This review highlights the main changes in the revised 2016 WHO Classification of Myeloid Neoplasms (published in 2017) that impact the diagnosis and management of patients with myelodysplastic syndrome (MDS). The revision was based on data accumulated since the 2008 WHO classification of MDS, much of which relates to new molecular genetic information about these neoplasms. The new information has led to some reorganization of the MDS disease categories, including a broadening of the subset of cases classified as MDS with ring sideroblasts, many of which have mutations in the spliceosome gene SF3B1. Other revisions have refined the definitions of some disease categories to improve disease risk stratification. The revised categories in the new classification ensure that MDS patients receive risk-adapted therapies based on the most recently available data.

Myelodysplastic syndromes (MDS) consist of clonal bone marrow diseases associated with ineffective hematopoiesis, manifesting as morphologic dysplasia in hemopoietic elements and peripheral cytopenias. The recently published 2016 (revised 4th edition) World Health Organization (WHO) Classification of Myeloid Neoplasms [1, 2] includes enhanced diagnostic information for many of the MDS subtypes from the 2008 (4th edition) WHO classification as well as some reorganization of the disease categories. This review highlights the main changes in the new WHO classification that impact the diagnosis of MDS.

Accurate diagnosis of MDS requires high-quality material for morphologic analysis, including review of a peripheral blood (PB) smear, a representative bone marrow aspirate (as well as an iron stain on the aspirate smear), and an adequate bone marrow biopsy. The diagnostician must also have access to ancillary testing results, including a full conventional bone marrow karyotype, relevant molecular genetic testing, flow cytometry immunophenotyping, and PB counts, as well as other clinical information such as any prior exposure to cytotoxic therapies. The 3 “pillars” that support a diagnosis of MDS are: 1. persistent and clinically unexplained cytopenia (an absolute requirement); 2. significant morphologic dysplasia of hematopoietic elements (with the exception of cases bearing certain qualifying cytogenetic aberrations); and 3. cytogenetic and/or molecular genetic evidence of clonal hematopoiesis (not required, but helpful adjunctive information).

In addition to establishing a primary diagnosis of MDS, a detailed assessment of blood and bone marrow morphology is essential to classify the disease, and provides critical information in patient risk stratification; a precise myeloblast percentage in blood and bone marrow is particularly important. Flow cytometry immunophenotyping can be helpful in supporting a diagnosis of MDS (discussed in another review article in this issue). Cytogenetic assessment for the del(5q) cytogenetic abnormality impacts MDS classification, and the overall karyotype holds important prognostic value used for risk stratification of patients and ultimately impacting therapeutic approaches to individual patients. A complete bone marrow karyotype should be sent whenever a bone marrow sample is taken for suspected MDS.

Finally, due to the rapid recent advances in next-generation sequencing (NGS) technologies, there has been a massive influx of data concerning the significance of mutations in the diagnosis and prognostication of MDS. Although the 2016 classification now incorporates the use of a single mutation, SF3B1, in the definition of a single MDS subtype, MDS with ring sideroblasts, there are still several challenges when applying NGS mutation data to a diagnosis. Many myeloid-associated somatic mutations have been identified in normal healthy elderly individuals [3, 4], and it may be difficult to determine if an identified mutation indicates MDS or is an incidental finding in the older patient population that is typically affected by MDS. As additional data accumulate, particular mutation patterns may, in the future, help to establish a primary diagnosis of MDS in cytopenic patients, just as certain specific cytogenetic abnormalities do at the present time [5].

Cytopenia

Some degree of cytopenia is a prerequisite for the diagnosis of MDS. Although most patients presenting with MDS have cytopenias below the cytopenia thresholds of 1.8 × 109/L for absolute neutrophil count, 10 g/dL for hemoglobin, and 100 × 109/L for platelets (derived from the original 1997 International Prognostic Scoring System [IPSS] [6]), a diagnosis of MDS can be made in patients with milder cytopenias, provided that other definitive features (clear-cut dysplasia and/or a defining cytogenetic abnormality) are present. In such cases, the cytopenia(s) should be below each individual laboratory’s reference range for hemoglobin, absolute neutrophil count, and platelets, and possible secondary causes of the cytopenia(s) should always be excluded. Regardless of the level of cytopenia(s), non-MDS etiologies should be carefully considered prior to rendering a diagnosis of MDS. These include non-MDS neoplasms (particularly hairy-cell leukemia and large granular lymphocytic leukemia) as well as nonneoplastic conditions such as metabolic deficiencies, hemolysis, immune thrombocytopenia, other autoimmune diseases, and the effects of drugs, alcohol intake, and infections.

