Pathology of Refractory Anemia With Ring Sideroblasts

Myelodysplastic syndromes (MDS), as defined by the World Health Organization (WHO), represent clonal hematopoietic stem cell disorders resulting in ineffective hematopoiesis. ^[1]

Refractory anemia with ring sideroblasts (RARS), a clonal hematopoietic neoplasm, is a low-grade MDS characterized by anemia, bone marrow erythrodysplasia, and ring sideroblasts; ring sideroblasts represent 15% or more of nucleated erythroid precursors. ^[1] RARS is associated with an increase in the risk of progression to overt acute myeloid leukemia (AML), as well as shortened overall survival, although these risks are not as great as with MDS of higher grades. See the image below.

See Myelodysplastic Syndromes: Classification, Features, Diagnosis, and Treatment Options, a Critical Images slideshow, to help identify, classify, work up, and treat these disorders.

The presence of ring sideroblasts alone is not sufficient for a diagnosis of RARS; complete evaluation of the nonerythroid lineages is necessary. Dysplasia in the granulocytic and megakaryocytic lineages is not present in RARS. Myeloblasts account for fewer than 5% of nucleated bone marrow cells, and Auer rods are absent.

Although the pathophysiologic mechanisms underlying RARS are not entirely elucidated, there is compelling evidence of mitochondrial dysfunction in early stages of erythroid maturation, leading to abnormal mitochondrial iron deposition and ineffective hematopoiesis. ^{[2, 3, 4]}

See Pathology of Refractory Anemia with Excess Blasts, Chronic Anemia, Chronic Pediatric Anemia, and Emergent Management of Acute Anemia for complete information on these topics.

Epidemiology

The epidemiologic features of refractory anemia with ring sideroblasts (RARS) are similar to those of most other types of myelodysplastic syndrome (MDS). The average age of patients at presentation is approximately 65-70 years. MDS, which is uncommon in the pediatric population, is most often associated with inheritable genetic syndromes and rarely presents as RARS.

In contrast with other categories of MDS, there is no sex predilection in RARS. ^[5] Exposure to agricultural chemicals and a history of smoking confer an increased risk of the development of RARS. ^[6]

Clinical features

Clinical features of the syndrome are nonspecific (eg, fatigue) and can be attributed to anemia associated with ineffective erythropoiesis.

The morphologic features of the peripheral blood and bone marrow are currently the gold standard for the diagnosis of myelodysplastic syndrome (MDS).

The peripheral blood shows a macrocytic or normocytic anemia (hemoglobin level < 10 g/dL) with anisocytosis. Although dysplasia is limited to the erythroid precursors, neutropenia or thrombocytopenia may be present.

Most often, the bone marrow is hypercellular with a significant increase in erythroid precursors. At least 10% of the erythroid lineage shows 1 or more of the dysplastic features (listed below). The granulocyte and megakaryocyte maturation is morphologically preserved. Ring sideroblasts (see the images below) represent at least 15% of the nucleated erythroid precursors.

Cases demonstrating Auer rods and/or increased myeloblasts (>5% in bone marrow) are not included in this group of disorders (see Refractory Anemia with Excess Blasts). As with all forms of MDS, other causes of myeloid dyspoiesis should be excluded before this diagnosis is assigned.

Dysplastic features of nucleated erythroid precursors

Cells are disproportionately large, corresponding to the normal stage of development (see the images below). Nuclear atypia includes binucleation and nuclear fragmentation/karyorrhexis. Single or multiple cytoplasmic vacuoles are present; on periodic acid-Schiff (PAS) staining, the cytoplasm demonstrates clumped or block like morphology. In contrast to normally maturing precursors with finely granular cytoplasmic morphology seen with PAS staining, severely dysplastic red cells show clumped cytoplasmic morphology.

Ring sideroblasts, defined as nucleated erythroid precursors with 5 iron granules or fewer surrounding at least one third of the nucleus, represent 15% or more of the erythroid lineage. Ring sideroblasts may be evaluated by use of commonly available iron preparations (eg, Prussian blue stain). Although ring sideroblasts are not distinguishable without appropriate iron stains, increased numbers of hemosiderin-laden macrophages can be appreciated by conventional staining methods (eg, Giemsa staining).

Findings of increased numbers of myeloblasts (>5%), the presence of Auer rods, and dysplasia of nonerythroid progenitors are not consistent with a diagnosis of refractory anemia with ring sideroblasts (RARS); cases with these features are more appropriately classified into other categories of MDS.

Diagnostic pitfalls

Morphologic changes identical to those of bona fide erythrodysplasia may occur as artifacts of poor preservation or inadequate storage of samples during transport; extra caution is advised when evaluating suboptimal specimens. Although not entirely specific for myelodysplasia, the presence of ring sideroblasts is necessary for the diagnosis of RARS; they are less likely to occur as artifacts.

Additionally, the presence of bone marrow hypoplasia does not exclude a diagnosis of this or any other MDS; hypoplastic forms are not uncommon in MDS, occurring in approximately 10% of confirmed cases.

