Telomerase in T lymphocytes: use it and lose it?

The enzyme telomerase counteracts telomere loss in proliferating cells and extends their capacity for replication. The importance of telomerase is highlighted by the award of the 2006 Albert Lasker Prize for Basic Medical Research for its discovery. Malignant cells subvert telomerase induction to their advantage, and up-regulation of this enzyme confers these populations with unlimited proliferative potential with obvious detrimental consequences. However this enzyme is also essential for the lifelong maintenance of normal cell populations that have a high rate of turnover. Thymic involution in early adulthood dictates that memory T cell populations have to be maintained by continuous proliferation. This highlights the inherent paradox that telomerase down-regulation in T cells may protect against malignancy yet also lead to replicative exhaustion of repeatedly activated memory T cells. In this article, we review the data on telomerase regulation in T lymphocytes and the implications this has for the maintenance of T cell memory.

Deficiencies in DNA damage repair limit the function of haematopoietic stem cells with age

A diminished capacity to maintain tissue homeostasis is a central physiological characteristic of ageing. As stem cells regulate tissue homeostasis, depletion of stem cell reserves and/or diminished stem cell function have been postulated to contribute to ageing. It has further been suggested that accumulated DNA damage could be a principal mechanism underlying age-dependent stem cell decline. We have tested these hypotheses by examining haematopoietic stem cell reserves and function with age in mice deficient in several genomic maintenance pathways including nucleotide excision repair, telomere maintenance and non-homologous end-joining. Here we show that although deficiencies in these pathways did not deplete stem cell reserves with age, stem cell functional capacity was severely affected under conditions of stress, leading to loss of reconstitution and proliferative potential, diminished self-renewal, increased apoptosis and, ultimately, functional exhaustion. Moreover, we provide evidence that endogenous DNA damage accumulates with age in wild-type stem cells. These data are consistent with DNA damage accrual being a physiological mechanism of stem cell ageing that may contribute to the diminished capacity of aged tissues to return to homeostasis after exposure to acute stress or injury.

Telomerase RNA levels limit the telomere length equilibrium

Small functional RNAs play essential roles in many biological processes. Regulating the level of these small RNAs can be as important as maintaining their function in cells. The telomerase RNA is maintained in cells at a steady-state level where small changes in concentration can have a profound impact on function. Cells that have half the level of the telomerase RNA cannot maintain telomeres through many cell divisions. People who are heterozygous for telomerase RNA mutations have the diseases dyskeratosis congenita and aplastic anemia, caused by short telomeres that result in loss of tissue renewal capacity. Mice heterozygous for telomerase RNA show haploinsufficiency in telomere length maintenance and also show loss of tissue renewal capacity. It is remarkable that small changes in the level of this functional RNA can have such profound effects in cells. This tight regulation highlights the importance of controlling the action of telomerase in cells.

Soothing the watchman: telomerase reduces the p53-dependent cellular stress response

In addition to conferring an indefinite replicative life span, telomerase renders p16(-) human mammary epithelial cells (HMEC) resistant to growth arrest by TGFbeta or by loss of EGF or insulin signaling. In contrast to earlier reports, we recently found that growth factor signaling was not directly affected by telomerase expression. Rather, short dysfunctional or near-dysfunctional telomeres in proliferating telomerase(-) HMEC sensitized the cells to p53-dependent signals for growth arrest. We showed that during serial passage and before any signs of replicative senescence, HMEC lacking telomerase experience enhanced p53 stability and DNA damage signaling, as determined by increased phosphorylation on p53-Ser15 and Chk2-Thr68, and formation of 53BP1/phosphorylated histone H2AX foci at chromosome ends. This heightened activity of the p53 pathway enhanced the efficiency with which cells arrested growth in response to TGFbeta or to EGF or insulin withdrawal, and was abolished by ectopic expression of hTERT, the catalytic subunit of telomerase. Telomerase elongated short telomeres, thereby reducing the basal level of activated p53 and raising cellular tolerance for other p53-dependent signals, including those emanating from non-genotoxic sources. These findings explain a number of observed effects of telomerase expression on cell growth and survival without postulating additional functions for telomerase.

