How I Diagnose Low-grade Myelodysplastic Syndromes

Alexa J. Siddon, MD; Robert P. Hasserjian, MD


Am J Clin Pathol. 2020;154(1):5-14. 

In This Article

Defining Myelodysplastic Syndromes

MDS are characterized by a combination of morphologic dysplasia of the various cell lines and ineffective hematopoiesis, leading to peripheral blood cytopenias. MDS have heterogeneous clinical behavior and many have a significant risk of transformation to acute myeloid leukemia (AML). The diagnosis of MDS first began to be defined by the 1982 French-American-British cooperative working group, in which five categories of MDS were proposed based entirely on morphologic criteria, including the bone marrow blast count.[1] Since then, the definitions have been modified and several subcategories have been added in the subsequent World Health Organization (WHO) classifications. In the current revised fourth edition, the WHO[2] describes the most up-to-date approach to the diagnosis of MDS, and here we discuss some of the diagnostic challenges and subtleties with a focus on low-grade MDS. Figure 1 presents a diagnostic algorithm for low-grade MDS based on the morphology and genetic testing results, assuming that the clinical situation has been taken into account and other causes of dysplasia considered. Specifically, low-grade MDS can be an especially difficult diagnosis to make, due to the differential diagnosis with the myriad nonneoplastic causes of cytopenia; interval rebiopsy of the patient may be required to establish a definitive diagnosis of MDS. It is particularly important to recognize that the presence of morphologic dysplasia does not necessarily equal a diagnosis of MDS. This article focuses on the diagnosis and differential diagnosis of MDS in adults and does not include specific details about pediatric MDS.

Figure 1.

Diagnostic algorithm for the classification of low-grade myelodysplastic syndromes (MDS) (without excess blasts). aOvert dysplasia constitutes at least 20% of erythroids/granulocytes and at least 30% of megakaryocytes showing dysplastic changes; any true micromegakaryocytes (size of a promyelocyte or less) would also constitute overt dysplasia.10 bIf there are 1% peripheral blood blasts documented on at least two occasions, a diagnosis of MDS unclassifiable is warranted. If there is an isolated del(5q) cytogenetic abnormality and other features are fulfilled, a diagnosis of MDS with isolated del(5q) is warranted. cBorderline dysplasia indicates that the dysplasia is close to 10% of the lineage. dCaution should be exercised if the karyotype is normal and no mutations are identified, or only a single clonal hematopoiesis of indeterminate potential–type mutation at low variant allele fraction is identified; a descriptive diagnosis with recommendation to repeat the marrow examination at a later date may be indicated. eIf there is pancytopenia and single-lineage dysplasia, a diagnosis of MDS unclassifiable is warranted. MDS-MLD, MDS with multilineage dysplasia; MDS-RS, MDS with ring sideroblasts; MDS-SLD, MDS with single-lineage dysplasia; MDS del(5q), MDS with isolated del(5q).

Clinical Features

The reported incidence of MDS in the United States is approximately 4 per 100,000 and peaks in the eighth decade. Patients with MDS generally present with varying symptomatology due to cytopenias. At least one cytopenia is necessary for the diagnosis of MDS, although more than one is not uncommon. Anemia is the most common cytopenia, seen in up to 85% of cases, with or without concomitant thrombocytopenia and/or neutropenia.[3,4] Pancytopenia in MDS is less common, with only 15% of patients presenting with this finding Table 1.[4] As anemia is the most common presentation, symptomatic patients generally experience fatigue, pallor, and/or weakness; bleeding can result from patients with thrombocytopenia.[2,6] Although the International Prognostic Scoring System[5,7] gives suggested prognostic cutoffs for cytopenias, a diagnosis of MDS may still be made with milder levels of cytopenia in patients with characteristic morphologic and/or cytogenetic findings.[2,8]

Morphology and Immunohistochemistry

In the workup of a cytopenic patient for possible MDS, the evaluation generally starts with examination of a peripheral blood smear. The peripheral smear findings of low-grade MDS vary from isolated anemia (commonly macrocytic) with unremarkable RBC morphology to significant RBC anisopoikilocytosis and basophilic stippling. If abnormal leukocyte morphology exists, it generally manifests as hyposegmented neutrophils (pseudo-Pelger-Huët cells), with or without cytoplasmic hypogranularity. Thrombocytopenia may be seen, with either normal-appearing platelets or hypogranular forms (Table 1).

