Telomere length, chromatin structure and chromosome fusigenic potential

Chromosomal Rearrangements and Origin of the Multiple XX/XY1Y2 Sex Chromosome System in Harttia Species (Siluriformes: Loricariidae)

The Neotropical genus Harttia comprises species with extensive chromosomal remodeling and distinct sex chromosome systems (SCSs). So far, three different SCSs with male heterogamety have been characterized in the group. In some species, the presence of the XX/XY₁Y₂ SCS is associated with a decrease in diploid numbers and several chromosomal rearrangements, although a direct relation to sex chromosome differentiation has not been shown yet. Here, we aimed to investigate the differentiation processes that have led to the establishment of the rare XX/XY₁Y₂ SCS and track its evolutionary history among other Harttia species. For that, four whole chromosome painting probes derived from chromosome 1 of H. torrenticola (HTO-1), chromosomes 9 and X of H. carvalhoi (HCA-9 and HCA-X), and chromosome X from H. intermontana (HIN-X) were applied in nine Harttia species. Homeologous chromosome blocks were located in Harttia species and demonstrated that Robertsonian (Rb) fusions originated HTO-1, HCA-9, and HCA-X chromosomes, while Rb fissions explain Y₁ and Y₂ sex chromosomes. Specifically, in H. intermontana, HCA-X, HCA-9, and the NOR-bearing chromosome demonstrated that homeologous blocks were used in the HIN-X and metacentric pair 2 origins. Consequently, diploid numbers changed between the studied species. Overall, the data also reinforce the existence of unstable genomic sites promoting chromosomal differentiation and remodeling within the genus Harttia.

Role of Xeroderma pigmentosum D (XPD) protein in genome maintenance in human cells under oxidative stress

Xeroderma pigmentosum D (XPD) protein plays a pivotal role in the nucleotide excision repair pathway. XPD unwinds the local area of the damaged DNA by virtue of constituting transcription factor II H (TFIIH) and is important not only for repair but also for basal transcription. Although cells deficient in XPD have shown to be defective in oxidative base-lesion repair, the effects of the oxidative assault on primary fibroblasts from patients suffering from Xeroderma Pigmentosum D have not been fully explored. Therefore, we sought to investigate the role of XPD in oxidative DNA damage-repair by treating primary fibroblasts derived from a patient suffering from Xeroderma Pigmentosum D, with hydrogen peroxide. Our results show dose-dependent increase in genotoxicity with minimal effect on cytotoxicity with H₂O₂ in XPD deficient cells compared to control cells. XPD deficient cells displayed increased susceptibility and reduced repair capacity when subjected to DNA damage induced by oxidative stress. XPD deficient fibroblasts exhibited increased telomeric loss after H₂O₂ treatment. In addition, we demonstrated that chronic oxidative stress induced accelerated premature senescence characteristics. Gene expression profiling revealed alterations in genes involved in transcription and nucleotide metabolisms, as well as in cellular and cell cycle processes in a more significant way than in other pathways. This study highlights the role of XPD in the repair of oxidative stress and telomere maintenance. Lack of functional XPD seems to increase the susceptibility of oxidative stress-induced genotoxicity while retaining cell viability posing as a potential cancer risk factor of Xeroderma Pigmentosum D patients.

Highly Rearranged Karyotypes and Multiple Sex Chromosome Systems in Armored Catfishes from the Genus Harttia (Teleostei, Siluriformes)

Harttia comprises an armored catfish genus endemic to the Neotropical region, including 27 valid species with low dispersion rates that are restricted to small distribution areas. Cytogenetics data point to a wide chromosomal diversity in this genus due to changes that occurred in isolated populations, with chromosomal fusions and fissions explaining the 2n number variation. In addition, different multiple sex chromosome systems and rDNA loci location are also found in some species. However, several Harttia species and populations remain to be investigated. In this study, Harttia intermontana and two still undescribed species, morphologically identified as Harttia sp. 1 and Harttia sp. 2, were cytogenetically analyzed. Harttia intermontana has 2n = 52 and 2n = 53 chromosomes, while Harttia sp. 1 has 2n = 56 and 2n = 57 chromosomes in females and males, respectively, thus highlighting the occurrence of an XX/XY₁Y₂ multiple sex chromosome system in both species. Harttia sp. 2 presents 2n = 62 chromosomes for both females and males, with fission events explaining its karyotype diversification. Chromosomal locations of the rDNA sites were also quite different among species, reinforcing that extensive rearrangements had occurred in their karyotype evolution. Comparative genomic hybridization (CGH) experiments among some Harttia species evidenced a shared content of the XY₁Y₂ sex chromosomes in three of them, thus pointing towards their common origin. Therefore, the comparative analysis among all Harttia species cytogenetically studied thus far allowed us to provide an evolutionary scenario related to the speciation process of this fish group.

Physical mapping of repetitive DNA suggests 2n reduction in Amazon turtles Podocnemis (Testudines: Podocnemididae)

Cytogenetic studies show that there is great karyotypic diversity in order Testudines (2n = 26-68), and that this may be mainly attributed to the presence/absence of microchromosomes. Members of the Podocnemididae family have the smallest diploid numbers of this order (2n = 26-28), which may be a derived condition of the group. Diverse studies suggest that repetitive-DNA-rich sites generally act as hotspots for double-strand breaks and chromosomal reorganization. In this context, we used fluorescent in situ hybridization (FISH) to map telomeric sequences (TTAGGG)n, 45S rDNA, and the genes encoding histones H1 and H3 in two species of genus Podocnemis. We also observed conservation of the 45S rDNA and H1 histone sequences (probable case of conserved synteny), but multiple conserved and non-conserved clusters of H3 genes, which colocalized with the interstitial telomeric sequences in the Podocnemis genome. Our results suggest that fusions have occurred between macro and microchromosomes or between microchromosomes, leading to the observed reduction in diploid number in the family Podocnemididae.

An intact putative mouse telomerase essential N-terminal domain is necessary for proper telomere maintenance

BACKGROUND INFORMATION

Naturally occurring telomerase reverse transcriptase (TERT) isoforms may regulate telomerase activity, and possibly function independently of telomeres to modulate embryonic stem (ES) cell self-renewal and differentiation.

