## Clinical Overview Refractory anemia with ring sideroblasts (RARS) is a distinct clinical entity within the spectrum of myelodysplastic syndromes (MDS), which are a group of clonal hematopoietic stem cell disorders characterized by ineffective hematopoiesis, peripheral blood cytopenias, and a variable risk of progression to acute myeloid leukemia (AML). RARS is specifically defined by the presence of significant anemia, isolated erythroid dysplasia (in its classic unilineage form), and a characteristic percentage of ring sideroblasts in the bone marrow. These ring sideroblasts are erythroid precursors containing pathological mitochondrial iron deposits that form a visible ring around the nucleus when stained with Prussian blue. ### Pathophysiology and Molecular Genetics The hallmark of RARS is the failure of iron utilization in the heme synthesis pathway within erythroid precursors. A major breakthrough in the understanding of this disease was the discovery of somatic mutations in the SF3B1 gene, which are found in 60% to 90% of RARS cases. SF3B1 encodes a critical component of the RNA spliceosome. Mutations in this gene result in the aberrant splicing of several downstream targets, including ABCB7, a gene that encodes a mitochondrial iron transporter. The resulting downregulation of ABCB7 leads to mitochondrial iron overload and oxidative stress, causing premature apoptosis of erythroid precursors (ineffective erythropoiesis) and the formation of the diagnostic ring sideroblasts. ### Clinical Presentation RARS primarily affects older adults, with the median age at diagnosis being over 70 years. The clinical presentation is typically dominated by signs and symptoms of chronic anemia. Patients usually describe an insidious onset of progressive fatigue, pallor, exertional dyspnea, and decreased exercise tolerance. Because the dysplasia and cytopenias in classic RARS are restricted to the erythroid lineage, patients are less likely to present with complications from neutropenia (such as infections) or thrombocytopenia (such as bruising or bleeding) unless the disease has progressed or is of the multilineage type (RCMD-RS). On physical examination, pallor is a common finding, while splenomegaly occurs in a minority of cases. ### Diagnostic Evaluation The diagnosis of RARS requires an integrated assessment of the peripheral blood and bone marrow. Key laboratory findings include a macrocytic or normocytic anemia and a low reticulocyte count, reflecting ineffective production. The bone marrow aspirate is essential and must demonstrate at least 15% ring sideroblasts among erythroid precursors. However, according to the revised World Health Organization (WHO) and International Consensus Classification (ICC) guidelines, if a somatic SF3B1 mutation is detected, a threshold of only 5% ring sideroblasts is sufficient to establish the diagnosis, provided other criteria for MDS are met. It is also vital to differentiate RARS from secondary causes of ring sideroblasts, including heavy alcohol consumption, copper deficiency, and the use of medications such as isoniazid or linezolid. ### Management and Prognosis RARS is generally categorized as a lower-risk MDS subtype according to the Revised International Prognostic Scoring System (IPSS-R). Patients often have a prolonged clinical course with a low rate of transformation to acute myeloid leukemia. Treatment is focused on alleviating the symptoms of anemia and improving quality of life. Erythropoiesis-stimulating agents (ESAs) are the standard first-line therapy. For patients who are refractory to ESAs or have a high transfusion burden, luspatercept—a TGF-beta ligand trap that promotes late-stage erythroid maturation—has become a standard of care. For those receiving frequent transfusions, iron chelation therapy may be used to prevent secondary hemochromatosis and its associated organ damage.