[Show abstract][Hide abstract] ABSTRACT: Breast cancer is the most common malignant disease in women, but some basic questions remain in breast cancer biology. To answer these, several cell models were developed. Recently, the use of improved cell-culture conditions has enabled the development of a new primary cell model with certain luminal characteristics. This model is relevant because, after the introduction of a specific set of genetic elements, the transformed cells yielded tumors resembling human adenocarcinomas in mice. The use of improved cell-culture conditions supporting the growth of these breast primary epithelial cells was expected to delay or eliminate stress-induced senescence and lead to the propagation of normal cells. However, no studies have been carried out to investigate these points. Propagation of breast primary epithelial cells was performed in WIT medium on Primaria plates. Immunofluorescence, western blot and qRT-PCR were used to detect molecular markers, and to determine the integrity of DNA damage-response pathways. Promoter methylation of p16 INK4a was assessed by pyrosequencing. In order to obtain a dynamic picture of chromosome instability over time in culture, we applied FISH methodologies. To better link chromosome instability with excessive telomere attrition, we introduced the telomerase reverse transcriptase human gene using a lentiviral vector. We report here that breast primary epithelial cells propagated in vitro with WIT medium on Primaria plates express some luminal characteristics, but not a complete luminal lineage phenotype. They undergo a p16-dependent stress-induced senescence (stasis), and the cells that escape stasis finally enter a crisis state with rampant chromosome instability. Chromosome instability in these cells is driven by excessive telomere attrition, as distributions of chromosomes involved in aberrations correlate with the profiles of telomere signal-free ends. Importantly, ectopic expression of the human TERT gene rescued their chromosomal instability phenotype. Essentially, our data show that contrary to what was previously suggested, improved culture conditions to propagate in vitro mammary epithelial cells with some luminal characteristics do not prevent stress-induced senescence. This barrier is overcome by spontaneous methylation of the p16 INK4a promoter, allowing the proliferation of cells with telomere dysfunction and ensuing chromosome instability.
Full-text · Article · Dec 2016 · Breast cancer research: BCR
[Show abstract][Hide abstract] ABSTRACT: Accurate chromosome segregation during cell division is essential to maintain genome stability, and chromosome segregation errors are causally linked to genetic disorders and cancer. An anaphase chromosome bridge is a particular chromosome segregation error observed in cells that enter mitosis with fused chromosomes/sister chromatids. The widely accepted Breakage/Fusion/Bridge cycle model proposes that anaphase chromosome bridges break during mitosis to generate chromosome ends that will fuse during the following cell cycle, thus forming new bridges that will break, and so on. However, various studies have also shown a link between chromosome bridges and aneuploidy and/or polyploidy. In this study, we investigated the behavior and properties of chromosome bridges during mitosis, with the idea to gain insight into the potential mechanism underlying chromosome bridge-induced aneuploidy. We find that only a small number of chromosome bridges break during anaphase, whereas the rest persist through mitosis into the subsequent cell cycle. We also find that the microtubule bundles (k-fibers) bound to bridge kinetochores are not prone to breakage/detachment, thus supporting the conclusion that k-fiber detachment is not the cause of chromosome bridge-induced aneuploidy. Instead, our data suggest that while the microtubules bound to the kinetochores of normally segregating chromosomes shorten substantially during anaphase, the k-fibers bound to bridge kinetochores shorten only slightly, and may even lengthen, during anaphase. This causes some of the bridge kinetochores/chromosomes to lag behind in a position that is proximal to the cell/spindle equator and may cause the bridged chromosomes to be segregated into the same daughter nucleus or to form a micronucleus.
[Show abstract][Hide abstract] ABSTRACT: In order to examine the relationship between accumulation of residual DNA double-strand breaks (DSBs) and cell death, we have used a control and an ATM (Ataxia-Telangiectasia Mutated) defective cell line, as Ataxia-Telangiectasia (AT) cells tend to accumulate residual DSBs at long times after damage infliction. After irradiation, AT cells showed checkpoint impairment and a fraction of cells displayed an abnormal centrosome number and tetraploid DNA content, and this fraction increased along with apoptosis rates. At all times analyzed, AT cells displayed a significantly higher rate of radiation-induced apoptosis than normal cells. Besides apoptosis, 70–85% of the AT viable cells (TUNEL-negative) carried ≥10
H2AX foci/cell, while only 12–27% of normal cells did. The fraction of AT and normal cells undergoing early and late apoptosis were isolated by flow cytometry and residual DSBs were concretely scored in these populations. Half of the
H2AX-positive AT cells undergoing early apoptosis carried ≥10
H2AX foci/cell and this fraction increased to 75% in late apoptosis. The results suggest that retention of DNA damage-induced
H2AX foci is an indicative of lethal DNA damage, as cells undergoing apoptosis are those accumulating more DSBs. Scoring of residual
H2AX foci might function as a predictive tool to assess radiation-induced apoptosis.
