Modeling chromosomal instability and epithelial carcinogenesis in the telomerase-deficient mouse

Chromosomal disruption and rearrangements during murine sarcoma development converge to stable karyotypic formation kept by telomerase overexpression

BACKGROUND

Tumor initiation presents a complex and unstable genomic landscape; one of the earliest hallmark events of cancer, and its progression is probably based on selection mechanisms under specific environments that lead to functional tumor cell speciation. We hypothesized that viable tumor phenotypes possess common and highly stable karyotypes and their proliferation is facilitated by an attuned high telomerase activity. Very few investigations have focused on the evolution of common chromosomal rearrangements associated to molecular events that result in functional phenotypes during tumor development.

RESULTS

We have used cytogenetic, flow cytometry and cell culture tools to investigate chromosomal rearrangements and clonality during cancer development using the murine sarcoma TG180 model, and also molecular biology techniques to establish a correlation between chromosome instability and telomerase activity, since telomeres are highly affected during cancer evolution. Cytogenetic analysis showed a near-tetraploid karyotype originated by endoreduplication. Chromosomal rearrangements were random events in response to in vitro conditions, but a stable karyotypic equilibrium was achieved during tumor progression in different in vivo conditions, suggesting that a specific microenvironment may stabilize the chromosomal number and architecture. Specific chromosome aberrations (marker chromosomes) and activated regions (rDNAs) were ubiquitous in the karyotype, suggesting that the conservation of these patterns may be advantageous for tumor progression. High telomerase expression was also correlated with the chromosomal rearrangements stabilization.

CONCLUSIONS

Our data reinforce the notion that the sarcoma cell evolution converges from a highly unstable karyotype to relatively stable and functional chromosome rearrangements, which are further enabled by telomerase overexpression.

In vivo models of brain tumors: roles of genetically engineered mouse models in understanding tumor biology and use in preclinical studies

Although our knowledge of the biology of brain tumors has increased tremendously over the past decade, progress in treatment of these deadly diseases remains modest. Developing in vivo models that faithfully mirror human diseases is essential for the validation of new therapeutic approaches. Genetically engineered mouse models (GEMMs) provide elaborate temporally and genetically controlled systems to investigate the cellular origins of brain tumors and gene function in tumorigenesis. Furthermore, they can prove to be valuable tools for testing targeted therapies. In this review, we discuss GEMMs of brain tumors, focusing on gliomas and medulloblastomas. We describe how they provide critical insights into the molecular and cellular events involved in the initiation and maintenance of brain tumors, and illustrate their use in preclinical drug testing.

Telomerase at the intersection of cancer and aging

Although cancer and aging have been studied as independent diseases, mounting evidence suggests that cancer is an aging-associated disease and that cancer and aging share many molecular pathways. In particular, recent studies validated telomerase activation as a potential therapeutic target for age-related diseases; in addition, abnormal telomerase expression and telomerase mutations have been associated with many different types of human tumor. Here, we revisit the connection between telomerase and cancer and aging in light of recent findings supporting a role for telomerase not only in telomere elongation, but also in metabolic fitness and Wnt activation. Understanding the physiological impact of telomerase regulation is fundamental given the therapeutic strategies that are being developed that involve telomerase modulation.

Heat induces gene amplification in cancer cells

BACKGROUND

Hyperthermia plays an important role in cancer therapy. However, as with radiation, it can cause DNA damage and therefore genetic instability. We studied whether hyperthermia can induce gene amplification in cancer cells and explored potential underlying molecular mechanisms.

MATERIALS AND METHODS

(1) Hyperthermia: HCT116 colon cancer cells received water-submerged heating treatment at 42 or 44°C for 30 min; (2) gene amplification assay using N-(phosphoacetyl)-L-aspartate (PALA) selection of cabamyl-P-synthetase, aspartate transcarbarmylase, dihydro-orotase (cad) gene amplified cells; (3) southern blotting for confirmation of increased cad gene copies in PALA-resistant cells; (4) γH2AX immunostaining to detect γH2AX foci as an indication for DNA double strand breaks.

RESULTS

(1) Heat exposure at 42 or 44°C for 30 min induces gene amplification. The frequency of cad gene amplification increased by 2.8 and 6.5 folds respectively; (2) heat exposure at both 42 and 44°C for 30 min induces DNA double strand breaks in HCT116 cells as shown by γH2AX immunostaining.

CONCLUSION

This study shows that heat exposure can induce gene amplification in cancer cells, likely through the generation of DNA double strand breaks, which are believed to be required for the initiation of gene amplification. This process may be promoted by heat when cellular proteins that are responsible for checkpoints, DNA replication, DNA repair and telomere functions are denatured. To our knowledge, this is the first study to provide direct evidence of hyperthermia induced gene amplification.

Mad2-induced chromosome instability leads to lung tumour relapse after oncogene withdrawal

Inhibition of an initiating oncogene often leads to extensive tumor cell death, a phenomenon known as oncogene addiction1. This has led to the search for compounds that specifically target and inhibit oncogenes as anti-cancer agents. Whether chromosomal instability (CIN) generated as a result of deregulation of the mitotic checkpoint pathway2,3, a frequent characteristic of solid tumors, has any effect on oncogene addiction, however, has not been explored systematically. We show here that induction of chromosome instability by overexpression of the mitotic checkpoint gene Mad2 does not affect the regression of Kras driven lung tumors upon Kras inhibition. However, tumors that experience transient Mad2 overexpression and consequent chromosome instability recur at dramatically elevated rates. The recurrent tumors are highly aneuploid and have varied activation of pro-proliferative pathways. Thus, early CIN may be responsible for tumor relapse after seemingly effective anti-cancer treatments.

