Connecting mitotic instability and chromosome aberrations in cancer—can telomeres bridge the gap?

Flexible Attachment and Detachment of Centromeres and Telomeres to and from Chromosomes

Accurate transmission of genomic information across multiple cell divisions and generations, without any losses or errors, is fundamental to all living organisms. To achieve this goal, eukaryotes devised chromosomes. Eukaryotic genomes are represented by multiple linear chromosomes in the nucleus, each carrying a centromere in the middle, a telomere at both ends, and multiple origins of replication along the chromosome arms. Although all three of these DNA elements are indispensable for chromosome function, centromeres and telomeres possess the potential to detach from the original chromosome and attach to new chromosomal positions, as evident from the events of telomere fusion, centromere inactivation, telomere healing, and neocentromere formation. These events seem to occur spontaneously in nature but have not yet been elucidated clearly, because they are relatively infrequent and sometimes detrimental. To address this issue, experimental setups have been developed using model organisms such as yeast. In this article, we review some of the key experiments that provide clues as to the extent to which these paradoxical and elusive features of chromosomally indispensable elements may become valuable in the natural context.

Kinesin Motor Protein KIFC1 Is a Target Protein of miR-338-3p and Is Associated With Poor Prognosis and Progression of Renal Cell Carcinoma

KIFC1 (kinesin family member C1) plays a critical role in clustering of extra centrosomes in various cancer cells and thus could be considered as a promising therapeutic target. However, whether KIFC1 is involved in the procession of renal cell carcinoma (RCC) still remains unclear. In this study, we found that KIFC1 was upregulated in RCC tissues and is responsible for RCC tumorigenesis (p < 0.001). The high expression of KIFC1 correlates with aggressive clinicopathologic parameters. Kaplan–Meier analysis suggested that KIFC1 was associated with poor survival prognosis in RCC. Silencing KIFC1 dramatically resulted in inhibition of proliferation, delayed the cell cycle at G₂/M phase, and suppressed cell invasion and migration in vitro. The antiproliferative effect of KIFC1 silencing was also observed in xenografted tumors in vivo. miR-338-3p could directly bind to the 3′-untranslated region (3′-UTR) of KIFC1, and ectopic miR-338-3p expression mimicked the inhibitory functions of KIFC1 silencing on RCC cells through inactivation of the PI3K/AKT signaling pathway. Therefore, these results revealed that KIFC1 may be a novel biomarker and an effective therapeutic target for the treatment of RCC.

Genetics of melanoma progression: the rise and fall of cell senescence

There are many links between cell senescence and the genetics of melanoma, meaning both familial susceptibility and somatic-genetic changes in sporadic melanoma. For example, CDKN2A, the best-known melanoma susceptibility gene, encodes two effectors of cell senescence, while other familial melanoma genes are related to telomeres and their maintenance. This article aimed to analyze our current knowledge of the genetic or epigenetic driver changes necessary to generate a cutaneous metastatic melanoma, the commonest order in which these occur, and the relation of these changes to the biology and pathology of melanoma progression. Emphasis is laid on the role of cell senescence and the escape from senescence leading to cellular immortality, the ability to divide indefinitely.

Aurora B prevents chromosome arm separation defects by promoting telomere dispersion and disjunction

The segregation of centromeres and telomeres at mitosis is coordinated at multiple levels to prevent the formation of aneuploid cells, a phenotype frequently observed in cancer. Mitotic instability arises from chromosome segregation defects, giving rise to chromatin bridges at anaphase. Most of these defects are corrected before anaphase onset by a mechanism involving Aurora B kinase, a key regulator of mitosis in a wide range of organisms. Here, we describe a new role for Aurora B in telomere dispersion and disjunction during fission yeast mitosis. Telomere dispersion initiates in metaphase, whereas disjunction takes place in anaphase. Dispersion is promoted by the dissociation of Swi6/HP1 and cohesin Rad21 from telomeres, whereas disjunction occurs at anaphase after the phosphorylation of condensin subunit Cnd2. Strikingly, we demonstrate that deletion of Ccq1, a telomeric shelterin component, rescued cell death after Aurora inhibition by promoting the loading of condensin on chromosome arms. Our findings reveal an essential role for telomeres in chromosome arm segregation.