[Show abstract][Hide abstract] ABSTRACT: Virtually all human cancers display chromosome instability (CIN), a condition in which chromosomes are gained or lost at a high rate. CIN occurs early in cancer development where it may undermine the advance of the neoplastic disease. With the aim of establishing the mechanisms underlying CIN in cancer, we investigated possible links between telomere-dysfunction and centrosome defects, which were seen to coincide in early in breast carcinogenesis using human mammary epithelial cells (HMECs). In this study, we show that TP53 proficient vHMECs cells develop centrosome aberrations when telomere-dysfunction genotoxic stress is produced in the presence of a defective p16INK4a setting and in parallel with an activation of the DNA damage checkpoint response. These aberrations consist of the accumulation of centrosomes in polyploid vHMECs, plus centriole overduplication in both diploid and polyploid cells, thus reflecting that distinct mechanisms underlie the generation of centrosome aberrations in vHMECs. Transduction of vHMEC with hTERT, which rescued the telomere dysfunction phenotype and consequently reduced DNA damage checkpoint activation, led to a progressive reduction of centrosome aberrations with cell culture, both in diploid and in polyploid vHMECs. Radiation-induced DNA damage also raised centrosome aberrations in vHMEC-hTERT. Collectively, our results, using vHMECs define a model where p16INK4a deficiency along with short dysfunctional telomeres cooperatively engenders centrosome abnormalities before p53 function is compromised.
[Show abstract][Hide abstract] ABSTRACT: Aging involves a deterioration of cell functions and changes that may predispose the cell to undergo an oncogenic transformation. The carcinogenic risks following radiation exposure rise with age among adults. Increasing inflammatory response, loss of oxidant/antioxidant equilibrium, ongoing telomere attrition, decline in the DNA damage response efficiency, and deleterious nuclear organization are age-related cellular changes that trigger a serious threat to genomic integrity. In this review, we discuss the mechanistic interplay between all these factors, providing an integrated view of how they contribute to the observed age-related increase in radiation sensitivity. As life expectancy increases and so it does the medical intervention, it is important to highlight the benefits of radiation protection in the elderly. Thus, a deep understanding of the mechanistic processes confining the threat of aging-related radiosensitivity is currently of foremost relevance.
[Show abstract][Hide abstract] ABSTRACT: The presence of γH2AX foci on apparently intact mitotic chromosomes is controversial because they challenge the assumed relationship between γH2AX foci and DNA double-strand breaks (DSBs). In this work, we show that after irradiation during interphase, a variety of γH2AX foci are scored in mitotic cells. Surprisingly, approximately 80% of the γH2AX foci spread over apparently undamaged chromatin at Terminal or Interstitial positions and they can display variable sizes, thus being classified as Small, Medium and Big foci. Chromosome and chromatid breaks that reach mitosis are spotted with Big (60%) and Medium (30%) Terminal γH2AX foci, but very rarely are they signaled with Small γH2AX foci. To evaluate if Interstitial γH2AX foci might be signatures of misrejoining, an mFISH analysis was performed on the same slides. The results show that Interstitial γH2AX foci lying on apparently intact chromatin do not mark sites of misrejoining, and that misrejoined events were never signaled by a γH2AX foci during mitosis. Finally, when analyzing the presence of other DNA-damage response (DDR) factors we found that all γH2AX foci—regardless their coincidence with a visible break—always colocalized with MRE11, but not with 53BP1. This pattern suggests that these γH2AX foci may be hallmarks of both microscopically visible and invisible DNA damage, in which an active, although incomplete or halted DDR is taking place.