Telomere dysfunction induces environmental alterations limiting hematopoietic stem cell function and engraftment

Cell-intrinsic checkpoints limit the proliferative capacity of primary cells in response to telomere dysfunction. It is not known, however, whether telomere dysfunction contributes to cell-extrinsic alterations that impair stem cell function and organ homeostasis. Here we show that telomere dysfunction provokes defects of the hematopoietic environment that impair B lymphopoiesis but increase myeloid proliferation in aging telomerase knockout (Terc(-/-)) mice. Moreover, the dysfunctional environment limited the engraftment of transplanted wild-type hematopoietic stem cells (HSCs). Dysfunction of the hematopoietic environment was age dependent and correlated with progressive telomere shortening in bone marrow stromal cells. Telomere dysfunction impaired mesenchymal progenitor cell function, reduced the capacity of bone marrow stromal cells to maintain functional HSCs, and increased the expression of various cytokines, including granulocyte colony-stimulating factor (G-CSF), in the plasma of aging mice. Administration of G-CSF to wild-type mice mimicked some of the defects seen in aging Terc(-/-) mice, including impairment of B lymphopoiesis and HSC engraftment. Conversely, inhibition of G-CSF improved HSC engraftment in aged Terc(-/-) mice. Taken together, these results show that telomere dysfunction induces alterations of the environment that can have implications for organismal aging and cell transplantation therapies.

Ink4a/Arf tumor suppressor does not modulate the degenerative conditions or tumor spectrum of the telomerase-deficient mouse

The Rb/p16(Ink4a) and p53/p19Arf tumor suppressor pathways have been linked to diverse cancer-relevant processes, including those governing the cellular responses to telomere dysfunction. In this study, we sought to provide direct genetic evidence of a role for the Ink4a/Arf tumor suppressor gene, encoding both p16(Ink4a) and p19(Arf), in modulating the cellular and tissue phenotypes associated with telomere dysfunction by using the mTerc Ink4a/Arf mouse model. In contrast to the rescue associated with p53 deficiency, Ink4a/Arf deficiency did not attenuate the degenerative phenotypes elicited by telomere dysfunction in the late-generation mTerc-/- mice. Furthermore, in contrast to accelerated cancer onset and increased epithelial cancers of late-generation mTerc-/- p53 mutant mice, late-generation mTerc-/- Ink4a/Arf mutant mice experienced a delayed tumor onset and maintained the lymphoma and sarcoma spectrum. Consistent with the negligible role of Ink4a/Arf in the telomere checkpoint response in vivo, late-generation mTerc-/- Ink4a/Arf-/- tissues show activated p53, and derivative tumor cell lines sustain frequent loss of p53 function, whereas all early generation mTerc Ink4a/Arf-/- tumor cell lines remain intact for p53. In addition, the late-generation mTerc-/- Ink4a/Arf-/- tumors showed activation of the alternative lengthening of telomere mechanism, underscoring the need for adaptation to the presence of telomere dysfunction in the absence of p16(Ink4a) and p19(Arf). These observations highlight the importance of genetic context in dictating whether telomere dysfunction promotes or suppresses age-related degenerative conditions as well as the rate of initiation and type of spontaneous cancers.

