Overview, Bone Marrow Failure Syndromes, and Pure Red Cell Aplasia

Hematopoietic stem cells (HSCs) reside in the bone marrow and are defined by two properties: self-renewal and multipotency. As multipotent cells, HSCs give rise to progenitor cells that produce all mature blood elements (leukocytes, erythrocytes, platelets). EPO drives erythrocytes, TPO drives platelets, and G-CSF/M-CSF drive granulocyte-monocyte production.

Severe aplastic anemia requires a hypocellular marrow plus at least two of three blood-count criteria: ANC <500/μL, platelet count <20,000/μL, and corrected reticulocyte count <1% (or absolute reticulocyte count <60,000/μL); very severe disease adds ANC <200/μL. The blood count — not the biopsy appearance alone — is the major prognostic determinant.

Bleeding is the most common early symptom of aplastic anemia — days to weeks of easy bruising, gum or nasal oozing, menorrhagia, and petechiae. Anemia symptoms (fatigue, weakness, dyspnea) are frequent. Infection is an UNcommon presenting symptom, and patients often look remarkably well despite drastically reduced counts; systemic complaints and weight loss should point toward other diagnoses.

Lymphadenopathy and splenomegaly are highly atypical of aplastic anemia and should redirect the differential toward leukemia, lymphoma, or hypersplenism (e.g., cirrhosis). Similarly, a "dry tap" on aspiration suggests marrow fibrosis or infiltration (myelophthisis), not AA.

Benzene (often an occupational solvent exposure) is the prototypical chemical cause of aplastic anemia. Viral causes include EBV, HIV, parvovirus B19, and especially seronegative hepatitis: post-hepatitis marrow failure accounts for ~5% of AA cases — classically a young man recovers from acute hepatitis and 1-2 months later develops very severe pancytopenia (non-A, non-B, non-C, presumed immune-mediated).

Fanconi anemia is an autosomal recessive defect in DNA interstrand cross-link repair (≥23 genes; FANCA most common). Features include short stature, café au lait spots, thumb/radial and genitourinary anomalies, progressive pancytopenia, and increased malignancy risk. Diagnosis is by chromosome breakage studies using diepoxybutane or mitomycin C.

Telomere biology disorders result from mutations in telomerase complex genes (TERT, TERC; X-linked DKC1) or shelterin proteins (e.g., TINF2). The pediatric syndrome carries the triad of oral leukoplakia, dystrophic nails, and reticular hyperpigmentation with early AA. Clues in adults: early hair graying, pulmonary fibrosis or hepatic cirrhosis, abnormal counts since childhood.

An acquired PIG-A mutation in an HSC produces progeny lacking glycosylphosphatidylinositol (GPI)-anchored surface proteins, including the complement-stabilizing proteins CD55 and CD59. Sensitive flow cytometry detects a PNH clone in one-half or more of AA patients at presentation, almost exclusively in immune marrow failure. Screen every AA patient at diagnosis; a clone supports immune pathophysiology and predicts responsiveness to immunosuppressive therapy.

The AA treatment algorithm pivots on age 50 and donor availability: patients <50 years with a suitable HLA-matched sibling donor receive allogeneic HSCT first-line (long-term survival >90% in children, >80% in young good-risk patients); patients >50 or without a suitable donor receive immunosuppressive therapy. HLA typing should be ordered at diagnosis, and transfusions from family members should be avoided in transplant candidates to prevent sensitization.

The first-line immunosuppressive regimen for severe aplastic anemia is horse ATG + cyclosporine + prednisone + eltrombopag, which induces hematologic recovery in 70-80% of patients. Eltrombopag, an oral TPO mimetic taken daily for about 6 months, raises overall (~80%) and complete (~50%) response rates; hepatotoxicity is its key adverse effect.

For first-line IST in aplastic anemia, use horse ATG — not rabbit ATG. Rabbit ATG is substantially less effective, possibly because it also depletes regulatory T cells whose recovery is needed. Serum sickness (fever, rash, arthralgias) typically appears ~10 days after starting ATG; short-course methylprednisolone is co-administered to blunt this reaction.

After immunosuppressive therapy for aplastic anemia, relapse is frequent (often as cyclosporine or eltrombopag is tapered), and clonal evolution — new cytogenetic abnormalities, MDS, or leukemia — occurs in ~10-15% of patients over a decade. Repeat marrow examination is warranted with any unfavorable change in counts. Patients who fail IST can still be salvaged with HSCT.

Pure red cell aplasia (PRCA) is a single-lineage marrow failure characterized by normocytic or macrocytic anemia with decreased reticulocytes and absent or markedly decreased erythrocyte precursors in the bone marrow, while leukocyte and platelet counts remain normal. A bone marrow biopsy showing selective decrease in erythrocyte precursors is required to diagnose idiopathic PRCA, which is commonly T cell-mediated and treated with immunosuppression (prednisone, cyclosporine, cyclophosphamide; azathioprine and ATG also effective).

Parvovirus B19 is directly cytotoxic to erythrocyte precursors, entering erythroid progenitors via the erythrocyte P antigen; the marrow classically shows giant pronormoblasts. Patients with chronic hemolysis (e.g., sickle cell disease) can suffer a dramatic transient aplastic crisis; immunocompromised patients may develop sustained viremia and prolonged anemia that responds to IVIG. Diagnosis in immunodeficient patients requires detection of viral DNA (serologic antibody tests may be falsely negative).

Some patients with PRCA harbor an occult thymoma — image the mediastinum in every case; excision is indicated, although the anemia does not necessarily improve with surgery. Large granular lymphocyte (LGL) leukemia, a T-cell lymphoproliferative disorder, is another important association identifiable by flow cytometry of peripheral blood. Red cell aplasia can also complicate CLL, and anti-EPO neutralizing antibodies from subcutaneous EPO are a rare iatrogenic cause.

Diamond-Blackfan anemia is the identical congenital syndrome of pure red cell aplasia that presents at birth or in early childhood (ribosomal protein gene mutations; often glucocorticoid-responsive) and is a pediatric diagnosis — in adults, PRCA is usually acquired.