Telomere length and the expression of natural telomeric genes in human fibroblasts

Progressive telomere shortening occurs with division of normal human cells, and eventually leads to replicative senescence. The mechanism by which the shortened telomeres cause growth arrest is largely unknown. Transcriptional silencing of genes adjacent to telomeres, also called telomere position effect, has been hypothesized as a possible mechanism of telomere-mediated senescence. However, there is no report regarding telomere position effect on natural telomeric genes in human cells. To address whether the expression of natural telomeric genes is regulated by telomere length, we combined quantitative RT-PCR with quantitative fluorescence in situ hybridization to comparatively analyze the expression of 34 telomeric genes and telomere length of their 24 corresponding chromosome ends in young and senescent human fibroblasts. We have demonstrated here that telomere length alone is not sufficient to determine the expression status of natural telomeric genes. An extended analysis of a tandem of eight telomeric genes on a single chromosome end revealed a discontinuous pattern of changed expression during telomere shortening and some of the changes are senescence-specific rather than non-dividing-related. These results suggest that the expression of natural telomeric genes may be influenced by alteration of local heterochromatin structure.

Telomeres in hematopoietic stem cells

Hematopoietic stem cells have an impressive regenerative potential, strikingly illustrated in transplantation experiments using limited number of cells. In mice, serial transplantation experiments suggest that individual hematopoietic cells are capable of extensive self-renewal and that any possible limitations in the replicative potential of individual hematopoietic stem cells are not affecting normal blood cell formation. The situation with human hematopoietic stem cells is less clear. Unlike the situation in the mouse, the telomere length in nucleated human blood cells shows a remarkable decline with age. Furthermore, even partial telomerase deficiency in humans typically results in marrow failure, whereas complete lack of telomerase is tolerated up to several generations in the mouse. The decline in telomere length in human leukocytes with age follows a cubic function and is much higher in lymphocytes than in granulocytes. This finding suggests that, under normal circumstances, telomere loss is more likely to compromise the function of lymphocytes than the function of hematopoietic stem cells. To reconcile differences in telomere biology between man and mice, it has been proposed that telomere shortening evolved as a tumor suppressor mechanism in long-lived species that may not exist in shorter-lived mammals. According to this model, telomeres in human cells are intimately involved in signaling cell cycle progression and cell division. Most likely, a minimum number of telomere repeats is required at each telomere to prevent activation of a "telomere checkpoint" and allow cell cycle progression. Telomere length measurements appear useful to distinguish between depletion and exhaustion of hematopoietic stem cells as a cause of marrow failure.

Normalization of previously shortened telomere length under treatment with imatinib argues against a preexisting telomere length deficit in normal hematopoietic stem cells from patients with chronic myeloid leukemia

Telomeres are composed of TTAGGG repeats and associated proteins. In somatic cells, telomere repeats are lost with each cell division, eventually leading to genetic instability and cellular senescence. In previous studies, we described substantial and disease stage-specific telomere shortening in peripheral blood (PB) leukocytes from patients with chronic myeloid leukemia (CML). Here, we sought to determine whether age-adjusted telomere length in PB granulocytes (deltaTEL(gran)) is associated with response to treatment with the selective tyrosine kinase inhibitor imatinib. A total of 517 samples from 206 patients in chronic phase (CP), accelerated phase (AP), and blast crisis (BC) before and up to 706 days after initiation of imatinib therapy (median: 144 days) were analyzed by quantitative fluorescence in situ hybridization of interphase cells in suspension (Flow-FISH); telomere fluorescence was expressed in molecular equivalents of soluble fluorochrome units (MESF). Telomere length in samples from start of treatment up to day 144 was significantly shorter (mean +/- SE; -1.5 +/- 0.3 kMESF) compared to samples from patients treated for more than 144 days (-0.8 +/- 0.3 kMESF, p = 0.035). In patients with repeated measurements, a significant increase in telomere length under treatment was observed. Median telomere length in major remission was found to be significantly longer compared to patients without response to treatment measured either by cytogenetics (n = 246, p < 0.05), interphase FISH (n = 204, p = 0.002), or quantitative RT-PCR (n = 371, p < 0.05). In conclusion, the increase in telomere length under treatment with imatinib reflects a shift from Ph+ to Ph- cells in the PB of patients with CML.