[Show abstract][Hide abstract] ABSTRACT: Most cancer genomes show abnormalities in chromosome structure and number, two types of aberrations that could share a common mechanistic origin through proliferation-dependent loss of telomere function. Impairment of checkpoints that limit cell proliferation when telomeres are critically short might allow unrestrained cell division. The resulting uncapped chromosomes can fuse to each other, forming unstable configurations that can bridge during mitosis. Chromatin bridges can break to generate new broken ends that will then fuse with other broken ends. Successive events of break and fusion will continuously generate unbalanced chromosomal rearrangements, leading to gene-copy gains and losses. However, chromosome bridges do not always break. Evidence has recently been obtained to suggest that telomere-dependent chromosome bridges remaining unbroken can hinder cytokinesis and yield tetraploid cells. This might constitute an unstable intermediate in tumorigenesis, as progressive losses of individual chromosomes due to geometrical defects during cell division result in subtetraploid karyotypes. Additionally, the presence of short dysfunctional telomeres in cells can also cause these cells to become sensitive to mutagens, and particularly to radiation exposure. Human individuals exhibit differences in their sensitivity to radiation, which can be relevant for choice of therapy. Telomere function may well be involved in cellular and organism responses to ionizing radiation. Since eroded telomeres are sensed and act as double-strand breaks, they can interact with radiation-induced breaks, sharply increasing the possibility of misjoining. Altogether, this scenario provides certain clues to understanding the important role of telomeres in maintaining genomic integrity.
No preview · Article · Jun 2014 · Political Communication
[Show abstract][Hide abstract] ABSTRACT: Phosphorylation of the H2AX protein is an early step in the double strand break (DSB) repair pathway; therefore, phosphorylated histone (γH2AX) foci scoring is widely used as a measure for DSBs. Foci scoring is performed either manually or semi-automatically using hand-operated capturing and image analysis software. In general, both techniques are laborious and prone to artifacts associated with manual scoring. While a few fully automated methods have been described in the literature, none of them have been used to quantify γH2AX foci in combination with a cell cycle phase analysis. Adding this feature to a rapid automated γH2AX foci quantification method would reduce the scoring uncertainty that arises from the variations in the background level of the γH2AX signal throughout the cell cycle. The method was set up to measure DNA damage induced in human mammary epithelial cells by irradiation under a mammogram device. We adapted a FISH (fluorescent in situ hybridization) Spot-counting system, which has a slide loader with automatic scanning and cell capture system throughout the thickness of each cell (z-stack), to meet our assay requirements. While scanning the sample, the system classifies the selected nuclei according to the signal patterns previously described by the user. For our purposes, a double staining immunofluorescence was carried out with antibodies to detect γH2AX and pericentrin, an integral component of the centrosome. We could thus distinguish both the number of γH2AX foci per cell and the cell cycle phase. Furthermore, restrictive settings of the program classifier reduced the "touching nuclei" problem described in other image analysis software. The automated scoring was faster than and as sensitive as its manually performed counterpart. This system is a reliable tool for γH2AX radio-induced foci counting and provides essential information about the cell cycle stage. It thus offers a more complete and rapid assessment of DNA damage.
Full-text · Article · Aug 2013 · International Journal of Molecular Sciences
[Show abstract][Hide abstract] ABSTRACT: Concerned about the risks of mammography screening in the adult population, we analyzed the ability of human mammary epithelial cells to cope with mammogram-induced DNA damage. Our study shows that an X-ray dose of 20 mGy, which is the standard dose received by the breast surface per two-view mammogram X-ray exploration, induces increased frequencies of DNA double-strand breaks to in vitro aged-but not to young-human mammary epithelial cells. We provide evidence that aged epithelial breast cells are more radiosensitive than younger ones. Our studies point to an inefficient damage response of aged cells to low-dose radiation, this being due to both delayed and incomplete mobilization of repair proteins to DNA strand breaks. This inefficient damage response is translated into an important delay in double-strand break disappearance and consequent accumulation of unrepaired DNA breaks. The result of this is a significant increase in micronuclei frequency in the in vitro aged mammary epithelial cells exposed to doses equivalent to a single mammogram X-ray exploration. Since our experiments were carried out in primary epithelial cell cultures in which cells age at the same time as they undergo replication-dependent telomere shortening, we needed to determine the contribution of these two factors to their phenotype. In this paper, we report that the exogenous expression of human telomerase retrotranscriptase in late population doubling epithelial cells does not rescue its delayed repair phenotype. Therefore, retarded DNA break repair is a direct consequence of cellular aging itself, rather than a consequence of the presence of dysfunctional telomeres. Our findings of long-lasting double strand breaks and incomplete DNA break repair in the in vitro aged epithelial cells are in line with the increased carcinogenic risks of radiation exposures at older ages revealed by epidemiologic studies.
