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Abstract
The myelodysplastic syndromes (MDS) constitute a challenge for the biologist as well as for the treating physician. In Section I, Dr. Willman reviews the current classifications and disease mechanisms involved in this heterogeneous clonal hematopoietic stem cell disorder. A stepwise genetic progression model is proposed in which inherited or acquired genetic lesions promote the acquisition of "secondary" genetic events mainly characterized by gains and losses of specific chromosome regions. The genetic risk to develop MDS is likely multifactorial and dependent on various constellations of risk-producing and -protecting alleles. In Section II Dr. Barrett with Dr. Saunthararajah addresses the immunologic factors that may act as important secondary events in the development of severe pancytopenia. T cells from patients with MDS may suppress autologous erythroid and granulocytic growth in vitro, and T cell suppression by antithymocyte globulin or cyclosporine may significantly improve cytopenia, especially in refractory anemia. Recent studies have also demonstrated an increased vessel density in MDS bone marrow, and a phase II trial of thalidomide showed responses in a subgroup of MDS patients especially in those with low blast counts. In Section III Dr. Hellström-Lindberg presents results of allogeneic and autologous stem cell transplantation (SCT), intensive and low-dose chemotherapy. The results of allogeneic SCT in MDS are slowly improving but are still poor for patients with unfavorable cytogenetics and/or a high score according to the International Prognostic Scoring System. A recently published study of patients between 55-65 years old showed a disease-free survival (DFS) at 3 years of 39%. Consolidation treatment with autologous SCT after intensive chemotherapy may result in long-term DFS in a proportion of patients with high-risk MDS. Low-dose treatment with 5-azacytidine has been shown to significantly prolong the time to leukemic transformation or death in patients with high-risk MSA. Erythropoietin and granulocyte colony-stimulating factor may synergistically improve hemoglobin levels, particularly in sideroblastic anemia. Recent therapeutic advances have made it clear that new biological information may lead to new treatment modalities and, in combination with statistically developed predictive models, help select patients for different therapeutic options.
I. Biologic and Genetic Features of the Myelodysplastic Syndromes
Cheryl L. Willman, M.D.*
Recent scientific advances have provided new insights into the etiology and pathogenesis of the myelodysplastic syndromes (MDS). Despite heterogeneous morphologic, genetic, biologic, and clinical features, all forms of MDS are clonal hematopoietic stem cell disorders characterized by ineffective hematopoiesis and peripheral cytopenias. Although a substantial proportion of MDS cases evolve to acute myeloid leukemia (AML), the natural history of these syndromes ranges from more indolent forms of disease spanning years to those with a rapid evolution to AML. Thus, MDS is best considered a preleukemic disorder in which the neoplastic clone that has been established may or may not fully progress to acute leukemia. Although the relationship between MDS and de novo AML has been controversial and current disease classification systems (Table 1) are considered unsatisfactory, most hematologists now consider MDS and AML as part of the same continuous disease spectrum rather than as distinct disorders. This review will briefly high-light current controversies in the classification of MDS and AML, the cytogenetic and molecular genetic features of MDS, the biologic features that characterize MDS including abnormal apoptosis and an altered marrow microenvironment, and new and highly interesting insights into the complex genetic predisposition to MDS. Excellent, well-referenced reviews are also available.1,2
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MDS and AML Disease Classification Systems: Unresolved
Controversies
The French-American-British (FAB) Classification, proposed in 1977,
provided hematologists with the first consistent framework for morphologic
classification of MDS (Table
1), the myeloproliferative disorders, and the acute
leukemias.3,4
However, the separation of MDS as a distinct disorder from AML in the FAB
classification scheme has been perceived by many to have scientifically
impeded our understanding of the full spectrum of leukemic
progression.1
Indeed, the initial failure to recognize and classify MDS as a
"neoplastic" pre-leukemic disorder and part of the same disease
spectrum as AML resulted in the exclusion of MDS cases from virtually all US
cancer registries and the NCI-sponsored Surveillance, Epidemiology, and End
Results (SEER) program (www.seer.cancer.gov). This has greatly impeded studies
of the true incidence, natural history, and epidemiology of MDS in the US.
Importantly, however, European epidemiologic studies suggest that the
incidence of MDS is at least as high as that of AML, particularly AML cases
that arise in older
individuals.5 In the
US, the age-specific incidence rate for AML in males aged 50 years at
diagnosis is 3.5 per 100,000, increasing dramatically to 15 at age 70 and 35
at age 90.6 (See
also www.seer.cancer.gov.) With the exponential increase in the incidence of
AML with age and the aging of our populations, the median age at diagnosis of
AML in the US is currently 63 years. Thus, the majority of AML cases, like
MDS, occur in older individuals. Further linking MDS and AML, several studies
have noted that the biologic, morphologic, and genetic features of AML arising
in older individuals are similar to 1) primary MDS; 2) AML arising secondary
to antecedent MDS; 3) AML arising secondary to prior therapy, particularly
alkylating agent exposure; and 4) AML cases that arise from documented
environmental or occupational exposures to agents such as benzene, petroleum,
organic solvents, and arsenical
pesticides.7,8,9,10,11,12,13
In the FAB Classification (Table
1), the two primary distinguishing features between the various
MDS subtypes, chronic myelomonocytic leukemia (CMML) and AML, are blast cell
percentage and the presence of dysplastic features. CMML is now considered a
myeloproliferative/leukemic-like disorder and frequently associated with
t(5;12)(q33;p13),1,14
and AML is defined as 
30% marrow blasts with the various MDS subtypes
ranging from < 5% to < 30% blasts. However, the previous distinction
between MDS and AML has become blurred with the recognition of several common
features of the two diseases: 1) MDS is now recognized to be a clonal
pre-leukemic hematopoietic stem cell disorder frequently associated with
specific recurrent cytogenetic
abnormalities15,16,17,18;
2) multi-lineage dysplasia is now recognized to occur in the majority of AML
cases presenting clinically as "de novo" disease in older
individuals7,19,20,21;
3) AML arising in older individuals and primary, secondary, or therapy-induced
MDS are now known to share strikingly similar biologic and genetic
features7,8,9,10,11,12,13;
4) de novo AML cases such as those with t(8;21) and inv(16) may present
clinically with less than 30% marrow blasts and may have dysplastic
features21; and 5)
transgenic and "knock-in" murine models of leukemia made with
fusion genes from translocations associated primarily with de novo AML
[t(15;17), t(8;21), inv(16)] are often characterized by hematopoietic
dysplasia or an MDS-like disease antecedent to the development of
AML.22,23,24,25
These more recent clinical and biologic studies indicate that disorders
previously classified as MDS are part of the same disease continuum as AML and
that MDS is best considered a pre-leukemic disorder with variable frequencies
and rates of progression to AML.
