Telomerase Mutations in Aplastic Anemia
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《新英格兰医药杂志》
Telomeres are DNA–protein complexes at the ends of linear eukaryotic chromosomes. They consist of double-stranded, short, repeated sequences of nucleotides, a 3' single-strand overhang of nucleotides, and telomere-binding proteins. Telomeric DNA typically consists of G-rich tandem repeats 5 to 8 bp long (in humans, TTAGGG). In their location at the termini of chromosomes, telomeres maintain chromosomal stability and integrity and protect chromosomal ends against fusion, the degradation of terminal DNA by exonucleases, and recombination events.1 Telomeric DNA also serves as a binding site for specific DNA-binding proteins that have critical functions in maintaining the function and structural integrity of telomeres.
Because of the inability of DNA polymerase to synthesize DNA in the 3'-to-5' direction, replication of the lagging DNA strand cannot be completed after the removal of the RNA primer. As a result, chromosomes shorten with each cell division, a phenomenon referred to as the "end-replication problem." In 1965, Hayflick described the limited replicative capacity of normal human fibroblasts in culture.2 After a characteristic number of cell divisions, known as the Hayflick limit, cells fail to divide in response to growth stimuli but remain metabolically active. Many types of somatic cells have a clock that limits their capacity to divide. This clock depends on cell division, rather than age, and is therefore called a "mitotic clock."3 Because of the end-replication problem, chromosomal DNA is gradually lost, and after 60 to 80 population doublings, cells become unable to pass from the G1 to the S phase of the cell cycle and hence become senescent. This process ultimately ends in the death of the cell by apoptosis.
Telomerase is a specialized ribonucleoprotein complex that is responsible for the synthesis and maintenance of telomere repeats. It consists of an RNA component (TERC) that serves as a template for the synthesis and addition of DNA repeats to the 3' terminus of the telomere, and a protein component, telomerase reverse transcriptase (TERT). Telomerase expression is low or absent in most human somatic tissues and in adult cells. The catalytic protein subunit of TERT is the key determinant of the enzymatic activity of human telomerase.
The heterogeneous length of telomeres (5 to 15 kb) is determined by factors that include the amount of telomeric DNA a cell inherits from the parent cell, the age and proliferative history of the cell, and the expression of telomerase. In hematopoietic cells, progressive telomere shortening is a consequence of normal replicative aging, during which there is a loss of approximately 50 to 200 bp of telomeric DNA (about 16 nucleotide repeats) per population doubling. Abnormal shortening of telomeres in blood cells could signal defective replication in hematopoietic stem cells. This kind of fault might occur in transplanted allogeneic stem cells, but one year after allogeneic hematopoietic stem-cell transplantation, the telomere lengths in recipient and donor cells are similar.4,5 Follow-up for more than 25 years after stem-cell transplantation has indicated that the difference between donor and recipient telomere lengths stabilizes in the range of 0.5 kb,6 suggesting a normal population of transplanted blood-cell precursors.
Short telomeres have been identified in several aging syndromes. Patients with autosomal dominant dyskeratosis congenita die prematurely, often as a result of a bone marrow failure syndrome. The discovery of 3' deletions in the gene encoding TERC has pointed to telomerase deficiency as a cause of the marrow failure.7 Mutations in the TERC gene have also been found in patients with apparently acquired aplastic anemia who did not have the overt clinical features or physical stigmata of dyskeratosis congenita.8 These observations suggest a critical role for telomerase in maintaining the replicative capacity of hematopoietic cells.
The existence of TERT mutations in aplastic anemia has been hypothesized for some time. In this issue of the Journal, Yamaguchi and colleagues report having found such mutations.9 They provide compelling evidence for the biologic relevance of the mutations and their role in causing marrow failure. It appears that patients with TERC or TERT mutations can present to a physician with late-onset aplastic anemia in a form that is clinically indistinguishable from truly acquired aplastic anemia. In light of this new information, which cases should be investigated for the presence of telomerase mutations? I believe that telomere length should be measured in patients with severe aplastic anemia who do not have a response to immunosuppressive therapy and in patients who have a family history of hematopoietic disorders; in those with short telomeres, telomerase-mutation analysis is warranted. One immediate clinical benefit of identifying telomerase deficiency in patients with aplastic anemia relates to the decision to proceed to allogeneic stem-cell transplantation. Delaying the decision while supporting the patient with transfusions can increase the risk of graft rejection because of sensitization to HLA and other antigens in transfused blood.