Telomerase abrogation dramatically accelerates TRF2-induced epithelial carcinogenesis

TRF2 is a telomere-binding protein with roles in telomere protection and telomere-length regulation. The fact that TRF2 is up-regulated in some human tumors suggests a role of TRF2 in cancer. Mice that overexpress TRF2 in the skin, K5TRF2 mice, show critically short telomeres and are susceptible to UV-induced carcinogenesis as a result of deregulated XPF/ERCC1 activity, a nuclease involved in UV damage repair. Here we demonstrate that, when in combination with telomerase deficiency, TRF2 acts as a very potent oncogene in vivo. In particular, we show that telomerase deficiency dramatically accelerates TRF2-induced epithelial carcinogenesis in K5TRF2/Terc-/- mice, coinciding with increased chromosomal instability and DNA damage. Telomere recombination is also increased in these mice, suggesting that TRF2 favors the activation of alternative telomere maintenance mechanisms. Together, these results demonstrate that TRF2 increased expression is a potent oncogenic event that along with telomerase deficiency accelerates carcinogenesis, coincidental with a derepression of telomere recombination. These results are of particular relevance given that TRF2 is up-regulated in some human cancers. Furthermore, these data suggest that telomerase inhibition might not be effective to cease the growth of TRF2-overexpressing tumors.

DNA-dependent protein kinase catalytic subunit is not required for dysfunctional telomere fusion and checkpoint response in the telomerase-deficient mouse

Telomeres are key structural elements for the protection and maintenance of linear chromosomes, and they function to prevent recognition of chromosomal ends as DNA double-stranded breaks. Loss of telomere capping function brought about by telomerase deficiency and gradual erosion of telomere ends or by experimental disruption of higher-order telomere structure culminates in the fusion of defective telomeres and/or the activation of DNA damage checkpoints. Previous work has implicated the nonhomologous end-joining (NHEJ) DNA repair pathway as a critical mediator of these biological processes. Here, employing the telomerase-deficient mouse model, we tested whether the NHEJ component DNA-dependent protein kinase catalytic subunit (DNA-PKcs) was required for fusion of eroded/dysfunctional telomere ends and the telomere checkpoint responses. In late-generation mTerc(-/-) DNA-PKcs(-/-) cells and tissues, chromosomal end-to-end fusions and anaphase bridges were readily evident. Notably, nullizygosity for DNA Ligase4 (Lig4)--an additional crucial NHEJ component--was also permissive for chromosome fusions in mTerc(-/-) cells, indicating that, in contrast to results seen with experimental disruption of telomere structure, telomere dysfunction in the context of gradual telomere erosion can engage additional DNA repair pathways. Furthermore, we found that DNA-PKcs deficiency does not reduce apoptosis, tissue atrophy, or p53 activation in late-generation mTerc(-/-) tissues but rather moderately exacerbates germ cell apoptosis and testicular degeneration. Thus, our studies indicate that the NHEJ components, DNA-PKcs and Lig4, are not required for fusion of critically shortened telomeric ends and that DNA-PKcs is not required for sensing and executing the telomere checkpoint response, findings consistent with the consensus view of the limited role of DNA-PKcs in DNA damage signaling in general.

Inhibition of gene amplification in telomerase deficient immortalized mouse embryonic fibroblasts

Mutations in genes important for the preservation of genome stability can increase the frequency of gene amplification, a process relevant to tumor development. To investigate whether telomerase, the enzyme deputed to telomere maintenance, also plays a role in gene amplification, we studied the amplification of the carbamyl-P-synthetase, aspartate transcarbamilase, dihydro-orotase (CAD) gene in immortalized embryonic fibroblasts derived from telomerase knockout mice (mTERC(-/-)) of the first and of the sixth generation. As expected, in 9 out of 10 N-(phosphonacetyl)-L-aspartate (PALA) resistant clones derived from wild-type cells, CAD was amplified; in contrast, in none of the 30 PALA resistant clones isolated from the three mTERC(-/-) cell lines we could detect CAD amplification, indicating that, in the absence of telomerase activity, gene amplification is inhibited. The causal relationship between mTERC deficiency and lack of gene amplification was demonstrated by the restoration of CAD gene amplification in two of the three deficient cell lines transfected with mTERC. The lack of amplification in mTERC deficient cells could be related to a defect in the stabilization of the ends of the amplified chromosomes in the absence of telomerase, to a more general effect of telomerase in the regulation of gene expression, including genes involved in amplification, or to a possible interaction of the telomerase RNA with proteins involved in gene amplification.