As we now recognize that MDS and AML are part of the same continuous biologic and genetic spectrum of disease, the use of arbitrary "thresholds" for the distinction of AML from MDS for the purposes of disease classification and therapeutic decision making has become particularly problematic. At what blast cell percentage should a clinician institute AML-based therapies in an MDS patient progressing to RAEB-T and from RAEB-T to AML? Should AML-based therapies be instituted in a patient whose marrow has dysplastic morphologic features, a blast cell percentage < 20%, and a t(8;21)-containing clonal population of cells? While the treshold of 30% blasts used by the FAB Classification to distinguish AML from MDS (Table 1) is clearly arbitrary, a reduction in this threshold to 20% blasts and the resultant elimination of RAEB-T as a distinct clinical stage in the evolution of MDS to AML as proposed in the new WHO Classification21 (Table 1) has been perceived by many hematologists to be even more problematic.26,27 While RAEB-T and AML arising clinically as "de novo" disease in older patients share highly similar cytogenetic features,13,26 they have differing clinical and biologic features and therapeutic responsiveness.26,27,28 Although not directly tested in a randomized fashion, in several, if not all, studies RAEB-T patients appear to have a worse response to intensive chemotherapy when compared historically to AML cases with similar biologic and cytogenetic features.28,29 Thus, it will be particularly important to retain the distinct RAEB-T MDS subtype in order to compare future therapeutic advances in AML/MDS to historical controls. Additional concerns that have arisen with the proposed WHO Classification (Table 1) include29: 1) the proposal that a diagnosis of RA and RARS be restricted to patients who have abnormalities solely involving the erythroid lineage, even though a diagnosis of MDS requires dysplasia in at least two hematopoietic lineages; 2) the creation of vague new MDS diagnostic categories ("refractory cytopenias with multilineage dysplasia (RCMD)" and "MDS, unclassifiable") which have no biologic, clinical, or genetic basis; and 3) the general lack of clinical and prognostic relevance in the proposed WHO classification scheme. Unfortunately for clinicians and diagnosticians alike, these controversies will likely continue until our knowledge has increased to the degree that disease classification systems can be developed on clinical features, genetics/genomics, and functional biology. And as our knowledge continues to evolve, classification systems will necessarily require constant revision.
Taking an alternative approach, others have worked to develop risk-based classification systems for MDS in order to facilitate clinical decision-making.30 The International Scoring System for Evaluating Prognosis (IPSS) in MDS assigns IPSS scores to varying degrees of those clinical and biologic features that today provide the most prognostic significance in MDS: marrow blast cell percentage, karyotype, and degree of cytopenia (Table 1). 29 An overall IPSS score developed using these variables has a strong correlation with predicted median survival.29 The IPSS system has proven to be a highly useful method for evaluating prognosis in MDS patients, has achieved international acceptance, and is being used to design clinical trials.
Genetic Features of MDS: Models for Genetic Progression and Clues to
Etiology
MDS is a clonal hematopoietic stem cell disorder characterized by
step-wise genetic progression
Initially demonstrated by studies of expression of the various isoforms of
the X-linked gene G6PD and more recently by molecular methods that detect
non-random patterns of X-inactivation, evidence for clonality has been found
in all forms of MDS, even in their very earliest
stages.17,18
Interestingly, cytogenetic and molecular data provide evidence, in some MDS
patients, for the existence of a clonal phase of disease prior to the
acquisition of the characteristic cytogenetic abnormalities associated with
MDS.18 Similarly,
MDS patients who evolve to acute leukemia may after therapy revert to a
cytogenetically normal but persistently clonal remission. Such findings have
led to the hypothesis that the recurrent cytogenetic abnormalities associated
with MDS, previously considered the "primary" cause of disease,
are actually "secondary" cytogenetic abnormalities that arise due
to cytogenetically undetectable initiating lesions in a clonal hematopoietic
stem cell
population.30 Such
initiating events are likely to be heterogeneous and could either be inherited
or result from acquired somatic DNA damage, genomic instability, defective DNA
repair, or perturbations in cell signal transduction pathways that give rise
to stem cell clones that have a growth or survival advantage. In contrast to
the de novo acute leukemias that occur primarily in younger patients
(particularly those associated with balanced translocations and inversions
such as t(8;21), t(15;17), inv(16), t(9;11), etc. lacking dysplastic
features), MDS and AML arising in older individuals appear to have a different
model of genetic progression (Figure
1).31,32
In this proposed model, initiating genetic lesions (which may be inherited or
acquired) promote the acquisition of "secondary" genetic events
that are primarily characterized by stepwise gains and losses of specific
chromosomal regions (particularly chromosome 3p-, 3q-, 5q-, 7q-, 12p-, -17,
-18, 20q11-12, +8). Such gross chromosomal changes are ultimately accompanied
by sub-microscopic DNA mutations of genes such as p53, FLT3, or
RAS, methylation of specific gene promoters, and in some cases by the
reciprocal translocations and inversions more frequently associated with AML.
This model of step-wise genetic progression for MDS and related AML is
strikingly reminiscent of those proposed for human solid tumors, such as colon
cancer. Three lines of evidence support this model and the existence of a
genetic predisposition to MDS: 1) the occurrence of AML and MDS in families
with inherited defects in DNA repair or neurofibromatosis-type
I33,34,35,36,37;
2) genetic mapping studies in the rare families with "familial"
MDS and
AML38,39,40,41;
and 3) studies of the association of various genetic polymorphisms with AML
and
MDS.42,43,44,45,46,47,48
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An inherited genetic predisposition to MDS
Support for an inherited predisposition to MDS and related AML has long
been evident from studies of inherited constitutional genetic defects (such as
Schwachman-Diamond syndrome, the defective DNA repair of Fanconi anemia, or
deregulation of the RAS signal transduction pathway in neurofibromatosis-type
1) that are present in a large proportion of children who develop MDS and
AML.33,34,35,36,37
Indeed, recent studies indicate that up to 30% of children affected by MDS and
related myeloproliferative disorders have an inherited constitutional genetic
disorder.34 The
original studies by Shannon and
colleagues35,36
of the genetic basis of familial MDS with chromosome 7q abnormalities (the
"monosomy 7 syndrome") are important in that they first revealed
that abnormalities of chromosome 7q were not the "primary" cause
of the syndrome; indeed, these investigators concluded that the predisposition
locus mapped to some other as yet unidentified chromosomal location. Thus, the
foundation was laid for the putative existence of genetic loci that could
"predispose" to chromosomal instability, secondary loss of
specific chromosomal regions (such as 5q, 7q, and 20q), and the ultimate
development of MDS and AML in adults and children. This original hypothesis
was recently supported the findings of Gilliland and colleagues who determined
the genetic basis of familial platelet disorder with leukemia (FPD/AML), an
autosomal-dominant congenital thrombocytopenia characterized by platelet
aggregation
abnormalities.38,39
Affected individuals in the seven pedigrees studied to date all have a
striking propensity to develop MDS, AML, and more rarely, chronic myelogenous
leukemia (CML). Interestingly, the MDS and AML cases that develop in these
pedigrees have the cytogenetic abnormalities classically associated with MDS,
particularly abnormalities of chromosomes 5q and 7q and complex abnormalities.
After mapping the FDP/AML predisposition locus to chromosome 21q22 in 1998,
Gilliland and colleagues went on to determine that the causative gene for this
disorder was CBFA2 (AML1), the gene whose function is most frequently
disrupted in the acute leukemias by various reciprocal translocations and
inversions, including the t(8;21), inv(16), t(3;21), and
t(12;21).49
Heterogeneous point mutations and small deletions of a single AML1 allele were
found in these different
pedigrees.39
Despite this molecular heterogeneity, each mutation characteristic of each
pedigree affected the DNA-binding domain of one AML1 allele, particularly
targeting the two arginine residues at positions 166 and 201 that bind to DNA.