[Show abstract][Hide abstract] ABSTRACT: Organisms are continuously exposed to DNA damaging agents, consequently, cells have developed an intricate system known as the DNA damage response (DDR) in order to detect and repair DNA lesions. This response has to be rapid and accurate in order to keep genome integrity. It has been observed that the condensation state of chromatin hinders a proper DDR. However, the condensation state of chromatin is not the only barrier to DDR. In this review, we have collected data regarding the presence of DDR factors on micronuclear DNA lesions that indicate that micronuclei are almost incapable of generating an effective DDR because of defects in their nuclear envelope. Finally, considering the recent observations about the reincorporation of micronuclei to the main bulk of chromosomes, we suggest that, under certain circumstances, micronuclei carrying DNA damage might be a source of chromosome instability.
Full-text · Article · Dec 2012 · International Journal of Molecular Sciences
[Show abstract][Hide abstract] ABSTRACT: Most cancer cells accumulate genomic abnormalities at a remarkably rapid rate, as they are unable to maintain their chromosome structure and number. Excessively short telomeres, a known source of chromosome instability, are observed in early human-cancer lesions. Besides telomere dysfunction, it has been suggested that a transient phase of polyploidization, in most cases tetraploidization, has a causative role in cancer. Proliferation of tetraploids can gradually generate subtetraploid lineages of unstable cells that might fire the carcinogenic process by promoting further aneuploidy and genomic instability. Given the significance of telomere dysfunction and tetraploidy in the early stages of carcinogenesis, we investigated whether there is a connection between these two important promoters of chromosomal instability. We report that human mammary epithelial cells exhibiting progressive telomere dysfunction, in a pRb deficient and wild-type p53 background, fail to complete the cytoplasmatic cell division due to the persistence of chromatin bridges in the midzone. Flow cytometry together with fluorescence in situ hybridization demonstrated an accumulation of binucleated polyploid cells upon serial passaging cells. Restoration of telomere function through hTERT transduction, which lessens the formation of anaphase bridges by recapping the chromosome ends, rescued the polyploid phenotype. Live-cell imaging revealed that these polyploid cells emerged after abortive cytokinesis due to the persistence of anaphase bridges with large intervening chromatin in the cleavage plane. In agreement with a primary role of anaphase bridge intermediates in the polyploidization process, treatment of HMEC-hTERT cells with bleomycin, which produces chromatin bridges through illegimitate repair, resulted in tetraploid binucleated cells. Taken together, we demonstrate that human epithelial cells exhibiting physiological telomere dysfunction engender tetraploid cells through interference of anaphase bridges with the completion of cytokinesis. These observations shed light on the mechanisms operating during the initial stages of human carcinogenesis, as they provide a link between progressive telomere dysfunction and tetraploidy.
[Show abstract][Hide abstract] ABSTRACT: The interplay between ATM and DNA-PKcs kinases during double strand breaks (DSBs) resolution is still a matter of debate. ATM and DNA-PKcs participate differently in the DNA damage response pathway (DDR), but important common aspects are indeed found: both of them are activated when faced with DSBs, they share common targets in the DDR and the absence of either kinase results in faulty DSB repair. Absence of ATM translates into timely repair that, nevertheless, is incomplete. On the other hand, DNA-PKcs absence translates into slower repair, which in turn gives rise to the accumulation of simple and complex reorganizations. These outcomes confirm that the function of both protein kinases is essential to guarantee genome integrity. Interestingly, V(D)J and CSR recombination events provide a powerful tool to study the interplay between both kinases in DSB repair. Although the physiological DSBs generated during V(D)J and CSR recombination are resolved by the non-homologous end-joining (NHEJ) repair pathway, ATM absence during these events translates into chromosome translocations. These results suggest that NHEJ accuracy is threatened in the absence of ATM, which may play a role in avoiding illegitimate repair by favouring the joining of the correct DNA ends. Indeed, simultaneous DNA-PKcs and ATM deficiency during V(D)J and CSR recombination translates into a synergistic increase in potentially dangerous chromosomal translocations and deletions. Although the exact nature of their interaction remains elusive, the evidence indicates that ATM and DNA-PKcs play complementary roles that allow complete and legitimate DSB repair to be reached. Faithful repair can only be achieved by the presence and correct functioning of both kinases: while DNA-PKcs ensures fast rejoining, ATM guarantees complete repair.
