Telomere shortening in hematopoietic stem cell transplantation: a potential mechanism for late graft failure?

Airway Telomere Length in Lung Transplant Recipients

Introduction

Chronic lung allograft dysfunction (CLAD) represents the major impediment to long term survival following lung transplantation. Donor and recipient telomere length have been shown to associate with lung transplant outcomes, including CLAD. In this study we aimed to measure the telomere lengths of bronchial and bronchiolar airway cells in lung allografts early after transplantation and to investigate associations with CLAD and all-cause mortality.

Methods

This prospective, longitudinal study was performed at The Prince Charles Hospital, Australia. Airway cells were collected via bronchial and bronchiolar airway brushings at post-transplant bronchoscopies. The relative telomere length in airway cells was determined by quantitative PCR based on the T/S ratio. All patients were censored for CLAD and all-cause mortality in August 2020.

Results

In total 231 bronchoscopies incorporating transbronchial brush and bronchial brush were performed in 120 patients. At the time of censoring, 43% and 35% of patients, respectively, had developed CLAD and had died. Airway bronchiolar and bronchial telomere lengths were strongly correlated (r=0.78, p<0.001), confirming conservation of telomere length with airway branch generation. Both the bronchiolar (r = -0.34, p<0.001) and bronchial (r = -0.31, p<0.001) telomere length decreased with age. Shorter airway telomere length was associated with older donor age and higher donor pack-year smoking history. Neither the bronchiolar nor the bronchial airway telomere length were associated with the development of CLAD (HR 0.39 (0.06-2.3), p=0.30; HR 0.66 (0.2-1.7), p=0.39, respectively) or all-cause mortality (HR 0.92 (0.2-4.5), p=0.92; HR 0.47 (0.1-1.9), p=0.28, respectively).

Conclusions

In this cohort, airway telomere length was associated with donor age and smoking history but was not associated with the future development of CLAD or all-cause mortality.

Influence of Donor and Recipient Gender on Telomere Maintenance after Umbilical Cord Blood Cell Transplantation: A Study by the Gruppo Italiano Trapianto Di Midollo Osseo

Physiologic loss of telomerase activity in adult life determines progressive telomere length (TL) shortening. Inflammation and oxidative damage are established causes of TL loss; moreover, males have shorter telomeres compared with females. Despite these notions, mechanisms regulating TL maintenance are poorly defined. Because umbilical cord blood (UCB) cells harbor very long telomeres, not yet exposed to environmental damages, UCB transplantation (UCBT) provides a unique experimental setting to study determinants of TL in humans. TL dynamics were analyzed on peripheral blood mononuclear cells (MNCs) from 36 patients (median age, 42 years) undergoing UCBT. TL was studied at a median of 20 months after UCBT. A significantly longer TL (mean, 8698 bp; range, 6521 to 11,960) was documented in UCBT recipients compared with age-matched healthy control subjects (mean, 7396 bp; range, 4375 to 11,108; P < .01). Among variables potentially influencing TL maintenance, including recipient features, graft type, transplant procedure, and engraftment kinetics, only donor-recipient gender combination was associated with TL, with the longest TL in women receiving male UCB (mean, 10,063 bp; range, 8381 to 11,960). To further investigate this trend, telomerase activation was assessed in vitro. Experiments showed that telomerase subunits were preferentially upregulated in male-derived bone marrow MNCs exposed ex vivo to estradiol as compared with female MNCs. This implies an increased sensitivity of male-derived MNCs to telomerase activation induced by estradiol. The results suggest that extrinsic and modifiable factors such as hormonal status and female milieu could be major determinants of TL in humans, providing the rationale for investigating hormonal-based approaches to counteract telomere erosion and aging-related diseases.

Mixing old and young: enhancing rejuvenation and accelerating aging

Donor age and recipient age are factors that influence transplantation outcomes. Aside from age-associated differences in intrinsic graft function and alloimmune responses, the ability of young and old cells to exert either rejuvenating or aging effects extrinsically may also apply to the transplantation of hematopoietic stem cells or solid organ transplants. While the potential for rejuvenation mediated by the transfer of youthful cells is currently being explored for therapeutic applications, aspects that relate to accelerating aging are no less clinically significant. Those effects may be particularly relevant in transplantation with an age discrepancy between donor and recipient. Here, we review recent advances in understanding the mechanisms by which young and old cells modify their environments to promote rejuvenation- or aging-associated phenotypes. We discuss their relevance to clinical transplantation and highlight potential opportunities for therapeutic intervention.