The core biopsy histology of low-grade MDS generally exhibits hypercellularity relative to the patient's age.[9,10] While hypocellular marrows can be seen in a minority of MDS, this is more frequently seen in pediatric MDS and in MDS following cytotoxic therapy (therapy-related myeloid neoplasm). In normal marrow, blasts are not increased, and the few blasts present are mostly close to the bone trabeculae, while in MDS, there may be abnormal localization of immature precursors,[11] in which clusters of blasts are seen away from bone trabeculae. Erythroid and myeloid lineage dysplasia is evaluated mainly on the bone marrow aspirate, while megakaryocytes can exhibit more obvious dysplasia on either the aspirate or the core biopsy specimen. In cases with a hemodiluted or inadequate aspirate, a definitive diagnosis of low-grade MDS based on morphology is difficult and should be made with caution. In the assessment of dysplasia, at least 10% of a lineage should be dysplastic to diagnose MDS.[2] However, it must be recognized that there are a wide variety of causes of secondary morphologic dysplasia, including infections, medications, nutritional deficiencies, toxins, bone marrow lymphomas and plasma cell neoplasms, and autoimmune diseases, and this list continues to grow Table 2.[12] Possible secondary causes of cytopenia and dysplasia should be excluded prior to making a definitive diagnosis of MDS, through careful clinical examination of the clinical history and laboratory parameters.

Erythroid dysplasia has a wide spectrum of findings, including megaloblastoid change, nuclear budding, cytoplasmic bridging, multinucleation, and nuclear/cytoplasmic dysynchrony (megaloblastoid change, mimicking the erythroid findings seen in megaloblastic anemia). Furthermore, an iron stain (ideally performed on a well-spiculated aspirate smear) must be assessed for the presence of ring sideroblasts.[10] Similar to the findings that can be seen in peripheral blood, the aspirate assessment for granulocytic dysplasia most frequently shows nuclear hypolobation and/or a subset of neutrophils with cytoplasmic hypogranularity. Dysplastic megakaryocytes are usually smaller than normal and have hypolobated or nonlobated nuclei, or separated nuclear lobes. Occasionally, if the megakaryocytes are particularly small (micromegkaryocytes), they may be difficult to visualize on the core biopsy specimen, and a CD61 stain can be helpful in their identification. Additional stains that should be considered are reticulin and CD34, as both increased fibrosis and clustering of CD34-positive blasts have been shown to portend a worse prognosis in MDS, including in patients without excess blasts.[13] While there is significant interobserver variability in diagnosing MDS in general, the interobserver variability is highest among low-grade MDS compared with MDS with increased blasts.[14,15] Finally, it is very important to recognize that in some low-grade MDS cases, little to no overt morphologic dysplasia may be present. In MDS with ring sideroblasts (while the ring sideroblasts themselves constitute dysplastic erythroid cells), only minimal dyserythropoiesis may be visualized on the Wright-Giemsa–stained aspirate smear. In a subset of MDS-unclassifiable cases, significant morphologic dysplasia is absent, and the diagnosis is made by identifying an MDS-defining cytogenetic abnormality.

Flow Cytometric Immunophenotyping

Immunophenotyping by flow cytometry serves as a useful adjunct for the diagnosis of MDS, particularly low-grade cases that may show subtle morphologic abnormalities. Although cytomorphology of the aspirate smear remains the definitive means of assessing blast percentage, flow cytometry provides valuable information about the overall immunophenotype of the blasts, such as aberrant expression of CD5, CD7, CD19, and/or CD56.[16,17] In addition, both myeloblasts and monocytic precursors have been shown to have aberrant expression of CD45, CD34, CD117, HLA-DR, CD13, and CD33, among others.[17] The European LeukemiaNet Working group suggests a minimum panel of flow markers that can be used for the marrow assessment of MDS that evaluates myeloid progenitors and erythroid, neutrophil, and monocyte components for signs of aberrancy.[17] It is recommended that in patients without significant morphologic dysplasia or cytogenetic findings diagnostic of MDS, finding three or more flow cytometric abnormalities warrants an interval repeat marrow analysis for further evaluation, as it raises strong suspicion of evolving MDS. In addition, it has been shown that multiparameter flow cytometry can be helpful in excluding MDS in patients with indeterminate morphology[18] and in distinguishing MDS from benign mimics.[19–21] In patients who have an established diagnosis of MDS, Alhan et al[22] showed that an MDS flow cytometric prognostic score can be applied that incorporates side scatter, CD117 expression on myeloid progenitors, and CD13 expression of monocytes. However, some overlap in the immunophenotypic profiles of MDS and non-MDS reactive conditions (particularly in maturing granulocytes) and a lack of consensus on which panels and criteria to use have limited the widespread application of flow cytometry to the diagnosis of MDS.[23]