DNA repair pathway selection caused by defects in TEL1, SAE2, and de novo telomere addition generates specific chromosomal rearrangement signatures

Whole genome sequencing of cancer genomes has revealed a diversity of recurrent gross chromosomal rearrangements (GCRs) that are likely signatures of specific defects in DNA damage response pathways. However, inferring the underlying defects has been difficult due to insufficient information relating defects in DNA metabolism to GCR signatures. By analyzing over 95 mutant strains of Saccharomyces cerevisiae, we found that the frequency of GCRs that deleted an internal CAN1/URA3 cassette on chrV L while retaining a chrV L telomeric hph marker was significantly higher in tel1Δ, sae2Δ, rad53Δ sml1Δ, and mrc1Δ tof1Δ mutants. The hph-retaining GCRs isolated from tel1Δ mutants contained either an interstitial deletion dependent on non-homologous end-joining or an inverted duplication that appeared to be initiated from a double strand break (DSB) on chrV L followed by hairpin formation, copying of chrV L from the DSB toward the centromere, and homologous recombination to capture the hph-containing end of chrV L. In contrast, hph-containing GCRs from other mutants were primarily interstitial deletions (mrc1Δ tof1Δ) or inverted duplications (sae2Δ and rad53Δ sml1Δ). Mutants with impaired de novo telomere addition had increased frequencies of hph-containing GCRs, whereas mutants with increased de novo telomere addition had decreased frequencies of hph-containing GCRs. Both types of hph-retaining GCRs occurred in wild-type strains, suggesting that the increased frequencies of hph retention were due to the relative efficiencies of competing DNA repair pathways. Interestingly, the inverted duplications observed here resemble common GCRs in metastatic pancreatic cancer.

Recent advances in the sequencing of human cancer genomes have revealed that some types of genome rearrangements are more common in specific types of cancers. Thus, these cancers may share defects in DNA repair mechanisms, which may play roles in initiation or progression of the disease and may be useful therapeutically. Linking a common rearrangement signature to a specific genetic or epigenetic alteration is currently challenging, because we do not know which rearrangement signatures are linked to which DNA repair defects. Here we used a genetic assay in the model organism Saccharomyces cerevisiae to specifically link two classes of chromosomal rearrangements, interstitial deletions and inverted duplications, to specific genetic defects. These results begin to map out the links between observed chromosomal rearrangements and specific DNA repair defects and in the present case, may provide insights into the chromosomal rearrangements frequently observed in metastatic pancreatic cancer.

Folate deficiency induces dysfunctional long and short telomeres; both states are associated with hypomethylation and DNA damage in human WIL2-NS cells

The essential role of dietary micronutrients for genome stability is well documented, yet the effect of folate deficiency or excess on telomeres is not known. Accordingly, human WIL2-NS cells were maintained in medium containing 30, 300, or 3,000 nmol/L folic acid (FA) for 42 days to test the hypothesis that chronic folate deficiency would cause telomere shortening and dysfunction. After 14 days, telomere length (TL) in FA-deficient (30 nmol/L) cultures was 26% longer than that of 3,000 nmol/L FA cultures; however, this was followed by rapid telomere attrition over the subsequent 28 days (P trend, P < 0.0001); both long and short telomere status was positively correlated with biomarkers of chromosome instability (P ≤ 0.003) and mitotic dysfunction (P = 0.01), measured by the cytokinesis-block micronucleus cytome (CBMN-cyt) assay. The early increase in TL was associated with FA-deficiency-induced global DNA hypomethylation (P = 0.05), with an effect size similar to that induced by the DNA methyltransferase inhibitor, 5-aza-2'-deoxycytidine. Quantitative PCR analysis indicated a negative association between FA concentration and uracil incorporation into telomeric DNA (r = -0.47, P = 0.1), suggesting a possible plausible mechanism for uracil as a cause of folate deficiency-induced telomere dysfunction or deletion. Peptide nucleic acid-FISH (PNA-FISH) analysis showed that FA deficiency resulted in 60% of micronuclei containing acentric terminal fragments, an observation consistent with the 3-fold increase in terminal deletions (P = 0.0001). Together, these results demonstrate the impact of folate deficiency on biomarkers of telomere maintenance and integrity, and provide evidence that dysfunctional long telomeres may be as important as critically short telomeres as a cause of chromosomal instability.

Senescence evasion in melanoma progression: uncoupling of DNA-damage signaling from p53 activation and p21 expression

The best-established function of the melanoma-suppressor p16 is mediation of cell senescence, a permanent arrest following cell proliferation or certain stresses. The importance of p16 in melanoma suggests indolence of the other major senescence pathway through p53. Little or no p53 is expressed in senescent normal human melanocytes, but p16-deficient melanocytes can undergo p53-mediated senescence. As p16 expression occurs in nevi but falls with progression toward melanoma, we here investigated whether p53-dependent senescence occurs at some stage and, if not, what defects were detectable in this pathway, using immunohistochemistry. Phosphorylated checkpoint kinase 2 (CHEK2) can mediate DNA-damage signaling, and under some conditions senescence, by phosphorylating and activating p53. Remarkably, we detected no prevalent p53-mediated senescence in any of six classes of lesions. Two separate defects in p53 signaling appeared common: in nevi, lack of p53 phosphorylation by activated CHEK2, and in melanomas, defective p21 upregulation by p53 even when phosphorylated.