Role of oxidative stress in telomere shortening in cultured fibroblasts from normal individuals and patients with ataxia-telangiectasia

Cells from patients with the autosomal recessive disorder ataxia-telangiectasia (A-T) display accelerated telomere shortening upon culture in vitro. It has been suggested that A-T cells are in a chronic state of oxidative stress, which could contribute to their enhanced telomere shortening. In order to examine this hypothesis, we monitored the changes in telomere length in A-T homozygous, heterozygous and control fibroblasts cultured in vitro under various conditions of oxidative stress using quantitative fluorescent in situ hybridization. Compared with normal cells, the rate of telomere shortening was 1.5-fold increased under 'normal' levels of oxidative stress in A-T heterozygous cells and 2.4-3.2-fold in A-T homozygous cells. Mild chronic oxidative stress induced by hydrogen peroxide increased the rate of telomere shortening in A-T cells but not in normal fibroblasts and the telomere shortening rate decreased in both normal and A-T fibroblasts if cultures were supplemented with the anti-oxidant phenyl-butyl-nitrone. Increased telomere shortening upon oxidative stress in A-T cells was associated with a significant increase in the number of extra-chromosomal fragments of telomeric DNA and chromosome ends without detectable telomere repeats. We propose that the ATM (A-T mutated) protein has a role in the prevention or repair of oxidative damage to telomeric DNA and that enhanced sensitivity of telomeric DNA to oxidative damage in A-T cells results in accelerated telomere shortening and chromosomal instability.

Telomere shortening in leukocyte subpopulations from baboons

To address questions about telomere length regulation in nonhuman primates, we studied the telomere length in subpopulations of leukocytes from the peripheral blood of baboons aged 0.2-26.5 years. Telomere length in granulocytes, B cells, and subpopulations of T cells all decreased with age. Overall, telomere length kinetics were lineage- and cell subset-specific. T cells showed the most pronounced, overall decline in telomere length. Levels of telomerase in stimulated T cells from old animals were lower than in corresponding cells from young animals. Memory T cells with very short telomeres accumulated in old animals. In contrast, the average telomere length values in B cells remained relatively constant from middle age onward. Individual B cells showed highly variable telomere length, and B cells with very long telomeres were observed after the ages of 1-2 years. In general, cell type-specific telomere kinetics in baboons were remarkably similar to those observed in humans.

The mammalian SIR2alpha protein has a role in embryogenesis and gametogenesis

The yeast Sir2p protein has an essential role in maintaining telomeric and mating type genes in their transcriptionally inactive state. Mammalian cells have a very large proportion of their genome inactive and also contain seven genes that have regions of homology with the yeast sir2 gene. One of these mammalian genes, sir2alpha, is the presumptive mammalian homologue of the yeast sir2 gene. We set out to determine if sir2alpha plays a role in mammalian gene silencing by creating a strain of mice carrying a null allele of sir2alpha. Animals carrying two null alleles of sir2alpha were smaller than normal at birth, and most died during the early postnatal period. In an outbred background, the sir2alpha null animals often survived to adulthood, but both sexes were sterile. We found no evidence for failure of gene silencing in sir2alpha null animals, suggesting that either SIR2alpha has a different role in mammals than it does in Saccharomyces cerevisiae or that its role in gene silencing in confined to a small subset of mammalian genes. The phenotype of the sir2alpha null animals suggests that the SIR2alpha protein is essential for normal embryogenesis and for normal reproduction in both sexes.

