Telomerase as a clinical target: current strategies and potential applications

Chromosome ends are capped by telomeres, protective DNA-protein complexes that distinguish natural ends from random DNA breaks. Telomeres erode with each successive cell division, and such divisions cease once telomeres become critically short. This proliferation limit is important as a tumor suppressive mechanism, but also contributes to the degenerative conditions associated with cellular aging. In cell types that require continuous renewal, transient expression of telomerase delays proliferation arrest by the de novo synthesis of telomere repeats. Data from our work and others' has shown that deficient telomerase activity has a negative impact on normal human physiology. In the bone marrow failure syndrome dyskeratosis congenita, telomerase enzyme deficiency leads to the premature shortening of telomeres. Premature telomere shortening most grievously affects tissues that have a rapid turnover, such as the hematopoietic and epithelial compartments. In the most severe cases, compromised renewal of hematopoietic stem cells leads to bone marrow failure and premature death. Telomerase activation/replacement shows potential as a therapy for telomere maintenance deficiency syndromes, and in tissue engineering for the degenerative conditions that are associated with normal aging. Conversely, clinical researchers are developing telomerase inhibition therapies to treat tumors, which overcome the short-telomere barrier to unrestricted proliferation by over-expressing telomerase.     

Elevated telomerase activity and minimal telomere loss in cord blood long-term cultures with extensive stem cell replication

Telomerase activity, telomere length, stem/progenitor cell production, and function of CD34+ cells from cord blood (CB), bone marrow, and mobilized peripheral blood were evaluated in long-term cultures. CB cells were cultured either on OP-9 stromal cells transduced with an adenovector expressing thrombopoietin (TPO) or stimulated by a cytokine cocktail in the absence of stroma, with, in one method, CD34+ cells reisolated at monthly intervals for passage. Continuous expansion of stem cells as measured by in vitro cobblestone area and secondary colony-forming assays was noted for 18 to 20 weeks and by severe combined immunodeficiency (SCID)-repopulating cells (SRCs), capable of repopulating and serially passage in nonobese diabetic/SCID mice, for 16 weeks. Despite this extensive proliferation, telomere length initially increased and only at late stages of culture was evidence of telomere shortening noted. This telomere stabilization correlated with maintenance of high levels of telomerase activity in the CD34+ cell population for prolonged periods of culture. Cytokine-stimulated cultures of adult CD34+ cells showed CD34+ and SRC expansion (6-fold) for only 3 to 4 weeks with telomere shortening and low levels of telomerase. There is clearly a clinical value for a system that provides extensive stem cell expansion without concomitant telomere erosion.     

Telomeres and telomerase: basic science implications for aging

Life expectancy in the United States and other developed nations has increased remarkably over the past century, and continues to increase. However, lifespan has remained relatively unchanged over this period. As life expectancy approaches maximum human lifespan, further increase in life expectancy would only be possible if lifespan could also be increased. Although little is known about the aging process, increasing lifespan and delaying aging are the research challenges of the new century, and have caused intense debate and research activities among biogerontologists. Many theories have been proposed to explain the aging process. However, damage to deoxyribonucleic acid (DNA) is the centerpiece of most of these. Recently telomere shortening has been described to be associated with DNA damage. Located at the ends of eukaryotic chromosomes and synthesized by telomerase, telomeres maintain the length of chromosomes. The loss of telomeres can lead to DNA damage. The association between cellular senescence and telomere shortening in vitro is well established. In the laboratory, telomerase-negative differentiated somatic cells maintain a youthful state, instead of aging, when transfected with vectors encoding telomerase. Many human cancer cells demonstrate high telomerase activity. Evidence is also accumulating that telomere shortening is associated with cellular senescence in vivo. What causes changes in expression of telomerase in different cell types and premature aging syndromes? Does the key to "youthfulness" lie in our ability to control the expression of telomerase? We have reviewed the contemporary literature to find answers to these questions and explore the association between aging, telomeres, and telomerase.     

Telomerase activity and its correlation with the proliferative potential of bone marrow in aplastic anemia in children

OBJECTIVE: To explore telomerase activity and its association with the proliferative potential of hematopoietic stem cells in bone marrow (BM) in patients with aplastic anemia (AA). METHODS: Telomerase activity of mononuclear cells separated from BM was determined with the TRAPeze kit. BM specimens from 22 cases with AA and 7 normal controls were included. SPSS10.0 was applied to analyze data derived from telomerase activity and colony-forming unit-granulocyte and monocyte. RESULTS: The median telomerase activity level of BM in AA was 2-fold higher than that in normal controls. There was an inverse correlation in AA between telomerase activity and colony-forming unit (r = 0.78, p < 0.05). BM of chronic AA expressed higher telomerase than that of acute AA. CONCLUSIONS: Telomerase activity in AA was increased and may be the result of the negative feedback of hematopoietic potential. Telomerase activity varied between the subtypes of AA. Telomerase activity was conversely correlated with the proliferative potential of BM in AA.     

Functional characterization of telomerase RNA variants found in patients with hematologic disorders

Human telomerase uses a specific cellular RNA, called hTERC, as the template to synthesize telomere repeats at chromosome ends. Approximately 10% to 15% of patients with aplastic anemia or other bone marrow failure syndromes are carriers of hTERC sequence variants whose functional significance, in most cases, is unknown. We screened 10 reported and 2 newly discovered hTERC variants from such patients and found that 10 of these negatively affected telomerase enzymatic function when they were used to reconstitute telomerase enzymatic function in human cells. Most functional deficits were due to perturbations of hTERC secondary structure and correlated well with the degrees of telomere shortening and reduced telomerase activity observed in peripheral blood lymphocytes of the representative patients. We also found no evidence of dominant-negative activity in any of the mutants. Therefore, loss of telomerase activity and of telomere maintenance resulting from inherited hTERC mutations may limit marrow stem cell renewal and predispose some patients to bone marrow failure.