Cord blood T cells retain early differentiation phenotype suitable for immunotherapy after TCR gene transfer to confer EBV specificity

Adoptive T cell therapy can be effective for Epstein-Barr virus (EBV)-associated posttransplant lymphoproliferative disease and melanoma. Transducing high-affinity TCR genes into T lymphocytes is an emerging method to improve potency and specificity of tumor-specific T cells. However, both methods necessitate in vitro lymphocyte proliferation, generating highly differentiated effector cells that display reduced survival and antitumor efficacy postinfusion. TCR-transduction of naive lymphocytes isolated from peripheral blood is reported to provide superior in vivo survival and function. We utilized cord blood (CB) lymphocytes, which comprise mainly naive cells, for transducing EBV-specific TCR. Comparable TCR expression was achieved in adult and CB cells, but the latter expressed an earlier differentiation profile. Further antigen-driven stimulation skewed adult lymphocytes to a late differentiation phenotype associated with immune exhaustion. In contrast, CB T cells retained a less differentiated phenotype after antigen stimulation, remaining CD57-negative but were still capable of antigen-specific polyfunctional cytokine expression and cytotoxicity in response to EBV antigen. CB T cells also retained longer telomeres and in general possessed higher telomerase activity indicative of greater proliferative potential. CB lymphocytes therefore have qualities indicating prolonged survival and effector function favorable to immunotherapy, especially in settings where donor lymphocytes are unavailable such as in solid organ and CB transplantation.

Explanation for excessive DNA single-strand breaks and endogenous repair foci in pluripotent mouse embryonic stem cells

Pluripotent mouse embryonic stem cells (mES cells) exhibit approximately 100 large gammaH2AX repair foci in the absence of measurable numbers of DNA double-strand breaks. Many of these cells also show excessive numbers of DNA single-strand breaks (>10,000 per cell) when analyzed using the alkaline comet assay. To understand the reasons for these unexpected observations, various methods for detecting DNA strand breaks were applied to wild-type mES cells and to mES cells lacking H2AX, ATM, or DNA-PKcs. H2AX phosphorylation and expression of other repair complexes were measured using flow and image analysis of antibody-stained cells. Results indicate that high numbers of endogenous gammaH2AX foci and single-strand breaks in pluripotent mES cells do not require ATM or DNA-PK kinase activity and appear to be associated with global chromatin decondensation rather than pre-existing DNA damage. This will limit applications of gammaH2AX foci analysis in mES cells to relatively high levels of initial or residual DNA damage. Excessive numbers of single-strand breaks in the alkaline comet assay can be explained by the vulnerability of replicating chromatin in mES cells to osmotic shock. This suggests that caution is needed in interpreting results with the alkaline comet assay when applied to certain cell types or after treatment with agents that make chromatin vulnerable to osmotic changes. Differentiation of mES cells caused a reduction in histone acetylation, gammaH2AX foci intensity, and DNA single-strand breakage, providing a link between chromatin structural organization, excessive gammaH2AX foci, and sensitivity of replicating mES cell chromatin to osmotic shock.

Mouse but not human embryonic stem cells are deficient in rejoining of ionizing radiation-induced DNA double-strand breaks

Mouse embryonic stem (mES) cells will give rise to all of the cells of the adult mouse, but they failed to rejoin half of the DNA double-strand breaks (dsb) produced by high doses of ionizing radiation. A deficiency in DNA-PK(cs) appears to be responsible since mES cells expressed <10% of the level of mouse embryo fibroblasts (MEFs) although Ku70/80 protein levels were higher than MEFs. However, the low level of DNA-PK(cs) found in wild-type cells appeared sufficient to allow rejoining of dsb after doses <20Gy even in G1 phase cells. Inhibition of DNA-PK(cs) with wortmannin and NU7026 still sensitized mES cells to radiation confirming the importance of the residual DNA-PK(cs) at low doses. In contrast to wild-type cells, mES cells lacking H2AX, a histone protein involved in the DNA damage response, were radiosensitive but they rejoined double-strand breaks more rapidly. Consistent with more rapid dsb rejoining, H2AX(-/-) mES cells also expressed 6 times more DNA-PK(cs) than wild-type mES cells. Similar results were obtained for ATM(-/-) mES cells. Differentiation of mES cells led to an increase in DNA-PK(cs), an increase in dsb rejoining rate, and a decrease in Ku70/80. Unlike mouse ES, human ES cells were proficient in rejoining of dsb and expressed high levels of DNA-PK(cs). These results confirm the importance of homologous recombination in the accurate repair of double-strand breaks in mES cells, they help explain the chromosome abnormalities associated with deficiencies in H2AX and ATM, and they add to the growing list of differences in the way rodent and human cells deal with DNA damage.

Short telomeres and high telomerase activity in T-cell prolymphocytic leukemia

To test the role of telomere biology in T-cell prolymphocytic leukemia (T-PLL), a rare aggressive disease characterized by the expansion of a T-cell clone derived from immuno-competent post-thymic T-lymphocytes, we analyzed telomere length and telomerase activity in subsets of peripheral blood leukocytes from 11 newly diagnosed or relapsed patients with sporadic T-PLL. Telomere length values of the leukemic T cells (mean+/-s.d.: 1.53+/-0.65 kb) were all below the 1st percentile of telomere length values observed in T cells from healthy age-matched controls whereas telomere length of normal T- and B cells fell between the 1st and 99th percentile of the normal distribution. Leukemic T cells exhibited high levels of telomerase and were sensitive to the telomerase inhibitor BIBR1532 at doses that showed no effect on normal, unstimulated T cells. Targeting the short telomeres and telomerase activity in T-PLL seems an attractive strategy for the future treatment of this devastating disease.

Dissociation of telomerase activity and telomere length maintenance in primitive human hematopoietic cells

Primitive human hematopoietic cells have low endogenous telomerase activity, yet telomeres are not maintained. In contrast, ectopic telomerase expression in fibroblasts and other cells leads to telomere length maintenance or elongation. It is unclear whether this disparity can be attributed to telomerase level or stems from fundamentally different telomere biology. Here, we show that telomerase overexpression does not prevent proliferation-associated telomere shortening in human hematopoietic cells, pointing to the existence of cell type-specific differences in telomere dynamics. Furthermore, we observed eventual stabilization of telomere length without detectable changes in telomerase activity during establishment of two leukemic cell lines from normal cord blood cells, indicating that additional cooperating events are required for telomere maintenance in immortalized human hematopoietic cells.