Synergistic role of Ku80 and poly(ADP-ribose) polymerase in suppressing chromosomal aberrations and liver cancer formation

Liver cancer is one of the major human tumors in the world. Basic and epidemiological studies have proposed that the major risk factors for liver cancer include alcohol and diet as well as infection with hepatitis B and C viruses. However, the mechanistic clues for the development of this type of cancer is largely unknown. Poly(ADP-ribose) polymerase (PARP-1) and a component of nonhomologous end-joining (NHEJ) machinery, Ku80, are two major DNA end-binding molecules that play a multifunctional role in DNA damage signaling and repair, recombination as well as the maintenance of genomic stability. Here we show that the interaction of PARP-1 and Ku80 is essential for development because PARP-1/Ku80 double null mice died at embryonic day (E) 9.5. Interestingly, haplo-insufficiency of Ku80 in PARP-1-/- mice promotes the development of hepatocellular adenoma and hepatocellular carcinoma (HCC). These tumors exhibited a multistage tumor progression associated with the loss of E-cadherin expression and the mutation of beta-catenin. Cytogenetic analysis revealed that Ku80 heterozygosity elevated chromosomal instability in PARP-1-/- cells and that these liver tumors harbored a high degree of chromosomal aberrations including fragmentations, end-to-end fusions, and recurrent nonreciprocal translocations (NRT). These features are reminiscent of human HCC. Taken together, these data implicate a synergistic function of Ku80 and PARP-1 in minimized chromosome aberrations and cancer development and suggest that defects in DNA end-processing molecules may be etiological factors in human HCC formation.

Telomere maintenance in human B lymphocytes

Telomere shortening has been causally linked to replicative senescence in human cells. To characterize telomere-length heterogeneity in peripheral blood cells of normal individuals, we analysed the mean length of telomeric repeat sequences in subpopulations of peripheral blood leucocytes, using fluorescence in situ hybridization and flow cytometry (flow-FISH). Although the telomere length of most haematopoietic subsets was within the same range, the mean telomere length was found to be 15% higher in B compared with T lymphocytes in adult peripheral blood. Whereas telomere loss with ageing corresponded to 33 base pairs (bp) per year in T cells, telomere shortening was slower in B cells, corresponding to 15 bp per year. Separation of adult B-lymphocyte subpopulations based on CD27 expression revealed that telomere length was almost 2 kb longer in CD19+CD27+ (memory) compared with CD19+CD27- (naive) cells. Furthermore, peripheral blood B cells were activated in vitro. Whereas B-cell activation with Staphylococcus aureus Cowan strain (SAC) did not increase telomere length, a striking telomere elongation was observed when cells were stimulated with SAC and interleukin 2 to induce plasma cell differentiation. Our observations support the concept that telomere dynamics in B cells are distinct from other haematopoietic cell lineages and that telomere elongation may play an essential role in the generation of long-term B memory cells.

Functional characterization of multiple domains involved in the subcellular localization of the hematopoietic Pbx interacting protein (HPIP)

We have previously reported the cloning of the Hematopoietic Pbx Interacting Protein (HPIP), a novel protein discovered through its interaction with Pbx1. HPIP is expressed in early hematopoietic precursors, can bind all members of the Pbx family and can inhibit the transcriptional activation of the oncogene E2A-Pbx. To further understand the function of HPIP, we have analysed its cellular localization and characterized its functional localization domains. Using fluorescence microscopy to follow the distribution of different HPIP sequences fused to GFP, we found that HPIP localizes predominantly to cytoskeletal fibers but has the potential ability to shuttle between the nucleus and the cytosol. The cytoskeletal localization of HPIP is mediated by an N-terminal leucine rich region (between aa 190-218) and can be disrupted by the microtubule destabilizing drug vincristine. The HPIP C-terminal domain (aa 443-731) bears a nuclear export activity that is blocked by the CRM1 inhibitor Leptomycin B. In addition, we found two basic amino acid regions located between aa 485-505 and aa 695-720 that contain nuclear import activities attenuated by nuclear export. These observations support a model in which the constitutive attachment of HPIP to the cytoskeleton could be modified by changes in functional domains implicated in nuclear export, import and cytoskeleton binding sequences, allowing the molecule to shuttle between the nucleus and the cytosol.