RESULTS

We report the characterisation of two novel mouse TERT (mTERT) splice variants, Ins-i1[1-102] (Insi1 for short) and Del-e12[1-40] (Dele12 for short) that have not been previously described. Insi1 represents an in-frame insertion of nucleotides 1-102 from intron 1, encoding a 34 amino acid insertion at amino acid 73. Based on known functions of this region in human and Tetrahymena TERTs, the insertion interrupts the RNA interaction domain 1 implicated in low-affinity RNA binding and the telomerase essential N-terminal domain implicated in DNA substrate interactions. Dele12 contains a 40 nucleotide deletion of exon 12 which generates a premature stop codon, and possible protein lacking the C-terminus. We found Insi1 expressed in adult mouse brain and kidney and Dele12 expressed in adult mouse ovary. Dele12 was inactive in vitro and in mTERT(-/-) ES cells and Insi1 retained 26-48% of telomerase activity reconstituted by wild-type mTERT in vitro and in mTERT(-/-) ES cells. The Insi1 variant exhibited reduced DNA substrate binding in vitro and both variants exhibited a reduction in binding the telomerase RNA, mTR, when expressed in mTERT(-/-) ES cells. Stable expression of Dele12 in the mouse fibroblast CB17 cell line inhibited telomerase activity and slowed cell growth, suggesting a potential dominant-negative effect. Levels of signal-free ends, representing short telomeres, and end-to-end fusions were higher in mTERT(-/-) ES cells expressing mTERT-Insi1 and mTERT-Dele12, compared with levels observed in mTERT(-/-) ES cells expressing wild-type mTERT. In addition, in mTERT(-/-) cells expressing mTERT-Insi1, we observed chromosomes that were products of repeated breakage-bridge-fusion cycles and other telomere dysfunction-related aberrations.

CONCLUSION AND SIGNIFICANCE

An intact mTERT N-terminus which contributes to mTR binding, DNA binding and telomerase activity is necessary for elongation of short telomeres and the maintenance of functional telomeres. It is reasonable to speculate that relative levels of mTERT-Insi1 may regulate telomere function in specific tissues.

Chromosomal mapping of repetitive DNAs in Gobionellus oceanicus and G. stomatus (Gobiidae; Perciformes): A shared XX/XY system and an unusual distribution of 5S rDNA sites on the Y chromosome

With nearly 2,000 species, Gobiidae is the most specious family of the vertebrates. This high level of speciation is accompanied by conspicuous karyotypic modifications, where the role of repetitive sequences remains largely unknown. This study analyzed the karyotype of 2 species of the genus Gobionellus and mapped 18S and 5S ribosomal RNA genes and (CA)15 microsatellite sequences onto their chromosomes. G. oceanicus (2n = 56; ♂ 12 metacentrics (m) + 4 submetacentrics (sm) + 1 subtelocentric (st) + 39 acrocentrics (a); ♀ 12m + 4sm + 2st + 38a) and G. stomatus (2n = 56; ♂ 20m + 14sm + 1st + 21a; ♀ 20m + 14sm + 2st + 20a) possess the highest diploid chromosome number among the Gobiidae and have different karyotypes. Both species share an XX/XY sex chromosome system with a large subtelocentric X and a small acrocentric Y chromosome which is rich in (CA)15 sequences and bears 5S rRNA sites. Although coding and noncoding repetitive DNA sequences may be involved in the genesis or differentiation of the sex chromosomes, the exclusive presence of 5S rDNA sites on the Y, but not on the X chromosome of both species, represents a novelty in fishes. In summary, the karyotypic differences, as well as new data on the sex chromosome systems in these 2 Gobiidae species, confirm the high chromosomal dynamism observed in this family.

Nucleoplasmic bridges and acrocentric chromosome associations as early markers of exposure to low levels of ionising radiation in occupationally exposed hospital workers

Ionising radiation, with the contribution of telomere shortening, induces DNA double-strand breaks that result in chromosome end fusion, nucleoplasmic bridges (NPBs) and chromosome aberrations (ChAbs) as well as dicentric chromosomes. In order to investigate the chromosomal damage induced by occupational ionising radiation at low exposure levels, and to find early markers of health hazard, peripheral lymphocytes of occupationally exposed hospital workers were cytogenetically analysed. Results showed a significant difference in the frequency of ChAbs in exposed subjects relative to controls. A significant number of NPBs between nuclei of binucleated cultured lymphocytes from exposed subjects were also observed, as well as a consistent amount of acrocentric chromosomes with associations of their short arms. Excluding confounding factors, the frequencies of all these three biological endpoints differed significantly in exposed subjects from those in controls. Because the absence of telomeres and/or their short length could be a common root for both the findings, we utilised fluorescence in situ hybridisation technique with telomeric repeat as probe to demonstrate that, in exposed subjects, chromatin of short arms of involved acrocentric chromosomes did not exhibit a telomeric shortening but appeared strongly decondensed. This finding suggests that NPBs and telomeric acrocentric association should be regarded as early markers of exposure to low levels of ionising radiation and their increase should be seen as an early warning for the health of the involved workers.

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.

The first cytogenetic description of Euleptes europaea (Gené, 1839) from Northern Sardinia reveals the highest diploid chromosome number among sphaerodactylid geckos (Sphaerodactylidae, Squamata)

The karyotype of a sphaerodactylid gecko Euleptes europaea (Gené, 1839) was assembled for the first time in this species. It is made of 2n = 42 gradually decreasing in size chromosomes, the highest chromosome number so far acknowledged in the family Sphaerodactylidae. The second chromosome pair of the karyotype appears slightly heteromorphic in the male individual. Accordingly, FISH with a telomeric probe revealed an uneven distribution of telomeric repeats on the two homologues of this pair, which may be indicative of an XY sex-determination system in the species, to be further investigated.

Different patterns of Robertsonian fusion pairing in Bovidae and the house mouse: the relationship between chromosome size and nuclear territories

Using a dataset of karyotypic changes reported for bovids and the house mouse (Mus musculus domesticus) together with information from the cattle (Bos taurus) and mouse genomes, we examined two principal variables that have been proposed to predict chromosomal positioning in the nucleus, chromosome size and GC content. These were expected to influence the distribution of Robertsonian (Rb) fusions, the predominant mode of chromosomal change in both taxa. We found the largest chromosomes to be most frequently involved in fusions in bovids, and confirm earlier reports that chromosomes of intermediate size were the most frequent fusers in mice. We then tested whether chromosomal positioning can explain Rb fusion frequencies. We classified chromosomes into groups by size and considered the frequency of interactions between specific groups. Among the interactions, mouse chromosomes showed a slight tendency to fuse with neighbouring chromosomes, in line with expectations of chromosomal positioning, but also resembling predictions from meiotic spindle-induced bias. Bovids, on the other hand, showed no trend in interactions, with small chromosomes being the least frequent partner for all size classes. We discuss the results in terms of nuclear organization at various cell cycle stages and the proposed mechanisms of Rb fusion formation, and note that the difference can be explained by (i) considering bovid species generally to be characterized by a greater intermingling of chromosomal size classes than the house mouse, or (ii) by the vastly different timescales underpinning their evolutionary histories.

