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Stem Cells 2002;20:198-204 www.StemCells.com
© 2002 AlphaMed Press

CONCISE REVIEW

Telomeres, X-Inactivation Ratios, and Hematopoietic Stem Cell Transplantation in Humans: A Review

Ian Thornley, Melvin H. Freedman

Division of Hematology/Oncology, The Hospital for Sick Children, University of Toronto, Ontario, Canada

Key Words. Hematopoietic stem cell • Hematopoietic stem cell transplantation • Telomere • X-inactivation ratio • Hematopoietic reconstitution

Ian Thornley, MBBS, MRCP(UK), Division of Hematology/Oncology, The Hospital for Sick Children, 555 University Avenue, Toronto, Ontario M5G 1X8, Canada. Telephone: 416-813-5977; Fax: 416-813-5327; e-mail: ihthornley{at}hotmail.com


   ABSTRACT
Top
 Abstract
Introduction
 Telomere Length and X...
 A Paradigm for Early...
 Conclusions and Future...
 References
 
The marrow repopulating hematopoietic stem cells (HSCs) in an auto- or allograft represent a small fraction of the normal complement of HSCs, yet are required to reconstitute hematopoiesis and sustain it for the lifetime of the recipient. Such a burden imposes a "replicative stress" upon hematopoietic stem/progenitor cells. The finding of accelerated telomere shortening in hematopoietic stem cell transplant (HSCT) recipients raised the specter of accelerated hematopoietic aging. Here, we review the HSCT telomere literature and other studies of surrogate markers of HSC behavior conducted in human HSCT recipients. We present a paradigm for posttransplant hematopoietic reconstitution and speculate on the fate of HSCs in the human transplant setting.


   INTRODUCTION
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 Abstract
 Introduction
Telomere Length and X...
 A Paradigm for Early...
 Conclusions and Future...
 References
 
THE ELUSIVE HEMATOPOIETIC STEM CELL
Hematopoietic stem cells (HSCs) are defined by their potential to affect sustained multilineage engraftment when infused into a myeloablated subject [1]. While they may be enriched by sorting bone marrow cells according to surface antigen expression, it is currently not possible to isolate a pure population for analysis. Human HSCs are enriched in the CD34hi [2], CD90 (Thy-1)+ [3–6], CD38dull [7–9], rhodamine-123lo [10], and CD34-Lin- [11,12] fractions of blood and marrow cells. CD133 [13] and, more recently, CD109 [14] have also been shown to be expressed on a subset of CD34+ cells with marrow-repopulating potential. Imperfect functionally defined surrogates have included cobblestone-area-forming cells [15] and long-term culture-initiating cells [16], both of which are recognized in vitro and engraft humanized severe combined immune-deficient (SCID-hu) mice [17] or nonobese diabetic (NOD)/SCID mice [18].

Though it is not possible to purify human HSCs, their behavior has been studied through serial examination of their more numerous differentiated progeny. There are two broad classes of HSC behavioral study: one characterized by the ability to follow the fate of a few discrete HSC clones, and another that nonspecifically captures the behavior of the entire HSC pool. Gene-marking studies, in which HSCs are transfected with retroviral vectors and individual clones distinguished by their possession of unique proviral insertion sites, are the prototype of the former class; studies of telomere length and X-inactivation ratios are examples of the latter.

To date, few studies have directly addressed the behavior of human HSCs in humans. It is increasingly recognized that findings in smaller experimental animals may not be easily extrapolated to humans [19]; not surprisingly, HSC behavior appears to be greatly influenced by the size and hematopoietic demands of an organism. Very few gene-marking studies have been performed in humans due to considerable technical and ethical constraints. This review focuses primarily on studies of leukocyte telomere length and X-inactivation ratios.

TELOMERES
The nonencoding regions of DNA at each end of eukaryotic chromosomes—telomeres—consist of stereotyped oligonucleotide repeats and associated protein [20–23]. Studies in various species have shown that telomeres mediate important chromosome/nuclear matrix interactions [24,25], may exert effects on regional subtelomeric gene transcription [26,27], protect encoding DNA from enzymatic breakdown [28,29], prevent dicentric fusion and other chromosomal aberrations [30,31], and interact critically with cell-cycle regulatory mechanisms [28,32,33]. Their length shows wide interindividual variation, and varies among cells in the same tissue and among chromosomes in the same cell [23].

Telomeres shorten with each round of cell division in normal human somatic cells, in contrast to the encoding DNA sequences [23]. Such shortening is due to the "end replication problem," whereby a stretch of DNA equivalent in length to an RNA primer is lost from the 5' end of each new DNA strand [34]. This DNA loss is estimated at 40-120 bp per division [23]. Thus, the telomere has been likened to a "mitotic clock" [35]. Its length is a marker of cellular senescence and replicative potential [23].

