Residency · Residency · Hematology Thrombosis

Aplastic Anemia and Bone Marrow Failure Syndromes

Introduction

Aplastic anemia is defined by the presence of pancytopenia in association with a hypocellular bone marrow in the absence of an abnormal infiltrate or significant fibrosis. The bone marrow failure syndromes encompass a broader group of both acquired and inherited conditions characterized by impaired hematopoiesis and resultant cytopenias. The distinction between acquired aplastic anemia, inherited bone marrow failure syndromes (IBMFS), and hypoplastic myelodysplastic syndromes is essential because these entities carry fundamentally different prognoses and require distinct therapeutic approaches, particularly with regard to the choice of conditioning regimen for hematopoietic stem cell transplantation.

Acquired Aplastic Anemia

Epidemiology and Etiology

The incidence of acquired aplastic anemia is approximately 2 to 6 cases per million per year, with a notably higher incidence in East Asian populations compared to Western countries. The majority of cases, approximately 70%, are classified as idiopathic and are presumed to result from a T-cell-mediated autoimmune attack against hematopoietic stem cells. Drug-induced aplastic anemia has been associated with numerous agents including chloramphenicol, nonsteroidal anti-inflammatory drugs, sulfonamides, antiepileptic medications, and gold compounds.

Viral infections represent an important category of triggers. Hepatitis-associated aplastic anemia is a well-recognized entity that follows an episode of seronegative hepatitis (notably not caused by hepatitis A, B, or C viruses) and characteristically responds well to immunosuppressive therapy. Other viral associations include Epstein-Barr virus, cytomegalovirus, parvovirus B19 (which more specifically causes pure red cell aplasia), and HIV. Environmental toxins including benzene, radiation exposure, and pesticides are additional recognized causes. Autoimmune diseases, particularly systemic lupus erythematosus and eosinophilic fasciitis, may be associated. Pregnancy-associated aplastic anemia has been described and may resolve following delivery. An important clinical overlap exists with paroxysmal nocturnal hemoglobinuria (PNH), with approximately 50% of aplastic anemia patients having detectable PNH clones at diagnosis.

Pathophysiology

The pathophysiology of acquired aplastic anemia is fundamentally immunologic. Oligoclonal CD8-positive cytotoxic T lymphocytes, producing interferon-gamma and tumor necrosis factor-alpha, drive hematopoietic stem cell apoptosis through the Fas/FasL pathway. The Th1 cytokine milieu created by elevated levels of IFN-gamma and TNF-alpha directly suppresses hematopoiesis and promotes stem cell destruction. Telomere shortening is present in approximately one-third of acquired aplastic anemia patients, which may indicate an underlying genetic predisposition to telomere biology disorders even in patients who present with apparently acquired disease.

Clonal hematopoiesis is increasingly recognized in aplastic anemia. Somatic mutations in genes such as BCOR and BCORL1 are considered favorable and are associated with good response to immunosuppressive therapy, while mutations in ASXL1, DNMT3A, and RUNX1 may indicate a higher risk of clonal evolution toward myelodysplastic syndromes or acute myeloid leukemia.

Severity Classification (Modified Camitta Criteria)

The Modified Camitta criteria provide a standardized framework for classifying the severity of aplastic anemia. Non-severe aplastic anemia (NSAA) is defined by a hypocellular bone marrow with cytopenias that do not meet the thresholds for severe disease. Severe aplastic anemia (SAA) requires bone marrow cellularity below 25% (or 25 to 50% with fewer than 30% residual hematopoietic cells) plus at least two of the following: absolute neutrophil count below 500 per microliter, platelet count below 20,000 per microliter, or absolute reticulocyte count below 60,000 per microliter. Very severe aplastic anemia (VSAA) meets all criteria for SAA with the additional requirement that the absolute neutrophil count is below 200 per microliter.

SAA requires marrow cellularity criterion plus at least 2 of 3 peripheral blood criteria.