Folate deficiency is associated with the formation of complex nuclear anomalies in the cytokinesis-block micronucleus cytome assay

Chromosomal instability (CIN) is an important hallmark to oncogenesis and can be diagnosed morphologically by the presence of nuclear anomalies such as micronuclei (MN), nucleoplasmic bridges (NPBs), and nuclear buds (NBuds). We have identified additional nuclear anomalies formed under folate-deficient conditions, defined as "fused" nuclei (FUS), "circular" nuclei (CIR), and "horse-shoe" nuclei (HS) and investigated their suitability for inclusion as additional CIN biomarkers in the lymphocyte cytokinesis-block micronucleus cytome (CBMN-Cyt) assay. Although the morphological appearance of FUS, CIR, and HS suggested an origin from multiple NPB in the fusion region between the two nuclei, the very low frequency of dicentric chromosomes in metaphase spreads from these cultures did not support this model. Fluorescence in situ hybridization (FISH) analysis of cytokinesis-blocked binucleated (BN) cells with peptide nucleic acid probes for telomeres and centromeres (PNA-FISH) revealed a high proportion of fusion regions contained both centromeric and telomeric DNA. This suggests that folate deficiency may disrupt the process of sister chromatid separation and chromosome segregation during mitosis. It was concluded that the FUS, CIR, and HS morphologies represent promising biomarkers of CIN that are sensitive to folate deficiency, and further validation and investigation of the mechanisms responsible for their formation is warranted.

Malignancy without immortality? Cellular immortalization as a possible late event in melanoma progression

Cell senescence is a permanent growth arrest following extended proliferation. Cultured cancer cells including metastatic melanoma cells often appear immortal (proliferate indefinitely), while uncultured benign nevi (moles) show senescence markers. Here, with new explantation methods, we investigated which classes of primary pigmented lesions are typically immortal. Nevi yielded a few proliferating cells, consistent with most nevus cells being senescent. No nevus culture (0/28) appeared immortal. Some thin and thick melanoma cultures proved immortal under these conditions, but surprisingly few (4/37). All arrested cultures displayed three senescence markers in some cells: β-galactosidase, nuclear p16, and heterochromatic foci/aggregates. However, melanoma cultures also showed features of telomeric crisis (arrest because of ultrashort telomeres). Moreover, crisis markers including anaphase bridges were frequent in uncultured vertical growth-phase (VGP) melanomas. Conversely, all immortal melanoma cultures expressed telomerase reverse transcriptase and telomerase, showing aneuploidy. The findings suggest that primary melanomas are typically precrisis, with immortalization/telomere maintenance as a late event.

Alternative lengthening of telomeres--an enhanced chromosomal instability in aggressive non-MYCN amplified and telomere elongated neuroblastomas

Telomere length alterations are known to cause genomic instability and influence clinical course in several tumor types, but have been little investigated in neuroblastoma (NB), one of the most common childhood tumors. In the present study, telomere-dependent chromosomal instability and telomere length were determined in six NB cell lines and fifty tumor biopsies. The alternative lengthening of telomeres (ALT) pathway was assayed by scoring ALT-associated promyelocytic leukemia (PML) bodies (APBs). We found a reduced probability of overall survival for tumors with increased telomere length compared to cases with reduced or unchanged telomere length. In non-MYCN amplified tumors, a reduced or unchanged telomere length was associated with 100% overall survival. Tumor cells with increased telomere length had an elevated frequency of APBs, consistent with activation of the ALT pathway. The vast majority of tumor biopsies and cell lines exhibited an elevated rate of anaphase bridges, suggesting telomere-dependent chromosomal instability. This was more pronounced in tumors with increased telomere length. In cell lines, there was a close correlation between lack of telomere-protective TTAGGG-repeats, anaphase bridging, and remodeling of oncogene sequences. Thus, telomere-dependent chromosomal instability is highly prevalent in NB, and may contribute to the complexity of genomic alterations as well as therapy resistance in the absence of MYCN amplification and in this tumor type.

Whole chromosome loss is promoted by telomere dysfunction in primary cells

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.

Dietary reference values of individual micronutrients and nutriomes for genome damage prevention: current status and a road map to the future

Damage to the genome is recognized as a fundamental cause of developmental and degenerative diseases. Several micronutrients play an important role in protecting against DNA damage events generated through endogenous and exogenous factors by acting as cofactors or substrates for enzymes that detoxify genotoxins as well as enzymes involved in DNA repair, methylation, and synthesis. In addition, it is evident that either micronutrient deficiency or micronutrient excess can modify genome stability and that these effects may also depend on nutrient-nutrient and nutrient-gene interaction, which is affected by genotype. These observations have led to the emerging science of genome health nutrigenomics, which is based on the principle that DNA damage is a fundamental cause of disease that can be diagnosed and nutritionally prevented on an individual, genetic subgroup, or population basis. In this article, the following topics are discussed: 1) biomarkers used to study genome damage in humans and their validation, 2) evidence for the association of genome damage with developmental and degenerative disease, 3) current knowledge of micronutrients required for the maintenance of genome stability in humans, 4) the effect of nutrient-nutrient and nutrient-genotype interaction on DNA damage, and 5) strategies to determine dietary reference values of single micronutrients and micronutrient combinations (nutriomes) on the basis of DNA damage prevention. This article also identifies important knowledge gaps and future research directions required to shed light on these issues. The ultimate goal is to match the nutriome to the genome to optimize genome maintenance and to prevent pathologic amounts of DNA damage.

