Spontaneous regression of cutaneous melanoma in sinclair swine is associated with defective telomerase activity and extensive telomere erosion

Recently we proposed the hypothesis that extensive telomeric association of chromosomes is an early manifestation of cell death and asked whether there are extensive telomeric associations present in metaphases of the spontaneously regressing Sinclair swine cutaneous melanoma (SSCM). Our results indicate that early passage SSCMs, in the accelerated growth phase, do not show telomeric associations but do have numerical and other specific structural abnormalities. However, the same melanoma cell lines at late passages or melanomas obtained from middle- and old-aged Sinclair swine show extensive telomeric associations in the form of dicentric, multicentric, and ring configurations. Such abnormal structures are present mostly in metaphases that are hyperploids. Increasing frequencies of apoptotic bodies were also observed in higher passage tumor cell lines obtained from younger animals or in melanomas obtained from older animals. The polymerase chain reaction (PCR)-based telomeric repeat amplification protocol (TRAP) assay shows no detectable telomerase activity in any of these regressing swine melanoma cell lines, neither in normal swine skin fibroblasts nor in nevi. However, the fetal swine (i.e., non-regressing) melanoma cells show telomerase activity. Fluorescence in situ hybridization (FISH) results using the commercially available human telomeric repeat DNA probe indicate a reduction of telomeric signals in metaphase and interphase cells of regressing melanomas. From these observations we conclude that spontaneous regression of SSCM is associated with the loss of telomerase activity and a reduction of telomeric repeats that results in the formation of multicentric and ring configurations. Such abnormal chromosome configurations are lost, following the breakage-fusion-bridge-cycles, and result in extensive DNA fragmentation, as shown by laddering experiments, and, finally, cell death.

Telomerase-deficient mice with short telomeres are resistant to skin tumorigenesis

Inhibition of telomerase is proposed to limit the growth of cancer cells by triggering telomere shortening and cell death. Telomere maintenance by telomerase is sufficient, in some cell types, to allow immortal growth. Telomerase has been shown to cooperate with oncogenes in transforming cultured primary human cells into neoplastic cells, suggesting that telomerase activation contributes to malignant transformation. Moreover, telomerase inhibition in human tumour cell lines using dominant-negative versions of TERT leads to telomere shortening and cell death. These findings have led to the proposition that telomerase inhibition may result in cessation of tumour growth. The absence of telomerase from most normal cells supports the potential efficacy of anti-telomerase drugs for tumour therapy, as its inhibition is unlikely to have toxic effects. Mice deficient for Terc RNA (encoding telomerase) lack telomerase activity, and constitute a model for evaluating the role of telomerase and telomeres in tumourigenesis. Late-generation Terc-/- mice show defects in proliferative tissues and a moderate increase in the incidence of spontaneous tumours in highly proliferative cell types (lymphomas, teratocarcinomas). The appearance of these tumours is thought to be a consequence of chromosomal instability in these mice. These observations have challenged the expected effectiveness of anti-telomerase-based cancer therapies. Different cell types may nonetheless vary in their sensitivity to the chromosomal instability produced by telomere loss or to the activation of telomere-rescue mechanisms. Here we show that late-generation Terc-/- mice, which have short telomeres and are telomerase-deficient, are resistant to tumour development in multi-stage skin carcinogenesis. Our results predict that an anti-telomerase-based tumour therapy may be effective in epithelial tumours.

Telomere dysfunction promotes non-reciprocal translocations and epithelial cancers in mice

Aged humans sustain a high rate of epithelial cancers such as carcinomas of the breast and colon, whereas mice carrying common tumour suppressor gene mutations typically develop soft tissue sarcomas and lymphomas. Among the many factors that may contribute to this species variance are differences in telomere length and regulation. Telomeres comprise the nucleoprotein complexes that cap the ends of eukaryotic chromosomes and are maintained by the reverse transcriptase, telomerase. In human cells, insufficient levels of telomerase lead to telomere attrition with cell division in culture and possibly with ageing and tumorigenesis in vivo. In contrast, critical reduction in telomere length is not observed in the mouse owing to promiscuous telomerase expression and long telomeres. Here we provide evidence that telomere attrition in ageing telomerase-deficient p53 mutant mice promotes the development of epithelial cancers by a process of fusion-bridge breakage that leads to the formation of complex non-reciprocal translocations--a classical cytogenetic feature of human carcinomas. Our data suggest a model in which telomere dysfunction brought about by continual epithelial renewal during life generates the massive ploidy changes associated with the development of epithelial cancers.