Comparative cytogenetics and heterochromatic patterns in two species of the genus Acanthostracion (Ostraciidae: Tetraodontiformes)

Some groups of fish, such as those belonging to the Order Tetraodontiformes, may differ significantly in the amount and location of heterochromatin in the chromosomes. There is a marked variation in DNA content of more than seven-fold among the families of this Order. However, the karyoevolutionary mechanisms responsible for this variation are essentially unknown. The largest genomic contents are present in species of the family Ostraciidae (2.20-2.60pg). The present study cytogenetically characterized two species of the family Ostraciidae, Acanthostracion polygonius and A. quadricornis, using conventional staining, C-bandings, Ag-NOR, CMA(3)/DAPI, AluI, PstI, EcoRI, TaqI and HinfI restriction enzymes (REs) and double FISH with 18S and 5S rDNA probes. The karyotypes of both species showed 2n=52 acrocentric chromosomes (FN=52; chromosome arms) and pronounced conserved structural characteristics. A significant heterochromatic content was observed equilocally distributed in pericentromeric position in all the chromosome pairs. This condition is unusual in relation to the karyotypes of other families of Tetraodontiformes and probability is the cause of the higher DNA content in Ostraciidae. Given the role played by repetitive sequences in the genomic reorganization of this Order, it is suggested that the conspicuous heterochromatic blocks, present in the same chromosomal position and with apparently similar composition, may have arisen or undergo evolutionary changes in concert providing clues about the chromosomal mechanisms which led to extensive variation in genomic content of different Tetraodontiformes families.

Chromosomal aberrations involving telomeres and interstitial telomeric sequences

Telomeres are specialised nucleoproteic complexes localised at the physical ends of linear eukaryotic chromosomes that maintain their stability and integrity. In vertebrate chromosomes, the DNA component of telomeres is constituted by (TTAGGG)n repeats, which can be localised at the terminal regions of chromosomes (true telomeres) or at intrachromosomal sites (interstitial telomeric sequences or ITSs, located at the centromeric region or between the centromere and the telomere). In the past two decades, the use of molecular cytogenetic techniques has led to a new spectrum of spontaneous and clastogen-induced chromosomal aberrations being identified, involving telomeres and ITSs. Some aberrations involve the chromosome ends and, indirectly, the telomeric repeats located at the terminal regions of chromosomes (true telomeres). A second type of aberrations directly involves the telomeric sequences located at the chromosome ends. Finally, there is a third class of aberrations that specifically involves the ITSs. The aims of this review are to provide a detailed description of these aberrations and to summarise the available data regarding their induction by physical and chemical mutagens.

Diet, nutrition and telomere length

The ends of human chromosomes are protected by DNA-protein complexes termed telomeres, which prevent the chromosomes from fusing with each other and from being recognized as a double-strand break by DNA repair proteins. Due to the incomplete replication of linear chromosomes by DNA polymerase, telomeric DNA shortens with repeated cell divisions until the telomeres reach a critical length, at which point the cells enter senescence. Telomere length is an indicator of biological aging, and dysfunction of telomeres is linked to age-related pathologies like cardiovascular disease, Parkinson disease, Alzheimer disease and cancer. Telomere length has been shown to be positively associated with nutritional status in human and animal studies. Various nutrients influence telomere length potentially through mechanisms that reflect their role in cellular functions including inflammation, oxidative stress, DNA integrity, DNA methylation and activity of telomerase, the enzyme that adds the telomeric repeats to the ends of the newly synthesized DNA.

Telomere attrition and genomic instability in xeroderma pigmentosum type-b deficient fibroblasts under oxidative stress

Xeroderma pigmentosum B (XPB/ERCC3/p89) is an ATP-dependent 3′→5′ directed DNA helicase involved in basal RNA transcription and the nucleotide excision repair (NER) pathway. While the role of NER in alleviating oxidative DNA damage has been acknowledged it remains poorly understood. To study the involvement of XPB in repair of oxidative DNA damage, we utilized primary fibroblasts from a patient suffering from XP with Cockayne syndrome and hydrogen peroxide (H₂O₂) to induce oxidative stress. Mutant cells retained higher viability and cell cycle dysfunction after H₂O₂ exposure. Cytokinesis blocked micronucleus assay revealed increased genome instability induced by H₂O₂. Single cell gel electrophoresis (comet) assay showed that the missense mutation caused a reduced repair capacity for oxidative DNA damage. Mutant fibroblasts also displayed decreased population doubling rate, increased telomere attrition rate and early emergence of senescent characteristics under chronic low dose exposure to H₂O₂. Fibroblasts from a heterozygous individual displayed intermediate traits in some assays and normal traits in others, indicating possible copy number dependence. The results show that a deficiency in functional XPB paradoxically renders cells more sensitive to the genotoxic effects of oxidative stress while reducing the cytotoxic effects. These findings have implications in the mechanisms of DNA repair, mutagenesis and carcinogenesis and ageing in normal physiological systems.

Growth defects in mouse telomerase RNA-deficient cells expressing a template-mutated mouse telomerase RNA

Cellular viability requires telomere maintenance, which, in mammals, is mainly mediated by the reverse transcriptase telomerase. Telomerase core components are a catalytic subunit TERT and an RNA subunit TR (hTR in humans, mTR in mouse) that carries the template to generate telomeres de novo. Telomere dysfunction can lead to senescence or apoptosis and impairs the continued growth of immortal cancerous cell lines. The introduction of a template-mutated hTR in telomerase-positive and telomerase-negative human cell lines results in dramatic growth defects. No study has addressed the consequences of expressing a template-mutated mTR in mouse immortal cell lines. Therefore, we analyzed the effects of long-term expression of a template-mutated mTR in the telomerase-positive and telomerase-negative murine cell lines CB17 and DKO301, respectively. Whereas the CB17 clones expressing the template-mutated mTR did not demonstrate any growth impairment, many of the DKO301 clones expressing the template-mutated mTR underwent growth and cell cycle defects and eventual cell death. These results suggest that in the absence of wild-type telomerase, the expression of the template-mutated mTR likely perturbs telomere function, leading to decreased cellular viability. Furthermore, whereas the expression of template-mutated hTR in telomerase-negative human cell lines leads to immediate cellular toxicity, the expression of the template-mutated mTR in the telomerase-negative mouse cell line did not.

Tissue culture triggers chromosome alterations, amplification, and transposition of repeat sequences in Allium fistulosum

Structural alterations in nuclei and chromosomes of cells derived from callus culture of Allium fistulosum have been studied with fluorescent in situ hybridization (FISH) using 5S ribosomal DNA (rDNA), 45S rDNA, and 375-bp repeat probes. A high frequency of chromosome abnormalities was found to be caused by the loss of telomere-located 375-bp repeats, chromosome fusion, and subsequent breakage-fusion-bridge cycles. Products of chromosome fusions and monocentric and regularly shaped chromosomes showed additional 375-bp repeat and 45S rDNA clusters at unusual sites, suggesting dynamic copy-number changes and transposition of these repeats. Southern hybridization revealed no differences in the 375-bp repeat and 45S rDNA repeat array order or the degree of methylation between DNA isolated from leaves or tissue-culture cells. In addition, protruding, spike-like structures positive for 375-bp repeats were identified on the surface of different-sized nuclei. Transmission electron microscopy analysis revealed the accumulation of densely packed chromatin within spike-like structures. Because root calyptra cells showed similar structures, it is likely that heterochromatic spike-like structures are a feature of nondividing cells at the onset of programmed cell death.