The change of arginine to glutamine resulted in a loss of DNA-binding
activity.38 These
data thus support the hypothesis that AML1 may surprisingly function as a
tumor suppressor gene and that loss of one AML1 allele (hemizygous loss) is
sufficient to initiate tumorigenesis and establish a neoplastic clone in
affected individuals. This loss of function of a single AML1 allele appears to
also confer a susceptibility to the acquisition of secondary mutations and the
gain and/ or loss of the chromosomal regions frequently associated with AML
and MDS. This discovery has led to a particularly attractive model for MDS/AML
whereby AML1 mutations predispose to chromosome instability leading to the
eventual loss of chromosomes 5q, 7q; such models are currently being developed
in mice in which the mutated AML1 allele has been introduced (Downing and
Gilliland, personal communication). These pivotal studies also further cement
a genetic and etiologic link between MDS and AML (and even CML). Not
unexpectedly, a low percentage of sporadic AML, ALL, and CML cases (5-8%) have
recently been reported to have similar AML1
mutations50;
whether such AML1 mutations can be observed in primary "sporadic"
MDS cases is currently under investigation. Whether AML and MDS cases with
AML1 mutations are indeed "sporadic" or represent AML and MDS
cases that have arisen in individuals with inherited AML1 mutations is as yet
unknown. Given the functional role and association of CBFß (mapping to
chromosome 16q22) with AML1 in normal and neoplastic
hematopoiesis,51 it
is tempting to further speculate that CBFß might be the causative gene
for the second predisposition locus in AML and MDS in those familial cases
where the predisposition has been mapped to chromosome
16q21-23.2.40
Models for the development of sporadic MDS: Cumulative environmental
exposures in genetically predisposed individuals
While genetic and familial mapping studies have clearly demonstrated that
mutations in a specific gene, such as AML1, NF1, or genes mediating DNA
repair, can predispose to the acquisition of secondary cytogenetic
abnormalities and MDS, it is likely that such inherited genetic mutations will
account for only a minority of MDS cases. How the majority of
"sporadic" MDS cases arise is as yet undetermined. However,
epidemiologic case-control studies of MDS (and related AML) have demonstrated
associations between MDS and smoking, exposure to chemical compounds
(particularly petroleum products and diesel derivatives, exhausts, organic
solvents, fertilizers, and nitro-organic explosives), semi-metals (arsenic and
thallium), stone dusts (such as silica), and cereal
dusts.52,53
In light of these epidemiologic studies, it is interesting that evidence is
increasing for a complex genetic predisposition to MDS involving naturally
occurring DNA polymorphisms in genes that mediate DNA repair and metabolize
environmental
carcinogens.42,43,44,45,46,47,48
These studies are leading to a model, also diagrammed in
Figure 1, in which MDS arises
as a result of cumulative environmental exposures in genetically predisposed
individuals. In this case, the genetic predisposition is a more complex
genetic trait: a constellation of genetic variants in critical polymorphic
genes. The initial attempts to dissect this complex genetic predisposition
have focused on the association of MDS with naturally occurring polymorphisms
in genes that mediate carcinogen
metabolism.42,43,44,45,46,47,48
Following initial reports of the association of a non-functioning
609C
T polymorphic allele of the NAD(P)H:Quinone
Oxidoreductase (NQO1) gene that plays a critical role in detoxifying benzene
metabolites with an increased incidence of hematologic malignancies in Chinese
workers exposed to
benzene,43 several
groups have attempted to determine the incidence of this polymorphism in
primary and secondary leukemia
cases.44,45
Larson and colleagues first reported an increased frequency of the NQ01
609CT polymorphism in patients with therapy-related AML,
particularly in AML patients with abnormalities involving chromosomes 5 and 7,
88% of whom were homozygous for the non-functional
allele.44
Interestingly, studies of de novo AML in
children45 and
adults (C. L. Willman et al, manuscript in preparation and M.A. Smith et al,
personal communication) have failed to demonstrate an association of this NQO1
polymorphism with abnormalities of chromosome 5 and 7, but have instead
demonstrated strong associations with balanced translocations and inversions,
particularly involving MLL and chromosome 11q23. While it is tempting to
speculate that the NQO1 609CT polymorphism could predispose
to the development of MDS, no such studies focusing on MDS cases have been
reported; our own limited studies of 120 primary MDS cases have failed to
reveal such an association. Similar complexities have arisen in studies of the
association of MDS and polymorphisms in the glutathione S-transferases (GST)
that mediate exposure to cytotoxic and genotoxic agents, specifically the
"null" variant allele GST theta 1
(GSTT1).46,47,48
Chen and colleagues initially reported that the frequency of the GSTT1 null
genotype was higher among MDS cases than
controls.46
However, Fenaux and
colleagues47 and
other groups48 did
not confirm these initial observations and actually reported that the
incidence of the GSTT1 null genotype tended to be higher in unexposed MDS
patients and controls. Thus, while the hypothesis and model that MDS arises
due to cumulative environmental exposures in genetically predisposed
individuals is indeed attractive, these studies of natural human genetic
variation and disease association are only in their infancy. Moreover, it is
likely that true genetic risk will not be simply determined through studies of
one gene, but through the constellation of risk-producing and risk-protecting
alleles present in each individual. Thus, it will ultimately be necessary to
study polymorphic variants in many human genes in a large number of affected
individuals and controls and carefully monitor environmental exposures in
order to dissect what is likely to be a very complex genetic
predisposition.
Cloning and identification of genes disrupted by the recurrent
cytogenetic abnormalities associated with MDS
The identification of potential "initiating" genetic lesions in
MDS and related AML patients has lead to the hypothesis that the cytogenetic
abnormalities traditionally associated with MDS (involving chromosomes 7q, 5q,
20q11-12, trisomy 8, 12p, and 3q) are "secondary" genetic events.
However, these secondary cytogenetic abnormalities are likely no less critical
for disease progression, and identification of the gene(s) involved in these
regions remains very important. Unfortunately, despite years of mapping and
definition of minimally deleted chromosomal regions on chromosomes 5, 7, and
20, no investigator has yet succeeded in identifying the "single"
tumor suppressor gene that is responsible for MDS and AML on any of these
chromosomes. Recent detailed cytogenetic and molecular mapping studies reveal
that rearrangements and deletions involving these chromosomes are very complex
and that multiple distinct regions may contribute to the disease phenotype or
progression: at least two different regions are implicated on chromosome 7q
(7q22 and 7q32-34) and more than four different regions may be involved on
chromosome 5q (5q11, 5q13-q21, 5q31, and distal
5q33-35).54,55,56,57,58,59,60,61
One issue with many mapping studies is that in most instances little attention
was paid to "phenotype" rather than "genotype." In
other words, patient samples were selected for molecular studies based on the
presence of a specific cytogenetic abnormality (such as a 5q- or a 7q- with or
without additional cytogenetic abnormalities) without regard to the specific
form of disease or mode of disease presentation (primary MDS, secondary AML,
de novo AML, or tAML/MDS). Both cytogenetic and molecular genetic studies are
not only revealing the tremendous heterogeneity in different breakpoints but
also the need to focus on a pure genotype and phenotype for mapping studies.
Very detailed cytogenetic studies by Pederson in 1996 revealed that while
chromosome region 5q31 was deleted in all patients with MDS, other chromosome
5 regions were deleted more often than 5q31 in AML patients; the chromosome
5q13-q21 region was particularly involved in the genetic progression of RA to
RAEB and 5q22-5q33 for further progression to
AML.61 Recent
studies by Westbrook and colleagues have focused on a single AML patient with
a very small deletion in the 5q31 region. In this patient, the D5S500-D5S594
region was identified to be the minimal deletion interval, and this interval
was shown to contain nine transcriptional units with five unknown expressed
sequence tags (ESTs) and the genes CDC25, HSPA9, EGR1, and
CTNNA1.58
While all of these sequences are interesting candidates and are expressed in
hematopoietic cells, none has yet been identified as "critical"
for disease. It also remains possible that loss of more than one gene in this
region, as well as the other distinct regions on chromosome 5, could actually
be responsible for the disease phenotype. Boultwood and
colleagues56,57
have focused on the identification of the gene(s) deleted in MDS patients who
have the isolated "5q- syndrome," a clinically distinct form of RA
associated with more indolent disease and a lower rate of progression to AML,
chronic macrocytic anemia, thrombocytosis, and dysplastic megakaryocytes.