Mice without telomerase: what can they teach us about human cancer?

Unicellular organisms, human cells and mice have provided insights into the processes of senescence, crisis, genomic instability and cancer in humans. Here, Artandi and DePinho discuss how studies in mice have uncovered a complex interplay between the ARF-p53 pathway, genomic instability due to telomere dysfunction, and the suppression or promotion of cancer.

Telomerase-dependent repeat divergence at the 3' ends of yeast telomeres

Yeast telomeres consist of approximately 300 nt of degenerate repeats with the consensus sequence G(2-3)(TG)(1-6). We developed a method for the amplification of a genetically marked telomere by PCR, allowing precise length and sequence determination of the G-rich strand including the 3' terminus. We examined wild-type cells, telomerase RNA deficient cells and a strain deleted for YKU70, which encodes for a protein involved in telomere maintenance and DNA double strand break repair. The 3' end of the G-rich strand was found to be at a variable position within the telomeric repeat. No preference for either thymine or guanine as the 3' base was detected. Comparison of telomere sequences from clonal populations revealed that telomeres consist of a centromere-proximal region of stable sequence and a distal region with differing degenerate repeats. In wild-type as well as yku70-Delta cells, variation in the degenerate telomeric repeats was detected starting 40-100 nt from the 3' end. Sequence divergence was abolished after deletion of the telomerase RNA gene. Thus, this region defines the domain where telomere shortening and telomerase-mediated extension occurs. Since this domain is much larger than the number of nucleo-tides lost per generation in the absence of telomerase, we propose that telomerase does not extend a given telomere in every cell cycle.

The role of telomerase expression and telomere length maintenance in human and mouse

The molecular regulation of telomere length has been well elucidated by a series of elegant studies over the past decade. More recently, experimental evidence has accrued that addresses the challenging question of if and how telomere length regulation may contribute to normal human aging or to human disease. Recent studies in mice have provided a mammalian precedent indicating that telomerase deficiency can lead to in vivo dysfunction, most probably as a consequence of progressive telomere shortening. In humans, the evidence that telomere shortening might lead to in vivo dysfunction is far less direct, although the recent description of telomerase deficiency and telomere shortening associated with the DKC syndrome is suggestive of such a link. Methodologies exist and continue to be developed that are increasingly capable of manipulating telomerase activity and telomere length in human cells. It remains to be determined whether scientifically rigorous and (equally important) medically ethical approaches will emerge to directly assess the ability of telomere length modulation to correct functional disorders of human cellular function ex vivo or more challenging still, in vivo.

Disruption of the telomerase catalytic subunit gene from Arabidopsis inactivates telomerase and leads to a slow loss of telomeric DNA

Telomerase is an essential enzyme that maintains telomeres on eukaryotic chromosomes. In mammals, telomerase is required for the lifelong proliferative capacity of normal regenerative and reproductive tissues and for sustained growth in a dedifferentiated state. Although the importance of telomeres was first elucidated in plants 60 years ago, little is known about the role of telomeres and telomerase in plant growth and development. Here we report the cloning and characterization of the Arabidopsis telomerase reverse transcriptase (TERT) gene, AtTERT. AtTERT is predicted to encode a highly basic protein of 131 kDa that harbors the reverse transcriptase and telomerase-specific motifs common to all known TERT proteins. AtTERT mRNA is 10-20 times more abundant in callus, which has high levels of telomerase activity, versus leaves, which contain no detectable telomerase. Plants homozygous for a transfer DNA insertion into the AtTERT gene lack telomerase activity, confirming the identity and function of this gene. Because telomeres in wild-type Arabidopsis are short, the discovery that telomerase-null plants are viable for at least two generations was unexpected. In the absence of telomerase, telomeres decline by approximately 500 bp per generation, a rate 10 times slower than seen in telomerase-deficient mice. This gradual loss of telomeric DNA may reflect a reduced rate of nucleotide depletion per round of DNA replication, or the requirement for fewer cell divisions per organismal generation. Nevertheless, progressive telomere shortening in the mutants, however slow, ultimately should be lethal.

G-strand overhangs on telomeres in telomerase-deficient mouse cells

Telomeres of eukaryotic chromosomes contain 3' overhangs which are thought to be essential for the maintenance of proper chromosome end structure and function. We examined the requirement for telomerase activity for the generation of these G-strand overhangs in mammalian cells. Using non-denaturing in-gel hybridization to both tissue and cultured cells from mice deficient for the telomerase RNA component, we found that G-strand overhangs exist in the absence of telomerase activity. Quantitation of overhang signal intensity showed no significant reduction in telomerase-deficient cells relative to wild-type. These results support a telomerase-independent mechanism for generating G-strand overhangs.