Robertsonian translocation as a result of telomere shortening during replicative senescence and immortalization of bovine oviduct epithelial cells

We investigated chromosome (Chr) aberrations in the process of replicative senescence and immortalization of cultured bovine oviduct epithelial cells (BOEC) before and after transfecting vectors SV40 large T or human telomerase reverse transcriptase (hTERT). We found that a gradual increase in the number of metacentric chromosomes occurred during replicative senescence but not immortalization of BOEC. The accumulation of metacentric chromosomes was concomitant with decreases in the number of acrocentric autosomes, strongly suggesting that Robertsonian (Rb) translocation frequently occurred in cultured BOEC. The process was also correlated with an accumulation of extremely shortened telomeres (<4 kb). The maximum number of metacentric chromosomes reached a plateau (8.75 +/- 0.53) in the senescent BOEC (approximately 48 population doublings), and the value was stably maintained in all immortalized lines. These results suggest that not all autosomes may be involved in Rb translocation. Fluorescence in situ hybridization analysis using probes specific for Chr1, Chr29, telomeres, and x-chromosomes of bovine confirmed the presence of t(1;29) with other unidentified fused chromosomes. There was no evidence for duplication of sex chromosomes. Because no detectable fluorescence in situ hybridization signals at the centromere for telomeres were indicative of no direct integration of telomere sequences in the Rb translocated chromosomes, these results raise a possibility that Rb translocation between certain autosomes of bovine cells is partly but critically dependent upon a physical state of telomere attrition. The cells and cell lines established in this study could provide a promising system for further studies on the mechanisms of chromosomal translocation because of centromeric fusion in bovine cells.

Polysomy 13 in a canine prostate carcinoma underlining its significance in the development of prostate cancer

The dog is a well-accepted model for prostate cancer in man because of the striking similarities between both species with respect to the clinical course of the disease as well as to its similar histopathology. Cytogenetic investigations of human prostate cancers has revealed the frequent occurrence of trisomies 7, 8, and 17. In this report, we present a case of prostate carcinoma in a dog characterized by polysomy 13 as the sole cytogenetic abnormality. Along with the known homology between canine chromosome 13 and human chromosome 8 these findings suggest that a homologous area on both chromosomes plays a crucial role in subsets of prostate cancer in both species.

Sex-specific telomere length profiles and age-dependent erosion dynamics of individual chromosome arms in humans

During aging, telomeres are gradually shortened, eventually leading to cellular senescence. By T/C-FISH (telomere/centromere-FISH), we investigated human telomere length differences on single chromosome arms of 205 individuals in different age groups and sexes. For all chromosome arms, we found a linear correlation between telomere length and donor age. Generally, males had shorter telomeres and higher attrition rates. Every chromosome arm had its individual age-specific telomere length and erosion pattern, resulting in an unexpected heterogeneity in chromosome-specific regression lines. This differential erosion pattern, however, does not seem to be accidental, since we found a correlation between average telomere length of single chromosome arms in newborns and their annual attrition rate. Apart from the above-mentioned sex-specific discrepancies, chromosome arm-specific telomere lengths were strikingly similar in men and women. This implies a mechanism that arm specifically regulates the telomere length independent of gender, thus leading to interchromosomal telomere variations.

Telomeres, interstitial telomeric repeat sequences, and chromosomal aberrations

Telomeres are specialized nucleoproteic complexes localized at the physical ends of linear eukaryotic chromosomes that maintain their stability and integrity. The DNA component of telomeres is characterized by being a G-rich double stranded DNA composed by short fragments tandemly repeated with different sequences depending on the species considered. At the chromosome level, telomeres or, more properly, telomeric repeats--the DNA component of telomeres--can be detected either by using the fluorescence in situ hybridization (FISH) technique with a DNA or a peptide nucleic acid (PNA) (pan)telomeric probe, i.e., which identifies simultaneously all of the telomeres in a metaphase cell, or by the primed in situ labeling (PRINS) reaction using an oligonucleotide primer complementary to the telomeric DNA repeated sequence. Using these techniques, incomplete chromosome elements, acentric fragments, amplification and translocation of telomeric repeat sequences, telomeric associations and telomeric fusions can be identified. In addition, chromosome orientation (CO)-FISH allows to discriminate between the different types of telomeric fusions, namely telomere-telomere and telomere-DNA double strand break fusions and to detect recombination events at the telomere, i.e., telomeric sister-chromatid exchanges (T-SCE). In this review, we summarize our current knowledge of chromosomal aberrations involving telomeres and interstitial telomeric repeat sequences and their induction by physical and chemical mutagens. Since all of the studies on the induction of these types of aberrations were conducted in mammalian cells, the review will be focused on the chromosomal aberrations involving the TTAGGG sequence, i.e., the telomeric repeat sequence that "caps" the chromosomes of all vertebrate species.

Heterochromatin differentiation shows the pathways of karyotypic evolution in Israeli mole rats (Spalax, Spalacidae, Rodentia)

C-banding, base-specific fluorochrome staining (CMA3/DA/DAPI), and comparative genomic hybridization (CGH) were used to analyze the constitutive heterochromatin in two Israeli Spalax species, S. galili (2n = 52) and S. judai (2n = 60). It was shown that C-positive centromeric heterochromatin and some telomeric sites comprise GC-rich DNA sequences in both species. Comparative genomic in situ hybridization revealed slight qualitative differences in highly repetitive sequences in the two Spalax species. Eight acrocentric pairs in S. judai that are involved in Robertsonian rearrangements, possessed composite heterochromatin with a preference of S. judai highly repetitive sequences in the proximal region. Heterochromatin of the sex chromosomes, two biarmed homologous pairs (4 and 5) in both species, and acrocentric chromosomes from the group with a variable centromere position in S. judai was entirely species-specific. The high level of homology in the composition of heterochromatin may relate to the recent divergence of Israeli Spalax. Interspecies heterochromatin differences are discussed in the context of possible mechanisms in the Spalax chromosome evolution.

