Achievements in Understanding and Treatment of
`Myelodysplastic Syndromes
`
`Eva Hellström-Lindberg (Chair), Cheryl Willman, A. John Barrett, and Yogen Saunthararajah
`
`The myelodysplastic syndromes (MDS) constitute a
`challenge for the biologist as well as for the treat-
`ing physician. In Section I, Dr. Willman reviews the
`current classifications and disease mechanisms
`involved in this heterogeneous clonal hematopoi-
`etic stem cell disorder. A stepwise genetic progres-
`sion 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 constella-
`tions 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 dem-
`onstrated an increased vessel density in MDS bone
`marrow, and a phase II trial of thalidomide showed
`responses in a subgroup of MDS patients espe-
`
`cially in those with low blast counts. In Section III
`Dr. Hellström-Lindberg presents results of alloge-
`neic and autologous stem cell transplantation
`(SCT), intensive and low-dose chemotherapy. The
`results of allogeneic SCT in MDS are slowly improv-
`ing 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 therapeu-
`tic 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 character-
`ized by ineffective hematopoiesis and peripheral cyto-
`penias. Although a substantial proportion of MDS cases
`evolve to acute myeloid leukemia (AML), the natural
`
`* University of New Mexico Cancer Center, 2325 Camino de Salud
`NE, Room 101, Albuquerque NM 87131
`
`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 pre-
`leukemic 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 unsatis-
`factory, 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 in-
`cluding abnormal apoptosis and an altered marrow mi-
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`Table 1. MDS Classification Systems.1
`
`FAB Classification System2
`
`WHO Classification System3
`
`IPSS Risk-Based
`Classification System4
`
`Refractory Anemia (RA): Cytopenia of one
`PB lineage; normo- or hypercellular marrow with
`dysplasias; < 1% PB Blasts and < 5% BM Blasts.
`
`Refractory Anemia with Ringed Sideroblasts
`(RARS): Cytopenia, dysplasia and the same %
`blast involvement in BM and PB as RA. Ringed
`sideroblasts account for >15% of nucleated
`cells in marrow.
`
`Refractory Anemia with Excess Blasts
`(RAEB): Cytopenia of two or more PB lineages;
`dysplasia involving all 3 lineages; < 5% PB
`blasts and 5-20% BM Blasts.
`
`Refractory Anemia with Excess Blasts in
`Transformation (RAEB-T): Hematologic features
`identical to RAEB. > 5% Blasts in PB or 21-30%
`Blasts in BM or the presence of Auer rods
`in the blasts.
`
`Chronic Myelomonocytic Leukemia (CMML):
`Monocytosis in PB (>1x109 per liter); < 5% blasts
`in PB and up to 20% BM blasts
`
`Juvenile Myelomonocytic
`
`Myelodysplastic Syndromes
`
`Overall IPSS Risk Score Based On:
`
`Refractory Anemia (RA)
`With ringed sideroblasts (RARS)
`Without ringed sideroblasts
`
`Refractory Cytopenia (MDS) with
`Multilineage Dysplasia (RCMD)
`
`Refractory Anemia with
`Excess Blasts (RAEB)
`
`5q- Syndrome
`
`Myelodysplastic syndrome,
`unclassifiable
`
`Myelodysplastic/Myeloproliferative
`Diseases
`
`Chronic Myelomonocytic
`Leukemia (CMML)
`
`Atypical Chronic Myelogenous
`Leukemia (aCML)
`
`Leukemia (JMML)
`
`Marrow Blast Percentage
`Blast %
`IPSS Score
`< 5
`0
`5-10
`0.5
`11-20
`1.5
`21-30
`2.0
`
`Cytogenetic Features5
`Karyotype
`Good prognosis
` (-Y, 5q-,20q-)
`Intermediate prognosis 0.5
`Poor prognosis
`1.0
` (abn. 7; Complex)
`
`IPSS Score
`0
`
`Cytopenias6
`Cytopenia
`None or 1 Type
`2 or 3 Types
`
`IPSS Score
`0
`0.5
`
`Overall IPSS Score and Survival
`
`Overall Score
`Low (0)
`Intermediate
` 1 (0.5 or 1.0)
` 2 (1.5 or 2.0)
`High (> 2.5)
`
`Median Survival
`5.7 Yrs.
`
`3.5 Yrs
`1.2 Yrs.
`0.4 Yrs.
`
`1 Abbreviations: PB, peripheral blood; BM, bone marrow; abn, abnormality
`2 References 3-4.
`3 Reference 21.
`4 Reference 30.
`5 IPSS Cytogenetic Classification28: Good prognosis: -Y only, normal, del(5q) only, del(20q) only; Intermediate prognosis: +8, Single
`miscellaneous abnormality, double abnormalities; Poor prognosis: Complex (i.e. > 3 abnormalities), any chromosome 7 abnormality.
`6 IPSS Types of Cytopenia28: Hemoglobin <10g per deciliter; Absolute neutrophil count <1500 per cubic millimeter; Platelet count
`< 100,000 per cubic millimeter.