Telomerase-dependent and -independent chromosome healing in mouse embryonic stem cells

Telomeres play an important role in protecting the ends of chromosomes and preventing chromosome fusion. We have previously demonstrated that double-strand breaks near telomeres in mammalian cells result in either the addition of a new telomere at the site of the break, termed chromosome healing, or sister chromatid fusion that initiates chromosome instability. In the present study, we have investigated the role of telomerase in chromosome healing and the importance of chromosome healing in preventing chromosome instability. In embryonic stem cell lines that are wild type for the catalytic subunit of telomerase (TERT), chromosome healing at I-SceI-induced double-strand breaks near telomeres accounted for 22 of 35 rearrangements, with the new telomeres added directly at the site of the break in all but one instance. In contrast, in two TERT-knockout embryonic stem cell lines, chromosome healing accounted for only 1 of 62 rearrangements, with a 23 bp insertion at the site of the sole chromosome-healing event. However, in a third TERT-knockout embryonic stem cell line, 10PTKO-A, chromosome healing was a common event that accounted for 20 of 34 rearrangements. Although this chromosome healing also occurred at the I-SceI site, differences in the microhomology at the site of telomere addition demonstrated that the mechanism was distinct from that in wild-type embryonic stem cell lines. In addition, the newly added telomeres in 10PTKO-A shortened with time in culture, eventually resulting in either telomere elongation through a telomerase-independent mechanism or loss of the subtelomeric plasmid sequences entirely. The combined results demonstrate that chromosome healing can occur through both telomerase-dependent and -independent mechanisms, and that although both mechanisms can prevent degradation and sister chromatid fusion, neither mechanism is efficient enough to prevent sister chromatid fusion from occurring in many cells experiencing double-strand breaks near telomeres.

Dual roles of telomere dysfunction in initiation and suppression of tumorigenesis

Human carcinomas arise through the acquisition of genetic changes that endow precursor cancer cells with a critical threshold of cancer-relevant genetic lesions. This complex genomic alterations confer upon precursor cancer cells the ability to grow indefinitely and to metastasize to distant sites. One important mechanism underlying a cell's tumorigenic potential is the status of its telomere. Telomeres are G-rich simple repeat sequences that serve to prevent chromosomal ends from being recognized as DNA double-strand breaks (DSBs). Dysfunctional telomeres resemble DSBs, leading to the formation of dicentric chromosomes that fuel high degrees of genomic instability. In the setting of an intact p53 pathway, this instability promotes cellular senescence, a potent tumor suppressor mechanism. However, rare cells that stochastically lose p53 function emerge from this sea of genomic instability and progress towards cancer. In this review, we describe the use of mouse models to probe the impact of dysfunctional telomeres on tumor initiation and suppression.

Tpp1/Acd maintains genomic stability through a complex role in telomere protection

Telomeres serve to protect the ends of chromosomes, and failure to maintain telomeres can lead to dramatic genomic instability. Human TPP1 was identified as a protein which interacts with components of a telomere cap complex, but does not directly bind to telomeric DNA. While biochemical interactions indicate a function in telomere biology, much remains to be learned regarding the roles of TPP1 in vivo. We previously reported the positional cloning of the gene responsible for the adrenocortical dysplasia (acd) mouse phenotype, which revealed a mutation in the mouse homologue encoding TPP1. We find that cells from homozygous acd mice harbor chromosomes fused at telomere sequences, demonstrating a role in telomere protection in vivo. Surprisingly, our studies also reveal fusions and radial structures lacking internal telomere sequences, which are not anticipated from a simple deficiency in telomere protection. Employing spectral karyotyping and telomere FISH in a combined approach, we have uncovered a striking pattern; fusions with telomeric sequences involve nonhomologous chromosomes while those lacking telomeric sequences involve homologues. Together, these studies show that Tpp1/Acd plays a vital role in telomere protection, but likely has additional functions yet to be defined.

Tumor-biology and current treatment of skull-base chordomas

Chordomas are rare, slow growing tumors of the axial skeleton, which derive from the remnants of the fetal notochord. They can be encountered anywhere along the axial skeleton, most commonly in the sacral area, skull base and less commonly in the spine. Chordomas have a benign histopathology but exhibit malignant clinical behavior with invasive, destructive and metastatic potential. Genetic and molecular pathology studies on oncogenesis of chordomas are very limited and there is little known on mechanisms governing the disease. Chordomas most commonly present with headaches and diplopia and can be readily diagnosed by current neuroradiological methods. There are 3 pathological subtypes of chordomas: classic, chondroid and dedifferentiated chordomas. Differential diagnosis from chondrosarcomas by radiology or pathology may at times be difficult. Skull base chordomas are very challenging to treat. Clinically there are at least two subsets of chordoma patients with distinct behaviors: some with a benign course and another group with an aggressive and rapidly progressive disease. There is no standard treatment for chordomas. Surgical resection and high dose radiation treatment are the mainstays of current treatment. Nevertheless, a significant percentage of skull base chordomas recur despite treatment. The outcome is dictated primarily by the intrinsic biology of the tumor and treatment seems only to have a secondary impact. To date we only have a limited understanding this biology; however better understanding is likely to improve treatment outcome. Hereby we present a review of the current knowledge and experience on the tumor biology, diagnosis and treatment of chordomas.