Telomere biology: integrating chromosomal end protection with DNA damage response

Telomeres play the key protective role at chromosomes. Many studies indicate that loss of telomere function causes activation of DNA damage response. Here, we review evidence supporting interdependence between telomere maintenance and DNA damage response and present a model in which these two pathways are combined into a single mechanism for protecting chromosomal integrity. Proteins directly involved in telomere maintenance and DNA damage response include Ku, DNA-PKcs, RAD51D, PARP-2, WRN and RAD50/MRE11/NBS1 complex. Since most of these proteins participate in the repair of DNA double-strand breaks (DSBs), this was perceived by many authors as a paradox, given that telomeres function to conceal natural DNA ends from mechanisms that detect and repair DSBs. However, we argue here that the key function of one particular DSB protein, Ku, is to prevent or control access of telomerase, the enzyme that synthesises telomeric sequences, to both internal DSBs and natural chromosomal ends. This view is supported by observations that Ku has a high affinity for DNA ends; it acts as a negative regulator of telomerase and that telomerase itself can target internal DSBs. Ku then directs other DSB repair/telomere maintenance proteins to either repair DSBs at internal chromosomal sites or prevent uncontrolled elongation of telomeres by telomerase. This model eliminates the above paradox and provides a testable scenario in which the role of DSB repair proteins is to protect chromosomal integrity by balancing repair activities and telomere maintenance. In our model, a close association between telomeres and different DNA damage response factors is not an unexpected event, but rather a logical result of chromosomal integrity maintenance activities.

Regulation of telomere length and suppression of genomic instability in human somatic cells by Ku86

Ku86 plays a key role in nonhomologous end joining in organisms as evolutionarily disparate as bacteria and humans. In eukaryotic cells, Ku86 has also been implicated in the regulation of telomere length although the effect of Ku86 mutations varies considerably between species. Indeed, telomeres either shorten significantly, shorten slightly, remain unchanged, or lengthen significantly in budding yeast, fission yeast, chicken cells, or plants, respectively, that are null for Ku86 expression. Thus, it has been unclear which model system is most relevant for humans. We demonstrate here that the functional inactivation of even a single allele of Ku86 in human somatic cells results in profound telomere loss, which is accompanied by an increase in chromosomal fusions, translocations, and genomic instability. Together, these experiments demonstrate that Ku86, separate from its role in nonhomologous end joining, performs the additional function in human somatic cells of suppressing genomic instability through the regulation of telomere length.

The stability of telomereless chromosome fragments in adult androgenetic rainbow trout

The study provides new data on the stability of gamma radiation-induced chromosome fragments of a putative maternal nuclear genome in an androgenetic vertebrate, rainbow trout (Oncorhynchus mykiss Walbaum). The fragments were found in five of 16 examined individuals and they were mostly centromeric parts of metacentric or subtelocentric chromosomes. Chromosome fragments were identical in all cells of a given androgenetic individual,indicating that segregation of chromosome fragments is active from the early cell divisions. Most of the fragments were telomereless, i.e. they had no telomeric sequences on their ends. This shows that telomeres are not necessary for stability of chromosomal structures in a vertebrate genome. In one individual, the interstitial telomeric sites were found in chromosomes, which could be the effect of joining chromosome fragments.

A centromeric satellite DNA may be involved in heterochromatin compactness in gobiid fishes

Centromere and telomere composition and organization were studied in various gobiid species exhibiting and not exhibiting chromosome polymorphisms involving Robertsonian rearrangements. In Gobius cobitis, we isolated an AT-rich centromeric DNA satellite, designated pCOB, and found that several sequences contain adenine stretches, various CA/TG dinucleotide steps, and a sequence 76% homologous to the yeast CDE III centromeric sequence. All of these traits are generally considered important for centromeric function, and the hypothesis has been advanced that some are involved in the control of DNA curvature and thus in the degree of centromeric chromatin compactness. Based on these features, and on the fact that they are found only in the species not exhibiting Robertsonian biarmed chromosomes, a role for pCOB in preventing centric fusions has been hypothesized. Our data also suggest that, as in other species, the formation of Robertsonian biarmed chromosomes is accompanied by the loss of telomeric sequences.

Cytogenetic analyses of chromosomal rearrangements in Mus minutoides/musculoides from North-West Zambia through mapping of the telomeric sequence (TTAGGG)n and banding techniques

Three specimens of M. minutoides/musculoides from Zambia were cytogenetically studied through G- and C-banding, DAPI staining and fluorescence in-situ hybridization (FISH) with a (TTAGGG)n telomeric sequence. Biarmed chromosomes were identified according to the current nomenclature as follows: Rb(2.7), Rb(3.12), Rb(4.5), Rb(6.8), Rb(9.16), and the sex chromosomes Rb(1.X), Rb(1.Y) and Rb(1.Xd), originated from the deleted X chromosome. One female showed the diploid number 2n = 24; in the two other individuals, the Rb(9.16) occurred in a heteromorphic condition, and, accordingly, the diploid number was 2n = 25. FISH showed the sites of telomeric sequences at telomeres of all the chromosomes, and in an interstitial position at the centromeres of all Robertsonian metacentrics, except one - the Rb(6.8), though the patterns of hybridization varied between chromosomes. Sex chromosome pairs, in the male and females, showed a similar C-banding pattern, but revealed clear differences after FISH. Traces of telomeric sequences were found dispersed in the whole-heterochromatic arm of the Rb(1.Xd). No visible bond between C-positive heterochromatin and telomeric sequences were detected in the other either bi- or uniarmed chromosomes, indicating that they may actually represent retained telomeres in the Robertsonian metacentrics.

Implication of telomere length as a proliferation-associated marker in schwannomas

BACKGROUND AND OBJECTIVES

Some schwannomas in the central nervous system may demonstrate relatively aggressive behavior in pathological findings and clinical course. We evaluate the diagnostic values of telomerase activity and telomere length in the clinicopathological behavior of schwannomas.

METHODS

Thirty surgical specimens from intracranial and intraspinal schwannomas were analyzed by polymerase chain reaction (PCR) and enzyme-linked immunosorbent assay (ELISA) for telomerase activity and terminal restriction fragments (TRFs) using Southern blot for telomere length. Proliferative indices were also studied.

RESULTS

Telomerase activity could not be detected in all schwannomas. Elongated telomere length (mean 17,101 +/- 259 bp) was found in four specimens (13.3%). Three of these four were found to have mitotic figures, high vascularity, cellularity, and pleomorphism in the pathological findings. The proliferative indices (35) showed correlative high values. One patient died of this disease, and one was found to have recurrence at follow-up evaluation. Those that displayed benign histopathological pictures showed relatively short telomere length (8,866 +/- 271 base pairs) and low proliferative indices (21). These is a significant difference between these two groups (P = 0.001).

CONCLUSIONS

Elongation of telomere length in schwannomas appears to predict aggressive clinicopathological behavior.