Conventional Cytogenetics

Conventional karyotyping is a cornerstone for both diagnosing MDS and assessing their prognosis. Specifically, certain specific karyotype abnormalities can establish clonality and allow a diagnosis of MDS in the absence of definitive morphologic dysplasia.[2] However, some of the most common cytogenetic abnormalities found in MDS, −Y, +8, and del(20q), also can be found in some nonneoplastic conditions such as aplastic anemia and are not considered diagnostic of MDS in the absence of sufficient morphologic dysplasia. Fluorescence in situ hybridization (FISH) studies assessing for the most common MDS-associated abnormalities can help confirm suspected karyotype abnormalities and allow for increased sensitivity in follow-up evaluations with known abnormalities. However, FISH studies are not necessary if 20 normal metaphases are obtained by conventional karyotyping.[24] Chromosomal abnormalities are found in approximately 50% of newly diagnosed MDS, with many cases exhibiting more than one abnormality; however, a normal karyotype is more common in some of the low-risk MDS subtypes, such as MDS with ring sideroblasts and MDS with single-lineage dysplasia. The only distinct WHO category based on a karyotype abnormality is MDS with isolated del(5q). This low-grade MDS subtype generally presents with anemia and thrombocytosis, an unusual combination in most other types of MDS. The bone marrow characteristically has abundant small, hypolobated megakaryocytes and no increase in blasts.[2] MDS with isolated del(5q) can be diagnosed with a single additional karyotypic abnormality, except for monosomy 7 or del(7q) (Figure 1).[2] Otherwise, the karyotype is mainly used to stratify patients with MDS into prognostic subgroups using the revised International Prognostic Scoring System[5,7] to estimate the median survival and risk of evolution to AML.[5,7,25]

Molecular Genetic Findings

In addition to conventional cytogenetics, high-throughput sequencing technologies have led to an explosion of new information about acquired DNA variants in myeloid neoplasms. Specifically, rapidly increasing access to NGS (also called massively parallel sequencing) has contributed to the body of literature about molecular changes within myeloid neoplasms. While not currently used for diagnosis alone, DNA variants can help guide knowledge about pathogenesis, prognosis, and treatment in MDS. A relatively small number of genes (approximately 40) are mutated in up to 90% of patients with MDS; thus, whole-genome or whole-exome sequencing typically is not employed in clinical NGS panels. The variants affect genes involved in RNA splicing, transcription, DNA methylation, chromatin modification, signal transduction, and DNA repair. Identifying these somatic variants can be particularly useful to prove clonality in cases of cytogenetically normal suspected MDS.

Only one variant is currently used to help classify one of the low-grade MDS subtypes—SF3B1. In the workup of MDS, if anemia is accompanied by erythroid dysplasia and 5% or more ring sideroblasts are identified, in conjunction with a pathogenic SF3B1 variant, a diagnosis of MDS with ring sideroblasts can be made (Figure 1);[2] in the absence of an SF3B1 mutation, at least 15% ring sideroblasts would be required. SF3B1-mutated cases of low-grade MDS have a particularly favorable prognosis.[26,27]

It is also important to note that many of the most common somatic DNA variants in MDS also can be found in healthy patients without overt evidence of MDS. In clonal hematopoiesis of indeterminate potential (CHIP), DNA variants (at a variant allele fraction [VAF] of at least 2%) can be identified in the peripheral blood or bone marrow of older individuals without cytopenias or other evidence of MDS. The rate of progression to MDS in patients with CHIP is approximately 1% to 2% per year. The most common genes seen in CHIP are DNMT3A, ASXL1, and TET2.[28] Additional genes less frequently involved are SF3B1, JAK2, TP53, BCORL1, and GNAS.[28] Clonal cytopenia of undetermined significance (CCUS) is similar to CHIP in the presence of somatic DNA variants, but these patients also manifest at least one unexplained peripheral blood cytopenia. In both CHIP and CCUS, examination of the bone marrow must lack sufficient morphologic evidence for a diagnosis of MDS and there must be no MDS-defining cytogenetic abnormality.[28] The risk of CCUS progression to MDS appears to be highest in patients with high mutated VAF (at least 10%) and with a DNMT3A, ASXL1, or TET2 variant in combination with at least one additional variant or spliceosome mutations such as SF3B1, U2AF1, SRSF2, or ZRSR2.[29]