Interestingly, the region on chromosome 5 specifically associated with this
disease presentation appears to be more distal to 5q31; novel transcriptional
units have also been recently identified in this distinct
region.57
In light of the complexity of this cytogenetic and molecular data, it is attractive to hypothesize that in MDS, an initiating abnormality gives rise to genome instability leading to the deletion and rearrangement of particularly susceptible chromosomal regions, such as those on chromosome 5q and 7q. Cytogenetic studies have revealed the continued instability of these regions during disease progression in individual patients.62 While it may be that loss of function of a single gene in each of these relatively large regions is responsible for disease progression, more recent studies have given more credence to the possibility that hemizygous loss of the function of several genes in each of these regions could contribute to the disease phenotype.
Molecular mutations and genome methylation in MDS
In addition to the complex cytogenetic abnormalities seen in MDS, several
molecular defects have also been reported. Studies of MDS in children, with or
without Neurofibromatosis-type 1, have provided evidence that abnormalities in
the RAS signal transduction pathway cooperate with a loss of genes on
chromosome 7 to promote myeloid
leukemogenesis.63
In adults with MDS, disease progression has been associated with the mutations
in genes such as RAS, p53, and
FLT364,65,66,67
and with progressive methylation and transcriptional inactivation of critical
cell cycle regulatory genes such as p15INK4b that normally
function to inhibit cyclin-dependent kinase activity at the G1
phase of the cell
cycle.68,69
MDS patients have also been shown to have defective activation of signal
transduction pathways, particularly involving STAT5, in response to
erythropoietin,70
which may account in part for the defective erythropoiesis and persistent
anemia. These molecular abnormalities associated with MDS all further
contribute to an escape from normal cell cycle regulation, a disruption of the
faithful maintenance of DNA integrity and repair leading to continued genetic
instability, and the altered activation of various signal transduction
pathways.
Biologic Features of MDS
An abnormal marrow microenvironment: Cytokines, adhesion,
apoptosis
While there is strong evidence supporting the view that MDS arises from an
intrinsic or acquired genetic defect in hematopoietic stem cells leading to
clonal expansion of a stem cell population, it is also clear that other
epigenetic abnormalities such as aberrant cytokine production, altered stem
cell adhesion, or an abnormal marrow microenvironment contribute to the
biology of the disease and may provide important therapeutic targets
(Figure 2). While
studies reporting aberrant cytokine expression profiles in MDS patients have
been criticized for their lack of controls, lack of a suitable ex vivo system
to study the true clonogenic hematopoietic stem cell and marrow stromal cell
interactions, and the failure to precisely identify the cellular origin of
particular cytokines, abnormalities and elevations in tumor necrosis
factor-
(TNF), transforming growth factor-ß (TGFß),
and interleukin-1ß (IL-1ß) have all been
reported.2 In
particular, an overabundance of IL-1ß and a relative lack of its
antagonist IL-1ß(ra) have been hypothesized to support the clonal
expansion of aberrant hematopoietic stem
cells.2
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Several recent studies have revealed that in its early stages, MDS is
characterized by accelerated apoptosis of hematopoietic progenitor
cells.71,72
While some controversy remains, most studies of MDS samples using various
techniques have demonstrated a lowered apoptotic threshold of marrow CD34+
cells to TNF-, interferon-
(IFN-), and anti-FAS
antibodies.70,71
Excessive apoptosis is an attractive explanation for how a clonal expansion of
marrow progenitor cells could result in effective hematopoiesis and marrow
failure. However, how such accelerated apoptosis is initiated or acquired is
not yet understood. Apoptosis may be triggered in cells by intrinsic DNA
damage or in cells that have an abnormal growth factor milieu; both mechanisms
could contribute to the accelerated apoptosis observed in MDS. Another
biologic mechanism that can induce apoptosis is a lack of appropriate adhesive
interactions between cells and their stromal support
(Figure 2). In the case of MDS,
perturbed adhesive interactions between clonogenic hematopoietic stem cells
and the underlying marrow stroma or endothelium could clearly trigger
excessive apoptosis. The migration, retention, and survival of hematopoietic
stem cells in the marrow microenvironment is controlled by critical adhesion
receptors including CD34, CD44, selectins, and integrins; ß1 integrins
play a particularly critical role in this process and virtually all
hematopoietic stem cells express high levels of 4, 5, 6,
and ß1
integrins.73,74
Unfortunately, this is a relatively unexplored area of MDS and is worthy of
continued investigation. Interestingly, while MDS is characterized by
excessive apoptosis, further genetic progression in this disease and the
transition to AML appears to be associated with a loss of functional
apoptosis, though additional studies documenting this progressive change in
individual patients with disease progression are clearly necessary.
Summary and Future Scientific Questions
Exciting scientific studies have provided phenomenal insights into the
striking biologic and genetic heterogeneity of MDS and related AML. As these
scientific studies lead to the further refinement of genetic models for the
development of MDS, we may gain new insights that will ultimately lead to the
design of more effective prevention strategies and therapeutic regimens. Our
ongoing attempts to improve our knowledge, classification, and therapy of MDS
and AML would be best served by appreciating that these disorders are linked
clonal diseases and represent stages in the progressive transformation of
clonal neoplastic hematopoietic stem cells. Particularly fruitful areas for
future investigation include the application of genomic and proteomic
technologies to understand alterations in the global patterns of gene
expression and protein function in MDS and the alterations in these patterns
during the variable progression of MDS to AML, studies of global genome
methylation and aberrant methylation in the development of MDS, continued
studies of the complex genetic predisposition to MDS and how such genetic
predispositions and environmental exposures contribute to disease etiology and
pathogenesis, studies of the adhesive interactions of hematopoietic stem cells
and their underlying stroma and how these interactions are perturbed in MDS,
and the role of neoangiogenesis in the development of MDS.
II. Immune Mechanisms and Modulation in MDS
A. John Barrett, M.D., FACP,* and Yogen Saunthararajah*
Despite improvements in the supportive care of patients with MDS with transfusions and antibiotics, nearly 50% of patients die before transformation to acute leukemia from complications of thrombocytopenic bleeding or infection. Treatment of the marrow failure alone should therefore bring survival advantages to a significant proportion of patients with MDS. To improve the treatment of the cytopenia associated with MDS requires an understanding of the pathophysiology of the disease. It is well accepted that MDS is a clonal stem cell disorder, characterized by cytopenia, progressive de-differentiation of myeloid lineages and ultimately unregulated blast proliferation. Recent research reveals a further dimension to this pathophysiology, with extrinsic immunological and microenvironmental factors compounding the intrinsic stem cell defect and contributing to the pancytopenia and possibly to leukemic progression. In this section, pathophysiological findings in MDS that underpin experimental trials of immune suppression, cytokine regulation and inhibition of angiogenesis are reviewed.