Telomerase is limiting the growth of acute myeloid leukemia cells

Telomeres play an important role in the proliferation and senescence of normal and malignant cells. To test the role of telomerase in acute myeloid leukemia (AML), we expressed the telomerase reverse transcriptase (hTERT) gene, a dominant-negative hTERT (DN-hTERT) (D868A, D869A) gene, or a gene encoding green fluorescence protein (GFP) in the leukemia cell line K562 and in primary AML cells from different patients, using retroviral vectors. Cells transduced with hTERT exhibited elevated levels of telomerase activity compared to GFP controls, whereas cells expressing DN-hTERT had decreased telomerase activity. K562 populations transduced with DN-hTERT showed reduced clonogenicity, telomere dysfunction and increased numbers of apoptotic cells compared to GFP- or hTERT-transduced cells. Two of four clones transduced with DN-hTERT died after 30 and 53 population doublings, respectively. Transduced AML cells were tested in primary colony-forming unit (CFU) and suspension culture assays. Relative to hTERT- and GFP-transduced controls, AML cells transfected with DN-hTERT produced fewer CFU and showed lower engraftment after transplantation into sublethally irradiated beta(2)-m(-/-) nonobese diabetic/severe combined immunodeficient mice. We conclude that telomerase is limiting the growth of the leukemic cell line K562 and primary AML progenitor cells. Our data warrant further studies of the therapeutic use of telomerase inhibitors in AML.

Effects of DNA nonhomologous end-joining factors on telomere length and chromosomal stability in mammalian cells

DNA repair by nonhomologous end-joining (NHEJ) relies on the Ku70:Ku80 heterodimer in species ranging from yeast to man. In Saccharomyces cerevisiae and Schizosaccharomyces pombe, Ku also controls telomere functions. Here, we show that Ku70, Ku80, and DNA-PKcs, with which Ku interacts, associate in vivo with telomeric DNA in several human cell types, and we show that these associations are not significantly affected by DNA-damaging agents. We also demonstrate that inactivation of Ku80 or Ku70 in the mouse yields telomeric shortening in various primary cell types at different developmental stages. By contrast, telomere length is not altered in cells impaired in XRCC4 or DNA ligase IV, two other NHEJ components. We also observe higher genomic instability in Ku-deficient cells than in XRCC4-null cells. This suggests that chromosomal instability of Ku-deficient cells results from a combination of compromised telomere stability and defective NHEJ.

Transfer of the human telomerase reverse transcriptase (TERT) gene into T lymphocytes results in extension of replicative potential

In most human somatic cells telomeres progressively shorten with each cell division eventually leading to chromosomal instability and cell senescence. The loss of telomere repeats with cell divisions may also limit the replicative life span of antigen-specific T lymphocytes. Recent studies have shown that the replicative life span of various primary human cells can be prolonged by induced expression of the telomerase reverse transcriptase (hTERT) gene. To test whether introduction of hTERT can extend the life span of primary human T lymphocytes, naive CD8(+) T lymphocytes were transfected with retroviral vectors containing the hTERT gene. Transduced T-cell clones expressed high levels of telomerase and either maintained or elongated their telomere lengths upon culture for extended periods of time. Two of the transduced subclones retained a normal cloning efficiency for more than 170 population doublings (PDs). In contrast, T-cell clones transfected with control vectors exhibited progressive telomere length shortening and stopped proliferation at around 108 PDs. Telomerase-positive T clones had a normal 46,XY karyotype, maintained their cytotoxic properties, and showed very little staining for the apoptotic marker annexin-V. These results indicate that ectopic hTERT gene expression is capable of extending the replicative life span of primary human CD8(+) cytotoxic T lymphocytes. (Blood. 2001;98:597-603)

CD8+ T cells in large granular lymphocyte leukemia are not defective in activation- and replication-related apoptosis

Persistent lymphocytosis in large granular lymphocyte leukemia (LGL) may result from defects in activation- or Fas crosslinking-induced cell death. Here we show that Fas crosslinking and CD3 activation causes apoptosis of in vitro activated CD8 T cells, but not of freshly isolated CD8 T cells. Death was partially blocked by a neutralizing antibody to FasL. Inhibition of metalloproteinase-mediated FasL solubilization significantly potentiated induction of cell death. Furthermore, CD3 plus CD28 stimulation resulted in telomeric erosion in LGL cells, and ultimately proliferation ceased. Together, these data indicate that activation- and proliferation-related cell death mechanisms are functional in LGL cells.

DNA strand break-sensing molecule poly(ADP-Ribose) polymerase cooperates with p53 in telomere function, chromosome stability, and tumor suppression

Genomic instability is often caused by mutations in genes that are involved in DNA repair and/or cell cycle checkpoints, and it plays an important role in tumorigenesis. Poly(ADP-ribose) polymerase (PARP) is a DNA strand break-sensing molecule that is involved in the response to DNA damage and the maintenance of telomere function and genomic stability. We report here that, compared to single-mutant cells, PARP and p53 double-mutant cells exhibit many severe chromosome aberrations, including a high degree of aneuploidy, fragmentations, and end-to-end fusions, which may be attributable to telomere dysfunction. While PARP(-/-) cells showed telomere shortening and p53(-/-) cells showed normal telomere length, inactivation of PARP in p53(-/-) cells surprisingly resulted in very long and heterogeneous telomeres, suggesting a functional interplay between PARP and p53 at the telomeres. Strikingly, PARP deficiency widens the tumor spectrum in mice deficient in p53, resulting in a high frequency of carcinomas in the mammary gland, lung, prostate, and skin, as well as brain tumors, reminiscent of Li-Fraumeni syndrome in humans. The enhanced tumorigenesis is likely to be caused by PARP deficiency, which facilitates the loss of function of tumor suppressor genes as demonstrated by a high rate of loss of heterozygosity at the p53 locus in these tumors. These results indicate that PARP and p53 interact to maintain genome integrity and identify PARP as a cofactor for suppressing tumorigenesis.