Prolonged damage to hematopoietic stem cells in aplastic anemia can induce residual, undamaged stem cells to undergo more than the usual number of cell divisions. As a result, an accelerated rate of telomere shortening in circulating blood cells would be expected. Indeed, an inverse correlation between age-adjusted telomere length and peripheral-blood counts has been observed in aplastic anemia.10 The telomere length in granulocytes from patients with aplastic anemia who have a response to immunosuppressive therapy is similar to that in controls — a finding consistent with the notion that stem cells are intrinsically normal in acquired aplastic anemia. Although their number is reduced by an immune attack, they remain able to proliferate and maintain telomere length. In patients who do not have a response to immunosuppressive therapy, by contrast, the granulocytes have much shorter telomeres.10 These patients may have an intrinsic stem-cell defect that becomes apparent only under conditions of excessive replicative stress. Patients with telomerase deficiency fall into the latter group.
The development of marrow failure in patients with telomerase deficiency may thus be a multistep process that starts with a genetically determined inability of hematopoietic stem cells to respond to an insult to their proliferative machinery. I can only speculate on the secondary factors that precipitate hematopoietic failure; they may include environmental causes and additional genetic events. Since adult family members carrying the mutations can have normal blood counts, their stem cells may have a considerable proliferative reserve capacity that delays the development of marrow failure. The acquisition of secondary genetic events may accelerate the process of replicative aging.
The bone marrow in patients with acquired aplastic anemia may exhibit signs of clonal hematopoiesis and clonal disorders, including paroxysmal nocturnal hemoglobinuria, myelodysplastic syndromes, and acute myeloid leukemia.11 Interestingly, patients with clonal hematopoiesis can have a response to immunosuppressive treatment. Clonal hematopoiesis in aplastic anemia with TERT mutations has not been reported, but it is possible that patients with clonal hematopoiesis and telomerase deficiency are at higher-than-usual risk for the development of hematologic neoplasms. The occurrence of myelodysplastic syndrome and acute myelogenous leukemia in the relatives of some patients with TERT mutations supports this idea.
The identification of TERT mutations in aplastic anemia is an important step toward defining the spectrum of aplastic anemia. This new finding may help in the development of individualized therapy at an early stage of the disease. Although telomerase mutations are found in only a small proportion of patients with aplastic anemia, they may represent the tip of an iceberg. The search for novel mutations in genes involved in the telomere-repair complex will therefore continue.
Source Information
From the Department of Hematology, Leiden University Medical Center, Leiden, the Netherlands.
References
Blackburn EH. Switching and signaling at the telomere. Cell 2001;106:661-673.
Hayflick L. The limited in vitro lifetime of human diploid cell strains. Exp Cell Res 1965;37:614-636.
Harley CB, Goldstein S. Cultured human fibroblasts: distribution of cell generations and a critical limit. J Cell Physiol 1978;97:Suppl 1:509-516.
Roelofs H, de Pauw ES, Zwinderman AH, et al. Homeostasis of telomere length rather than telomere shortening after allogeneic peripheral blood stem cell transplantation. Blood 2003;101:358-362.
Rufer N, Brummendorf TH, Chapuis B, Helg C, Lansdorp PM, Roosnek E. Accelerated telomere shortening in hematological lineages is limited to the first year following stem cell transplantation. Blood 2001;97:575-577.
de Pauw ES, Otto SA, Wijnen JT, et al. Long-term follow-up of recipients of allogeneic bone marrow grafts reveals no progressive telomere shortening and provides no evidence for haematopoietic stem cell exhaustion. Br J Haematol 2002;116:491-496.
Vulliamy T, Marrone A, Goldman F, et al. The RNA component of telomerase is mutated in autosomal dominant dyskeratosis congenita. Nature 2001;413:432-435.
Vulliamy T, Marrone A, Dokal I, Mason PJ. Association between aplastic anaemia and mutations in telomerase RNA. Lancet 2002;359:2168-2170.
Yamaguchi H, Calado RT, Ly H, et al. Mutations in TERT, the gene for telomerase reverse transcriptase, in aplastic anemia. N Engl J Med 2005;352:1413-1424.