`
`croenvironment, and new and highly interesting insights
`into the complex genetic predisposition to MDS. Excel-
`lent, well-referenced reviews are also available.1,2
`
`MDS and AML Disease Classification Systems:
`Unresolved Controversies
`The French-American-British (FAB) Classification, pro-
`posed in 1977, provided hematologists with the first con-
`sistent framework for morphologic classification of MDS
`(Table 1), the myeloproliferative disorders, and the acute
`leukemias.3,4 However, the separation of MDS as a dis-
`tinct disorder from AML in the FAB classification scheme
`has been perceived by many to have scientifically im-
`peded our understanding of the full spectrum of leuke-
`mic progression.1 Indeed, the initial failure to recognize
`and classify MDS as a “neoplastic” pre-leukemic disor-
`der and part of the same disease spectrum as AML re-
`sulted 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 stud-
`ies of the true incidence, natural history, and epidemiol-
`ogy 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-spe-
`cific 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 popu-
`lations, 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 aris-
`ing secondary to antecedent MDS; 3) AML arising sec-
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`ondary to prior therapy, particularly alkylating agent ex-
`posure; and 4) AML cases that arise from documented
`environmental or occupational exposures to agents such
`as benzene, petroleum, organic solvents, and arsenical
`pesticides.7-13
`In the FAB Classification (Table 1), the two primary
`distinguishing features between the various MDS sub-
`types, chronic myelomonocytic leukemia (CMML) and
`AML, are blast cell percentage and the presence of dys-
`plastic features. CMML is now considered a myelopro-
`liferative/leukemic-like disorder and frequently associ-
`ated with t(5;12)(q33;p13),1,14 and AML is defined as >
`30% marrow blasts with the various MDS subtypes rang-
`ing 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 abnormali-
`ties15-18; 2) multi-lineage dysplasia is now recognized to
`occur in the majority of AML cases presenting clinically
`as “de novo” disease in older individuals7,19-21; 3) AML
`arising in older individuals and primary, secondary, or
`therapy-induced MDS are now known to share strikingly
`similar biologic and genetic features7-13; 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 ante-
`cedent to the development of AML.22-25 These more re-
`cent clinical and biologic studies indicate that disorders
`previously classified as MDS are part of the same dis-
`ease continuum as AML and that MDS is best consid-
`ered 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 distinc-
`tion of AML from MDS for the purposes of disease clas-
`sification 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 fea-
`tures, a blast cell percentage < 20%, and a t(8;21)-con-
`taining clonal population of cells? While the threshold
`of 30% blasts used by the FAB Classification to distin-
`guish AML from MDS (Table 1) is clearly arbitrary, a
`reduction in this threshold to 20% blasts and the result-
`ant 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 fea-
`tures and therapeutic responsiveness.26-28 Although not
`directly tested in a randomized fashion, in several, if not
`all, studies RAEB-T patients appear to have a worse re-
`sponse to intensive chemotherapy when compared his-
`torically to AML cases with similar biologic and cytoge-
`netic 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 re-
`stricted to patients who have abnormalities solely involv-
`ing 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 con-
`tinue until our knowledge has increased to the degree
`that disease classification systems can be developed on
`clinical features, genetics/genomics, and functional biol-
`ogy. And as our knowledge continues to evolve, classifica-
`tion 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 Inter-
`national 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 per-
`centage, 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 clini-
`cal 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 vari-
`ous isoforms of the X-linked gene G6PD and more re-
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`cently by molecular methods that detect non-random pat-
`terns 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 exist-
`ence 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 consid-
`ered the “primary” cause of disease, are actually “sec-
`ondary” cytogenetic abnormalities that arise due to cyto-
`genetically undetectable initiating lesions in a clonal he-
`matopoietic stem cell population.30 Such initiating events
`are likely to be heterogeneous and could either be inher-
`ited or result from acquired somatic DNA damage, ge-
`nomic 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 pri-
`marily 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 re-
`lated 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 re-
`pair or neurofibromatosis-type I33-37; 2) genetic mapping
`studies in the rare families with “familial” MDS and
`AML38-41; and 3) studies of the association of various
`genetic polymorphisms with AML and MDS.42-48
`
`An inherited genetic predisposition to MDS
`Support for an inherited predisposition to MDS and re-
`lated AML has long been evident from studies of inher-
`ited 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.33-37 Indeed, recent studies indicate that up to 30%
`of children affected by MDS and related myeloprolifera-
`tive disorders have an inherited constitutional genetic
`
`Figure 1. Proposed model for genetic progression of MDS to AML.
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`disorder.34 The original studies by Shannon and col-
`leagues35,36 of the genetic basis of familial MDS with chro-
`mosome 7q abnormalities (the “monosomy 7 syndrome”)
`are important in that they first revealed that abnormali-
`ties 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, second-
`ary 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 dis-
`order with leukemia (FPD/AML), an autosomal-domi-
`nant congenital thrombocytopenia characterized by plate-
`let 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 de-
`letions of a single AML1 allele were found in these dif-
`ferent 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 associ-
`ated 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 unexpect-
`edly, a low percentage of sporadic AML, ALL, and CML
`cases (5–8%) have recently been reported to have simi-
`lar 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 rep-
`resent AML and MDS cases that have arisen in individu-
`als with inherited AML1 mutations is as yet unknown.