Genetic analysis of myc and telomerase interactions in vivo

Myc is a transcription factor with pleiotropic effects on tumorigenesis which are likely to be mediated by its target genes. A known Myc transcriptional target is the catalytic subunit of telomerase, Tert. However, the contribution of Tert activation to Myc-induced tumorigenesis in vivo remains unknown. In this study, we addressed the role of telomerase in Myc-induced skin papillomatosis by using compound mice with a switchable Myc gene, Inv-MycERTAM mice, in combination with either telomerase deficiency (Terc-/-) or telomerase overexpression (K5-mTert) in the skin. We first demonstrated that Myc activates telomerase in the skin. With Inv-MycERTAM x Terc-/- mice, we further showed that this telomerase activation is partially required to elicit a full hyperplastic Myc-induced response. The presence of critically short telomeres in late-generation Inv-MycERTAM x Terc-/- mice further reduced the skin lesion induced by Myc. On the other hand, telomerase overexpression in the skin of K5-mTert mice augments Myc-induced hyperplasia in the absence of changes in telomere length, suggesting a direct role of telomerase in the Myc protumorigenic response. Taken together, these results highlight telomerase as a mediator of Myc-induced papillomatosis and suggest telomerase as a putative therapeutic target for Myc-dependent lesions.

Telomere-related genome instability in cancer

Genome instability is a hallmark of most human cancers. Although a mutator phenotype is not required for tumorigenesis, it can foster mutations that promote tumor progression. Indeed, several inherited cancer-prone syndromes are due to mutations in DNA repair pathways. However, sporadic tumors are usually proficient in DNA repair, making it unlikely that unrepaired lesions are a major source of genome instability in sporadic cancers. A decade ago, I argued in another CSHL Press publication that a "collapse in telomere function can explain a significant portion of the genetic instability in tumors" (de Lange 1995). Since that time, the structure of mammalian telomeres has been analyzed, the consequences of telomere dysfunction have been determined, a mouse model for cancer-relevant aspects of telomere biology has been developed, and the nature and magnitude of cancer genome rearrangements have been revealed. In light of these developments, this is an opportune time to revisit the conjecture that telomere dysfunction contributes to genome instability in human cancer.

Mice Deficient in Telomerase Activity Develop Hypertension Because of an Excess of Endothelin Production

Background— Telomere shortening has been related to vascular dysfunction and hypertension. In the present study, we analyzed the influence of telomerase deficiency and telomere shortening on arterial pressure (AP).

Methods and Results— AP was evaluated in 6-month-old mice lacking the RNA component of the telomerase ( terc −/− ) at the first generation and third generation (G3). First generation and G3 mice showed higher AP than wild-type (WT) mice. To analyze the mechanisms involved, mean AP and vascular resistance in response to vasoactive substances were measured in G3 and WT mice. These mice showed similar responses to acetylcholine, N G -nitro- l -arginine methyl ester, angiotensin II, and losartan administration. Mean AP did not increase after endothelin-1 (ET-1) administration in G3 mice, but it did in WT animals. Bosentan treatment decreased mean AP only in G3 mice. Serum and urine concentrations of ET-1 were higher in terc −/− than in WT mice. Endothelin-converting enzyme (ECE-1) mRNA expression was higher in terc −/− animals than in the WT group. FR901533, an ECE antagonist, decreased blood pressure in conscious G3 mice. Studies in mouse embryonic fibroblasts from G3 mice suggest that ECE-1 overexpression could be mediated by reactive oxygen species in an AP-1–dependent mechanism, in which some kinases such as PI3-kinase, Akt, erk1/2, and Jun Kinase could be involved. An increased activity of nicotinamide adenine dinucleotide phosphate oxidase seems to be the main source of reactive oxygen species.