Brummendorf TH, Maciejewski JP, Mak J, Young NS, Lansdorp PM. Telomere length in leukocyte subpopulations of patients with aplastic anemia. Blood 2001;97:895-900.
Tichelli A, Gratwohl A, Wursch A, Nissen C, Speck B. Late haematological complications in severe aplastic anaemia. Br J Haematol 1988;69:413-418.(Willem E. Fibbe, M.D., Ph)
Because of the inability of DNA polymerase to synthesize DNA in the 3'-to-5' direction, replication of the lagging DNA strand cannot be completed after the removal of the RNA primer. As a result, chromosomes shorten with each cell division, a phenomenon referred to as the "end-replication problem." In 1965, Hayflick described the limited replicative capacity of normal human fibroblasts in culture.2 After a characteristic number of cell divisions, known as the Hayflick limit, cells fail to divide in response to growth stimuli but remain metabolically active. Many types of somatic cells have a clock that limits their capacity to divide. This clock depends on cell division, rather than age, and is therefore called a "mitotic clock."3 Because of the end-replication problem, chromosomal DNA is gradually lost, and after 60 to 80 population doublings, cells become unable to pass from the G1 to the S phase of the cell cycle and hence become senescent. This process ultimately ends in the death of the cell by apoptosis.
Telomerase is a specialized ribonucleoprotein complex that is responsible for the synthesis and maintenance of telomere repeats. It consists of an RNA component (TERC) that serves as a template for the synthesis and addition of DNA repeats to the 3' terminus of the telomere, and a protein component, telomerase reverse transcriptase (TERT). Telomerase expression is low or absent in most human somatic tissues and in adult cells. The catalytic protein subunit of TERT is the key determinant of the enzymatic activity of human telomerase.
The heterogeneous length of telomeres (5 to 15 kb) is determined by factors that include the amount of telomeric DNA a cell inherits from the parent cell, the age and proliferative history of the cell, and the expression of telomerase. In hematopoietic cells, progressive telomere shortening is a consequence of normal replicative aging, during which there is a loss of approximately 50 to 200 bp of telomeric DNA (about 16 nucleotide repeats) per population doubling. Abnormal shortening of telomeres in blood cells could signal defective replication in hematopoietic stem cells. This kind of fault might occur in transplanted allogeneic stem cells, but one year after allogeneic hematopoietic stem-cell transplantation, the telomere lengths in recipient and donor cells are similar.4,5 Follow-up for more than 25 years after stem-cell transplantation has indicated that the difference between donor and recipient telomere lengths stabilizes in the range of 0.5 kb,6 suggesting a normal population of transplanted blood-cell precursors.
Short telomeres have been identified in several aging syndromes. Patients with autosomal dominant dyskeratosis congenita die prematurely, often as a result of a bone marrow failure syndrome. The discovery of 3' deletions in the gene encoding TERC has pointed to telomerase deficiency as a cause of the marrow failure.7 Mutations in the TERC gene have also been found in patients with apparently acquired aplastic anemia who did not have the overt clinical features or physical stigmata of dyskeratosis congenita.8 These observations suggest a critical role for telomerase in maintaining the replicative capacity of hematopoietic cells.
The existence of TERT mutations in aplastic anemia has been hypothesized for some time. In this issue of the Journal, Yamaguchi and colleagues report having found such mutations.9 They provide compelling evidence for the biologic relevance of the mutations and their role in causing marrow failure. It appears that patients with TERC or TERT mutations can present to a physician with late-onset aplastic anemia in a form that is clinically indistinguishable from truly acquired aplastic anemia. In light of this new information, which cases should be investigated for the presence of telomerase mutations? I believe that telomere length should be measured in patients with severe aplastic anemia who do not have a response to immunosuppressive therapy and in patients who have a family history of hematopoietic disorders; in those with short telomeres, telomerase-mutation analysis is warranted. One immediate clinical benefit of identifying telomerase deficiency in patients with aplastic anemia relates to the decision to proceed to allogeneic stem-cell transplantation. Delaying the decision while supporting the patient with transfusions can increase the risk of graft rejection because of sensitization to HLA and other antigens in transfused blood.