The ribonucleoprotein reverse transcriptase, telomerase, may restore telomere length [36,37]. It is found in human germ line cells, most cancer cells, and cells immortalized in vitro, and in all these cells, it appears to preserve the length of telomeres [38,39]. The replicative demands on the human hematopoietic system are formidable, with approximately a trillion new cells required daily in an adult. Telomerase is variably present in hematopoietic progenitors and in peripheral blood monocytes and lymphocytes [40], but in these cells, its presence does not appear to completely attenuate telomeric shortening [35,41–43].

Telomere length has been assessed in peripheral blood leukocytes. The length of a leukocyte's telomeres is determined by the following: A) the telomere length in the HSC from which it is derived; B) the number of cell divisions required to complete its development; C) the extent of telomere loss with each round of mitosis, and D) the degree of attenuation of telomere loss by telomerase. Cross-sectional studies of normal subjects beyond infancy have shown a yearly decrease in peripheral blood leukocyte telomere length of approximately 30-60 bp [42,44–46]. Given steady-state hematopoietic conditions, it has been inferred that this represents the yearly aggregate telomere loss in HSCs.

Few studies have prospectively assessed changes in leukocyte telomere length within individual hematologically normal subjects. Aside from age-associated decline, telomere length has fluctuated over time in most leukocyte subsets studied [43,47,48]. In the setting of steady-state hematopoiesis and constant rates of postmaturational telomere loss, such fluctuations can be attributed to differences in the clonal origin of the cells analyzed.

Brief mention should be made of the different methods currently used to measure telomeres. The "gold standard" has been Southern hybridization or the related "in-gel" hybridization method. Here, integral DNA derived from at least a million cells is digested to completion with a combination of restriction enzymes (e.g., RsaI and HinfI) to yield the telomeres and a few kilobases of subtelomeric DNA. Digests are run alongside labeled size markers, and hybridization is accomplished with a labeled telomeric probe. Telomeres are then quantified using densitometry [45]. The literature is divided on what unit of measurement to use, with some papers reporting mean terminal restriction fragment (TRF) length, and others reporting modal TRF length. While the two derivations clearly differ, results have tended to concur [49]. These methods have been shown to reliably detect differences in telomere length exceeding 270-320 bp [50,51]. Telomere length may be assessed more simply by flow cytometry after staining fixed cells with a fluorescent peptide nucleic acid telomeric probe [52]. Like the Southern hybridization method, this method generally requires the analysis of at least a million cells. Telomere length may also be assessed using a quantitative fluorescent in situ hybridization (Q-FISH) technique [53]. The advantage of Q-FISH is that it allows rare cells to be studied; however, accounting for normal background variability in telomere length is an inherent challenge.

X-INACTIVATION RATIOS
Polymorphisms existing at distinct loci on the X chromosome can be used to distinguish paternally and maternally derived X chromosomes. The active and inactive X chromosome may be distinguished by differential DNA methylation patterns at particular loci [54]. The human androgen receptor (HUMARA) locus has been especially useful for X-inactivation studies, since it is amenable to polymerase chain reaction-based analysis, and approximately 90% of women are heterozygous and, therefore, informative [54,55]. Skewed X-inactivation ratios (arbitrarily defined as activation of one allele in more than 75% of cells) have been demonstrated in leukocytes of hematologically normal women [55]. The incidence of such skewing rises with age. It is likely that most cases of acquired skewing in apparently healthy individuals are due to a growth advantage conferred by parent-specific X-linked alleles (hemizygous selection) [56]. Other causes, such as stem cell depletion and the expansion of premalignant clones, have been suggested [57,58]. In their longitudinal studies of hematologically normal women, Prchal et al. [59] established that X-inactivation ratios remained remarkably constant in multiple blood lineages over a period of 2-3 years. They were able to estimate the number of stem cells required to be engaged in active hematopoiesis in order to ensure ratio stability. Of note, their mathematical model did not take into account the effect of hemizygous selection. That clonal instability could reflect small stem cell numbers was elegantly demonstrated by Abkowitz et al. through studies of X-inactivation ratios in transplanted Safari cats and computer modeling of stem cell behavior [57,60]. Thus, both the ratio itself and its change over time may yield important information about HSCs.