Diagnostic usefulness of fluorescent cytogenetics in differentiating chromophobe renal cell carcinoma from renal oncocytoma: a validation study combining metaphase and interphase analyses

We investigated the usefulness of interphase fluorescence in situ hybridization (FISH) analysis to differentiate between 11 chromophobe renal carcinomas and 12 renal oncocytomas, showing different clinical outcomes, when compared with conventional metaphase cytogenetics by karyotyping. Karyotypically, 3 chromophobe renal cell carcinomas showed losses of chromosomes, 3 were polyploid, 1 was normal, and 4 failed to grow. Of 12 oncocytomas, 5 showed a normal numeric karyotype and 6 additional structural rearrangements. FISH on chromophobe renal cell carcinomas showed a high percentage of cases (10/11 [91%]) with multiple numeric losses among chromosomes 1, 2, 6, 10, and 17; this interphase pattern was observed irrespective of the 3 different metaphase karyotypes. Of 12 oncocytomas, 11 (92%) revealed a normal numeric chromosomal status showing at least 2 chromosomes without aneusomy by interphase FISH. The study demonstrates that indeed FISH performed on formalin-fixed, paraffin-embedded tissue can provide clinically useful information more reliably than karyotyping of most of these tumors.

Cytogenetic analysis of 101 giant cell tumors of bone: nonrandom patterns of telomeric associations and other structural aberrations

Giant cell tumor of bone (GCTB) is a benign but locally aggressive tumor with metastatic potential. We performed cytogenetic analysis on 101 GCTB from 92 patients. Karyotypes were obtained from 95 tumors, 47 of which had clonal aberrations. The majority of the cytogenetically abnormal GCTB had multiple, up to 28 per tumor, clones. Clonal telomeric associations (tas) and other structural and numerical changes were found in about 70, 60, and 30%, respectively, of clonally abnormal tumors. Forty-seven aberrations were recurrent, of which 35 are novel. The vast majority of the recurrent aberrations were tas, confirming the important role of telomeric fusions in the development of GCTB. The frequency of tas in GCTB cultures increased with passaging, suggesting a selective advantage of tas-positive cells in vitro. The termini most frequently involved in tas were 22p, 13p, 15p, 21p, 14p, 19q, 1q, 12p, 11p, and 20q. The frequency of tas (irrespective of their clonality) was significantly higher in tumors carrying clonal changes, indicating that tas are precursors of other types of aberrations. In line with this assumption, the chromosomes preferentially involved in tas in a given tumor were also the ones most often affected by other rearrangements. We did not find the previously reported amplicon in 20q11.1, assessed by fluorescence in situ hybridization in 10 tumors. Nor did we find any association between cytogenetic features and adverse clinical outcome. Thus, local recurrences probably depend more on the adequacy of surgical treatment than on the intrinsic biology of the tumors.

Large liver cell change in hepatitis B virus-related liver cirrhosis

Large liver cell change (LLCC) refers to microscopic lesions often found in various chronic liver diseases; however, its nature is still controversial. Thirty-four formalin-fixed and 19 fresh frozen hepatitis B virus (HBV)-related cirrhosis samples were examined for the presence of LLCC, small liver cell change (SLCC), and hepatocellular carcinoma (HCC). The cell cycle checkpoint status (p21, p27, p16, Tp53), cell dynamics (proliferating cell nuclear antigen, Ki-67, terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick-end labeling, M30), DNA damage (gamma-H2AX [H2A histone family, member X]), telomere lengths, chromosomal instability (micronuclei index), and senescence-associated beta-galactosidase (SA-beta-Gal) activity were evaluated using an in situ approach and compared to those in normal liver (n = 5) and liver with chronic cholestasis (34 cases of hepatolithiasis and three cases of primary biliary cirrhosis). In HBV-related cirrhosis, the p21, p27, and p16 cell cycle checkpoint markers were activated in normal-looking cirrhotic hepatocytes (NLCH), but diminished gradually from LLCC, SLCC, to HCC, with an increase in Tp53 expression. There was a general decrease in telomere length from NLCH, LLCC, SLCC, to HCC. Micronuclei, gamma-H2AX foci, and net cellular gain were significantly increased from normal hepatocytes, NLCH, LLCC, SLCC, to HCC. The SA-beta-Gal activity was weaker in LLCC compared to NLCH and absent in SLCC and HCC. In contrast, cholestatic LLCC showed retained expression of cell cycle checkpoint markers and decreased net cellular gain compared to adjacent normal-looking hepatocytes. HBV-related LLCC showed significantly higher Tp53 labeling index, gamma-H2AX labeling index, and micronuclei index; shorter telomere length; decreased SA-beta-Gal activity; and increased net cellular gain compared to cholestatic LLCC.

