What is Sideroblastic Anemia
Sideroblastic anemia is a heterogeneous group of inherited and acquired disorders characterized by ineffective erythropoiesis, microcytic hypochromic anemia, and the pathognomonic finding of ring sideroblasts in the bone marrow—abnormal erythroblasts with iron-loaded mitochondria forming a perinuclear ring visible on Prussian blue staining. 1, 2, 3
Pathophysiologic Mechanism
The fundamental defect in all forms of sideroblastic anemia involves disrupted heme biosynthesis or mitochondrial iron metabolism, leading to paradoxical mitochondrial iron accumulation despite inadequate hemoglobin synthesis 4, 5. This creates the characteristic clinical picture of:
- Microcytic hypochromic anemia due to reduced hemoglobin synthesis, with severity ranging from mild in X-linked forms to severe transfusion-dependent anemia in congenital forms like SLC25A38 defects 4, 6
- Systemic iron overload affecting liver, heart, and endocrine organs despite anemia, with increased transferrin saturation and ferritin reflecting iron loading despite impaired erythroid iron utilization 4, 6
- Ineffective erythropoiesis resulting in anemia that cannot be corrected by iron supplementation alone 2, 3
Classification and Genetic Basis
Inherited Forms
X-Linked Sideroblastic Anemia (XLSA) is the most common congenital form, caused by mutations in ALAS2 (erythroid-specific δ-aminolevulinate synthase), which encodes the first enzyme in heme biosynthesis 4, 3, 7. Affected males present with mild-to-moderate microcytic anemia, variable phenotypic expression even with identical mutations, and approximately 40-60% respond to pyridoxine (vitamin B6) supplementation 1, 3.
Autosomal recessive forms include:
- SLC25A38 defects: severe congenital sideroblastic anemia presenting in childhood with transfusion-dependent microcytic hypochromic anemia, marked iron overload, and ring sideroblasts on bone marrow examination 1, 4
- GLRX5 mutations: impair Fe-S cluster biosynthesis, leading to increased cytosolic IRP1 activity, repression of ALAS2, and reduced heme synthesis 1, 4
- ABCB7 defects: cause X-linked sideroblastic anemia with ataxia due to disrupted mitochondrial Fe-S complex export 1
Iron metabolism defects that can mimic or coexist with sideroblastic anemia:
- SLC11A2 (DMT1) mutations: cause defective enterocyte and erythroid iron uptake yet paradoxically lead to systemic iron loading, likely through alternative heme absorption pathways or low hepcidin levels 1, 4
- STEAP3 mutations: impair ferroreductase activity, preventing Fe³⁺ to Fe²⁺ reduction in erythroblast endosomes 1, 4
Acquired Forms
Refractory anemia with ring sideroblasts (RARS) is a myelodysplastic syndrome subtype characterized by 3, 8:
- Indolent clinical course with progressive anemia requiring eventual transfusion dependence
- >90% carry somatic SF3B1 mutations affecting RNA splicing machinery, causing misrecognition of 3' splice sites and multifactorial pathogenesis 3, 8
- Ineffective erythropoiesis as the primary mechanism of anemia 3
Secondary acquired causes include drugs, toxins, copper deficiency, chronic neoplastic disease, and alcohol 8, 5.
Critical Diagnostic Pitfall
The hallmark paradox of sideroblastic anemia is iron overload coexisting with microcytic anemia—iron cannot be properly utilized for heme synthesis despite adequate or excessive body iron stores, making iron supplementation contraindicated in most forms 4, 6. This distinguishes sideroblastic anemia from iron-deficiency anemia, where iron supplementation is therapeutic.
Diagnostic Approach
When microcytic anemia fails to respond to oral iron therapy and presents with high serum ferritin and high transferrin saturation, perform bone marrow examination for ring sideroblasts to confirm the diagnosis 6, 9. The American Society of Hematology recommends genetic testing for disorders like SLC11A2, STEAP3, SLC25A38, ALAS2, or ABCB7 if extreme microcytosis (MCV <70 fL), family history of refractory anemia, or elevated ferritin with abnormal iron studies are present 6, 9.
Treatment Principles
X-Linked Sideroblastic Anemia (ALAS2 Defects)
Initial pyridoxine (vitamin B6) 50-200 mg daily is effective in improving anemia and iron overload in all responsive patients 1, 6. Once response is obtained, lifelong maintenance at 10-100 mg daily prevents neurotoxicity from excessive doses 1, 6. Because iron overload may compromise mitochondrial function and heme biosynthesis, patients should not be considered pyridoxine-refractory until iron stores are normalized 1. Most patients can be treated with phlebotomy for iron overload because anemia is mild, and hemoglobin typically increases rather than decreases after reversal of iron overload 1, 9.
SLC25A38 Defects
Hematopoietic stem cell transplantation (HSCT) is the only curative treatment 1, 6, 9. Symptomatic treatment consists of erythrocyte transfusions and iron chelation 1, 6. HSCT was performed in 8 of 29 patients and resulted in disease-free survival in 4 patients with follow-up <5 years 1.
SLC11A2 (DMT1) Defects
Three patients received oral iron supplementation, which increased hemoglobin and led to transfusion independence in one patient 1, 9. Three patients received erythropoietin (EPO), resulting in hemoglobin increase but not prevention of liver iron loading 1. Erythrocyte transfusions and iron supplementation cause additional liver iron loading, and chelation was not effective in reducing liver iron 1.
Acquired Sideroblastic Anemia (MDS-RS)
Management includes erythroid maturation agents (Luspatercept), red blood cell transfusions for symptomatic patients, and iron chelation therapy for iron overload 2, 3. Inhibitors of transforming growth factor-β superfamily molecules target ineffective erythropoiesis and ameliorate anemia in RARS patients 3.
Monitoring Requirements
Regular monitoring of complete blood count to assess response to therapy is essential 9. Periodic assessment of iron status (ferritin, transferrin saturation) is necessary, and for patients receiving iron supplementation or transfusions, monitor for iron overload 9. MRI of the liver may be considered to detect toxic iron loading early 6. Consider genetic counseling and family screening once a specific genetic disorder is identified 9.