Telomere dysfunction increases mutation rate and genomic instability

Telomere length assessment in tissue sections by quantitative FISH: image analysis algorithms

BACKGROUND

Telomeres are tandem repeated DNA sequences at the ends of every chromosome, which cap, stabilize, and prevent chromosome fusions and instability. Telomere regulation is an important mechanism in cellular proliferation and senescence in normal diploid and neoplastic cells. Quantitative methods to assess telomere lengths are essential to understanding how telomere dynamics play a role in these processes.

METHODS

Telomere lengths have been conventionally measured using terminal restriction fragment (TRF), quantitative fluorescence in situ hybridization (QFISH), and flow FISH. In this study, we have applied QFISH to measure average telomere lengths in cultured cells and human tissues of the GI tract. Importantly, this method can be used to analyze telomere lengths in sections using confocal microscopy. We describe and compare three image analysis algorithms: a simple pixel histogram calculation of background corrected fluorescence, a telomere spot-finding method, and a background curve subtraction algorithm.

RESULTS

Using normal human diploid fibroblasts (NHDF) either dropped on slides or sectioned after agar embedding, similar telomere length shortening is evident with increasing population doubling levels (PDLs), using peptide nucleic acid (PNA) and an N3'-P5'-phosphoamidate (PA) oligonucleotide probe for all three methods. Validation of these in situ telomere quantification methods showed excellent agreement with the commonly used telomere repeat fragment-Southern blot method. Telomere length reductions can also be demonstrated in tissue sections from histologically normal mucosa from patients with chronic ulcerative colitis (with dysplasia or cancer elsewhere in the colon), in colon adenomas, and in mucosal biopsies from patients with Barrett's esophagus. Both on slides and in tissue sections, the telomere spot-finding method has the greatest variability, while intra- and inter-biopsy variability in telomere length assessment using the other methods is relatively low.

CONCLUSIONS

Accurate and reproducible telomere length measurements can be made in tissue sections using QFISH and confocal microscopy. The simplest methods proved the most reliable and make these methods readily accessible to many laboratories. The use of these methods will enhance the ability to measure telomere lengths in tissue samples and aid in the understanding of the role of telomere length in aging and disease.

Telomere protection without a telomerase; the role of ATM and Mre11 in Drosophila telomere maintenance

The conserved ATM checkpoint kinase and the Mre11 DNA repair complex play essential and overlapping roles in maintaining genomic integrity. We conducted genetic and cytological studies on Drosophila atm and mre11 knockout mutants and discovered a telomere defect that was more severe than in any of the non-Drosophila systems studied. In mutant mitotic cells, an average of 30% of the chromosome ends engaged in telomere fusions. These fusions led to the formation and sometimes breakage of dicentric chromosomes, thus starting a devastating breakage-fusion-bridge cycle. Some of the fusions depended on DNA ligase IV, which suggested that they occurred by a nonhomologous end-joining (NHEJ) mechanism. Epistasis analyses results suggest that ATM and Mre11 might also act in the same telomere maintenance pathway in metazoans. Since Drosophila telomeres are not added by a telomerase, our findings support an additional role for both ATM and Mre11 in telomere maintenance that is independent of telomerase regulation.

Ty1 mobilizes subtelomeric Y' elements in telomerase-negative Saccharomyces cerevisiae survivors

When telomerase is inactivated in Saccharomyces cerevisiae, telomeric DNA shortens with every cell division, and cells stop dividing after approximately 100 generations. Survivors that form in these senescent populations and resume growing have variably amplified arrays of subtelomeric Y' elements. We marked a chromosomal Y' element with the his3AI retrotransposition indicator gene and found that Y'HIS3 cDNA was incorporated into the genome at approximately 10- to 1,000-fold-higher frequencies in survivors compared to telomerase-positive strains. Y'HIS3 cDNA mobility was significantly reduced if assayed at 30 degrees C, a nonpermissive temperature for Ty1 retrotransposition, or in the absence of Tec1p, a transcription factor for Ty1. Microarray analysis revealed that Y' RNA is preferentially associated with Ty1 virus-like particles (VLPs). Genomic copies of Y'HIS3 cDNA typically have downstream oligo(A) tracts, followed by a complete Ty1 long terminal repeat and TYA1 or TYB1 sequences. These data are consistent with the use of Ty1 cDNA to prime reverse transcription of polyadenylated Y' RNA within Ty1 VLPs. Unmarked Y'-oligo(A)-Ty1 cDNA was also detected in survivors, reaching copy numbers of approximately 10(-2) per genome. We propose that Y'-oligo(A)-Ty1 cDNA recombines with Y' elements at eroding telomeres in survivors and may play a role in telomere maintenance in the absence of telomerase.

Microarray TRAP—a high-throughput assay to quantitate telomerase activity

Telomeric repeat amplification protocol (TRAP)--a sensitive, PCR-based assay to detect telomerase activity was quintessential to the evaluation of telomerase role in telomere maintenance, cell proliferation, tumour development, and cell immortalization. The assay, however, suffers from many limitations. The most significant are: lack of telomerase activity quantification, changes of the enzyme activity product size and/or ratio, and complex post-amplification procedures which limit the assay throughput. Here we report the development of the microarray TRAP (MTRAP) assay which combines advantages of microarray technology with a modified TRAP assay. The MTRAP was designed and optimized on rice cell suspension telomerase extract to enable telomerase specific, reliable, and linear quantification in high throughput mode, with sensitivity comparable to those of radioisotope-based TRAP assays. The MTRAP has a built-in system guaranteeing the amplification of telomerase activity products unchanged in length and/or ratio and built-in control for false negatives. Thus, our MTRAP assay provides new reliable tool for experiments requiring massive quantitation of telomerase activity.

Non-neutral role of replicative senescence in tissue homeostasis and tumorigenesis

Normal somatic cells divide only a limited number of times reaching a state known as replicative senescence. This restraint in reproductive potential has been proposed as a mechanism evolved in higher eukaryotes to protect the organism from developing cancer. However, despite this protection there is a positive correlation between tumor incidence and organism aging when cells are potentially closer to their replication limit. We use simple mathematical models derived from quasispecies theory to analyse the role of senescence in various scenarios with different cell types according to their replicative capacity. The models predict that a situation with cells launching more often the senescence response plays against tissue homeostasis favoring tumor initiation. It is also shown that cancer cells arising early in organism life are more sensitive to genetic instabilities progressing less often toward tissue invasion. The passage of cells through crisis emerges as a mechanism to maintain tissue homeostasis that is weakened in aged individuals. The models introduced, though simple, help to integrate experimental information relating tumorigenesis with cellular and organism aging phenomena.