CONCLUSION

The nature of LLCC is rather heterogeneous depending on the biological setting. The characteristics of HBV-related LLCC are more consistent with dysplastic rather than merely reactive hepatocytes, whereas cholestatic LLCC more likely represents reactive change with more stringent cell cycle checkpoint control.

Stabilization of dicentric translocations through secondary rearrangements mediated by multiple mechanisms in S. cerevisiae

Background

The gross chromosomal rearrangements (GCRs) observed in S. cerevisiae mutants with increased rates of accumulating GCRs include predicted dicentric GCRs such as translocations, chromosome fusions and isoduplications. These GCRs resemble the genome rearrangements found as mutations underlying inherited diseases as well as in the karyotypes of many cancers exhibiting ongoing genome instability

Methodology/Principal Findings

The structures of predicted dicentric GCRs were analyzed using multiple strategies including array-comparative genomic hybridization, pulse field gel electrophoresis, PCR amplification of predicted breakpoints and sequencing. The dicentric GCRs were found to be unstable and to have undergone secondary rearrangements to produce stable monocentric GCRs. The types of secondary rearrangements observed included: non-homologous end joining (NHEJ)-dependent intramolecular deletion of centromeres; chromosome breakage followed by NHEJ-mediated circularization or broken-end fusion to another chromosome telomere; and homologous recombination (HR)-dependent non-reciprocal translocations apparently mediated by break-induced replication. A number of these GCRs appeared to have undergone multiple bridge-fusion-breakage cycles. We also observed examples of chromosomes with extensive ongoing end decay in mec1 tlc1 mutants, suggesting that Mec1 protects chromosome ends from degradation and contributes to telomere maintenance by HR.

Conclusions/Significance

HR between repeated sequences resulting in secondary rearrangements was the most prevalent pathway for resolution of dicentric GCRs regardless of the structure of the initial dicentric GCR, although at least three other resolution mechanisms were observed. The resolution of dicentric GCRs to stable rearranged chromosomes could in part account for the complex karyotypes seen in some cancers.

Distinct patterns of structural and numerical chromosomal instability characterize sporadic ovarian cancer

Sporadic ovarian cancer is a particularly aggressive tumor characterized by highly abnormal karyotypes exhibiting many features of genomic instability. More complex genomic changes in tumors arise as a consequence of chromosomal instability (CIN), which can generate both numerical [(N)-CIN] and structural chromosomal instability [(S)-CIN]. In this study, molecular cytogenetic analysis was used to evaluate the relative levels of both (N)-CIN and (S)-CIN. Six tumors had a near-diploid chromosome number, two were near-tetraploid, and two were near-triploid. (N)-CIN levels increased as a function of overall tumor genomic content, with near-diploid tumors exhibiting numerical instability indices ranging from 7.0 to 21.0 and near-tetraploid and triploid tumors exhibiting instability indices ranging from 24.9 to 54.9. In contrast, the extent of (S)-CIN was generally more evident in the diploid tumors compared with the near-tetraploid tumors. To determine whether the associated chromosomal constitution and/or ploidy changes were influenced by mitotic segregation errors, centrosome analyses were performed on all 10 tumors. The near-diploid tumors, with the lowest numerical change, were observed to possess fewer cells with centrosome abnormalities (5.5% to 14.0%), whereas the near-tetraploid tumors possessed much higher levels of (N)-CIN and were characterized by a trend of elevating percentages of cells with abnormal centrosomes (16.0% to 20.5%). These observations suggest that two distinct processes governing genome stability may be disrupted in ovarian cancer: those that impact on numerical segregation and ploidy of chromosomes and those that affect the fidelity of DNA repair and lead to structural aberrations.

Apoptotic cell death, long-term persistence, and neuronal differentiation of aneuploid cells generated in the adult brain of teleost fish

Aneuploidy, caused by segregation defects during mitosis, has previously been identified in adult-born cells of mammals and teleosts. In the present study, we have examined the fate of these cells in the brain of the teleost fish Apteronotus leptorhynchus. By immunostaining against active caspase-3, we have shown that both cells with normal nuclear morphology and cells with mitotic segregation defects undergo apoptosis, but the relative number of apoptotic cells is higher among cells of the latter category. Long-term survival of cells with mitotic segregation defects could be demonstrated by incorporation of 5-bromo-2'-deoxyuridine into newly synthesized DNA during the S-phase of mitosis, and by employment of postadministration survival times of up to 860 days. Moreover, by combining 5-bromo-2'-deoxyuridine immunolabeling with immunostaining against the neuron-specific marker protein Hu, we have shown that among the long-term persistent cells with mitotic segregation defects a similar portion develops into neurons as does among the long-term persistent cells without such defects. It is possible that aneuploid cells play a role in the regulation of gene expression by somatic genomic alterations during postnatal development.