Longitudinal studies of telomere length in feline blood cells: implications for hematopoietic stem cell turnover in vivo

OBJECTIVE

To address questions about stem cell turnover in relation to telomere length dynamics, we analyzed telomere length in serial blood samples from cats.

MATERIALS AND METHODS

Lymphocytes and granulocytes from two newborn kittens, a 2-year-old cat, a 10-year-old recipient of a double autologous stem cell transplant, and a 10-year-old control animal were analyzed by fluorescence in situ hybridization and flow cytometry at 2-week intervals over a 1-year period.

RESULTS

At study onset, long telomeres were found in granulocytes and lymphocytes from the two kittens (mean +/- SD: 70.2 +/- 3.1 and 72.5 +/- 3.1 telomere fluorescence units [TFU], respectively) compared with the 2-year-old cat (55.6 +/- 2.5 and 64.1 +/- 4.3 TFU, respectively) and the two adult animals (49.6 +/- 1.5 and 45.4 +/- 0.8 TFU, respectively). The rate of telomere shortening in both granulocytes and lymphocytes was most rapid in the kittens (slope: -16.7 +/- 1.4 and -15.6 +/- 0.2 TFU/year, respectively). As in humans, telomere shortening with age was more rapid in lymphocytes than in granulocytes. An average rate of telomere attrition of -0.52 +/- 0.03 TFU per cell division was calculated for cultured lymphocytes from the two kittens, approximately 5-fold higher than the rate observed in human cells.

CONCLUSIONS

The average telomere length in cats is 5- to 10-fold longer than in humans, but the rate of telomere shortening is much higher both in vivo and in vitro. These observations are compatible with similar stem cell kinetics in both species.

Disruption of dog-1 in Caenorhabditis elegans triggers deletions upstream of guanine-rich DNA

Genetic integrity is crucial to normal cell function, and mutations in genes required for DNA replication and repair underlie various forms of genetic instability and disease, including cancer. One structural feature of intact genomes is runs of homopolymeric dC/dG. Here we describe an unusual mutator phenotype in Caenorhabditis elegans characterized by deletions that start around the 3' end of polyguanine tracts and terminate at variable positions 5' from such tracts. We observed deletions throughout genomic DNA in about half of polyguanine tracts examined, especially those containing 22 or more consecutive guanine nucleotides. The mutator phenotype results from disruption of the predicted gene F33H2.1, which encodes a protein with characteristics of a DEAH helicase and which we have named dog-1 (for deletions of guanine-rich DNA). Nematodes mutated in dog-1 showed germline as well as somatic deletions in genes containing polyguanine tracts, such as vab-1. We propose that DOG-1 is required to resolve the secondary structures of guanine-rich DNA that occasionally form during lagging-strand DNA synthesis.

Feature analysis and centromere segmentation of human chromosome images using an iterative fuzzy algorithm

Classification of homologous chromosomes is essential to advanced studies of cancer genetics. Centromere intensities are believed to be an important differentiating feature between homologs. Therefore, segmentation of centromeres is a major step toward the realization of homolog classification. This paper describes an iterative fuzzy algorithm which successfully segments centromeres from images of human chromosomes prepared using fluorescence in-situ hybridization technique. The algorithm is based on assigning a fuzzy membership value to each pixel in the centromere image. An iterative algorithm then updates and minimizes a defined error function. Chromosome 22, a highly heteromorphic chromosome, is used to verify the centromere segmentation method. Homologs of this chromosome are classified based on their segmented centromere intensities as well as their morphological differences. The classification results of these two methods agree completely and are used to validate our developed algorithm.