Dysplasia, Apoptosis and Cytokines
Dysplasia of erythroid, granulocytic and megakaryocytic lineages are the
diagnostic hallmarks of MDS. Despite increased proliferation of the marrow,
there is an increased rate of programmed cell death. Indeed some of the
dysplastic appearance may be explained by apoptotic
changes.1,2
Kinetically the apoptosis prevails over the increased proliferation, causing
the peripheral
cytopenia.3,4
Cytokines derived from unselected marrow mononuclear cells are believed to be
extrinsic factors predisposing to apoptosis in
MDS.5,6,7,8,9,10
In MDS many studies link overexpression of TNF- to cell
death.7,11,12
TNF- produced by MDS mononuclear cells is inhibitory to both normal and
MDS colony growth indicating that residual normal hematopoiesis can also be
blocked in
MDS.7,8
IFN-, IL-1, and
TGF-ß4,13
as well as undefined factors produced by stromal cells have also been
implicated in causing
apoptosis14 but
their role in causing marrow failure is not well established. The
identification of TNF- as a key cytokine in cell death regulation and
the increased susceptibility of MDS cells to TNF- is the basis for
several clinical trials of TNF-
inhibitors.4
Immune Dysfunction in MDS
Hamblin and others have pointed out an association of MDS with an
autoimmune process, noting the occurrence of autoantibody production, and
monoclonal gammopathy in some patients with
MDS.15 The
occasional finding of T cell clonality in MDS has been interpreted as evidence
of T cell involvement in the stem cell
disorder16;
however, more recent evidence suggests that clonal T cell expansion is a
feature of autoimmunity: we have observed a high frequency of T cell receptor
(TCR) V-ß repertoire skewing in MDS patients indicating the presence of a
persisting clonally-expanded T cell population, characteristic of an
autoimmune process. Interestingly, it is not uncommon to find MDS in
association with T-cell large granular lymphocyte (T-LGL) leukemia, a
condition characterized by oligoclonal or clonal T cell expansions arising in
a background of autoimmune
disease.17 MDS also
shares some of the features of acquired aplastic anemia (AA), a disease with
an established autoimmune
pedigree.18 Both in
AA and MDS, plasma TNF- and IFN- levels are high and T
cell-mediated myelosuppression
occurs.19 Three
groups studying MDS identified T cells inhibitory to autologous
granulocyte20,21
or erythroid22
colony growth. These observations strongly suggest that, as in AA, an
autoimmune T cell-mediated myelosuppression contributes to the cytopenia of
MDS. The use of immunosuppressive treatment to restore marrow function in
patients with AA has been the stimulus to consider similar immunosuppressive
treatment in
MDS.23,24,25,32
Evidence for immune-based myelosuppression in MDS is summarized in
Table 2.
|
Marrow Microenvironment and Angiogenesis
In MDS the marrow shows a non-homogeneous distribution of cell types, seen
as islands of pure erythroid cells, granulocytes,
megakaryocytes26
or blast cells termed "abnormally localized immature precursors"
(ALIPS).27 This
appearance may reflect an underlying abnormality in the marrow stroma. In MDS,
vascular endothelial growth factor (VEGF) is strongly expressed especially in
megakaryocytes, and investigators have found an increased density of blood
vessels and an association of increasing marrow vascularity with leukemic
transformation.28,29
This is the basis for using angiogenesis inhibitors to correct abnormalities
in MDS and possibly to retard disease progression.
A Model of Pathophysiological Changes of MDS
A hypothetical model of MDS pathophysiology and the role of experimental
treatments is presented in Figure
3.1
Transformation of normal stem cells induces danger signals or antigenic change
and hence an autoimmune T cell response directed against the marrow. Both MDS
and normal marrow cells at various stages of differentiation are directly
inhibited by CD8+ CTL causing varying degrees of stem cell failure. The MDS
cells may be relatively resistant to this immune attack than normal stem
cells, and consequently the abnormal immune environment may provide selective
pressure in favor of the abnormal
cells.2 T cell
expansions are clonal and can become autonomous in some individuals as
illustrated by the coincidence of MDS with T cell
LGL.3 The
persisting autoimmune attack results in chronic overproduction of
pro-apoptotic cytokines, especially TNF-. This affects cells
differentiated at or beyond the CD34+ stage of differentiation and may
contribute to a dysplastic morphology and increased apoptosis in the marrow.
Despite the increased cell proliferation in MDS, the marrow fails to export
sufficient cells into the blood because intramedullary apoptosis mechanism
prevails over proliferation. Increased TNF- levels lead to changes in
the MDS cells: increased FAS, downregulation of Fap-1, and an increase in
caspases causing apoptosis. Non-hematopoietic cells in the marrow
microenvironment contribute to the process in two ways: (1) stromal cells
induce apoptosis by a non TNF-dependent mechanism; (2) VEGF production by the
MDS cells may stimulate angiogenesis that could favor disease progression by
altering the microenvironment to favor unregulated cell growth.
|
Experimental Treatments for MDS
Previous research in MDS has focused on increasing marrow cell
proliferation and differentiation by growth factors and differentiating
agents. Perhaps because of the importance of inhibitory factors extrinsic to
the MDS stem cell itself, these treatment approaches have produced only
partial beneficial effects.
The results of experimental treatments that affect extrinsic mechanisms of cytopenia and disease progression in MDS are outlined below.
It is important to recognize that MDS represents a diversity of subtypes with different degrees of severity of cytopenia and different rates of progression to leukemia. Determining if a novel treatment has altered the natural disease progression requires a knowledge of the rate of progression and survival probability of MDS subtypes with comparable prognostic features. Currently the IPSS provides the most widely accepted yardstick to measure disease progression and survival.30 Numerical changes in cell counts are frequently used to describe responses to cytopenia. However, even the finding of a 50% increase in neutrophils or platelets is only of clinical relevance if treatment raised the count from a level associated with a high risk of infection or bleeding to a safe level. Furthermore, only sustained responses, not peak counts achieved, correlate with prolonged clinical benefit. Reliable end-points of major clinical significance are independence from transfusion of blood and platelets and a recovery of neutrophils from below 500 to > 1000/mm3.
Immunosuppressive Treatments
Prednisone
Bagby et al reported in 1980 that prednisone alone improved low blood
counts in a minority of patients with MDS and that the response could be
predicted in vitro by enhancement of granulocyte macrophage colony-forming
units (CFU-GM)
growth.31 However,
although steroids have frequently been used in treatment, low response rates
and increased risk fo infection make corticosteroids unattractive agents in
MDS.
Antithymocyte globulin
Because of the similarity of hypoplastic MDS to SAA, antithymocyte globulin
(ATG) has been used occasionally to treat hypoplastic MDS with severe
cytopenia. A summary of case reports and small series reveals hematological
responses in 8/13 hypoplastic MDS
patients.23,24
Based on these reports, unpublished observations, and the hypothesis that a T
cell-mediated process may cause pancytopenia, we evaluated ATG as
immunosuppressive treatment to improve marrow function in
MDS.25 This study,
now completed, involved 61 patients. Study entry criteria were red cell or
platelet transfusion-dependence and no concurrent treatment with
immunosuppressives, chemotherapy or growth factors. Thirty-seven had
refractory anemia (RA), 12 had RA with excess of blasts (RAEB) and 10 RA with
ring-sideroblasts (RARS). Marrow cellularity varied from hypo- to
hyper-cellular. Most patients (75%) had failed previous treatment with single
or multiple agents, including cyclosporine, amifostine, and growth factors.
The interval between diagnosis and entry to the study was 3-300 months, with a
median of 24 months. Twenty-three cases had hypoplastic MDS, seven had the PNH
abnormality by flow cytometry, and 23 had karyotypic abnormalities
characteristic of MDS. Patients received ATG at 40 mg/kg/day for 4 days and
were periodically evaluated for 38 (range 20-58) months. The main criterion
for response was independence from the requirement for red cell transfusions.
Twenty-one (33%) patients became red cell transfusion-independent within 8
months of treatment (median 75 days). Transfusion-independence was maintained
in 76% of responding patients for a median of 32 months (range 20-58).