A mutation in separase causes genome instability and increased susceptibility to epithelial cancer

Proper chromosome segregation is essential for maintenance of genomic integrity and instability resulting from failure of this process may contribute to cancer. Here, we demonstrate that a mutation in the mitotic regulator separase is responsible for the cell cycle defects seen in the zebrafish mutant, cease&desist (cds). Analysis of cds homozygous mutant embryos reveals high levels of polyploidy and aneuploidy, spindle defects, and a mitotic exit delay. Carcinogenesis studies demonstrated that cds heterozygous adults have a shift in tumor spectrum with an eightfold increase in the percentage of fish bearing epithelial tumors, indicating that separase is a tumor suppressor gene in vertebrates. These data strongly support a conserved cross-species role for mitotic checkpoint genes in genetic stability and epithelial carcinogenesis.

TGF-beta and cancer: is Smad3 a repressor of hTERT gene?

Transforming growth factor beta (TGF-beta) carries out tumor suppressor activity in epithelial and lymphoid cells, whereas telomerase is required for most cancers. Although the molecular mechanisms by which TGF-beta acts as a tumor suppressor are yet to be fully established, a link between TGFb and its tumor suppressor activity by telomerase has been suggested. Recently, we have noted a novel mode of action for TGF-beta through which human telomerase reverse transcriptase (hTERT) gene is repressed in immortal and neoplastic cells, confirming that one of the mechanisms underlying TGF-beta suppression of tumor growth may be through inhibiting hTERT gene transcription. Moreover, the inhibition of hTERT gene by TGF-beta suggests a cis action of the TGF-beta signaling molecule Smad3 on hTERT promoter directly. This article examines our current understanding and investigation of TGF-beta regulation of telomerase activity, and presents a model in which Smad3 participates in regulating hTERT gene transcription by acting as a repressor directly. Engineering the interface between Smad3 and hTERT gene may lead to a new strategy to inhibit telomerase activity in cancer.

Telomeres and chromosome instability

Genomic instability has been proposed to play an important role in cancer by accelerating the accumulation of genetic changes responsible for cancer cell evolution. One mechanism for chromosome instability is through the loss of telomeres, which are DNA-protein complexes that protect the ends of chromosomes and prevent chromosome fusion. Telomere loss can occur as a result of exogenous DNA damage, or spontaneously in cancer cells that commonly have a high rate of telomere loss. Mouse embryonic stem cells and human tumor cell lines that contain a selectable marker gene located immediately adjacent to a telomere have been used to investigate the consequences of telomere loss. In both cell types, telomere loss is followed by either the addition of a new telomere on to the end of the broken chromosome, or sister chromatid fusion and prolonged breakage/fusion/bridge (B/F/B) cycles that result in DNA amplification and large terminal deletions. The regions amplified by B/F/B cycles can then be transferred to other chromosomes, either through the formation of double-minute chromosomes that reintegrate at other sites, or through end-to-end fusions between chromosomes. B/F/B cycles eventually end when a chromosome acquires a new telomere by one of several mechanisms, the most common of which is translocation, which can involve either nonreciprocal transfer or duplication of all or part of an arm of another chromosome. Telomere acquisition involving nonreciprocal translocations results in the loss of a telomere on the donor chromosome, which subsequently becomes unstable. In contrast, translocations involving duplications do not destabilize the donor chromosome, although they result in allelic imbalances. Thus, the loss of a single telomere can generate a wide variety of chromosome alterations commonly associated with human cancer, not only on the chromosome that originally lost its telomere, but other chromosomes as well. Factors promoting spontaneous telomere loss and the resulting B/F/B cycles are therefore likely to be important in generating the karyotypic changes associated with human cancer.

Gene amplification in cancer

Gene amplification is a copy number increase of a restricted region of a chromosome arm. It is prevalent in some tumors and is associated with overexpression of the amplified gene(s). Amplified DNA can be organized as extrachromosomal elements, as repeated units at a single locus or scattered throughout the genome. Common chromosomal fragile sites, defects in DNA replication or telomere dysfunction might promote amplification. Some regions of amplification are complex, yet elements of the pattern are reproduced in different tumor types. A genetic basis for amplification is suggested by its relative frequency in some tumor subtypes, and its occurrence in "early" preneoplastic lesions. Clinically, amplification has prognostic and diagnostic usefulness, and is a mechanism of acquired drug resistance.

Telomeres, chromosome instability and cancer

Telomeres are composed of repetitive G-rich sequence and an abundance of associated proteins that together form a dynamic cap that protects chromosome ends and allows them to be distinguished from deleterious DSBs. Telomere-associated proteins also function to regulate telomerase, the ribonucleoprtotein responsible for addition of the species-specific terminal repeat sequence. Loss of telomere function is an important mechanism for the chromosome instability commonly found in cancer. Dysfunctional telomeres can result either from alterations in the telomere-associated proteins required for end-capping function, or from alterations that promote the gradual or sudden loss of sufficient repeat sequence necessary to maintain proper telomere structure. Regardless of the mechanism, loss of telomere function can result in sister chromatid fusion and prolonged breakage/fusion/bridge (B/F/B) cycles, leading to extensive DNA amplification and large terminal deletions. B/F/B cycles terminate primarily when the unstable chromosome acquires a new telomere, most often by translocation of the ends of other chromosomes, thereby providing a mechanism for transfer of instability from one chromosome to another. Thus, the loss of a single telomere can result in on-going instability, affect multiple chromosomes, and generate many of the types of rearrangements commonly associated with human cancer.

Dynamics of metastasis suppressor gene inactivation

For most cancer cell types, the acquisition of metastatic ability leads to clinically incurable disease. Twelve metastasis suppressor genes (MSGs) have been identified that reduce the metastatic propensity of cancer cells. If these genes are inactivated in both alleles, metastatic ability is promoted. Here, we develop a mathematical model of the dynamics of MSG inactivation and calculate the expected number of metastases formed by a tumor. We analyse the effects of increased mutation rates and different fitness values of cells with one or two inactivated alleles on the ability of a tumor to form metastases. We find that mutations that are negatively selected in the main tumor are unlikely to be responsible for the majority of metastases produced by a tumor. Most metastases-causing mutations will be present in all (or most) cells in the main tumor.