<image>A diagnostic algorithm for aplastic anemia workup. Start with "Pancytopenia with Hypocellular Bone Marrow." First step: exclude other causes (MDS, hairy cell leukemia, mycobacterial infection, myelofibrosis with reticulin stain). Second step: classify severity using Modified Camitta Criteria with specific cutoff values for ANC, platelets, and reticulocytes (show as a table within the flowchart). Third step: workup for etiology - show parallel testing tracks: (1) PNH flow cytometry (FLAER, CD59), (2) Chromosomal breakage analysis (DEB/MMC) for Fanconi anemia, (3) Telomere length measurement for dyskeratosis congenita, (4) Hepatitis panel, (5) Cytogenetics/FISH (monosomy 7, trisomy 8), (6) NGS myeloid panel for clonal hematopoiesis. Fourth step: age-based treatment algorithm branching into <40 years (HLA-matched sibling donor available vs. not) and >40 years (IST first-line). Include treatment boxes showing ATG+CsA vs. allogeneic HSCT. Clinical decision-tree format with clear decision points.</image>

Treatment of Acquired Aplastic Anemia

Treatment Algorithm - Age and Donor-Based

The treatment of acquired aplastic anemia follows an algorithm based on patient age and donor availability. For patients younger than 40 years who have an HLA-matched sibling donor, upfront allogeneic hematopoietic stem cell transplantation is the preferred approach, with overall survival exceeding 90%. For patients younger than 40 without a sibling donor, or for patients aged 40 to 60, first-line immunosuppressive therapy (IST) is recommended, with matched unrelated donor transplantation reserved for those who are refractory to or relapse after IST. For patients older than 60, IST is generally the first-line treatment, as transplant-related toxicity is typically prohibitive, though reduced-intensity transplantation may be considered in select fit individuals.

Immunosuppressive Therapy (IST)

The standard first-line immunosuppressive regimen for acquired aplastic anemia consists of horse antithymocyte globulin (ATG, ATGAM) combined with cyclosporine. Horse ATG is administered at 40 mg/kg/day intravenously for 4 consecutive days and is preferred over rabbit ATG based on the landmark NIH randomized trial that demonstrated a 6-month overall response rate of 68% with horse ATG compared to only 37% with rabbit ATG. Cyclosporine is initiated concurrently at 5 to 6 mg/kg/day orally in divided doses, with trough levels targeted to 200 to 400 ng/mL initially. Cyclosporine should be continued for at least 12 to 24 months, with a very gradual taper thereafter to minimize the risk of relapse. The overall response rate to this combination is 60 to 75% at 6 months, with approximately 30% of patients achieving a complete response.

The addition of eltrombopag, a thrombopoietin receptor agonist, to upfront IST has represented a major advance. In the pivotal NIH randomized study, eltrombopag at 150 mg daily added to horse ATG and cyclosporine beginning on day 1 produced an overall response rate of 94% compared to 66% with IST alone, and a complete response rate of 58% compared to 10%. Eltrombopag is now considered standard as an addition to upfront IST for severe and very severe aplastic anemia. The drug is typically continued for 6 months and then tapered in responding patients. An important concern is the risk of clonal evolution, particularly the development of monosomy 7, which has been observed in approximately 5 to 10% of eltrombopag-treated patients and necessitates regular cytogenetic monitoring.

Serum sickness from ATG manifests 7 to 14 days after infusion as fever, rash, and arthralgias, and is managed with a course of corticosteroids, typically prednisone at 1 mg/kg tapered over 2 weeks.

Allogeneic HSCT

Matched sibling donor (MSD) transplantation is the first-line therapy for young patients under 40 years of age. Conditioning with cyclophosphamide alone or cyclophosphamide plus ATG achieves overall survival exceeding 90% in children and 85 to 90% in young adults, with low rates of graft-versus-host disease when cyclophosphamide-only conditioning is used. Matched unrelated donor (MUD) transplantation is reserved for patients who fail IST and employs conditioning with fludarabine, cyclophosphamide, and ATG, achieving overall survival of 80 to 85% in modern series, though GVHD rates are higher than with MSD transplant.

Haploidentical HSCT is an expanding option made feasible by the use of post-transplant cyclophosphamide (PT-Cy) for GVHD prophylaxis, with recent series reporting overall survival of 70 to 80%. The major advantage of the haploidentical approach is that virtually every patient has an available haploidentical donor, typically a parent, sibling, or child. The Baltimore PT-Cy protocol adapted for aplastic anemia is increasingly used as an alternative to MUD transplantation. An ongoing clinical trial, BMT CTN 2202, is comparing upfront alternative donor HSCT (MUD or haploidentical) against IST plus eltrombopag as first-line therapy, with results that may reshape the treatment algorithm.