Donor telomere length and causes of death after unrelated hematopoietic cell transplantation in patients with marrow failure

Previous studies have suggested that longer donor leukocyte telomere length (TL) is associated with improved survival after hematopoietic cell transplantation (HCT) in severe aplastic anemia (SAA). This study aimed to determine whether cell-specific lymphocyte TL is associated with certain post-HCT causes of death. We used flow cytometry and fluorescence in situ hybridization to measure TL in donor total lymphocytes and subsets: naïve enriched T cells (CD45RA⁺CD20^-), memory enriched T cells (CD45RA^-CD20^-), natural killer (NK) fully differentiated T cells (CD45RA⁺CD57⁺), and B cells (CD45RA⁺CD20⁺). Competing risk survival regression was used for cause-specific death analyses. Clinical data and biospecimens were available from the Center for International Blood and Marrow Transplant Research database and biorepository. The study included 197 patients who underwent unrelated-donor HCT for SAA between 1988 and 2004. The median age at HCT was 15 years (range, 0.5-40 years), and the median follow-up was 5 years (range, <1 month to 20.7 years). Longer donor TL in all cell subsets was associated with lower risk of all-cause mortality (P < .01). In cause-specific mortality analyses, longer TL in B cells (hazard ratio [HR], 0.63; 95% confidence interval [CI], 0.46-0.87; P = .006) and possibly NK fully differentiated T cells (HR, 0.7; 95% CI, 0.51 to 0.97; P = .03) was associated with lower risk of infection-related death. Donor TL in other tested lymphocyte subsets was not statistically significantly associated with death resulting from graft-versus-host disease or graft failure (P > .05). However, a trend toward excess risk of graft-versus-host mortality was noted (HR for total lymphocyte TL, 1.26; P = .15). In conclusion, longer donor TL was associated with reduced rate of infection-related deaths after HCT for SAA.

Allogeneic hematopoietic stem cell transplantation for dyskeratosis congenita

PURPOSE OF REVIEW

Dyskeratosis congenita is an inherited bone marrow failure syndrome caused by defects in telomere maintenance. Hematopoietic stem cell transplantation (HSCT) is the only curative treatment for bone marrow failure because of dyskeratosis congenita. The present review summarizes the literature with respect to the diagnosis and treatment of patients with dyskeratosis congenita who received HSCT, and discusses the recent progress in the management of dyskeratosis congenita.

RECENT FINDINGS

The recent systematic review of the literature showed poor long-term outcome, with 10-year survival estimates of only 23% in 109 patients with dyskeratosis congenita who received HSCT. Multivariate analysis identified age greater than 20 years at HSCT, HSCT before 2000, and alternative donor source to be poor prognostic markers. HSCT for dyskeratosis congenita is characterized by a marked decline in long-term survival because of late deaths from pulmonary complications. However, a prospective study using danazol showed promising results in gain in telomere length and hematologic responses.

SUMMARY

A recent prospective study may support the recommendation that HSCT is not indicated for patients with dyskeratosis congenita; instead, they should receive androgen, particularly danazol, as a first-line therapy. Another option may be routine use of androgen after HSCT for the prophylaxis of pulmonary fibrosis.

Telomere Biology of Human Hematopoietic Stem Cells

BACKGROUND

Telomeres are protein DNA structures present at the ends of chromosomes and are essential for genetic stability and cell replication. Telomerase is the enzyme complex that maintains telomere integrity. Hematopoietic stem cells express telomerase and contain long telomeres, which become shorter as cells differentiate and mature. The extent of telomere shortening and the level of telomerase activity often correlate with the presence and severity of some hematopoietic diseases.

METHODS

The fundamentals of telomeres and telomerase are reviewed, and the telomere biology of human hematopoietic cells is discussed.

RESULTS

Telomere length and telomerase activity are important in the self-renewal of hematopoietic stem cells. Changes within these compartments affect both normal hematopoietic cells and the generation of hematopoietic disease. Telomere length provides information pertaining to the proliferative history and potential of a hematopoietic cell.

CONCLUSIONS

The role of telomerase and telomeres within the hematopoietic compartment needs further clarification. Advances in our knowledge in this field may improve clinical outcomes for the treatment of hematologic disease.

Telomerase and telomere biology in hematological diseases: A new therapeutic target

Telomeres are structures confined at the ends of eukaryotic chromosomes. With each cell division, telomeric repeats are lost because DNA polymerases are incapable to fully duplicate the very ends of linear chromosomes. Loss of repeats causes cell senescence, and apoptosis. Telomerase neutralizes loss of telomeric sequences by adding telomere repeats at the 3' telomeric overhang. Telomere biology is frequently associated with human cancer and dysfunctional telomeres have been proved to participate to genetic instability. This review covers the information on telomerase expression and genetic alterations in the most relevant types of hematological diseases. Telomere erosion hampers the capability of hematopoietic stem cells to effectively replicate, clinically resulting in bone marrow failure. Furthermore, telomerase mutations are genetic risk factors for the occurrence of some hematologic cancers. New discoveries in telomere structure and telomerase functions have led to an increasing interest in targeting telomeres and telomerase in anti-cancer therapy.