Telomere length dynamics in normal individuals and in patients with hematopoietic stem cell-associated disorders

The telomere length in nucleated peripheral blood (PB) cells indirectly reflects the mitotic history of their precursors: the hematopoietic stem cells (HSCs). The average length of telomeres in PB leukocytes can be measured using fluorescence in situ hybridization and flow cytometry (flow FISH). We previously used flow FISH to characterize the age-related turnover of HSCs in healthy individuals. In this review, we describe results of recent flow FISH studies in patients with selected hematopoietic stem cell-associated disorders: chronic myelogenous leukemia (CML) and several bone marrow failure syndromes. CML is characterized by a marked expansion of myeloid Philadelphia chromosome positive (Ph+) cells. Nevertheless, nonmalignant (Ph-) HSCs typically coexist in the bone marrow of CML patients. We analyzed the telomere length in > 150 peripheral blood leukocytes (PBLs) and bone marrow samples of patients with CML as well as samples of Ph- T-lymphocytes. Compared to normal controls, the overall telomere fluorescence in PBLs of patients with CML was significantly reduced. However, no telomere shortening was observed in Ph- T-lymphocytes. Patients in late chronic phase (CP) had significantly shorter telomeres than those assessed earlier in CP. Our data suggest that progressive telomere shortening is correlated with disease progression in CML. Within the group of patients with bone marrow failure syndromes, we only found significantly shortened telomeres (compared to age-adjusted controls) in granulocytes from patients with aplastic anemia (AA). Strikingly, the telomere length in granulocytes from AA patients who had recovered after immunosuppressive therapy (recAA) did not differ significantly from controls, whereas untreated patients and nonresponders with persistent severe pancytopenia (sAANR) showed marked and significant telomere shortening compared to healthy donors and patients with recAA. Furthermore, an inverse correlation between age-adjusted telomere length and peripheral blood counts was found in support of a model in which the degree of cytopenia and the amount of telomere shortening are correlated. These results support the concept of extensive proliferation of HSCs in subgroups of AA patients and suggest a potential use of telomere-length measurements as a prognostic tool in this group of disorders as well.

Limited telomere shortening in hematopoietic stem cells after transplantation

The number of cell divisions in hematopoietic stem cells (HSCs) following transplantation of bone marrow or mobilized peripheral blood into myelo-ablated recipients is unknown. This number is expected to depend primarily on the number of transplanted stem cells, assuming that stem cells do not differ in engraftment potential and other functional properties. In a previous study, we found that the telomere length in circulating granulocytes in normal individuals shows a biphasic decline with age, most likely reflecting age-related changes in the turnover of HSCs. In order to study HSCs' proliferation kinetics following stem cells transplantation, we analyzed the telomere length in donor-derived nucleated blood cells in four HLA-matched bone marrow transplant recipients relative to comparable cells from the sibling donors. In each case, the telomeres in granulocytes were shorter in the recipient than in the donor. This difference was established in the first year post transplantation and did not change after that. The telomere length in naïve and memory T cells showed marked differences after transplantation, complicating the interpretation of telomere length data using unseparated nucleated blood cells. Interestingly, the telomere length in naïve T cells that were first observed six months post transplantation was very similar in donor and recipient pairs. Our observations are compatible with a limited number of additional cell divisions in stem cell populations after bone marrow transplantations and support the idea that different populations of stem cells contribute to short-term myeloid and long-term lympho myeloid hematopoiesis.

Extra-chromosomal telomeric DNA in cells from Atm(-/-) mice and patients with ataxia-telangiectasia

Ataxia-telangiectasia (AT) is an autosomally recessive human genetic disease with pleiotropic defects such as neurological degeneration, immunodeficiency, chromosomal instability, cancer susceptibility and premature aging. Cells derived from AT patients and ataxia-telangiectasia mutated (ATM)-deficient mice show slow growth in culture and premature senescence. ATM, which belongs to the PI3 kinase family along with DNA-PK, plays a major role in signaling the p53 response to DNA strand breaks. Telomere maintenance is perturbed in yeast strains lacking genes homologous to ATM and cells from patients with AT have short telomeres. We examined the length of individual telomeres in cells from ATM(-/-) mice by fluorescence in situ hybridization. Telomeres were extensively shortened in multiple tissues of ATM(-/-) mice. More than the expected number of telomere signals was observed in interphase nuclei of ATM(-/-) mouse fibroblasts. Signals corresponding to 5-25 kb of telomeric DNA that were not associated with chromosomes were also noticed in ATM(-/-) metaphase spreads. Extrachromosomal telomeric DNA was also detected in fibroblasts from AT patients and may represent fragmented telomeres or by-products of defective replication of telomeric DNA. These results suggest a role of ATM in telomere maintenance and replication, which may contribute to the poor growth of ATM(-/-) cells and increased tumor incidence in both AT patients and ATM(-/-) mice.

Telomere length in leukocyte subpopulations of patients with aplastic anemia

In most human cells, the average length of telomere repeats at the ends of chromosomes provides indirect information about their mitotic history. To study the turnover of stem cells in patients with bone marrow failure syndromes, the telomere length in peripheral blood granulocytes and lymphocytes from patients with aplastic anemia (AA, n = 56) and hemolytic paroxysmal nocturnal hemoglobinuria (n = 6) was analyzed relative to age-matched controls by means of fluorescence in situ hybridization and flow cytometry. The telomere lengths in granulocytes from patients with AA were found to be significantly shorter than those in age-adjusted controls (P =.001). However, surprisingly, telomere length in granulocytes from AA patients who had recovered after immunosuppressive therapy did not differ significantly from controls, whereas untreated patients and nonresponders with persistent severe pancytopenia showed marked and significant telomere shortening. These results support extensive proliferation of hematopoietic stem cells in subgroups of AA patients. Because normal individuals show significant variation in telomere length, individual measurements in blood cells from AA patients may be of limited value. Whether sequential telomere length measurements can be used as a prognostic tool in this group of disorders remains to be clarified. (Blood. 2001;97:895-900)

Accelerated telomere shortening in hematological lineages is limited to the first year following stem cell transplantation

Using quantitative fluorescence in situ hybridization and flow cytometry, the telomere length of telomere repeat sequences after stem cell transplantation (SCT) were measured. The study included the telomeres of peripheral blood monocytes that should reflect the length of telomeres in stem cells and the telomeres of T lymphocytes that could shorten as a result of peripheral expansion. The loss of telomeres in monocytes and in memory T cells, although accelerated initially, became comparable to the loss of telomeres in healthy controls from the second year after transplantation. In addition, the telomere length in the naive T cells that were produced by the thymus was comparable to the telomere length in the naive T cells of the donor. Compared to the total length of telomeres available, the loss of telomere repeats in leukocytes after SCT resembles the accelerated shortening seen in early childhood and remains, therefore, relatively insignificant.

Multicolor fluorescence in situ hybridization with peptide nucleic acid probes for enumeration of specific chromosomes in human cells

In previous studies, we showed that peptide nucleic acid (PNA) probes have significant advantages over conventional synthetic RNA or DNA probes in FISH procedures for detecting telomeric and trinucleotide repeat sequences. Here, we report that directly labeled PNA probes recognizing chromosome-specific repeat sequences are also powerful tools for detecting and enumerating specific chromosomes in interphase and metaphase cells. This is illustrated by multicolor FISH experiments with cells from normal individuals and patients with numerical sex chromosome aberrations.