Conclusions— Mice lacking telomerase activity show hypertension as a result of an increase in plasma ET-1 levels, which is a consequence of ECE-1 overexpression. A direct link between telomerase activity and hypertension is reported.

Cooperative interactions of p53 mutation, telomere dysfunction, and chronic liver damage in hepatocellular carcinoma progression

Hepatocellular carcinoma is among the most common and lethal cancers in humans. Hepatocellular carcinoma is commonly associated with physical or functional inactivation of the p53 tumor suppressor, high levels of chromosomal instability, and disease conditions causing chronic cycles of hepatocyte death and regeneration. Mounting evidence has implicated regeneration-induced telomere erosion as a potential mechanism fueling genome instability. In mouse models of hepatocellular carcinoma, telomere dysfunction has been shown to enhance initiation of hepatic neoplasias yet constrain full malignant progression of these neoplasms possibly due to activation of a p53-dependent checkpoint and/or intolerable levels of genomic instability. Here, in a hepatocellular carcinoma-prone model brought about through toxin-induced hepatocyte injury and regeneration, we sought to determine the cooperative interactions of germ line p53 mutation and telomere dysfunction [produced by telomerase reverse transcriptase (mTERT) gene knockout]. In the setting of intact telomeres, p53 mutation had no effect on hepatocarcinogenesis, whereas in the setting of telomere dysfunction, p53 mutation enabled advanced hepatocellular carcinoma disease. Notably, there was no evidence of deletion or mutation of the wild-type p53 allele in the late generation mTert(-/-)p53(+/-) mice, suggesting that reduced levels of p53 potently enable hepatocellular carcinoma progression in the setting of telomere dysfunction. Thus, this study supports a model that, in the face of chronic liver damage, attenuated p53 function and telomere-induced chromosomal instability play critical and cooperative roles in the progression of hepatocellular carcinoma.

Immunodetection of human telomerase reverse-transcriptase (hTERT) re-appraised: nucleolin and telomerase cross paths

The involvement of telomerase in cellular immortalization and senescence has often been assessed by means of telomerase expression at the RNA level and quantification of telomerase activity by the telomeric repeat amplification protocol assay. However, these methods either neglected the existence of various telomerase splice variants, or ignored the nonconventional functions of telomerase independent of its ability to elongate and maintain telomere length. Immunodetection of telomerase is now being recognized as a necessary approach to precisely elucidate its roles in oncogenesis and senescence. A few antibodies directed against the catalytic subunit of the human telomerase (hTERT) are currently used but their specificity is not always demonstrated. A survey of the literature showed inconsistencies and led us to comparatively re-evaluate the most frequently used antibodies. Surprisingly, mass spectrometry, two-dimensional gel analysis and immunofluorescent experiments revealed that the most frequently used hTERT immunoprobe, a mouse monoclonal antibody that was claimed to be directed against an hTERT protein epitope, in fact recognizes nucleolin rather than telomerase. Our findings have interesting implications regarding the biology of nucleolin and telomerase in the context of pathophysiological investigations recently carried out.

Telomerase- and Alternative Telomere Lengthening–Independent Telomere Stabilization in a Metastasis-Derived Human Non–Small Cell Lung Cancer Cell Line: Effect of Ectopic hTERT