   TELOMERE LENGTH AND X-INACTIVATION RATIOS IN HUMAN TRANSPLANT RECIPIENTS
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 Abstract
 Introduction
 Telomere Length and X...
A Paradigm for Early...
 Conclusions and Future...
 References
 
Telomeres Post-HSCT
Excessive telomere shortening in leukocytes has been observed in virtually all short- and long-term survivors of autologous and allogeneic HSC transplants (HSCT) studied and has amounted to between 300 and 2,000 bp [43,45,50,51,61–65]. It occurs in neutrophils, monocytes, and T cells [43,50,63]. Notaro et al. found an inverse relationship between the extent of post-HSCT telomere loss and the number of nucleated cells in the graft [50]. Another study found a correlation between donor age and the extent of telomere shortening in allogeneic recipients [61]. Other studies have failed to identify these, or any other, associations [45,62,63]. Most investigators have invoked HSC replicative stress as the cause of HSCT-induced accelerated telomere shortening. Some have concluded that transplantation induces accelerated HSC "aging."

Recently, we prospectively examined changes in neutrophil telomere length and cycling of marrow progenitors in 25 fully engrafted allogeneic HSCT recipients [51]. Donors were sampled during stem cell harvest, and recipients at engraftment, 2-6 months, and 12 months post-HSCT. Telomere length was measured by an in-gel hybridization technique, and cell cycle status determined by flow cytometric analysis of Hoechst 33342- and pyronin Y-stained CD34+CD90+ and CD34+CD90- marrow cells. Compared with their respective donors, telomeres were shortened in recipients at engraftment (-424 bp; p < 0.0001), 6 months (-495 bp; p = 0.0001), and 12 months post-HSCT (-565 bp; p < 0.0001). Considering recipient telomere length in isolation from donor values, there were no significant differences among telomere lengths at engraftment, 6, and 12 months. The slope of the regression line derived from these telomere lengths was not different from zero (p = 0.16). To determine if a nonlinear model would better fit the data, residuals from the regression line expressing change in telomere lengths were plotted against time; their random distribution suggested random determination of the pattern of change in telomere length. The variation in the pattern of change in telomere length observed among this cohort [51], and in other reports [66,67], has been striking. Many recipients have had marked, seemingly random, fluctuations in telomere length over the 12-month period of observation; in some, pronounced fluctuations have been observed in the first 60 days post-HSCT [51]. These fluctuations, which exceed 270-320 bp and thus fall within the method's range of reliable detection [50,51], are greater than those previously reported in a few normal subjects [43,47,48]. The propensity for such wide fluctuations is probably conferred by a significant reduction in the number of HSC clones contributing to hematopoiesis early post-HSCT.

It is possible that many factors (e.g., graft cellularity, graft-versus-host disease [GVHD], infection, and the use of cytokines and other drugs) may interact to modulate posttransplant hematopoietic replicative stress and associated leukocyte telomere shortening. Using univariable and multivariable linear regression, we sought associations among the degree of telomere shortening at 12 months post-HSCT and donor age, nucleated cell dose, and the extent of GVHD [51]. No significant associations were identified. In addition, univariable and multivariable analyses were performed to assess associations among these factors and the pattern of change in telomere length. Again, no significant associations were identified. Given the weight of evidence for stochastic determination of HSC behavior [60], it is likely that the pattern is determined randomly.

We found the proportion of CD34+CD90+ progenitors in S/G2/M to be 4.3% in donors and 15.7% in recipients at 2-6 months (p < 0.0001). The proportion was 11.5% in recipients 12 months post-HSCT (p < 0.0001, versus donors; p = 0.04, versus 2-6 months) [51]. Cycling of CD34+CD90- progenitors was largely unchanged over the period of study. If an increase in the number of cell divisions during differentiation from HSC to mature neutrophil was the sole factor underlying the excessive telomere shortening in HSCT recipients, changes in telomere length would be expected to parallel changes in the mitotic activity of bone marrow progenitors. No such relationship has been demonstrated [51].

Telomerase activity in bone marrow progenitors has not been assessed in HSCT recipients and, therefore, impaired telomerase expression or function cannot be excluded as causes of the telomere shortening. However, there is indirect evidence to suggest that telomerase activity may be normal or upregulated in HSCT recipients: telomerase has been found to be upregulated in response to ex vivo expansion of bone marrow progenitors [41, 68] and in the setting of accelerated telomere shortening in Fanconi's anemia [69] and HIV infection [70].

The demonstration of excessive telomere shortening in multiple leukocyte lineages of HSCT recipients [43, 50, 63] strongly suggests that accelerated proliferation at the level of the pluripotent stem cell is the dominant cause of HSCT-induced telomere shortening.

