Thalassemia and Hemoglobinopathies

Alpha- and beta-thalassemia pathophysiology, transfusion-dependent management, iron chelation, luspatercept, and curative gene therapy with Zynteglo

Key Points

• Thalassemias are autosomal recessive disorders of globin chain synthesis — alpha-thalassemia from HBA1/HBA2 deletions; beta-thalassemia from HBB point mutations — causing ineffective erythropoiesis, hemolytic anemia, and extramedullary hematopoiesis.
• Transfusion-dependent thalassemia (TDT) requires chronic RBC transfusions every 2–4 weeks to maintain hemoglobin ≥9–10 g/dL and suppress ineffective erythropoiesis — but each unit delivers ~200–250 mg of iron, inevitably causing iron overload.
• Iron chelation therapy with deferasirox (oral), deferoxamine (subcutaneous/IV), or deferiprone (oral) is essential for all chronically transfused patients — liver iron concentration (LIC) by MRI T2* and cardiac MRI T2* are the gold-standard monitoring tools.
• Luspatercept (Reblozyl), an activin receptor ligand trap reducing aberrant TGF-β signaling, is FDA-approved for TDT (2020) and non-transfusion-dependent thalassemia (NTDT, 2023) — it reduces transfusion burden in TDT and raises hemoglobin in NTDT.
• Betibeglogene autotemcel (Zynteglo), an FDA-approved (August 2022) lentiviral gene therapy adding a functional β-globin gene (βA-T87Q) to autologous HSCs, has achieved transfusion independence in the majority of TDT patients without a suitable donor.
• Allogeneic HSCT from a matched sibling donor (MSD) remains curative with event-free survival >90% in young patients — it is the established standard for eligible pediatric TDT patients who have an MSD.

Genetics, Pathophysiology, and Classification

**Alpha-thalassemia:** Caused by deletions (most commonly) or point mutations in the α-globin genes HBA1 and HBA2 on chromosome 16p13.3. Humans carry 4 α-globin gene copies (2 per chromosome). The spectrum of disease correlates with the number of deleted/non-functional genes: - **1 gene deleted (αα/α-):** Silent carrier. No clinical anemia; normal CBC. Common in populations of African, Southeast Asian, and Mediterranean descent. - **2 genes deleted (α-/α- or αα/--):** Alpha-thalassemia trait. Mild hypochromic microcytic anemia (Hgb 10–13 g/dL). Often misdiagnosed as iron deficiency.…

Transfusion Therapy and Iron Overload

**Chronic transfusion in TDT:** The goal is to suppress endogenous (ineffective) erythropoiesis while maintaining adequate oxygen delivery. Standard targets: - Pre-transfusion hemoglobin (nadir): ≥9–10 g/dL - Post-transfusion hemoglobin: ≤14–15 g/dL (to avoid hyperviscosity) - Interval: typically every 2–4 weeks; adjust based on nadir Hgb, transfusion volume, and splenomegaly - Unit selection: leukoreduced, extended antigen-matched (C, E, K at minimum; Fy, Jk, MNS in highly alloimmunized patients) to reduce alloimmunization. Alloimmunization develops in 20–30% of chronically transfused…

Luspatercept — Erythroid Maturation Agent

Luspatercept (Reblozyl) is a first-in-class activin receptor ligand trap — a fusion protein of the extracellular domain of activin receptor type IIB (ActRIIB) linked to an IgG1 Fc domain. It binds and sequesters select TGF-β superfamily ligands (GDF11, GDF8, activin B) that aberrantly accumulate in thalassemia marrow, suppressing late-stage erythroid maturation. By blocking these signals, luspatercept promotes terminal erythropoiesis and increases functional red cell output. **FDA approvals in thalassemia:** *Transfusion-dependent thalassemia (TDT) — FDA-approved August 2020:* BELIEVE trial…

Gene Therapy and Allogeneic Transplantation

**Betibeglogene autotemcel (Zynteglo) — FDA-approved August 2022 for TDT:** Zynteglo is a one-time autologous gene therapy that uses a lentiviral vector (BB305) to insert a modified β-globin gene (βA-T87Q) into the patient's own hematopoietic stem cells. The βA-T87Q globin is specifically engineered to have anti-sickling properties as well as functional oxygen transport, and it can be distinguished from endogenous hemoglobin by HPLC. **Procedure:** Mobilization with G-CSF + plerixafor → leukapheresis → ex vivo transduction (gene insertion) → myeloablative busulfan conditioning → autologous…