Telomere length heterogeneity and chromosome instability

Chromosome aberrations are the hallmark of cancer cells. Although a few specific chromosome aberrations are frequently detected in some types of cancer, the majority of karyotypic abnormalities tend to differ between different histological types and between individuals with the same type of cancer. Recent work indicates that telomeres may be directly involved in shaping the karyotypes of tumor cells. In particular, the heterogeneity of telomere lengths within cells may have direct influence on the frequency with which chromosomes engage in telomeric fusions and in subsequent breakage-fusion-bridge cycles. Since telomere length distribution among chromosome arms is a polymorphic trait, difference in distributions between individuals may account, at least in part, for the karyotypic differences found among tumors of the same type. Conversely, if single telomere lengths happen to be inherited, the segregation of particularly short telomeres in families may increase the incidence of specific chromosome aberrations during tumor evolution, and perhaps contribute, along with other factors, to cancer pre-disposition.

Capping of radiation-induced DNA breaks in mouse early embryos

The aim of the study was to investigate the spectrum and frequencies of chromosome aberrations induced by the exposure of different mouse spermatogenic germ cell stages to ionizing radiation. Male mice were exposed in vivo to X-rays. Chromosome aberrations were analyzed in first- and second-embryonic cleavages obtained from mating irradiated males with nonirradiated females at different periods after radiation exposure. A combination of telomeric and centromeric labeling as well as whole Y chromosome painting was used to characterize the rejoining pattern and the telomere status of the radiation-induced DNA breaks. The frequency of chromosome aberrations observed in eggs fertilized with sperm irradiated at the early spermatid stage was markedly higher than the frequency in eggs fertilized with sperm irradiated at the other spermatogenic stages when reference was made to the chromosome aberrations recovered in early embryos. At the first division postirradiation, distal rejoining of broken chromosome ends (in regard to the position of the centromere) was more frequent than proximal rejoining; thus compound acentric fragments were more frequently observed than dicentric chromosomes. The presence of additional telomere signals at the broken chromosome ends in mouse germ cells and early embryos, compatible with de novo formation of telomeres, was not frequent.

Genetic susceptibility to tobacco-related cancer

Although cigarette smoking is the dominant risk factor for several epithelial cancers, only a small fraction of individuals with tobacco exposure develop cancer. The underlying hypothesis is that genetic factors may render certain smokers more susceptible to cancer than others. Genetic alterations in critical regulatory pathways may predispose cells to carcinogenesis. These pathways include regulation of xenobiotic metabolism; control of genomic stability, including DNA repair mechanisms, cell-cycle checkpoints, apoptosis and telomere length; and control of microenvironmental factors, such as matrix metalloproteinases, inflammation and growth factors. In addition, epigenetic events, such as promoter hypermethylation and loss of imprinting, are also involved in carcinogenesis. In this review, we will summarize recent advances in genetic susceptibility to tobacco-related cancer. Emphasizing on risk assessment, we will describe how genetic variations in the above-mentioned genetic pathways modify the tobacco-related cancer risk. In addition, we will discuss how genetic variations may assist in predicting clinical outcome, such as the natural history of cancer and treatment response. The measurements of genetic susceptibility by both genotypic and phenotypic assays are covered in the text. Finally, we present a number of current concerns that need to be addressed as the exciting field of molecular cancer epidemiology advances rapidly.

End-to-end fusion of linear deleted chromosomes initiates a cycle of genome instability in Streptomyces ambofaciens

Two mutant strains harbouring a linear chromosome whose size reached 13 Mb (versus approximately 8 Mb for the wild type) were characterized. This chromosomal structure resulted from the fusion in inverted orientation of two chromosomes partially deleted on the same arm. The fusion occurred by illegitimate recombination between 6 bp repeats. This chromosomal structure was inherited in strict association with a high level of genetic instability (30% of mutants in a single progeny, phenomenon also called hypervariability) and chromosomal instability. In contrast, derivatives, which did not retain the chromosome fusion, showed a wild-type-like instability frequency (c. 1%). Stabilization of the chromosomal structure occurred by chromosome arm replacement or circularization. A high variability of the terminal inverted repeat (TIR) length in the rescued chromosomes (from 5 kb to approximately 1.4 Mb for linear derivatives) was observed. Mutant lineages harbouring the chromosomal fusion are characterized by a highly heterogeneous distribution of DNA in the spores, by the presence of spores without DNA as well as aberrant sporulation figures, and by the production of spores with a low germination rate. The wild-type characteristics were restored in the descendants, which lost the chromosomal fusion. Thus, the fusion of deleted chromosomes initiates a cycle of chromosome instability sharing several levels of analogy with the behaviour of dicentric chromosomes in eukaryotes. We propose that the high instability of the fused chromosomes results from the duplication of a region involved in partitioning of the chromosomes (parAB-oriC ).

Phosphorylation of H2AX at short telomeres in T cells and fibroblasts

Eukaryotic cells undergo arrest and enter apoptosis in response to short telomeres. T cells from late generation mTR(-/-) mice that lack telomerase show increased apoptosis when stimulated to enter the cell cycle. The increased apoptosis was not inhibited by colcemid, indicating that the response did not result from breakage of dicentric chromosomes at mitosis. The damage response protein gamma-H2AX localized to telomeres in metaphases from T cells and fibroblasts from mTR(-/-) cells with short telomeres. These data suggest that the major mechanism for induction of apoptosis in late generation mTR(-/-) cells is independent of chromosome segregation and that loss of telomere function through progressive telomere shortening in the absence of telomerase leads to recognition of telomeres as DNA breaks.

Reconstitution of active telomerase in primary human foreskin fibroblasts: effects on proliferative characteristics and response to ionizing radiation

PURPOSE

Telomere shortening has been proposed to trigger senescence, and since most primary cells do not express active telomerase, reactivation of telomerase activity was proposed as a safe and non-transforming way of immortalizing cells. However, to study radiation responses, it is as yet unclear whether cells immortalized by telomerase reactivation behave in a similar manner as their parental primary cells.

MATERIALS AND METHODS

Primary human foreskin fibroblasts were transfected with the human catalytic subunit of telomerase, the reverse transcriptase (hTERT), and their growth characteristics and response to DNA damage were characterized.

RESULTS

The sole expression of the human hTERT was sufficient to immortalize the human foreskin fibroblasts. With time in culture, the immortalized cells almost doubled their average telomeric length and the clonal population contained almost no post-mitotic fibroblasts anymore. Up to 300 population doublings, no alterations compared with the parental primary cells were seen in terms of clonogenic radiosensitivity, DNA double-strand break repair, radiation-induced increases in p53 and p21(WAF-1,CIP-1) expression, and the G1/S and G2/M cell cycle checkpoints. Moreover, mitogen-induced mitotic arrest of fibroblasts was still possible in the hTERT-immortalized clones.

CONCLUSIONS

Immortalizing fibroblasts by reconstitution of active telomerase seems a good, reliable manner to generate a large source of cells with a radiation damage response similar to the primary cells.