Management of IST Failure/Relapse

For patients who fail first-line IST, a second course of immunosuppression using rabbit ATG plus cyclosporine achieves response rates of approximately 30 to 40%. Eltrombopag monotherapy produces responses in approximately 40% of refractory patients. Transplantation from an alternative donor (MUD, haploidentical, or cord blood) is a definitive option for younger patients with refractory disease. Clonal evolution to myelodysplastic syndromes or acute myeloid leukemia is a long-term risk of IST-treated aplastic anemia, occurring in approximately 15% of patients at 10 years, and necessitates ongoing surveillance with annual cytogenetics and myeloid next-generation sequencing panels.

Inherited Bone Marrow Failure Syndromes (IBMFS)

Fanconi Anemia (FA)

Fanconi anemia is the most common inherited bone marrow failure syndrome and is predominantly inherited in an autosomal recessive pattern, with the exception of FANCB, which is X-linked. Twenty-three Fanconi anemia genes have been identified, with FANCA mutations accounting for approximately 65% of cases. These genes encode proteins involved in the interstrand crosslink DNA repair pathway, and their deficiency renders cells hypersensitive to DNA-damaging agents. The diagnostic test of choice is the chromosomal breakage assay using diepoxybutane (DEB) or mitomycin C (MMC), which demonstrates increased chromosomal breaks and characteristic radial chromosome formations in affected cells, with a sensitivity exceeding 90%.

The clinical features of Fanconi anemia include short stature, skeletal anomalies (particularly absent or hypoplastic thumbs and radial ray defects), cafe-au-lait spots, microcephaly, renal anomalies, and hypogonadism. However, it is critical to recognize that 25 to 40% of patients with Fanconi anemia have no physical anomalies and are diagnosed only upon presentation with bone marrow failure or cancer. Hematologic manifestations include progressive pancytopenia, with a median age of onset of approximately 7 years. Thrombocytopenia and macrocytosis are often the earliest hematologic findings. The cancer risk is substantial, with a cumulative incidence of AML or MDS of 30 to 40% by age 40 and squamous cell carcinomas (particularly of the head, neck, esophagus, and vulva) with a cumulative incidence of 15 to 20% by age 40.

HSCT is the only cure for the bone marrow failure component of Fanconi anemia. Critically, conditioning regimens must be reduced-intensity because Fanconi anemia cells are hypersensitive to alkylating agents and radiation; fludarabine-based regimens have become the standard. Lifelong surveillance includes annual bone marrow examination for MDS or AML and annual cancer screening for squamous cell carcinomas.

Dyskeratosis Congenita (DC) / Telomere Biology Disorders

Dyskeratosis congenita and the related telomere biology disorders result from mutations in genes essential for telomere maintenance. The most commonly mutated gene is DKC1 (encoding dyskerin), which follows X-linked inheritance, but autosomal dominant (TERC, TERT, TINF2, RTEL1) and autosomal recessive (CTC1 and others) forms also exist. The classic clinical triad, present in fewer than 50% of patients, consists of dystrophic nails, lacy reticular skin pigmentation, and oral leukoplakia. The clinical spectrum ranges from the severe Hoyeraal-Hreidarsson syndrome (characterized by cerebellar hypoplasia, intrauterine growth restriction, and immunodeficiency) to mild presentations with isolated pulmonary fibrosis or hepatic disease.

Diagnosis is established by telomere length measurement using flow cytometry and fluorescence in situ hybridization (flow-FISH), with values below the first percentile for age being diagnostic. Genetic confirmation follows. Hematologic manifestations include progressive bone marrow failure and an increased risk of MDS and AML. Extrahematologic complications are a major source of morbidity and mortality and include pulmonary fibrosis (the leading cause of death in adults), liver cirrhosis and hepatoportal sclerosis, squamous cell cancers, and vascular disease. Treatment options include androgens (danazol or oxymetholone), which can lengthen telomeres and improve blood counts temporarily, serving as a bridge to transplantation. HSCT is indicated for severe bone marrow failure but must employ reduced-intensity conditioning due to the high risk of pulmonary and hepatic toxicity with standard myeloablative regimens.