Polyclonal hematopoiesis with variable telomere shortening in human long-term allogeneic marrow graft recipients

Donor-derived hematopoiesis was assessed in 17 patients who received allogeneic marrow grafts from HLA-matched siblings between 1971 and 1980. Complete blood counts were normal or near normal in all patients except one. Chimerism analyses, using either dual-color XY-chromosome fluorescence in situ hybridization (FISH) or analysis of variable number tandem repeat loci, indicated that 15 out of 16 patients had greater than 97% donor-derived hematopoiesis, whereas 1 patient had indeterminate chimerism. All 12 recipients of grafts from female donors exhibited polyclonal hematopoiesis by X-linked clonal analysis with the use of molecular probes. Of the 17 recipients, 9 exhibited a less than 1.0-kilobase shortening of granulocyte telomere length compared with their respective donors, according to terminal restriction fragment analysis or flow-FISH with a fluorescein-labeled peptide nucleic acid probe. These data suggest that under standard transplantation conditions, the stem cell proliferative potential is not compromised during hematopoietic reconstitution. (Blood. 2000;96:3991-3994)

The telomerase reverse transcriptase is limiting and necessary for telomerase function in vivo

Mammalian telomerase is essential for the maintenance of telomere length [1-5]. Its catalytic core comprises a reverse transcriptase component (TERT) and an RNA component. While the biochemical role of mammalian TERT is well established [6-11], it is unknown whether it is sufficient for telomere-length maintenance, chromosome stability or other cellular processes. Cells from mice in which the mTert gene had been disrupted showed progressive loss of telomere DNA, a phenotype similar to cells in which the gene encoding the telomerase RNA component (mTR) has been disrupted [1,12]. On prolonged growth, mTert-deficient embryonic stem (ES) cells exhibited genomic instability, aneuploidy and telomeric fusions. ES cells heterozygous for the mTert disruption also showed telomere attrition, a phenotype that differs from heterozygous mTR cells [12]. Thus, telomere maintenance in mammals is carried out by a single, limiting TERT.

Telomerase-associated protein TEP1 is not essential for telomerase activity or telomere length maintenance in vivo

TEP1 is a mammalian telomerase-associated protein with similarity to the Tetrahymena telomerase protein p80. Like p80, TEP1 is associated with telomerase activity and the telomerase reverse transcriptase, and it specifically interacts with the telomerase RNA. To determine the role of mTep1 in telomerase function in vivo, we generated mouse embryonic stem (ES) cells and mice lacking mTep1. The mTep1-deficient (mTep1(-/-)) mice were viable and were bred for seven successive generations with no obvious phenotypic abnormalities. All murine tissues from mTep1(-/-) mice possessed a level of telomerase activity comparable to that in wild-type mice. In addition, analysis of several tissues that normally lack telomerase activity revealed no reactivation of telomerase activity in mTep1(-/-) mice. Telomere length, even in later generations of mTep1(-/-) mice, was equivalent to that in wild-type animals. ES cells deficient in mTep1 also showed no detectable alteration in telomerase activity or telomere length with increased passage in culture. Thus, mTep1 appears to be completely dispensable for telomerase function in vivo. Recently, TEP1 has been identified within a second ribonucleoprotein (RNP) complex, the vault particle. TEP1 can also specifically bind to a small RNA, vRNA, which is associated with the vault particle and is unrelated in sequence to mammalian telomerase RNA. These results reveal that TEP1 is an RNA binding protein that is not restricted to the telomerase complex and that TEP1 plays a redundant role in the assembly or localization of the telomerase RNP in vivo.

Oligoclonal expansions in the CD8(+)CD28(-) T cells largely explain the shorter telomeres detected in this subset: analysis by flow FISH

We have previously reported that CD8(+)CD28(-) T cells have relatively shorter telomeres compared with CD8(+)CD28(+) T cells. Oligoclonal expansion is a common feature of CD8(+) T cells in human peripheral blood, and these expansions predominantly occur in the CD57(+)/CD28(-) population. We studied the telomere length in subsets of CD8(+) T cells using quantitative fluorescence in situ hybridization and flow cytometry (flow FISH). Our results confirm that CD8(+)CD28(-) T cells have shorter telomeres as compared with their CD28(+) counterpart cells. In addition, the oligoclonally expanded cells within the CD8(+)CD28(-) T cell subset generally have even shorter telomeres than the CD28(-) subset as a whole. We conclude that the presence of clonal expansions in the CD8(+)CD28(-) T cell population largely explain the shorter telomeres in this subset. These clonally expanded CD8(+)CD28(-) T cells generally have characteristics of terminally differentiated effector cells. Nevertheless, there is considerable individual variation in the degree of telomere shortening in these cells, which may reflect host genetic factors as well as the type and timing of the antigenic exposure.

Repair of telomeric DNA prior to replicative senescence

The average length of telomere repeats at the ends of chromosomes in most normal human somatic cells has been found to decrease by 50-200 base pairs with each cell division. The loss of telomere repeats has been causally linked to replicative senescence by the demonstration that overexpression of the enzyme telomerase can result in the elongation or maintenance of telomeres and immortalization of somatic cells with a diploid and apparently normal karyotype. Major questions that remain are related to the actual mechanism by which telomere shortening induces replicative senescence and the importance of telomere shortening and replicative senescence in the homeostasis of cells in renewal tissues and aging. This perspective is concerned with the consequences of telomere shortening at individual chromosomes in individual cells. Experimental evidence indicates that short telomeres accumulate prior to senescence and that replicative senescence is not triggered by the first telomere to reach a critical minimal threshold length. These observations are compatible with limited repair of short telomeres by telomerase-dependent or telomerase-independent DNA repair pathways. Deficiencies in telomere repair may result in accelerated senescence and aging as well as genetic instability that facilitates malignant transformation. Examples of molecules that may have a role in the repair of telomeric DNA prior to replicative senescence include ATM, p53, PARP, DNA-PK, Ku70/80, the human hRad50-hMre11-p95 complex, BRCA 1 and 2 and the helicases implicated in Bloom's and Werner's syndrome.