In the majority of human malignancies, maintenance of telomeres is achieved by reactivation of telomerase, whereas a smaller fraction uses an alternative telomere lengthening (ALT) mechanism. Here, we used 16 non-small cell lung cancer (NSCLC) cell lines to investigate telomere stabilization mechanisms and their effect on tumor aggressiveness. Three of 16 NSCLC cell lines (VL-9, SK-LU-1, and VL-7) lacked telomerase activity, correlating with significantly reduced tumorigenicity in vitro and in vivo. Of the three telomerase-negative cell lines, only SK-LU-1 displayed characteristics of an ALT mechanism (i.e., highly heterogeneous telomeres and ALT-associated promyelocytic leukemia bodies). VL-9 cells gained telomerase during in vitro propagation, indicating incomplete immortalization in vivo. In contrast, NSCLC metastasis-derived VL-7 cells remained telomerase and ALT negative up to high passage numbers and following transplantation in severe combined immunodeficient mice. Telomeres of VL-7 cells were homogeneously short, and chromosomal instability (CIN) was comparable with most telomerase-positive cell lines. This indicates the presence of an efficient telomere stabilization mechanism different from telomerase and ALT in VL-7 cells. To test the effect of ectopic telomerase reverse transcriptase (hTERT) in these unique ALT- and telomerase-negative tumor backgrounds, hTERT was transfected into VL-7 cells. The activation of telomerase led to an excessively rapid gain of telomeric sequences resulting in very long ( approximately 14 kb), uniform telomeres. Additionally, hTERT expression induced a more aggressive growth behavior in vitro and in vivo without altering the level of CIN. These data provide further evidence for a direct oncogenic activity of hTERT not based on the inhibition of CIN.

Semi-conservative DNA replication through telomeres requires Taz1

Telomere replication is achieved through the combined action of the conventional DNA replication machinery and the reverse transcriptase, telomerase. Telomere-binding proteins have crucial roles in controlling telomerase activity; however, little is known about their role in controlling semi-conservative replication, which synthesizes the bulk of telomeric DNA. Telomere repeats in the fission yeast Schizosaccharomyces pombe are bound by Taz1, a regulator of diverse telomere functions. It is generally assumed that telomere-binding proteins impede replication fork progression. Here we show that, on the contrary, Taz1 is crucial for efficient replication fork progression through the telomere. Using two-dimensional gel electrophoresis, we find that loss of Taz1 leads to stalled replication forks at telomeres and internally placed telomere sequences, regardless of whether the telomeric G-rich strand is replicated by leading- or lagging-strand synthesis. In contrast, the Taz1-interacting protein Rap1 is dispensable for efficient telomeric fork progression. Upon loss of telomerase, taz1Delta telomeres are lost precipitously, suggesting that maintenance of taz1Delta telomere repeats cannot be sustained through semi-conservative replication. As the human telomere proteins TRF1 and TRF2 are Taz1 orthologues, we predict that one or both of the human TRFs may orchestrate fork passage through human telomeres. Stalled forks at dysfunctional human telomeres are likely to accelerate the genomic instability that drives tumorigenesis.

Increased p53 activity does not accelerate telomere-driven ageing

There is a great interest in determining the impact of p53 on ageing and, for this, it is important to discriminate among the known causes of ageing. Telomere loss is a well-established source of age-associated damage, which by itself can recapitulate ageing in mouse models. Here, we have used a genetic approach to interrogate whether p53 contributes to the elimination of telomere-damaged cells and its impact on telomere-driven ageing. We have generated compound mice carrying three functional copies of the p53 gene (super-p53) in a telomerase-deficient background and we have measured the presence of chromosomal abnormalities and DNA damage in several tissues. We have found that the in vivo load of telomere-derived chromosomal damage is significantly decreased in super-p53/telomerase-null mice compared with normal-p53/telomerase-null mice. Interestingly, the presence of extra p53 activity neither accelerates nor delays telomere-driven ageing. From these observations, we conclude that p53 has an active role in eliminating telomere-damaged cells, and we exclude the possibility of an age-promoting effect of p53 on telomere-driven ageing.