Association of Donor and Recipient Telomere Length with Clinical Outcomes following Lung Transplantation

Background

Patients with short telomere syndromes and pulmonary fibrosis have increased complications after lung transplant. However, the more general impact of donor and recipient telomere length in lung transplant has not been well characterized.

Methods

This was an observational cohort study of patients who received lung transplant at a single center between January 1^st 2012 and January 31^st 2015. Relative donor lymphocyte telomere length was measured and classified into long (third tertile) and short (other tertiles). Relative recipient lung telomere length was measured and classified into short (first tertile) and long (other tertiles). Outcome data included survival, need for modification of immunosuppression, liver or kidney injury, cytomegalovirus reactivation, and acute rejection.

Results

Recipient lung tissue telomere lengths were measured for 54 of the 79 patients (68.3%) who underwent transplant during the study period. Donor lymphocyte telomeres were measured for 45 (83.3%) of these recipients. Neither long donor telomere length (hazard ratio [HR] = 0.58, 95% confidence interval [CI], 0.12–2.85, p = 0.50) nor short recipient telomere length (HR = 1.01, 95% CI = 0.50–2.05, p = 0.96) were associated with adjusted survival following lung transplant. Recipients with short telomeres were less likely to have acute cellular rejection (23.5% vs. 58.8%, p = 0.02) but were not more likely to have other organ dysfunction.

Conclusions

In this small cohort, neither long donor lymphocyte telomeres nor short recipient lung tissue telomeres were associated with adjusted survival after lung transplantation. Larger studies are needed to confirm these findings.

Outcomes of allogeneic hematopoietic cell transplantation in patients with dyskeratosis congenita

We describe outcomes after allogeneic transplantation in 34 patients with dyskeratosis congenita who underwent transplantation between 1981 and 2009. The median age at transplantation was 13 years (range, 2 to 35). Approximately 50% of transplantations were from related donors. Bone marrow was the predominant source of stem cells (24 of 34). The day-28 probability of neutrophil recovery was 73% and the day-100 platelet recovery was 72%. The day-100 probability of grade II to IV acute GVHD and the 3-year probability of chronic graft-versus-host disease were 24% and 37%, respectively. The 10-year probability of survival was 30%; 14 patients were alive at last follow-up. Ten deaths occurred within 4 months from transplantation because of graft failure (n = 6) or other transplantation-related complications; 9 of these patients had undergone transplantation from mismatched related or from unrelated donors. Another 10 deaths occurred after 4 months; 6 of them occurred more than 5 years after transplantation, and 4 of these were attributed to pulmonary failure. Transplantation regimen intensity and transplantations from mismatched related or unrelated donors were associated with early mortality. Transplantation of grafts from HLA-matched siblings with cyclophosphamide-containing nonradiation regimens was associated with early low toxicity. Late mortality was attributed mainly to pulmonary complications and likely related to the underlying disease.

Concise review: hematopoietic stem cell aging, life span, and transplantation

Self-renewal and multilineage differentiation of stem cells are keys to the lifelong homeostatic maintenance of tissues and organs. Hematopoietic aging, characterized by immunosenescence, proinflammation, and anemia, is attributed to age-associated changes in the number and function of hematopoietic stem cells (HSCs) and their microenvironmental niche. Genetic variants and factors regulating stem cell aging are correlatively or causatively associated with overall organismal aging and longevity. Translational use of HSCs for transplantation and gene therapy demands effective methods for stem cell expansion. Targeting the molecular pathways involved in HSC self-renewal, proliferation, and homing has led to enhanced expansion and engraftment of stem cells upon transplantation. HSC transplantation is less effective in elderly people, even though this is the demographic with the greatest need for this form of treatment. Thus, understanding the biological changes in the aging of stem cells as well as local and systematic environments will improve the efficacy of aged stem cells for regenerative medicine and ultimately facilitate improved health and life spans.

Age-adjusted recipient pretransplantation telomere length and treatment-related mortality after hematopoietic stem cell transplantation

Telomere attrition induces cell senescence and apoptosis. We hypothesized that age-adjusted pretransplantation telomere length might predict treatment-related mortality (TRM) after hematopoietic stem cell transplantation (HSCT). Between 2000 and 2005, 178 consecutive patients underwent HSCT from HLA-identical sibling donors after myeloablative conditioning regimens, mainly for hematologic malignancies (n = 153). Blood lymphocytes' telomere length was measured by real-time quantitative PCR before HSCT. Age-adjusted pretransplantation telomere lengths were analyzed for correlation with clinical outcomes. After age adjustment, patients' telomere-length distribution was similar among all 4 quartiles except for disease stage. There was no correlation between telomere length and engraftment, GVHD, or relapse. The overall survival was 62% at 5 years (95% confidence interval [CI], 54-70). After a median follow-up of 51 months (range, 1-121 months), 43 patients died because of TRM. The TRM rate inversely correlated with telomere length. TRM in patients in the first (lowest telomere length) quartile was significantly higher than in patients with longer telomeres (P = .017). In multivariate analysis, recipients' age (hazard ratio, 1.1; 95% CI, .0-1.1; P = .0001) and age-adjusted telomere length (hazard ratio, 0.4; 95% CI; 0.2-0.8; P = .01) were independently associated with TRM. In conclusion, age-adjusted recipients' telomere length is an independent biologic marker of TRM after HSCT.