The lack of correlation between the extent of leukocyte telomere shortening and time from HSCT has been a consistent finding [43, 45, 50, 51, 61–65]. This implies that HSCT-induced acceleration in leukocyte telomere loss is neither progressive nor sustained. Most of the cohort of allogeneic HSCT recipients we studied entered their second year post-HSCT with no evidence of sustained acceleration in telomere loss [51]. Another recent longitudinal study that specifically addressed the kinetics of telomere shortening after HSCT was that of Rufer et al. [43]. They examined telomere length in monocytes and T cells obtained from four allogeneic HSCT recipients up to 12 years after transplant and found that telomeres in the recipients shortened at a comparable rate with that of their donors from the second year post-HSCT.

X-Inactivation Ratios Post-HSCT
Autologous HSCT experiments in female Safari cats have shown that low stem cell doses may result in an extended period of skewed and highly unstable X-inactivation ratios posttransplant [57], suggestive of oligoclonal hematopoiesis. The incidence of profoundly skewed X-inactivation ratios in women with non-Hodgkin's lymphoma presenting for autologous HSCT was found to be 13.5% [71]. For these patients, the risk of post-HSCT myelodysplasia was as high as 40%; for nonskewed patients, the risk was less than 5%. Thus, in this setting, profound skewing of the X-inactivation ratio appears to signal latent myelodysplasia and/or profound stem cell depletion.

Studies of X-inactivation ratios have revealed polyclonal hematopoiesis in the majority of human allogeneic HSCT recipients studied, with oligoclonal hematopoiesis reported in 0%-14% of patients [65, 72, 73]. One case report describes donor-derived oligoclonal hematopoiesis in a recipient of a hypocellular graft [74]. The patient ultimately experienced graft failure, leading the authors to suggest that oligoclonal reconstitution be considered an indication for graft augmentation with a second stem cell dose. In 11 fully engrafted allogeneic HSCT recipients, we found that donor X-inactivation ratios (determined by the HUMARA assay) were faithfully reproduced and maintained with striking consistency over the first 12 months posttransplant [51].


   A PARADIGM FOR EARLY HEMATOPOIETIC RECONSTITUTION AFTER STEM CELL TRANSPLANTATION IN HUMANS
Top
 Abstract
 Introduction
 Telomere Length and X...
 A Paradigm for Early...
Conclusions and Future...
 References
 
Immediately postgrafting, maximal proliferation of infused HSCs is induced in a myeloablated recipient. This results in accelerated telomere loss. The contribution of individual HSCs to active hematopoiesis (through differentiation) or hematopoietic repopulation (through extensive self-renewal) is determined stochastically. When HSCs with shortened telomeres contribute to hematopoiesis, the accelerated telomere loss may be detected in circulating leukocytes. The duration of the contribution of an HSC to hematopoiesis, or to hematopoietic repopulation, varies. It is unclear whether this variation, well documented by gene-marking studies in primates [75] and NOD/SCID mice [76], is stochastically or intrinsically determined.

In the majority of human recipients, peripheral blood cell counts and bone marrow cellularity normalize in the first 2 to 6 months post-HSCT [51]. Despite this superficial appearance of complete hematopoietic reconstitution, the number of HSC clones contributing to mature blood cell formation is substantially reduced compared with normal, as evidenced by a propensity for marked fluctuations in neutrophil telomere length and a nearly fourfold increase in the mitotic rate of CD34+CD90+ marrow progenitors [77].

By 12 months, hematopoietic repopulation is more advanced. The mitotic rate of CD34+CD90+ bone marrow progenitors, though still significantly higher than in steady-state marrow, is lower than at 6 months post-HSCT because more HSC clones are now contributing to mature blood cell formation at any given time [51]. HSC proliferative rates have slowed, and leukocyte telomere loss is no longer accelerated [45, 51]. However, a marked reduction in the proportion of CD34+ cells expressing the CD90 antigen [51] betrays a persistent proximal hematopoietic deficit—a deficit exposed by studies of the clonogenic potential of marrow from HSCT recipients [78–80].


   CONCLUSIONS AND FUTURE DIRECTIONS
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 Abstract
 Introduction
 Telomere Length and X...
 A Paradigm for Early...
 Conclusions and Future...
References
 
There is now strong evidence that HSCT-induced accelerated telomere loss, occurring at the HSC level, is limited with respect to its duration and magnitude. The magnitude of telomere loss is within the normal interindividual range at any given age [42, 44–46] and may be less than the normal telomere loss experienced in the first year of life [42, 47, 81, 82]. Neither the telomere loss nor persistent HSC deficit is likely to pose a threat to graft survival or appreciably increase the risk of clonal hematopoietic disorders in the majority of HSCT recipients.

Until it is possible to purify human HSCs, it is likely that combined studies of telomere length, X-inactivation ratios, and gene-marked progeny in humans will offer the greatest insights into their behavior.