Highly efficient photochemical 2'-deoxyribonolactone formation at the diagonal loop of a 5-iodouracil-containing antiparallel G-quartet

To explore the structure-dependent hydrogen abstraction in antiparallel and parallel G-quartet DNA structures, the photochemical reactivity of 5-iodouracil ((I)U)-containing human telomeric DNA 22-mers was investigated under the 302 nm UV irradiation conditions. We discovered that only antiparallel ODN 4, in which the second T residue in the diagonal loop of the antiparallel G-quartet is substituted with (I)U, was rapidly consumed as compared with parallel ODN 4 and the other (I)U-containing 22-mers under the irradiation conditions. Product analysis of the photolyzate of antiparallel ODN 4 indicated that a 2'-deoxyribonolactone residue was effectively produced at the 5' side of the (I)U residue in the diagonal loop. Photochemical 2'-deoxyribonolactone formation was also found in the (I)U-containing diagonal loop of antiparallel G-quartets d(GGGGTTT(I)UGGGG)(2) and d(GGGGTT(I)UTGGGG)(2), whereas the reaction did not occur at other DNA structures, including the single-stranded form, the loop region of the hairpin, and linear four-stranded G-quartets. The results clearly indicate that this type of 2'-deoxyribonolactone formation efficiently occurrs only in the diagonal loop of the antiparallel G-quartet. Furthermore, we found that 2'-deoxyribonolactone was formed at the (I)U-containing G-rich sequence of the IgG switch regions and the 5' termini of the Rb gene, suggesting the formation of an antiparallel G-quartet with a diagonal loop in these sequences. These results suggest that the present photochemical method can be used as a specific probe for the antiparallel G-quartet with the diagonal loop.

Dynamics of protein binding to telomeres in living cells: implications for telomere structure and function

Telomeric proteins have an essential role in the regulation of the length of the telomeric DNA tract and in protection against end-to-end chromosome fusion. Telomere organization and how individual proteins are involved in different telomere functions in living cells is largely unknown. By using green fluorescent protein tagging and photobleaching, we investigated in vivo interactions of human telomeric DNA-binding proteins with telomeric DNA. Our results show that telomeric proteins interact with telomeres in a complex dynamic fashion: TRF2, which has a dual role in chromosome end protection and telomere length homeostasis, resides at telomeres in two distinct pools. One fraction ( approximately 73%) has binding dynamics similar to TRF1 (residence time of approximately 44 s). Interestingly, the other fraction of TRF2 binds with similar dynamics as the putative end-protecting factor hPOT1 (residence time of approximately 11 min). Our data support a dynamic model of telomeres in which chromosome end-protection and telomere length homeostasis are governed by differential binding of telomeric proteins to telomeric DNA.

Strain-specific telomere length revealed by single telomere length analysis in Caenorhabditis elegans

Terminal restriction fragment analysis is the only method currently available for measuring telomere length in Caenorhabditis elegans. Its limitations include low sensitivity and interference by the presence of interstitial telomeric sequences in the C.elegans genome. Here we report the adaptation of single telomere length analysis (STELA) to measure the length of telomeric repeats on the left arm of chromosome V in C.elegans. This highly sensitive PCR-based method allows telomere length measurement from as few as a single worm. The application of STELA to eight wild-type C.elegans strains revealed considerable strain-specific differences in telomere length. Within individual strains, short outlying telomeres were observed that were clearly distinct from the bulk telomere length distributions, suggesting that processes other than end-replication losses and telomerase-mediated lengthening may generate telomere length heterogeneity in C.elegans. The utility of this method was further demonstrated by the characterization of telomere shortening in mrt-2 mutants. We conclude that STELA appears to be a valuable tool for studying telomere biology in C.elegans.

Telomere length abnormalities occur early in the initiation of epithelial carcinogenesis

PURPOSE

Telomeres help maintain chromosomal integrity. Dysfunctional telomeres can cause genetic instability in vitro and an increased cancer incidence in telomerase knock out mouse models. We recently reported that telomere shortening was a prevalent alteration in human prostate, pancreas, and breast cancer precursor lesions. In the present study, we address whether the previous findings are broadly applicable to human epithelial cancer precursors in general.

EXPERIMENTAL DESIGN

Surgical specimens of epithelial cancer precursor lesions from the urinary bladder, esophagus, large intestine, oral cavity, and uterine cervix were examined using a recently developed technique for direct in situ telomere length assessment in formalin-fixed human tissue specimens.

RESULTS

Widespread telomere length abnormalities were nearly universal (97.1% of cases) in the preinvasive stages of human epithelial carcinogenesis in all sites examined in this series, with telomere shortening the predominant abnormality (88.6% of cases).

CONCLUSIONS

Telomere length abnormalities appear to be one of the earliest and most prevalent genetic alterations acquired in the multistep process of malignant transformation. These findings support a model whereby telomere dysfunction induces chromosomal instability as an initiating event in many, perhaps most, human epithelial cancers. Together with previous findings from the prostate and pancreas, the percentage of intraepithelial neoplasia lesions showing telomere length abnormalities is 95.6%. The implications of these findings include the potential that telomere length assessment in situ may be a widely useful biomarker for monitoring disease prevention strategies and for improved early diagnosis.

Recombination and the Tel1 and Mec1 checkpoints differentially effect genome rearrangements driven by telomere dysfunction in yeast

In telomerase-deficient Saccharomyces cerevisiae, telomeres are maintained by recombination. Here we used a S. cerevisiae assay for characterizing gross chromosomal rearrangements (GCRs) to analyze genome instability in post-senescent telomerase-deficient cells. Telomerase-deficient tlc1 and est2 mutants did not have increased GCR rates, but their telomeres could be joined to other DNAs resulting in chromosome fusions. Inactivation of Tel1 or either the Rad51 or Rad59 recombination pathways in telomerase-deficient cells increased the GCR rate, even though telomeres were maintained. The GCRs were translocations and chromosome fusions formed by nonhomologous end joining. We observed chromosome fusions only in mutant strains expressing Rad51 and Rad55 or when Tel1 was inactivated. In contrast, inactivation of Mec1 resulted in more inversion translocations such as the isochromosomes seen in human tumors. These inversion translocations seemed to be formed by recombination after replication of broken chromosomes.

The shortest telomeres drive karyotype evolution in transformed cells

Maintenance of telomeres is essential for chromosome stability. In the absence of telomerase, telomeres shorten with cell division until they approach a stability threshold, at which point cells enter senescence. When senescence-signaling pathways are inactive, further telomere shortening leads to chromosome instability characterized by telomeric fusions and breakage-fusion-bridge (BFB) cycles. Since the distribution of telomere lengths among chromosome extremities is heterogeneous, we wondered about the impact of such variability on the stability of particular chromosome arms. We correlated the initial length of individual telomeres in telomerase-negative-transformed cells with the stability of the corresponding chromosome arms during the precrisis period. We show that arms carrying the shortest telomeres are the first to become unstable and this instability affects the chromosome homologues with shorter telomeres almost exclusively. The analysis of several postcrisis cell populations, which had stabilized their telomeres by re-expressing telomerase, showed that the karyotypic outcome is strongly influenced by the initial telomere length heterogeneity. The timing of telomerase re-expression also seems to play a role in limiting the extent of karyotypic changes, probably by reducing the frequency of telomeric fusions and hence BFB. Since the distribution of telomere lengths within somatic cells is proper to every individual, our results predict that the risk for a particular chromosome arm of becoming unstable early in tumorigenesis will differ between individuals and contribute directly to the heterogeneity of chromosome aberrations found in tumors.