Telomere biology in hematopoiesis and stem cell transplantation

Telomeres are long (TTAGGG)(n) nucleotide repeats and an associated protein complex located at the end of the chromosomes. They shorten with every cell division and, thus are markers for cellular aging, senescence, and replicative capacity. Telomere dysfunction is linked to several bone marrow disorders, including dyskeratosis congenita, aplastic anemia, myelodysplastic syndrome, and hematopoietic malignancies. Hematopoietic stem cell transplantation (HSCT) provides an opportunity in which to study telomere dynamics in a high cell proliferative environment. Rapid telomere shortening of donor cells occurs in the recipient shortly after HSCT; the degree of telomere attrition does not appear to differ by graft source. As expected, telomeres are longer in recipients of grafts with longer telomeres (e.g., cord blood). Telomere attrition may play a role in, or be a marker of, long term outcome after HSCT, but these data are limited. In this review, we discuss telomere biology in normal and abnormal hematopoiesis, including HSCT.

The long and the short of telomeres in bone marrow recipient SCID patients

Telomeres are noncoding DNA regions at the end of the chromosomes that are crucial for genome stability. Since telomere length decreases with cell division, they can be used as a signature of cell proliferation history. T-cell reconstitution in severe combined immunodeficiency (SCID) subjects, recipients of T-cell-depleted, allogeneic-related bone marrow cells, is due to the development and maturation of donor T-cell precursors in the infant's vestigial thymus and to homeostatic proliferation of mature T cells in the peripheral organs. Since T-cell function, thymic output, and T-cell clonal diversity are maintained long term in these patients, we investigated whether donor T-cell engraftment resulted in increased telomere shortening. Our study of seven SCID patients, following successful bone marrow transplantation, demonstrates that the patients' peripheral T cells did not exhibit greater than normal telomere shortening.

Predicting clonal self-renewal and extinction of hematopoietic stem cells

A single hematopoietic stem cell (HSC) can generate a clone, consisting of daughter HSCs and differentiated progeny, which can sustain the hematopoietic system of multiple hosts for a long time. At the same time, this massive expansion potential must be restrained to prevent abnormal, leukemic proliferation. We used an interdisciplinary approach, combining transplantation assays with mathematical and computational methods, to systematically analyze the proliferative potential of individual HSCs. We show that all HSC clones examined have an intrinsically limited life span. Daughter HSCs within a clone behaved synchronously in transplantation assays and eventually exhausted at the same time. These results indicate that each HSC is programmed to have a finite life span. This program and the memory of the life span of the mother HSC are inherited by all daughter HSCs. In contrast, there was extensive heterogeneity in life spans between individual HSC clones, ranging from 10 to almost 60 mo. We used model-based machine learning to develop a mathematical model that efficiently predicts the life spans of individual HSC clones on the basis of a few initial measurements of donor type cells in blood. Computer simulations predict that the probability of self-renewal decays with a logistic kinetic over the life span of a normal HSC clone. Other decay functions lead to either graft failure or leukemic proliferation. We propose that dynamical fate probabilities are a crucial condition that leads to self-limiting clonal proliferation.

Short telomeres resulting from heritable mutations in the telomerase reverse transcriptase gene predispose for a variety of malignancies

Telomeres are composed of long arrays of TTAGGG repeats and associated proteins that act as a protective cap for chromosome ends. The length of telomere repeats is set in the germline but decreases in somatic cells, primarily as a function of DNA replication. Progressive telomere shortening limits stem cell divisions and probably acts as a tumor suppressor mechanism. Using a sensitive PCR method to detect the length of individual telomere repeats on specific chromosomes, we confirmed that telomere length decreases from primitive to more differentiated human cell types within the hematopoietic hierarchy. Genetic mutations in the components of telomerase (the RNA template sequence hTERC, reverse transcriptase hTERT, and Syskerin DKC1) have recently been implicated in a variety of bone marrow failure syndromes, idiopathic pulmonary fibrosis, and more recently, acute myeloid leukemia (AML). The majority of mutations discovered in AML patients were heritable and resulted in partial loss of telomerase activity, a finding counterintuitive to the requirement of telomerase in cancer cells. We have found heritable hypomorphic TERT mutations in other cancers as well, and we propose that such mutations result in short telomeres and premature loss of stem cells. Loss of normal stem cells could provide strong selection for abnormal cells incapable of responding to DNA damage signals originating from short telomeres. Such cells will have a DNA repair defect resulting in genomic instability and a mutator phenotype. Our findings point to an intimate connection between senescence and cancer and highlight the important role of telomeres in the biology of normal and malignant human cells.