   ACKNOWLEDGMENT
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We are indebted to Robert Wynn, Robert Sutherland, and Hans Messner for their collaboration and insights.


   REFERENCES
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 Abstract
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 A Paradigm for Early...
 Conclusions and Future...
 References
 

  1. Till JE, McCulloch EA. Hemopoietic stem cell differentiation. Biochim Biophys Acta 1980;605:431–459.[Medline]

  2. DiGiusto D, Chen S, Combs J et al. Human fetal bone marrow early progenitors for T, B, and myeloid cells are found exclusively in the population expressing high levels of CD34. Blood 1994;84:421–432.[Abstract/Free Full Text]

  3. Baum CM, Weissman IL, Tsukamoto AS et al. Isolation of a candidate human hematopoietic stem-cell population. Proc Natl Acad Sci USA 1992;89:2804–2808.[Abstract/Free Full Text]

  4. Craig W, Kay R, Cutler RL et al. Expression of Thy-1 on human hematopoietic progenitor cells. J Exp Med 1993;177:1331–1342.[Abstract/Free Full Text]

  5. Sutherland DR, Yeo EL, Stewart AK et al. Identification of CD34+ subsets after glycoprotease selection: engraftment of CD34+/Thy-1+/Lin- stem cells in fetal sheep. Exp Hematol 1996;24:795–806.[Medline]

  6. Murray L, Chen B, Galy A et al. Enrichment of human hematopoietic stem cell activity in the CD34+Thy-1+Lin- subpopulation from mobilized peripheral blood. Blood 1995;85:368–378.[Abstract/Free Full Text]

  7. Terstappen LWMM, Huang S, Safford M et al. Sequential generations of hematopoietic colonies derived from single nonlineage-committed CD34+CD38- progenitor cells. Blood 1991;77:1218–1227.[Abstract/Free Full Text]

  8. Bhatia M, Wang JCY, Kapp U et al. Purification of primitive human hematopoietic cells capable of repopulating immune-deficient mice. Proc Natl Acad Sci USA 1997;94:5320–5325.[Abstract/Free Full Text]

  9. Zanjani ED, Almeida-Porada G, Ascensao JL et al. Transplantation of hematopoietic stem cells in utero. STEM CELLS 1997;15(suppl 1):79–92.

  10. Bertoncello I, Hodgson GS, Bradley TR. Multiparameter analysis of transplantable hemopoietic stem cells: I. The separation and enrichment of stem cells homing to marrow and spleen on the basis of rhodamine-123 fluorescence. Exp Hematol 1985;13:999–1006.[Medline]

  11. Zanjani ED, Almeida-Porada G, Livingston AG et al. Human bone marrow CD34- cells engraft in vivo and undergo multilineage expression that includes giving rise to CD34+ cells. Exp Hematol 1998;26:353–360.[Medline]

  12. Bhatia M, Bonnet D, Murdoch B et al. A newly discovered class of human hematopoietic cells with SCID-repopulating activity. Nat Med 1998;4:1038–1045.[CrossRef][Medline]

  13. Yin AH, Miraglia S, Zanjani ED et al. AC133, a novel marker for human hematopoeitic stem and progenitor cells. Blood 1997;90:5002–5012.[Abstract/Free Full Text]

  14. Murray LJ, Bruno E, Uchida N et al. CD109 is expressed on a subpopulation of CD34+ cells enriched in hematopoietic stem and progenitor cells. Exp Hematol 1999;27:1282–1294.[CrossRef][Medline]

  15. Ploemacher RE, van der Sluijs JP, Voerman JS et al. An in vitro limiting-dilution assay of long-term repopulating hematopoietic stem cells in the mouse. Blood 1989;74:2755–2763.[Abstract/Free Full Text]

  16. Eaves CJ, Sutherland HJ, Udomsakdi C et al. The human hematopoietic stem cell in vitro and in vivo. Blood Cells 1992;18:301–307.[Medline]

  17. McCune JM, Namikawa R, Kaneshima H et al. The SCID-hu mouse: murine model for the analysis of human hematolymphoid differentiation and function. Science 1988;241:1632–1639.[Abstract/Free Full Text]

  18. Larochelle A, Vormoor J, Hanenberg H et al. Identification of primitive human hematopoietic cells capable of repopulating NOD/SCID mouse bone marrow: implications for gene therapy. Nat Med 1996;2:1329–1337.[CrossRef][Medline]

  19. Abkowitz JL, Golinelli D, Harrison DE et al. In vivo kinetics of murine hemopoietic stem cells. Blood 2000;96:3399–3405.[Abstract/Free Full Text]