Telomerase activity and telomere length in primary and metastatic tumors from pediatric bone cancer patients

The presence of telomerase activity has been analyzed in almost all tumor types and tumor-derived cell lines. However, there are very few studies that focus on the presence of telomerase activity in bone tumors, and most of them report analysis on very few samples or bone-derived cell lines. The objective of this study was to analyze the telomere length and telomerase activity in primary tumors and metastatic lesions from pediatric osteosarcoma and Ewing's sarcoma patients. The presence of telomerase activity was analyzed by the telomeric repeat amplification protocol assay, and the telomere length was measured by Southern blot. Results were related to survival and clinical outcome. Telomerase activity was detected in 85% of the bone tumor metastases (100% Ewing's sarcomas and 75% osteosarcomas) but only in 12% of the primary tumors (11.1% osteosarcomas and 12.5% Ewing's sarcomas). Bone tumor tissues with telomerase activity had mean telomere lengths 3 kb shorter than those with no detectable telomerase activity (p = 0.041). The presence of telomerase activity was associated with survival (p = 0.009), and longer event-free survival periods were found in patients who lacked telomerase activity compared with those who had detectable telomerase activity levels in their tumor tissues (p = 0.037). The presence of longer telomeres in primary pediatric bone tumors than in metastases could be indicative of alternative mechanisms of lengthening of telomeres for their telomere maintenance rather than telomerase activity. Nevertheless, the activation of telomerase seems to be a crucial step in the malignant progression and acquisition of invasive capability of bone tumors.

Those dam-aged telomeres!

Telomere integrity plays a crucial role in the capacity for continuous cell proliferation. In some circumstances, shortened telomeres contribute to cell arrest or death, but in others, shortened telomeres may actually enhance the incidence and spectrum of tumors. Resolution of this apparent paradox requires a more detailed understanding of a non-functional telomere. Recent evidence reveals that critically shortened or uncapped telomeres share molecular hallmarks of damaged DNA. It is likely that the cellular response to this DNA damage, influenced by the nature of the damage itself, affects the outcome of loss of telomere function.

Mutant telomere sequences lead to impaired chromosome separation and a unique checkpoint response

Mutation of the template region in the RNA component of telomerase can cause incorporation of mutant DNA sequences at telomeres. We made all 63 mutant sequence combinations at template positions 474-476 of the yeast telomerase RNA, TLC1. Mutants contained faithfully incorporated template mutations, as well as misincorporated sequences in telomeres, a phenotype not previously reported for Saccharomyces cerevisiae telomerase template mutants. Although growth rates and telomere profiles varied widely among the tlc1 mutants, chromosome separation and segregation were always aberrant. The mutants showed defects in sister chromatid separation at centromeres as well as telomeres, suggesting activation of a cell cycle checkpoint. Deletion of the DNA damage response genes DDC1, MEC3, or DDC2/SML1 failed to restore chromosome separation in the tlc1 template mutants. These results suggest that mutant telomere sequences elicit a checkpoint that is genetically distinct from those activated by deletion of telomerase or DNA damage.

Chromosome rearrangements resulting from telomere dysfunction and their role in cancer

Telomeres play a vital role in protecting the ends of chromosomes and preventing chromosome fusion. The failure of cancer cells to properly maintain telomeres can be an important source of the chromosome instability involved in cancer cell progression. Telomere loss results in sister chromatid fusion and prolonged breakage/fusion/bridge (B/F/B) cycles, leading to extensive DNA amplification and large deletions. These B/F/B cycles end primarily when the unstable chromosome acquires a new telomere by translocation of the ends of other chromosomes. Many of these translocations are nonreciprocal, resulting in the loss of the telomere from the donor chromosome, providing a mechanism for transfer of instability from one chromosome to another until a chromosome acquires a telomere by a mechanism other than nonreciprocal translocation. B/F/B cycles can also result in other forms of chromosome rearrangements, including double-minute chromosomes and large duplications. Thus, the loss of a single telomere can result in instability in multiple chromosomes, and generate many of the types of rearrangements commonly associated with human cancer.

Indecent Exposure

The protective "cap" that assembles at chromosome ends recruits and controls an intricate network of biochemical activities, each one critical for telomere structure and the maintenance of genomic stability. Recent studies have uncovered the components of telomere caps and have started to define the pathways that lead from telomere dysfunction to chromosomal catastrophe.

Upregulation of the Catalytic Telomerase Subunit by the Transcription Factor ER81 and Oncogenic HER2/Neu, Ras, or Raf

One hallmark of tumor formation is the transcriptional upregulation of human telomerase reverse transcriptase, hTERT, and the resultant induction of telomerase activity. However, little is presently understood about how hTERT is differentially activated in tumor cells versus normal somatic cells. Specifically, it is unclear if oncoproteins can directly elicit hTERT expression. To this end, we now show that three oncoproteins, HER2/Neu, Ras, and Raf, stimulate hTERT promoter activity via the ETS transcription factor ER81 and ERK mitogen-activated protein (MAP) kinases. Mutating ER81 binding sites in the hTERT promoter or suppression of ERK MAP kinase-dependent phosphorylation of ER81 rendered the hTERT promoter unresponsive to HER2/Neu. Further, expression of dominant-negative ER81 or inhibition of HER2/Neu significantly attenuated telomerase activity in HER2/Neu-overexpressing SKBR3 breast cancer cells. Moreover, HER2/Neu, Ras, and Raf collaborated with ER81 to enhance endogenous hTERT gene transcription and telomerase activity in hTERT-negative, nonimmortalized BJ foreskin fibroblasts. Accordingly, hTERT expression was increased in HER2/Neu-positive breast tumors and breast tumor cell lines relative to their HER2/Neu-negative counterparts. Collectively, our data elucidated a mechanism whereby three prominent oncoproteins, HER2/Neu, Ras, and Raf, may facilitate tumor formation by inducing hTERT expression in nonimmortalized cells via the transcription factor ER81.

Order of genetic events is critical determinant of aberrations in chromosome count and structure

A sequential acquisition of genetic events is critical in tumorigenesis. A key step is the attainment of infinite proliferative potential. Acquisition of this immortalization requires the activation of telomerase in addition to other activities, including inactivation of TP53 and the retinoblastoma family of tumor-suppressor proteins. However, the importance of the order in which these genetic events occur has not been established. To address this question, we used a panel of normal mammary fibroblasts and endothelial cultures that were immortalized after transduction with the catalytic subunit of telomerase (hTERT) and a temperature-sensitive mutant of the SV40 large-tumor (tsLT) oncoprotein in different orders in early- and late-passage stocks. These lines were maintained in continuous culture for up to 90 passages, equivalent to >300 population doublings (PDs) post-explantation during 3 years of continuous propagation. We karyotyped the cultures at different passages. Cultures that received hTERT first followed by tsLT maintained a near-diploid karyotype for more than 150 PDs. However, in late-passage stocks (>200 PDs), metaphase cells were mostly aneuploid. In contrast, the reverse order of gene transduction resulted in a marked early aneuploidy and chromosomal instability, already visible after 50 PDs. These results suggest that the order of genetic mutations is a critical determinant of chromosome count and structural aberration events.