Principles of the highly ordered arrangement of metaphase I bivalents in spermatocytes of Agrodiaetus (Insecta, Lepidoptera)

We have investigated the nature of highly ordered bivalent arrangement in lepidopteran spermatocytes by analysing and comparing the patterns of bivalent distribution in intact metaphase I plates of 24 closely related species of the genus Agrodiaetus (Lycaenidae). The studied species greatly differed in haploid chromosome numbers (from n = 13 to n = 90) and in the structure of their karyotypes. We found that the larger the bivalent, the closer to the centre of the metaphase plate it was situated. In species with a high chromosome number and asymmetrical karyotype structure, the largest bivalent was located in the centre of the circular metaphase plate. Bivalents of equal size were approximately equidistant from the centre of the metaphase plate and formed concentric circles around the largest bivalent. These principles are diametrically different from those known in the majority of other animals and plants, in which the smallest elements of the chromosome set are situated in the centre of metaphase plate. The only exception from the above principles was observed in spermatocytes of A. surakovi which were heterozygous for reciprocal translocation involving two or three chromosome pairs. In addition to one large bivalent, the heterozygous cells had a multivalent, the size of which was comparable to or even exceeded that of the largest bivalentin the karyotype. In spite of thelarge size, the multivalent was always situated at the periphery of metaphase plate. This indicated that the chromosome size itself is not the only factor determining the bivalent position. We also found that the structure of the metaphase plate is fundamentally different in mitotic and meiotic cells of Agrodiaetus. In spermatogonial metaphase, chromosomes were tightly brought together, forming a dense compact disk, whereas during metaphase I of spermatocytes, all bivalents were clearly separated from each other, and the distance between adjacent bivalents varied from 0.4 to 1.5 microm. Based on the above findings, we proposed a model of bivalent distribution in the Lepidoptera. According to the model, during congregation in the prometaphase stage there is a centripetal movement of bivalents made by a force directed to the centre of the metaphase plate transverse to the spindle. This force is proportional to the kinetochore size of a particular bivalent. The Lepidoptera have a special near-holokinetic type of chromosome organisation. Therefore, large bivalents having large kinetochores are situated in the central part of metaphase plate. Another possible factor affecting the bivalent position is the interaction of bivalents with the cisternae of the membrane system compartmentalising the intraspindle space.

Different genomic evolutionary rates in the various reptile lineages

Although Reptiles occupy a strategic position among terrestrial vertebrates, studies of the composition and evolution of their genome are scarce. The cytogenetic analysis of nearly 1400 species evidenced different karyotypical evolutionary rates and different G-banding structures in turtles and crocodiles on the one hand and squamates on the other. A similar dichotomy was also identified through the study of the quantitative and compositional characteristics of the genome. The different evolutionary rates of chromosome morphology and genome size and composition and the diversification of coding and non-coding sequences bear an interesting relationship to the number of extant species and the extinction rates of the reptilian orders and suborders studied, suggesting a large role for such different evolutionary rates in the phylogenesis of this class. The different molecular and structural organisation of chromosomes could be an important, though by no means the sole, factor affecting the genome's evolutionary rate.

Telomere length assessment in human archival tissues: combined telomere fluorescence in situ hybridization and immunostaining

A method was developed to assess human telomere lengths at the individual cell level in tissue sections from standard formalin-fixed paraffin-embedded tissues. We coupled this method with immunofluorescence to allow the simultaneous identification of specific cell types. Validation of this in situ quantification method showed excellent agreement with the commonly used telomere repeat fragment-Southern blot method. The assay requires very few cells ( approximately 10 to 15). Thus, small tissue samples, including clinical biopsies, can be easily accommodated. In addition, the cells under study need not be actively cycling and there is no requirement for tissue disaggregation or cell culture. This method provides a more accurate assessment of telomere lengths than Southern blotting because confounding contributions from undesired cell types within tissue samples are avoided. Using this technique, we were able to perform the first comparison of relative telomere lengths in matched tumor versus normal epithelial cells within archival human prostate tissues.

Telomere elongation (Tel), a new mutation in Drosophila melanogaster that produces long telomeres

In most eukaryotes telomeres are extended by telomerase. Drosophila melanogaster, however, lacks telomerase, and telomere-specific non-LTR retrotransposons, HeT-A and TART, transpose specifically to chromosome ends. A Drosophila strain, Gaiano, that has long telomeres has been identified. We extracted the major Gaiano chromosomes into an Oregon-R genetic background and examined the resulting stocks after 60 generations. In situ hybridization using HeT-A and TART sequences showed that, in stocks carrying either the X or the second chromosome from Gaiano, only the Gaiano-derived chromosomes display long telomeres. However, in stocks carrying the Gaiano third chromosome, all telomeres are substantially elongated, indicating that the Gaiano chromosome 3 carries a factor that increases HeT-A and TART addition to the telomeres. We show that this factor, termed Telomere elongation (Tel), is dominant and localizes as a single unit to 69 on the genetic map. The long telomeres tend to associate with each other in both polytene and mitotic cells. These associations depend on telomere length rather than the presence of Tel. Associations between metaphase chromosomes are resolved during anaphase, suggesting that they are mediated by either proteinaceous links or DNA hydrogen bonding, rather than covalent DNA-DNA bonds.

Multiple manifestations of X-ray-induced genomic instability in Chinese hamster ovary (CHO) cells

Carcinogenesis is postulated to follow a multistep cascade in which the first genetic event may destabilize cells and thereby facilitate the induction of subsequent mutations within the same cell. It has recently been shown that exposure to ionizing radiation can in itself induce a persistent, heritable genetic instability in cells. To further investigate this phenomenon, we utilized a mutationally unstable population derived from a single Chinese hamster ovary (CHO) cell that survived X irradiation. We exposed these cells to a second dose of radiation, selected hypoxanthine phosphoribosyl transferase (HPRT) mutant subclones, and identified the type of mutations involved. We found complete deletions, continuous tract partial deletions, single-exon deletions, discontinuous-exon deletions ("skip mutations"), and point mutations (changes of less than 100 bp) among the isolated HPRT mutants. We hypothesized that the skip mutation clones might be more likely to demonstrate genomic instability. To test this hypothesis, mutant subclones were screened for three markers of genetic instability: alteration of minisatellite sequences, change in telomere length, and induction of chromosomal aberrations. Clones with skip mutations and single-exon deletions possessed elevated frequencies of minisatellite alterations and chromosomal aberrations, particularly rings and dicentrics. All mutant clones showed longer telomere terminal restriction fragment lengths than did wild-type cells. These results are consistent with the hypothesis that irradiation may induce a global instability phenotype, since the multiple alterations observed are mechanistically distinct, heritable cellular modifications that arose in the clonogenic progeny of the irradiated cells. Skip mutations may be one manifestation of this instability, but their presence was not specifically associated with the other genetic alterations.

Intrachromosomal distribution of telomeric repeats in Eumops glaucinus and Euntops perotis (Molossidae, Chiroptera)

The location of chromosomal telomeric repeats (TTAGGG)n was investigated in two species of the Molossidae family, Eumops glaucinus and Eumops perotis. The diploid chromosome number (2n) is 40 in E. glaucinus and 48 in E. perotis and the fundamental numbers (FN) are 64 and 58, respectively. It has been suggested that the E. glaucinus karyotype has evolved from the E. perotis karyotype through Robertsonian fusion events. In the present study, the telomeric sequences were detected at the termini of chromosomes in both species. In addition, E. glaucinus also displayed telomeric repeats in centromeric and pericentromeric regions in almost all biarmed chromosomes. Conversely, in E. perotis pericentromeric signals were only observed in two biarmed chromosomes. In both E. glaucinus and E. perotis, such telomeric sequences were observed as part of the heterochromatin. The interstitial sites of telomeric sequences suggest that they are remnants of telomeres of ancestral chromosomes that participated in the fusion event.