Expression of mTert in primary murine cells links the growth-promoting effects of telomerase to transforming growth factor-β signaling

Here, we show that ectopic expression of the catalytic subunit of mouse telomerase (mTert) confers a growth advantage to primary murine embryonic fibroblasts (MEFs), which have very long telomeres, as well as facilitates their spontaneous immortalization and increases their colony-forming capacity upon activation of oncogenes. We demonstrate that these telomere length-independent growth-promoting effects of mTert overexpression require catalytically active mTert, as well as the formation of mTert/Terc complexes. The gene expression profile of mTert-overexpressing MEFs indicates that telomerase enhances growth in these cells through the repression of growth-inhibiting genes of the transforming growth factor-beta (TGF-beta) signaling network. We functionally validate this result by showing that mTert abrogates the growth-inhibitory effect of TGF-beta in MEFs, thus demonstrating that telomerase increments the proliferative potential of primary mouse embryonic fibroblasts by targeting the TGF-beta pathway.

Telomere dynamics in genome stability

The past several years have seen an increasing interest in telomere recombinational interactions that provide many functions in telomere capping, in telomere size homeostasis and in overcoming the catastrophic effects of telomerase deficiency. Several key recombination mechanisms have emerged from recent investigations. In the yeasts, these mechanisms include exchange between subtelomeric regions and telomere sequences, rapid telomere expansion and telomere deletion. These processes proceed by pathways that use both the cellular recombination machinery and novel mechanisms such as rolling circle replication. The insights gained from recent studies extend our understanding of similar processes in higher eukaryotes and suggest that the recombinational dynamics of telomeres have additional roles that contribute to genomic stability and instability.

Pathophysiologic mechanisms in acquired aplastic anemia

Aplastic anemia, an unusual hematologic disease, is the paradigm of the human bone marrow failure syndromes. Absence of hematopoietic cells has been recognized from the characteristic morphology for a century; an immune pathophysiology has been inferred from improvement in blood counts with immunosuppressive therapy in the majority of patients. Molecular mechanisms underlying both T cell effector cells and the target marrow stem and progenitor cells are now being identified. Activated type 1 cytotoxic T cells and type 1 cytokines have been implicated in cell culture experiments; clues to the molecular basis of the aberrant immune response include cytokine gene polymorphisms and abnormalities in the regulatory pathways for gamma-interferon. For stem cell depletion, mutations in genes of the telomere repair complex are present in some patients with apparently acquired aplastic anemia. Telomerase deficiency is associated with short telomeres and a quantitative reduction in marrow progenitors and likely also a qualitative deficiency in the repair capacity of hematopoietic tissue.

Haploinsufficiency of telomerase reverse transcriptase leads to anticipation in autosomal dominant dyskeratosis congenita

Dyskeratosis congenita is a rare inherited disorder characterized by abnormal skin manifestations. Morbidity and mortality from this disease is usually due to bone marrow failure, but idiopathic pulmonary fibrosis and an increased cancer predisposition also occur. Families with autosomal dominant dyskeratosis congenita display anticipation and have mutations in the telomerase RNA gene. We identified a three-generation pedigree with autosomal dominant dyskeratosis congenita, anticipation, and telomere shortening. We show that a null mutation in motif D of the reverse transcriptase domain of the protein component of telomerase, hTERT, is associated with this phenotype. This mutation leads to haploinsufficiency of telomerase, and telomere shortening occurs despite the presence of telomerase. This finding emphasizes the importance of telomere maintenance and telomerase dosage for maintaining tissue proliferative capacity and has relevance for understanding mechanisms of age-related changes.

Telomere sister chromatid exchange in telomerase deficient murine cells

We have recently demonstrated that several types of genomic rearrangements (i.e., telomere sister chromatid exchange (T-SCE), genomic-SCE, or end-to-end fusions) were more often detected in long-term cultured murine telomerase deficient embryonic stem (ES) cells than in freshly prepared murine splenocytes, even through they possessed similar frequencies of critically short telomeres. The high rate of genomic rearrangements in telomerase deficient ES cells, when compared to murine splenocytes, may reflect the cultured cells' gained ability to protect chromosome ends with eroded telomeres allowing them to escape "end crisis". However, the possibility that ES cells were more permissive to genomic rearrangements than other cell types or that differences in the microenvironment or genetic background of the animals might consequentially determine the rate of T-SCEs or other genomic rearrangements at critically short telomeres could not be ruled out.