Telomere maintenance in telomerase-deficient mouse embryonic stem cells: characterization of an amplified telomeric DNA

Telomere dynamics, chromosomal instability, and cellular viability were studied in serial passages of mouse embryonic stem (ES) cells in which the telomerase RNA (mTER) gene was deleted. These cells lack detectable telomerase activity, and their growth rate was reduced after more than 300 divisions and almost zero after 450 cell divisions. After this growth crisis, survivor cells with a rapid growth rate did emerge. Such survivors were found to maintain functional telomeres in a telomerase-independent fashion. Although telomerase-independent telomere maintenance has been reported for some immortalized mammalian cells, its molecular mechanism has not been elucidated. Characterization of the telomeric structures in one of the survivor mTER(-/-) cell lines showed amplification of the same tandem arrays of telomeric and nontelomeric sequences at most of the chromosome ends. This evidence implicates cis/trans amplification as one mechanism for the telomerase-independent maintenance of telomeres in mammalian cells.

Extension of cell life-span and telomere length in animals cloned from senescent somatic cells

The potential of cloning depends in part on whether the procedure can reverse cellular aging and restore somatic cells to a phenotypically youthful state. Here, we report the birth of six healthy cloned calves derived from populations of senescent donor somatic cells. Nuclear transfer extended the replicative life-span of senescent cells (zero to four population doublings remaining) to greater than 90 population doublings. Early population doubling level complementary DNA-1 (EPC-1, an age-dependent gene) expression in cells from the cloned animals was 3.5- to 5-fold higher than that in cells from age-matched (5 to 10 months old) controls. Southern blot and flow cytometric analyses indicated that the telomeres were also extended beyond those of newborn (<2 weeks old) and age-matched control animals. The ability to regenerate animals and cells may have important implications for medicine and the study of mammalian aging.

Accumulation of short telomeres in human fibroblasts prior to replicative senescence

The loss of telomere repeats has been causally linked to in vitro replicative senescence of human diploid fibroblasts (HDFs). In order to study the mechanism(s) by which telomere shortening signals cell senescence, we analyzed the telomere length at specific chromosome ends at cumulative population doublings in polyclonal and clonal HDFs by quantitative fluorescence in situ hybridization. The rate of telomere shortening at individual telomeres varied between 50 and 150 bp per population doubling and short telomeres with an estimated 1-2 kb of telomere repeats accumulated prior to senescence. The average telomere length in specific chromosome ends was remarkably similar between clones. However, some exceptions with individual telomeres measuring 0.5-1 kb were observed. In the fibroblast clones, the onset of replicative senescence was significantly correlated with the mean telomere fluorescence but, strikingly, not with chromosomes with the shortest telomere length. The accumulation of short telomeres in late passages of cultured HDFs is compatible with selection of cells on the basis of telomere length and limited recombination between telomeres prior to senescence.

Prognostic implications of differences in telomere length between normal and malignant cells from patients with chronic myeloid leukemia measured by flow cytometry

Chronic myeloid leukemia (CML) is a clonal, multilineage myeloproliferative disorder characterized by the Philadelphia chromosome (Ph) and a marked expansion of myeloid cells. Previous studies have indicated that the telomere length in blood cells may indicate their replicative history. However, the large variation in telomere length between individuals complicates the use of this parameter in CML and other hematologic disorders. To circumvent this problem, we compared the telomere length in peripheral blood or bone marrow cells with purified normal (Ph(-)) T lymphocytes from the same CML patient using fluorescence in situ hybridization and flow cytometry. Overall telomere fluorescence was significantly reduced in Ph(+) cells from patients with CML compared to blood leukocytes from normal individuals (P < 0.001) or normal (Ph(-)) T lymphocytes from the same individuals (n = 51, P < 0.001). Cells from patients in accelerated phase or blast phase (AP/BP) showed significantly shorter average telomere length than cells from patients in chronic phase (CP, P = 0.02) or cytogenetic remission (CR, P = 0.03). Patients in CP who subsequently developed BP within 2 years had significantly shorter telomeres than those who did not develop BP for at least 2 years (P < 0.05). Accelerated replication-dependent telomere shortening in Ph(+ )versus Ph(-) leukocytes supports previous evidence that Ph(+) stem cells cycle more actively than their counterparts in normal individuals. Our data further suggest that telomere shortening may serve as a surrogate marker of disease progression in patients with CP CML, supporting a mechanistic link between CML stem cell turnover, genetic instability, and malignant evolution in this disease. (Blood. 2000;95:1883-1890) (Blood. 2000;95:1883-1890)

Accelerated telomere shortening in the human inactive X chromosome

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

Functions of poly(ADP-ribose) polymerase in controlling telomere length and chromosomal stability

In most eukaryotes, poly(ADP-ribose) polymerase (PARP) recognizes DNA strand interruptions generated in vivo. DNA binding by PARP triggers primarily its own modification by the sequential addition of ADP-ribose units to form polymers; this modification, in turn, causes the release of PARP from DNA ends. Studies on the effects of the disruption of the gene encoding PARP (Adprt1, formerly Adprp) in mice have demonstrated roles for PARP in recovery from DNA damage and in suppressing recombination processes involving DNA ends. Telomeres are the natural termini of chromosomes and are, therefore, potential targets of PARP. Here, by the use of two different techniques, we show that mice lacking PARP display telomere shortening compared with wild-type mice. Telomere shortening is seen in different genetic backgrounds and in different tissues, both from embryos and adult mice. In vitro telomerase activity, however, is not altered in Adprt1-/- mouse fibroblasts. Furthermore, cytogenetic analysis of mouse embryonic fibroblasts reveals that lack of PARP is associated with severe chromosomal instability, characterized by increased frequencies of chromosome fusions and aneuploidy. The absence of PARP does not affect the presence of single-strand overhangs, naturally present at the ends of telomeres. This study therefore reveals an unanticipated role for PARP in telomere length regulation and provides insights into its functions in maintaining genomic integrity.

Telomere length measurements using digital fluorescence microscopy

BACKGROUND

The ends of chromosomes (telomeres) are important to maintain chromosome stability, and the loss of telomere repeat sequences has been implicated in cellular senescence and genomic instability of cancer cells. The traditional method for measuring the length of telomeres (Southern analysis) requires a large number of cells (>10(5)) and does not provide information on the telomere length of individual chromosomes. Here, we describe a digital image microscopy system for measurements of the fluorescence intensity derived from telomere repeat sequences in metaphase cells following quantitative fluorescence in situ hybridization (Q-FISH).

METHODS

Samples are prepared for microscopy using Q-FISH with Cy3 labeled peptide nucleic acid probes specific for (T(2)AG(3))(n) sequences and the DNA dye DAPI. Separate images of Cy3 and DAPI fluorescence are acquired and processed with a dedicated computer program (TFL-TELO). With the program, the integrated fluorescence intensity value for each telomere, which is proportional to the number of hybridized probes, is calculated and presented to the user.