  20. Blackburn EH, Gall JG. A tandemly repeated sequence at the termini of the extrachromosomal ribosomal RNA genes in Tetrahymena. J Mol Biol 1978;120:33–53.[CrossRef][Medline]

  21. Conrad MN, Wright JH, Wolf AJ et al. RAP1 protein interacts with yeast telomeres in vivo: overproduction alters telomere structure and decreases chromosome stability. Cell 1990;63:739–750.[CrossRef][Medline]

  22. Zakian VA. Telomeres: beginning to understand the end. Science 1995;270:1601–1607.[Abstract/Free Full Text]

  23. McEachern MJ, Krauskopf A, Blackburn EH. Telomeres and their control. Annu Rev Genet 2000;34:331–358.[CrossRef][Medline]

  24. Walker CL, Cargile CB, Floy KM et al. The Barr body is a looped X chromosome formed by telomere association. Proc Natl Acad Sci USA 1991;88:6191–6195.[Abstract/Free Full Text]

  25. Mathog D, Hochstrasser M, Gruenbaum Y et al. Characteristic folding pattern of polytene chromosomes in Drosophila salivary gland nuclei. Nature 1984;308:414–421.[CrossRef][Medline]

  26. Levis R, Hazelrigg T, Rubin GM. Effects of genomic position on the expression of transduced copies of the white gene of Drosophila. Science 1985;229:558–561.[Abstract/Free Full Text]

  27. Gottschling DE, Aparicio OM, Billington BL et al. Position effect at S. cerevisiae telomeres: reversible repression of PolII transcription. Cell 1990;63:751–762.[CrossRef][Medline]

  28. Sandell LL, Zakian VA. Loss of a yeast telomere: arrest, recovery, and chromosome loss. Cell 1993;75:729–739.[CrossRef][Medline]

  29. White CI, Haber JE. Intermediates of recombination during mating type switching in Saccharomyces cerevisiae. EMBO J 1990;9:663–673.[Medline]

  30. Smith CD, Blackburn EH. Uncapping and deregulation of telomeres lead to detrimental cellular consequences in yeast. J Cell Biol 1999;145:203–214.[Abstract/Free Full Text]

  31. van Steensel B, Smogorzewska A, de Lange T. TRF2 protects human telomeres from end-to-end fusions. Cell 1998;92:401–413.[CrossRef][Medline]

  32. Weinert TA, Hartwell LH. The RAD9 gene controls the cell cycle response to DNA damage in Saccharomyces cerevisiae. Science 1988;241:317–322.[Abstract/Free Full Text]

  33. Schiestl RH, Reynolds P, Prakesh S et al. Cloning and sequence analysis of the Saccharomyces cerevisiae RAD9 gene and further evidence that its product is required for cell cycle arrest induced by DNA damage. Mol Cell Biol 1989;9:1882–1896.[Abstract/Free Full Text]

  34. Olovnikov AM. A theory of marginotomy: the incomplete copying of template margin in enzymatic synthesis of polynucleotides and biological significance of the phenomenon. J Theor Biol 1973;41:181–190.[CrossRef][Medline]

  35. Vaziri H, Dragowska W, Allsopp RC et al. Evidence for a mitotic clock in human hematopoietic stem cells: loss of telomeric DNA with age. Proc Natl Acad Sci USA 1994;91:9857–9860.[Abstract/Free Full Text]

  36. Greider CW, Blackburn EH. The telomere terminal transferase of Tetrahymena is a ribonucleoprotein enzyme with two kinds of primer specificity. Cell 1987;51:887–898.[CrossRef][Medline]

  37. Bodnar AG, Ouellette M, Frolkis M et al. Extension of life-span by introduction of telomerase into normal human cells. Science 1998;279:349–352.[Abstract/Free Full Text]

  38. Ulaner GA, Giudice LC. Developmental regulation of telomerase activity in human fetal tissues during gestation. Mol Hum Reprod 1997;3:769–773.[Abstract/Free Full Text]

  39. Kim NW, Piatyszek MA, Prowse KR et al. Specific association of human telomerase activity with immortal cells and cancer. Science 1994;266:2011–2015.[Abstract/Free Full Text]

  40. Hiyama K, Hirai Y, Kyoizumi S et al. Activation of telomerase in human lymphocytes and hematopoietic progenitor cells. J Immunol 1995;155:3711–3715.[Abstract]

  41. Engelhardt M, Kumar R, Albanell J et al. Telomerase regulation, cell cycle, and telomere stability in primitive hematopoietic cells. Blood 1997;90:182–193.[Abstract/Free Full Text]