Full-text · Article · Jan 2012 · Mutation Research/Fundamental and Molecular Mechanisms of Mutagenesis
[Show abstract][Hide abstract] ABSTRACT: The nucleoprotein complexes that cap the very ends of the eukaryotic chromosomes, named telomeres, are indispensable for cell viability. Telomeric DNA shortens in each cell division until it cannot exert end-protective functions in human somatic cells. Additionally, several proteins have been described to play a key role in telomere homeostasis preventing chromosome extremities to be recognized as double-stranded breaks (DSBs). When telomeres become dysfunctional, either through excessive shortening or due to defects in the proteins that form its structure, they trigger p53/pRb pathways what limits proliferative lifespan. Impairment of telomere function together with a compromised senescence/apoptosis response leads to chromosome instability. Fusions between dysfunctional telomeres or even between dysfunctional telomeres and DSBs can initiate breakage-fusion-bridge (BFB) cycles. Initially, telomere fusions were proposed to cause only structural abnormalities. Nevertheless, changes in chromosome number have also emerged as a possible consequence of alterations in end capping. Here we review the main aspects of telomeres and telomere-based chromosome instability, highlighting why they have been proposed as a driving force for tumourigenesis.
No preview · Article · Jan 2012 · Frontiers in Bioscience
[Show abstract][Hide abstract] ABSTRACT: Most solid tumors are unable to maintain the stability of their genomes at the chromosome level. Indeed, cancer cells display highly rearranged karyotypes containing translocations, amplifications, deletions, and gains and losses of whole chromosomes, which reshuffle steadily. This chromosomal instability most likely occurs early in the development of cancer, and may represent an important step in promoting the multiple genetic changes required for the initiation and/or progression of the disease. Different mechanisms may underlie chromosome instability in cancer cells, but a prominent role for telomeres, the tip of linear chromosomes, has been determined. Telomeres are ribonucleoprotein structures that prevent natural chromosome ends being recognized as DNA double-strand breaks, by adopting a loop structure. Loss of telomere function appears from either alteration on telomere-binding proteins or from the progressive telomere shortening that normally occurs under physiological conditions in the majority of cells in tissues. Importantly, unmasked telomeres may either trigger the senescent phenotype that has been linked to the aging process or may initiate the chromosome instability needed for cancer development, depending on the integrity of the DNA damage checkpoint responses. Telomere dysfunction contributes to chromosome instability through end-to-end chromosome fusions entering breakage-fusion-bridge (BFB) cycles. Resolution of chromatin bridge intermediates is likely to contribute greatly to the generation of segmental chromosome amplification events, unbalanced chromosome rearrangements, and whole chromosome aneuploidy. Noteworthy is the fact that telomere length heterogeneity among individuals may directly influence the scrambling of the genome at tumor initiation. However, reiterated BFB cycles would randomly reorganize the cell karyotype, thus increasing the genetic diversity that characterizes tumor cells. Even though a direct link is still lacking, multiple evidence lead one to believe that telomere dysfunction directly contributes to cancer development in humans. The expansion of highly unstable cells due to telomere dysfunction enhances the genetic diversity needed to fuel specific mutations that may promote cell immortalization and the acquisition of a tumor phenotype.