Telomere-dependent chromosomal instability

Telomeres are specialized DNA-protein structures at the ends of the linear chromosomes. In mammalian cells, they are composed of multifold hexameric TTAGGG repeats and a number of associated proteins. The double-stranded telomeric DNA ends in a 3' single stranded overhang of 150 to 300 base pair (bp) which is believed to be required for a higher order structure (reviewed in (Blackburn, 2001)). One important model is that the telomeres form loop structures, the T-loops, and by invasion of the 3' overhang into the duplex region of the double stranded part protect the DNA against degradation and hinder the cellular machinery to recognize the ends as broken DNA, thus providing chromosomal integrity (Griffith et al, 1999). If telomeres become critically short they loose their capping function, become sticky, and are prone to illegitimate chromosome end-to-end fusions. The resulting dicentric chromosomes are highly unusable and because of bridge-fusion-breakage cycles they give rise to chromosomal translocations, deletions, and amplifications. Thus, critically short telomeres are thought to be responsible for the onset of genomic instability. In addition, we provide evidence that in a length-independent manner telomeres can confer to genomic instability by forming telomericaggregates which through chromosomal dys-locations contribute to chromosomal aberrations.

Chromosomal radiosensitivity in two cell lineages derived from clinically radiosensitive cancer patients

PURPOSE

Despite its prominent contribution to cancer cure and palliation, around 1% to 5% of cancer patients suffer serious side effects from radiotherapy. A cardinal goal in the fields of radiobiology and oncology is to predict normal tissue radiosensitivity of a cancer patient before radiotherapy. Higher tumor control rates are likely if radiotherapy individualization could be achieved by applying predictive approaches.

EXPERIMENTAL DESIGN

Here, we make use of the cytokinesis block micronucleus assay to assess radiosensitivity in cell lines derived from two different cell lineages obtained from clinically radiosensitive patients. We determined the micronucleus frequency after graded doses of ionizing radiation to primary fibroblasts and lymphoblast cell lines derived from 36 highly radiosensitive cancer patients.

RESULTS

Many cell lines, following exposure to ionizing radiation, from patients with severe clinical reactions to radiotherapy showed statistically significantly higher frequencies of micronuclei than those from patients who had normal reactions to radiotherapy. One individual revealed significantly higher micronucleus frequencies in both cell lineages. Interestingly, lymphoblast cell lines from one patient showed micronucleus frequencies similar to ataxia telangiectasia mutated-deficient cells.

CONCLUSIONS

These results indicate that the micronucleus assay may have use for identifying predisposition to clinical radiosensitivity, at least in a subset of patients as a component of a pretreatment radiosensitivity assay for use in the clinic.

Non-melanoma skin cancer: what drives tumor development and progression?

Non-melanoma skin cancer, i.e. basal cell carcinoma (BCC) and squamous cell carcinoma (SCC) are the most frequent tumors and their number is still increasing worldwide. Furthermore, immunosuppression in organ transplant patients strongly contributes to the increase in skin cancer incidence--being 65-250 times more frequent than in the general population. Often these patients suffer from a second and third lesion and the severity of these tumors is linked to their number. SCCs in transplant recipients also appear to be more aggressive. They tend to grow rapidly, show a higher rate of local recurrences and metastasize in 5-8% of the patients (all reviewed in Ref. 2). This largely differs from BCCs which are more frequent in the general population--at a ratio of 4:1 as compared with SCCs--but the number is only increased by a factor of 10 in transplant recipients. This may suggest that 'dormant' SCC precursor cells/lesions are present at a high frequency in the population but they are well controlled by the immune system. BCC, on the other hand, may be less dependent on immune surveillance thereby underlining its different etiology. While for BCC development the genetic hallmark is abrogation of the ptch-sonic hedgehog pathway, little is known about the causal alterations of SCCs. However, the complexity of the genetic alterations (numerical and structural aberration profiles) in SCCs argues for several levels of genomic instability involved in the generation and progression of skin cancer.

Population genetics of tumor suppressor genes

Cancer emerges when a single cell receives multiple mutations. For example, the inactivation of both alleles of a tumor suppressor gene (TSG) can imply a net reproductive advantage of the cell and might lead to clonal expansion. In this paper, we calculate the probability as a function of time that a population of cells has generated at least one cell with two inactivated alleles of a TSG. Different kinetic laws hold for small and large populations. The inactivation of the first allele can either be neutral or lead to a selective advantage or disadvantage. The inactivation of the first and of the second allele can occur at equal or different rates. Our calculations provide insights into basic aspects of population genetics determining cancer initiation and progression.

