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Blood, 1 August 2001, Vol. 98, No. 3, pp. 888-890
CORRESPONDENCE
To the editor:
Does telomere shortening count?
Telomeres are specialized chromosomal end structures of
eukaryotic chromosomes that consist of 2- to 15-kb repeat
sequences of TTAGGG in humans. Since telomeres shorten with subsequent
cell divisions and decline with age by 50-100 bp per year, they
reflect the proliferative history of cells and serve as a mitotic clock on our way to senescence.1-3 The extent of telomere loss
has been subject to extensive in vitro and in vivo analyses, since short telomeres limit the remaining replicative capacity of cells and
may be responsible for subsequent changes and potential deteriorating effects, such as acquisition of secondary myelodysplastic syndrome (MDS) or acute myelogenous leukemia (AML) after life-saving
therapies such as stem cell transplantation (SCT).1-4 We and others have shown that hematopoietic cells lose telomeres after
extensive proliferative stress despite activation of telomerase, which
is insufficient to prevent telomere shortening upon massive
cell expansion and proliferation in vitro.1
Moreover, we have demonstrated considerable telomere loss after
repetitive chemotherapy cycles or high-dose chemotherapy followed by
autologous or allogeneic SCT2,3,5,6; this loss was higher
after extensive chemotherapy cycles (defined as 6 or more cycles) than after a lower number of cycles (fewer than 6),5
correlated with chemotherapy-dose intensity with a 3- to 4-fold-higher
telomere loss after high-dose chemotherapy (autologous SCT, n = 17)
than after standard-dose (n = 13) chemotherapy, and was comparable after autologous (n=17) and allogeneic (n=11) SCT with an approximately 1- to 2-kb telomere restriction fragment (TRF) loss in
mononuclear cells (MNCs) and granulocytes (GRs).6 The finding that telomeres shorten after chemotherapy and autologous
SCT is not very surprising since extensive cell renewal and
reconstitution occur after nonablative and myeloablative therapies. A
telomere loss of 1-2 kb after autologous transplantations will account
for 20-40 years of premature aging in the autologous recipient, since
patients receive their own stem cell support instead of donor stem cell
support. This leaves a 60-year-old with 6 kb prior to SCT and
with 4-5 kb after SCT, thereby closely reaching the Hayflick limit
(disregarding the effect of prior chemotherapy, subsequent infections,
or other factors). This is very likely to effect the observed
subsequent changes, such as altered colony-forming unit (CFU) or
long-term colony-initiating cell (LTC-IC) efficiency, as well as the
potential development of secondary MDS/AML in some patients. With
allogeneic transplantations, the situation is somewhat different, since
the patient's malignant hematopoietic system is replaced by the
healthy donor's. Telomere loss has been observed, nevertheless, due to the extensive cell regeneration after reinfusion of approximately 1% of donor stem cells necessary to fully
reconstitute the patient's blood count. With an
observed telomere loss of 1 kb and a telomere loss of 50 kb per
population doubling (PD), hematopoietic cells have to undergo 20 PDs
after allogeneic transplantations.6-8 In a recent paper, Rufer et al measured telomeres of peripheral blood
monocytes and T lymphocytes in 4 patients and their respective donors
by flow cytometry after T-cell-depleted HLA-identical allogeneic
SCT.8 In the first year after transplantation,
they found an accelerated telomere loss that was later comparable to the loss in healthy donors and controls. Although this work fits with
our1-3,5,6 and other7-10 reports
demonstrating a telomere loss immediately after strong proliferative
stress such as SCT, whereas later this slows down significantly in most
patients or may in rare patients even show a telomere
elongation2,3,5,6 (either due to telomere regenerative
mechanisms such as telomerase activation or repopulation of genuine
pluripotent stem cells that have not undergone extensive telomere
shortening11), some critical points of Rufer et
al's paper may need to be pointed out: First, the number of patients and donors tested (although at various
time points and with long-term follow-up) are fairly low (n = 4).