Activation of a LTR-retrotransposon by telomere erosion

Retrotransposons can facilitate repair of broken chromosomes, and therefore an important question is whether the host can activate retrotransposons in response to chromosomal lesions. Here we show that Ty1 elements, which are LTR-retrotransposons in Saccharomyces cerevisiae, are mobilized when DNA lesions are created by the loss of telomere function. Inactivation of telomerase in yeast results in progressive shortening of telomeric DNA, eventually triggering a DNA-damage checkpoint that arrests cells in G2/M. A fraction of cells, termed survivors, recover from arrest by forming alternative telomere structures. When telomerase is inactivated, Ty1 retrotransposition increases substantially in parallel with telomere erosion and then partially declines when survivors emerge. Retrotransposition is stimulated at the level of Ty1 cDNA synthesis, causing cDNA levels to increase 20-fold or more before survivors form. This response is elicited through a signaling pathway that includes Rad24, Rad17, and Rad9, three components of the DNA-damage checkpoint. Our findings indicate that Ty1 retrotransposons are activated as part of the cellular response to telomere dysfunction.

End resection initiates genomic instability in the absence of telomerase

Telomere dysfunction causes genomic instability. However, the mechanism that initiates this instability when telomeres become short is unclear. We measured the mutation rate and loss of heterozygosity along a chromosome arm in diploid yeast that lacked telomerase to distinguish between mechanisms for the initiation of instability. Sequence loss was localized near chromosome ends in the absence of telomerase but not after breakage of a dicentric chromosome. In the absence of telomerase, the increase in mutation rate is dependent on the exonuclease Exo1p. Thus, exonucleolytic end resection, rather than chromosome fusion and breakage, is the primary mechanism that initiates genomic instability when telomeres become short.

Telomere length and telomerase activity during expansion and differentiation of human mesenchymal stem cells and chondrocytes

Chondrocyte ex vivo expansion currently performed to replace damaged articular surfaces is associated with a loss of telomeric repeats similar to decades of aging in vivo. This might affect the incidence or time of onset of age-related disorders within transplanted cells or tissues. This study examined whether more immature progenitor cells, such as mesenchymal stem cells (MSC), which can be expanded and subsequently differentiated into chondrocytes is advantageous regarding telomere-length related limitations of expansion protocols. Primary chondrocytes and bone-marrow-derived MSC were isolated from 12 donors, expanded separately to 4 x 10(6) cells, and (re-)differentiated as three-dimensional chondrogenic spheroids. Cells were collected during expansion, after three-dimensional culturing and chondrogenic differentiation, and sequential analyses of telomere length and telomerase activity were performed. Surprisingly, telomeres of expanded MSC were significantly shorter than those from expanded chondrocytes from the same donor (11.4+/-2.5 vs. 13.4+/-2.2 kb) and tended to remain shorter after differentiation in chondrogenic spheroids (11.9+/-1.8 vs. 13.0+/- kb). While telomere lengths in native chondrocytes and MSC were not related to the age of the donor, significant negative correlations with age were observed in expanded (136 bp/year), three-dimensionally reconstituted (188 bp/year), and redifferentiated (229 bp/year) chondrocytes. Low levels of telomerase activity were found in MSC and chondrocytes during expansion and after (re-)differentiation to chondrogenic spheroids. In terms of replicative potential, as determined by telomere length, ex vivo expansion followed by chondrogenic differentiation of MSC did not provide a benefit compared to the expansion of adult chondrocytes. However, accelerated telomere shortening with age during expansion and redifferentiation argues for an "age phenotype" in chondrocytes as opposed to MSC and suggests an advantage for the use of MSC especially in older individuals and protocols requiring extensive expansion

A genomewide screen in Saccharomyces cerevisiae for genes that suppress the accumulation of mutations

A genomewide screen of a collection of 4,847 yeast gene deletion mutants was carried out to identify the genes required for suppressing mutations in the CAN1 forward-mutation assay. The primary screens and subsequent analysis allowed (i) identification of 18 known mutator mutants, providing a solid means for checking the efficiency of the screen, and (ii) identification of a number of genes not known previously to be involved in suppressing mutations. Among the previously uncharacterized mutation-suppressing genes were six genes of unknown function including four (CSM2, SHU2, SHU1, and YLR376c) encoding proteins that interact with each other and promote resistance to killing by methyl methanesulfonate, one gene (EGL1) previously identified as suppressing Ty1 mobility and recombination between repeated sequences, and one gene (YLR154c) that was not associated with any known processes. In addition, five genes (TSA1, SOD1, LYS7, SKN7, and YAP1) implicated in the oxidative-stress responses were found to play a significant role in mutation suppression. Furthermore, TSA1, which encodes thioredoxin peroxidase, was found to strongly suppress gross chromosomal rearrangements. These results provide a global view of the nonessential genes involved in preventing mutagenesis. Study of such genes should provide useful clues in identification of human genes potentially involved in cancer predisposition and in understanding their mechanisms of action.

Genetic regulation of telomere-telomere fusions in the yeast Saccharomyces cerevisae

Yeast strains with mutations in both TEL1 and MEC1 have short telomeres and elevated rates of chromosome deletions. By using a PCR assay, we demonstrate that mec1 tel1 strains also have telomere-telomere fusions (T-TFs). T-TFs require Lig4p (a ligase required for nonhomologous end-joining DNA repair). The highest rates of T-TFs are found in strains with combination of mutations that affect telomere length and DNA damage checkpoints (mec1 tel1, mec3 tel1, mre11 mec1, and ddc1 tel1 strains). Examining many mutant genotypes, we find good agreement between the level of T-TFs and the rate of chromosomal deletions. In addition, if telomeres are elongated in a mec1 tel1 strain, we eliminate T-TFs and reduce the deletion rate. The correlation between the level of T-TFs and the rate of deletions argues that many of these deletions reflect a cycle of T-TF formation (resulting in dicentric chromosomes), followed by chromosome breakage.