FISH characterization of a dicentric Yq (p11.32) isochromosome in an azoospermic male

The most common structural rearrangements of the Y chromosome result in the production of dicentrics. In this work, we analyze an abnormal Y chromosome, detected as a mosaic in an azoospermic male ascertained for infertility. FISH with seven different DNA probes specific for Y chromosome sequences (Y alpha-satellite, Y alpha-satellite III, non-alpha-satellite centromeric Y, SRY gene, subtelomeric Yp, subtelomeric Yq, and PNA-tel) and CGH analysis were performed. FISH results showed that the abnormal Y chromosome was a dicentric Yq isochromosome and that the breakpoint was distally in band Yp11.32. Lymphocyte chromosomes showed a mosaicism with 46,X,idicY(qter-->p11.32::p11.32-->qter) (51.7%), 46,XY (45.6%), and other cell lines (2.7%). In oral interphase cells, the mosaicism was 46,XidicY (62.8%), 46,XY (25.7%), 45,X (6.6%), and others (4.9%). The possible origin of this dicentric Yq isochromosome is discussed. Finally, we compare differences in mosaicism and phenotype among three reported cases with the breakpoint at Yp11.32

Telomere attrition predominantly occurs in precursor lesions during in vivo carcinogenic process of the uterine cervix

Although human papillomavirus (HPV) has been defined as the pathogen for cervical carcinomas, molecular events underlying the oncogenic process are unclear. As telomere dysfunction-mediated chromosomal instability and telomerase activation have been suggested as key events in carcinogenesis, we dissected the dynamic changes in telomere length, checkpoint response, and temporal profile of telomerase expression during the evolution from precursor lesions (cervical intraepithelial neoplasia, CINs) to invasive cancers of the uterine cervix in sequential samples from 16 patients. Telomeres were significantly shortened in all CIN samples and no further substantial attritions occurred in most cases with the acquisition of malignant phenotype. Very short telomeres were coupled with constitutive activation of the DNA damage response pathway (Chk2 phosphorylation) and increased cellular proliferation in those cervical specimens. Telomerase reverse transcriptase (hTERT) expression was preferably induced at advanced CINs or invasive cancers. The present finding demonstrates that excessive telomere shortening predominantly occurs in the early carcinogenesis of the uterine cervix largely prior to telomerase activation. Widespread over-erosion of telomeres or telomere dysfunction in very early stages of cervical tumorigenesis might fuel transformation processes by driving chromosomal instability.

Telomeres and telomerase: a dual role in hepatocarcinogenesis

Telomere shortening limits the proliferative capacity of primary human cells and restrains the regenerative capacity of organ systems during chronic diseases and aging. Telomere shortening apparently has a dual role in tumor development and progression. On the one hand, it induces chromosomal instability and the initiation of cancer; on the other hand, tumor progression requires stabilization of telomeres. The predominant mechanism of telomere stabilization in tumor cells is the activation of the telomere-synthesizing enzyme telomerase. The potential use of telomerase activators for the treatment of regenerative disorders will ultimately depend on their effects on tumorigenesis. This review focuses on the role of telomere shortening and telomerase in carcinogenesis with a special focus on hepatocellular carcinoma.

Tumor-specific low molecular weight forms of cyclin E induce genomic instability and resistance to p21, p27, and antiestrogens in breast cancer

The deregulated expression of cyclin E as measured by the overexpression of its low molecular weight (LMW) isoforms is a powerful predictor of poor outcome in patients with breast cancer. The mechanism by which these LMW forms give tumor cells a growth advantage is not known and is the subject of this article. In this article, we provide the pathobiological mechanisms of how these LMW forms are involved in disease progression. Specifically, we show that overexpression of the LMW forms of cyclin E but not the full-length form in MCF-7 results in (a) their hyperactivity because of increased affinity for cdk2 and resistance to inhibition by the cyclin-dependent kinase inhibitors p21 and p27, (b) resistance to the growth inhibiting effects of antiestrogens, and (c) chromosomal instability. Lastly, tumors from breast cancer patients overexpressing the LMW forms of cyclin E are polyploid in nature and are resistant to endocrine therapy. Collectively, the biochemical and functional differences between the full-length and the LMW isoforms of cyclin E provide a molecular mechanism for the poor clinical outcome observed in breast cancer patients harboring tumors expressing high levels of the LMW forms of cyclin E. These properties of the LMW forms cyclin E suggest that they are not just surrogate markers of poor outcome but bona fide mediators of aggressive disease and potential therapeutic targets for patients whose tumors overexpress these forms.

Telomere erosion and chromosomal instability in cells expressing the HPV oncogene 16E6

Progression to advanced-stage cervical carcinomas is characterized by a recurrent pattern of chromosomal rearrangements. Structural chromosome rearrangements are generated through the fusion of broken chromosome ends. These chromosome breaks may be induced by mutagenic agents such as ionizing radiation, or chromosome ends may be exposed through extensive telomere shortening. The human papilloma virus oncogene 16E6 induces telomerase activity in human keratinocytes, a model system for cervical tumor formation. The present study explores the relationship between 16E6 expression, telomerase activity, and chromosomal instability. We show that the frequency of anaphase bridges is dependent on the level of telomerase activity in 16E6/E7-expressing clones, and is the result of telomere shortening. High frequencies of anaphase bridges, associated with low telomerase activity, correlate with increased chromosome instability. Anaphase bridge formation is also associated with the presence of micronuclei, which are shown to contain unstable chromosomes frequently involved in rearrangements. As anaphase bridges are observed in both high and low telomerase 16E6/E7 clones, but not in hTERT-expressing control clones, expression of 16E6 in these immortalized clones is not sufficient to stabilize shortened telomeres completely. We suggest a model in which HPV-induced tumorigenesis may be dependent on persistent bridge-breakage-fusion cycles that allow for continued genomic rearrangements.