Twenty-three of 41 (56%) severely thrombocytopenic patients had sustained
platelet count increases between 25,000-290,000/µl and 18/41 (44%) severely
neutropenic patients achieved sustained neutrophil counts > 1000/µl. At
last follow-up 39 patients were alive with an actuarial survival of 64% at a
median of 34 (range 18-72) months. Of the 21 responders, 20 survived and one
died following progression to AML
(Figure 4). In the
others no significant alteration in the bone marrow appearance or cellularity
was observed and cytogenetic abnormalities, present in four, persisted after
treatment. Three relapsed with transfusion-dependence but one regained red
cell transfusion-independence after a second course of ATG. Of the 40
non-responders 21 died, 15 from cytopenia and 7 from progression to AML.
|
Factors Predicting a Response to Immunosuppression
In a multivariate analysis of 82 RA, RARS or RAEB patients treated with
either ATG alone or cyclosporine alone, younger age, shorter duration of red
cell transfusion dependence and the presence of HLA DRB1 15 predicted a
response to immunosuppression. In other words, to maximize the probability of
response to immunosuppression, patients should be treated sooner rather than
later and this strategy appears to be particularly appropriate and effective
in RA or RAEB patients who are young and/or have HLA DRB1 15. Pretreatment
variables associated with a response to immunosuppression are summarized in
Table 3. Responses were
associated with a significant survival benefit at 4 years (95% versus 38% for
non-responders, p = 0.006). In the subset of 41/61 patients with INT-1 IPSS
given ATG, responders had 100% survival at 3 years and no disease progression
versus 45% survival (p < 0.0004) and 51% probability of disease progression
in non-responders (p = 0.02). Since we previously found correlation between
response to ATG treatment and normalization of the T cell repertoire, we
studied 15 patients with MDS who received immunosuppression for biological
markers of response. Abnormalities in the TCR Vß repertoire were common
and occurred both in responders and non-responders, rendering Vß analysis
of no prognostic value.
|
Cyclosporine
Jonasova and co-workers reported a high rate of hematological responses
following cyclosporine treatment in MDS. Sixteen patients with RA and one with
RAEB were given standard doses of cyclosporine for 5-31 months. Substantial
hematological responses were
observed,32 mostly
occurring around 3 months. Transfusion independence was achieved in all 12
patients requiring red cells before cyclosporine administration, and
significant increases in leucocyte and platelet counts also occurred.
Responses occurred in RAEB as well as RA patients and in hyper- or
normocellular MDS as well as in hypoplastic MDS (6/8 and 7/9 responders
respectively). Of six patients with abnormal karyotype, three responded (all
were 5q-). These clinical studies provide further strong evidence to support
an immune mechanism of marrow suppression in patients with MDS.
New trials of immunosuppressive agents in MDS
Based on these preliminary findings, several trials evaluating
immunosuppressive treatment for MDS are underway in the US and in Europe
(Table 4). Collectively
these studies should better define the potential benefit for immunosuppression
by comparing ATG treatment with a control arm of conventionally supported
patients. Prospective randomized trials will evaluate the relative effects of
different ATG types and different immunosuppressive combinations in improving
marrow function and prolonging survival.
|
Cytokine inhibition
Amifostine is a phosphorylated organic thiol that is metabolized
to intermediates with antioxidant activity. The drug has two actions: (1) It
protects cells from oxidative stress after exposure to cytokines including
TNF-; (2) it suppresses inflammatory cytokine release. Since amifostine
reduces apoptotic marrow cell death following chemotherapy it may also reduce
apoptosis in
MDS.33 Indeed,
incubation of MDS marrow cells with amifostine was found to improve colony
growth.34 In a
study reported by List et
al,35 15 of 18
patients with MDS given amifostine had single or multilineage hematological
responses. These results were recently updated for 75 patients; there were 40%
platelet responses, 24% ANC responses and 20% with > 50% reduction in need
for
transfusions.36
There are ongoing, not yet reported studies on the use of amifostine in
combination with growth factors and chemotherapeutic agents that should
provide more information about the usefulness of amifostine.
Pentoxifylline
The observation that the three pro-inflammatory cytokines TNF-,
TGF-ß and IL-1ß are implicated in the increased apoptosis of
MDS4 led Raza et al
to treat MDS patients with pentoxifylline (PTX), a xanthine derivative known
to interfere with the lipid-signalling pathway used by these
cytokines.37
Ciprofloxacin was added because it reduced the hepatic degradation of PTX, and
dexamethasone was used to further downregulate TNF- production by
reducing translation of TNF- mRNA. Eighteen of 43 patients had
hematopoietic responses that correlated with a reduction in blood levels of
TNF-. In a subsequent study a four-drug combination of amifostine,
pentoxyfylline, ciprofloxacin and dexamethasone was used to optimize the
anti-cytokine
effect.38 Of 29
evaluable patients given this combination (evaluable because they survived 12
or more weeks), 22 (76%) showed partial responses, 19 with improvement in
neutrophils, 11 with a reduction in transfusion requirement or a rise in
hemoglobin, and 7 with improvements in platelet counts. However, these
improvements were not sustained or statistically significant at 24 weeks and
are similar to those reported for amifostine alone. There were no complete
responses or conversions to transfusion independence. In contrast, in a study
of 14 patients given PTX and ciproflovacin, Neumatis et al found a trend to
lower TNF- levels with treatment but failed to demonstrate a
hematological response in any
patient.39
Soluble TNF- receptor (Enbrel)
Excessive amounts of soluble receptor can effectively block the function of
TNF- by competitive binding. In a recent report the soluble receptor
Enbrel was reported to be well tolerated and to reduce plasma TNF-
concentrations. However, clinical responses were
modest.40
Thalidomide
Thalidomide is an immunosuppressive drug. It switches T helper cells from a
Th1 to a Th2 cytokine profile, inhibiting production of
TNF-.41 It
selectively inhibits TNF- production by
monocytes.42 The
drug has also been found to strongly inhibit angiogenesis in animal
experiments and in in vitro cultures of human
cells.43,44
Thalidomide has produced surprisingly substantial responses in patients with
myeloma, possibly because of its effects on marrow angiogenesis. Both the
immunosuppression and the angiogenesis inhibition of thalidomide might be
beneficial in MDS. Raza and colleagues have recently completed a study of
thalidomide in 83 patients with
MDS.45 The drug
was started at a dose of 100 mg daily and increased to a maximum, when
tolerated, of 400 mg daily. Despite this approach, 26 patients stopped
treatment within 12 weeks because of intolerance. Twenty-one of the 57
patients who continued treatment responded, 15 showing an erythroid, 13 a
platelet and 7 a neutrophil response, with a median time to response of 10
weeks. By intention-to-treat analysis, 37% of patients became responders. Of
note, eight patients of the erythroid responders became
transfusion-independent. There was no evidence that thalidomide favorably or
unfavorably affected disease progression, but follow-up was short. Patients
most likely to respond to thalidomide had fewer blast cells in the marrow.
These findings suggest that thalidomide can improve marrow function in some
patients with MDS.
Future directions
We still know very little about the interaction of the immune system, the
marrow microenvironment and the MDS stem cell. The antigens evoking the T cell
response are unknown and the mechanism of T cell-mediated myelosuppression is
not defined. The etiology of MDS remains unclear. Similarities between MDS and
AA, including the response to immunosuppressive treatment, raise questions
about the relationship between AA and
MD.20 Are they
different diseases sharing a common autoimmune pathology or are they two ends
of a spectrum of a marrow failure syndrome differing only in their tendency to
evolve to acute leukemia? Provocatively, it has been proposed that the
abnormal cytokine milieu in AA is the initiator of genetic instability in the
aplastic stem cell leading to clonal evolution and
leukemia.46 Also,
this abnormal milieu may act as selective pressure in favor of the abnormal
MDS clone. In these scenarios immune modulation at an early stage of clonal
evolution might be expected to maintain disease stability. Finally the
relationship between the MDS stem cell and the marrow microenvironment
deserves further study. In this regard it will be important to determine
whether thalidomide exerts its beneficial effect through inhibition of
angiogenesis as well as through immunosuppression.