Telomere associations in interphase nuclei: possible role in maintenance of interphase chromosome topology

The relative sizes of individual telomeres in cultured human cells under conditions of cell cycling, replicative quiescence, cell transformation and immortalization were determined using quantitative fluorescence in situ hybridization (Q-FISH) with a telomere-specific peptide nucleic acid (PNA) probe. Results obtained from analysis of telomere length profiles (TLPs), which display the distribution of relative telomere lengths for individual cells, confirmed telomere length heterogeneity at the single cell level and proportional shortening of telomere length during replicative aging of virus-transformed cells. TLPs also revealed that some telomeric ends of chromosomes are so closely juxtaposed within interphase nuclei that their fluorescent signals appear as a single spot. These telomeric associations (TAs) were far more prevalent in interphase nuclei of noncycling normal and virus-transformed cells than in their cycling counterparts. The number of interphase TAs per nucleus observed in late-passage E6/E7-transformed cells did not increase during progression to crisis, suggesting that telomere shortening does not increase the frequency of interphase TAs. Furthermore, interphase TAs were rarely observed in rapidly cycling, telomerase-positive, immortalized cells that exhibit somewhat shortened, but stabilized, telomere length through the activity of telomerase. Our overall results suggest that the number of interphase TAs is dependent more on whether or not cells are cycling than on telomere length, with TAs being most prominent in the nuclei of replicatively quiescent cells in which nonrandom (even preferred) chromosome spatial arrangements have been observed. We propose that interphase TAs may play a role in the generation and/or maintenance of nuclear architecture and chromosome positional stability in interphase nuclei, especially in cells with a prolonged G(1)/G(0) phase and possibly in terminally differentiated cells.

Implication of telomerase activity and alternations of telomere length in the histologic characteristics of intracranial meningiomas

BACKGROUND

Telomerase activity and telomere length have been shown to be involved in the control of cell proliferation and regulation of cell senescence. The expression of telomerase activity may endow cells with the capacity of unlimited proliferation and immortality. The authors examined the telomerase activity and telomere length of intracranial meningiomas to determine the relation between the results and the clinicopathologic behavior of these tumors.

METHODS

Sixty-two specimens of meningiomas including 13 atypical and malignant tumors were used in this study. Telomerase activity was measured with polymerase chain reaction and enzyme-linked immunosolvent assay. Telomere length was measured by detecting the terminal restriction fragments using Southern blots.

RESULTS

Detectable telomerase activity was found in 4 of 13 (30.8%) malignant or atypical meningiomas and only 1 in 49 benign meningiomas (P = 0.006). Elongated telomere length was measured in 6 of 13 (46.1%) patients with malignant or atypical meningiomas and only 1 of 48 (2.1%) in those with benign tumors (P = 0.0002). Three of 4 (75%) of malignant or atypical meningiomas with detectable telomerase activity revealed shortened telomere length, and all tumors with elongated telomere length displayed undetectable telomerase activity. The percentage of malignant or atypical meningiomas with detectable telomerase activity or elongated telomere were significantly higher (76.9%) than that of benign tumors (4.0%). The proliferative index was calculated as the percentage of tumor cell nuclei immunoreactive for Ki-67 to total tumor nuclei. The mean values of proliferative index in benign, atypical, and malignant meningiomas were 1.2, 11.0, and 30.0, respectively.

CONCLUSIONS

The results indicate that telomerase activation may be a critical step in the pathogenesis of malignant or atypical meningioma. Elongation of the telomere length also implicates the high potential for malignant behavior in these tumors.

Mammalian Ku86 protein prevents telomeric fusions independently of the length of TTAGGG repeats and the G-strand overhang

Ku86 together with Ku70, DNA-PKcs, XRCC4 and DNA ligase IV forms a complex involved in repairing DNA double-strand breaks (DSB) in mammals. Yeast Ku has an essential role at the telomere; in particular, Ku deficiency leads to telomere shortening, loss of telomere clustering, loss of telomeric silencing and deregulation of the telomeric G-overhang. In mammals, Ku proteins associate to telomeric repeats; however, the possible role of Ku in regulating telomere length has not yet been addressed. We have measured telomere length in different cell types from wild-type and Ku86-deficient mice. In contrast to yeast, Ku86 deficiency does not result in telomere shortening or deregulation of the G-strand overhang. Interestingly, Ku86-/- cells show telomeric fusions with long telomeres (>81 kb) at the fusion point. These results indicate that mammalian Ku86 plays a fundamental role at the telomere by preventing telomeric fusions independently of the length of TTAGGG repeats and the integrity of the G-strand overhang.

Chromosome-specific telomeric associations in Chinese hamster embryonic cells

Telomeric associations (TAs) represent an important cytogenetic marker of human tumor cells. It has been thought that the primary cause of TAs is telomere shortening. However, we report here a surprising aspect of telomere maintenance in primary Chinese hamster embryonic (CHE) cells: relatively high frequencies of TAs in spite of normal telomere length. These TAs are present in both interphase and metaphase cells, suggesting that metaphase TAs may be relics of interphase chromosome organization. In addition, some TAs observed here are chromosome-specific and recurrent in at least three consecutive cell cycles in two different CHE cell strains. In spite of relatively high frequencies of TAs, none of the CHE strains show chromosome instability resulting from breakage-fusion-bridge cycles, as would be expected from tumor cell studies. It appears that TAs in CHE cells may be reversible events. These results are discussed in light of current understanding of telomere biology.

PRINS analysis of the telomeric sequence in seven lemurs

We examined the chromosomal localization of the telomeric sequence, (TTAGGG)n, in seven species of the lemurs and one greater galago, as an outgroup, using the primed in-situ labeling (PRINS) technique. As expected, the telomeric sequence was identified at both ends of all chromosomes of the eight prosimians. However, six species showed a signal at some pericentromeric regions involving constitutive heterochromatin as well. The pericentromeric region of chromosome 1 of Verreaux's sifaka (Propithecus verreauxi verreauxi) was labeled with a large and intense signal. The range of the signal considerably exceeded the area of DAPI positive heterochromatin. On the other hand, in the five lemurs, a large signal was detected also in the short arm of acrocentric chromosomes. Acquisition of the large block of the telomeric sequence into the acrocentric short arm might be interpretable in terms of the tandem growth of the heterochromatic short arm and the reciprocal translocation between heterochromatic short arms involving the telomeric sequence. Subsequently, it was postulated that meta- or submetacentric chromosomes possessing the telomeric sequence at the pericentromeric region could be formed by centric fusion between such acrocentric chromosomes.