Telomere cap components influence the rate of senescence in telomerase-deficient yeast cells

Cells lacking telomerase undergo senescence, a progressive reduction in cell division that involves a cell cycle delay and culminates in "crisis," a period when most cells become inviable. In telomerase-deficient Saccharomyces cerevisiae cells lacking components of the nonsense-mediated mRNA decay (NMD) pathway (Upf1,Upf2, or Upf3 proteins), senescence is delayed, with crisis occurring approximately 10 to 25 population doublings later than in Upf+ cells. Delayed senescence is seen in upfDelta cells lacking the telomerase holoenzyme components Est2p and TLC1 RNA, as well as in cells lacking the telomerase regulators Est1p and Est3p. The delay of senescence in upfDelta cells is not due to an increased rate of survivor formation. Rather, it is caused by alterations in the telomere cap, composed of Cdc13p, Stn1p, and Ten1p. In upfDelta mutants, STN1 and TEN1 levels are increased. Increasing the levels of Stn1p and Ten1p in Upf+ cells is sufficient to delay senescence. In addition, cdc13-2 mutants exhibit delayed senescence rates similar to those of upfDelta cells. Thus, changes in the telomere cap structure are sufficient to affect the rate of senescence in the absence of telomerase. Furthermore, the NMD pathway affects the rate of senescence in telomerase-deficient cells by altering the stoichiometry of telomere cap components.

Cell cycle-dependent 3D distribution of telomeres and telomere repeat-binding factor 2 (TRF2) in HaCaT and HaCaT-myc cells

Telomeres are specialized structures at the ends of the chromosomes that, with the help of proteins--such as the telomere repeat-binding factor TRF2 -, form protective caps which are essential for chromosomal integrity. Investigating the structure and three-dimensional (3D) distribution of the telomeres and TRF2 in the nucleus, we now show that the telomeres of the immortal HaCaT keratinocytes are distributed in distinct non-overlapping territories within the inner third of the nuclear space in interphase cells, while they extend more widely during mitosis. TRF2 is present at the telomeres at all cell cycle phases. During mitosis additional TRF2 protein concentrates all around the chromosomes. This change in staining pattern correlates with a significant increase in TRF2 protein at the S/G2 transition as seen in Western blots of synchronized cells and is paralleled by a cell cycle-dependent regulation of TRF2 mRNA, arguing for a specific role of TRF2 during mitosis. The distinct territorial localization of telomeres is abrogated in a HaCaT variant that constitutively expresses c-Myc--a protein known to contribute to genomic instability. These cells are characterized by overlapping telomere territories, telomeric aggregates (TAs), that are accompanied by an overall irregular telomere distribution and a reduced level in TRF2 protein. These TAs which are readily detectable in interphase nuclei, are similarly present in mitotic cells, including cells in telophase. Thus, we propose that TAs, which subsequently also cluster their respective chromosomes, contribute to genomic instability by forcing an abnormal chromosome segregation during mitosis.

Overexpressed pituitary tumor-transforming gene causes aneuploidy in live human cells

The mammalian securin, pituitary tumor-transforming gene (PTTG), is overexpressed in several tumors and transforms cells in vitro and in vivo. To test the hypothesis that PTTG overexpression causes aneuploidy, enhanced green fluorescent protein (EGFP)-tagged PTTG (PTTG-EGFP) was expressed in human H1299 cancer cells (with undetectable endogenous PTTG expression) and mitosis of individual live cells observed. Untransfected cells and cells expressing EGFP alone exhibited appropriate mitosis. PTTG-EGFP markedly prolonged prophase and metaphase, indicating that PTTG blocks progression of mitosis to anaphase. In cells that underwent apparently normal mitosis (35 of 65 cells), PTTG-EGFP was degraded about 1 min before anaphase onset. Cells that failed to degrade PTTG-EGFP exhibited asymmetrical cytokinesis without chromosome segregation (18 of 65 cells) or chromosome decondensation without cytokinesis (9 of 65 cells), resulting in appearance of a macronucleus. Fifty-one of 55 cells expressing a nondegradable mutant PTTG exhibited asymmetrical cytokinesis without chromosome segregation, and some (4 of 55) decondensed chromosomes, both resulting in macronuclear formation. During this abnormal cytokinesis, all chromosomes and spindles and both centrosomes moved to one daughter cell, suggesting potential chaos in the subsequent mitosis. In conclusion, failure of PTTG degradation or enhanced PTTG accumulation, as a consequence of overexpression, inhibits mitosis progression and chromosome segregation but does not directly affect cytokinesis, resulting in aneuploidy. These results demonstrate that PTTG induces aneuploidy in single, live, human cancer cells.

Homozygous deletions scanning in tumor cell lines detects previously unsuspected loci

High rates of loss of heterozygosity commonly affect multiple chromosomes in individual tumor types, yet the number of known tumor suppressor genes (TSGs) systematically mutated in the corresponding tumors is usually low. The search for homozygously deleted genome segments in tumor samples or cell lines has become a method of choice to identify major TSGs or to reveal their influence on the development of a given tumor type. Here, we report a detailed homozygous deletion (HD) profiling for 246 critical loci on a panel of 89 tumor cell lines containing significant subsets of lung, ovarian and head and neck squamous cell carcinomas. We found a total of 53 HDs affecting 17 loci. The major target for HDs was p16-INK4A/p14-ARF (23/89, 26% of cases). Among the remaining alterations, HDs affecting TP73 or telomeric markers have never been previously described, whereas other HDs represent the first examples associating lesions of certain TSGs with a given tumor type (NF2 in lung and ovarian cells, STK11 in HELA cells). Overall, tumor cell lines established from ovarian or lung carcinomas displayed a surprising diversity of loci targeted by HDs with 7 and 6 loci involved, respectively. Our data suggest that, beside allelotyping or transcriptome/proteome studies, extensive HD profiling represents a promising approach for the detection of hitherto not implicated signalling pathways of tumorigenesis.

Telomere dysfunction: a potential cancer predisposition factor

BACKGROUND

Genetic instability associated with telomere dysfunction (i.e., short telomeres) is an early event in tumorigenesis. We investigated the association between telomere length and cancer risk in four ongoing case-control studies.