Telomere length measurement by fluorescence in situ hybridization and flow cytometry: tips and pitfalls

BACKGROUND

Telomeres containing noncoding DNA repeats at the end of the chromosomes are essential for chromosomal stability and are implicated in regulating the replication and senescence of cells. The gradual loss of telomere repeats in cells has been linked to aging and tumor development and methods to measure telomere length are of increasing interest. At least three methods for measuring the length of telomere repeats have been described: Southern blot analysis and quantitative fluorescence in situ hybridization using either digital fluorescence microscopy (Q-FISH) or flow cytometry (flow-FISH). Both Southern blot analysis and Q-FISH have specific limitations and are time-consuming, whereas the flow-FISH technique requires relatively few cells (10(5)) and can be completed in a single day. A further advantage of the flow-FISH method is that data on the telomere length from individual cells and subsets of cells (lymphocytes and granulocytes) can be acquired from the same sample. In order to obtain accurate and reproducible results using the flow-FISH technique, we systematically explored the influence of various steps in the protocol on telomere length values and established an acceptable range for the most critical parameters.

METHODS

Isolated leukocytes from whole blood are denatured by heat and 70%/75% formamide, then hybridized with or without a telomere-specific fluorescein isothiocyante (FITC)-conjugated peptide nucleic acid probe (PNA). Unbound telomere PNA is washed away, the DNA is counterstained, and telomere fluorescence is measured on a flow cytometer using an argon ion laser (488 nm) to excite FITC. For each sample, duplicates of telomere PNA-stained and unstained tubes are analyzed.

RESULTS

Cell counts and flow-FISH telomere length measurements were performed on leukocytes and thymocytes of humans and other species. Leukocyte suspensions were prepared by two red blood cell lysis steps with ammonium chloride. Optimal denaturation of DNA was achieved by heating at 85-87 degrees C for 15 min in a solution containing 70%/75% formamide. Hybridization was performed at room temperature with a 0.3 microg/ml telomere-PNA probe for at least 60-90 min. Unbound telomere-PNA probe was diluted at least 4,000-40,000 times with wash steps containing 70%/75% formamide at room temperature. LDS 751 and DAPI were suitable as DNA counterstains as they did not show significant interference with telomere length measurement.

CONCLUSIONS

The use of flow-FISH for telomere length measurements in nucleated blood cells requires tight adherence to an optimized protocol. The method described here can be used to determine rapidly the telomere length in subsets of nucleated blood cells.

Stem cells: hype and reality

This update discusses what is known regarding embryonic and adult tissue-derived pluripotent stem cells, including the mechanisms underlying self-renewal without senescence, differentiation in multiple cell types both in vitro and in vivo, and future potential clinical uses of such stem cells. In Section I, Dr. Lansdorp reviews the structure and function of telomerase, the enzyme that restores telomeric ends of chromosomes upon cell division, highly present in embryonic stem cells but not adult stem cells. He discusses the structure and function of telomerase and signaling pathways activated by the enzyme, with special emphasis on normal and leukemic hematopoietic stem cells. In Section II, Dr. Pera reviews the present understanding of mammalian pluripotent embryonic stem cells. He discusses the concept of pluripotentiality in its embryonic context, derivation of stem cells from embryonic or fetal tissue, the basic properties of the stem cells, and methods to produce specific types of differentiated cell from stem cells. He examines the potential applications of stem cells in research and medicine and some of the barriers that must be crossed to achieve these goals. In Section III, Dr. Verfaillie reviews the present understanding of pluripotency of adult stem cells. She discusses the concept of stem cell plasticity, a term used to describe the greater potency described by several investigators of adult tissue-derived stem cells, critically reviews the published studies demonstrating stem cell plasticity, and possible mechanisms underlying such plasticity, and examines the possible role of pluripotent adult stem cells in research and medicine.

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.