Intranuclear anchoring of repetitive DNA sequences: centromeres, telomeres, and ribosomal DNA

Centromeres, telomeres, and ribosomal gene clusters consist of repetitive DNA sequences. To assess their contributions to the spatial organization of the interphase genome, their interactions with the nucleoskeleton were examined in quiescent and activated human lymphocytes. The nucleoskeletons were prepared using "physiological" conditions. The resulting structures were probed for specific DNA sequences of centromeres, telomeres, and ribosomal genes by in situ hybridization; the electroeluted DNA fractions were examined by blot hybridization. In both nonstimulated and stimulated lymphocytes, centromeric alpha-satellite repeats were almost exclusively found in the eluted fraction, while telomeric sequences remained attached to the nucleoskeleton. Ribosomal genes showed a transcription-dependent attachment pattern: in unstimulated lymphocytes, transcriptionally inactive ribosomal genes located outside the nucleolus were eluted completely. When comparing transcription unit and intergenic spacer, significantly more of the intergenic spacer was removed. In activated lymphocytes, considerable but similar amounts of both rDNA fragments were eluted. The results demonstrate that: (a) the various repetitive DNA sequences differ significantly in their intranuclear anchoring, (b) telomeric rather than centromeric DNA sequences form stable attachments to the nucleoskeleton, and (c) different attachment mechanisms might be responsible for the interaction of ribosomal genes with the nucleoskeleton.

DNA double-strand break repair proteins are required to cap the ends of mammalian chromosomes

Recent findings intriguingly place DNA double-strand break repair proteins at chromosome ends in yeast, where they help maintain normal telomere length and structure. In the present study, an essential telomere function, the ability to cap and thereby protect chromosomes from end-to-end fusions, was assessed in repair-deficient mouse cell lines. By using fluorescence in situ hybridization with a probe to telomeric DNA, spontaneously occurring chromosome aberrations were examined for telomere signal at the points of fusion, a clear indication of impaired end-capping. Telomeric fusions were not observed in any of the repair-proficient controls and occurred only rarely in a p53 null mutant. In striking contrast, chromosomal end fusions that retained telomeric sequence were observed in nontransformed DNA-PK(cs)-deficient cells, where they were a major source of chromosomal instability. Metacentric chromosomes created by telomeric fusion became even more abundant in these cells after spontaneous immortalization. Restoration of repair proficiency through transfection with a functional cDNA copy of the human DNA-PK(cs) gene reduced the number of fusions compared with a negative transfection control. Virally transformed cells derived from Ku70 and Ku80 knockout mice also displayed end-to-end fusions. These studies demonstrate that DNA double-strand break repair genes play a dual role in maintaining chromosomal stability in mammalian cells, the known role in repairing incidental DNA damage, as well as a new protective role in telomeric end-capping.

Accelerated telomere shortening in the human inactive X chromosome

Telomeres are nucleoprotein complexes at the end of eukaryotic chromosomes, with important roles in the maintenance of genomic stability and in chromosome segregation. Normal somatic cells lose telomeric repeats with each cell division both in vivo and in vitro. To address a potential role of nuclear architecture and epigenetic factors in telomere-length dynamics, the length of the telomeres of the X chromosomes and the autosomes was measured in metaphases from blood lymphocytes of human females of various ages, by quantitative FISH with a peptide nucleic-acid telomeric probe in combination with an X-chromosome centromere-specific probe. The activation status of the X chromosomes was simultaneously visualized with antibodies against acetylated histone H4. We observed an accelerated shortening of telomeric repeats in the inactive X chromosome, which suggests that epigenetic factors modulate not only the length but also the rate of age-associated telomere shortening in human cells in vivo. This is the first evidence to show a differential rate of telomere shortening between and within homologous chromosomes in any species. Our results are also consistent with a causative role of telomere shortening in the well-documented X-chromosome aneuploidy in aging humans.

Telomerase activity and telomere detection during early bovine development

The ends of mammalian chromosomes are composed of repeated DNA sequences of (TTAGGG)(n) known as telomeres. Telomerase is a ribonucleoprotein that synthesizes telomeric DNA to replenish the 50-200 bp lost during cell replication. Cellular aging and senescence are associated with a lack of telomerase activity and a critical shortening of the telomere. The objectives of this study were to confirm the presence of TTAGGG repeats on the chromosomes of bovine embryos using in situ hybridization and assess the relative amounts of telomerase activity using a telomeric repeat amplification protocol (TRAP) during oocyte maturation and early embryo development. Applying a telomere DNA probe to the chromosomes of blastocysts and adult fibroblasts, telomeres were identified on the terminal ends of the p and q arms of chromosomes in all cells examined. Immature oocytes, matured oocytes, zygotes, 2- to 5-cell embryos, 6- to 8-cell embryos, morulae, and blastocysts were lysed in NP-40 lysis buffer and telomerase activity was assayed using the TRAP assay. Telomerase activity was detected in all developmental stages examined. Relative telomerase activity (based on telomerase internal standards and positive controls) appeared to decrease during oocyte maturation and subsequent development to the 8-cell stage but significantly increased (P < 0.05) by approximately 40-fold at the morula and blastocyst stages. It was concluded that the telomeres of bovine chromosomes contain TTAGGG repeats and that telomerase activity is up-regulated in morulae and blastocysts.

The XRCC2 and XRCC3 repair genes are required for chromosome stability in mammalian cells

The irs1 and irs1SF hamster cell lines are mutated for the XRCC2 and XRCC3 genes, respectively. Both show heightened sensitivity to ionizing radiation and particularly to the DNA cross-linking chemical mitomycin C (MMC). Frequencies of spontaneous chromosomal aberration have previously been reported to be higher in these two cell lines than in parental, wild-type cell lines. Microcell-mediated chromosome transfer was used to introduce complementing or non-complementing human chromosomes into each cell line. irs1 cells received human chromosome 7 (which contains the human XRCC2 gene) or, as a control, human chromosome 4. irs1SF cells received human chromosome 14 (which contains the XRCC3 gene) or human chromosome 7. For each set of hybrid cell lines, clones carrying the complementing human chromosome recovered MMC resistance to near-wild-type levels, while control clones carrying noncomplementing chromosomes remained sensitive to MMC. Fluorescence in situ hybridization with a human-specific probe revealed that the human chromosome in complemented clones remained intact in almost all cells even after extended passage. However, the human chromosome in noncomplemented clones frequently underwent chromosome rearrangements including breaks, deletions, and translocations. Chromosome aberrations accumulated slowly in the noncomplemented clones over subsequent passages, with some particular deletions and unbalanced translocations persistently transmitted throughout individual subclones. Our results indicate that the XRCC2 and XRCC3 genes, which are now considered members of the RAD51 gene family, play essential roles in maintaining chromosome stability during cell division. This may reflect roles in DNA repair, possibly via homologous recombination.