  42. Robertson JD, Gale RE, Wynn RF et al. Dynamics of telomere shortening in neutrophils and T lymphocytes during ageing and the relationship to skewed X chromosome inactivation patterns. Br J Haematol 2000;109:272–279.[CrossRef][Medline]

  43. Rufer N, Brummendorf TH, Chapuis B et al. Accelerated telomere shortening in hematological lineages is limited to the first year following stem cell transplantation. Blood 2001;97:575–577.[Abstract/Free Full Text]

  44. Vaziri H, Schachter F, Uchida I et al. Loss of telomeric DNA during aging of normal and trisomy 21 human lymphocytes. Am J Hum Genet 1993;52:661–667.[Medline]

  45. Wynn RF, Cross MA, Hatton C et al. Accelerated telomere shortening in young recipients of allogeneic bone marrow transplants. Lancet 1998;351:178–181.[CrossRef][Medline]

  46. Ball SE, Gibson FM, Rizzo S et al. Progressive telomere shortening in aplastic anemia. Blood 1998;91:3582–3592.[Abstract/Free Full Text]

  47. Zeichner SL, Palumbo P, Feng Y et al. Rapid telomere shortening in children. Blood 1999;93:2824–2830.[Abstract/Free Full Text]

  48. Wolthers KC, Bea G, Wisman A et al. T cell telomere length in HIV-1 infection: no evidence for increased CD4+ T cell turnover. Science 1996;274:1543–1547.[Abstract/Free Full Text]

  49. Engelhardt M, Mackenzie K, Drullinsky P et al. Telomerase activity and telomere length in acute and chronic leukemia, pre- and post-ex vivo culture. Cancer Res 2000;60:610–617.[Abstract/Free Full Text]

  50. Notaro R, Cimmino A, Tabarini D et al. In vivo telomere dynamics of human hematopoietic stem cells. Proc Natl Acad Sci USA 1997;94:13782–13785.[Abstract/Free Full Text]

  51. Thornley I, Sutherland DR, Wynn R et al. Early hematopoietic reconstitution after clinical stem cell transplantation: evidence for stochastic stem cell behavior and limited acceleration in telomere loss. Blood 2002;99:2387–2396.[Abstract/Free Full Text]

  52. Rufer N, Dragowska W, Thornbury G et al. Telomere length dynamics in human lymphocyte subpopulations measured by flow cytometry. Nat Biotechnol 1998;16:743–747.[CrossRef][Medline]

  53. Poon SS, Martens UM, Ward RK et al. Telomere length measurements using digital fluorescence microscopy. Cytometry 1999;36:267–278.[CrossRef][Medline]

  54. Allen RC, Zoghbi HY, Moseley AB et al. Methylation of HpaII and HhaI sites near the polymorphic CAG repeat in the human androgen-receptor gene correlates with X chromosome inactivation. Am J Hum Genet 1992;51:1229–1239.[Medline]

  55. Busque L, Mio R, Mattioli J et al. Nonrandom X-inactivation patterns in normal females: lyonization ratios vary with age. Blood 1996;88:59–65.[Abstract/Free Full Text]

  56. Abkowitz JL, Taboada M, Shelton GH et al. An X chromosome gene regulates hematopoietic stem cell kinetics. Proc Natl Acad Sci USA 1998;95:3862–3866.[Abstract/Free Full Text]

  57. Abkowitz JL, Persik MT, Shelton GH et al. Behavior of hematopoietic stem cells in a large animal. Proc Natl Acad Sci USA 1995;92:2031–2035.[Abstract/Free Full Text]

  58. Busque L, Gilliland DG. X-inactivation analysis in the 1990s: promise and potential problems. Leukemia 1998;12:128–135.[CrossRef][Medline]

  59. Prchal JT, Prchal JF, Belickova M et al. Clonal stability of blood cell lineages indicated by X-chromosomal transcriptional polymorphism. J Exp Med 1996;183:561–567.[Abstract/Free Full Text]

  60. Abkowitz JL, Catlin SN, Guttorp P. Evidence that hematopoiesis may be a stochastic process in vivo. Nat Med 1996;2:190–197.[CrossRef][Medline]

  61. Akiyama M, Hoshi Y, Sakurai S et al. Changes of telomere length in children after hematopoietic stem cell transplantation. Bone Marrow Transplant 1998;21:167–171.[CrossRef][Medline]

  62. Lee JJ, Kook H, Chung IJ et al. Telomere length changes in patients undergoing hematopoietic stem cell transplantation. Bone Marrow Transplant 1999;24:411–415.[CrossRef][Medline]

  63. Wynn RF, Thornley I, Freedman MH et al. Telomere shortening in leucocyte subsets of long-term survivors of allogeneic bone marrow transplantation. Br J Haematol 1999;105:997–1001.[CrossRef][Medline]

