidy for 3 randomly selected chromosomes (autosomes 8, 15, and 16) in each sample. For each tested chromosome, our results showed a significantly higher level of aneuploid cells in the samples from the patients than in those from controls, with no difference between the patient groups. The mean level of aneuploid cells (percentage) for all 3 tested autosomes was almost twice as high in the patient samples as in the control samples. The aneuploidy level was mainly due to monosomy, which was significantly higher in the samples from the patients than in those from controls for each one of the tested chromosomes, with no difference between the patient groups. The mean level of monosomic cells (percentage) for all 3 tested chromosomes was almost twice as high in the patient samples as in the control samples. Our study shows that various constitutional autosomal trisomies are associated with an increased frequency of non-chromosome specific aneuploidy and is a continuation of the previous study documenting sporadic aneuploidy in Turner's sample cells. It is possible that primary aneuploid cells destabilize their own genome resulting in variable aneuploidy of other chromosomes. It is also possible that one or several common factor(s) is/are involved in both constitutional and sporadic aneuploidy.