Adult neurogenesis and neuronal regeneration in the brain of teleost fish

Whereas adult neurogenesis appears to be a universal phenomenon in the vertebrate brain, enormous differences exist in neurogenic potential between "lower" and "higher" vertebrates. Studies in the gymnotiform fish Apteronotus leptorhynchus and in zebrafish have indicated that the relative number of new cells, as well as the number of neurogenic sites, are at least one, if not two, orders of magnitude larger in teleosts than in mammals. In teleosts, these neurogenic sites include brain regions homologous to the mammalian hippocampus and olfactory bulb, both of which have consistently exhibited neurogenesis in all species examined thus far. The source of the new cells in the teleostean brain are intrinsic stem cells that give rise to both glial cells and neurons. In several brain regions, the young cells migrate, guided by radial glial fibers, to specific target areas where they integrate into existing neural networks. Approximately half of the new cells survive for the rest of the fish's life, whereas the other half are eliminated through apoptotic cell death. A potential mechanism regulating development of the new cells is provided by somatic genomic alterations. The generation of new cells, together with elimination of damaged cells through apoptosis, also enables teleost fish rapid and efficient neuronal regeneration after brain injuries. Proteome analysis has identified a number of proteins potentially involved in the individual regenerative processes. Comparative analysis has suggested that differences between teleosts and mammals in the growth of muscles and sensory organs are key to explain the differences in adult neurogenesis that evolved during phylogenetic development of the two taxa.

Overexpression of Separase induces aneuploidy and mammary tumorigenesis

Separase is an endopeptidase that separates sister chromatids by cleaving cohesin Rad21 during the metaphase-to-anaphase transition. Conditional expression of Separase in tetracycline-inducible diploid FSK3 mouse mammary epithelial cells with both p53 WT and mutant (Ser-233-234) alleles of unknown physiological significance develops aneuploidy within 5 days of Separase induction in vitro. Overexpression of Separase induces premature separation of chromatids, lagging chromosomes, and anaphase bridges. In an in vivo mouse mammary transplant model, induction of Separase expression in the transplanted FSK3 cells for 3-4 weeks results in the formation of aneuploid tumors in the mammary gland. Xenograft studies combined with histological and cytogenetic analysis reveal that Separase-induced tumors are clonal in their genomic complements and have a mesenchymal phenotype suggestive of an epithelial-mesenchymal transition. Induction of Separase resulted in trisomies for chromosomes 8, 15, and 17; monosomy for chromosome 10; and amplification of the distal region of chromosomes 8 and 11. Separase protein is found to be significantly overexpressed in human breast tumors compared with matched normal tissue. These results collectively suggest that Separase is an oncogene, whose overexpression alone in mammary epithelial cells is sufficient to induce aneuploidy and tumorigenesis in a p53 mutant background.

Telomeric 3' overhangs in chronic HBV-related hepatitis and hepatocellular carcinoma

Telomeric 3' overhang is a key component of telomere structure, but little is known about its role in hepatocarcinogenesis. We examined the 3' overhang and telomere length, mRNA levels of hTERT, POT1, TRF1 and cytokeratin 19 (CK19) in 41 hepatitis B virus (HBV)-related hepatocellular carcinomas (HCCs) and adjacent non-HCCs of B viral chronic hepatitis/cirrhosis. 3' overhang length was positively correlated with telomere length (p < 0.001). In non-HCCs, the 3' overhang shortened with increasing age (p = 0.043). Twenty-six HCCs had shorter and 15 HCCs had longer 3' overhangs than the adjacent non-HCCs. The mRNA levels of hTERT, POT1 and TRF1 were upregulated in HCCs than in non-HCCs. HCCs with lengthened 3' overhangs expressed higher hTERT mRNA levels than those with shortened 3' overhangs, when compared to 3' overhangs in non-HCCs (p = 0.044). POT1 and TRF1 showed no significant difference according to the 3' overhangs. HCCs with long 3' overhangs had higher mitosis (p = 0.046) and more frequent multipolar mitosis compared to those with short 3' overhangs (p = 0.034). HCCs with high cytokeratin 19 mRNA levels, a marker for hepatic progenitor cells, had longer 3' overhangs than HCCs with low cytokeratin 19 mRNA levels (p= 0.019). In conclusion, the 3' overhang erosion might be closely related to the number of cell divisions in telomerase-negative hepatocytes of chronic hepatitis/cirrhosis. In telomerase-positive HCCs, an altered 3' overhang are involved in HBV-related hepatocarcinogenesis and hTERT might be involved in regulation of 3' overhang.