Disease states associated with telomerase deficiency appear earlier in mice with short telomeres

Mice deficient for the mouse telomerase RNA (mTR-/-) and lacking telomerase activity can only be bred for approximately six generations due to decreased male and female fertility and to an increased embryonic lethality associated with a neural tube closure defect. Although late generation mTR-/- mice show defects in the hematopoietic system, they are viable to adulthood, only showing a decrease in viability in old age. To assess the contribution of genetic background to the effect of telomerase deficiency on viability, we generated mTR-/- mutants on a C57BL6 background, which showed shorter telomeres than the original mixed genetic background C57BL6/129Sv. Interestingly, these mice could be bred for only four generations and the survival of late generation mTR-/- mice decreased dramatically with age as compared with their wild-type counterparts. Fifty percent of the generation 4 mice die at only 5 months of age. This decreased viability with age in the late generation mice is coincident with telomere shortening, sterility, splenic atrophy, reduced proliferative capacity of B and T cells, abnormal hematology and atrophy of the small intestine. These results indicate that telomere shortening in mTR-/- mice leads to progressive loss of organismal viability.

Longevity, stress response, and cancer in aging telomerase-deficient mice

Telomere maintenance is thought to play a role in signaling cellular senescence; however, a link with organismal aging processes has not been established. The telomerase null mouse provides an opportunity to understand the effects associated with critical telomere shortening at the organismal level. We studied a variety of physiological processes in an aging cohort of mTR-/- mice. Loss of telomere function did not elicit a full spectrum of classical pathophysiological symptoms of aging. However, age-dependent telomere shortening and accompanying genetic instability were associated with shortened life span as well as a reduced capacity to respond to stresses such as wound healing and hematopoietic ablation. In addition, we found an increased incidence of spontaneous malignancies. These findings demonstrate a critical role for telomere length in the overall fitness, reserve, and well being of the aging organism.

Telomere shortening in mTR-/- embryos is associated with failure to close the neural tube

Mice genetically deficient for the telomerase RNA (mTR) can be propagated for only a limited number of generations. In particular, mTR-/- mice of a mixed C57BL6/129Sv genetic background are infertile at the sixth generation and show serious hematopoietic defects. Here, we show that a percentage of mTR-/- embryos do not develop normally and fail to close the neural tube, preferentially at the forebrain and midbrain. The penetrance of this defect increases with the generation number, with 30% of the mTR-/- embryos from the fifth generation showing the phenotype. Moreover, mTR-/- kindreds in a pure C57BL6 background are only viable up to the fourth generation and also show defects in the closing of the neural tube. Cells derived from mTR-/- embryos that fail to close the neural tube have significantly shorter telomeres and decreased viability than their mTR-/- littermates with a closed neural tube, suggesting that the neural tube defect is a consequence of the loss of telomere function. The fact that the main defect detected in mTR-/- embryos is in the closing of the neural tube, suggests that this developmental process is among the most sensitive to telomere loss and chromosomal instability.

Telomere length dynamics and chromosomal instability in cells derived from telomerase null mice

To study the effect of continued telomere shortening on chromosome stability, we have analyzed the telomere length of two individual chromosomes (chromosomes 2 and 11) in fibroblasts derived from wild-type mice and from mice lacking the mouse telomerase RNA (mTER) gene using quantitative fluorescence in situ hybridization. Telomere length at both chromosomes decreased with increasing generations of mTER-/- mice. At the 6th mouse generation, this telomere shortening resulted in significantly shorter chromosome 2 telomeres than the average telomere length of all chromosomes. Interestingly, the most frequent fusions found in mTER-/- cells were homologous fusions involving chromosome 2. Immortal cultures derived from the primary mTER-/- cells showed a dramatic accumulation of fusions and translocations, revealing that continued growth in the absence of telomerase is a potent inducer of chromosomal instability. Chromosomes 2 and 11 were frequently involved in these abnormalities suggesting that, in the absence of telomerase, chromosomal instability is determined in part by chromosome-specific telomere length. At various points during the growth of the immortal mTER-/- cells, telomere length was stabilized in a chromosome-specific man-ner. This telomere-maintenance in the absence of telomerase could provide the basis for the ability of mTER-/- cells to grow indefinitely and form tumors.