Cellular senescence of white blood cells in very long-term survivors after allogeneic hematopoietic stem cell transplantation: the role of chronic graft-versus-host disease and female donor sex

In this single-center, cross-sectional study, we evaluated 44 very long-term survivors with a median follow-up of 17.5 years (range, 11-26 years) after hematopoietic stem cell transplantation. We assessed the telomere length difference in human leukocyte antigen-identical donor and recipient sibling pairs and searched for its relationship with clinical factors. The telomere length (in kb, mean +/- SD) was significantly shorter in all recipient blood cells compared with their donors' blood cells (P < .01): granulocytes (6.5 +/- 0.9 vs 7.1 +/- 0.9), naive/memory T cells (5.7 +/- 1.2 vs 6.6 +/- 1.2; 5.2 +/- 1.0 vs 5.7 +/- 0.9), B cells (7.1 +/- 1.1 vs 7.8 +/- 1.1), and natural killer/natural killer T cells (4.8 +/- 1.0 vs 5.6 +/- 1.3). Chronic graft-versus-host disease (P < .04) and a female donor (P < .04) were associated with a greater difference in telomere length between donor and recipient. Critically short telomeres have been described in degenerative diseases and secondary malignancies. If this hypothesis can be confirmed, identification of recipients at risk for cellular senescence could become part of monitoring long-term survivors after hematopoietic stem cell transplantation.

Probing the mitotic history and developmental stage of hematopoietic cells using single telomere length analysis (STELA)

In most human somatic cells, telomeres shorten as a function of DNA replication. Telomere length is therefore an indirect measure of the replicative history of cells. We measured the telomere lengths at XpYp chromosomes in purified human hematopoietic populations enriched for stem cells (Lin(-)CD34(+)CD38(-)Rho(-)) and successively more mature cells. The average telomere length showed expected length changes, pointing to the utility of this method for classifying novel differentiation markers. Interestingly, the frequency of abruptly shortened telomeres increased in terminally differentiated adult populations, suggesting that damage to telomeric DNA occurs or is not repaired upon hematopoietic differentiation. When Lin(-)CD34(+)CD38(-)Rho(-) cord blood cells were transplanted into immunodeficient mice, the telomeres of the most primitive regenerated human hematopoietic cells lost approximately 3 kb, indicative of more than 30 cell divisions. Further losses in differentiating cells were similar to those observed in pretransplantation cell populations. These results indicate extensive self-renewal divisions of human hematopoietic stem cells are the primary cause of telomere erosion upon transplantation rather than added cell divisions in downstream progenitors.

Flow cytometry and FISH to measure the average length of telomeres (flow FISH)

Telomeres have emerged as crucial cellular elements in aging and various diseases including cancer. To measure the average length of telomere repeats in cells, we describe our protocols that use fluorescent in situ hybridization (FISH) with labeled peptide nucleic acid (PNA) probes specific for telomere repeats in combination with fluorescence measurements by flow cytometry (flow FISH). Flow FISH analysis can be performed using commercially available flow cytometers, and has the unique advantage over other methods for measuring telomere length of providing multi-parameter information on the length of telomere repeats in thousands of individual cells. The accuracy and reproducibility of the measurements is augmented by the automation of most pipetting (aspiration and dispensing) steps, and by including an internal standard (control cells) with a known telomere length in every tube. The basic protocol for the analysis of nucleated blood cells from 22 different individuals takes about 12 h spread over 2-3 days.

Telomeres and telomerase in normal and leukemic hematopoietic cells

Telomere length and telomerase have an important role in normal and malignant hematopoiesis. Telomere erosion can lead to chromosome end fusion and thereby contribute to genomic instability during tumorigenesis. Thus, like complex chromosomal aberrations, telomere length may be a prognostic factor in hematopoietic malignancies. A paper by Sieglova et al. in this issue of Leukemia Research reports on the prognostic impact of telomere shortening in bone marrow (BM) and peripheral blood (PB) specimens of myelodysplastic syndrome (MDS) and MDS converted-AML patients (pts). Their results underline the importance to study telomere biology together with cytogenetics, genomic and proteomic profiling as prognostic factors, in order to improve risk-adapted therapy of MDS and AML pts.