Genome-health nutrigenomics and nutrigenetics: nutritional requirements or ‘nutriomes’ for chromosomal stability and telomere maintenance at the individual level

It is becoming increasingly evident that (a) risk for developmental and degenerative disease increases with more DNA damage, which in turn is dependent on nutritional status, and (b) the optimal concentration of micronutrients for prevention of genome damage is also dependent on genetic polymorphisms that alter the function of genes involved directly or indirectly in the uptake and metabolism of micronutrients required for DNA repair and DNA replication. The development of dietary patterns, functional foods and supplements that are designed to improve genome-health maintenance in individuals with specific genetic backgrounds may provide an important contribution to an optimum health strategy based on the diagnosis and individualised nutritional prevention of genome damage, i.e. genome health clinics. The present review summarises some of the recent knowledge relating to micronutrients that are associated with chromosomal stability and provides some initial insights into the likely nutritional factors that may be expected to have an impact on the maintenance of telomeres. It is evident that developing effective strategies for defining nutrient doses and combinations or ‘nutriomes’ for genome-health maintenance at the individual level is essential for further progress in this research field.

High telomerase activity and long telomeres in advanced hepatocellular carcinomas with poor prognosis

Telomerase reactivation and telomere maintenance are crucial in carcinogenesis and tumor progression. In this study, the relationships between telomere parameters, chromosomal instability and clinicopathological features were evaluated in hepatocellular carcinomas (HCCs). Telomere length (TL), telomerase activity (TA) and human telomerase reverse transcriptase (hTERT) mRNA levels were measured in 49 hepatitis B virus (HBV)-related HCCs and corresponding non-tumorous tissues. The results were compared with clinicopathological data, including differentiation, multipolar mitosis (MM), anaphase bridge, immunohistochemical stain results for cytokeratin 19 (CK19) and patient outcome. TL of HCCs ranged from 4.7 to 13.1 kb, and 44.4% of HCCs showed telomere lengthening. hTERT mRNA levels and TA were closely related (P=0.008), and were significantly higher in HCCs than non-tumorous tissues. TL was significantly higher in HCCs with strong TA (P=0.048), high hTERT mRNA levels (P=0.001) and poor differentiation (P=0.041). Frequent MM was associated with poor differentiation (P=0.007) and advanced stage (P<0.001). TA was positively correlated with MM, anaphase bridges and advanced stage (P=0.019, P=0.017 and P=0.029). Thirteen (28.3%) HCCs were CK19+ and demonstrated longer telomeres than CK19- HCCs (P=0.046). Overall survival was poor in HCCs with MM >0.4 per field (P=0.016), high TA (P=0.009) and high TL ratio (HCC/non-HCC) >0.8 (P=0.044). Our results show that long telomeres, high TA and high mitotic instability are poor prognostic markers for HBV-related HCCs and their close association suggests that telomere maintenance may be important for the progression of HCCs with high chromosomal instability to more aggressive ones.

Stable inheritance of host species-derived microchromosomes in the gynogenetic fish Poecilia formosa

B chromosomes are additional, usually unstable constituents of the genome of many organisms. Their origin, however, is often unclear and their evolutionary relevance is not well understood. They may range from being deleterious to neutral or even beneficial. We have followed the genetic fate of B chromosomes in the asexual, all-female fish Poecilia formosa over eight generations. In this species, B chromosomes come in the form of one to three tiny microchromosomes derived from males of the host species that serve as sperm donors for this gynogenetic species. All microchromosomes have centromeric heterochromatin but usually only one has a telomere. Such microchromosomes are stably inherited, while the telomereless are prone to be lost in both the soma and germline. In some cases the stable microchromosome carries a functional gene lending support to the hypothesis that the B chromosomes in P. formosa could increase the genetic diversity of the clonal lineage in this ameiotic organism and to some degree counteract the genomic decay that is supposed to be connected with the lack of recombination.

Telomere instability and cancer

Telomeres are required to preserve genome integrity, chromosome stability, nuclear architecture and chromosome pairing during meiosis. Given that telomerase activity is limiting or absent in most somatic tissues, shortening of telomeres during development and aging is the rule. In vitro, telomere length operates as a mechanism to prevent uncontrolled cell growth and therefore defines the proliferation potential of a cell. In vitro, in somatic cells that have lost proliferation control, shortening of telomeres becomes the main source of genome instability leading to genetic or epigenetic changes that may allow cells to become immortal and to acquire tumor phenotypes. In vivo, mice models have indisputably shown both the protective and the promoting role of very short telomeres in cancer development. In humans, although telomere shortening and other types of telomere dysfunction probably contribute to the genome instability often detected in tumors, the specific contributions of such instability to the development of cancer remain largely undetermined.

SKY analysis of childhood neural tumors and cell lines demonstrates a susceptibility of aberrant chromosomes to further rearrangements

Malignant solid tumors are commonly characterized by a large number of complex structural and numerical chromosomal alterations, which often reflect the level of genomic instability and can be associated with disease progression. The aim of this study was to evaluate whether chromosomes that harbor primary aberrations have a higher susceptibility to accumulate further alterations. We used spectral karyotyping (SKY), to compare the individual chromosomal instability of two chromosome types: chromosomes that have a primary aberration and chromosomes without an aberration, in 13 primary childhood neural tumors and seven cell lines. We found that chromosomes that contain a primary aberration are significantly (p-value<0.001) more likely to gain further structural rearrangements or to undergo numerical changes (22.6%, 36 of 159 chromosomes) than chromosomes with no initial aberration (4.9%, 54 of 1099 chromosomes). These results are highly suggestive that aberrant chromosomes in solid tumors have a higher susceptibility to accumulate further rearrangements than "normal" chromosomes.