Both in this study8 and a previous study by this group,7 telomeres were studied under standard
transplantation conditions, here after cyclophosphamide and total body
irradiation (TBI) conditioning, graft-versus-host disease (GVHD)
prophylaxis with cyclosporine A alone, and no acute or chronic GVHD
development in any patient. Before concluding that telomere
shortening is relatively insignificant after SCT, as Rufer et al have
done,8 it would be interesting to study telomere loss
within a larger patient cohort, focusing especially on patients and
donors with respect to their age, possible differences between
peripheral blood stem cell transplantation (PBSCT) and bone marrow
transplantation (BMT), possible differences between non- and
HLA-identical transplants, possible differences between standard and
reduced conditioning regimens, the effects of acute and chronic GVHD,
and the stem cell dose. Second, mean or median values of telomere losses in patients and donors
would have been more important than individual numbers alone,
especially since the telomere loss in Figures 1 and 2 seems fairly
unintelligible to nonexperts. Nevertheless, from their figures it is
conceivable that the telomere loss during the first year after SCT is
substantial and decreases considerably thereafter, which is consistent
with our analyses showing a loss of 0.9 kb in MNCs and 0.5 kb in GRs
immediately (1-2 months) after autologous SCT and of 0.4 kb in
both after allogeneic SCT (after 6 months, 0.6 kb in MNCs and 0.5 kb in
GRs).6 This might, however, increase in
HLA-mismatched SCT, with acute or chronic GVHD, after cytomegalovirus (CMV) or Epstein-Barr virus (EBV) infections as recently
shown,12 low cell numbers reinfused,9,10 and
repetitive conditioning such as tandem transplantations or extensive
previous chemotherapy regimens.2,3,5,6 Finally, their conclusion that "the transplantation-induced loss of
telomeres represents only 20% of the `telomere reserve' ... should ... have little consequence for the function of different hematological lineages"8(p577) stands in
contrast to several observations of potential detrimental effects after
SCT, such as the significant decrease of both CFU and LTC-IC
capacity13,14 and development of secondary MDS/AML that in
a series of 552 autologous transplant recipients showed an estimated
incidence at 10 years of 19.8% and continues to be a major concern for
patients who survive long-term.4 These adverse events seem
less worrisome after allogeneic transplantations than after autologous
transplantations since a new hematopoietic system is established.
Nevertheless, this must undergo extensive proliferation and is further
challenged with immunosuppressive therapy, acquisition of infections,
GVHD, and so forth. These critical points aside, the essential observation of a TRF
shortening of the stem cell compartment is worth reporting. This should
alert us, however, that the telomere loss will have a potential adverse
effect, especially in older individuals, with older donors and
transplantation complications. Therefore, we recommend a more
critical and cautious interpretation of the transplantation-induced telomere losses than underestimating their effects.
Monika Engelhardt and Jürgen Finke
Correspondence: Monika Engelhardt, University of Freiburg
Medical Center, Hematology/Oncology Department, Hugstetterstr 55, 79106 Freiburg, Germany
References
1.
Engelhardt M, Kumar R, Albanell J, Pettengell R, Han W, Moore MAS.
Telomerase regulation, cell cycle, and telomere stability in primitive hematopoietic cells.
Blood.
1997;90:182-193[Abstract/Free Full Text].
2.
Engelhardt M, Ozkaynak MF, Drullinsky P, et al.
Telomerase activity and telomere length in pediatric patients with malignancies undergoing chemotherapy.
Leukemia.
1998;12:13-24[CrossRef][Medline]
[Order article via Infotrieve].
3.
Engelhardt M, Mackenzie K, Drullinsky P, Silver RT, Moore MAS.
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].
4.
Friedberg JW, Neuberg D, Stone RM, et al.
Outcome of patients with myelodysplastic syndrome after autologous bone marrow transplantation for non-Hodgkin`s lymphoma.
J Clin Oncol.
1999;17:3128-3135[Abstract/Free Full Text].
5.
Engelhardt M, Lange W, Henschler R, Moore MAS, Mertelsmann R.
Effect of prolonged myelosuppressive chemotherapy treatment on apheresis results and telomere length [abstract].
Blood.
1997;90(suppl):211a.
6.
Engelhardt M, Guo Y, Finke J.
Telomere shortening in hematopoietic cells of patients undergoing allogeneic stem cell transplantation [abstract].
Blood.
2000;96(suppl):762a.
7.
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].
8.
Rufer N, Brümmendorf T, 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[Abstract/Free Full Text].
9.
Notaro R, Cimmino A, Tabarini D, Rotoli B, Luzzatto L.
In vivo telomere dynamics of human hematopoietic stem cells.
Proc Natl Acad Sci U S A.
1997;94:13782-13785[Abstract/Free Full Text].
10.
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]
[Order article via Infotrieve].
11.
Krafft T, Aswald J, Dankbar B, et al.
Durable increase of telomerase activity in CD34+ cells after autologous PBSCT [abstract].
Blood.
1999;94(suppl):132a.
12.
Plunkett FJ, Soares MVD, Annels N, et al.
The flow cytometric analysis of telomere length in antigen-specific CD8+ T cells during acute Epstein-Barr virus infection.
Blood.
2001;97:700-707[Abstract/Free Full Text].
13.
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]
[Order article via Infotrieve].
14.