III. Therapeutic Advances in MDS
Eva Hellström-Lindberg, M.D., Ph.D.*
Increased biological understanding of different subtypes of MDS has resulted in new therapeutic alternatives for groups of patients in whom, hitherto, only conservative treatment was available. Achievements in the techniques for stem cell transplantation (SCT) have made it possible to cure an increasing, but still small proportion of the patients. Allogeneic SCT results for patients in older age groups are constantly improving, which will increase the proportion of MDS patients eligible for this treatment. A large phase II trial suggests that a proportion of patients with high-risk MDS with complete remission after highdose chemotherapy may benefit from consolidation with autologous stem cell transplantation. Growth factors may support hemoglobin levels and neutrophil counts in subgroups of patients, but is an ineffective and expensive treatment for others, which encourages the development of decision models for this type of treatment. While erythropoietin (Epo) as monotherapy has shown efficacy mainly in RA patients with low serum Epo-levels, the combination of G-CSF + Epo may induce long-lasting normalization of hemoglobin levels, in particular in patients with RARS. For RA, again, new therapeutic approaches, such as ATG (reviewed in Section II), seem promising. Low-dose chemotherapy may improve peripheral blood counts, but has previously been shown not to improve long-term outcome. Two DNA-hypomethylating agents, 5-azacytidine and 2,5-deoxycytidine may however change the role of low-dose chemotherapy, since 5-azacytidine has been shown to prolong time to leukemic transformation or death, compared to untreated patients.
The IPSS scoring system is at present the most useful tool to predict long-term outcome in cohorts of untreated patients and should be included as a variable in studies describing the effects of different treatment approaches.1 A new MDS classification has recently been proposed, but its capacity to improve the clinical characterization and management of MDS patients is under debate.2
Allogeneic Transplantation
Allogeneic bone marrow transplantation (allo-BMT) is a curative therapeutic
option for younger patients with MDS
(Table 5). The outcome
of treatment is highly dependent on the selection of patients, and it is
therefore difficult to evaluate the effect of different conditioning regimens
and other treatment approaches. In a recent review from Fred Hutchinson Cancer
Research Center, the results of allogeneic BMT in 251 patients with MDS were
reported.3 The
overall median disease-free survival (DFS) was 40% after a median follow-up of
6 years. Important predictors for long-term survival were age, morphology and
cytogenetics. While patients below 20 years of age (i.e. pediatric MDS and
young secondary MDS) showed a DFS of almost 60%, DFS in patients > 50 years
of age was below 20%, mostly due to high transplant-related mortality.
Increasing disease duration before transplant significantly increased the risk
for non-relapse mortality but did not influence disease-free survival. A
Canadian study reported the outcome of 60 adult patients with
MDS.4 The 7-year
event-free survival was 29% for all patients, > 60% for patients with
RA/RARS, 20% for patients with 5% blasts, and 6% for patients in the poor
cytogenetic subgroup. An update of the EBMT experience of SCT in 1378 patients
with MDS has recently been
published.5
Estimated DFS and relapse risk at 3 years were both 36% for 885 patients
transplanted with stem cells from matched siblings. DFS and relapse rate in
RA/RARS was 55% and 13%, respectively, while the corresponding figures for
more advanced MDS was 28% and 43%. DFS in patients with advanced MDS treated
to CR was 44%. This analysis did not indicate a significantly better DFS in
patients transplanted < 1 year from diagnosis.
|
Recently, Deeg et al reported results on 50 MDS patients, aged 55-66, receiving allogeneic BMT with stem cells from matched siblings (36), unrelated volunteers (6), HLA-nonidentical family members (4), and identical twins (4).6 The Kaplan-Meier (KM) estimate of relapse-free survival at 3 years was 39% for all patients and 47% for patients with primary MDS transplanted with stem cells from an HLA-identical sibling. As in previous studies, cytogenetic risk group and IPSS score were highly predictive for the outcome of treatment. Moreover, conditioning regimen with cyclophosphamide + targeted busulfan showed an advantage compared to other conditioning regimens. The study shows that results in selected groups of older patients are beginning to improve.
Although the results from allogeneic SCT for MDS seem to improve over time, the outcome for patients with MDS is still worse than for those with other myeloid diagnoses, mostly due to a high transplant-related mortality and relapse rate. Future studies will need to focus on optimizing pre-treatment schedules, conditioning regimens and post-transplant immune-modulation. Case reports have shown an effect of donor lymphocyte infusion also in MDS.7 Slavin reported one patient with MDS who entered complete remission after non-myeloablative stem cell transplantation,8 and approximately 60 patients with MDS and secondary leukemia have been included in the protocols from Jerusalem, Seattle and NIH (Shimon Slavin, Rainer Storb, Ghulam Mufti, and John Barrett, personal communication), and at the Huddinge transplantation unit. Complete remissions lasting for 1-2 years have been observed, which encourages further investigations.
Autologous Stem Cell Transplantation for MDS
Conventional high-dose chemotherapy for MDS may lead to complete remission,
but only to cure in extremely rare cases. For patients without a suitable
donor, or with age and medical conditions making allogeneic stem cell
transplantation unsuitable, alternative methods to maintain CR and obtain cure
need to be developed. The presence of polyclonal peripheral blood stem cells
after high-dose chemotherapy constitutes the theoretical basis for autologous
stem cell
transplantation,9,10
and several European groups have evaluated the effect of ABMT/APSCT in
high-risk
MDS.11,12
Encouraged by early findings that DFS seemed to enter a plateau phase
approximately 3 years after transplantation, more studies have followed. The
EBMT and EORTC working groups have conducted a large phase II trial, in which
184 evaluable patients with high-risk MDS and AML following MDS were given
induction therapy to obtain CR. In CR, patients with a histocompatible sibling
donor were candidates for allogeneic transplantation, while the remaining
patients were planned for autologous stem cell transplantation after
consolidation therapy and stem cell harvest (Th. de Witte et al, submitted).
The complete remission rate was 54%, with a median remission duration of 12
months, and DFS was 29% at 4 years. Thirty-five of 57 patients without a donor
received APSCT in first CR, and 13 of these (37%) were in continuous complete
remission at follow-up. The benefit of APSCT compared to conventional
high-dose chemotherapy was also supported by a study in which 184 patients
from the EORTC-EBMT trials were compared with 216 patients from M.D. Anderson
Cancer Research Center receiving only combined
chemotherapy.13
Preliminary results indicate a possible benefit of using autologous
transplantation as post-remission consolidation, since the survival curve of
the transplanted patients showed a plateau, while that of the
chemotherapy-treated patients continued to decline. An ongoing study within
the EORTC-EBMT working group compares the effect of APSCT with the effect of a
second consolidation with high-dose cytosine arabinoside. Careful studies of
residual clonality in responding patients will serve as a tool to predict the
usefulness of this treatment in individual patients. Although the relapse rate
after APSCT is higher than for de novo AML, the results suggest that there is
a subset of patients with MDS that may be cured by intensive chemotherapy in
combination with stem cell support.
Intensive Chemotherapy
The results of AML-type induction regimens in high-risk MDS have improved
during recent years, but are still poorer than for de novo AML. Estey et al
compared 158 patients with MDS with a cohort of patients with
AML.14 The CR
rates were comparable in the MDS and AML groups (62-66%), but patients with
RAEB showed significantly shorter event-free survival (EFS) than both those
with RAEB-t and AML. Poor prognostic karyotype, length of MDS phase and age
predicted for a poorer outcome to treatment. A French group compared highdose
chemotherapy given with or without
quinine.15
In-patients expressing P-glycoprotein (PGP) the response rate to chemotherapy
+ quinine was 52% compared to only 18% in those treated with chemotherapy
only, indicating that one important reason for the lower CR rates in MDS is
functional drug resistance. Bernasconi et al randomized 105 patients to
chemotherapy, with or without
G-CSF.16 The
G-CSF-treated group showed a better response rate (74% vs 52%, p < 0.05),
but no effect on long-term survival could be observed unless an allo-BMT was
given. In another study, 112 patients with RAEB-t, MDS-AML and secondary AML
(median age 58 years, range 22-75) were treated with intensive chemotherapy in
combination with
G-CSF.17 Overall
response rate was 62%, but median duration of response was only 8 months. DFS
was 15% at 28 months. The CR rate was higher in patients < 60 years (68% vs
55%), and patients who obtained CR showed a DFS of 25% at 36 months. Outcome
was better compared with a previous trial in which patients were treated with
the same chemotherapy, but without G-CSF.