RESULTS

Indirect tests of our method were performed using simulated as well as defined tests objects. The precision and consistency of human telomere length measurements was then analyzed in a number of experiments. It was found that by averaging the results of less than 30 cells, a good indication of the telomere length (SD of 10-15%) can be obtained.

CONCLUSIONS

We demonstrate that accurate and repeatable fluorescence intensity measurements can be made from Q-FISH images that provide information on the length of telomere repeats at individual chromosomes from limited number of cells.

Asymmetric cell divisions in hematopoietic stem cells

In order to study cell kinetics involved in long-term hematopoiesis, we studied single sorted candidate hematopoietic stem cells (HSC) from fetal liver cultured in the presence of a mixture of stimulatory cytokines. After 8-10 days in culture, the number of cells varied from less than a hundred to more than ten thousand cells. Single cells in slowly growing colonies were recloned upon reaching a 100-200-cell stage. Strikingly, the number of cells in subclones varied widely again. These results are indicative of asymmetric divisions in primitive hematopoietic cells in which the proliferative potential and cell cycle properties are unevenly distributed among daughter cells. The continuous generation of heterogeneity in cell cycle properties among the clonal progeny of HSC appears a relevant mechanism to maintain long-term maintenance of hematopoiesis in vitro and in vivo.

An 18q- syndrome breakpoint resides between the duplicated serpins SCCA1 and SCCA2 and arises via a cryptic rearrangement with satellite III DNA

The 18q-syndrome is representative of a group of terminal deficiency or macrodeletion syndromes characterized by mental retardation and congenital malformations. To gain insight into the mechanism of chromosomal loss and stabilization in these disorders, we cloned a putative terminal deletion breakpoint from an 18q-syndrome patient. The 18q21.3 breakpoint occurred between two nearly identical serine protease inhibitor (serpin) genes, SCCA1 and SCCA2. Although cytogenetic studies suggested that this chromosomal aberration was formed by a simple terminal deletion, DNA sequence analysis, pulsed-field gel electrophoresis and fluorescence in situ hybridization showed that the breakpoint was contiguous with a 35 bp filler sequence followed by a satellite III DNA-containing telomeric fragment of 475-1000 kb. This type of satellite III DNA sequence was not detected on the normal chromosome 18, but was highly homologous with types of satellite III DNA sequences normally located on the short arms (p11) of the acrocentric chromosomes and other heterochromatic regions. This DNA sequence analysis suggested that the terminal deficiency in this 18q-syndrome patient arose via illegitimate (non-homologous) recombination. Moreover, these data raise the possibility that a subset of chromosomal aberrations appearing cytogenetically and molecularly as simple terminal truncations or deletions are caused by small (<1000 kb) cryptic rearrangements.

Absence or low number of telomere repeats at junctions of dicentric chromosomes

Human ovarian surface epithelial (HOSE) cells transfected with the E6 and E7 oncogenes of the human papilloma virus (PV) do not express measurable telomerase activity. Relative to untransfected control cells, HOSE-PV cells have an extended in vitro lifespan characterized by a very high frequency of telomeric associations (TAs) of chromosomes. In order to study the role of telomere shortening in the formation of TAs, we studied the telomere length in 120 dicentric chromosomes in HOSE-PV cells by using quantitative fluorescence in situ hybridization. Forty percent of the dicentric chromosomes had no fluorescence signal at the junction site, and in the remainder the fluorescence at the junction was less than at corresponding unjoined ends. These observations support a critical role of telomere shortening in the development of TAs and the subsequent genetic instability observed in a majority of tumor cells.

Dissociation among in vitro telomerase activity, telomere maintenance, and cellular immortalization

The immortalization of human cells is a critical step during tumorigenesis. In vitro, normal human somatic cells must overcome two proliferative blockades, senescence and crisis, to become immortal. Transformation with viral oncogenes extends the life span of human cells beyond senescence. Such transformed cells eventually succumb to crisis, a period of widespread cellular death that has been proposed to be the result of telomeric shortening. We now show that ectopic expression of the telomerase catalytic subunit (human telomerase reverse transcriptase or hTERT) and subsequent activation of telomerase can allow postsenescent cells to proliferate beyond crisis, the last known proliferative blockade to cellular immortality. Moreover, we demonstrate that alteration of the carboxyl terminus of human telomerase reverse transcriptase does not affect telomerase enzymatic activity but impedes the ability of this enzyme to maintain telomeres. Telomerase-positive cells expressing this mutant enzyme fail to undergo immortalization, further tightening the connection between telomere maintenance and immortalization.

Asymmetric cell divisions sustain long-term hematopoiesis from single-sorted human fetal liver cells

Hematopoietic stem cells (HSCs) in adult marrow are believed to be derived from fetal liver precursors. To study cell kinetics involved in long-term hematopoiesis, we studied single-sorted candidate HSCs from fetal liver that were cultured in the presence of a mixture of stimulatory cytokines. After 8-10 d, the number of cells in primary cultures varied from <100 to >10,000 cells. Single cells in slow growing colonies were recloned upon reaching a 100-200 cell stage. Strikingly, the number of cells in subclones varied widely again. These results are indicative of asymmetric divisions in primitive hematopoietic cells in which proliferative potential and cell cycle properties are unevenly distributed among daughter cells. The continuous generation of functional heterogeneity among the clonal progeny of HSCs is in support of intrinsic control of stem cell fate and provides a model for the long-term maintenance of hematopoiesis in vitro and in vivo.

Stem cell biology for the transfusionist

The limited life-span of most blood cells requires continuous production of cells which in adults may exceed 1012 cells/day. This impressive production of cells (approximately 4.1015 cells over a life time) is achieved by the proliferation and differentiation of committed progenitor cells which themselves are derived from a population of pluripotent stem cells with self-renewal potential. In adults, the large majority of stem cells are found in the bone marrow among cells with a CD34 + CD38- phenotype. Interestingly, small but significant numbers of such cells can be found in the circulation. The frequency of circulating CD34 + CD38- cells can be dramatically increased by treatment with certain compounds including cytokines. Such "mobilized" peripheral blood stem cells have become an important alternative to bone marrow in stem cell transplantation procedures primarily because engraftment is more rapid. The latter is almost certainly related to the increased numbers of primitive CD34 + CD38- cells capable of engrafting the bone marrow in blood versus bone marrow stem cell grafts [1]. Paradoxically, the large majority of "candidate" stem cells in adult bone marrow are quiescent cells. One possibility is that stem cells, like other somatic cells, have only a limited replicative potential (< 100 divisions). This hypothesis is supported by two key observations and the consideration that, in theory, 52 divisions can yield 4.1015 cells. First, it was shown that "candidate" stem cells purified from fetal and adult tissue display marked functional differences in turn-over time and the ability to produce cells with stem cell properties [2]. Secondly, these functional differences were found to correlate with a measurable loss of telomere repeats [3], despite the presence of low but readily detectable levels of telomerase in all purified cell fractions [4,5]. In order to address questions about the role of telomeres in normal and malignant hematopoiesis, we developed quantitative fluorescence in situ hybridization [6]. With this technique the length of telomere repeats at individual chromosome ends can be reliably estimated using optical density measurements from digital images of metaphase chromosomes after fluorescence in situ hybridization with directly labeled (CCCTAA)3--Peptide Nucleic Acid Probe [6,7]. Furthermore, we recently showed that this method can be adapted to measure the total telomere repeat content of cells by flow cytometry [8]. Here some issues in studies of hematopoietic stem cells are discussed in relation to rapidly accumulating information about telomere biology.