Extra-chromosomal telomere repeat DNA in telomerase-negative immortalized cell lines

We found novel extra-chromosomal telomere repeat (ECTR) DNAs in telomerase-negative immortalized KMST-6 cells, by staining these cells with a (TTAGGG)n probe using both cycling oligonucleotide-primed in situ synthesis and by fluorescence in situ hybridization. Relatively small amounts of ECTR DNAs were also observed in telomerase-negative VA13 and SUSM-1 cells, but not observed in telomerase-positive immortalized HeLa cells. The ECTR DNAs existed mainly in the nucleoplasm with a small amount in the cytoplasm. The nucleoplasm ECTR DNAs were co-stained with an antibody directed to the telomeric-repeat binding factor 1 (TRF1), suggesting that they exist as a complex with TRF1. In consistent with these cytological studies, Southern blot analysis showed the existence of small telomere repeat DNAs. The ECTR DNA may provide an insight into the elucidation of the mechanisms responsible for the maintenance of telomeres in telomerase-negative immortalized cells.

Essential role of mouse telomerase in highly proliferative organs

We have investigated the role of the enzyme telomerase in highly proliferative organs in successive generations of mice lacking telomerase RNA. Late-generation animals exhibited defective spermatogenesis, with increased programmed cell death (apoptosis) and decreased proliferation in the testis. The proliferative capacity of haematopoietic cells in the bone marrow and spleen was also compromised. These progressively adverse effects coincided with substantial erosion of telomeres (the termini of eukaryotic chromosomes) and fusion and loss of chromosomes. These findings indicate an essential role for telomerase, and hence telomeres, in the maintenance of genomic integrity and in the long-term viability of high-renewal organ systems.

[1997, a good year for telomerase?]

Telomerase is a ribonucleoprotein enzyme specialized in telomere elongation which may be involved in the control of cell proliferation, and consequently could play a role in oncogenesis, 1997 was a very exciting year for telomerase understanding. First, the sequence of the human telomerase catalytic subunit revealed strong homologies with known reverse transcriptases. Second, the production of viable mice deleted for the RNA moiety of the enzyme showed that telomerase is essential to maintain telomere length in germ cells but is not required for tumour formation.

Telomere maintenance without telomerase

Telomeres are nucleoprotein structures at the ends of eukaryotic chromosomes that perform a number of vital functions. They allow a cell to distinguish between natural chromosome ends and chromosome breaks in order to delay the cell cycle and repair the broken end. Telomeres also compensate for the inability of DNA polymerase to replicate the chromosome completely. In most eukaryotes a special reverse transcriptase, telomerase, adds telomeric DNA repeats to the chromosome ends using an internal RNA template. However, evidence is accumulating for alternative elongation mechanisms in a variety of eukaryotes. In the yeast Saccharomyces cerevisiae, and possibly in humans, both of which normally use telomerase, a different mechanism can be used for chromosome length maintenance when telomerase is inactive or inactivated. Yeast apparently uses recombination for this purpose; the mechanism in humans is not known. Some insect and plant species, on the other hand, do not use telomerase as their primary mechanism for maintaining chromosome length. Drosophila makes use of specific retrotransposons for this purpose, while other dipterans use recombination. We summarize here the current knowledge of these alternative telomere elongation mechanisms.

Telomere elongation in immortal human cells without detectable telomerase activity

Immortalization of human cells is often associated with reactivation of telomerase, a ribonucleoprotein enzyme that adds TTAGGG repeats onto telomeres and compensates for their shortening. We examined whether telomerase activation is necessary for immortalization. All normal human fibroblasts tested were negative for telomerase activity. Thirteen out of 13 DNA tumor virus-transformed cell cultures were also negative in the pre-crisis (i.e. non-immortalized) stage. Of 35 immortalized cell lines, 20 had telomerase activity as expected, but 15 had no detectable telomerase. The 15 telomerase-negative immortalized cell lines all had very long and heterogeneous telomeres of up to 50 kb. Hybrids between telomerase-negative and telomerase-positive cells senesced. Two senescent hybrids demonstrated telomerase activity, indicating that activation of telomerase is not sufficient for immortalization. Some hybrid clones subsequently recommenced proliferation and became immortalized either with or without telomerase activity. Those without telomerase activity also had very long and heterogeneous telomeres. Taken together, these data suggest that the presence of lengthened or stabilized telomeres is necessary for immortalization, and that this may be achieved either by the reactivation of telomerase or by a novel and as yet unidentified mechanism.