Topotecan, a topoisomerase I inhibitor, has been used as single therapy in high-risk MDS. In a cohort of 47 patients with RAEB/RAEB-t and CMML, the CR rate was 28%, with a response rate in previously untreated patients of 31-38%.18 Toxicity, however, was significant with 19% of patients dying during induction treatment. In a recent review by Estey, the effects of topotecan, alone and in combination with cytosine arabinoside (ara-C), was compared with the effect of ara-C in other combinations, such as the FLAG regimen.19 In RAEB/RAEB-t, but not in CMML, ara-C seemed to add to the effect of topotecan as single therapy. Ara-C + topotecan did not show any overall benefit compared to high-dose ara-C alone, but in patients with high-risk (5/7) chromosomal aberrations the combination of ara-C + topotecan showed better results than both topotecan alone, high-dose ara-C, and the FLAG regimen. None of these differences translated into a difference in survival.
Thus, high-dose results are slowly improving over time but long-term survival is still very poor for the majority of patients. It is, however, important to continue to aim for better response rates, since CR is the starting point for curative approaches.
Low-Dose Chemotherapy
Non-curative treatments must aim at improving quality of life and reducing
morbidity, but they may also carry hope for improved survival and reduced risk
of leukemic transformation. Low-dose chemotherapy may improve peripheral blood
values and reduce blast counts, and can be an effective and inexpensive
alternative for certain patients with MDS.
The drug that has been most extensively used in low doses is ara-C. Low-dose ara-C may induce complete or partial remissions in approximately 30% of patients with RAEB, RAEB-t and MDS-AML.20,21 However, the only prospective randomized phase III trial comparing low-dose ara-C with supportive care failed to show a difference in survival between the two alternatives.22 Moreover, side effects (mainly bone marrow hypoplasia and pancytopenia) can be pronounced, with therapy-related deaths having a frequency of 7-19%.20 The addition of growth factors, mostly GM-CSF, to ara-C has not been successful.23 Based upon a large phase II trial, pretreatment bone marrow cellularity, chromosome aberrations and ringed sideroblasts were used to formulate a predictive model for the use of low-dose ara-C in MDS and MDS-AML.21 This model identified patient groups with a > 50% probability of response and those with no response to treatment.
Melphalan
Two recently published studies have reported the effects of low-dose
melphalan (2 mg/day until progression/toxicity or response) in patients with
high-risk MDS. In the first study, 8 of 21 patients with RAEB or RAEB-t (38%)
achieved a complete (7) or partial (1) response with a median survival of 27
months for CR patients and 6.5 months for the
rest.24 No severe
side effects were observed in any patients. Recently, these results were
confirmed by a European
group,25 who
reported a response rate of 40% and suggested a better response in patients
with hypocellular MDS. The difference from other low-dose regimens seems to be
that melphalan relatively frequently causes improvement without preceding
cytopenia. Other advantages are the extremely low cost of the drug and its
simple administration.
5-azacytidine and 5-aza-2' -deoxycytidine
Several trials have investigated the effect of low doses of two DNA
hypomethylating pyrimidine analogues, 5-azacytidine (5-aza) and 5-aza-2'
-deoxycytidine (DAC), in high-risk MDS. In spite of the similar structure of
these drugs, their clinical effects differ in several ways. In phase II
studies, treatment with 5-aza led to improved peripheral blood values and
reduction of bone marrow blasts in 49-54% of the
patients.26
Inspired by this relatively high response rate and moderate toxicity, 5-aza
was evaluated by CALGB in a large randomised phase III study in which 5-aza,
given subcutaneously 7/28 days, was studied against observation in a
cross-over
model.27 The
overall response rate was 63% (6% CR, 10% PR, 47% improved). This is not
better than for other chemotherapeutic compounds, but the median time to
leukemic transformation or death was 22 months in the 5-aza arm versus 12
months for those on supportive care (p = 0.003). Median survival for 5-aza and
observation was 18 and 14 months, respectively (p = 0.1). Moreover,
treatment-related mortality was low, quality of life was enhanced in
responding patients, and toxic side effects
few.28 This
promising study suggests that 5-aza may alter the natural course of MDS.
However, confirmatory studies are warranted.
DAC, given intravenously, has been evaluated in two pilot trials and a large recently published multi-center phase II trial, in which primary endpoints were response rate and toxicity.29 The overall response rate was 49%, with 64% response in patients with an IPSS high-risk score. The actuarial median survival time from start of therapy was 15 months. Myelosuppression was common, and 7% therapy-related deaths were reported. Noticeable was the rapid and distinct platelet response, which occurred after the first course in the responding patients. Compared to 5-aza, DAC seems to be more efficient in inducing CR, it has more severe toxicity, and has not yet been investigated in a trial with survival as endpoint. It is, however, likely that both 5-aza and DAC will add to the arsenal of clinically useful therapies for MDS.
Growth factor treatment
The bone marrow of patients with RA and RARS is mainly characterized by
ineffective hematopoiesis. This is paralleled by reduced clonogenic growth in
vitro, and the presence of an increased proportion of apoptotic bone marrow
precursors.30,31,32
Recent studies suggest that the underlying mechanisms behind apoptosis in MDS
may differ between the subtypes of
MDS.33,34
Colony culture studies of myelodysplastic bone marrow have shown that the
impaired erythroid growth may respond to Epo alone, but that it may be further
improved if Epo is combined with other growth
factors.35 The
ineffective hematopoiesis and cytopenia in RA, RARS and RAEB with < 10%
blasts is considered to respond poorly to chemotherapy, and a number of
clinical trials have therefore addressed the use of growth factors in these
subtypes (Table 6).
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|
Erythropoietin
Erythropoietin (Epo) may improve the anemia in MDS. All 17 studies (205
patients) published before 1995 were included in a meta-analysis, which showed
an overall response rate of
16%.40 Information
about FAB group, transfusion need and serum Epo levels (S-Epo) were useful to
characterize groups of patients with different outcomes to treatment. Patients
with RARS responded significantly less well to treatment than the other
subgroups (8% versus 21% in non-RARS). The response rates in patients with
S-Epo below or above 200 U/l, were 21% and 10%, respectively, and 10% in
patients with pretreatment transfusion need. Rose et al treated 116 patients
with Epo.41 The
response rate was 28%, S-Epo levels < 100 U/l predicited for a better
response, and patients with RA had a better response than those with RARS (39%
versus 17.5%). The Italian cooperative group performed a placebo-controlled
randomized study, which showed that Epo significantly improved hemoglobin
levels in MDS. However, the response was significant only in patients with RA
and in patients who did not need transfusion before
treatment.42
Finally, Hast et al reported a series of 18 patients responding to Epo, who
continued on maintenance treatment and showed a median response duration of 15
months. The dose of Epo could be safely reduced or withdrawn, since the
majority of patients who then relapsed with anemia responded well to
reintroduction of Epo or to an increase of the
dose.43
Erythropoietin in combination with other growth factors
Several clinical trials have studied the combination of Epo and G-CSF,
GM-CSF or IL-3. The response rate to Epo + G-CSF has been shown to be around
40% in MDS patients with < 10% bone marrow blasts, i.e. twice as much as
for Epo alone. A large US phase II study and the trials within the
Scandinavian MDS Group have demonstrated evidence of a clear synergistic
effect between the drugs in
vivo.44,45
The US study showed that 50% of the patients with a response lost it when
G-CSF was withdrawn and regained it when G-CSF was reintroduced, and the
Scandinavian study showed that addition of G-CSF could induce erythroid
responses in Epo-resistant patients. The synergistic effect was most
pronounced in patients with RARS, since this subgroup responded poorly