Prolonged damage to hematopoietic stem cells in aplastic anemia can induce residual, undamaged stem cells to undergo more than the usual number of cell divisions. As a result, an accelerated rate of telomere shortening in circulating blood cells would be expected. Indeed, an inverse correlation between age-adjusted telomere length and peripheral-blood counts has been observed in aplastic anemia.10 The telomere length in granulocytes from patients with aplastic anemia who have a response to immunosuppressive therapy is similar to that in controls — a finding consistent with the notion that stem cells are intrinsically normal in acquired aplastic anemia. Although their number is reduced by an immune attack, they remain able to proliferate and maintain telomere length. In patients who do not have a response to immunosuppressive therapy, by contrast, the granulocytes have much shorter telomeres.10 These patients may have an intrinsic stem-cell defect that becomes apparent only under conditions of excessive replicative stress. Patients with telomerase deficiency fall into the latter group.
The development of marrow failure in patients with telomerase deficiency may thus be a multistep process that starts with a genetically determined inability of hematopoietic stem cells to respond to an insult to their proliferative machinery. I can only speculate on the secondary factors that precipitate hematopoietic failure; they may include environmental causes and additional genetic events. Since adult family members carrying the mutations can have normal blood counts, their stem cells may have a considerable proliferative reserve capacity that delays the development of marrow failure. The acquisition of secondary genetic events may accelerate the process of replicative aging.
The bone marrow in patients with acquired aplastic anemia may exhibit signs of clonal hematopoiesis and clonal disorders, including paroxysmal nocturnal hemoglobinuria, myelodysplastic syndromes, and acute myeloid leukemia.11 Interestingly, patients with clonal hematopoiesis can have a response to immunosuppressive treatment. Clonal hematopoiesis in aplastic anemia with TERT mutations has not been reported, but it is possible that patients with clonal hematopoiesis and telomerase deficiency are at higher-than-usual risk for the development of hematologic neoplasms. The occurrence of myelodysplastic syndrome and acute myelogenous leukemia in the relatives of some patients with TERT mutations supports this idea.
The identification of TERT mutations in aplastic anemia is an important step toward defining the spectrum of aplastic anemia. This new finding may help in the development of individualized therapy at an early stage of the disease. Although telomerase mutations are found in only a small proportion of patients with aplastic anemia, they may represent the tip of an iceberg. The search for novel mutations in genes involved in the telomere-repair complex will therefore continue.
Source Information
From the Department of Hematology, Leiden University Medical Center, Leiden, the Netherlands.
References
Blackburn EH. Switching and signaling at the telomere. Cell 2001;106:661-673.
Hayflick L. The limited in vitro lifetime of human diploid cell strains. Exp Cell Res 1965;37:614-636.
Harley CB, Goldstein S. Cultured human fibroblasts: distribution of cell generations and a critical limit. J Cell Physiol 1978;97:Suppl 1:509-516.
Roelofs H, de Pauw ES, Zwinderman AH, et al. Homeostasis of telomere length rather than telomere shortening after allogeneic peripheral blood stem cell transplantation. Blood 2003;101:358-362.
Rufer N, Brummendorf TH, Chapuis B, Helg C, Lansdorp PM, Roosnek E. Accelerated telomere shortening in hematological lineages is limited to the first year following stem cell transplantation. Blood 2001;97:575-577.
de Pauw ES, Otto SA, Wijnen JT, et al. Long-term follow-up of recipients of allogeneic bone marrow grafts reveals no progressive telomere shortening and provides no evidence for haematopoietic stem cell exhaustion. Br J Haematol 2002;116:491-496.
Vulliamy T, Marrone A, Goldman F, et al. The RNA component of telomerase is mutated in autosomal dominant dyskeratosis congenita. Nature 2001;413:432-435.
Vulliamy T, Marrone A, Dokal I, Mason PJ. Association between aplastic anaemia and mutations in telomerase RNA. Lancet 2002;359:2168-2170.
Yamaguchi H, Calado RT, Ly H, et al. Mutations in TERT, the gene for telomerase reverse transcriptase, in aplastic anemia. N Engl J Med 2005;352:1413-1424.
Brummendorf TH, Maciejewski JP, Mak J, Young NS, Lansdorp PM. Telomere length in leukocyte subpopulations of patients with aplastic anemia. Blood 2001;97:895-900.
Tichelli A, Gratwohl A, Wursch A, Nissen C, Speck B. Late haematological complications in severe aplastic anaemia. Br J Haematol 1988;69:413-418.(Willem E. Fibbe, M.D., Ph)