Conditional telomerase induction causes proliferation of hair follicle stem cells

TERT, the protein component of telomerase, serves to maintain telomere function through the de novo addition of telomere repeats to chromosome ends, and is reactivated in 90% of human cancers. In normal tissues, TERT is expressed in stem cells and in progenitor cells, but its role in these compartments is not fully understood. Here we show that conditional transgenic induction of TERT in mouse skin epithelium causes a rapid transition from telogen (the resting phase of the hair follicle cycle) to anagen (the active phase), thereby facilitating robust hair growth. TERT overexpression promotes this developmental transition by causing proliferation of quiescent, multipotent stem cells in the hair follicle bulge region. This new function for TERT does not require the telomerase RNA component, which encodes the template for telomere addition, and therefore operates through a mechanism independent of its activity in synthesizing telomere repeats. These data indicate that, in addition to its established role in extending telomeres, TERT can promote proliferation of resting stem cells through a non-canonical pathway.

Telomerase-independent cell survival in Arabidopsis thaliana

Telomerase is the reverse transcriptase responsible for the maintenance of telomeric repeat sequences in most species that have been studied. Inactivation of telomerase causes telomere shortening and results in the loss of the telomere's protective function, which in mammals leads to cell-cycle arrest and apoptosis. Experiments performed on Arabidopsis thaliana mutants lacking telomerase activity revealed their unusually high tolerance for genome instability. Here we present molecular and cytogenetic analysis of two cell lines (A and B) derived from seeds of late-generation telomerase-deficient A. thaliana. These cultures have survived for about 3 years and are still viable. However, neither culture has adapted mechanisms to maintain terminal telomeric repeats. One culture (B) suffers from severe growth irregularities and a high degree of mortality. Karyological analysis revealed dramatic genomic rearrangements, a large variation in ploidy, and an extremely high percentage of anaphase bridges. The second cell line (A) survived an apparent crisis and phenotypically appears wild-type with respect to growth and morphology. Despite these indications of genome stabilization, a high percentage of anaphase bridges was observed in the A line. We conclude that the restructured chromosome termini provide the A line with partial protection from end-joining repair activities, thus allowing normal growth.

An increase in telomere sister chromatid exchange in murine embryonic stem cells possessing critically shortened telomeres

Telomerase deficiency leads to a progressive loss of telomeric DNA that eventually triggers cell apoptosis in human primary cells during prolonged growth in culture. Rare survivors can maintain telomere length through either activation of telomerase or recombination-based telomere lengthening, and thus proliferate indefinitely. We have explored the possibility that telomeres may be maintained through telomere sister chromatid exchange (T-SCE) in murine telomere reverse transcriptase-deficient (mTert-/-) splenocytes and ES cells. Because telomerase deficiency leads to gradual loss of telomeric DNA in mTert-/- splenocytes and ES cells and eventually to chromosomes with telomere signal-free ends (SFEs), we examined these cell types for evidence of sister chromatid exchange at telomeres, and observed an increase in T-SCEs only in a subset of mTert-/- splenocytes or ES cells that possessed multiple SFEs. Furthermore, T-SCEs were more often detected in ES cells than in splenocytes that harbored a similar frequency of SFEs. In mTert heterozygous (mTert+/-) ES cells or splenocytes, which are known to exhibit a decrease in average telomere length but no SFEs, no increase in T-SCE was observed. In addition to T-SCE, other genomic rearrangements (i.e., SCE) were also significantly increased in mTert-/- ES cells possessing critically short telomeres, but not in splenocytes. Our results suggest that animals and cell culture differ in their ability to carry out genomic rearrangements as a means of maintaining telomere integrity when telomeres become critically shortened.