METHODS

All studies had equal numbers of case patients and matched control subjects (92 for head and neck cancer, 135 for bladder cancer, 54 for lung cancer, and 32 for renal cell carcinoma). Telomere length was measured in peripheral blood lymphocytes from study participants. Genetic instability was assessed with the comet assay. Patient and disease characteristics were collected and analyzed for associations with risk for these cancers. All statistical tests were two-sided.

RESULTS

Telomeres were statistically significantly shorter in patients with head and neck cancer (6.5 kilobases [kb]) than in control subjects (7.4 kb) (difference = 0.9 kb, 95% confidence interval [CI] = 0.5 to 1.2 kb; P<.001). Nine percent of patients with head and neck cancer were in the longest quartile of telomere length, whereas 59% were in the shortest quartile. Similar patterns were observed for lung, renal cell, and bladder cancer. When subjects were categorized into telomere length quartiles defined by the distribution in control subjects, the following inverse relationship between telomere length and cancer risk was observed: adjusted odds ratios [ORs] for decreasing quartiles = 0.84 (95% CI = 0.36 to 1.97), 1.77 (95% CI = 0.72 to 4.36), and 5.11 (95% CI = 1.90 to 13.77). In stratified analysis, we found a suggestive greater-than-additive interaction between smoking status and telomere length: for ever smokers with short telomeres, OR = 25.05 (95% CI = 6.91 to 90.73); for never smokers with short telomeres, OR = 6.18 (95% CI = 1.72 to 22.13); and for ever smokers with long telomeres, OR = 6.49 (95% CI = 1.54 to 27.38). Telomere length was statistically significantly and inversely associated with baseline and mutagen-induced genetic instability.

CONCLUSION

Short telomeres appear to be associated with increased risks for human bladder, head and neck, lung, and renal cell cancers.

Epithelial cancer in Fanconi anemia complementation group D2 (Fancd2) knockout mice

Fanconi anemia (FA) is a genetic disorder characterized by hypersensitivity to DNA damage, bone marrow failure, congenital defects, and cancer. To further investigate the in vivo function of the FA pathway, mice with a targeted deletion in the distally acting FA gene Fancd2 were created. Similar to human FA patients and other FA mouse models, Fancd2 mutant mice exhibited cellular sensitivity to DNA interstrand cross-links and germ cell loss. In addition, chromosome mispairing was seen in male meiosis. However, Fancd2 mutant mice also displayed phenotypes not observed in other mice with disruptions of proximal FA genes. These include microphthalmia, perinatal lethality, and epithelial cancers, similar to mice with Brca2/Fancd1 hypomorphic mutations. These additional phenotypes were not caused by defects in the ATM-mediated S-phase checkpoint, which was intact in primary Fancd2 mutant fibroblasts. The phenotypic overlap between Fancd2-null and Brca2/Fancd1 hypomorphic mice is consistent with a common function for both proteins in the same pathway, regulating genomic stability.

Loss of p53 function accelerates acquisition of telomerase activity in indefinite lifespan human mammary epithelial cell lines

We describe novel effects of p53 loss on immortal transformation, based upon comparison of immortally transformed human mammary epithelial cell (HMEC) lines lacking functional p53 with closely related p53(+) lines. Our previous studies of p53(+) immortal HMEC lines indicated that overcoming the stringent replicative senescence step associated with critically short telomeres (agonescence), produced indefinite lifespan lines that maintained growth without immediately expressing telomerase activity. These telomerase(-) 'conditionally immortal' HMEC underwent an additional step, termed conversion, to become fully immortal telomerase(+) lines with uniform good growth. The very gradual conversion process was associated with slow heterogeneous growth and high expression of the cyclin-dependent kinase inhibitor p57(Kip2). We now show that p53 suppresses telomerase activity and is necessary for the p57 expression in early passage p53(+) conditionally immortal HMEC lines, and that p53(-/-) lines exhibit telomerase reactivation and attain full immortality much more rapidly. A p53-inhibiting genetic suppressor element introduced into early passages of a conditionally immortal telomerase(-) p53(+) HMEC line led to rapid induction of hTERT mRNA, expression of telomerase activity, loss of p57 expression, and quick attainment of uniform good growth. These studies indicate that derangements in p53 function may impact malignant progression through direct effects on the conversion process, a potentially rate-limiting step in HMEC acquisition of uniform unlimited growth potential. These studies also provide evidence that the function of p53 in suppression of telomerase activity is separable from its cell cycle checkpoint function.

Mutations and aneuploidy: co-conspirators in cancer?

The role of intragenic point mutations in human cancer is well established. However, the contribution of massive genomic changes collectively known as aneuploidy is less certain. Recent experimental work suggests that aneuploidy is required for sporadic carcinogenesis in mice and that it may collaborate with intragenic mutations during tumorigenesis. The genomic plasticity afforded by aneuploidy could facilitate emergence of protumorigenic gene dosage changes and accelerate accumulation of oncogenes and loss of tumor suppressor genes. These new findings force us to rethink the pathogenesis of carcinoma in ways that have significant implications for diagnosis and therapy.

Telomere dysfunction and the initiation of genome instability

Tumour growth is an evolutionary process that is characterized by the selection of clonal populations of cells that acquire distinct genetic changes. Many cancer therapies aim to exploit the specific changes that occur in cancer cells, but understanding the underlying mechanisms of genomic instability that cause these mutations could lead to more effective therapies. If common mechanisms exist for initiating genomic instability in tumours, selection could explain the differences in specific gene mutations that accumulate in different tumour types. The cause of genomic instability in human tumours is unclear, although there is evidence to indicate that telomere dysfunction could make an important contribution.