Morphologic Dysplasia

Although not pathognomonic for MDS, dysplastic morphology remains a critical feature in establishing the diagnosis. Experienced observers can generally agree on the presence of overt morphologic dysplasia [7]; however, there is inherent subjectivity to interpreting and quantifying dysplasia, leading to significant interobserver variability in assessing milder or borderline degrees [8]. The 2016 WHO classification recommends that at least 10% of cells in a lineage demonstrate dysplastic features to be considered significant [9]. However, dysplastic features involving > 10% of a hematopoietic lineage are frequently seen in patients with reactive secondary cytopenia [7, 10-12]. Thus, even if significant morphologic dysplastic changes are identified in a cytopenic patient, the possible underlying secondary causes of cytopenia and hematopoietic dysplasia must be carefully excluded prior to rendering a diagnosis of MDS. Interestingly, the type of dysplasia does not always agree with the type of cytopenia in MDS [13-15]. For these reasons, the MDS nomenclature was changed to remove the reference to cytopenia from the disease names (e.g., “refractory anemia with excess blasts” in prior WHO classifications).

The blast threshold for separating MDS from acute myeloid leukemia (AML) is now always 20%, with the exception of cases bearing the AML-defining cytogenet ic abnormalities PML-RARA, inv(16)/t(16; 16);CBFB-MYH11, or t(8; 21);RUNX1-RUNXT1. The rule in the previous 2008 WHO classification that excluded erythroid precursors from the denominator when erythroid precursors exceeded 50% of marrow cells, allowing a diagnosis of AML (acute erythroid leukemia of the erythroid/myeloid subtype) based on the nonerythroid blast percentage, has been eliminated; these cases have clinical, pathologic, and genetic features that are more akin to MDS than to AML [1, 16]. Thus, stratification of myeloid neoplasms into entities lacking excess blasts, MDS with excess blasts-1, MDS with excess blasts-2, and AML is always based on myeloid blasts counted as a percentage of all nucleated marrow cells, irrespective of whether or not there is a prominent erythroid proliferation. The diagnostician must be careful to accurately identify the myeloid blasts and not erroneously include pronormoblasts; the latter may have a similar size and immature chromatin to myeloid blasts but have round nuclei and deeply basophilic cytoplasm.

Genetic Evidence of Clonality

Conventional karyotyping should be performed in all MDS cases at diagnosis, as cytogenetic abnormalities are seen in 50–60% of patients and can help in establishing clonality; > 90% of therapy-related MDS cases show an abnormal karyotype [17]. According to the 2016 WHO classification, the presence of certain MDS-associated cytogenetic abnormalities, the most common being –7, del(7q), del(5q), and i(17q), is sufficient to confirm a diagnosis of MDS in a cytopenic patient, even if significant dysplasia is lacking [2]. However, some of the most common cytogenetic abnormalities seen in MDS, i.e., del(20q), +8, and –Y, are specifically excluded from this list, as they can be seen in normal aged individuals or in patients with non-MDS causes of cytopenia [18-20].

A large amount of data is now available regarding NGS findings in MDS, with several commonly recurring mutations. Targeted sequencing of a limited number of genes can detect mutations in up to 90% of MDS patients, most commonly involving splicing factors and epigenetic modifiers. However, acquired mutations in many MDS-associated genes are also found in normal aging individuals, and the natural history of such “clonal hematopoiesis of indeterminate potential” (CHIP) is still not fully understood (discussed further in another article in this issue). For these reasons, in the current 2016 WHO classification, the presence of genetic mutations cannot be used to make a diagnosis of MDS in the absence of other diagnostic features, even in a cytopenic patient.