Induction of telomerase activity by in vivo X-irradiation of mouse splenocytes and its possible role in chromosome healing

Telomeres serve as protective caps for the chromosome ends. They are one of the functional elements required for the stable transmission of eukaryotic chromosomes. Telomerase, a ribonucleoprotein, stabilises the telomere length by adding telomere repeats on to chromosome ends. Telomeres and telomerase can play a role in the formation of chromosome aberrations and especially in healing of the chromosome or chromatid breaks produced by radiation-induced DNA damage. Telomerase-independent processes also appear to be capable of capping broken chromosome ends. We have studied the expression of telomerase, telomere status and chromosome rearrangements in mouse splenocytes following different doses (0.5, 1.0, 2.0 or 3.0 Gy) of X-irradiation in vivo up to 224 days post-exposure. A dose-dependent increase in telomerase activity up to 2 Gy X-ray exposure was observed immediately after irradiation. The increased enzyme activity was detected even up to day 224 post-irradiation, the last time point studied, especially at higher doses (2 Gy and 3 Gy). A significant difference in average telomere length, measured by quantitative fluorescence in situ hybridisation (Q-FISH) on metaphase chromosomes, noticed immediately after irradiation indicates terminal deletion or altered telomere chromatin. However, telomere length was not statistically significant from the control at the later time points studied. Presence of telomere repeats at the chromosomal breakage sites revealed by FISH with peptide nucleic acid (PNA) telomeric probe indicates a possible role of telomerase-dependent or independent processes in chromosome healing and telomere capture in mammalian cells. We found that approximately 25 to 50% of the newly formed telomeres at the breakage sites are in the range of 200 bp to 1 kb, which might suggest that these repeats could have been added by telomerase which showed a corresponding increase following irradiation.

Telomere length dynamics in human lymphocyte subpopulations measured by flow cytometry

To measure the average length of telomere repeats at chromosome ends in individual cells we developed a flow cytometry method using fluorescence in situ hybridization (flow FISH) with labeled peptide nucleic acid (PNA) probes. Results of flow FISH measurements correlated with results of conventional telomere length measurements by Southern blot analysis (R = 0.9). Consistent differences in telomere length in CD8+ T-cell subsets were identified. Naive and memory CD4+ T lymphocytes in normal adults differed by around 2.5 kb in telomere length, in agreement with known replicative shortening of telomeres in lymphocytes in vivo. T-cell clones grown in vitro showed stabilization of telomere length after an initial decline and rare clones capable of growing beyond 100 population doublings showed variable telomere length. These results show that flow FISH can be used to measure specific nucleotide repeat sequences in single cells and indicate that the very large replicative potential of lymphocytes is only indirectly related to telomere length.

Telomere length regulation in mice is linked to a novel chromosome locus

Little is known about the mechanisms that regulate species-specific telomere length, particularly in mammalian species. The genetic regulation of telomere length was therefore investigated by using two inter-fertile species of mice, which differ in their telomere length. Mus musculus (telomere length >25 kb) and Mus spretus (telomere length 5-15 kb) were used to generate F1 crosses and reciprocal backcrosses, which were then analyzed for regulation of telomere length. This analysis indicated that a dominant and trans-acting mechanism exists capable of extensive elongation of telomeres in somatic cells after fusion of parental germline cells with discrepant telomere lengths. A genome wide screen of interspecific crosses, using M. spretus as the recurrent parent, identified a 5-centimorgan region on distal chromosome 2 that predominantly controls the observed species-specific telomere length regulation. This locus is distinct from candidate genes encoding known telomere-binding proteins or telomerase components. These results demonstrate that an unidentified gene(s) mapped to distal chromosome 2 regulates telomere length in the mouse.

Biology of human umbilical cord blood-derived hematopoietic stem/progenitor cells

Reported in 1989, studies by Broxmeyer, Gluckman, and colleagues demonstrated that umbilical cord blood (UCB) is a rich source of hematopoietic stem/progenitor cells (HSPC) and that UCB could be used in clinical settings for hematopoietic cell transplantation. Since then, a great interest has been generated on the biological characterization of these cells. Over the last nine years, several groups have focused on the study of UCB HSPC, addressing different aspects, such as the frequency of these cells in UCB, the identification of different HSPC subsets based on their immunophenotype, their ability to respond to hematopoietic cytokines, the factors that control their proliferation and expansion potentials, and their capacity to reconstitute hematopoiesis in animal models. Most of these studies have shown that significant functional differences exist between HSPC from UCB and adult bone marrow (i.e., the former possess higher proliferation and expansion potential than the latter). It is also noteworthy that genetic manipulation of UCB HSPC has been achieved by several groups and that genetically modified UCB cells have already been used in the clinic. In spite of the significant advances in the characterization of these cells, we are still in the process of trying to fully understand their biology, both at the cellular and the molecular levels. In the present article, we describe and discuss what is currently known about the biology of UCB HSPC.

Induction of telomerase activity and chromosome aberrations in human tumour cell lines following X-irradiation

Telomerase, a ribonucleoprotein enzyme, has been detected in immortalised cells and in majority of human cancers. Numerical and structural chromosome aberrations are commonly observed in tumour cell lines. To study the expression of telomerase and its influence on the formation of chromosomal aberrations, human colon carcinoma cell line (SW480) and human nonpolyposis colorectal carcinoma (HNPCC) cell lines (NA50600, NA59 and NA61) were exposed to 2 or 4 Gy X-rays. Increased telomerase activity was observed in all these cell lines at 24 h postirradiation and a 3 to 7 fold increase was seen at 4 Gy dose as detected by Telomere Repeat Amplification Protocol. Chromosomal rearrangements (dicentrics, translocations and breaks/fragments) analysed by Giemsa staining and chromosome painting were increased significantly following X- Quantitative fluorescence in situ hybridisation using a peptide nucleic acid telomeric probe to measure telomere length at irradiation chromosomal level revealed that all cell lines have very short telomeres in the range of 0.29 to 2.1 kb. Following X-irradiation, an increase in the chromosome end-to-end (telomere) associations was observed. The present results demonstrate that presence or upregulation of telomerase activity did not prevent the formation of chromosome aberrations and/or telomere associations in tumour cell lines after X-irradiation.