Telomere length dynamics in normal hematopoiesis and in disease states characterized by increased stem cell turnover

Telomeres both reflect and limit the replicative lifespan of normal somatic cells. Immature sub-populations of human CD34+38- hematopoietic stem cell (HSC) can be identified in vitro based on their growth kinetics and telomere length. Fluorescence in situ hybridization and flow cytometry (flow-FISH) has been used to characterize telomere length dynamics as a surrogate marker for HSC turnover in vivo. Investigations in normal steady-state hematopoiesis provided the basis for follow-up studies in model scenarios characterized by increased HSC turnover. Disorders with underlying malignant transformation of HSC (e.g., chronic myeloid leukemia (CML)) can be discriminated from disease states with increased HSC turnover rates secondary to depletion of the stem cell compartment, for example, as in defined bone marrow failure syndromes. In some of these model scenarios, the degree of telomere shortening can be correlated with disease duration, disease stage and severity as well as with response to disease-modifying treatment strategies. Whether increased telomere shortening represents a causal link between HSC turnover, replicative senescence and/or the induction of genetic instability in acquired HSC disorders remains to be shown. However, data from congenital disorders, like dyskeratosis congenita (DKC), suggest that disturbed telomere maintenance may play a role for replicative exhaustion of the HSC pool in vivo.

Telomere length changes after umbilical cord blood transplant

BACKGROUND

The establishment of donor-derived hematopoiesis in the recipients of hematopoietic stem cell (HSC) transplants involves extensive proliferation and differentiation of HSCs. Data from long-term survivors of HSC transplants suggest that these transplanted HSCs may experience a debilitating replicative senescence. A significant posttransplant shortening of peripheral blood mononuclear cell (PBMNC) telomeres has been observed in both marrow transplant and peripheral blood progenitor cell transplant recipients. Similar studies have not been performed for umbilical cord blood (UCB) HSC transplants, which might be expected to exhibit increased posttransplant replicative potential due to their inherently greater telomere length.

STUDY DESIGN AND METHODS

Blood was obtained from donor-recipient pairs of allogeneic PBHSC transplant and UCB HSC transplant, both before transplant and at follow-up treatments (minimum 1 year after transplant) after engraftment. Telomere restriction fragment length (TRFL) analysis was performed on the blood samples. The mean TRFL and posttransplant changes in the mean TRFL were analyzed.

RESULTS

Measurements of telomere lengths in the PBMNCs of transplant patients revealed a significant net decrease in telomere length in all transplant recipients compared with their respective donors. Our results also revealed that the PBMNCs of umbilical cord stem cell transplant patients retain a significantly longer posttransplant telomere length.

CONCLUSION

The significantly longer telomeres observed in the allogeneic UCB HSC transplant recipients compared to the allogeneic PBHSC transplant recipients in our study may be indicative of a replicative advantage inherent in the use of UCB HSC for transplant.

The importance of the telomere and telomerase system in hematological malignancies

Telomeres are specialized chromosomal end structures composed of repeat TTAGGG sequences in humans. They shorten with each cell division and thus serve as the "mitotic clock" of the cell. One of their main functions is the maintenance of chromosomal integrity and their excessive shortening is associated with DNA instability. Telomerase, a unique reverse transcriptase, is inactive in most somatic human cells and is up-regulated in most cancer cells. Recently, the biology of the telomere/telomerase system has attracted much attention because of its possible role in carcinogenesis and aging. In this article we review the biology of this system and its relevance to normal and malignant hematopoietic cells. The biological, diagnostic and prognostic value of telomere/telomerase biology is discussed, as well as its potential future applications in cancer therapeutics.

Senescence of hematopoietic stem cells and bone marrow failure

Functional failure in hematopoietic stem cells (HSCs) may bring fatal consequences because HSCs are the ultimate source of mature blood cells, which need continuous replenishment. One potential cause of HSC dysfunction is senescence, in which HSCs and progenitor cells enter a state of proliferative arrest. HSC senescence is genetically regulated and one particular regulator is the telomerase gene. Mutations in the telomerase gene complex have been found in patients with bone marrow failure syndromes. During a normal lifetime, HSC clones function over the long term and may not show any functional loss under normal circumstances. However, pathologic environments may limit HSC proliferation, accelerate HSC turnover, and shorten the functional life of HSCs, leading to HSC clonal exhaustion and senescence.

Nonmyeloablative conditioning does not prevent telomere shortening after allogeneic stem cell transplantation

BACKGROUND

Stem cell transplantation (SCT) may be associated with premature aging of the hematopoietic stem cells. Telomere length reflects the proliferative history of a cell. In most studies published so far on telomere dynamics after myeloablative allogeneic SCT, recipients had shorter telomeres than their respective donors, thus reflecting "accelerated aging" of hematopoietic cells. We evaluated telomere dynamics in patients who underwent transplantation with nonmyeloablative protocols, assuming that the decreased intensity of chemotherapy might prevent telomere attrition.