A major change in the disease hierarchy involving MDS is the reorganization of MDS with ring sideroblasts into a single disease group (MDS-RS), with or without multilineage dysplasia. This is due to the remarkable recent discovery of mutations in the spliceosome gene SF3B1 that are associated with ring sideroblasts, thus solving the enigma of the association of ring sideroblasts with MDS [21-24]. The shared biology and generally favorable prognosis associated with this mutation has led to this expanded category of MDS-RS [25]. In the 2016 WHO classification, a diagnosis of MDS-RS can be made in the presence of an SF3B1 mutation with at least 5% ring sideroblasts, while 15% ring sideroblasts are required in the absence of an SF3B1 mutation or if the mutation status is unknown.

MDS-RS includes cases with single-lineage dysplasia (MDS-RS-SLD) and multilineage dysplasia (MDS-RS-MLD), considered subtypes of this MDS category. By definition, the erythroid lineage is dysplastic in all MDS-RS cases, since ring sideroblasts constitute a form of erythroid dysplasia. Thus, any additional (nonerythroid) dysplastic lineages mandate classification as MDS-RS-MLD, which has an inferior prognosis to MDS-RS-SLD [26-28]. It is critical to note that excess blood or bone marrow blasts, Auer rods, or features consistent with MDS with isolated del(5q) supercede the presence of ring sideroblasts or the SF3B1 mutation, and the presence of any such factors would exclude a case from the MDS-RS category. Cases with pancytopenia are excluded from the MDS-RS-SLD category, and cases with 1% PB blasts are also excluded from MDS-RS; such cases are put in the MDS-U category since they appear to have more aggressive behavior. Cases with 1% blood blasts have a prognosis that is akin to MDS with excess blasts [29]. Of note, the new classification mandates that this 1% PB blast measurement be documented on at least 2 separate occasions, to ensure that this is not a transient phenomenon. The disease names and changes in the 2016 WHO Classification are shown in Table 1. Figure 1 illustrates (in a schematic form) the reallocation of cases in the new classification based on the new diagnostic criteria.

Table 1.

Myelodysplastic syndrome entities in the 2016 WHO classification and major changes since the 2008 WHO classification

Myelodysplastic syndrome entities in the 2016 WHO classification and major changes since the 2008 WHO classification
Myelodysplastic syndrome entities in the 2016 WHO classification and major changes since the 2008 WHO classification
Fig. 1.

Changes in nomenclature and case allocation between the original (2008) and revised (2016) 4th editions of the WHO Classification of Myelodysplastic Syndrome. a Allocation of MDS cases without excess blasts. RCUD, refractory cytopenia with unilineage dysplasia; RARS, refractory anemia with ring sideroblasts; RCMD, refractory cytopenia with multilineage dysplasia. b Allocation of MDS cases with excess blasts (EB) versus acute myeloid leukemia (AML). AEL, acute erythroid leukemia, erythroid/myeloid subtype; RAEB, refractory anemia with excess blasts. a, b RS, ring sideroblasts; PB, peripheral blood; other disease abbreviations as defined in Table 1.

Fig. 1.

Changes in nomenclature and case allocation between the original (2008) and revised (2016) 4th editions of the WHO Classification of Myelodysplastic Syndrome. a Allocation of MDS cases without excess blasts. RCUD, refractory cytopenia with unilineage dysplasia; RARS, refractory anemia with ring sideroblasts; RCMD, refractory cytopenia with multilineage dysplasia. b Allocation of MDS cases with excess blasts (EB) versus acute myeloid leukemia (AML). AEL, acute erythroid leukemia, erythroid/myeloid subtype; RAEB, refractory anemia with excess blasts. a, b RS, ring sideroblasts; PB, peripheral blood; other disease abbreviations as defined in Table 1.

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The only cytogenetic abnormality which defines a specific MDS subtype in the 2016 WHO classification is an isolated del(5q) abnormality, reflecting its strong association with a particular disease phenotype and biology, responsiveness to a specific therapy (lenalidomide), and favorable prognosis [30]. Recent studies have shown that the del(5q) abnormality in MDS is prognostically similar whether it is isolated or occurs with one additional low-risk cytogenetic aberration [31, 32], which has led to an expansion of MDS with isolated del(5q) to include cases with one additional cytogenetic abnormality, with the exception of monosomy 7 or del(7q).