Numerical chromosome variation and mitotic segregation defects in the adult brain of teleost fish

Teleost fish are distinguished by their enormous potential for the generation of new cells in both the intact and the injured adult brain. Here, we present evidence that these cells are a genetic mosaic caused by somatic genomic alteration. Metaphase chromosome spreads from whole brains of the teleost Apteronotus leptorhynchus revealed an euploid complement of 22 chromosomes in only 22% of the cells examined. The rate of aneuploidy is substantially higher in brain cells than in liver cells, as shown by both metaphase chromosome spreads and flow cytometric analysis. Among the aneuploid cells in the brain, approximately 84% had fewer, and the remaining 16% more, than 22 chromosomes. Typically, multiple chromosomes were lost or gained. The aneuploidy is putatively caused by segregation defects during mitotic division. Labeling of condensed chromosomes of M-phase cells by phosphorylated histone-H3 revealed laggards, anaphase bridges, and micronuclei, all three of which indicate displaced mitotic chromosomes. Quantitative analysis has shown that in the entire brain on average 14% of all phosphorylated histone-H3-labeled cells exhibit such signs of segregation defects. Together with the recent discovery of aneuploidy in the adult mammalian brain, the results of the present investigation suggest that the loss or gain of chromosomes might provide a mechanism to regulate gene expression during development of new cells in the adult vertebrate brain.

Genomic mechanisms and measurement of structural and numerical instability in cancer cells

The progression to cancer is often associated with instability and the acquisition of genomic heterogeneity, generating both clonal and non-clonal populations. Chromosomal instability (CIN) describes the excessive rate of numerical and structural genomic change in tumors. Mitotic segregation errors strongly influences copy number, while structural aberrations can occur at unstable genomic regions, or through aberrant DNA repair or methylation. Combined molecular cytogenetic analyses can evaluate cell-to-cell variation, and define the complexity of numerical and structural alterations. Because structural change may occur independently of numerical alteration, we propose the term structural chromosomal instability [(S)-CIN] to distinguish numerical from structural CIN.

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.

HTLV-1 propels untransformed CD4 lymphocytes into the cell cycle while protecting CD8 cells from death

Human T cell leukemia virus type 1 (HTLV-1) infects both CD4+ and CD8+ lymphocytes, yet it induces adult T cell leukemia/lymphoma (ATLL) that is regularly of the CD4+ phenotype. Here we show that in vivo infected CD4+ and CD8+ T cells displayed similar patterns of clonal expansion in carriers without malignancy. Cloned infected cells from individuals without malignancy had a dramatic increase in spontaneous proliferation, which predominated in CD8+ lymphocytes and depended on the amount of tax mRNA. In fact, the clonal expansion of HTLV-1-positive CD8+ and CD4+ lymphocytes relied on 2 distinct mechanisms--infection prevented cell death in the former while recruiting the latter into the cell cycle. Cell cycling, but not apoptosis, depended on the level of viral-encoded tax expression. Infected tax-expressing CD4+ lymphocytes accumulated cellular defects characteristic of genetic instability. Therefore, HTLV-1 infection establishes a preleukemic phenotype that is restricted to CD4+ infected clones.

Telomere dysfunction and loss of p53 cooperate in defective mitotic segregation of chromosomes in cancer cells

Aneuploidy is a fundamental principle of many cancer cells and is mostly related to defects in mitotic segregation of chromosomes. Many solid tumors as well as some preneoplastic lesions have been shown to contain polyploid chromosome numbers. The exact mechanisms behind whole-genome duplications are not known but have been linked to compromised mitotic checkpoint genes. We now report that the telomere checkpoint plays a key role for polyploidy in colon cancer cells. Telomerase suppression by a dominant-negative mutant of hTERT and consecutive telomere dysfunction in wild-type HCT116 colon cancer cells resulted in only minor stable chromosomal alterations. However, higher ploidy levels with up to 350 chromosomes were found when the cell-cycle checkpoint proteins p53 or p21 were absent. These findings indicate that telomere dysfunction in the absence of cell-cycle control may explain the high frequency of alterations in chromosome numbers found in many solid tumors.

Telomere dysfunction: a new player in radiation sensitivity

Human individuals often exhibit important differences in their sensitivity to ionising radiation. Extensive literature links radiation sensitivity with impaired DNA repair which is due to a lack of correct functioning in many proteins involved in DNA-repair pathways and/or in DNA-damage checkpoint responses. Given that ionising radiation is an important and widespread diagnostic and therapeutic tool, it is important to investigate further those factors and mechanisms that underlie individual radiosensitivity. Recently, evidence is accumulating that telomere function may well be involved in cellular and organism responses to ionising radiation, broadening still further the currently complex and challenging scenario.