  64. Akiyama M, Asai O, Kuraishi Y et al. Shortening of telomeres in recipients of both autologous and allogeneic hematopoietic stem cell transplantation. Bone Marrow Transplant 2000;25:441–447.[CrossRef][Medline]

  65. Mathioudakis G, Storb R, McSweeney PA et al. Polyclonal hematopoiesis with variable telomere shortening in human long-term allogeneic marrow graft recipients. Blood 2000;96:3991–3994.[Abstract/Free Full Text]

  66. Roelofs H, de Pauw ESD, Tanke HJ et al. Telomere length homeostasis after allogeneic peripheral blood stem cell transplantation. Blood 2000;96:203a.

  67. Engelhardt M, Guo Y, Finke J. Telomere shortening in hematopoietic cells of patients undergoing allogeneic stem cell transplantation. Blood 2000;96:762a.

  68. Kobari L, Pflumio F, Giarratana M et al. In vitro and in vivo evidence for the long-term multilineage (myeloid, B, NK, and T) reconstitution capacity of ex vivo expanded human CD34+ cord blood cells. Exp Hematol 2000;28:1470–1480.[CrossRef][Medline]

  69. Leteurtre F, Li X, Guardiola P et al. Accelerated telomere shortening and telomerase activation in Fanconi's anaemia. Br J Haematol 1999;105:883–893.[CrossRef][Medline]

  70. Wolthers KC, Otto SA, Wisman GB et al. Normal T-cell telomerase activity and upregulation in human immunodeficiency virus-1 infection. Blood 1999;93:1011–1019.[Abstract/Free Full Text]

  71. Mach-Pascual S, Legare RD, Lu D et al. Predictive value of clonality assays in patients with non-Hodgkin's lymphoma undergoing autologous bone marrow transplant: a single institution study. Blood 1998;91:4496–4503.[Abstract/Free Full Text]

  72. Nash R, Storb R, Neiman P. Polyclonal reconstitution of human marrow after allogeneic bone marrow transplantation. Blood 1988;72:2031–2037.[Abstract/Free Full Text]

  73. Turhan AG, Humphries RK, Phillips GL et al. Clonal hematopoiesis demonstrated by X-linked DNA polymorphisms after allogeneic bone marrow transplantation. N Engl J Med 1989;320:1655–1661.[Abstract]

  74. Nakao S, Yamaguchi M, Takamatsu H et al. Clonal hematopoiesis detected in an allogeneic marrow transplant recipient with poor engraftment. Transplantation 1994;57:1266–1268.[Medline]

  75. Kim HJ, Tisdale JF, Wu T et al. Many multipotential gene-marked progenitor or stem cell clones contribute to hematopoiesis in nonhuman primates. Blood 2000;96:1–8.[Abstract/Free Full Text]

  76. Guenechea G, Gan OI, Dorrell C et al. Distinct classes of human stem cells that differ in proliferative and self-renewal potential. Nat Immunol 2001;2:75–82.[CrossRef][Medline]

  77. Thornley I, Sutherland DR, Nayar R et al. Replicative stress after allogeneic bone marrow transplantation: changes in cycling of CD34+CD90+ and CD34+CD90- hematopoietic progenitors. Blood 2001;97:1876–1878.[Abstract/Free Full Text]

  78. Novitzky N, Mohammed R. Alterations in the progenitor cell population follow recovery from myeloablative therapy and bone marrow transplantation. Exp Hematol 1997;25:471–477.[Medline]

  79. Podesta M, Piaggio G, Frassoni F et al. Deficient reconstitution of early progenitors after allogeneic bone marrow transplantation. Bone Marrow Transplant 1997;19:1011–1017.[CrossRef][Medline]

  80. Selleri C, Maciejewski JP, De Rosa G et al. Long-lasting decrease of marrow and circulating long-term culture initiating cells after allogeneic bone marrow transplant. Bone Marrow Transplant 1999;23:1029–1037.[CrossRef][Medline]

  81. Frenck RW Jr, Blackburn EH, Shannon KM. The rate of telomere sequence loss in human leukocytes varies with age. Proc Natl Acad Sci USA 1998;95:5607–5610.[Abstract/Free Full Text]

  82. Rufer N, Brummendorf TH, Kolvraa S et al. Telomere fluorescence measurements in granulocytes and T lymphocyte subsets point to a high turnover of hematopoietic stem cells and memory T cells in early childhood. J Exp Med 1999;190:157–167.[Abstract/Free Full Text]

Received December 13, 2001; accepted for publication February 1, 2002.



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