Regarding MDS in pediatric patients, studies have shown that refractory cytopenia of childhood (RCC) has reproducible morphology and can be accurately distinguished from severe aplastic anemia [33]. However, there is considerable clinical and genetic heterogeneity in RCC and a potential overlap with inherited bone marrow failure syndromes such as Fanconi anemia [34]. Because of the difficulty in distinguishing RCC from other inherited and acquired bone marrow failure syndromes, RCC remains as a provisional entity in the updated classification, as it was in the original (2008) 4th edition classification. Many cases of pediatric MDS (and a subset of adult MDS cases that is probably currently underrecognized) arise in the setting of a germline predisposition condition. The diagnostician should be aware of this possibility and consider the possibility of an inherited predisposing condition when approaching any MDS case (germline predispositions to myeloid neoplasms are discussed in a separate article in this issue). If present, the germline condition should be appended to the MDS disease subcategory in the diagnosis. Figures 2, 3 illustrate algorithms to approach the diagnosis of MDS cases without excess blood or bone marrow blasts.

Fig. 2.

Diagnostic algorithm for myelodysplastic syndrome (MDS) cases without excess blasts and with single-lineage dysplasia. * Except –7 or del(7q). BM, bone marrow; PB, peripheral blood, ANC, absolute neutrophil count; HGB, hemoglobin; PLT, platelet count; RS, ring sideroblasts; other disease abbreviations as defined in Table 1.

Fig. 2.

Diagnostic algorithm for myelodysplastic syndrome (MDS) cases without excess blasts and with single-lineage dysplasia. * Except –7 or del(7q). BM, bone marrow; PB, peripheral blood, ANC, absolute neutrophil count; HGB, hemoglobin; PLT, platelet count; RS, ring sideroblasts; other disease abbreviations as defined in Table 1.

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Fig. 3.

Diagnostic algorithm for myelodysplastic syndrome (MDS) cases without excess blasts and with multilineage (2–3 hematopoietic lineages) dysplasia. * Except –7 or del(7q). BM, bone marrow; PB, peripheral blood, ANC, absolute neutrophil count; HGB, hemoglobin; PLT, platelet count; RS, ring sideroblasts; other disease abbreviations as defined in Table 1.

Fig. 3.

Diagnostic algorithm for myelodysplastic syndrome (MDS) cases without excess blasts and with multilineage (2–3 hematopoietic lineages) dysplasia. * Except –7 or del(7q). BM, bone marrow; PB, peripheral blood, ANC, absolute neutrophil count; HGB, hemoglobin; PLT, platelet count; RS, ring sideroblasts; other disease abbreviations as defined in Table 1.

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Beyond classifying MDS into one of the defined WHO categories, the pathologist should optimally provide any relevant morphologic and genetic information for additional prognostic guidance. The blast percentage in MDS remains a critical variable that should be assessed carefully in both blood and bone marrow in every case. The 2012 revised IPSS (IPSS-R) [35] introduced an additional stratum of 2% for bone marrow blast count, such that low blast-count MDS cases (< 5% blasts) are now divided into 2 groups [35]. While the 2016 WHO classification does not subdivide cases with < 5% blasts based on the bone marrow blast percentage, the diagnostician should provide the precise bone marrow blast percentage (i.e., not merely state “< 5% blasts”) so that the IPSS-R schema can be applied.

Current studies are focusing on applying mutational profiles to specific clinical situations or disease subtypes. The TP53 mutation is associated with complex karyotype, therapy-related disease, and adverse prognosis in all MDS disease subtypes. Specifically, TP53 predicts a worse prognosis in MDS with isolated del(5q), a subgroup that is otherwise associated with a favorable outcome [36, 37]. For these reasons, the 2016 WHO classification recommends that TP53 mutation status be specifically assessed in MDS with isolated del(5q). This mutation is generally assessed as a part of broad NGS panels, but may also be evaluated indirectly by immunohistochemistry: strong immunostaining for p53 protein in > 1% of the bone marrow cells correlates with the presence of a TP53 mutation [38].

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