Diamond-Blackfan Anemia (DBA)

Diamond-Blackfan anemia is caused by mutations in ribosomal protein genes, with RPS19 being the most commonly affected (approximately 25% of cases). Inheritance is autosomal dominant with variable penetrance. The disease presents as a pure red cell aplasia, with onset in the first year of life in more than 90% of cases. The characteristic laboratory findings include macrocytic anemia, reticulocytopenia, elevated erythrocyte adenosine deaminase (eADA), and elevated fetal hemoglobin. Physical anomalies are present in approximately 50% of patients and include craniofacial abnormalities (such as Pierre Robin sequence), thumb anomalies, cardiac defects, and short stature.

Treatment begins with corticosteroids, with prednisone at 2 mg/kg/day producing an initial response in approximately 80% of patients. Responding patients are maintained on the lowest effective steroid dose to minimize long-term toxicity. Patients who are steroid-refractory or steroid-dependent at unacceptable doses require chronic red cell transfusion therapy with iron chelation. HSCT is curative for the hematologic manifestations. An increased risk of malignancies including AML, osteosarcoma, and colon cancer has been documented.

Shwachman-Diamond Syndrome (SDS)

Shwachman-Diamond syndrome results from biallelic mutations in the SBDS gene and is inherited in an autosomal recessive pattern. The clinical triad consists of exocrine pancreatic insufficiency (which characteristically improves with age), neutropenia (with or without accompanying anemia and thrombocytopenia), and skeletal anomalies, particularly metaphyseal dysostosis. The lifetime risk of MDS and AML is substantial, estimated at 20 to 30%. Treatment includes pancreatic enzyme replacement therapy for the exocrine insufficiency, granulocyte colony-stimulating factor (G-CSF) for symptomatic neutropenia, and HSCT for patients with severe cytopenias or progression to MDS or AML.

Distinguishing AA from Hypoplastic MDS

The distinction between aplastic anemia and hypoplastic MDS is one of the most challenging diagnostic problems in hematology, as approximately 10 to 15% of MDS cases present with a hypocellular bone marrow that can closely mimic aplastic anemia. Features that favor a diagnosis of MDS include morphologic dysplasia in 10% or more of cells in at least one lineage, increased blast percentage, cytogenetic abnormalities (particularly deletions of chromosomes 7 and 5q and complex karyotype), ring sideroblasts, and mutations in TP53 or SF3B1. Features that support a diagnosis of aplastic anemia include the presence of a PNH clone (which supports an immune-mediated process), normal cytogenetics, the absence of dysplasia or excess blasts, and the presence of BCOR or BCORL1 mutations as a form of favorable clonal hematopoiesis.

Key Clinical Pearls

• Horse ATG (not rabbit ATG) is the standard of care for first-line IST in acquired AA based on the NIH randomized trial
• Eltrombopag added to upfront IST dramatically improves response rates and should be considered standard in SAA/VSAA
• ALWAYS screen for inherited BMF syndromes before age 40 (and consider even in older patients): chromosomal breakage (FA), telomere length (DC); failure to diagnose IBMFS before HSCT can lead to fatal conditioning toxicity
• PNH clones are found in ~50% of AA patients at diagnosis; small clones may predict favorable response to IST
• Clonal evolution (MDS/AML) is a long-term risk of AA treated with IST; monitor with annual cytogenetics and consider myeloid NGS panel
• Danazol can improve cytopenias in telomere biology disorders through telomere elongation; may serve as a bridge to HSCT

References

1. Young NS. Aplastic Anemia. N Engl J Med. 2018;379(17):1643-1656.
2. Townsley DM, et al. Eltrombopag added to standard immunosuppression for aplastic anemia. N Engl J Med. 2017;376(16):1540-1550.
3. Scheinberg P, et al. Horse versus rabbit antithymocyte globulin in acquired aplastic anemia. N Engl J Med. 2011;365(5):430-438.
4. Alter BP. Fanconi anemia and the development of leukemia. Best Pract Res Clin Haematol. 2014;27(3-4):214-221.
5. Savage SA, et al. Dyskeratosis congenita and telomere biology disorders. Hematology Am Soc Hematol Educ Program. 2022;2022(1):637-648.

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