`Given the functional role and association of CBF␤ (map-
`ping to chromosome 16q22) with AML1 in normal and
`neoplastic hematopoiesis,51 it is tempting to further specu-
`late that CBF␤ might be the causative gene for the sec-
`ond predisposition locus in AML and MDS in those fa-
`milial 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 pre-
`dispose to the acquisition of secondary cytogenetic ab-
`normalities and MDS, it is likely that such inherited ge-
`netic 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 demon-
`strated associations between MDS and smoking, expo-
`sure to chemical compounds (particularly petroleum prod-
`ucts 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 ge-
`netic predisposition to MDS involving naturally occurring
`DNA polymorphisms in genes that mediate DNA repair
`and metabolize environmental carcinogens.42-48 These stud-
`ies are leading to a model, also diagrammed in Figure 1,
`in which MDS arises as a result of cumulative environ-
`mental exposures in genetically predisposed individuals.
`In this case, the genetic predisposition is a more com-
`plex genetic trait: a constellation of genetic variants in
`critical polymorphic genes. The initial attempts to dis-
`sect this complex genetic predisposition have focused on
`the association of MDS with naturally occurring poly-
`morphisms in genes that mediate carcinogen metabo-
`lism.42-48 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 malignan-
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`cies 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 in-
`creased frequency of the NQ01 609C➝ T 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-func-
`tional 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 communica-
`tion) have failed to demonstrate an association of this
`NQO1 polymorphism with abnormalities of chromosome
`5 and 7, but have instead demonstrated strong associa-
`tions with balanced translocations and inversions, par-
`ticularly involving MLL and chromosome 11q23. While
`it is tempting to speculate that the NQO1 609C➝ T poly-
`morphism could predispose to the development of MDS,
`no such studies focusing on MDS cases have been re-
`ported; our own limited studies of 120 primary MDS cases
`have failed to reveal such an association. Similar com-
`plexities 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).46-48 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 ini-
`tial observations and actually reported that the incidence
`of the GSTT1 null genotype tended to be higher in unex-
`posed MDS patients and controls. Thus, while the hy-
`pothesis and model that MDS arises due to cumulative
`environmental exposures in genetically predisposed in-
`dividuals 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 vari-
`ants in many human genes in a large number of affected
`individuals and controls and carefully monitor environ-
`mental 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 hy-
`pothesis that the cytogenetic abnormalities traditionally
`associated with MDS (involving chromosomes 7q, 5q,
`20q11-12, trisomy 8, 12p, and 3q) are “secondary” ge-
`netic events. However, these secondary cytogenetic ab-
`
`normalities are likely no less critical for disease progres-
`sion, 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 cy-
`togenetic and molecular mapping studies reveal that re-
`arrangements 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).54-61 One issue with many map-
`ping studies is that in most instances little attention was
`paid to “phenotype” rather than “genotype.” In other
`words, patient samples were selected for molecular stud-
`ies based on the presence of a specific cytogenetic ab-
`normality (such as a 5q- or a 7q- with or without addi-
`tional cytogenetic abnormalities) without regard to the
`specific form of disease or mode of disease presentation
`(primary MDS, secondary AML, de novo AML, or t-
`AML/MDS). Both cytogenetic and molecular genetic
`studies are not only revealing the tremendous heteroge-
`neity 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 re-
`gion. 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 iden-
`tified 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 actu-
`ally 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 iso-
`lated “5q- syndrome,” a clinically distinct form of RA
`associated with more indolent disease and a lower rate of
`progression to AML, chronic macrocytic anemia, throm-
`bocytosis, and dysplastic megakaryocytes. Interestingly,
`
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`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 epige-
`netic abnormalities such as aberrant cytokine production,
`altered stem cell adhesion, or an abnormal marrow mi-
`croenvironment 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 iden-
`tify the cellular origin of particular cytokines, abnormali-
`ties and elevations in tumor necrosis factor-␣ (TNF␣),
`transforming growth factor-␤ (TGF␤), and interleukin-
`1␤ (IL-1␤) have all been reported.2 In particular, an over-
`abundance of IL-1␤ and a relative lack of its antagonist
`IL-1␤(ra) have been hypothesized to support the clonal
`expansion of aberrant hematopoietic stem cells.2
`Several recent studies have revealed that in its early
`stages, MDS is characterized by accelerated apoptosis of
`hematopoietic progenitor cells.71-72 While some contro-
`versy remains, most studies of MDS samples using vari-
`ous techniques have demonstrated a lowered apoptotic
`threshold of marrow CD34+ cells to TNF-␣, interferon-
`␥ (IFN-␥), and anti-FAS antibodies.70,71 Excessive apopto-
`sis is an attractive explanation for how a clonal expan-
`sion of marrow progenitor cells could result in ineffec-
`tive 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
`
`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 iden-
`tified in this distinct region.57
`In light of the complexity of this cytogenetic and mo-
`lecular 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 chro-
`mosome 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 rela-
`tively large regions is responsible for disease progres-
`sion, 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