No preview · Article · Dec 2011 · Advances in Cancer Research
[Show abstract][Hide abstract] ABSTRACT: When damage is inflicted in nuclear DNA, cells activate a hierarchical plethora of proteins that constitute the DNA damage response machinery. In contrast to the cell nucleus, the ability of micronuclear DNA lesions to activate this complex network is controversial. In order to determine whether the DNA contained in micronuclei is protected by the cellular damage response system, we studied the recruitment of excision repair factors to photolesions inflicted in the DNA of radiation-induced micronuclei. To perform this analysis, primary human dermal fibroblasts were exposed to UV-C light to induce photolesions in nuclear and micronuclear DNA. By means of immunofluorescence techniques, we observed that most micronuclei were devoid of NER factors. We conclude that UV photoproducts in micronuclei are mostly unable to generate an effective DNA damage response. We observed that the micronuclear envelope structure is a determinant factor that influences the repair of the DNA lesions inside micronuclei. Therefore, our results allow us to conclude that photolesions in radiation-induced micronuclei are poorly processed because the repair factors are unable to reach the micronuclear chromatin when a micronucleus is formed or after a genotoxic insult.
Full-text · Article · Sep 2011 · Mutation Research/Fundamental and Molecular Mechanisms of Mutagenesis
[Show abstract][Hide abstract] ABSTRACT: Chromosomal instability occurs early in the development of cancer and may represent an important step in promoting the multiple genetic changes required for the initiation and/or progression of the disease. Telomere erosion is one of the factors that contribute to chromosome instability through end-to-end chromosome fusions entering BFB (breakage-fusion-bridge) cycles. Uncapped chromosomes with short dysfunctional telomeres represent an initiating substrate for both pre- and post-replicative joining, which leads to unstable chromosome rearrangements prone to bridge at mitotic anaphase. Resolution of chromatin bridge intermediates is likely to contribute greatly to the generation of segmental chromosome amplification events, unbalanced chromosome rearrangements and whole chromosome aneuploidy. Accordingly, telomere-driven instability generates highly unstable genomes that could promote cell immortalization and the acquisition of a tumour phenotype.
Preview · Article · Dec 2010 · Biochemical Society Transactions
[Show abstract][Hide abstract] ABSTRACT: Errors in chromosome segregation during mitosis result in aneuploidy, which in humans may play a role in the onset of neoplasia by changing gene dosage. Nearly all solid tumors exhibit genomic instability at the chromosomal level, showing both structural and numerical chromosome abnormalities. Chromosomal instability occurs early in the development of cancer and may represent an important step in the initiation and/or progression of the disease. Telomere integrity appears to be a critical element in the genesis of structural chromosome imbalances, but it is still not clear whether it can also generate numerical chromosome aberrations. We investigated the possible relationship between telomere shortening and aneuploidy formation in human mammary epithelial cells using the cytokinesis-block micronucleus assay combined with fluorescent DNA probes. In this cell system, uncapped chromosomes fuse with each other resulting in dicentric chromosomes, which are known to be a source of new structural chromosome rearrangements. Here, we show that in primary epithelial cells, the chromosomes with short telomeres are more frequently involved in missegregation events than chromosomes of normal telomere length. Whole chromosome aneuploidy occurs through both nondisjunction and anaphase lagging of dicentric chromatids, which suggests that pulling anaphase bridges toward opposite poles can generate the necessary force for detaching a chromosome from the microtubules of one or both spindle poles. Therefore, telomere-driven instability can promote not only the appearance of chromosomal rearrangements but also the appearance of numerical chromosome aberrations that could favor cell immortalization and the acquisition of a tumor phenotype.
No preview · Article · Apr 2010 · Genes Chromosomes and Cancer
[Show abstract][Hide abstract] ABSTRACT: Micronuclei are good markers of genotoxic exposure in humans and their scoring has been extensively used to identify potential genotoxic agents. Micronuclei are also indicators of chromosomal instability, since the frequency of micronuclei is higher in tumour cells and cells with a defective DNA damage repair system or disrupted cell cycle checkpoint machinery. Despite the widespread use of this biomarker, information on the basic biology of micronuclei and the impact of micronuclei on the cell is relatively controversial. In some cell systems, micronuclei are considered to be genetic material that is lost for the cell; whereas other studies suggest that micronuclear DNA is actively transcribed and its genes are fully expressed. Recently, evidence has accumulated suggesting that damaged DNA entrapped in micronuclei induces a defective cell cycle checkpoint arrest and DNA repair response, and that micronuclear content can be degraded without inducing an immediate cell cycle arrest or causing the cell to enter apoptosis. Overall, these findings emphasise the important consequences of micronucleus formation in terms of chromosomal instability in general and gene loss in particular.
Full-text · Article · Mar 2010 · Mutation Research/Fundamental and Molecular Mechanisms of Mutagenesis