METHODS

Telomere length was measured using FISH-FACS method. Telomeres of recipients were compared to their respective donors. Twenty-three consecutive patients after nonmyeloablative SCT were evaluated. A control group consisted of 10 donor-recipient pairs after conventional myeloablative transplantation.

RESULTS

There was significant telomere shortening in both recipients of nonmyeloablative and myeloablative conditioning (0.487+/-0.65 kb, P=0.003; 0.361+/-0.50 kb, P=0.047 respectively). The extent of telomere shortening in the two groups was not different (P=0.64). There was no correlation between the degree of shortening and parameters such as time interval from transplant, age of donor or recipient, and the number of infused cells.

CONCLUSIONS

This is the first study on telomere dynamics after nonmyeloablative conditioning SCT. The study demonstrates significant shortening of telomeres in recipients in spite of decreased intensity conditioning. Results of this study suggest that the main mechanism following transplantation is the proliferative stress imposed upon the stem cells and not direct damage by cytotoxic drugs. The different kinetics of restoration of hematopoiesis and the probable ongoing process of graft-versus-leukemia in the bone marrow do not prevent the attrition of telomeric ends of chromosomes.

Role of Telomerase in Hematopoietic Stem Cells

Studies in hematopoietic stem cell (HSC) biology are often focused on "self-renewal" and differentiation. Implicit in the word self-renewal is that the two daughter cells generated by a self-renewal division are identical to the parental cell. Strictly speaking, this is not possible because DNA is continuously damaged and repaired by DNA-repair mechanisms that are not 100% efficient. It is important to note that the efficiency of DNA repair varies greatly among different stem cell types. For example, embryonic stem cells are quite resistant to DNA damage and maintain the length of telomere repeats on serial passage, whereas HSCs are quite sensitive to DNA damage and less able to maintain telomere length. Most likely, differences between stem cell types in DNA repair and telomere maintenance pathways coevolved with cell mass, turnover, reproductive strategy, and life span. This idea has given rise to the notion that many aspects of normal aging could primarily reflect limitations in DNA repair and telomere-maintenance pathways in the (stem) cells of the soma. In humans, levels of telomerase in HSCs are under extremely tight control, as is illustrated by the marrow failure in patients with (mild) telomerase deficiencies. Here, the role of telomerase in human HSC biology is reviewed, and it is proposed that telomerase has an important role in the repair of G-rich DNA.

Influence of telomere length on short-term recovery after allogeneic stem cell transplantation

OBJECTIVE

Telomeres shorten in somatic cells during aging and states of increased turnover, including hematopoietic stem cell transplantation. Fast hematopoietic recovery is critical for the patients' course after hematopoietic stem cell transplantation. It is unknown whether telomere length in hematopoietic stem cells (HSCs) predicts short-term hematopoietic recovery.

METHODS

We quantified telomere length by flow fluorescence in situ hybridization analysis in HSCs and granulocytes of healthy stem cell donors and monitored time to peripheral blood cell recovery in transplanted hosts. Furthermore, we measured in vitro repopulation potency of HSCs by assaying for colony-forming units granulocyte-macrophage (CFU-GM).

RESULTS

Telomere length in HSC shortens continuously in vivo and is comparable to telomere length in granulocytes from the same individual. Numbers of in vitro formed CFU-GM per HSC show an inverse relationship to age and telomere length. However, telomere length in HSCs was not correlated with short-term recovery after HSC transplantation.

CONCLUSION

These findings suggest that healthy stem cell donors have sufficient telomere length reserve to repopulate a myeloablatively treated host, despite continuous aging of HSCs in vivo and decreased repopulation ability of HSCs from older donors in vitro.

Graft failure following allogeneic blood and marrow transplant: evidence-based nursing case study review

Despite the advances made since the earliest days of transplant therapy, graft failure following allogeneic blood and marrow transplant is still a life-threatening complication. This article reviews the science of graft failure and uses a case study presentation to address how an oncology nursing staff was motivated by a patient's experience of graft failure. An evidence-based literature review was undertaken to answer three relevant clinical questions: (a) What factors contribute to graft failure in patients receiving allogeneic hematopoietic stem cell transplants? (b) What interventions are appropriate for these patients? and (c) How can this information assist nursing staff in providing improved care for these patients? An example of the table of evidence is provided.