3rd Geneva aging workshop 2002: cancer, apoptosis and aging

The topics of the 3rd Geneva Aging Workshop were cancer, apoptosis, and aging. As the risk for cancer is increasing with age, synergies that could emerge from this encounter could yield significant insights into the molecular mechanisms underlying both the etiology of cancer and the aging process. Several pathways that play a role in aging are thought to have an impact on the initiation and progression of cancer, with apoptosis being the key player. Which are the factors that are correlated with age-dependent cancer incidence? The regulators of apoptosis and genetic pathways that lead to apoptosis were discussed. This included genes that are clearly involved in aging, such as WS and BS, and genes that act in repair, such as BRCA1 and BARD1. The functions of telomere shortening and telomerase, extra cellular matrix, or epigenetic changes were presented in the light of their influence on aging and cancer risk.

Molecular cytogenetic parameters in Ewing sarcoma

To evaluate possible genomic instability and possible random aneuploidy, we applied comparative genomic hybridization and fluorescence in situ techniques, and evaluated telomerase activity in 16 cases of Ewing sarcoma (EWS) and compared the results to 7 controls. Common secondary aberrations (gains of chromosomes 8 and 12) were found in the study group. There was a direct correlation between the detection of random aneuploidy and development of tumor relapse (P = 0.0047). Other detectable abnormal parameters (secondary) and high telomerase activity were also more common among the cases with relapse but did not reach a statistical significance (probably because of the small sample size). In EWS, the detection of random aneuploidy seems to be a sensitive parameter in the prediction of tumor relapse.

Telomerase in the human organism

The intent of this review is to describe what is known and unknown about telomerase in somatic cells of the human organism. First, we consider the telomerase enzyme. Human telomerase ribonucleoproteins undergo at least three stages of cellular biogenesis: accumulation, catalytic activation and recruitment to the telomere. Next, we describe the patterns of telomerase regulation in the human soma. Telomerase activation in some cell types appears to offset proliferation-dependent telomere shortening, delaying but not defeating the inherent mitotic clock. Finally, we elaborate the connection between telomerase misregulation and human disease, in the contexts of inappropriate telomerase activation and telomerase deficiency. We discuss how our current perspectives on telomerase function could be applied to improving human health.

Cell senescence and cancer

Historically, the senescent state has been associated with, and was named after, the cell-cycle arrest that occurs after cells have undergone an intrinsically defined number of divisions in vitro. More recently, however, it has been shown that extrinsic factors, including those encountered in normal tissue-culture environments, can prematurely induce an indistinguishable senescent phenotype. In this review, we discuss the pathways of cell senescence, the mechanisms involved and the role that these pathways have in regulating the initiation and progression of cancer.

How carcinogens (or telomere dysfunction) induce genetic instability: associated-selection model

Carcinogens induce carcinogen-specific genetic instability (defects in DNA repair). According to the 'direct-selection' model, defects in DNA repair per se provide an immediate growth advantage. According to the 'associated-selection' model, carcinogens merely select for cells with adaptive mutations. Like any mutations, adaptive mutations occur predominantly in genetically unstable cells. The 'associated-selection' model predicts that carcinogen-driven selection minimizes cytotoxic but maximizes mutagenic effects of carcinogens. A purely mutagenic (neither cytotoxic, nor cytostatic) environment will favor effective DNA repair, whereas any growth-limiting conditions (telomerase deficiency, anticancer drugs) will select for genetically unstable cells. Genetic instability is a postmark of selective pressure rather than a hallmark of cancer per se. Once selected, genetic instability facilitates the development of resistance to any other growth-limiting conditions. As an example, a putative link between prior exposure to carcinogens and the ability to develop a telomerase-independent growth is discussed.

Mechanisms of ultraviolet (UV) B and UVA phototherapy

Ultraviolet (UV) radiation has been used for decades with great success and at a constantly increasing rate in the management of skin diseases, becoming an essential part of modern dermatologic therapy (Krutmann et al, 1999). For phototherapy, irradiation devices emitting either predominantly middlewave UV (UVB, 290-315 nm) or longwave UV (UVA, 315-400 nm) radiation are employed. In former years, patients were treated with broad-band UVB, broad-band UVA, or combination regimens. Broad-band UV phototherapy, however, is being replaced more frequently by the use of irradiation devices that allow treatment of patients' skin with selected emission spectra. Two such modalities which have their origin in European Photodermatology are 311 nm UVB phototherapy (which uses long-wave UVB radiation above 300 nm rather than broadband UVB) and high-dose UVA1 therapy (which selective employs long-wave UVA radiation above 340 nm). In Europe, 311 nm UVB phototherapy has almost replaced classical broad-band UVB phototherapy and has significantly improved therapeutic efficacy and safety of UVB phototherapy (van Welden et al, 1988; Krutmann et al, 1999). The constantly increasing use of UVA-1 phototherapy has not only improved UVA phototherapy for established indications such as atopic dermatitis (Krutmann et al, 1992a, 1998; Krutmann, 1996), but has also provided dermatologists with the opportunity to successfully treat previously untractable skin diseases, e.g., connective tissue diseases (Stege et al, 1997; Krutmann, 1997). These clinical developments have stimulated studies about the mechanisms by which UVB and UVA phototherapy work. The knowledge obtained from this work is an indispensable prerequisite to make treatment decisions on a rationale rather than an empirical basis. Modern dermatologic phototherapy has started to profit from this knowledge, and it is very likely that this development will continue and provide dermatologists with improved phototherapeutic modalities and regimens for established and new indications. This review aims to provide an overview about current concepts of the mode of action of dermatologic phototherapy. Special emphasis will be given on studies that have identified previously unrecognized immunosuppressive/anti-inflammatory principles of UV phototherapy.