Virulence-related surface glycoproteins in the yeast pathogen Candida glabrata are encoded in subtelomeric clusters and subject to RAP1- and SIR-dependent transcriptional silencing

Candida glabrata is an important opportunistic pathogen causing both mucosal and bloodstream infections. C. glabrata is able to adhere avidly to mammalian cells, an interaction that depends on the Epa1p lectin. EPA1 is shown here to be a member of a larger family of highly related genes encoded in subtelomeric clusters. Subtelomeric clustering of large families of surface glycoprotein-encoding genes is a hallmark of several pathogens, including Plasmodium, Trypanosoma, and Pneumocystis. In these other pathogens, a single surface glycoprotein is expressed, whereas other genes in the family are transcriptionally silent. Similarly, whereas EPA1 is expressed in vitro, EPA2-5 are transcriptionally repressed. This repression is shown to be due to regional silencing of the subtelomeric loci. In Saccharomyces cerevisiae, subtelomeric silencing is initiated by Rap1p binding to the telomeric repeats and subsequent recruitment of the Sir complex by protein-protein interaction. We demonstrate here that silencing of the subtelomeric EPA loci also depends on functional Sir3p and Rap1p. This identification and analysis of the EPA gene family provides a compelling example in an ascomycete of chromatin-based silencing of natural subtelomeric genes and provides for the first time in a pathogen, molecular insight into the transcriptional silencing of large subtelomeric gene families.

Elg1 Forms an Alternative PCNA-Interacting RFC Complex Required to Maintain Genome Stability

BACKGROUND

Genome instability is a hallmark of cancer and plays a critical role in generating the myriad of phenotypes selected for during tumor progression. However, the mechanisms that prevent genome rearrangements remain poorly understood.

RESULTS

To elucidate the mechanisms that ensure genome stability, we screened a collection of candidate genes for suppressors of gross chromosomal rearrangements (GCRs) in budding yeast. One potent suppressor gene encodes Elg1, a conserved but uncharacterized homolog of the large RFC subunit Rfc1 and the alternative RFC subunits Ctf18/Chl12 and Rad24. Our results are consistent with the hypothesis that Elg1 forms a novel and distinct RFC-like complex in both yeast and human cells. We find that Elg1 is required for efficient S phase progression and telomere homeostasis in yeast. Elg1 interacts physically with the PCNA homolog Pol30 and the FEN-1 homolog Rad27. The physical and genetic interactions suggest a role for Elg1 in Okazaki fragment maturation. Furthermore, Elg1 acts in concert with the alternative Rfc1-like proteins Rad24 and Ctf18 to enable Rad53 checkpoint kinase activation in response to replication stress.

CONCLUSIONS

Collectively, these results reveal that Elg1 forms a novel and conserved alternative RFC complex. Furthermore, we propose that genome instability arises at high frequency in elg1 mutants due to a defect in Okazaki fragment maturation.

Telomere dysfunction: a potential cancer predisposition factor

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSION

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

Dramatic increase of telomerase activity during dendritic cell differentiation and maturation

Telomerase, the reverse transcriptase that maintains telomere DNA, is usually undetectable in most adult tissues but is positive in embryonic tissues and in cancers. In addition, freshly islolated or in vitro-activated lymphocytes were shown to express high levels of telomerase activity, although its expression in myeloid cells including dendritic cells (DCs) is largely unknown. Here, we investigated telomerase activity during the differentiation and maturation process of DCs. In vitro culture of bone marrow (BM) cells with granulocyte macrophage-colony stimulating factor and interleukin-4 induced a dramatic increase of telomerase activity accompanied with their differentiation into DCs. Furthermore, stimulation with microbial components such as lipopolysaccharide (LPS), which triggers maturation of DCs, augmented the activity. In vivo responses of telomerase activity were also observed in splenic DCs by injection of LPS intraperitoneally. It is interesting that in old mice, telomerase activity of splenic DCs was significantly higher than young mice but rather decreased after LPS stimulation. By measuring expression of cell-surface activation markers, splenic DCs of old mice responded poorly to LPS stimulation. Such poor responses to LPS were also observed in BM-derived DCs. These different features of DCs between young and old mice may contribute to a pathogenesis to microbial infections.

The genetic basis of renal cell carcinoma

The recognition of hereditary forms of renal cancer and the development of high-throughput genetic analysis have led to the identification of genes responsible for familial renal epithelial tumors of differing histologies and cytogenetic features. Some of these genes (VHL) are known to have an important role in sporadic renal neoplasia. This article describes the various epithelial renal tumors most commonly encountered by the urologist, the molecular and cytogenetic distinctions between them, and the hereditary syndromes that predispose to these tumors. Consideration of these syndromes is important for proper treatment when one encounters patients with multiple renal tumors, tumors at an early age of onset, or patients with a positive family history of renal cell carcinoma.

Telomere dysfunction and the initiation of genome instability

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

Telomerase-independent proliferation is influenced by cell type in Saccharomyces cerevisiae

Yeast strains harboring mutations in genes required for telomerase function (TLC1 and the EST genes) exhibit progressive shortening of telomeric DNA and replicative senescence. A minority of cells withstands loss of telomerase through RAD52-dependent amplification of telomeric and subtelomeric sequences; such survivors are now capable of long-term propagation with telomeres maintained by recombination rather than by telomerase. Here we report that simultaneous expression in haploid cells of both MATa and MATalpha information suppresses the senescence of telomerase-deficient mutants, with suppression occurring via the RAD52-dependent survivor pathway(s). Such suppression can be mimicked by deletion of SIR1-SIR4, genes that function in transcriptional silencing of several loci including the silent mating-type loci. Furthermore, telomerase-defective diploid strains that express only MATa or MATalpha information senesce at a faster rate than telomerase-defective diploids that are heterozygous at the MAT locus. This suggests that the RAD52-dependent pathway(s) for telomere maintenance respond to changes in the levels of recombination, a process regulated in part by the hierarchy of gene control that includes MAT regulation. We propose that cell-type-specific regulation of recombination at human telomeres may similarly contribute to the tissue-specific patterns of disease found in telomerase-deficient tumors.

Telomerase-immortalized sheep fibroblasts can be reprogrammed by nuclear transfer to undergo early development

Telomere shortening and lack of telomerase activity have been implicated in cellular senescence in human fibroblasts. Expression of the human telomerase catalytic reverse transcriptase subunit (hTERT) in these cells reconstitutes telomerase activity and immortalizes the cells without tumor transformation. In this report, we show that sheep fibroblasts are similar to human cells. They do not have detectable telomerase activity and undergo only a finite numbers of cell divisions before replicative senescence. Telomere lengths in sheep fibroblasts are similar to those reported for human cells and shorten at a rate of 50-200 base pairs (bp) each cell division. Expression of the human telomerase catalytic subunit restored the telomerase activity in the sheep cells and extended their proliferative life span. None of the telomerase positive sheep fibroblasts exhibited a transformed phenotype after 200 days of continuous culture, and the higher hTERT expressing cells maintained their telomere lengths and normal cell characteristics for more than 500 days in culture. In cloning experiments using one of these cell lines as a nuclear donor, the reconstructed karyoplasts were reprogrammed and developed to the blastocyst stage at a similar frequency to that observed with the parental, telomerase negative cell line. After embryo transfer the blastocysts exhibited a relatively high frequency of implantation, early fetal development, and organogenesis. No fetuses survived beyond 40 days of development, however, showing that although these cells could be substantially reprogrammed, they were not fully competent for nuclear transfer.