Exclusion of alternative causes of sideroblastic anemia

Common causes of acquired sideroblastic anemias, particularly alcohol- and drug- associated forms, should be excluded before a diagnosis of RARS is issued. The morphologic findings in RARS are not entirely specific, and both dyserythropoiesis and ring sideroblasts may be seen in a variety of acquired and inherited conditions., as listed below.

Acquired factors and conditions that may be associated with ring sideroblasts include the following:

• Nutritional factors - Folate deficiency, vitamin B6 deficiency, copper deficiency

• Alcohol (impairment of the B6-dependent step in porphyrin production)

• Drug toxicities (impairment of the B6-dependent step in porphyrin production) – Isoniazid, chloramphenicol, pyrazinamide, azathioprine

• Heavy metal toxicities – Lead, zinc, arsenic

• Other forms of MDS associated with ring sideroblasts - Refractory cytopenia with multilineage dysplasia (RCMD), refractory anemia with excess blasts (RAEB)

• Myelodysplastic/myeloproliferative neoplasm, unclassifiable - Refractory anemia with ring sideroblasts associated with marked thrombocytosis

Hereditary factors and conditions that may be associated with ring sideroblasts include the following:

• X-linked sideroblastic anemia

• Mitochondrial cytopathies, including Pearson syndrome and Wolfram syndrome

Immunohistochemical studies are not necessary for the diagnosis of refractory anemia with ring sideroblasts (RARS), because in RARS, the morphologic features are paramount.

Nevertheless, staining for CD34, a relatively specific marker of hematopoietic progenitors in bone marrow biopsy sections, may be used to exclude the presence of an increased number of blasts—a finding characteristic of forms of myelodysplastic syndrome (MDS) other than RARS, including refractory anemia with excess blasts (RAEB). However, one should keep in mind that not all myeloblasts express CD34; thus, although an increased percentage of CD34 cells would not be seen in RARS, an absence of CD34+ cells does not exclude a higher-grade MDS.

The use of flow cytometry in the diagnosis of myelodysplasia has progressively evolved from identifying myeloblast populations to recognizing aberrant immunophenotypic patterns of myeloid and erythroid maturation. Because the diagnosis of RARS and other forms of MDS is complicated by the lack of morphologic specificity, there is an ever increasing body of literature supporting the application of flow cytometry to fill diagnostic gaps. ^[7]

Evidence suggests that a limited panel of markers, including glycophorin A, CD71 (transferrin receptor), CD105 (endoglin), cytosolic ferritin subunits, and mitochondrial ferritin, may be used to accurately assess for erythroid dysplasia and ring sideroblasts. ^[8]

Cytogenetic abnormalities are recognized in approximately one third of refractory anemia with ring sideroblasts (RARS) cases. When recognized, the aberrations most often involve a single chromosome or translocation, although none are specific for RARS. ^[9]

Although chromosomal abnormalities are not diagnostic of RARS in isolation, they are prognostically significant. Accordingly, cytogenetic analysis of all bone marrow specimens should be performed.

Malcovati et al performed a comprehensive mutation analysis in 293 patients with myeloid neoplasm and 1% or more ring sideroblasts. The study concluded that SF3B1 mutation identifies a distinct myelodysplastic syndrome subtype that is unlikely to develop detrimental subclonal mutations and is characterized by indolent clinical course and favorable outcome. ^[10]

Refractory anemia with ring sideroblasts (RARS) is considered an indolent form of myelodysplastic syndrome (MDS); it is associated with a prolonged median survival—on the order of 5-10 years—with only 1-2% of cases progressing to acute leukemia. ^{[5, 11, 12]} Among RARS patients, there is multifactorial prognostic heterogeneity, such that patients with additional peripheral cytopenias show shortened survival. ^{[12, 13]}

The widely accepted International Prognostic Scoring System (IPSS) was introduced by the Myelodysplastic Syndrome Working Group in 1997. The IPSS stresses the importance of cytogenetic findings. By use of the IPSS, patients may be successfully stratified on the basis of the percentage of bone marrow myeloblasts, karyotype abnormality, and cytopenia. ^[14]

The IPSS separates patients into 4 risk groups (low, intermediate-1, intermediate-2, and high); the 4 groups represent increasing likelihood of progression to acute leukemia and decreasing survival. Scoring is based on the number and type of cytogenetic abnormality, the percentage of bone marrow blasts, and the number of peripheral blood cytopenias. The majority of patients who have been diagnosed as having RARS on the basis of the World Health Organization (WHO) criteria fall into the low and intermediate-1 risk groups.

Additionally, the IPSS has independent prognostic significance for patients with RARS. ^{[5, 13, 14]} The prognostic value of a recently proposed WHO Prognostic Scoring System (WPSS), which considers the impact of transfusion requirements and morphologic classification, has also been established. ^[15]

The significance and impact of long-term transfusion therapy on patients with RARS has not been well established. Recent data suggest that in patients with RARS, unlike patients with other pathologic states requiring long-term transfusion therapy, increasing numbers of red cell transfusions and an increase in the serum ferritin level do not confer a worse prognosis. ^[16]