Podesta M, Piaggio G, Frassoni F, et al.
The assessment of the hematopoietic reservoir after immunosuppressive therapy or bone marrow transplantation in severe aplastic anemia.
Blood.
1998;91:1959-1965[Abstract/Free Full Text].
Response:
Consequences of stem cell transplantation-induced
telomere shortening
Engelhardt et al ask the question whether telomere shortening
counts or, more precisely, whether the stem cell transplantation (SCT)-induced telomere shortening in hematopoietic cells could be
serious enough to be responsible for some of the adverse alterations in
the hematologic lineages. This is an important question1 that remains unresolved despite Engelhardt et al's claims that the
conclusions from our study stand in sharp contrast to several observations. What is the argument about? We found that following allogeneic
transplantation, telomere shortening is limited to the first year
after transplantation.2 We believe it to be unlikely that the observed telomere shortening per se will result in adverse consequences for the sustenance of long-term hematopoiesis in these
patients. The reason for this is simple. If having 1- to 2-kb-shorter
telomeres would have considerable consequences for the remaining
replicative potential, one would expect to see a much higher
heterogeneity in the hematopoiesis of healthy individuals, because
already at birth the telomere length in different individuals varies between 4.5 and 14 kb.3 For this reason, we
believe that Engelhardt et al's statement that the
transplantation-induced telomere loss "leaves a 60-year-old with
6 kb prior to SCT with 4-5 kb after SCT, thereby closely reaching the
Hayflick limit" is misleading. Indeed, previous calculations
translating telomere shortening directly into "years of
aging" are subject to the same criticism. In general, we believe that
one should be extremely cautious explaining posttransplantation
complications by a concomitant telomere loss. This might be illustrated
best by the example that Engelhardt et al use to support their view. It
could indeed be that, in certain patients, replicative exhaustion
contributes to the development of secondary myelodysplastic
syndrome/acute myelogenous leukemia (MDS/AML).1 But in
such patients the number and quality of stem cells that are available
for transplantation, engraftment, and sustained hematopoiesis may
all have been severely compromised by the underlying disease and/or the
prior treatment. Most of Engelhardt et al's other comments refer to telomere
shortening after allogeneic SCT, where the quality of the graft is not
influenced by the clinical status of the patient. Here, their main
criticism is that our conclusion "that telomere shortening is
relatively insignificant after SCT" is not legitimate because our
data come from patients under standard transplantation conditions without complications such as graft-versus-host disease (GVHD) or
cytomegalovirus (CMV) infection that could possibly induce further
telomere shortening. This critique seems to lack the necessary accuracy. In our study,2 we showed that after 6 months to
1 year of an accelerated rate of telomere loss, patients'
telomeres shortened at a rate comparable to that of their donor. Based
on this observation, we concluded that the high rate of telomere loss
during the first year is entirely responsible for the difference of 1-2 kb observed through the entire transplantation period. Obviously, we
did not say that the loss itself was insignificant. We only concluded
that, because the accelerated loss is limited to this initial period,
the difference in telomere length of 1-2 kb between the patient and his
donor would remain stable and, therefore, would most likely be without
severe consequences for the function of the patient's hematologic
lineages. Whether this difference would be more considerable in
patients with GVHD or viral infections remains an open question. But
even if it were, such correlations could be indirect because these
complications may induce a massive expansion of lymphocytes with a
subsequent impact on the average size of the telomeres in the
patient's leukocytes. In conclusion, we think it unlikely that the telomere shortening
observed after allogeneic SCT is substantial enough to cause replicative exhaustion that would lead to subsequent complications. Importantly, correlations between short telomeres and a less beneficial outcome may be found even when telomere shortening and replicative exhaustion are not the cause of such complications. In recipients of
autografts, this can be the case because patients with a poor prognosis
often receive transplants of lower numbers of stem cells. In allograft
recipients, correlations between short telomeres and a less beneficial
outcome may be found in patients who develop GVHD or severe viral
infections. In both situations a substantial expansion of T
lymphocytes may occur that could be accompanied by significant telomere shortening.
Nathalie Rufer, Tim H. Brümmendorf, Bernard Chapuis, Claudine Helg, Peter M. Lansdorp, and Eddy Roosnek
Correspondence: Eddy Roosnek, Division of Immunology and
Allergology, University of Geneva, Switzerland
References
1.
Lansdorp PM.
Telomere length and proliferation potential of hematopoietic stem cells.
J Cell Sci.
1995;108:1-6[Abstract].
2.
Rufer N, Brümmendorf TH, Chapuis B, et al.
Accelerated telomere shortening is limited to the first year following stem cell transplantation.
Blood.
2001;97:575-577.
3.
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].
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