Saccharomyces cerevisiae chromatin-assembly factors that act during DNA replication function in the maintenance of genome stability

Some spontaneous gross chromosomal rearrangements (GCRs) seem to result from DNA-replication errors. The chromatin-assembly factor I (CAF-I) and replication-coupling assembly factor (RCAF) complexes function in chromatin assembly during DNA replication and repair and could play a role in maintaining genome stability. Inactivation of CAF-I or RCAF increased the rate of accumulating different types of GCRs including translocations and deletion of chromosome arms with associated de novo telomere addition. Inactivation of CAF-I seems to cause damage that activates the DNA-damage checkpoints, whereas inactivation of RCAF seems to cause damage that activates the DNA-damage and replication checkpoints. Both defects result in increased genome instability that is normally suppressed by these checkpoints, RAD52-dependent recombination, and PIF1-dependent inhibition of de novo telomere addition. Treatment of CAF-I- or RCAF-defective cells with methyl methanesulfonate increased the induction of GCRs compared with that seen for a wild-type strain. These results indicate that coupling of chromatin assembly to DNA replication and DNA repair is critical to maintaining genome stability.

Telomere-driven genomic instability in cancer cells

Telomeres, the ends of linear chromosomes, play a major role in the maintenance of genome integrity. Telomerase or alternative lengthening of telomeres (ALT) mechanisms exist in most cancer cells in order to stabilize telomere length by the addition of telomeric repeats. Telomere loss can be dramatically mutagenic. Chromosomes lacking one telomere remain unstable until they are capped, generating chromosomal instability, gene amplification via breakage/fusion/bridge (B/F/B) cycles and resulting in chromosome imbalances. The chronology of the occurrence of gene amplification and chromosome imbalances detected in human tumors is still unknown. All of the aberrations that occur prior to, during or after activation of a telomere maintenance mechanism promote the development of cancer.

Replication proteins influence the maintenance of telomere length and telomerase protein stability

We investigated the effects of fission yeast replication genes on telomere length maintenance and identified 20 mutant alleles that confer lengthening or shortening of telomeres. The telomere elongation was telomerase dependent in the replication mutants analyzed. Furthermore, the telomerase catalytic subunit, Trt1, and the principal initiation and lagging-strand synthesis DNA polymerase, Polalpha, were reciprocally coimmunoprecipitated, indicating these proteins physically coexist as a complex in vivo. In a polalpha mutant that exhibited abnormal telomere lengthening and slightly reduced telomere position effect, the cellular level of the Trt1 protein was significantly lower and the coimmunoprecipitation of Trt1 and Polalpha was severely compromised compared to those in the wild-type polalpha cells. Interestingly, ectopic expression of wild-type polalpha in this polalpha mutant restored the cellular Trt1 protein to the wild-type level and shortened the telomeres to near-wild-type length. These results suggest that there is a close physical relationship between the replication and telomerase complexes. Thus, mutation of a component of the replication complex can affect the telomeric complex in maintaining both telomere length equilibrium and telomerase protein stability.

NEJ1 prevents NHEJ-dependent telomere fusions in yeast without telomerase

In a search for genes involved in cell-type-dependent chromosome instability, we have found a role for NEJ1, a regulator of nonhomologous end joining (NHEJ), in cells that survive in the absence of telomerase. In yeast, NHEJ is regulated by mating-type status through NEJ1, which is repressed in a/alpha cells. For efficient NHEJ, NEJ1 is required as part of a complex with LIF1 and DNL4, which catalyzes DNA ligation. In haploid cells without telomerase, we find that the absence of NEJ1 results in high frequencies of circular chromosomes in type II survivors (i.e., those typified by lengthened telomere repeat tracts). These telomere fusion events are DNL4 dependent. NEJ1 therefore has a role in protecting telomeres from end fusions by NHEJ in the absence of telomerase that contrasts with its role in promoting repair at sites of DNA double-strand breaks.

Short telomeres induce a DNA damage response in Saccharomyces cerevisiae

Telomerase-deficient Saccharomyces cerevisiae cells show a progressive decrease in telomere length. When grown for several days in log phase, the tlc1Delta cells initially display wild-type growth kinetics with subsequent loss of growth potential after which survivors are generated via RAD52-dependent homologous recombination. We found that chromosome loss in these telomerase-deficient cells only increased after a significant decline in growth potential of the culture. At earlier stages of growth, as the telomerase-deficient cells began to show loss of growth potential, the cells arrested in G2/M and showed RNR3 induction and Rad53p phosphorylation. These responses were dependent on RAD24 and MEC1, suggesting that short telomeres are recognized as DNA damage and signal G2/M arrest.

Telomerase activity and genetic alterations in primary breast carcinomas

It has been proposed that the structural and numerical chromosome abnormalities recorded in breast cancer could be the result of telomere dysfunction and that telomerase is activated de novo to provide a survival mechanism curtailing further chromosomal aberrations. However, recent in vivo and in vitro data show that the ectopic expression of telomerase promotes tumorigenesis via a telomere length-independent mechanism. In this study, the relation between telomerase expression and the extent of chromosomal aberrations was investigated in 62 primary breast carcinomas. Telomerase activity was measured using a polymerase chain reaction-based telomeric repeat amplification protocol assay and 92% of the tumors were found to express telomerase with a relative activity ranging from 0 to 3839.6. Genetic alterations were determined by G-banding and comparative genomic hybridization analysis and 97% of the tumors exhibited chromosomal aberrations ranging from 0 to 44 (average: 10.98). In the overall series, the relationship between telomerase activity levels and genetic changes could be best described by a quadratic model, whereas in tumors with below-average genetic alteration numbers, a significant positive association was recorded between the two variables (coefficient=0.374, P=.017). The relationship between telomerase activity levels and the extent of genetic alteration may reflect the complex effect of telomerase activation upon tumor progression in breast carcinomas.

Telomeric protein distributions and remodeling through the cell cycle in Saccharomyces cerevisiae

In Saccharomyces cerevisiae, telomeric DNA is protected by a nonnucleosomal protein complex, tethered by the protein Rap1. Rif and Sir proteins, which interact with Rap1p, are thought to have further interactions with conventional nucleosomic chromatin to create a repressive structure that protects the chromosome end. We showed by microarray analysis that Rif1p association with the chromosome ends extends to subtelomeric regions many kilobases internal to the terminal telomeric repeats and correlates strongly with the previously determined genomic footprints of Rap1p and the Sir2-4 proteins in these regions. Although the end-protection function of telomeres is essential for genomic stability, telomeric DNA must also be copied by the conventional DNA replication machinery and replenished by telomerase, suggesting that transient remodeling of the telomeric chromatin might result in distinct protein complexes at different stages of the cell cycle. Using chromatin immunoprecipitation, we monitored the association of Rap1p, Rif1p, Rif2p, and the protein component of telomerase, Est2p, with telomeric DNA through the cell cycle. We provide evidence for dynamic remodeling of these components at telomeres.