Marked telomere shortening in mobilized peripheral blood progenitor cells (PBPC) following two tightly spaced high-dose chemotherapy courses with G-CSF

The purpose of the study was to compare telomere length (TL) in peripheral blood progenitor cells (PBPC) collected after two tightly spaced high-dose (hd) chemotherapy courses. We assessed 37 previously untreated lymphoma patients undergoing a hd-chemotherapy program with autografting. They sequentially received hd-cyclophosphamide (CY) and hd-Ara-C, both followed by PBPC harvesting. Both post-CY and post-Ara-C harvests were assessed for TL by Southern blot analysis. In 12 patients, the assay was also performed on purified CD34+ cells. All patients displayed high PBPC mobilization following both hd-CY and hd-Ara-C. In all but one patient, TL was shorter in PBPC collected after Ara-C compared to CY: 7226bp (range: 4135-9852) vs 8282 bp (range 4895-14860) (P < 0.0001). This result was confirmed on CD34+ cells. Platelet recovery in patients receiving post-Ara-C PBPC was significantly slower compared to those receiving post-CY PBPC. In conclusion, (i) administration of tightly spaced hd-chemotherapy courses induces marked telomere shortening on harvested PBPC; (ii) engraftment kinetics seem slower, with delayed platelet recovery, in patients autografted with PBPC suffering marked TL erosion; (iii) long-term follow-up is required to verify whether PBPC with shortened telomeres display defective engraftment stability and/or risk of secondary leukemia; (iv) TL evaluation is advisable whenever new mobilization procedures are developed.

Senescence and functional failure in hematopoietic stem cells

Maintaining normal function of hematopoietic stem cells (HSCs) is critical to blood coagulation, oxygen transportation, and host defense against infection. A potential cause of HSC dysfunction is senescence, in which HSCs and progenitor cells suffer from proliferative arrest. Studies on humans and various animal models have indicated that HSCs can become senescent, showing a significant decline in functional ability with increasing age. There are genetic elements mapped to specific chromosomal sites that regulate HSC senescence. These elements may differ among species, strains, and even individuals. HSC senescence and related pathological effects may not be obvious during normal lifetime under most circumstances since individual primitive HSC clones can function long-term to produce progeny that sustain life-long mature blood cell production. Shortening of telomeres at the chromosomal ends could contribute to HSC senescence, especially when HSCs are stressed under certain pathological conditions. Future studies should define the molecular elements that are important in the regulation of HSC senescence.

Telomere length predicts neutrophil recovery in the absence of G-CSF after autologous peripheral blood stem cell transplantation

SUMMARY

Haemopoietic regeneration after autologous peripheral blood progenitor cell (PBPC) transplantation can be delayed in some patients despite adequate infusion of CD34(+) cells. This suggests variability in the proliferation potential of the implanted cells, a capacity that may be predicted by their telomere length. To test this theory, telomere length was measured on stored apheresis samples from 36 patients aged 46.6+/-11.1 years, who had undergone successful autologous PBPC transplantation with a median of 5.6 x 10(6)/kg (1.3 x 10(6)-36.1 x 10(6)/kg) CD34(+) cells. The mean PBPC telomere length for the cohort was 9.4+/-2.3 kbp. For patients who did not receive G-CSF post transplantation (n=7), days to absolute neutrophil recovery (ANC), >/=0.1, 0.5 and 1.0 x 10(9) cells/l, were significantly inversely correlated with telomere length of the infused PBPC (r=-0.88, -0.81, -0.77, respectively; P<0.05,). However, no correlation was found for patients who received G-CSF from day 1 post transplantation (n=20). These data suggest that for transplantation with sufficient CD34(+) cells, neutrophil recovery is less efficient in patients receiving infusions of cells with short telomeres, but this deficiency can be corrected with adequate post transplantation administration of G-CSF. Bone Marrow Transplantation (2004) 34, 439-445. doi:10.1038/sj.bmt.1704607 Published online 19 July 2004

Telomere length in subpopulations of human hematopoietic cells

In order to test the hypothesis that the telomere length in human hematopoietic cells correlates with their proliferative potential, we analyzed the telomere length in highly purified subpopulations of bone marrow cells. Cells were sorted on the basis of CD34 and CD38 cell surface markers, and two samples were additionally sorted on the basis of Hoechst 33342 dye efflux allowing isolation of side population (SP) cells. The telomere length in limiting numbers of sorted cells was analyzed using a newly developed fluorescence in situ hybridization (flow-FISH) method in which hybridization of telomere probe in cells of interest is measured relative to control cells in the same tube. In all seven bone marrow samples analyzed, the telomere length in CD34(+)CD38(-) cells was longer than in CD34(+)CD38(+) cells from the same donor (p < 0.02). Results with sorted SP cells were less clear: the telomere fluorescence in these cells was very heterogeneous, and a reproducible difference in telomere length relative to CD34(+)CD38(-) cells could not be observed. We conclude that the telomere length in subpopulations of hematopoietic cells does appear to be correlated with the known proliferative potential of such cells and that further characterization of cells on the basis of telomere length is warranted for enrichment of very rare precursors of hematopoietic and other tissues.