Telomerase activity in candidate stem cells from fetal liver and adult bone marrow

Telomerase is a ribonucleoprotein polymerase that synthesizes telomeric repeats onto the 3' ends of eukaryotic chromosomes. Activation of telomerase may prevent telomeric shortening and correlates with cell immortality in the germline and certain tumor cells. Candidate hematopoietic stem cells (HSC) from adult bone marrow express low levels of telomerase, which is upregulated with proliferation and/or differentiation. To address this issue, we stimulated purified candidate HSC from human adult bone marrow with stem cell factor (SCF), interleukin-3 (IL-3), and Flt3-ligand (FL). After 5 days in culture, activity was detected in total cell extracts from IL-3-, SCF + FL-, SCF + IL-3-, FL + IL-3-, and SCF + IL-3 + FL-stimulated cultures, but not from cells cultured in SCF or FL alone. Within the CD34(+) fraction of the cultured cells, significant activity was found in the CD34(+)CD71(+) fraction. In addition, PKH26 staining confirmed that detectable telomerase activity was present in dividing PKH26(lo) cells, whereas nondividing PKH26(hi) cells were telomerase negative. Because in these experiments no distinction could be made between cycling "candidate" stem cells that had retained or had lost self-renewal properties, fetal liver cells with a CD34(+)CD38(-) phenotype, highly enriched for cycling stem cells, were also examined and found to express readily detectable levels of telomerase activity. Given the replication-dependent loss of telomeric DNA in hematopoietic cells, these observations suggest that the observed telomerase activity in candidate stem cells is either expressed in a minor subset of stem cells or, more likely, is not sufficient to prevent telomere shortening.

Telomeres in the haemopoietic system

The limited life span of most blood cells requires the continuous production of cells, which in adults exceeds 10(12) cells/day. This impressive production of cells (approximately 4 x 10(16) cells over a lifetime) is achieved by the proliferation and differentiation of committed progenitor cells, which themselves are derived from a population of pluripotent stem cells with self-renewal potential. Paradoxically, the large majority of stem cells in adult bone marrow are quiescent cells. One possibility is that stem cells, like other somatic cells, have only a limited replicative potential (< 100 divisions). This hypothesis is supported by two key observations and the consideration that, in theory, 55 divisions can yield 4 x 10(16) cells. First, it was shown that 'candidate' stem cells purified from fetal and adult tissue showed dramatic functional differences in turn-over time and the ability to produce cells with stem cell properties, Second, these functional differences were found to correlate with a measurable loss of telomere repeats despite the presence of low but readily detectable levels of telomerase in all purified cell fractions. In order to address questions about the role of telomeres in normal and malignant haemopoiesis, we developed a quantitative fluorescence in situ hybridization technique. Here we review the characteristics of this novel tool to assess the number of telomere repeats at the end of individual chromosomes and provide an overview of recent observations.

Short telomeres on human chromosome 17p

Human chromosomes terminate in a series of T2AG3 repeats, which, together with associated proteins, are essential for chromosome stability. In somatic cells, these sequences are known to be gradually lost through successive cells divisions; however, information about changes on specific chromosomes is not available. Individual telomeres could mediate important biological effects as was shown in yeast, in which loss of a single telomere results in cell-cycle arrest and chromosome loss. We now demonstrate by quantitative fluorescence in situ hybridization (Q-FISH; ref. 7) that the number of T2AG3 repeats on specific chromosome arms is very similar in different tissues from the same donor and varies only to some extent between donors. In all sixteen individuals studied, telomeres on chromosome 17p were shorter than the median telomere length--a finding confirmed by analysis of terminal restriction fragments from sorted chromosomes. These observations provide evidence of chromosome-specific factors regulating the number of T2AG3 repeats in individual telomeres and raise the possibility that the relatively short telomeres on chromosome 17p contribute to the frequent loss of 17p alleles in human cancers.

Intrinsic control of stem cell fate

Efforts to expand the number of stem cells ex vivo are based on the assumption that the self-renewal properties of stem cells are subject to extrinsic control. Although loss of stem cell properties ex vivo has been well-documented, conclusive evidence for a gain of stem cell function in vitro has not been forthcoming. In this short discussion paper, some issues related to the self-renewal of stem cells in relation to future experimental strategies are presented.

Telomere shortening and tumor formation by mouse cells lacking telomerase RNA

To examine the role of telomerase in normal and neoplastic growth, the telomerase RNA component (mTR) was deleted from the mouse germline. mTR-/- mice lacked detectable telomerase activity yet were viable for the six generations analyzed. Telomerase-deficient cells could be immortalized in culture, transformed by viral oncogenes, and generated tumors in nude mice following transformation. Telomeres were shown to shorten at a rate of 4.8+/-2.4 kb per mTR-/- generation. Cells from the fourth mTR-/- generation onward possessed chromosome ends lacking detectable telomere repeats, aneuploidy, and chromosomal abnormalities, including end-to-end fusions. These results indicate that telomerase is essential for telomere length maintenance but is not required for establishment of cell lines, oncogenic transformation, or tumor formation in mice.

Self-renewal of stem cells

The mechanisms that regulate the fate of hematopoietic stem cells are poorly understood. Hematopoietic growth factors and factors in the microenvironment are clearly essential for ensuring the survival and differentiation of hematopoietic stem cells, but their role in the selection between self-renewal and lineage commitment options is unclear. Differences in the functional behavior of purified stem cells at different stages of development suggest that developmentally-regulated intrinsic factors may play an important role in directing stem cell fate. Recent studies strongly implicate homeobox genes in these processes and have further emphasized the link between developmental and stem cell biology. Changes in stem cell function during development correlate with measurable changes in telomere length, and loss of telomere repeats may limit the replicative potential of stem cells. In order to reconcile developmental changes in stem cell properties with loss of telomeric DNA, the intrinsic timetable model of stem cell biology is introduced. In this model, the self-renewal properties of stem cells are relative and their replicative potential is limited to less than 100 cell divisions.