Role of constitutively activated protein tyrosine kinases in malignant myeloproliferative disorders: an update

Modern molecular technology helped identify more than 10 protein tyrosine kinases related to myeloid malignancies, which allowed the development of small molecule inhibitors targeting deregulated protein tyrosine kinase activity. Protein tyrosine kinase deregulation can occur as a consequence of fusion gene formation because of chromosomal translocations, or as distinct gain-of-function point mutations. Although the tyrosine kinase inhibitor imatinib mesylate (Gleevec) targeting the ABL protein tyrosine kinase has revolutionized current chronic myeloid leukemia therapy, it became rapidly evident that overcoming the multiple cellular resistance mechanisms will be very challenging. To develop efficient therapeutic alternatives, one must understand the complex signal transduction mechanisms involved in transformation by deregulated protein tyrosine kinases. This article reviews the most recently identified molecular mechanisms involved in cell transformation by the BCR/ABL protein tyrosine kinase fusion and presents new members of the increasing family of deregulated protein tyrosine kinases involved in myeloproliferative disorders. In addition, the article discusses new, promising small molecule protein tyrosine kinase inhibitors and the molecular mechanism that may lead to resistance to these drugs. Finally, the article highlights putative alternative strategies that could be used to block signal transduction pathways of deregulated protein tyrosine kinase activity.

Arsenite induces delayed mutagenesis and transformation in human osteosarcoma cells at extremely low concentrations

Arsenite is a human multisite carcinogen, but its mechanism of action is not known. We recently found that extremely low concentrations (</=0.1 microM) of arsenite transform human osteosarcoma TE85 (HOS) cells to anchorage-independence. In contrast to other carcinogens which transform these cells within days of exposure, almost 8 weeks of arsenite exposure are required for transformation. We decided to reexamine the question of arsenite mutagenicity using chronic exposure in a spontaneous mutagenesis assay we previously developed. Arsenite was able to cause a delayed increase in mutagenesis at extremely low concentrations (</=0.1 microM) in a dose-dependent manner. The increase in mutant frequency occurred after almost 20 generations of growth in arsenite. Transformation required more than 30 generations of continuous exposure. We also found that arsenite induced gene amplification of the dihydrofolate reductase (DHFR) gene in a dose-dependent manner. Since HOS cells are able to methylate arsenite at a very low rate, it was possible that active metabolites such as monomethylarsonous acid (MMA(III)) contributed to the delayed mutagenesis and transformation in these cells. However, when the assay was repeated with MMA(III), we found no significant increase in mutagenesis or transformation, suggesting that arsenite-induced delayed mutagenesis and transformation are not caused by arsenite's metabolites, but by arsenite itself. Our results suggest that long-term exposure to low concentrations of arsenite may affect signaling pathways that result in a progressive genomic instability.

Biology and treatment of chronic lymphocytic leukemia

Major advances have occurred in our understanding of the biology, immunology, and opportunities for treatment of chronic lymphocytic leukemia (CLL) in recent times. Surface antigen analysis has helped us define classical CLL and differentiate it from variants such as marginal zone leukemia, mantle cell leukemia, and prolymphocytic leukemia. An important observation has been that the B-cells in indolent types of CLL, which do not require therapy, have undergone somatic hypermutation and function as memory B-lymphocytes whereas those more likely to progress have not undergone this process. Section I by Dr. Nicholas Chiorazzi encompasses emerging elements of the new biology of CLL and will address the types of somatic hypermutation that occur in CLL cells and their correlation with other parameters such as telomere length and ZAP70 status. In addition he addresses the concept of which cells are proliferating in CLL and how we can quantitate the proliferative thrust using novel methods. The interaction between these parameters is also explored. Section II by Dr. Thomas Kipps focuses on immune biology and immunotherapy of CLL and discusses new animal models in CLL, which can be exploited to increase understanding of the disease and create new opportunities for testing the interaction of the CLL cells with a variety of elements of the immune system. It is obvious that immunotherapy is emerging as a major therapeutic modality in chronic lymphocytic leukemia. Dr. Kipps addresses the present understanding of the immune status of CLL and the role of passive immunotherapy with monoclonal antibodies such as rituximab, alemtuzumab, and emerging new antibodies. In addition the interaction between the CLL cells and the immune system, which has been exploited in gene therapy with transfection of CLL cells by CD40 ligand, is discussed. In Section III, Dr. Michael Keating examines the question "Do we have the tools to cure CLL?" and focuses on the fact that we now have three distinct modalities, which are able to achieve high quality remissions with polymerase chain reaction (PCR) negativity for the immunoglobulin heavy chain in CLL. These modalities include initial chemoimmunotherapy with fludarabine, cyclophosphamide, and rituximab, the use of alemtuzumab for marrow cytoreduction in minimal residual disease and allogeneic bone marrow transplants. The emergence of non-ablative marrow transplants in CLL has led to the broadening of the range of opportunities to treat older patients. The addition of rituximab to the chemotherapy preparative regimens appears to be a significant advance. The combination of our increased understanding of the biology, immune status, and therapy of CLL provides for the first time the opportunity for curative strategies.

DNA amplification by breakage/fusion/bridge cycles initiated by spontaneous telomere loss in a human cancer cell line

The development of genomic instability is an important step in generating the multiple genetic changes required for cancer. One consequence of genomic instability is the overexpression of oncogenes due to gene amplification. One mechanism for gene amplification is the breakage/fusion/bridge (B/F/B) cycle that involves the repeated fusion and breakage of chromosomes following the loss of a telomere. B/F/B cycles have been associated with low-copy gene amplification in human cancer cells, and have been proposed to be an initiating event in high-copy gene amplification. We have found that spontaneous telomere loss on a marker chromosome 16 in a human tumor cell line results in sister chromatid fusion and prolonged periods of chromosome instability. The high rate of anaphase bridges involving chromosome 16 demonstrates that this instability results from B/F/B cycles. The amplification of subtelomeric DNA on the marker chromosome provides conclusive evidence that B/F/B cycles initiated by spontaneous telomere loss are a mechanism for gene amplification in human cancer cells.

Segmental polymorphisms in the proterminal regions of a subset of human chromosomes

The subtelomeric domains of chromosomes are probably the most rapidly evolving structures of the human genome. The highly variable distribution of large duplicated subtelomeric segments has indicated that frequent exchanges between nonhomologous chromosomes may have been taking place during recent genome evolution. We have studied the extent and variability of such duplications using in situ hybridization techniques and a set of well-defined subtelomeric cosmid probes that identify discrete regions within the subtelomeric domain. In addition to reciprocal translocation and illegitimate recombination events that could explain the observed mosaic pattern of subtelomeric regions, it is likely that homology-based recombination mechanisms have also contributed to the spread of distal subtelomeric sequences among particular groups of nonhomologous chromosome arms. The frequency and distribution of large-scale subtelomeric polymorphisms may have direct implications for the design of chromosome-specific probes that are aimed at the identification of cryptic subtelomeric deletions. Furthermore, our results indicate that the relevance of some of the telomere closures proposed within the present Human Genome Sequence draft are restricted to specific allelic variants of unknown frequencies.